WO2023008516A1 - 鋼板及びその製造方法 - Google Patents

鋼板及びその製造方法 Download PDF

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Publication number
WO2023008516A1
WO2023008516A1 PCT/JP2022/029080 JP2022029080W WO2023008516A1 WO 2023008516 A1 WO2023008516 A1 WO 2023008516A1 JP 2022029080 W JP2022029080 W JP 2022029080W WO 2023008516 A1 WO2023008516 A1 WO 2023008516A1
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Prior art keywords
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steel sheet
ferrite
content
rolling
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PCT/JP2022/029080
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English (en)
French (fr)
Japanese (ja)
Inventor
拓 宮川
恭平 石川
卓史 横山
健悟 竹田
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日本製鉄株式会社
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Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to CN202280050160.XA priority Critical patent/CN117751205A/zh
Priority to JP2023538615A priority patent/JPWO2023008516A1/ja
Priority to US18/576,123 priority patent/US20240327965A1/en
Priority to MX2024000890A priority patent/MX2024000890A/es
Priority to KR1020247002063A priority patent/KR20240025615A/ko
Priority to EP22849577.6A priority patent/EP4379083A1/en
Publication of WO2023008516A1 publication Critical patent/WO2023008516A1/ja

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    • 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
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    • C21D1/84—Controlled slow cooling
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a steel sheet and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2021-122923 filed in Japan on July 28, 2021, the content of which is incorporated herein.
  • Patent Document 1 describes a steel sheet excellent in elongation, hole expansibility, bending workability and delayed fracture resistance, in mass%, C: 0.15 to 0.25% , Si: 1.00 to 2.20%, Mn: 2.00 to 3.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.01 to 0.50%, N: 0.010% or less, B: 0.0003 to 0.0050%, Ti: 0.005 to 0.05%, Cu: 0.003 to 0.50%, Ni: 0.003 to Contains one or more selected from 0.50%, Sn: 0.003 to 0.50%, Co: 0.003 to 0.05%, Mo: 0.003 to 0.50% In addition, the balance is Fe and unavoidable impurities, and the microstructure has a volume fraction of 15% or less (including 0%) of ferrite having an average crystal grain size of 2 ⁇ m or less, and an average crystal grain size of 2 ⁇ m.
  • the following retained austenite is 2 to 15% in volume fraction, martensite with an average crystal grain size of 3 ⁇ m or less is 10% or less in volume fraction (including 0%), and the balance is bainite with an average crystal grain size of 6 ⁇ m or less and
  • a high-strength TRIP steel sheet is disclosed which is tempered martensite and contains an average of 10 or more cementite grains having a grain size of 0.04 ⁇ m or more in bainite and tempered martensite grains.
  • Patent Document 2 discloses that a steel sheet having both high tensile strength (TS): 980 MPa or more and excellent bendability has a specific chemical composition, a ferrite phase area ratio of 30% or more and 70% or less, martensite The area ratio of the phase is 30% or more and 70% or less, the average grain size of the ferrite grains is 3.5 ⁇ m or less, the standard deviation of the grain size of the ferrite grains is 1.5 ⁇ m or less, and the average aspect ratio of the ferrite grains is 1.8
  • high-strength steel having a specific steel structure such as an average grain size of martensite grains of 3.0 ⁇ m or less and an average aspect ratio of martensite grains of 2.5 or less, and a tensile strength of 980 MPa or more A cold rolled steel sheet is disclosed.
  • a steel sheet having a yield strength (YS) of 780 MPa or more, a tensile strength (TS) of 1180 MPa or more, and excellent spot weldability, ductility and bending workability has a C content of 0.15% or less
  • the area ratio of ferrite is 8 to 45%
  • the area ratio of martensite is 55 to 85%
  • the ratio of martensite adjacent to ferrite alone to the entire structure is 15% or less
  • the average grain size of ferrite and martensite is 10 ⁇ m or less
  • the area ratio of ferrite having a crystal grain size of 10 ⁇ m or more among the ferrite existing in the range of 20 ⁇ m depth from the steel plate surface to 100 ⁇ m depth from the steel plate surface is less than 5%.
  • Patent Document 4 as a steel sheet with little variation in mechanical properties (especially strength and ductility), in mass%, C: 0.10 to 0.25%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.05%, N: 0.01% or less, and the balance is iron and unavoidable impurities, and contains 20 to 50% by area ratio of ferrite, which is the first soft phase, and the remainder is the hard second phase, and has a structure composed of tempered martensite and/or tempered bainite.
  • the total area of particles with an average particle size of 10 to 25 ⁇ m accounts for 80% or more of the total area of all the ferrite particles, and is present in all the ferrite particles.
  • Disclosed is a high-strength cold-rolled steel sheet having a dispersed state of cementite particles having an equivalent circle diameter of 0.3 ⁇ m or more, which is more than 0.15 and 1.0 or less per 1 ⁇ m 2 of the ferrite, and having a tensile strength of 980 MPa or more.
  • Patent Documents 1 to 4 Although it is mentioned to achieve high strength and to have good ductility and bendability, there is a reference to suppressing the occurrence of cracks when press molding is performed under such severe conditions. there is room for improvement. Therefore, it is an object of the present invention to provide a steel sheet that can suppress the occurrence of cracks during press forming (excellent rupture resistance) even when a strain amount exceeding the uniform elongation is introduced, and a method for manufacturing the same.
  • the present inventors have found that when a steel sheet containing ferrite and/or bainite and martensite and/or tempered martensite is subjected to a strain amount exceeding the uniform elongation, cracking is suppressed (excellent fracture resistance is achieved). (obtained) method was examined. As a result, it was found that cracking during press forming can be suppressed if a high true stress can be maintained even at a strain amount equal to or greater than the uniform elongation.
