JP2018090877A - High strength steel sheet and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet having all of the tensile strength, a product of tensile strength and total elongation, the deep drawability, the hole expandability, the limitation bulging height and the cross tensile strength of a spot welding part at high level, and a production method thereof.SOLUTION: A high strength steel sheet includes C: 0.15% by mass to 0.35% by mass, a 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% by mass or smaller, and S: 0.01% by mass or smaller, the balance Fe and inevitable impurities, and a steel structure has a ferrite fraction of 5% or smaller, a total fraction of annealed martensite and annealed bainite of 60% or larger, an amount of retained austenite of 10% by mass or larger, an average size of the retained austenite of 1.0 μm or smaller, an amount of the retained austenite having a size of 1.5 μm or larger of 2% or larger of an amount of the total retained austenite, an average size of MA of 1.0 μm or smaller, and a half value width of a concentration distribution of Mn in a carbon enriched region of the same amount as the austenite amount of 0.3% by mass or larger.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength steel sheet that can be used for various applications including automobile parts.

  Steel sheets used for automotive parts and the like are required to have improved strength in order to achieve both collision safety and weight reduction, and are excellent for processing into complex skeleton parts. Molding processability is required.

  Patent Document 1 contains C: 0.02 to 0.15% by mass, Si: 0.3% or less, Mn: 1.0 to 3.5%, with the balance being Fe and inevitable impurities. It discloses a high-strength thin steel sheet exhibiting a tensile strength of 780 MPa or more and a good deep drawability by recrystallizing a slab after cold rolling into a ferrite single phase structure.

JP 2007-092130 A

However, in various applications including automotive parts, it has not only high tensile strength and excellent deep drawability but also excellent strength ductility balance, excellent hole expansion ratio and excellent stretch formability. It has been demanded.
Specifically, the following is required for each of tensile strength, strength ductility balance, deep drawability, hole expansion ratio, and stretch formability.

The tensile strength (TS) is required to be 980 MPa or more.
Regarding the strength ductility balance, the product of TS and total elongation (EL) (TS × EL) is required to be 23000 MPa ·% or more. Furthermore, in order to ensure the moldability at the time of component molding, the LDR indicating deep drawability is 2.05 or more, the hole expansion ratio λ indicating hole expandability is 20% or more, and the stretch formability is It is also required that the limit overhang height shown is 20 mm or more. 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.

  However, in the high-strength steel sheet disclosed in Patent Document 1, it is difficult to satisfy all of these requirements, and a high-strength steel sheet that can satisfy all of these requirements has been demanded.

  The present invention has been made to meet such demands, and includes tensile strength (TS), product of tensile strength (TS) and total elongation (EL) (TS × EL), deep drawability (LDR). ), The hole expansion ratio (λ), the limit overhang height, and the cross tensile strength (SW cross tension) of the spot welds are all at high levels, and an object thereof is to provide a high-strength steel sheet and a method for producing the same.

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 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,
The average size of MA is 1.0 μm or less,
This is a high-strength steel sheet having a half-value width of 0.3% by mass or more of the Mn concentration distribution in the carbon enriched region, which is equal to the amount of retained austenite.

Aspect 2 of the present invention
Total of one or more of Cu, Ni, Mo, Cr and B: 1.0% by mass or less,
The high-strength steel sheet according to aspect 1, further comprising:

  Aspect 3 of the present invention is the high-strength steel sheet according to Aspect 1 or 2, further comprising a total of one or more of V, Nb, Ti, Zr and Hf: 0.2% by mass or less.

  Aspect 4 of the present invention is the high-strength steel sheet according to any one of Aspects 1 to 3, further comprising a total of one or more of Ca, Mg, and REM: 0.01% or less.

