| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high tensil strength hot rolled steel plate extremely excellent in ductility, toughness and fatigue strength. SOLUTION: A steel contg., by weight, 0.05 to 0.30% C, <=2.0% Si, 1.0 to 2.5% Mn and <=0.05% Al and contg. one or two kinds of 0.05 to 0.3% Ti and <=0.10% Nb, and the balance Fe with inevitable impurities is heated at 950 to 1100 deg.C, is thereafter applied with rolling reduction so as to regulated the draft to >=20% for at least >= two times and is subjected to hot rolling so as to regulate the finishing temp. to the Ar3 transformation point or above. After that, it is cooled in the temp. range of the Ar3 transformation point or above to 750 deg.C at a rate of >=20C/sec, is successively retained in the temp. range of <750 to 600 deg.C for 5 to 20sec, is again cooled to <=550 deg.C at a rate of >=20C/sec and is coiled round a coil at <=550 deg.C to obtain an extremely fine structure in which the ferrite volume rate is regulated to >=80% and the average ferrite grain size to <10μm.
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