Effect of the bainitic transformation temperature on retained austenite fraction and stability in Ti microalloyed TRIP steels

TRIP properties in four hot-rolled microalloyed steels were studied as a function of bainitic domain entrance temperature. It was found that this temperature affects the size and morphology of the retained austenite grains. Size and morphology was shown to play a role on the stability of the retained austenite. This stability was shown to be the major factor affecting the mechanical properties of these steels. In the case of the lowest bainitic temperature, the TRIP effect is hindered due to the premature transformation of austenite into martensite probably due to an insufficient carbon enrichment of the small austenite grains. Moreover the bainitic treatment performed at the highest temperature contained a very low volume fraction of retained austenite that does not transform even under deformation. Using TEM and the Kikuchi-line method, it was shown that the sample that did not undergo a phase transformation during FIB milling had a carbon concentration in its retained austenite grains of approximately 1.7%.