Systematic study on orientation relationships between acicular ferrite and Ti-Mg oxide at different cooling rates in low-carbon steel

In this paper, the influence of cooling rate on acicular ferrite (AF) nucleation and growth on Ti-Mg oxide was studied using high-temperature confocal laser scanning microscopy. The compositions and elemental distributions of the inclusions were studied via electron probe microanalysis, and the crystallographic orientation relationships between AF and inclusions were investigated systematically via electron back scattered diffraction. The results indicated that the initial transformation temperature of AF decreased from similar to 656 to similar to 609 degrees C and the growth rate of AF raised from 6.74 to 55.52 mu m/s as the cooling rate increased from 1 to 12 degrees C/s. The area fraction and the nucleation probability of AF reached a maximum at 2 degrees C/s, which was the most suitable cooling rate for AF formation. The Ti-Mg-Mn-O, Ti-Mg-Mn-O.TiN and Ti-Mg-Mn-O.MnS inclusions in Ti-Mg deoxidized steel were all effective for AF nucleation. For the four samples at different cooling rates, all AF grains had four kinds of orientation relationships with Ti-Mg-Mn-O oxides: Baker-Nutting (B-N), Nishiyama-Wasserman (N-W), {100}(Ferrite)parallel to{110}(Oxide), and irrational orientation relationships. The {100}(Ferrite)parallel to{110}(Oxide) orientation was also a common parallel relationship between the B-N and N-W orientation relationships. The lattice mismatch between the AF and oxide was low, and the interface between the AF and oxide was close to a coherent interface when the first three kinds of orientation relationships were adopted. The formation of a Mn-depleted zone (MDZ) around Ti-Mg-Mn-O oxide could increase the driving force of ferrite formation, and it contributed more to the nucleation of those AF grains that had no orientation relationships with oxides.