Statistical Analysis of Influential Factors on the Stability of Retained Austenite in Low Alloy TRIP Steel

Abstract

The factors representing the stability of retained austenite (γ_R) in low-alloy TRIP steel at a plastic strain of up to 0.04 were statistically analyzed by evaluating the size, strain level, crystal orientation, morphology, and precipitation site of a large number of γ_R grains (>1000). The ratio (R_area) of the area of γ_R after deformation to that before deformation was adopted as a representative parameter for the stability of γ_R. The frequency of low R_area in fine γ_R was significantly high, indicating that they were unstable. Moreover, it was indicated that γ_R in the high-strain regions tended to be unstable, whereas γ_R in the bainite region was stable. The crystal orientation and aspect ratio did not correlate with R_area in TRIP steel. The strain level and precipitation sites in coarse γ_R were uncorrelated with R_area, indicating that the size of the γ_R has a greater effect on its stability. The stability of γ_R was not dominated by the carbon concentration but was affected by other factors. The size of γ_R may represent the stress conditions at the γ_R/ferrite or bainite boundaries, which is why this factor represents the stability of γ_R in the TRIP steel at a plastic strain of up to 0.04.

Fig. 13 Schematic illustration of the relationship among the stability of γ_R, stress concentration and each representative factor.