Abstract: An automobile undercarriage part of the present invention is an automobile undercarriage part including a welded joint in which a first steel sheet and a second steel sheet are overlapped and a fillet weld is formed between an end surface of the first steel sheet and a surface of the second steel sheet, in which a chemical composition of a weld metal that forms the welded joint contains, with respect to a total mass of the weld metal, by mass %, C: 0.02% to 0.20%, Si: more than 0% to less than 0.10%, Mn: 0.3% to 2.0%, Al: 0.002% to 0.30%, Ti: 0.005% to 0.30%, P: more than 0% to 0.015%, and S: more than 0% to 0.030%, and the following formula (1) and formula (2) are satisfied. [Al]+[Ti]>0.05 ??Formula (1) 7×[Mn]?112×[Ti]?30×[Al]?4.

Abstract: A high-strength steel sheet includes a predetermined chemical composition, a microstructure contains, by volume percentage, 95% or more of tempered martensite and bainite in total, and a remainder consists of ferrite and pearlite, the microstructure contains 5.0×109 pieces/mm or more of, per unit volume, precipitate having an equivalent circle diameter of 5.0 nm or less and containing Ti, Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20 ?m from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the surface is 0.85 or more, a tensile strength is 980 MPa or more, and a product of the tensile strength and elongation is 12,000 MPa×% or more.

Abstract: An automobile undercarriage part of the present invention has a welded joint formed by base steel plate, wherein the chemical composition of a weld metal contains, with respect to a total mass of the weld metal, by mass %, C: 0.02% to 0.30%, Si: 0.10% to less than 1.0%, Mn: 1.2% to 3.0%, Al: 0.002% to 0.30%, Ti: 0.005% to 0.30%, P: more than 0% to 0.015%, and S: more than 0% to 0.030%, the following formula (1A), formula (1B), formula (2), and formula (3) are satisfied, and slag in a toe portion of the fillet weld satisfies a formula (4). [Al]+[Ti]>0.05??Formula (1A) [Ti]/[Al]>0.9??Formula(1B) 7×[Si]+7×[Mn]?112×[Ti]?30×[Al]?12??Formula (2) 2.0<[Si]+[Mn]??Formula (3) [Ti content on slag surface]>[Si content on slag surface]??Formula (4).

Abstract: A high-strength steel sheet includes a predetermined chemical composition, a microstructure contains, by volume percentage, 80% or more of tempered martensite, and a remainder consists of ferrite and bainite, the microstructure contains 5.0×1011 pieces/mm3 or more of, per unit volume, precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less, Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20 ?m from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the surface is 0.85 or more, and the tensile strength is 1,180 MPa or more.

Abstract: A steel sheet includes, as a chemical composition, by mass %: C: 0.0015% to 0.0400%; Mn: 0.20% to 1.50%; P: 0.010% to 0.100%; Cr: 0.001% to 0.500%; Si: 0.200% or less; S: 0.020% or less; sol. Al: 0.200% or less; N: 0.0150% or less; Mo: 0% to 0.500%; B: 0% to 0.0100%; Nb: 0% to 0.200%; Ti: 0% to 0.200%; Ni: 0% to 0.200%; Cu: 0% to 0.100%; and a remainder including iron and impurities, in which a metallographic structure in a surface layer region includes ferrite having a volume fraction of 90% or more, and in the surface layer region, an average grain size of the ferrite is 1.0 ?m to 15.0 ?m, and a texture in which an XODF{001}/{111}, S as a ratio of an intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.30 or more and less than 3.50 is included.

Abstract: A steel sheet has a predetermined chemical composition, in which a metallographic structure in a surface layer region ranging from a surface to a position of 20 ?m from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%, the metallographic structure in an internal region ranging from a position of more than 20 ?m from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%, the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, and in the surface layer region, the average grain size of the secondary phase is 0.5 ?m to 4.0 ?m, and a texture in which an XODF{001}/{111} as the ratio of the intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.70 to 2.

Abstract: There is provided a hot-rolled steel sheet in which a composition contains: in mass %, C: 0.01% to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; P: 0.10% or less; S: 0.03% or less; Al: 0.001% to 2.0%; N: 0.01% or less; O: 0.01% or less; Ti: 0.01 to 0.30%; and the balance being composed of iron and impurities and a structure is composed of by volume fraction, 90% or more of tempered martensite with an average aspect ratio of 2 or less, or 90% or more in total of both tempered martensite and lower bainite.

