Abstract: A high strength steel sheet includes a steel sheet containing, by mass, C: 0.030%-0.500%, Si: 0.50%-2.50%, Mn: 1.50%-5.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 1.000%, N: not more than 0.0100%, O: not more than 0.0100%, and Nb: 0.005%-0.100%, with a balance consisting of Fe and inevitable impurities, an amount of martensite is not less than 70%, an amount of retained austenite is 3%-20%, a total amount of ferrite and bainitic ferrite is not more than 10%, an instability index k of retained austenite is less than 6.1, and an instability index d of retained austenite in an initial stage of working is less than 5.7.

Abstract: A high strength steel sheet contains, by mass, C: 0.030%-0.500%, Si: 0.50%-2.50%, Mn: 1.50%-5.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 1.000%, N: not more than 0.0100%, O: not more than 0.0100%, and Nb: 0.005%-0.100%, with a balance consisting of Fe and inevitable impurities, an amount of martensite is not less than 60%, an amount of retained austenite is 3%-20%, and a total amount of ferrite and bainitic ferrite is more than 10%, an instability index k of retained austenite is less than 5.9, and an instability index d of retained austenite in an initial stage of working is less than 5.8.

Abstract: Provided is a steel sheet that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. The steel sheet has a defined chemical composition and a microstructure where an area fraction of tempered martensite is 83% or more, an area fraction of retained austenite is less than 3%, a total area fraction of bainitic ferrite and ferrite 5 % or more and less than 15%, and a 20° or greater grain boundary density in tempered martensite is 1.0 ?m/?m2 or more, satisfying the following Expression ? ( 1 ) ? KAM ? ( S ) / KAM ? ( C ) > 1.

Abstract: Provided is a steel sheet that has a TS of 1320 MPa or more, as well as excellent stretch flangeability, and excellent toughness and crash properties after paint baking. The steel sheet has a defined chemical composition and a microstructure where an area fraction of tempered martensite is 95% or more, an area fraction of retained austenite is less than 3%, a total area fraction of bainitic ferrite and ferrite is less than 5%, and a 20° or greater grain boundary density in tempered martensite is 1.0 ?m/?m2 or more, satisfying the following Expression ? ( 1 ) ? KAM ? ( S ) / KAM ? ( C ) > 1.

Abstract: A steel sheet, a member, and a method for producing them are disclosed. The steel sheet has a chemical composition containing, in mass %: C: not less than 0.15% and not more than 0.45%; Si: not less than 0.3% and not more than 2.0%; Mn: not less than 1.7% and not more than 4.0%; P: not more than 0.10%; S: not more than 0.01%; sol. Al: not more than 0.50%; N: not more than 0.010%; and B: not less than 0.0008% and not more than 0.0100%, with the balance being Fe and incidental impurities, wherein the steel sheet has a steel microstructure where the area ratio of martensite to the entire microstructure is not less than 85% and less than 95%, and the area ratio of retained austenite to the entire microstructure is not less than 5% and not more than 15%, and wherein the number density A of precipitates having an equivalent circular diameter of 500 nm or more satisfies the formula: A (number/mm2)?8.5×105×[B].

Abstract: Provided is a dehydrogenation method capable of efficiently reducing hydrogen content in steel for thick or complexly-shaped steel materials and steel products in general. In a dehydrogenation method for a steel material, in a series of steel material production process including: a process of supplying a steel raw material; a process of subjecting the steel raw material to hot working; a process of inspecting a steel material obtained from the steel raw material; and a process of shipping the steel material, at least one of the steel raw material and the steel material at any stage from the supply process to the shipment process is subjected to, at least once, a sound wave irradiation treatment so that a sound pressure level at a surface of the at least one of the steel raw material and the steel material will be 30 dB or more.

Abstract: A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below: KAM ? ( S ) / KAM ? ( C ) < 1. ( 1 ) wherein KAM (S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM (C) is a KAM value of a central portion of the steel sheet, Hv ? ( Q ) - Hv ? ( S ) ? 8 ( 2 ) wherein Hv (Q) indicates the hardness of a portion at ¼ sheet thickness and Hv (S) indicates the hardness of a superficial portion of the steel sheet.

Abstract: There is provided a high-strength steel sheet in which content amounts of a specific steel composition are limited and manufacturing conditions are optimized and that has a steel structure that contains ferrite and a harder phase and in which ferrite accounts for 30% to 70% in terms of area ratio, the harder phase contains bainitic ferrite, tempered martensite, martensite (including 0%), and retained austenite (including 0%), the maximum grain diameter of ferrite dmax is 2.5 ?m or less, and the area ratio of ferrite AF and the average grain diameter of ferrite dF satisfy AF [%]/dF [?m]?15.

