Effect of Lanthanum Content on the Formation of Acicular Ferrite

The microstructure and inclusions in the heat-affected zone (HAZ) of steel with two different contents of rare earth (RE) lanthanum (La) were investigated. The results indicated that when the content of La was 0.010 wt pct in steel, the area percent of acicular ferrite (AF) was 62.9 pct and the main composition of inclusions was La–O–S–Ti–Mg + MnS, whereas in steel with 0.068 wt pct La, the area percent of AF decreased to 36.9 pct and the main composition of inclusions changed to La–O–P + La–P. Both the mean size and the number of inclusions in the steel with high content of La were larger than those in the steel with low content of La. The ratios of effective inclusions, which can act as the nucleus of AF in the two steels, were 59.2 and 36.5 pct, respectively. In the two steels, the size of effective inclusions was concentrated in 1 to 4 μ m. Although the main effective inclusions in the two steels were both RE inclusions, in the steel with low RE content, the effective RE inclusions were mainly composites of La–O–S–Ti–Mg and MnS, while in the steel with high RE content, these effective RE inclusions were La–O–P and La–O–P + TiN. The effective inclusions in low RE content steel were more capable than those in high RE content steel, and the mean numbers of AF laths induced by effective inclusions in the two steels were 2.9 and 2.3, respectively.