Design of carbide-free bainite in Si/Al-free steel by chemical heterogeneity
Instead of conventional bainite austempering(CB)process,a bainite austempering(PB)process purposely based on Mn-partitioned pearlite to investigate the effect of Mn distribution on bainite transformation was proposed.In present study,carbide-free bainite by Mn-heterogeneous austenite which was originated from the Mn-partitioned pearlite in the Fe-0.39C-3.69Mn(wt.％)steel was obtained.The results show that the Mn-heterogeneous austenite can tailor the austenite-to-bainite transformation during austempering,leading to the formation of ghost pearlite which was assembled by bainite ferrite lath and film retained austenite.During the austempering stage,Mn-depleted austenite was transformed into bainite ferrite quickly,while Mn-enriched austenite can be stabilized to the austempering temperature.Specifically,carbide precipitation was not detected in ghost pearlite without Si and Al addition.This is mainly due to the strong interaction between carbon and manganese elements,which drives the diffusion of C from the carbon-depleted region to the manganese-enriched region.Moreover,the nano-scale lamellar structure of ghost pearlite reduced the diffusion distance,resulting in an increased C content in the manganese-enriched austenite,and further inhibited the bainite transformation and carbide precipitation.As a result,the PB samples significantly improve ductility than CB samples under the same austempering condition,with a total elongation of 11.5％,uniform elongation of 7.6％.Meanwhile,PB samples keep a constant strength with CB samples,with an ultimate tensile strength of 1 043 MPa.
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