STEEL SHEET

Abstract

A steel sheet comprising a steel microstructure containing, by volume fraction, tempered martensite: 85% or more, retained austenite: from 5% or more to less than 15%, and ferrite, pearlite, bainite, and freshly tempered martensite less than 10% in total, where the Mn and C contents in the retained austenite are denoted as MnA and CA and the Mn and C contents in the matrix are denoted as MnM and CM, respectively, satisfies the following formulas (1) to (3), and the number of carbides having an equivalent circle radius of 0.1 µm or more is 100 or less in a measurement region of 20,000 µm², and the steel sheet has a tensile strength of 1100 MPa or more. The steel sheet is excellent in terms of impact resistance and formability. MnA / MnM=1.2 (1) CA / CM=5.0 (2) CA=1.0 (3).

Description

STEEL SHEET TECHNICAL FIELD The present invention relates to a steel sheet. BACKGROUND OF THE STATE OF THE ART To ensure the safety of a car in a collision and a reduction in weight, it is required that the members or elements of the car's structure establish compatibility between high strength and excellent impact resistance. Patent document 1 (WO 2016 / 021193) describes an invention relating to a high-strength steel sheet having a predetermined chemical composition; The high-strength steel sheet includes a steel microstructure containing 59.2% or more to 80% or less of ferrite and bainitic ferrite in total and 3% or more to 20% or less of martensite in area fraction, 10% or more of retained austenite in volume fraction and 10% or less of a remaining structure in area fraction, the retained austenite having an average grain diameter of 2 pm or less, an average amount of Mn (% by mass) in the retained austenite is 1.2 times or more an amount of Mn (% by mass) in steel and the steel sheet includes a steel microstructure in which an area fraction of retained austenite, which has an average amount of C (% by mass) that is 2.1 time or more of an amount of C (% by mass) in the steel, represents 60% or more of a fraction of the area of ​​the total retained austenite. Patent document 2 (WO 2018 / 073919) describes an invention relating to a metal-coated steel sheet having a predetermined chemical composition; the metal-coated steel sheet includes a steel microstructure containing more than 5.0% by volume of retained austenite and more than 5.0% by volume of tempered martensite and an amount of C in the retained austenite is 0.85% by mass or more. LIST OF STATE OF THE ART DOCUMENTS PATENT DOCUMENT Patent document 1: WO 2016 / 021193 Patent document 2: WO 2018 / 073919 CC Ln / Lznz / E / YILI SUMMARY OF THE INVENTION TECHNICAL PROBLEM The invention described in patent document 1 relates to a steel sheet comprising a composite steel microstructure in which retained austenite, which primarily provides ductility, and martensite, which provides strength, are dispersed within a steel microstructure containing primarily ferrite, which is ductility-rich and soft. The invention, according to patent document 1, causes carbon (C) to concentrate in austenite, thereby maintaining a certain amount of retained austenite, by performing a heat treatment from 820°C or higher to 950°C or lower, specifically in a single-phase austenite region, and by performing a heat treatment from 740°C or higher to 840°C or higher, i.e., an intercritical ferrite-austenite region. However, retained austenite with a high carbon concentration is a steel microstructure that undergoes strain-induced transformation to improve formability when a component is machined. However, after undergoing strain-induced transformation to become martensite, retained austenite becomes very hard and can serve as a fracture initiation point, potentially degrading the properties of a component after machining. This can reduce the formability of a steel sheet. Patent document 2 describes a heating step of a steel sheet to a point Aci or higher or a point Acj or higher, a cooling step of the steel sheet to 500°C or less, a galvanizing step of the steel sheet, a cooling step of the steel sheet to 300°C or less, a step of performing a thermal refining roll on the steel sheet, and a reheating and holding step of the steel sheet from 200°C to 600°C, which are carried out in this order, and patent document 2 describes a metal-coated steel sheet including a steel microstructure including retained austenite in which C is concentrated, obtained by performing a reheating step after galvanizing. However, heating a steel sheet to the Ac3o point plus only once before galvanizing causes coarse carbides to remain in large quantities, resulting in a decrease in the limiting hole expansion ratio. An objective of the present invention is to provide a steel sheet that has high strength (specifically, a tensile strength of 1100 MPa or more) and excellent impact or shock resistance and also has excellent forming capability. QJ CC Ln / Lznz / E / YILI SOLUTION TO THE PROBLEM To achieve this objective, the present inventors studied how to stabilize retained austenite by concentrating the Mn, rather than relying solely on concentrating the C. Generally, a transformation-induced plasticity (TRIP) steel containing approximately 5% Mn by mass is known to contain Mn. However, increasing the Mn content is disadvantageous from the standpoint of productivity and weldability. The present inventors hypothesized that retained austenite would stabilize by producing regions with locally high Mn concentrations while restricting an increase in the total amount of Mn contained in a steel sheet (the amount of Mn in a matrix). For example, to concentrate Mn in carbides after hot rolling, it is conceivable to roll a hot-rolled steel sheet at a high temperature. However, the carbides in which the Mn concentrates are difficult to dissolve, form thick, and can serve as the starting point for fracture. It is difficult to dissolve such thick carbides by normal heat treatment (performing the heat treatment only once). Therefore, the present inventors conducted intensive studies on how to dissolve carbides in which Mn is concentrated and based on a multi-stage heat treatment. Specifically, the first heat treatment is performed to form a steel microstructure consisting primarily of freshly quenched or tempered martensite. Martensite is a steel microstructure containing a large number of grain boundaries and dislocations. In grain boundary diffusion, where the grain boundaries serve as diffusion pathways, and in dislocation diffusion, where the dislocations serve as diffusion pathways, elements diffuse faster than in intraparticle diffusion, where elements diffuse within the grains.The dissolution of carbides is a phenomenon attributable to the diffusion of elements. In multi-stage heat treatment, a material is made to contain a large amount of grain boundaries and dislocations after the first heat treatment. During the second heat treatment, grain boundary diffusion and dislocation diffusion tend to occur. Consequently, carbides dissolve readily through multi-stage heat treatment. Furthermore, during the second heat treatment, carbides containing concentrated manganese (Mn) dissolve sufficiently. Even after these Mn-containing carbides have dissolved sufficiently, some Mn remains concentrated in the areas where the carbides form because the diffusion rate of Mn is low compared to that of carbon (C). As can be seen from the above, retained austenite tends to form in regions where Mn is concentrated.Therefore, it is possible to stabilize the retained austenite without excessively increasing the carbon concentration. In other words, it is possible to increase the amount of retained austenite, which is unlikely to act as a fracture starting point even after deformation-induced transformation. Furthermore, since the carbides dissolve sufficiently before multi-stage heat treatment, it is also possible to limit the impact strength degradation caused by carbides. As a result, a high-strength steel sheet is obtained that has excellent impact resistance and formability. The present invention was made based on such discoveries and the essence of the present invention is a steel sheet described below. A steel sheet including a steel microstructure containing, by volume fraction, tempered martensite: 85% or more, retained austenite: from 5% or more to less than 15%, and ferrite, pearlite, bainite as freshly quenched martensite being less than 10% in total, the chemical composition of the steel sheet consisting, by mass %: C: from 0.18% or more to 0.38% or less, Yes: from 0.80% or more to 2.50% or less, Mn: from 0.6% or more to 5.0% or less, P: 0.0200% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, To: from 0% or more to 1,000% or less, Cr: from 0% or more to 2.0% or less, Mo: from 0% or more to 0.50% or less, Ti: from 0% or more to 0.10% or less, Nb: from 0% or more to 0.10% or less, B: from 0% or more to 0.0100% or less, V: from 0% or more to 0.50% or less, Cu: from 0% or more to 0.50% or less, W: from 0% or more to 0.100% or less, Ta: from 0% or more to 0.100% or less, Ni: from 0% or more to 1.00% or less, Co: from 0% or more to 0.50% or less, Sn: from 0% or more to 0.050% or less, Sb: from 0% or more to 0.050% or less, As: from 0% or more to 0.050% or less, QJ CC Ln / Lznz / E / YILI Mg: from 0% or more to 0.050% or less, Ca: from 0% or more to 0.050% or less, And: from 0% or more to 0.050% or less, Zr: from 0% or more to 0.050% or less, The: from 0% or more to 0.050% or less, Ce: from 0% or more to 0.050% or less, and the remainder: Fe and unavoidable impurities, when a content of Mn and a content of C in the retained austenite are denoted as Mπα and Ca, respectively, and when a content of Mn and a content of C in a matrix are denoted as Mπμ and Cm, respectively, the following formulas (1) to (3) are satisfied and when a region measuring 20000 pm2 and centered around the point t / 4 (t denotes a thickness of the steel sheet) of a surface of the steel sheet is observed, the number of carbides having an equivalent circle radius of 0.1 pm or more is 100 or less and the steel sheet has a tensile strength of 1100 MPa or more. MnA / MnM>L2(1) Ca / Cm<5.0(2) CA< LO(3) The steel sheet described above may include a galvanized layer, a galvanized and annealed layer, or an electrogalvanized layer on its surface. ADVANTAGEOUS EFFECTS OF THE INVENTION According to the present invention, a steel sheet is obtained that has high strength (specifically, a tensile strength of 1100 MPa or more) and excellent impact resistance and also has excellent forming capacity. DESCRIPTION OF THE MODALITIES The following will describe one embodiment of the present invention. Steel microstructure The steel microstructure of the steel sheet, according to the present modality, contains, in volume fraction, tempered martensite: 85% or more, retained austenite: from 5% or more to less than 15% and ferrite, pearlite, bainite and freshly tempered martensite being less than 10% in total. QJ CC Ln / ίΖΠΖ / Β / ΥΙΛΙ By achieving