Abstract: Disclosed is an Fe-based electroplated steel sheet including: a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; and an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of 5.0 g/m2 or more, in which in an intensity profile measured by glow discharge optical emission spectrometry, a peak of emission intensity at wavelengths indicating Si is detected within a range from a surface of the Fe-based electroplating layer to more than 0.2 ?m in a thickness direction and not more than a thickness of the Fe-based electroplating layer, and an average value of C concentration in a region ranging from 10 ?m to 20 ?m in the thickness direction from the surface of the Fe-based electroplating layer is 0.10 mass % or less.

Abstract: A high-strength cold-rolled steel sheet has a chemical composition containing C: 0.150 to 0.350 mass %, Si: 0.80 to 3.00 mass %, Mn: 1.50 to 3.50 mass %, P: 0.100 mass % or less, S: 0.0200 mass % or less, Al: 0.100 mass % or less, N: 0.0100 mass % or less, and O: 0.0100 mass % or less, with a remaining part consisting of Fe and impurities. The amount of diffusible hydrogen is 0.50 mass ppm or less, the area ratio of tempered martensite and bainite is 55 to 95%, the area ratio of retained austenite is 5 to 30%, a prior austenite grain has an average circle equivalent diameter of 15.0 ?m or less, and the ratio b/a is 0.80 or less, where a circumferential length of the prior austenite grain is a, and a circumferential length of a portion of the prior austenite grain having a carbon concentration of 0.6 mass % or more is b.

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: To provide an Fe-based electroplated steel sheet that not only has excellent chemical convertibility or excellent coating appearance when subjected to hot-dip galvanizing, but also has excellent resistance to cracking in resistance welding. Disclosed is an Fe-based plated steel sheet including: a cold-rolled steel sheet having a chemical composition containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; and an Fe-based electroplating layer formed on one or both surfaces of the cold-rolled steel sheet with a coating weight per surface of 1.0 g/m2 or more, in which a thickness of an internal oxidation layer is 2.00 ?m or less, and an average value of C concentration in a range of 10 ?m to 20 ?m in a thickness direction from the surface of the Fe-based electroplating layer is 0.10 mass % or less.

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 an alloyed galvanized steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if crystal orientations of an Fe-based electroplating layer and a cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. The alloyed galvanized steel sheet has a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % to 3.0 mass %; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface exceeding 20.0 g/m2, and an alloyed galvanized layer formed on the Fe-based electroplating layer, where crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50% at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.

Abstract: Disclosed is a galvannealed steel sheet including: a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet; and a galvannealed layer formed on the Fe-based electroplating layer, in which in an intensity profile measured by glow discharge optical emission spectrometry, ISi,Fe/ISi,bulk is 0.30 or more, and an average value of C concentration in a region ranging from 10 ?m to 20 ?m in the thickness direction from the interface between the galvannealed layer and the Fe-based electroplating layer towards the Fe-based electroplating layer is 0.10 mass % or less.

Abstract: Provided is a galvanized steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if crystal orientations of an Fe-based electroplating layer and a cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. The galvanized steel sheet has a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % to 3.0 mass %; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of more than 20.0 g/m2, and a galvanized layer formed on the Fe-based electroplating layer, where crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50 % at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.

Abstract: Provided is a steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if the crystal orientations of an Fe-based electroplating layer and a Si-containing cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet. Provided is an Fe-based electroplated steel sheet having a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; and an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of more than 20.0 g/m2, where the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50% at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.

Abstract: To provide a steel sheet with excellent resistance to cracking in resistance welding at a welded portion. Disclosed is a galvanized steel sheet including: a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of 5.0 g/m2 or more; and a galvanized layer formed on the Fe-based electroplating layer, in which in an intensity profile measured by glow discharge optical emission spectrometry, ISi,Fe/ISi,bulk is 0.50 or more, and an average value of C concentration in a region ranging from 10 ?m to 20 ?m from an interface between the galvanized layer and the Fe-based electroplating layer towards the Fe-based electroplating layer is 0.10 mass % or less.

