Formation and evolution of non-metallic inclusions in 20CrMnTiH gear steel during LF-RH-CC process
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摘要: 通过工业试验分析了20CrMnTiH在精炼过程中非金属夹杂物的演变行为，并采用FactSage8.0热力学软件计算了20CrMnTiH在钙处理、二次氧化和冷却凝固过程中夹杂物的转变趋势。结果表明，LF精炼过程中夹杂物由Al2O3向MgO-Al2O3和CaO-MgO-Al2O3转变，经过钙处理后，夹杂物改性为CaO-MgO-Al2O3和CaO-Al2O3-CaS，随后经过RH真空精炼，在真空精炼结束时进行第二次钙处理，夹杂物转变为高CaS含量的CaO-Al2O3-CaS和CaO-MgO-Al2O3，连铸过程中钢液发生再氧化，夹杂物中CaS含量进一步升高。热力学计算表明，在钢液中加入0.003%～0.0005%的[Ca]时，钢液中会生成高熔点的MgO-Al2O3，当[Ca]的加入量在0.0005%～0.0007%时，夹杂物主要为液态夹杂物，当[Ca]含量大于0.0007%时，钢液中开始有CaS生成。此外，二次氧化的发生会使得钢液中的液态夹杂物更多地转变为固态夹杂物。随着温度的降低，夹杂物中CaO、CaS含量在逐渐降低，而MgO、Al2O3含量逐渐增加，最终形成高CaS含量的CaS-CaO-MgO-Al2O3。Abstract: The formation and evolution of non-metallic inclusions in 20CrMnTiH gear steel during LF-RH-CC process were analyzed by industrial trials, and the transformation trend of non-metallic inclusions during reoxidation and cooling solidification in 20CrMnTiH gear steel was analyzed by thermodynamic calculations. During LF refining, inclusions experience evolutions from Al2O3 to MgO-Al2O3 and CaO-MgO-Al2O3. After calcium treatment, inclusions are modified into CaO-MgO-Al2O3 and CaO-Al2O3-CaS. After calcium treatment at the end of RH vacuum refining, inclusions are modified into CaO-Al2O3-CaS with high CaS contents and CaO-MgO-Al2O3. During continuous casting, the content of CaS in inclusions increase with the reoxidation of steel. Thermodynamic calculations reveal that when 0.000 3% to 0.000 5% [Ca] is added to the steel, MgO-Al2O3 would be formed in the steel. When 0.000 5% to 0.000 7% [Ca] is added to the steel, liquid calcium aluminates would be largely formed. When the content of [Ca] exceeds 0.000 7%, CaS would be formed. Moreover, liquid inclusions would be gradually transformed into solid inclusions with the reoxidation of steel. With the decrease of temperature, the contents of CaO and CaS in inclusions decrease and the contents of MgO and Al2O3 in inclusions increase, leading to the formation of CaS-CaO-MgO-Al2O3 with high CaS contents eventually.
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Key words:
- 20CrMnTiH /
- inclusions /
- calcium treatment /
- reoxidation
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图 1 精炼过程 T[O] 和 [N] 变化
Figure 1. Variation of mass percent of T[O] and [N] during refining processes
图 2 精炼过程夹杂物的化学成分
Figure 2. Chemical compositions of inclusions at different stages of refining
图 6 钢液冷却凝固过程中夹杂物的转变
Figure 6. Calculated equilibrium state of inclusions at different temperatures
图 7 仅考虑 CaS 析出的成分转变
Figure 7. Calculated equilibrium contents of inclusions considering only CaS precipitation at different temperatures
表 1 精炼过程20CrMnTiH的主要化学成分
Table 1. Main chemical compositions of 20CrMnTiH during refining
% 节点 C Si Mn P S Cr Ti [Al] [Ca] [Mg] L1 0.13 0.13 0.89 0.011 0.0245 1.11 0.005 0.024 0.0002 L2 0.15 0.19 0.94 0.011 0.0159 1.11 0.073 0.0558 0.0018 0.0003 R1 0.15 0.19 0.94 0.011 0.0159 1.11 0.073 0.0558 0.0018 0.0003 R2 0.19 0.26 0.96 0.011 0.022 1.11 0.075 0.041 0.0005 0.0003 R3 0.19 0.26 0.96 0.011 0.018 1.11 0.066 0.038 0.0018 0.0003 T1 0.19 0.26 0.96 0.011 0.018 1.11 0.066 0.038 0.0018 0.0003 T2 0.19 0.26 0.96 0.011 0.018 1.11 0.066 0.038 0.0018 0.0003 T3 0.19 0.26 0.96 0.011 0.018 1.11 0.066 0.038 0.0018 0.0003 
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表 2 夹杂物的尺寸分布
Table 2. Size distribution of the inclusions
节点 占比/% 1～2 μm 2～5 μm 5～10 μm 10～20 μm >20 μm L1 72.12 26.06 0.91 0.61 0.30 L2 75.68 19.82 2.25 1.35 0.90 R1 73.02 25.55 0.85 0.43 0.14 R2 54.90 38.24 5.23 0.98 0.65 R3 71.84 22.82 3.88 0.49 0.97 T1 70.53 26.39 2.96 0.11 0.00 T2 67.36 29.66 2.78 0.20 0.00 T3 64.39 31.94 3.50 0.17 0.00
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