an tempered martensite volume fraction of 85% or higher, the steel sheet can have sufficient strength. The tempered martensite volume fraction is preferably 87% or higher. It is noted that, from the standpoint of limiting the deterioration of the steel sheet's formability, the tempered martensite volume fraction is preferably 95% or lower. Retained austenite contributes to improving the formability of steel sheets through strain-induced transformation. To achieve this effect, the volume proportion of retained austenite must be 5% or more. The remainder of the steel microstructure consists of at least one or more types of ferrite, pearlite, bainite, and freshly tempered martensite. With less than 10% of these in total, the deterioration of tensile strength and formability of the steel sheet can be restricted due to insufficient tempered martensite or retained austenite. Amount of Mn in retained austenite In steel sheets, according to the present modality, when the Mn content in the retained austenite is denoted as Mπα and the Mn content in a matrix excluding the retained austenite in the steel sheet is denoted as Mπμ, “Miu / Mum” must be 1.2 or greater and preferably 1.5 or greater. “Mua / Mum” is considered an index indicating the Mn concentration in the retained austenite. With “Mua / Mum” of 1.2 or greater, the Mn concentration in the retained austenite is considered sufficient. The stability of retained austenite with an insufficient Mn concentration can be maintained by increasing the C concentration. Retained austenite with an excessive C concentration becomes hard when subjected to a strain-induced transformation to martensite and can serve as the starting point for fracture. Amount of C in retained austenite In steel sheets, according to the present modality, when the carbon content in the retained austenite is denoted as Ca and the carbon content in a matrix excluding the retained austenite in the steel sheet is denoted as Cm, “Ca / Cm” must be 5.0 or less and Ca must be 1.0 or less. When “Ca / Cm” is 5.0 or less and Ca is 10 or less, the excessive carbon concentration in the retained austenite is considered restricted. Therefore, retained austenite that hardens when subjected to a strain-induced transformation to martensite and can serve as the starting point for a fracture, as described above, is considered restricted. “Ca / Cm” is preferably 4.5 or less. “Ca / Cm” has no specific lower limit. The number of carbides In the steel sheet, according to the present modality, when observing a region measuring 20000 pm2 and centered around a point t / 4 (t denotes a thickness of the steel sheet) from a surface of the steel sheet, it is observed that the number of carbides having a radius The number of carbides with an equivalent circle radius of 0.1 pm or more should be 100 or less. If the number of carbides with an equivalent circle radius of 0.1 pm or more is excessively large, sufficient bore expansion capacity cannot be maintained, and impact resistance deteriorates. For this reason, the number of carbides should be 100 or less in an area of ​​20,000 pm². The number is preferably 80 or less, and more preferably 70 or less. Even more preferably, the number is 50 or less. It should be noted that the amount of carbides is measured based on an image of a steel microstructure captured with a scanning electron microscope. Before observation, an observation surface of a sample for the steel microstructure is wet-polished with sandpaper and then polished with a diamond abrasive having an average particle size of 1 µm to a mirror finish. Subsequently, the microstructure is chemically etched with a saturated alcoholic solution of picric acid. A field of view centered on a point with a thickness of t / 4 the sheet thickness is observed at a magnification of 5000x. A plurality of randomly selected points are captured, such that the total area of ​​the points becomes 20,000 µm².The captured image is analyzed using image analysis software such as WinROOF, manufactured by MITANI CORPORATION, and the areas of carbides contained within a 20,000 pm2 region are measured in detail. Assuming that each carbide has a circular shape, a radius for each carbide (equivalent circle radius) is determined from the areas determined by the image analysis, and the number of carbides with a radius of 0.1 pm or more is calculated. Tensile strength The steel sheet, according to the present embodiment, has a tensile strength of 1100 MPa or more. The tensile strength of the steel sheet, according to the present invention, is determined by a tensile test. Specifically, the tensile test is performed in accordance with JIS Z 2241 (2011) and using JIS No. 5 test coupons, which are taken from the steel sheet in a direction perpendicular to the rolling direction of the steel sheet, and the maximum of the measured tensile strengths is determined as the tensile strength of the steel sheet. Chemical composition The chemical composition of the steel sheet, according to this specification, will be described below. It should be noted that the symbol “%” for the content of each element means “% by mass”. C: from 0.18% or more to 0.38% or less Carbon (C) is an element that maintains a predetermined amount of martensite to improve the strength of steel sheets. A C content of 0.18% or more produces the predetermined amount of martensite, facilitating an increase in the strength of steel sheets to 1100 MPa or more. The C content should preferably be 0.22% or more. Alternatively, the C content should be 0.38%. CC Ln / ίZΖΠZ / Β / YΙΛΙ or less from the point of view of limiting the embrittlement caused by an excessive increase in the strength of the steel sheet. Yes: from 0.80% or more to 2.50% or less Silicon (Si) is an element that acts as a deoxidizer. Si also influences carbide morphology and the production of retained austenite after heat treatment. Furthermore, Si is useful for increasing the strength of steel sheets through the utilization of retained austenite. To restrict carbide production and produce a desired amount of retained austenite to maintain the workability of the steel sheet, the Si content should be 0.80% or higher. Conversely, the Si content should be 2.50% or lower to limit the reduction in the workability of the steel sheet due to steel embrittlement. Mn: from 0.6% or more to 5.0% or less Manganese (Mn) is an element that acts as a deoxidizer. Mn also improves hardenability. To obtain sufficiently tempered martensite with Mn, the Mn content should be 0.6% or higher. On the other hand, the Mn content should be 5.0% or lower to limit the formation of coarse Mn oxide, which can be the starting point for fracture during press casting. P: 0.0200% or less Phosphorus (P) is an impurity element that segregates in the central portion of the steel sheet thickness, reducing toughness and embrittlement in the weld zone. The P content is preferably as low as possible to limit the reduction in workability and impact resistance of the steel sheet. Specifically, the P content should be 0.0200% or less. Ideally, it should be 0.0100% or less. However, if the P content of a practical steel sheet is reduced to less than 0.00010%, the production costs increase significantly, which is economically disadvantageous. For this reason, the P content can be 0.00010% or higher. S: 0.0200% or less Sulfur (S) is an impurity that impairs weldability and reduces productivity in casting and hot rolling. S also forms thick MnS deposits, which can negatively impact hole expansion. To minimize the reduction in weldability, productivity, and impact strength, the S content should ideally be as low as possible. Specifically, the S content should be 0.0200% or less. More preferably, 0.0100% or less. However, if the S content of a practical steel sheet is reduced to less than 0.000010%, the sheet's production costs will decrease. CC Ln / įZРZ / B / YILI of steel increase significantly, which is economically disadvantageous. For that reason, the S content can be 0.000010% or more. N: 0.0200% or less Nitrogen (N) is an element that forms coarse nitride, which degrades the formability and impact resistance of steel sheet and causes pitting to develop during welding. For this reason, the N content should preferably be 0.0200% or less. O: 0.0200% or less Oxygen (O₂) is an element that forms a thick oxide which degrades the formability and impact resistance of steel sheet and causes pitting to develop during welding. For this reason, the O₂ content is preferably 0.0200% or less. To: from 0% or more to 1,000% or less Aluminum (Al) is an element that acts as a deoxidizer and is added to steel sheet when necessary. To achieve this effect and ensure the steel sheet contains Al, the Al content is preferably 0.02% or more. However, the Al content is preferably 1.000% or less to prevent the formation of coarse Al oxide, which would reduce the workability of the steel sheet. Cr: from 0% or more to 2.0% or less As with manganese (Mn), chromium (Cr) is a useful element for improving the strength of steel sheet by increasing hardenability. Although the Cr content can be 0%, to achieve the desired effect and make the steel sheet contain Cr, the Cr content is preferably 0.10% or higher. Furthermore, the Cr content is preferably 2.0% or lower to prevent the formation of coarse Cr carbide, which reduces cold forming capacity. Mo: from 0% or more to 0.50% or less As with manganese (Mn) and chromium (Cr), molybdenum (Mo) is a useful element for improving the strength of steel sheets. Although the Mo content can be 0%, to achieve the desired effect and make the steel sheet moly resistant, the Mo content is preferably 0.01% or higher. Furthermore, the Mo content is preferably 0.50% or lower to prevent the formation of coarse Mo carbide, which would reduce cold workability. Ti: from 0% or more to 0.10% or less Titanium (Ti) is an important element in controlling carbide morphology. Ti can accelerate the increase in ferrite strength. Furthermore, Ti tends to form coarse Ti oxide (TiN), decreasing the workability of the steel sheet. Therefore, from the perspective of maintaining the workability of the steel sheet, the Ti content is QJ CC Ln / ίZΖΠZ / Β / YΙΛΙ preferably as low as possible, is preferably 0.10% or less and can be 0%. However, the Ti content can be 0.001% or more because decreasing the Ti content to less than 0.001% leads to an excessive increase in refining costs. Nb: from 0% or more to 0.10% or less As with titanium, niobium (Nb) is a useful element for controlling carbide morphology and is also effective in improving the toughness of steel sheets by refining the steel's microstructure. For this reason, steel sheets may contain niobium when necessary. Although the niobium content can be 0%, to achieve the desired effects, it should preferably be 0.001% or higher. However, 0.10% or lower is preferable to prevent the precipitation of hard, fine niobium carbide, which would increase the strength of the steel sheet while degrading its ductility. B: