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WO2018025674A1 - High-strength steel plate and manufacturing method thereof - Google Patents

High-strength steel plate and manufacturing method thereof Download PDF

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Publication number
WO2018025674A1
WO2018025674A1 PCT/JP2017/026557 JP2017026557W WO2018025674A1 WO 2018025674 A1 WO2018025674 A1 WO 2018025674A1 JP 2017026557 W JP2017026557 W JP 2017026557W WO 2018025674 A1 WO2018025674 A1 WO 2018025674A1
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mass
temperature
retained austenite
cooling
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PCT/JP2017/026557
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French (fr)
Japanese (ja)
Inventor
エライジャ 柿内
村上　俊夫
茂生 大谷
二村　裕一
忠夫 村田
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株式会社神戸製鋼所
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Priority to US16/321,544 priority Critical patent/US20190218640A1/en
Priority to CN201780046030.8A priority patent/CN109477181B/en
Priority to EP17836780.1A priority patent/EP3495522B1/en
Publication of WO2018025674A1 publication Critical patent/WO2018025674A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • This disclosure relates to a high-strength steel sheet that can be used for various applications including automobile parts.
  • Patent Document 1 by heating a slab to 1210 ° C. or higher and controlling hot rolling conditions, fine TiN particles having a size of 0.5 ⁇ m or less are generated, and a particle size of 1 ⁇ m or more serving as a starting point for low-temperature fracture.
  • a high-strength steel sheet that has improved impact resistance properties by suppressing the formation of AlN particles is disclosed.
  • the C content is more than 0.45%, 0.77% or less
  • the Mn content is 0.1% or more, 0.5% or less
  • the Si content is 0.5% or less
  • the addition amount of Cr, Al, N, O is specified.
  • a high-strength steel sheet is disclosed in which the impact resistance is improved by forming a network structure in which 50% or more of the ferrite grain size is in contact with the hard phase.
  • high strength is achieved by adding 3.5 to 10% of Mn so that the amount of retained austenite is 10% or more and the average interval of retained austenite is 1.5 ⁇ m or less to improve the impact resistance.
  • a steel sheet is disclosed.
  • Patent Document 4 discloses a high-strength steel sheet having a tensile strength of 980 to 1180 MPa and showing good deep drawability.
  • the tensile strength is required to be 980 MPa or more.
  • a high yield strength (YS) in addition to a high tensile strength (TS).
  • TS tensile strength
  • an improvement in the thickness reduction rate of the fractured portion at the time of the tensile test is required as an evaluation index that substitutes the fracture characteristics.
  • the joint strength of a spot welded part is also calculated
  • the cross tensile strength of the spot weld is required to be 6 kN or more.
  • the product of TS and total elongation (EL) (TS ⁇ EL) is required to be 20000 MPa% or more. Furthermore, in order to ensure the moldability at the time of component molding, it is also required that the LDR indicating deep drawability is 2.05 or more and the hole expansion ratio ⁇ indicating hole expandability is 20% or more. .
  • Patent Documents 1 to 4 are difficult to satisfy all of these requirements, and a high-strength steel sheet that can satisfy all of these requirements has been demanded.
  • the embodiment of the present invention has been made to meet such a demand, and is the product of tensile strength (TS), yield ratio (YR), (TS) and total elongation (EL) (TS ⁇ EL).
  • TS tensile strength
  • YiR yield ratio
  • TS total elongation
  • EL total elongation
  • LDR hole expansion ratio
  • RA plate thickness reduction rate
  • SW cross tension cross tensile strength
  • Aspect 1 of the present invention C: 0.15% by mass to 0.35% by mass, Total of Si and Al: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.05 mass% or less, S: 0.01% by mass or less, And the balance consists of Fe and inevitable impurities, Steel structure
  • the ferrite fraction is 5% or less,
  • the total fraction of tempered martensite and tempered bainite is 60% or more,
  • the amount of retained austenite is 10% or more,
  • the average size of MA is 1.0 ⁇ m or less,
  • the average size of retained austenite is 1.0 ⁇ m or less, Residual austenite having a size of 1.5 ⁇ m or more is 2% or more of the total retained austenite amount,
  • This is a high-strength steel sheet having a scattering intensity of 1.0 cm ⁇ 1 or less when the q value in X-ray small angle scattering is 1 nm ⁇ 1 .
  • Aspect 2 of the present invention is the high-strength steel sheet according to Aspect 1, wherein the C content is 0.30% by mass or less.
  • Aspect 3 of the present invention is the high-strength steel sheet according to Aspect 1 or 2, wherein the Al content is less than 0.10% by mass.
  • Aspect 4 of the present invention includes C: 0.15% by mass to 0.35% by mass, Si and Al total: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% Including a mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, with the balance being made of Fe and inevitable impurities, Heating the rolled material to a temperature of Ac 3 point or higher to austenite; After the austenitization, cooling is performed at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second between 650 ° C. and 500 ° C., and 10 ° C./second or less within a range of 300 ° C. to 500 ° C.
  • Aspect 5 of the present invention is the manufacturing method according to Aspect 4, wherein the residence is held at a constant temperature within a range of 300 ° C to 500 ° C.
  • Aspect 6 of the present invention is the manufacturing method according to Aspect 4 or 5, wherein the tempering parameter is 11000 to 14000 and the holding time is 1 to 150 seconds.
  • tensile strength (TS), yield ratio (YR), product of (TS) and total elongation (EL) (TS ⁇ EL), LDR, hole expansion ratio ( ⁇ ), tensile test It is possible to provide a high-strength steel sheet and a method for manufacturing the same, in which both the thickness reduction ratio (RA) (impact resistance) of the fractured portion and the cross tensile strength (SW cross tension) of the spot welded portion are high. .
  • RA thickness reduction ratio
  • SW cross tension cross tensile strength
  • FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to an embodiment of the present invention, particularly heat treatment.
  • the steel structure has a ferrite fraction of 5% or less, a total fraction of tempered martensite and tempered bainite: 60% or more, and remains.
  • ⁇ amount 10% or more, average size of MA: 1.0 ⁇ m or less, average size of retained austenite: 1.0 ⁇ m or less, and residual austenite of size 1.5 ⁇ m or more: 2% or more of the total retained austenite amount, X-ray small angle Scattering intensity at a scattering q value of 1 nm ⁇ 1 : 1.0 cm ⁇ 1 or less, so that the product of tensile strength (TS), yield ratio (YR), (TS) and total elongation (EL) ( TS ⁇ EL), LDR, hole expansion rate ( ⁇ ), plate thickness reduction rate (RA) (impact resistance) at the time of tensile test, and cross tensile strength (SW cross tension) of spot welds are all high High strength at level It was found that it is possible to obtain a steel plate.
  • TS tensile strength
  • YR yield ratio
  • TS total elongation
  • EL total elongation
  • LDR hole expansion rate
  • RA plate
  • Ferrite fraction 5% or less
  • ferrite is generally excellent in workability, it has a problem of low strength. As a result, the yield ratio decreases when the amount of ferrite is large. Therefore, the ferrite fraction is set to 5% or less (5% by volume or less).
  • the ferrite fraction is preferably 3% or less, more preferably 1% or less.
  • the ferrite fraction can be obtained by observing with a light microscope and measuring a white region by a point calculation method. That is, the ferrite fraction can be obtained by an area ratio (area%) by such a method. And the value calculated
  • Total fraction of tempered martensite and tempered bainite 60% or more Both high strength and high hole expansibility are achieved by setting the total fraction of tempered martensite and tempered bainite to 60% or more (60% by volume or more). it can.
  • the total fraction of tempered martensite and tempered bainite is preferably 70% or more.
  • the amount of tempered martensite and tempered bainite (total fraction) is measured by SEM observation of the cross-section subjected to nital corrosion, and the fraction of MA (that is, the sum of residual austenite and as-quenched martensite) is measured. It can be obtained by subtracting the above-mentioned ferrite fraction and MA fraction from the entire structure.
  • Residual austenite amount 10% or more Residual austenite causes a TRIP phenomenon that transforms into martensite by processing-induced transformation during processing such as press processing, and can obtain a large elongation. Further, the formed martensite has a high hardness. Therefore, an excellent strength-ductility balance can be obtained.
  • the amount of retained austenite is preferably 15% or more.
  • MA is an abbreviation for martensite-austenite constituent and is a composite (composite structure) of martensite and austenite.
  • the amount of retained austenite can be obtained by calculating the diffraction intensity ratio of ferrite (including tempered martensite and untempered martensite in X-ray diffraction) and austenite by X-ray diffraction.
  • Co-K ⁇ rays can be used as the X-ray source.
  • MA 1.0 ⁇ m or less MA is a hard phase, and the vicinity of the interface between the mother phase and the hard phase acts as a void formation site during deformation.
  • the coarser the MA size the more concentrated the strain on the matrix / hard phase interface, and the more likely the fracture starts from voids formed in the vicinity of the matrix / hard phase interface.
  • the hole expansion ratio ⁇ can be improved by making the MA size, particularly the MA average size as fine as 1.0 ⁇ m or less, and suppressing breakage.
  • the average size of MA is preferably 0.8 ⁇ m or less.
  • the average size of MA is observed by observing three or more fields of view at 3000 times or more by SEM with a SEM, drawing a straight line of 200 ⁇ m or more at an arbitrary position in the photograph, and measuring a section length where the straight line and the MA intersect, It can be obtained by calculating an average value of the intercept lengths.
  • Average size of retained austenite 1.0 ⁇ m or less, and retained austenite having a size of 1.5 ⁇ m or more: 2% or more of the total amount of retained austenite
  • the average size of retained austenite is 1.0 ⁇ m, and the size is 1.5 ⁇ m or more. It has been found that excellent deep drawability can be obtained when the ratio (volume ratio) of the retained austenite to the total retained austenite is 2% or more.
  • inflow stress of the flange portion is smaller than the tensile stress of the vertical wall portion formed at the time of deep drawing, drawing forming will easily proceed and good deep drawing properties will be obtained.
  • compressive stress is strongly applied from the board surface direction and the circumference, and therefore, the flange portion is deformed in a state where isotropic compressive stress is applied.
  • martensitic transformation is accompanied by volume expansion, martensitic transformation is less likely to occur under isotropic compressive stress. Therefore, the work-induced martensitic transformation of the retained austenite at the flange portion is suppressed and work hardening is reduced. As a result, the deep drawability is improved. The larger the size of retained austenite, the greater the effect of suppressing martensitic transformation.
  • the present inventors set the average size of retained austenite to 1.0 ⁇ m, and the ratio (volume ratio) of the amount of retained austenite having a size of 1.5 ⁇ m or more to the total retained austenite amount to 2% or more, It has been found that a high work hardening rate can be maintained during deformation and an excellent deep drawability (LDR) can be obtained.
  • LDR deep drawability
  • the martensite structure formed by the process-induced transformation is hard and acts as a starting point for fracture. Larger martensite structures are more likely to be the origin of destruction.
  • the average size of retained austenite and the ratio of the amount of retained austenite with a size of 1.5 ⁇ m or more to the total austenite amount are created using the EBSD (Electron Back Scatter Diffraction Patterns) method which is a crystal analysis method using SEM. Can be obtained. From the obtained Phase map, the area of each austenite phase (residual austenite) is determined, the circle equivalent diameter (diameter) of each austenite phase is determined from the area, and the average value of the determined diameters is the average size of the retained austenite. To do.
  • EBSD Electro Back Scatter Diffraction Patterns
  • the ratio of the retained austenite of size 1.5 ⁇ m or more to the total austenite can be obtained.
  • the ratio of the retained austenite having a size of 1.5 ⁇ m or more to the total austenite thus obtained is an area ratio, but is equivalent to a volume ratio.
  • X-ray small angle scattering refers to the scattering of X-rays transmitted through a steel sheet by irradiating the steel sheet with X-rays.
  • the size distribution of fine particles for example, cementite particles dispersed in the steel plate
  • the size distribution of cementite particles which are fine particles dispersed in tempered martensite, can be determined by X-ray small angle scattering.
  • the size and fraction of cementite particles can be analyzed using the q value and the scattering intensity.
  • the q value is an index of the size of particles (for example, cementite particles) in the steel sheet. “The q value is 1 nm ⁇ 1 ” corresponds to a cementite particle having a particle diameter of about 1 nm.
  • Scattering intensity is an index of the volume fraction of particles (for example, cementite particles) in a steel plate. The stronger the scattering intensity, the greater the volume fraction of cementite.
  • the scattering intensity at a certain q value semi-quantitatively indicates the volume fraction of cementite particles having a size corresponding to the q value.
  • the scattering intensity at a q value of 1 nm ⁇ 1 is shown semi-quantitatively by the volume fraction of fine cementite particles of about 1 nm. That is, a large scattering intensity at a q value of 1 nm ⁇ 1 indicates that the volume fraction of fine cementite particles of about 1 nm is large.
  • the volume fraction of fine cementite particles of about 1 nm existing in the steel sheet has a predetermined value (scattering intensity of 1.0 cm Is a value equal to or less than ⁇ 1 ).
  • the steel sheet having a “q value of 1 nm ⁇ 1 or less and a scattering intensity of 1.0 cm ⁇ 1 or less” has a low volume fraction of about 1 nm of cementite and thus has excellent collision resistance. It is thought that.
  • the steel sheet according to the embodiment of the present invention is tempered martensite by keeping the volume fraction of fine cementite low, more specifically, by reducing the scattering intensity when the q value is 1 nm ⁇ 1 to 1 cm ⁇ 1 or less. By reducing the fine carbides formed in the lath, the deformability in martensite is enhanced. Thereby, it is suppressed that a steel plate destroys at the time of a collision, and the impact resistance characteristic of a steel plate is improved.
  • X-ray small angle scattering was measured using a Nano-viewer and Mo tube manufactured by RIGAKU.
  • a disk-like sample having a diameter of 3 mm was cut out from a steel plate, and a sample having a thickness of 20 ⁇ m was cut out from around 1 ⁇ 4 of the thickness.
  • Data with a q value of 0.1-10 nm ⁇ 1 were collected. Among them, the absolute intensity was obtained for a q value of 1 nm ⁇ 1 .
  • steel structures other than the above-described ferrite, tempered martensite, tempered bainite retained austenite, and cementite are not particularly defined.
  • pearlite, tempered bainite, untempered martensite, and the like may exist in addition to the steel structure such as ferrite. If the steel structure such as ferrite satisfies the above-described structure condition, the effect of the present invention is exhibited even if pearlite or the like is present in the steel.
  • composition The composition of the high-strength steel sheet according to the embodiment of the present invention will be described below.
  • the basic elements C, Si, Al, Mn, P and S will be mainly described.
  • unit% display of a component composition means the mass% altogether.
  • C 0.15 to 0.35%
  • C is an indispensable element for ensuring high strength-ductility balance (TS x EL balance) by increasing the amount of desired structure, especially residual ⁇ . Therefore, it is necessary to add 0.15% or more.
  • more than 0.35% is not suitable for welding.
  • it is 0.18% or more, More preferably, it is 0.20% or more. Further, it is preferably 0.30% or less.
  • the C content is 0.25% or less, welding can be performed more easily.
  • Si and Al have a function of suppressing precipitation of cementite and leaving residual austenite, respectively.
  • Si and Al in total of 0.5% or more.
  • the total of Si and Al exceeds 3.0%, the deformability of the steel decreases, and TS ⁇ EL decreases.
  • it is 0.7% or more, More preferably, it is 1.0% or more.
  • Al may be added in an amount that functions as a deoxidizing element, that is, less than 0.10% by mass, and is 0 for the purpose of, for example, suppressing the formation of cementite and increasing the amount of retained austenite. A larger amount such as 7% by mass or more may be added.
  • Mn 1.0 to 4.0% Mn suppresses the formation of ferrite. In order to exhibit such an action effectively, it is necessary to add 1.0% or more. However, if it exceeds 4.0%, the MA becomes coarse and the hole expansibility deteriorates. Preferably it is 1.5% or more, More preferably, it is 2.0% or more. Moreover, preferably 3.5. % Or less.
  • P 0.05% or less P is unavoidably present as an impurity element. If P exceeds 0.05%, EL and ⁇ deteriorate. Therefore, the P content is 0.05% or less (including 0%). Preferably, it is 0.03% or less (including 0%).
