| 摘要 |
<p>PROBLEM TO BE SOLVED: To stably provide high collapse strength by computing initial cooling temp. so that the inside circumferential residual compressive stress of a steel pipe, resultant from warm straightening, becomes equal to residual tensile stress at water cooling and controlling the amount of cooling water. SOLUTION: A quenched-and-tempered steel pipe is subjected to warm straightening at a temp. between 150 deg.C and tempering temp. The inside circumferential residual compressive stress of the steel pipe, resultant from warm straightening, is determined from the relation with previously determined rolling reduction of outside diameter at straightening, and the initial steel pipe cooling temp., where the inside circumferential residual tensile stress of the steel pipe at water cooling becomes equal to the computed residual compressive stress, is computed on the basis of an equation. The amount of cooling water is controlled on the basis of the above, and the final inside circumferential residual compressive stress of the steel pipe is reduced. The residual outside-tensile and inside-compressive stress of the steel tube, resultant from warm straightening, is compensated with the residual outside-compressive and inside-tensile stress of the steel pipe, resultant from water cooling, and, as a result, the final residual stress of the steel pipe can be reduced and collapse strength can be improved.</p> |
