TRANSFORMATIONS DURING INTERCRITICAL ANNEALING OF A TRIP-ASSISTED STEEL

During intercritical annealing of cold rolled TRIP-assisted steel, fer rite matrix recrystallizes and austenite forms. After subsequent quenc hing, part of that austenite may remain at room temperature as retaine d austenite, while the other part transforms into martensite. Based on their respective origin, recrystallized ferrite grains were recognize d as PSN (particle stimulated nucleation) and non-PSN types. Near rand omized PSN grains nucleated somewhat preferentially (i.e. at the earli er stages of recrystallization), which may explain the slight drop in non-PSN orientations at higher annealing temperatures. Increased inter critical soaking periods coarsened the austenite grains and increased the austenite volume fraction (till 37%), while the austenite stabilit y dropped (i.e. M-s temperature increased). On the other hand, both th e retained austenite volume fraction and its carbon content decreased with prolonged intercritical soaking. Stability of the initial retaine d austenite particles was ascertained by their small sizes and high ca rbon 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 te xture was observed. After the transformation was complete and a drop i n retained austenite percentage was recorded, again a randomized textu re for retained austenite was noted.