Abstract: A projection welded joint is formed by joining a steel sheet having a tensile strength of 980 MPa or higher and a nut by performing projection welding. In a hardness distribution from the steel sheet to the nut through a weld metal zone, and a hardened region exists on each of the steel sheet side and the nut side. A hardness of each hardened region is higher than that of the base metal on the corresponding side, and each hardened region extends continuously for a distance of 500 µm or more. The maximum hardness NHmax on the nut side is higher than the maximum hardness SHmax on the steel sheet side. A region where the maximum hardness SHmax on the steel sheet side exists is formed of a martensite microstructure having a prior austenite grain size of 20 µm or less.

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 hot-pressed member having a specified chemical composition in which Ti/Sb 2.0?Ti/Sb?20.0 is satisfied and a method for producing the hot-pressed member. The hot-pressed member has a microstructure in which a volume fraction of martensite is 95% or more, an average grain size of prior austenite is 7 ?m or less, the number of cementite grains with a grain size of 0.10 ?m or more is 2 grains/?m2 or more, and the number of Ti-based carbide grains with a grain size of 0.10 ?m or less is 0.20 grains/?m2 or more, and a tensile strength of 2100 MPa or higher.

Abstract: Main current passage and subsequent current passage are performed, with a cooling time of 0.01 s or more being provided between the main current passage and the subsequent current passage. Ip/I which is a ratio of a current value of the subsequent current passage to a current value of the main current passage is controlled to satisfy a predetermined relationship depending on a constant A defined by Mn content and P content of a steel sheet as a part to be welded, in relation to a welding time Tp of the subsequent current passage, a cooling time T, and the constant A.

Abstract: The high-strength thin steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, and N, with the balance being Fe and inevitable impurities, and a complex structure containing ferrite, tempered martensite, and bainite, where a volume fraction of a total of tempered martensite and bainite containing five or more carbides with a particle size of 0.1 ?m or more and 1.0 ?m or less in a grain with respect to a total of the tempered martensite and the bainite is 85% or more, and C mass % and Mn mass % in a region of 20 ?m or less in a thickness direction from a surface of the steel sheet are each 20% or less with respect to C mass % and Mn mass % in a region of 100 ?m or more and 200 ?m or less from the surface of the steel sheet.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing of 1780 MPa or more, excellent delayed fracture resistance, and high cross tensile strength after resistance spot welding by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 8 ?m or less, a volume fraction of martensite is 90% or more, and at least 10 cementite grains having a grain size of 0.05 ?m or more are present on average per 200 ?m2 of a cross section parallel to a thickness direction of the member, and such that at least 10 Ti-based precipitates having a grain size of less than 0.10 ?m are present on average per 100 ?m2 of the cross section parallel to the thickness direction of the member in a range of 100 ?m in the thickness direction from a surface of the member.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing, excellent delayed fracture resistance, and high tensile shear stress after resistance spot welding by properly adjusting its chemical composition and its microstructure such that at least 20 Nb-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, a prior austenite average grain size is 8 ?m or less, an average aspect ratio of prior austenite grains is 2.5 or less, and a volume fraction of martensite is 90% or more, and such that a standard deviation of Vickers hardness measured every 200 ?m on a surface of the member is 40 or less.

Abstract: Provided is a hot-pressed member that combines both high strength of 1850 MPa or more in TS and excellent delayed fracture resistance. A hot-pressed member comprises: a predetermined chemical composition; a microstructure in which a prior austenite average grain size is 8 ?m or less, a volume fraction of martensite is 95% or more, and a volume fraction of granular carbide of 0.1 ?m or more in grain size is 0.10% to 4.0%; a Ni diffusion region of 2.0 ?m or more in a depth direction in a surface layer; and a tensile strength of 1850 MPa or more.

Abstract: A hot press member includes excellent indentation peel strength which has a tensile strength of 1780 MPa or more. A plating layer has at a surface thereof a 10-point average roughness Rzjis of 25 ?m or less, and a steel sheet contains, in mass %, not less than 0.25% but less than 0.50% of C, 1.5% or less of Si, 1.1-2.4% of Mn, 0.05% or less of P, 0.005% or less of S, 0.01-0.50% of Al, 0.010% or less of N, 0.001-0.020% of Sb, 0.005-0.15% of Nb, and 0.005-0.15% of Ti, the balance being Fe and incidental impurities. The average crystal grain size of prior austenite is 7 ?m or less and the volume proportion of martensite is 90% or more, within 50 ?m in the thickness direction from the surface of the steel sheet excluding the plating layer.

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.