A Ni steel has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 12.0 μm, an average aspect ratio of the prior austenite grains is 2.6 to 10.0, a plate thickness is 4.5 mm to 20 mm, a yield stress at room temperature is 590 MPa to 710 MPa, and a tensile strength at the room temperature is 690 MPa to 810 MPa, when the plate thickness is more than 16 mm, the Ni steel contains Ni: 11.5% or more, and when the plate thickness is 16 mm or less and the Ni steel contains Ni: less than 11.5%, the average grain size of the prior austenite grains is 6.0 μm or less.

A grain-oriented electrical steel sheet having excellent iron loss property is produced from a steel slab containing, by mass %, C: 0.002-0.15%, Si: 2.5-6.0%, Mn: 0.01-0.80%, Al: 0.010-0.050% and N: 0.003-0.020%, when: a heating rate between 500-700° C. in decarburization annealing is not less than 80° C./s, a surface roughness Ra of a work roll in final cold rolling is 0.01-3.0 μm, a total content of alkaline earth metals to MgO in annealing separator is 0-5 mass %, an average length L in rolling direction of passed crystal grains is not more than 25 mm, a ratio of crystal grains having a length in rolling direction of not more than 5 mm is 40-90%, and an existence ratio of alkaline earth metal sulfides having a size corresponding to circle of not less than 0.5 μm in a cross section of forsterite film is not more than 0.2/μm per unit length in sheet width direction.

With a hot-rolled steel sheet for electrical steel sheet production having a scale layer on the surface, where the surface of the steel sheet has a lightness L* as defined in JIS Z 8781-4: 2013 satisfying 30≤L*≤50, and chromaticities a* and b* as defined in JIS Z 8781-4: 2013 satisfying −1≤a*≤2 and −5≤b*≤3 respectively, and with one end portion in the longitudinal direction of a coil as a reference, a color difference ΔE.sub.ab* as defined in JIS Z 8781-4: 2013 at the central portion and at the opposite end portion satisfies ΔE.sub.ab*≤8, it is possible to obtain a grain-oriented electrical steel sheet where the variation of properties in a product coil is small.

A production method for a high-strength steel sheet having a tensile strength TS of 780 MPa or more is provided. The production method comprises: heating a steel slab having a predetermined chemical composition; hotrolling the steel slab; coiling the hot-rolled sheet; subjecting the hot-rolled sheet to pickling treatment; holding the hot-rolled sheet in a pre-determined temperature range for predetermined time; cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; subjecting the cold-rolled sheet to first annealing treatment; cooling the cold-rolled sheet at a pre-determined average cooling rate; cooling the cold-rolled sheet to room temperature; reheating the clod-rolled sheet to perform second annealing treatment; cooling the cold-rolled sheet at a first average cooling rate; cooling the cold-rolled sheet at a second average cooling rate; reheating the cold-rolled sheet to a predetermined reheating temperature range; and holding the cold-rolled sheet in the reheating temperature range.

A hot-rolled steel sheet has, as a chemical composition, by mass %: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, in which among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an EPMA in a region of 250 μm×250 μm on a surface, the percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.

A steel sheet includes a predetermined composition, in which a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet stretched by 10% at the ¼ thickness position from the surface in the sheet thickness direction, a number density of voids having a maximum diameter of 1.0 μm or more is 1.0×10.sup.9 pieces/m.sup.2 or less.

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.

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 %).

A steel for mining chain and a manufacturing method thereof, wherein the steel has compositions by weight percentage: C: 0.20-0.28%, Si: 0.01-0.40%, Mn: 0.50-1.50%, P≤0.015%, S≤0.005%, Cr: 0.30-2.00%, Ni: 0.50-2.00%, Mo: 0.10-0.80%, Cu: 0.01-0.30%, Al: 0.01-0.05%, Nb: 0.001-0.10%, V: 0.001-0.10%, H≤0.00018%, N≤0.0150%, O≤0.0020%, and the balance is Fe and inevitable impurities. The manufacturing method comprises steps of smelting, refining and vacuum treatment, casting, heating, forging or rolling, and quenching and tempering heat treatment processes. The steel in the present invention has high strength and good impact toughness, good elongation and reduction of area. The steel can also resist stress corrosion cracking and have good weather resistance, wear resistance and fatigue resistance, which can be used in scenarios where the steel having high strength and toughness is required, such as construction machinery and marine engineering.

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.