EP4379083A1 - Steel sheet and method for producing same - Google Patents

Steel sheet and method for producing same Download PDF

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Publication number
EP4379083A1
EP4379083A1 EP22849577.6A EP22849577A EP4379083A1 EP 4379083 A1 EP4379083 A1 EP 4379083A1 EP 22849577 A EP22849577 A EP 22849577A EP 4379083 A1 EP4379083 A1 EP 4379083A1
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EP
European Patent Office
Prior art keywords
steel sheet
ferrite
less
content
bainite
Prior art date
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Pending
Application number
EP22849577.6A
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German (de)
English (en)
French (fr)
Inventor
Taku MIYAKAWA
Kyohei ISHIKAWA
Takafumi Yokoyama
Kengo Takeda
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
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Nippon Steel Corp
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Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4379083A1 publication Critical patent/EP4379083A1/en
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    • 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
    • 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
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    • 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
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    • 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
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    • 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
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a steel sheet and a method for producing the steel sheet.
  • Patent Document 1 discloses a high strength TRIP steel sheet that is a steel sheet having excellent elongation, hole expansibility, bending workability, and delayed fracture resistance property, the steel sheet including, as a composition, by 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%, one or more selected from Ti: 0.005% to 0.05%, Cu: 0.003% to 0.50%, Ni: 0.003% to 0.50%, Sn: 0.003% to 0.50%, Co: 0.003% to 0.05%, and Mo: 0.003% to 0.50%, and a remainder consisting of Fe and unavoidable impurities, in which a microstructure contains 15% or less (including 0%) of ferrite having an average grain size of 2 ⁇ m
  • Patent Document 2 discloses a high strength cold-rolled steel sheet that is a steel sheet having both a high strength of 980 MPa or greater in terms of tensile strength (TS) and excellent bendability, the steel sheet having a specific composition and a specific steel structure in which the area ratio of ferrite is 30% or greater and 70% or less, the area ratio of martensite is 30% or greater and 70% or less, the average grain size of ferrite grains is 3.5 ⁇ m or less, a standard deviation of grain sizes of the ferrite grains is 1.5 ⁇ m or less, the average aspect ratio of the ferrite grains is 1.8 or less, the average grain size of martensite grains is 3.0 ⁇ m or less, and the average aspect ratio of the martensite grains is 2.5 or less, in which a tensile strength is 980 MPa or greater.
  • TS tensile strength
  • Patent Document 3 discloses a high strength steel sheet that is a steel sheet that has a yield strength (YS) of 780 MPa or greater and a tensile strength (TS) of 1,180 MPa or greater, and is excellent in spot weldability, ductility, and bending workability, in which a C content is 0.15% or less, the area ratio of ferrite is 8% to 45%, the area ratio of martensite is 55% to 85%, a ratio of martensite adjacent only to ferrite is 15% or less in a total structure, the average grain size of ferrite and martensite is 10 ⁇ m or less, and the area ratio of ferrite having a grain size of 10 ⁇ m or greater is less than 5% in ferrite present in a range from a depth of 20 ⁇ m to a depth of 100 ⁇ m from a steel sheet surface.
  • YS yield strength
  • TS tensile strength
  • Patent Document 4 discloses a high strength cold-rolled steel sheet that is a steel sheet having a small variation in mechanical properties (strength and ductility in particular), the steel sheet including, as a composition, by 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 a remainder consisting of iron and unavoidable impurities, in which a structure containing, by area ratio, 20% to 50% of ferrite as a soft first phase, and a remainder consisting of tempered martensite and/or tempered bainite as a hard second phase is provided, a total area of ferrite grains having an average grain size of 10 to 25 ⁇ m occupies 80% or greater of a total area of all the ferrite grains, the number of cementite grains dispersed, that are present in all the ferrite grains and have a circle equivalent diameter
  • steel sheets to be used for vehicle components or the like are formed by pressing, punching, or the like when being formed into components.
  • Patent Documents 1 to 4 mention increasing the strength and having good ductility and bendability, there is no mention about the suppression of crack occurrence in a case where press forming is performed under such strict conditions, and there is room for improvement.
  • an object of the present invention is to provide a steel sheet in which it is possible to suppress crack occurrence during press forming (excellent fracture resistance) even in a case where a strain amount exceeding uniform elongation is introduced and a method for producing the steel sheet.
  • the present inventors have conducted studies on a method of suppressing crack occurrence (of obtaining excellent fracture resistance) in a case where a strain amount exceeding uniform elongation is introduced to a steel sheet containing: ferrite and/or bainite; and martensite and/or tempered martensite. As a result, it has been found that it is possible to suppress crack occurrence during press forming as long as a state in which the true stress is high can be maintained even with a strain amount equal to or greater than uniform elongation.
  • the present invention has been contrived in view of the above problems.
  • the gist of the present invention is as follows.
  • FIG. 1 is a diagram explaining a method of obtaining ⁇ .
