Research And Development Of Fluorine-free Mold Flux For Casting Titanium-bearing Steels

Ti-bearing steel is widely used in many fields of automobile,shipbuilding and offshore industry,due to its excellent properties such as high strength and toughness,as well as excellent weldability.However,Ti in high temperature steel is more active and has strong affinity for oxygen and nitrogen,then it is easy to form Ti O₂,Ti N and other inclusions during the casting process,resulting in the problems of clogging and floater in mold,which largely affects the smooth production of continuous casting.In addition,the inclusions absorbed by molten slag also deteriorates the performances of the mold flux and affects its metallurgical function,leading to the problems of slab quality like cracks,slag inclusions,and depressions,etc.On the other hand,the Ti in molten steel will also react with the Si O₂ in liquid mold flux,which will also deteriorate the performances of the mold flux and affect the quality of the casted slab.Besides,a certain amount of fluoride in conventional mold flux tends to cause environmental pollution and corrosion of the casting machine.Thus,it is an inevitable trend to develop fluorine-free（F-free）mold flux.Therefore,this paper focused on the problems of Ti-bearing steels during the casting process,and developed a special mold flux for casting Ti-bearing steels,and synergistically realized the mold flux without fluorine.First of all,the comprehensive properties of the commercial fluorine-containing（F-containing）mold fluxes for casting Ti-bearing steel were characterized using laboratory methods,the problems occurred in casting process of the Ti-bearing steel used the F-containing mold fluxes were also ascertained.The results showed that the F-containing mold fluxes have a weaker ability to absorb the inclusions of Ti O₂ and Ti N,and the dissolved Ti in the molten slag tends to promote the precipitation of high melting point phases like perovskite and magnesium-containing crystals,resulting in the formation of big slag rim and affects the infiltration of the molten slag.As a result,the temperature and heat flux during the casting stage show abnormal fluctuations,and the quality problem of depression obviously occurs in the casted slab.Focusing on the above problems,a Ca O-Si O₂-B₂O₃ based F-free mold flux was designed in the present study.Then,the difference in the interfacial properties between the designed F-free and conventional F-containing mold flux with Ti-bearing steel was investigated by Sessile Drop Method.And the influence of the major components of Ca O/Si O₂and the key component of B₂O₃ on the steel-slag interfacial properties was clarified.The results showed that they are no significant slag reaction at the interface between the F-free and F-containing mold flux with Ti-bearing steel,and there forms an oxidized interfacial layer of only 10μm.And the designed Ca O-Si O₂-B₂O₃ based F-free mold flux has better interfacial wettability and ability to absorb inclusions.In the present slag system,Ca O/Si O₂ increasing from 1.05 to 1.25 will make the slag interface wettability better,but increasing to 1.45 will precipitate high melting point material at 1200 ^oC.Besides,the mold flux with B₂O₃content of 6%has better interfacial stability,while that of 4%cannot inhibit the precipitation of high melting point crystal,that of 8%increases volatility significantly.Besides,the dissolution behavior of Ti O₂ and Ti N inclusions absorbed by the F-free mold flux was investigated by using Single Hot Thermocouple Technique with in situ observation,and the changes of properties of the mold flux after absorbing the inclusions were characterized.The results showed that the F-free mold flux can effectively absorb and dissolve Ti O₂ inclusions,and the dissolved Ti O₂ in the present slag system decreases the viscosity and increases the fluidity of the molten slag when it is less than 10%.While the Ti O₂ content exceeds 10%,Ca Ti O₃ crystals will precipitate in the melt,which makes the molten slag in a solid-liquid mixed state,thus the viscosity of the molten slag increases sharply and the fluidity deteriorates.Likewise,the F-free mold flux can effectively absorb Ti N inclusions,but it cannot effectively dissolve Ti N.Ti N inclusion mainly as solid particles uniformly dispersed in the slag,so the viscosity of the molten slag increases and its flowability decreases.Adding Mn O to the slag can oxidize Ti N to Ti O₂ and increase the dissolvability in the molten slag,but the dissolved Ti will precipitate of Ca₂Ti₂O₆ crystal.On the above basis,a Ca O-Si O₂-5%B₂O₃-4.5%Mn O based F-free mold flux with basicity of 0.83 for casting Ti-bearing steel was optimized and designed.Then,the comprehensive properties of the F-free mold flux were characterized,and the mold simulation experiment and plant trial were also conducted.The results showed that the melting temperature range of the F-free mold flux is 1136-1403 ^oC,its break temperature is about 1137 ^oC,the viscosity is 0.188 Pa?s,the interfacial tension is 1490m N/m,and the heat flux is 479 KW/m².Compared with the commercial F-containing mold flux,the F-free mold flux has a lower viscosity and interfacial tension,indicating it has a better lubrication and wettability.It can be seen from the mold simulation experiment and plant trials that,the designed F-free mold flux has a better ability to absorb and dissolve Ti N inclusions,and it also can inhibit the precipitation of high melting point Ti-containing phase likes Ca Ti O₃ crystal.During the casting process,the infiltration of the liquid slag is smooth,and the fluctuation of the temperature and heat flow show strong regularity and stability;the billet surface vibration marks are clear and regular,and there are no obvious defects on the billet surface.The quality of the cast billets is better than that of the billets casted by the F-containing mold flux.