  • a steel sheet according to an aspect of the present invention has, in mass %, C: 0.07 to 0.15%, Si: 0.01 to 2.00%, Mn: 1.5 to 3.0%, P: 0-0.020%, S: 0-0.0200%, Al: 0.001-1.000%, N: 0-0.0200%, O: 0-0.0200%, Co: 0 ⁇ 0.500%, Ni: 0-1.000%, Cu: 0-0.500%, Mo: 0-1.000%, Cr: 0-2.000%, Ti: 0-0.5000% , Nb: 0-0.50%, V: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, B: 0-0.0100%, Mg: 0- 0.050%, Ca: 0-0.050%, Zr: 0-0.050%, REM: 0-0.100%, Sn: 0-0.050%, Sb: 0-0.050%, It has a chemical composition
  • the area is 6 ⁇ m 2 or less, the number ratio of ferrite and bainite crystal grains is 40% or more, and the area is 50 ⁇ m 2 or more;
  • the ratio of the number of crystal grains of ferrite and bainite is 5% or less, and from the interface between the ferrite and the martensite or the tempered martensite, in a direction perpendicular to the interface and toward the grain inner side of the ferrite
  • the maximum Mn content in the region up to 0.5 ⁇ m is lower than the average Mn content of the steel sheet by 0.30% by mass or more.
  • the average aspect ratio of the crystal grains of the ferrite and the bainite having an area of 6 ⁇ m 2 or less may be 1.0 or more and 2.0 or less.
  • the steel sheet according to [1] or [2] may have a coating layer containing zinc, aluminum, magnesium or an alloy thereof on the surface.
  • a method for producing a steel sheet according to another aspect of the present invention in mass%, C: 0.07 to 0.15%, Si: 0.01 to 2.00%, Mn: 1.5 to 3 0%, P: 0-0.020%, S: 0-0.0200%, Al: 0.001-1.000%, N: 0-0.0200%, O: 0-0.0200% , Co: 0-0.500%, Ni: 0-1.000%, Cu: 0-0.500%, Mo: 0-1.000%, Cr: 0-2.000%, Ti: 0- 0.5000%, Nb: 0-0.50%, V: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, B: 0-0.0100%, Mg: 0-0.050%, Ca: 0-0.050%, Zr: 0-0.050%, REM: 0-0.100%, Sn: 0-0.050%, Sb: 0-0 050%, As: 0 to 0.050%, and the balance: Fe and impurities.
  • finish rolling is performed using a rolling mill having four or more stands, the first stand is the first stand, and the final stand is the nth stand.
  • the thickness reduction rate at each stand from the stand to the n-th stand is set to 30% or more, and the rolling temperature at the n-th stand is set to 900° C. or less.
  • a coating layer containing zinc, aluminum, magnesium, or an alloy thereof may be formed on the surface of the steel sheet.
  • a steel sheet according to one embodiment of the present invention (steel sheet according to the present embodiment) and a method for manufacturing the same will be described below.
  • the steel sheet according to the present embodiment has a predetermined chemical composition described later, has a tensile strength of 780 MPa or more, has a microstructure with a ferrite area ratio of 5% or more, and a total area ratio of ferrite and bainite.
  • the total area ratio of martensite and tempered martensite is 10% or more and 90% or less
  • the total area ratio of pearlite and retained austenite is 0% or more and 10% or less
  • the ratio of the number of ferrite and bainite crystal grains having an area of 6 ⁇ m 2 or less to the total number of ferrite and bainite crystal grains is 40% or more, and the number of ferrite and bainite crystal grains having an area of 50 ⁇ m 2 or more
  • the number ratio is 5% or less, and the maximum in the region from the interface between the ferrite and the martensite or the tempered martensite to 0.5 ⁇ m in the direction perpendicular to the interface and toward the grain inner side of the ferrite
  • the Mn content is 0.30% by mass or more lower than the average Mn content of the steel sheet.
  • % of the content of each element means % by mass.
  • numerical values displayed between "-" include the values at both ends as the lower limit or the upper limit in the range. For example, 0.07 to 0.15% indicates 0.07% or more and 0.15% or less.
  • C 0.07-0.15%
  • C is an element necessary to secure a predetermined amount of martensite and improve the strength of the steel sheet. If the C content is less than 0.07%, it is difficult to obtain a predetermined amount of martensite, and a tensile strength of 780 MPa or more cannot be secured. Therefore, the C content is made 0.07% or more.
  • the C content is preferably 0.09% or more.
  • the C content exceeds 0.15%, the formation of ferrite is suppressed, which causes a decrease in elongation and a deterioration in ductility of the punched end face. Therefore, the C content is made 0.15% or less.
  • the C content is preferably 0.13% or less.
  • Si 0.01-2.00% Si has a function of increasing the strength of the steel sheet as a solid-solution strengthening element, and is also an effective element for obtaining a structure containing martensite, bainite, and retained ⁇ .
  • the Si content is set to 0.01% or more.
  • the Si content may be 0.10% or more.
  • the Si content is set to 2.00% or less.
  • Mn 1.5-3.0%
  • Mn is an element that contributes to improving the strength of the steel sheet.
  • Mn is an element that has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. If the Mn content is less than 1.5%, these effects are not sufficiently exhibited, and ferrite with an area ratio exceeding the required area is generated, making it impossible to obtain a tensile strength of 780 MPa or more. Therefore, the Mn content is set to 1.5% or more.
  • the Mn content is preferably 1.7% or more, more preferably 1.9% or more.
  • the Mn content is set to 3.0% or less.
  • the Mn content is preferably 2.7% or less.
