Abstract: A cold-rolled steel sheet has a chemical composition of C: 0.12% to 0.3%, Si: 0.5% or less, Mn: less than 1.5%, Al: 0.15% or less, N: 0.01% or less, P: 0.02% or less, and S: 0.01% or less, with the remainder including iron and inevitable impurities and has a martensite single-phase structure as its steel microstructure. In a surface region of the steel sheet from the surface to a depth one-tenth the gauge, the number density of n-ary groups of inclusions determined by specific n-th determinations is 120 or less per 100 cm2 of a rolling plane, where the distance in steel sheet rolling direction between outermost surfaces of two outermost particles of the group of inclusions is 100 ?m or more. The steel sheet is a high-strength cold-rolled steel sheet which has a sufficiently minimized rate of bending fracture starting from inclusions and thereby has excellent bending workability.

Abstract: Disclosed is a cold-rolled steel sheet having a specific steel composition and having a composite steel structure including a ferrite structure and a martensite-containing second phase. In a surface region of the steel sheet from the surface to a depth one-tenth the gage, the number density of n-ary groups of inclusions determined by specific n-th determinations is 120 or less per 100 cm2 of a rolling plane, in which the distance in steel sheet rolling direction between outermost surfaces of two outermost particles of the group of inclusions is 80 ?m or more. Also disclosed is a cold-rolled steel sheet having a specific steel composition and having a steel structure of a martensite single-phase structure. In the surface region, the number density of groups of inclusions, in which the distance between the outermost surfaces is 100 ?m or more, is 120 or less per 100 cm2 of a rolling plane.

Abstract: The ultrahigh-strength steel sheet contains, in mass %, C: 0.05-0.25%, Si: 1.00-2.5%, Mn: 2.0-4.0%, P: 0.1% or below (not inclusive of 0%), S: 0.05% or below (not inclusive of 0%), Al: 0.01-0.15%, Ti: 0.003-0.10%, N: 0.01% or below (not inclusive of 0%), the balance comprising iron with inevitable impurities, and is a composite structure steel sheet comprising 10-50 area % ferrite and 50 area % or above martensite, in which the average circle-equivalent grain diameter of ferrite grains is 2.0 ?m or below, the average aspect ratio of ferrite grains is 2.0 or below, and the tensile strength is 1,100 MPa or above. Accordingly, the ultrahigh-strength steel sheet is excellent in hydrogen embrittlement resistance and workability.

Abstract: Disclosed is a hot-dip galvanized steel sheet, in which a steel sheet of the hot-dip galvanized steel sheet before galvanization contains, on the percent by mass basis, 0.06% to 0.15% carbon (C); 1% to 3% manganese (Mn); 0.01% to 0.05% phosphorus (P); 0.03% to 1% chromium (Cr); 0.03% to 1% molybdenum (Mo); and 0.02% to 0.15% aluminum (Al). The steel sheet before galvanization has a silicon (Si) content of 0.2% or less (inclusive of 0%) and a sulfur (S) content of 0.03% or less (inclusive of 0%) and has a K value of ?2.0 or more, in which the K value is calculated according to following Equation (1): K value=3×[Mo]?5×[Cr]??(1) wherein “[Element]” represents the content (percent by mass) of the element.

Abstract: Disclosed is a high-strength electric resistance welded steel pipe which is manufacture from a steel sheet containing, on mass basis, 0.2% to 0.4% C, 0.05% to 0.5% Si, 0.5% to 2.5% Mn, 0.025% or less P, 0.01% or less S, 0.01% to 0.15% or Al, 0.01% to 2% Cu, 0.05% to 2% Cr, 0.005% to 0.2% Ti, and 0.0002% to 0.005% B, with the remainder being iron and inevitable impurities. The steel pipe has a tensile strength of 1750 N/mm2 or more, a 0.1%-proof stress of 1320 N/mm2 or more, and a Charpy impact value at a testing temperature of minus 40° C. of 50 J/cm2 or more.

Abstract: A high-strength hot-dip galvanized steel sheet is provided which comprises a composite structure consisting essentially of ferrite and martensite. The steel comprises, by mass %, C: 0.05 to 0.12%, Si: not more than 0.05%, Mn: 2.7 to 3.5%, Cr: 0.2 to 0.5%, Mo: 0.2 to 0.5%, Al: not more than 0.10%, P: not more than 0.03%, and S: not more than 0.03%. The high-strength hot-dip galvanized steel sheet has not only excellent spot weldability, but also excellent “stability of material properties”, including tensile strength, total elongation, and yield strength, in a high range of strengths from 780 to 1180 MPa, even if the manufacturing condition (especially, the condition of the cooling process after annealing the steel sheet) is changed.

Abstract: A high-strength hot-dip galvanized steel sheet is provided which comprises a composite structure consisting essentially of ferrite and martensite. The steel comprises, by mass %, C: 0.05 to 0.12%, Si: not more than 0.05%, Mn: 2.7 to 3.5%, Cr: 0.2 to 0.5%, Mo: 0.2 to 0.5%, Al: not more than 0.10%, P: not more than 0.03%, and S: not more than 0.03%. The high-strength hot-dip galvanized steel sheet has not only excellent spot weldability, but also excellent “stability of material properties”, including tensile strength, total elongation, and yield strength, in a high range of strengths from 780 to 1180 MPa, even if the manufacturing condition (especially, the condition of the cooling process after annealing the steel sheet) is changed.

Abstract: A superhigh-strength dual-phase steel sheet containing ferritic microstructure and a martensitic microstructure—containing composite-phase steel sheet containing: C: 0.08-0.20% (mass% here and hereinafter), Si: 0.5% or less (inclusive of 0%) Mn: 3.0% or less (exclusive of 0%) P: 0.02% or less (inclusive of 0%) S: 0.02% or less (inclusive of 0%), and Al: 0.001-0.15%, and further containing Mo: 0.05-1.5%, and Cr: 0.05-1.5%, and which satisfying that: the average Vickers hardness of the ferritic microstructure is 150 Hv or more and the average Vickers hardness of the martensitic microstructure is 500 Hv or more, the superhigh-strength dual-phase steel sheets being of excellent fatigue characteristic in a spot welded joint.

Abstract: A superhigh-strength dual-phase steel sheet containing ferritic microstructure and a martensitic microstructure—containing composite-phase steel sheet containing: