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.

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 dual-phase stainless steel or dual-phase stainless steel seamless pipe has a certain composition, the dual-phase stainless steel or dual-phase stainless steel seamless pipe having a microstructure containing 20 to 70% austenitic phase and 30 to 80% ferritic phase by volume, the dual-phase stainless steel or dual-phase stainless steel seamless pipe having a yield strength, YS, of 448 MPa or more, and containing oxide inclusions of which oxide inclusions having an average particle diameter of 1 μm or more have a number density of 15/mm.sup.2 or less, and at most 50 mass % of the oxide inclusions having an average particle diameter of 1 μm or more are oxide inclusions containing aluminum.

An electric resistance welded steel pipe having excellent formability and torsional fatigue resistance and a method for manufacturing the same. The electric resistance welded steel pipe includes a seam region and a base metal region, the seam region having a range of ±10 degrees in a pipe circumferential direction with respect to an electric resistance welded seam formed in a pipe longitudinal direction, the base metal region being a region other than the seam region. The electric resistance welded steel pipe has an r-value in the pipe longitudinal direction of 1.0 or greater, H (mm) and W (mm) satisfy a specified formula, and Ts.sub.(MAX) (mm) and Tb.sub.(Ave) (mm) satisfy a specified formula.

A sliding element, in particular a piston ring, includes a base material of martensitic or austenitic stainless steel having a chromium content of at least 6.0% by mass and a nitrided layer having a surface hardness of up to 950 HV1. A method of producing such a sliding layer is also provided.

Provided herein is a stainless steel seamless pipe having a composition that contains, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.5% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.1% or more and 3.5% or less, Ni: 3.0% or more and 6.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and Sn: 0.001% or more and 1.000% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the predetermined formula, and the balance is Fe and incidental impurities, the stainless steel seamless pipe having a microstructure containing 30% or more martensitic phase, 65% or less ferrite phase, and 40% or less retained austenite phase by volume.

A steel pipe for pressure piping can be subjected to autofrettage. When an outer diameter of the pipe is D, an inner diameter is d, and a yield stress is σ.sub.y, and when a measured value of an outer surface residual stress is σ.sub.o1, a measured value of an outer surface residual stress after halving is σ.sub.o2, and a measured value of an inner surface residual stress after the halving is σ.sub.i2, D/d is 1.2 or more, an estimated value σ.sub.i1 of inner surface residual stress is [σ.sub.i1=(−σ.sub.i2)/(A×(t/T).sup.2−1)], where [t/T=((σ.sub.o2−σ.sub.o1)/(A×(σ.sub.o2−σ.sub.o1)−C×σ.sub.i2)).sup.1/2], [A=3.9829×exp(0.1071×(D/d).sup.2)], and [C=−3.3966×Exp(0.0452×(D/d).sup.2)] satisfies [1.1×F×σ.sub.y≤α.sub.i1≤0.8×F×σ.sub.y], and (F=(0.3×(3−D/d).sup.2−1) when 1.2≤D/d≤3.0, and F=−1 when D/d>3.0).

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength and toughness that is maintained through post-weld heat treatment, including heating to a temperature of 600° C. or more.

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.