Abstract: A martensitic stainless steel for welded structures including by mass %, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.03% or less, REM: 0.0005 to 0.1%, Cr: 8 to 16%, Ni: 0.1 to 9% and sol. Al: 0.001 to 0.1%; and further including one or more elements selected from among Ti: 0.005 to 0.5%, Zr: 0.005 to 0.5%, Hf: 0.005 to 0.5%, V: 0.005 to 0.5% and Nb: 0.005 to 0.5%; and O: 0.005% or less, N: 0.1% or less, with the balance being Fe and impurities; and the P and REM content satisfies: P?0.6×REM. This steel possesses excellent SCC (stress corrosion cracking) resistance in welded sections in Sweet environments.

Abstract: The chemical composition of a stainless steel in accordance with the present invention consists of C: not more than 0.05%, Si: not more than 0.5%, Mn: 0.01 to 0.5%, P: not more than 0.04%, S: not more than 0.01%, Cr: more than 16.0 and not more than 18.0%, Ni: more than 4.0 and not more than 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to 0.10%, and N: not more than 0.050%, the balance being Fe and impurities, and satisfies Formulas (1) and (2). Also, the micro-structure thereof contains a martensitic phase and a ferritic phase having a volume ratio of 10 to 40%, and the ferritic phase distribution ratio is higher than 85%. Cr+Cu+Ni+Mo?25.5??(1) ?8?30(C+N)+0.5Mn+Ni+Cu/2+8.2?1.

Abstract: The problem to be solved is the provision of a high-strength stainless steel pipe having a sufficient corrosion resistance in a high-temperature carbonic acid gas environment and having an excellent sulfide stress cracking resistance at normal temperature. A high-strength stainless steel pipe consist of by mass %, C: 0.05% or less, Si: 1.0% or less, P: 0.05% or less, S: less than 0.002%, Cr: more than 16% and 18% or less, Mo: more than 2% and 3% or less, Cu: 1% to 3.5%, Ni: 3% or more and less than 5%, Al: 0.001% to 0.1% and O: 0.01% or less, Mn: 1% or less and N: 0.05% or less, and Mn and N in the above ranges satisfy formula (1), and the balance being Fe and impurities; and the metal micro-structure of the stainless steel pipe mainly includes a martensitic phase and comprises 10 to 40% of a ferritic phase by volume fraction and 10% or less of a retained ?-phase by volume fraction. [Mn]×([N]?0.0045)?0.

Abstract: A stainless steel for an oil country tubular good according to the invention includes, in percent by mass, 0.001% to 0.05% C, 0.05% to 1% Si, at most 2% Mn, at most 0.03% P, less than 0.002% S, 16% to 18% Cr, 3.5% to 7% Ni, more than 2% and at most 4% Mo, 1.5% to 4% Cu, 0.001% to 0.3% rare earth metal, 0.001% to 0.1% sol. Al, 0.0001% to 0.01% Ca, at most 0.05% O, and at most 0.05% N, and the balance consists of Fe and impurities. The stainless steel according to the invention includes REM and therefore has high SCC resistance in a high temperature chloride aqueous solution environment.

Abstract: An oil country tubular good for expansion according to the invention is expanded in a well. The oil country tubular good for expansion has a composition containing, in percentage by mass, 0.05% to 0.08% C, at most 0.50% Si, 0.80% to 1.30% Mn, at most 0.030% P, at most 0.020% S, 0.08% to 0.50% Cr, at most 0.01% N, 0.005% to 0.06% Al, at most 0.05% Ti, at most 0.50% Cu, and at most 0.50% Ni, and the balance consisting of Fe and impurities, and a structure having a ferrite ratio of at least 80%. The oil country tubular good for expansion has a yield strength in the range from 276 MPa to 379 MPa and a uniform elongation of at least 16%. Therefore, the oil country tubular good according to the invention has a high pipe expansion characteristic.

Abstract: A martensitic stainless steel for welded structures including by mass %, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.03% or less, REM: 0.0005 to 0.1%, Cr: 8 to 16%, Ni: 0.1 to 9% and sol. Al: 0.001 to 0.1%; and further including one or more elements selected from among Ti: 0.005 to 0.5%, Zr: 0.005 to 0.5%, Hf: 0.005 to 0.5%, V: 0.005 to 0.5% and Nb: 0.005 to 0.5%; and O: 0.005% or less, N: 0.1% or less, with the balance being Fe and impurities; and the P and REM content satisfies: P?0.6×REM. This steel possesses excellent SCC (stress corrosion cracking) resistance in welded sections in Sweet environments.

Abstract: A metal pipe according to the invention has a plurality of ridges that have different heights, extend in the axial direction, and are arranged in the circumferential direction at its inner circumferential surface. If the Reynolds number changes and the streak structure and the scale of a hairpin vortex change, the streak and the hairpin vortex each match any one of the ridges. Therefore, the fluid friction can be reduced in a wide Reynolds number range.

Abstract: The martensitic stainless steel according to the invention includes, in percent by mass, 0.010% to 0.030% C, 0.30% to 0.60% Mn, at most 0.040% P, at most 0.0100% S, 10.00% to 15.00% Cr, 2.50% to 8.00% Ni, 1.00% to 5.00% Mo, 0.050% to 0.250% Ti, at most 0.25% V, at most 0.07% N, and at least one of at most 0.50% Si and at most 0.10% Al, the balance consists of Fe and impurities, and the martensitic stainless steel satisfies Expression (1) and has a yield stress in the range from 758 MPa to 862 MPa. In this way, the martensitic stainless steel has a yield stress of 110 ksi grade (a yield stress in the range from 758 MPa to 862 MPa) and the value produced by subtracting the yield stress from the tensile stress is not less than 20.7 MPa. 6.0?Ti/C?10.

