| To guarantee the superior corrosion resistance of stainless steel containing titanium and aluminum(such as 1Cr21Ni5Ti,0Cr18Ni10Ti),people add appropriate amount of titanium content which prevents intergranular corrosion into the steel.As a deoxidizer during the process of steelmaking or electroslag remelting,aluminum is added into and remains in the ingot.Being strong ferrite forming elements,excessive titanium and aluminum content can increase ferrite content in austenitic stainless steel,intensify the crack sensitivity and lower the plasticity of its hot-working.Therefore,it is important to control the titanium and aluminum content during the electroslag remelting process,which has not been resolved well so far.In view of the above characteristics of aluminum and titanium in steel,it is significant to control titanium and aluminum composition range in aluminum-titanium-steel smelting for improving the corrosion resistance of the steel and hot-workability.As electroslag remelting is the terminal process in the production of this kind of special steel with high quality,strict control of aluminum and titanium content and their uniform axial distribution in the ignot during the remelting process plays an important part in improving the organization and performance of this type of steel.The debate over aluminum and titanium control technology can be investigated from three directions.First and foremost,the appropriate slag used for steel with high titanium and low aluminum content should be designed during the ESR process.Secondly,the mass transfer model of each alloy element during the ESR process should be established to investigate the effect of remelting process on the concentration of aluminum,titanium and silicon in ingot under argon atmophere.Moreover,a thermodynamic model for deoxidation in 1Cr21Ni5Ti system is established based on ion and molecule coexistence theory,thermodynamics equilibrium and mass conservation to control aluminum,titanium and silicon content during electroslag remelting under air atmosphere.Firstly,the study of thermodynamic equilibrium between 1Cr21Ni5Ti and several slags are investigated in an electrical resistance furnace.Metal samples were taken from the molten metal at different time after the slag was added on the surface of molten metal at metallurgical temperature.Slag design rules are determined after investigating the effect of different slags on aluminum and titanium combined with thermodynamic analysis.The results of slag-metal experiments indicate that the slag with low CaO content can make the aluminum and titanium in steel change very little.With the increase of CaO mass fraction in slag,the titanium in steel decreases significantly,whereas,significantly increases for aluminum in steel.The calculated results of ion and molecule coexistence theory of slag indicate that when temperature T and △G are fixed,the la(aTiO23/aAl2O32)decreases with the increase of CaO content,which makes the lg([Ti]3/[Al]4)have a decreasing trend and the phenomenon of ’the increase of aluminum and decrease of titanium’ happen.Thus,the slag with low CaO content is suitable for electroslag remelting of 1Cr21Ni5Ti stainless steel with high titanium and low aluminum content.Secondly,based on penetration and film theories,the mass transfer models used for the reactions between the metal and slag at the tip of consumable electrode,the molten metal droplet,and the molten metal pool are established.And then the change of each alloying element concentration with height of ingot is calculated by the software of Matlab.Two experiments were carried out under slags containing 5%CaO and 18%CaO content by using a 50 kg electroslag remelting furnace,and the chemical composition of slag,electrode and ingot were investigated.The results validate that the slag with low CaO content is suitable for electroslag remelting of steel containing high titanium and low aluminum content.The calculated results based on mass transfer model are in line with the measured results.The temperature of slag increases at the beginning of the elecroslag remelting process,and becomes stable after the the ESR system reaches the heat equilibrium.The increase of slag temperature at the beginning of ESR process can change the thermodynamic equilibrium constant of reaction 3[Ti]+ 2(Al2O3)= 4[Al]+ 3(TiO2),which would arise the macrosegregation of aluminum and titanium content along the height of ingot.In order to make the concentrations of alloying elements in ingot uniform,the titania content in initial slag should be calculated at 1477 ℃,and the extral titania(the value is the difference of titania content in slag calculated by reaction 3[Ti]+ 2(Al2O3)= 4[Al]+3(TiO2)thermodynamic equilibrium between 1677 ℃ and 1477 ℃)should be continuously added into molten slag during the first temperature-rising period in ESR process.In the present work,the new electroslag remelting heat was carried out by using the 1Cr21Ni5Ti steel containing 0.03%Al and 0.66 Ti,slag of CaF2:CaO:Al2O3:MgO:TiO2 =59:5:18:10:8,and 6%TiO2 constantly added into molten slag in the first 13 minutes during the process.The aluminum content along the height of ingot ranges from 0.033%to 0.048%,and titanium content ranges from 0.53%to 0.61%.This alloying elements control technology can improve the homogeneity of chemical composition along the height of ingot.At last,oxidation weight model of consumable electrode during the ESR process shows that the activation energy of 1Cr21Ni5Ti stainless steel is 231.433 kJ/mol,and the iron oxide increment of consumable electrode is IFeO=0.068 g/cm2 under the condition of electrode with 60 mm diameter combined with 66 kg/h remelting rate.Alloying element controlling thermodynamic model based on ion and molecule coexistence theory and conservation law of element atoms is established to improve the macrosegregation of aluminum and titanium content in ESR ingot under air atmosphere.The results obtained show that the utilization ratio of deoxidizer aluminum under air atmosphere is 80%.The extra titania constantly added into molten slag in the first temperature-rising period can improve the macrosegregation of alloying element during the ESR with 1Cr21Ni5Ti under air atmosphere.In order to prevent excessive aluminum dissolved in slag[Al]in slag from increasing the alunium content in ingot,the deoxidizer aluminum should not be added too much during the remelting process. |
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