| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high strength and high toughness steel sheet having≥550 MPa yield strength at a normal temperature and an intermediate temperature zone, especially suitable to the use as a high strength and high toughness welded steel tube for vapor piping. SOLUTION: This production method is performed as the followings, with which the steel composed by mass% of 0.04-0.08% C, 0.05-0.2% Si, 1.5-2% Mn,≤0.02% P,≤0.002% S, 0.05-0.3% Mo, 0.03-0.07% Nb,≤0.02% Ti,≤0.04% Al,≤0.015% REM,≤0.006% N and further, one or more kinds among Cu, Ni, Cr, V, Ca and the balance Fe with inevitable impurities, is heated to 1100-1200°C, and after hot-rolling at≤850°C rolling-finish temperature in≥50% accumulated rolling reduction ratio at≤900°C, the accelerated cooling is performed to 400-550°C at≥5°C/sec cooling speed and thereafter, immediately, reheating is performed to 550-700°C at≥0.5°C/s temperature-rising speed. COPYRIGHT: (C)2007,JPO&INPIT
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