  • P is an impurity element, and is an element that segregates in the central portion of the plate thickness of the steel plate to lower the toughness. Moreover, P is an element that embrittles the weld zone when the steel plate is welded. If the P content exceeds 0.020%, the weld zone strength, hole expansibility, and ductility of the punched end face are significantly reduced. Therefore, the P content is set to 0.020% or less. The P content is preferably 0.010% or less. The P content is preferably as low as possible, and may even be 0%. However, if the P content is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the P content may be 0.0001% or more.
  • S 0-0.0200%
  • S is an impurity element and is an element that lowers weldability and manufacturability during casting and hot rolling. Moreover, S is also an element that forms coarse MnS and reduces the hole expansibility. If the S content exceeds 0.0200%, weldability, hole expandability, and ductility of the punched end face are significantly lowered. Therefore, the S content should be 0.0200% or less.
  • the S content is preferably 0.0050% or less. The lower the S content, the better, and it may even be 0%. However, if the S content is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost will increase significantly, which will be economically disadvantageous. Therefore, the S content may be 0.0001% or more.
  • Al 0.001-1.000%
  • Al is an element that acts as a deoxidizing agent for steel and stabilizes ferrite. In order to obtain these effects, the Al content is made 0.001% or more. On the other hand, when the Al content exceeds 1.000%, coarse Al oxides are formed and the ductility is lowered. Therefore, the content is made 1.000% or less.
  • the Al content is preferably 0.500% or less.
  • N 0 to 0.0200%
  • N is an element that forms coarse nitrides and lowers bendability and hole expansibility.
  • N is an element that causes blowholes during welding. If the N content exceeds 0.0200%, coarse nitrides are formed, resulting in significant reduction in formability and occurrence of blowholes. Therefore, the N content is made 0.0200% or less. The lower the N content, the better, and it may even be 0%. However, if the N content is reduced to less than 0.0005% in a practical steel sheet, the manufacturing cost will increase significantly, which will be economically disadvantageous. Therefore, the N content may be 0.0005% or more.
  • O 0 to 0.0200%
  • O is an element that forms coarse oxides and degrades formability and fracture resistance.
  • O is an element that causes blow holes during welding. If the O content exceeds 0.0200%, the presence of coarse oxides will significantly deteriorate the formability and the ductility of the punched end face and cause blowholes. Therefore, the O content is set to 0.0200% or less. The lower the O content, the better, and it may even be 0%. However, if the O content is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost will increase significantly, which will be economically disadvantageous. Therefore, the O content may be 0.0001% or more.
  • Co is an effective element for increasing the strength of the steel sheet.
  • the Co content may be 0%, the Co content is preferably 0.001% or more, more preferably 0.010% or more, in order to obtain the above effect.
  • the Co content is 0.500% or less.
  • Ni 0 to 1.000% Ni, like Co, is an element effective in increasing the strength of the steel sheet.
  • the Ni content may be 0%, the Ni content is preferably 0.001% or more, more preferably 0.010% or more, in order to obtain the above effect.
  • the Ni content is too high, the ductility of the steel sheet may be lowered and the formability may be lowered. Therefore, the Ni content is 1.000% or less.
  • Cu 0-0.500%
  • Cu is an element that contributes to improving the strength of the steel sheet.
  • the Cu content may be 0%, the Cu content is preferably 0.001% or more in order to obtain the above effects.
  • the Cu content is 0.500% or less.
  • Mo 0-1.000% Mo, like Mn, is an element that contributes to increasing the strength of the steel sheet. Although 0% of Mo content may be sufficient, when obtaining the said effect, it is preferable that Mo content is 0.010% or more. On the other hand, when the Mo content exceeds 1.000%, coarse Mo carbides are formed, which may deteriorate the cold formability of the steel sheet. Therefore, the Mo content is 1.000% or less.
  • Cr 0 to 2.000% Cr, like Mn and Mo, is an element that contributes to increasing the strength of the steel sheet.
  • the Cr content may be 0%, the Cr content is preferably 0.001% or more, more preferably 0.100% or more, in order to obtain the above effect.
  • the Cr content exceeds 2.000%, coarse Cr nitrides are formed, which may deteriorate the cold formability of the steel sheet. Therefore, the Cr content is 2.000% or less.
  • Ti is an element that is effective in strengthening ferrite, is an element that is effective in controlling the morphology of carbides, and is an element that is effective in refining the structure and improving the toughness of the steel sheet.
  • the Ti content may be 0%, the Ti content is preferably 0.0001% or more, more preferably 0.0010% or more, in order to obtain the above effect.
  • the Ti content is excessive, coarse Ti oxides or TiN may be formed, which may deteriorate the formability of the steel sheet. Therefore, from the viewpoint of ensuring the formability of the steel sheet, the Ti content is 0.5000% or less.
  • Nb 0-0.50% Nb, like Ti, is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel sheet.
  • the Nb content may be 0%, the Nb content is preferably 0.001% or more, more preferably 0.01% or more, in order to obtain the above effect.
  • the Nb content is excessive, a large number of fine and hard Nb carbides are precipitated, and the strength of the steel sheet is increased, resulting in significant deterioration in ductility and deterioration in formability of the steel sheet. Therefore, the Nb content is 0.50% or less.
  • V 0-0.500% V, like Ti and Nb, is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel sheet.
  • the V content may be 0%, the V content is preferably 0.001% or more to obtain the above effect.
  • the V content is 0.500% or less.
  • W 0-0.100%
  • W is also an element effective in controlling the morphology of carbides.
  • W is an element effective in improving the strength of the steel sheet.
  • the W content may be 0%, the W content is preferably 0.001% or more in order to obtain the above effect.
  • the W content is 0.100% or less.
  • Ta 0-0.100% Ta, like W, is an element effective in controlling the morphology of carbides and improving the strength of the steel sheet.
