Theoretical Research And Application Of Continuous Casting Mold Fluxes For High Ti-bearing Alloy Steel

Titanium provides many desirable properties to steel such as the exceptional strength and resistance to corrosion.The production of Ti-bearing alloy steel for automotive industry or corrosion-resisting materials is often associated with higher than normal material losses,and casting or operational difficulties associated with the formation of Ti containing oxides or nitrides.These can,for instance,take the form of nozzle clogging or floater steel at the caster,which can restrict the effectiveness of continuous casting?CC?process.In particular,mold fluxes play an important role in slab quality during the CC of high-titanium alloy steel,and then inevitably involve reactions between steel and traditional CaO-SiO₂ type flux with higher SiO₂ content in the mold.As the CC is processed,properties of CaO-SiO₂ type flux were gradually deteriorated,which was prone to accidents such as steel sticking and break-out.Mold fluxes have been used for more than 50 years and the knowledge of how they work and perform has increased steadily over this time.However,it is still inexplicable about the mechanism of floater formation in the mold during the CC of high-Ti bearing alloy steel and the developed mold flux still cannot settle many technical problems and achieve multi-heats during CC.In order to improve the development of high-titanium steel,a series of research was conducted in this study,including the development of new-type mold flux,mechanism of steel-slag reaction and verification of industrial application which is of great significance both in theory of mold flux and in practice.In this study,the thermodynamics and kinetics of the steel-slag reaction were studied.The mechanism and conditions of the formation of the floater in mold were explored,which provided an important theoretical basis and instructive idea for the mold flux of high Ti-bearing alloy steel.The behavior characteristics and mechanism of high-titanium steel-slag reaction,and the variation in the performance of mold fluxes during the reaction was systematically studied in order to eliminate floater in mold and realize the smooth casting.The effect of the addition of oxidants on the reaction of steel-slag and its mechanism in mold flux was investigated.The following results were obtained.Firstly,through theoretical calculations and experimental studies,the mechanism of the floater formation in mold during CC of high-Ti alloy steel was proposed.The major inclusion is TiN in high Ti-bearing steel,in addition to a small amount of MgO,MgO·Al₂O₃ and other inclusions.The?-Fe has a small lattice disregistry with titanium nitride,which could act as an effective nucleating agent for?-Fe and accelerate the precipitation of?-Fe.This results in a higher liquidus temperature,a higher iron content and a lower density of the floater steel than Ti-bearing steel,so the floater was floating between the molten steel and slag in mold.Secondly,thermodynamic calculation and verification experiments were carried out to study the oxidation mechanism of different oxidants in mold fluxes,which could provide the selection of oxidants for the mold flux of high-Ti bearing alloy steel,and realize the oxidation of TiN inclusions from the mold.The results show that pure Fe₂O₃,Cu₂O,MnO₂,Mn₂O₃ and Mn₃O₄ can react with TiN at 1000?¹400?,and SiO₂ does not react with TiN.The oxidability of oxidant in mold flux from strong to weak is Mn₂O₃?Fe₂O₃>Mn₃O₄>Cu₂O;The reactivity from strong to weak between three mold fluxes with different basicities and Ti-bearing steel is CaO-SiO₂ type>CaO-SiO₂-Al₂O₃type>CaO-Al₂O₃ type.The Fe₂O₃ and Cu₂O in mold flux could be reduced to Fe or Cu by TiN,which destroys the apparent properties of slag;while the MnO₂,Mn₂O₃ and Mn₃O₄ are reduced to low-valent compounds,mainly in the form of Mn²⁺in mold flux,which is stable and exists as the phase of Li₂Mn₂O₄,Mn₂TiO₄,MnAl₂O₄ or MnO.Finally,the Mn₂O₃ and Fe₂O₃ was considered as the appropriate oxidants after critical evaluation of its stabilization and oxidizability.Then,based on the theoretical exploration and experiments of oxidant,the effect of TiN or TiO₂ inclusion in steel,and oxidants Mn₂O₃ or Fe₂O₃ on the fundamental properties of mold flux was studied.The mold flux with stable performance and excellent ability of absorbing inclusions was obtained.The results show that TiN has a robust negative effect on properties of CaO-SiO₂-type slag.This verified the deterioration of CaO-SiO₂-type slag and entrapment of slag in floater steel during CC of Ti-bearing steel.TiO₂ could bring a large variation in the properties of CaO-Al₂O₃-type slag.At TiO₂ levels of>10wt%,the perovskite was the main phase,which severely demotes lubricity property of liquid slag and increase occurrence risk of breakout.Mn₂O₃ orFe₂O₃ can effectively reduce the viscosity of slag;For CaO-SiO₂-Al₂O₃-type and CaO-Al₂O₃-type slag with Fe₂O₃,the melting point and solidification temperature of slag was increased after the reaction with TiN.After the reaction between TiN and CaO-SiO₂-Al₂O₃-type slag with Mn₂O₃,the stable phases were NaF,CaF₂ and MnAl₂O₄,etc.and the performance of slag is much more stable than that of CaO-Al₂O₃-type slag.At the same time,in order to solve the problem of interfacial reactivity of steel-slag in the mold,the reactivity between the components of SiO_2,Na₂O,B₂O₃,Fe₂O₃ and Mn₂O₃,etc.in mold fluxes with different basicities,and[Al],[Ti]and[TiN]in steel was conducted by thermodynamic calculation,laboratory experiments of steel-slag and industrial test.The results showed that the main phase of CaO-SiO2-type slag after the steel-slag reaction was composed of perovskite,nepheline and cuspidine,and the slag emerged serious crust status during the experiment.The phase with low-melting point increased with the addition of Mn₂O₃ in CaO-SiO₂-type slag,but the crust was still there.TiN was discovered in the slag after the reaction in spite that the CaO-SiO₂-Al₂O₃-type slag showed a satisfactory condition during the steel-slag reaction.TiN in the steel can be preferentially oxidized with the addition of Mn₂O₃ in CaO-SiO₂-Al₂O₃type slag.The content of Mn₂O₃ in slag was suggested less than 8wt%in order to keep the stability.The industrial mold flux CaO-SiO₂-Al₂O₃+5wt%Mn₂O₃ was designed for the casting of 825 alloy.During the casting process,TiN was not found in the liquid slag,which was with stable properties in mold such as melting point and viscosity.A decalescence and discharge heat model was estalibied in this study,which can be utilized to calculate the comprehensive reaction heat of steel-slag reaction.It shows the comprehensive reaction heat between slag and Ti-bearing steel is endothermic.However,the heat absorption is not sufficient to solidify the molten steel.For the slag without Mn₂O₃,the composition involved in the steel-slag reaction is SiO₂,B₂O₃,and Fe₂O₃.With the addition of Mn₂O₃,the reaction sequence of the components in slag could be changed,and the reaction between TiN and Mn₂O₃ is the main reaction instant of SiO₂ and B₂O₃,etc.,and the oxidation ratio of TiN was increased.Lastly,based on theoretical research,combined with conditions of the CC process and industrial application,the preliminary developed mold flux for high-titanium alloy steel has been applied successfully.The mold flux had a delightful melting performance,normal consumption,stable heat flow in the mold,and reduced the frequency of breakout alarms during the industrial test.The surface quality of the slab had been considerably improved,and the yield of slab was 10%higher than that of traditional CaO-SiO₂-type slag.