AUSTENITIC GRAIN-SIZE EVOLUTION AND CONTINUOUS COOLING TRANSFORMATIONDIAGRAMS IN VANADIUM AND TITANIUM MICROALLOYED STEELS

The evolution of the austenitic grain size in medium carbon steels mic roalloyed with vanadium and titanium was studied as a function of rehe ating temperature, heating rate, and titanium content. High resolution dilatometric techniques were used to determine the continuous cooling transformation (CCT) diagrams for two different austenitization tempe ratures. The microstructure and hardness were determined for different cooling rates. The results revealed a significant effect of titanium concentration on the austenitic grain growth control. The smallest gra in size was found in the steel with a Ti concentration = 0.019 wt %. L ow heating rates produced smaller grain sizes than high heating rates although an abnormal grain growth took place. In these steels, at temp eratures above 1050 degrees C the influence of the reheating temperatu re on their hardness for cooling rates around 2 degrees C . s(-1) was negligible. The higher reheating temperatures caused a slight increase in their hardenability. Finally, it was found that the greater the ti tanium content, the greater the hardness of these steels, but only whe n the titanium percentages were higher than 0.020 wt%.