In aspect 5 of the present invention, 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% wt%, P: 0.05 wt% or less, S: 0.01 wt% or less, hints, and providing a rolled material balance being Fe and inevitable impurities, said rolled material Ac ₁ point and 0. Holding at a temperature between 2 × Ac ₁ point + 0.8 × Ac ₃ points for 5 seconds or more, then heating to a temperature of Ac ₃ points or more and 950 ° C. or less and holding for 5 to 600 seconds to austenite; After the austenitization, cooling is performed at an average cooling rate of 15 ° C./second or more to a temperature in the range of 300 to 500 ° C., and within a range of 300 to 500 ° C., at a cooling rate of 10 ° C./second or less, 250 seconds or more. Staying for less than a second,
After the residence, cooling at an average cooling rate of 10 ° C./second or more from a temperature of 300 ° C. or higher to a cooling stop temperature between 100 ° C. and 300 ° C .;
It is a manufacturing method of the high strength steel plate including heating from the said cooling stop temperature to the reheating temperature which exists in the range of 300-500 degreeC, and hold | maintaining for 50-1200 second.

  Aspect 6 of the present invention is the manufacturing method according to Aspect 5, wherein the retention includes holding at a constant temperature within a range of 300 to 500 ° C.

  According to the present invention, tensile strength (TS), product of tensile strength (TS) and total elongation (EL) (TS × EL), deep drawability (LDR), hole expansion rate (λ), limit overhang height Further, it is possible to provide a high-strength steel plate having a high level of cross tensile strength of spot welds and a method for producing the same.

It is a diagram explaining the manufacturing method of the high strength steel plate which concerns on this invention, especially heat processing.

As a result of intensive studies, the present inventors have determined that the steel structure (metal structure) in the steel having a predetermined component has a ferrite fraction of 5% or less, a total fraction of tempered martensite and tempered bainite: 60% or more, Residual austenite amount: 10% or more, retained austenite average size: 1.0 μm or less, retained austenite size of 1.5 μm or more, 2% or more of total retained austenite amount, MA average size: 1.0 μm or less, and residual The half-value width of the Mn concentration distribution in the carbon enriched region, which is equal to the austenite amount: 0.3 mass% or more, so that the tensile strength (TS), tensile strength (TS) and total elongation (EL) A high strength steel sheet with high levels of product (TS x EL), deep drawability (LDR), hole expansion ratio (λ), limit overhang height, and cross tensile strength of spot welds. I found out that I could get it.
Details of the high-strength steel sheet and the manufacturing method thereof according to the present invention will be described below.

1. Steel structure Details of the steel structure of the high-strength steel sheet according to 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) Ferrite fraction: 5% or less Although ferrite is generally excellent in workability, it has a problem of low strength. Moreover, when there is much ferrite content, hole expansibility (stretch flangeability) will fall. Therefore, the ferrite fraction is set to 5% or less (5% by volume or less). Further, when the ferrite fraction is 5% or less, an excellent hole expansion ratio λ can be obtained.
The ferrite fraction is preferably 3% or less, more preferably 1% or less, and most preferably 0%.
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) 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 repeller 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) 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 working, and can obtain a large elongation. Further, the formed martensite has a high hardness. For this reason, the outstanding intensity-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 23000 MPa ·% or more.
The amount of retained austenite is preferably 15% or more.

In the high-strength steel sheet 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 bainite, tempered bainite, 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) 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.

  The average size of the MA is observed by observing 3 or more fields of view with a SEM at 3000 times or more by SEM, drawing a straight line of 200 μm or more in any position in the photograph, and measuring the length of the section where the straight line and the MA intersect, It can be obtained by calculating an average value of the intercept lengths.

(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.

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.

  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. Then, the present invention has an average size of retained austenite of 1.0 μm and a ratio (volume ratio) of the amount of retained austenite having a size of 1.5 μm or more to the total amount of retained austenite is 2% or more. It has been found that a high work hardening rate can be maintained and excellent deep drawability (LDR) can be obtained.

  Further, 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 starting point 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.

  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 prepared by using the EBSD (Electron Back Scatter Diffraction Patterns) method, which is a crystal analysis technique 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) The half-value width of Mn concentration distribution in the carbon-enriched region, which is equal to the amount of retained austenite, is 0.3 mass% or more. As described above, most of retained austenite exists in the form of MA. It is difficult to identify only retained austenite with a microscope or SEM. Residual austenite has a carbon solid solubility limit larger than that of ferrite or the like, so that the heat treatment described later causes carbon to concentrate in the retained austenite. Therefore, the element mapping of carbon is performed using EPMA, and the measurement points having the same amount as the amount of retained austenite obtained by the above-mentioned X-ray diffraction in order from the measurement points having the highest carbon concentration are defined as the carbon concentration region. The region can be determined as retained austenite. That is, for example, when the amount of retained austenite is 15% by volume, by selecting 15% from the higher carbon concentration of the measurement points where the carbon content was measured by element mapping, these high carbon concentration measurement points (carbon concentration It can be determined that the crystallization region is retained austenite.
Therefore, the “carbon enriched region having an amount equal to the amount of retained austenite” means a region corresponding to (corresponding to) retained austenite.

