Steel plate and their method of production used to it and the low yield ratio column for steel pipe for construction

摘要

PROBLEM TO BE SOLVED: To obtain a steel tube excellent in economy and shocking characteristics, in the case of making the steel tube for low yield ratio column for building by cold-forming an extremely thick steel plate having ≥40 mm plate thickness.;SOLUTION: The manufacturing method includes: heating a rolling stock; hot-rolling it; then subjecting the hot-rolled steel plate to any one of heat treatments shown in the followings (1) to (3); manufacturing a steel plate without performing subsequent heat treatment; subsequently cold-forming it into the steel pipe; then holding it at 400 to 650°C which are lower than the Ac₁ transformation temperature, for at least five minutes; and then cooling it at a cooling rate of 10°C/second or less. The heat treatments includes: (1) after rolling, directly quenching the rolled plate from an Ar₃ temperature or lower to ≤400°C; (2) after the rolling, directly quenching the rolled plate from a temperature of Ar₃+50°C or lower to ≤400°C, further reheating it, and quenching it at a temperature between Ac₁ to Ac₃; and (3) after the rolling, air-cooling the rolled plate, further reheating it, then quenching it at a temperature of Ar₃ or higher, further reheating it, and then quenching it at a temperature between Ac₁ and Ac₃.;COPYRIGHT: (C)2009,JPO&INPIT