Abstract: A heat-treated steel sheet member having a chemical including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn: 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: optional amounts of Ni, Cu, Mo, V, Ca, Al, Nb, and REM, with the balance: Fe and impurities. The steel sheet member has a microstructure comprising: mainly martensite; and retained austenite of which a volume ratio is 5.0% or higher, a number density of retained carbide in the steel sheet member having circle-equivalent diameters of 0.1 mm or larger is 4.0×103/mm2 or lower, [(log fg0?log fg(0.02))/0.02<20.0] is satisfied when mechanical properties are measured using a sheet specimen specified in ASTM E8, a tensile strength is 1.4 GPa or higher, and a total elongation is 8.0% or higher.

Abstract: When a zone surrounded by a grain boundary that is measured to be 5.0° or more by an EBSD analysis is assumed to be a grain, and when a K value is a value obtained by multiplying an average value of Image Qualities in a grain by 10?3, a Y value is an average crystal misorientation (°) in the grain, a metallic phase 1 is a metallic phase the K value of which is less than 4.000, a metallic phase 2 is a metallic phase the K value of which is 4.000 or more and the Y value of which is 0.5 to 1.0, a metallic phase 3 is a metallic phase the K value of which is 4.000 or more and the Y value of which is less than 0.5, and a metallic phase 4 is a metallic phase that falls under none of metallic phases 1 to 3, there is provided a steel sheet that has a predetermined chemical composition and includes a microstructure including, in area percent, a metallic phase 1: 1.0% or more and less than 35.0%, a metallic phase 2: 30.0% or more and 80.0% or less, a metallic phase 3: 5.0% or more and 50.

Abstract: This wire for gas-shielded arc welding is a wire for joining a plurality of thin steel sheets by gas-shielded arc welding, the wire including, in mass %, with respect to a total mass of the wire: C: 0.06 to 0.15%; Si: more than 0 to 0.18%; Mn: 0.3 to 2.2%; Ti: 0.06 to 0.30%; Al: 0.001 to 0.30%; and B: 0.0030 to 0.0100%, in which Si, Mn, Ti, and Al satisfy Expressions (1) and (2). Si×Mn?0.30??Expression (1) (Si+Mn/5)/(Ti+Al)?3.

Abstract: This high-strength cold-rolled steel sheet has a predetermined chemical composition, a microstructure at a ¼ position of a sheet thickness from a surface includes, by a volume percentage, 70.0% or more of tempered martensite, more than 3.0% and less than 10.0% of residual austenite, a total of 25.0% or less of ferrite and bainite, and 5.0% or less of martensite, a microstructure at a position 25 ?m from the surface includes, by the volume percentage, a total of 70% or more of ferrite and bainite and a total of 30% or less of martensite and tempered martensite, in the position 25 ?m from the surface, an average grain size of the martensite and the tempered martensite is 5.0 ?m or less, a tensile strength is 1,310 MPa or more, a uniform elongation is 5.0% or more, and Rh is 5.0 or less, the Rh/t being a ratio of a limit bend radius R in 90° V-bending to a sheet thickness t.

Abstract: In a cold-rolled steel sheet having a predetermined chemical composition, a metallographic structure contains 40.0% or more and less than 60.0% of a polygonal ferrite, 30.0% or more of a bainitic ferrite, 10.0% to 25.0% of a residual austenite, and 15.0% or less of a martensite, by an area ratio, in the residual austenite, a proportion of the residual austenite in which an aspect ratio is 2.0 or less, a length of a long axis is 1.0 ?m or less, and a length of a short axis is 1.0 ?m or less, is 80.0% or more, in the bainitic ferrite, a proportion of the bainitic ferrite in which an aspect ratio is 1.7 or less and an average value of a crystal orientation difference in a region surrounded by a boundary in which a crystal orientation difference is 15° or more is 0.5° or more and less than 3.0°, is 80.0% or more, and a connection index D value of the martensite, the bainitic ferrite, and the residual austenite is 0.70 or less.

Abstract: A steel sheet for heat treatment having a chemical composition including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn; 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: 0 to 1.0%; Ni: 0 to 2.0%; Cu: 0 to 1.0%; Mo: 0 to 1.0%; V: 0 to 1.0%; Ca: 0 to 0.01%; Al: 0 to 1.0%; Nb: 0 to 1.0%; REM: 0 to 0.1%; and the balance: Fe and impurities, wherein a maximum height roughness Rz on a surface of the steel sheet is 3.0 to 10.0 ?m, and a number density of carbide being present in the steel sheet and having circle-equivalent diameters of 0.1 ?m or larger is 8.0×103/mm2 or lower.

Abstract: A steel sheet according to an aspect of the present invention has predetermined chemical composition, in which a structure at a thickness ¼ portion includes, in terms of volume ratios, tempered martensite: 30% to 70% and one or both of ferrite and bainite: a total of 20% to 70%, in the structure at the thickness ¼ portion, a volume ratio of residual austenite is less than 10%, a volume ratio of fresh martensite is 10% or less, a volume ratio of pearlite is 10% or less, and a total volume ratio of the residual austenite, the fresh martensite, and the pearlite is 15% or less, a number density of iron-based carbides having a major axis of 5 nm or more in the tempered martensite at the thickness ¼ portion is 5×107 particles/mm2 or more, a ratio of the number of ?-type carbides in the number of the iron-based carbides having the major axis of 5 nm or more at the thickness ¼ portion is 20% or more, and a tensile strength is 780 MPa or higher.

