Mechanisms Of Inclusion Evolution And SEN Clogging In Ultra-pure Ferritic Stainless Steels

By reducing C and N contents and adding Ti as a stabilizing element,mechanical properties and corrosion resistance are significantly improved in ultra-pure ferritic stainless steels(UPFSS)in comparison to conventional ferrite stainless steels.However,complex oxide inclusions containing Al-Ti are easily generated during refining,thereby affecting the Ti yield and the castability of steels.To improve the Ti yield and minimize the nozzle clogging,laboratory-scale experiments combined with theoretical analyses were carried out on complex oxide inclusions containing Al-Ti in 20wt%Cr UPFSS,to investigate the evolution mechanism of inclusion behaviors during refining and the clogging mechanism during continuous casting.The main work and key results are summarized as follows:The evolution mechanism of inclusion behaviors during Al-Ti complex deoxidation in UPFSS was investigated using ASPEX analysis and electrolytic extraction technique.The results show that immediately after Ti addition,transien TiOx was readily generated on the existing Al2O3 inclusions and disappeared as time progressed.The transient TiO,is not stable for the current experimental compositions.Its formation results form the interfacial behaviors between molten steel and Al2O3 inclusions.The main factors for the interfacial behaviors shortly after Ti addition include the inhomogeneity of the solute element Ti,the interfacial reaction between molten steel and Al2O3 inclusions,and the wettability between Ti-bearing molten steel and Al2O3 inclusions.The morphology of existing inclusions in UPFSS also affects the formation of transient TiOx.To minimize Ti loss caused by the formation of TiOx,removal of large particles,including aggregates,clusters,and flower-shaped inclusions should be promoted by stirring before Ti addition.The evolution of inclusion behaviors and the modification mechanism of Al2O3 and MgO·Al2O3 spinel inclusions by Ca treatment were investigated in Ti-stabilized UPFSS.The results show that,with Al contents ranging between 0.02wt%and 0.05wt%,Al2O3 inclusions were modified into liquid CaO-Al2O3-TiOx inclusions after Ca addition,whereas the MgO·Al2O3 spinel inclusions were modified into liquid inclusions and spherical high-liquid inclusions consisting of MgO.Al2O3 and CaO-Al2O3-TiOx.In the present study,Ca treatment was performed prior to the Ti wire feeding.After Ti addition,the dissolved Ti in molten steel partially reduced the Al present in the existing CaO-Al2O3 inclusions and combined with CaO-Al2O3 inclusions in the form of TiOx to form CaO-Al2O3-TiOx.An appropriate increase in A1 content and adjustment of Ca content based on the steel composition can guarantee the content of liquid phase in inclusions,reduce the TiOx content in CaO-Al2O3-TiOx inclusions and improve the modification of MgO·Al2O3 spinel inclusions.The inspection results of the plant-used submerged entry nozzle(SEN)in UPFSS production show that the deposit profile on the nozzle wall can be divided into three major layers,i.e.,eroded SEN,initial adhesive layer and loose multiphase deposits.The initial adhesive layer was composed of two sub-layers i.e.,Al2O3-ZrO2 composite layer and dense Al2O3-based deposit layer.The Al2O3-ZrO2 composite layer was deposited close to the refractory and consisted of fibrous Al2O3,frozen steel and ZrO2,whereas the dense Al2O3-based deposit layer on the side of steel consisted of compact polygonal Al2O3,frozen steel and some CaO TiO2.The formation of Al2O3 inclusions observed in the initial adhesive layer is likely to be related to the precipitation caused by a decrease in temperature when high-Cr molten steel flowed through the SEN wall.The loose multiphase deposits mainly consisted of MgO·Al2O3,CaO·Al2O3 and CaO·TiO2.MgO·Al2O3 spinel inclusions did not stick directly to the eroded refractory,but were entrapped during the deposit growth.The nozzle buildup in the loose multiphase deposit layer originates from existing particles present in the molten steel.To analyze the adhesion and detachment forces of inclusions at the interface between the liquid steel and SEN wall,a calculation model based on the cavity theory was developed.The calculation model systematically analyzes the influences of the inclusion behaviors,refractory types,and hydrodynamic conditions on the adhesion of the inclusions on the SEN wall.This provides a better understanding of the formation mechanism of SEN clogging,allowing for the development of effective countermeasures.The results of the calculation model show that Al2O3 and MgO·Al2O3 spinel inclusions with a high melting point are more easily to be entrapped to the nozzle wall under the following conditions:smaller particle size,higher contact angle between molten steel and refractory,and lower casting speed.Unfortunately,it is difficult to achieve the detachment of inclusions adhering to the wall by steel flows.For the SEN clogging in UPFSS,the formation of the initial adhesive layer is caused by the adhesion and sintering of newly generated Al2O3 at the interface between the liquid steel and SEN wall.Subsequently,the existing un-well modified MgO Al2O3 spinel inclusions continue to adhere to the initial adhesive layer and sinter together.The adhesion force on existing MgO Al2O3 spinel inclusions provides the driving force for the multiphase deposit growth.Therefore,to avoid the SEN clogging,it is important to improve the modification of MgO Al2O3 spinel inclusions by Ca treatment during refining.Based on the evolution mechanism of inclusion behaviors during refining and the clogging mechanism during continuous casting,an effective countermeasure to prevent clogging is to increase the Al content above 0.05wt%and optimize the Ca content based on the steel composition.