Investigation of Lanthanum Content on Nonmetallic Inclusions and Mechanical Properties of Low Alloy Wear-Resistant Steel Containing P and As Elements Through In Situ Tensile Test

In the present study, low alloy wear-resistant steels (LAWRS) with four kinds of total lanthanum contents (T.La), i.e., 0, 25, 40, and 96 ppm, were produced in a vacuum induction furnace. The ingots were hot forged into bars and the subjected to heat treatment. The influence of different T.La contents on the evolution of inclusions and mechanical properties of the steel was experimentally investigated. In the absence of La, the initial inclusion in the steel was CaO–Al₂O₃–CaS and pure CaS. After adding 25 ppm T.La, uniform La–O–S–Ca system inclusions could be observed. When the T.La content was more than 40 ppm, La–O–S–(Ca) inclusions surrounded by an outer layer containing P and As were found. The homogeneous inclusions La–O–P–As could be detected as the T.La content was added to 96 ppm. The number density and area fraction of inclusions were minimized when the T.La content reached 25 ppm, whereas the addition of T.La in the steel had little influence on the average diameter of inclusions. Regarding mechanical properties, the optimized tensile strength exhibited 1783 (±10) MPa as T.La content was 25 ppm. In-situ tensile testing revealed that the single-phase La–O–S inclusions with small size induced smaller voids and crack propagation compared to larger CaO–Al₂O₃–CaS inclusions without the addition of T.La in the steel. The results indicated that the enhancement in the tensile strength could be attributed to the modification of initial CaO–Al₂O₃–CaS inclusions and the reduction of the number density and the size of rare earth La–O–S–Ca inclusions after the addition of 25 ppm T.La.