In Situ Investigation of the Bainitic Transformation from Deformed Austenite During Continuous Cooling in a Low Carbon M
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UCTION
CONTINUOUSLY cooled bainitic steels for hot-rolled bars or forged components have been a topic of growing interest over the past 10 years.[1] In general, Si or Al alloyed steels are considered for such applications, since these elements strongly inhibit the carbide formation and lead to carbide-free bainite with an increased amount of retained austenite.[2] The combination of strength from the microstructural constituent bainitic ferrite and ductility related to retained austenite of the new generation of low carbon bainitic steels can lead to a broad range of mechanical properties. Therefore, bainitic steels have the potential to replace conventional quenching and tempering processes applied after hot forging and uncontrolled cooling. Through the use of a continuous cooling transformation process, which is one primary interest
WILLIAM LEMOS BEVILAQUA and ALEXANDRE DA SILVA ROCHA are with Post-graduation Program in Mining, Metallurgical and Materials Engineering (PPGE3M), Federal University of Rio Grande do Sul (UFRGS), Av. Bento Gonc¸alves, 9500. Porto Alegre, CEP 91501-970, Brazil. Contact e-mail: [email protected] JE´RE´MY EPP and HEINER MEYER are with the Leibniz Institute for Materials Engineering IWT, Badgasteiner Str. 3, 28359 Bremen, Germany. HANS ROELOFS is with Swiss Steel AG, Emmenbru¨cke, Switzerland. Manuscript submitted September 28, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
in industrial schedules,[3] a substantial reduction of energy consumption in manufacturing routes can be achieved, especially for the low and medium carbon content steels due to rapid bainite formation kinetics. On the other hand, the use of hot deformation can affect the bainitic transformation in a complex way, leading to different results obtained in the literature that sometimes look contradictory,[4] mainly considering the effect of prior deformation on the kinetics of phase transformation. In fact, the effects of deformation on the bainitic transformation, and the resulting microstructure are less studied than the effects of transformation temperature.[5] Additionally, the carbon partitioning into austenite can be affected by the previous hot deformation, which has not been accurately studied in the literature during continuous cooling. Few investigations using in situ high-energy X-ray diffraction (HEXRD) were reported on bainitic transformation in low carbon steels. Studies conducted with isothermal treatments[6] show that carbon in the austenite becomes heterogeneous as the transformation progresses, leading to the different austenite morphologies (films and blocks). Similar behavior was also found for continuous cooling transformation,[7] in which the overall transformation was described as a four-stage process when cooling rates lower than 0.75 C/s are applied.
The effects of hot deformation on the bainitic transformation by continuous cooling have been only discussed in terms of dilatometry data, which is not capable of describing in detail the bainite formation and kinetics due to anisotropic dilatat
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