2007 Volume 47 Issue 9 Pages 1341-1350
The microstructural characteristics of the bainite constituent associated with TRIP steels were investigated in detail by means of transmission electron microscopy (TEM). Two Si–Mn steels were selected for the study, i.e. a 0.6C–1.5Mn–1.5Si and a 0.18C–1.55Mn–1.7Si (wt%). Following austenitizing treatment the high carbon steel was cooled directly to bainite reaction temperature, whereas the low carbon steel was subjected to an intercritical holding step, i.e. in order to form polygonal ferrite prior to bainite transformation. Partially reacted samples at the bainite stage were examined in particular to determine the tendency of carbide precipitation at different transformation temperatures. Cementite particles were found in most of the analyzed conditions despite the high Si content of the specimens. The orientation relationship between cementite and bainitic ferrite was determined to be different from the frequently observed Bagaryatski relationship. The formation of cementite was rationalized in terms of its thermodynamic affinity as well as from a kinetic viewpoint. The findings strongly suggest that the common perception of carbide-free bainite pertinent to TRIP steels is not necessarily a valid assumption and thus carbide precipitation should not be neglected in a kinetic model and for designing the processing parameters for this class of advanced high strength steel.