PURPOSE: To regulate segregation and A type inclusions in a steel plate and to obtain a steel plate hardly causing defects detected by ultrasonic flaw detection due to heat-affected zone cracking in the high heat input welded square joint part of a steel-frame box pillar by subjecting steel having a specified compsn. to continuous casting, specified heating and rolling.
CONSTITUTION: Molten steel consisting of, by weight, 0.10-0.20% C, 0.05-0.50% Si, 0.50-1.80% Mn, ≤0.010% P, ≤0.002% S, 0.0008-0.0035% Ca, 0.020-0.080% Al and the balance Fe with inevitable impurities is continuously cast into a slab and this slab is heated at ≥1,280°C for ≥8hr and rolled. By this method, the difference in hardness Hv between the nonsegregated and segregated parts is regulated to ≤30 and the amt. of A type inclusions in the segregated part measured in accordance with JIS G0555 (microscopic test of nonmetallic inclusions in steel) is regulated to ≤0.008%. After the rolling, accelerated cooling or normalizing is preferably carried out.
SHIMIZU MASATO
IWAI KIYOSHI
OKANO SHIGEO
| JPS6256519A | 1987-03-12 |