SIMULATION OF THE HOT-ROLLING AND ACCELERATED COOLING OF A C-MN FERRITE-BAINITE STRIP STEEL

By means of torsion testing, the microstructures and mechanical proper ties produced in a 0.14 pet C-1.18 pet Mn steel were investigated over a wide range of hot-rolling conditions, cooling rates, and simulated coiling temperatures. The austenite grain size present before accelera ted cooling was varied from 10 to 150 mu m by applying strains of 0 to 0.8 at temperatures of 850 degrees C to 1050 degrees C. Two cooling r ates, 55 degrees C/s and 90 degrees C/s, were used. Cooling was interr upted at temperatures ranging from 550 degrees C to 300 degrees C. Opt ical microscopy and transmission electron microscopy (TEM) were employ ed to investigate the microstructures. The mechanical properties were studied by means of tensile testing. When a fine austenite grain size was present before cooling and a high cooling rate (90 degrees C/s) wa s used, the microstructure was composed of ferrite plus bainite and a mixture of ferrite and cementite, which may have formed by an interpha se mechanism. The use of a lower cooling rate (55 degrees C/s) led to the presence of ferrite and fine pearlite. In both cases, the cooling interruption temperature and the amount of prior strain had little inf luence on the mechanical properties. Reheating at 1050 degrees C, whic h led to the presence of very coarse austenite, resulted in a stronger influence of the interruption temperature. A method developed at Inst itut de Recherche Siderurgique (IRSID, St. Germain-en-Laye, France) fo r deducing the Continuous-Cooling-Transformation (CCT) diagrams from t he cooling data was adapted to the present apparatus and used successf ully to interpret the observed influence of the process parameters.