| 摘要 |
PURPOSE: To provide high strength and excellent flange workability and resistance weldability by specifying a composition and controlling a rolled structure. CONSTITUTION: This steel plate has a composition containing, by weight ratio, 0.04-0.12% C, 0.1-0.6% Si, 1-3% Mn, <=0.006% S, 0.15-0.3% Ti, 0.02-0.008% Nb, 0.01-0.1% sol.Al, and <=0.05% P. Simultaneously, in the structure of this steel plate, the area ratio of polygonal ferrite of <=6μm average grain size is regulated to >=10% or the area ratio of polygonal ferrite of >=10μm grain size is regulated to >=20% and also the total area ratio of polygonal ferrite is regulated to >=30%, and further, bainite of <=10μm average grain size is contained by >=5% by area ratio, and moreover, tensile strength is regulated to >=740N/mm<2> . In order to obtain this steel plate, it is preferable that, at the time of rolling, the steel is heated to 1240-1340 deg.C, finish-rolled at 800-950 deg.C, cooled down to <=600 deg.C at >=20 deg.C/s average cooling rate, and coiled at 400-550 deg.C.
|
