Abstract: A hot-pressed member has a predetermined chemical composition. In the hot-pressed member, a steel sheet has a microstructure in which a prior austenite average grain diameter is 8 ?m or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 ?m of a surface; a Ni diffusion region having a thickness of 0.5 ?m or greater exists in a surface layer; a standard deviation of Vickers hardness values is 35 or less; Mndif (mass %) in a sheet thickness direction ?0.20, where Mndif (mass %) is a degree of Mn segregation; and a tensile strength is 1780 MPa or greater.

Abstract: Provided is a resistance spot welding method. The resistance spot welding method for joining a sheet set including a plurality of lapped steel sheets includes: holding the sheet set between a pair of electrodes; and energizing the sheet set under application of electrode force to thereby join the steel sheets together. At least one of the plurality of lapped steel sheets is a surface-treated steel sheet including a metal coating layer on a surface thereof. The energizing includes: a primary energizing step of performing energization to form a nugget portion; a non-energizing step in which, after the primary energizing step, the energization is suspended for an energization suspension time Tc (cycles); and a secondary energizing step of, after the non-energizing step, performing energization for reheating while the nugget portion is prevented from growing. During the energizing, the relations of a particular formula are satisfied.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, excellent resistance to resistance welding cracking, and excellent delayed fracture resistance after resistance welding by having a specific chemical composition, and a microstructure such that a prior austenite average grain size is 7 ?m or less, a volume fraction of martensite is 90% or more, and at least 5 Nb-based precipitates having a grain size of less than 0.08 ?m are present on average per 100 ?m2 of a cross section parallel to a thickness direction of the member within a range of 100 ?m in the thickness direction from a surface of the member, and such that a Ni diffusion region having a thickness of 0.5 ?m or more is present in a surface layer of the member.

Abstract: A high-strength cold rolled steel sheet has a specific composition and a steel structure that is, by volume %, 10%-70% ferrite, 1%-10% retained austenite, 10%-60% bainite, and 2%-50% martensite, the average crystal grain size of the ferrite being no more than 6.0 ?m, the average crystal grain size of the retained austenite being no more than 4.0 ?m, the average crystal grain size of the bainite being no more than 6.0 ?m, and the average crystal grain size of the martensite being no more than 4.0 ?m. The concentration ratio of the average concentration of Si to a depth of 10 ?m from the surface of the high-strength cold-rolled steel sheet to the average concentration of Si throughout the high-strength cold-rolled steel sheet is, by mass ratio, greater than 1.00 but less than 1.30.

Abstract: A hot-pressed member having excellent bending collapsibility, a method for manufacturing the same, and a method for manufacturing a steel sheet for the hot-pressed member. The hot-pressed member includes a steel sheet as a base material, the steel sheet having a specified chemical composition. The hot-pressed member has a microstructure in which a martensite microstructure is present in a volume fraction of 70% or greater, and a number density of inclusions having a longest diameter of 25 ?m or greater is 0.02/mm2 or less. The hot-pressed member has a tensile strength of 1.8 GPa or greater.

Abstract: A spot welded member includes a spot weld formed by holding a sheet stack of multiple steel sheets between a pair of electrodes and spot-welding the sheet stack, in which at least one of the multiple steel sheets is a high-strength zinc-based coated steel sheet having a tensile strength of 780 MPa or more, the high-strength zinc-based coated steel sheet having a coating with an Al content of 0.5% or more by mass, and in which the heat shock region of the spot weld outwardly extending from an edge of a corona bond area includes a coated layer including an FeAl alloy layer having an average thickness of 0.3 ?m or more and a zinc-based coated layer having an average thickness of 2.0 ?m or more on the FeAl alloy layer at the interface between the base steel sheet of the high-strength zinc-based coated steel sheet and the coating.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent indentation peeling strength at projection welds by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7 ?m or less within a range of 50 ?m or less in a thickness direction from a surface of the member, a volume fraction of martensite is 90% or more, and an average intergrain distance of Nb and Ti carbonitrides having a grain size of less than 0.10 ?m within a depth range of 20 ?m to 100 ?m in the thickness direction from the surface of the member is 5 ?m or less.

