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Abstract

A method for producing seamless steel tubes suitable for use as grades of casing and line pipe having yield strengths in excess of 70,000 psi, without being heat treated. Such steels are made of an alloy comprising, by weight, about 0.10% to 0.18% carbon, about 0.10% to 2.0% manganese, about 0.10% to 0.16% vanadium, about 0.008% to 0.012% titanium and about 150 parts per million to 220 parts per million nitrogen, the balance comprising iron and incidental impurities. The subject method comprises the steps of alloying a steel of the aforesaid chemical composition, forming the steel into a billet of steel, reheating the billet in a reheating furnace, passing the billet through a piercing mill to form a steel shell, elongating the steel shell within a mandrel mill, and reducing the diameter of the elongated shell in a stretch reducing mill. Strains are applied to the shell in the stretch reducing mill below the T.sub.nr of the steel and above the A.sub.r3 to provoke dynamic recrystallization. The subject method may also include the steps of cooling the steel shell after it exits the mandrel mill to a temperature below its A.sub.r1 temperature, prior to reheating, and/or accelerated cooling (optional, special cooling may not be necessary for thinner walls) after exiting the stretch reducing mill at a rate between 3.degree. C. to 5.degree. C. per second. The nitrogen and vanadium are preferably introduced to the steel during alloying in the form of a VN alloying agent.

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