Ferrite Transformation in C-Mn Steel Containing High Volume Fraction Oxide Particles

摘要

Some oxide inclusions are known to work as a nucleating agent of ferrite in the austenite-to-ferrite transformation of steel, and thus the understanding of this nucleation event is essential to achieve further refinement of microstructure in steels and their welds. In this context, effects of the chemistry and volume fraction of oxide particles on microstructure development in oxide-dispersed steel were investigated in the present study. Steels containing oxide particles of Al_2O_3 or Ti_2O_3 up to 1.0 vol. %, produced by a powder metallurgy route, were subjected to the austenite-to-ferrite transformation by continuous cooling or isothermal holding at different temperatures. It was found that, while the dispersion of Al_2O_3 had little effect on ferrite formation in steel during either continuous cooling or isothermal holding, the dispersion of Ti_2O_3 had a striking effect on microstructure refinement by promoting intragranular polygonal ferrite and acicular ferrite. The size of acicular ferrite was decreased with increasing volume fraction and number density of Ti_2O_3 particles, but the trend toward the refinement became sluggish after reaching a certain level. EBSP analyses indicated that, while polygonal ferrite in Ti_2O_3-dispersed steel did not hold any specific orientation relationship with prior austenite, acicular ferrite in Ti_2O_3-dispersed steel exhibited either the Kurdjumov-Sachs orientation relationship (K-S) or an orientation relationship close to but different from the K-S with prior austenite. The latter can be achieved by rotating the K-S orientation around <111>_(fee) while keeping {111}_(fce)//{011}_(bcc). This particular orientation relationship was not identified in either Widmanstatten ferrite or bainite at all, but was always observed in acicular ferrite microstructure, and thus seems to characterize the formation mechanism of acicular ferrite from oxides as well as the trend of microstructure refinement.