| 摘要 |
PROBLEM TO BE SOLVED: To stably produce a hot rolled plate of high strength dual-phase steel, having >=780 MPa tensile strength and excellent in workability and fatigue characteristic. SOLUTION: The steel plate has a composition consisting of, by mass, 0.03-0.15% C, 0.5-2.0% Si, 0.6-2.0% Mn, 0.005-0.1% Al, Ti and Nb in the amounts satisfying inequality 0.006<=Nb+Ti<0.04, and the balance Fe with inevitable impurities. Further, this steel plate is a high strength hot rolled steel plate with dual-phase structure consisting of 60 to 95%, by area ratio, of polygonal ferrite and a hard phase of martensite, bainite, etc., formed at low temp. This steel plate can be produced by forming a slab by continuous casting, rolling the slab or heating the slab up to <=1280 deg.C in a heating furnace and then hot- rolling it, finishing finish rolling in the temp. range of±50 deg.C of the Ar3 transformation point, and coiling the resultant hot rolled plate at <=250 deg.C. At this time, the average cooling rate in the former stage and that in the latter stage are regulated to (1 to 50) deg.C/sec and (10 to 100) deg.C/sec, respectively, and further the average cooling rate in the former stage is lower than that in the latter stage.
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