from 0% or more to 0.0100% or less Boron (B) is an element that prevents ferrite and pearlite from forming during the cooling process from austenite and accelerates the formation of low-temperature transformation structures such as bainite and martensite. Furthermore, boron is beneficial for improving the strength of steel sheets. For this reason, boron may be incorporated into steel sheets when necessary. When boron is present in steel sheets, the content is preferably 0.0001% or less. It should be noted that boron concentrations below 0.0001% require meticulous analysis for identification and reach the lower detection limit for some analytical instruments. On the other hand, a boron content of 0.0100% or less is preferable to limit the production of coarse boron nitride, which can contribute to void formation during the press molding of steel sheets. V: from 0% or more to 0.50% or less As with Ti and Nb, V (vanadium) is a useful element for controlling carbide morphology and is also effective in improving the toughness of steel sheets by refining the steel's microstructure. For this reason, V can be incorporated into steel sheets when necessary. When V is incorporated, the V content is preferably 0.001% or higher. On the other hand, the V content is preferably 0.50% or lower to prevent excessive precipitation of fine V carbide, which increases the strength of the steel sheet and reduces its ductility. Cu: from 0% or more to 0.50% or less Copper (Cu) is a useful element for improving the strength of steel sheets. Although a Cu content can be 0%, to achieve the desired effect and make the steel sheet appear to contain Cu, the Cu content is preferably 0.001% or higher. Furthermore, the Cu content is preferably CC Ln / ίZΖΠZ / Β / YΙΛΙ 0.50% or less from the point of view of preventing productivity from decreasing due to hot shortages in hot rolling. W: from 0% or more to 0.100% or less As with Nb and V, W (tungsten) is a useful element for controlling carbide morphology and increasing the strength of steel sheets. Although the W content can be 0%, to achieve the desired effect and ensure the steel sheet contains W, the content is preferably 0.001% or higher. On the other hand, the W content is preferably 0.100% or lower to prevent excessive precipitation of fine W carbide, which increases the strength of the steel sheet but reduces its ductility. Ta: from 0% or more to 0.100% or less As with Nb, V, and W, tantalum (Ta) is a useful element for controlling carbide morphology and increasing the strength of steel sheets. Although the Ta content can be 0%, to achieve the desired effect and ensure the steel sheet contains Ta, the Ta content is preferably 0.001% or more, and more preferably 0.002% or more. Furthermore, the Ta content is preferably 0.100% or less, and more preferably 0.080% or less, to prevent excessive precipitation of fine Ta carbide, which increases the strength of the steel sheet while degrading its ductility. Ni: from 0% or more to 1.00% or less Nickel (Ni) is an element that is useful for improving the strength of steel sheets. Although the Ni content can be 0%, to achieve the desired effect and make the steel sheet contain Ni, the Ni content is preferably 0.001% or higher. On the other hand, the Ni content is preferably 1.00% or lower from the perspective of limiting the reduction in the ductility of the steel sheet. Co: from 0% or more to 0.50% or less As with nickel (Ni), cobalt (Co) is a useful element for improving the strength of steel sheets. Although the Co content can be 0%, to achieve the desired effect and make the steel sheet contain Co, the Co content is preferably 0.001% or higher. On the other hand, the Co content is preferably 0.50% or lower from the perspective of limiting the reduction in the ductility of the steel sheet. Sn: from 0% or more to 0.050% or less Tin (Sn) is an element that may be present in steel sheets when scrap is used as the raw material. The Sn content is preferably as low as possible and can be 0%. To prevent a decrease in cold forming capacity due to ferrite embrittlement, the Sn content is preferably 0.050% or less. CC Ln / Lznz / E / YILI preferably 0.040% or less. However, the Sn content may be 0.001% or more from the point of view of limiting the increase in refining costs. Sb: from 0% or more to 0.050% or less As with tin (Sn), antimony (Sb) is an element that may be present in steel sheet when scrap is used as the raw material. The Sb content is preferably as low as possible and can be 0%. From the perspective of limiting the reduction in the cold forming capacity of the steel sheet, the Sb content is preferably 0.050% or less, and more preferably 0.040% or less. However, the Sb content can be 0.001% or more from the perspective of limiting the increase in refining costs. As: from 0% or more to 0.050% or less As with Sn and Sb, As (arsenic) is an element that may be present in steel sheet when scrap is used as the raw material. The As content is preferably as low as possible and can be 0%. From the perspective of limiting the reduction in the cold forming capacity of the steel sheet, the As content is preferably 0.050% or less, and more preferably 0.040% or less. However, the As content can be 0.001% or more from the perspective of limiting the increase in refining costs. Mg: from 0% or more to 0.050% or less Magnesium (Mg) is an element that controls the morphology of sulfides and oxides, contributing to improved workability when bending steel sheets. Although the Mg content can be 0%, to achieve the desired effect and ensure the steel sheet contains Mg, the content is preferably 0.0001% or more, and more preferably 0.0005% or more. Furthermore, to prevent a decrease in cold forming capacity due to the formation of coarse inclusions, the Mg content is preferably 0.050% or less, and more preferably 0.040% or less. Ca: from 0% or more to 0.050% or less As with magnesium, calcium (Ca) is an element that, in trace amounts, can control sulfide morphology. Although the Ca content can be 0%, to achieve the desired effect and ensure the steel sheet contains Ca, the Ca content is preferably 0.001% or higher. Furthermore, to prevent a reduction in the cold-formability of the steel sheet due to the production of coarse calcium oxide, the Ca content is preferably 0.050% or lower, and more preferably 0.040% or lower. And: from 0% or more to 0.050% or less As with Mg and Ca, Yttrium (Y) is an element that, in trace amounts, can control sulfide morphology. Although the Y content can be 0%, to achieve the desired effect and make the steel sheet contain Y, the Y content is preferably 0.001% or higher. CC Ln / įZРZ / B / YILI side, from the point of view of preventing the reduction of the cold forming capacity of the steel sheet by the production of coarse oxide Y, the content of Y is preferably 0.050% or less and more preferably 0.040% or less. Zr: from 0% or more to 0.050% or less As with Mg, Ca, and Y, Zr (zirconium) is an element that, in trace amounts, can control sulfide morphology. Although the Zr content can be 0%, to achieve the desired effect and ensure the steel sheet contains Zr, the Zr content is preferably 0.001% or higher. Furthermore, to prevent a decrease in the cold-formability of the steel sheet due to the production of coarse Zr oxide, the Zr content is preferably 0.050% or lower, and more preferably 0.040% or lower. The: from 0% or more to 0.050% or less Lanthanum (La) is an element that, in trace amounts, is useful for controlling sulfide morphology. Although the La content can be 0%, to achieve the desired effect and ensure the steel sheet contains La, the La content is preferably 0.001% or higher. Furthermore, to prevent a reduction in the cold-formability of the steel sheet due to the production of coarse La oxide, the La content is preferably 0.050% or lower, and more preferably 0.040% or lower. Ce: from 0% or more to 0.050% or less As with lamina, cerium (Ce) is an element that, in trace amounts, is useful for controlling sulfide morphology. Although the Ce content can be 0%, to achieve the desired effect and ensure the steel sheet contains Ce, the Ce content is preferably 0.001% or higher. On the other hand, from the perspective of preventing a decrease in the steel sheet's forming capacity through Ce oxide production, the Ce content is preferably 0.050% or lower, and more preferably 0.040% or lower. The remainder of the chemical composition of the steel sheet, according to the present embodiment, is Fe (iron) and impurities. Examples of impurities may include elements that are inevitably contained in steel raw materials or scrap, or inevitably contained in a steelmaking process, and are permitted to be contained within ranges in which the steel sheet, according to the present invention, can exert the effects according to the present invention. Metal-coated steel sheet The steel sheet, according to this specification, may include a metallic coating on its surface. The metallic coating may be any of, for example, a galvanized layer, a galvanized and annealed layer, or an electrogalvanized layer. QJ CC Ln / Lznz / E / YILI Production method The following describes a method for producing steel sheet in accordance with this modality. The production method described below is an example of a method for producing steel sheet in accordance with this modality, and is not limited to the method described below. A casting is produced that has the chemical composition and, from the casting obtained, the steel sheet can be produced, according to the present modality, by the following production method. Casting step There are no specific restrictions on the method for producing the casting from molten steel of the specified chemical composition; for example, the casting can be produced using a typical method such as continuous slab casting and thin slab casting. Hot rolling step There are also no specific restrictions on hot rolling conditions. For example, in a hot rolling pass, it is preferable that the casting first be heated to 1100°C or higher and held for 20 minutes or more. This is to promote the remelting of coarse inclusions. The heating temperature is more preferably 1200°C or higher, and the holding time is more preferably 25 minutes or more. Alternatively, the heating temperature is preferably 1350°C or lower, and the holding time is preferably 60 minutes or less. In the hot rolling step, when the heated casting, as described above, undergoes hot rolling, it is preferable that the casting undergo finish rolling within a temperature range of 850°C or higher to 1000°C or lower. A preferred lower temperature limit is 860°C and a preferred upper temperature limit is 950°C. Winding pitch Hot-rolled steel sheet undergoing finish rolling is wound into a coil at temperatures above 550°C, at 700°C, or below. This accelerates the concentration of alloying elements such as Mn and Cr in the carbide produced during the winding step. Setting the winding temperature above 550°C facilitates increasing the concentration of