Microstructure Evolution During High-Temperature Partitioning of a Medium-Mn Quenching and Partitioning Steel

Abstract

a r t i c l e i n f o Keywords:Quenching and partitioning Medium manganese steel High-temperature partitioning Carbon partitioning Austenite stability a b s t r a c t Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high temperature, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to investigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C-600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C-600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium-Mn steels requires microstructure design strategies to minimize or suppress competitive reactions. (C. Celada-Casero), J.Sietsma@tudelft.nl (J. Sietsma), M.J.SantofimiaNavarro@tudelft.nl (M.J. Santofimia).cess. The CCE model is characterized by two assumptions: a) the carbon partitioning from martensite to austenite is finalized when the chemical potential of carbon in both phases is equal and b) the austenite/martensite interface is immobile during the partitioning step as the number of iron atoms in each phase are conserved. Typical partitioning temperatures in the Q&P process (350 °C-450 °C) are relatively low and the diffusivities of substitutional alloying elements during the partitioning step can be ignored at the time ranges that are normally considered. Therefore, most studies are concentrated on studying the stabilization of austenite by carbon [2 , 5-8] .Recently, the idea of stabilizing the austenite through Q&P heat treatments in which the partitioning stage takes place at temperatures high enough to stimulate the partitioning of substitutional alloying elements has been proposed by some authors [9][10][11][12][13] . This possibility stems from the observation of an apparent partitionin...