| 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 Ac<SB>1</SB>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 Ar<SB>3</SB>temperature or lower to≤400°C; (2) after the rolling, directly quenching the rolled plate from a temperature of Ar<SB>3</SB>+50°C or lower to≤400°C, further reheating it, and quenching it at a temperature between Ac<SB>1</SB>to Ac<SB>3</SB>; and (3) after the rolling, air-cooling the rolled plate, further reheating it, then quenching it at a temperature of Ar<SB>3</SB>or higher, further reheating it, and then quenching it at a temperature between Ac<SB>1</SB>and Ac<SB>3</SB>. COPYRIGHT: (C)2009,JPO&INPIT
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