  Further, the concentration distribution of Mn in the carbon enrichment region, which is equal to the amount of retained austenite, particularly the half width of the Mn concentration distribution, can also be measured using EPMA. The distribution of the Mn amount at the measurement point determined to be the carbon enrichment region is graphed, and the half width can be obtained therefrom.

  The larger the half width of this Mn concentration distribution, the larger the variation in Mn concentration in the retained austenite (the wider the range of Mn concentration distribution). In the high-strength steel sheet of the present invention, the half width of the Mn concentration distribution is 0.3% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, More preferably, it is 0.75 mass% or more.

  Thus, by varying the amount of Mn contained in the retained austenite (carbon-enriched region), it is possible to form retained austenite having a wide range of stability, from low-stability residual austenite to high-stability residual austenite. Residual austenite with low stability causes a work-induced transformation with a small amount of strain and becomes martensite. Residual austenite having high stability does not cause a processing-induced transformation at a small strain amount, and does not cause a processing-induced transformation until a large strain amount is applied, and becomes martensite. Accordingly, if there is residual austenite having a wide range of stability, the machining-induced transformation will occur continuously from when the amount of strain immediately after the start of processing is small to when processing is advanced and the amount of strain is large. As a result, the n value can be increased over a wide strain range, and the strain dispersibility can be enhanced to achieve high overhang workability.

(7) Other steel structures:
In the present specification, the steel structure other than the above-described ferrite, tempered martensite, tempered bainite and retained austenite is not particularly defined. However, in addition to steel structures such as ferrite, pearlite, untempered bainite, untempered martensite, and the like may exist. 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.

2. Composition The composition of the high-strength steel sheet according to the present invention will be described below. First, basic elements C, Si, Al, Mn, P and S will be described, and further elements that may be selectively added will be described.
In addition, unit% display of a component composition means the mass% altogether.

(1) C: 0.15 to 0.35%
C is an essential element for obtaining a desired structure and ensuring high characteristics (TS × EL) and the like. In order to exhibit such an action effectively, 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. If the C content is 0.30% or less, welding can be performed more easily.

(2) Total of Si and Al: 0.5 to 3.0%
Si and Al each have a function of suppressing the precipitation of cementite and promoting the formation of retained austenite. In order to exhibit such an action effectively, it is necessary to add 0.5% or more in total of Si and Al. However, if the total of Si and Al exceeds 3.0%, MA becomes coarse. Preferably it is 0.7% or more, More preferably, it is 1.0% or more. Preferably it is 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 to 4.0%
Mn is an element necessary for suppressing the formation of ferrite and stabilizing austenite. In order to exhibit such an action effectively, it is necessary to add 1.0% or more. The amount of Mn is preferably 1.3% or more, more preferably 1.6% or more. However, if it exceeds 4.0%, MA becomes coarse and the hole expandability deteriorates. Therefore, the amount of Mn is 4.0% or less. The amount of Mn is preferably 3.5% or less, more preferably 3.0% or less.

(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% (including 0%) or less.

(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% (including 0%) or less.

(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.

  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 of the present invention can be maintained. Other elements that can be selectively contained as described above are exemplified below.

(7) One or more of Cu, Ni, Mo, Cr and B: 1.0% or less in total These elements are useful as steel strengthening elements, and also stabilize residual austenite and ensure a predetermined amount. It is an effective element. In order to effectively exhibit such an action, it is recommended that these elements are contained in a total amount of 0.001% or more, and further 0.01% or more. However, even if these elements are contained excessively, the above effect is saturated and is economically wasteful. Therefore, these elements are added in a total amount of 1.0% or less, and further 0.5% or less. Is preferred.

