The high-strength thin steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, and N, with the balance being Fe and inevitable impurities, and a complex structure containing ferrite, tempered martensite, and bainite, where a volume fraction of a total of tempered martensite and bainite containing five or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less in a grain with respect to a total of the tempered martensite and the bainite is 85% or more, and C mass % and Mn mass % in a region of 20 μm or less in a thickness direction from a surface of the steel sheet are each 20% or less with respect to C mass % and Mn mass % in a region of 100 μm or more and 200 μm or less from the surface of the steel sheet.

The present invention relates to a steel material for hot stamping with ultra-fine grains and a process of making the same, a hot stamping process and a hot stamped component. The steel material for hot stamping comprises the following components by weight: 0.27 to 0.40% of C; 0.2 to 3.0% of Mn; 0.11 to 0.4% of V; 0 to 0.8% of Si; 0 to 0.5% of Al; 0 to 2% of Cr; 0 to 0.15% of Ti; 0 to 0.15% of Nb; 0 to 0.004% B; a total of less than 2% of Mo, Ni, Cu and other alloying elements that are beneficial to improving the hardenability, and other impurity elements. After hot stamping or equivalent heat treatment, the steel material or the formed component of the present invention can achieve a yield strength of 1300 MPa to 1700 MPa, a tensile strength of 1800 to 2200 MPa, and an elongation of 6 to 9% after direct hot stamping quenching and without tempering, which properties cannot be achieved by a material with a composition in the prior art in the situation of direct quenching (no tempering). After the tempering treatment of the present invention, preferably 1500 MPa-1900 MPa-8%, and 1600 MPa-2100 MPa-7% can be reached.

A cold rolled martensitic steel sheet including the following elements, expressed in percentage by weight: 0.1%≤C≤0.2%; 1.5%≤Mn≤2.5%; 0.1%≤Si≤0.25%; 0.1%≤Cr≤1%; 0.01%≤Al≤0.1%; 0.001%≤Ti≤0.1%; 0%≤S≤0.09%; 0%≤P≤0.09%; 0%≤N≤0.09%; and can contain one or more of the following optional elements 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤Mo≤0.4%; 0%≤Nb≤0.1%; 0%≤V≤0.1%; 0%≤B≤0.05%; 0%≤Sn≤0.1%; 0%≤Pb≤0.1%; 0%≤Sb≤0.1%; 0.001%≤Ca≤0.01%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel including, by area percentage, at least 95% of martensite, a cumulated amount of ferrite and bainite between 1% and 5%, and an optional amount of residual austenite between 0% and 2%.

An aluminum-based coating of a flat steel product is applied in a hot-dipping method and comprises a mass percentage of silicon within a given range. The coating for a flat steel product, in particular for press mold hardening components, offers a shortened required minimum oven dwell time and a sufficiently large processing window when heating in an oven. This is achieved in that the surface of the coating has a degree of absorption for thermal radiation ranging between 0.35 and 0.95 prior to an annealing treatment, where the degree of absorption relates to an oven temperature ranging from 880 to 950° C. during the austenitizing annealing treatment. The invention additionally relates to an improved method for producing a flat steel product with an aluminum-based coating, to an inexpensive method for producing press-hardened components from such flat steel products, and to a press-hardened component made of such flat steel products.

A cold rolled steel strip or sheet has a composition consisting of (in wt. %): C 0.15-0.25; Si 0.5-1.6; Mn 2.2-3.2; Cr≤0.8; Mo≤0.2; Al 0.03-1.0; Nb≤0.04; V≤0.04; Ti 0.01-0.04; B 0.001-0.010; TUB 5-30; Cu≤0.15; Ni≤0.15; Ca≤0.01; balance Fe apart from impurities, where the cold rolled steel has a multiphase microstructure comprising a matrix mainly composed of tempered martensite and has a tensile strength (R.sub.m) of at least 1380 MPa.

A high strength steel strip having medium amounts of C, Mn, Si, Cr and Al, wherein the steel strip has a microstructure consisting of, in vol. %: ferrite and bainite together 50-90%, martensite <15%, retained austenite 5-15%, the remainder being pearlite, cementite, precipitates and inclusions together up to 5%.

A hot-dip galvanized steel sheet including: a hot-dip galvanizing layer on at least one side of a base steel sheet. The hot-dip galvanizing layer has a Fe content of more than 0% to 3.0% and an Al content of more than 0% to 1.0%. The hot-dip galvanized steel sheet includes a Fe—Al alloy layer provided on an interface between the hot-dip galvanizing layer and the base steel sheet and a fine-grain layer provided in the base steel sheet and directly in contact with the Fe—Al alloy layer. The Fe—Al alloy layer has a thickness of 0.1-2.0 μm. The fine-grain layer has an average thickness of 0.1-5.0 μm, includes a ferrite phase with an average grain diameter of 0.1-3.0 μm, and contains oxides of one or more out of Si and Mn, a maximum diameter of the oxides being 0.01-0.4 μm.

A flat steel product consisting of (in wt %) 0.1-0.5% C, 1.0-3.0% Mn, 0.9-1.5% Si, ≤1.5% Al, ≤0.008% N, ≤0.020% P, ≤0.005% S, 0.01-1% Cr and optionally one or more of: ≤0.2% Mo, ≤0.01% B, ≤0.5% Cu, ≤0.5% Ni and optionally a total of 0.005-0.2% microalloying elements, the remainder iron and unavoidable impurities. The steel has a structure consisting of ≥80 area % martensite, where ≥75 area % is tempered and ≤25 area % is non-tempered, ≥5 volume % residual austenite, 0.5-10 area % ferrite, ≤5 area % bainite, and carbides with a length of ≤250 nm, wherein in the phase boundary between tempered martensite and residual austenite, there is a low-Mn ferrite seam having a width of 4.Math.12 nm and a Mn content of ≤50% of the average Mn content. Also, a method for producing the flat steel in which the structural characteristics of the flat steel product are set by suitable heat treatment.

Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent resistance to resistance welding cracking by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7.5 μm or less, a volume fraction of martensite is 95% or more, and at least 10 Nb-based and Ti-based precipitates having a grain size of less than 0.10 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 or less in the thickness direction from the surface of the member, and such that a B concentration in prior austenite grain boundaries is at least 3.0 times a B concentration at a position 5 nm away from the grain boundaries.

A high-strength hot-dip coated hot-rolled steel sheet excellent in terms of surface appearance quality and coating adhesiveness and a method for manufacturing. The method includes performing hot rolling followed by pickling on steel to form a pickled steel sheet, the steel having a chemical composition containing, by mass %, C: 0.02% or more and 0.30% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.001% or more and 0.20% or less, and Fe and inevitable impurities. The method further includes performing rolling with a rolling reduction ratio of 1% or more and 10% or less, and a hot-dip coating treatment. The obtained steel sheet has an arithmetic average roughness Ra of 2.0 μm or less on the surface of the steel sheet, and a tensile strength of 590 MPa or more.