Effect of Cooling Rate on Phase Transformation and Microstructure of Nb-Ti Microalloyed Steel

The cooling rate is a key factor of controlling the slab surface microstructures during continuous casting of steel. The effect of cooling rate on phase transformation and microstructure of Nb-Ti microalloyed steel was investigated by a confocal laser scanning microscopy and a Gleeble-3800 thermal simulation machine. The process of phase transformation can be analyzed through in situ observation. A critical cooling rate of 5 K.s(-1) was revealed, below which the proeutectoid ferrite along austenite grain boundaries and widmanstatten structures were observed, and carbonitrides precipitated were also observed in the proeutectoid ferrite. With the increase of cooling rate, the quantity of the precipitates decreases while the width of the proeutectoid ferrite becomes smaller. The carbonitrides precipitated along the austenite grain boundary result in the decrease of the carbon concentration near the grain boundary, which is more favorable to form the proeutectoid ferrite as well as to change its width. When the cooling rate was greater than or equal to 5 K.s(-1), the precipitates were dispersed uniformly in the grain, and the bainite was observed mainly.