| Automotive industry has a large demand for spring steel,which account for about 60%of all spring steel consumption.In recent years,with the rapid development of automotive industry in China,the demand for high quality and performance of spring steel is continuously increasing.Furthermore,lightweight and high intensification will be the development direction for automotive spring steel in the future.Therefore,much stricter requirement and challenge will be clearly proposed for the variety,performance and quality for the spring steel.Fatigue fracture is the main failure mode of spring steel which is primarily caused by the hard and non-deformable inclusions.Consequently,how to improve the plastic deformation performance and reduce the number of inclusions as well as the size becomes an important technical problem during the production process of spring steel.In this article,the formation,evolution and control mechanism of inclusions were investigated systematically during the whole production process of 54SiCr6 automotive suspension spring steel under the theme of non-metallic inclusion plasticization control.Liquid steel and inclusion composition was obtained in the 54SiCr6 mill bar with an optimal inclusion plasticization produced in Saarstahl steel company after analyzing several mill bars from different companies.In addition,the deficiencies and differences were found out in inclusion plasticizing technology for domestic products.The steel-slag reaction experiments were conducted in the Si-Mo resistance furnace in order to study the influence of slag basicity and Al2O3 content on the composition changes of steel liquid and oxides inclusions.The results shows that both increasing slag basicity and Al2O3 content can lead to the increase of[Al]s,in which the latter is much easier to raise the[Al]s.The Al2O3 content in oxide inclusions rises with the increasing basicity and Al2O3 content in slag.The low melting control of inclusions can be achieved by adjusting the slag composition with the basicity of 1.5 and Al2O3 content less than 3%in laboratory study.Two industrial tests were carried out based on the laboratory study.The result indicated that the composition of oxide inclusions could be controlled in the 1400?low melting area(LMA)with basicity in the range of 0.8-1,Al2O3 content in slag less than 3%and[Al]s in liquid steel less than 0.0005%,while it would deviate from the LMA when the basicity and[Al]s increased to 1.25 and 0.0010%respectively.In addition,the residual aluminium containing in the alloy for LF refining process is the main source of the increased[Al]s in liquid steel.According to the results,the addition of 1770kg low-titanium ferrosilicon and 1000kg low-titanium high carbon ferrochrome to 1 OOt liquid steel can increase[Al]s by 0.0019%and 0.0006%respectively so that the residual aluminium in alloy must be controlled strictly.After RH vacuum refining,total oxygen and nitrogen contents decreased significantly as well as the inclusion number density arriving the lowest value,which clearly affirms the metallurgical efficiency of RH for inclusion removal and degassing.However,[Al]s increase and T.Ca decrease were prone to happen during RH refining with a remarkable variation and the deviation of the inclusion composition in ternary diagram is more obvious when vacuum remains longer.Therefore,RH vacuum treatment time should not be too long(<13min according to the tests).Followed by tundish pouring process,T.O,[N]and inclusion number density show various increasing trends,indicating that liquid steel was contaminated by second oxidation,tundish fluxes and related refractory.Meanwhile,it is quite necessary to have a protective casting to continuously keep the remarkable effective inclusion removal of RH and avoid recontamination of liquid steel.Factsage thermodynamic calculation was used to optimize the composition of inclusions.For SiO2-Al2O3-CaO system,the appropriate range of Al2O3 content variation is 5.6%-26%corresponding to below 1400? liquidus with controlling the value of w(CaO)/(SiO2)at 0.50-1.00.For SiO2-Al2O3-CaO-MgO system,it is advantageous to the inclusion low melting control with the MgO content of 15%,and the appropriate range of Al2O3 content variation is 0%-26.2%corresponding to below 1400? liquidus with controlling the value of w(CaO)/(SiO2)at 0.2-0.7.For SiO2-Al2O3-CaO-MnO system,it is advantageous to the inclusion low melting control with the MnO content of 30%,and the appropriate range of Al2O3 content variation is 0%-30.6%corresponding to below 1400? liquidus with controlling the value of w(CaO)/(SiO2)at 0.1-1.22.Compared the inclusion composition distribution of mill bars from Nangang,Saarstahl and Xingcheng,it can be seen that higher Al2O3 content is the main reason for the deviation from LMA.The evolution mechanism of calcium aluminum silicate inclusion during production process was systematically studied.Thermodynamic calculations shows that compound oxide inclusions can be formed by the intrinsic reactions inside liquid steel and in order to guarantee the Al2O3 content less than 20%,the corresponding[Al]s should be less than 0.0005%,which is consistent with the industrial results.It was found that slag-typed inclusions were initially originated form the coalescence between deoxidation products and emulsified slag because of violent agitation of steel flow during tapping.As the solidification processing,the average size of inclusions increased gradually from 3.74?m in tundish to 5.14?m in slab as well as the Al2O3 content rising from 29.87%to 35.21%.The combination of inclusion growth and collision between newly precipitated inclusions and already existed inclusions jointly changed the composition of original inclusions.Inclusions detected along the rolling direction were classified three categories according to the morphology:(1)single particles with small deformation,(2)inclusion fracture with small extension,(3)inclusion strings extending along the rolling direction.The lower plastic deformation of inclusions mainly caused by higher Al2O3 content,lower CaO content and magnesium aluminate spinel precipitation with high melting point and hardness. |
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