Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

High strength steel sheet and method for manufacturing the same

Disclosed are an ultra-high-strength steel sheet having both a tensile strength of as high as 1400MPa or above and excellent formability and an advantageous process for manufacturing the same. A high-strength steel sheet having both a composition which contains by mass C: 0.12 to 0.50%, Si: 2.0% or less, Mn: 1.0 to 5.0%, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less with the balance being Fe and unavoidable impurities, and a structure which comprises, in terms of area fraction, autotempered martensite: 80% or more, ferrite: less than 5%, bainite: 10% or less, and retained austenite: 5% or less and in which the average number of precipitated iron carbide particles of 5nm to 0.5[mu]m in the autotempered martensite is 5OE04 or above per mm2.

High-strength steel sheet and process for production thereof

A high strength pressed member has excellent ductility and stretch flangeability and tensile strength of 780-1400 MPa, with a predetermined steel composition and steel microstructure relative to the entire microstructure of steel sheet, where area ratio of martensite 5-70%, area ratio of retained austenite 5-40%, area ratio of bainitic ferrite in upper bainite 5% or more, and total thereof is 40% or more, 25% or more of martensite is tempered martensite, polygonal ferrite area ratio is above 10% and below 50% to the entire microstructure of steel sheet, and average grain size is 8 μm or less, average diameter of a group of polygonal ferrite grains is 15 μm or less, the group of polygonal ferrite grains represented by a group of ferrite grains of adjacent polygonal ferrite grains, and average carbon content in retained austenite is 0.70 mass % or more and tensile strength is 780 MPa or more.

High-strength steel sheet and method for manufacturing same

The following sheet and method are provided: a high-strength galvanized steel sheet having excellent mechanical properties such as a TS of 1200 MPa or more, an El of 13% or more, and a hole expansion ratio of 50% or more and a method for manufacturing the same. A high-strength galvanized steel sheet excellent in formability contains 0.05% to 0.5% C, 0.01% to 2.5% Si, 0.5% to 3.5% Mn, 0.003% to 0.100% P, 0.02% or less S, and 0.010% to 0.5% Al on a mass basis, the remainder being Fe and unavoidable impurities, and has a microstructure which contains 0% to 10% ferrite, 0% to 10% martensite, and 60% to 95% tempered martensite on an area basis as determined by structure observation and which further contains 5% to 20% retained austenite as determined by X-ray diffractometry.

High-strength hot-dip galvanized steel sheet with excellent processability and process for producing the same

Provided are a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent in terms of bending workability and a method for manufacturing the steel sheet. A high-strength hot-rolled steel sheet having a tensile strength TS of 980 MPa or more is manufactured by controlling the steel sheet to have a chemical composition containing C: more than 0.1% and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.15% or less, V: more than 0.10% and 0.30% or less, and the balance being Fe and inevitable impurities. Surface regions have a microstructure including mainly a ferrite phase, and an inner region has a microstructure including mainly a bainite phase. The proportions of the surface regions in the thickness direction of the steel sheet are 1.0% or more and 5.0% or less of the whole thickness respectively from the upper and lower surfaces of the steel sheet.

High strength hot rolled steel sheet and method for producing same

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet reduced in dispersion in mechanical properties and its manufacturing method. SOLUTION: The high strength cold rolled steel sheet has a composition which consists of 0.06 to 0.15% C, 0.5 to 1.5% Si, 1.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, 0.5 to 1.5% Al and the balance Fe with inevitable impurities and in which the value of A, defined by A=Si+9×Al, satisfies 6.0≤A≤20.0. Moreover, the steel sheet has a structure which is composed of a dual-phase structure of ferrite and martensite and in which the area ratio of the ferrite is 40 to 90%. When manufacturing this steel sheet, recrystallization annealing/tempering treatment is carried out by carrying out: holding at a temperature ranging from the Ac 1 point and the Ac 3 point for ≥10 sec; cooling down to 500 to 750°C at ≤20°C/sec cooling rate; rapid cooling down to ≤100°C at ≥100°C/sec cooling rate; and tempering at 300 to 500°C. COPYRIGHT: (C)2007,JPO&INPIT

High strength cold rolled steel sheet reduced in dispersion in mechanical characteristic, and its manufacturing method