  • a steel sheet according to the present embodiment has a predetermined chemical composition to be described below, a tensile strength is 780 MPa or greater, in a microstructure, the area ratio of ferrite is 5% or greater, the total of the area ratio of ferrite and an area ratio of bainite is 10% or greater and 90% or less, the total of the area ratio of martensite and an area ratio of tempered martensite is 10% or greater and 90% or less, and the total of an area ratio of pearlite and the area ratio of residual austenite is 0% or greater and 10% or less, the number proportion of crystal grains of ferrite and bainite having an area of 6 ⁇ m 2 or less is 40% or greater to a total number of crystal grains of the ferrite and the bainite, and a number proportion of crystal grains of ferrite and bainite having an area of 50 ⁇ m 2 or greater is 5% or less to the total number of crystal grains of the ferrite and the bainite, and the maximum Mn content in a region up to 0.5 ⁇
  • the symbol % indicating the amount of each element means mass%.
  • numerical values at both ends of a range, with “to” in between, are included as a lower limit and an upper limit. For example, “0.07% to 0.15%” indicates “0.07% or greater and 0.15% or less”.
  • C is an element necessary to secure a predetermined amount of martensite and to improve the strength of the steel sheet.
  • the C content is set to 0.07% or greater.
  • the C content is preferably 0.09% or greater.
  • the C content is set to 0.15% or less.
  • the C content is preferably 0.13% or less.
  • Si has a function of increasing the strength of the steel sheet as a solid solution strengthening element, and is an effective element for obtaining a structure containing martensite, bainite, residual y, and the like.
  • the Si content is set to 0.01% or greater.
  • the Si content may be set to 0.10% or greater.
  • the Si content is set to 2.00% or less.
  • the Si content is preferably set to 1.20% or less.
  • Mn is an element that contributes to an improvement in strength of the steel sheet.
  • Mn is an element acting to suppress ferritic transformation that occurs during a heat treatment in a continuous annealing facility or a continuous hot-dip galvanizing facility.
  • the Mn content is set to 1.5% or greater.
  • the Mn content is preferably 1.7% or greater, and more preferably 1.9% or greater.
  • 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 a sheet thickness center part of the steel sheet and causes a decrease in toughness.
  • P is an element that causes embrittlement of a welded part in a case where the steel sheet is welded.
  • 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 small as possible and may be 0%. However, in a case where the P content is reduced to less than 0.0001 % in a practical steel sheet, the production cost increases significantly, which is economically disadvantageous. Therefore, the P content may be set to 0.0001% or greater.
  • S is an impurity element, and is an element that decreases weldability and also decreases producibility during casting and hot rolling. In addition, S is also an element that forms coarse MnS, thereby decreasing hole expansibility. In a case where the S content is greater than 0.0200%, the weldability, the hole expansibility, and the ductility of a punched end surface significantly decrease. Therefore, the S content is set to 0.0200% or less.
  • the S content is preferably 0.0050% or less.
  • the S content is preferably as small as possible and may be 0%. However, in a case where the S content is reduced to less than 0.0001 % in a practical steel sheet, the production cost increases significantly, which is economically disadvantageous. Therefore, the S content may be set to 0.0001% or greater.
  • 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 set to 0.001% or greater.
  • the content is set to 1.000% or less.
  • the Al content is preferably 0.500% or less.
  • N is an element that forms a coarse nitride and decreases bendability and hole expansibility.
  • N is an element that causes the generation of blowholes during welding.
  • the N content is set to 0.0200% or less.
  • the N content is preferably as small as possible and may be 0%.
  • the N content may be set to 0.0005% or greater.
  • O is an element that forms a coarse oxide and deteriorates formability and fracture resistance.
  • O is an element that causes the generation of blowholes during welding.
  • the O content is set to 0.0200% or less.
  • the O content is preferably as small as possible and may be 0%.
  • the O content may be set to 0.0001% or greater.
  • Co is an effective element for increasing the strength of the steel sheet.
  • the Co content may be 0%, but in a case where the above effects are obtained, the Co content is preferably 0.001% or greater, and more preferably 0.010% or greater.
  • the Co content is 0.500% or less.
  • Ni is an effective element for increasing the strength of the steel sheet.
  • the Ni content may be 0%, but in a case where the above effects are obtained, the Ni content is preferably 0.001% or greater, and more preferably 0.010% or greater.
  • the Ni content is 1.000% or less.
  • Cu is an element that contributes to an improvement in strength of the steel sheet.
  • the Cu content may be 0%, but in a case where the above effects are obtained, the Cu content is preferably 0.001% or greater.
  • the Cu content is 0.500% or less.
  • Mo is an element that contributes to high-strengthening of the steel sheet.
  • the Mo content may be 0%, but in a case where the above effects are obtained, the Mo content is preferably 0.010% or greater.
  • the Mo content is greater than 1.000%, there is a concern that a coarse Mo carbide may be formed, and the cold formability of the steel sheet may decrease. Therefore, the Mo content is 1.000% or less.
  • Cr is an element that contributes to high-strengthening of the steel sheet.
  • the Cr content may be 0%, but in a case where the above effects are obtained, the Cr content is preferably 0.001% or greater, and more preferably 0.100% or greater.
  • the Cr content is 2.000% or less.
  • Ti is an effective element for strengthening ferrite.
  • Ti is an effective element for controlling the morphology of carbide, and is also an effective element for improving the toughness of the steel sheet by refining the structure.
  • the Ti content may be 0%, but in a case where the above effects are obtained, the Ti content is preferably 0.0001% or greater, and more preferably 0.0010% or greater.
  • the Ti content is 0.5000% or less.
  • Nb 0% to 0.50%
  • Nb is an effective element for controlling the morphology of carbide and is also an effective element for improving the toughness of steel sheet by refining the structure.
