| Super duplex stainless steel 2507 is widely used in many industries,such as oil and gas well production,offshore oil production platform facilities,oil and gas transmission pipelines and other harsh chlorine-containing environments due to its good mechanical properties and corrosion resistance.Compared with carbon steel,super duplex stainless steel has high chromium content and strong affinity with oxygen,which makes it difficult to deoxidize.Aluminum deoxidation can effectively reduce the oxygen content in steel,but there is a risk that AlN will be formed and the properties of super duplex stainless steel will be deteriorated if the aluminum content is too high,so it is urgent to optimize the aluminum content in super duplex stainless steel.In addition,after aluminum deoxidation,the content of Al2O3 in LF refining slag can be greatly increased,which is very beneficial to reduce the melting point and viscosity of refining slag,and correspondingly can reduce the amount of CaF2.Therefore,from the perspective of reducing refractory erosion and environmental hazards,it is urgent to reoptimize the content of CaF2 in refining slag.In this paper,the effect of CaF2 content on the viscosity and precipitation behavior of high basicity CaO-18%Al2O3-SiO2-10%MgO-CaF2(C/S=6)refining slag was studied.By comparing with the typical fluorine-free high basicity CaO-30%Al2O3-SiO2-10%MgO(C/S=6)refining slag in terms of viscosity and steel cleanliness,the content of CaF2 in medium aluminum high basicity refining slag was optimized;Based on the equilibrium experiment of AlN precipitation,the solubility product formula of AlN precipitation in super duplex stainless steel 2507 was constructed,and the growth process of AlN precipitation phase was observed The influence of AlN precipitation on the mechanical properties and corrosion resistance of super duplex stainless steel was studied by means of tensile testing machine,impact testing machine,electrochemical workstation and other testing equipment,combined with metallographic microscope and electron probe to analyze the microstructure.The law and mechanism of action.The results are as follows:The addition of CaF2 significantly reduces the viscosity of refining slag,but with the increase of temperature and CaF2 content,the effect becomes insignificant.When the content of CaF2 exceeds 10%,the viscosity of refining slag only decreases slightly with the further increase of CaF2 content.Compared with Al2O3,CaF2 is more effective in reducing the viscosity of refining slag.The viscosity of refining slag containing 6%CaF2 is lower than that of fluorinefree refining slag containing 30%Al2O3,especially in the low temperature range.In addition,there is no significant difference between refining slag containing 6%CaF2 and typical fluorinefree refining slag containing 30%Al2O3 on the cleanliness of 2507 steel.Therefore,for high basicity refining slag with CaO-18%Al2O3-SiO2-10%MgO-CaF2(C/S=6),6%CaF2 content can meet the refining requirements;Thermodynamic calculation and high temperature quenching experiment show that m-CaO phase and m-MgO phase precipitate from fluorine-containing slag in the viscosity measurement temperature range.With the increase of CaF2 content,the content of m-MgO phase increases slightly,but the content of m-CaO phase decreases significantly.The main reason for CaF2 to reduce the viscosity of refining slag is that CaF2 releases[SiO4]silicate units from the[AlO4]aluminate grid,thus destroying the network structure of refining slag.The secondary reason is that CaF2 reduces the solid content in refining slag.By measuring the solubility of Al in 2507 steel at 900℃,1050℃ and 1200℃,the solubility product formula of AlN precipitated phase was constructed:log[Al·N]=-712/T-5.2544.The temperature range of hot working and heat treatment of super duplex stainless steel is between 900℃ and 1200℃.According to the solubility product formula of AlN precipitation phase,in order to avoid the risk of AlN precipitation,it is suggested to control the Al content in 2507 steel below 0.03%.Compared with the thermodynamic calculation results of Thermo-Calc and FactSage,the maximum precipitation amount of AlN obtained by AlN solubility product during the cooling process of Al-3#2507 steel(0.066%Al)has no obvious difference,both are about 0.1%,but the precipitation rate of AlN obtained based on AlN solubility product is the slowest;With the increase of holding time at 1000℃,the average diameter and area fraction of AlN precipitates in 2507 steel increase significantly,and approach the maximum at 120 min.In addition,the shape of AlN precipitates is elliptical,nearly spherical and elongated,among which nearly spherical accounts for the vast majority.With the increase of Al content,the yield strength and tensile strength at room temperature of 2507 steel do not change obviously,but the elongation decreases continuously,which indicates that AlN precipitation will worsen the plasticity at room temperature of 2507 steel The low-temperature impact energy of 2507 steel increases first and then decreases,which is consistent with the refinement degree of austenite phase,that is,the quantity density of AlN precipitates affects the size of austenite phase,and the size of austenite phase affects the lowtemperature impact energy of 2507 steel;The open circuit potential of 2507 steel decreases from-0.8686 V to-0.9349 V,the self-corrosion potential decreases from-0.865 mV to-0.924 mV,and the pitting potential decreases from-0.653 mV to-0.660 mV.The obvious corrosion phenomenon occurs around the AlN precipitates,which indicates that the AlN precipitates will reduce the corrosion resistance of 2507 steel. |
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