Prior Microstructure Modification and Synergic Effect of Deformation in TRIP Steel

Co-authors MURÁNSKY Ondrej, ŠITTNER Petr

Low alloyed TRIP (Transformation Induced Plasticity) steels remain one of the most challenging goals of the research. These steels are a class of multiphase steels offering an attractive combination of high strength and ductility. Their microstructure usually consists of a mixture of polygonal (equiaxed) ferrite, bainite (bainitic ferrite) and metastable retained austenite. Most publications on TRIP-assisted steels highlight the role of the retained austenite, which transforms to martensite during plastic deformation and thus contributes to the enhanced strength and formability of steel. The paper presents results of in-situ neutron diffraction experiments aimed on monitoring the phase evolution and load distribution in TRIP steel when subjected to tensile loading. Tensile deformation behaviour of two TRIP-assisted multiphase steel with slightly different microstructures due to different thermo-mechanical treatments conditions applied was investigated by in situ neutron diffraction. The steel with lower austenite volume fraction (fγ=0.04) and higher volume fraction of needle-like bainite in the -matrix exhibits higher yield stress (sample B, 600MPa) but considerably lower elongation in comparison to the steel with higher austenite volume fraction (fγ=0.08), granular bainite and polygonal ferrite matrix (sample A, 500 MPa). The neutron diffraction results have shown that the applied tensile load is redistributed at the yielding point in such a way that the retained austenite bears a significantly larger load than the -matrix during the TRIP-assisted steel deformation. Steel sample with higher volume fraction of retained austenite and less strong ferrite matrix proved to be a better TRIP steel with respect to strength, ductility and the side effect of the strain induced austenite-martensite transformation. The transforming retained austenite in time of loading provides potential for higher ductility of experimental TRIP steel but at the same time acts as a reinforcement phase during the further plastic deformation.