Microstructural analysis of a TRIP assisted steel obtained through three different heat treatments

ABSTRACT

The automotive industry is always interested in decreasing cars’ weight, in order to promote fuel economy and, consequently, less emission of toxic gases into the atmosphere. In this context, TRIP steels stand out since they present an excellent combination of high strength and ductility. The union of these characteristics in the same steel is due to its multiphase microstructure and to the TRIP effect, characterized by the transformation of retained austenite into martensite induced by plastic deformation. In this work, the effects of three different thermal processing routes and the austempering time on the final microstructure and on the hardness of a low carbon steel were analyzed. One cycle (CD) was based on an intercritical stage followed by a subsequent austempering process, while the others included stages prior to the intercritical step - quenching (CT cycle) and complete austenitization (CC cycle). It was observed that different thermal processing cycles resulted in microstructures whose constituents had different distribution, dimensions and morphologies. The increase in austempering time favored the formation of bainite, but caused a reduction in the amount of MA constituent. In addition, the longer this time, the greater the proportion of retained austenite and its carbon content and the lower the hardness values were verified for the material.

Keywords
Advanced high-strength steels; TRIP effect; Multiphase microstructure; Retained austenite