The present invention relates to a high strength steel tube. In addition, the invention relates to a method of manufacturing a high strength steel tube. The method is characterized in that a hot rolled pre-tube is subjected to at least two hardening steps with a final tempering step, the pre-tube is heated to a quenching temperature of at least Ac3 temperature for hardening and is heated to a tempering temperature in the range of 400 to 600° C. for tempering.

This electric resistance-welded steel tube for a hollow stabilizer is an electric resistance-welded steel tube for a hollow stabilizer including a base material portion and a weld, in which the base material portion has predetermined chemical components, a wall thickness of the base material portion is 2.0 to 6.0 mm, an outer diameter of the electric resistance-welded steel tube is 10 to 40 mm, in a C direction cross section of the electric resistance-welded steel tube, a recessed bead cut is present in a region including the weld on an inner surface side of the electric resistance-welded steel tube, when an imaginary line is drawn from one opening edge to the other opening edge of the bead cut in a shortest distance, a maximum depth from the imaginary line to a bottom of the bead cut is 300 μm or less, a maximum inclusion diameter that is included in the base material portion is 300 μm or less, in the base material portion of the electric resistance-welded steel tube, a surface roughness of the inner surface side is 300 μm or less in terms of a maximum profile valley depth Rv, and maximum hardness of the electric resistance-welded steel tube including the weld is 300 Hv or less.

A stainless steel seamless pipe having high strength, excellent low-temperature toughness and corrosion resistance, and a composition including, 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: 14.0% or more and 17.0% or less, Mo: more than 3.80% and 6.0% or less, Cu: more than 1.03% and 3.5% or less, Ni: 3.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and the balance is Fe and incidental impurities. Elements C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a predetermined relationship. The stainless steel seamless pipe has a yield strength of 862 MPa or more and has a microstructure that contains at least 40% martensitic phase, at most 60% ferrite phase, and at most 30% retained austenite phase by volume.

A method for manufacturing an electric resistance welded steel tube for high-strength thin hollow stabilizers is provided. A steel with a certain chemical composition is heated to a temperature of 1000 to 1300° C., hot rolled under conditions where the rolling finish temperature is 750 to 950° C., cooled, coiled into a coil at 500 to 650° C., and skin pass rolled with a rolling reduction ratio of not less than 0.3%. The resultant hot rolled sheet is electric resistance welded into an electric resistance welded steel tube, which is then reheated to a temperature of 800 to 1100° C. and hot stretch-reducing rolled under conditions where the rolling finish temperature is not more than 850° C. and the cumulative diameter reduction is not more than 75%. Consequently, a thin electric resistance welded steel tube is produced.

A steel pipe for a fuel injection pipe has a chemical composition consisting of, by mass %: C: 0.17 to 0.27%, Si: 0.05 to 0.40%, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.0100% or less, O: 0.0040% or less, Ca: 0.0010% or less, Al: 0.005 to 0.060%, N: 0.0020 to 0.0080%, Ti: 0.005 to 0.015%, Nb: 0.015 to 0.045%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.15%, and the balance: Fe and impurities. The metal micro-structure consists substantially of tempered martensite, or tempered martensite and tempered bainite. The hardness is within the range of 350 to 460 HV1. A lattice spacing of a (211) diffraction plane measured by CoKα characteristic X-ray diffraction is 1.1716 Å or less, and a half-value width of the (211) diffraction plane is 1.200° or less. The number density of cementite having a diameter of 50 nm or more is 20/μm.sup.2 or less.

Provided herein is a high-strength seamless steel pipe containing a particular chemical composition. The volume fraction of tempered martensite is 90% or more in terms of a volume fraction. The number of nitride inclusions with a particle diameter of 4 μm or more is 50 or less per 100 mm.sup.2, the number of nitride inclusions with a particle diameter of less than 4 μm is 500 or less per 100 mm.sup.2, the number of oxide inclusions with a particle diameter of 4 μm or more is 40 or less per 100 mm.sup.2, and the number of oxide inclusions with a particle diameter of less than 4 μm is 400 or less per 100 mm.sup.2 in a cross section perpendicular to a rolling direction.

Provided herein is a high-strength stainless steel seamless pipe for oil country tubular goods. The high-strength stainless steel seamless pipe having a yield strength of 862 MPa or more contains, in mass %, C:0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, B: 0.0005 to 0.0100%, and the balance Fe and unavoidable impurities, and in which the composition satisfies specific formulas. The stainless steel pipe has more than 45% martensite phase, 10 to 45% ferrite phase, and 30% or less retained austenite phase. The ferrite grains have a maximum crystal grain size of 500 μm or less.

The present invention deals with alloyed steels having yield strength of at least 862 MPa (125 Ksi) and exhibiting outstanding hardness and toughness behavior, especially under stringent conditions which may be subjected to frost-heave and thaw settlement cycles, namely at subzero temperatures. The invention also relates to a seamless pipe comprising said steel and a method of production of said pipe thereof.

A process for preparing a multi-thickness welded steel vehicle rail, the process comprises the steps of: (a) forming a first tube having a first outer diameter, an inner diameter and a first wall thickness; (b) forming a second tube having the first outer diameter, a second inner diameter and a second wall thickness different than the first wall thickness; (c) swaging a first end of the first tube to a second outer diameter less than the second inner diameter of the second tube; (d) inserting the swaged first end of the first tube into an end of the second tube to form a joint; (e) welding the first tube and the second tube together to form a weld at the joint to form a tube blank with a heat affected zone of lower metal strength in the area of the weld; (f) preheating the tube blank to create a common crystalline microstructure along a length of the tube blank; (g) introducing the tube blank into a blow molding tool having inner molding walls; (h) molding the tube blank at an elevated temperature by expanding the tube blank against the inner molding walls of the molding tool by injecting a pressurized medium into an interior cavity of the tube blank; and (i) quenching the tube blank by replacing the pressurized medium with a cooling medium through the molding tool and the tube blank to achieve a rapid cooling effect on the tube blank and to create a completed vehicle rail with essentially uniform material strength across the weld. A completed vehicle rail has an overlapped welded structure and uniform microcrystalline structure along the length of the rail.

The high-strength seamless steel pipe has a volume fraction of tempered martensite of 95% or more, and a prior austenite size number of 8.5 or more, and contains nitride inclusions having a size of 4 μm or more and whose number is 100 or less per 100 mm.sup.2, nitride inclusions having a size of less than 4 μm and whose number is 700 or less per 100 mm.sup.2, oxide inclusions having a size of 4 μm or more and whose number is 60 or less per 100 mm.sup.2, and oxide inclusions having a size of less than 4 μm and whose number is 500 or less per 100 mm.sup.2, in a cross section perpendicular to a rolling direction.