Abstract
Microstructural changes during thermal processing of a medium manganese steel containing (in wt%) 0.19C and 4.39 Mn were evaluated in situ with a high energy X-ray diffraction system (HEXRD). Samples with an initial fully martensitic microstructure were heated to intercritical annealing (IA) temperatures of 600 or 650°C, held for 30 min, and cooled to room temperature. Diffraction data were analyzed to determine the variations in austenite and ferrite phase fractions and phase lattice constants throughout the ICA cycles. On heating, the 2 vol. pct of austenite present in the starting microstructure decomposed, and cementite precipitation then occurred. During isothermal holding, the austenite fraction increased, up to 20% for the sample annealed at 650°C. The measured austenite fractions were less than those calculated by Thermo-Calc for equilibrium conditions, indicating that the 30-min hold time was insufficient to achieve near-equilibrium conditions. Observed changes in lattice parameters during isothermal holding were interpreted to reflect composition changes due to redistribution of the C and Mn between austenite and ferrite. The results are discussed in relation to the potential for controlling austenite stability during ambient temperature deformation.
Highlights
Medium manganese steels represent one of the third-generation advanced high strength steels (3Gen AHSS) which have attracted significant attention from the global automotive industry and steel researchers as potential enablers for vehicle light weighting (Abu-Farha et al, 2018)
In the current work, advanced in situ high energy X-ray diffraction system (HEXRD) observation of the austenite characteristics during intercritical annealing has been performed using a Linkam 1,500 heating stage positioned on the high energy synchrotron X-ray beamline
The dissolution of the small amount of austenite is observed during initial heating at around of 375°C, with the associated appearance of cementite in the microstructure
Summary
Medium manganese steels represent one of the third-generation advanced high strength steels (3Gen AHSS) which have attracted significant attention from the global automotive industry and steel researchers as potential enablers for vehicle light weighting (Abu-Farha et al, 2018). The intercritical annealing process reheats the material to a temperature between AC1 and AC3 and forms a two-phase microstructure of ferrite (α) and austenite (γ). The observed variations in ductility can be attributed to the different microstructural characteristics (e.g., volume fractions of retained austenite, the sizes, morphologies and distributions of the austenite and ferrite, etc.) which result from the different annealing conditions. The sample which exhibited the highest elongation was annealed at 600°C resulting in the highest austenite volume fraction (around 40%) after cooling to room temperature. In addition to high elongations, medium Mn steels may exhibit substantial yield point elongations (Abu-Farha et al, 2018)
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