Abstract: An oil country tubular good for expansion according to the invention is expanded in a well. The oil country tubular good for expansion has a composition containing, in percentage by mass, 0.05% to 0.08% C, at most 0.50% Si, 0.80% to 1.30% Mn, at most 0.030% P, at most 0.020% S, 0.08% to 0.50% Cr, at most 0.01% N, 0.005% to 0.06% Al, at most 0.05% Ti, at most 0.50% Cu, and at most 0.50% Ni, and the balance consisting of Fe and impurities, and a structure having a ferrite ratio of at least 80%. The oil country tubular good for expansion has a yield strength in the range from 276 MPa to 379 MPa and a uniform elongation of at least 16%. Therefore, the oil country tubular good according to the invention has a high pipe expansion characteristic.

Abstract: An oil country tubular good for expansion according to the invention is expanded in a well. The oil country tubular good for expansion according to the invention is formed of duplex stainless steel having a composition containing, in percentage by mass, 0.005% to 0.03% C, 0.1% to 1.0% Si, 0.2% to 2.0% Mn, at most 0.04% P, at most 0.015% S, 18.0% to 27.0% Cr, 4.0% to 9.0% Ni, at most 0.040% Al, and 0.05% to 0.40% N, and the balance consisting of Fe and impurities, a structure including an austenite ratio in the range from 40% to 90%. The oil country tubular good for expansion according to the invention has a yield strength from 256 MPa to 655 MPa, and a uniform elongation more than 20%. Therefore, the oil country tubular good for expansion according to the invention has a high pipe expansion characteristic.

Abstract: Martensitic stainless steel according to the invention contains, by mass, 0.001% to 0.01% C, at most 0.5% Si, 0.1% to 3.0% Mn, at most 0.04% P, at most 0.01% S, 10% to 15% Cr, 4% to 8% Ni, 2.8% to 5.0% Mo, 0.001% to 0.10% Al, at most 0.07% N, 0% to 0.25% Ti, 0% to 0.25% V, 0% to 0.25% Nb, 0% to 0.25% Zr, 0% to 1.0% Cu, 0% to 0.005% Ca, 0% to 0.005% Mg, 0% to 0.005% La, and 0% to 0.005% Ce, with the balance being Fe and impurities, and the steel satisfies Expressions (1) and (2) and has a yield stress in the range from 758 MPa to 860 MPa. In this way, in the martensitic stainless steel according to the invention, the tempering temperature range at which a yield stress in the range from 758 MPa to 860 MPa can be obtained is increased. 922.6?554.5C?50.9Mn+2944.8P+1.056Cr?81.1Ni+95.8Mo?125.1Ti?1584.9Al?376.1N?600 ??(1) 30C+0.5Mn+Ni+0.5Cu?1.5Si?Cr?Mo+7.

Abstract: A martensitic stainless steel for oil wells, which comprises specified quantities of C, Si, Mn, P, S, Cr, Al, Ni, Cu, Mo, Ti and N, and the balance being Fe and impurities, has not only a satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide but also an excellent low temperature toughness. Therefore, it can be used in an environment containing hydrogen sulfide, carbon dioxide and chloride ion in cold areas. This steel pipe stands good for a use in the above-mentioned severe environment can be easily manufactured by the present invention method.

Abstract: The present invention provides a martensitic stainless steel in which specified elements in a steel composition are limited. The martensitic stainless steel can have high strength of 0.2% proof stress of 860 MPa or more and excellent carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance by limiting the steel composition of specified elements and defining Mo content in the steel by relationships with IM values as well as by forming microstructure of the steel with main tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as a Laves phase, a ? phase and the like. As a result the martensitic stainless steels of the present invention can be applied to practical steels, which can be widely used in oil well tubes and the like under environment including carbon dioxide gas, hydrogen sulfide, chlorine ions or two or more of them, in wide fields.

Abstract: A steel for oil well pipe, which has excellent resistance to localized corrosion in CO2 environments and corrosion in seawater, and a seamless pipe made of the steel. The steel includes, in weight %, more than 0.10 to 0.30% C, 0.10 to 1.0% Si, 0.1 to 3.0% Mn, 2.0 to 9.0% Cr, 0.01 to 0.10% Al and optionally 0.05 to 0.5% Cu, and the balance including Fe and incidental impurities including not more than 0.03% P and not more than 0.01% S. The steel has a substantially single phase martensitic structure in the as-quenched or as-normalized condition, and yield strength of not lower than 552 MPa in the as-quenched-tempered or as-normalized-tempered condition.

Abstract: A process for producing a seamless steel pipe wherein pipe manufacturing steps and the heat treatment steps are carried out in one production line. The properties of the pipe are comparative or superior to those of the pipe manufactured in the conventional reheating, quenching and tempering process. The process is characterized by using the billet of a low alloy steel containing C: 0.15-0.50%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, Al: 0.005-0.50%, Ti: 0.005-0.50% and Nb: 0.003-0.50%, and comprising the following steps (1) to (5).(1) hot rolling with 40% or more of cross sectional reduction ratio,(2) finishing the hot rolling in a temperature range of 800-1100.degree. C.,(3) putting the manufactured steel pipe promptly in a complementary heating apparatus after the finish rolling, and complementarily heating at the temperature and time satisfying the following formula (a).