  • the Nb content may be 0%, but in a case where the above effects are obtained, the Nb content is preferably 0.001% or greater, and more preferably 0.01% or greater.
  • the Nb content is 0.50% or less.
  • V 0% to 0.500%
  • V is also an effective element for controlling the morphology of carbide and is also an effective element for improving the toughness of steel sheet by refining the structure.
  • the V content may be 0%, but in a case where the above effects are obtained, the V content is preferably 0.001% or greater.
  • the V content is 0.500% or less.
  • W is also an effective element for controlling the morphology of carbide.
  • W is also an effective element for improving the strength of the steel sheet.
  • the W content may be 0%, but in order to obtain the above effects, the W content is preferably 0.001% or greater.
  • the W content is 0.100% or less.
  • Ta is also an effective element for controlling the morphology of carbide and improving the strength of the steel sheet.
  • the Ta content may be 0%, but in a case where the above effects are obtained, the Ta content is preferably 0.001% or greater.
  • the Ta content is 0.100% or less.
  • the Ta content is preferably 0.020% or less, and more preferably 0.010% or less.
  • B is an element that suppresses the formation of ferrite and pearlite in a cooling process from an austenite temperature range and accelerates the formation of a low temperature transformation structure such as bainite or martensite.
  • B is an effective element for high-strengthening of steel.
  • the B content may be 0%, but in a case where the above effects are obtained, the B content is preferably 0.0001% or greater.
  • the B content is 0.0100% or less.
  • Mg is an element that controls the morphologies of sulfide and oxide and contributes to an improvement in bendability of the steel sheet.
  • the Mg content may be 0%, but in a case where the above effects are obtained, the Mg content is preferably 0.0001 % or greater, and more preferably 0.001% or greater.
  • the Mg content is 0.050% or less.
  • the Mg content is preferably 0.040% or less.
  • Ca is an element capable of controlling the morphology of sulfide with a trace amount.
  • the Ca content may be 0%, but in order to obtain the above effect, the Ca content is preferably 0.001% or greater.
  • the Ca content is 0.050% or less.
  • the Ca content is preferably 0.030% or less.
  • Zr is an element capable of controlling the morphology of sulfide with a trace amount.
  • the Zr content may be 0%, but in a case where the above effects are obtained, the Zr content is preferably 0.001% or greater.
  • the Zr content is 0.050% or less.
  • the Zr content is preferably 0.040% or less.
  • the REM is an effective element for controlling the morphology of sulfide with a trace amount.
  • the REM content may be 0%, but in order to obtain the above effects, the REM content is preferably 0.001% or greater.
  • the REM content is 0.100% or less.
  • the REM content is preferably 0.050% or less.
  • REM represents a rare earth metal (rare earth element) and is a general term for 17 elements, including 2 elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu).
  • the term "REM” is composed of one or more selected from these rare earth elements, and the term "REM content” is a total amount of the rare earth elements.
  • REM is often added as mischmetal and contains, in addition to La and Ce, the above-described lanthanoid-series element in complex in some cases. Even in a case where these lanthanoid-series elements other than La and Ce are contained as the impurities, the effects of the present embodiment are exhibited. Even in a case where the metal La or Ce is added, the effects of the present embodiment are exhibited.
  • Sn is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet.
  • Sn is an element that may cause a decrease in cold formability of the steel sheet attributed to the embrittlement of ferrite.
  • the Sn content is 0.050% or less.
  • the Sn content is preferably 0.040% or less.
  • the Sn content is preferably as small as possible and may thus be 0%. However, the reduction of the Sn content to less than 0.001% leads to an excessive increase in refining cost. Therefore, the Sn content may be set to 0.001% or greater.
  • Sb is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet.
  • Sb is an element that may be strongly segregated in grain boundaries and may cause the embrittlement of grain boundaries, a decrease in ductility, and a decrease in cold formability.
  • the Sb content is greater than 0.050%, adverse effects thereof are significant. Therefore, the Sb content is 0.050% or less.
  • the Sb content is preferably 0.040% or less.
  • the Sb content is preferably as small as possible and may thus be 0%. However, the reduction of the Sb content to less than 0.001% leads to an excessive increase in refining cost. Therefore, the Sb content may be set to 0.001% or greater.
  • the As content is preferably as small as possible and may thus be 0%. However, the reduction of the As content to less than 0.001% leads to an excessive increase in refining cost. Therefore, the As content may be set to 0.001% or greater.
  • a remainder except the above-described elements consists of Fe and impurities.
  • Impurities are elements that are incorporated from steel raw materials and/or in a steelmaking process, and are allowed within the range that does not impair the properties of the steel sheet according to the present embodiment, and are elements that are not components intentionally added to the steel sheet.
  • the above-described chemical composition can be measured using, for example, a spark discharge emission analysis method (Spark-OES, commonly known as Quantovac) or an ICP emission spectroscopic/mass analysis device (ICP-OES/ICP-MS). Those measured by this method are the average content in the steel sheet.
  • spark-OES spark discharge emission analysis method
  • ICP-OES/ICP-MS ICP emission spectroscopic/mass analysis device
  • microstructure (metallographic structure) of the steel sheet according to the present embodiment will be described.
  • a microstructural fraction is expressed by area ratio
  • the unit "%" of the microstructural fraction means area%.
  • At least a microstructure in a range of 1/8 to 3/8 of the thickness, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet, is as follows.
