Abstract
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.
Highlights
Medium Mn steels containing 5–10 wt% Mn have been listed as one of the third-generation advanced high-strength steels for automobile application due to their attractive combination of strength and plasticity [1,2,3,4,5,6,7,8,9]
The superior mechanical properties are mainly originated from the transformation of metastable austenite into hard martensite during deformation, namely the transformation-induced plasticity (TRIP) effect [4,7,10,11,12,13]
An ultrafine and globular austenite and ferrite microstructure will be formed after the above heat treatment process, leading to discontinuous yielding accompanied by Lüders strain during tensile deformation
Summary
Medium Mn steels containing 5–10 wt% Mn have been listed as one of the third-generation advanced high-strength steels for automobile application due to their attractive combination of strength and plasticity [1,2,3,4,5,6,7,8,9]. The excellent mechanical properties of medium Mn steels are highly dependent on the partitioning of C and Mn from martensite/ferrite to austenite. An ultrafine and globular austenite and ferrite microstructure will be formed after the above heat treatment process, leading to discontinuous yielding accompanied by Lüders strain during tensile deformation. This mechanical performance is not favorable to the formability of the steel sheet due to severe localized thinning and results in a rough surface of the stamping parts [21,22,23,24,25]
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