| 摘要 |
PURPOSE:To manufacture a high ductility and high strength steel sheet having superior spot weldability and a composite structure by not rolling coiling a steel slab having a specified compsn. under specified conditions and by successively carrying out heating, slow cooling an rapid cooling to form the specified composite structure. CONSTITUTION:A steel slab consisting of, by weight, 0.12-0.25% C, 1.5-3.0% Si, 1.1-2.0% Mn, <0.005% S, 0.02-0.50% sol, Al and the balance Fe with inevitable impurities is hot rolled at the Ar2 transformation temp. or above coiled at <650 deg.C. Continuous annealing is then carried out by heating, holding at >=800 deg.C in the austenite+ferrite two-phase for >=4min and cooling followed by holding at 350-450 deg.C for 1-5min. The cooling is carried out by slow cooling from the holding temp. to 600-800 deg.C at <=30 deg.C/sec cooling rate and rapid cooling to 350-450 deg.C at >=30 deg.C/sec cooling rate of form a composite structure consisting of martensite, bainite, ferrite and retained austenite.
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