Study On The Source And Evolution Mechanism Of Inclusions In Bearing Steel During Steelmaking Process

Though the equipments and technologies of domestic steelmaking plants have reached international advanced level,there is still a big gap in controlling the quality of bearing steel,which is mainly related to total oxygen content and inclusion controlling.In order to improve the quality of bearing steel,the effects of BOF process,refractory and ladle glaze on the inclusions and total oxygen content were studied by thermodynamic analysis,industrial and laboratory experiments.The study is expected to illuminate the effects of BOF process,refractory and ladle glaze on the cleanliness of bearing steel,and provide more theoretical support for improving the quality of bearing steel.The main related conclusions are listed as follows.(1)The types of inclusions in BOF crude steel were studied by industrial experiments,and their evolution during Al deoxidation was also investigated by laboratory experiment in this study.The results show that the BOF crude steel is an important source of inclusions.Three types of inclusions((Fe,Mn)O inclusions,calcium silicates as well as calcium silicates with some solid particles)were found in BOF crude steel,among which(Fe,Mn)O inclusions are the main types of inclusions.Calcium silicates come from BOF slag while(Fe,Mn)O inclusions form due to the reduction of molten steel.During Al deoxidation,alumina cluster could be formed because of homogeneous nucleation,while some singular alumina particles can also be generated on(Fe,Mn)O inclusions due to heterogeneous nucleation.Besides,(Fe,Mn)O inclusions would be reduced to form alumina by dissolved Al completely.Calcium silicates would transform into calcium aluminates with the help of dissolved Al in liquid steel.(2)The effects of different refractories(Al2O3 refractory,MgO refractory and glazed Al2O3 refractory)on the formation of spinel inclusions and calcium aluminate inclusions were studied by laboratory experiments in this study.It is found that the inclusions in bearing steel were hardly affected by Al2O3 refractory,while alumina inclusions would transform into spinel inclusions with the help of MgO refractory or glazed Al2O3 refractory.Meanwhile,glazed Al2O3 refractory could also promote the transformation of spinel inclusions into calcium aluminate inclusions.In contrast to ladle glaze,MgO refractory would paly a more important role in the formation of dissolved Mg.The formation of dissolved Mg and Ca decides the evolution sequence of alumina,and alumina would transform into spinel inclusions preferentially due to the faster formation of dissolved Mg,then calcium aluminate inclusions form with the formation of dissolved Ca.Ladle glaze is also an important source of calcium aluminate inclusions.It is the reaction between refractory and liquid steel that contributes to the rapid formation of spinel when the new MgO-bearing ladle lining is used,while the reason for the rapid generation of spinel in the subsequent heats is the interaction of refractory-ladle glaze-liquid steel.(3)Different ladle usage was simulated by MgO refractory rods with different glazes to investigate the effects of ladle usage on the cleanliness of bearing steel.The results show that the effect of different ladle usage on the cleanliness of steel varies greatly.The total oxygen content of steel would increase with the decrease of glaze basicity which increases from 5.7 ppm to 6.4 ppm,9.5 ppm and 11.7 ppm,respectively.The ladle glaze with a lower basicity would deteriorate the cleanliness of the following production more severely.Meanwhile,inclusions can be generated by the flush-off of ladle glaze,and the initial glaze plays an important role in the evolution of inclusions in the following heats.In order to reduce the negative effect of different ladle usage and improve the steel cleanliness as much as possible,specialized ladles should be used when the bearing steel are to produce.MgO in refractory would be turned into ladle glaze by the mechanical and chemical erosion of ladle glaze.The erosion of refractory would be more seriously by ladle glaze with lower basicity and longer immersion time.Crack formed during serving period could decrease service life of refractory and be very harmful to steel cleanliness.(4)The source of inclusions and their evolution were investigated during industrial procedure.Besides,industrial experiments were carried out to study the effect of BOF slag on the removal of nitrogen.It is found that during deoxidation procedure,a large number of alumina inclusions were generated by the reaction of[Al]and[O].At the same time,(Fe,Mn)O inclusions were also transformed into alumina inclusions by dissolved Al.Calcium silicates in crude steel and inclusions formed by ladle glaze were evolved into calcium aluminates with the help of dissolved Al.During refining process,some alumina inclusions were removed by refining slag and the rest would transform into spinel inclusions,then calcium aluminates.Calcium aluminates with larger size would appear during VD refining.Alumina inclusions formed again due to reoxidation of steel after VD refining.Moreover,ladle filler sand would become an important source of inclusions in steel during procedure.The good foaming slag in BOF can promote the removal of nitrogen in steel.The collision between CO bubbles and metal droplets in foaming slag can form new reaction interfaces to remove nitrogen in droplets,resulting in the decrease of nitrogen content in the bath.In the present study,the average nitrogen content in the end of BOF decreases to 13.7 ppm,and the removal rate of nitrogen reaches 74.2%in the condition of good foaming slag.The formation of calcium aluminate inclusions can hardly be avoided,but it is possible to reduce the quantity and size of these inclusions during the production.