Nippon Steel Corporation

Yoichi Yazaki

Haruyuki Nagayoshi

Kazushi Maruyama

Kyohei Murata

Heiji Morise

Etuji Kajiki

Quenched steel pipe is first held at a tempering temperature for a given period of time, then cooled relatively rapidly. While carrying the pipe in the direction of its length, cooling water is sprayed from outside onto its external surface at an average water flux of not lower than 0.05 m.sup.3 /min.m.sup.2 and not higher than 2 m.sup.3 /min.m.sup.2. This cooling is started when the pipe temperature is between 400.degree. C. and 700.degree. C. and ended between room temperature and 350.degree. C. Also, the cooling is effected at a mean cooling rate of not lower than 5.degree. C./sec. and not higher than 40.degree. C./sec. This cooling method improves the collapse strength of a pipe by providing an appropriate tensile residual stress to the internal surface of the pipe and reduces the cooling floor area without sacrificing the collapse strength of the pipe.

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Kitakyushu, Japan

Yoichi Yazaki