Study On The Refinement And Homogenized Distribution Of Inclusions In Al Killed Steel

Composition,number,size and distribution of non-metallic inclusions affect the performance and fatigue life of steel directly and sometimes they play a decisive role.When a great number of large inclusions segregate in a certain part of steel,they will wreck continuity of steel base in which crack initiates and propagates because of stress concentration during the processing and operating,and breakage occurs finally.Therefore,the study on the refinement and homogenized distribution of inclusions has a great significance on the improvement of steel quality.High temperature experiments were designed based on calculated steel composition,type and weight of generated inclusions at 1873 K in the equilibrated liquid steel obtained by thermodynamic software FACTSAGE.The transformation of three-dimensional size distribution from two-dimension was realized with stereological method of modified Schwartz-Saltykov.The distribution of inclusions was reflected directly with the inclusion information about two-dimensional coordinate and dispersivity of inclusions was characterized quantitatively.The composition,morphology,size,number density,volume fraction,distribution,dispersivity of inclusions and the change rule of those with time during the Al,Al-Mg,Al-Ti,Al-Ca deoxidation process under the condition of no melt stirring were studied based on those methods.According to classic nucleation theory,Ostwald ripening theory and collision model,figures showing the relationships between content of solute elements with critical size of nuclei,nucleation rate and coarsening rate caused by Ostwald ripening,and the change rule of inclusion collision rate were obtained.The change rule of different type inclusion behavior was analyzed by CLSM and the calculation of capillary force,Van der Waals force and cavity bridge force.The mechanism and control methods on the refinement and homogenized distribution of inclusions were revealed and mastered based on the results of high temperature experiment and the theoretical calculation.The results of high temperature experiments indicate:1)The size of three-dimensional inclusions in these four experiments at 3900s with[%Al]1= 0.05 was in the range of 1.4-1.6 μm which was smallest,the number of those was above 0.35×105 mm-3,and the degree of homogeneity in inclusion dispersion was more than 1.8,including the experiments with[%Mg]i=0.03(adding Mg after Al deoxidized),[%Ti]i=0.78,2.22,and[%Ca]i=0.78.The inclusions in Ca deoxidized steels distributed more uniformly.2)It is conductive to decreasing the inclusion size by increasing the addition of Al,Ti and Ca,and controlling the MgO content of spinel in the range of 16%-28%.3)The size of inclusions and the interfacial distance between inclusions increased in general,and number and volume fraction of inclusions decreased with holding time.4)The change trend of degree of homogeneity in inclusion dispersion was same as that of interfacial distance between inclusions,and contrary to that of inclusion number density.5)The sequence of adding Mg,Ti or Ca after Al deoxidized was conductive to the refinement and homogenized distribution of inclusions.The analyzation of inclusion nucleation,growth,and force between inclusion indicates:1)The critical size of inclusion nuclei was smaller than 3 nm.The critical size of inclusion nuclei decreased with the increase of activity of solute element.The activity of oxygen affected the nucleation rate greatly.2)The smaller critical size of inclusion nuclei was,the larger nucleation rate.The size distribution curve tended to be narrow with the increase of nucleation rate.3)The determinate factor of Ostwald ripening varied with the change of type of inclusion and steel composition.4)The collision rate increased with the increase of inclusion density,the decrease of interfacial distance between inclusions,and the decrease of nucleation rate.5)Under the condition of no melt stirring,the change of inclusion size was mainly dependent on Ostwald ripening at early stage of deoxidation,and on collision at later stage of deoxidation.6)The aggregation of inclusions was affected by collision and the force.The average force acted on the different inclusions on the surface of liquid steel decreased in the following sequence:Al2O3>Al2O3-TiOx>Al2O3-MgO>Al2O3-CaO.7)The activity of oxygen,wettability and size of inclusions,and interfacial distance between inclusions had an influence on the capillary force,Van der Waals force and cavity bridge force acted on the inclusions.8)The smaller capillary force was,larger Van der Waals force was,and smaller cavity bridge force was,the harder inclusions would attract and the denser aggregates would be.Therefore,the inclusions would distribute uniformly.The mechanism on the refinement and homogenized distribution of inclusions indicates that decreasing the content of oxygen and metallic element,controlling the composition of inclusion,increasing the critical size of inclusion nuclei and nucleation rate,changing the oxygen activity,wettability and size of inclusions,interfacial distance between inclusions were conductive to the refinement and homogenized distribution of inclusions under the condition of no melt stirring.