Experimental Study On High Cleanness GCr15 Bearing Steel Produced By Electroslag Remelting

In the early 1960s,China began to use electroslag remelting process to produce military bearing steel.ANF-6 slag has been widely used in the production process.It has good comprehensive metallurgical properties and can effectively remove non-metallic inclusions and sulfur and other impurities in steel.However,a large amount of fluoride gas will be volatilized from the slag during smelting to pollute the environment.The high conductivity of slag results in high power consumption in smelting process.The composition of slag changes greatly during melting process,which affects the stability of remelting process.Slag contains about 30%Al2O3,aluminum deoxidized bearing steel smelting,affects the removal Al2O3 type of inclusions.At present,in the process of producing bearing steel with ANF-6 slag,the inclusion rating exceeds the standard in a steel plant.Therefore,this project has carried out experimental research on the production of high cleanness GCr15 bearing steel by electroslag remelting.Firstly,oxygen mass fraction and inclusion control level of GCr15 consumable electrode were investigated.Further,aiming at the problems of inclusion rating exceeding the standard in the process of electroslag remelting in the production of bearing steel with ANF-6 slag at present,this paper designed different slags with FactSage thermodynamic software,and compared the melting temperature,viscosity,density,conductivity and other properties of slags.Then the wettability tests of slags with the aluminum oxide inclusion were carried out,and the dissolution tests with the aluminum oxide inclusion rod and the calcium hexaaluminate inclusion rod were carried out.Then the balance experiments with S1 slag and S2 slag were carried out,and the change of oxygen mass fraction and the formation mechanism of inclusions were analyzed.Finally,the industrial tests of S1 slag and S2 slag were carried out,and the oxygen mass fraction and inclusion control of electroslag ingots after the test were analyzed and compared.The following conclusions are drawn from the above experiments:The GCr15 self-consuming electrode produced by EAF?LF?VD?CC processes is adopted in the steel plant.There are certain differences in oxygen mass fraction and non-metallic inclusions in different parts.Alumina granular inclusions larger than 10 microns have poor mass at the center of the consumable electrode,and there are shrinkage holes around them.This kind of inclusion belongs to D type inclusion in the grading process,which is the main reason to reduce the fatigue life of bearing steel.At the same time,due to the generation of shrinkage hole,the detection of oxygen mass fraction in bearing steel has great interference,and the detection values of oxygen mass fraction from the edge to the center were 0.00069%,0.00103%and 0.00144%,respectively.The mass fractions of impurity elements S,P and Ti in the consumable electrode were 0.003%,0.0097%and 0.001%,respectively.For the purpose of improving the ability of electroslag remelting slags to remove alumina inclusions,CaF2-Al2O3-CaO-MgO quaternary slag system was selected as the main component.FactSage software was used to calculate the appropriate MgO addition amount of 5%in the slag system,and three new slag systems were designed.The melting temperature,viscosity,conductivity and density of the new slags and the existing ANF-6 slag were compared.The results show that the new design of slags are more conducive to remelting GCr 15 bearing steel.The wetting angle of S1?S4 slag with aluminum oxide inclusion are 52°,77°,44°,46°,respectively;The dissolution rates of S1?S4 slag on alumina inclusion are 0.022 g/min,0.142 g/min,0.101 g/min and 0.114 g/min,respectively.The dissolution rates of S1?S4 slag on the inclusion bars of calcium hexaaluminate are 0.38 g/min,0.5 g/min,0.37 g/min and 0.34 g/min,respectively.Therefore,the optimal slag is S2.The slag steel balance experiments were carried out on the existing slag S1 and the optimal slag S2.The results show that the oxygen mass fraction in steel increase gradually with the extension of the reaction time of slag steel,and the oxygen mass fraction in steel are 0.0016%and 0.00145%respectively after the equilibrium reaction with the two groups of slags for 30 min.At the end of equilibrium reaction with S2 slag,inclusions in steel are mainly composite calcium aluminate inclusions,calcium aluminum-magnesium inclusions covered by calcium sulfide,titanium nitride inclusions,etc.The inclusions are distributed around 2.5 ?m,most of the shapes of inclusions are spherical,but titanium nitride inclusions are angular rhomboids.Electroslag remelting experiments were carried out on the existing slag S1 and the optimal slag S2.After the test,the oxygen mass fraction in the electroslag ingots are 0.0019%and 0.0022%,respectively.Due to the increase of oxygen mass fraction in the remelting process,Al element and Si element in the steel are burned to different degrees.The removal rates of sulfur in electroslag remelting are 53%and 63%respectively.Inclusion rating results show that D type fineness in electroslag ingots with S1 slag remelted is 1.0 grade,and with S2 slag remelted is 0.5 grade.The reasons for the increase of oxygen mass fraction in electroslag remelting process were analyzed from the perspective of thermodynamics.The increase of oxygen mass fraction in steel is mainly due to the high oxygen potential in slag caused by the failure of protective atmosphere in remelting process.In order to obtain lower oxygen mass fraction in bearing steel production,the proportion of calcium oxide in S2 slag should be appropriately increased and protective atmosphere should be adopted.