A method of forming a shaped steel object is provided. The method includes cutting a blank from an alloy composition including 0.05-0.5 wt. % carbon, 4-12 wt. % manganese, 1-8 wt. % aluminum, 0-0.4 wt. % vanadium, and a remainder balance of iron. The method also includes heating the blank until the blank is austenitized to form a heated blank, transferring the heated blank to a press, forming the heating blank into a predetermined shape to form a stamped object, and decreasing the temperature of the stamped object to a temperature between a martensite start (Ms) temperature of the alloy composition and a martensite final (Mf) temperature of the alloy composition to form a shaped steel object comprising martensite and retained austenite.

Provided is a high-strength steel sheet including: 0.12% to less than 0.17% of carbon (C), 0.3% to 0.8% of silicon (Si), 2.5% to 3.0% of manganese (Mn), 0.4% to 1.1% of chromium (Cr), 0.01% to 0.3% of aluminum (Al), 0.01% to 0.03% of niobium (Nb), 0.01% to 0.03% of titanium (Ti), 0.001% to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S): 0.01% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities. The contents of C, Si, and Al satisfy: [C]+[Si]+[Al])/5≤0.35 wt. A microstructure includes more than 1% to 4% or less of retained austenite, more than 10% to 20% or less of fresh martensite, 5% or less (excluding 0%) of ferrite, more than 50% to 70% or less of tempered martensite, and a balance of bainite.

A high-strength cold-rolled steel sheet having a high yield ratio and excellent stretch flangeability and a method for manufacturing the steel sheet. The high-strength cold-rolled steel sheet has a chemical composition including, by mass %, C: 0.10 to 0.30%, Si: 0.50 to 2.00%, Mn: 2.5 to 4.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, and B: 0.0003 to 0.0030%, with the balance being Fe and incidental impurities. N and Ti satisfy a specified formula, and the total area fraction of martensite and bainite is 95% or more. The number density of bainite grains having an area of 3 μm.sup.2 or more and a carbon concentration of less than 0.7C is 1200 grains/mm.sup.2 or less.

The present invention is to provide a steel plate with physical properties superior to existing steel plates used in fields such as industrial machinery, especially excellent low-temperature impact toughness along with high strength and high hardness, and a method for manufacturing same.

A method for producing a steel strip with a multiphase structure by which the production of complex geometries with a high energy-absorption capacity and high resistance to edge cracking is provided achieving a high yield strength or high yield-strength ratio and a high elongation at break, comprising producing a rolled steel strip of particular elements, and first annealing the steel strip at a temperature of between 750° C. and 950° C., and subsequently first cooling of the steel strip to a temperature of between 200° C. and 500° C. at an average cooling rate of 2 K/s to 150 K/s, further cooling of the steel strip to a supercooling temperature below 100° C. at an average cooling rate of 1 K/s to 50 K/s, final annealing of the steel strip with a Hollomon-Jaffe parameter, and final cooling of the steel strip to room temperature at an average cooling rate of 1 K/s to 160 K/s.

A high-strength steel sheet with a tensile strength of 1,180 MPa or more has a predetermined chemical composition and a steel microstructure in which the area fraction of ferrite is 5% or less, the area fraction of martensite is 2% to 10%, the area fraction of bainite is 5% to 37%, the area fraction of tempered martensite is 42% to 65%, the volume fraction of retained austenite is 3% to 15%, the average grain size of ferrite and bainite is 3 μm or less, in a region extending 50 μm from a surface of the steel sheet in a through-thickness direction, and the average grain size of prior austenite grains is 10 μm or less, the average grain size of the prior austenite grains in the through-thickness direction is 0.9 or less of the average grain size thereof in a rolling direction.

The present invention relates to steel used for a sash component and the like of a vehicle and, more specifically, to a hot-rolled steel sheet for a high-strength electric resistance welded steel pipe having excellent expandability and a method for manufacturing same, the hot-rolled steel sheet having a smaller decrease in the strength of a welding heat-affected zone (HAZ) formed during electric resistance welding, in comparison with a base material.

A steel sheet with a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a member, and a method for producing them. In a region of the steel sheet within 4.9 μm in the thickness direction, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more. The lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 μm from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy the following formula (1):

L.sub.Si+L.sub.Mn≤(T.sub.Si+T.sub.Mn)/4  (1).

This high-strength cold-rolled steel sheet has a predetermined chemical composition, wherein a microstructure at a ¼ position of a sheet thickness from a surface includes predetermined ranges of volume percentages of tempered martensite, residual austenite, ferrite and bainite, and martensite, wherein a microstructure at a position 25 μm from the surface includes, predetermined ranges of volume percentage of ferrite and bainite, and martensite and tempered martensite, wherein in the position 25 μm from the surface, an average grain size of the martensite and the tempered martensite is 5.0 μm or less, a tensile strength is 1,310 MPa or more, and a uniform elongation is 5.0% or more, and R/t is 5.0 or less, the R/t being a ratio of a limit bend radius R in 90° V-bending to a sheet thickness t.

This high-strength cold-rolled steel sheet has a predetermined chemical composition, wherein a microstructure at a ¼ position of a sheet thickness from a surface includes predetermined ranges of volume percentages of tempered martensite, residual austenite, ferrite and bainite, and martensite, wherein a microstructure at a position 25 μm from the surface includes, predetermined ranges of volume percentage of ferrite and bainite, and martensite and tempered martensite, wherein in the position 25 μm from the surface, an average grain size of the martensite and the tempered martensite is 5.0 μm or less, a tensile strength is 1,310 MPa or more, and a uniform elongation is 5.0% or more, and R/t is 5.0 or less, the R/t being a ratio of a limit bend radius R in 90° V-bending to a sheet thickness t.