(8) One or more of V, Nb, Ti, Zr, and Hf: 0.2% or less in total These elements have effects of precipitation strengthening and structure refinement, and are useful elements for increasing the strength. In order to effectively exhibit such an action, it is recommended that these elements are contained in a total amount of 0.01% or more, and further 0.02% or more. However, even if these elements are contained excessively, the above effects are saturated and economically useless. Therefore, these elements are 0.2% or less in total amount, and further 0.1% or less. It is preferable to do this.

(9) One or more of Ca, Mg and REM: 0.01% or less in total These elements are effective elements for controlling the form of sulfides in steel and improving workability. Here, examples of the REM (rare earth element) used in the present invention include Sc, Y, and lanthanoid. In order to effectively exhibit the above action, it is recommended to contain these elements in a total amount of 0.001% or more, and further 0.002% or more. However, even if these elements are contained excessively, the above effects are saturated and economically useless, so these elements are 0.01% or less in total amount, further 0.005% or less. It is preferable to do this.

3. Characteristics As described above, the high-strength steel sheet of the present invention has high levels of TS, TS × EL, LDR, λ, limit overhang height, and cross tensile strength of spot welds. These characteristics of the high-strength steel sheet of the present invention will be described in detail below.

(1) Tensile strength (TS)
It has a TS of 980 MPa or more. Thereby, sufficient strength can be secured.

(2) Product of TS and total elongation (EL) (TS x EL)
TS × EL is 23000 MPa ·% or more. By having TS × EL of 23000 MPa ·% or more, it is possible to obtain a high level of strength ductility balance having both high strength and high ductility. Preferably, TS × EL is 25000 MPa ·% or more.

(3) 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 process 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 sample diameter (maximum diameter D) that has been drawn without breaking.

  The high-strength steel sheet of the present invention has an LDR of 2.05 or more, preferably 2.10 or more, and has excellent deep drawability.

(4) Overhang formability (limit overhang height)
The limit overhang height is an index used for evaluating the overhang formability. The limit overhang height is defined as the punch stroke at the time of occurrence of breakage in which the load decreases rapidly in the load-stroke diagram.
More specifically, a φ120 mm test piece was used, a φ53.4 mm die with a shoulder radius of 8 mm and a φ50 mm ball head punch, a polysheet for lubrication was sandwiched between the punch and the steel plate, and a blank holding force of 1000 kgf The limit overhang height is obtained by measuring the height at break (punch stroke).

  The high strength steel sheet of the present invention has a limit overhang height of 20 mm or more, preferably 21 mm or more.

(5) Hole expansion rate (λ)
The hole expansion ratio λ is obtained according to Japanese Industrial Standard (JIS Z 2256). 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

The high-strength steel sheet 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) 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). Manufacturing Method Next, a manufacturing method of the high strength steel plate according to the present invention will be described.
The inventors of the present invention obtain a high-strength steel sheet having the above-described desired steel structure and, as a result, the above-described desired characteristics, by performing a heat treatment described below on a rolled material having a predetermined composition. I found out.
Details will be described below.

FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to 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) Austenitization As shown in [2] in FIG. 1, the austenitization step is a two-phase coexistence region between Ac ₁ point and Ac ₃ point, more specifically Ac ₁ point and 0.2 × Ac _1. After holding at temperature T ₁ (Ac ₁ ≦ T ₁ ≦ 0.2 × Ac ₁ + 0.8 × Ac ₃ ) between the point + 0.8 × Ac ₃ points for 5 seconds or more, [3] in FIG. As shown in [4], it is heated to a temperature T ₂ (Ac ₃ ≦ T ₂ ) of Ac ₃ points or higher, and held at temperature T ₂ for 5 to 600 seconds to austenite.

The holding temperature T ₁ may be held at a constant temperature between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point as shown in [2] in FIG. such as gradually heated between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point, varies between ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point Ac You may let them. Further, to hold more than 5 seconds and heated to a temperature T _1. Preferably, the holding time is 900 seconds or less.
As described above, Mn is distributed to the austenite side of the coexisting ferrite and austenite by holding for a sufficient time in a relatively low temperature range in the two-phase coexisting region of ferrite and austenite. Can be obtained. And since the Mn concentration of the austenite formed in this Mn-concentrated region and remaining as austenite after heat treatment is high, it becomes possible to increase the variation in the Mn concentration in the carbon-concentrated region, and to realize high stretch formability. .

