The present invention relates to a steel material including, in mass %: 0.310≤C≤0.410; 0.001≤Si≤0.35; 0.45≤V≤0.70; Cr≤6.00; 6.25≤Mn+Cr; Mn/Cr≤0.155; Cu+Ni≤0.84; 0.002≤P≤0.030; 0.0003≤S≤0.0060; P+5S≤0.040; 2.03<Mo<2.40; 0.001≤Al≤0.050; and 0.003≤N≤0.050, with the balance being Fe and unavoidable impurities.

A duplex stainless steel and method of manufacturing the same, said steel having an amount of Cr in an extraction residue [Cr] of 0.005 to 0.050% and an amount of Nb in an extraction residue [Nb] of 0.001 to 0.080%, the [Nb]/[Cr] ratio being 0.2 or more. By slow cooling down to 800° C., then fast cooling down to 600° C., it is possible to control the precipitation of chromium nitrides and niobium nitrides, and by making the ratio [Nb]/[Cr] 0.2 or more, it is possible to raise the corrosion resistance. Further, by reducing Mn to less than 2.0% and N to 0.25% or less, then adding a trace amount of Nb, the effect of raising the critical pitting temperature CPT is obtained.

A duplex stainless steel and method of manufacturing the same, said steel having an amount of Cr in an extraction residue [Cr] of 0.005 to 0.050% and an amount of Nb in an extraction residue [Nb] of 0.001 to 0.080%, the [Nb]/[Cr] ratio being 0.2 or more. By slow cooling down to 800° C., then fast cooling down to 600° C., it is possible to control the precipitation of chromium nitrides and niobium nitrides, and by making the ratio [Nb]/[Cr] 0.2 or more, it is possible to raise the corrosion resistance. Further, by reducing Mn to less than 2.0% and N to 0.25% or less, then adding a trace amount of Nb, the effect of raising the critical pitting temperature CPT is obtained.

This austenitic stainless steel sheet contains, as a chemical composition, by mass %, C: 0.030% or less, Si: 1.0% or less, Mn: 1.5% or less, Cr: 15.0% or more and 20.0% or less, Ni: 6.5% or more and 9.0% or less, N: 0.030% or more and 0.150% or less, any one of Nb, V, and Ti or two or more thereof in total: 0.030% or more and 0.300% or less, Mo: 0% or more and 2.0% or less, Cu: 0% or more and 1.5% or less, Co: 0% or more and 1.0% or less, P: 0.10% or less, S: 0.010% or less, and Al: 0.10% or less, in which a remainder includes Fe and impurities, the average grain size is 5.0 μm or less, and the non-recrystallization rate is more than 3% and 20% or less.

This austenitic stainless steel sheet contains, as a chemical composition, by mass %, C: 0.030% or less, Si: 1.0% or less, Mn: 1.5% or less, Cr: 15.0% or more and 20.0% or less, Ni: 6.5% or more and 9.0% or less, N: 0.030% or more and 0.150% or less, any one of Nb, V, and Ti or two or more thereof in total: 0.030% or more and 0.300% or less, Mo: 0% or more and 2.0% or less, Cu: 0% or more and 1.5% or less, Co: 0% or more and 1.0% or less, P: 0.10% or less, S: 0.010% or less, and Al: 0.10% or less, in which a remainder includes Fe and impurities, the average grain size is 5.0 μm or less, and the non-recrystallization rate is more than 3% and 20% or less.

Steel material whose constituent grains comprise a matrix in which precipitates are incorporated, the precipitates comprising at least one metallic element selected from a metallic element M, a metallic element M′, a metallic element M″ or mixtures thereof; the microstructure of the steel being such that the grains are equiaxed and the average grain size being such that the average of their largest dimension “Dmax” and/or the average of their smallest dimension “Dmin” is comprised between 10 μm and 50 μm. The steel material has optimized, stable and isotropic mechanical properties, in particular so that the steel material can best withstand mechanical and/or thermal stresses.

Steel material whose constituent grains comprise a matrix in which precipitates are incorporated, the precipitates comprising at least one metallic element selected from a metallic element M, a metallic element M′, a metallic element M″ or mixtures thereof; the microstructure of the steel being such that the grains are equiaxed and the average grain size being such that the average of their largest dimension “Dmax” and/or the average of their smallest dimension “Dmin” is comprised between 10 μm and 50 μm. The steel material has optimized, stable and isotropic mechanical properties, in particular so that the steel material can best withstand mechanical and/or thermal stresses.

A seamless steel pipe of the present invention is a seamless steel pipe having a composition including, in mass %, C: 0.01 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.040% or less, Al: 0.010 to 0.100%, Cu: 0.03 to 0.80%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, Sb: 0.002 to 0.50%, Cr: 0.004% or less, W: 0.002% or less, and the balance Fe and incidental impurities, and a structure including a ferrite phase having an area percentage of 50 to 65%, a pearlite phase having an area percentage of 2% or less, and one or both of a bainite phase and a martensitic phase representing the remainder, the seamless steel pipe having a yield strength of 230 MPa or more, and a tensile strength of 380 MPa or more.

A seamless steel pipe of the present invention is a seamless steel pipe having a composition including, in mass %, C: 0.01 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.040% or less, Al: 0.010 to 0.100%, Cu: 0.03 to 0.80%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, Sb: 0.002 to 0.50%, Cr: 0.004% or less, W: 0.002% or less, and the balance Fe and incidental impurities, and a structure including a ferrite phase having an area percentage of 50 to 65%, a pearlite phase having an area percentage of 2% or less, and one or both of a bainite phase and a martensitic phase representing the remainder, the seamless steel pipe having a yield strength of 230 MPa or more, and a tensile strength of 380 MPa or more.

A stainless steel seamless pipe having high strength and excellent corrosion resistance. The stainless steel seamless pipe has a specified composition in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a predetermined formula, a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% retained austenite phase by volume, and a yield strength of 758 MPa or more.