Cleanliness Control Mechanism And Process During Electroslag Remelting Of G20CrNi2Mo Bearing Steel

G20CrNi2Mo is one of the typical carburized bearing steels,usually used in the bearing ring of railway wagon due to its high strength and toughness.Steel cleanliness is a key factor influencing the fatigue life of bearing steel.With the development of high-speed and heavy-load railway,a higher requirement for the steel cleanliness has been put forward.Electroslag remelting(ESR)can remove the large inclusions effectively and improve the solidification structure of remelted ingot.The Ministry of Railway stipulates that the special steel plant in China should utilize ESR to produce G20CrNi2Mo.According to the new standard(Q/CR592-2017)proposed by Ministry of Railway,the oxygen content in G20CrNi2Mo ingots shoud be controlled below 20ppm since 2017.Xining Special Steel Group,Co.,Ltd is the largest production base of G20CrNi2Mo in China.Statistically,the oxygen qualified rate of G20CrNi2Mo ingots was only 65%in 2017.Therefore,it is urgent to develop low-oxygen technology to meet the requirement for the high-cleanliness bearing steel.The thesis regards G20CrNi2Mo as the research project,focusing on the control of oxygen and inclusions during electroslag remelting process.Firstly,the thesis defines the cleanliness characteristics under current technological conditions,then studies the increase mechanism of slag potential and soluble oxygen during droplet formation and falling.After that,a thermodynamic model to design the equilibrium slag compositions is established.With the model,the target slag compositions are proposed.Then,the evolution mechanism of inclusions during ESR is fully studied,and the effect of consumable electrode and slag on the ingot cleanliness is defined.Finally,in light of the previous studies,the key technologies refining the high-cleanliness bearing steel are proposed and applied to actual production successfully.The conclusions of the thesis are listed as follows:(1)The results of industrial experiments show that the content of Al decreases and oxygen increases from consumable electrode to bottom and top of ingot,reflecting the feature of "Al-loss,oxygen-increase".With the increase of oxygen potential of slag,the extent of Al-oxidation and oxygen increase of ingot increase.Therefore,the control of oxygen potential of slag is of great importance.(2)The kinetic models of slag oxidation and electrode oxidation show that when the initial FeO content in slag is 0.20%and oxygen pressure is 0.15atm,the final FeO content resulted from slag oxidation is 0.69%,occupying by 27%of total mass fraction;while the FeO content resulted from electrode oxidation is 2.30%,occupying by 73%of total mass fraction.The oxidation of consumable electrode plays a dominant role in increasing the oxygen potential of slag.(3)The kinetic model of soluble oxygen increase in molten steel shows that the increase of soluble oxygen mainly occurs during droplet formation and falling.By comparing the soluble oxygen calculated by the kinetic model and equilibrium oxygen calculated by Al-oxygen equilibrium equation,a thermodynamic state between Al and oxygen could be established at the metal film.During droplet falling,the reaction between A1 and oxygen is far from thermodynamic state.The soluble oxygen content is much influenced by the temperature of slag pool.When the temperature increases from 1960K(1687?)to 2020K(1747?),the theoretical soluble oxygen content increases 4ppm.(4)Based on the thermodynamic model to design the equilibrium slag compositions,the equilibrium Al2O3,Si02,MnO and FeO could be obtained.The variation extent of steel composition is determined by the difference between the measured and equilibrium value of each component in the slag.In light of the slag design model and industrial test results,the target slag compositions are proposed as follows:40-45%CaF2,10-15%CaO,30-35%Al2O3,5%MgO,1-3%Si02.(5)The main inclusions in consumable electrode are low-melting-point CaO-MgO-Al2O3,in droplet are single Al2O3 and spherical CaO-MgO-Al2O3,in ingot are high-melting-point CaO-MgO-Al2O3 with small amount of CaO.The inclusion characteristics obviously change after remelting.ESR is a process of Ca-decrease and Mg-decrease.The reason for the decrease of Ca lies in that the absorption of Ca-containing inclusions while the decrease of Mg lies in that the oxidation of soluble Mg in the liquid film.(6)The ingot oxygen content shows a rising trend with the increase of low-melting-point inclusions,mainly lies in that part low-melting-point inclusions are remained in the steel due to the low interfacial energy between inclusions and steel,futher dissolve into steel during droplet falling,which would be part of total oxygen in ingot.The characteristics of inclusions in ingot is much influenced by the slag composition.When the ingot is remelted by the slag with 20%CaO,a certain amount of globular CaAl4O7 inclusions are generated.When the ingot is remelted by the slag without MgO,large Al2O3 inclusions are prone to be generated.The slag with 10-15%CaO,5%MgO exhibits greater capacity to control the inclusion characteristics.(7)The non-ideal protection effect,unstable control of voltage,metallurgical quality of consumable electrode,unreasonable design of slag are the main reasons for the high oxygen content of ingot and appearance of the large-sized inclusions.After optimizing the technologies,the current stability is obviously improved,the power consumption per ton of steel is reduced by 140 kW h,the average oxygen content of ingots is 17.3 ppm,and the qualified rate of oxygen content(?20 ppm)is more than 95%.