| 摘要 |
PURPOSE: A method is provided to produce a high-strength thick steel strip having low yield ratio by obtaining a compound structure of fine ferrite and perlite. CONSTITUTION: A steel ingot consists of 0.01 to 0.15wt% of C, less than 0.50wt% of Si, 0.5 to 2.0wt% of Mn, less than 0.020wt% of P, less than 0.010wt% of S, 0.005 to 0.050wt% of Al and the remain of Fe and incidental impurities. Herein, the steel ingot contains more than one among 0.08 to 0.50wt% of Cu, 0.08 to 0.50wt% of Ni, 0.005 to 0.050wt% of each Ti, Nb and V, 0.01 to 1.0wt% of each Cr and Mo and 0.0010 to 0.0050wt% of Ca. The steel ingot is heated in a temperature of 1000 to 1200°C and hot rolled. Then, the ingot is cooled at more than 7°C/sec of speed within a temperature more than Ar.sub.3 transformation point to a normal temperature and cooled again with air. The ingot is reheated and hot rolled with 10 to 80% of depressing ratio in a temperature between Ac.sub.1 to Ac.sub.3 transformation point.
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