  • the reason why the microstructure is regulated in this range is that it is a representative structure of the steel sheet and has a high correlation with the properties.
  • Ferrite and bainite are soft structures and are thus easily deformed, which contribute to an improvement in fracture resistance.
  • the sum of ferrite and bainite is 10% or greater, sufficient elongation can be obtained and the formability is improved. Therefore, the sum of the area ratios of ferrite and bainite is set to 10% or greater.
  • the sum of the area ratios of ferrite and bainite is preferably 20% or greater, and more preferably 25% or greater.
  • the sum of the area ratios of ferrite and bainite is set to 90% or less.
  • the sum of the area ratios of ferrite and bainite is preferably 70% or less, and more preferably 50% or less.
  • ferrite is softer and more excellent in ductility than bainite, and thus contributes to an improvement in fracture resistance more than bainite.
  • the area ratio of ferrite is set to 5% or greater.
  • the area ratio of ferrite is preferably greater than 5%, more preferably 7% or greater, and even more preferably 10% or greater.
  • Martensite (so-called fresh martensite) and tempered martensite are hard structures and thus contribute to an improvement in tensile strength.
  • the sum of martensite and tempered martensite is set to 10% or greater, high-strengthening can be achieved, and it becomes easy to secure a tensile strength of 780 MPa or greater.
  • the sum of the area ratios of martensite and tempered martensite is preferably 45% or greater, more preferably 50% or greater, and even more preferably 55% or greater.
  • the sum of the area ratios of martensite and tempered martensite is preferably 70% or greater, and more preferably 80% or greater.
  • the sum of the area ratios of martensite and tempered martensite is set to 90% or less.
  • the sum of the area ratios of martensite and tempered martensite is preferably 85% or less, and more preferably 80% or less.
  • Pearlite that is a structure containing hard cementite, becomes a starting point of the occurrence of voids during press forming, thereby deteriorating fracture resistance.
  • Residual austenite is a structure that contributes to an improvement in elongation by transformation induced plasticity (TRIP).
  • TRIP transformation induced plasticity
  • martensite that is formed by transformation induced plasticity of the residual austenite is extremely hard, and becomes a starting point of the occurrence of voids, thereby deteriorating fracture resistance.
  • the sum of the area ratios of pearlite and residual austenite is set to 10% or less.
  • the sum of the area ratios is preferably 5% or less.
  • the area ratio of residual austenite is preferably 5% or less, and more preferably less than 3%.
  • the steel sheet according to the present embodiment may not include pearlite and residual austenite. That is, the sum of the area ratios may be 0%.
  • each microstructure structure and the calculation of the area and the area ratio can be performed by using a scanning electron microscope by electron back scattering diffraction (EBSD), an X-ray diffraction method, or by observing a region of 100 ⁇ m ⁇ 100 ⁇ m on a cross section of the steel sheet that is parallel to the rolling direction and perpendicular to the sheet surface which is etched using a Nital reagent or a LePera reagent at a magnification of 1,000 to 50,000 times, according to the target structure.
  • EBSD electron back scattering diffraction
  • X-ray diffraction method or by observing a region of 100 ⁇ m ⁇ 100 ⁇ m on a cross section of the steel sheet that is parallel to the rolling direction and perpendicular to the sheet surface which is etched using a Nital reagent or a LePera reagent at a magnification of 1,000 to 50,000 times, according to the target structure.
  • EBSD electron back scattering diffraction
  • the area and area ratio of crystal grains of ferrite can be measured by the following method. That is, a range of 1/8 to 3/8 of the sheet thickness from the surface, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet, is measured at intervals of 0.2 ⁇ m (pitch) by EBSD attached to a scanning electron microscope.
  • the value of grain average misorientation (GAM) is calculated from the measured data. Then, a region where the value of grain average misorientation is less than 0.5° is defined as ferrite, and the area and area ratio thereof are measured.
  • the grain average misorientation is a value obtained by: calculating an orientation difference between adjacent measurement points in a region surrounded by grain boundaries having a crystal orientation difference of 5° or greater; and averaging the orientation differences among all measurement points in the crystal grains.
  • a sample is collected so that a cross section in the sheet thickness direction that is parallel to the rolling direction of the steel sheet serves as an observation surface, the observation surface is polished and etched with a Nital reagent, a range of 1/8 to 3/8 of the sheet thickness from the surface, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface, is observed by a field emission scanning electron microscope (FE-SEM), and the calculation is performed using known image analysis software.
  • FE-SEM field emission scanning electron microscope
  • the area ratio can be calculated.
  • “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 and does not contain therein an iron-based carbide having a major axis of 20 nm or greater, or contains therein an iron-based carbide having a major axis of 20 nm or greater where the carbide belongs to a single variant, that is, an iron-based carbide group elongated in the same direction.
  • the iron-based carbide group stretched in the same direction means a group in which a difference in stretching direction of the iron-based carbide group is within 5°.
  • the bainite surrounded by grain boundaries having an orientation difference of 15° or greater is counted as one bainite crystal grain.
  • the area ratios of martensite and tempered martensite can be calculated as follows: a sample is collected so that a cross section in the sheet thickness direction that is parallel to the rolling direction of the steel sheet serves as an observation surface, the observation surface is polished and etched with a LePera reagent, a range of 1/8 to 3/8 of the sheet thickness from the surface, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface, is observed and photographed by a FE-SEM, and the area ratio of residual austenite measured using X-rays to be described later (to be described in detail) is subtracted from the area ratio of an uncorroded region.
