Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%, and a tensile strength is 1180 MPa or more.

Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%, and a tensile strength is 1180 MPa or more.

This disclosure is related to high yield strength steel where yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation.

This disclosure is related to high yield strength steel where yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation.

An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).

An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).

Provided are a material for hot stamping, wherein the material includes: a steel sheet including carbon (C) in an amount of 0.19 wt % to 0.25 wt %, silicon (Si) in an amount of 0.1 wt % to 0.6 wt %, manganese (Mn) in an amount of 0.8 wt % to 1.6 wt %, phosphorus (P) in an amount less than or equal to 0.03 wt %, sulfur (S) in an amount less than or equal to 0.015 wt %, chromium (Cr) in an amount of 0.1 wt % to 0.6 wt %, boron (B) in an amount of 0.001 wt % to 0.005 wt %, an additive in an amount less than or equal to 0.1 wt %, balance iron (Fe), and other inevitable impurities; and fine precipitates distributed within the steel sheet. The additive includes at least one of titanium (Ti), niobium (Nb), and vanadium (V), and the fine precipitates include nitride or carbide of at least one of titanium (Ti), niobium (Nb), and vanadium (V) and trap hydrogen.

Provided are a material for hot stamping, wherein the material includes: a steel sheet including carbon (C) in an amount of 0.19 wt % to 0.25 wt %, silicon (Si) in an amount of 0.1 wt % to 0.6 wt %, manganese (Mn) in an amount of 0.8 wt % to 1.6 wt %, phosphorus (P) in an amount less than or equal to 0.03 wt %, sulfur (S) in an amount less than or equal to 0.015 wt %, chromium (Cr) in an amount of 0.1 wt % to 0.6 wt %, boron (B) in an amount of 0.001 wt % to 0.005 wt %, an additive in an amount less than or equal to 0.1 wt %, balance iron (Fe), and other inevitable impurities; and fine precipitates distributed within the steel sheet. The additive includes at least one of titanium (Ti), niobium (Nb), and vanadium (V), and the fine precipitates include nitride or carbide of at least one of titanium (Ti), niobium (Nb), and vanadium (V) and trap hydrogen.

The invention relates to a surface-hardened, rotationally symmetrical workpiece, to a hardening method and to a hardening apparatus. The proposed hardening apparatus comprises a machine frame on which two coaxially arranged rotary bearings designed to support a rotationally symmetrical workpiece are arranged, at least one rotary bearing being operatively connected to a drive device to generate rotation of the workpiece; and at lease one laser apparatus for generating focused, high-energy radiation is arranged on said rotary bearing, said laser apparatus being movable in the axial direction, and the radiation being directed toward the workpiece.

The invention relates to a surface-hardened, rotationally symmetrical workpiece, to a hardening method and to a hardening apparatus. The proposed hardening apparatus comprises a machine frame on which two coaxially arranged rotary bearings designed to support a rotationally symmetrical workpiece are arranged, at least one rotary bearing being operatively connected to a drive device to generate rotation of the workpiece; and at lease one laser apparatus for generating focused, high-energy radiation is arranged on said rotary bearing, said laser apparatus being movable in the axial direction, and the radiation being directed toward the workpiece.