| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of manufacture of a high strength hot rolled steel plate having an excellent strength-ductility balance satisfying TS×El=24000MPa.% and minimal in fluctuations is mechanical properties in a coil, by means of regulation of the volume ratio of retained austenite to >=5%. SOLUTION: A slab, having a composition consisting of, by weight, 0.10-0.25% C, 0.01-1.00 Si, 1.00-2.00% Mn, 0.50-2.00% sol.Al, and the balance Fe with inevitable impurities, is used. This slab is heated to a two-phase mixed region of ferrite and austenite and finish-rolled in the two-phase mixed region of ferrite and austenite. The resultant hot rolled steel strip is air-cooled at a rate of <=15 deg.C/s in the front stage of a run-out table after the above finish rolling and water-cooled at >=50 deg.C/s in the rear stage and then coiled at 350-500 deg.C to form the microstructure of the resultant steel strip into a structure consisting of polygonal ferrite and retained austenite of >=5% by volume ratio, or, after finish rolling, the resultant hot rolled steel strip is cooled at >=20 deg.C/s down to 650-750 deg.C in the initial stage and held for >=1s and is then cooled at >=20 deg.C/s in the finish stage and coiled at 350-500 deg.C.
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