When the temperature T ₁ is lower than the Ac ₁ point, the amount of Mn-enriched austenite becomes small, variation in Mn concentration in the retained austenite (carbon-enriched region) becomes small, and sufficient stretch formability can be obtained. Can not.
When the temperature T ₁ is higher than 0.2 × Ac ₁ point + 0.8 × Ac ₃ points, the Mn concentration of austenite becomes low, and the variation of Mn concentration in the retained austenite (carbon enriched region) becomes small, which is sufficient. The stretch formability cannot be obtained.
The holding time at the temperature T ₁ is from time shorter 5 seconds, insufficient time for Mn is diffused, Mn enrichment of the austenite is insufficient, variation in Mn in residual austenite (carbon concentrated region) It becomes small and sufficient stretch formability cannot be obtained.
The holding time at the holding temperature T ₁ is preferably long, but 900 seconds or less is preferable from the viewpoint of productivity.

Preferably, the temperature T ₁ is between 0.9 × Ac ₁ point + 0.1 × Ac ₃ points and 0.3 × Ac ₁ point + 0.7 × Ac ₃ points, and the holding time at the temperature T ₁ Is 10 seconds or more and 800 seconds or less. More preferably, the temperature T ₁ is between 0.8 × Ac ₁ point + 0.2 × Ac ₃ points and 0.4 × Ac ₁ point + 0.6 × Ac ₃ points, and is maintained at the temperature T _1. The time is 30 seconds or more and 600 seconds or less.
Incidentally, it is shown as [1] in Figure 1, heating rate of up to temperatures _{T 1} is preferably 5 to 20 ° C. / sec.

Next, as shown in [3] and [4] in FIG. 1, the temperature was raised to a temperature _{T 2} of the following 950 ° C. or higher ₃ points Ac, austenitizing held in temperature _{T 2.} A holding time 5-600 seconds at the temperature _{T 2.}

By heating to Ac ₃ points or more and 950 ° C. or less, when heated to temperature T ₁ , the portion that was ferrite also becomes austenite. In this newly transformed part to austenite, Mn is not concentrated. For this reason, in the austenite, there is a region where Mn is not concentrated together with the above-described Mn concentration region, and in the high-strength steel sheet after the heat treatment, there is a variation in Mn concentration in the retained austenite (carbon concentrated region). It becomes possible to increase the size, and high stretch formability can be realized.

When the temperature T ₂ is lower than the Ac ₃ point or the holding time at the temperature T ₂ is shorter than 5 seconds, the ferrite fraction of the obtained high-strength post-steel sheet exceeds 5%, and the hole expansion ratio decreases. .
When temperature T ₂ exceeds 950 ° C., diffused Mn of Mn concentrated region formed earlier, variation in Mn concentration is reduced, bulging formability is lowered. Therefore, temperature _{T 2} is 950 ° C. or less.
If the holding time at the temperature T ₂ is longer than 600 seconds, Mn concentration of Mn concentrated region by diffusion is low, the variation of the Mn concentration in the retained austenite is reduced, bulging formability is lowered. Holding time at that for temperature T ₂ is less than 600 seconds.
If the holding time at the temperature T ₂ is shorter than 5 seconds, the ferrite fraction of the obtained high strength steel sheet is more than 5%, the hole expanding ratio is lowered.

Preferably, the temperature T ₂ is Ac ₃ point + 10 ° C. or higher, and the holding time at the temperature T ₂ is 10 to 450 seconds. More preferably, temperature _{T 2} is the Ac ₃ point + 20 ° C. or higher, holding time at temperature _{T 2} is 20 to 300 seconds.
Shown in [3] in FIG. 1, the heating from temperature _{T 1} of the temperature _{T 2} is, 0.1 ° C. / sec or more, is preferably carried out at a heating rate of less than 10 ° C. / sec.

In addition, about Ac ₁ point and Ac ₃ point, although you may obtain | require by a measurement, you may calculate by the calculation formula generally known using the composition.
For example, Ac ₁ point and Ac ₃ point can be calculated by using the following formulas (1) and 2 (formulas) (see, for example, “Leslie Steel Material Science” Maruzen, (1985)).

