Inclusions evolution in 20CrMnTiH gear steel during BOF-LF-VD-CC process
20CrMnTiH gear steel produced by a plant has a large amount of CaS inclusions during smelting,which affects the quality of casting billet.In order to study the evolution law of oxide inclusions in 20CrMnTiH gear steel and propose reasonable calcium treatment process,to reduce the precipitation of CaS in clusions,the 20CrMnTiH gear steel produced by BOF-LF-RH-CC process in a factory was sampled in the whole refining process.The evolution law and mechanism of oxide inclusions were studied through electron microscopy,inclusion automatic scanning analysis system and thermodynamic cal-culation.The results show that there is secondary oxidation of molten steel in the sulfur feeding line at the later stage of LF refining.The evolution law of oxide inclusions in refining process is as follows,pure Al2O3 inclusions→magnesia alumina spinel and some Al2O3-MgO-CaO composite inclusions→Al2O3-MgO-CaO and Al2O3-CaO composite inclusions after calcium treatment→a large number of Al2O3-MgO-CaO-CaS and Al2O3-CaO-CaS composite inclusions produced after feeding sulfur wire.According to the experiment and thermodynamic calculation,the calcium content in calcium treatment process is slightly lower than the precipitation condition of 12CaO·7Al2O3(C12 A7)at the present stage.In order to modify the Al2O3 inclusion into 12CaO·7Al2O3 inclusion as much as possible,and achieve the ideal calcium treatment effect and save cost,the mass frac-tion of Ca required in the calcium treatment process should be controlled within 0.002 26％-0.003 00％.The composition of molten steel after sulfur feeding line can easily meet the precipitation conditions of CaS inclusions.During the production process,the time between calcium feeding line and sulfur feeding line should be extended,and feed the sulfur line after the cal-cium content decreasing,then move the sulfur feeding process in RH refining to reduce the precipitation of CaS inclusions.The research provides a theoretical basis for improving the cleanliness of sulfur-containing gear steel in industrial production processes.
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