| In recent years,China is becoming the largest vehicle producing country in the world,and the output of cars is increasing every year.As vehicle suspension spring production materials,suspension spring steel has broad prospects for development.But with the requirements of energy saving and environmental protection,suspension spring steel is developing towards light weight,high stress and high reliability,and its performance is more and more strict.The brittle and hard inclusions of large particles have a serious effect on the properties of spring steel,which is an main cause for the fatigue fracture of spring steel.The production and development level of spring steel in China are still lagging behind.The control technology of spring steel inclusions needs to be further improved compared with the imported materials.The 55SiCr high strength suspension spring steel is taken as the research object.This part is the calculation of the low melting point,the component activity,Iso-[O]lines and Iso-[Al]by FactSage thermodynamic software.For the CaO-SiO2-Al2O3-MgO(10%)quaternary inclusions,there are two low melting point areas.When the molten steel and inclusions are in equilibrium,the content of oxygen and aluminum ranges from 0.0015%to 0.0085%,0.00001%to 0.0040%in the low melting point area near plagioclase.The changes of inclusions in 55SiCr spring steel under two smelting conditions were studied experimentally:silicon deoxidization combined with low basicity 0.5,0.8,1.0 and 1.2 refining slag smelting and aluminum deoxidization with high basicity 5.0,8.0 and 11.0 refining slag smelting.Before the study of the influence of slag on the steel inclusion of spring steel,the melting properties of the slag were studied and the melting point experiment was carried out.In silicon deoxidation with low basicity refining slag smelting experiments,when basicity was 0.8,the melting point of refining slag decreases with the content of Al2O3 increasing in slag.When the Al2O3 content is 5.0%,the melting point of refining slag increases with the increasing of basicity of refining slag.In high basicity refining slag,when the basicity of refining slag increases gradually,the melting point of refining slag is getting lower and lower.According to the study,the effect of refining slag on the inclusion in 55SiCr spring steel was different in two smelting conditions.In silicon deoxidation with low basicity 0.5,0.8,1.0 and 1.2 refining slag smelting experiment,inclusions are mainly the similar types of CaO-SiO2Al2O3-MgO four element composite inclusions,SiO2 inclusions,part of MnS,a small amount of SiO2-Al2O3-MgO-MnO,VN inclusions,and some VC inclusions.When the refining slag basicity unchanged and Al2O3 content gradually increases,the content of Al2O3 in the inclusions has a tendency of increasing.The inclusions with more than 5 ?m are significantly reduced,and the size of inclusions is mainly at 2 to 5 ?m.When the Al2O3 content in the refining slag unchanged and refining slag basicity increased from 0.5 to 1.2,the content of CaO in the composite inclusion has a tendency of increasing.Meanwhile,the size of the inclusions increases,and the number of large inclusions with more than 5 ?m is obviously increasing.In the experiments of refining slag with high basicity 5.0,8.0 and 11.0 and aluminum deoxidization,the types of inclusions in the experimental steels are also similar.The inclusions in steel are mainly magnesium alumina spinel,part of MnS inclusions,and other external package MnS with magnesium aluminum spinel compound inclusions.The size of inclusion is obviously smaller than that in the steel with low basicity refining slag.The inclusions in the range of 2 to 5 ?m increase obviously,and the inclusions with 5 ?m or more decrease with the increasing of the basicity of the refining slag.Finally,the effects of inclusions on mechanical properties of spring steel were studied.It is found that the inclusions have little effect on the rockwell hardness under laboratory conditions.In silicon deoxidation spring steel with low basicity refining slag smelting,with the increasing of Al2O3 content in the refining slag,steel tensile strength increases,and the plasticity becomes worse.When the content of Al2O3 in the refining slag remains constant,the tensile strength of steel decreases gradually with the increasing of the basicity of the refining slag,but the plasticity is better.In the spring steel produced by aluminum deoxidization and high basicity refining slag,the tensile strength is obviously higher than that of the spring steel produced by the low basicity slag,but the plasticity is poor.In comparison,both smelting techniques are feasible.In the process of aluminum deoxidization with high basicity refining slag,the size and distribution of inclusions in steel should be strictly controlled.In the process of silicon deoxidization and low basicity refining slag,when the content of Al2O3 is 5.0%and the basicity ranges from 0.8 to 1.0,refining slag has the best effect on inclusions with all factors taken into consideration. |
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