Ac ₁ point (° C.) = 723 + 29.1 × [Si] −10.7 × [Mn] + 16.9 × [Cr] −16.9 × [Ni] (1)
Ac ₃ points (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] −11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (2)
Here, [] shows content shown by the mass% of the element described in it.

(2) Cooling and residence in a temperature range of 300 ° C. to 500 ° C. After the above austenite formation, cooling is performed and, as shown in [6] of FIG. 1, 10 ° C./second or less within a temperature range of 300 to 500 ° C. The liquid is allowed to stay at a cooling rate of 10 seconds or more and less than 250 seconds.
Cooling to 500 ° C. after austenitization is at least at an average cooling rate of 15 ° C./second or more. This is because the formation of ferrite during cooling is suppressed by setting the average cooling rate to 15 ° C./second or more. The average cooling rate is preferably less than 200 ° C./second. This is because excessive heat distortion due to rapid cooling can be prevented by setting the temperature to less than 200 ° C./second.

It is allowed to stay for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 to 500 ° C. That is, in a temperature range of 300 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 T ₃ (ie, the cooling rate is 0 ° C./second) as shown in [6] 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.

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 enrichment region becomes small, the amount of coarse retained austenite becomes 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 250 seconds or more, the carbon concentration region becomes too large, and not only the retained austenite but also the 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.

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 to 500 ° C. It is preferable to retain 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) Cooling to a cooling stop temperature T ₄ between 100 ° C. and 300 ° C. After the above-mentioned residence, as shown in [7] of FIG. Cooling is performed at an average cooling rate of 10 ° C./second or more to a cooling stop temperature T ₄ between the following. In one of the preferred embodiments, as shown in [7] of FIG. 1, the end temperature of the above-mentioned residence 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. Cooling stop temperature T ₄ of 100 ° C. or higher, by controlling within a temperature range of 300 ° C. or less, by adjusting the amount of austenite remaining without transformation to martensite, to control the final amount of retained austenite.

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 exceeds 300 ° C., coarse untransformed austenite increases and remains even after cooling, so that the MA size becomes coarse and the hole expansion ratio λ decreases.
In addition, a preferable cooling rate is 15 degreeC / second 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./second or more, and a more preferable cooling stop temperature is 140 ° C. or more and 260 ° C. or less.

  As shown in [8] in FIG. 1, the cooling stop temperature may be maintained. 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) Reheating to a temperature range of 300 to 500 ° C. As shown in [9] in FIG. 1, heating is performed from the cooling stop temperature T ₄ to a reheating temperature T _{5 in} the range of 300 to 500 ° C. The heating rate is not particularly limited. After reaching the reheating temperature _{T 5,} the temperature holding 50 to 1200 seconds, as shown in [10] in FIG.

By this reheating, the carbon in the martensite is expelled to promote the carbon concentration of the surrounding austenite, and the austenite can be stabilized. Thereby, the amount of retained austenite finally obtained can be increased.
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. Further, if the holding is not performed or the holding time is shorter than 50 seconds, the carbon diffusion is similarly insufficient. For this reason, holding for 50 seconds or more is performed at the reheating temperature.
When the reheating temperature is higher than 500 ° C., carbon is precipitated as cementite, and a sufficient amount of retained austenite cannot be obtained, so TS × EL is lowered. When the holding time is longer than 1200 seconds, carbon is similarly precipitated as cementite. For this reason, the holding time is 1200 seconds or less.
The preferable reheating temperature is 320 to 480 ° C. In this case, the upper limit of the holding time is preferably 900 seconds or less. A more preferable reheating temperature is 320 to 460 ° C. In this case, the upper limit of the holding time is preferably 600 seconds or less.

After reheating, as shown in [11] of 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.
The high-strength steel sheet of the present invention can be obtained by the above heat treatment.

  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 this invention by the manufacturing method different from the manufacturing method mentioned above by trial and error can be obtained. There is a possibility.

