| 摘要 |
PURPOSE:To manufacture hot rolled steel plate having both toughness and low yield ratio practically and inexpensively, by controlling hot rolling condition of steel of which Nb is added. CONSTITUTION:Steel contg. by weight, 0.01-0.2% C, 0.01-0.7% Si, 0.3-2.0% Mn, <=0.003% S, 0.001-0.003% Al, 0.005-0.020% Nb is hot rolled under the following condition. Namely, the steel is held at >=1,150 deg.C temp. to enter Nb carbonitride is steel into solid soln. completely. Next, finish rolling in hot rolling of the steel is ended in austenitic range, e.g. finish rolling in the range is carried out under >=1,100 deg.C rolling starting temp., <=1,000 deg.C finish rolling inlet side temp., >=850 deg.C finish rolling end temp. The steel after completion of rolling is cooled at <=15 deg.C/sec rate and coiled at 550-680 deg.C conventionally. In this way, steel having both low temp. toughness and low yield ratio is obtd.
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