Mn in retained austenite after the multi-step heat treatment described later. Furthermore, from a productivity standpoint, the winding temperature is preferably set at 700°C or below. QJ CC Ln / Lznz / E / YILI When necessary, hot-rolled steel sheet can be subjected to reheating treatment to soften it. Pickling step The coiled hot-rolled steel sheet is uncoiled and pickled. Pickling removes oxide scale from the surface of the hot-rolled steel sheet, improving its chemical treatment properties and the properties of the metallic coating. Pickling can be performed once or multiple times. “Cold rolling process” Pickled hot-rolled steel sheet is cold-rolled with a rolling reduction of 30% or more to 80% or less. Setting the rolling reduction at 30% or more helps maintain the flatness of the steel sheet and limits the decrease in the ductility of the finished product. Conversely, setting the rolling reduction at 80% or less prevents excessive cold rolling loads, thus facilitating the cold rolling process. A preferred lower limit for the rolling reduction is 45%, and a preferred upper limit is 70%. There are no specific restrictions on the number of rolling passes or the rolling reduction per pass. “Multi-step heat treatment process After the cold rolling step, the steel sheet, according to the present invention, is subjected to at least two heat treatments for its production. First heat treatment In a primary heat treatment, the steel sheet is first subjected to a heating step in which the steel sheet is heated to a temperature of point A or higher and held for 10 seconds or more. This is followed by a cooling step to cool the steel sheet to the following conditions 1) or 2). 1) The steel sheet is cooled to a temperature range of 25°C or more to 300°C or less at an average cooling rate of 20°C / s or more. 2) The steel sheet is cooled to a stop-cooling temperature of 600°C or more to 750°C or less at an average cooling rate of 0.5°C / s more to less than 20°C / s (first cooling stage) and then cooled to a stop-cooling temperature of 25°C or more to 300°C or less at an average cooling rate of 20°C / s more. It should be noted that the Acs point is determined by the following formula (a). In formula (a), each element symbol indicates a content of the element (% by mass). A symbol for an element that is not contained in steel should be replaced by zero. CC Ln / Lznz / E / YILI Ac3(°C) Point = 901 - 203x^C - 15.2xNi + 44.7xSi +104xV +31.5xMo + 13.1xW Formula (a) Through the first heat treatment step, the steel microstructure of the steel sheet is formed into a steel microstructure consisting primarily of freshly quenched or tempered martensite. Martensite is a steel microstructure containing a large number of grain boundaries and dislocations. In grain boundary diffusion, where the grain boundaries serve as diffusion pathways, and in dislocation diffusion, where the dislocations serve as diffusion pathways, elements diffuse faster than in intraparticle diffusion, where elements diffuse within the grains. After the first heat treatment, a large amount of carbide remains. However, since carbide dissolution is a phenomenon attributable to the diffusion of elements, the more grain boundaries are present, the more readily the carbide will dissolve during the second heat treatment. Setting the heating temperature at the Ac3o point facilitates obtaining a sufficient amount of austenite during heating and facilitates obtaining a sufficient amount of tempered martensite after cooling. Setting the holding time to 10 seconds or more during heating facilitates obtaining a sufficient amount of austenite and facilitates obtaining a sufficient amount of tempered martensite after cooling. In the cooling step described in 1), adjusting the average cooling rate by 20°C / s or more results in sufficient tempering, making it easy to obtain martensite. Therefore, after the first heat treatment, this allows sufficient carbide dissolution to progress in the second heat treatment, which will be described later. Setting the cooling stop temperature to 25°C or higher helps to slow the decline in productivity. Setting the cooling stop temperature to 300°C or lower facilitates obtaining a sufficient amount of martensite. This allows sufficient carbide dissolution to progress in the second heat treatment, which will be described later. The cooling step described in 2) is performed, for example, in a case where the steel sheet is rapidly cooled through a slow cooling zone. In the first cooling stage, setting the average cooling rate to less than 20°C / s makes it possible to produce ferrite and pearlite. However, given the chemical composition, ferrite and pearlite transformations are unlikely to occur, which can help to limit excessive ferrite and pearlite production. Adjusting the cooling rate in the first cooling stage to 20°C / s or higher leads to the same results as in the case where the cooling step described in 1) is performed, and the material quality of the steel sheet is not necessarily compromised. At the same time, setting the average cooling rate in the first cooling stage to 0.5°C / s CC Ln / ίZΖΠΖ / E / YΙΛΙ or more restricts the excessive progress of ferrite transformation and pearlite transformation, making it easy to obtain a predetermined amount of martensite. Second heat treatment In the second heat treatment, the steel sheet first undergoes a heating step in which it is reheated to a temperature of point Ac.3 or higher and held for 10 seconds or more up to 600 seconds or less. This is followed by a cooling step to cool the steel sheet to the following conditions: 1) or 2). 1) The steel sheet is cooled to a temperature range of 25°C or more to 300°C or less at an average cooling rate of 20°C / s or more. 2) The steel sheet is cooled to a stop-cooling temperature of 600°C or more to 750°C or less at an average cooling rate of 0.5°C / s more to less than 20°C / s (first cooling stage) and then cooled to a stop-cooling temperature of 25°C or more to 300°C or less at an average cooling rate of 20°C / s more. The first heat treatment step forms a steel microstructure consisting primarily of freshly quenched or tempered martensite, where elements diffuse readily. The second heat treatment step further develops the steel microstructure and sufficiently dissolves the coarse carbide in which manganese (Mn) is concentrated within the steel sheet (specifically, carbide with an equivalent circle radius of 0.1 pm or greater). Even after the Mn-containing carbides dissolve, some Mn remains concentrated in the carbide formation sites because Mn's diffusion rate is low compared to carbon (C). As seen above, retained austenite tends to form in regions where Mn is concentrated. Therefore, it is possible to stabilize the retained austenite without excessively increasing the carbon concentration.In other words, it is possible to increase the amount of retained austenite, which is unlikely to serve as the starting point for fracture, even after deformation-induced transformation. Furthermore, since the carbides dissolve sufficiently before the multi-stage heat treatment, it is also possible to limit the impact strength degradation initiated by the carbides. As a result, a high-strength steel sheet is obtained that has excellent impact resistance and formability. Setting the heating temperature at point Ac.3 or higher facilitates obtaining a sufficient amount of austenite during heating and a sufficient amount of tempered martensite after cooling. Setting the holding time to 10 seconds or more during heating also facilitates obtaining a sufficient amount of austenite and a sufficient amount of tempered martensite after cooling. Setting the holding time to 600 seconds or less during heating allows for further restriction. CC Ln / ίZΖΠZ / Β / YΙΛΙ so that the Mn diffuses, causing the regions where Mn is concentrated to disappear after the coarse carbides dissolve. This facilitates obtaining the desired amount of retained austenite. In the cooling step described in 1), setting the average cooling rate to 20°C or higher results in sufficient tempering, making it easy to obtain the desired tempered martensite. For this reason, the tensile strength of the steel sheet can be increased to 1100 MPa or more. Setting the cooling stop temperature to 25°C or higher helps to slow the decline in productivity. Adjusting the cooling stop temperature to 300°C or lower makes it easy to obtain the desired tempered martensite. For this reason, the tensile strength of the steel sheet can be increased to 1100 MPa or more. The cooling step described in 2) is performed, for example, in a case where the steel sheet is rapidly cooled through a slow cooling zone. In the first cooling stage, setting the average cooling rate to less than 20°C / s makes it possible to produce ferrite and pearlite. However, given the chemical composition, ferrite and pearlite transformations are unlikely, which can help to limit excessive ferrite and pearlite production. Adjusting the cooling rate in the first cooling stage to 20°C / s or higher leads to the same results as the cooling step described in 1) and does not necessarily deteriorate the quality of the steel sheet material. At the same time, setting the average cooling rate in the first cooling stage to 0.5°C / so more restricts the excessive progress of ferrite transformation and pearlite transformation, making it easy to obtain a predetermined amount of martensite. Although the effects of the multi-stage heat treatment step are sufficiently exerted by performing the two heat treatments, three or more heat treatment steps can be performed in total, performing the second heat treatment step a plurality of times after the first heat treatment step. Maintenance step After quenching in the final step of a multi-step heat treatment process, the steel sheet is held at a temperature range of 200°C or higher to 450°C or lower for 10 seconds or more to 600 seconds or less. In this holding step, the steel sheet can be maintained at a constant temperature or heated and cooled midway through the step, as appropriate. Through this holding step, the newly quenched martensite obtained by quenching can be tempered, and the carbon can be concentrated in the retained austenite. Setting the holding temperature to 200°C or higher allows the tempering process to progress sufficiently, facilitating the production of a sufficient amount of tempered martensite. Holding the temper at 450°C or less prevents excessive tempering progress. Setting the holding time to 10 seconds or more allows sufficient tempering progress. Setting the holding time to 600 seconds or less prevents excessive tempering progress. Tempering step After the holding step, the steel sheet can be tempered. This tempering step can be a step in which the steel sheet is held or reheated to a predetermined temperature halfway through the room-temperature cooling, or it can be a step in which the steel sheet is reheated to the predetermined temperature after