| 摘要 |
PROBLEM TO BE SOLVED: To provide a manufacturing method of a square seamless steel tube for obtaining a product having small roundness in corner parts and flat sides even when its wall is thick. SOLUTION: At the time of forming a cylindrical tube stock which is obtained by piercing a cast steel billet with seamless steel tube manufacturing equipment into a square shape, successively, by hot-rolling the tube stock with multistage forming mill the caliber 4 of the roll of each stand of which is changed from a cylindrical shape to a square shape, the cylindrical tube stock is hot-rolled so that in the stand having a first square caliber, the rate of reduction RL of a peripheral length li is larger than Bt<2> /L(S-At) which is determined from the length L of the side, thickness (t) and the upper limit value of S value of the square steel tube of the final products. Where, S value = (1/2)×(L-LF), L is the length of the side of the cross section of the steel tube (mm), LF is the length of the flat part of the side of the cross section of the steel tube (mm) and both A and B are constants.
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