OPTIMIZATION OF HOT-ROLLING SCHEDULE FOR DIRECT CHARGING OF THIN SLABS OF NB-V MICROALLOYED STEEL

A recently developed continuous casting simulator and the 'Wumsi' hot deformation simulator have been used to carry out laboratory simulation tests to determine the as cast microstructure and the recrystallisation behaviour of a Nb-V microalloyed steel during the process of direct charging. By variation of the initial specimen thickness (between 25 and 60 mm) different values of total strain phi(SIGMA) could be imposed to improve the coarse as cast microstructure. For a series of deformation schedules the total strain was divided systematically into two components: an austenite grain refining strain phi(gammaGF) (above the recrystallisation stop temperature T(RS) and an austenite strengthening strain phi(gammaS) (below T(RS). After hot deformation slow and accelerated cooling with simulated coiling were employed. It was found that a total strain phi(SIGMA) > 1.4 is required to ensure mechanical properties that were comparable or even superior to those found using the conventional cold charging process. The coarse as cast austenite microstructure can be refined significantly when phi(gammaGF) = 0.3-0.6. The austenite strengthening strain phi(gammaS) represents the dominant component of the total strain if a satisfactory toughness is to be achieved. Strength properties are less sensitive to the applied strain.