A ferritic stainless steel with chemical composition includes, in mass %, Cr: 10.5 to 25.0%; Al: 0.01 to 0.20%; Ti: 0.15% to 0.35%; O: 0.0001 to 0.0030%; and Mg: 0.008×[% Al] or more, in which oxysulfide-containing inclusions are present in the steel, a number ratio of the oxysulfide-containing inclusions whose minor axis is 3 μm or more is 5 pieces/mm.sup.2 or less, and a number ratio of the oxysulfide-containing inclusions whose minor axis is 15 μm or more is 0.05 pieces/mm.sup.2 or less. 75% or more of the inclusions whose minor axis is 3 μm or more have an oxysulfide part whose composition satisfies formulae (1) and (2),

CaO+Al.sub.2O.sub.3+MgO≥90%  formula (1),

Al.sub.2O.sub.3/MgO≤1.25  formula (2).

To provide a high-strength seamless stainless steel pipe for oil well that has high strength, is excellent in hot workability, has excellent carbon dioxide gas corrosion resistance, and is excellent in SSC resistance under a low temperature environment. A high-strength seamless stainless steel pipe for oil well having a composition containing the particular components, the balance being Fe and unavoidable impurities, and satisfying certain expressions, having a number density of an inclusion having a major axis of 5 μm or more and 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 of 0.5 per mm.sup.2 or more and 3 per mm.sup.2 or less, and having a yield strength of 655 MPa or more, wherein in 0.5<Ti/(Ti+Al+Mg+Ca)<1.0, Ti, Al, Mg, and Ca represent the contents (% by mass) of the elements in the inclusion, and an element that is not contained is designated as 0.

To provide a high-strength seamless stainless steel pipe for oil well that has high strength, is excellent in hot workability, has excellent carbon dioxide gas corrosion resistance, and is excellent in SSC resistance under a low temperature environment. A high-strength seamless stainless steel pipe for oil well having a composition containing the particular components, the balance being Fe and unavoidable impurities, and satisfying certain expressions, having a number density of an inclusion having a major axis of 5 μm or more and 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 of 0.5 per mm.sup.2 or more and 3 per mm.sup.2 or less, and having a yield strength of 655 MPa or more, wherein in 0.5<Ti/(Ti+Al+Mg+Ca)<1.0, Ti, Al, Mg, and Ca represent the contents (% by mass) of the elements in the inclusion, and an element that is not contained is designated as 0.

The invention relates to a method for producing a steel material, particularly a corrosion-resistant steel material for pumps and similar, in which a steel corresponding to the following analysis (in wt. %) is smelted: C<0.050; Si<0.70; Mn<1.00; P<0.030; S<0.010; Cr=14-15.50; Mo=0.30-0.60; Ni=4.50-5.50; V<0.20; W<0.20; Cu=2.50-4.00; Co<0.30; Ti<0.05; Al<0.05; Nb<0.05; Ta<0.05; N<0.05.

The invention relates to a method for producing a steel material, particularly a corrosion-resistant steel material for pumps and similar, in which a steel corresponding to the following analysis (in wt. %) is smelted: C<0.050; Si<0.70; Mn<1.00; P<0.030; S<0.010; Cr=14-15.50; Mo=0.30-0.60; Ni=4.50-5.50; V<0.20; W<0.20; Cu=2.50-4.00; Co<0.30; Ti<0.05; Al<0.05; Nb<0.05; Ta<0.05; N<0.05.

A stainless steel seamless pipe having high strength and excellent corrosion resistance, and a method for producing the same. The stainless steel seamless pipe has a specified composition and satisfies a predetermined formula. The stainless steel seamless pipe has a microstructure containing at least 30% martensitic phase, at most 60% ferrite phase, and at most 40% retained austenite phase by volume, the stainless steel seamless pipe having a yield strength of 758 MPa or more.

An alloy includes: an average Ni concentration of 1.5 at.% or more and 15.5 at.% or less; an average Co concentration of 0 at.% or more and 10.0 at.% or less; an average B concentration of 3.0 at.% or more and 16.0 at.% or less; an average P concentration of 0.5 at.% or more and 10.0 at.% or less; an average Cu concentration of 0 at.% or more and 2.0 at.% or less; an average Si concentration of 0 at.% or more and 6.0 at.% or less; an average C concentration of 0 at.% or more and 6.0 at.% or less; a total of average concentrations of Nb, Mo, Zr, W, V, Hf, Ta, Al, Ti, and Cr of 0 at.% or more and 6.0 at.% or less; and a total of an average Fe concentration, the average Ni concentration, and the average Co concentration of 78.0 at.% or more and 88.0 at.% or less.

An alloy includes: an average Ni concentration of 1.5 at.% or more and 15.5 at.% or less; an average Co concentration of 0 at.% or more and 10.0 at.% or less; an average B concentration of 3.0 at.% or more and 16.0 at.% or less; an average P concentration of 0.5 at.% or more and 10.0 at.% or less; an average Cu concentration of 0 at.% or more and 2.0 at.% or less; an average Si concentration of 0 at.% or more and 6.0 at.% or less; an average C concentration of 0 at.% or more and 6.0 at.% or less; a total of average concentrations of Nb, Mo, Zr, W, V, Hf, Ta, Al, Ti, and Cr of 0 at.% or more and 6.0 at.% or less; and a total of an average Fe concentration, the average Ni concentration, and the average Co concentration of 78.0 at.% or more and 88.0 at.% or less.

A valve spring which has an excellent fatigue limit is provided. A chemical composition of the valve spring according to the present embodiment contains, in mass %, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less. S: 0.020% or less: Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, Ca: 0.0001 to 0.0050%, and N: 0.0100% or less, with the balance being Fe and impurities. In the valve spring, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces/μm.sup.3, and a numerical proportion of Ca sulfides with respect to a total number of oxide-based inclusions and sulfide-based inclusions is 0.20% or less.

A valve spring which has an excellent fatigue limit is provided. A chemical composition of the valve spring according to the present embodiment contains, in mass %, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less. S: 0.020% or less: Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, Ca: 0.0001 to 0.0050%, and N: 0.0100% or less, with the balance being Fe and impurities. In the valve spring, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces/μm.sup.3, and a numerical proportion of Ca sulfides with respect to a total number of oxide-based inclusions and sulfide-based inclusions is 0.20% or less.