Study On Key Process And Inclusion Control Of GCr15 Bearing Steel Based On Non Aluminum Deoxidation

In recent years,the production technology of high-quality bearing steel in China has made important progress,and the physical quality in some enterprises has reached the international advanced level;however,there is still a gap in terms of the stability of the physical quality.At present,aluminum deoxidation process is mainly used to produce bearing steel around the world.By controlling deoxidation conditions and high basicity slag,the oxygen content and inclusions can be quickly reduced.The total oxygen content(T.O)in some enterprises can be controlled below 5 ppm.However,there still is the problem of fatigue failure caused by Ds-type inclusions.And there are still some problems in the production of bearing steel by billet continuous casting with aluminum deoxidation process,such as nodulation and blockage of nozzle,difficult control of stable ultra-low T.O and titanium content,unstable control of production process and so on.In order to avoid the above problems,this study proposes to smelt bearing steel by non aluminum combined deoxidation method(Si/Mn pre-deoxidation+diffusion deoxidation+carbon deoxidation with vacuum condition)with low basicity slag to mainly change the main type of inclusions and control the plasticity of inclusions,which effectively solved the problem of bad fluidity of liquid steel and reduced production cost.However,the following problems need to be solved:(1)How to ensure low/ultra-low T.O;(2)Control of key elements and inclusions under the condition of Si/Mn alloy deoxidation;(3)Effect of deoxidation products on fatigue life and comparison with aluminum deoxidation bearing steel.The control of oxygen content in the whole process of non aluminum deoxidation process mainly includes three stages:direct deoxidation by adding Si/Mn alloy during converter tapping;Si containing deoxider is added to the slag surface for diffusion deoxidation during LF refining when oxygen content can be controlled at 20-32 ppm;and deoxidation by C-O reaction in RH vacuum refining process,and finally the T.O in steel can be controlled at about 5-9 ppm to realize the low oxygen control of liquid steel.The evolution path of inclusions this process is Si-Mn-O?Si-Ca-Al-Mg-Mn-O?Si-Ca-Al-Mg-O that finally formed an inclusion system dominated by low-melting-point inclusions,realized the plasticity of inclusions.Therefore,the number of continuous casting can reach 8-17 heats without nozzle nodulation in the billet continuous casting production process.The ultra-high cycle fatigue life and failure mechanism of two deoxidized high-quality bearing steels were studied by means of ultrasonic testing machine.It was found that calcium aluminate inclusion is the main cause of cracks in aluminum deoxidized steel while in non aluminum deoxidized steel,only 39%of fatigue cracks are caused by different types of inclusions.The order of critical size of fatigue fracture caused by different types of inclusions is:calcium aluminate>silicate>spinel>titanium nitride.And experiments show that non aluminum deoxidized bearing steel still has high ultra-high cycle fatigue life when the oxygen content is higher than that in aluminum deoxidized bearing steel.Through inclusion detection and ultra-high cycle fatigue mechanism analysis,it is also verified that non aluminum deoxidized bearing steel can allow high total oxygen content under the condition of ensuring ultra-high cycle fatigue life.The advantage of non aluminum deoxidized bearing steel is that the main oxide type is plastic inclusion(silicate),and it effectively reduces the amount of Ds and other aluminum-containing inclusions in the steel which improved the fluidity of liquid steel realized the integration of bearing steel billet continuous casting and high-speed wire rod one fire technology,which significantly reduces the process energy consumption and production cost.In this paper,the production of high quality small section bearing steel by non aluminum combined deoxidation(Si/Mn pre-deoxidation+diffusion deoxidation+carbon deoxidation with vacuum condition)is discussed and applied to industrial production.The method of producing bearing steel by this method not only improves the fluidity of liquid steel,but also realizes the process of producing bearing steel by billet high-speed wire rod.However,its quality still lags behind the most advanced production level of bearing steel in the world,including the poor desulfurization effect caused by the use of low basicity slag in the refining process,which needs to be further studied.