| Efficient power generation is very important to reduce CO2 emissions and improve energy efficiency.The rotor with dual performance for combination cycle steam turbine needs to work in harsh and complex environment for a long time,which puts forward the very high requirements for the performance of rotor materials.The cleanliness has a fatal effect on the performance and quality of steel.As a special refining process,electroslag remelting is widely applied in the manufacture of special steel and high-end alloys because it can effectively remove inclusions and provide the directional solidification conditions for ingots.However,it is very difficult for conventional electroslag remelting process to reduce the total oxygen content in steel to the very low level.Hence,we focus on the preparation of ultra clean ingot by vacuum electroslag remelting technology,hoping to provide a theoretical basis for the manufacture of ultra clean ingot with dual performance by vacuum electroslag remelting.In this paper,the effect of current on the structure,elemental distribution and cleanliness of dual alloy ingot was studied by remelting and joining alloy steels with two different compositions in conventional electroslag furnace under atmospheric atmosphere.Then the vaporization behavior of CaF2-CaO-Al2O3-SiO2 slag at elevated temperature was analyzed by TG.On this basis,the measures to reduce the vaporization of high fluoride containing slag for electroslag remelting were put forward.Finally,electrode steel was remelted in vacuum electroslag furnace under argon and vacuum conditions,respectively.The slag-steel-inclusion-gas phase interactions during vacuum electroslag remelting process were studied by chemical composition analysis,thermodynamic calculation and various characterization methods.The physical and chemical mechanisms of deoxidation and removal of inclusions under vacuum electroslag remelting were explored.The conclusions are listed as follows:?1?There is a chemical compositional transition zone in the dual alloy ingot processed by electroslag remelting,and the transition zone does not show discontinuity in structure.During the electroslag remelting of dual alloy ingot,the Cr content in the molten metal pool gradually increases.At the same height,the edge of molten metal pool solidifies firstly,then the central area solidifies,the Cr content in the centre of ingot is higher.With the increase of current,the gap of Cr content between the center and the edge of ingot become larger,and the total oxygen content and number,size and area fraction of inclusions increase.To improve the cleanliness of ingot,the low current and inert gas protection or vacuum condition should be applied during electroslag remelting.?2?At 1400 oC,the vaporization product of CaF2-CaO-Al2O3 slag is AlF3.The vaporization rate of 70 wt%CaF2-30 wt%Al2O3 slag is largest due to the best thermodynamic conditions for vaporization reaction.For 70 wt%CaF2 containing slag,with the CaO/Al2O3 increasing from 0 to 1 and 2,the fluoride vaporization rate firstly decrease,then increase.For slag with same CaO and Al2O3 contents,with the decrease of CaF2 content from 68.9 wt%to 48.1 wt%and 33.3 wt%,the fluoride vaporization rate firstly increase,then decrease.For the CaF2-CaO-Al2O3-SiO2 slag with same CaF2,CaO and Al2O3 contents,with the increase of SiO2 content,the structure of slag melts becomes more complex and the viscosity of slag melts increases.With the SiO2 content increasing to 6.1 wt%,the vaporization rate of fluoride increases because the SiF4possessing higher vapor pressure is formed and the SiO2 addition can promote the formation of AlF3.As SiO2 content is further increased to 8.6 wt%,the vaporization rate of fluoride decreases because the mass transfer becomes slower.At the temperature of molten slag pool during electroslag remelting,the differences of slag viscosity and complex molecule number would be greatly reduced.To reduce the fluoride vaporization during electroslag remelting,the activities of CaF2 and Al2O3 in slag melts should be reduced as much as possible.?3?At 1600 oC,with the remelting conditions changing from air atmosphere to Ar atmosphere and vacuum condition,the cleanliness of remelted ingot is gradually improved.Under vacuum condition,carbon deoxidation in molten steel does not bring in new inclusions,which is very advantageous to improve cleanliness.In addition,the CO gas formed by C deoxidation strengthens the flows of molten steel,which accelerates the diffusion of elements in molten steel and improves the desulfurization rate.?4?During electroslag remelting with 50 wt%CaF2-25 wt%CaO-25 wt%Al2O3 slag,the Al content in ingot increase,resulting from that Si in liquid steel reacts with Al2O3 in molten slag because the activity of Al in liquid steel and the activity of SiO2 in molten slag are relatively low and the stability of Al2O3 in slag decreases at the temperature of molten slag pool.The Al content of ingot remelted under Ar atmosphere increases from0.016 wt%to 0.051 wt%,but the Al content of ingot remelted under vacuum condition increases to 0.027 wt%.Compared with the ingot remelted under Ar atmosphere,the Al content pickup of ingot remelted under vacuum condition is less because the fluoride vaporization from molten slag becomes severer under vacuum condition.The vaporization reaction decreases the content of Al2O3 in slag and increases the content of CaO,which decreases the activity of Al2O3 in molten slag.?5?Compared with the ingot remelted under Ar atmosphere,the inclusion size of ingot remelted under vacuum condition is finer.After remelting,the differences in inclusion types of two ingots are little.The inclusions in both ingots are Al-Ca oxides and Al-Ca-Mg oxides as well as?Ti,V?N.The CaS inclusions are not found in both ingots.The inclusions in ingots do not contain SiO2 because the affinity of Al and O in liquid steel is stronger than that of Si and O.In the temperature range of electroslag remelting under Ar atmosphere,only Al deoxidation occurs in molten steel.The Al2O3inclusions formed by Al deoxidation have more time to grow,and the total oxygen content in the ingot remelted under Ar atmosphere is higher.A new equilibrium among liquid steel,inclusions and gas phase is established in the temperature range of electroslag remelting at 20 kPa.Upon 1635 oC,the equilibrium among liquid steel,inclusions and gas phase proceeds in the direction of CO formation.The dissolved oxygen content in liquid steel equilibrized with C is 7.1×10-4 wt%,which is lower than that in liquid steel equilibrized with Al under Ar atmosphere.During vacuum electroslag remelting,less dissolved oxygen in liquid steel combines with Al to form Al2O3inclusions and the inclusion size is finer at low temperature.Vacuum are beneficial to reduce the secondary dendrite arm spacing and the size of primary carbides in ingots.?6?In order to reduce the fluoride vaporization while ensuring metallurgical properties of slag,the compostion range of slag used for vacuum electroslag remelting should be on the CaF2-12CaO·7Al2O3 line in the ternary phase diagram of CaF2-CaO-Al2O3 system.The present experimental results show that 50 wt%CaF2-25wt%CaO-25 wt%Al2O3 slag can be applied in pressure conditions above 20 kPa.If further pressure reduction is needed,it can be achieved by reducing CaF2 content in slag.During the vacuum electroslag remelting,the new steel-slag and alloy elements-inclusion-gas phase equilibriums are established,creating better conditions for deoxidation of steel.Vacuum electroslag remelting can effectively reduce the total oxygen content and inclusion size in steel,and improve the cleanliness and refine the structure of electroslag ingots,which is beneficial to the preparation of ultra clean ingots. |
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