| Generally,the cleanliness of Chinese gear steel is poor,so the improvement of the cleanliness of gear steel is of significance to the quality of final gear products.In China,20CrMnTi is the most widely used gear steel grade.On this basis,the formation and evolution of the inclusions in 20CrMnTi gear steel during refining were studied by industrial and laboratory experiments in this paper.Different kinds of clogging behaviors of submerged entry nozzles(SEN)were investigated.The types and sources of the macro-inclusions in this steel grade were analyzed by Slime Method.Industrial experiments(e.g.slag adjustment and calcium treatment modification)were conducted to optimize the refining process of gear steel in a steel plant.This study could provide a theoretical guidance for the control of inclusion in 20CrMnTi gear steel.The main contents and conclusions are listed as follows:(1)The formation and evolution of inclusions in 20CrMnTi gear steel during secondary refining were studied.It is found that before titanium addition,the inclusions transform along the route of "Al2O3?MgO·.Al2O3 spinel?CaO-Al2O3(-MgO)calcium aluminate" due to the formation of dissolved Mg and Ca in liquid steel.The addition of titanium in the secondary refining process affects the evolution of MgO-Al2O3 spinel and calcium aluminate inclusions.When there is no sufficient Ca in liquid steel,the addition of titanium would lead to the transformation of MgO-Al2O3 spinel into MgO-Ti2O3 inclusions;when there is a small amount of Ca in liquid steel,CaO-Al2O3-TiOx system inclusions would be formed.At the same time,the addition of Ti would result in the increase of TiOx in CaO-Al2O3-TiOx system inclusions.Heavy calcium treatment will lead to the formation of CaO·TiO2 inclusions and weaken the castability of Ti-bearing Al-killed steel.In addition,calcium treatment can increase the frequency of macro-inclusions in liquid steel and decline the quality of steel.(2)Four different types of attachments on submerged entry nozzles were investigated.It is found that three of them are different in color and morphology,but no big difference can be found in their compositions.Calcium aluminates with MgO-Al2O3 spinel islands are the main phase(s)of the attachments,which also contain a small amount of TiO2,Na2O and SiO2.The three attachments are mainly generated by the attachment of liquid inclusions on the inner wall of SEN.In addition,the fourth type of attachment is mainly from mold flux.(3)The source of macro-inclusions in 20CrMnTi gear steel was analyzed by Slime Method.It is found that there is no obvious difference between the types of the macro-inclusions in sulfur-containing gear steel and the conventional gear steel.The types of these inclusions are mainly calcium aluminate inclusions,TiOx-SiO2-Al2O3-CaO system inclusions and a small amount of SiO2 inclusions.Calcium aluminate inclusions may be caused by calcium treatment and strong stirring during VD treatment;TiOx-SiO2-Al2O3-CaO system inclusions and SiO2 inclusions may come from ladle filler sands.At the same time,ladle glaze is also an important source of macro-inclusions.(4)The refining process of gear steel in a plant was optimized by industrial experiments.Due to the adjustment of the composition of refining slag,the melting point and viscosity of the refining slag decreased,and the fluidity is improved evidently.By reducing the feeding amount of calcium wires,the possibility of the formation of macro-inclusions in steel is much lower.Although the yield of Ti decreases slightly when adding ferrotitanium alloy earlier,it is still benifit to the cleanliness of steel as well.After optimization,the much lower contents of T.[O]and N in the rolled steel confirm the advantages of process optimization obviously. |
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