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Ferrite grain refinement in dual phase steels Hazra, Sujoy S.

Abstract

Dual phase steels with microstructure of hard martensite islands dispersed in ductile ferrite matrix, possess an optimum balance of strength and ductility alongwith a high work hardening rate. With this attractive combination of mechanical properties, dual phase steels are the candidate materials for structural and reinforcement auto-body parts, where refining the ferrite-martensite microstructure is expected to further improve the properties. In order to produce fine-grained dual phase steels, this study examines refinement of ferrite grain size using Deformation Induced Ferrite Transformation (DIFT) technique in two low carbon microalloyed steels, one with combined addition of Nb & Mo and the other with only Nb. In this thermo-mechanical processing technique, the steels have been rapidly cooled from austenitization temperature to approximately 25-50 °C above austenite to ferrite transformation start temperature without prestrain, to produce highly undercooled austenite, followed by heavy deformation, and subsequent rapid cooling thereby facilitating transformation to dual-phase microstructure consisting of fine grained ferrite with martensite. The effect of prior austenite grain size, degree of undercooling, amount of deformation and microalloying additions on the final microstructure of the steels has been studied with tests performed on a Gleeble 3500 thermomechanical simulator. Microstructures have been quantitatively characterized with Scanning Electron Microscopy and to a limited extent by EBSD technique. The maximum ferrite grain refinement has been observed at the highest amount of deformation employed with a true strain of 0.6 for austenitization temperature of 950 °C. Here, the microstructure consists of at least 60% ultrafine ferrite (UFF) grains with a mean grain size of 1-2 μm and rest martensite. The results indicate that increase in undercooling leads to an increase in UFF fraction, although the effect of the same on UFF grain size is insignificant. Strain rate seems to have a secondary effect on the DIFT potential. Further, though the post deformation isothermal hold after highest employed deformation at the highest undercooling did not register any significant change in terms of UFF grain size and fraction in the final dual phase microstructure, transition from a quasi-polygonal to polygonal UFF morphology has been observed. The combined addition of Mo and Nb leads to enhanced retardation of the austenite to ferrite transformation. Hence the single addition of Nb seems to be more beneficial in terms of obtaining optimum UFF fraction in fine grained DP steel.

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