Strength and toughness of austenite decomposition products in high strength Cu-bearing steel during continuous cooling

The effects of cooling rate on the strength and toughness of austenite decomposition products in high strength Cu-bearing steels HSLA80 and HSLA100 were investigated during continuous cooling. Characterization and ripening of Cu precipitates formed dur-ing continuous cooling were discussed. Continuous cooling experiments from 0.1 to 20℃·s^-1 were carried out by Gleeble thermo-simu-lation. Microstructures and Cu precipitates were observed by scanning electron microscopy and transmission electron microscopy. It is found that the microstructure of austenite decomposition products in HSLA80 steel changes from polygonal ferrite to granular ferrite and bainite with increasing cooling rate. Cu precipitation occurs during continuous cooling within the cooling rate of 0.1 to 1℃·s^-1. Due to the microstructure and Cu precipitates, the hardness changes by stages and the toughness increases gradually as the cooling rate in-creases. The microstructure of austenite decomposition products in HSLA100 steel is mainly bainite and Cu precipitation does not oc-cur, so that the hardness does not change on the whole, but the toughness varies dramatically with increasing cooling rate. Ripening of Cu precipitates formed during continuous cooling obeys the Ostwald law. The radius of Cu precipitates increases with aging time t^1/3.