Distribution of Alloying Elements and Phase Transformation Behavior of Retained Austenite in CMnSiAl TRIP Steels

Abstract

Transformation induced plasticity (TRIP) steels have excellent combination of strength and ductility compared to other commercial steels. Thus, TRIP steels have been widely investigated for automotive application by many researchers. However, there have been some difficulties in the expanding industrial applications of the TRIP steels. It is well known that in conventional CMnSi TRIP steels, the silicon stabilizes the retained austenite by inhibiting carbide precipitation. Unfortunately, high silicon content in TRIP steels can result in the formation of a very strong oxide layer, which can prevent hot dip galvanizing coating. Therefore, a reduction in silicon contents and/or a partial substitution of silicon with another alloying element is required. The other challenges faced to date during research on TRIP steel has been to evaluate the quantitative distribution the alloying elements and precisely understand the property of the individual phase, especially retained austenite.The present study is aimed at observation of microstructural evolution depending on aluminum content and quantification of alloying elements in constituent phases of the CMnSiAl TRIP steels. Also, the emphasis has been placed on understanding decomposition and phase transformation behavior of retained austenite as one of the constituents. Especially, in order to characterize individual retained austenite and find out the relationship among their morphology, chemical composition and mechanical properties, various advanced analysis techniques such as TEM, EBSD, APT and nanoindenter were employed.In present study, CMnSiAl TRIP steels with different aluminum content (0.04 wt.%, 1.00 wt.%, and 2.00 wt.% ) were fabricated through thermomechanical process, and the samples were designated as 0.04-Al, 1.00-Al, and 2.00-Al steel, respectively. Experimental results show the microstructure, phase volume fraction and its mechanical properties vary significantly depending on added aluminum, content. Aluminum additions greatly affect the Fe-C phase diagram, expanding the α+γ region and increasing both the Ae1 and Ae3 temperature. Additions of 1.00 wt.% Al increases the Ae3 temperature close to 1000°C. 2.00 wt.%. Al content shifts Ae1 temperatures to significantly higher value (770°C) and completely eliminates fully austenitic region. 0.04-Al steel showed smaller ferrite grain due to newly formed ferrite phases at low temperature (~400°C). In the case of 2.00-Al steels, coarse ferrite bands were observed. It is expected that this unique band structure was resulted from initial microstructure formed before cold rolling. XRD and EBSD results revealed that 1.00-Al steel shows the maximum volume fraction of retained austenite. As a result, the greatest elongation could be obtained in 1.00-Al steelsFrom the APT results, elemental distribution in the constituent phases of CMnSiAl TRIP steels could be quantified successfully. In 0.04-Al steels, fine carbide was observed indicating that low levels of aluminum content cannot completely suppress carbide formation in isothermal bainitic transformation process. Also, it can be concluded that aluminum atoms diffused into ferrite from austenite only during intercritical annealing process. In the case of carbon atoms, they were enriched in retained austenite during isothermal bainitic transformation under para-equilibrium condition.In order to characterize properties of retained austenite intensively, we have investigated the decomposition and phase transformation behavior of retained austenite in 1.00-Al steels, which have optimized microstructure and the greatest elongation. Tempering experimental revealed that retained austenite phases began to decompose at 400°C due to significantly increased diffusivity of carbon in austenite. Also, film type retained austenite containing high carbon content is decomposed more readily compared to blocky type. Retained austenite was decomposed into ferrite and M3C (M: Fe, Mn). In observed carbide, cross diffusion of manganese and silicon across α/θ interface was observed. Especially, manganese atoms were accumulated at the phase boundary due to difference of diffusivity in ferrite and cementite.Based on stepwise straining EBSD and TEM observation, it can be concluded that large retained austenite containing various defects was easily transformed into martensite in relatively low strain region. However, ultrafine austenite particles (<~500 nm) hardly transformed to martensite, even if fracture occurred. Nanoindentation results provided nanohardness values of constitutive phases. Also, the comparison of nanoindenting behavior in various shaped retained austenite phases revealed that film type austenite is more stable than blocky ones against mechanical deformation. These results were related with carbon content of individual retained austenite. APT results supported that film shaped retained austenite contained higher carbon content. Therefore, it is expected that high carbon content in film shaped retained austenite might hinder the phase transformation during the indenting. Ion beam bombarding on the steel promoted phase transformation of retained austenite into martenstie because defects induced by ion implantation can be acted as nuclei for transformation. Results revealed that the degree of transformation was affected by ion dose and orientation of retained austenite. It was found that austenite grains with orientations aligned favorably for channeling were more resistant to transformation.