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The effect of retained austenite stability on microstructure evolution and mechanical properties of a new cold-rolled medium-manganese steel (0.12C, 7.69Mn, 1.45Si, 2.76Al wt-%) under intercritical annealing Q&P process at different partitioning temperatures was investigated. The evolution of retained austenite was characterized and statistically analysed using SEM, TEM, EBSD, and XRD techniques. The results show that the stability of retained austenite depends to a large extent on the applied heat treatment and is mainly attributed to the retained austenite content and morphology, whose carbon content is not the main influence. More retained austenite content and the formation of heterogeneous austenite structures (granular, massive, lamellar) are observed at a partitioning temperature of 150°C. The retained austenite of different morphologies has different stabilities and can sustain the TRIP effect at different strain stages, obtaining the best comprehensive mechanical properties with 34.5% elongation and the product of tensile strength and elongation is up to 43.5 GPa•%.

Simultaneously, the benefits of the traditional quenching and partitioning (Q&P) process (carbon distribution) and the austenite reverse transformation (ART) process (manganese diffusion) are combined to apply the Q&P process to the production of medium-manganese steels. [2][3][4] The elements C and Mn are broken down from ferrite/martensite and stabilized in austenite during reverse phase transformation. the microstructure of medium manganese steels, therefore, consists of a two-or three-phase microstructure of ultrafine grain ferrite, substable austenite, and/or fresh martensite (from austenite transformation). [5] However, the amount and stability of retained austenite are the most important elements influencing the mechanical characteristics of medium-manganese steels. Furthermore, the effects of chemical composition, [6] morphology, [7] grain size, [6] surrounding phases, [8] grain orientation, [9] and strain distribution between phases on retained austenite must be taken into account. At the same time, creating automobile components is a complicated process that involves many stretching, bending, and forming processes to finish. [10] Because these forming processes are similar to prestraining the sheet to varying degrees, studying the effects of prestrain on the microstructure and properties of the sheet can serve as a guide to the actual manufacture of the part. In recent years, studies on the prestrain of low-alloy high-strength duplex steels under unidirectional tensile stress have been published. The unidirectional stretching process pre-strain was used by Qiao, [11] with different sizes of prestrain (0%, 5%, 10%, and 15%) for DP780 duplex steel. The yield strength and tensile strength both rose when the prestrain increased, although the change in tensile strength was less. The mechanical properties and hardening behavior of transformation induced plasticity (TRIP) steels under different prestrain were studied by Hu. [12] As the tensile prestrain rose, the material's work-hardening capability was found to be significantly reduced. The prestretching, cupping, and reaming tests on high-strength steel DH780 were carried out by Liu. [13] It was discovered that when the prestrain rose, the retained austenite content steadily reduced, the yield strength and tensile strength both

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