| 20CrMnTi steel is one of the most widely used steel for gears in multiple fields,and one predominant production for steel companies.However,large high-melting inclusions are generated during the production process of gear steel so that the contact stress,impact or friction force,and pulsating bending stress of the variable load during use can result into the gear fatigue,damage or even failure.Therefore,in this paper the on-site investigation,theoretical calculation,experimental research and tracking feedback during the BOF-LF-VD-CC process of 20CrMnTi were investigated and optimized for one domestic steel mill.The main results are as follows:It was found that during the refining process the reaction between steel and slag was violent in the VD furnace which can result in the loss of the aluminum and titanium in the steel,increment of inclusions and production cost of the steel.Meanwhile,nozzle clogging occurs frequency during the casting and the clogging is mainly composed of calcium aluminate and spinel inclusions by XRD analysis.The inclusions of electrolytic castings are mainly composed of CaO-MgO-Al2O3 inclusions,SiO2-Al2O3-MgO,and tiny CaS and TiO2.Some of the inclusion are in sharp-edged shape and others are spherical.There are four different types of inclusions in the rolled steel by EDS-SEM analysis:O-Ca-Al-S-Mg inclusions with chain shape,xCaO?yAl2O3 inclusions with spherical shape,Al2O3 and TiN inclusions with irregular shape.Establishing the coupled model of inclusions in the solidification process of molten steel,it is found that under the present composition of steel,only Al2O3 inclusions and TiN inclusions are precipitated during the solidification process,and Al2O3 inclusions increases with the increase of solidification fraction.However,TiN inclusion is mainly precipitated at the end of solidification.Analysis of the change of nitrogen content in steel,LF furnace slag heating process,VD broken into the middle package process and encapsulation into the slab process molten steel nitrogen increase.The higher the initial nitrogen content in the steel,the higher the total precipitation of TiN inclusions during solidification,and the larger the size.The precipitation content of each step of TiN inclusion increases and then decreases gradually.The content of nitrogen in steel should be controlled to reduce the secondary oxidation of molten steel,improve the cleanliness of molten steel,avoid the formation of large-sized TiN in steel,and reduces the damage of TiN inclusions to molten steel.Under the condition of[Al]=0.01-0.03%in steel,the influence of the composition and physical properties of the refining slag on the absorption capacity of inclusions in steel was discussed.The results show that when the alkalinity of the refining slag is at the range of 4 to 6,range 1.4 to 1.8 of the CaO/Al2O3 and range 0.1 to 0.2of the MI index,the activity of Al2O3 in the slag is the smaller,while the activity of CaO is the larger.The absorption capacity of Al2O3 inclusion is strong,which can beneficial to reduce the T.O.content.At the same time,to reduce the loss of titanium and the formation of titanium dioxide caused by the severe slag reaction,ferrotitanium should be added before the LF exits.For reducing the erosion of the slag on the magnesium carbon steel lining,the composition of the refining slag needs to be guaranteed with a certain amount of MgO.So the target refining slag composition range of 20CrMnTi steel is controlled as follows:CaO=50-54%,SiO2≤10%,Al2O3=30-34%,MgO≈6%,and（FeO+MnO）<1%.When the content of[Al]in steel is in the range of 0.020.03%,the a[O]should be in the range of 1020ppm,and[Ca]is between 1530ppm,then Al2O3 in the steel can turn into liquid 12CaO?7Al2O3 phase.During the VD treatment,the refractory material is eroded and magnesium is entrapped into the steel continually.MgO?Al2O3 spinal inclusion can be easily formed as the increasement of Al2O3 inclusion,which leads to the nozzle clogging during pouring process and reduce the pourability property of molten steel.Meanwhile,the thermodynamic study on the formation of Ca-Al-Mg-O inclusions shows that when the[Al]content in steel is 0.03%and the dissolved[Ca]content is2030ppm,the[Mg]content in steel should be controlled of less 2ppm.By optimizing processes of the refining slag composition and calcium treatment,reducing ladle slag,the average T.O.in steel decreases 6ppm from 19ppm to 13ppm,the yield of titanium increases 14.8%from 51.2%to 66%,and the loss of aluminum in steel decreases 144ppm from 275ppm to 131ppm,and the absorbability of inclusions of refining slag is strengthened.After 10 heats of the continuous casting,the inner wall of the nozzle is still smooth with small amount of clogging,and the liquid level fluctuation of mold is also stable.The size of inclusions in the billet is mainly in the range of 0 to 10μm,and extraneous inclusions with the size of more than 50μm are not found in the steel. |