Abstract: A high strength steel sheet having 1180 MPa or higher tensile strength and a method for manufacturing the same are disclosed. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of martensite is 80% or more, the volume fraction of retained austenite is 3% or more and 15% or less, the area fraction of the total of ferrite and bainitic ferrite is 10% or less, the average grain size of prior austenite is 20 ?m or less, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

Abstract: A high strength steel sheet having 1180 MPa or higher tensile strength and 85% or more yield ratio and a method for manufacturing the same are disclosed. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of tempered martensite is 90% or more, the volume fraction of retained austenite is less than 3%, the area fraction of the total of ferrite and bainitic ferrite is less than 10%, the average grain size of prior austenite is 20 ?m or less, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

Abstract: A high strength steel sheet having 980 MPa or higher tensile strength and 10% or more elongation and a method for manufacturing the same are disclosed. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of martensite is 60% or more, the volume fraction of retained austenite is 3% or more and 15% or less, the area fraction of the total of ferrite and bainitic ferrite is more than 10%, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

Abstract: A high strength steel sheet having 980 MPa or higher tensile strength and a method for manufacturing the same are provided. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of tempered martensite is 38% or more and less than 90%, the volume fraction of retained austenite is less than 3%, the area fraction of the total of ferrite and bainitic ferrite is 10% or more and 60% or less, the average grain size of prior austenite is 20 ?m or less, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

Abstract: A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 4% or less (including 0%), as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an amount of 7% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 71% and less than 93%; and BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an equivalent circular diameter of 1 ?m or less is present in an area fraction of 4% or more and 50% or less, and BCC iron that has a misorientation of more than 1° is present in an area fraction of 25% or more and 85% or less.

Abstract: A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below: KAM ? ( S ) / KAM ? ( C ) < 1. ( 1 ) wherein KAM(S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM(C) is a KAM value of a central portion of the steel sheet, Hv ? ( Q ) - Hv ? ( S ) ? 8 ( 2 ) wherein Hv(Q) indicates the hardness of a portion at ¼ sheet thickness and Hv(S) indicates the hardness of a superficial portion of the steel sheet.

Abstract: The steel sheet comprising a chemical composition containing C, Si, Mn, P, S, Al, N, Ti, Nb, and B, with the balance being Fe and inevitable impurities, and satisfying predetermined formula (1), in which the total area ratio of martensite and bainite is 95% or more, the grain size of prior austenite grains is 10 ?m or less, the B concentration at a prior austenite grain boundary is 0.10% or more in mass %, the C concentration at the prior austenite grain boundary is 1.5 times or more than the C content in the steel, the amount of precipitated Fe is 200 mass ppm or less, and for a defined in predetermined formula (2), a ratio of adislocation on dislocation to agrain boundary on the prior austenite grain boundary: adislocation/agrain boundary is 1.3 or more.

Abstract: Provided is a high strength steel sheet with high stretch flangeability and high YR in the rolling direction as well as in the direction orthogonal to the rolling direction with TS of 1180 MPa or more. The high strength steel sheet has a prescribed chemical composition and steel microstructure. In particular, an average grain size of the hard phase is 5.3 ?m or less, the volume fraction-carbon concentration ratio of retained austenite (?) is 0.10 or more and 0.45 or less, and the integration degree of {112} <111> orientation is 1.0 or more.

Abstract: The steel sheet comprising a chemical composition containing C, Si, Mn, P, S, Al, N, Ti, Nb, and B, with the balance being Fe and inevitable impurities, and satisfying predetermined formula, in which the total area ratio of martensite and bainite is 95% or more, the average grain size of prior austenite grains is 10 ?m or less, the B concentration at a prior austenite grain boundary is 0.10% or more in mass %, a C-concentrated region is provided along a martensitic grain boundary, and the C-concentrated region has a C concentration of 4.0 times or more than the C content in the steel, and the C-concentrated region has a concentration width of 3 nm or more and 100 nm or less in a direction perpendicular to the martensitic grain boundary and a length of 100 nm or more in a direction parallel to the martensitic grain boundary.

Abstract: A steel sheet has a chemical composition containing a predetermined amount of C, Si, Mn, Cu, P, S, Al, and N, and optionally a predetermined amount of one or more selected from the group consisting of Ti, B, Nb, Cr, V, Mo, Ni, As, Sb, Sn, Ta, Ca, Mg, Zn, Co, Zr, and REM, with the balance being Fe and inevitable impurities; a microstructure comprising, in volume fraction, tempered martensite: 90% or more, retained austenite: 1% to 7%, one or both of bainitic ferrite and fresh martensite: 3% to 9% in total, and ferrite: 0% to 5%, where the retained austenite has a carbon concentration of 0.35% or more; a tensile strength TS of 1470 MPa to 1650 MPa, and a yield strength YS of 1100 MPa or more.

Abstract: A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ?: 95% to 100% by area percentage, and retained ?: 4% to 15% by volume percentage. Retained ?UB has a specified area percentage S?UB, retained ?LB has a specified distribution number N?LB, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained ? grains has a specified equivalent circular grain diameter and aspect ratio.

Abstract: A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below: KAM ? ( S ) / KAM ? ( C ) < 1. ( 1 ) wherein KAM (S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM (C) is a KAM value of a central portion of the steel sheet, Hv ? ( Q ) - Hv ? ( S ) ? 8 ( 2 ) wherein Hv (Q) indicates the hardness of a portion at ¼ sheet thickness and Hv (S) indicates the hardness of a superficial portion of the steel sheet.