INFLUENCES OF COOLING RATES ON PRECIPITATION BEHAVIOR AND AUSTENITE PHASE TRANSFORMATION IN HIGH Ti MICROALLOYED STEEL

摘要

The precipitation behavior and austenite phase transformation during cooling process were in-situ observed using a high temperature confocal laser scanning microscopy (CLSM). The observation result revealed that two obvious precipitation processes of the embossments (precipitations I and precipitations II) were discovered during cooling process, the precipitations I were identified as titanium nitrides particles through the microdissection of focused ion beam (FIB). With the increase of the cooling rate, the precipitation temperature of titanium nitrides particles was delayed. Moreover, the size of precipitations I decreased and the distribution of precipitations I changed from along the austenitic grain boundaries to within the grains with the increase of the cooling rate. Meanwhile, increasing cooling rate was able to make the pro-eutectoid ferrite film to be smaller and even disappear. To improve the hot ductility of high Ti microalloyed steel and eliminate the film like pro-eutectoid ferrites or the precipitations along the austenite grain boundaries, the cooling rate should be above 5°C/s.