Influence of lanthanum on inclusions and as-cast microstructures in a low-alloy high-strength steel

Advanced high-strength steel ingots with total lanthanum (TLa) contents of 0, 15 9 10–6, 86 9 10–6 and 360 9 10–6 were prepared through laboratory experiments. The modification of inclusions and the variation of the as-cast microstructure with the content of lanthanum in the high-strength steel were analyzed. The result showed that with the increase in the TLa content in the steel from 0 to 360 9 10–6, the modification path of inclusions in the as-cast steel was Al2O3 and calcium aluminate ? LaAlO3 ? La2O2S ? La2O2S–La2O3. The addition of La in the high-strength steel significantly refined the solidification structures. With the increase in the TLa content in the steel from 0 to 360 9 10–6, the ratio of the equiaxed crystal region in the macrostructure increased from 30.1% to 50.7%, the proportion of the high-angle grain boundary in the microstructure increased from 36.9% to 69.8%, and the area fraction of the acicular ferrite and the bainite increased from 0 to 93.3%. Inclusions of LaAlO3, La2O2S and La2O3 in the La-containing steel could act as heterogeneous nucleation cores of a-Fe during the solidification. With the increase in the TLa content in the steel, the number density of inclusions that could act as effective heterogeneous nucleation cores in the steel gradually increased, which enlarged the ratio of the equiaxed crystal region and the proportion of intragranular acicular ferrite, and refined the as-cast microstructure of the high-strength steel.