The production method of a seamless steel pipe includes a heating step of heating an Nb-containing steel material to 800 to 1030° C., a pipe-making step of producing a hollow shell by performing piercing-rolling or elongation-rolling on the Nb-containing steel material, by using a piercing mill including a plurality of skewed rolls, a plug disposed between the plurality of skewed rolls, and a mandrel bar, and a cooling step immediately after rolling, of carrying out cooling using a cooling liquid on a hollow shell portion that passes between rear ends of the plurality of skewed rolls, in the hollow shell, so as to reduce an outer surface temperature of the hollow shell portion to 700 to 1000° C. within 15.0 seconds after the hollow shell portion passes between the rear ends of the plurality of skewed rolls.

The steel material according to the present disclosure has a chemical composition consisting of, in mass %, C: 0.10 to 0.60%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, V: 0.01 to 0.60%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0020 to 0.0100%, and O: 0.0100% or less, with the balance being Fe and impurities. A dislocation density ρ is 3.5×10.sup.15 m.sup.−2 or less. Among fine precipitates, the numerical proportion of precipitates for which a ratio of the Mo content is not more than 50% is 15% or more. The yield strength is in a range of 655 to 1172 MPa.

The martensitic stainless steel material according to the present disclosure consists of C: less than 0.030%, Si: 1.00% or less, Mn: 0.05 to 2.00%, Cr: 11.50 to 14.00%, Ni: 5.00 to 7.50%, Mo: 1.10 to 3.50%, Cu: 0.50 to 3.50%, Co: 0.01 to 0.30%, Al: 0.001 to 0.100%, N: 0.001 to 0.100%, and the balance: Fe and impurities. The microstructure is composed of retained austenite in an amount of 0 to 15 vol%, and ferrite in an amount of 0 to 10 vol%, with the balance being martensite. The yield strength is 862 MPa or more, and a number density of Cu precipitates is 3.0 x 10.sup.21 to 50.0 x 10.sup.21 /m.sup.3.

A duplex stainless steel having excellent carbon dioxide corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress cracking resistance. The duplex stainless steel comprises, by mass %, C: 0.03% or less, Si: 1.0% or less, Mn: 0.10 to 1.5%, P: 0.030% or less, S: 0.005% or less, Cr: 20.0 to 30.0%, Ni: 5.0 to 10.0%, Mo: 2.0 to 5.0%, Cu: 2.0 to 6.0%, N: less than 0.07%, at least one selected from Al: 0.05 to 1.0%, Ti: 0.02 to 1.0%, and Nb: 0.02 to 1.0%, and the balance being Fe and unavoidable impurities, and has a structure that is 20 to 70% austenite phase, and 30 to 80% ferrite phase in terms of a volume fraction.

Provided herein is a stainless steel seamless pipe for oil country tubular goods. A method for manufacturing such a stainless steel seamless pipe is also provided. The stainless steel seamless pipe has: a composition that contains, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 14.0%, Mo: 0.5 to 2.8%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, and in which the balance is Fe and incidental impurities; a microstructure containing at most 20 non-metallic inclusions having a predetermined composition ratio of CaO and Al.sub.2O.sub.3 and a major axis of 5 μm or more per 100 mm.sup.2; and a yield stress of 655 MPa or more.

Manufacturing method for tubular products made of zirconium-based alloy includes melting an ingot by multiple vacuum arc remelting, mechanical processing of the ingot, heating, multi-stage hot forging of the ingot for production of a forged piece, subsequent mechanical processing of the forged piece for production of the a round-profile blank, manufacturing of tubular billets, their quenching and tempering, application of a protective coating, heating to a hot pressing temperature, hot pressing, removal of the protective coating, vacuum thermal treatment, multiple cold rolling steps in order to produce tubular products, with intermediate vacuum thermal treatment after each cold rolling, and a final vacuum thermal treatment being carried out at a final size with subsequent final finishing operations. The tubular products can be used as the structural components of a core in water-cooled nuclear reactors. The method can provide increased processibility, high strength, and corrosion resistance of tubular products.

A hot and cold composite formed square and rectangular steel tube and a production method for the same are provided. The radius of an outer corner of the square and rectangular steel tube meets the following conditions: when t is less than or equal to 6 mm, R is greater than 0 and less than 2.0 t; when t is greater than 6 mm and less than or equal to 10 mm, R is greater than 0 and less than 2.5 t; when t is greater than 10 mm, R is greater than 0 and less than 3.0 t, wherein t is the wall thickness of a straight tube part of the square and rectangular steel tube; R is the radius of each of the outer corners of the four corners of the square and rectangular steel tube; and the wall thickness of each corner of the square and rectangular steel tube is between 1.0 t and 1.8 t.

A method of manufacturing a seamless stainless steel pipe for Oil Country Tubular Goods by heating a billet having a specified chemical composition including forming the billet into a seamless steel pipe by applying hot working to the billet, cooling the seamless steel pipe to a room temperature at a cooling rate of air cooling or more, thereafter, performing quenching by heating the seamless steel pipe to a temperature of 850° C. or above, subsequently, cooling the seamless steel pipe to a temperature of 100° C. or below at a cooling rate of air cooling or more, and subsequently, applying tempering to the seamless steel pipe at a temperature of 700° C. or below for a specific holding time.

Provided is steel for nitrocarburizing with excellent surface fatigue strength. The steel has a nitride compound layer with a thickness of 5.0 μm to 30.0 μm and a hardened layer in an order from a steel surface to steel inside, where a thickness of a porous layer on an outermost surface of the compound layer is 3.0 μm or less and 40.0% or less of a compound layer's thickness, the hardened layer has hardness of HV600 or more, HV400 or more and HV250 or more at 50 μm inward from the steel surface, from the steel surface to the steel inside of 400 μm, and from the steel surface to the steel inside of 600 μm, respectively, an unhardened portion excluding the compound and hardened layers has a predetermined chemical composition, and the hardened layer has a chemical composition with a higher N content than the unhardened portion.

A high-strength thick-walled electric resistance welded steel pipe has excellent low-temperature toughness and excellent HIC resistance and a yield strength of 400 MPa or more. The steel has a chemical composition consisting of C: 0.025% to 0.084%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.80%, controlled amounts of P, S, Al, N, and O, Nb: 0.001% to 0.065%, V: 0.001% to 0.065%, Ti: 0.001% to 0.033%, and Ca: 0.0001% to 0.0035% on a mass percent basis and the remainder being Fe and incidental impurities, and satisfies Pcm of 0.20 or less.