  • S 0.01% or less S is unavoidably present as an impurity element. If S exceeding 0.01% is present, sulfide inclusions such as MnS are formed, which becomes a starting point of cracking and lowers ⁇ . Therefore, the S content is 0.01% or less (including 0%). Preferably, it is 0.005% or less (including 0%).
  • the balance is iron and inevitable impurities.
  • inevitable impurities mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed.
  • trace elements for example, As, Sb, Sn, etc.
  • P and S it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.
  • the high-strength steel plate according to the embodiment of the present invention has high levels of TS, YR, TS ⁇ EL, LDR, ⁇ , impact resistance characteristics, and SW cross tension. These characteristics of the high-strength steel sheet according to the embodiment of the present invention will be described in detail below.
  • Tensile strength (TS) It has a TS of 980 MPa or more. Preferably, TS is 1180 MPa or more. If TS is less than 980 MPa, excellent fracture characteristics can be obtained more reliably, but this is not preferable because the load resistance at the time of collision becomes low.
  • Yield ratio (YR) It has a yield ratio of 0.75 or more. Thereby, combined with the above-described high tensile strength, high yield strength can be realized, and the final product obtained by processing such as deep drawing can be used under high stress. Preferably, it has a yield ratio of 0.80 or more.
  • TS ⁇ EL Product of TS and total elongation (EL) (TS x EL) TS ⁇ EL is 20000 MPa% or more.
  • TS ⁇ EL a high level of strength-ductility balance having both high strength and high ductility can be obtained.
  • TS ⁇ EL is 23000 MPa% or more.
  • LDR Deep drawability
  • the high-strength steel plate according to the embodiment of the present invention has an LDR of 2.05 or more, preferably 2.10 or more, and has excellent deep drawability.
  • the hole expansion ratio ⁇ is obtained in accordance with Japan Iron and Steel Federation standard JFS T1001.
  • d is measured and obtained from the following equation.
  • ⁇ (%) ⁇ (d ⁇ d 0 ) / d 0 ⁇ ⁇ 100
  • the high-strength steel plate according to the embodiment of the present invention has a hole expansion ratio ⁇ of 20% or more, preferably 30% or more. Thereby, excellent workability such as press formability can be obtained.
  • Cross tensile strength of spot welding was evaluated in accordance with JIS Z 3137.
  • the spot welding was performed by stacking two 1.4 mm steel plates.
  • Spot welding was performed with a dome radius type electrode with a pressurizing force of 4 kN and a current increased by 0.5 kA in the range from 6 kA to 12 kA, and the current value (minimum current value) at which dust was generated during welding was examined.
  • a cross joint spot-welded at a current 0.5 kA lower than the minimum current value was used as a sample for measuring the cross tensile strength.
  • a cross tensile strength of 6 kN or more was defined as “good”.
  • the cross tensile strength is preferably 8 kN or more, more preferably 10 kN or more.
  • the cross tensile strength is 6 kN or more, when automobile parts and the like are manufactured from a steel plate, a part having high joint strength during welding can be obtained.
  • FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to an embodiment of the present invention, particularly heat treatment.
  • the rolled material to be heat-treated is usually produced by hot rolling followed by cold rolling.
  • the present invention is not limited to this, and either one of hot rolling and cold rolling may be performed.
  • the conditions for hot rolling and cold rolling are not particularly limited.
  • the rolled material is austenitized by heating the rolled material to a temperature of Ac 3 points or higher and heating it for a predetermined heating time. To do.
  • the heating time at this heating temperature is, for example, 1 to 1800 seconds.
  • the upper limit of the heating temperature is preferably Ac 3 points or more and Ac 3 points + 100 ° C. or less. This is because coarsening of crystal grains can be suppressed by setting the temperature to Ac 3 points + 100 ° C. or lower.
  • the heating temperature is more preferably Ac 3 points + 10 ° C. or higher, Ac 3 points + 90 ° C. or lower, and further preferably Ac 3 points + 20 ° C. or higher, Ac 3 points + 80 ° C.
  • heating at the time of austenitization shown by [1] in FIG. 1 may be performed at an arbitrary heating rate, a preferable average heating rate is 1 ° C./second or more, more preferably 20 ° C./second.
  • Cooling and retention in the temperature range of 300 ° C. to 500 ° C. After the above austenite formation, cooling is performed and, as shown in [5] of FIG. 1, 10 ° C./second within the temperature range of 300 ° C. to 500 ° C. It is allowed to stay for 10 seconds or more and less than 300 seconds at the following cooling rate.
  • the cooling is performed at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second at least between 650 ° C. and 500 ° C. This is because the formation of ferrite during cooling is suppressed by setting the average cooling rate to 15 ° C./second or more.
  • cooling rate into less than 200 degrees C / sec As a preferable example of such cooling, as shown in [3] in FIG. 1, a relatively low average cooling of 0.1 ° C./second or more and 10 ° C./second or less is performed up to a rapid cooling start temperature of 650 ° C. or more. Cooling at a rate, and as shown in [4] of FIG. 1, cooling is performed at an average cooling rate of 20 ° C./second or more and less than 200 ° C./second from a rapid cooling start temperature to a residence start temperature of 500 ° C. or less. be able to.
  • the sample is retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 ° C. to 500 ° C. That is, in the temperature range of 300 ° C. to 500 ° C., the cooling rate is set to 10 ° C./second or less for 10 seconds or more.
  • the state where the cooling rate is 10 ° C./second or less includes the case where the cooling rate is maintained at a substantially constant temperature (that is, the cooling rate is 0 ° C./second) as shown in [5] in FIG. Due to this residence, bainite is partially formed. And since bainite has a lower carbon solid solubility limit than austenite, it expels carbon beyond the solid solubility limit. As a result, an austenite region enriched with carbon is formed around bainite. This region becomes slightly coarse retained austenite after cooling and reheating described later. By forming this slightly coarse retained austenite, the deep drawability can be enhanced as described above.
  • the retention temperature is higher than 500 ° C., the carbon concentration region becomes too large, and not only retained austenite but also MA becomes coarse, so that the hole expansion rate decreases.
  • the retention temperature is lower than 300 ° C., the carbon concentration region is small, the amount of coarse retained austenite is insufficient, and the deep drawability deteriorates.
  • the residence time is shorter than 10 seconds, the area of the carbon-enriched region is reduced, the amount of coarse retained austenite is insufficient, and the deep drawability is deteriorated.
  • the residence time is 300 seconds or more, the carbon enrichment region becomes too large, and not only retained austenite but also MA becomes coarse, so the hole expansion rate decreases. Further, if the cooling rate during the residence is higher than 10 ° C./second, sufficient bainite transformation does not occur, and therefore a sufficient carbon enriched region is not formed, and the amount of coarse retained austenite is insufficient.
  • a preferable cooling rate is 15 degreeC / degrees C or more, and a preferable cooling stop temperature is 120 degreeC or more and 280 degrees C or less. A more preferable cooling rate is 20 ° C./s or more, and a more preferable cooling stop temperature is 140 ° C. or more and 260 ° C. or less.
  • a preferable holding time in the case of holding can be 1 to 600 seconds. Even if the holding time is increased, there is almost no influence on the characteristics, but if the holding time exceeds 600 seconds, the productivity is lowered.
  • the tempering parameter P represented by the following formula (1) is 10000 or more and 14500 or less and the holding time is 1 to 150 seconds.
  • the tempering parameter P of the steel plate of this embodiment is represented by the following formula (1).
  • P T (K) ⁇ (20 + log (t / 3600) (1)
  • T is a tempering temperature (K)
  • t is a holding time (seconds).
  • Cementite particles present in martensite are likely to be the starting point of collision fracture, which causes a decrease in collision resistance. Therefore, at the time of reheating, it is desirable to perform a reheating treatment that promotes carbon diffusion from martensite to austenite while suppressing the precipitation of carbide (cementite) in the martensite lath. Therefore, it is effective to perform rapid heating and heat treatment at a high temperature in a short time.
  • the tempering parameter P is necessary to control the tempering parameter P as a factor of the combination of temperature and time within a certain range. If the tempering parameter P is less than 10,000, carbon diffusion from martensite to austenite does not occur sufficiently, the austenite becomes unstable, and the amount of retained austenite cannot be ensured, resulting in insufficient TS ⁇ EL balance. On the other hand, if the tempering parameter P is larger than 14500, the formation of carbides cannot be prevented even in a short time treatment, the amount of retained austenite cannot be secured, and the TS ⁇ EL balance deteriorates.
  • the amount of carbide in the martensite lath can be determined from the scattering intensity of X-ray small angle scattering.
  • the reheating temperature is lower than 300 ° C., the diffusion of carbon is insufficient and a sufficient amount of retained austenite cannot be obtained, resulting in a decrease in TS ⁇ EL.
  • the reheating temperature is higher than 500 ° C., the retained austenite is decomposed into cementite and ferrite and the retained austenite is insufficient, and the characteristics cannot be ensured.
  • the holding is not performed or the holding time is shorter than 1 second, there is a possibility that the carbon diffusion is insufficient. For this reason, it is preferable to hold for 1 second or more at the reheating temperature. If the holding time is longer than 150 seconds, similarly, carbon may be precipitated as cementite. For this reason, the holding time is preferably 150 seconds or less.
  • a preferable reheating temperature is 320 to 480 ° C., and a more preferable reheating temperature is 340 to 460 ° C.
  • the tempering parameter P is 10500 to 14500, and a preferable holding time at this time is 1 to 150 seconds.
  • a more preferable tempering parameter P is 11000 to 14000, and a preferable holding time at this time is 1 to 100 seconds, and more preferably 1 to 60 seconds.
  • the high-strength steel plate according to the embodiment of the present invention can be obtained.
  • sample no. 1, 4, 7 and 26 are samples which were not retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 to 500 ° C. in the process corresponding to [5] in FIG.
  • sample no. Reference numerals 1 and 26 are samples that are rapidly cooled to 200 ° C. after starting rapid cooling at 700 ° C.
  • Sample No. 9 is a sample (steps corresponding to [6] to [8] in FIG. 1) that is held at that temperature after being cooled to the reheating temperature instead of being cooled to a cooling stop temperature between 100 ° C. and less than 300 ° C. Sample skipped).
  • the reheating corresponding to [8] was performed by an electric heating method.
  • Tables 1 to 4 numerical values marked with an asterisk (*) indicate that they are out of the scope of the embodiment of the present invention.
  • Sample No. Reference numerals 13, 15, 18, 21, and 28 to 36 are examples that satisfy all the requirements (composition, manufacturing conditions, and steel structure) defined in the embodiment of the present invention. All of these samples have a tensile strength (TS) of 980 MPa or more, a yield ratio (YR) of 0.75 or more, TS ⁇ EL of 20000 MPa% or more, an LDR of 2.05 or more, a hole expansion ratio of 20% or more ( ⁇ ), SW cross tension of 6 kN or more, and R5 tensile thickness reduction ratio (RA) of 50% or more.
  • sample no. No. 1 was not retained in the temperature range of 300 ° C. to 500 ° C. after the austenite formation, so the amount of retained austenite with a size of 1.5 ⁇ m or more was not sufficient, and as a result, sufficient deep drawability could not be obtained. It was. [7] Since the holding time was as long as 300 seconds, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
  • Sample No. No. 5 had a high [5] holding temperature of 550 ° C., and therefore the average MA size was excessive. As a result, a sufficient hole expansion ratio and a sufficient deep drawability could not be obtained.
  • Sample No. No. 16 had a long [9] holding time of 300 seconds, so carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
  • Sample No. No. 22 had a small amount of C, an insufficient amount of retained austenite, and an insufficient amount of retained austenite having a size of 1.5 ⁇ m or more. As a result, sufficient TS ⁇ EL and deep drawability could not be obtained. Sample No. No. 23 had a large amount of Mn and a short amount of retained austenite, and as a result, sufficient TS ⁇ EL was not obtained.
  • Sample No. No. 24 has a small amount of Mn, an excessive amount of ferrite, and a total amount of tempered martensite and tempered bainite is insufficient. As a result, sufficient tensile strength and yield ratio were not obtained.
  • Sample No. No. 26 had an excessive amount of C and was not retained in the temperature range of 300 ° C. to 500 ° C. after austenitization, so that sufficient SW cross tensile strength could not be obtained.
  • the steel sheet satisfying the composition and steel structure defined in the embodiment of the present invention has a tensile strength (TS), a yield ratio (YR), a product of (TS) and total elongation (EL) (TS ⁇ EL). ), LDR, hole expansion rate ( ⁇ ), plate thickness reduction rate (RA) of the fractured portion during the tensile test, and cross tensile strength of the spot welded portion were all confirmed to be at high levels. Moreover, according to the manufacturing method which concerns on embodiment of this invention, it has confirmed that the steel plate which satisfy

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Abstract

This high-strength steel plate contains C: 0.15-0.35 mass%, total of Si and Al: 0.5-3.0 mass%, Mn: 1.0-4.0 mass%, P: 0.05 mass% or less and S: 0.01 mass% or less, the remainder being Fe and unavoidable impurities. In the steel composition, the ferrite fraction is 5% or less, the total fraction of tempered martensite and tempered bainite is 60% or higher, the amount of retained austenite is 10% or greater, the average size of MA is 1.0 μm or less, the average size of the retained austenite is 1.0 μm or less, the retained austenite with a size of greater than or equal to 1.5 μm is at least 2% of the total amount of retained austenite, and the scattering intensity in small-angle X-ray scattering with a q-value of 1 nm-1 is less than or equal to 1.0 cm-1.

Description

高強度鋼板およびその製造方法High strength steel plate and manufacturing method thereof
　本開示は、自動車部品をはじめとする各種の用途に使用可能な高強度鋼板に関する。 This disclosure relates to a high-strength steel sheet that can be used for various applications including automobile parts.
　自動車用部品等に供される鋼板は、軽量化と衝突安全性とを共に実現するために、強度の向上と耐衝撃特性の向上とを両立することが求められている。
　例えば特許文献１は、スラブを１２１０℃以上に加熱し、熱間圧延条件を制御することで、０．５μｍ以下の微細なＴｉＮ粒子を生成させて、低温破壊の起点となる粒径１μｍ以上のＡｌＮ粒子の生成を抑制することにより、耐衝撃特性の向上を図った高強度鋼板が開示されている。 Steel sheets used for automobile parts and the like are required to achieve both strength improvement and impact resistance improvement in order to achieve both weight reduction and collision safety.
For example, in Patent Document 1, by heating a slab to 1210 ° C. or higher and controlling hot rolling conditions, fine TiN particles having a size of 0.5 μm or less are generated, and a particle size of 1 μm or more serving as a starting point for low-temperature fracture. A high-strength steel sheet that has improved impact resistance properties by suppressing the formation of AlN particles is disclosed.
　特許文献２には、C量を0.45%超、0.77%以下、Mn量を0.1%以上、0.5%以下、Si量を0.5%以下および、Cr, Al, N, O添加量を規定しつつ、フェライト粒径の50%以上を硬質相と接する網目状組織とすることで耐衝突特性の向上を図った高強度鋼板が開示されている。
　特許文献３には、3.5～10%のMnを添加することで残留オーステナイトの量を10%以上、残留オーステナイトの平均間隔を1.5μm以下とすることで、耐衝突特性の改善を図った高強度鋼板が開示されている。 In Patent Document 2, the C content is more than 0.45%, 0.77% or less, the Mn content is 0.1% or more, 0.5% or less, the Si content is 0.5% or less, and the addition amount of Cr, Al, N, O is specified. A high-strength steel sheet is disclosed in which the impact resistance is improved by forming a network structure in which 50% or more of the ferrite grain size is in contact with the hard phase.
In Patent Document 3, high strength is achieved by adding 3.5 to 10% of Mn so that the amount of retained austenite is 10% or more and the average interval of retained austenite is 1.5 μm or less to improve the impact resistance. A steel sheet is disclosed.
　特許文献４は９８０～１１８０ＭＰａの引張強さを有し、かつ良好な深絞り性を示す高強度鋼板が開示している。 Patent Document 4 discloses a high-strength steel sheet having a tensile strength of 980 to 1180 MPa and showing good deep drawability.