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 includes _three Ac values 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 plate is subjected to multi-stage rolling. The thickness was 2.5 mm. At this time, the end temperature of the hot rolling was 880 ° C. Then, it cooled to 500 degreeC at 30 degree-C / sec, stopped cooling, and after inserting into the furnace heated at 500 degreeC, it hold | maintained for 30 minutes, and then cooled in the furnace, and it was set as the hot rolled sheet steel.
The hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to 1.5 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. 9 is a sample that was 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 [6] in FIG. That is, sample No. Reference numeral 9 is a sample (a sample in which the steps corresponding to [6] and [7] in FIG. 1 are skipped) which is rapidly cooled to 200 ° C. at a rapid temperature after being austenitized. Sample No. Reference numeral 14 denotes a sample that was not held for 5 seconds or more at a temperature between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point in the step corresponding to [2] in FIG. That is, sample No. Reference numeral 14 denotes a sample obtained by heating the cold-rolled steel sheet at a stretch to the austenitizing temperature (850 ° C.) (a sample skipping the steps corresponding to [2] and [3] in FIG. 1).
In Tables 1 to 4, the underlined numerical values indicate that they are out of the scope of the present invention.

2. Steel structure According to the method described above for each sample, the ferrite fraction, the total fraction of tempered martensite and tempered bainite (described in Table 3 as “tempered M / B”), the amount of retained austenite, the average size of MA, and the retained austenite Mean size, ratio of residual austenite of size 1.5 μm or more to total austenite (described in Table 3 as “residual austenite ratio of 1.5 μm or more”), and half width of Mn concentration distribution in carbon enriched region Asked. 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.

3. Mechanical characteristics About the obtained sample, TS and EL were measured using the tensile tester, and TSxEL was computed. Further, the hole expansion ratio λ, LDR, the limit overhang height, and the cross tensile strength (SW cross tension) of the spot weld were obtained by the above-described methods. Table 4 shows the obtained results. In Table 4, the properties of the steel sheet after the heat treatment are as follows: tensile strength: 980 MPa or more, TS × EL: 23000 MPa% or more, deep drawability (LDR): 2.05 or more, limit height in stretch forming: 20 mm or more, And the sample which satisfy | fills all of the hole expansion rate (lambda): 20% or more was set as the pass ((circle)), and the sample other than that was set as the disqualification (x).

4). Summary Consider the results in Table 4. Sample No. 2, 3, 5, 6, 12, 13, 15, 17, 27 to 29 and 36 to 55 are examples that satisfy all the requirements (composition, production conditions, and steel structure) defined in the present invention. All of these samples have a tensile strength (TS) of 980 MPa or more, TS × EL of 23000 MPa or more, LDR of 2.05 or more, a hole expansion ratio (λ) of 20% or more, a limit overhang height of 20 mm or more, 6 kN The cross tensile strength of the spot weld is achieved.

  In contrast, sample no. No. 1 [6] Since the primary stop temperature is as low as 280 ° C., the carbon concentration region is reduced and the deep drawability is reduced.

  Sample No. 4 and 7 had [6] long primary cooling time, so the average size of MA became coarse, and as a result, the hole expansion ratio decreased.

  Sample No. No. 8 [6] Since the primary stop temperature was as high as 520 ° C., the MA average size became coarse, and as a result, the hole expansion rate decreased.

  Sample No. 9 is cooling after austenitization, and it did not retain for 10 seconds or more at the temperature of 300-500 degreeC. As a result, the amount of coarse retained austenite decreased, and the deep drawability deteriorated.

Sample No. _No. 10 [2] Since the primary holding temperature was lower than the Ac ₁ point, variation in the Mn concentration in the carbon concentration region was reduced, and the stretch formability was lowered.

Sample No. _No. 11 [2] Since the primary holding temperature was higher than 0.2 × Ac ₁ point + 0.8 × Ac ₃ points, the variation in Mn concentration in the carbon concentration region was reduced, and the stretch formability was lowered.

Sample No. _No. 14 did not hold at a temperature between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point. Therefore, the variation of Mn in the carbon concentration region was reduced, and the stretch formability was lowered.

  Sample No. No. 16 [8] Since the stop temperature is high, the MA average size becomes coarse, and the hole expansion ratio λ decreases.

  Sample No. No. 18 [8] Since the stop temperature was low, the amount of retained austenite was insufficient, and although the TS increased, the EL decreased and the TS × EL balance decreased.