  • the Ta content may be 0%, the Ta content is preferably 0.001% or more to obtain the above effect.
  • the Ta content is 0.100% or less.
  • the Ta content is preferably 0.020% or less, more preferably 0.010% or less.
  • B 0 to 0.0100%
  • B is an element that suppresses the formation of ferrite and pearlite in the cooling process from the austenite temperature range and promotes the formation of a low temperature transformation structure such as bainite or martensite.
  • B is an element effective for increasing the strength of steel.
  • the B content may be 0%, the B content is preferably 0.0001% or more in order to obtain the above effect.
  • the B content is 0.0100% or less.
  • Mg 0-0.050% Mg is an element that controls the morphology of sulfides and oxides and contributes to the improvement of the bendability of steel sheets.
  • the Mg content may be 0%, the Mg content is preferably 0.0001% or more, more preferably 0.001% or more, in order to obtain the above effects.
  • the Mg content is 0.050% or less.
  • the Mg content is preferably 0.040% or less.
  • Ca 0-0.050% Ca, like Mg, is an element capable of controlling the morphology of sulfides with a trace amount.
  • the Ca content may be 0%, the Ca content is preferably 0.001% or more in order to obtain the above effects.
  • the Ca content is 0.050% or less.
  • the Ca content is preferably 0.030% or less.
  • Zr 0-0.050% Zr, like Mg and Ca, is an element capable of controlling the morphology of sulfides with a trace amount.
  • the Zr content may be 0%, the Zr content is preferably 0.001% or more to obtain the above effects.
  • the Zr content is 0.050% or less.
  • the Zr content is preferably 0.040% or less.
  • REM 0-0.100% REM is an element effective in controlling the morphology of sulfides even in a very small amount.
  • the REM content may be 0%, the REM content is preferably 0.001% or more in order to obtain the above effects.
  • the REM content is 0.100% or less.
  • the REM content is preferably 0.050% or less.
  • REM is a RareEarthMetal (rare earth element), and is a general term for 17 elements: two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu).
  • "REM" is composed of one or more selected from these rare earth elements, and "REM content” is the total amount of rare earth elements.
  • REM is often added as a misch metal, and in addition to La and Ce, it may contain the above lanthanide series elements in combination. Even if a lanthanide series element other than La and Ce is contained as an impurity, the effect of the present embodiment is exhibited. Even if metal La or Ce is added, the effect of this embodiment is exhibited.
  • Sn 0-0.050%
  • Sn is an element that can be contained in a steel sheet when scrap is used as a raw material for the steel sheet. Further, Sn is an element that may cause deterioration of the cold formability of the steel sheet due to embrittlement of ferrite. If the Sn content exceeds 0.050%, the adverse effects are significant, so the Sn content is 0.050% or less.
  • the Sn content is preferably 0.040% or less. As the Sn content is preferably as small as possible, it may be 0%, but reducing the Sn content to less than 0.001% causes an excessive increase in refining cost. Therefore, the Sn content may be 0.001% or more.
  • Sb 0-0.050%
  • Sb is an element that can be contained in a steel sheet when scrap is used as a raw material for the steel sheet.
  • Sb is an element that strongly segregates at grain boundaries and may cause embrittlement of grain boundaries, deterioration of ductility, and deterioration of cold formability. If the Sb content exceeds 0.050%, the adverse effects are significant, so the Sb content is 0.050% or less.
  • the Sb content is preferably 0.040% or less. Since the Sb content is preferably as small as possible, it may be 0%, but reducing the Sb content to less than 0.001% causes an excessive increase in refining cost. Therefore, the Sb content may be 0.001% or more.
  • As is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet. As is an element that strongly segregates at grain boundaries, and is an element that may cause deterioration of cold formability. If the As content exceeds 0.050%, the adverse effects are significant, so the As content is 0.050% or less. As content is preferably 0.040% or less. Since the As content is preferably as small as possible, it may be 0%, but reducing the As content to less than 0.001% causes an excessive increase in refining cost. Therefore, the As content may be 0.001% or more.
  • the remainder excluding the above elements is Fe and impurities.
  • Impurities are elements that are allowed to exist within the range that is mixed from the steel raw material and/or during the steelmaking process and does not impair the characteristics of the steel sheet according to the present embodiment, and is an ingredient that is intentionally added to the steel sheet. It is an element that means that it is not
  • the above chemical composition can be measured by using, for example, spark discharge emission spectrometry (Spark-OES, commonly known as Cantback) or ICP emission spectrometry/mass spectrometry (ICP-OES/ICP-MS). What is measured by this method is the average content in the steel sheet.
  • spark-OES spark discharge emission spectrometry
  • ICP-OES/ICP-MS ICP emission spectrometry/mass spectrometry
  • microstructure metal structure of the steel sheet according to this embodiment.
  • the structure fraction is expressed in terms of area ratio
  • the unit "%" of the structure fraction means area %.
  • at least the microstructure in the range of 1/8 to 3/8 thickness centering on the position of 1/4 of the thickness from the surface of the steel sheet is as follows. The reason why the microstructure within this range is defined is that it is a representative structure of steel sheets and is highly correlated with the properties.
  • Total area ratio of ferrite and bainite 10% or more and 90% or less
  • Ferite area ratio 5% or more
  • the total area ratio of ferrite and bainite is set to 10% or more.
  • the total area ratio of ferrite and bainite is preferably 20% or more, more preferably 25% or more.
  • the total area ratio of ferrite and bainite should be 90% or less.
  • the total area ratio of ferrite and bainite is preferably 70% or less, more preferably 50% or less.
  • the area ratio of ferrite is set to 5% or more.