Abstract: A high-strength steel sheet having a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si: 1.00% to 2.20%, Mn: 2.00% to 3.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.01% to 0.50%, N: 0.010% or less, B: 0.0003% to 0.0050%, one, two, or more selected from Ti: 0.005% to 0.05%, Cu: 0.003% to 0.50%, Ni: 0.003% to 0.50%, Sn: 0.003% to 0.50%, Co: 0.003% to 0.05%, and Mo: 0.003% to 0.50%, and the balance being Fe and inevitable impurities and a microstructure including, in terms of volume fraction, 15% or less (including 0%) of ferrite, 2% to 15% of retained austenite, 10% or less (including 0%) of martensite, and the balance being bainite and tempered martensite, in which the average number of cementite grains having a grain diameter of 0.04 ?m or more existing in the bainite and tempered martensite grains is 10 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: 6 to 90% in terms of an area fraction, and a constituent formed of at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ?: 10 to 94% in terms of an area fraction, and retained ?: 3 to 15% in terms of a volume fraction. A total area fraction of SC-enriched is 0.1 to 5%, a total area fraction of S?Block of at least one of (i) fresh martensite having a specified equivalent circular grain diameter and aspect ratio and (ii) retained ? grains having a specified equivalent circular grain diameter and aspect ratio is 5% or less, and prior ? in a surface layer has a grain diameter of 2 to 12 ?m.

Abstract: A high-strength cold rolled steel sheet having a specified chemical composition and a method for producing the same. The steel sheet having a microstructure including, in terms of volume fraction, 35% or less of ferrite, 1% or more and 10% or less of retained austenite, 2% or more and 12% or less of as-quenched martensite, and a total of 25% to 70% of bainite and tempered martensite. The microstructure has an average crystal grain diameter of the ferrite: 5.0 ?m or less, an average crystal grain diameter of the retained austenite: 2.0 ?m or less, an average crystal grain diameter of the as-quenched martensite: 3.0 ?m or less, an average crystal grain diameter of the bainite and the tempered martensite phase: 4.0 ?m or less, and an average intergrain distance of the as-quenched martensite of 1.0 ?m or more.

Abstract: A high-strength steel sheet having a specified chemical composition and a method for producing the same. The steel sheet including a multi-phase structure including, by volume fraction, ferrite of 35% to 65%, residual austenite of 7% or less (including 0%), martensite of 20% to 60%, bainite of 20% or less (including 0%), the ferrite having an average crystal grain size of 8 ?m or less and an average aspect ratio of 3.5 or less, the martensite having an average crystal grain size of 3 ?m or less, and the bainite having an average crystal grain size of 3 ?m or less when contained. A standard deviation of Vickers hardness of the steel sheet being 30 or less when measured at 200 ?m intervals from a position of 100 ?m from a surface to a central portion in a thickness direction of the sheet.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent resistance to resistance welding cracking by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7.5 ?m or less, a volume fraction of martensite is 95% or more, and at least 10 Nb-based and Ti-based precipitates having a grain size of less than 0.10 ?m are present on average per 100 ?m2 of a cross section parallel to a thickness direction of the member within a range of 100 ?m or less in the thickness direction from the surface of the member, and such that a B concentration in prior austenite grain boundaries is at least 3.0 times a B concentration at a position 5 nm away from the grain boundaries.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more and excellent delayed fracture resistance after projection welding by properly adjusting its chemical composition and its microstructure such that at least 5 Ti-based precipitates having a grain size of 0.10 ?m or less are present on average per 100 ?m2 of a cross section parallel to a thickness direction of the member within a range of 100 ?m in a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 ?m to 100 ?m in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 ?m are present on average in a prior austenite grain.

Abstract: A high-strength steel sheet having a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si: 1.00% to 2.20%, Mn: 2.00% to 3.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.01% to 0.50%, N: 0.010% or less, B: 0.0003% to 0.0050%, one, two, or more selected from Ti: 0.005% to 0.05%, Cu: 0.003% to 0.50%, Ni: 0.003% to 0.50%, Sn: 0.003% to 0.50%, Co: 0.003% to 0.05%, and Mo: 0.003% to 0.50%, and the balance being Fe and inevitable impurities and a microstructure including, in terms of volume fraction, 15% or less (including 0%) of ferrite, 2% to 15% of retained austenite, 10% or less (including 0%) of martensite, and the balance being bainite and tempered martensite, in which the average number of cementite grains having a grain diameter of 0.04 ?m or more existing in the bainite and tempered martensite grains is 10 or more.

