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Microstructure evolution during processing of dual phase and trip steels Huang, Jin

Abstract

In this study, a systematic experimental study on the microstructure evolution for a dual phase and a TRIP steel during intercritical annealing and intermediate temperature holding (for the TRIP steel) was conducted. In the reheating stage, recrystallization occurs for the cold-rolled materials, which were characterized by the microhardness and metallography. A mathematical description of the recrystallization kinetics can be made using the Avrami equation. It was found that for dual phase steel, ferrite recrystallization may extend into the austenite formation region thus overlapping with the austenite formation reaction. While for the TRIP steel, which has faster recrystallization rates, the recrystallization and austenite formation processes are sequential for the processing parameters (i.e. heating rates of 1, 10 and 100 °C/s) considered in this study. In the intercritical annealing stage, the materials were subjected to both continuous and ramp plus isothermal hold heat treatments with various heating rates. The kinetics of austenite formation was determined by dilatometry. Very significant effects of heating rate on both the fraction of austenite and its spatial distribution and morphology were observed. The material behaviour during austenite formation can be understood by considering the effect of heating rate on the nucleation and growth of austenite. The basic trends can be rationalized by the competing mechanisms for nucleation and growth of austenite and how these depend on the starting microstructure (i.e. spatial distribution of pearlite and the degree of ferrite recrystallization). The interaction between ferrite recrystallization and austenite formation is strong and it affects not only the kinetics of austenite formation but also the spatial distribution and morphology of austenite. A new approach using neutron Bragg-edge transmission (BET) was applied to studying the austenite decomposition which occurs when the material is cooled to the intermediate hold stage (i.e. during processing of TRIP steel). The neutron BET technique allows for the analysis of the evolution of volume fraction for the participating phases; and also reveals unique, 'msitu' carbon redistribution information which may be derived from the change in the lattice parameter. The volume fraction results were compared to the results from traditional XRD, dilatometry and optical metallography. Excellent agreement between these results was observed. In addition, carbon enrichment in the austenite phase during bainite transformation is clearly observed from this technique.

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