Effect of austenite grain size on the acicular ferrite transformation in Ti–Zr treated steel

The composite oxides of Ti and Zr can effectively induce acicular ferrite nucleation and refine austenite grain size. To study the transformation mechanism of acicular ferrite in Ti–Zr treated steel, the mass fraction of 0.038% titanium and 0.008% zirconium were added to low alloy steel by melting in a 25 kg vacuum induction furnace. The effect of austenitizing temperature on acicular ferrite transformation behavior was observed in-situ using a high-temperature laser confocal microscope: the samples were heated to 1250, 1300, 1350, and 1400 ℃ at a heating rate of 5 ℃·s⁻¹ and then cooled to 400 ℃ at a cooling rate of 3 ℃·s⁻¹ after holding for 300 s. The ferrite transformation behavior of samples during the above process was observed using a high-temperature confocal microscope. The inclusion composition of Ti–Zr treated steel and the nucleation of acicular ferrite on the inclusion surface were observed using a scanning electron microscope. The variation in microstructure at different austenitizing temperatures was observed using an optical microscope. The austenite grain size was found to increase from 125.6 to 279.8 μm with increasing austenitizing temperature from 1250 to 1400 ℃. The initial transformation temperature of acicular ferrite and side-plate ferrite increased, reached a maximum at 1350 ℃, and then decreased. The volume fraction of acicular ferrite increased from 39.6% to 83.6%. In Ti–Zr treated steel, the size of complex inclusion with Zr–Ti–O in core and Al–Ti–Zr–O in exterior and MnS precipitated on the surface was mainly concentrated in 1–3 μm. It could effectively promote acicular ferrite nucleation. The Mn-poor region and the good lattice relationship between complex inclusions and ferrite were the mechanisms by which the type of inclusions in the steel could promote acicular ferrite nucleation. Using classical nucleation theory, the nucleation potential of acicular ferrite under different conditions was calculated. The results showed that when the austenitizing temperature was 1300 ℃, the nucleation potential of acicular ferrite was the strongest, reaching 191.7 mm⁻². The calculation results were consistent with the variated law of acicular ferrite volume fraction. An increase in austenite grain size led to a decrease in polygonal ferrite nucleation sites, an increase in acicular ferrite nucleation space, and the formation of many inclusions that effectively induced nucleation of acicular ferrite treated by titanium and zirconium, which increased the acicular ferrite volume fraction.