| Ultra pure ferritic stainless steel has advantages properties over traditional ferritic stainless steel in toughness, corrosion resistance, weldability, etc, and it’s the ideal resource-saving environmentally friendly materials for not bearing nickel which leads allergy to the human body. Therefore, ultra pure ferritic stainless steel has become one of important development directions in the modern ferritic stainless steel process. But ultra pure ferritic stainless steel’s surface and inner quality are easily impacted by non-metallic inclusions, such as high melting spinel inclusions causing strip defects in cold and hot rolled stainless steel, moreover, the steel’s mechanical properties will be effected by hard aluminate inclusions. Control of inclusions in ferritic stainless steel is always the central issue in metallurgical field.Analysis the on-site sample, the average diameter of inclusions is about two micron, irregular, high melting point and easy deformed inclusions predominant majority in the staninless steel, the surface quality is easy to be impacted by these inclusions, and is not conducive to aggregate grew up and float removed.This study was simulated as VOD position to refining ultra pure ferritic stainless steel with the content of alumina (0.3%-0.9%), low silicon (<0.2%) and using CaO-SiO2-Al2O3-MgO-CaF2refining slag with aluminum as deoxidizing addition. And the the amount of aluminum addition is two kilograms per ton. Experiment studied the influence of total oxygen as well as the iclusions affected by refining slag alkalinity and alumina content on439and443NT ultra-pure ferritic stainless steel, In addition, investigated the inclusion and total oxygen influenced by wire feeding process of calcium and the different sequences of feeding titanium and calcium.Thermodynamic calculations show that a wide spinel generation region will emerge when using aluminum as deoxidizing agent in439stainless steel consist of higher aluminum and lower silicon. Even a minute amount of magnesium in molten steel will produce spinel. And there is a complex mixture of titanium oxide, and transformation or coexistence is shown by titania, titanium sesquioxide, trititanium pentoxide. Typically titanium dioxide will not generate, with the higher need of titanium and oxygen content. Titanium nitride inclusions generate above the liquidus temperature. It is difficult for Titanium carbide inclusions to generate due to formed during cooling. Also there will be no single titanium carbide inclusions. In443NT stainess steel with similar Al and Si constents of439, titanium nitride also generate above the liquidus temperature. Considering solidification segregation titanium carbide generates at two-phase region of liquid and solid, titanium carbide will genertes below solidus temperature when not considering solidification segregation. In443NT stainess steel, titanium nitride is also generated above the liquidus temperature, titanium carbide generates at two-phase region of liquid and solid, and then niobium nitride, niobium carbide will be separated out in order below solidus temperature. The calculating results show that it’s able to play the effect of calcium treatment, Calcium treatment will work, when aluminum and calcium content percentage account for0.03%to0.06%,0.002%-0.004%, respectively. At equilibrium of mayenite, the higher temperature of molten steel, the higher calcium content of formation of liquid calcium aluminate is needed.Experimental studies have found that increasing the basicity of refining slag benefits reducing total oxygen content in steel, and total oxygen was merly affcted by changing alumina in refining slag. The average diameter of inclusions was lower with by increasing the basicity and alumina, which indicated this measure helped absorbing large size of alumina inclusions.Most of inclusions in439and443NT stainlees steel were spherical inclusions and most inclusions had low melting points by calcium treatment. And the average diameter of inclusion was about1.5μm, and the proportion of size distribution below5μm reprensented about99%. Study also indicated that increasing alkalinity and alumina contribute to improving modification effects of calcium treatment.The process of first feeding calcium weir then feeding ferrotitanium weir had lower total oxygen, larger diameter than the process with opposite order. The former process was better for glomerating and deformation inclusion and decreasing the adverse effect on steel.Overall, on basis of the comprehensive investigations on the controlling of total oxygen content, the shape of inclusions, and composition, the author advices, first, it’s able to use aluminum as deoxidation addition to produce ultra pure ferritic stainless steel with the content of high aluminum and low silicon, and it’s necessary to treat inclusions by calcium treatment, however the calcium addition should be larger and relatively adjusted on the ground of this study. Second suggesting the process with sequence of first titanium weir then calcium weir can be decent candidate for ultra pure ferritic stainless steel process. |
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