Interaction Beahviors Among Steel-Slag-Refractory During Secondary Refining Process Of Medium-and High-Mn Steel Grades

The interaction behaviors between conventional Al-killed steel with low alloy steel and molten slag,refractory and other aulxiliary materials were studied by many researchers,and some great achievements were obtained,which are very helpful for the improvement of quality of Al-killed steel.Recently,intensive attention has been focused on medium-and high-manganese steel due to its outstanding mechanical properties.The reaction behaviors of medium-and high-manganese steel with molten slag,refractory and other aulxiliary materials and the formation and evolution of non-metallic inclusions are of significant difference from the conventional Al-killed steel,because of high alloyed elements in the steel grades.However,most of the researches on medium-and high-manganese steel were devoted to the improvement of materials properties,while the work on steelmaking and continuous casting of the steel grades is less reported.In order to understand the reaction behaviors of medium-and high-manganese steel with molten slag and refractory and formation and evolution mechanism of inclusions,in the present paper some reaction behaviors on medium-and high-manganese steel during steelmaking process were studied with the methods of laboratory experiments,industrial experiments and thermodynamic calculation.The main contents and conclusions are listed as follows.(1)Laboratory experiments were carried out to study the reaction behaviors between medium-and high-manganese steel and refining slag.Four kinds of steel with different manganese and six kinds of refining slag with different basicities were applied.The results showed that[Mn]in the steel would react with SiO₂ in refining slag to generate MnO.After the reaction,the content of MnO in the slag depended on the content of dissolved[Mn]and the basicity of slag.The content of MnO in the slag decreased with the increase of slag basicity and reduce of dissolved[Mn].The increase of MnO would decrease MgO solubility in the slag.Meanwhile the increase of MnO would also lead to a rise in the oxidizability of slag.This would produce adverse impacts on the desulfurization and steel cleanliness.Furthermore,the increase of MnO was able to improve melting properties of slag to some extent.In order to lower the content of MnO,the basicity of slag ought to be greater than 4 for the steelmaking process of medium-and high-Mn steel grades.(2)Based on industrial and laboratory experiments,the formation and evolution mechanism of non-metallic inclusions in medium-Mn steel during secondary steelmaking process was researched.In industrial experiments,with the generation of dissolved Mg and Ca in liquid steel from the reaction of dissolved[Al]with molten slag and refractory,the inclusions in liquid steel would transform along with the route of "Al₂O₃ inclusions?MgO·Al₂O₃ spinel inclusions?(Mn,Mg)O·Al₂O₃ spinel inclusions?CaO-MgO-MnO-Al₂O₃ system calcium aluminate inclusions".As a different type of inclusions compared with conventional Al-killed steel grades it is found that(Mn,Mg)O·Al₂O₃ spinel with a high MnO content would transform into calcium aluminate,and the MnO content in calcium aluminate inclusions became very low.Laboratory experiments were employed to explain the generation of(Mn,Mg)O·Al₂O₃ spinel inclusions.It is found that the crystal structure of MgO·Al₂O₃ spinel phase would help the formation of(Mn,Mg)O·Al₂O₃ spinel inclusions,while Al₂O₃ inclusions can not react with dissolved Mn in steel to form(Mn,Mg)O·Al₂O₃ spinel inclusions,even the Mn content was around 5.5 mass%in steel.Consequently,(Mn,Mg)O·Al₂O₃ spinel inclusions formed after the formation of MgO·Al₂O₃ spinel inclusions in industrial practice.(3)Some laboratory experiments were conducted at 1873 K(1600?)to understand the reaction mechanism between medium manganese steel and different refractories.Al₂O₃,MgO and MgO·Al₂O₃ refractory were used.The results show that dissolved[Mn]in liquid medium manganese steel could not directly react with Al₂O₃ refractory,but it was able to react with MgO refractory to produce a(Mn,Mg)O layer at the boundary between the refractory and liquid steel,then MgO in the(Mn,Mg)O layer would react with dissolved[Al]to form a layer of(Mn,Mg)O·Al₂O₃ spinel.Similar to MgO refractory,the dissolved[Mn]could react with MgO·Al₂O₃ refractory as well,and a layer of(Mn,Mg)O·Al₂O₃ was also generated after reaction.It is found that the formation of(Mn,Mg)O·Al₂O₃ at the edge of the refractory was a major source of(Mn,Mg)O·Al₂O₃ inclusions in liquid steel.The flush-off of the(Mn,Mg)O·Al₂O₃ layer would result in the formation of(Mn,Mg)O·Al₂O₃ inclusions.(4)In order to understand the reaction mechanism of Al-killed medium-Mn steel with glazed MgO refractory,some laboratory experiments were conducted at 1873 K(1600?).The refractory rods were glazed by the slag with four different basicity,and inserted into liquid steel(Mn:5.5 mass pct)for different times.The results show that MgO refractory can react with medium-Mn steel to generate(Mn,Mg)O solid solution even(Mn,Mg)O·Al₂O₃ spinel at the boundary of the refractory.When a glaze layer is formed at the edge of a MgO refractory,the glaze layer will supply inclusions into liquid steel and may act as a protection layer for the refractory.Although the glaze could also react with dissolved Mn in steel to generate MnO in both the glaze and refractory,the MnO content in both the glaze and refractory is evidently lower than that without glaze.The reaction between glazed MgO refractory and liquid steel is influenced by the basicity of the glaze layer.High-basicity slag would hinder the generation of MnO in both the glaze and refractory.So,lower-basicity glaze needs to be avoided for the steelmaking process of medium-Mn steel grades.