Low-alloy oil-well steel pipe includes a composition consisting, in mass %, of C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.15% or less, Cr: 0.40 to 1.50%, Mo: 0.50 to 2.50%, V: 0.05 to 0.25%, Ti: 0 to less than 0.01%, Nb: 0.01 to 0.2%, sol. Al: 0.010 to 0.100%, N: 0.006% or less, B: 0 to 0.0015%, and Ca: 0 to 0.003%, the balance being Fe and impurities. The structure has tempered martensite and 0 to less than 2% volume ratio of retained austenite. A grain size number of prior-austenite grain in the structure is 9.0 or more. An equivalent circular diameter of a sub-structure surrounded by a boundary having a crystal orientation difference of 15 or more from a packet boundary, a block boundary and a lath boundary is 3 m or less for the tempered martensite.

Provided is a round billet capable of reducing damage on a piercing plug in a method of producing a seamless metal tube with a Mannesmann process. The round billet (5), for use in a seamless metal tube, to be produced into a seamless metal tube with a Mannesmann process includes a body having a hole (6) formed in an axial direction of the body. The hole (6) includes an aperture (6a) opening at least at one end face of the round billet (5), and a tapered portion (61) continued to the aperture (6a) and having a diameter gradually increasing toward the aperture (6a).

Provided is a round billet capable of reducing damage on a piercing plug in a method of producing a seamless metal tube with a Mannesmann process. The round billet (5), for use in a seamless metal tube, to be produced into a seamless metal tube with a Mannesmann process includes a body having a hole (6) formed in an axial direction of the body. The hole (6) includes an aperture (6a) opening at least at one end face of the round billet (5), and a tapered portion (61) continued to the aperture (6a) and having a diameter gradually increasing toward the aperture (6a).

The seamless pipe in which a thin-walled portion in a pipe circumferential direction is formed in a pipe axial direction, in which a line segment formed by connecting one end and the other end of the thin-walled portion along a pipe surface with a shortest distance in a formation direction of the thin-walled portion is inclined at an angle of 5.0 or more with respect to the pipe axial direction. It is preferable that one end and the other end of the thin-walled portion are set from a region in a pipe selected with a shorter length between a length of 1.0 m in the pipe axial direction and 90% of a length in the pipe axial direction where the thin-walled portion turns once in the pipe circumferential direction.

A corrosion resistant alloy suitable for use as a seamless tubular is described. The corrosion resistant alloy includes 13-15 wt. % chromium, 5-7 wt. % nickel, and 2.5-4.5 wt. % molybdenum. The balance of the corrosion resistant alloy is iron.

A corrosion resistant alloy suitable for use as a seamless tubular is described. The corrosion resistant alloy includes 13-15 wt. % chromium, 5-7 wt. % nickel, and 2.5-4.5 wt. % molybdenum. The balance of the corrosion resistant alloy is iron.

Provided is a method for producing a plug for use in a piercing-rolling mill for producing a seamless steel tube/pipe, and the method for producing the plug for piercing-rolling comprises shotblasting a surface of the plug, and arc-spraying a spray wire so as to form a film on a surface of a base metal of the plug that is shotblasted. In the arc-spraying step, the arc spraying is carried out by using, as the spray wire, a cored wire whose iron sheath tube is charged with at least iron oxide particles among iron particles and the iron oxide particles (one or more of FeO particles, Fe.sub.3O.sub.4 particles, and Fe.sub.2O.sub.3 particles), so as to form the film containing iron oxide and Fe. Accordingly, it is possible to enhance the production efficiency of the plug, and to produce the plug for piercing-rolling capable of securing the steady enhancement of the durability life of the plug during the piercing-rolling.

Provided is a method for producing a plug for use in a piercing-rolling mill for producing a seamless steel tube/pipe, and the method for producing the plug for piercing-rolling comprises shotblasting a surface of the plug, and arc-spraying a spray wire so as to form a film on a surface of a base metal of the plug that is shotblasted. In the arc-spraying step, the arc spraying is carried out by using, as the spray wire, a cored wire whose iron sheath tube is charged with at least iron oxide particles among iron particles and the iron oxide particles (one or more of FeO particles, Fe.sub.3O.sub.4 particles, and Fe.sub.2O.sub.3 particles), so as to form the film containing iron oxide and Fe. Accordingly, it is possible to enhance the production efficiency of the plug, and to produce the plug for piercing-rolling capable of securing the steady enhancement of the durability life of the plug during the piercing-rolling.

A method for preparing a high-performance difficult-to-deform metal precision seamless pipe includes steps of performing a heat treatment; drilling; externally grinding; internally grinding; straightening; performing four-roller warm-rolling; performing warm-drawing to reduce a diameter; performing warm-expansion to reduce a wall thickness and increase the diameter; performing precise cold-rolling; degreasing; brightening; performing surface grinding; cleaning dust; detecting flaws; testing metal structure performance; and sizing and packaging. By cycling the warm-drawing, the warm-expansion, and the precision cold-rolling, key indicators such as product dimensional accuracy, surface quality, material properties, and crystal grain size can be collaboratively controlled, to achieve higher accuracy, better performance, and more outstanding extreme specifications. Product requirements of different hard-to-deform metal materials and different product specifications can be satisfied, to flexibly prepare metal pipe products with different material characteristics, which greatly improves production efficiency and effectively reduces production costs.

A method for preparing a high-performance difficult-to-deform metal precision seamless pipe includes steps of performing a heat treatment; drilling; externally grinding; internally grinding; straightening; performing four-roller warm-rolling; performing warm-drawing to reduce a diameter; performing warm-expansion to reduce a wall thickness and increase the diameter; performing precise cold-rolling; degreasing; brightening; performing surface grinding; cleaning dust; detecting flaws; testing metal structure performance; and sizing and packaging. By cycling the warm-drawing, the warm-expansion, and the precision cold-rolling, key indicators such as product dimensional accuracy, surface quality, material properties, and crystal grain size can be collaboratively controlled, to achieve higher accuracy, better performance, and more outstanding extreme specifications. Product requirements of different hard-to-deform metal materials and different product specifications can be satisfied, to flexibly prepare metal pipe products with different material characteristics, which greatly improves production efficiency and effectively reduces production costs.