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EP4407062A1 - Tôle d'acier laminée à froid à très haute résistance ayant une excellente extensibilité de trou et son procédé de fabrication - Google Patents

Tôle d'acier laminée à froid à très haute résistance ayant une excellente extensibilité de trou et son procédé de fabrication Download PDF

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Publication number
EP4407062A1
EP4407062A1 EP22873199.8A EP22873199A EP4407062A1 EP 4407062 A1 EP4407062 A1 EP 4407062A1 EP 22873199 A EP22873199 A EP 22873199A EP 4407062 A1 EP4407062 A1 EP 4407062A1
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EP
European Patent Office
Prior art keywords
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steel sheet
rolled steel
excluding
cold
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP22873199.8A
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German (de)
English (en)
Inventor
Min-Seo KOO
Eun-Young Kim
Sang-Hyun Kim
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP4407062A1 publication Critical patent/EP4407062A1/fr
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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
    • 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
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—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
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to an ultra-high strength cold-rolled steel sheet having excellent expandability and a method for manufacturing the same and, more specifically, to an ultra-high strength cold-rolled steel sheet having excellent hole expandability which can mainly be used for automobile collision and structural members, and to a method for manufacturing the same.
  • a steel sheet for automobiles should be lighter to preserve the global environment, while the steel sheet for automobiles needs to meet the conflicting goal of securing collision safety for the safety of passengers.
  • various steel sheets for automobiles such as dual phase (DP) steel, transformation induced plasticity (TRIP) steel, complex (CP) steel, and the like, are being developed.
  • DP dual phase
  • TRIP transformation induced plasticity
  • CP complex
  • tensile strength that can be realized in advanced high strength steel (AHSS) is limited to about 1200Mpa. Accordingly, when manufacturing structural members to secure collision safety, a hot press forming method securing final strength through rapid cooling (water cooling) through direct contact with a die after being formed at high temperature, is in the spotlight, but expansion of application thereof is not significant due to high facility investment costs and high heat treatment and processing costs.
  • a roll forming method which is more productive than general press forming and hot press forming, is a method of producing complex shapes through multi-stage roll forming, which is generally applied to forming parts of ultra-high strength materials with low elongation, and the application thereof is also expanding.
  • a steel sheet applied to such a roll forming method is mainly manufactured in a continuous annealing facility equipped with a water cooling facility.
  • shape quality is deteriorated due to deviation in temperatures thereof in a width direction and a length direction during water cooling, so that there is a disadvantage in that workability is deteriorated and material deviation for each location occurs when the roll forming method is applied. Therefore, there is a need to devise an alternative to a rapid cooling method through water cooling.
  • An aspect of the present disclosure is to provide an ultra-high strength cold-rolled steel sheet having excellent hole expandability and a method for manufacturing the same.
  • an ultra-high strength cold-rolled steel sheet having excellent hole expandability including by weight: C: 0.2-0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0-2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities, wherein a microstructure includes a single-phase structure of tempered martensite structure or a mixed structure of martensite and tempered martensite, wherein the microstructure has an F HAGB of 60% or more by area and L HAGB of 8 mm
  • F HAGB is a fraction of grains with high-hardness angle grain boundaries
  • L HAGB is a total length of grain boundaries with high-hardness angle grain boundaries
  • the high-hardness grain boundaries refer to grain boundaries with a mismatch angle between adjacent grains of 15° or more.
  • a method for manufacturing an ultra-high cold-rolled steel sheet having excellent hole expandability including operations of: heating a steel slab including by weight: C: 0.2-0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0-2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities, to a temperature in a range of 1100 to 1300°C; finish hot rolling the heated steel slab at a temperature of Ar3 or higher; coiling the hot-rolled steel sheet at a temperature
  • an ultra-high cold-rolled steel sheet having excellent hole expandability and a tensile strength of 1470 MPa or more and a method for manufacturing the same may be provided.
