Study Of The Formation Mechanism And Control Of DS-type Inclusions In Large Castings And Forgings Of 15-5PH Stainless Steel

15-5PH(0Cr15Ni5Cu3Nb)steel is a martensitic precipitation hardening stainless steel with excellent comprehensive mechanical properties,good corrosion resistance and fatigue resistance,so it is often used in harsh environments such as shale gas development.With the continuous development of China’s economic and social construction,the demand for 15-5PH stainless steel is increasing.However,non-metallic inclusions in steel,especially DS-type inclusions(round or nearly round inclusions with a diameter greater than or equal to 13μm),seriously affect their fatigue life.Therefore,it is of great significance to study the formation mechanism of DS inclusions and the key metallurgical control process in the smelting process of 15-5PH stainless steel.In this paper,based on the 15-5PH stainless steel produced by IM-VOD/VDLF-VC process,the formation mechanism and key metallurgical process of DStype inclusions in 15-5PH stainless steel were systematically studied by means of scanning electron microscopy(SEM-EDS)and inclusion automatic analysis system,combined with the inclusion movement model at the steel-slag interface and the slag-steel equilibrium model.The characteristics of DS-type inclusions in the smelting process were analyzed,and the influence mechanism of liquid steel composition and refining slag on the formation and removal of DS-type inclusions was studied.(1)The CaO-Al2O3-SiO2-MgO-CaS inclusions in the molten steel during the smelting process are the main reason for the excessive DS-type inclusions.The formation mechanism of DS-type inclusions in 15-5PH stainless steel was revealed:After adding Si-Al to VD,the typical inclusions are MgO Al2O3 and Al2O3-SiO2MgO inclusions with Al2O3as the main chemical composition.After adding Ca-Si during LF2 refining,the typical inclusions are liquid phase or two-phase CaOAl2O3-SiO2-MgO.and CaO-Al2O3-SiO2-MgO-CaS spherical inclusions are formed in the casting due to the segregation of S element and the decrease of solubility during solidification.The control range of[Ca]content that inhibits the formation of DS type inclusions in the smelting of 15-5PH stainless steel is obtained:[%Ca]<(0.00209×[%Al]1.73)/(0.000239+[%Al]1.73)-0.00083(2)The Al-O equilibrium relationship of 15-5PH stainless steel was obtained by laboratory deoxidation equilibrium experiment and Wagner thermodynamic model.The results show that the deoxidation behavior of 15-5PH stainless steel is closely related to the dissolved[Al]content.When[Al]content<0.001%,the[O]content in molten steel is controlled by Al-Si composite deoxidation,and the deoxidation product is Al2O3-SiO2-MnO-CrOx.When[Al]content≥0.001%,the aluminum-oxygen balance is the main oxygen control factor,and the deoxidation product is MgO·Al2O3.When[Al]content<0.02%,the formation of Al2O3-SiO2MnO-CrOx low melting point deoxidation products and MgO·Al2O3 interface layer of MgO crucible wall reduces the activity of Al2O3 in molten steel,which is the reason why the thermodynamic calculation results of deoxidation are higher than the experimental results.When[Al]content≥0.02%,the experimental results of deoxidation equilibrium are basically consistent with the calculated values.This indicates that the first-order and second-order interaction parameters introduced in this paper are reasonable for the numerical expression of Al-O equilibrium in 155PH stainless steel.According to the oxygen content control requirements of 155PH stainless steel,the range of[Al]content in steel is obtained:0.044%～0.26%,which provides theoretical guidance for the design of 15-5PH stainless steel deoxidization process.(3)Based on the stress analysis of inclusions,the movement model of DS-type inclusions at the steel-slag interface in 15-5PH stainless steel was constructed,and the removal characteristics of CaO-SiO2-MgO-Al2O3 inclusions,MgO·Al2O3 inclusions and Al2O3 inclusions were compared.The results show that the CaOSiO2-MgO-Al2O3 inclusions are below 90μm and cannot be removed from the molten steel,but oscillate at the initial stage and finally stabilize at the steel slag interface.Under the same steel slag condition,the interfacial tension between MgO·Al2O3 and Al2O3 inclusions and molten steel is much larger than that between CaO-SiO2-MgO-Al2O3 inclusions and molten steel,which leads to the increase of the rebound force of MgO·Al2O3 and Al2O3 inclusions and the increase of the resultant force.The 10 μm Al2O3 inclusions can be removed within 1.6 × 10-6s.(4)Based on IMCT-Wagner and steel-slag equilibrium principle,a slag-steel equilibrium model of 15-5PH stainless steel was established,and the rationality of the model was verified by laboratory slag-steel equilibrium experiments.The effects of[Al]content in molten steel and CaO and Al2O3 content in refining slag on the equilibrium[O],[Mg],[Ca]content in molten steel and equilibrium SiO2 content in refining slag were discussed.In order to prevent the formation of DStype inclusions in molten steel,the optimum[Al]content and refining slag composition of 15-5PH stainless steel were obtained:[Al]content 0.02～0.03%;refining slag 50～55%CaO-15～20%SiO2-15～20%Al2O3-5%CaF2-5%MgO.(5)Through laboratory experiments and thermodynamic calculations,the key metallurgical processes for controlling DS inclusions in 15-5PH stainless steel were formed,including the setting of key parameters in VOD/VD vacuum refining process,deoxidation system and LF refining slag composition design.The main component of inclusions in 15-5PH stainless steel produced by the optimized process is Al2O3,and the main type is MnS composite inclusions wrapped with Al2O3·MgO as the core.The diameter of inclusions is less than 19μm,and the inclusion rating result is grade 1,it is much better than that before optimization(inclusion rating is 3.0).