  • the area ratio of ferrite is preferably over 5%, more preferably 7% or more, and still more preferably 10% or more.
  • Total area ratio of martensite and tempered martensite 10% or more and 90% or less
  • martensite and tempered martensite are hard structures, they contribute to improvement in tensile strength.
  • the strength can be increased, and a tensile strength of 780 MPa or more can be easily secured.
  • it is preferable to increase the total area ratio of martensite and tempered martensite.
  • the total area ratio of martensite and tempered martensite is preferably 45% or more, more preferably 50% or more, and still more preferably 55% or more.
  • the total area ratio of martensite and tempered martensite is preferably 70% or more, more preferably 80% or more.
  • the total area ratio of martensite and tempered martensite exceeds 90%, sufficient elongation cannot be obtained, resulting in deterioration of formability. Therefore, the total area ratio is set to 90% or less. From the viewpoint of formability, the total area ratio of martensite and tempered martensite is preferably 85% or less, more preferably 80% or less.
  • Total area ratio of pearlite and retained austenite 0% or more and 10% or less
  • Pearlite is a structure containing hard cementite, and becomes a starting point for the generation of voids during press molding, degrading the fracture resistance.
  • Retained austenite is a structure that contributes to the improvement of elongation by transformation induced plasticity (TRIP).
  • TRIP transformation induced plasticity
  • martensite generated by deformation-induced transformation of retained austenite is extremely hard and becomes a starting point for void generation, deteriorating fracture resistance. Therefore, the total area ratio of pearlite and retained austenite is set to 10% or less.
  • the total area ratio is preferably 5% or less.
  • the area ratio of retained austenite is preferably 5% or less, more preferably less than 3%.
  • the steel sheet according to the present embodiment may not contain pearlite and retained austenite. That is, the total area ratio may be 0%.
  • Identification of ferrite, bainite, martensite, tempered martensite, pearlite and retained austenite, and calculation of area and area ratio will be described.
  • the identification of each metal structure and the calculation of the area and area ratio are performed by EBSD (Electron Back Scattering Diffraction), X-ray diffraction, or the rolling direction corroded using a Nital reagent or Repeller reagent, depending on the target structure. This can be done by observing a 100 ⁇ m ⁇ 100 ⁇ m region of the steel plate cross section parallel to the plate surface and perpendicular to the plate surface using a scanning electron microscope at a magnification of 1000 to 50000 times. When measuring the area ratio of any tissue, three measurement points are used and the average value is calculated.
  • the area and area ratio of ferrite crystal grains can be measured by the following method. That is, with the EBSD attached to the scanning electron microscope, the range of 1/8 to 3/8 of the plate thickness from the surface centered at the position of 1/4 of the plate thickness from the surface of the steel plate is measured at intervals of 0.2 ⁇ m ( pitch). A value of the local orientation difference average (Grain Average Misorientation: GAM) is calculated from the measured data. A region with a grain average misorientation value of less than 0.5° is regarded as ferrite, and its area and area ratio are measured.
  • GAM Garnier Average Misorientation
  • the average local misorientation means that in a region surrounded by grain boundaries with a difference in crystal orientation of 5° or more, the misorientation between adjacent measurement points is calculated and averaged for all measurement points in the crystal grain. This is the converted value.
  • a sample is taken with a thickness cross-section parallel to the rolling direction of the steel sheet as an observation surface, the observation surface is polished, etched with a nital reagent, and 1/th of the plate thickness from the surface.
  • a range of 1/8 to 3/8 of the plate thickness from the surface centered at the position of 4 is observed with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), and a known image analysis software.
  • FE-SEM Field Emission Scanning Electron Microscope
  • Calculated using The area ratio can be calculated using, for example, "ImageJ” as image analysis software.
  • “ImageJ” is open source and public domain image processing software, and is widely used by those skilled in the art.
  • Bainite is an aggregate of lath-shaped crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbide is a single variant, That is, they belong to a group of iron-based carbides elongated in the same direction.
  • the iron-based carbide group extending in the same direction means that the difference in the extending direction of the iron-based carbide group is within 5°.
  • a bainite surrounded by grain boundaries with an orientation difference of 15° or more is counted as one bainite crystal grain.
  • the area ratio of martensite and tempered martensite is obtained by taking a sample with a thickness cross section parallel to the rolling direction of the steel plate as an observation surface, polishing the observation surface, etching with a repeller reagent, and measuring 1/4 of the thickness from the surface.
  • a range of 1/8 to 3/8 of the plate thickness from the surface centered on the position of is observed and photographed by FE-SEM, and from the area ratio of the uncorroded area, the residual measured using X-rays described later It can be calculated by subtracting the area ratio of austenite (details will be described later).
  • the observation range on the observation plane is, for example, a square range with a side of 100 ⁇ m.
  • the area ratio of retained austenite is reduced by electrolytic polishing or chemical polishing from the surface to the position of 1/8 to 3/8 of the plate thickness.
  • the polished surface is subjected to X-ray diffraction using MoK ⁇ rays as characteristic X-rays, and the obtained bcc phase (200), (211) and fcc phase (200), (220), (311)
  • the area ratio of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks and taken as the value at the position of 1/4 of the plate thickness.
  • the observation range on the observation plane is, for example, a square range with a side of 100 ⁇ m.
  • the ratio of the number of ferrite and bainite crystal grains with an area of 6 ⁇ m 2 or less to the total number of ferrite and bainite crystal grains is 40% or more
  • the ratio (N 6 /N T ) of the number (N 6 ) of ferrite and bainite crystal grains having an area of 6 ⁇ m 2 or less to the total number (N T ) of ferrite and bainite crystal grains is the steel sheet according to the present embodiment.