  • the observation range in the observation surface is, for example, a square range having a side of 100 ⁇ m.
  • the thickness is reduced by electrolytic polishing or chemical polishing up to a position that is at a depth of 1/8 to 3/8 of the sheet thickness from the surface.
  • the polished surface is subjected to X-ray diffraction using MoK ⁇ rays as characteristic X-rays, and from the integrated intensity ratios of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained, the area ratio of residual austenite is calculated and set as the value at the position that is at a depth of 1/4 of the sheet thickness.
  • the area ratio of pearlite is obtained as follows: a sample is collected so that a cross section in the sheet thickness direction that is parallel to the rolling direction of the steel sheet serves as an observation surface, the observation surface is polished and etched with a Nital reagent, and a range of 1/8 to 3/8 of the thickness, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet, is observed using a secondary electron image obtained by a scanning electron microscope. The region photographed with high contrast in the secondary electron image is defined as pearlite, and the area ratio thereof is calculated using the above-described image analysis software "ImageJ".
  • the observation range in the observation surface is, for example, a square range having a side of 100 ⁇ m.
  • the ratio (N 6 /N T ) of the number (N 6 ) of crystal grains of ferrite and bainite having an area of 6 ⁇ m 2 or less to the total number (N T ) of crystal grains of ferrite and bainite is one index having a great influence on the fracture resistance of the steel sheet according to the present embodiment.
  • the ratio (N 6 /N T ) of the number of crystal grains (fine grains) having an area of 6 ⁇ m 2 or less to the total number of crystal grains of ferrite and bainite increases, voids are less likely to occur in the vicinity of a forming part during forming. In addition, the occurred voids are less likely to be connected to each other, and the fracture resistance is thus improved.
  • the ratio (N 6 /N T ) of the number of crystal grains having an area of 6 ⁇ m 2 or less in the ferrite and bainite is set to 40% or greater.
  • (N 6 /N T ) is preferably 50% or greater, and more preferably 55% or greater.
  • the ratio of the number of crystal grains having an area of 6 ⁇ m 2 or less in the ferrite and bainite may be set to 90% or less from the viewpoint of suppressing yield point elongation.
  • the ratio (N 50 /N T ) of the number (N 50 ) of crystal grains of ferrite and bainite having an area of greater than 50 ⁇ m 2 to the total number (N T ) of crystal grains of ferrite and bainite is one index having a great influence on the fracture resistance of the steel sheet according to the present embodiment.
  • the ratio (N 50 /N T ) of the number of crystal grains having an area of greater than 50 ⁇ m 2 is set to 5% or less.
  • (N 50 /N T ) is preferably 3% or less. Since (N 50 /N T ) is preferably as small as possible, the lower limit thereof is not particularly set. However, from the viewpoint of suppressing an increase in production cost due to fine control, (N 50 /N T ) may be set to 1 % or greater.
  • the grain sizes and the number proportions of crystal grains of ferrite and bainite are calculated from the results of image analysis obtained in the same visual field using the above-described "EBSD” and "ImageJ".
  • the present inventors have found that in a steel sheet containing soft ferrite and hard martensite and tempered martensite, in a case where the Mn content in the ferrite near the interface between ferrite and martensite or tempered martensite (the interface between ferrite and martensite and the interface between ferrite and tempered martensite) is high, the fracture resistance decreases.
  • the maximum Mn content in a region up to 0.5 ⁇ m from the interface between ferrite and martensite or tempered martensite in a direction perpendicular to the interface (in a case where the interface is not a straight line, a direction perpendicular to a tangent to the interface at the position, and the same shall apply hereinafter) and toward the inside of the ferrite grains (that is, a range extending 0.5 ⁇ m from the interface in the ferrite) is set to be 0.30 mass% or more lower than the average Mn content of the steel sheet.
  • Whether the maximum Mn content in the region up to 0.5 ⁇ m from the interface between ferrite and martensite or tempered martensite in a direction perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet is determined by the following method.
  • the range extending 0.5 ⁇ m or greater from the interface between two, i.e., ferrite and martensite or ferrite and tempered martensite facing each other at a distance of 3.0 ⁇ m or less in a direction perpendicular to the interface and toward the inside of the ferrite grains is subjected to linear analysis using an EPMA.
  • the difference between the maximum Mn content in the ferrite in the range extending 0.5 ⁇ m from the interface obtained by the linear analysis and the average Mn content of the steel sheet (average Mn content of steel sheet - maximum Mn content in range extending 0.5 ⁇ m from interface) is denoted by ⁇ Mn.
  • the linear analysis is performed at 10 points, and in a case where the average of ⁇ Mn at the 10 points is 0.30 (mass%) or greater, the Mn content in the region up to 0.5 ⁇ m from the interface between ferrite and martensite or tempered martensite in a direction perpendicular to the interface and toward the inside of the ferrite grains is determined to be 0.30 mass% or more lower than the average Mn content of the steel sheet ( ⁇ Mn ⁇ 0.30).
  • Average Aspect Ratio of Crystal Grains of Ferrite and Bainite Having Area of 6 ⁇ m 2 or Less is 1.0 or Greater and 2.0 or Less
  • the average aspect ratio of crystal grains of ferrite and bainite having an area of 6 ⁇ m 2 or less is also one index having an influence on the fracture resistance.