  • Carbon (C) is an element added to secure strength of martensite, and it is preferable that C is preferably added in an amount of 0.2% or more for the above effect. However, if the C content exceeds 0.4%, weldability may be poor. Therefore, it is preferable that the C content is in a range of 0.2 to 0.4%. A lower limit of the C content is more preferably 0.21%, and even more preferably 0.22%. An upper limit of the C content is more preferably 0.3%, even more preferably 0.29%, and most preferably 0.28%.
  • Silicon (Si), a ferrite stabilizing element, has a disadvantage of weakening strength by promoting formation of ferrite during slow cooling after annealing in a continuous annealing furnace with a slow cooling section.
  • the Si content is in a range of 0.5% or less.
  • the Si content is more preferably 0.4% or less, and even more preferably 0.3% or less.
  • Manganese (Mn) is an element which suppresses ferrite formation and facilitates austenite formation.
  • Mn content is less than 1.0%, ferrite is easily formed during slow cooling, while when the Mn content exceeds 2.0%, bending workability, delayed fracture resistance, and weldability may be reduced. Therefore, the Mn content is preferably in a range of 1.0 to 2.0%.
  • a lower limit of the Mn content is more preferably 1.3%, and even more preferably 1.5%.
  • Phosphorus (P) is an impurity element, and if a P content exceeds 0.03%, weldability decreases and a risk of steel brittleness increases, and a possibility of causing dent defects increases, so an upper limit of the P content is limited to be 0.03%.
  • the P content is more preferably 0.025% or less, and even more preferably 0.02% or less.
  • S Sulfur
  • S is an impurity element that impairs ductility and weldability of a steel sheet. If the S content exceeds 0.015%, a possibility of impairing the ductility and weldability of the steel sheet is high, so an upper limit of the S content is preferably limited to be 0.015%.
  • the S content is more preferably 0.01% or less, and even more preferably 0.005% or less.
  • Aluminum (Al) is an element that expands a ferrite transformation section, and when using a continuous annealing process with a slow cooling section as in the present disclosure, there is a disadvantage of promoting ferrite formation, and high-temperature hot rolling properties may be reduced due to AlN formation. Therefore, an upper limit thereof is limited to 0.1%.
  • the Al content is more preferably 0.07% or less, and even more preferably 0.05% or less.
  • Chromium (Cr) is an alloy element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation, and when using a continuous annealing process with slow cooling as in the present disclosure, there is an advantage of suppressing ferrite formation.
  • the Cr content exceeds 0.5%, delayed fracture resistance may deteriorate, carbides such as CrC, or the like may be formed, to impair hole expandability and bending workability, and costs may increase due to excessive alloy input. Therefore, the Cr content is preferably in a range of 0.5% or less. The Cr content is more preferably 0.4% or less, and even more preferably 0.3% or less.
  • Molybdenum has an effect of improving quenchability of steel, an effect of generating fine carbides containing Mo, serving as a hydrogen trap site, and an effect of improving delayed fracture resistance by refining martensite.
  • Mo content is 0.2% or more, phosphatability may be deteriorated, and there is a problem of increased cost, so it is preferable to limit the range. Therefore, the Mo content is preferably in a range of less than 0.2%.
  • a lower limit of the Mo content is more preferably 0.03%, even more preferably 0.05%, and most preferably 0.1%.
  • Titanium (Ti) is a nitride forming element, and is an element scavenging by precipitating N in steel into TiN.
  • Ti When Ti is not added, there is a possibility that cracks may occur during continuous casting due to AlN formation.
  • the Ti content exceeds 0.1%, the strength of martensite may be reduced by additional carbide precipitation in addition to removal of dissolved N, and hole expandability and bending workability may be impaired by the formation of carbides and nitrides such as TiC and TiN. Therefore, the Ti content is preferably in a range of 0.1% or less.
  • the Ti content is more preferably 0.07% or less, and even more preferably 0.05% or less.
  • Ti may be added in a chemical equivalent amount of 48/14*[N] or more.
