Study On The Behavior And Control Of Inclusions In Al-killed Steel During Refining And Casting Processes

In the past decades,many researchers studied the behavior and control of non-metallic inclusions in steel,and got many significant achievements.In fact,there are still many research results,which are lack of good consideration of real industrial practice,even are quite different from real industrial production.In order to bridge the gap between fundamental researches and industrial practice,some factors which have been questioned or ignored were focused on,and the interactions among liquid steel,slag,refractory and atmosphere were comprehensively considered in present study.The behavior and control of non-metallic inclusions in Al-killed low alloy steel during refining and casting processes were studied by thermodynamic analysis,industrial and laboratory experiments,and CFD calculations.Theresults are expected to help metallurgists to understand the mechanisms behind the process control in secondary steelmaking.The main contents and related conclusions are listed as follows.(1)Based on industrial measured data and thermodynamic calculations,the deoxidation mechanism was proposed by the comparison between measured and calculated values of dissolved oxygen activity,and the choice of refining slag was also discussed.It is found that during refining process,the alumina activity in refining slag doesn’t have an obvious effect on the dissolved oxygen activity.In order to keep low dissolved oxygen activity,a certain dissolved aluminum content should be guaranteed.On the other hand,the dissolved oxygen activity at the interface between liquid steel and refining slag is controlled by FeO in slag,and the value of oxygen activity is much higher than that in liquid bulk steel.The oxygen therefore could transfer from slag into liquid steel,and then slagging during refining process shows its significance.Besides,in order to reduce aluminum loss,stable refining slag is also required for low oxygen steel.A small amount of MgO can improve the stability of refining slag,and its content in slag should be controlled to less than 10 mass%.At the same time,the slag basicity of 3～4 is recommended to refine low oxygen Al-killed steels.(2)Based on industrial experiments,the evolution of inclusions and the formation of dissolved Mg and Ca during the refining of Al-killed low alloy steel were investigated,and the evolution mechanism of inclusions in real melting process was proposed.It is found that with the reaction between steel and slag(or refractory),the inclusions transform along with the route of "Al2O3 inclusions→MgO-Al2O3 system inclusions→CaO-MgO-Al2O3 system inclusions",and finally become globular inclusions.Since compared with CaO,MgO has lower stability and higher activity in slag,dissolved Mg with more Mg sources forms earlier and faster than dissolved Ca.MgO-Al2O3 inclusions therefore generate before the formation of CaO-Al2O3 system inclusions.The evolution mechanism of MgO-Al2O3 inclusions into CaO-MgO-Al2O3 system inclusions is the substitution of Ca element for the Mg element in MgO-Al2O3 inclusions.Meanwhile,in the outer layer of CaO-Al2O3(without MgO),Ca element also substitutes for the Al element in the inclusion.(3)Based on industrial trials,the control standard of calcium treatment for low sulfur and low oxygen steels was discussed,and the effect of the arrangement of calcium treatment on total oxygen content was also studied.The results show that the double calcium treatment method could impro,ve the grade of D-Thin inclusions,but couldn’t help RH further reduce total oxygen content.It means this method is not suitable for ultra-low oxygen steel grades.For low oxygen and low sulfur steels,the value of T.[Ca]/T.[O]is more proper than the value of T.[Ca]/w[Al]s to control calcium treatment.The value of T.[Ca]/T.[O]should be larger than 0.91 in order to get good treatment results.In present work,the value is suggested in the range of 0.91～1.25.With the purpose to improve castability only,calcium treatment is strongly suggested after refining(before casting);while with the focus on inclusion modification(inclusion morphology),double calcium treatment method is a good choice;during LF+RH refining process,calcium treatment before RH refining(after LF refining)is not recommended due to its weak efficiency.For the ultra-low oxygen steel grades refined by LF+RH route,liquid inclusions need to be avoided as many as possible to reach better inclusion removal rate during RH process.Too high slag basicity therefore needs to be controlled.(4)Laboratory experiments were carried out to study the sintering behavior of ladle filler sand with liquid steel,and the possibility of the formation of macro inclusions from filler sands was also discussed based on industrial results.The results show that the liquid formation due to the reacation between chromite and silica is the main mechanism of the sintering.The content and size of silica phase in the sand have a big influence on the sintering.The existence of liquid steel accelerates the formation of liquid phase in sand,resulting in serious sintering.Higher temperature and longer holding time would also enhance the sintering.The factors,such as steel grade,temperature,holding time and the cotent and size of silica,should be taken into account when choosing filler sand.Besides,filler sands are very possible to become macro inlcusions when they are carried to mold by the flow of liquid steel.The removal of the filler sand particles at the beginning of teeming therefore becomes a very important task to improve steel cleanliness.(5)The possibility of liquid inclusion attachment on submerged entry nozzle(SEN)was investigated,and the attachment behavior of alumina inclusions on SEN was studied based on industrial experiments and CFD calculations.It is found that the roughness of the surface of SEN has a large impact on the attachment of liquid inclusions.Although the composition is very similar,the attachment behaviors of liquid inclusions on the inner wall of SEN are still different when the surface properties of SEN are quite different.When with large grain size and big holes in the inner wall refractory,liquid calcium aluminate inclusions could attach on the inner wall and form a liquid attachment layer.The cluster alumina attachments on the inner wall of SEN in the case of ultra-low carbon steel is due to the alumina inclusions accumulated and attached on the inner wall.High oxygen refining slag is the main reason to cause alumina attachments in this case.In the case of high strength low alloy steel,the alumina attachments are not from the inclusions already existed in steel,but the crystal growth due to oxygen absorption(reoxidation)of liquid steel during casting process.The one-direction supply of oxygen by convective flow is the main reason to make alumina grow into plate-like shape.In order to control this kind of attachments,enhanced operation to avoid reoxidation during casting process is the key factor rather than calcium treatment.