PURPOSE: To produce a steel plate with mechanical damage resistance, where weldability is secured and which has superior bendability, by subjecting a steel slab with a temp. not lower than the Ac3 point to specific cooling, to rolling, and to specific cooling and providing specific hardness to the surface and the rear surface layer part.
CONSTITUTION: A steel slab or a steel plate, having a temp. not lower than the Ac3 point, is cooled rapidly down to a temp. not higher than the Ar1 point at a rate of 2°C/sec, through the region between the surface layer and a position of 1×t0/t to 0.3t0 [where t0 means plate thickness at the time of water cooling in the course of rolling and (t) means product plate thickness] in the direction of plate thickness. Rolling is started or started again from the point of time when the temp. in the surface layer part reaches ≤Ar3 and finished at a temp. between (Ac3-50°C) and Ac3. Then the surface layer part is cooled at least to the Ar1 point at a rate of ≥1°C/sec without recuperating the steel plate up to a temp. not lower than the Ac3 point. Further, the steel plate is regulated to a temp. not lower than the Ac3 while being held at a temp. in the range between Ac1-100°C to Ac1 for ≥120sec, followed by than the Ac, while being held at a temp. in the range rapid cooling at a rate of ≤10°C/ sec. By this method, the steel plate can be regulated so that the part ranging from the surface and the rear surface layer part to the position at a depth of ≥1mm from the surface and the rear surface layer part and also ≤30% of the plate thickness is composed of old austenitic grains of ≥50μm and also the hardness in this part becomes higher than the hardness in the inner part, containing ≥80% martensite, by ≥50 by hardness Hvic.
NOMIYAMA YUJI
TAKEZAWA HIROSHI
| JPS5380317A | 1978-07-15 |