the room-temperature cooling is complete. The method for heating the steel sheet in the tempering step is not limited to a specific method. However, from the perspective of limiting the decrease in the strength of the steel sheet, the holding or heating temperature in the tempering step is preferably 500°C or lower. Metal coating step Steel sheet can undergo metallic coating treatments such as electroplating and deposition, and can be further annealed and galvanized after coating. Steel sheet can also undergo surface treatments such as organic film coating, film rolling, organic or inorganic salt treatment, and chromium deposition. In the case of a galvanizing treatment applied to a steel sheet as a metallic coating, the steel sheet is heated or cooled to a temperature of (galvanizing bath temperature -40°C) to (galvanizing bath temperature +50°C) and immersed in a galvanizing bath. The galvanizing treatment results in a steel sheet with a galvanized layer on its surface, i.e., a galvanized steel sheet. For example, the galvanized layer can have a chemical composition containing Fe: 7% by mass or more to 15% by mass or less, with the remainder expressed as Zn, Al, and impurities. Alternatively, the galvanized layer can be made of a zinc alloy. In cases where annealing and galvanizing follow galvanizing, the galvanized steel sheet is heated to a temperature of 460°C or higher, or 600°C or lower, for example. Setting this heating temperature to 460°C or higher ensures the steel sheet is sufficiently galvanized. Setting this heating temperature to 600°C or lower prevents over-annealing and over-galvanizing, which would compromise corrosion resistance. Such annealing and galvanizing treatment results in a sheet of QJ CC Ln / įZРZ / В / YILI steel with an annealed and galvanized layer on its surface, i.e., an annealed and galvanized steel sheet. EXAMPLE 1 An example of the present invention is described below; however, the conditions described in the example are merely illustrative of the conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example. Various conditions may be adopted in the present invention, provided that they allow the objective of the present invention to be achieved without departing from its essence. The castings, which had the chemical compositions shown in Tables 1 to 4, were hot-rolled under the conditions shown in Tables 5 to 8 and subsequently coiled. The resulting hot-rolled steel sheets were cold-rolled under the conditions shown in Tables 5 to 8. The resulting cold-rolled steel sheets were then heat-treated under the conditions shown in Tables 5 to 8. Some of the steel sheets were metallic-coated by a conventional method, and some of the metallic-coated steel sheets were annealed and galvanized by a conventional method.The steel sheets obtained in this way were subjected to identification of their steel microstructures and measurement of their Mn and C content in retained austenite, their tensile strength, and their impact strength using the following methods. The results are shown in Tables 9 to 12. It should be noted that because the Mn and C content in a casting matrix is ​​substantially the same as that in the chemical composition of the casting, the Mn and C content in the chemical composition of the casting is considered to be that of the matrix. In the present invention, the identification of steel microstructures and the calculation of their volume fractions are performed as follows. Ferrite First, a sample is taken that includes a cross-section of the sheet thickness parallel to the rolling direction of a steel sheet, and this cross-section is designated as an observation surface. From the observation surface, a 100 µm x 100 µm region centered around a point 1 / 4 of the sheet thickness from a surface of the steel sheet is designated as an observation region. An electron channeling contrast image, seen by observing this observation region under a scanning electron microscope at a magnification of QJ CC Ln / ίΖΠΖ / Β / ΥΙΛΙ At 1000x to 50000x magnification, this image illustrates a difference in crystal orientation between grains in the form of a contrast difference. In this electron channeling contrast image, a uniform contrast area represents ferrite. A ferrite area fraction identified in this way is then calculated using a point-counting procedure (in accordance with ASTM E562). The ferrite area fraction calculated in this way is considered a ferrite volume fraction. Perlite First, the observation surface is chemically etched with Nital reagent. From the chemically etched observation surface, a 100 µm x 100 µm region centered around a point 1 / 4 of the sheet thickness from a surface of the steel sheet is identified as the observation region. This observation region is viewed under an optical microscope at a magnification of 1000x to 50000x, and in the observed image, a dark contrast area is identified as pearlite. A pearlite area fraction identified in this manner is then calculated using the point-counting procedure. This calculated pearlite area fraction is considered the pearlite volume fraction. Bainite and aged martensite An observation region obtained by chemical etching with Nital reagent is observed under a field emission scanning electron microscope (FE-SEM) at a magnification of 1000 to 50000x. In this observation region, bainite and tempered martensite are identified from the positions and arrangement of cementite grains included within a steel microstructure, as follows. Bainite exists in two states: one where cementite or retained austenite grains are present at the boundaries of the bainitic ferrite band, and the other where the cementite is present within the bainitic ferrite band. In cases where cementite or retained austenite grains are present at the boundaries of the bainitic ferrite band, the bainitic ferrite boundaries are found, allowing for the identification of bainite. In cases where cementite is present within the bainitic ferrite band, the ratio of crystal orientations between the bainitic ferrite and cementite is one, and the cementite grains share this orientation, thus enabling bainite identification. The area fraction of bainite identified in this way is calculated using the point-counting method. This area fraction is considered a volume fraction of bainite. In tempered martensite, cementite grains are present within the martensite bands; the number of crystal orientation relationships between the martensite and cementite bands is two or more, and the cementite has a plurality of variants, so that tempered martensite can be identified. An area fraction of tempered martensite identified in this way is calculated using the CC Ln / Lznz / E / YILI point counting procedure. The area fraction of tempered martensite is considered a volume fraction of tempered martensite. freshly tempered martensite First, an observation surface similar to the one used for ferrite identification is chemically etched with LePera reagent, and a region similar to that used for ferrite identification is determined as the observation region. During the chemical etching with LePera reagent, the martensite and retained austenite are not chemically attacked. Therefore, the chemically etched observation region is observed under FE-SEM, and the areas that are not chemically etched are considered to be martensite and retained austenite. Subsequently, a fraction of the total area of ​​martensite and retained austenite identified in this way is calculated using the point-counting procedure, and this area fraction is considered to be a fraction of the total volume of martensite and retained austenite. Next, from the total volume fraction, a volume fraction of retained austenite is subtracted, calculated as follows, so that a volume fraction of freshly quenched martensite can be calculated. Retained austenite In the present invention, a retained austenite area fraction is determined by X-ray measurement as follows. First, a portion of the steel sheet, from its surface to 1 / 4 of its sheet thickness, is removed by mechanical and chemical polishing. Next, a chemically polished surface is measured using MoKa X-rays as the characteristic X-rays. Subsequently, based on an integrated intensity ratio between diffraction peaks of (200) and (211) of a body-centered cubic (bcc) lattice phase and diffraction peaks of (200), (220), and (311) of a face-centered cubic (fcc) lattice phase, a retained austenite area fraction Sy is calculated using the following formula. The retained austenite area fraction Sy calculated in this manner is considered a retained austenite volume fraction.Sy = (I200f + I220f +1311 f) / (1200b +1211 b) x 100. Here, I200f, I220f and 131 If represent diffraction peak intensities of (200), (220) and (311) of an fcc phase, respectively, and 1200b and 1211b represent diffraction peak intensities of (200) and (211) of a bcc phase, respectively. Measurement of the amount of Mn in retained austenite The amount of Mn in the retained austenite is measured using an electron probe microanalyzer (ERMA). First, to capture the locations of the retained austenite in a measurement region, crystal orientation information is obtained in an observation region. QJ CC Ln / ίZРZ / Β / YILI is obtained by electron backscatter diffraction (EBSD). A sample of steel sheet is taken such that its cross-section, parallel to the rolling direction, serves as the observation surface. This surface is wet-polished with sandpaper, polished with a diamond abrasive having an average particle size of 1 µm, and then chemically polished. To determine the observation region, indentations are made along a 50 x 50 µm square centered around a 1 / 4-thick portion of the sheet in a Vickers hardness test. These indentations are used as markers. A field emission scanning electron microscope (FE-SEM) is then used to obtain crystal orientations at 0 µm intervals.At 5:00 PM, within a region surrounded by indentations that serve as markers, the software “OIM Data Collection TM (ver. 7)” developed by TSL Solutions KK was used to obtain data on crystal orientations. The crystal orientation information obtained was divided into BCC and FCC phases using the software “OIM Analysis TM (ver. 7)” developed by TSL Solutions KK. The FCC phases correspond to retained austenite. An ERMA was then used to measure the amount of Mn in the retained austenite. The instrument used for this measurement was the JXA-8500F manufactured by JEOL, Ltd. The crystal orientation information was obtained under conditions that included an accelerating voltage of 7 kV and a measurement point spacing of 80 nm to measure portions identified as FCC within the region.Based on the data obtained through this measurement, the amount of Mn in the retained austenite is determined using the calibration curve method. Measured amount of C in the retained austenite The carbon concentration “Cy” in retained austenite can be determined by X-ray diffraction. First, a portion of the steel sheet, from its surface to a point 1 / 