A method of manufacturing a high-strength stainless steel pipe includes forming a steel pipe having a predetermined size, the steel having a composition comprising by mass % 0.005 to 0.05% C, 0.05 to 1.0% Si, 0.2 to 1.8% Mn, 0.03% or less P, 0.005% or less S, 14 to 20% Cr, 1.5 to 10% Ni, 1 to 5% Mo, 0.5% or less V, 0.15% or less N, 0.01% or less O, 0.002 to 0.1% Al, and Fe and unavoidable impurities as a balance; applying a quenching treatment two times or more to the steel pipe where the steel pipe is quenched by reheating to a temperature of 750 C. or above and cooling to a temperature of 100 C. or below at a cooling rate equal to or higher than an air-cooling rate; and applying a tempering treatment where the steel pipe is tempered at a temperature of 700 C. or below.

A piercing plug of the present invention includes: a plug body; a NiCr layer formed on a surface of the plug body; and a sprayed coating formed on a surface of the NiCr layer. The plug body contains, by mass %, 20 to 30% of Cr, 30 to 55% of Ni, 0.005 to 0.5% of C, 0.1 to 1.0% of Si, 0.2 to 1.5% of Mn, and at least one of Mo and W which satisfy a following conditional expression (A), and remainder including Fe and impurities.
1.5%Mo+0.5W8.5%(A)

A piercing plug of the present invention includes: a plug body; a NiCr layer formed on a surface of the plug body; and a sprayed coating formed on a surface of the NiCr layer. The plug body contains, by mass %, 20 to 30% of Cr, 30 to 55% of Ni, 0.005 to 0.5% of C, 0.1 to 1.0% of Si, 0.2 to 1.5% of Mn, and at least one of Mo and W which satisfy a following conditional expression (A), and remainder including Fe and impurities.
1.5%Mo+0.5W8.5%(A)

There are provided: a seamless steel pipe formed from a cylindrical steel material billet through a hot isostatic extrusion step, wherein a depth of a contiguous flaw formed on an inner periphery surface and an outer periphery surface of the steel pipe is 50 m or less; a hollow spring obtained by forming a hollow body in a shape of a coil or a bar or a bar with curved part from the seamless steel pipe made of spring steel and applying a surface treatment to the hollow body so that the hollow body has compressive residual stress; and a method for producing seamless steel pipe including: a billet molding step; a first heating step; a hot isostatic extrusion step; a second heating step; an extension step; a third heating step; and a pickling step.

There are provided: a seamless steel pipe formed from a cylindrical steel material billet through a hot isostatic extrusion step, wherein a depth of a contiguous flaw formed on an inner periphery surface and an outer periphery surface of the steel pipe is 50 m or less; a hollow spring obtained by forming a hollow body in a shape of a coil or a bar or a bar with curved part from the seamless steel pipe made of spring steel and applying a surface treatment to the hollow body so that the hollow body has compressive residual stress; and a method for producing seamless steel pipe including: a billet molding step; a first heating step; a hot isostatic extrusion step; a second heating step; an extension step; a third heating step; and a pickling step.

An online-control cooling process for seamless steel tube for effectively refining grains, comprising the following steps: when the temperature of a crude tube is higher than Ar3, evenly spraying water along the circumferential direction of the tube so as to continuously cool the tube to T1? C.?T2? C., the cooling rate being controlled to be N1? C./s?N2? C./s, wherein T1=810?360C?80(Mn+Cr)?37Ni?83Mo, T2=T1+115? C., N1=55-80?C, N2=168*(0.8?C), and C, Mn, Cr, Ni and Mo in the equations each represent the mass percentage of corresponding elements in the seamless steel tube; then, cooling to the room temperature at cooling rate no more than 10? C./s. Correspondingly, also provided are a method for manufacturing seamless steel tube for effectively refining grains, and a seamless steel tube. The online-control seamless steel tube cooling process does not require adding too many alloying elements, which is simple and can yield seamless steel tubes with good grain refinement and better toughness.

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?423C?30.4Mn, T2=780?270C?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 seamless steel tube with high strength and toughness, comprising the following chemical elements by mass: 0.1-0.25% of C, 0.1-0.5% of Si, 0.01-0.1% of Al, 0.6-2% of Mn, the balance of Fe and other unavoidable impurities, wherein C+Mn/6?0.35. Also provided is a method for preparing a seamless steel tube.

An process for the on-line quenching of seamless steel tube using residual heat, a method for manufacturing a seamless steel tube, and a seamless steel tube. The process for the on-line quenching of a seamless steel tube comprises the following steps: when the temperature of a tube is higher than Ar3, evenly spraying water along a circumferential direction of the tube so as to continuously cool the tube to be not higher than T? C., the cooling rate being controlled to be E1? C./s to E2? C./s to obtain a microstructure with martensite as the main composition, wherein T=Ms?95? C., Ms represents the martensitic phase transition temperature, E1=20?(0.5?C)+15?(3.2?Mn)?8?Cr?28?Mo?4?Ni?2800?B, and E2=96?(0.45?C)+12?(4.6?Mn), and the C, Mn, Cr, Ni, B and Mo in the equations each represents the mass percentages of corresponding elements in the seamless steel tube.

An process for the on-line quenching of seamless steel tube using residual heat, a method for manufacturing a seamless steel tube, and a seamless steel tube. The process for the on-line quenching of a seamless steel tube comprises the following steps: when the temperature of a tube is higher than Ar3, evenly spraying water along a circumferential direction of the tube so as to continuously cool the tube to be not higher than T? C., the cooling rate being controlled to be E1? C./s to E2? C./s to obtain a microstructure with martensite as the main composition, wherein T=Ms?95? C., Ms represents the martensitic phase transition temperature, E1=20?(0.5?C)+15?(3.2?Mn)?8?Cr?28?Mo?4?Ni?2800?B, and E2=96?(0.45?C)+12?(4.6?Mn), and the C, Mn, Cr, Ni, B and Mo in the equations each represents the mass percentages of corresponding elements in the seamless steel tube.