  • the ratio of the number of crystal grains having an area of 6 ⁇ m 2 or less (N 6 /N T ) is set to 40% or more.
  • (N 6 /N T ) is preferably 50% or more, more preferably 55% or more.
  • the ratio of the number of crystal grains having an area of 6 ⁇ m 2 or less in ferrite and bainite may be 90% or less from the viewpoint of suppressing yield point elongation.
  • the ratio of the number of ferrite and bainite crystal grains with an area of 50 ⁇ m 2 or more to the total number of ferrite and bainite crystal grains is 5% or less
  • the ratio (N 50 /N T ) of the number (N 50 ) of ferrite and bainite crystal grains having an area of more than 50 ⁇ m 2 with respect to the total number (N T ) of ferrite and bainite crystal grains is the steel sheet according to the present embodiment.
  • the ratio of the number of crystal grains having an area of more than 50 ⁇ m 2 (N 50 /N T ) is set to 5% or less.
  • (N 50 /N T ) is preferably 3% or less.
  • the grain size and number ratio of ferrite and bainite crystal grains are calculated from the results of image analysis using the above-mentioned "EBSD” and "ImageJ" performed within the same field of view.
  • the maximum Mn content in the region from the interface between ferrite and martensite or tempered martensite to 0.5 ⁇ m in the direction perpendicular to the interface and toward the grain inner side of ferrite is the average Mn content of the steel sheet lower than 0.30% by mass
  • the interface between ferrite and martensite or tempered martensite (the interface between ferrite and martensite and the ferrite) It was found that when the Mn content in the ferrite in the vicinity of the interface between the steel and the tempered martensite is high, the rupture resistance deteriorates.
  • the direction perpendicular to the interface if the interface is not a straight line, the direction perpendicular to the tangent to the interface at that position
  • the maximum Mn content in the region up to 0.5 ⁇ m toward the inside of the ferrite grain that is, the range from the interface to 0.5 ⁇ m in the ferrite
  • the maximum Mn content in the above region is larger than (average Mn content of steel sheet - 0.30% by mass), a sufficient effect of improving rupture resistance cannot be obtained.
  • the maximum Mn content in the region from the interface between ferrite and martensite or tempered martensite to 0.5 ⁇ m in the direction perpendicular to the interface and toward the inner side of the ferrite grain is 0.5 ⁇ m with respect to the average Mn content of the steel sheet. Whether or not it is lower than 30% by mass is judged by the following method.
  • Line analysis is performed using EPMA in a range of 0.5 ⁇ m or more in the direction perpendicular to the interface and toward the inner side of the ferrite grains.
  • the difference between the maximum Mn content in ferrite in the range of 0.5 ⁇ m from the interface obtained by line analysis and the average Mn content of the steel sheet (average Mn content of steel sheet - maximum Mn content in the range of 0.5 ⁇ m from the interface amount) is ⁇ Mn.
  • the average aspect ratio of ferrite and bainite crystal grains having an area of 6 ⁇ m 2 or less is 1.0 or more and 2.0 or less
  • the average aspect ratio of ferrite and bainite crystal grains having an area of 6 ⁇ m 2 or less is also one of the indexes that affect fracture resistance.
  • the average aspect ratio of ferrite and bainite crystal grains is preferably 1.0 or more and 2.0 or less. If the average aspect ratio exceeds 2.0, it is difficult to obtain this effect.
  • the average aspect ratio is more preferably 1.0 or more and 1.5 or less.
  • the aspect ratio refers to the ratio between the longest diameter (major diameter) of a ferrite crystal grain and the longest diameter (minor diameter) of the ferrite diameter perpendicular thereto.
  • Crystal grains with an area of more than 6 ⁇ m 2 are not particularly limited because their contribution to void connectivity is relatively small, but from the viewpoint of reducing stress concentration on the interface, elongated grains are preferable.
  • the ratio may be greater than 2.0 and less than or equal to 5.0.
  • the steel sheet according to the present embodiment has a tensile strength of 780 MPa or more in consideration of contribution to weight reduction of automobiles by application to automobile parts. Considering the contribution to weight reduction of automobiles, the tensile strength is preferably 980 MPa or more, more preferably 1180 MPa or more. On the other hand, although it is not necessary to limit the upper limit of the tensile strength, since elongation and hole expandability may decrease as the tensile strength increases, the tensile strength may be set to 1500 MPa or less.
  • ⁇ is set to be 50 MPa or less. More preferably, ⁇ is 40 MPa or less.
  • the tensile strength ⁇ 1 of the steel plate and the stress ⁇ 2 at uniform elongation +1.0% are obtained by performing a tensile test according to JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011.
  • the steel sheet according to the present embodiment described above may have a coating layer containing zinc, aluminum, magnesium, or alloys thereof on the surface.
  • the presence of the coating layer on the surface of the steel sheet improves corrosion resistance.
  • the coating layer may be a known coating layer.
  • One of the purposes of increasing the strength of steel sheets is to reduce the weight by making them thinner. Therefore, even if a high-strength steel sheet is developed, its application is limited if the corrosion resistance is low.
  • the corrosion resistance is improved and the applicable range is widened.
  • the steel sheet has a coating layer (for example, a plating layer) on the surface, the “surface” in “the range of 1/8 to 3/8 thickness centering on the position of 1/4 of the thickness from the surface of the steel sheet” It means the base iron surface excluding layers.
  • the plate thickness of the steel plate according to the present embodiment is not limited to a specific range, but considering strength, versatility, and manufacturability, it is preferably 0.3 to 6.0 mm.
  • the steel plate according to the present embodiment can be obtained by a manufacturing method including the following steps, although the manufacturing method is not particularly limited.
