Modeling of Loading-path Dependent Martensitic Transformation in a Low-alloy TRIP Steel

The aim of this paper is to predict the deformation-induced martensitic transformation of the retained austenite in steels under various deformations, including loading-path changes, by using mesoscopic finite element analyses (FEAs). First, a TRIP steel was subjected to monotonic uniaxial tension and compression, as well as a couple of two-stage loadings to investigate the effect of loading direction and loading-path on transformation behavior experimentally. In monotonic loading, tension induced transformation at higher rate than compression did. Whereas, in two-stage loadings, the transformation progress was suspended immediately after the start of secondary loading. As the secondary tension proceeded, the transformation resumed and gradually accelerated toward the transformation rate for monotonic tension. These experimental results were analyzed by FEAs with a two-dimensional image of microstructure. The transformation rates under monotonic loading are well predicted by the simulation. It is also suggested that the difference in the transformation rate between tension and compression is mainly due to the volumetric expansion associated with martensitic transformation, and that the transformation behavior of the untransformed austenite is dominated by the distribution of the hard transformed martensite. In addition, the prediction of the transformation rate in secondary tension after pre-compression required the consideration of back stress in the austenite. The reproducibility of the transformation behavior just after the onset of secondary deformation was improved by the hypothesis that the equivalent value of back stress tensor at the transformation needs to exceed its maximum value in the past.