4 of the sheet thickness, is removed by mechanical and chemical polishing. Then, a chemically polished surface is measured using MoKa X-rays as the characteristic X-rays. Based on the diffraction peak positions of (200), (220), and (311) in an fcc phase, a lattice constant “dy” of retained austenite is determined. Additionally, the chemical component values ​​of each sample, obtained by chemical analysis, are used to calculate the carbon concentration (Cy) of the retained austenite using the following formula. In the following formula, each element symbol indicates the mass percent of that element in a sample. Cy = (100xdy - 357.3 - 0.095xMn + 0.02xNi - O.OóxCr - 0.31xMo - 0.18xV - 2.2xN - 0.56xAl + 0.04xCo - 0.15xCu - 0.51xNb - 0.39xTi - 0.18xW) / 3.3 QJ CC Ln / Lznz / E / YILI It should be mentioned that the carbon Cy concentration of the retained austenite does not include an amount of carbon present in the form of carbides. Measurement of the number of carbides The number of carbides is measured based on an image of a steel microstructure captured with a scanning electron microscope. Before observation, an observation surface of a steel microstructure sample is wet-polished with sandpaper and then polished with diamond abrasive having an average particle size of 1 µm to a mirror finish. Subsequently, the microstructure is chemically etched with a saturated alcoholic solution of picric acid. A field of view centered on a point t / 4 the sheet thickness is observed with an observation set magnification of 5000x, where a plurality of randomly selected points are captured such that the total area of ​​the points becomes 20,000 µm².The captured image is analyzed using image analysis software such as WinROOF, manufactured by MITANI CORPORATION, and the areas of carbides included in a region of 20,000 pm2 are measured in detail. Assuming that each carbide has a circular shape, a radius for each carbide (equivalent circle radius) is determined from the areas determined by the image analysis, and the number of carbides that have a radius of 0.1 pm or more is calculated. Measurement of tensile strength TS and elongation The The measurement was performed in accordance with JIS Z 2241 (2011) and using No. 5 test coupons taken from the steel sheet in a direction perpendicular to the rolling direction of the steel sheet and the tensile strengths TS (MPa) and elongations El (%). were determined Flexion test The bending capacity was evaluated in accordance with the VDA standard (VDA238-100) defined by the German Association of the Automotive Industry under the following measurement conditions. In the present invention, a maximum bending angle α was determined by converting a displacement at a maximum load obtained through the bending test to an angle in accordance with the VDA standard. A test sample with a maximum bending angle α (degrees) of 2.37t² - 14t + 65 or greater was considered good. Here, t denotes a sheet thickness (mm). Dimensions of the test sample: 60 mm (rolling direction) x 60 mm (direction perpendicular to the rolling direction) Ridge line bending: a punch was pressed so that a ridge line bend extended in a direction perpendicular to the rolling direction. Test method: support lamination, punch press CC Ln / Lznz / E / YILI Distance between rollers: φ 30 mm Punch shape: tip R = 0.4 mm Distance between supports: 2.0 x sheet thickness (mm) + 0.5 mm Pressing speed: 20 mm / min Test machine: SIMADZU AUTOGRAPH 20kN Measuring the expansion limit ratio of the orifice In measuring the hole expansion limit (λ), a sheet metal piece measuring 90 mm ± 10 mm on each side was first cut, and a 10 mm diameter hole was punched in the center of the sheet metal piece. This was used to prepare a test specimen for hole expansion. The punch clearance was set at 12.5%. The test specimen was positioned so that the distance between the tip of a cone-shaped template for hole expansion and the center of the punched hole was within ± 1 mm, and a hole expansion value was measured in accordance with JIS Z 2256 (2010). Impact resistance assessment For the evaluation of impact or shock resistance, a test sample with a maximum bending angle a (degrees) of 2.37t2- 14t + 65 or more, TS x El of 15000 or more and TS x λ of 33000 or more was rated as “O” and a test sample that did not meet any of them was rated as “x”. CC Ln / ίZΖΠZ / Β / YΙΛΙ > ar ch ccco Table 1 Chemical composition (% by mass, the remainder: Fe and impurities) 1W. C Si Mn PSN 0 Al B Ti Nb V Mo Cr Co Ni Cu w Ta Sn Sb As Mg Ca Y Zr La Ce 1 0.20 1.14 3.1 0.0176 0.0016 0.0009 0.0020 0.0012 0.02 0.02 - 8 0.20 . 0.0017 0.0010 0.0070 0.0193 0.101 0.0020 949 - 861 3 0.31 2.34 0.8 0.0157 0.0062 0.0003 0.0001 0.3 0.393 - 834 - 861 0.95 1.1 0.0023 0.0030 0.0162 0.0080 0.095 949 - 825 5 0.20 2.26 0.8 0.0023 0.0022 0.0006 0.0003 0.0023 0.994 -9 6 0.21 1.25 1.4 0.0029 0.0164 0.0002 0.0014 0.184 - 0.03 - 0.10 - - - - 0.32 - - - - - - - - - 875 Ί 0.32 1.74 0.010.010. 0.0020 0.0012 0.030 - - 0.02 - - - - - - - - - - - - - - 0.006 864 8 0.28 2.15 1.1 0.0014 0.0112 0.0025 0.0009 - 0.009 - - -0 -6 - - - - - - - - - 0.030 - - - - 889 9 0.19 0.99 0.8 0.0020 0.0021 0.0138 0.0033 0.258 - 0.002 857 10 0.33 0.97 3.2 0.072 0.020.0.040. 0.0061 0.760 - 0.008 - - - - - 828 11 0.19 0.94 0.8 0.0111 0.0172 0.0035 0.0027 0.082 - - - 0.10 - - - 0.20 - - 0.0 -2 -0 -2 -0 -2 -0 -8 0.98 0.9 0.0014 0.0017 0.0036 0.0142 0.032 0.0020 - - - 0.3 - - - 0.022 - - - - - - - - - 855 13 0.20 0.95 0.8 0.0033 0.0013 0.0161 0.0146 0.055 0.0011 - - - - - 0.15 - - - - - - 0.040 - - - - 850 14 0.21 0.90 2.9 0.0179 0.0174 0.0060 0.0007 0.796 - - 0.04 - - - - - - - 0.030 - - - - - - - 849 15 0.23 2.22 4.6 0.0061 0.0037 0.0039 0.0031 0.096 - 0.03 - 0.020 - - - 903 16 0.32 1.74 2.2 0.0142 0.0013 0.0035 0.0001 - - - - - - - 0.10 - - - - - - - - - - - 864 17 0.25 2.29 0.9 0.0169 0.0027 0.0041 0.0019 0.653 0.0022 - - 0.08 - - - - - - - 0.007 - - - - - - 905 18 0.25 1.20 2.3 0.0154 0.0106 0.0023 0.0016 0.139 0.0018 0.03 - - - 0.15 - 0.20 - - - - - - - - - - 853 19 0.24 1.19 2.8 0.0036 0.0160 0.0173 0.0019 943 - 869 20 0.22 1.52 3.0 0.0020 0.0020 0.0021 0.0180 0.050 - - - - - - - - - - - - - - - 0.050 - - 874. > ar ch ccco Table 2 Chemical composition (% by mass, the remainder: Fe and impurities) 1W. C Si Mn PSN 0 Al B Ti Nb V Mo Cr Co Ni Cu W Ta Sn Sb As Mg Ca Y Zr La Ce rtL.í 21 0.28 2.13 1.7 0.0022 0.0169 0.0026 0.0019 0.211 - 0.050 -20 -20 -8 -2 -8 0.20 2.30 2.6 0.0104 0.0032 0.0032 0.0042 0.065 - - - - 0.30 - - - - - - - - - 914 23 0.19 2.36 1.0 0.0026 0.0.027 -020 -0.02 -0. 1.0 - - - - - - - - - - 917 24 0.19 1.54 2.4 0.0048 0.0003 0.0028 0.0033 0.082 - 0.6 - - - - - - - - 880 25 0.28 0.190.030. 0.0013 0.0032 0.077 0.0015 - 836 26 0.33 1.19 1.0 0.0011 0.0008 0.0152 0.0003 0.060 - 0.50 - - 0.15 - - 8 -29 -2 -2 -5 1.00 1.8 0.0039 0.0012 0.0104 0.0151 0.170 0.08 - 836 28 0.30 1.41 3.0 0.0011 0.0032 0.0004 0.0016 0.8038 - 2980 0.95 1.8 0.0001 0.0159 0.0011 0.0021 0.800 - 830 30 0.26 2.04 2.9 0.0010 0.0003 0.0017 0.0134 0.559 - 1891 1.96 1.0 0.0009 0.0028 0.0021 0.0015 - - 0.03 0.10 - 906 32 0.19 0.84 1.7 0.0018 0.0014 0.0024 - 0.0030 0. - - - - - - 0 - 2 - 0. -0. 848 33 0.20 1.56 3.1 0.0017 0.0016 0.0029 0.0007 0.156 0.020 - 881 34 0.19 1.67 1.4 0.0036 0.0012 0.0008 0.0023 0.188 0.0030 - 887 35 0.20 2.24 2.5 0.0023 0.0015 0.0004 0.0065 0.173 - 910 36 0.21 0.99 2.6 0.0020 0.0006 0.0018 0.0017 0.900 - - 0.05 - 855 37 0.20 1.98 0.9 0.0029 0.0012 0.0036 0.0034 0.038 - 0.5 - - - - - - - - - 900 38 0.20 1.20 3.3 0.0035 0.0029 0.0182 0.0004 0.803 - - - 0.05 0.10 - - - - - - - - 863 39 0.20 2.32 3.0 0.0021 0.0013 0.0021 0.0178 0.144 - 913 40 0.19 1.24 1.3 0.0031 0.0036 0.0056 0.0148 0.047 0.0023 - 868. Table 3 > ahchc cco Chemical composition (% by mass, the remainder: Fe and impurities) 1W. C Si Mn PSNO Al B Ti Nb V Mo Cr Co Ni Cu w Ta Sn Sb As Mg Ca Y Zr La Ce rtL.i 41 0.21 1.63 1.6 0.0029 0.0019 0.0020 0.0008 0.487 0.0020 - 0.020 - 882 42 0.19 2.15 3.0 0.0021 0.0035 0.0186 0.0042 0.179 - 908 43 0.19 1.87 2.0 0.0032 0.0021 0.0021 0.0054 0.138 - 0.10 - 896 44 0.19 1.35 0.9 0.0006 0.0022 0.0035 0.0021 0.194 - - - - 1.3 - 873 45 0.19 1.61 2.2 0.0006 0.0014 0.0035 0.0011 0.036 - - 0.05 - 885 46 0.20 1.94 2.3 0.0037 0.0008 0.0121 0.0033 - - - - - - - - 0.050 - 900 47 0.21 1.73 2.2 0.0014 0.0009 0.0022 0.0018 0.071 - - - - 0.10 - 886 48 0.21 0.89 1.5 0.0036 0.0005 0.0031 0.0011 0.091 - 0.04 - 848 49 0.20 1.24 1.9 0.0026 0.0024 0.0022 0.0064 0.156 - 866 50 0.19 1.81 2.8 0.0010 0.0009 0.0007 0.0018 0.143 - 894 51 0.19 1.36 3.2 0.0034 0.0015 0.0013 0.0050 0.165 - 872 52 0.10 0.87 3.1 0.0168 0.0155 0.0031 0.0079 0.009 - 876 53 0.39 1.16 1.0 0.0027 0.0038 0.0023 0.0188 0.008 - 826 54 0.19 0.40 1.5 0.0022 0.0015 0.0125 0.0090 0.440 - 830 55 0.28 2.55 1.0 0.0018 0.0159 0.0031 0.0013 0.918 - 907 56 0.32 2.28 W 0.0018 0.0019 0.0148 0.0028 0.040 - 889 57 0.29 2.23 55 0.0024 0.0017 0.0022 0.0156 0.442 - 891 58 0.22 1.59 2.2 0.0206 0.0019 0.0140 0.0006 0.769 - 878 59 0.20 0.97 0.8 0.0019 0.0208 0.0021 0.0015 - 854 60 0.31 1.43 1.0 0.0029 0.0075 0.0207 0.0024 0.005 - 852. An underlined value indicates that it does not meet the declared range. > ar ch ccco Table 4 Chemical composition (% by mass, the remainder: Fe and impurities) 1NU. C Si Mn PSN 0 Al B Ti Nb V Mo Cr Co Ni Cu w Ta Sn Sb As Mg Ca Y Zr La Ce 61 0.25 2.26 1.1 0.0166 0.0014 0.0018 0.0087 1.034 - 901 62 0.33 0.93 0.8 0.0050 0.0038 0.0035 0.0148 0.444 0.0104 - 826 63 0.19 2.06 3.1 0.0011 0.0174 0.0119 0.0113 0.099 - 0.10 - 905 64 0.20 0.99 0.9 0.0013 0.0165 0.0187 0.0024 0.010 040 - 855 65 0.21 2.27 3.2 0.0148 0.0111 0.0138 0.0021 0.112 044 - 962 66 0.33 0.91 0.9 0.0019 0.0025 0.0107 0.0206 0.205 - 825 67 0.20 1.05 0.8 0.0025 0.0169 0.0156 0.0024 0.195 - 875 68 0.20 0.94 0.9 0.0165 0.0061 0.0049 0.0018 0.234 M - 851 69 0.19 1.05 0.9 0.0178 0.0021 0.0016 0.0033 0.576 041 - 859 70 0.19 0.98 2.9 0.0128 0.0020 0.0035 0.0014 0.150 OOP - 841 71 0.19 1.34 0.9 0.0165 0.0018 0.0067 0.0005 0.164 - 872 72 0.20 0.94 0.7 0.0068 0.0024 0.0023 0.0029 0404 - 854 73 0.21 1.75 1.1 0.0148 0.0013 0.0015 0.0010 0.028 - 0.103 - 886 74 0.19 2.24 3.1 0.0016 0.0014 0.0113 0.0031 0.229 - 0.052 - 912 75 0.19 0.97 3.1 0.0183 0.0018 0.0011 0.0034 0.172 - 0.051 - 856 76 0.22 2.27 0.9 0.0037 0.0022 0.0035 0.0009 0.736 - 0.051 - 908 77 0.20 2.19 2.3 0.0019 0.0181 0.0141 0.0028 - 0.052 - 909 78 0.20 1.10 0.9 0.0011 0.0023 0.0027 0.0024 0.856 - 0.051 - 859 79 0.20 1.06 1.2 0.0015 0.0020 0.0014 0.0030 0.135 - 0.051 - 859 80 0.31 0.92 1.7 0.0167 0.0059 0.0173 0.0021 0.118 - 0.052 - 828 81 0.23 0.99 ll 0.0105 0.0177 0.0049 0.0027 0.673 - 0.052 849 82 0.33 1.25 0.9 0.0017 0.0037 0.0131 0.0013 0.020 - 0.052 841. An underlined value indicates that it does not meet the declared range. Table 5 No. Hot rolling Cold rolling First heat treatment Second heat treatment Tempering Metallic coating Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Working temp. (°C) Working time (s) Heating temp. (°C) Working time (s) Stock Heating temp. CQ Finishing temp. (°C) Winding temp. (°C) Rolling reduction (%) Cooling