Study On Inclusion Removal And Modification Mechanism In High Speed Heavy Rail Steel

High-speed heavy rail steel U75V is an excellent pearlite steel with high strength,toughness and wear resistance,which is widely used in high-speed railway tracks.The properties of heavy rail steel are greatly affected by inclusions.Currently,there are issues with the A(MnS)and C(CaO-SiO2-Al2O3-MgO)inclusion ratings for heavy rail steel produced by a certain domestic steel plant.This paper focuses on the research of U75V high-speed heavy rail steel,and conducts laboratory-based research and theoretical analysis around two key points:the effect of refining slag composition on oxide inclusion removal and the modification of MnS inclusions by Ce/Mg alone and CeMg composite treatment.The main work and achievements are summarized as follows:Through the investigation of the entire process cleanliness,it was found that the removal effect of inclusions during the smelting process is good.As the smelting progresses,the number density of inclusions gradually decreases.However,there is a slight increase in the number density of inclusions due to secondary oxidation during ladle pouring,and the company should strengthen the control of protective atmosphere.The size of inclusions fluctuates to some extent during the smelting process,with the average size of oxide inclusions in the ingot being between 4-7μm.During the refining process,typical inclusions gradually transform from CaOSiO2-Al2O3-MnO to CaO-Al2O3-SiO2-MgO.The typical inclusions in the ingot are CaO-Al2O3-SiO2-MgO and MnS,which may cause the rating of C and A class inclusions to exceed the standard after rolling,so the main control targets for inclusions in U75V heavy rail steel are the removal of oxide inclusions and modification of MnS inclusions.In order to enhance the effect of refining slag on removing oxide inclusions,the influence of refining slag basicity(1.5～3)and Al2O3 content(5～20%)on steel cleanliness was studied under laboratory conditions.The results showed that increasing the basicity appropriately is conducive to deoxidation,desulfurization and inclusion removal;excessive Al2O3 content can lead to an increase in aluminum and inclusions in the steel.Based on the force situation of inclusions at the steelslag interface,a crossing model of inclusions at the steel-slag interface was established.According to the model,it was found that smaller inclusion size,higher refining slag viscosity,and greater interfacial tension are not conducive to inclusion removal.Combined with laboratory data on steel slag experiments,it was revealed that the overall wettability between inclusions and refining slag and steel is the main factor affecting inclusion removal.As the overall wettability increases,inclusions are easier to remove,and the total oxygen content is lower.The influence of Ce/Mg single treatment and cooling method on the modification of MnS inclusions in heavy rail steel was studied through hightemperature laboratory experiments.The evolution mechanism of inclusions after Ce/Mg single treatment was studied using ASPEX scanning electron microscopy and anhydrous organic solvent electrolysis technology.The results showed that Ce and Mg added to the steel form small-sized Ce2O2S or MgO(-Al2O3),which can become nucleation points for MnS precipitation,significantly reducing the precipitation of elongated MnS and increasing the number density of inclusions while reducing the average size of inclusions.However,the Ce content and Mg content should not exceed 139 ppm and 41 ppm,respectively.Otherwise,the number density of inclusions will be too high,increasing the collision probability,and the size of inclusions will increase instead.By comparing the morphology and statistical characteristics of inclusions treated with Mg under different cooling rates,it was found that the aspect ratio of inclusions was the smallest when the solidification cooling rate of the steel was 67.7℃/s.Finally,using a gleeble thermal simulation machine,it was found that the increase in aspect ratio of inclusions after Ce and Mg single treatment decreased from 45%to about 10%and 20%,respectively,indicating that both Ce and Mg can suppress the deformation of MnS during rolling.The effect of CeMg composite treatment on the modification of MnS inclusions in heavy rail steel was studied through high-temperature laboratory experiments,and the evolution mechanism of inclusions after CeMg composite treatment was proposed.The results showed that CeMg composite treatment can generate Ce2O2S-MgO composite inclusions,which also reduces the precipitation of elongated MnS,increases the number density of inclusions,and reduces the size of inclusions.Compared with single Ce or Mg treatment,CeMg composite treatment can achieve MnS modification at lower contents,and the density of Ce2O2S-MgO composite inclusions is lower than that of Ce2O2S,avoiding the disadvantage of similar density between Ce2O2S and steel liquid and difficult upward floatation.Through hot compression experiments,it was found that the increase in aspect ratio of inclusions in heavy rail steel after CeMg composite treatment decreased from 45%to about 10%,indicating that CeMg composite treatment is also a way to suppress the deformation of MnS during rolling.When the Ce content is 59 ppm and the Mg content is 15 ppm,MnS can be modified well.