In addition to manganese, carbon partitioning has been proposed in a new medium Mn-TRIP steel by two-step partitioning during the first batching annealing and the final continuous annealing. In the second-step partitioning, the cementite dissolves and blocky austenite forms with carbon enrichment, while the partition of manganese is negligible from prior lath austenite back into ferrite due to short duration. The combined partition of carbon and manganese improves both fraction and stability of retained austenite. The alloy has exhibited the product of strength and elongation of approximately 50 GPa%. It is highlighted that there is no Lüder strain in tensile curves. The microstructure evolution and relationship of microstructure and properties have been investigated and characterised carefully in this research. This paper is part of a Thematic Issue on Medium Manganese Steels.
Cracking generates usually due to the internal stress during martensitic transformation in high carbon steels, for instance the ‘1C–1.5Cr’ bearing steels. A novel low-density ‘1.2C–1.5Cr–5Al’ alloy has been designed recently for bearing application in previous research. The effect of morphology of martensite plates on the transformation cracking has been proposed and investigated in comparison between the ‘1.2C–1.5Cr–5Al’ and ‘1C–1.5Cr’ steels in this research. Based on the mechanisms proposed, the tendency for transformation cracking in the novel alloy with more carbon addition has been discussed compared with the ‘1C–1.5Cr’ steel. This paper is part of a Themed Issue on Recent developments in bearing steels.
The narrow process window during intercritical annealing and discontinuous yielding have limited the commercialization of medium Mn steels. In this study, a double-annealing process based on the commercial continuous annealing line is proposed. The cold-rolled medium Mn steels were first fully austenitized and quenched during the first annealing, followed by intercritical annealing for reverted austenite transformation. The microstructure of duplex lath-shaped austenite and ferrite is produced and steel exhibits a desirable continuous yielding during tensile deformation. Al is added into the medium Mn steel to enlarge the process window and to improve the partitioning efficiency of Mn. The produced steel is more robust with temperature fluctuation during the industrial process due to the enlarged intercritical region. Mn partitioning is more efficient owing to the elevated annealing temperature, which results in the improvement of ductility in the Al-added steel with increased austenite stability.
The austenite/ferrite interface movement during intercritical annealing of 0.15C–8.0Mn–2.1Al (wt-%) steel was simulated by DICTRA software under local equilibrium and then confirmed experimentally. The simulation results show that the austenite volume fraction exceeds the thermodynamic equilibrium amount during isothermal annealing and then decreases because the interface returns from ferrite to austenite. The reverse interface movement is verified in simulated batch annealing experiment for the first time by ex-situ determining the austenite volume fraction using characterisation of XRD as well as by in-situ measuring the bulk volume change led by phase transformation using dilatometric curves.
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