Prediction and Validation of the Austenite Phase Fraction upon Intercritical Annealing of Medium Mn Steels

Abstract

In this research, the effects of Mn and Si concentration and that of the isothermal intercritical holding temperature on the austenite-to-ferrite (c fi a) and the martensite-to-austenite (a¢ fi c) phase transformations are studied for a series of Fe-C-Mn-Si steels with up to 7 wt pct Mn. The model is based on the local equilibrium (LE) concept. The model predictions are compared to experimental observations. It is found that the austenite volume fraction at the end of intercritical annealing depends significantly on the initial microstructure. For Mn concentrations between 3 and 7 wt pct, the LE model is qualitatively correct. However, at higher Mn levels the discrepancy between the predicted austenite fractions and the experimental values increases, in particular for the a¢ fi c transformation. Intragrain nucleation is held responsible for the higher austenite fractions observed experimentally. Silicon is found have a much smaller effect on the kinetics of the intercritical annealing than Mn.

Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels

“…Therefore, Setup A + M is applied to the same steel intercritically annealed at two other temperatures as well as several other steels. The simulated austenite fractions are compared to experimental data from the literature [8,9,18,19] in Figure 12. In all simulations, a 200 nm cell with 1 nm cementite is adopted since no experimental information on the microstructure is provided by the references.…”

“…In most of the simulations, a diffusion couple of austenite and martensite is used. [2][3][4]11,[17][18][19][20][21] In this setup, the simulated temporal evolution of austenite volume fraction has three stages, i.e., a rapid increase under non-partitioning local equilibrium (NPLE) controlled by rapid carbon diffusion, a slow increase under partitioning local equilibrium (PLE) controlled by relatively slow diffusion of Mn in martensite, and a decrease to the equilibrium level under PLE due to homogenization of all alloying elements. When performed on medium Mn steels, such simulations predict much faster austenite formation than experimentally observed after short intercritical Article published online February 7, 2018 annealing times, e.g., less than 1 hour, [7,9] primarily attributed to the NPLE stage.…”

An In-Situ LSCM Study on Bainite Formation in a Fe-0.2C-1.5Mn-2.0Cr Alloy

“…The occurrence of high volume fractions of retained austenite in the microstructure is realized by specific thermal processing routes and fine tuning of the Mn concentration. A key feature of these multi-phase steels is their primarily bainitic microstructure containing retained austenite volume fractions between 10 and 30% [12] at overall Mn concentrations between 1.5 and 8 mass% [13][14][15][16][17][18][19]. The 3rd Gen AHSS family includes the so-called medium Mn steels, to which the steel to be researched in this publication belongs.…”

Determination of Mode Switching in Cyclic Partial Phase Transformation in Fe-0.1C-xMn Alloys as a Function of the Mn Concentration

“…The differences in the Mn profile can explain well the kinematic features in the interface migration behavior by considering the transition from negligible partitioning local equilibrium (NPLE) to partitioning local equilibrium (PLE) mode at transformation interfaces. 1,23,30,31 The Mn concentration profiles after the CC and IC treatments show a Mn spike at the interface position as a result the LE condition at the interface. The position of these Mn spikes is consistent with the positions of the interface in the corresponding curves in Fig.…”