718H Pre-hardened mold steels with different Rare Earth (RE) contents were prepared to investigate the influence of RE on microstructure evolution and mechanical properties through a series of experiments and theoretical analysis. The results indicated that the toal oxygen (T.O) content decreased from 15 ppm to 6 ppm with 0.022 wt% RE addition, which is attributed to the active chemical properties of RE elements. For test steels, RE additions of 0.012 wt% and 0.022 wt% were significantly effective for refining inclusions by eliminating 11.5% large-sized inclusions with diameter exceeding 10 μm compared with that of 0RE steel. RE addition contributed to modify irregular MnS and Al₂O₃ inclusions into ellipsoidal RE-inclusions (RE₂O₃, RES, RE₂O₂S and REAlO₃). The purification of molten steel and the modification of inclusions by RE treatment have significant effects on improvement of the fatigue crack growth tests (FCG) inhibition ability and impact energy as well as the isotropy. However, excessive addition of RE elements (0.07 wt%) seriously reduced the impact energy, ultimate tensile strength and FCG inhibition ability due to rapidly increase of the volume fraction of large-sized inclusions. In addition to the inclusions formed by RE treatment, trace solid solution RE atoms improve the stability of undercooled austenite, resulting in the transformation region of bainite and perlite of 0.07RE steel shifting to the bottom right and prolonging the incubation period compared with that of 0RE steel.

通过一系列实验和理论分析，制备了不同稀土（RE）含量的718H预硬化模具钢，以研究RE对微观组织演变和力学性能的影响。结果表明，添加0.022 wt％的RE时，总氧（TO）含量从15 ppm降至6 ppm，这归因于RE元素的活性化学性质。对于试验钢，与0RE钢相比，添加0.012 wt％和0.022 wt％的RE通过消除直径超过10μm的11.5％大尺寸夹杂物，对精炼夹杂物非常有效。稀土元素的添加有助于将不规则的MnS和Al ₂ O ₃夹杂物改性成椭圆形的稀土夹杂物（RE ₂ O ₃，RES，RE ₂ O ₂ S和REAlO ₃）。钢水的净化和通过RE处理对夹杂物的改性对改善疲劳裂纹扩展试验（FCG）的抑制能力和冲击能以及各向同性具有重要影响。然而，由于大尺寸夹杂物体积分数的快速增加，过量添加稀土元素（0.07 wt％）会严重降低冲击能，极限抗拉强度和FCG抑制能力。除了通过RE处理形成的夹杂物外，微量固溶体RE原子还改善了过冷奥氏体的稳定性，导致0.07RE钢的贝氏体和珍珠岩的转变区域移至右下角，并且与0RE相比延长了潜伏期钢。