  • III a holding step of holding the hot-rolled steel sheet after the winding step for a holding time of 2 to 8 hours in a temperature range from the winding temperature to the winding temperature ⁇ 50 ° C.;
  • a slab having a predetermined chemical composition (when obtaining the steel sheet according to the present embodiment, the same chemical composition as the steel sheet according to the present embodiment) is heated and hot rolled to perform hot rolling.
  • the slab to be heated may be one obtained by continuous casting or casting/slabbing rolling, or may be one obtained by subjecting them to hot working or cold working.
  • the heating temperature is not limited, if the temperature is less than 1100° C., the carbides and sulfides generated during casting may not be solid-dissolved and may coarsen, thereby deteriorating the press formability. Therefore, the heating temperature is preferably 1100° C. or higher, more preferably 1150° C. or higher.
  • the finish rolling is performed using a rolling mill having four or more stands, and the first stand is the first stand and the final stand is the nth stand.
  • the thickness reduction rate at each stand from the n-th stand is set to 30% or more, and the rolling temperature at the final stand (n-th stand) is set to 900° C. or less. That is, for example, in a rolling mill with seven stands, the thickness reduction rate at the fourth stand, the fifth stand, the sixth stand, and the seventh stand is set to 30% or more, and the rolling at the seventh stand is The temperature is set to 900°C or less.
  • the austenite grain size is refined by recrystallization during rolling, and by introducing a large amount of strain into the austenite, the number of sites where ferrite nuclei are generated increases, and the crystal grains of the hot-rolled steel sheet become finer. to make it better. If even one sheet thickness reduction rate at each stand is less than 30%, or if the rolling temperature at the n-th stand is higher than 900°C, the hot-rolled structure becomes coarse and mixed grains, and the annealing process described later. Later tissues also coarsen. If the hot rolling completion temperature is lower than 830° C., the rolling reaction force increases, making it difficult to stably obtain the target thickness. Therefore, the rolling temperature at the final stand is preferably 830° C. or higher.
  • each of the plate thickness reduction rates at the n-3th stand to the nth stand is 50% or less.
  • finish rolling is performed using a rolling mill having four or more stands in order to perform continuous rolling with a short time between the final four passes of rolling. This is because if the time between passes is long, the strain recovers between passes and the strain does not accumulate sufficiently even if the rolling is performed at a large thickness reduction rate.
  • the hot-rolled steel sheet after the hot rolling step is cooled at an average cooling rate of 30°C/sec or higher to a coiling temperature of 650°C or lower and 450°C or higher, and coiled at the coiling temperature.
  • a coiling temperature 650°C or lower and 450°C or higher
  • coiled at the coiling temperature By rapidly cooling after hot rolling, the transformation to ferrite and pearlite at high temperatures is suppressed, and by causing ferrite transformation at low temperatures where the driving force for transformation is high, fine ferrite and its grain boundaries are formed. It is possible to obtain a structure having fine cementite and fine grains.
  • the average cooling rate is less than 30°C/sec or the cooling stop temperature (coiling temperature) is more than 650°C, coarse ferrite and pearlite containing coarse carbides are unevenly formed. Since coarse carbides are difficult to dissolve in the annealing process, the structure after annealing becomes coarse and mixed grains. On the other hand, if the coiling temperature is less than 450° C., the strength of the hot-rolled steel sheet becomes excessive, the cold-rolling load increases, and the productivity deteriorates.
  • the average cooling rate is preferably 100° C./sec or less in order to stably obtain the target cooling stop temperature.
  • the steel sheet after the winding step is held in a temperature range from the winding temperature to -50°C for a holding time of 2 to 8 hours.
  • Mn mainly diffuses through ferrite grain boundaries and concentrates in cementite. Refining the ferrite grains as described above increases the number of diffusion paths and promotes the concentration of Mn in cementite (for example, the Mn content in cementite exceeds 3.0%).
  • Mn is concentrated in cementite, a Mn-depleted layer with a low Mn content is formed in the vicinity thereof.
  • the holding time in the temperature range from the coiling temperature to ⁇ 50° C. exceeds 8 hours, the cementite coarsens.
  • the retention time is set within 8 hours.
  • the holding time in the temperature range from the coiling temperature to ⁇ 50° C. is 2 hours or more in order to sufficiently concentrate Mn in the cementite.
  • the hot-rolled steel sheet after the holding step is cooled to a temperature of 300° C. or lower at an average cooling rate of 0.1° C./sec or higher. If the average cooling rate to the cooling stop temperature of 300° C. or lower after the holding step is less than 0.1° C./sec, the cementite may coarsen. When the average cooling rate is high, hard martensite tends to form when untransformed ⁇ remains. In this case, there is a concern that the strength of the hot-rolled sheet is increased and the cold-rolling load is increased. Therefore, the average cooling rate is preferably 12.0° C./sec or less.
  • the hot-rolled steel sheet after the cooling process is cold-rolled at a sheet thickness reduction rate of 20 to 80% to obtain a cold-rolled steel sheet. If the sheet thickness reduction rate is less than 20%, the strain accumulation in the steel sheet is insufficient, and the austenite nucleation sites become uneven. In this case, in the post-annealing process, the grain size becomes coarse or mixed grains are formed, so that the number ratio of ferrite and bainite crystal grains with an area of 6 ⁇ m 2 or less and the area is 50 ⁇ m 2 or more. Certain ferrite and bainite crystal grain number ratios do not fall within the desired range. In addition, the aspect ratio of crystal grains also increases.
  • the plate thickness reduction rate is set to 20% or more and 80% or less.
  • the plate thickness reduction rate is preferably 30% or more and 80% or less.
