Research On Inclusion Control Of Bearing Steel Low-basicity Slag Refining Process

Aluminum is often used as a deoxidizer for sedated steel because of its strong deoxidizing ability and relative low price.However,if the Al₂O₃inclusions and composite inclusions containing Al₂O₃formed after aluminum deoxidation can not be removed before continuous casting,the residues in the steel may cause serious harm to the performance of the steel.At present,the solution for cord steel,cutting wire steel and heavy rail steel,which is not to use aluminum deoxidation in the production process,while strictly controlling the amount of aluminum brought by non-aluminum deoxidizers and alloys.However,some steel grades with clear requirements for aluminum content,dissolved oxygen and total oxygen content(such as bearing steel,etc.)must be added with aluminum to meet the requirements of strong deoxidation and a certain aluminum content;therefore,through deoxidation and refining process control to produce plasticity.A good composite inclusion containing Al₂O₃is a possible idea to solve this problem.This paper takes GCr15 bearing steel as the research object to study the plasticization control effect and mechanism of inclusions in low-alkalinity slag refined steel.First,Fact Sage 7.2 thermodynamic software was used to calculate the plasticity control conditions of Mn O-SiO₂-Al₂O₃and CaO-SiO₂-Al₂O₃inclusions based on the composition of the bearing steel.For Mn O-SiO₂-Al₂O₃system:The requirement for inclusions to be controlled in the plastic zone is to control[Al]in steel below 200 ppm,and a_[O]in steel should be controlled between 2.8×10^-5?1.31×10^-4.For CaO-SiO₂-Al₂O₃series:The requirement for inclusions in the plastic zone is to control[Al]in steel below600 ppm,and a_[O]in steel should be controlled between 1.2×10^-5and 1.65×10^-4.Thermodynamic calculations show that a_[O]required for inclusion plasticization control is higher than the total oxygen content of high-quality bearing steel.In order to achieve the plasticity control of inclusions as much as possible and ensure the low total oxygen requirement of bearing steel.Based on the inclusion plastic zone is basically equivalent to the range of the 1400°C isotherm,this paper defines the 1500°C isotherm and the inclusion plastic zone provided in the literature.The middle area is the secondary plastic zone.Secondly,according to the results of thermodynamic calculations and the refining requirements of the bearing steel industry,under laboratory conditions,low-alkalinity slag was used to conduct plasticization control experiments on inclusions in GCr15bearing steel.The top slag refining test uses different alkalinity(R=1.0,1.3,1.6)and different Al additions(In actual production,a part of Al will be consumed in the refining process.So the Al content in the steel is required to be 0.050%?0.060%before the ladle reaches LF.So the amount of Al added in this experiment is set to be 600 ppm and 800ppm)for the top slag reaction experiment.After the experiment,an automatic inclusion analysis system is used to analyze and count the experimental samples.The results show:(1)Mn O-SiO₂-Al₂O₃inclusions in the original bearing steel all fall on the Mn O-Al₂O₃and SiO₂-Al₂O₃systems,and the CaO-SiO₂-Al₂O₃inclusions mostly fall on the CaO-Al₂O₃system,with a small CaO-SiO₂-Al₂O₃composite inclusions;(2)When the Al addition is 600 ppm,and R=1.0,1.3,1.6,the number of inclusions in the plastic zone of the CaO-SiO₂-Al₂O₃system accounts for the total number of inclusions respectively.They are 23.5%,16.7%,4.2%,respectively 38.8%,31.3%,11.3%in the subplastic zone;as the alkalinity increases,the plasticization effect of inclusions gradually becomes worse;(3)In the top slag basicity R=1.0,the amount of Al added is 600 ppm and 800ppm,the number of inclusions in the plastic zone of the CaO-SiO₂-Al₂O₃system accounts for 23.5%and 2.5%of the total number of inclusions,respectively,and the secondary plastic zone is 38.8%and 8.9,respectively%;As the amount of Al added increases,the plasticization effect of inclusions gradually becomes worse;(4)Combining the characteristics of industrial production,the basicity of low-alkalinity refining slag is controlled at about 1.0,and the amount of Al added is controlled below600 ppm,which may achieve better Good plasticization effect of inclusions.