12th A M M E ’ 2 0 0 3 Thermo-mechanical processing of low alloy TRIP steel L. Skálová, R. Divišová, D. Jandová University of West Bohemia in Pilsen, Univerzitní 8, 306 00 Pilsen, Czech Republic Low alloy TRIP (transformation induced plasticity) steel was investigated after different thermo-mechanical processing (TMP) consisting of high temperature deformation followed by isothermal holding in the bainite region and cooling to room temperature. The microstructure of processed specimens was characterised by the means of light and electron microscopy. Varying TMP changed volume fraction of retained austenit, which was measured using X-ray diffractometer. 1. INTRODUCTION TRIP effect is used for production of steels with high strength and high formability. The remarkable strength to ductility balance results from strain-induced transformation of retained austenite to martensite during plastic deformation. The retention of austenite is obtained by combination of appropriate chemical composition and thermo-mechanical processing of steel. The presence of austenite in the initial microstructure appears to be critical to the achievement of the desired properties. The retained austenite volume fraction is a key factor in controlling final mechanical properties, but there are also other important parameters, such as the morphology, particle size and mechanical stability of retained austenite. TRIP effect was first recognised by Zackay [1] in metastable austenitic stainless steels. These steels had high additions of Cr and Ni, which made them very expensive and limited their use only to special applications. Increasing attention has been recently paid to low alloy C-Mn-Si steels with a multiphase microstructure consisting of a ferrite matrix, dispersion of bainite, metastable retained austenite and martensite, quite different from previously used fully austenitic TRIP steels [2]. Low alloy TRIP steel properties are very desirable for thin sheets applied in automobile industry, in order to improve productivity and reduce auto body weight. The need of materials, which would be used in special forming technologies, has been increasing. 2. EXPERIMENTAL PROGRAM The low alloy multiphase steel with following chemical composition was used in this study: 0,18% C, 1,45% Mn, 1,9% Si, 0,02% P, 0,07% S. Seven different TMP methods were applied (Table 1). TMP consisted of austenitization at temperatures of 850, 900 and 950 °C,