Transformation and Decomposition Behavior of Retained Austenite (RA) in High-Al TRIP Steels

Abstract

Recently, there has been increased interest in the development of the “Third Generation” of advanced high strength steels(AHSS) i.e. steels with strength-ductility combinations significantly better than exhibited by the first generation AHSS but at a cost significantly less than required for second generation AHSS. In addition, the steel body for automotive application would be required to have light-weight containing low density element, such as Mn and Al. In the alloying range of disordered Fe-Al solid solution, these steels can be differentiated to various types. Among the rest, the duplex microstructure containing ferrite and austenite utilizing the transformation induced plasticity (TRIP) is one of the strong candidates for newly developed steel body. Therefore, we have focused on the TRIP steels with high Al addition. The present study is aimed at the finding the correlation between the chemical composition, heat-treatment, microstructure and mechanical properties in high-Al TRIP steels. Also, the emphasis has been placed on understanding transformation and decomposition behavior of retained austenite as important constituents in TRIP steels. Especially, in order to characterize individual retained austenite and find out the relationship among the grain size, chemical composition, distribution of retained austenite and mechanical properties, various advanced analytical techniques such as transmission electron microscopy(TEM), electron backscatter diffraction(EBSD), atom probe tomography(APT) and nano-indenter were employed. In present study, high-Al TRIP steels (contains 3 wt.% Mn, 5.5 wt.% Al) with different carbon contents (0.1 wt.%, 0.3 wt.%, 0.5 wt.%) were investigated. As rolled state, the volume fraction and thickness of k-carbide band increases with increasing carbon contents. Due to high-Al addition, the entirely microstructure is composed of ferrite band and second phase such as k-carbide, austenite, and martensite align to parallel to rolling direction. For dissolution of k-carbide to austenite in as-rolled state steel, inter-critical annealing temperature would be selected to 850°C and 950°C. The elevated intercritical annealing temperature causes the grain growth of δ-ferrite and retained austenite resulting in low yield strength and high strain-hardening rate at the initial stage of deformation. However, the microstructure inhomogeneity of present steel causes the large deviation of retained austenite grain size from average value after intercritical annealing. The chemical composition of individual retained austenite depending on the annealing temperature (i.e., grain size) were measured by atom probe tomography. The carbon and manganese contents increases with decreasing austenite grain size. However, the aluminum contents were nearly constant regardless of austenite grain size. Step-wise EBSD and nano-indentation test revealed that the small grain of austenite with high carbon content have high resistance to martensite transformation. However, the mechanical stability of retained austenite is also determined by adjacent microstructure. At the region near the interface of enlarged δ-ferrite and austenite band, it is expected to relatively low probability of interaction between retained austenite and dislocation formed by grain boundary. In order to investigate the decomposition behavior of retained austenite depending on the chemical composition and grain size, 0.3C steel annealed at 850°C and 950°C were selected. As the tempering temperature increases, the thickness of carbides gets thicker. For further elevated temperature tempering conducted, the morphology of carbide is getting to spherodized in order to reducing the interfacial energy. Tempering experiment revealed that retained austenite with small size and high carbon contents began to decompose at the 400°C for a long time. DTA analysis also revealed that small size of retained austenite was readily transformed to k-carbide and ferrite mixture at the relatively low temperature. In order to analyze the chemical composition of carbide and ferrite decomposed from retained austenite, APT experiment was carried out. Very thin carbide with plate type were transformed at the 0.3C steel annealed at 950°C tempered at 500°C for 1h. (early stage). From the results, these carbides were identified as (Fe,Mn)xAlxC (M: Fe, Mn) type carbide(k-carbide). The manganese and carbon content were strongly partitioned to k-carbide compared to aluminum, and Mn diffuse out from carbide to ferrite under further tempering for prolonged time. From the DFT calculations, it is considered that formation enthalpy of Fe2MnAlC is negatively larger than that of Fe3AlC resulting in high Mn contents at the early stage of tempering.