Effect Of Calcium Treatment And Slagging Process On Inclusions In 20CrMnTi Gear Steel

At present,20CrMnTi is the most widely used gear steel in China,the purity of gear steel in China is low,and there is still a certain gap between China and developed countries.In the production of 20CrMnTi gear steel in a factory,the dispersion degree of non-metallic inclusions is not enough,the distribution is uneven,and there are many large inclusions,which seriously affect the service performance.In this paper,the inclusion in 20CrMnTi gear steel produced by a factory is taken as the research object,the thermodynamic calculation of calcium treatment process is carried out on the basis of factory infestation,and the influence of calcium treatment process on inclusions in gear 20CrMnTi is analyzed through the optimization of calcium treatment process.Based on the design and calculation of CaO-SiO2-Al2O3-MgO quaternary refining slag by using factsage thermodynamics software,the effect of different components of CaO-SiO2-Al2O3-MgO quaternary refining slag on 20CrMnTi inclusion of gear steel was studied by slag steel reaction experiment in laboratory.The main conclusions are as follows:(1)The investigation shows that the oxidation of slag is high,the control of refining slag is not stable,and there is air suction in smelting and casting process.The main component of large-scale inclusions is CaO-Al2O3,part of which comes from slag,and the size is generally less than 50?m.CaO/Al2O3 is higher in inclusions,and the mass fraction of MgO and SiO2 is very low;Some of them come from the clogging in the nozzle,with the size of 30?100 ?m.CaO/Al2O3 is relatively low and contains some MgO.(2)The results of inclusion detection in different processes show that the inclusions in argon station are mainly MnS and A12O3.TiN and calcium aluminate are the main inclusions in LF refining,VD,tundish and slab processes.After LF refining calcium treatment,some oxides are found in VD,tundish and billet outside the low melting point zone.(3)The thermodynamic calculation shows that only a small amount of heat can transform inclusions into C12A7 in LF refining,after the diffusion deoxidization of aluminum particles in LF refining is changed to SiC and a small amount of aluminum particles,the content of total oxygen and nitrogen in the slab is similar to the original process,and the inclusions can be transformed into C12A7 inclusions with low melting point after calcium treatment.(4)The results of FactSage thermodynamic calculation shows that the area of high basicity and low melting point zone of CaO-SiO2-Al2O3-MgO quaternary refining slag increases first and then decreases with the increase of MgO mass fraction.The effect is better when the mass fraction is not more than 6%.When CaO/Al2O3 in slag is constant,with the increase of basicity,the activity of A12O3 decreases,the activity of CaO increases,the activity of SiO2 decreases,and the mass fraction of dissolved oxygen decreases at equilibrium;When the basicity is constant,with the increase of CaO/A12O3,the activity of A12O3 decreases,the activity of CaO increases,the activity of SiO2 decreases,and the mass fraction of dissolved oxygen decreases at equilibrium.(5)The equilibrium experiments of different basicity and different CaO/Al2O3 refining slag liquid steel were carried out in the laboratory,the center of inclusions in the experimental furnace was CaO-SiO2-Al2O3-MgO and outside was(Ca,Mn)S-clad layer.When CaO/Al2O3 in refining slag is 1.5,with the increase of basicity,SiO2 in inclusion center decreases gradually,and MnS in inclusion layer changes into CaS.When the basicity of refining slag is 7,with the increase of CaO/Al2O3 in refining slag,MnS in the coating gradually changes to CaS,and the mass fraction of CaO and SiO2 in the oxide inclusion center increases;The melting point of refining slag decreased,the number of inclusions per unit area decreased,and the proportion of inclusions larger than 8 ?m decreased.(6)The industrial test results show that there is no significant difference between the original process and the new process in different processes and the total oxygen content of the billet,and the control level of sulfide and TiN inclusions in the steel are close.The average size and quantity of oxide inclusions in the LF refining,VD,tundish and slab of the new process are obviously smaller than that of the original process.The number of large-size inclusions with a unit area of more than 30?m in the new process is also smaller than that in the original process,among them,the large-size inclusions from nozzle clogging are obviously less than those of the original process.The results show that the new process has a certain effect on the reducing nozzle clogging and has a good effect on the control of oxide inclusions in steel.