Abstract: A steel sheet according to an aspect of the present invention has predetermined chemical composition, in which a structure at a thickness ¼ portion includes, in terms of volume ratios, tempered martensite: 70% or more and one or both of ferrite and bainite: a total of less than 20%, in the structure at the thickness ¼ portion, a volume ratio of residual austenite is less than 10%, a volume ratio of fresh martensite is 10% or less, a volume ratio of pearlite is 10% or less, and a total volume ratio of the residual austenite, the fresh martensite, and the pearlite is 15% or less, a number density of iron-based carbides having a major axis of 5 nm or more in the tempered martensite at the thickness ¼ portion is 5×107 particles/mm2 or more, a ratio of the number of ?-type carbides in the number of the iron-based carbides having the major axis of 5 nm or more at the thickness ¼ portion is 20% or more, and a tensile strength is 780 MPa or higher.

Abstract: A steel sheet includes a predetermined chemical composition, and includes a steel structure represented by, in a volume fraction, tempered martensite and bainite: 70% or more and less than 92% in total, retained austenite: 8% or more and less than 30%, ferrite: less than 10%, fresh martensite: less than 10%, and pearlite: less than 10%. A number density of iron-base carbides in tempered martensite and lower bainite is 1.0×106 (pieces/mm2) or more, and an effective crystal grain diameter of tempered martensite and bainite is 5 ?m or less.

Abstract: A hot-dip galvanized steel sheet includes: a predetermined chemical composition; and a steel structure represented by: in terms of area ratio, polygonal ferrite: 10% or less; upper bainite: 20% or less; retained austenite: 5% or less; and martensite: 70% or more, in which: martensite having Fe carbides at a number density of 1×106/mm2 or more is contained by 50% or more, in terms of area ratio, with respect to the entire amount of martensite; and the steel structure has an average effective crystal grain diameter of 5.0 ?m or less.

Abstract: A heat-treated steel sheet member having a composition including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn: 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: 0 to 1.0%; Ni: 0 to 2.0%; Cu; 0 to 1.0%; Mo: 0 to 1.0%; V: 0 to 1.0%; Ca: 0 to 0.01%; Al: 0 to 1.0%; Nb: 0 to 1.0%; REM: 0 to 0.1%; and the balance: Fe and impurities. The steel sheet member has a microstructure comprising mainly martensite and retained austenite of which a volume ratio is 0.2 to 1.0%, a number density of retained carbide in the steel sheet member having circle-equivalent diameters of 0.1 mm or larger is 4.0×103/mm2 or lower, a tensile strength is 1.4 GPa or higher, and a yield ratio is 0.65 or higher.

Abstract: A high-strength cold-rolled steel sheet includes, as a component composition, by mass %: C: 0.075% to 0.300%; Si: 0.30% to 2.50%; Mn: 1.30% to 3.50%; P: 0.001% to 0.050%; S: 0.0001% to 0.0100%; Al: 0.001% to 1.500%; and N: 0.0001% to 0.0100%, in which a surface microstructure contains residual austenite of 3% to 10% and ferrite of 90% or less by volume fraction, an inner microstructure at a depth of t/4 from the surface assuming that a sheet thickness is t contains residual austenite of 3% to 20% by volume fraction, a ratio Hvs/Hvb between a surface hardness Hvs of the steel sheet surface and a hardness Hvb at a depth of ¼ of the thickness is more than 0.75 to 0.90, and a maximum tensile strength is 700 MPa or more.

Abstract: When a zone surrounded by a grain boundary that is measured to be 5.0° or more by an EBSD analysis is assumed to be a grain, and when a K value is a value obtained by multiplying an average value of Image Qualities in a grain by 10?3, a Y value is an average crystal misorientation (°) in the grain, a metallic phase 1 is a metallic phase the K value of which is less than 4.000, a metallic phase 2 is a metallic phase the K value of which is 4.000 or more and the Y value of which is 0.5 to 1.0, a metallic phase 3 is a metallic phase the K value of which is 4.000 or more and the Y value of which is less than 0.5, and a metallic phase 4 is a metallic phase that falls under none of metallic phases 1 to 3, there is provided a steel sheet that has a predetermined chemical composition and includes a microstructure including, in area percent, a metallic phase 1: 1.0% or more and less than 35.0%, a metallic phase 2: 30.0% or more and 80.0% or less, a metallic phase 3: 5.0% or more and 50.