Microstructural Characterization of Thermo-Mechanical Treated TRIP Steels

Abstract

The increasing demand for the reduction of automobiles CO2 emissions for environmental preservation leads the automotive industries towards the mechanical components weight reduction. Sheet steels with multiphase microstructures exhibit favourable combinations of strength and ductility. The so called TRIP steels have a metastable microstructure that consists of a continuous ferrite matrix containing a dispersion of hard second phases martensite and bainite. These steels also contain retained austenite, at room temperature, that represents the source of the TRansformation Induced Plasticity effect. When the material is subjected to deformation step, the retained austenite transforms itself into martensite; the produced martensite delays the onset of necking resulting in a product with high total elongation, excellent formability and high crash energy absorption. In the present research the steel TRIP 800 zinc coated has been subjected to different thermo–mechanical treatments in order to evaluate the relation between microstructure of material and TRIP effects. Whit this aim the microstructural analysis has been performed and the evaluation of content of different phases has been made by means of the image analysis techniques. The relation among the strain level, the content of different phases, the thermal treatments and the work hardening properties of materials have been valued. Furthermore, it has been also highlighted the dependence of the bake hardening properties of material on the different thermo-mechanical treatments.