| The present study is aimed at developing the high strength, high formability low alloy steels having retained austenite as one of the constituents. The main emphasis has been put on understanding the effects of processing variables and alloying elements (Cu, Nb) on the microstructure and mechanical properties of hot-rolled Fe-C-Mn-Si base TRIP (Transformation Induced Plasticity) steels. Experimental results show that microstructure of hot-rolled alloys consists of ferrite, bainite, martensite and retained austenite, whose volume fraction and morphology vary depending on the hot-rolling conditions and alloy compositions. The increase in the quench-finish temperature results in a homogeneous distribution of fine granular particles of retained austenite and hard second phases in a ferrite matrix and accordingly a good combination of strength and ductility. The Cu-containing alloy has been found to improve mechanical properties over the base alloy due to the precipitation of fine ε-Cu in ferrite grains. Nb addition increases the dynamic recrystallization temperature of austenite, resulting in the formation of coarse pan-caked bainite packets. Based on these results, the optimum microstructure for a better combination of strength and ductility is suggested to be the one having granular particles of retained austenite and hard phase (martensite or bainite) in a fine ferrite matrix. |
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