THE EFFECT OF COOLING RATE AND AUSTENITE GRAIN SIZE ON THE AUSTENITE TO FERRITE TRANSFORMATION TEMPERATURE AND DIFFERENT FERRITE MORPHOLOGIES IN MICROALLOYED STEELS

Abstract: transformation temperature and different ferrite morphologies in one Nb-microalloyed (HSLA) steel has beeninvestigated. Three different austenite grain sizes were selected and cooled at two different cooling rates for obtainingaustenite to ferrite transformation temperature. Moreover, samples with specific austenite grain size have beenquenched, partially, for investigation on the microstructural evolution.In order to assess the influence of austenite grain size on the ferrite transformation temperature, a temperaturedifferences method (TDM) is established and found to be a good way for detection of austenite to ferrite, pearlite andsometimes other ferrite morphologies transformation temperatures.The results obtained in this way show that increasing of austenite grain size and cooling rate has a significant influenceon decreasing of the ferrite transformation temperature.Micrographs of different ferrite morphologies show that at high temperatures, where diffusion rates are higher, grainboundary ferrite nucleates. As the temperature is lowered and the driving force for ferrite formation increases,intragranular sites inside the austenite grains become operative as nucleation sites and suppress the grain boundaryferrite growth. The results indicate that increasing the austenite grain size increases the rate and volume fraction ofintragranular ferrite in two different cooling rates. Moreover, by increasing of cooling rate, the austenite to ferritetransformation temperature decreases and volume fraction of intragranular ferrite increases.The effect of different austenite grain size and different cooling rates on the austenite to ferrite