Sumitomo Metal Industries, Inc.

Kazuo Tsumura

Yasutaka Okada

Masaru Nishiguchi

Tatsuro Kunitake

Iwao Saito

Masahiro Nishio

A method of producing a tube with an ultra-high tensile strength over 255 kg/mm.sup.2 and remarkably improved ductility and toughness is provided, which comprises preparing a mother tube through hot extrusion of a steel consisting essentially of 15.0 - 18.5% by weight of Ni, 12.5 - 15.0% by weight of Co, 5.0 - 6.9% by weight of Mo, 1.00 - 1.28% by weight of Ti, 0.01 - 0.20% by weight of Al and the balance essentially iron, cold working the mother tube with a reduction in wall thickness of 5 - 25%, then raising the temperature of the resulting tube to a temperature of 800 - 950.degree. C in a period of from 20 minutes to 2 hours and maintaining the heated tube at this temperature for from 30 minutes to 3 hours, and, after cooling to or below room temperature, ageing the cooled tube at a temperature of 450.degree. - 550.degree. C for 1 - 10 hours.

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Ibaraki, Japan

Kazuo Tsumura