| Non-metallic inclusions not only affect the mechanical properties of steels,i.e,strength,toughness and plasticity,but also influence the casting process.Strict control of the components,morphology,diameter and distribution of the inclusions in steels are required for the clean steel metallurgy.Magnesium treatment can have a function of control the components and diameter of the inclusions.Metallurgy effect and evolution mechenism of magnesium in molten steel system should be studied systematically,which is very valuable to the theoretical developments and industrial application of the magnesium treatment technology.Combined with the real molten steel system,this paper firstly took thermodynamic research on Mg-contained molten steel(Fe-Mg-Al-O-S,Fe-Si-Mn-Al-Mg-O and Fe-Mg-Al-Ca-0 system).Based on it,evolution behavior and effect mechanism of the inclusions in Mg-contained molten steel was investigated through high temperature experiment.In Fe-Mg-Al-O-S system,thermodynamic research results indicate that Al2O3 was changed into MgO Al2O3 in Fe-Mg-Al-O-S system by adding a little bit of magnesium,when Al content ranging from 0.01 to 0.1%.If the ratio[Mg]/[Al]is just larger than 0.1,MgO·Al2O3 inclusions can be modified to MgO.In low-S content Al deoxidation melt,Al2O3 inclusions were changed into MgO·Al2O3 spinel inclusions after Mg addition.With the increase of reaction time,Mg atomic percent of the inclusions gradually increased.After Mg treatment proceeded for 5 min,inclusions whose size was smaller than 2 microns increased to 90.55%from 80.23%,and the inclusions whose size was among 0.8～1 μm reduced to 37.41%from 32.82%.At this time,magnesium can raise formation temperature of the sulphide in high-S content A1 deoxidation melt,and there were a large number of pure sulfide precipitated separately.To S-contained steel,Mg treatment can improve the distribution of sulfide,decrease the amount of chain-typed sulfide,and enhance mechanical property of the steel.In Fe-Si-Mn-Al-Mg-0 system,It is of great interest a very small amount of Mg([Mg]=0.0001%)can widen the liquid phase in Fe-Si-Mn-Al-Mg-0 system,but the region of liquid inclusion formation narrows as the Mg content in molten iron increases owing to solid inclusion phase(2MgO·SiO2,MgO Al2O3)through thermodynamic research.Individual modification mechanism was induced by a specified deoxidant addition sequence.When Si-Fe and Mn-Fe were added into molten steel prior to Mg,Mg reacts with liquid phase Sio2-MnO-Al2O3 inclusion,and oxide inclusions were modified quickly.However,when Ni-Mg alloy was introduced into the melt prior to Si-Fe and Mn-Fe,Si02-MnO-Al2O3 complex liquid inclusions were possible to precipitate,stretch and cover the solid MgO inclusion.Then the MgO inclusions were gradually dissolved from outside to inside by the contaction with liquid phase inclusion.After Mg treatment proceeded for 10 min,inclusions whose size was smaller than 2 microns increased to 92.34%from 80.90%.Increasing the strength of Mg treatment can reduce the inclusion size,but superfluous MgO raise the melting point of the complex inclusions,and then worsen the steel castability.To Si-Mn deoxidized steel,proper amount of Mg treatment can ensure the formation of the liquid phase inclusions and control the size of the inclusions,so as to enhance the processability and mechanical property of the steel.In Fe-Mg-Al-Ca-0 system,It is of great interest a very small amount of Mg can widen the liquid phase,but the region of liquid inclusion formation narrows as the Mg content in molten iron increases owing to MgO·Al2O3 phase through thermodynamic research.Modification mechanism was individual by a specified deoxidant addition amount.When Ca addition is excess,liquid calcium aluminate inclusions formed at intial stage.If there is a trace of dissolved magnesium in liquid steel,the liquid inclusions will be unstable and change into CaO-MgO-Al2O3 system inclusions.When Ca addition is insufficient,magnesium can also modify solid calcium aluminate inclusions in the melts similarly,but there are unreacted cores in the inclusions after Mg treatment 1min.The results showed that magnesium can control the size of oxide inclusions in molten steel well.After Mg treatment,the size of inclusions in melts was decreased obviously,average size increased to 1.5μ.m from 2.4μm.If increasing the strength of Mg treatment,solid CaO-Al2O3-MgO inclusions formed,which had a smaller size of 1.2μm at later stages of Ca treatment,Mg treatment can minimize the size of the inclusions,so as to avoid residual of large size inclusion. |
