The effect of La on inclusions and fracture toughness of 40CrNi2Si2MoVA steel was investigated via the optical microscope, scanning electron microscope, image software and electronic universal testing machine. The results reveal that the inclusions in steel without La are mainly MnS and Al₂O₃–MnS, while the inclusions in steels with La primarily contain La–O–S, La–S and other rare earth complex inclusions contain P and As. La–O–S and La–S are formed under the steelmaking temperature and act as the nucleation core of rare earth complex inclusions containing P and As. According to the segregation model, La–O–S–P–As and La–S–P–As are formed through chemical reactions during the solidification stage. As La content in steels increases from 0 to 0.032 mass%, the average spacing of inclusions is gradually increased from 5.28 to 15.91 μm. The volume fraction of inclusions in steels containing less than 0.018 mass% La approaches 0.006%; however, it is significantly improved to 0.058% when La content is increased to 0.032 mass%. With the increase in La content, the fracture toughness is firstly improved from 63.1 to 80.0 MPa m^1/2 due to the increase in average spacing of inclusions and then decreases to 69.6 MPa m^1/2 owing to the excessive increase in volume fraction of inclusions. The optimal fracture toughness is found in 40CrNi2Si2MoVA steel with 0.018 mass% La.

通过光学显微镜，扫描电子显微镜，图像软件和电子万能试验机研究了La对40CrNi2Si2MoVA钢夹杂物和断裂韧性的影响。结果表明，不含La的钢中夹杂物主要为MnS和Al ₂ O _3。-MnS，而含La的钢中的夹杂物主要包含La-O-S，La-S和其他稀土元素夹杂物包含P和As。La–O–S和La–S是在炼钢温度下形成的，并充当包含P和As的稀土复合夹杂物的成核核心。根据偏析模型，La–O–S–P–As和La–S–P–As是在凝固阶段通过化学反应形成的。随着钢中La含量从0增加到0.032质量％，夹杂物的平​​均间距从5.28μm逐渐增加。La含量小于0.018质量％的钢中夹杂物的体积分数接近0.006％；然而，当La含量增加到0.032质量％时，其显着提高到0.058％。随着La含量的增加，断裂韧性首先从63.1 MPa提高到80.0 MPa m。^1/2是由于夹杂物平均间距的增加，然后由于夹杂物的体积分数的过度增加而降至69.6 MPa m ^1/2。在具有0.018质量％La的40CrNi2Si2MoVA钢中发现了最佳的断裂韧性。