| 摘要 |
PROBLEM TO BE SOLVED: To stably mass-produce low yield ratio steel pipes at a low cost by subjecting a steel having a specified componental compsn. to each treatment of hot rolling, air cooling, quenching, tempering and forming in succession under specified conditions. SOLUTION: A steel contg., by weight, 0.05 to 0.20% C, 0.01 to 0.5% Si, 0.5 to 2.0% Mn, <=0.05% P, <=0.01% S, 0.005 to 0.10% sol.Al and 0.001 to 0.010% N and furthermore contg. one or more kinds among 0.005 to 0.05% Nb, 0.01 to 0.10% V and 0.005 to 0.10% Ti is subjected to hot rolling in the temp. range of less than the recrystallization temp. to more than the Ar3 at >=30% cumulative draft. Next, the steel sheet obtd. by subjecting the same to air cooling at the Ar3 -50 deg.C±30 deg.C, thereafter executing quenching to an ordinary temp. at a cooling rate of 10 to 40 deg.C/sec and subjecting it to tempering treatment at 450 to 600 deg.C is formed into a steel pipe, and the expanding rate in the final stage in pipe making is regulated to >=0.4%. Thus, the low yield ratio 60kilo class steel pipe in which the pipe thickness is regulated to about <=50mm, the external size is regulated to about 400 to 2000mm and the yield ratio in the pipe axial direction in a tensile test is regulated to <=80% can stably be produced.
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