Coupling Effect of Prior Austenite Grain Size and Inclusion Characteristics on Acicular Ferrite Formation in Ti-Zr Deoxidized Low Carbon Steel

The coupling effect of prior austenite grain size and inclusion characteristics on acicular ferrite (AF) formation was investigated in Ti-Zr deoxidized low carbon steel by utilizing the high temperature confocal laser scanning microscope (HT-CLSM), optical microscope (OM), and scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS). The results indicated that with the target heating temperature increased from 1100 °C to 1350 °C, the average size of prior austenite grain varied from 58.22 to 237.40 μ m, and the average grain size increased rapidly when the temperature was above 1200 °C. For inclusion characteristics, different target heating temperatures had no obvious effect on inclusion types, but had a great influence on the average size and number density of each, especially for the intragranular effective inclusions. In addition, as the increase of target heating temperature, the types of microstructure were identical, but both AF volume fraction and AF relative nucleation ability increased first and then decreased. When the target heating temperature of sample was 1250 °C, the AF volume fraction reached the maximum of 49.48 pct. However, the AF relative nucleation ability reached the maximum of 474.5 at the target heating temperature 1200 °C, at this time, the AF volume fraction was 47.92 pct, only 1.56 pct smaller than that at 1250 °C. Therefore, considering the AF volume fraction and AF relative nucleation ability, the optimal target heating temperature for AF formation in this study was 1200 °C, and the corresponding prior austenite grain size was 69.58 μ m.