An austenitic stainless steel material that has a passivation film on a surface is provided. The austenitic stainless steel material has a chemical composition consisting of, in mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 8.0-10.0%, P: 0.030% or less, S: 0.003% or less, Cr: 15.0-18.0%, Ni: 7.0-9.0%, N: 0.15-0.25%, Al: 0.005-0.20%, Ca: 0.0005-0.01%, Cu: less than 1.0%, Mo: less than 1.0%, B: 0-0.0050%, Nb: 0-0.50%, Ti: 0-0.50%, V: 0-0.50%, W: 0-0.50%, Zr: 0-0.50%, Co: 0-0.50%, Mg: 0-0.005%, Ga: 0-0.005%, Hf: 0-0.10%, REM: 0-0.10%, and the balance: Fe and impurities. An f value, namely, [Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 0.53Cu + 12.93C + 7.55N], is more than 29.5 and less than 32.5.

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 Sb: 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.

The present invention relates to a formed product used in vehicle components and the like, and to a method for manufacturing the same. The present invention provides heat treatable steel, a formed product using the same having ultra-high strength and excellent durability, and a method for manufacturing the same, wherein the heat treatable steel contains, in wt %, C (0.22-0.42%), Si (0.05-0.3%), Mn (1.0-1.5%), Al (0.01-0.1%), P (0.01% or less (including 0), S (0.005% or less), Mo (0.05-0.3%), Ti (0.01-0.1%), Cr (0.05-0.5%), B (0.0005-0.005%), N (0.01% or less), the balance Fe, and other inevitable impurities, Mn and Si satisfying Relationship formula (1), below, Mo/p satisfying Relationship formula (2), below: [Relationship formula 1] Mn/Si≧5 [Relationship formula 2] Mo/P≧15.

Provided herein is a dual-phase stainless steel having excellent carbon dioxide corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress cracking resistance. The dual-phase stainless steel contains, in 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%, and the balance 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.

A heating system for drill steel pipe billet includes a feedback device configured to control distances d.sub.1 and d.sub.2 of three flamethrowers, a propulsion device, a positioning device, a heating device, a temperature measuring device and a conveying device. Heating temperature of the flamethrowers is controlled by an oxygen distribution box and a gas distribution box. A heating method for the drill steel pipe billet is also provided, in which a rolling forming method of gradient flame heating is adopted to control density of the rolled pieces and avoid internal defects of the drill steel caused by the same deformation of the traditional uniformly heated pipe billets. A radial temperature of the drill steel pipe billet is accurately controlled through the feedback device. Only one set of flamethrowers corresponding to drill steel pipe billets of different dimensions is required, and other devices are universal components, which will not be replaced.

Provided herein is a high-strength seamless steel pipe, and a method for manufacturing same. A high-strength seamless steel pipe of the present invention has a steel microstructure with a prior austenite grain size of 11.0 or more in terms of a grain size number in compliance with ASTM E112, and has a yield strength of 862 MPa or more and 965 MPa or less.

An anti-collapse oil casing with high strength and a manufacturing method therefor, comprising the following chemical elements in percentage by mass: C:0.08%-0.18%; Si:0.1%-0.4%; Mn:0.1%-0.28%; Cr:0.2%-0.8%; Mo:0.2%-0.6%; Nb:0.02%-0.08% b; V:0.01%-0.15%; Ti:0.02%-0.05%; B:0.0015%-0.005%; and Al:0.01%-0.05%. The manufacturing method for the anti-collapse oil casing with high strength comprises the steps of: (1) smelting and continuous casting; (2) perforating, rolling, and sizing; (3) controlled cooling: the initial cooling temperature being Ar3+50° C. and the final cooling temperature being ≤80° C.; the cooling step being performed only to the outer surface of the casing without performing to the inner wall of the casing; and the rate of the controlled cooling being 30-70° C./s; (4) tempering; and (5) thermal straightening. The anti-collapse oil casing with high strength according to the present invention has reasonable chemical composition and process design, which not only has excellent economic efficiency, but also has high strength, high toughness and high anti-collapse performance.

The duplex stainless steel seamless pipe according to the present disclosure has the chemical composition described in the description and a microstructure consisting of 30 to 55% of ferrite, and austenite. In a square observation field of view region with sides of 250 μm including a center portion of the wall thickness and including a T direction and a C direction, a number of intersections NT which is a number of intersections between the line segment T1 to T4 described in the description and ferrite interfaces is 65 or more. A number of intersections NC which is a number of intersections between the line segments C1 to C4 described in the description and ferrite interfaces is 50 or more.

Provided is a steel component with excellent surface fatigue strength. The steel component 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 component surface to a component inside, where a thickness of a porous layer on an outermost surface of the nitride compound layer is 3.0 μm or less and 40.0% or less of a thickness of the nitride compound layer, and the hardened layer has a hardness of HV600 or more at a position of 50 μm inward from the component surface, a hardness of HV400 or more at a position from the component surface to the component inside of 400 μm, and a hardness of HV250 or more at a position from the component surface to the component inside of 600 μm.

There is provided a duplex stainless steel tube including a chemical composition consisting of, in mass %, C: 0.008 to 0.030%, Si: 0.10 to 0.70%, Mn: 0.80 to 2.60%, P: 0.030% or less, S: 0.0001 to 0.0050%, O: 0.0004 to 0.0150%, Sn: 0.0001% or more to less than 0.0100%, Cu: 0.10 to 2.50%, Ni: more than 2.50 to 5.50% or less, Cr: 21.5 to 25.5%, Mo: 0.10 to 0.50%, N: 0.050 to 0.200%, Al: 0.200% or less, and optional elements, with the balance: Fe and impurities, wherein 4S+8O+Sn is 0.0040 to 0.0900, and 4S+Sn is 0.0180 or less.