  • Niobium is an element that segregates at austenite grain boundaries and suppresses coarsening of austenite grains during annealing heat treatment.
  • the Nb content is preferably in a range of 0.1% or less.
  • the Nb content is more preferably 0.08% or less, and even more preferably 0.06% or less.
  • B Boron
  • B is an element that suppresses formation of ferrite, and accordingly, in the present disclosure, B has an advantage of suppressing the formation of ferrite during cooling after annealing.
  • the B content is preferably in a range of 0.005% or less.
  • the B content is more preferably 0.004% or less, and most preferably 0.003% or less.
  • N Nitrogen
  • the N content is more preferably 0.008% or less, and most preferably 0.006% or less.
  • the remaining component of the present disclosure is iron (Fe).
  • Fe iron
  • the component since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
  • the cold-rolled steel sheet of the present disclosure may further include one or more of Cu: 0.5% or less and Ni: 0.5% or less.
  • Copper (Cu) improves corrosion resistance, and has an effect of suppressing hydrogen intrusion by being coated on a surface of the steel sheet. However, if the Cu content exceeds 0.5%, it may cause surface defects. Therefore, the Cu content is preferably in a range of 0.5% or less. The Cu content is more preferably 0.4% or less, and even more preferably 0.3% or less.
  • cold-rolled steel sheet of the present disclosure may further include Sb: 0.05% or less.
  • Sb is an element that contributes to high strength and improved delayed fracture resistance by suppressing oxidation and nitridation of a surface layer.
  • the Sb content is preferably in a range of 0.05% or less.
  • the Sb content is more preferably 0.04% or less, and even more preferably 0.03% or less.
  • the microstructure of the cold-rolled steel sheet of the present disclosure preferably includes a single-phase structure of tempered martensite or a mixed structure of martensite and tempered martensite.
  • the microstructure since the microstructure includes a tempered martensite a single-phase structure of tempered martensite or a mixed structure of martensite and tempered martensite, an effect of high yield strength and excellent hole expandability may be obtained.
  • the microstructure of the present disclosure is a single-phase structure of tempered martensite, but since tempering does not occur completely during the manufacturing process, the microstructure of the present disclosure may include a mixed structure of martensite and tempered martensite.
  • a fraction of the mixed structure of martensite and tempered martensite is not particularly limited, but for example, the mixed structure may have a fraction of tempered martensite of 80% or more by area, and more preferably 90% or more by area.
  • the microstructure of the present disclosure preferably has an F HAGB of 60% or more by area and an L HAGB of 8 mm or more per unit area of 45 ⁇ m ⁇ 45 ⁇ m.
  • the F HAGB is a fraction of grains with high-hardness angle grain boundaries
  • the L HAGB is a total length of grain boundaries with high-hardness angle grain boundaries
  • the high-hardness grain boundaries refer to grain boundaries having a mismatch angle between adjacent grains of 15° or more.
  • the F HAGB is less than 60% by area or the L HAGB is less than 8mm, there is a disadvantage in that hole expandability is inferior.
  • the cold-rolled steel sheet of the present disclosure may have an average particle size of prior austenite of 6 ⁇ m or less.
  • the average particle size of the prior austenite exceeds 6 ⁇ m, there may be a disadvantage in that hole expandability and bending workability are inferior.
  • the cold-rolled steel sheet of the present disclosure provided as described above may have a tensile strength (TS) of 1470 MPa or more, and a value of a tensile strength (TS) (MPa) ⁇ a hole expansion ratio (HER) (%) of 73,500 MPa ⁇ % or more, so that ultra-high strength and excellent hole expandability may be secured at the same time.
  • TS tensile strength
  • HER hole expansion ratio
  • a slab satisfying the above-described alloy composition is heated to a temperature in a range of 1100 to 1300°C.
  • the heating temperature is less than 1100°C, a problem in that a hot rolling load increases rapidly may occur, and when the heating temperature exceeds 1300°C, an amount of surface scales increases, which may result in material loss. Therefore, the heating temperature of the steel slab is preferably in a range of 1100 to 1300°C.
