Retained austenite characteristics in thermomechanically processed Si-Mn transformation-induced plasticity steels
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I.
INTRODUCTION
WHEN low alloy steels are quenched and tempered to produce higher tensile and yield strengths, the true stress– true strain curve shapes are similar to the as-hot-rolled condition, but necking begins at lower strains. The start of plastic instability at lower strains in quenched and tempered steels indicates that the rate of strain hardening produced by dislocation interactions is inadequate to compensate for the increase in stress in the region of the neck. Therefore, effects stronger than increases in dislocation density must be introduced during plastic straining if the onset of necking is to be delayed, so that the ductility, as indicated by the reduction of area, can be increased by increasing the region of uniform strain. Such barriers must be introduced during plastic straining, not before; otherwise, the yield strength would be increased without necessarily increasing the strain hardening rate. It is well known that deformation-induced martensite formation can significantly strengthen the deforming alloy,[1] thus increasing the strain to necking. This is the principle of transformation induced plasticity (TRIP) steels. The role of this phenomenon in postponing and stabilizing the necking was first recognized by Banerjee et al.[2] They suggested that, when necking takes place, the retained austenite in the neck region transforms to martensite in response to the applied deformation. The transformed region possesses a much higher flow stress than the adjacent untransformed regions. Thus, the progress of necking stops, and the A. ZAREI HANZAKI, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, The Faculty of Engineering, Tehran University, Tehran, Iran. P.D. HODGSON, Principal Research Engineer, is with BHP Research, Melbourne Laboratories, Melbourne, Australia Victoria 3170. S. YUE, Professor, is with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Manuscript submitted September 30, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
regions immediately adjacent to the neck begin to deform, in turn, transforming to martensite. In steels containing high concentrations (.1 pct) of Si, a significant amount of austenite can be retained where bainite is the predominant microcomponent.[3] Thus, steels with appropriate amounts of Si and Mn have been classified as Si-Mn TRIP steels.[4,5] The effects of different intercritical annealing conditions on various aspects of these steels have been extensively studied in previous investigations.[6–9] On the other hand, thermomechanical processing (TMP), which is a combination of heat treatment and deformation, is also a powerful method to control the microstructure, but has not been explored in the context of TRIP steel processing. In order to establish the effectiveness of TMP in the processing of TRIP steels, the effects of TMP parameters on the transformation characteristics of TRIP steels must be first considered, since these ultimately control the retained auste
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