The Study On Morphological Control Of Inclusion Using Mg-treatment In Si-Mn Deoxidized Steel

The morphological control of non-metallic inclusions (including the composition, quantity, size, distribution, etc.) is one of the important tasks for clean steel production. Some steels (such as cord steel, bearing steel, etc.) are sensitive to Al2O3 hard inclusions, and Si-Mn complex deoxidizing is adopted to avoid high melting point inclusions. MnO-SiO2-(Al2O3) inclusions of low melting point have a good ability of deformation during the process of rolling. But Si-Mn complex deoxidation products are easy aggregating, growing up. The residual inclusions have adverse effect in steel performance.Studies at home and abroad have shown that adding magnesium into Al killed steel can deep purifying molten steel, and magnesium deoxidizing/desulfurization products are smaller and dispersion, which can effectively control inclusions morphological. So far, there are no reports about the applications of metal magnesium in Si/Mn complex killed steel at home and abroad. Therefore, study on morphological control of inclusion using Mg-treatment in Si-Mn Deoxidized Steel, not only can broaden the scope of magnesium treatment technology in the application of clean steel, but also can provide experimental basis for the production.This paper simulated the production process of using Mg-treatment in Si-Mn complex deoxidization steel under laboratory conditions. Using quantitative metallography, SEM and energy spectrum analysis and chemical analysis methods. The effects of different alloy addition sequence and alloy content on inclusion in the composition, morphology and size distribution are researched. Through experiments, can get the following conclusions:(1) The thermodynamics calculation shows that deoxidization ability of metal magnesium is greater than aluminum, and much greater than the silicon manganese. In Si-Mg complex deoxidation system, stable area of 2MgO·SiO2 is larger, there is no liquid phase zone exists, inclusions containing a small amount of MgO did not reduce their melting point. In Si-Mn-Al complex deoxidation system, Mn-Si-Al-0 complex inclusions are homogeneous. When Al2O3 content is not more than 25%, the composition of inclusions are still at the low melting liquid phase area.(2) In the process of using Mg-treatment in Si-Mn deoxidization steel, the formation of inclusions is closely related to the alloy added order in steel. When Ni-Mg alloy were inserted into the melt after Si-Fe and Mn-Fe, magnesium react with liquid Si-Mn-Al complex inclusion to product homogenous phase inclusions and Mg diffusion in the liquid inclusions. It generated Si-Mn-Al-Mg-0 inclusions and SiO2 content of inclusions is about 47%, MgO content is about 5%, Al2O3 content is 20%. When Ni-Mg alloy were inserted into the melt before Si-Fe and Mn-Fe, liquid complex inclusions were generated and package solid MgO style at the beginning stage of the reaction. As holding time goes by, homogeneous Si-Mn-Al-Mg-0 complex inclusions were generated finally. SiO2 mass fraction of inclusions is 45%, Al2O3 mass fraction is 10%. Comparing the two kinds of the formation speed of the inclusion, the former one way more quickly.(3) The particle size of SiO2-MnO-Al2O3 type complex inclusions is mostly located in 5μm. As Ni-Mg alloy were inserted into the melts, inclusion particle size reduce to about 1μm. When Mg treatment for 10 minutes, inclusion size proportion is smaller than 2μm increased to 92.34% from 80.10% after Si-Mn-Al deoxidizing, among 0.8～1μm of inclusions by Si-Mn-Al deoxidizing increased 19.65% from 16.84%.