rate (°C / s) Stopping temp. (°C) Cooling rate (°C / s) Temp. of unemployment (»C) A 1,250 900 577 31 931 360 - 88 134 870 263 - 64 180 280 185 - None B 1,253 862 645 47 986 111 - 28 273 944 524 - 99 250 289 207 - None C 1,160 989 632 63 900 388 - 181 212 900 407 102 190 411 138 None D 1,107 958 674 55 987 75 - 178 64 893 350 - 84 180 313 129 - Existing E 1.175 892 693 34 936 169 - 58 262 960 305 - 137 250 250 299 - None F 1,250 901 581 44 886 343 - 170 175 915 85 - 90 220 320 221 - None G 1,198 949 583 74 865 248 - 102 151 880 252 - 142 140 376 125 - None H 1,132 855 630 74 900 299 - 42 231 932 74 - 197 200 292 225 - Existing 1 1,175 851 697 70 916 255 - 40 97 879 182 - 59 260 270 118 - None J 1,221 876 593 47 895 472 - 43 49 955 282 - 148 110 353 167 - None K 1,298 916 632 58 886 117 5 680 178 238 860 37 - 189 255 330 289 - None L 1,300 950 673 31 947 129 - 114 189 969 225 - 164 245 310 194 - None M 1,235 918 572 55 976 495 - 84 67 900 117 - 67 250 310 143 300 100 None N 1,132 964 600 66 945 31 - 166 93 962 314 - 170 180 300 105 - None 0 1,257 922 617 68 910 371 - 180 298 920 406 - 183 165 340 132 - None P 1.107 861 569 33 880 35 - 56 43 890 60 - 195 150 350 270 - None Q 1,122 866 557 34 920 100 - 40 41 910 56 3 650 91 220 320 107 - None R 1,112 860 564 35 860 17 - 197 40 860 51 - 125 180 262 122 - Existing (Galvanized and annealed) S 1,103 866 562 31 890 12 - 67 25 875 32 - 66 165 342 176 - None T 1,115 870 551 30 880 29 - 174 40 880 36 - 91 170 350 275 - None. Table 6 No. Hot rolling Cold rolling First heat treatment Second heat treatment Tempering Metallic coating Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Working temp. (°C) Working time (s) Heating temp. (°C) Working time (s) Stock Heating temp. (°C) Finishing temp. (°C) Winding temp. (°C) Rolling reduction (%) Cooling rate (°C / s) Stopping temp. (°C) Cooling rate (°C / s) Temp. Stop temperature (°C) U 1,121 870 551 45 900 75 - 80 31 900 31 - 159 185 349 258 - Existing V Lili 885 552 31 920 37 • 43 41 930 76 • 43 195 332 100 250 300 None w 1,114 856 557 33 932 10 - 148 53 940 36 - 182 230 333 223 - None X 1,101 865 554 36 880 42 - 169 100 890 74 - 182 190 293 148 - None Y 1.153 864 564 32 860 68 - 156 50 840 55 - 104 200 308 82 - None z 1.160 853 557 35 877 135 - 160 32 870 150 - 165 175 408 32 - None AA 1,122 858 580 33 841 55 - 131 200 853 33 - 117 175 285 32 - None AB 1,111 857 551 31 915 198 3 690 112 36 890 73 2 650 77 135 263 80 - None AC 1,125 869 554 50 864 46 - 163 25 838 75 - 123 165 401 84 - None AD 1.121 860 567 55 900 46 - 81 33 900 72 - 141 155 268 37 - None AE 1.121 860 567 64 900 46 - 81 33 900 72 - 141 155 268 37 - Existing AF 1.121 860 567 38 900 46 - 81 33 900 72 - 141 155 268 37 - Existing (Galvanized and annealed) AG 1.226 855 AH 1.149 937 400 45 970 460 - 57 47 910 202 - 129 220 300 104 - None to 1.169 921 670 12 - AJ 1,185 941 660 81 - AK 1,261 883 633 70 750 430 - 157 44 934 475 - 59 180 300 297 - None AL 1,128 919 565 50 890 0 - 200 48 947 109 - 178 190 300 282 - None AM 1,206 885 352 56 912 620 - 28 110 920 629 - 182 205 300 258 - None AN 1,250 980 572 35 902 285 0J 627 136 88 958 282 - 114 170 262 101 - None AO ​​1,277 860 588 63 910 66 5 500 39 50 906 87 • 94 180 320 170 • None. An underlined value indicates that it does not meet the recommended condition. Board? No. Hot rolling Cold rolling First heat treatment Second heat treatment Tempering Metallic coating Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Working temp. (°C) Working time (s) Heating temp. (°C) Working time (s) Stock Heating temp. (°C) Finishing temp. (°C) Winding temp. (°C) Rolling reduction (%) Cooling rate (°C / s) Stopping temp. (°C) Cooling rate (°C / s) Temp. unemployment (»C) AP 1,202 867 614 62 909 228 - 15 138 881 434 - 102 185 310 140 - None AQ 1,293 982 556 49 930 349 - 72 400 978 376 - 29 195 310 240 - None AR 1,141 994 607 54 920 67 - 130 82 700 109 - 162 220 294 233 - None AS 1,273 950 613 76 936 441 - 137 83 875 0 04 728 97 195 303 239 - None AT 1,189 967 687 60 964 174 - 151 116 909 113 6 550 154 220 300 144 - None AU 1.116 894 605 75 920 425 - 165 146 951 209 - 12 200 280 153 - None AV 1,129 876 554 69 951 43 - 61 103 984 178 - 25 450 291 119 - None AW 1,178 949 629 48 935 143 - 192 136 927 183 - 49 20 270 107 - None AX 1,225 955 605 35 914 61 - 183 57 950 140 - 29 200 500 200 - None AY 1,138 946 686 56 933 337 - 119 234 960 167 - 72 220 300 5 - None AZ 1,265 914 662 75 890 399 - 165 250 984 403 - 145 220 333 650 - None BA 1,188 894 635 38 867 15 - 40 207 990 398 - 24 190 338 315 - None BB 1,203 924 672 38 976 463 - 83 245 878 282 - 66 170 280 334 463 195 None BC 1,247 898 599 46 992 258 - 183 73 922 267 - 75 225 250 310 233 618 None BD 1,116 959 696 48 - 900 156 - 74 170 264 215 - None BE 1,139 978 670 74 947 59 - 52 57 978 85 - 30 230 250 125 - None BF 1,149 904 580 59 918 70 - 54 43 838 75 - 55 175 294 114 - None BG 1.266 905 635 35 852 77 - 43 35 960 53 - 37 220 290 269 - None BH 1,230 959 656 48 923 68 - 33 43 968 81 - 50 115 300 163 - None B1 1241 951 627 75 929 67 - 60 37 992 90 - 54 200 329 327 - None BJ 1,233 950 607 45 895 32 - 57 55 945 62 - 48 135 380 303 - None. An underlined value indicates that it does not meet the recommended condition. Table 8 No. Hot rolling Cold rolling First heat treatment Second heat treatment Tempering Metallic coating Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Heating temp. (°C) Working time (s) First cooling stage Cooling rate (°C / s) Stopping temp. (°C) Working temp. (°C) Working time (s) Heating temp. (°C) Working time (s) Stock Heating temp. (°C) Finishing temp. (°C) Winding temp. (°C) Rolling reduction (%) Cooling rate (°C / s) Stopping temp. (°C) Cooling rate (°C / s) Temp. Stop temperature (°C) BK 1.257 936 684 69 911 60 - 55 69 895 31 - 53 190 300 172 - None BL 1.150 939 579 37 903 93 - 53 68 921 28 - 33 220 316 235 - None BM 1.209 868 563 50 860 77 - 45 56 871 98 - 50 190 253 258 - None BN 1.152 974 591 40 925 60 - 31 50 931 37 - 44 180 352 347 - None BO 1.132 903 639 47 893 39 - 34 39 897 50 - 47 200 425 243 - None BP 1.179 884 629 47 910 65 - 42 73 935 91 - 48 165 260 133 - None BQ 1,165 943 622 65 867 73 - 49 62 967 58 - 44 185 276 189 - None BR 1,125 949 650 58 968 43 - 43 73 968 31 - 31 180 333 346 - None BS 1,194 960 607 63 848 71 - 37 49 929 70 - 43 200 341 108 - None BT 1,214 899 602 47 911 94 - 59 70 886 31 - 47 250 320 105 - None BU 1,241 940 584 41 904 46 - 59 63 915 40 - 43 170 294 297 - None BV 1,133 930 586 41 963 53 • 60 69 861 55 • 33 250 253 252 • None BW 1,209 888 621 56 851 88 - 46 56 904 77 - 39 180 290 273 - None BX 1,228 975 613 74 991 51 - 49 71 958 31 - 52 240 310 220 - None BY 1,251 877 672 37 941 85 - 53 66 860 84 - 33 195 300 217 - None BZ 1,183 913 693 68 945 65 - 51 51 901 67 - 44 180 300 242 - None CA 1.191 957 563 57 942 57 - 54 38 979 88 - 54 195 300 209 - None CB 1,161 974 687 75 860 41 - 59 57 892 57 - 49 185 200 129 - None CC 1,226 900 602 70 991 28 - 43 68 923 98 - 41 245 300 188 - None CD 1,284 874 685 55 967 53 - 60 72 920 93 - 52 215 300 206 - None CE 1,281 890 664 70 892 31 - 55 56 953 53 - 34 185 329 200 - None CF 1,132 875 619 51 933 62 - 31 50 950 21 - 51 225 280 112 - None CG 1,123 969 611 60 931 68 - 53 35 867 64 - 34 180 322 279 - None CH 1,271 885 585 62 945 98 - 44 36 879 21 - 39 220 290 324 - None C1 1,197 909 663 40 974 48 - 44 38 852 45 - 32 205 251 297 - None CJ 1.250 900 577 31 870 263 - 64 190 300 185 - None. An underlined value indicates that it does not meet the recommended condition. Table 9 Test No. 1 Steel No. 1 Production No. 0 q Ó & ω Microstructure Volume Fraction (%) Number of Carbide Radii §0.1 gm ( / 20000 μητ) TS (MPa) El (%) Retained γ Content Matrix Content MnA / MnM CA / Cm λ (%) Impact Strength Observations FPB Retained γ M TM Mn (MnA) C (Ca) Mn (MnM) C (Cm) a (grad) ® TSxEl T5χλ Evaluation 1 1 A 1.4 - - 5 8 87 40 1,206 13.2 3.8 0.7 3.1 0.20 1.2 3.7 40 75 25 15,956 48,222 O Steel of the invention 2 2 B 1.4 - - 4 8 - 87 43 1,186 13.6 2.0 0.8 0.8 0.20 2.5 3.9 38 68 18 16,156 45,062 0 Steel of the invention 3 3 C 1.4 - - 4 8 87 65 1,262 13.5 1.5 1.0 0.8 0.31 1.9 3.2 38 73 23 17.018 47,960 0 Steel of the invention 4 4 D 1.4 - - 6 7 - 87 32 1,531 12.7 2.4 0.8 1.1 0.33 2.2 2.3 38 65 15 19,515 58,191 0 Steel of the invention 5 5 E 1.4 - - 3 9 - 88 43 1,241 14.4 2.0 0.7 0.8 0.20 2.5 3.7 37 69 19 17,891 45,924 0 Steel of the invention 6 6 F 1.2 - - 3 9 88 26 1,171 14.2 2.3 0.8 1.4 0.21 1.7 4.0 29 68 16 16.602 33.958 0 Steel of the invention 7 7 G 1.4 - - 4 8 - 88 38 1,382 13.1 2.8 0.9 2.3 0.32 1.2 2.8 34 62 12 18,111 46,993 0 Steel of the invention 8 8 H 1.4 - - 3 9 - 88 25 1,390 13.9 1.8 0.8 1.1 0.28 1.6 2.8 33 65 15 19,344 45,878 0 Steel of the invention 9 9 1 1.4 - - 6 7 87 33 1,198 12.8 1.9 0.7 0.8 0.19 2.4 3.6 41 73 23 15.334 49.118 0 Steel of the invention 10 10 1 1.4 - - 4 8 - 87 61 1,464 13.6 4.0 0.9 3.2 0.33 1.2 2.7 35 64 14 19,883 51,245 0 Steel of the invention 11 11 K 1.4 - - 4 9 - 87 10 1,145 14.2 1.6 0.9 0.8 0.19 2.0 4.6 29 65 15 16.226 33,205 0 Steel of the invention 12 12 L 1.4 - - 4 8 87 30 1,151 13.6 1.9 0.8 0.9 0.20 2.1 4.0 30 66 16 15.678 34,533 0 Steel of the invention 13 13 M 1.4 - - 5 7 - 87 40 1,158 13.0 1.9 0.8 0.8 0.20 2.4 3.8 32 68 18 15.056 37,060 0 Steel of the invention 14 14 N 1.4 - - 6 7 87 19 1,192 12.6 3.9 0.7 2.9 0.21 1.3 3.6 33 68 18 15.019 39.336 0 Steel of the invention 15 15 0 1.4 - - 6 7 - 87 68 1,162 12.9 5.6 0.8 4.6 0.23 1.2 3.7 34 70 20 15,027 39,512 0 Steel of the invention 16 16 P 1.6 - - 3 10 - 87 10 1,430 15.1 3.4 0.9 2.2 0.32 1.5 2.9 42 65 16 21,619 60,078 0 Steel of the invention 17 17 Q 1.4 - - 5 7 88 14 1,251 12.5 1.1 0.8 0.9 0.25 1.2 3.2 36 64 14 15,688 45,020 0 Steel of the invention 18 18 R 1.4 - - 6 7 - 87 6 1,349 12.4 3.0 0.7 2.3 0.25 1.3 2.6 40 68 18 16,692 53,956 0 Steel of the invention. 