Research Progress and Mechanism of Rare Earth in Steel Casting

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1. Mechanism of Rare Earth in Steel Casting

1.1 Purification of Molten Steel

Rare earth (RE) elements significantly enhance the purity of steel casting by reacting with oxygen, sulfur, and harmful trace elements. The generated RE oxides/sulfides exhibit high melting points, low density, and stability (Table 1), enabling their removal via slag or utilization as nucleation sites during solidification. Key reactions include:

$$ \text{La (s)} + \text{H}_2\text{(g)} \rightarrow \text{LaH}_2\text{(s)}, \quad \Delta G = -145070 + 177.979T $$

This reaction fixes hydrogen, mitigating hydrogen embrittlement. Similarly, RE reduces nitrogen activity and forms stable REN compounds.

Table 1. Physical properties of rare earth compounds
Compound Melting Point (°C) Density (g/cm³) Formation Heat (-ΔH298, kJ/mol)
La2O3 2315 6.51 1913.4
Ce2O3 1690 6.86 1863.1
Pr2O3 2199 6.80 1838.0
Nd2O3 2271 7.24 1808.7

1.2 Modification of Inclusions

RE transforms harmful elongated MnS and Al2O3 inclusions into spherical RE oxysulfides. The optimal [RE]/[S] ratio ensures inclusion spheroidization and dispersion. Excessive RE (>0.22 wt.%) causes inclusion clustering, degrading mechanical properties.

1.3 Microstructure Refinement

RE reduces interfacial energy and induces compositional undercooling, promoting grain refinement. The relationship between grain size (d) and RE content follows:

$$ d = k \cdot [\text{RE}]^{-n} $$

where k and n are material constants. RE additions increase ferrite content while reducing pearlite spacing.

1.4 Microalloying Effects

Solid-solution RE atoms (atomic radius: 0.182-0.204 nm) distort the Fe matrix, enhancing strength through lattice strain:

$$ \sigma_{ss} = MG\varepsilon^{3/2}c^{1/2} $$

where M = Taylor factor, G = shear modulus, ε = strain, and c = RE concentration.

Steel casting production process

2. Advancements in RE-Enhanced Steel Casting

Recent studies demonstrate remarkable improvements in steel casting performance through RE alloying:

Table 2. Mechanical properties of Ce-modified steel casting
Ce Content (wt.%) YS (MPa) UTS (MPa) Impact Energy (J) Hardness (HB)
0 600 800 72 230
0.052 750 880 130 260
0.22 700 820 95 240

Optimal Ce addition (0.05-0.12 wt.%) enhances yield strength by 25% and impact toughness by 81% through dual mechanisms:

  1. Inclusion spheroidization reducing stress concentration
  2. Grain boundary purification suppressing impurity segregation

3. Industrial Applications

RE-treated steel castings exhibit superior performance in:

  • Mining machinery: Wear resistance increased by 20-40%
  • Power generation: Creep rupture life extended 2-3× at 600°C
  • Marine equipment: Corrosion rate reduced by 35-50%

4. Challenges and Perspectives

Key unresolved issues include:

$$ \text{RE yield} = \frac{[\text{RE}]_{\text{steel}}}{[\text{RE}]_{\text{added}}} \times 100\% $$

Current industrial RE yields remain below 60% due to oxidation losses. Future research directions focus on:

  • Developing protective RE addition techniques
  • Establishing quantitative RE-process-property relationships
  • Exploring RE recycling from steel casting slag

The strategic integration of RE elements presents a transformative approach for manufacturing high-performance steel castings, particularly for critical applications in extreme environments.

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