特許第５２４０４２１号明細書Japanese Patent No. 5240421 特開２０１５－１０５３８４号公報JP2015-105384A 特開２０１２－２５１２３９号公報JP 2012-251239 A 特開２００９－２０３５４８号公報JP 2009-203548 A
　自動車用部品に使用される鋼板のさらなる軽量化を実現するために、より薄くしつつ、十分な強度と耐衝撃特性を備えている必要がある。つまり、より高い引張強度と優れた衝撃特性を有する鋼板が求められている。
　また、自動車用部品をはじめとする各種用途において、高い引張強度と衝撃特性を有するだけでなく、さらに優れた強度－延性バランス、高い降伏比、優れた深絞り性および優れた穴広げ率を有することが求められている。
　引張強度、強度－延性バランス、降伏比、深絞り特性および穴広げ率それぞれについて、具体的には、以下のことが求められている。 In order to realize further weight reduction of steel plates used for automobile parts, it is necessary to have sufficient strength and impact resistance characteristics while being thinner. That is, a steel sheet having higher tensile strength and excellent impact characteristics is required.
In addition, it has not only high tensile strength and impact properties, but also excellent strength-ductility balance, high yield ratio, excellent deep drawability, and excellent hole expansion ratio in various applications including automotive parts. It is demanded.
Specifically, the following are required for each of tensile strength, strength-ductility balance, yield ratio, deep drawing characteristics, and hole expansion ratio.
　引張強度については、９８０ＭＰａ以上であることが求められている。使用中に負荷できる応力を高くするためには、高い引張強度（ＴＳ）に加えて高い降伏強度（ＹＳ）を有する必要がある。また、衝突安全性等を確保する観点から、鋼板の降伏強度を高めること、また衝突した際に強度特性を安定して発現させるために変形時に破断を抑制する特性も必要である。このため、具体的には０．７５以上の降伏比（ＹＲ＝ＹＳ／ＴＳ）と共に、破壊特性を代替する評価指標として引張試験時の破断部の板厚減少率向上が求められている。また、自動車用鋼板の基本性能としてスポット溶接部の継手強度も求められる。具体的には、スポット溶接部の十字引張強度は６ｋＮ以上であることが求められている。 The tensile strength is required to be 980 MPa or more. In order to increase the stress that can be applied during use, it is necessary to have a high yield strength (YS) in addition to a high tensile strength (TS). In addition, from the viewpoint of ensuring collision safety and the like, it is also necessary to increase the yield strength of the steel sheet, and to suppress fracture at the time of deformation in order to stably develop the strength characteristics when a collision occurs. For this reason, specifically, with a yield ratio (YR = YS / TS) of 0.75 or more, an improvement in the thickness reduction rate of the fractured portion at the time of the tensile test is required as an evaluation index that substitutes the fracture characteristics. Moreover, the joint strength of a spot welded part is also calculated | required as basic performance of the steel plate for motor vehicles. Specifically, the cross tensile strength of the spot weld is required to be 6 kN or more.
　強度－延性バランスについては、ＴＳと全伸び（ＥＬ）との積（ＴＳ×ＥＬ）が２００００ＭＰａ％以上であることが求められている。さらに部品成形時の成形性を確保するために、深絞り性を示すＬＤＲが２．０５以上であること、および穴広げ性を示す穴広げ率λが２０％以上であることも求められている。 Regarding the strength-ductility balance, the product of TS and total elongation (EL) (TS × EL) is required to be 20000 MPa% or more. Furthermore, in order to ensure the moldability at the time of component molding, it is also required that the LDR indicating deep drawability is 2.05 or more and the hole expansion ratio λ indicating hole expandability is 20% or more. .
　しかし、特許文献１～４が開示する高強度鋼板では、これらの要求全てを満足することは困難であり、これらの要求全てを満足できる高強度鋼板が求められていた。 However, the high-strength steel sheets disclosed in Patent Documents 1 to 4 are difficult to satisfy all of these requirements, and a high-strength steel sheet that can satisfy all of these requirements has been demanded.
　本発明の実施形態はこのような要求に応えるためになされたものであって、引張強度（ＴＳ）、降伏比（ＹＲ）、（ＴＳ）と全伸び（ＥＬ）との積（ＴＳ×ＥＬ）、ＬＤＲ、穴広げ率（λ）、引張試験時の破断部の板厚減少率（ＲＡ）およびスポット溶接部の十字引張強度（ＳＷ十字引張）が何れも高いレベルにある高強度鋼板およびその製造方法を提供することを目的とする。 The embodiment of the present invention has been made to meet such a demand, and is the product of tensile strength (TS), yield ratio (YR), (TS) and total elongation (EL) (TS × EL). , LDR, hole expansion ratio (λ), plate thickness reduction rate (RA) at fracture during tensile test, and cross tensile strength (SW cross tension) at spot welds are all at high levels and their manufacture It aims to provide a method.
　本発明の態様１は、
　Ｃ　：０．１５質量％～０．３５質量％、
　ＳｉとＡｌの合計：０．５質量％～３．０質量％、
　Ｍｎ：１．０質量％～４．０質量％、
　Ｐ　：０．０５質量％以下、
　Ｓ　：０．０１質量％以下、
を含み、残部がＦｅおよび不可避不純物からなり、
　鋼組織が、
　　フェライト分率が５％以下であり、
　　焼戻しマルテンサイトと焼戻しベイナイトの合計分率が６０％以上であり、
　　残留オーステナイト量が１０％以上であり、
　　ＭＡの平均サイズが１．０μｍ以下であり、
　　残留オーステナイトの平均サイズが１．０μｍ以下であり、
　　サイズ１．５μｍ以上の残留オーステナイトが全残留オーステナイト量の２％以上であり、
　　Ｘ線小角散乱でのｑ値が１ｎｍ－１での散乱強度が１．０ｃｍ－１以下である高強度鋼板である。 Aspect 1 of the present invention
C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 0.5% by mass to 3.0% by mass,
Mn: 1.0% by mass to 4.0% by mass,
P: 0.05 mass% or less,
S: 0.01% by mass or less,
And the balance consists of Fe and inevitable impurities,
Steel structure
The ferrite fraction is 5% or less,
The total fraction of tempered martensite and tempered bainite is 60% or more,
The amount of retained austenite is 10% or more,
The average size of MA is 1.0 μm or less,
The average size of retained austenite is 1.0 μm or less,
Residual austenite having a size of 1.5 μm or more is 2% or more of the total retained austenite amount,
This is a high-strength steel sheet having a scattering intensity of 1.0 cm −1 or less when the q value in X-ray small angle scattering is 1 nm −1 .
　本発明の態様２は、Ｃ量が０．３０質量％以下である態様１に記載の高強度鋼板である。 Aspect 2 of the present invention is the high-strength steel sheet according to Aspect 1, wherein the C content is 0.30% by mass or less.
　本発明の態様３は、Ａｌ量が０．１０質量％未満である態様１または２に記載の高強度鋼板である。 Aspect 3 of the present invention is the high-strength steel sheet according to Aspect 1 or 2, wherein the Al content is less than 0.10% by mass.
　本発明の態様４は、Ｃ：０．１５質量％～０．３５質量％、ＳｉとＡｌの合計：０．５質量％～３．０質量％、Ｍｎ：１．０質量％～４．０質量％、Ｐ：０．０５質量％以下、Ｓ：０．０１質量％以下、を含み、残部がＦｅおよび不可避不純物からなる圧延材を用意することと、
　前記圧延材をＡｃ３点以上の温度に加熱しオーステナイト化することと、
　前記オーステナイト化後、６５０℃～５００℃の間を平均冷却速度１５℃／秒以上、２００℃／秒未満で冷却し、３００℃～５００℃の範囲内で１０℃／秒以下の冷却速度で１０秒以上、３００秒未満滞留させることと、
　前記滞留の後、３００℃以上の温度から１００℃以上、３００℃未満の間の冷却停止温度まで１０℃／秒以上の平均冷却速度で冷却することと、
　前記冷却停止温度から３００℃～５００℃範囲にある再加熱温度まで３０℃／秒以上の平均加熱速度で加熱することと、
　前記再加熱温度において、式（１）で規定される焼戻しパラメータPが１００００～１４５００かつ保持時間１～３００秒を満たすように保持することと、
　前記保持の後、前記再加熱温度から２００℃まで１０℃／秒以上の平均冷却速度で冷却すること、
を含む、高強度鋼板の製造方法である。
　　P=T×(20+log(t/3600))・・・（１）
　ここで、T: 温度(K)、t: 時間(秒)である。 Aspect 4 of the present invention includes C: 0.15% by mass to 0.35% by mass, Si and Al total: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% Including a mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, with the balance being made of Fe and inevitable impurities,
Heating the rolled material to a temperature of Ac 3 point or higher to austenite;
After the austenitization, cooling is performed at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second between 650 ° C. and 500 ° C., and 10 ° C./second or less within a range of 300 ° C. to 500 ° C. For more than a second and less than 300 seconds,
After the residence, cooling at a mean cooling rate of 10 ° C / second or more from a temperature of 300 ° C or more to a cooling stop temperature of 100 ° C or more and less than 300 ° C;
Heating at an average heating rate of 30 ° C./second or more from the cooling stop temperature to a reheating temperature in the range of 300 ° C. to 500 ° C .;
Holding at the reheating temperature so that the tempering parameter P defined by the formula (1) satisfies 10000 to 14500 and a holding time of 1 to 300 seconds;
After the holding, cooling from the reheating temperature to 200 ° C. at an average cooling rate of 10 ° C./second or more,
Is a method for producing a high-strength steel sheet.
P = T × (20 + log (t / 3600)) (1)
Here, T: temperature (K), t: time (second).
　本発明の態様５は、前記滞留が３００℃～５００℃の範囲内の一定温度で保持することを含む態様４に記載の製造方法である。 Aspect 5 of the present invention is the manufacturing method according to Aspect 4, wherein the residence is held at a constant temperature within a range of 300 ° C to 500 ° C.
　本発明の態様６は、前記焼戻しパラメータが１１０００～１４０００、保持時間が１～１５０秒である態様４または５に記載の製造方法である。 Aspect 6 of the present invention is the manufacturing method according to Aspect 4 or 5, wherein the tempering parameter is 11000 to 14000 and the holding time is 1 to 150 seconds.
　本発明の実施形態によれば、引張強度（ＴＳ）、降伏比（ＹＲ）、（ＴＳ）と全伸び（ＥＬ）との積（ＴＳ×ＥＬ）、ＬＤＲ、穴広げ率（λ）、引張試験時の破断部の板厚減少率（ＲＡ）（耐衝撃特性）およびスポット溶接部の十字引張強度（ＳＷ十字引張）が何れも高いレベルにある高強度鋼板およびその製造方法を提供することができる。 According to the embodiment of the present invention, tensile strength (TS), yield ratio (YR), product of (TS) and total elongation (EL) (TS × EL), LDR, hole expansion ratio (λ), tensile test It is possible to provide a high-strength steel sheet and a method for manufacturing the same, in which both the thickness reduction ratio (RA) (impact resistance) of the fractured portion and the cross tensile strength (SW cross tension) of the spot welded portion are high. .
図１は本発明の実施形態に係る高強度鋼板の製造方法、とりわけ熱処理を説明するダイアグラムである。FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to an embodiment of the present invention, particularly heat treatment.
　本発明者らは鋭意検討した結果、所定の成分を有する鋼において、鋼組織（金属組織）を、フェライト分率：５％以下、焼戻しマルテンサイトと焼戻しベイナイトの合計分率：６０％以上、残留γ量：１０％以上、ＭＡの平均サイズ：１．０μｍ以下、残留オーステナイトの平均サイズ：１．０μｍ以下、およびサイズ１．５μｍ以上の残留オーステナイト：全残留オーステナイト量の２％以上、Ｘ線小角散乱でのｑ値が１ｎｍ－１での散乱強度：１．０ｃｍ－１以下とすることで、引張強度（ＴＳ）、降伏比（ＹＲ）、（ＴＳ）と全伸び（ＥＬ）との積（ＴＳ×ＥＬ）、ＬＤＲ、穴広げ率（λ）、引張試験時の破断部の板厚減少率（ＲＡ）（耐衝撃特性）およびスポット溶接部の十字引張強度（ＳＷ十字引張）が何れも高いレベルにある高強度鋼板を得ることができることを見いだしたのである。 As a result of intensive studies, the present inventors have found that in steel having a predetermined component, the steel structure (metal structure) has a ferrite fraction of 5% or less, a total fraction of tempered martensite and tempered bainite: 60% or more, and remains. γ amount: 10% or more, average size of MA: 1.0 μm or less, average size of retained austenite: 1.0 μm or less, and residual austenite of size 1.5 μm or more: 2% or more of the total retained austenite amount, X-ray small angle Scattering intensity at a scattering q value of 1 nm −1 : 1.0 cm −1 or less, so that the product of tensile strength (TS), yield ratio (YR), (TS) and total elongation (EL) ( TS × EL), LDR, hole expansion rate (λ), plate thickness reduction rate (RA) (impact resistance) at the time of tensile test, and cross tensile strength (SW cross tension) of spot welds are all high High strength at level It was found that it is possible to obtain a steel plate.
１．鋼組織
　以下に本発明の実施形態に係る高強度鋼板の鋼組織の詳細を説明する。
　以下の鋼組織の説明では、そのような組織を有することにより各種の特性を向上できるメカニズムについて説明している場合がある。これらは本発明者らが現時点で得られている知見により考えたメカニズムであるが、本発明の技術的範囲を限定するものではないことに留意されたい。 1. Steel Structure Details of the steel structure of the high-strength steel sheet according to the embodiment of the present invention will be described below.
In the following description of the steel structure, a mechanism that can improve various properties by having such a structure may be described. It should be noted that these are the mechanisms considered by the present inventors based on the knowledge obtained at the present time, but do not limit the technical scope of the present invention.
（１）フェライト分率：５％以下
　フェライトは、一般的に加工性に優れるものの、強度が低いという問題を有する。その結果、フェライト量が多いと降伏比が低下する。このため、フェライト分率を５％以下（５体積％以下）とした。
　フェライト分率は好ましくは３％以下であり、さらに好ましくは１％以下である。
　フェライト分率は光学顕微鏡で観察し、白い領域を点算法で測定することにより求めることができる。すなわち、このような方法により、フェライト分率を面積比（面積％）で求めることができる。そして、面積比で求めた値をそのまま体積比（体積％）の値として用いてよい。 (1) Ferrite fraction: 5% or less Although ferrite is generally excellent in workability, it has a problem of low strength. As a result, the yield ratio decreases when the amount of ferrite is large. Therefore, the ferrite fraction is set to 5% or less (5% by volume or less).
The ferrite fraction is preferably 3% or less, more preferably 1% or less.
The ferrite fraction can be obtained by observing with a light microscope and measuring a white region by a point calculation method. That is, the ferrite fraction can be obtained by an area ratio (area%) by such a method. And the value calculated | required by area ratio may be used as a value of volume ratio (volume%) as it is.
（２）焼戻しマルテンサイトと焼戻しベイナイトの合計分率：６０％以上
　焼戻しマルテンサイトと焼戻しベイナイトの合計分率を６０％以上（６０体積％以上）とすることで高強度と高い穴広げ性を両立できる。焼戻しマルテンサイトと焼戻しベイナイトの合計分率は好ましくは７０％以上である。
　焼戻しマルテンサイトおよび焼戻しベイナイト量（合計分率）は、ナイタール腐食を行った断面のＳＥＭ観察を行い、ＭＡ（すなわち、残留オーステナイトと焼入れたままのマルテンサイトの合計）の分率を測定し、鋼組織全体から上述のフェライト分率とＭＡ分率を引くことにより求めることができる。 (2) Total fraction of tempered martensite and tempered bainite: 60% or more Both high strength and high hole expansibility are achieved by setting the total fraction of tempered martensite and tempered bainite to 60% or more (60% by volume or more). it can. The total fraction of tempered martensite and tempered bainite is preferably 70% or more.