  • the cold-rolled steel sheet is heated to an annealing temperature of 740 to 900°C at an average heating rate of 5°C/sec or more, and held at the annealing temperature (740 to 900°C) for 60 seconds or more. If the average heating rate is less than 5° C./sec, Mn concentrated to cementite ( ⁇ ) in the hot-rolled steel sheet may diffuse into the Mn-depleted layer with a low Mn content, and the Mn-depleted layer may disappear. . Therefore, the average heating rate to the annealing temperature is set to 5° C./second or more. In particular, when the temperature exceeds 550° C., diffusion of Mn tends to occur.
  • the average temperature increase rate is 5° C./second or more in the temperature range of 550° C. or higher. Excessive equipment investment is required to control the average heating rate to exceed 50°C/sec. Therefore, from the viewpoint of economy, the average heating rate is preferably 50° C./sec or less. If the annealing temperature is less than 740°C, the amount of austenite is small, the area ratio of martensite after annealing and tempered martensite is less than 10%, and the tensile strength is less than 780 MPa. On the other hand, if the annealing temperature exceeds 900°C, the metal structure becomes coarse and the fracture resistance deteriorates. Therefore, the annealing temperature should be 740° C.
  • the annealing temperature is preferably 780°C or higher and 850°C or lower. If the holding time (residence time) at the annealing temperature is less than 60 seconds, sufficient austenite is not generated, the area ratio of martensite after annealing and tempered martensite is less than 10%, and the tensile strength is less than 780 MPa. . Therefore, the holding time at the annealing temperature should be 60 seconds or more. The retention time is preferably 70 seconds or longer, more preferably 80 seconds or longer. On the other hand, if the annealing time exceeds 300 seconds, the crystal grains become coarse. Therefore, the annealing time is set to 300 seconds or less.
  • the cooling rate after heating in the annealing process is not limited, but after slow cooling to achieve the desired ferrite fraction, rapid cooling is performed to generate martensite.
  • a holding or reheating step for tempering the martensite may be included.
  • a hot-rolled steel sheet having fine cementite and Mn-deficient layers at ferrite grain boundaries is cold-rolled and then annealed under the conditions described above. It is thought that the steel sheet according to the present embodiment has a structure by dissolving the cementite and becoming martensite (or retained austenite) with a high Mn content.
  • a coating layer such as a plating layer containing zinc, aluminum, magnesium, or alloys thereof may be formed on the surface of the steel sheet.
  • hot dip plating may be formed by immersing the steel sheet in a plating bath during cooling after holding.
  • this hot-dip plating may be heated to a predetermined temperature and alloyed to form alloyed hot-dip plating.
  • the plating layer may further contain Fe, Al, Mg, Mn, Si, Cr, Ni, Cu, and the like. Any of the above methods may be used as the plating layer for the purpose of enhancing corrosion resistance.
  • the plating conditions and the alloying conditions known conditions may be applied according to the composition of the plating.
  • the temperature is 300 ° C. or less. cooled to room temperature.
  • the hot-rolled steel sheets after cooling were pickled and then cold-rolled at the sheet thickness reduction rates shown in Tables 2-3 and 2-4 to obtain cold-rolled steel sheets with a thickness of 1.4 mm.
  • the obtained cold-rolled steel sheets were annealed under the conditions shown in Tables 2-5 and 2-6. Some of the steel sheets were immersed in a galvanizing bath during cooling after annealing to form a hot-dip galvanized layer on the surface.
  • the metal structure (ferrite, bainite, martensite, tempering Martensite, pearlite and retained austenite (retained ⁇ )) area ratio, the number of ferrite and bainite crystal grains with an area of 6 ⁇ m 2 or less with respect to the total number of ferrite and bainite crystal grains NT in 1/4 part of the sheet thickness
  • the ratio of the number N 6 (N 6 /N T ), the ratio of the number N 50 of the ferrite and bainite grains having an area of more than 50 ⁇ m 2 to the total number N T of the ferrite and bainite grains (N 50 /N T ), the difference ⁇ Mn from the maximum value of the Mn concentration in the region from the interface between ferrite and martensite to 0.5 ⁇ m toward the inside of the ferrite grain in the direction perpendicular to the interface, the area at 1/4 part of the plate thickness
  • ⁇ which is the difference between stress ⁇ 2 and tensile strength ⁇ 1 at uniform elongation +1.0%, tensile strength (TS) ( ⁇ 1), uniform elongation (u-El), and uniform elongation +1.0%
  • TS tensile strength
  • u-El uniform elongation
  • ⁇ uniform elongation
  • the invention examples (test No. 1 to No. 37, No. 58) in which both the chemical composition and the manufacturing conditions are within the scope of the present invention have a microstructure fraction , the characteristics of the structure ((N 6 /N T ), (N 50 /N T ), ⁇ Mn, the average aspect ratio of ferrite and bainite having an area of 6 ⁇ m 2 or less) and properties are both within the scope of the invention, and the strength , was excellent in formability and fracture resistance.
  • the comparative examples (Test Nos.
  • Test No. 38 to No. No. 47 is a comparative example in which the manufacturing conditions were within the scope of the invention, but the chemical composition was outside the scope of the invention, and was inferior in at least one of strength, moldability, and rupture resistance.
  • Test no. 48 to No. No. 57 is a comparative example in which the chemical composition was within the scope of the present invention, but one of the conditions in the manufacturing method was outside the scope of the present invention.
  • Test no. In No. 48 the temperature of the final stand of hot rolling was too high, so that N 6 / NT and N 50 / NT could not be sufficiently secured, and the formation of an Mn depleted layer was insufficient ( ⁇ Mn was As a result, ⁇ , which is an index of rupture resistance, did not meet the target.
  • Test no. 49 is the final stand for hot rolling; No. 50 because the plate thickness reduction rate from the n-3th stand to the n-1th stand was too small. In No.

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