When a non-oriented electrical steel sheet is produced by hot rolling a slab containing, by mass %, C: not more than 0.0050%, Si: 1.5-5.0%, Mn: 0.20-3.0%, sol. Al: not more than 0.0050%, P: not more than 0.2%, S: not more than 0.0050% and N: not more than 0.0040% to form a hot rolled sheet, cold rolling the hot rolled sheet without conducting a hot band annealing and then subjecting to a finish annealing, a compositional ratio of CaO in oxide-based inclusions existing in the slab defined by CaO/(SiO.sub.2+Al.sub.2O.sub.3+CaO) is set to not less than 0.4 and/or a compositional ratio of Al.sub.2O.sub.3 defined by Al.sub.2O.sub.3/(SiO.sub.2+Al.sub.2O.sub.3+CaO) is set to not less than 0.3, and a coiling temperature in the hot rolling is set to not lower than 650° C.

The invention relates to a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R.sub.e/R.sub.m ratio of less than 0.75 and an elongation at break of greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10%≦C≦0.25%; 1%≦Mn≦3%; Al≧0.015%; Si≦1.985%; Mo≦0.30%; Cr≦1.5%; S≦0.015%; P≦0.1%; Co≦1.5%; B≦0.005%; it being understood that 1%≦Si+Al≦2%; Cr +(3×Mo)≦0.3%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the steel consisting of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.

The invention relates to a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R.sub.e/R.sub.m ratio of less than 0.75 and an elongation at break of greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10%≦C≦0.25%; 1%≦Mn≦3%; Al≧0.015%; Si≦1.985%; Mo≦0.30%; Cr≦1.5%; S≦0.015%; P≦0.1%; Co≦1.5%; B≦0.005%; it being understood that 1%≦Si+Al≦2%; Cr +(3×Mo)≦0.3%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the steel consisting of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.

A bolt of the present invention has a composition comprising: 0.50 mass % or greater and 0.65 mass % or less of carbon (C), 1.5 mass % or greater and 2.5 mass % or less of silicon (Si), 1.0 mass % or greater and 2.0 mass % or less of chromium (Cr), 0.2 mass % or greater and 1.0 mass % or less of manganese (Mn), 1.5 mass % or greater and 5.0 mass % or less of molybdenum (Mo), wherein a total amount of phosphorous (P) and sulfur (S) as impurities is 0.03 mass % or less, the remaining is iron (Fe), and the bolt comprises an iron based oxide film with a film thickness of 3 μm or greater and 20 μm or less on the surface thereof. The bolt has excellent delayed fracture resistance and reliably provides a fastening axial force.

A metal powder for powder metallurgy contains Fe as a principal component, Ni in a proportion of 5 mass % or more and 20 mass % or less, Si in a proportion of 0.3 mass % or more and 5 mass % or less, and C in a proportion of 0.005 mass % or more and 0.3 mass % or less, and when one element selected from the group consisting of Ti, V, Y, Zr, Nb, Hf, and Ta is defined as a first element, and one element selected from the group and having a higher group number in the periodic table than that of the first element or having the same group number in the periodic table as that of the first element and a higher period number in the periodic table than that of the first element is defined as a second element.

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 %).

The present invention relates to a coated steel sheet having a thin aluminium alloy coating and a coating method thereof The coated steel sheet of the present invention is used for hot stamping. The coating thickness of the coated steel sheet is 5˜14 μm, wherein the aluminium alloy coating comprises a FeAlSi inhibitive layer adjacent to a substrate steel sheet and an Al alloy layer outside the FeAlSi inhibitive layer, wherein the thickness of the FeAlSi inhibitive layer is no more than 60% of the coating thickness and is 1.5˜6.0 μm. The diameters of Kirkendall voids within 2 μm from an interface between the FeAlSi inhibitive layer and the substrate steel to the interior of the substrate steel are no more than 2.5 μm, wherein the number of Kirkendall voids with a diameter of no less than 0.5 μm and no more than 2.5 μm does not exceed 15 per 35 μm. The present invention also discloses a coating method for coating a thin aluminium alloy coating on a substrate steel sheet for hot stamping. It can eliminate skip coating and enable a hot stamped component obtained by the coated steel sheet to have excellent resistance spot welding performance.

The invention relates to a process for producing an at least partly quenched and tempered sheet steel component, where the process comprises the following steps: providing a sheet steel, at least partly austenitizing the sheet steel at a temperature of at least Ac1, at least partly hardening the at least partly austenitized sheet steel to give an at least partly hardened sheet steel component, where the at least partly austenitized sheet steel is cooled to a temperature below Ms, at least partly annealing the at least partly hardened sheet steel component at a temperature of less than Ac1 for producing an at least partly quenched and tempered sheet steel component. A further subject of the invention is an at least partly quenched and tempered sheet steel component.

This steel sheet has a specific chemical composition, the tensile strength is 1300 MPa or more, the ratio (R/t) of the limit bend radius to the sheet thickness is less than 3.5, when a depth position of 30 μm from the surface in the sheet thickness direction is defined as a position A and a depth position of ¼ of the sheet thickness from the surface in the sheet thickness direction is defined as a position B, the number density of AIN at the position A is 3000 pieces/mm.sup.2 or more and 6000 pieces/mm.sup.2 or less, a metallographic structure at the position B includes 90% or more of martensite by volume percentage, and the hardness at the position A is 1.20 times or higher than the hardness at the position B.

This hot-rolled steel sheet has a predetermined chemical composition, a microstructure includes 80% or more of tempered martensite by a volume percentage and a remainder consisting of one or more of ferrite, pearlite, bainite, fresh martensite, and residual austenite, the tempered martensite includes 5×10.sup.9 pieces/mm.sup.3 or more of precipitates containing Ti and having an equivalent circle diameter of 5 nm or less per unit volume, in a surface layer region that is a range from a surface to a 1/10 position of a sheet thickness, a sum of an average pole density of a crystal orientation group consisting of {211}<111> to {111}<112> and a pole density in a crystal orientation of {110}<001> is 6.0 or less, and a tensile strength is 980 MPa or more.