  • the heated steel slab is subjected to finish hot rolling at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet.
  • the Ar3 temperature is a temperature at which ferrite begins to appear when austenite is cooled.
  • the finish hot rolling temperature is more preferably 800°C or higher, even more preferably 850°C or higher, and most preferably 900°C or higher.
  • the hot rolled steel sheet is coiled at a temperature of 720°C or lower.
  • a temperature of 720°C or lower exceeds 720°C, an oxide film may be excessively generated on a surface of the steel sheet, which may cause defects.
  • the strength of the hot-rolled steel sheet increases, which may cause a disadvantage of increasing a rolling load of cold rolling, which is a post-process, but since this is not a factor that makes actual production impossible, in the present disclosure, a lower limit of the coiling temperature is not specifically limited.
  • the coiling temperature is more preferably 700°C or lower, even more preferably 680°C or lower, and most preferably 650°C or lower.
  • the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • the cold rolling process there is no particular limitation on the cold rolling process, and all processes commonly used in the technical field may be used. Meanwhile, a pickling process may be further performed prior to the cold rolling process.
  • the cold-rolled steel sheet is subjected to an annealing heat treatment at a temperature in a range of 780 to 900°C.
  • the annealing heat treatment temperature is less than 780°C, the strength may be reduced due to formation of a large amount of ferrite.
  • material deviation may occur due to temperature gradients at the top and end of the steel material of the present disclosure.
  • the annealing heat treatment temperature exceeds 900°C, durability of a continuous annealing furnace may deteriorate, resulting in a difficulty in product production. Therefore, the annealing heat treatment temperature is preferably in a range of 780 to 900°C.
  • a lower limit of the annealing heat treatment temperature is more preferably 800°C, even more preferably 820°C, and most preferably 840°C.
  • An upper limit of the annealing heat treatment temperature is more preferably 880°C, and even more preferably 860°C.
  • a continuous annealing furnace has a slow cooling section after annealing heat treatment. That is, after the annealing heat treatment process described above, slow cooling is performed for a certain period.
  • a slow cooling section of 100 to 200 m after annealing, and a soft phase such as ferrite is formed by slow cooling after annealing at a high temperature, resulting in a disadvantage in that it is difficult to manufacture ultra-high steel.
  • a slow cooling section of 160 m when there is a slow cooling section of 160 m, when a sheet-passing speed of steel sheet is 160 m per minute, a time maintained in the slow cooling section is 60 seconds, and when an annealing temperature is 830°C and a final temperature of the slow cooling section is 60 seconds.
  • a cooling rate in the slow cooling section is very low at 3°C/s. Thereby, a possibility of generating a soft phase such as ferrite is very high.
  • an additional cooling device should be introduced, which may cause problems such as manufacturing costs, equipment replacement, or the like.
  • the slowly-cooled cold-rolled steel sheet is rapidly cooled to a temperature of 150°C or lower at a cooling rate of 40°C/s or more.
  • the microstructure may be transformed into martensite.
  • the rapid cooling rate is more preferably 50°C/s or more, more preferably 60°C/s or more, and most preferably 70°C/s or more.
  • the rapid cooling end temperature is more preferably 140°C or lower, and even more preferably 130°C or lower.
  • the rapidly-cooled cold-rolled steel sheet is reheated and is subjected to an overaging heat treatment at a temperature in a range of 180 to 240°C.
  • an overaging heat treatment at a temperature in a range of 180 to 240°C.
  • martensite obtained through the above-described rapid cooling process may be transformed into tempered martensite.
  • the reheating and overaging heat treatment temperature is less than 180°C, tempering is not sufficiently performed, resulting in a disadvantage in that low yield strength is low and insufficient toughness may not be achieved, and when the reheating and overaging heat treatment temperature exceeds 240°C, a large amount of carbides are precipitated and coarsened, resulting in a disadvantage in that bending workability may be inferior.
  • a lower limit of the reheating and overaging heat treatment temperature is more preferably 190°C, and even more preferably 200°C.