The amount of tempered martensite and tempered bainite (total fraction) is measured by SEM observation of the cross-section subjected to nital corrosion, and the fraction of MA (that is, the sum of residual austenite and as-quenched martensite) is measured. It can be obtained by subtracting the above-mentioned ferrite fraction and MA fraction from the entire structure.
（３）残留オーステナイト量：１０％以上
　残留オーステナイトは、プレス加工等の加工中に加工誘起変態により、マルテサイトに変態するＴＲＩＰ現象を生じ、大きな伸びを得ることができる。また、形成されるマルテンサイトは高い硬度を有する。このため、優れた強度－延性バランスを得ることができる。残留オーステナイト量を１０％以上（１０体積％以上）とすることでＴＳ×ＥＬが２００００ＭＰａ％以上と優れた強度－延性バランスを実現できる。
　残留オーステナイト量は好ましくは１５％以上である。 (3) Residual austenite amount: 10% or more Residual austenite causes a TRIP phenomenon that transforms into martensite by processing-induced transformation during processing such as press processing, and can obtain a large elongation. Further, the formed martensite has a high hardness. Therefore, an excellent strength-ductility balance can be obtained. By setting the amount of retained austenite to 10% or more (10% by volume or more), it is possible to realize an excellent strength-ductility balance with TS × EL of 20000 MPa% or more.
The amount of retained austenite is preferably 15% or more.
　本発明の実施形態に係る高強度鋼板では、残留オーステナイトの多くは、ＭＡの形態で存在する。ＭＡとは、martensite-austenite constituentの略であり、マルテンサイトとオーステナイトの複合体（複合組織）である。
　残留オーステナイト量は、Ｘ線回折によりフェライト（Ｘ線回折では焼戻しマルテンサイトおよび未焼戻しのマルテンサイトを含む）とオーステナイトの回折強度比を求めて算出することにより得ることができる。Ｘ線源としてはＣｏ－Ｋα線を用いることができる。 In the high-strength steel plate according to the embodiment of the present invention, most of retained austenite exists in the form of MA. MA is an abbreviation for martensite-austenite constituent and is a composite (composite structure) of martensite and austenite.
The amount of retained austenite can be obtained by calculating the diffraction intensity ratio of ferrite (including tempered martensite and untempered martensite in X-ray diffraction) and austenite by X-ray diffraction. Co-Kα rays can be used as the X-ray source.
（４）ＭＡの平均サイズ：１．０μｍ以下
　ＭＡは硬質相であり、変形時に母相／硬質相界面近傍がボイド形成サイトとして働く。ＭＡサイズが粗大になるほど、母相／硬質相界面への歪集中が起こり、母相／硬質相界面近傍に形成されたボイドを起点とした破壊を生じ易くなる。
　このため、ＭＡサイズ、とりわけＭＡ平均サイズを１．０μｍ以下と微細にし、破壊を抑制することで穴広げ率λを向上させることができる。
　ＭＡの平均サイズは好ましくは０．８μｍ以下である。 (4) Average size of MA: 1.0 μm or less MA is a hard phase, and the vicinity of the interface between the mother phase and the hard phase acts as a void formation site during deformation. The coarser the MA size, the more concentrated the strain on the matrix / hard phase interface, and the more likely the fracture starts from voids formed in the vicinity of the matrix / hard phase interface.
For this reason, the hole expansion ratio λ can be improved by making the MA size, particularly the MA average size as fine as 1.0 μm or less, and suppressing breakage.
The average size of MA is preferably 0.8 μm or less.
　ＭＡの平均サイズは、ナイタール腐食した断面をＳＥＭにより３０００倍以上で３視野以上観察し、写真中の任意の位置に合計２００μｍ以上の直線を引き、その直線とＭＡが交わる切片長を測定し、それら切片長の平均値を算出することで求めることができる。 The average size of MA is observed by observing three or more fields of view at 3000 times or more by SEM with a SEM, drawing a straight line of 200 μm or more at an arbitrary position in the photograph, and measuring a section length where the straight line and the MA intersect, It can be obtained by calculating an average value of the intercept lengths.
（５）残留オーステナイトの平均サイズ：１．０μｍ以下、およびサイズ１．５μｍ以上の残留オーステナイト：全残留オーステナイト量の２％以上
　残留オーステナイトの平均サイズを１．０μｍとし、かつサイズ１．５μｍ以上の残留オーステナイトの全残留オーステナイトに占める比率（体積比）を２％以上とすることで、優れた深絞り性が得られることを見いだした。 (5) Average size of retained austenite: 1.0 μm or less, and retained austenite having a size of 1.5 μm or more: 2% or more of the total amount of retained austenite The average size of retained austenite is 1.0 μm, and the size is 1.5 μm or more. It has been found that excellent deep drawability can be obtained when the ratio (volume ratio) of the retained austenite to the total retained austenite is 2% or more.
　深絞り成形時に形成されるたて壁部の引張応力に対してフランジ部の流入応力の方が小さいと、絞り成形が容易に進行することになり、良好な深絞り性が得られる。フランジ部の変形挙動は盤面方向、円周から圧縮応力が強くかかるため、等方的な圧縮応力が付与された状態で変形することとなる。一方、マルテンサイト変態は体積膨張を伴うため、等方的な圧縮応力下ではマルテンサイト変態は起こりにくくなる。よって、フランジ部での残留オーステナイトの加工誘起マルテンサイト変態が抑制されて加工硬化が小さくなる。
　この結果、深絞り性が改善される。残留オーステナイトのサイズが大きいほど、マルテンサイト変態を抑制する効果が大きく発現する。 If the inflow stress of the flange portion is smaller than the tensile stress of the vertical wall portion formed at the time of deep drawing, drawing forming will easily proceed and good deep drawing properties will be obtained. As the deformation behavior of the flange portion, compressive stress is strongly applied from the board surface direction and the circumference, and therefore, the flange portion is deformed in a state where isotropic compressive stress is applied. On the other hand, since martensitic transformation is accompanied by volume expansion, martensitic transformation is less likely to occur under isotropic compressive stress. Therefore, the work-induced martensitic transformation of the retained austenite at the flange portion is suppressed and work hardening is reduced.
As a result, the deep drawability is improved. The larger the size of retained austenite, the greater the effect of suppressing martensitic transformation.
　また、深絞り成形により形成されるたて壁部の引張応力を高めるためには、変形中に高い加工硬化率を持続させることが必要である。比較的低い応力下で容易に加工誘起変態する不安定な残留オーステナイトと高い応力下でないと加工誘起変態を起こさない安定な残留オーステナイトとを混在させて、広い応力範囲に亘って加工誘起変態を起こさせることで変形中に高い加工硬化率を持続させることができる。そのために粗大で不安定な残留オーステナイトと微細で安定な残留オーステナイトとをそれぞれ所定量含むような鋼組織を得ることを検討した。そして、本発明者らは、残留オーステナイトの平均サイズを１．０μｍとし、かつサイズ１．５μｍ以上の残留オーステナイト量の全残留オーステナイト量に占める比率（体積比）を２％以上とすることで、変形中に高い加工硬化率を持続させ、優れた深絞り性（ＬＤＲ）を得ることができることを見いだした。 Also, in order to increase the tensile stress of the vertical wall formed by deep drawing, it is necessary to maintain a high work hardening rate during deformation. Unstable residual austenite that easily undergoes processing-induced transformation under relatively low stress and stable residual austenite that does not cause processing-induced transformation unless under high stress cause processing-induced transformation over a wide stress range. By doing so, a high work hardening rate can be maintained during deformation. For this purpose, we studied to obtain a steel structure containing a predetermined amount of coarse and unstable retained austenite and fine and stable retained austenite. And, the present inventors set the average size of retained austenite to 1.0 μm, and the ratio (volume ratio) of the amount of retained austenite having a size of 1.5 μm or more to the total retained austenite amount to 2% or more, It has been found that a high work hardening rate can be maintained during deformation and an excellent deep drawability (LDR) can be obtained.
　また、上述のように、残留オーステナイトが加工誘起変態する際にＴＲＩＰ現象を生じ大きな伸びを得ることができる。一方で、加工誘起変態により形成されたマルテンサイト組織は硬く破壊の起点として作用する。より大きなマルテンサイト組織ほど破壊の起点となりやすい。残留オーステナイトの平均サイズを１．０μｍ以下として、加工誘起変態により形成されるマルテンサイトの大きさを小さくすることで破壊を抑制する効果も得ることができる。 Also, as described above, when the retained austenite undergoes processing-induced transformation, a TRIP phenomenon occurs and a large elongation can be obtained. On the other hand, the martensite structure formed by the process-induced transformation is hard and acts as a starting point for fracture. Larger martensite structures are more likely to be the origin of destruction. By setting the average size of retained austenite to 1.0 μm or less and reducing the size of martensite formed by processing-induced transformation, an effect of suppressing fracture can be obtained.
　残留オーステナイトの平均サイズおよびサイズ１．５μｍ以上の残留オーステナイト量の全オーステナイト量に占める比率は、ＳＥＭを用いた結晶解析手法であるＥＢＳＤ（Electron Back Scatter Diffraction Patterns）法を用いてＰｈａｓｅマップを作成することにより求めることができる。得られたＰｈａｓｅマップから、個々のオーステナイト相（残留オーステナイト）の面積を求め、その面積から個々のオーステナイト相の円相当径（直径）を求め、求めた直径の平均値を残留オーステナイトの平均サイズとする。また、円相当径が１．５μｍ以上のオーステナイト相の面積を積算し、オーステナイト相の総面積に対する比率を求めることにより、サイズ１．５μｍ以上の残留オーステナイトの全オーステナイトに占める比率を得ることができる。なお、このようにして求めたサイズ１．５μｍ以上の残留オーステナイトの全オーステナイトに占める比率は面積比であるが、体積比と等価である。 The average size of retained austenite and the ratio of the amount of retained austenite with a size of 1.5 μm or more to the total austenite amount are created using the EBSD (Electron Back Scatter Diffraction Patterns) method which is a crystal analysis method using SEM. Can be obtained. From the obtained Phase map, the area of each austenite phase (residual austenite) is determined, the circle equivalent diameter (diameter) of each austenite phase is determined from the area, and the average value of the determined diameters is the average size of the retained austenite. To do. Further, by integrating the area of the austenite phase having an equivalent circle diameter of 1.5 μm or more and determining the ratio to the total area of the austenite phase, the ratio of the retained austenite of size 1.5 μm or more to the total austenite can be obtained. . The ratio of the retained austenite having a size of 1.5 μm or more to the total austenite thus obtained is an area ratio, but is equivalent to a volume ratio.
（６）Ｘ線小角散乱のｑ値が１ｎｍ－１での散乱強度が１．０ｃｍ－１以下
　Ｘ線小角散乱とは、Ｘ線を鋼板に照射して、鋼板を透過したＸ線の散乱を測定することにより、鋼板中に含まれる微細粒子（例えば、鋼板中に分散したセメンタイト粒子）のサイズ分布を求めることができる。本発明の実施形態に係る鋼板では、Ｘ線小角散乱により、焼戻しマルテンサイト中に分散した微細粒子であるセメンタイト粒子のサイズ分布を求めることができる。具体的には、Ｘ線小角散乱では、ｑ値と散乱強度を用いてセメンタイトの粒子のサイズとその分率を解析することができる。
　ｑ値は鋼板中の粒子（例えばセメンタイト粒子）のサイズの指標である。「ｑ値が１ｎｍ－１」とは、粒子径が約１ｎｍのセメンタイト粒子に対応する。散乱強度は、鋼板中の粒子（例えばセメンタイト粒子）の体積分率の指標である。散乱強度が強いほどセメンタイトの体積分率が大きいことを示している。 (6) Scattering intensity when the q-value of X-ray small angle scattering is 1 nm −1 is 1.0 cm −1 or less X-ray small angle scattering refers to the scattering of X-rays transmitted through a steel sheet by irradiating the steel sheet with X-rays. By measuring, the size distribution of fine particles (for example, cementite particles dispersed in the steel plate) contained in the steel plate can be obtained. In the steel sheet according to the embodiment of the present invention, the size distribution of cementite particles, which are fine particles dispersed in tempered martensite, can be determined by X-ray small angle scattering. Specifically, in X-ray small angle scattering, the size and fraction of cementite particles can be analyzed using the q value and the scattering intensity.
The q value is an index of the size of particles (for example, cementite particles) in the steel sheet. “The q value is 1 nm −1 ” corresponds to a cementite particle having a particle diameter of about 1 nm. Scattering intensity is an index of the volume fraction of particles (for example, cementite particles) in a steel plate. The stronger the scattering intensity, the greater the volume fraction of cementite.
　あるｑ値における散乱強度は、そのｑ値に対応するサイズのセメンタイト粒子の体積分率を半定量的に示す。例えば、ｑ値が１ｎｍ－１における散乱強度は、約１ｎｍの微細なセメンタイト粒子の体積分率半定量的に示す。
　すなわち、ｑ値が１ｎｍ－１における散乱強度が大きいことは、約１ｎｍの微細なセメンタイト粒子の体積分率が大きいことを示している。「ｑ値が１ｎｍ－１での散乱強度１．０ｃｍ－１以下」の鋼板では、その鋼板中に存在する約１ｎｍの微細なセメンタイト粒子の体積分率が、所定の値（散乱強度１．０ｃｍ－１に相当する値）以下であることを意味している。以下に説明するように、「ｑ値が１ｎｍ－１での散乱強度１．０ｃｍ－１以下」の鋼板は、約１ｎｍのセメンタイトの体積分率が低く抑えられているので、耐衝突特性に優れていると考えられる。 The scattering intensity at a certain q value semi-quantitatively indicates the volume fraction of cementite particles having a size corresponding to the q value. For example, the scattering intensity at a q value of 1 nm −1 is shown semi-quantitatively by the volume fraction of fine cementite particles of about 1 nm.
That is, a large scattering intensity at a q value of 1 nm −1 indicates that the volume fraction of fine cementite particles of about 1 nm is large. In a steel sheet having a “q value of 1 nm −1 or less and a scattering intensity of 1.0 cm −1 or less”, the volume fraction of fine cementite particles of about 1 nm existing in the steel sheet has a predetermined value (scattering intensity of 1.0 cm Is a value equal to or less than −1 ). As will be described below, the steel sheet having a “q value of 1 nm −1 or less and a scattering intensity of 1.0 cm −1 or less” has a low volume fraction of about 1 nm of cementite and thus has excellent collision resistance. It is thought that.
　残留γを含む高延性鋼においては、炭素が残留オーステナイトに集まっている状態で、理想的にはセメンタイトが存在しないことが好ましい。鋼材中に分散している粒径１ｎｍ程度の微細なセメンタイトは、転位の移動を妨げて鋼材の変形能を低下させ得る。そのため、粒径約１ｎｍのセメンタイトの体積分率が多い鋼材では、変形時の破壊が促進されて、耐衝突特性が低下し得る。
　本発明の実施形態に係る鋼板は、微細なセメンタイトの体積分率を低く抑えること、より具体的には、ｑ値が１ｎｍ－１の散乱強度を１ｃｍ－１以下にすることにより、焼戻しマルテンサイトのラス内に形成される微細な炭化物を減少させて、マルテンサイト中の変形能を高めている。これにより、鋼板が衝突時に破壊するのを抑制して、鋼板の耐衝突特性を向上させる。 In the high ductility steel containing residual γ, it is ideal that no cementite exists ideally in a state where carbon is gathered in residual austenite. Fine cementite having a particle diameter of about 1 nm dispersed in the steel material can hinder the movement of dislocations and reduce the deformability of the steel material. Therefore, in a steel material having a particle size of about 1 nm and a large volume fraction of cementite, the fracture at the time of deformation is promoted, and the collision resistance can be lowered.
The steel sheet according to the embodiment of the present invention is tempered martensite by keeping the volume fraction of fine cementite low, more specifically, by reducing the scattering intensity when the q value is 1 nm −1 to 1 cm −1 or less. By reducing the fine carbides formed in the lath, the deformability in martensite is enhanced. Thereby, it is suppressed that a steel plate destroys at the time of a collision, and the impact resistance characteristic of a steel plate is improved.
