Quantitative assessment of the effects of microstructure on the stability of retained austenite in TRIP steels

Abstract

The stability of retained austenite (RA) in transformation-induced plasticity steels (TRIP) is affected by many factors, including chemical composition, RA grain size, neighboring microconstituents of RA and temperature. The composition and microstructure factors are interrelated, so it is difficult to separate out the influence of each individually. In this investigation, methods were developed to study the effects of RA grain size and neighboring microconstituents in a silicon-alloyed low-carbon (0.2C–1.6Si–1.6Mn) TRIP steel. Uniaxial tensile tests, performed at −20°C, room temperature (20°C) and 40°C, were interrupted at several strain levels. Scanning electron micrographs were obtained from each condition, and RA was quantified using a novel technique called the categorical chord-length distribution (CCLD), which enables microstructural quantification based on specific neighboring microconstituents. The results show that RA adjacent to bainitic ferrite (BF) and fine RA grain size are correlated with higher RA stability. A modified Burke–Matsumura–Tsuchida stability model was developed to kinetically analyze the effects of microstructure on RA transformation. The CCLD and kinetic modeling analysis indicates that the stability of RA inside BF is less sensitive to the testing temperature than RA inside polygonal ferrite (PF), and thermodynamic analysis of the driving force for transformation as a function of temperature and carbon content implies that there is a higher carbon content in RA inside BF. Additionally, nanohardness tests showed that the hardnesses of BF and PF are not significantly different after moderate amounts of deformation. Thus, the enhanced stability of RA inside BF compared to PF is more strongly related to the elevated carbon content of RA inside BF rather than stress partitioning differences for RA adjacent to BF or PF.