  Sample No. No. 19 [4] Since the heating temperature was high, the variation in the Mn concentration in the carbon enriched region was reduced, and the stretch formability was reduced.

  Sample No. No. 20 [4] Since the heating time was long, the variation in the Mn concentration in the carbon enriched region was reduced, and the stretch formability was reduced.

Sample No. No. 21 [4] Since the heating temperature was lower than Ac ₃ point, the ferrite fraction exceeded 5%, and the hole expansion rate decreased.

  Sample No. No. 22 had a short [10] holding time, so that the amount of retained austenite was insufficient due to insufficient carbon diffusion, and TS × EL decreased.

  Sample No. Since No. 23 had a long [10] retention time, carbon precipitated as cementite, the amount of retained austenite was insufficient, and TS × EL decreased.

  Sample No. No. 24, [5] Since the cooling rate to the primary stop temperature was small, ferrite was generated, TS decreased, and λ also decreased.

  Sample No. In No. 25, since the [10] holding temperature was high, carbon precipitated as cementite, the amount of retained austenite was insufficient, and TS × EL decreased.

  Sample No. No. 26 had a low [10] holding temperature, so that the amount of retained austenite was insufficient due to insufficient carbon diffusion, and TS × EL decreased.

  Sample No. No. 30 had a low C content, so a desired structure was not obtained and TS × EL was lowered.

  Sample No. No. 31 had a large Mn content, so the MA average size was large and the hole expandability was reduced.

  Sample No. No. 32 has a low Mn content, so the dispersion of Mn in the carbon-concentrated region was reduced, and λ was lowered.

  Sample No. In No. 33, since the total content of Si and Al was small, carbon precipitated as cementite, and the amount of retained austenite was insufficient, resulting in a decrease in TS × EL.

  Sample No. No. 34 has a high C content, resulting in a low SW cross tensile strength.

  Sample No. No. 35 had a large total content of Si and Al, so the MA became coarse and λ decreased.

Thus, a steel sheet satisfying the composition and steel structure defined in the present invention has a tensile strength (TS), a product of (TS) and total elongation (EL) (TS × EL), LDR, and hole expansion ratio (λ). It was confirmed that the stretch formability and the cross tensile strength of the spot welded portion were both high.
Moreover, according to the manufacturing method of this invention, it has confirmed that the steel plate which satisfy | fills the composition and steel structure prescribed | regulated to this invention can be manufactured.

Claims (6)

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 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,
The average size of MA is 1.0 μm or less,
A high-strength steel sheet having a half-value width of 0.3% by mass or more of the concentration distribution of Mn in the carbon enriched region, which is equal to the amount of retained austenite. Total of one or more of Cu, Ni, Mo, Cr and B: 1.0% by mass or less,
The high-strength steel sheet according to claim 1, further comprising: Total of one or more of V, Nb, Ti, Zr and Hf: 0.2% by mass or less,
The high-strength steel sheet according to claim 1 or 2, further comprising: Total of one or more of Ca, Mg and REM: 0.01% or less,
The high-strength steel sheet according to any one of claims 1 to 3, further comprising: C: 0.15% by mass to 0.35% by mass, Si and Al in total: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.0. Preparing a rolled material comprising 05% by mass or less, S: 0.01% by mass or less, and the balance consisting of Fe and inevitable impurities;
The rolled material is held at a temperature between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point for 5 seconds or more, and then heated to a temperature of Ac ₃ point or more and 950 ° C. or less, Holding for 600 seconds to austenite,
After the austenitization, it is cooled to a temperature in the range of 300 to 500 ° C. at an average cooling rate of 15 ° C./second or more, and is 10 to 250 seconds at a cooling rate of 10 ° C./second or less in the range of 300 to 500 ° C. Staying less than,
After the residence, cooling at an average cooling rate of 10 ° C./second or more from a temperature of 300 ° C. or higher to a cooling stop temperature between 100 ° C. and 300 ° C .;
Heating from the cooling stop temperature to a reheating temperature in the range of 300-500 ° C. and holding for 50-1200 seconds;
A method for producing a high-strength steel sheet including   The manufacturing method according to claim 5, wherein the retention includes holding at a constant temperature within a range of 300 to 500 ° C.

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