　Ｘ線小角散乱の測定は、RIGAKU社製 Nano-viewer、Mo管球を用いて測定した。試料は鋼板からΦ３ｍｍのディスク状サンプルを切り出し、板厚１／４付近から２０μｍ厚さのサンプルを削り出して用いた。ｑ値、０．１～１０ｎｍ－１のデータを採取した。そのうち、ｑ値が１ｎｍ－１について絶対強度を求めた。 X-ray small angle scattering was measured using a Nano-viewer and Mo tube manufactured by RIGAKU. As a sample, a disk-like sample having a diameter of 3 mm was cut out from a steel plate, and a sample having a thickness of 20 μm was cut out from around ¼ of the thickness. Data with a q value of 0.1-10 nm −1 were collected. Among them, the absolute intensity was obtained for a q value of 1 nm −1 .
（７）その他の鋼組織：
　本明細書においては、前記したフェライト、焼戻しマルテンサイト、焼戻しベイナイト残留オーステナイトおよびセメンタイト以外の鋼組織は特に規定していない。しかしながら、それらフェライト等の鋼組織以外にも、パーライト、焼き戻されていないベイナイトおよび焼き戻されていないマルテンサイトなどが存在することがある。フェライト等の鋼組織が、前述した組織条件を満たしていれば、鋼中にパーライト等が存在しても、本発明の効果は発揮される。 (7) Other steel structures:
In the present specification, steel structures other than the above-described ferrite, tempered martensite, tempered bainite retained austenite, and cementite are not particularly defined. However, pearlite, tempered bainite, untempered martensite, and the like may exist in addition to the steel structure such as ferrite. If the steel structure such as ferrite satisfies the above-described structure condition, the effect of the present invention is exhibited even if pearlite or the like is present in the steel.
２．組成
　以下に本発明の実施形態に係る高強度鋼板の組成について説明する。主に、基本となる元素、Ｃ、Ｓｉ、Ａｌ、Ｍｎ、ＰおよびＳについて説明する。
　なお、成分組成について単位の％表示は、すべて質量％を意味する。 2. Composition The composition of the high-strength steel sheet according to the embodiment of the present invention will be described below. The basic elements C, Si, Al, Mn, P and S will be mainly described.
In addition, unit% display of a component composition means the mass% altogether.
（１）Ｃ：０．１５～０．３５％
　Ｃは所望の組織、特に残留γの量を増加させることで、高い強度－延性バランス（ＴＳ×ＥＬバランス）等の特性を確保するために必須の元素であり、このような作用を有効に発揮させるためには０．１５％以上添加する必要がある。ただし、０．３５％超は溶接に適さない。好ましくは０．１８％以上、さらに好ましくは０．２０％以上である。また、好ましくは０．３０％以下である。Ｃ量が０．２５％以下だとより容易に溶接することができる。 (1) C: 0.15 to 0.35%
C is an indispensable element for ensuring high strength-ductility balance (TS x EL balance) by increasing the amount of desired structure, especially residual γ. Therefore, it is necessary to add 0.15% or more. However, more than 0.35% is not suitable for welding. Preferably it is 0.18% or more, More preferably, it is 0.20% or more. Further, it is preferably 0.30% or less. When the C content is 0.25% or less, welding can be performed more easily.
（２）ＳｉとＡｌの合計：０．５～３．０％
　ＳｉとＡｌは、それぞれ、セメンタイトの析出を抑制し、残留オーステナイトを残存させる働きを有する。このような作用を有効に発揮させるためにはＳｉとＡｌを合計で０．５％以上添加する必要がある。ただし、ＳｉとＡｌの合計が３．０％を超えると鋼の変形能が低下して、ＴＳ×ＥＬが低下する。好ましくは０．７％以上、さらに好ましくは１．０％以上である。また、好ましくは２．５％以下である。
　なお、Ａｌについては、脱酸元素として機能する程度の添加量、すなわち０．１０質量％未満であってよく、また、例えばセメンタイトの形成を抑制し、残留オーステナイト量を増加させる目的等ために０．７質量％以上のようなより多くの量を添加してもよい。 (2) Total of Si and Al: 0.5 to 3.0%
Si and Al have a function of suppressing precipitation of cementite and leaving residual austenite, respectively. In order to exhibit such an action effectively, it is necessary to add Si and Al in total of 0.5% or more. However, if the total of Si and Al exceeds 3.0%, the deformability of the steel decreases, and TS × EL decreases. Preferably it is 0.7% or more, More preferably, it is 1.0% or more. Moreover, it is preferably 2.5% or less.
Al may be added in an amount that functions as a deoxidizing element, that is, less than 0.10% by mass, and is 0 for the purpose of, for example, suppressing the formation of cementite and increasing the amount of retained austenite. A larger amount such as 7% by mass or more may be added.
（３）Ｍｎ：１．０～４．０％
　Ｍｎはフェライトの形成を抑制する。このような作用を有効に発揮させるためには１．０％以上添加する必要がある。ただし、４．０％を超えるとＭＡが粗大になり穴拡げ性が劣化する。好ましくは１．５％以上、さらに好ましくは２．０％以上である。また、好ましくは３．５．％以下である。 (3) Mn: 1.0 to 4.0%
Mn suppresses the formation of ferrite. In order to exhibit such an action effectively, it is necessary to add 1.0% or more. However, if it exceeds 4.0%, the MA becomes coarse and the hole expansibility deteriorates. Preferably it is 1.5% or more, More preferably, it is 2.0% or more. Moreover, preferably 3.5. % Or less.
（４）Ｐ：０．０５％以下
　Ｐは不純物元素として不可避的に存在する。０．０５％を超えたＰが存在するとＥＬおよびλが劣化する。このため、Ｐの含有量は０．０５％以下（０％を含む）とする。好ましくは、０．０３％以下（０％を含む）である。 (4) P: 0.05% or less P is unavoidably present as an impurity element. If P exceeds 0.05%, EL and λ deteriorate. Therefore, the P content is 0.05% or less (including 0%). Preferably, it is 0.03% or less (including 0%).
（５）Ｓ：０．０１％以下
　Ｓは不純物元素として不可避的に存在する。０．０１％を超えたＳが存在するとＭｎＳ等の硫化物系介在物を形成し、割れの起点となってλを低下させる。このため、Ｓの含有量は０．０１％以下（０％を含む）とする。好ましくは、０．００５％以下（０％を含む）である。 (5) S: 0.01% or less S is unavoidably present as an impurity element. If S exceeding 0.01% is present, sulfide inclusions such as MnS are formed, which becomes a starting point of cracking and lowers λ. Therefore, the S content is 0.01% or less (including 0%). Preferably, it is 0.005% or less (including 0%).
（６）残部
　好ましい１つの実施形態では、残部は、鉄および不可避不純物である。不可避不純物としては、原料、資材、製造設備等の状況によって持ち込まれる微量元素（例えば、Ａｓ、Ｓｂ、Ｓｎなど）の混入が許容される。なお、例えば、ＰおよびＳのように、通常、含有量が少ないほど好ましく、従って不可避不純物であるが、その組成範囲について上記のように別途規定している元素がある。このため、本明細書において、残部を構成する「不可避不純物」という場合は、別途その組成範囲が規定されている元素を除いた概念である。 (6) Balance In one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. In addition, for example, like P and S, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.
　しかし、この実施形態に限定されるものではない。本発明の実施形態に係る高強度鋼板の特性を維持できる限り、任意のその他の元素を更に含んでよい。 However, it is not limited to this embodiment. Any other element may be further included as long as the characteristics of the high-strength steel sheet according to the embodiment of the present invention can be maintained.
３．特性
　上述のように本発明の実施形態に係る高強度鋼板は、ＴＳ、ＹＲ、ＴＳ×ＥＬ、ＬＤＲ、λ、耐衝突特性およびＳＷ十字引張が何れも高いレベルにある。本発明の実施形態に係る高強度鋼板のこれらの特性について以下に詳述する。 3. Characteristics As described above, the high-strength steel plate according to the embodiment of the present invention has high levels of TS, YR, TS × EL, LDR, λ, impact resistance characteristics, and SW cross tension. These characteristics of the high-strength steel sheet according to the embodiment of the present invention will be described in detail below.
（１）引張強度（ＴＳ）
　９８０ＭＰａ以上のＴＳを有する。好ましくは、ＴＳは１１８０ＭＰａ以上である。ＴＳが９８０ＭＰａ未満だとより確実に優れた破断特性が得られるが、衝突時の耐荷重が低くなるために好ましくないためである。 (1) Tensile strength (TS)
It has a TS of 980 MPa or more. Preferably, TS is 1180 MPa or more. If TS is less than 980 MPa, excellent fracture characteristics can be obtained more reliably, but this is not preferable because the load resistance at the time of collision becomes low.
（２）降伏比（ＹＲ）
　０．７５以上の降伏比を有する。これにより上述の高い引張強度と相まって高い降伏強度を実現でき、深絞り加工等の加工により得た最終製品を高い応力下で使用することができる。好ましくは、０．８０以上の降伏比を有する。 (2) Yield ratio (YR)
It has a yield ratio of 0.75 or more. Thereby, combined with the above-described high tensile strength, high yield strength can be realized, and the final product obtained by processing such as deep drawing can be used under high stress. Preferably, it has a yield ratio of 0.80 or more.
（３）ＴＳと全伸び（ＥＬ）との積（ＴＳ×ＥＬ）
　ＴＳ×ＥＬが２００００ＭＰａ％以上である。２００００ＭＰａ％以上のＴＳ×ＥＬを有することで、高い強度と高い延性とを同時に有する、高いレベルの強度－延性バランスを得ることができる。好ましくは、ＴＳ×ＥＬは２３０００ＭＰａ％以上である。 (3) Product of TS and total elongation (EL) (TS x EL)
TS × EL is 20000 MPa% or more. By having TS × EL of 20000 MPa% or more, a high level of strength-ductility balance having both high strength and high ductility can be obtained. Preferably, TS × EL is 23000 MPa% or more.
（４）深絞り性（ＬＤＲ）
　ＬＤＲは深絞り性の評価に用いられている指標である。円筒絞り成形において、得られる円筒の直径をｄとし、１回の深絞り加工で破断を生じずに円筒を得ることができる円盤状の鋼板（ブランク）の最大直径をＤとし、Ｄ／ｄをＬＤＲ(Limiting Drawing Ratio)という。より詳細には、板厚１．４ｍｍで各種径を有する円盤状の試料を、パンチ径５０ｍｍ、パンチ角半径６ｍｍ、ダイ径５５．２ｍｍ、ダイ角半径８ｍｍの金型で円筒深絞りを行い、破断することなく絞り抜けた円盤状試料の試料直径のうち最大の試料直径（最大直径Ｄ）を求めることによりＬＤＲを求めることができる。 (4) Deep drawability (LDR)
LDR is an index used for evaluation of deep drawability. In cylindrical drawing, the diameter of the obtained cylinder is d, and the maximum diameter of a disk-shaped steel plate (blank) that can be obtained without breaking by one deep drawing is D, and D / d is It is called LDR (Limiting Drawing Ratio). More specifically, a cylindrical sample having a plate thickness of 1.4 mm and various diameters is subjected to cylindrical deep drawing with a die having a punch diameter of 50 mm, a punch angle radius of 6 mm, a die diameter of 55.2 mm, and a die angle radius of 8 mm. The LDR can be obtained by obtaining the maximum sample diameter (maximum diameter D) of the sample diameters of the disk-like sample drawn out without breaking.
　本発明の実施形態に係る高強度鋼板は、ＬＤＲが２．０５以上であり、好ましくは２．１０以上であり、優れた深絞り性を有している。 The high-strength steel plate according to the embodiment of the present invention has an LDR of 2.05 or more, preferably 2.10 or more, and has excellent deep drawability.
（５）穴広げ率（λ）
　穴広げ率λは、日本鉄鋼連盟規格　ＪＦＳ　Ｔ１００１に従って求める。試験片に直径ｄ０（ｄ０＝１０ｍｍ）の打ち抜き穴を空け、先端角度が６０°のポンチをこの打ち抜き穴に押し込み、発生した亀裂が試験片の板厚を貫通した時点の打ち抜き穴の直径ｄを測定し、下記の式より求める。
　　λ（％）＝｛（ｄ－ｄ０）／ｄ０｝×１００ (5) Hole expansion rate (λ)
The hole expansion ratio λ is obtained in accordance with Japan Iron and Steel Federation standard JFS T1001. A punched hole having a diameter d 0 (d 0 = 10 mm) is formed in the test piece, a punch having a tip angle of 60 ° is pushed into the punched hole, and the diameter of the punched hole when the generated crack penetrates the plate thickness of the test piece. d is measured and obtained from the following equation.
λ (%) = {(d−d 0 ) / d 0 } × 100
　本発明の実施形態に係る高強度鋼板は、穴広げ率λが２０％以上、好ましくは３０％以上である。これによりプレス成形性等の優れた加工性を得ることができる。 The high-strength steel plate according to the embodiment of the present invention has a hole expansion ratio λ of 20% or more, preferably 30% or more. Thereby, excellent workability such as press formability can be obtained.
（６）引張試験での板厚減少率（Ｒ５引張板厚減少率）
　５号試験片に半径５ｍｍの円弧形の切欠きを設けた試験片を用い、引張試験の変形速度を１０ｍｍ／ｍｉｎとして試験を行い、試料を破断させた。その後、破面観察を行い、破面の板厚方向の厚さｔ１を元の板厚ｔ０で割った値（ｔ１／ｔ０）を、板厚減少率とした。
　この試験での板厚減少率は、５０％以上、好ましくは５２％以上、より好ましくは５５％以上である。これにより、衝突時に大きく変形しても破断しにくくなるので、優れた耐衝撃特性を有する鋼板を得ることができる。 (6) Plate thickness reduction rate in the tensile test (R5 tensile plate thickness reduction rate)
Using a test piece in which an arc-shaped notch with a radius of 5 mm was provided on a No. 5 test piece, the test was performed at a deformation rate of 10 mm / min in the tensile test, and the sample was broken. Thereafter, the fracture surface was observed, and a value (t 1 / t 0 ) obtained by dividing the thickness t 1 of the fracture surface in the thickness direction by the original thickness t 0 was defined as the thickness reduction rate.
The plate thickness reduction rate in this test is 50% or more, preferably 52% or more, more preferably 55% or more. Thereby, even if it deform | transforms greatly at the time of a collision, since it becomes difficult to fracture | rupture, the steel plate which has the outstanding impact resistance characteristic can be obtained.
（７）スポット溶接の十字引張強度
　スポット溶接の十字引張強度はＪＩＳ　Ｚ　３１３７に則って評価した。スポット溶接の条件は１．４ｍｍの鋼板を２枚重ねたものを用いた。ドームラジアス型の電極で加圧力４ｋＮ、電流を６ｋＡから１２ｋＡまでの範囲で０．５ｋＡずつ増加してスポット溶接を行い、溶接時にちりが発生する電流値（最低電流値）を調べた。その最低電流値より０．５ｋＡ低い電流でスポット溶接した十字継ぎ手を、十字引張強度の測定用の試料とした。十字引張強度が６ｋＮ以上を「良好」とした。なお、十字引張強度は、好ましくは８ｋＮ以上、さらに好ましくは１０ｋＮ以上である。
　十字引張強度が６ｋＮ以上であると、鋼板から自動車用部品等を製造したとき、溶接時の接合強度の高い部品を得ることができる。 (7) Cross tensile strength of spot welding The cross tensile strength of spot welding was evaluated in accordance with JIS Z 3137. The spot welding was performed by stacking two 1.4 mm steel plates. Spot welding was performed with a dome radius type electrode with a pressurizing force of 4 kN and a current increased by 0.5 kA in the range from 6 kA to 12 kA, and the current value (minimum current value) at which dust was generated during welding was examined. A cross joint spot-welded at a current 0.5 kA lower than the minimum current value was used as a sample for measuring the cross tensile strength. A cross tensile strength of 6 kN or more was defined as “good”. The cross tensile strength is preferably 8 kN or more, more preferably 10 kN or more.
When the cross tensile strength is 6 kN or more, when automobile parts and the like are manufactured from a steel plate, a part having high joint strength during welding can be obtained.