  • the average aspect ratio is more preferably 1.0 or greater and 1.5 or less.
  • the aspect ratio refers to the ratio of the longest diameter (major axis) of the ferrite crystal grain to the longest diameter (minor axis) among the diameters of the ferrite orthogonal thereto. The same applies to the aspect ratio of the bainite crystal grain.
  • Crystal grains having an area of greater than 6 ⁇ m 2 are not particularly limited since the contribution thereof to void connection is relatively small. However, since it is preferable that the crystal grains be stretched grains from the viewpoint of reducing the stress concentration at the interface, the average aspect ratio may be greater than 2.0 and 5.0 or less.
  • the tensile strength is set to 780 MPa or greater in consideration of contribution to the weight reduction of a vehicle by application of the steel sheet to a vehicle component.
  • the tensile strength is preferably 980 MPa or greater, and more preferably 1,180 MPa or greater.
  • the tensile strength may be set to 1,500 MPa or less.
  • ⁇ is an index indicating the amount of voids formed, that reduce true stress, and has a good correlation with the crack occurrence during press forming.
  • target ⁇ is 50 MPa or less. ⁇ is more preferably 40 MPa or less.
  • the tensile strength ⁇ 1 of the steel sheet 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 an alloy of these metals on its surface. Due to the presence of the coating layer on the surface of the steel sheet, corrosion resistance is improved.
  • the coating layer may be a known coating layer.
  • the steel sheet is used under an environment where it corrodes, perforation or the like may occur, and thus it may not be possible to reduce the thickness to a certain sheet thickness or less even in a case where the strength is increased.
  • One purpose of high-strengthening of the steel sheet is to reduce the weight by making the steel sheet thinner. Accordingly, even in a case where a high strength steel sheet is developed, the site where the steel sheet is to be applied is limited in a case where the steel sheet has low corrosion resistance. It is preferable that a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals be provided on the surface since the corrosion resistance is improved and the range where the steel sheet is applicable is widened.
  • the term "surface” in the “range of 1/8 to 3/8 of the thickness, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet, means a base metal surface excluding the coating layer.
  • the sheet thickness of the steel sheet according to the present embodiment is not limited to a specific range, but is preferably 0.3 to 6.0 mm in consideration of strength, versatility, and producibility.
  • the method for producing the steel sheet according to the present embodiment is not particularly limited, but the steel sheet can be obtained by a producing method including the following steps:
  • a slab having a predetermined chemical composition in a case where the steel sheet according to the present embodiment is obtained, the same chemical composition as the steel sheet according to the present embodiment) is heated and hot-rolled to obtain a hot-rolled steel sheet.
  • the slab to be heated may be a slab obtained by continuous casting or casting and blooming or may also be a slab obtained by additionally performing hot working or cold working on the above-described slab.
  • the heating temperature is not limited. However, in a case where the temperature is lower than 1,100°C, a carbide or sulfide formed during casting does not form a solid solution and becomes coarse, whereby press formability may deteriorate. Therefore, the heating temperature is preferably 1,100°C or higher, and more preferably 1,150°C or higher.
  • finish rolling is performed using a rolling mill having four or more stands, and in a case where an initial stand is defined as a first stand and a final stand is defined as an n-th stand, the ratio of sheet thickness reduction in each of stands ranging from an (n-3)-th stand to the n-th stand is set to 30% or greater, and the rolling temperature in the final stand (the n-th stand) is set to 900°C or lower. That is, for example, in a rolling mill having seven stands, the ratio of sheet thickness reduction in each of a fourth stand, a fifth stand, a sixth stand, and a seventh stand is set to 30% or greater, and the rolling temperature in the seventh stand is set to 900°C or lower.
  • the austenite grain size is refined by recrystallization during rolling, and by introducing a large amount of strain into the austenite, sites where ferrite nuclei are generated are increased and the crystal grains of the hot-rolled steel sheet are refined.
  • the hot-rolled structure becomes coarse and duplex grain sized, and the structure after the annealing step to be described later also becomes coarse.
  • the rolling temperature in the final stand is preferably 830°C or higher.
  • the ratio of sheet thickness reduction in each of the (n-3)- to n-th stands is preferably set to 50% or less.
  • the finish rolling is continuous rolling in which the interpass time between final four passes for rolling is short, the finish rolling is performed using a rolling mill having four or more stands. This is because, in a case where the interpass time is long, the strain is recovered between the passes and does not sufficiently accumulate even in a case where the reduction is performed at a large ratio of sheet thickness reduction.
  • the hot-rolled steel sheet after the hot rolling step is cooled to a coiling temperature of 650°C or lower and 450°C or higher at an average cooling rate of 30 °C/sec or higher and coiled at the coiling temperature.
  • the average cooling rate is lower than 30 °C/sec or the cooling stop temperature (coiling temperature) is higher than 650°C
  • coarse ferrite and pearlite including a coarse carbide are formed non-uniformly. Since the coarse carbide is not easily dissolved in the annealing step, the structure after annealing becomes coarse and duplex grain sized.
  • the average cooling rate is preferably 100 °C/sec or lower in order to stably obtain a target cooling stop temperature.
  • the steel sheet after the coiling step is held so that a holding time in a temperature range from the coiling temperature to the coiling temperature - 50°C is 2 to 8 hours.
