Deformation-Induced Transformation of Retained Austenite in Transformation Induced Plasticity-Aided Steels: A Thermodyna
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I.
INTRODUCTION
LOW-ALLOY transformation induced plasticity (TRIP)[1]–aided steels offer an excellent combination of high strength and ductility at affordable cost.[2] These unique properties can be attributed to the complex microstructure of these steels. Conventional polygonal ferrite (PF) matrix TRIP-aided steel (PF steel) consists of polygonal ferrite, carbide free bainite, retained austenite, and in some cases, a small fraction of martensite. These steels are produced by a two-step heat-treatment process that involves intercritical annealing followed by isothermal bainitic transformation treatment.[2,3] Two more varieties of TRIP-aided steels have been developed in recent years in response to the complex formability requirements of the present day autobody manufacturers.[4,5] These steels are based on bainitic ferrite (BF) and annealed martensite (AM) matrices and are termed as BF steel and AM steel, respectively.[4–6] The TRIP-aided steels with bainitic ferrite as the matrix phase and retained austenite as the second phase (BF steels) can be produced by a two-step heat-treatment process, where complete austenitization is followed by an isothermal bainitic transformation treatment.[4] The third variety of TRIP-aided steels called AM steels consists of ‘‘annealed martensite’’ as the matrix phase and carbide free bainite and retained austenite as the other phases.[5] In this case, the steel is subjected to an additional austenitizing and oil quenching step prior to the conventional treatment of intercritical annealing and
isothermal bainitic transformation. As a result, the microstructure at the beginning of intercritical annealing is martensite, which is different from that in the conventional treatment where the starting microstructure before intercritical annealing is ferrite + pearlite. The resultant lath type of matrix microstructure has been named annealed martensite by Sugimoto et al.[5] They found that intercritical annealing for 1200 seconds resulted only in recovery and decrease in the dislocation density of the martensite formed during prior heat treatment but did not result in the nucleation of polygonal ferrite. The most important constituent of the complex microstructure of the TRIP-aided steels is 10 to 20 vol pct of retained austenite. Deformation causes progressive transformation of retained austenite to martenite, which increases the strain hardening rate and delays the onset of necking. A large amount of work has been dedicated to study the deformation-induced transformation of austenite to martensite. As a result, a number of emperical as well as semiemperical models have been formulated to predict the kinetics of this transformation.[3,7–14] The aim of the present work was to develop a thermodynamic-based model for the deformationinduced transformation of austenite to martensite so that the amount of martensite or the amount of austenite left untransformed after a certain amount of strain can be predicted.
II. MONIDEEPA MUKHERJEE, Researcher, is with Research and Development, Tata Steel, Ja
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