INFLUENCE OF AUSTENITIZING TEMPERATURE ON THE MICROSTRUCTURE AND IMPACT TOUGHNESS OF A HIGH STRENGTH LOW ALLOY HSLA100 STEEL

The effect of austenitizing temperature on the microstructures and -20 ¤ impact toughness of HSLA100 steel was investigated by Gleeble-3500 thermal simulator.Its microstructures were observed by SEM and EBSD, and the relevant transformation kinetics was also analyzed by means of dilatometer.The results show that the microstructure of HSLA100 steel changes gradually from granular to lath bainite with increasing austenitizing temperature.The highest impact toughness of samples was achieved at austenitizing temperature of 1000 ¤, in which martensite-austenite (M/A) islands are finer and dispersed and the density of high angle boundaries is maximum.M/A islands, however, become coarser and this density lowers below 1000 ¤, beyond 1000 ¤, these islands are refined, being accompanied by a dramatic decrease of this density of high angle boundaries.Kinetics analysis indicates that with increasing austenitizing temperature, the transformation start temperature decreases but the transformation rate increases.Both lower start temperature and faster rate would facilitate M/A islands refining.All the transformation occurring in samples might be divided into two stages: bainite and martensite stages.The highest transformed fraction of bainite is achieved in the bainite stage at about 1000 ¤, resulting in the best impact toughness of HSLA100 steel.The