In Situ Observation and Prediction of the Transformation of Acicular Ferrites in Ti-Containing HLSA Steel

The evolution of different types of ferrites in a Ti-bearing high-strength low alloy (HSLA) steel was investigated using a high-temperature confocal laser scanning microscopy (HT-CLSM). As the austenitizing temperature increased from 1250 °C to 1400 °C, the volume fraction of acicular ferrites (AFs) increased from 17.6 to 23.2 pct in the steel containing 0.0070 pct Ti and decreased from 78.7 to 52.5 pct in the steel containing 0.0370 pct Ti. The volume fraction of the AF in the steel varied with the start transformation temperature of the AF and the ferrite side plate (FSP). A linear dependency of the Gibbs free energy variation on the grain size of austenites for the transformation of AFs was obtained. The relationship between the volume fraction ratio of AFs to FSPs and the size of inclusions and the austenite grains was established through the classical nucleation theory and was validated by HT-CLSM measurements.