A method for manufacturing a steel product, comprising the steps of: providing a heated steel starting product at a temperature comprised between 380° C. and 700° C., having a metastable fully austenitic structure, with a composition comprising, in percent by weight: 0.15%≤C≤0.40%, 1.5%≤Mn≤4.0%, 0.5%≤Si≤2.5%, 0.005%≤Al≤1.5%, with 0.8%≤Si+Al≤2.5%, S≤0.05%, P≤0.1%, at least one element chosen among Cr and Mo, such that: 0%≤Cr≤4.0%, 0%≤Mo≤0.5%, and 2.7%≤Mn+Cr+3 Mo≤5.7%, and optionally one or several elements chosen among: Nb≤0.1%, Ti≤0.1%, Ni≤3.0%, 0.0005%≤B≤0.005%, 0.0005%≤Ca≤0.005%, the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting; subjecting said starting product to a hot forming step at a temperature comprised between 700° C. and 380° C., with a cumulated strain ε.sub.b between 0.1 and 0.7, in at least one location of said heated steel starting product, to obtain a fully austenitic hot-formed steel product; then quenching the hot-formed steel product by cooling it down, at a cooling rate VR.sub.2 superior to the critical martensitic cooling rate, to a quenching temperature QT lower than Ms in order to obtain a structure containing between 40% and 90% of martensite, the rest of the structure being austenite; then maintaining at, or reheating the product up to a holding temperature PT between QT and 470° C. and holding it at said temperature PT for a duration Pt between 5 s and 600 s.

The invention relates to a method for producing a sheet steel component by means of a press hardening or form hardening process, the sheet steel component being produced by virtue of the fact that a sheet bar composed of at least one region made of a highly hardenable carbon/manganese/boron steel and at least one dual-phase steel is cold-formed, then heated, and then quenched in a cooling press or a sheet bar composed of at least one region made of a highly hardenable carbon/manganese/boron steel and at least one region made of a dual-phase steel is heated to a temperature above the austenitization temperature of the highly hardenable steel material and is then formed into the sheet steel component in a single stroke or in a plurality of strokes in a forming and cooling press, wherein as a softer material and as a partner for the highly hardenable carbon/manganese/boron steel, a dual-phase steel is used, whose Ac3 value is increased until at the required annealing temperatures, with the austenitization of the carbon/manganese/boron steel, only a partial austenitization of the dual-phase steel takes place so that when loaded into the cooling press, the dual-phase steel has a ferritic matrix, and in addition to this, austenite is present.

Provided is a steel sheet which is suitably used for an automobile structural member, etc., and more specifically, to: a steel sheet having excellent uniform elongation and strain hardening rate, while having high strength; and a method for producing same.

A high-strength steel sheet having a TS of 780 MPa or more, excellent stretch flangeability, and excellent in-plane anisotropy of TS is provided. A high-strength steel sheet comprises: a predetermined chemical composition; a steel microstructure including, in area fraction, ferrite: 20% or more and 50% or less, lower bainite: 5% or more and 40% or less, martensite: 1% or more and 20% or less, and tempered martensite: 20% or less, and including, in volume fraction, retained austenite: 5% or more, the retained austenite having an average grain size of 2 μm or less; and a texture having an inverse intensity ratio of γ-fiber to α-fiber of 3.0 or less.

High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 μM and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of more than 0.60 time and 0.90 time or less the average Vickers hardness of the sheet thickness 1/2 position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.

A method for manufacturing a hot-stamped part includes reheating a steel slab at a temperature of 1,200° C. to 1,250° C., the steel slab including, by wt %, 0.20 to 0.50% carbon (C), 0.05 to 1.00% silicon (Si), 0.10 to 2.50% manganese (Mn), more than 0% and not more than 0.015% phosphorus (P), more than 0% and not more than 0.005% sulfur (S), 0.05 to 1.00% chromium (Cr), 0.001 to 0.009% boron (B), 0.01 to 0.09% titanium (Ti), and a balance of iron (Fe) and inevitable impurities; finish-rolling the reheated steel slab at a temperature of 880° C. to 950° C.; cooling the hot-rolled steel plate without using water, and coiling the cooled steel plate at a temperature of 680° C. to 800° C. to form a hot-rolled decarburized layer on a surface of the steel plate; pickling the coiled steel plate, followed by cold rolling; annealing the cold-rolled steel plate in a reducing atmosphere; plating the annealed steel plate; and hot-stamping the plated steel plate.

The high-strength hot-dip galvanized steel sheet, which includes a hot-dip galvanized coating layer on a surface of the steel sheet, has a component composition containing, in mass %, C: 0.07% to 0.20%, Si: 0.1% to 2.0%, Mn: 2.0% to 3.5%, P: 0.05% or less, S: 0.05% or less, and sol. Al: 0.005% to 0.1%, with the balance being Fe and incidental impurities; and has a steel microstructure containing, in area fraction, 60% or less of ferrite, 40% or more of tempered martensite, and 10% or less of fresh martensite and having a void number density of 1,500/mm.sup.2 or less in a bent portion in the VDA bending test.

Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, excellent resistance to resistance welding cracking, and excellent delayed fracture resistance after resistance welding by having a specific chemical composition, and a microstructure such that a prior austenite average grain size is 7 μm or less, a volume fraction of martensite is 90% or more, and at least 5 Nb-based precipitates having a grain size of less than 0.08 μm are present on average per 100 μm.sup.2 of a cross section parallel to a thickness direction of the member within a range of 100 μm in the thickness direction from a surface of the member, and such that a Ni diffusion region having a thickness of 0.5 μm or more is present in a surface layer of the member.

A high-strength steel sheet that has a predetermined component composition, that has a steel microstructure in which, in a thickness cross-section in a rolling direction, an area percentage of ferrite ranges from 5% to 30%, a total area percentage of tempered martensite and bainite ranges from 40% to 90%, pearlite constitutes 0% to 5%, a total area percentage of fresh martensite and retained γ ranges from 5% to 30%, a ratio of a total area percentage of the fresh martensite and the retained γ to a total area percentage of the tempered martensite, bainite, and pearlite is 0.5 or less, and a ratio of the fresh martensite and the retained γ adjacent to the ferrite with respect to the fresh martensite and the retained γ is 30% or more in total area percentage, and that has a yield strength of 550 MPa or more.

A high-strength steel sheet that has a predetermined component composition, that has a steel microstructure in which an area percentage of ferrite ranges from 5% to 50% in a thickness cross-section in a rolling direction, a total area percentage of fresh martensite and retained γ ranges from 2% to 30%, each of the fresh martensite and the retained γ has an average grain size of 5 μm or less, and a ratio of the fresh martensite and the retained γ adjacent only to ferrite with respect to the fresh martensite and the retained γ from a surface to 200 μm in the thickness direction is 30% or less in total area percentage, and that has a yield strength of 550 MPa or more.