| TiO2-bearing slags have been successfully applied to solve titanium loss from liquid metal and composition inhomogeneity of remelted ingot during electroslag remelting(ESR)process,but enclosed slag and ripple defects are often observed on the surface of remelted ingot.Moreover,for producing heavy remelted ingot,the TiO2 content in the slag gradually increases during ESR process because of the oxidation of titanium in the liquid metal.It results in the change of the physical properties of slag and the slag-metal reactivity,which causes the poor surface quality and composition inhomogeneity of remelted ingot.These defects are closely related to the physical properties of TiO2-bearing slags and the slag-metal reactivity.Therefore,it is of great guiding significance for producing high-quality produces to explore the mechanism of the TiO2 content in the ESR-type slag on the physical properties of the slag,the chemical composition and inclusion of remelted ingot.Increasing TiO2 content from 4.2 mass%to 16.8 mass%in the CaF2-Al2O3-CaO-TiO2 slag decreased the crystallization temperature of the slag.The liquidus temperature of the slag decreased from 1439? to 1290?.The crystalline phases precipitated in the slag during continuous cooling process were11CaO·7Al2O3·CaF2,CaTiO3 and CaF2.The dominant crystalline phase changed from 11CaO·7Al2O3·CaF2 to CaTiO3 with increasing the TiO2 content in the slag.The crystallization of the primary crystalline phase originated from constant nucleation rate,interface reaction controlled and one-dimensional growth.The decrease in the crystallization tendency of the slag with increasing TiO2 content was attributed to the decrease in the activity of CaO in the slag which was the main component of 11 CaO·7Al2O3·CaF2.It was favorable for forming thin and uniform slag skin and stabilizing heat flux across the slag skin,which improved the surface quality of the remelted ingot.The viscosity of the CaF2-Al2O3-CaO-TiO2 slag decreased with increasing TiO2 content from 4.2 mass%to 16.8 mass%in the slag.TiO2 played a network-modifier role in relatively more complex Al-O-Al band and Q4 units by forming Q2 units and less complex Ti2O64-chain unit.The amount of non-bridging oxygens and bridging oxygens increased and decreased,respectively,resulting in the decrease in the polymerization degree of the slag.The variation in slag structure was consistent with the change in the viscosity,which revealed the mechanism that the viscosity of the slag decreased with increasing TiO2 content in the slag.The reaction between Ti in Fe-25Ni-15Cr alloy and Al2O3 and SiO2 in the CaF2-Al2O3-CaO-TiO2 slag resulted in the loss of Ti and pickup of A1 and Si in the alloy.The loss ratio of Ti in the alloy was 11%-21%.The increasing ratios of Al and Si pickup in the alloy were 93%-111%and about 5.9%,respectively.A kinetic model of slag-metal reactions was established based on penetration theory.The developed model accurately predicted the contents of Ti,Al and Si in the remelted ingot.The average deviation of the predicted and measured values was 4.1%.For the reactions of[Ti]-(TiO2)and[AI]-(Al2O3),the controlling step of the reactions at the slag-metal interface at electrode tip was the mass transfer of Ti and Al in the liquid metal.These reactions at the interface of slag-droplet and slag-liquid metal pool were controlled by the mass transfer of components in the liquid metal and molten slag.The controlling step of the[Si]-(SiO2)reaction was the mass transfer of SiO2 in the slag.On the condition of SiO2<0.43 mass%,FeO<0.81 mass%in the slag,the consumable electrode with 2.4-2.6 mass%Ti,Al<0.1 mass%and Si<0.3 mass%can ensure the standard chemical composition of Fe-25Ni-15Cr alloy ingot.The Ti loss,Al and Si pickup decreased with increasing TiO2 content from 12.41 mass%to 17.52 mass%in the slag.Further increasing TiO2 content in slag has no further influence on the contents of Ti,Al and Si in the remelted ingot.Increasing TiO2 content in the CaF2-Al2O3-CaO-TiO2 slag had no influence on the amount,size and types of inclusions in remelted ingots.The inclusions in both consumable electrode and remelted ingots were mainly 1 to 3 ?m in size.The inclusions in electrode are TiN,Al2O3-Ti2O3,Al2O3-Ti2O3 with a surrounded TiN layer.The inclusions in liquid metal pool and remelted ingots were TiN,MgO-Al2O3-Ti2O3 surrounded by TiN,MgO·Al2O3,MgO·Al2O3 with an outer Ti2O3-rich layer.Original TiN inclusions in electrode could not be completely dissociated at the electrode tip,and TiN inclusions in remelted ingots mainly generated in liquid metal pool during ESR process.The determining factor in the presence of TiN inclusions in remelted ingot is the initial N content in the consumable electrode.The remaining Al2O3-Ti2O3 inclusions in liquid alloy reacted with Mg dissolved from the slag to form MgO-Al2O3-Ti2O3 inclusions which served as the nucleation sites for TiN inclusions formation.MgO·Al2O3 inclusions in the remelted ingots precipitated in the liquid metal pool during ESR process,which reacted with soluble Ti in liquid alloy to form an outer Ti2O3-rich layer on unreacted MgO-Al2O3 core. |
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