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet in which TS is 690 to 980 MPa, El is ≥27%, λ is ≥50%, and also, the variation of the TS in the steel sheet ΔTS stably reaches ≤15 MPa, and to provide a method for producing the same. SOLUTION: The high strength hot rolled steel sheet has a composition comprising, by mass, 0.060 to 0.150% C, ≤0.1% Si, 0.8 to 1.8% Mn, ≤0.030% P, ≤0.005% S, 0.005 to 0.1% Al, ≤0.005% N and 0.032 to 0.120% Ti, and the balance Fe with inevitable impurities, and in which the C, Ti and N are prescribed by numerical equations, and has a microstructure composed of a ferrite (F) phase and a second phase including a martensite (M) phase, and in which the area ratio of the F phase is 65 to 80%, the total area ratio of the F phase and the M phase is ≥95%, the variation in the area ratio of the F phase ΔSF is ≤2%, and the Vickers hardness difference ΔHv between the F phase and the second phase is ≤250. COPYRIGHT: (C)2011,JPO&INPIT

High strength hot rolled steel sheet and method for producing the same

PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet having a high tensile strength of TS≥980 MPa and being excellent in workability and weldability. SOLUTION: The high-strength hot-dip galvanized steel sheet has a composition comprising not less than 0.05% and less than 0.12% C, not less than 0.01% and less than 0.35% Si, 2.0-3.5% Mn, 0.001-0.020% P, 0.0001-0.0030% S, 0.005-0.1% Al, 0.0001-0.0060% N, more than 0.5% and 2.0% or less Cr, 0.01-0.50% Mo, 0.010-0.080% Ti, 0.010-0.080% Nb, and 0.0001-0.0030% B with the balance being Fe and unavoidable impurities; has a structure containing a ferrite phase having a volume fraction of 20-70% and an average crystal grain diameter of 5 μm or less; and is coated with a hot-dip galvanized layer of 20-150 g/m 2 coating weight (per one surface) on the steel sheet surface. COPYRIGHT: (C)2009,JPO&INPIT

High-strength hot-dip galvanized steel sheet excellent in workability and weldability, and its manufacturing method

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet having excellent formability for an extension flange while retaining characteristics equivalent to those of a conventional DP steel; also to provide a hot-dip galvanized steel sheet having the same formability; and a method for manufacturing those SOLUTION: The high-strength cold-rolled steel sheet comprises 003-015% C, 14-35% Mn, 005% P or less, 001% S or less, 015% Al or less, 001% N or less, 0005-005% Ti, 0005-004% Nb, 00003-00020% B, and the balance Fe with unavoidable impurities; and has a structure that contains a ferrite phase and a low-temperature transforming austenite phase in which the area ratio A ML /A M is 030 or higher, where the A ML is an area of the low-temperature transforming austenite phase having a grain size 15 times larger than the average crystal grain size d α of the ferrite phase, and the A M is the total area of the low-temperature transforming austenite phase The hot-dip galvanized steel sheet has the same composition and structure COPYRIGHT: (C)2007,JPO&INPIT

High-strength cold-rolled steel sheet having excellent formability for extension flange, hot-dip galvanized steel sheet having the same formability, and method for manufacturing those

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet having tensile strength (TS) of 1,180 MPa or more and exhibiting improved elongation, stretch flangeability and bendability by adjusting the metal structure in a component system which does not contain any expensive alloying element such as Nb, V, Cu, Ni, Cr and Mo.SOLUTION: A high-strength cold-rolled steel sheet has a componential composition including, by mass, 0.12 to 0.22% C, 0.8 to 1.8% Si, 2.2 to 3.2% Mn, ≤0.020% P, ≤0.0040% S, 0.005 to 0.08% Al, ≤0.008% N, 0.001 to 0.040% Ti and 0.0001 to 0.0020% B, and the balance Fe with inevitable impurities and a structure including, in volume fraction, 40 to 60% of ferrite phases, 10 to 30% of bainite phases, 20 to 40% of tempered martensite phases, and 5 to 20% of retained austenite phases and in which tempered martensite phases having major-axis lengths of 5 μm or less account for 80 to 100% of the total volume fraction of the tempered martensite phases.

英尚川邉

Papers

High strength cold rolled steel sheet reduced in dispersion in mechanical characteristic, and its manufacturing method

High strength hot rolled steel sheet and method for producing the same

High-strength hot-dip galvanized steel sheet excellent in workability and weldability, and its manufacturing method

High-strength cold-rolled steel sheet having excellent formability for extension flange, hot-dip galvanized steel sheet having the same formability, and method for manufacturing those

High-strength cold-rolled steel sheet having excellent formability and method for manufacturing the same

英尚 川邉

Papers

High strength cold rolled steel sheet reduced in dispersion in mechanical characteristic, and its manufacturing method

High strength hot rolled steel sheet and method for producing the same

High-strength hot-dip galvanized steel sheet excellent in workability and weldability, and its manufacturing method

High-strength cold-rolled steel sheet having excellent formability for extension flange, hot-dip galvanized steel sheet having the same formability, and method for manufacturing those

High-strength cold-rolled steel sheet having excellent formability and method for manufacturing the same

英尚川邉