An in-situ microscopy study on nucleation and growth of acicular ferrite in Ti-Ca-Zr deoxidized low-carbon steel

Abstract

We describe here an in-situ microscopy study on the nucleation and growth of acicular ferrite in Ti-Ca-Zr deoxidized low-carbon steel as a function of cooling rate. The study indicated the starting transformation temperature of acicular ferrite decreased with increasing cooling rate. Moreover, the average growth rate of acicular ferrite was increased from ~2.2 μm/s to ~14.4 μm/s with the increase of cooling rate from 2 °C/s to 15 °C/s. In Ti-Ca-Zr deoxidized steel, (Ti, Ca, Zr, Al)-O-Mn-S oxide inclusions effectively promoted the formation of intragranular acicular ferrite. A significantly high proportion of acicular ferrite was characterized by high angle grain boundaries and reduced grain size. When the cooling rate was low at 5 °C/s, the proportion of acicular ferrite was high at ~85%, with ~48% high angle grain boundaries and small average grain size of ~2.7 μm. Additionally, MnS precipitated in the vicinity of (Ti, Ca, Zr, Al)-O-Mn-S oxide particles, which promoted the nucleation of acicular ferrite.