Transformations during Intercritical Annealing of a TRIP-assisted Steel.

Abstract

During intercritical annealing of cold rolled TRIP-assisted steel, ferrite matrix recrystallizes and austenite forms. After subsequent quenching, part of that austenite may remain at room temperature as retained austenite, while the other part transforms into martensite. Based on their respective origin, recrystallized ferrite grains were recognized as PSN (particle stimulated nucleation) and non-PSN types. Near randomized PSN grains nucleated somewhat preferentially (i.e. at the earlier stages of recrystallization), which may explain the slight drop in non-PSN orientations at higher annealing temperatures. Increased intercritical soaking periods coarsened the austenite grains and increased the austenite volume fraction (till 37%), while the austenite stability dropped (i.e. Ms temperature increased). On the other hand, both the retained austenite volume fraction and its carbon content decreased with prolonged intercritical soaking. Stability of the initial retained austenite particles was ascertained by their small sizes and high carbon content, while smaller dimensions of the austenitic domains were possibly crucial in their stability/survival after prolonged soaking. At the initial stages of transformation, the retained austenite showed randomized orientations. During the subsequent stages, a weak cube texture was observed. After the transformation was complete and a drop in retained austenite percentage was recorded, again a randomized texture for retained austenite was noted.