The effect of hot rolling condition and chemistry of steels on the onset temperature of r-a transformation after hot rolling was investigated using a thermal analyzer developed for the measurement of Ar3 temperature after hot rolling. The changes of Ar3 temperature and austenitic microstructure in connection with reheating temperature, rolling temperature and reduction rate were investigated in a Si-Mn and a Nb-bearing steels. The results were analyzed based on the changes of the effective interfacial area per unit volume (Sv) that included both of the recrystallized and unrecrystallized grain boundaries and deformation band as nucleation sites for ferrite. Although the Sv value increased with the refinement of recrystallized r grain or the increase of rolling reduction below recrystallization temperature of austenite, the latter resulted in a much greater change of Ar3 temperature in the Nb steel. This was considered to be due to a reduced amount of dissolved Nb atoms around the grain boundary or deformation band through the strain induced precipitation of Nb(CN). The effect of chemistry and plate thickness on the Ar3 temperature after controlled rolling was studied in a large number of steels with different chemistry, and the relation between the Ar3 and chemistry was quantitatively established based on the multiple regression analysis.

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