Effect of heating rate on nucleation and growth of austenite in cold rolled dual phase steel

In this study, the effect of heating rate on austenite nucleation and growth in a cold rolled dual phase (DP) steel during annealing was investigated by microstructural analysis and electron probe microanalysis (EPMA). Two heating rates, 5 and 300 K s(-1), and different holding times ranging from 1 to 300 s were applied to study the austenisation progress. It was found that the austenite nucleated at recrystallised ferrite grain boundaries and grew along them at low heating rate (5 K s(-1)), and a high carbon rim is observed in the formed austenite at low heating rate. An increase in heating rate resulted in austenite nucleating at the carbide/ferrite interface and growing into deformed ferrite. In addition, a higher carbon concentration existed at original carbide particle area. Higher heating rate could improve the strength. But long holding time minimises the strength difference caused by a difference in heating rate.