Provided is a low alloy oil-well steel pipe having a yield strength of 827 MPa or more, and an excellent SSC resistance. The low alloy oil-well steel pipe according to the present invention consisting of: in mass %, C: more than 0.35 to 0.65%; Si: 0.05 to 0.50%; Mn: 0.10 to 1.00%; Cr: 0.40 to 1.50%; Mo: 0.50 to 2.00%; V: 0.05 to 0.25%; Nb: 0.01 to 0.040%; sol.Al: 0.005 to 0.10%; N: 0.007% or less; Ti: 0 to 0.012%; Ca: 0 to 0.005%; and a balance being Fe and impurities, the impurities including: P: 0.020% or less; S: 0.002% or less; O: 0.006% or less; Ni: 0.10% or less; Cu: 0.03% or less; and B: 0.0005% or less, wherein in a microstructure, a number of cementite particles each of which has an equivalent circle diameter of 200 nm or more is 200 particles/100 μm.sup.2 or more, and a yield strength is 827 MPa or more.

A forming system forming a metal pipe by expanding a metal pipe material, includes: a main body part having a forming die for forming the metal pipe; an electrode causing a current to flow through the metal pipe material disposed in the forming die such that the metal pipe material is heated; a power supply unit disposed at a position separated from the main body part and supplying power to the electrode; and a power supply line connecting the power supply unit and the electrode, in which the power supply line includes a lower-side passing portion passing through a lower side of a placing surface on which the main body part is placed, a first connection portion drawn to an upper side of the placing surface and connecting the lower-side passing portion and the electrode, and a second connection portion connecting the lower-side passing portion and the power supply unit.

Improved methods for quenching a metal tube are disclosed. A method of manufacturing a metal tube generally comprises solution heat treating a metal tube at an elevated temperature, rapidly cooling the metal tube from the elevated temperature, raising the open end of the metal tube to an elevated position, and lowering the open end of the metal tube to a downward facing position, wherein the metal tube comprises an open end and an opposing closed end, wherein the immersing step comprises at least partially filling the metal tube with the cooling liquid, and developing an evolved gas inside the metal tube, wherein the raising comprises releasing at least some of the evolved gas from the metal tube via the open end, and wherein the lowering comprises draining cooling liquid from the metal tube via the open end.

A pipe having chemical composition contains, by mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15% or more and 1.0% or less, Cr: 13.5% or more and 15.4% or less, Ni: 3.5% or more and 6.0% or less, Mo: 1.5% or more and 5.0% or less, Cu: 3.5% or less, W: 2.5% or less, and N: 0.15% or less so that the relationship −5.9×(7.82+27C−0.91 Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0 is satisfied.

The invention is intended to provide a duplex stainless steel and a method for manufacturing same. A duplex stainless steel pipe is also provided. A duplex stainless steel of the present invention has a specific composition, and has a microstructure containing an austenitic phase and a ferrite phase. The duplex stainless steel satisfies the following contents for C, Si, Mn, Cr, Mo, Ni, N, Cu, and W in the formula (1) below, and has a yield strength YS of 655 MPa or more, and an absorption energy vE.sub.−10 of 40 J or more as measured by a Charpy impact test at a test temperature of −10° C.

0.55[% C]−0.056[% Si]+0.018[% Mn]−0.020[% Cr]−0.087[% Mo]+0.16[% Ni]+0.28[% N]−0.506[% Cu]−0.035[% W]+[% Cu*F]≤0.94  (1)

A steel pipe for fuel injection pipe has a tensile strength of 500 to 900 MPa and a yield ratio of 0.50 to 0.85, and has a critical internal pressure (IP) satisfying [IP≥0.41×TS×α] (α=[(D/d).sup.2−1]/[0.776×(D/d).sup.2], where TS: tensile strength (MPa) of the steel pipe, D: steel pipe outer diameter (mm), and d: steel pipe inner diameter (mm)), wherein a circumferential-direction residual stress on an inner surface of the pipe is −20 MPa or lower after the steel pipe is split in half in a pipe axis direction.

A method for manufacturing a bainite high-strength seamless steel tube, comprising the following steps: smelting, manufacturing a billet, heating, perforating, rolling, stretch reducing or sizing to obtain tube, and cooling. In the cooling step, the quenching starting temperature is controlled to be at least 20° C. higher than the Ar3 temperature of the steel grade; the finish cooling temperature is controlled to be within a range between T1 and T2, where T1=519-423 C-30.4Mn, T2=780-270 C-90Mn, and the units of the T1 and the T2 are ° C.; in the formulas, C and Mn respectively represent the mass percents of element C and element Mn of the steel grade, the content of the element C is 0.06-0.2%, and the content of the element Mn is 1-2.5%; the cooling rate is controlled to be 15-80° C./s; and the finished product of the bainite high-strength seamless steel tube is directly obtained after the cooling step. The manufacturing of a bainite high-strength seamless steel tube using the method requires neither the addition of precious alloying elements nor the subsequent heat treatment. Therefore the production costs are low.

A system for air quenching a heat treated element comprises a tubular component, an internal air quench device moveably disposed within the interior of the tubular component, and an external air quench device moveably disposed about the tubular component. The internal air quench device comprises a nozzle configured to induce an airflow within the tubular component. The external air quenching device can comprise an annular ring disposed about the tubular component that is configured to generate a cone of air about the tubular component.

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.

The steel material according to the present disclosure has a chemical composition consisting of, in mass %, C: 0.10 to 0.60%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, V: 0.01 to 0.60%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0020 to 0.0100%, and O: 0.0100% or less, with the balance being Fe and impurities. A dislocation density ρ is 3.5×10.sup.15 m.sup.−2 or less. Among fine precipitates, the numerical proportion of precipitates for which a ratio of the Mo content is not more than 50% is 15% or more. The yield strength is in a range of 655 to 1172 MPa.