Abstract: Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 ?m or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

Abstract: A high-strength steel sheet having a tensile strength of 1320 MPa or more is disclosed. The above-described high-strength steel sheet includes a specific component composition including Ti and the like, wherein a diffusible hydrogen amount in steel is 0.50 ppm by mass or less, tempered martensite and bainite are 70.0 to 95.0%, fresh martensite is 15.0% or less, retained austenite is 5.0 to 15.0%, an average grain size of a precipitate A, which is a carbide, nitride, or carbonitride containing at least one selected from the group consisting of Ti, Nb, and V is 0.001 to 0.050 ?m, a number density NS of the precipitate A having a major axis of 0.050 ?m or less is 10/?m2 or more, and a ratio of the number density NS and a number density NL of the precipitate A having a major axis of more than 0.050 ?m is 10.0 or more.

Abstract: A high-strength steel sheet having a tensile strength of 1320 MPa or more is disclosed. The above-described high-strength steel sheet includes a specific component composition, wherein a diffusible hydrogen in steel is 0.50 ppm by mass or less, tempered martensite and bainite are 70.0% or more, fresh martensite is 15.0% or less, iron-based carbides existing in the tempered martensite and the bainite have an average equivalent circular radius of 0.10 ?m or less and an average aspect ratio of 4.5 or less, and a proportion p of tempered martensite and bainite including iron-based carbides, which have major axes facing the same direction within a range of 0 to 10°, at a number density of 7 to 35/?m2 among the tempered martensite and the bainite is 25 to 70%.

Abstract: Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, martensite: 5% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 ?m or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

Abstract: Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, martensite: 5% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 ?m or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

Abstract: A high strength steel sheet has a yield-point elongation of 1% or greater and a tensile strength of 980 MPa or greater. The high strength steel sheet has a specific chemical composition and microstructure. A ratio of retained austenite grains adjoining a retained austenite grain having a different crystal orientation to total retained austenite grains is 0.60 or greater, the ferrite has an average grain size of 5.0 ?m or less, and the retained austenite has an average grain size of 2.0 ?m or less. A value obtained by dividing a volume fraction V?a by a volume fraction V?b is 0.40 or greater, where the volume fraction V?a is a volume fraction of retained austenite in a fractured portion of a tensile test specimen after a warm tensile test at 150° C., and the volume fraction V?b is a volume fraction of retained austenite before the warm tensile test at 150° C.

Abstract: Provided is a continuous annealing line capable of producing a steel sheet excellent in hydrogen embrittlement resistance. A continuous annealing line 100 includes: a payoff reel 10 to feed a cold-rolled steel sheet S; an annealing furnace 20 to continuously anneal the cold-rolled steel sheet S and including a heating zone 22, a soaking zone 24, and a cooling zone 26; a downstream line 30; a tension reel 50 configured to coil the cold-rolled steel sheet S; and a vibration application device 60 or 70 configured to apply vibration to the cold-rolled steel sheet S being passed from the cooling zone 26 to the tension reel 50 so that the cold-rolled steel sheet is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 ?m 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: Provided are a steel sheet dehydrogenation apparatus, a steel sheet production system, and a steel sheet production method capable of producing a steel sheet excellent in hydrogen embrittlement resistance without changing the mechanical properties of the steel sheet. A dehydrogenation apparatus comprises: a housing configured to house a steel sheet coil obtained by coiling a steel strip; and a vibration application device configured to apply vibration to the steel sheet coil housed in the housing so that the steel sheet coil is caused to vibrate at a frequency of 100 Hz to 100,000 Hz and a maximum amplitude of 10 nm to 500 ?m.

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: Provided is a resistance spot welding method suitable for manufacturing a welded joint exhibiting excellent delayed fracture resistance. The resistance spot welding method includes sandwiching two or more overlapped steel sheets between a pair of welding electrodes, applying current to the steel sheets while pressing the steel sheets, forming a nugget on overlapping surfaces of the steel sheets to join the steel sheets, and after the joining, applying vibration directly or indirectly to the nugget so that the steel sheets are caused to vibrate at a frequency of 100 Hz or more and a maximum amplitude of 10 nm to 500 ?m.

Abstract: A high strength steel sheet with a chemical composition containing C, Si, Mn, P, S, Al, N, Nb, Ti, and B, where [% Si]/[% Mn] is 0.10 or more and 0.60 or less, and free Ti content determined from Formula (1), below, is 0.001 mass % or more, with the balance being Fe and inevitable impurity. In a steel sheet surface layer, a total amount of Nb solute and 100 nm or smaller precipitate is 0.002 mass % or more. free ? Ti ? content ? ( % ) = [ % ? Ti ] - ( 47.9 / 14. ) × [ % ? N ] - ( 47.9 / 32.

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: A high strength galvanized steel sheet is disclosed having a tensile strength of 1,320 MPa or more. A chemical composition of a steel sheet contains, by mass, C: 0.150 to 0.450%, Si: 0.80 to 3.00%, Mn: 2.00 to 4.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 0.100%, O: not more than 0.0100%, and N: not more than 0.0100%, with a balance of Fe and inevitable impurities. An amount of diffusible hydrogen in steel is not more than 0.60 mass ppm. A total of tempered martensite and bainite is 60 to 95%, retained austenite is 5 to 30%, and retained austenite having an aspect ratio of 5.5 or higher with respect to an entire retained austenite is not more than 50%, and fresh martensite having a diameter of 2.0 ?m or less is not more than 20%.

Abstract: Provided is a hot-dip galvanized steel sheet that has a high YR, as well as high ductility, stretch flangeability, and bendability, improved shear workability, and a TS of 1,180 MPa or more. The base steel sheet of the hot-dip galvanized steel sheet has a defined chemical composition and steel microstructure. In particular, a number ratio of retained austenite (y) having an aspect ratio of 2.0 or less is more than 50%, a number of bins of hardness frequency: 0.25 or more is 1, the area ratio of quenched martensite in a surface layer is 80% or less, a surface layer softening thickness is 10 ?m or more and 100 ?m or less, and the amount of low temperature range diffusible hydrogen is 0.015 mass ppm or less.

Abstract: A high-strength steel sheet is disclosed having a specified chemical composition and a steel microstructure composed of, on an area fraction basis, ferrite: 1% to 40%, fresh martensite: 1% to 20%, bainite and tempered martensite in total: 35% to 90%, and retained austenite: 6% or more, wherein a value obtained by dividing an average Mn content (% by mass) of the retained austenite by an average Mn content (% by mass) of the ferrite is 1.1 or more, and a value obtained by dividing an average C content (% by mass) of retained austenite with an aspect ratio of 2.0 or more by an average C content (% by mass) of the ferrite is 3.0 or more, and a value obtained by dividing a C content of all retained austenite by a C content of a T0 composition is 1.0 or more.

Abstract: A high-strength steel sheet is disclosed having a specified chemical composition and a steel microstructure composed of, on an area fraction basis, ferrite: 1% to 40%, fresh martensite: less than 1.0%, bainite and tempered martensite in total: 40% to 90%, and retained austenite: 6% or more, wherein a value obtained by dividing an average Mn content (% by mass) of the retained austenite by an average Mn content (% by mass) of the ferrite is 1.1 or more, and a value obtained by dividing an average C content (% by mass) of retained austenite with an aspect ratio of 2.0 or more by an average C content (% by mass) of the ferrite is 3.0 or more, and a diffusible hydrogen content of steel is 0.3 ppm by mass or less.