| 摘要 |
PURPOSE:To obtain a non-refined steel plate having much higher strength and toughness than conventional products in techniques for accelerated cooling of steel plates after hot rolling by controlling the cooling patterns specifically in a specified temp. range after rolling. CONSTITUTION:After the hot rolling of a steel plate is finished, the steel plate is cooled forcibly from the temp. for initiation of the cooling wherein Ar3 is a lower limit value down to the temp. for stopping the cooling wherein Ms is a lower limit value. Here, in a cooling stage RA1 of a fore stage from the temp. (t) above the Ar3 down to around Ar1, the cooling rate is made higher than an average cooling rate (curve C) as shown by a curve A. On the other hand, in a cooling stage RA2 of a post stage from around the Ar1 down to a temp. t2 above the Ms, the cooling rate is made lower than the average cooling rate. Then, the driving force that accelerates the formation of nuclei for ferrite is increased in the fore stage. Further the stage from the formation of ferrite up to the finishing thereof is quick, and the growth thereof is suppressed, whereby fining is accelerated. The reason for decreasing the cooling rate in the post stage lies in that it is effective to retard the cooling below the temp. for initiation of bainite formation in order to decrease the condensing of C, Mn, etc. which contribute to the formation of island martensite. |
