Precipitation And Evolution Of Magnesium-containing Inclusions In Steel

Alumina（Al₂O₃）inclusions in aluminum deoxidized steel easily aggregate and grow in the molten steel,which seriously affects the quality and performance of the steel.Controlling the composition,morphology and size distribution of inclusions in aluminum deoxidized steel is the key to improving the quality of aluminum killed steel.It has been found through research that the addition of magnesium to steel can transform irregular Al₂O₃ into smaller,approximately spherical magnesium-aluminum spinel（MgAl₂O₄）,thereby reducing the harmful effects of alumina on steel quality.Therefore,this article takes low carbon steel as the research object to study the precipitation and evolution behavior of magnesium-containing inclusions under the condition of Mg-Al deoxidation.Field emission scanning electron microscope（FESEM）was used to observe the single-particle MgAl₂O₄,multi-particle aggregate MgAl₂O₄ and MgAl₂O₄-MnS composite inclusions in the sample.Thermodynamic calculations show that MgAl₂O₄ inclusions precipitate in the liquid phase and MnS inclusions precipitate in the solid phase.By calculating and analyzing the magnitude and action mechanism of van der Waals force,capillary force and cavity bridge force between MgAl₂O₄ inclusion particles in molten steel,it is found that the cavity bridge force is the main force that causes the aggregation of MgAl₂O₄ particles,and the capillary force determined by the interface energy is weak.By calculating the two-dimensional mismatch between MgAl₂O₄ and MnS crystals,it was found that MnS is easier to nucleate on the（110）and（100）faces of the MgAl₂O₄ surface.Through the automatic analysis system for inclusions,the size distribution of inclusions on the cross section and sides of the steel sample was calculated.The number of inclusions in the range of 1-5 μm accounted for more than 80% of the total number of inclusions.The high temperature confocal laser scanning microscopy was used to in-situ observe the aggregation process of MgAl₂O₄ inclusions.The aggregation process can be divided into three consecutive steps: metastable structure connection aggregation,metastable structure fracture separation,and hard agglomeration.The analysis found that the cavity bridge force has a linear relationship with the inclusion radius: When0.14 R_p <d <0.24 R_p,the metastable structure formed between MgAl₂O₄ inclusions will be broken by cavity bridge force;When d = 0.14 R_p,a stable cavity bridge will be formed between the inclusions and a stable structure will be formed,the MgAl₂O₄ inclusion aggregates will not break apart again.SiO₂,2MgO·SiO₂,3Al₂O₃·2SiO₂,2MgO·2Al₂O₃·5SiO₂ and their compositeinclusions were found in the sample.The formation process of different Si-containing inclusions was analyzed by thermodynamics,and two formation pathways were obtained.