Abstract: In one aspect of the present invention, a hot-dip galvannealed steel sheet includes a steel sheet and a hot-dip galvannealed layer on the surface of the steel sheet. The steel sheet has a predetermined composition and has an average oxygen concentration of 0.10 mass % or less in the region of 1 ?m from the interface between the steel sheet and the hot-dip galvannealed layer toward the steel sheet. The metal microstructure of the steel sheet at a position of t/4 where t represents the sheet thickness of the hot-dip galvannealed steel sheet includes 50 to 85 area % of martensite, 15 to 50 area % of bainite, and 5 area % or less of ferrite.

Abstract: In one aspect of the present invention, a hot-dip galvannealed steel sheet includes a steel sheet and a hot-dip galvannealed layer on the surface of the steel sheet. The steel sheet has a predetermined composition and has an average oxygen concentration of 0.10 mass % or less in the region of 1 ?m from the interface between the steel sheet and the hot-dip galvannealed layer toward the steel sheet. The metal microstructure of the steel sheet at a position of t/4 where t represents the sheet thickness of the hot-dip galvannealed steel sheet includes 50 to 85 area % of martensite, 15 to 50 area % of bainite, and 5 area % or less of ferrite.

Abstract: Provided is a method for manufacturing a hot press formed steel member that has high strengths, an excellent balance between strength and ductility, and good deformation properties upon crush on collision (crashworthiness). The manufacturing method is highly efficient and allows a high degree of freedom with respect to the shape to be formed. The method manufactures a steel member by heating a steel sheet having a specific chemical composition and subjecting the steel sheet to at least one time of hot press forming. In the method, the heating temperature is equal to or higher than the Ac3 transformation temperature, and a starting temperature of the hot press forming is in the range from the heating temperature to martensite start (Ms) temperature. Cooling from [(Ms temperature)?150° C.] down to 80° C. is performed so that a tempering parameter (?) specified by Expression (1) is in the range from 7100 to 8030.

Abstract: Provided is a method for manufacturing a hot press formed steel member that has high strengths, an excellent balance between strength and ductility, and good deformation properties upon crush on collision (crashworthiness). The manufacturing method is highly efficient and allows a high degree of freedom with respect to the shape to be formed. The method manufactures a steel member by heating a steel sheet having a specific chemical composition and subjecting the steel sheet to at least one time of hot press forming. In the method, the heating temperature is equal to or higher than the Ac3 transformation temperature, and a starting temperature of the hot press forming is in the range from the heating temperature to martensite start (Ms) temperature. Cooling from [(Ms temperature)?150° C.] down to 80° C. is performed so that a tempering parameter (?) specified by Expression (1) is in the range from 7100 to 8030.

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: 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: The present invention provides a high strength steel sheet with 780 MPa class tensile strength excellent in bending workability and fatigue strength. The high strength steel sheet is (1) a steel sheet whose steel composition contains: C: 0.05-0.20%; Si: 0.6-2.0%; Mn: 1.6-3.0%; P: 0.05% or below; S: 0.01% or below; Al: 0.1% or below; and N: 0.01% or below, the balance comprising iron and inevitable impurities, in which (2) a microstructure comprises a polygonal ferrite structure and a structure formed by low-temperature transformation, in which, when a sheet plane located at a depth of 0.1 mm from a surface of the steel sheet is in the observation under a scanning electron microscope with respect to twenty sights in total in different positions in the sheet-width direction, the maximum value of the areal proportion of the polygonal ferrite (Fmax) and the minimum value of the areal proportion of the ferrite (Fmin) in a 50 ?m×50 ?m area in each sight satisfy Fmax?80%, Fmin?10%, and Fmax?Fmin?40%.

Abstract: The present invention provides a high-strength steel material capable of suppressing formation of blowholes during welding even if the strength thereof is improved by nitriding, and a method of manufacturing such a high-strength steel material by using a low rolling force. A high-strength steel of the present invention contains 0.05% (percent by mass unless otherwise specified in describing chemical composition) or below C, 1% or below Si, 1.5% or below Mn, 0.05% or below P, 0.05% or below S, 0.05% or below Al, 0.02 to 0.3% Ti, and 0.020% or below N. The high-strength steel material has metallographic structure of a single phase of ferrite and contains Ti nitride grains having a maximum size of 20 nm or below and coherently precipitated in a density of 250 grains/?m2 or above. In the high-strength steel material, the ratio of the number of Ti nitride grains having a maximum size of 6 nm or below to that of Ti nitride grains having a maximum size of 20 nm or below is 80% or above.

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 present invention provides a high strength steel sheet with 780 MPa class tensile strength excellent in bending workability and fatigue strength. The high strength steel sheet is (1) a steel sheet whose steel composition contains: C: 0.05-0.20%; Si: 0.6-2.0%; Mn: 1.6-3.0%; P: 0.05% or below; S: 0.01% or below; Al: 0.1% or below; and N: 0.01% or below, the balance comprising iron and inevitable impurities, in which (2) a microstructure comprises a polygonal ferrite structure and a structure formed by low-temperature transformation, in which, when a sheet plane located at a depth of 0.1 mm from a surface of the steel sheet is in the observation under a scanning electron microscope with respect to twenty sights in total in different positions in the sheet-width direction, the maximum value of the areal proportion of the polygonal ferrite (Fmax) and the minimum value of the areal proportion of the ferrite (Fmin) in a 50 ?m×50 ?m area in each sight satisfy Fmax?80%, Fmin?10%, and Fmax?Fmin?40%.

Abstract: The present invention provides a high-strength steel material capable of suppressing formation of blowholes during welding even if the strength thereof is improved by nitriding, and a method of manufacturing such a high-strength steel material by using a low rolling force. A high-strength steel of the present invention contains 0.05% (percent by mass unless otherwise specified in describing chemical composition) or below C, 1% or below Si, 1.5% or below Mn, 0.05% or below P, 0.05% or below S, 0.05% or below Al, 0.02 to 0.3% Ti, and 0.020% or below N. The high-strength steel material has metallographic structure of a single phase of ferrite and contains Ti nitride grains having a maximum size of 20 nm or below and coherently precipitated in a density of 250 grains/?m2 or above. In the high-strength steel material, the ratio of the number of Ti nitride grains having a maximum size of 6 nm or below to that of Ti nitride grains having a maximum size of 20 nm or below is 80% or above.