Grain refinement by cold deformation and recrystallization of bainite and acicular ferrite structures of C–Mn steels

Abstract

The propensity of bainite and acicular ferrite structures of experimental C–Mn steels for enhanced grain refinement by combining phase transformation and plastic deformation has been investigated. Formation of acicular ferrite structures were stimulated with a small amount of titanium and titanium oxide nanoparticles added into the molten steels of high Mn concentrations. Isothermal transformations into the bainite and acicular ferrite structures were performed for 1.8ks at 823K after preliminary austenitization for 1.8ks at 1523K. Cold rolling for 50% thickness reduction was conducted on the isothermally transformed structures. Subsequent annealing of the deformed structures was conducted for 3.6ks at 773, 873 and 973K. Optical microscopy, scanning electron microscopy and tensile test were used for characterization of the studied steels. Cold rolling and annealing of the transformed structures at 873K resulted in strengthening at the expense of ductility where an initial stage of recrystallization is realized. Acicular ferrite obtained by the addition of titanium into the molten steel exhibited the remarkable improvement of tensile properties. Discontinuous recrystallization of the deformed structures at 973K leads to the formation of fine grains wherein acicular structures represented more enhanced grain refinement than bainite.