Formation Mechanism And Key Metallurgical Process Of Large-size Inclusions In High-end Bearing Steel

With the domestication of high-end equipment,the manufacturing requirements of high-end bearings,which are key components,have been continuously increasing,especially the research and production of high-end bearing steel with high fatigue life has become one of the urgent problems that need to be solved in China’s bearing industry.The cleanliness level directly affects the fatigue life of bearing steel,which is a very important factor.Improving cleanliness mainly involves minimizing impurity elements and eliminating harmful inclusions.Currently,China’s production of bearing steel has reached or approached the international level in terms of minimizing impurities such as oxygen and titanium.However,the problem of large-size inclusions,ranging from tens of micrometers to several hundred micrometers or even millimeters,is becoming increasingly prominent in the steel,which still lags behind foreign counterparts.Therefore,this paper focuses on the formation mechanism and key metallurgical processes of large-size inclusions in high-end bearing steel,based on the Baowu Shaoguan Special Steel production process of "Converter-Argon Station-LF Refining-RH Vacuum Degassing-Continuous Casting",in order to provide theoretical and practical guidance for the efficient and stable production of high-quality bearing steel.Through inspection and confirmation of the characteristics of large-size inclusions in bearing steel,it was found that there are mainly two types of largesize inclusions in bearing steel,namely,individual ball-shaped DS-type micro-scale inclusions and millimeter-scale elongated string-type macro-scale inclusions.The sources and formation mechanisms of these large-size inclusions were further identified,and key improvement processes were proposed and applied in actual production,achieving significant improvement effects.The main research results of this paper are as follows:(1)The DS-type micro-scale inclusions that exceeded the standard are mainly low-melting CaO-Al2O3 inclusions,with a CaO mass fraction greater than 30%,a size range of 30 μm-50 μm,and a two-dimensional morphology that is close to circular.The distribution of Ca and Al elements in the inclusions is relatively uniform,and some inclusions have characteristics of containing Si elements.The millimeter-scale elongated string-type macro-scale inclusions are mainly semi-solid CaO-Al2O3 inclusions,with a CaO mass fraction of less than 30%and a length greater than 5 mm.The two-dimensional and three-dimensional morphology information shows that the large-size inclusions are formed by the agglomeration of different small-sized CaO-Al2O3-MgO inclusions ranging from several micrometers to tens of micrometers.(2)Based on the results of thermodynamic and mathematical modeling,it was found that the low-melting CaO-Al2O3 inclusions with SiO2 content,which cause DS exceeding in steel,have a size larger than 30 μm and a SiO2 mass fraction greater than 2.4%.These inclusions are formed by the slag entrapment.The formation of this slag entrapment is related to the low basicity slag added after the steel is tapped from the converter,which contains high SiO2.This slag,which is added simultaneously with lime(CaO),has a slow slagging rate and is easily rolled into the steel liquid.As refining progresses,the average SiO2 mass fraction in inclusions formed by slag entrapment decreases from 27.5%at the argon station to 3.0%at the rolling stage.(3)DS type micro-sized inclusions without SiO2 and millimeter-scale elongated macro-sized inclusions are similar in composition,both being CaO-Al2O3 type inclusions,but with different specific contents.The CaO/Al2O3 mass ratio of DS type micro-sized inclusions is 0.5<CaO/Al2O3<1.2,while that of millimeterscale elongated macro-sized inclusions is CaO/Al2O3<0.5.The essential mechanism behind this difference is the variation in the inclusion composition caused by different Ca contents in the steel melt.When the Ca content in the steel melt is high,the CaO content in the corresponding inclusions is also high.The Ca content in the steel melt is related to the transfer of Ca from the slag to the steel melt during the LF refining process.(4)Thermodynamic calculations show that under the conditions of Mg=5 ppm and O=10 ppm,when the Ca content in the steel is greater than 4.4 ppm,all the equilibrium inclusions are low-melting CaO-Al2O3 inclusions.By controlling the Ca content in the steel to less than 4 ppm and the Al content to a mass fraction of 0.04%,the generation of low-melting CaO-Al2O3 inclusions can be effectively suppressed.The calculation results of the equilibrium Ca content between the steel and slag show that when the Al mass fraction in the steel is 0.035%and the CaO mass fraction in the slag is less than 52%.the equilibrium Ca content can be controlled to be less than 4 ppm.and the mass fraction of CaO in the inclusions can be less than 17%,which is beneficial for the inclusions to float up and be removed(5)The results of the study on the collision and aggregation model of liquid inclusions show that the critical diameter for liquid inclusions to collide and aggregate increases from 11.8 μm to 40.3 μm as the interfacial tension between the inclusions and the steel increases from 0.1 N/m to 2.0 N/m.The interfacial tension has a significant effect on the collision and aggregation of liquid inclusions.The study on the aggregation model of inclusions that are not wetted by steel shows that the cavity bridge force caused by the cavities is 103 to 104 times greater than van der Waals force,which is the main force for CaAl2O4 particles to adhere to each other.The inclusions that have been in contact can sinter at 1673 K in 4.2 seconds.The sintered inclusions further form clusters,which eventually become macroscopic inclusions in the form of millimeter-sized elongated clusters after rolling.(6)The control process for large-size inclusions mainly includes the use of low-melting and low-viscosity slag-making agents during converter tapping;gradually reducing the Al content in the steel from above 0.05%at the argon station to above 0.02%at the end of the ladle furnace refining process;and adding lime in stages during LF refining to gradually increase the CaO content in the slag from 52%to 57%.After improvement,the number and size of large-size inclusions containing SiO2 and low-melting CaO-Al2O3 types in the smelting process were reduced,and the proportion of inclusions with a CaO/Al2O3 mass ratio of less than 0.12 in the hot-rolled bar is as high as 90.9%,.The proportion of DS rating ≤0.5 increased from 71%to 86.38%,and the qualified rate of high-frequency ultrasonic water immersion testing by batch increased from 48.1%to 85.7%.The improvement effect of the process was significant.