Abstract:
transformation temperature and different ferrite morphologies in one Nb-microalloyed (HSLA) steel has been
investigated. Three different austenite grain sizes were selected and cooled at two different cooling rates for obtaining
austenite to ferrite transformation temperature. Moreover, samples with specific austenite grain size have been
quenched, partially, for investigation on the microstructural evolution.
In order to assess the influence of austenite grain size on the ferrite transformation temperature, a temperature
differences method (TDM) is established and found to be a good way for detection of austenite to ferrite, pearlite and
sometimes other ferrite morphologies transformation temperatures.
The results obtained in this way show that increasing of austenite grain size and cooling rate has a significant influence
on decreasing of the ferrite transformation temperature.
Micrographs of different ferrite morphologies show that at high temperatures, where diffusion rates are higher, grain
boundary 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 boundary
ferrite growth. The results indicate that increasing the austenite grain size increases the rate and volume fraction of
intragranular ferrite in two different cooling rates. Moreover, by increasing of cooling rate, the austenite to ferrite
transformation temperature decreases and volume fraction of intragranular ferrite increases.
