| 摘要 |
Disclosed are a high strength hot rolled steel plate, preventing a deterioration in bore expandability and ductility involved in an increase in strength to not less than 690 N/mm2 and, despite high strength, providing a high level of bore expandability and a high level of ductility, and a producing process thereof. The steel plate according to the first aspect is a steel plate, comprising a steel comprising, by mass, C:0.01-0.15%; Si:0.30-2.00%; Mn:0.50-3.00%; P.ltoreq.0.03%; S.ltoreq.0.005%; Ti:0.01-0.50% and/or Ni:0.01-0.05%; and the balance consisting of Fe and unavoidable impurities, not less than 80% of all grains being accounted for by grains having a ratio (ds/dl) of minor axis (ds) to major axis (dl) of not less than 0.1, the steel plate having a steel structure comprising not less than 80% of ferrite and the balance consisting of bainite, the steel plate having a strength of not less than 690 N/mm2. The steel plate according to the second aspect is a steel plate wherein, in the steel plate having the above composition, the steel plate has a ferrite-bainite duplex steel structure, in which the proportion of ferrite having a grain diameter of not less than 2 .mu.m is not less than 80%, and has a strength of not less than 690 N/mm2. The steel plate according to the third aspect is a steel plate comprising a steel wherein, in the steel having the above composition, the contents of C, Si, Mn, Ti and Nb satisfying a requirement represented by formula: 115 .ltoreq. (917 - 480 [C%] + 100 [Si%] - 100 [Mn%]) - (790 .times. ([Ti%] + [Nb%]/2)0.05) .ltoreq. 235, and the steel plate has a steel structure comprising not less than 80% of ferrite and the balance consisting of bainite and has a strength of not less than 770 N/mm2.
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