  • Mn mainly diffuses at ferrite grain boundaries and is concentrated in cementite.
  • the concentration of Mn to cementite is promoted (for example, the Mn content in cementite becomes greater than 3.0%).
  • Mn-deficient layer having a low Mn content is formed in the vicinity of the part where Mn is concentrated.
  • the holding time in a temperature range from the coiling temperature to (the coiling temperature- 50)°C is longer than 8 hours, the cementite becomes coarse.
  • the coarse carbide is not easily dissolved in the annealing step. Accordingly, in a case where the carbide becomes coarse, the structure after annealing becomes coarse and duplex grain sized. Therefore, the holding time is set to 8 hours or shorter.
  • the holding time in a temperature range from the coiling temperature to the coiling temperature-50°C is 2 hours or longer.
  • 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.
  • the average cooling rate after the holding step until the cooling stop temperature of 300°C or lower is lower than 0.1 °C/sec, there is a concern that cementite may become coarse.
  • the average cooling rate is high, hard martensite is likely to be formed in a case where untransformed ⁇ remains. In this case, there is a concern that the strength of the hot-rolled sheet may increase and the cold rolling load may increase. Therefore, the average cooling rate is preferably 12.0 °C/sec or lower.
  • the hot-rolled steel sheet after the cooling step is cold-rolled at a ratio of sheet thickness reduction of 20% to 80% to obtain a cold-rolled steel sheet.
  • the strain does not sufficiently accumulate in the steel sheet, and sites where austenite nuclei are generated become non-uniform.
  • the grain size becomes coarse or duplex grain sized are formed in the subsequent annealing step, whereby the number proportion of the crystal grains of ferrite and bainite having an area of 6 ⁇ m 2 or less and the number proportion of the crystal grains of ferrite and bainite having an area of 50 ⁇ m 2 or greater to the total number of crystal grains of the ferrite and the bainite are not within a desired range.
  • the aspect ratio of the crystal grains also increases. As a result, the fracture resistance deteriorates.
  • the ratio of sheet thickness reduction is set to 20% or greater and 80% or less.
  • the ratio of sheet thickness reduction is preferably 30% or greater and 80% or less.
  • the cold rolling method is not limited, and the number of rolling passes and the rolling reduction per pass may be set as appropriate.
  • Pickling may be performed under known conditions before the cold rolling.
  • the cold-rolled steel sheet is heated to an annealing temperature of 740°C to 900°C at an average temperature rising rate of 5 °C/sec or higher and held at the annealing temperature (740°C to 900°C) for 60 seconds or longer.
  • the average temperature rising rate is set to 5 °C/sec or higher.
  • the average temperature rising rate in a temperature range of 550°C or higher be set to 5 °C/sec or higher.
  • the average temperature rising rate is preferably 50 °C/sec or lower from the viewpoint of economic efficiency.
  • the annealing temperature is lower than 740°C
  • the amount of austenite is small, the area ratio of martensite and tempered martensite after annealing becomes less than 10%, and the tensile strength becomes less than 780 MPa.
  • the annealing temperature is higher than 900°C, the microstructure structure becomes coarse and the fracture resistance deteriorates.
  • the annealing temperature is set to 740°C or higher and 900°C or lower.
  • the annealing temperature is preferably 780°C or higher and 850°C or lower.
  • the holding time (retention time) at the annealing temperature is shorter than 60 seconds, austenite is not sufficiently formed, the area ratio of martensite and tempered martensite after annealing becomes less than 10%, and the tensile strength becomes less than 780 MPa. Therefore, the holding time at the annealing temperature is set to 60 seconds or longer.
  • the holding time is preferably 70 seconds or longer, and more preferably 80 seconds or longer.
  • the annealing time is set to 300 seconds or shorter.
  • the cooling rate after heating in the annealing step is not limited. However, the steel sheet is slowly cooled to achieve a desired ferrite fraction, and then rapidly cooled to form martensite. A holding step or a step of increasing the temperature again may be included in order to temper the martensite.
  • the Mn-deficient layer having a low Mn content becomes ferrite after annealing, the fine cementite is dissolved and becomes martensite (or residual austenite) having a high Mn content, and thus the structure of the steel sheet according to the present embodiment is obtained.
  • a coating layer such as a plating layer containing zinc, aluminum, magnesium, or an alloy of these metals may be formed on the surface of the steel sheet from the viewpoint of increasing the corrosion resistance of the steel sheet.
  • the steel sheet may be immersed in a plating bath during cooling after holding to form a hot-dip plating.
  • the hot-dip plating may be heated to a predetermined temperature and alloyed to obtain an 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-described methods may be adopted.
  • the plating conditions and alloying conditions known conditions may be applied depending on the composition of the plating.
  • Tables 1-1 and 1-2 Various slabs having chemical compositions shown in Tables 1-1 and 1-2 (subsequent to 1-1) were prepared by casting.
  • Tables 1-1 and 1-2 the blank space indicates that the corresponding element was not added intentionally.
  • the unit of each slab component was mass%, and the remainder of the slab was Fe and impurities.
  • the slabs were heated to a temperature range of 1,150°C to 1,250°C, and hot-rolled under conditions shown in Tables 2-1 and 2-2 to obtain hot-rolled steel sheets.
  • the hot-rolled steel sheets were cooled to a coiling temperature under conditions shown in Tables 2- 3 and 2-4, and coiled.
