The precipitation, dissolution, and coarsening of different carbides at 680 °C in G18CrMo2–6 steel was investigated experimentally combined with Jmatpro simulation. The G18CrMo2–6 steel was normalized at 940 °C, followed by tempering at different times at a constant temperature of 680 °C. During the tempering process, there are mainly two kinds of carbide, namely M3C and M23C6. Through characterization of microstructural evolution, thermodynamic calculation, and kinetic simulation, it was observed that during the tempering process, the stable M23C6 carbide was growing, whereas the metastable M3C carbide was disappearing. At the end, the M3C carbide was dissolved and the M23C6 carbide was in equilibrium with the matrix.
Microstructure and mechanical properties of medium-Mn steel (Fe–0.14C–5Mn–1Al–Ce) processed by different austenite reverted transformation-annealing temperatures vary from 580 °C to 740 °C were studied. It was found that the austenite reverted transformation-annealing temperature has a strong effect on microstructure evolution. The martensite structure was transformed into austenite by austenite reverted transformation during the austenite reverted transformation-annealing process. The orientation relationship between the austenite and the matrix was dominated by the Kennicutt–Schmidt relation. With the increase of the austenite reverted transformation-annealing temperature, the content of retained austenite first increases and then decreases at room temperature. The tensile strength first decreases and then increases, while the elongation first increases and then decreases. An excellent combination of tensile strength and elongation (Rm × A) was obtained in the Fe–0.14C–5Mn–1Al–Ce steel by austenite reverted transformation-annealing at 640 °C.
The effects of intercritical annealing time on microstructure evolution and mechanical properties of a novel medium Mn steel (Fe-0.14C-5Mn-1Al-Ce) were investigated. The microstructure composed of lamellar ferrite and retained austenite (RA)/α’-martensite mixed phases after intercritical annealing. With the extension of intercritical annealing holding time, the volume fraction of RA first increases and then decreases, and RA is always formed at the high-angle grain boundaries of the ferrite. Both the product of Rm*A and the total elongation increase as the volume fraction of RA increases. The greater volume fraction of RA, the greater total elongation and Rm*A. The enrichment of carbon in RA was investigated by XRD and DICTRA. As intercritical annealing holding time increases, the carbon concentration in austenite decreases, while the change of the carbon concentration will affect the volume fraction of RA after intercritical annealing.
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