| High melting point Al2O3 inclusions are common in aluminum killed steels.On the one hand,such inclusions will cause nodules at the submerged nozzle of the mold and affect the continuous casting production.On the other hand,due to its poor plasticity and poor deformation,it will deteriorate the performance of steel.Especially for the deep-drawn sheet steel,the surface quality is seriously affected.Metallurgical workers try to change the composition,size and distribution of high melting point inclusions in steel through inclusion modification technology to reduce their negative impact on steel properties.In recent years,with the improvement of magnesium alloy cored wire system composition design and stable wire feeding process,magnesium treatment has gradually developed into an emerging control technology for high melting point Al2O3 inclusion modification treatment in steel.After the magnesium alloy cored wire is added to the molten steel,the Al2O3 inclusions can be adjusted to small and dispersed MgAl2O4 inclusions,which can exert the metallurgical effect of the fine inclusions to refine the as-cast structure and achieve the control effect of seeking advantages and avoiding disadvantages.Aiming at the positive effect of inclusion refinement and microstructure control during magnesium treatment,this paper systematically analyzes the change law of inclusion composition,size and distribution in aluminum-killed steel under the condition of magnesium-calcium composite treatment.The critical conditions for the liquefaction control of magnesium inclusions can provide a new solution to the problem of submerged nozzle nodules.At the same time,based on the theory of lattice mismatch degree,the effect of magnesium treatment on the as-cast equiaxed crystallinity in single-phase ferrite solidification mode and austenite solidification mode steel was systematically studied,and the effect of magnesium treatment on the solidification structure refinement of steel was analyzed.mechanism.The main findings of this paper are as follows.(1)A large number of spherical CaO-Al2O3-MgO composite inclusions(1 μm～2 μm)with MgAl2O4 as the core can be formed in the magnesium-calcium composite treated steel,indicating that they are liquid in the molten steel.Comparing the effects of different addition orders of magnesium wires and calcium wires on inclusions,it is found that since MgAl2O4 and CaO·xAl2O3 can be transformed into each other in steel,the addition order of magnesium wires and calcium wires in the composite treatment process will not affect the stability of inclusions in as-cast steel.composition,but the calcium-first-magnesium process can obtain composite inclusions of smaller size(27%smaller than that of magnesium first and then calcium process).(2)Based on the lattice mismatch theory,the magnesium-rich oxides MgAl2O4 and MgO particles formed by magnesium treatment can serve as effective heterogeneous nucleation cores for TiN.The content of Ti and N elements in steel can be characterized by the[Ti][N]product to characterize their effects on the precipitation behavior of TiN.Under the same[Ti][N]product conditions,increasing the number density of MgAl2O4 and MgO particles can provide more effective nucleation cores for the nucleation of TiN,and the limit size of a single TiN will also decrease,that is,the TiN particles are refined..Increasing the Ti content alone can cause the coarsening of TiN particles,while increasing the Mg content can increase the number density of TiN in the steel and refine the TiN particle size.Under the condition of high[Ti][N]product,the size of tin in quenched samples decreased by 33.6%and the quantity density increased by 496.6%.(3)There are significant differences in the improvement effect of magnesium treatment on the proportion of as-cast equiaxed grains obtained by different solidification modes.By comparing the refining effect of magnesium-treated austenitic stainless steel SUS316L and ferritic stainless steel SUS439,the effect of magnesium treatment on the solidification structure of Ti-stabilized ferritic stainless steel is more significant.Before and after magnesium treatment,the equiaxed grain rate of SUS316L stainless steel increased from 0 to 15%,and that of sus439 stainless steel increased from 47%to 91%..And comparing the refining effect of different grain refiners on the solidification structure of ferritic stainless steel tube,it is found that single Mg treatment>RE treatment>Mg-Y treatment>Mg-RE treatment.The reason lies in the refining effect of MgAl2O4 and MgO particles formed by magnesium treatment on TiN,and the further refined TiN particles have a good heterogeneous nucleation effect on the ferrite phase δ,and finally obtain iron with a high proportion of axial crystals.Solid stainless steel. |
PDF Full Text Request