19 19 S 1.4 - - 4 8 - 88 5 1,189 13.7 3.5 0.9 2.8 0.24 1.2 3.6 31 66 16 16,246 36,872 0 Steel of the invention 20 20 T 1.4 - - 2 10 - 87 8 1,132 15.2 3.6 0.9 3.0 0.22 1.2 4.2 30 68 18 17.206 33,956 0 Steel of the invention Each symbol in the microstructure means the following: F: ferrite, P: pearlite, B: bainite, and retained: retained austenite, TM: tempered martensite, M: freshly tempered martensite. @ means the calculated value of “a-(2.37t2-14t+65)” and the value is good if it is 0 or more. “ - ” means that the microstructure was not observed. Table 10Test No. Steel No. | Production No. h id Volume fraction of microstructure (%) Number of carbide radii 0.1 pm ( / 20000 μπτ) TS (MPa) El (%) Content in and retained Content in matrix MnA / Mum Ca / Cm λ (%) Impact strength Remarks FPB and retained M TM Mn (MnJ C (Ca) Mn (MnM) C (CM) a (grad) © TSxEl TS4 Evaluation 21 21 u 1.4 3 10 87 9 1,285 14.9 2.2 0.9 1.7 0.28 1.3 3.3 34 65 15 19,196 43,689 O Steel of the invention 22 22 V 1.4 6 7 87 12 1,210 12.4 3.3 0.8 2.6 0.20 1.3 4.1 28 62 12 15,055 33,880 0 Steel of the invention 23 23 w 1.4 4 8 88 4 1,190 13.4 2.1 0.9 LO 0.19 2.1 4.4 30 63 13 15,946 35,700 0 Steel of the invention 24 24 X 1.4 5 7 88 10 1,177 12.9 3.3 0.7 2.4 0.19 1.4 3.8 34 68 18 15,188 40,004 0 Steel of the invention 25 25 Y 1.4 6 6 87 14 1,342 12.1 2.0 0.7 1.3 0.28 1.5 2.7 38 70 20 16,235 51,015 0 Steel of the invention 26 26 z 1 7 6 87 24 1,333 11.5 1.5 0.9 1.0 0.33 1.5 2.9 40 70 17 15,390 53,324 0 Steel of the invention 27 27 AA 1.4 7 6 87 6 1,437 11.4 2.3 0.7 1.8 0.29 1.3 2.3 41 65 15 16,363 58,927 0 Steel of the invention 28 28 AB 1.4 7 6 87 22 1,488 11.9 3.7 0.6 3.0 0.30 1.2 2.1 45 71 21 17.690 66,976 0 Steel of the invention 29 29 AC 1.4 6 7 87 9 1,296 12.5 2.3 1.0 1.8 0.32 1.3 3.0 34 66 16 16,149 44,074 0 Steel of the Invention 30 30 AD 1.4 7 6 87 11 1,357 11.4 4.3 0.6 2.9 0.26 1.5 2.5 38 67 17 15,517 51,570 0 Steel of the invention 31 30 AE 1.4 7 6 87 12 1,357 11.4 3.6 0.6 2.9 0.26 1.2 2.5 35 66 16 15,517 47,499 0 Steel of the invention 32 30 AF 1.4 7 6 87 13 1,357 11.4 3.7 0.6 2.9 0.26 1.3 2.5 34 66 16 15,517 46,141 0 Steel of the invention 33 31 AG Cannot be examined. Comparative steel 34 32 AH 1.4 6 7 88 62 1,152 12.4 1.8 0.7 1.8 0.19 LO 3.9 26 62 12 14,270 29,950 X Comparative steel 35 32 Al Cannot be examined. Comparative steel 36 32 AJ Cannot be examined. Comparative steel 37 33 AK 1.4 7 6 87 139 1,167 11.8 3.2 L2 3.1 0.20 LO 4.2 23 58 8 13,737 26,846 X Comparative steel 38 34 AL 1.4 2 6 92 92 1,200 12.2 2.5 L2 1.4 0.19 1.8 63 18 57 7 14,638 21,600 X Comparative steel. 39 35 AM 1.4 5 9 86 108 1,168 14.3 3.2 U 2.5 0.20 1.3 12 28 65 15 16,662 32,711 X Comparative Steel 40 36 AN 1.4 4 6 90 53 1,274 12.3 2.7 0.7 2.6 0.21 LO 3.2 21 61 11 15,711 26,746 X Comparative Steel > ahchc cco An underlined value indicates that it does not meet the stated range, recommended condition, or target performance. Each symbol in the microstructure means the following: F: ferrite, P: pearlite, B: bainite, and retained: retained austenite, TM: tempered martensite, M: freshly tempered martensite. @ means the calculated value of “a-(2.37t2-14t+65)” and the value is good if it is 0 or more. “ - ” means that the microstructure was not observed. Table 11 Test No. c Z 0 LU u < Production No. 0 Q ¢) ca EMC Microstructure Volume Fraction (%) Number of Carbide Radii ^0.1 gm ( / 20000 μην) TS (MPa) El (%) Content in and retained Content in matrix MnA MnM Ca / Cm λ (%) Impact Strength Observations FPB and retained M TM Mn (MnA) C (Ca) Mn (MnM) C (CM) a (grad) © TSxEI TS4 Evaluation 41 31 AO 1.4 1 5 94 17 1,182 11.5 1.0 1.0 0.9 0.20 El 4.9 18 68 18 13,579 21,272 X Comparative Steel 42 38 AP 1.4 - 6 7 2 85 53 1,142 12.7 3.4 0.9 3.3 0.20 10 4.6 42 74 24 14,475 47,956 X Comparative Steel 43 39 AQ 1.4 6 2 7 85 69 1,142 8.4 3.4 El 3.0 0.20 El 51 35 67 17 9,560 39,970 X Comparative Steel 44 40 AS 1.4 30 8 0 62 57 1,193 17.0 2.0 U 1.3 0.19 1.5 17 17 78 28 20,234 20,282 X Comparative Steel 45 41 AT 1.4 35 10 0 55 31 1,193 18.4 1.6 1.0 1.6 0.21 10 4.8 24 75 25 21,945 28,624 X Comparative Steel 46 42 AU 1.4 4 7 4 85 59 1,177 12.7 3.8 1.0 3.0 0.19 1.3 12 34 79 29 14,910 40,026 X Comparative Steel 47 43 AV 1.4 - 5 68 7 20 16 750 7.1 2.6 0.8 2.0 0.19 1.3 4.4 51 95 45 5,339 38,250 X Comparative Steel 48 44 AW 1.4 - 5 95 32 1,273 11.6 1.5 1.0 0.9 0.19 1.7 13 17 68 18 14,764 21,636 X Comparative Steel 49 45 AX 1.4 3 8 89 19 865 13.5 2.9 E6 2.2 0.19 1.3 83 15 89 39 11,679 12,975 X Comparative Steel 50 46 AY 1.4 - 1 0 13 86 56 1,190 6.9 2.5 0.2 2.3 0.20 U 1.2 41 85 35 8,187 48,790 X Comparative Steel 51 47 AZ 1.4 - 5 10 85 87 1,109 14.8 3.9 19 2.2 0.21 1.8 11 22 67 17 16,382 24,400 X Comparative Steel 52 48 BA 1.4 4 5 91 16 1,197 12.6 2.8 0.9 1.5 0.21 1.9 4.3 26 68 18 15,082 31,122 0 Steel of the Invention 53 49 BB 1.4 - 2 6 92 66 1,279 13.2 2.9 0.9 1.9 0.20 1.5 4.5 25 66 16 16,883 31,975 0 Steel of the invention 54 50 BC 1.4 6 9 3 82 52 1,198 13.5 3.5 0.8 2.8 0.19 1.2 4.3 48 81 31 16,173 57,504 0 Steel of the invention 55 51 BD 1.4 - 4 8 88 162 1,232 13.5 3.0 1.0 3.2 0.19 10 4.9 21 62 12 16,671 25,862 X Comparative Steel 56 52 BE 1.4 - 5 7 2 86 17 1,043 12.3 4.7 0.7 3.1 0.10 1.5 63 18 72 22 12,811 18,775 X Comparative Steel 57 53 BF 1.4 6 7 87 19 1,864 8.9 1.8 12 1.0 0.39 1.8 3.1 28 42 16,590 52,192 X Comparative Steel 58 54 BG 1.4 - 2 9 89 18 1,195 14.5 2.7 14 1.5 0.19 1.8 17 17 42 17,375 20,309 X Comparative Steel. > Q l\ chc cco Q l\ ch 59 55 BH 1.4 - 6 5 9 80 21 1,393 11.1 1.9 0.9 1.0 0.28 1.9 3.2 18 43 -1 15,493 25,079 X Comparative Steel 60 56 B1 1.4 - 4 6 90 18 1,480 12.0 0.3 U 0.1 0.32 3.3 4.1 14 40 40 17,730 20,714 X Comparative Steel An underlined value indicates that it did not meet the stated range, recommended condition, or target performance. Each symbol in the microstructure means the following: F: ferrite, P: pearlite, B: bainite, and retained: retained austenite, TM: tempered martensite, M: freshly tempered martensite. (T) means the calculated value of “a-(2.37t2-14t+65)” and the value is good if it is 0 or more. “ - ” means that the microstructure was not observed. Table 12 Test No. 1 Steel No. 1 Production No. Thickness (mm) Microstructure Volume Fraction (%) Number of Carbide Radii Pimi ( / 20000 μπτ) TS (MPa) El (%) Content in and retained Content in matrix MnA ¡ MnM Ca / Cm λ (%) Impact Strength Observations FPB and retained M TM Mn (MnA) C (Ca) Mn (MnM) C (Cm) a (grad) ® TSxEl TS4 Evaluation 61 57 BJ 1.4 4 11 - 85 13 1,231 15.8 4.7 lá 3.6 0.29 1.3 4.9 30 46 4 19,471 36,938 X Comparative Steel 62 58 BK 1.4 4 8 - 88 9 1,222 13.3 3.8 0.8 2.2 0.22 1.7 3.5 19 52 2 16,297 23,217 X Comparative Steel 63 59 BL 1.4 4 5 - 91 12 1,173 11.3 1.8 0.8 0.8 0.20 2.3 4.2 17 49 d 13,253 19,941 X Comparative Steel 64 a BM 1.4 4 9 - 88 14 1,520 9.8 2.1 0.7 1.0 0.31 2.1 2.3 28 54 4 14,896 42,560 X Comparative Steel 65 61 BN 1.4 8 - 92 10 1,232 13.7 1.8 1.0 1.1 0.25 1.6 3.9 17 56 6 16,934 20,943 X Comparative Steel 66 62 BO 1.4 5 9 - 86 9 1,298 11.3 1.2 U 0.8 0.33 1.5 3.3 28 58 8 14,668 36,346 X Comparative Steel 67 63 BP 1.4 4 7 - 89 13 1,240 12.7 4.6 0.7 3.1 0.19 1.5 3.4 16 48 2 15,709 19,845 X Comparative Steel 68 64 BQ 1.4 8 - 92 15 1,259 13.6 1.2 0.7 0.9 0.20 1.4 3.6 17 49 d 17,185 21,406 X Comparative Steel 69 65 BR 1.4 3 11 - 85 7 1,130 15.9 4.5 0.9 3.2 0.21 1.4 4.3 19 53 3 17,944 21,463 X Comparative Steel 70 66 BS 1.4 8 7 - 85 12 1,421 10.1 1.4 0.8 0.9 0.33 1.5 2.5 31 54 4 14,352 44,051 X Comparative Steel 71 67 BT 1.4 7 7 - 86 9 1,120 12.4 1.3 0.8 0.8 0.20 1.6 4.0 19 48 2 13,869 21,276 X Comparative Steel 72 68 BU 1.4 3 5 - 92 9 1,235 11.4 1.8 0.8 0.9 0.20 1.9 3.9 10 42 d 14,031 12,354 X Steel Comparative 73 69 BV 1.4 4 8 - 88 8 1,243 13.8 1.6 0.7 0.9 0.19 1.7 3.6 15 46 4 17,155 18,647 X Comparative Steel 74 70 BW 1.4 5 9 - 86 16 1,161 14.3 4.6 0.8 2.9 0.19 1.6 4.1 16 48 2 16,652 18,578 X Comparative Steel 75 71 BX 1.4 2 9 - 89 9 1,156 14.1 1.5 0.8 0.9 0.19 1.7 4.2 13 48 2 16,320 15,033 X Comparative Steel 76 72 BY 1.4 6 - 94 19 1,225 12.3 1.9 0.8 0.7 0.20 2.8 4.0 13 47 4 15,056 15,931 X Comparative steel 77 73 BZ 1.4 1 6 - 93 13 1,254 12.3 1.6 0.8 1.1 0.21 1.5 3.8 9 40 dO 15,373 11,284 X Comparative steel 78 74 CA 1.4 7 8 - 85 16 1,142 13.5 4.1 0.8 3.1 0.19 1.3 4.0 19 54 4 15,457 21,689 X Comparative steel. 79 75 CB 1.4 8 5 - 87 8 1,451 10.1 5.1 0.1 3.1 0.19 1.6 0.5 28 58 8 14,654 40,625 X Comparative Steel 80 76 CC 1.4 5 8 - 87 12 1,207 13.4 2.1 0.8 0.9 0.22 2.3 3.5 22 56 6 16,179 26,554 X Comparative Steel 81 77 CD 1.4 6 8 - 86 18 1,152 13.6 4.2 0.8 2.3 0.20 1.8 4.0 20 54 4 15,647 23,049 1,218 12.5 2.5 0.7 1.2 0.20 2.1 3.5 19 51 1 15,256 23,139 1.8 2.7 28 56 6 14,618 40,128 X Comparative Steel 85 il CH 1.4 1 10 - 89 17 1,269 12.3 1.7 0.8 1.1 0.23 1.5 3.6 19 45 .5 15,607 24,108 X Comparative Steel 86 82 Cl 1.4 6 9 - 85 18 1,543 9.6 1.6 0.7 0.9 0.33 1.8 2.2 31 53 3 14,813 47,833 X Comparative Steel 87 1 CJ 2.4 6 9 - 85 173 1,203 13.2 3.8 0.7 3.1 0.20 1.2 3.7 18 57 12 15,880 21,654 X Comparative Steel 88 1 AR 1.4 82 18 - 320 832 17.2 - 3.1 0.2 43 102 52 14,310 35,776 X Comparative Steel. An underlined value indicates that it does not meet the declared range, recommended condition, or target performance. Each symbol in the microstructure means the following: F: ferrite, P: pearlite, B: bainite, γ retained: retained austenite, TM: tempered martensite, M: freshly tempered martensite. @ means the calculated value of “a-(2.37t2-14t+65)” and the value is good if it is 0 or more. “ - ” means that the microstructure was not observed. As shown in Tables 9 to 12, tests No. 1 to 32, which met the definition according to the present invention, exhibited high strength and excellent impact resistance. Conversely, tests No. 33 to 88, which did not meet any or more of the requirements for steel microstructure, chemical composition, macrohardness, and microhardness according to the present invention, were poor, at least in terms of impact resistance. INDUSTRIAL APPLICABILITY According to the present invention, a steel sheet is obtained that has high strength (specifically a tensile strength of 1100 MPa or more), high work capacity, and excellent impact resistance.

Claims

1. A steel sheet comprising a steel microstructure containing, by volume fraction, tempered martensite: 85% or more, retained austenite: from 5% or more to less than 15%, and ferrite, pearlite, bainite, and freshly quenched martensite less than 10% in total, the chemical composition of the steel sheet consisting, by mass %, of: C: from 0.18% or more to 0.38% or less, Si: from 0.80% or more to 2.50% or less, Mn: from 0.6% or more to 5.0% or less, P: 0.0200% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, Al: from 0% or more to 1.000% or less, Cr: from 0% or more to 2.0% or less, Mo: from 0% or more to 0.50% or less, Ti: from 0% or more to 0.10% or less, Nb: from 0% or more to 0.10% or less, B: from 0% or more to 0.0100% or less, V: from 0% or more to 0.50% or less, Cu: from 0% or more to 0.50% or less, W: from 0% or more to 0.100% or less, Ta: from 0% or more to 0.100% or less, Ni: from 0% or more to 1.00% or less, Co: from 0% or more to 0.50% or less, Sn: from 0% or more to 0.050% or less, Sb: from 0% or more to 0.0.50% or less, As: from 0% or more to 0.050% or less, Mg: from 0% or more to 0.050% or less, Ca: from 0% or more to 0.050% or less, Y: from 0% or more to 0.050% or less, Zr: from 0% or more to 0.050% or less, La: from 0% or more to 0.050% or less, CC Ln / ίZϖ / B / YILI Ce: from 0% or more to 0.050% or less and the remainder: Fe and unavoidable impurities, when a Mn content and a C content in retained austenite are denoted as Mπα and Ca, respectively, and when a Mn content and a C content in a matrix are denoted as Mum and Cm, respectively, satisfy the following formulas (1) to (3) and when observing a region measuring 20000 pm2 and centered approximately at the point t / 4 (t denotes a thickness of the steel sheet) from a surface of the steel sheet, the number of carbides having an equivalent circle radius of 0.1 pm or more is 100 or less and the steel sheet has a tensile strength of 1100 MPa or more. MnA / MnM>1.2(1) Ca / Cm < 5.0(2) Ca<1.0(3).

2. The steel sheet, according to claim 1, comprising a galvanized layer, an annealed and galvanized layer, or an electrogalvanized layer on its surface.