Abstract
The work presents results of phase transformation kinetics of hot-rolled 5% Mn steel subjected to different heat treatments. Three different schedules were introduced: isothermal holding in a bainite region, coiling simulation and intercritical annealing. The evolution of microstructure components was investigated using dilatometric and metallographic analyses. According to obtained results, the medium-Mn steel exhibits high resistance for γ/α transformation during the bainite transformation and coiling simulation (upon cooling from the austenite region). During 5 h isothermal holding, no bainite and/or ferrite formation was detected. This results in the formation of martensite upon cooling to room temperature. Differently, when the steel was subjected to the intercritical annealing at 720 and 700 °C (upon heating from room temperature), a final microstructure consisted of ferrite, martensite and retained austenite. At 700 °C, no fresh martensite formation was detected upon cooling to room temperature. This means that the austenite was enriched in carbon during the intercritical annealing step enough to keep its thermal stability.
Highlights
Medium manganese steels are some of the most promising grades of steel in the automotive industry
The problem is the price of such steels, which limits their application in car body construction
The aim of the study was to determine the effect of different heat treatments on the formation and stability of retained austenite in the steel containing 5% Mn and the increased Al addition
Summary
Medium manganese steels are some of the most promising grades of steel in the automotive industry. The reason for this is a good compromise between material cost and mechanical properties. The metastable retained austenite, one of the main phases, is responsible for high combinations of strength and ductility [1,2]. This phase during deformation transforms locally into martensite increasing both strength and ductility of the steel. This prevents the strain localization increasing the deformation potential of steel elements. The medium manganese steels have recently become of particular scientific interest [4,5,6]
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