  • a rolling mill having four or more stands was used for finish rolling.
  • the hot-rolled steel sheets were held so that a holding time in a temperature range from the coiling temperature to the coiling temperature - 50°C was as shown in Tables 2-3 and 2-4, the hot-rolled steel sheets were cooled to a temperature range of 300°C or lower.
  • the hot-rolled steel sheets after cooling were pickled, and then cold-rolled at a ratio of sheet thickness reduction shown in Tables 2-3 and 2-4, and cold-rolled steel sheets having a sheet thickness of 1.4 mm were obtained.
  • the obtained cold-rolled steel sheets were annealed under conditions shown in Tables 2-5 and 2-6. Some steel sheets were immersed in a galvanizing bath during cooling in the annealing to form a hot-dip galvanized layer on their surfaces. In addition, among the plated steel sheets, some steel sheets were further subjected to an alloying treatment to turn the hot-dip galvanized layer into a hot-dip galvannealed layer.
  • the phrase "Presence or Absence of Plating" indicates whether hot-dip galvanizing was performed in the continuous annealing step, and the phrase “Presence or Absence of Alloying” indicates whether the alloying treatment was performed after hot-dip galvanizing.
  • the cooling rate was adjusted in order to set the area ratios of ferrite, martensite, and tempered martensite within preferable ranges.
  • Annealing Step Remarks Average Temperature Rising Rate until Annealing Temperature (°C/sec) Average Temperature Rising Rate at 550°C or Higher (°C/sec) Annealing Temperature (°C) Holding Time (sec) Presence or Absence of Plating Presence or Absence of Alloying 1 8 6 860 220 Presence Presence Invention Example 2 9 6 839 102 Presence Absence 3 15 15 811 82 Absence Absence 4 9 6 890 87 Presence Presence 5 14 4 794 263 Absence Absence 6 10 8 895 89 Presence Absence 7 10 8 877 248 Absence Absence 8 7 6 805 111 Presence Presence 9 9 4 883 80 Absence Absence 10 9 8 804 102 Presence Presence 11 16 15 866 280 Absence Absence 12 19 17 815 78 Presence Presence 13 22 15 826 241 Presence Presence 14 12 9 830 122 Presence Absence 15 12 10 745 258 Absence Absence 16 8 7 763 91 Presence Presence 17
  • Annealing Step Remarks Average Temperature Rising Rate until Annealing Temperature (°C/sec) Average Temperature Rising Rate at 550°C or Higher (°C/sec) Annealing Temperature (°C) Holding Time (sec) Presence or Absence of Plating Presence or Absence of Alloying 38 8 6 845 98 Absence Absence Comparative Example 39 9 7 889 158 Presence Presence 40 10 7 885 79 Presence Presence 41 11 8 868 77 Absence Absence 42 31 29 845 76 Absence Absence 43 9 7 840 79 Presence Presence 44 8 6 832 97 Presence Absence 45 8 8 860 243 Presence Presence 46 8 6 820 215 Presence Presence 47 11 8 891 254 Presence Absence 48 7 6 856 90 Presence Absence Comparative Example 49 7 5 798 79 Presence Absence 50 11 7 880 76 Absence Absence 51 24 13 840 82 Presence Absence 52 - 53 15 9 858 132 Presence Presence 54 9 6 800 77
  • the area ratio of the microstructure structure (ferrite, bainite, martensite, tempered martensite, pearlite, and residual austenite (residual ⁇ )) in a range (thickness 1/4 part) of 1/8 to 3/8 of the thickness, centered at a position that was at a depth of 1/4 of the sheet thickness from the surface; the ratio (N 6 /N T ) of the number N 6 of crystal grains of ferrite and bainite having an area of 6 ⁇ m 2 or less to the total number NT of crystal grains of ferrite and bainite and the ratio (N 50 /N T ) of the number N 50 of crystal grains of ferrite and bainite having an area of greater than 50 ⁇ m 2 to the total number NT of crystal grains of ferrite and bainite in the thickness 1/4 part; the difference ⁇ Mn between the maximum value of the Mn concentration in a region up to 0.5 ⁇ m from an interface between ferrite and martensite in a direction per
  • the tensile strength (TS) ( ⁇ 1), the uniform elongation (u-El), and ⁇ , that is a difference ( ⁇ 1 - ⁇ 2) between the stress ⁇ 2 at uniform elongation + 1.0% and the tensile strength ⁇ 1 were evaluated as follows: a JIS No. 5 test piece was collected from the steel sheet so that a longitudinal direction thereof was perpendicular to the rolling direction of the steel sheet, and a tensile test was performed thereon according to JIS Z 2241: 2011.
  • Steel sheets having a tensile strength (TS) of 780 MPa or greater were determined as acceptable in terms of tensile strength.
  • steel sheets in which ⁇ was 50 MPa or less were determined to be excellent in fracture resistance.
  • Test Nos. 38 to 47 are comparative examples in which the production conditions are within the ranges of the present invention, but the chemical composition is outside the ranges of the present invention, and at least one of the strength, formability, or fracture resistance thereof is inferior.
  • Test Nos. 48 to 57 are comparative examples in which the chemical composition is within the ranges of the present invention, but any of the conditions in the producing method is outside the ranges of the present invention.
  • the present invention it is possible to provide a steel sheet having excellent fracture resistance and a method for producing the steel sheet. Therefore, the present invention has high industrial applicability.

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