４．製造方法
　次に本発明の実施形態に係る高強度鋼板の製造方法について説明する。
　本発明者らは、所定の組成を有する圧延材に詳細を後述する熱処理（マルチステップのオーステンパー処理）を行うことにより、上述の所望の鋼組織を有し、その結果、上述の所望の特性を有する高強度鋼板を得ることを見いだしたのである。
　以下にその詳細を説明する。 4). Manufacturing Method Next, a manufacturing method of the high strength steel plate according to the embodiment of the present invention will be described.
The inventors of the present invention have the above-mentioned desired steel structure by performing heat treatment (multi-step austemper treatment), which will be described in detail later, on the rolled material having a predetermined composition, and as a result, the above-mentioned desired characteristics. It was found that a high-strength steel plate having
Details will be described below.
　図１は本発明の実施形態に係る高強度鋼板の製造方法、とりわけ熱処理を説明するダイアグラムである。
　熱処理を施す圧延材は、通常、熱間圧延後、冷間圧延を行って製造する。しかし、これに限定されるものでなく熱間圧延および冷間圧延のいずれか一方を行って製造してもよい。また、熱間圧延および冷間圧延の条件は特に限定されるものではない。 FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to an embodiment of the present invention, particularly heat treatment.
The rolled material to be heat-treated is usually produced by hot rolling followed by cold rolling. However, the present invention is not limited to this, and either one of hot rolling and cold rolling may be performed. The conditions for hot rolling and cold rolling are not particularly limited.
（１）オーステナイト化処理
　図１の［１］および［２］に示すように、圧延材をＡｃ３点以上の温度に加熱して、所定の加熱時間で加熱することにより、圧延材をオーステナイト化する。この加熱温度での加熱時間は、例えば１～１８００秒である。加熱温度の上限は、好ましくは、Ａｃ３点以上、Ａｃ３点＋１００℃以下である。Ａｃ３点＋１００℃以下の温度とすることで結晶粒の粗大化を抑制できるからである。加熱温度は、より好ましくはＡｃ３点＋１０℃以上、Ａｃ３点＋９０℃以下、さらに好ましくは、Ａｃ３点＋２０℃以上、Ａｃ３点＋８０℃以下である。より完全にオーステナイト化しフェライトの形成を抑制できるとともに、結晶粒の粗大化をより確実に抑制できるからである。
　図１の［１］で示す、オーステナイト化時の加熱は任意の加熱速度で行ってよいが、好ましい平均加熱速度として１℃／秒以上、より好ましくは２０℃／秒を挙げることができる。 (1) Austenitizing treatment As shown in [1] and [2] of FIG. 1, the rolled material is austenitized by heating the rolled material to a temperature of Ac 3 points or higher and heating it for a predetermined heating time. To do. The heating time at this heating temperature is, for example, 1 to 1800 seconds. The upper limit of the heating temperature is preferably Ac 3 points or more and Ac 3 points + 100 ° C. or less. This is because coarsening of crystal grains can be suppressed by setting the temperature to Ac 3 points + 100 ° C. or lower. The heating temperature is more preferably Ac 3 points + 10 ° C. or higher, Ac 3 points + 90 ° C. or lower, and further preferably Ac 3 points + 20 ° C. or higher, Ac 3 points + 80 ° C. or lower. This is because the austenite can be more completely suppressed and the formation of ferrite can be suppressed, and the coarsening of crystal grains can be more reliably suppressed.
Although heating at the time of austenitization shown by [1] in FIG. 1 may be performed at an arbitrary heating rate, a preferable average heating rate is 1 ° C./second or more, more preferably 20 ° C./second.
（２）冷却と３００℃～５００℃の温度域での滞留
　上記のオーステナイト化後、冷却し、図１の［５］に示すように、３００℃～５００℃の温度範囲内で１０℃／秒以下の冷却速度で１０秒以上、３００秒未満滞留させる。
　冷却は、少なくとも６５０℃～５００℃の間は、平均冷却速度１５℃／秒以上、２００℃／秒未満で冷却する。平均冷却速度１５℃／秒以上とすることで、冷却中のフェライトの形成を抑制するためである。また、冷却速度を２００℃／秒未満とすることで急激な冷却よる過大な熱歪みの発生を防止できる。このような冷却の好ましい例として、図１の［３］に示すように、６５０℃以上である急冷開始温度までは、０．１℃／秒以上、１０℃／秒以下の比較的低い平均冷却速度で冷却し、図１の［４］に示すように、急冷開始温度から、５００℃以下である滞留開始温度まで平均冷却速度２０℃／秒以上、２００℃／秒未満で冷却することを挙げることができる。 (2) Cooling and retention in the temperature range of 300 ° C. to 500 ° C. After the above austenite formation, cooling is performed and, as shown in [5] of FIG. 1, 10 ° C./second within the temperature range of 300 ° C. to 500 ° C. It is allowed to stay for 10 seconds or more and less than 300 seconds at the following cooling rate.
The cooling is performed at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second at least between 650 ° C. and 500 ° C. This is because the formation of ferrite during cooling is suppressed by setting the average cooling rate to 15 ° C./second or more. Moreover, generation | occurrence | production of the excessive thermal distortion by rapid cooling can be prevented by making a cooling rate into less than 200 degrees C / sec. As a preferable example of such cooling, as shown in [3] in FIG. 1, a relatively low average cooling of 0.1 ° C./second or more and 10 ° C./second or less is performed up to a rapid cooling start temperature of 650 ° C. or more. Cooling at a rate, and as shown in [4] of FIG. 1, cooling is performed at an average cooling rate of 20 ° C./second or more and less than 200 ° C./second from a rapid cooling start temperature to a residence start temperature of 500 ° C. or less. be able to.
　３００℃～５００℃の温度範囲内で１０℃／秒以下の冷却速度で１０秒以上滞留させる。すなわち、３００℃～５００℃の温度範囲内において、冷却速度が１０℃／秒以下の状態に１０秒以上置かれる。冷却速度が１０℃／秒以下の状態は、図１の［５］のように、実質的に一定の温度で保持する（すなわち、冷却速度が０℃／秒）場合も含む。
　この滞留により、部分的にベイナイトを形成させる。そして、ベイナイトはオーステナイトより炭素の固溶限が低いことから、固溶限を超えた炭素をはき出す。この結果、ベイナイト周囲に炭素が濃化したオーステナイトの領域が形成される。
　この領域が後述する冷却、再加熱を経て、やや粗大な残留オーステナイトとなる。このやや粗大な残留オーステナイトを形成することで、上述のように深絞り性を高くすることができる。 The sample is retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 ° C. to 500 ° C. That is, in the temperature range of 300 ° C. to 500 ° C., the cooling rate is set to 10 ° C./second or less for 10 seconds or more. The state where the cooling rate is 10 ° C./second or less includes the case where the cooling rate is maintained at a substantially constant temperature (that is, the cooling rate is 0 ° C./second) as shown in [5] in FIG.
Due to this residence, bainite is partially formed. And since bainite has a lower carbon solid solubility limit than austenite, it expels carbon beyond the solid solubility limit. As a result, an austenite region enriched with carbon is formed around bainite.
This region becomes slightly coarse retained austenite after cooling and reheating described later. By forming this slightly coarse retained austenite, the deep drawability can be enhanced as described above.
　滞留させる温度が５００℃より高いと、炭素濃化領域が大きくなりすぎて、残留オーステナイトだけでなく、ＭＡも粗大になるために、穴広げ率が低下する。一方、滞留させる温度が３００℃より低いと、炭素濃化領域が小さく、粗大な残留オーステナイトの量が不足し、深絞り性が低下する。
　また、滞留時間が１０秒より短いと、炭素濃化領域の面積が小さくなり、粗大な残留オーステナイトの量が不足し、深絞り性が低下する。一方、滞留時間が３００秒以上になると、炭素濃化領域が大きくなりすぎて、残留オーステナイトだけでなく、ＭＡも粗大になるため、穴広げ率が低下する。
　また、滞留中の冷却速度が１０℃／秒より大きいと十分なベイナイト変態が起こらず、従って、十分な炭素濃化領域が形成されず、粗大な残留オーステナイトの量が不足する。 If the retention temperature is higher than 500 ° C., the carbon concentration region becomes too large, and not only retained austenite but also MA becomes coarse, so that the hole expansion rate decreases. On the other hand, if the retention temperature is lower than 300 ° C., the carbon concentration region is small, the amount of coarse retained austenite is insufficient, and the deep drawability deteriorates.
On the other hand, if the residence time is shorter than 10 seconds, the area of the carbon-enriched region is reduced, the amount of coarse retained austenite is insufficient, and the deep drawability is deteriorated. On the other hand, when the residence time is 300 seconds or more, the carbon enrichment region becomes too large, and not only retained austenite but also MA becomes coarse, so the hole expansion rate decreases.
Further, if the cooling rate during the residence is higher than 10 ° C./second, sufficient bainite transformation does not occur, and therefore a sufficient carbon enriched region is not formed, and the amount of coarse retained austenite is insufficient.
　従って、３００℃～５００℃の温度範囲内で１０℃／秒以下の冷却速度で１０秒以上滞留させる。好ましくは３２０～４８０℃の温度範囲内で８℃／秒以下の冷却速度で１０秒以上滞留させ、その間、一定温度で３～８０秒保持することが好ましい。
　更に好ましくは３４０～４６０℃の温度範囲内で３℃/秒以下の冷却速度で１０秒以上滞留させ、その間、一定温度で５～６０秒保持する。 Therefore, it is retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 ° C. to 500 ° C. It is preferable to hold for 10 seconds or more at a cooling rate of 8 ° C./second or less in a temperature range of 320 to 480 ° C., and hold at a constant temperature for 3 to 80 seconds.
More preferably, it is retained for 10 seconds or more at a cooling rate of 3 ° C./second or less within a temperature range of 340 to 460 ° C., and during that time, it is held at a constant temperature for 5 to 60 seconds.
（３）１００℃以上、３００℃未満の間の冷却停止温度まで冷却
　上述の滞留後、図１の［６］に示すように３００℃以上の第２冷却開始温度から１００℃以上、３００℃未満の間の冷却停止温度まで１０℃／秒以上の平均冷却速度で冷却する。好ましい実施形態の１つでは、図１の［６］に示すように、上述の滞留の終了温度（例えば、図１の［５］に示す保持温度）を第２冷却開始温度とする。
　この冷却により、上述の炭素濃化領域をオーステナイトとして残したまま、マルテンサイト変態を起こさせる。冷却停止温度を１００℃以上、３００℃未満の温度範囲内で制御することで、マルテンサイトに変態せずに残存するオーステナイトの量を調整して、最終的な残留オーステナイト量を制御する。 (3) Cooling to a cooling stop temperature between 100 ° C. and less than 300 ° C. After the above-mentioned residence, as shown in [6] of FIG. 1, from the second cooling start temperature of 300 ° C. or more to 100 ° C. or more and less than 300 ° C. Cooling is performed at an average cooling rate of 10 ° C./second or more until the cooling stop temperature during In one of the preferred embodiments, as shown in [6] of FIG. 1, the above-mentioned end temperature of residence (for example, the holding temperature shown in [5] of FIG. 1) is set as the second cooling start temperature.
By this cooling, the martensitic transformation is caused while leaving the above-described carbon-enriched region as austenite. By controlling the cooling stop temperature within a temperature range of 100 ° C. or more and less than 300 ° C., the amount of austenite remaining without transformation to martensite is adjusted, and the final amount of retained austenite is controlled.
　冷却速度が、１０℃／秒より遅いと、冷却中に炭素濃化領域が必要以上に広がり、ＭＡが粗大になるために、穴広げ率が低下する。冷却停止温度が１００℃より低いと、残留オーステナイト量が不足する。この結果、ＴＳは高くなるものの、ＥＬが低下し、ＴＳ×ＥＬバランスが不足する。
　冷却停止温度が３００℃以上だと、粗大な未変態オーステナイトが増え、その後の冷却でも残存することで、最終的にＭＡサイズが粗大になり、穴広げ率λが低下する。
　なお、好ましい冷却速度は１５℃／℃以上であり、好ましい冷却停止温度は１２０℃以上、２８０℃以下である。更に好ましい、冷却速度は２０℃／ｓ以上であり、更に好ましい冷却停止温度は１４０℃以上、２６０℃以下である。 When the cooling rate is slower than 10 ° C./second, the carbon concentration region spreads more than necessary during cooling, and the MA becomes coarse, so the hole expansion rate decreases. When the cooling stop temperature is lower than 100 ° C., the amount of retained austenite is insufficient. As a result, although TS increases, EL decreases and TS × EL balance is insufficient.
When the cooling stop temperature is 300 ° C. or higher, coarse untransformed austenite increases and remains even after the subsequent cooling, resulting in coarse MA size and a decrease in the hole expansion ratio λ.
In addition, a preferable cooling rate is 15 degreeC / degrees C or more, and a preferable cooling stop temperature is 120 degreeC or more and 280 degrees C or less. A more preferable cooling rate is 20 ° C./s or more, and a more preferable cooling stop temperature is 140 ° C. or more and 260 ° C. or less.
　図１の［７］に示すように、冷却停止温度で保持してもよい。保持する場合の好ましい保持時間として、１～６００秒を挙げることができる。保持時間が長くなっても特性上の影響はほとんどないが、６００秒を超える保持時間は生産性を低下させる。 As shown in [7] of FIG. 1, it may be held at the cooling stop temperature. A preferable holding time in the case of holding can be 1 to 600 seconds. Even if the holding time is increased, there is almost no influence on the characteristics, but if the holding time exceeds 600 seconds, the productivity is lowered.
（４）３００℃～５００℃の温度範囲まで再加熱
　図１の［８］に示すように、上述の冷却停止温度から３００℃～５００℃範囲にある再加熱温度まで、３０℃／秒以上の再加熱速度で加熱する。急速に加熱することにより炭化物の析出および成長が促進される温度域での滞在時間を短くすることができ、微細な炭化物の形成を抑制することができる。好ましい再加熱速度は、６０℃／ｓ以上、より好ましくは７０℃／ｓである。
　このような急速加熱は、例えば高周波加熱、通電加熱などの方法で達成することができる。 (4) Reheating to a temperature range of 300 ° C. to 500 ° C. As shown in [8] of FIG. 1, from the above-mentioned cooling stop temperature to a reheating temperature in the range of 300 ° C. to 500 ° C., 30 ° C./second or more Heat at reheat rate. By rapidly heating, the residence time in the temperature range where precipitation and growth of carbides are promoted can be shortened, and formation of fine carbides can be suppressed. A preferable reheating rate is 60 ° C./s or more, more preferably 70 ° C./s.
Such rapid heating can be achieved by a method such as high-frequency heating or electric heating.
　再加熱温度に到達した後は、図１の［９］に示すようにその温度で保持する。そのとき、以下の式（１）で表される焼戻しパラメータPが１００００以上、１４５００以下となるように、かつ、保持時間が１～１５０秒とするのが好ましい。本実施形態の鋼板の焼戻しパラメータPは以下の式（１）で表される。
　　P=T(K)×（20+log(t/3600）・・・（１）
ここで、Tは焼戻し温度(K)、tは保持時間(秒)である。 After reaching the reheating temperature, the temperature is maintained as shown in [9] of FIG. At that time, it is preferable that the tempering parameter P represented by the following formula (1) is 10000 or more and 14500 or less and the holding time is 1 to 150 seconds. The tempering parameter P of the steel plate of this embodiment is represented by the following formula (1).
P = T (K) × (20 + log (t / 3600) (1)
Here, T is a tempering temperature (K), and t is a holding time (seconds).
　再加熱の時、マルテンサイト中に過飽和に固溶している炭素の再分配が起こる。具体的には、マルテンサイトからオーステナイトへの炭素拡散と、マルテンサイトのラス中での炭化物（セメンタイト）の析出の２つの現象が起こる。この二つの現象のうち、炭化物の析出は、低温で長時間の保持を行うと起こりやすい。また、高温で保持する場合であっても、加熱速度が遅い場合や、保持時間が長すぎると、炭化物が析出する。一方、マルテンサイトからオーステナイトへの炭素拡散は、拡散速度に強く依存するために、高温で短時間の処理で十分に行うことができる。 During reheating, redistribution of carbon that is supersaturated in martensite occurs. Specifically, two phenomena occur: carbon diffusion from martensite to austenite and precipitation of carbide (cementite) in the martensite lath. Of these two phenomena, carbide precipitation is likely to occur when holding at low temperature for a long time. Moreover, even if it is a case where it hold | maintains at high temperature, when a heating rate is slow or holding time is too long, a carbide | carbonized_material will precipitate. On the other hand, carbon diffusion from martensite to austenite strongly depends on the diffusion rate, and can be sufficiently performed at a high temperature in a short time.