  • An upper limit of the reheating and overaging heat treatment temperature is more preferably 230°C, and even more preferably 220°C.
  • the overaging heat treatment may be performed for more than 400 seconds.
  • the overaging heat treatment time is less than 400 seconds, there is a disadvantage in that the yield strength is low because tempering is not sufficiently achieved.
  • the overaging heat treatment time is not specifically limited, but it is difficult to exceed 1000 seconds due to the characteristics of continuous annealing equipment.
  • the lower limit of the overaging heat treatment time is more preferably 500 seconds, and even more preferably 600 seconds.
  • Molten steel having the alloy composition shown in Table 1 below was cast into an ingot and then sized and rolled to prepare a steel slab.
  • the steel slab was heated to a temperature of 1200°C, maintained for 1 hour, and then was subjected to finish hot rolling at a temperature of 900°C, charged into a furnace, which is pre-heated to a temperature of 550°C, maintained for 1 hour, and then furnace-cooled to simulate hot rolling.
  • cold rolling was performed at a cold rolling reduction rate of 50%, and then was subjected to an annealing heat treatment, slow cooling, rapid cooling, reheating, and overaging heat treatment under the conditions shown in Table 2 below to form a cold-rolled steel sheet.
  • Fm (Fk ⁇ 10 6 ) / ( (0.67n + z) ⁇ V 2 ).
  • Fm is an average grain size of prior austenite
  • Fk is a total area of an image of the microstructure
  • Z is the number of grains inside a circle
  • n is the number of grains spanning the circle
  • V is magnification when measuring the microstructure.
  • F HAGB and L HAGB were obtained by measuring a microstructure within a measurement area of 45 ⁇ m ⁇ 45 ⁇ m and a measurement interval of 0.75 um using electron backscattering diffraction (EBSD), and then analyzing the same based on a threshold of 15° using TSL-OIM software.
  • EBSD electron backscattering diffraction
  • a tensile strength (TS) and yield strength (YS) were measured by collecting a JIS No. 5 tensile test sample in a direction perpendicular to a rolling direction and then performing a tensile test at a strain rate of 0.01/s.
  • a hole expansion ratio was measured according to ISO 16630 standards. A sample dimension was 120mm ⁇ 120mm, and an initial hole diameter was 10mm based on a clearance standard of 12%. A punching holding load was 20 tons, and a test speed was 12 mm/min.
  • the cold-rolled steel sheet was processed into a sample having a width of 100mm * a length of 30 mm, then was subjected to a 90° bending test, and then a minimum bending radius R at which no cracks occur was divided by a thickness (t) of a test sample by checking cracks in a bending portion using a microscope, to obtain an R/t value.
  • a thickness (t) of a test sample by checking cracks in a bending portion using a microscope, to obtain an R/t value.
  • FIG. 1 is a photograph of Inventive Example 5 and Comparative Example 5 observed with an optical microscope. As can be seen from FIG. 1 , in Inventive Example 5, an average grain size of prior austenite is fine, while in Comparative Example 5, an average grain size of prior austenite is relatively large.
  • FIG. 2 is a photograph illustrating a microstructure of Inventive Example 5 and Comparative Example 5 using electron backscattering diffraction attached to a scanning electron microscope and then analyzing high-hardness angle grain boundaries and low-hardness angle grain boundaries.
  • FIG. 2 in Inventive Example 5, it can be seen that F HAGB and L HAGB had high values, while in Comparative Example 5, F HAGB and L HAGB had low values.

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EP22873199.8A 2021-09-23 2022-09-23 Tôle d'acier laminée à froid à très haute résistance ayant une excellente extensibilité de trou et son procédé de fabrication Pending EP4407062A1 (fr)

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PCT/KR2022/014244 WO2023048495A1 (fr) 2021-09-23 2022-09-23 Tôle d'acier laminée à froid à très haute résistance ayant une excellente extensibilité de trou et son procédé de fabrication

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