Abstract: A method for producing a hot-rolled steel sheet, a method for producing a cold-rolled full-hard steel sheet, and a method for producing a heat-treated sheet are provided herein. The methods comprising hot rolling a steel material of a composition comprising, in mass %, C: 0.05 to 0.12%, Si: 0.80% or less, Mn: 1.30 to 2.10%, P: 0.001 to 0.050%, S: 0.005% or less, Al: 0.01 to 0.10%, N: 0.010% or less, one or more selected from Cr in an amount of 0.05 to 0.50%, and Mo in an amount of 0.05 to 0.50%, one or more selected from Ti in an amount of 0.01 to 0.10%, Nb in an amount of 0.01 to 0.10%, and V in an amount of 0.01 to 0.10%, and the balance Fe and unavoidable impurities.

Abstract: Disclosed is a hot-pressed steel sheet member having a tensile strength of 1780 MPa or more and excellent bending collapsibility. The hot-pressed steel sheet member includes: a specific chemical composition; a microstructure in which an average grain size of prior austenite grains is 8 ?m or less, a volume fraction of martensite is 90% or more, and a solute C content is 25% or less of a total C content; and a tensile strength of 1780 MPa or more.

Abstract: Provided herein is a plated steel sheet having a yield ratio, strength (tensile strength), elongation, hole expansibility, and CTS. A method for producing the plated steel sheet is also provided, among others. A steel sheet of a specific composition is provided that has a micro structure containing 70 to 90% of ferrite, 5 to 20% of martensite, 5% or less of retained austenite, 10% or less of bainite, and 5% or less of perlite by volume. The ferrite has an average crystal grain diameter of 20 ?m or less. The martensite has an average crystal grain diameter of 5 ?m or less. The retained austenite has an average crystal grain diameter of 5 ?m or less. The bainite has an average crystal grain diameter of 7 ?m or less. The steel sheet has a tensile strength of 590 MPa or more.

Abstract: Provided is a resistance spot welding method that inhibits, in accordance with the degree of axis misalignment between electrodes, the occurrence of cracking in a weld regardless of the steel grade. In resistance spot welding methods according to the present invention, H (ms) is an electrode force retaining time after completion of current passage, D (mm) is an amount of axis misalignment between the electrodes, t (mm) is a total sum of sheet thicknesses of a plurality of overlapping steel sheets, T (MPa) is a tensile strength of a steel sheet having a highest tensile strength among the plurality of steel sheets, F (N) is an electrode force, and d (mm) is a tip diameter of one electrode of the pair of electrodes that has a smaller tip diameter. The electrode force retaining time H is specified to be a predetermined value or greater.

Abstract: Provided is an automotive member, or particularly, a steel sheet having a tensile strength exceeding 900 MPa. The automotive member has a resistance weld for fixing two or more steel sheets containing a predetermined composition, in which a maximum hardness (HVBM) in a heat-affected zone of the resistance weld is at least 1.1 times hardness (HVW) of a nugget in the resistance weld formed in a softest steel sheet of a sheet set during resistance welding, and furthermore, an average grain size of a steel sheet structure of the heat-affected zone within 2 mm in a direction at a right angle to a sheet thickness from an end part of the nugget of the high-strength steel sheet is 3 ?m or less, and a minimum hardness (HVmin) in the heat-affected zone is at least 90% of hardness (HV?) of the high-strength steel sheet before the resistance welding.

Abstract: Provided herein is a plated steel having high strength with excellent elongation, excellent hole expansibility, and excellent material uniformity, and a method for producing such a plated steel. The steel sheet provided herein has a specific composition, and a steel structure that contains ferrite as a primary phase, and 2 to 12% of perlite, and 3% or less of martensite by volume, and in which the remainder is a low-temperature occurring phase. The ferrite has an average crystal grain diameter of 25 ?m or less. The perlite has an average crystal grain diameter of 5 ?m or less. The martensite has an average crystal grain diameter of 1.5 ?m or less. The perlite has a mean free path of 5.5 ?m or more. The steel sheet has a tensile strength of 440 MPa or more.