| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of manufacture of a high tensile strength steel tube for construction use, capable of stably and inexpensively mass-producing a 60kg-class low-yield-ratio steel tube of <=50 mm tube thickness, used for a concrete-filling-type composite steel tube pillar, etc., while obviating the necessity of plate heat treatment after rolling and heat treatment after cold forming. SOLUTION: A steel, having a composition consisting of, by weight. 0.06-0.17% C, 0.06-0.5% Si, 0.5-1.6% Mn, <=0.05% P, <=0.01% S, <=0.07% Al, <=0.006% N, further one or <=2 kinds among 0.05-0.5% Mo, 0.01-0.1% V, and 0.005-0.015% Ti, and the balance Fe with inevitable impurities, is heated to >=1000 deg.C and hot rolled in the temp. region not lower than the Ar3 point. The resultant steel plate is subjected to accelerated cooling down to the temp. region of Ar3 -50 deg.C±30 deg.C at a rate of (1 to 40) deg.C/sec. Subsequently, the acceleratedly cooled steel plate is held in the temp. region of Ar3 -50 deg.C±30 deg.C for 1-150 sec and subjected to accelerated cooling down to 400-600 deg.C at a rate of (1 to 40 deg.C)/sec. The resultant steel plate is cold-formed into steel tube, and further, the tube expanding rate in the final stage of tube manufacture is regulated to >=0.8%.
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