　マルテンサイト中に存在するセメンタイトの粒子は衝突破壊の起点になりやすく、耐衝突特性を低下させる原因になる。よって、再加熱の際には、マルテンサイトのラス内での炭化物（セメンタイト）の析出を抑制しつつ、マルテンサイトからオーステナイトへの炭素拡散を促進させるような再加熱処理を行うことが望ましい。そこで、急速加熱と、高温かつ短時間での熱処理を施すことが有効である。 Cementite particles present in martensite are likely to be the starting point of collision fracture, which causes a decrease in collision resistance. Therefore, at the time of reheating, it is desirable to perform a reheating treatment that promotes carbon diffusion from martensite to austenite while suppressing the precipitation of carbide (cementite) in the martensite lath. Therefore, it is effective to perform rapid heating and heat treatment at a high temperature in a short time.
　ただし、十分な炭素拡散を生じさせて所望の引張強度を得るためには、温度と時間の組合せの因子としての焼戻しパラメータPを一定の範囲内に制御することが必要となる。
　焼戻しパラメータPが１００００より小さいと、マルテンサイトからオーステナイトへの炭素拡散が十分に起こらず、オーステナイトが不安定になり、残留オーステナイト量が確保できないために、ＴＳ×ＥＬバランスが不足する。また、焼戻しパラメータPが１４５００より大きいと、短時間処理でも炭化物の形成を防止できず、残留オーステナイト量が確保できず、ＴＳ×ＥＬバランスが劣化する。なお、焼戻しパラメータが適正でも、加熱速度が低すぎる、時間が長すぎると、マルテンサイトラス内に炭化物が形成されて、衝突変形時の亀裂進展が起こりやすくなり、耐衝突特性が劣化する。マルテンサイトラス内の炭化物の量は、Ｘ線小角散乱の散乱強度から求めることができる。 However, in order to obtain sufficient tensile strength by causing sufficient carbon diffusion, it is necessary to control the tempering parameter P as a factor of the combination of temperature and time within a certain range.
If the tempering parameter P is less than 10,000, carbon diffusion from martensite to austenite does not occur sufficiently, the austenite becomes unstable, and the amount of retained austenite cannot be ensured, resulting in insufficient TS × EL balance. On the other hand, if the tempering parameter P is larger than 14500, the formation of carbides cannot be prevented even in a short time treatment, the amount of retained austenite cannot be secured, and the TS × EL balance deteriorates. Even if the tempering parameters are appropriate, if the heating rate is too low and the time is too long, carbides are formed in the martensite lath, and crack propagation is likely to occur at the time of impact deformation, and the impact resistance characteristics deteriorate. The amount of carbide in the martensite lath can be determined from the scattering intensity of X-ray small angle scattering.
　再加熱温度が３００℃より低いと、炭素の拡散が不足して十分な残留オーステナイト量が得られずＴＳ×ＥＬが低下する。再加熱温度が５００℃より高いと、残留オーステナイトがセメンタイトとフェライトに分解して残留オーステナイトが不足し特性が確保できない。
　保持を行わないまたは保持時間が１秒より短いと、同様に炭素の拡散が不足する虞がある。このため、再加熱温度で１秒以上の保持を行うのが好ましい。保持時間が１５０秒より長いと、同様に、炭素がセメンタイトとして析出する虞がある。このため、保持時間は１５０秒以下であることが好ましい。
　好ましい再加熱温度は、３２０～４８０℃であり、更に好ましい再加熱温度は、３４０～４６０℃である。
　好ましくは、焼戻しパラメータPは、１０５００～１４５００であり、このときの好ましい保持時間は１～１５０秒である。さらに好ましい焼戻しパラメータPは１１０００～１４０００であり、このときの好ましい保持時間は１～１００秒、より好ましくは１～６０秒である。 If the reheating temperature is lower than 300 ° C., the diffusion of carbon is insufficient and a sufficient amount of retained austenite cannot be obtained, resulting in a decrease in TS × EL. When the reheating temperature is higher than 500 ° C., the retained austenite is decomposed into cementite and ferrite and the retained austenite is insufficient, and the characteristics cannot be ensured.
If the holding is not performed or the holding time is shorter than 1 second, there is a possibility that the carbon diffusion is insufficient. For this reason, it is preferable to hold for 1 second or more at the reheating temperature. If the holding time is longer than 150 seconds, similarly, carbon may be precipitated as cementite. For this reason, the holding time is preferably 150 seconds or less.
A preferable reheating temperature is 320 to 480 ° C., and a more preferable reheating temperature is 340 to 460 ° C.
Preferably, the tempering parameter P is 10500 to 14500, and a preferable holding time at this time is 1 to 150 seconds. A more preferable tempering parameter P is 11000 to 14000, and a preferable holding time at this time is 1 to 100 seconds, and more preferably 1 to 60 seconds.
　再加熱の後、図１の［１０］に示すように、例えば室温のような２００℃以下の温度まで冷却してよい。２００℃以下までの好ましい平均冷却速度として１０℃／秒を挙げることができる。
　以上の熱処理により本発明の実施形態に係る高強度鋼板を得ることができる。 After the reheating, as shown in [10] in FIG. 1, it may be cooled to a temperature of 200 ° C. or lower such as room temperature. A preferable average cooling rate up to 200 ° C. or less can be 10 ° C./second.
By the above heat treatment, the high-strength steel plate according to the embodiment of the present invention can be obtained.
　以上に説明した本発明の実施形態に係る高強度鋼板の製造方法に接した当業者であれば、試行錯誤により、上述した製造方法と異なる製造方法により本発明の実施形態に係る高強度鋼板を得ることができる可能性がある。 If it is those skilled in the art who contacted the manufacturing method of the high-strength steel plate which concerns on embodiment of this invention demonstrated above, the high-strength steel plate which concerns on embodiment of this invention by the manufacturing method different from the manufacturing method mentioned above by trial and error is demonstrated. There is a possibility that can be obtained.
１．サンプル作製
　表１に記載した化学組成を有する鋳造材を真空溶製で製造した後、この鋳造材を熱間鍛造で板厚３０ｍｍの鋼板にした後、熱間圧延を施した。なお、表１には組成から計算したＡｃ３点も記載した。
　熱間圧延の条件は本特許の最終組織および特性に本質的な影響を施さないが、１２００℃に加熱した後、多段圧延で板厚２．５ｍｍとした。この時、熱間圧延の終了温度は８８０℃とした。その後、６００℃まで３０℃／秒で冷却し、冷却を停止し、６００℃に加熱した炉に挿入後、３０分保持し、その後、炉冷し、熱延鋼板とした。
　この熱延鋼板に酸洗を施して表面のスケールを除去した後、１．４ｍｍまで冷間圧延を施した。この冷間圧延板に熱処理を行い、サンプルを得た。熱処理条件を表２に示した。なお、表２中の例えば、[２]のように[　]を内に示した番号は、図１中に[　]内に示した同じ番号のプロセスに対応する。表２において、サンプルＮｏ．１、４、７および２６は、図１の［５］に相当する工程において、３００～５００℃の温度範囲内で１０℃／秒以下の冷却速度で１０秒以上滞留させなかったサンプルである。特に、サンプルＮｏ．１および２６は、７００℃で急冷を開始後、２００℃まで一気に冷却したサンプル（図１で［５］、［６］に相当する工程をスキップしたサンプル）である。サンプルＮｏ．９は、１００℃以上、３００℃未満の間の冷却停止温度まで冷却する代わりに、再加熱温度まで冷却した後にその温度で保持したサンプル（図１で［６］～［８］に相当する工程をスキップしたサンプル）である。
　［８］に相当する再加熱は通電加熱法により行った。
　なお，表１～表４において、アスタリスク（＊）を付した数値は、本発明の実施形態の範囲から外れていることを示している。 1. Sample Production After a cast material having the chemical composition shown in Table 1 was manufactured by vacuum melting, this cast material was hot forged into a steel plate having a thickness of 30 mm, and then subjected to hot rolling. Table 1 also shows three points of Ac calculated from the composition.
The hot rolling conditions do not substantially affect the final structure and characteristics of this patent, but after heating to 1200 ° C., the sheet thickness is 2.5 mm by multi-stage rolling. At this time, the end temperature of the hot rolling was 880 ° C. Then, it cooled to 600 degreeC at 30 degree-C / sec, stopped cooling, and after inserting into the furnace heated at 600 degreeC, it hold | maintained for 30 minutes, and then cooled in the furnace, and it was set as the hot-rolled steel plate.
The hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to 1.4 mm. The cold-rolled plate was heat-treated to obtain a sample. The heat treatment conditions are shown in Table 2. In Table 2, for example, a number indicated in [] as in [2] corresponds to a process having the same number indicated in [] in FIG. In Table 2, sample no. 1, 4, 7 and 26 are samples which were not retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 to 500 ° C. in the process corresponding to [5] in FIG. In particular, sample no. Reference numerals 1 and 26 are samples that are rapidly cooled to 200 ° C. after starting rapid cooling at 700 ° C. (samples in which steps corresponding to [5] and [6] in FIG. 1 are skipped). Sample No. 9 is a sample (steps corresponding to [6] to [8] in FIG. 1) that is held at that temperature after being cooled to the reheating temperature instead of being cooled to a cooling stop temperature between 100 ° C. and less than 300 ° C. Sample skipped).
The reheating corresponding to [8] was performed by an electric heating method.
In Tables 1 to 4, numerical values marked with an asterisk (*) indicate that they are out of the scope of the embodiment of the present invention.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
２．鋼組織
　それぞれのサンプルについて上述した方法により、フェライト分率、焼戻しマルテンサイトと焼戻しベイナイトの合計分率（表３には「焼戻しＭ／Ｂ」と記載）、残留オーステナイト量（残留γ量）、ＭＡの平均サイズ、残留オーステナイトの平均サイズ（残留γ平均サイズ）、サイズ１．５μｍ以上の残留オーステナイトの全オーステナイトに占める比率（表３には、「１．５μｍ以上の残留γ比率」と記載）、Ｘ線小角散乱のｑ値が１ｎｍ－１での散乱強度を求めた。残留オーステナイト量の測定には、株式会社リガク製２次元微小部Ｘ線回折装置（ＲＩＮＴ－ＲＡＰＩＤII）を用いた。得られた結果を表３に示す。
　なお、本実施例において、表３に記載された鋼組織以外の鋼組織（残組織）は、サンプルＮｏ．９を除いたサンプルでは焼き戻されていないマルテンサイトであり、サンプルＮｏ．９では焼き戻されていないベイナイトである。 2. Steel structure By the method described above for each sample, the ferrite fraction, the total fraction of tempered martensite and tempered bainite (described as “tempered M / B” in Table 3), the amount of retained austenite (residual γ amount), MA Average size, average size of residual austenite (residual γ average size), ratio of residual austenite of size 1.5 μm or more to total austenite (in Table 3, described as “residual γ ratio of 1.5 μm or more”), The scattering intensity when the q-value of X-ray small angle scattering was 1 nm −1 was determined. For the measurement of the amount of retained austenite, a two-dimensional micro part X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku Corporation was used. The obtained results are shown in Table 3.
In this example, the steel structures (remaining structures) other than the steel structures described in Table 3 are sample Nos. Samples other than 9 are martensite that has not been tempered. 9 is a bainite that has not been tempered.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
３．機械的特性
　得られたサンプルについて、引張試験機を用いて、ＹＳ、ＴＳ、ＥＬを測定し、ＹＲおよびＴＳ×ＥＬを算出した。また、上述の方法により穴拡げ率λ、深絞り性ＬＤＲ、スポット溶接部の十字引張強度（ＳＷ十字引張）およびＲ５引張板厚減少率を求めた。得られた結果を表４に示す。 3. Mechanical properties About the obtained sample, YS, TS, and EL were measured using the tensile tester, and YR and TSxEL were computed. Further, the hole expansion rate λ, deep drawability LDR, cross-tension strength (SW cross-tension) of the spot welded portion, and R5 tensile plate thickness reduction rate were determined by the above-described methods. Table 4 shows the obtained results.
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　表４の結果を考察する。サンプルＮｏ．１３、１５、１８、２１および２８～３６は、本発明の実施形態で規定する全ての要件（組成、製造条件および鋼組織）を満たす実施例である。これらの試料はいずれも、９８０ＭＰａ以上の引張強度（ＴＳ）、０．７５以上の降伏比（ＹＲ）、２００００ＭＰａ％以上のＴＳ×ＥＬ、２．０５以上のＬＤＲ、２０％以上の穴広げ率（λ）、６ｋＮ以上のＳＷ十字引張および５０％以上のＲ５引張板厚減少率（ＲＡ）を達成している。 Consider the results in Table 4. Sample No. Reference numerals 13, 15, 18, 21, and 28 to 36 are examples that satisfy all the requirements (composition, manufacturing conditions, and steel structure) defined in the embodiment of the present invention. All of these samples have a tensile strength (TS) of 980 MPa or more, a yield ratio (YR) of 0.75 or more, TS × EL of 20000 MPa% or more, an LDR of 2.05 or more, a hole expansion ratio of 20% or more ( λ), SW cross tension of 6 kN or more, and R5 tensile thickness reduction ratio (RA) of 50% or more.
　これに対して、サンプルＮｏ．１は、オーステナイト化後、３００℃～５００℃の温度範囲内で滞留させなかったことから、サイズ１．５μｍ以上の残留オーステナイト量が十分でなく、この結果、十分な深絞り性が得られなかった。さらに[7]保持時間が３００秒と長かったため、炭化物(セメンタイト)が析出した。また、Ｘ線小角散乱の散乱強度が大きいことから、約１ｎｍのセメンタイトの体積分率が大きいといえる。その結果、耐衝突特性(板厚減少率)が低下した。 In contrast, sample no. No. 1 was not retained in the temperature range of 300 ° C. to 500 ° C. after the austenite formation, so the amount of retained austenite with a size of 1.5 μm or more was not sufficient, and as a result, sufficient deep drawability could not be obtained. It was. [7] Since the holding time was as long as 300 seconds, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
　サンプルＮｏ．２は、[5]保持温度が３００秒と長いため、ＭＡ平均サイズが過大となり、この結果、十分な穴広げ率が得られなかった。 Sample No. In [2], since the [5] holding temperature was as long as 300 seconds, the MA average size was excessive, and as a result, a sufficient hole expansion rate was not obtained.
　サンプルＮｏ．３は、[6]冷却速度が１℃／秒と遅いため、ＭＡ平均サイズが過大となり、この結果、十分な穴広げ率が得られなかった。さらに[7]保持時間が３００秒と長かったため、炭化物(セメンタイト)が析出した。また、Ｘ線小角散乱の散乱強度が大きいことから、約１ｎｍのセメンタイトの体積分率が大きいといえる。その結果、耐衝突特性(板厚減少率)が低下した。 Sample No. No. 3 [6] Since the cooling rate was as low as 1 ° C./second, the average MA size was excessive, and as a result, a sufficient hole expansion rate was not obtained. [7] Since the holding time was as long as 300 seconds, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
　サンプルＮｏ．４は、[5]保持時間が３秒と短いため、サイズ１．５μｍ以上の残留オーステナイト量が十分でなく、十分な深絞り性が得られなかった。 Sample No. No. 4 [5] Since the holding time was as short as 3 seconds, the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and sufficient deep drawability could not be obtained.
　サンプルＮｏ．５は、[5]保持温度が５５０℃と高いため、ＭＡ平均サイズが過大となり、この結果、十分な穴広げ率および十分な深絞り性が得られなかった。 Sample No. No. 5 had a high [5] holding temperature of 550 ° C., and therefore the average MA size was excessive. As a result, a sufficient hole expansion ratio and a sufficient deep drawability could not be obtained.
