This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L.sub.60/L.sub.7 of a length L.sub.60 of a grain boundary having a crystal misorientation of 60° to a length L.sub.7 of a grain boundary having a crystal misorientation of 7° about a<110> direction of less than 0.60, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

Thick high-strength steel having a high yield ratio and excellent durability, and a method for manufacturing same are provided. The thick high-strength steel having a high yield ratio and excellent durability of the present invention comprises, in percentage by weight, C: 0.05 to 0.15%, Si: 0.01 to 1.0%, Mn: 1.0 to 2.3%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.005 to 0.07%, Ti 0.005 to 0.11%, Fe and unavoidable impurities.

A ferritic stainless steel sheet includes a base metal and a nitrided layer that is formed on a surface of the base metal, a chemical composition of the base metal contains, in mass %, C: 0.001 to 0.020%, Si: 0.01 to 1.50%, Mn: 0.01 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.010%, Cr: 16.0 to 25.0%, N: 0.001 to 0.030%, Ti: 0.01 to 0.30%, and optional elements, with the balance: Fe and unavoidable impurities, a steel microstructure of the base metal includes, in volume ratio, 95% or more of a ferritic phase, the nitrided layer is a layer that is present in a region from a surface of a rolled surface to a 0.05 μm depth position in a sheet thickness direction, and an average nitrogen concentration in the nitrided layer is, in mass %, 0.80% or more.

A steel sheet has a predetermined chemical composition, in which a microstructure in a ¼ width portion, a microstructure in a ½ width portion, and a microstructure in a ¾ width portion, include, by area %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, in which a proportion of unrecrystallized ferrite in the ferrite is 5% to 60%, an average grain size of carbonitrides is 6.0 nm to 30.0 nm, and Expressions (2) to (5) are satisfied.

Δ.sub.SF/μ.sub.SF≤0.10  (2)

Δ.sub.dF/μ.sub.dF≤0.20  (3)

Δ.sub.SUF≤20  (4)

Δ.sub.dC/μ.sub.dC≤0.50  (5)

A method for manufacturing of steel tube from a long steel strip, including providing a length of steel strip material to the process, forming a tube of the steel strip material, welding the formed tube in longitudinal direction, giving the tube a heat treatment wherein the mentioned steps are performed in one continuous in-line manufacturing line and the heat treatment includes a heating regime such that in successive cross-sections of the tube a microstructure is achieved which holds at least 50 vol % austenite and a cooling trajectory to re-introduce ferrite, and/or bainite in desired volume fractions.

A high-strength high-elongation tinned primary plate and a double cold reduction method therefor. The tinned primary plate comprises the following components by weight from 0.065 to 0.12% of carbon, from 0.2 to 0.8% of manganese, from 0.003 to 0.015% of nitrogen, the remainder being iron and the inevitable trace impurities. The tinned primary plate is necessarily subjected to double cold reduction at a reduction of 5˜13% and a rolling tension of 50˜100 MPa. The tinned primary plate has a yield strength of Rp.sub.0.2≥520 MPa, and percentage elongations in rolling direction RD, 45° direction and perpendicular direction TD, which are all greater than or equal to 10% after bake-hardening.

A steel sheet includes: a predetermined chemical composition; and a steel structure represented by, in area %, first martensite in which two or more iron carbides each having a circle-equivalent diameter of 2 nm to 500 nm are contained in each lath: 20% to 95%, ferrite: 15% or less, retained austenite: 15% or less, and the balance: bainite, or second martensite in which less than two iron carbides each having a circle-equivalent diameter of 2 nm to 500 nm are contained in each lath, or the both of these, in which the total area fraction of ND//<111> orientation grains and ND//<100> orientation grains is 40% or less, and the content of solid-solution C is 0.44 ppm or more.

A method of shaping an article from a zinc or zinc alloy coated steel blank, including the steps of: a) providing a blank of the zinc or zinc alloy coated steel; b) reheating of the blank obtained in step a) to a reheating temperature T.sub.RH in the range Ac3-200° C. of the steel; c) soaking the blank for a time up to 3 minutes at the reheating temperature T.sub.RH; d) shaping the article in a press; and e) cooling the article. The steel includes (in wt. %) C: 0.01-0.2; Mn: 3.1-9.0; Al: 0.5-3.0; and optionally further alloying elements selected from Si, Cr, V, Nb, Ti and Mo; inevitable impurities and the balance is Fe.

A high-strength steel having excellent low-yield-ratio characteristics, according to one embodiment of the present invention, comprises, by wt %, 0.06-0.12% of C, 0.2-0.5% of Si, 1.5-2.0% of Mn, 0.003-0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05-0.5% of Cr, 0.05-0.5% of Mo, 0.01-0.05% of Nb, 0.0005-0.005% of Ca and the balance of Fe and other inevitable impurities, and comprises polygonal ferrite as a microstructure, wherein the area fraction of the polygonal ferrite is 10-30% and the average hardness of the polygonal ferrite can be 180 Hv or less.

Provided is a high-strength steel sheet having a tensile strength of 780 MPa or higher. The high-strength steel sheet has a low yield ratio and excellent ductility (El) and strain hardening exponent (n) and thus has enhanced processability.