Dilatometric study of continuous cooling transformation of intercritical austenite in cold rolled AHSS-DP steels

The austenite phase transformations under continuous cooling from intercritical auste-nitization at 800 degrees C and full austenitization at 910 degrees C were investigated in a cold rolled advanced high strength dual phase steel (AHSS-DP) strip. Experimental continuous cooling transformation (CCT) diagrams were obtained using induction-quenching dilatometry and the corresponding microstructures were characterized by scanning electron microscopy (SEM) analysis and Vickers microhardness measurements. In general, it was shown that the experimental behavior of phase transformations in current DP steels depends on the dynamic changes that occur in the hardenability of austenite during continuous cooling at different rates. The main results reveal that the martensite-start transformation temperature (Ms) depends on the cooling rate, particularly when an intercritical austenitization and slow cooling rates conditions coincide. The unusual behavior of Ms temperature is explained in terms of solute distribution, which changes the chemical composition of metastable austenite (local enrichment in carbon content) during cooling, causes changes in hardenability, lowers both the Ms temperature and the critical cooling rate at which martensite is formed. In general terms, the behavior of Ms at slow cooling rates is relevant information that has not been reported yet in the specific literature because it is generally known that Ms does not depend on the cooling rate when quenching is performed from the austenite phase field.