　サンプルＮｏ．６は、[5]保持温度が２５０℃と低いため、サイズ１．５μｍ以上の残留オーステナイト量が十分でなく、この結果、十分な深絞り性が得られなかった。 Sample No. No. 6 [5] Since the holding temperature was as low as 250 ° C., the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and as a result, sufficient deep drawability was not obtained.
　サンプルＮｏ．７は、[6]冷却停止温度が３５０℃と高いため、焼戻しマルテンサイトと焼戻しベイナイトの合計量が不足し、ＭＡ平均サイズが過大で、かつ残留オーステナイトの平均サイズも過大となった。この結果、十分な穴広げ率および深絞り性が得られなかった。 Sample No. No. 7 [6] Since the cooling stop temperature was as high as 350 ° C., the total amount of tempered martensite and tempered bainite was insufficient, the MA average size was excessive, and the average size of retained austenite was excessive. As a result, a sufficient hole expansion rate and deep drawability could not be obtained.
　サンプルＮｏ．８は、[1]加熱温度が７８０℃と低いため、フェライト量が過大となり、かつ焼戻しマルテンサイトと焼戻しベイナイトの合計量が不足し、この結果、十分な引張強度および降伏比が得られなかった。 Sample No. No. 8 [1] Since the heating temperature was as low as 780 ° C., the amount of ferrite was excessive, and the total amount of tempered martensite and tempered bainite was insufficient. As a result, sufficient tensile strength and yield ratio were not obtained. .
　サンプルＮｏ．９は、[6]冷却停止温度が４００℃と高いため、マルテンサイトおよびベイナイトが形成されず、ＭＡ平均サイズが過大で、かつ残留オーステナイトの平均サイズも過大となった。この結果、十分な引張強度および降伏比が得られなかった。さらに、その温度で３００秒（[9]保持時間）保持しているため炭化物の形成も少ない。これらの結果、λが低下した。 Sample No. No. 9 [6] Since the cooling stop temperature was as high as 400 ° C., martensite and bainite were not formed, the average size of MA was excessive, and the average size of retained austenite was also excessive. As a result, sufficient tensile strength and yield ratio were not obtained. Furthermore, since it is held at that temperature for 300 seconds ([9] holding time), the formation of carbides is small. As a result, λ decreased.
　サンプルＮｏ．１０は、[5]冷却停止温度が２０℃と低かったため残留γ量が少なくなり、かつサイズ１．５μｍ以上の残留オーステナイト量が十分でない。この結果、十分なＴＳ×ＥＬの値および十分な深絞り性が得られなかった。 Sample No. No. 10 [5] Since the cooling stop temperature was as low as 20 ° C., the amount of residual γ decreased, and the amount of residual austenite having a size of 1.5 μm or more was not sufficient. As a result, sufficient TS × EL value and sufficient deep drawability could not be obtained.
　サンプルＮｏ．１１は、[8]再加熱速度が３０℃／秒と遅かったため、炭化物(セメンタイト)が析出した。また、Ｘ線小角散乱の散乱強度が大きいことから、約１ｎｍのセメンタイトの体積分率が大きいといえる。その結果、耐衝突特性(板厚減少率)が低下した。 Sample No. No. 11 [8] Since the reheating rate was as low as 30 ° C./second, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
　サンプルＮｏ．１２は、[4]急冷開始温度が５８０℃と低いため、フェライト量が過大となり、かつ焼戻しマルテンサイトと焼戻しベイナイトの合計量が不足し、この結果、十分な引張強度および降伏比が得られなかった。 Sample No. No. 12 [4] Since the quenching start temperature is as low as 580 ° C., the amount of ferrite becomes excessive, and the total amount of tempered martensite and tempered bainite is insufficient. As a result, sufficient tensile strength and yield ratio cannot be obtained. It was.
　サンプルＮｏ．１４は、[4]冷却速度が８℃／秒と遅いため、フェライト量が過大となり、焼戻しマルテンサイトと焼戻しベイナイトの合計量が不足し、かつＭＡ平均サイズが過大となった。この結果、十分な引張強度および降伏比が得られなかった。 Sample No. No. 14 [4] Since the cooling rate was as low as 8 ° C./second, the amount of ferrite was excessive, the total amount of tempered martensite and tempered bainite was insufficient, and the MA average size was excessive. As a result, sufficient tensile strength and yield ratio were not obtained.
　サンプルＮｏ．１６は、[9]保持時間が３００秒と長かったため、炭化物(セメンタイト)が析出した。また、Ｘ線小角散乱の散乱強度が大きいことから、約１ｎｍのセメンタイトの体積分率が大きいといえる。その結果、耐衝突特性(板厚減少率)が低下した。 Sample No. No. 16 had a long [9] holding time of 300 seconds, so carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
　サンプルＮｏ．１７は、[8]再加熱速度が１５℃／秒と遅かったため、炭化物(セメンタイト)が析出した。また、Ｘ線小角散乱の散乱強度が大きいことから、約１ｎｍのセメンタイトの体積分率が大きいといえる。その結果、耐衝突特性(板厚減少率)が低下した。 Sample No. No. 17 [8] Since the reheating rate was as low as 15 ° C./second, carbide (cementite) precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
　サンプルＮｏ．１９は、[7]再加熱温度が５５０℃高かったため、パラメータが１４６０４と高くなった。そのため、残留γ量が少なくなり、かつサイズ１．５μｍ以上の残留オーステナイト量が十分でない。その結果としてＴＳ×ＥＬおよび深絞り性が低下した。また、Ｘ線小角散乱の散乱強度が大きいことから、約１ｎｍのセメンタイトの体積分率が大きいといえる。その結果、耐衝突特性(板厚減少率)が低下した。 Sample No. No. 19 [7] Since the reheating temperature was 550 ° C. higher, the parameter became 14604 higher. Therefore, the amount of residual γ is reduced and the amount of retained austenite having a size of 1.5 μm or more is not sufficient. As a result, TS × EL and deep drawability decreased. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.
　サンプルＮｏ．２０は、[8]再加熱温度が２５０℃と低かったため、パラメータが９２８０と低くなった。そのため、炭素の拡散が不足し、残留γ量が少なくなり、かつサイズ１．５μｍ以上の残留オーステナイト量が十分でない。その結果、ＴＳ×ＥＬおよび深絞り性が低下した。 Sample No. No. 20, [8] The reheating temperature was as low as 250 ° C., so the parameter was as low as 9280. Therefore, the diffusion of carbon is insufficient, the amount of residual γ is reduced, and the amount of residual austenite having a size of 1.5 μm or more is not sufficient. As a result, TS × EL and deep drawability decreased.
　サンプルＮｏ．２２は、Ｃ量が少なく、残留オーステナイト量が不足し、かつサイズ１．５μｍ以上の残留オーステナイト量が十分でなく、この結果、十分なＴＳ×ＥＬおよび深絞り性が得られなかった。
　サンプルＮｏ．２３は、Ｍｎ量が多く、残留オーステナイト量が不足し、この結果、十分なＴＳ×ＥＬが得られなかった。 Sample No. No. 22 had a small amount of C, an insufficient amount of retained austenite, and an insufficient amount of retained austenite having a size of 1.5 μm or more. As a result, sufficient TS × EL and deep drawability could not be obtained.
Sample No. No. 23 had a large amount of Mn and a short amount of retained austenite, and as a result, sufficient TS × EL was not obtained.
　サンプルＮｏ．２４は、Ｍｎ量が少なく、フェライト量が過大で、焼戻しマルテンサイトと焼戻しベイナイトの合計量が不足している。この結果、十分な引張強度および降伏比が得られなかった。
　サンプルＮｏ．２５は、Ｓｉ＋Ａｌ量が少なく、焼戻しマルテンサイトと焼戻しベイナイトの合計量が不足し、残留オーステナイトが少なく、ＭＡ平均サイズが過大で、かつ残留オーステナイトの平均サイズも過大となった。この結果、十分なＴＳ×ＥＬ、穴広げ率および深絞り性が得られなかった。 Sample No. No. 24 has a small amount of Mn, an excessive amount of ferrite, and a total amount of tempered martensite and tempered bainite is insufficient. As a result, sufficient tensile strength and yield ratio were not obtained.
Sample No. In No. 25, the amount of Si + Al was small, the total amount of tempered martensite and tempered bainite was insufficient, the amount of retained austenite was small, the MA average size was excessive, and the average size of residual austenite was excessive. As a result, sufficient TS × EL, hole expansion ratio and deep drawability were not obtained.
　サンプルＮｏ．２６はＣ量が過大で、かつオーステナイト化後、３００℃～５００℃の温度範囲内で滞留させなかったことから、十分なＳＷ十字引張強度が得られなかった。 Sample No. No. 26 had an excessive amount of C and was not retained in the temperature range of 300 ° C. to 500 ° C. after austenitization, so that sufficient SW cross tensile strength could not be obtained.
　サンプルＮｏ．２７は、Ｓｉ＋Ａｌ量が過多であり、十分なＴＳ×ＥＬが得られなかった。 Sample No. In No. 27, the amount of Si + Al was excessive, and sufficient TS × EL was not obtained.
４．まとめ
　このように、本発明の実施形態に規定する組成と鋼組織を満たす鋼板は、引張強度（ＴＳ）、降伏比（ＹＲ）、（ＴＳ）と全伸び（ＥＬ）との積（ＴＳ×ＥＬ）、ＬＤＲ、穴広げ率（λ）、引張試験時の破断部の板厚減少率（ＲＡ）およびスポット溶接部の十字引張強度が何れも高いレベルとなることが確認できた。
　また、本発明の実施形態に係る製造方法によれば、本発明の実施形態に規定する組成と鋼組織を満たす鋼板を製造することができることが確認できた。 4). Summary As described above, the steel sheet satisfying the composition and steel structure defined in the embodiment of the present invention has a tensile strength (TS), a yield ratio (YR), a product of (TS) and total elongation (EL) (TS × EL). ), LDR, hole expansion rate (λ), plate thickness reduction rate (RA) of the fractured portion during the tensile test, and cross tensile strength of the spot welded portion were all confirmed to be at high levels.
Moreover, according to the manufacturing method which concerns on embodiment of this invention, it has confirmed that the steel plate which satisfy | fills the composition and steel structure which prescribe | regulate in embodiment of this invention can be manufactured.
　本出願は、出願日が２０１６年８月３日である日本国特許出願、特願２０１６－１５３１０７号を基礎出願とする優先権主張を伴う。特願２０１６－１５３１０７号は参照することにより本明細書に取り込まれる。 This application is accompanied by a priority claim based on Japanese patent application No. 2016-153107, filed on August 3, 2016. Japanese Patent Application No. 2016-153107 is incorporated herein by reference.

Claims (6)

  1. 　Ｃ　：０．１５質量％～０．３５質量％、
    　ＳｉとＡｌの合計：０．５質量％～３．０質量％、
    　Ｍｎ：１．０質量％～４．０質量％、
    　Ｐ　：０．０５質量％以下、
    　Ｓ　：０．０１質量％以下、
    を含み、残部がＦｅおよび不可避不純物からなり、
    　鋼組織が、
    　　フェライト分率が５％以下であり、
    　　焼戻しマルテンサイトと焼戻しベイナイトの合計分率が６０％以上であり、
    　　残留オーステナイト量が１０％以上であり、
    　　ＭＡの平均サイズが１．０μｍ以下であり、
    　　残留オーステナイトの平均サイズが１．０μｍ以下であり、
    　　サイズ１．５μｍ以上の残留オーステナイトが全残留オーステナイト量の２％以上であり、
    　　Ｘ線小角散乱でのｑ値が１ｎｍ－１での散乱強度が１．０ｃｍ－１以下である高強度鋼板。     C: 0.15% by mass to 0.35% by mass,
    Total of Si and Al: 0.5% by mass to 3.0% by mass,
    Mn: 1.0% by mass to 4.0% by mass,
    P: 0.05 mass% or less,
    S: 0.01% by mass or less,
    And the balance consists of Fe and inevitable impurities,
    Steel structure
    The ferrite fraction is 5% or less,
    The total fraction of tempered martensite and tempered bainite is 60% or more,
    The amount of retained austenite is 10% or more,
    The average size of MA is 1.0 μm or less,
    The average size of retained austenite is 1.0 μm or less,
    Residual austenite having a size of 1.5 μm or more is 2% or more of the total retained austenite amount,
    A high-strength steel sheet having a scattering intensity of 1.0 cm -1 or less when the q value in X-ray small angle scattering is 1 nm -1 .
  2. 　Ｃ量が０．３０質量％以下である請求項１に記載の高強度鋼板。 The high-strength steel sheet according to claim 1, wherein the C content is 0.30 mass% or less.
  3. 　Ａｌ量が０．１０質量％未満である請求項１または２に記載の高強度鋼板。 The high-strength steel sheet according to claim 1 or 2, wherein the Al amount is less than 0.10% by mass.
  4. 　Ｃ：０．１５質量％～０．３５質量％、ＳｉとＡｌの合計：０．５質量％～３．０質量％、Ｍｎ：１．０質量％～４．０質量％、Ｐ：０．０５質量％以下、Ｓ：０．０１質量％以下、を含み、残部がＦｅおよび不可避不純物からなる圧延材を用意することと、
    　前記圧延材をＡｃ３点以上の温度に加熱しオーステナイト化することと、
    　前記オーステナイト化後、６５０℃～５００℃の間を平均冷却速度１５℃／秒以上、２００℃／秒未満で冷却し、３００℃～５００℃の範囲内で１０℃／秒以下の冷却速度で１０秒以上、３００秒未満滞留させることと、
    　前記滞留の後、３００℃以上の温度から１００℃以上、３００℃未満の間の冷却停止温度まで１０℃／秒以上の平均冷却速度で冷却することと、
    　前記冷却停止温度から３００℃～５００℃範囲にある再加熱温度まで３０℃／秒以上の平均加熱速度で加熱することと、
    　前記再加熱温度Tにおいて、式（１）で規定される焼戻しパラメータPが１００００～１４５００かつ保持時間tが１～１５０秒を満たすように保持することと、
    　前記保持の後、前記再加熱温度から２００℃まで１０℃／秒以上の平均冷却速度で冷却すること、
    を含む、高強度鋼板の製造方法。
    　　P=T×(20+log(t/3600))・・・（１）
    　ここで、T: 再加熱温度(K)、t: 保持時間(秒)である。     C: 0.15% by mass to 0.35% by mass, Si and Al total: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.0% by mass. Preparing a rolled material comprising 05% by mass or less, S: 0.01% by mass or less, and the balance being Fe and inevitable impurities;
    Heating the rolled material to a temperature of Ac 3 point or higher to austenite;
    After the austenitization, cooling is performed at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second between 650 ° C. and 500 ° C., and 10 ° C./second or less within a range of 300 ° C. to 500 ° C. For more than a second and less than 300 seconds,
    After the residence, cooling at a mean cooling rate of 10 ° C / second or more from a temperature of 300 ° C or more to a cooling stop temperature of 100 ° C or more and less than 300 ° C;
    Heating at an average heating rate of 30 ° C./second or more from the cooling stop temperature to a reheating temperature in the range of 300 ° C. to 500 ° C .;
    Holding at the reheating temperature T such that the tempering parameter P defined by the formula (1) is 10000 to 14500 and the holding time t is 1 to 150 seconds;
    After the holding, cooling from the reheating temperature to 200 ° C. at an average cooling rate of 10 ° C./second or more,
    A method for producing a high-strength steel sheet, comprising:
    P = T × (20 + log (t / 3600)) (1)
    Here, T: reheating temperature (K), t: holding time (second).
  5. 　前記の滞留が３００℃～５００℃の範囲内の一定温度で保持することを含む請求項４に記載の製造方法。 The manufacturing method according to claim 4, wherein the retention includes holding at a constant temperature within a range of 300 ° C to 500 ° C.
  6. 　前記焼戻しパラメータが１１０００～１４０００、保持時間が１～１５０秒である請求項４または５に記載の製造方法。 The manufacturing method according to claim 4 or 5, wherein the tempering parameter is 11000 to 14000 and the holding time is 1 to 150 seconds.
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