The Technologies And Theory Of Desulfurization And Inclusion Control For 304 Stainless Steel On GOR Process

304 stainless steel is a kind of stainless steel containing Cr and Ni, which has the largest usage amount and most widely used field, such as industrial production, furniture decoration, food and medical. Sulfur is a kind of harmful element, which can lead to heat brittle and act the starting point of pitting corrosion. Inclusion can cause product surface defects of stainless steel, such as line squamous defects. AOD and VOD process has been generally used for the production of 304 stainless steel. GOR converter is blowing at the bottom and the way of tapping is that the steel is frist out and then the slag, the studies of desulfurization and inclusion control based on GOR process are little few. In order to achieve the 304 stainless steel desulfurization and the improvement of the defects of rolled products based on GOR process, desulfurization process of GOR and LF, property tests of slag with Cr2O3, composition design of FeSi alloy, formation and composition evolution of inclusion were studied by sampling for industrial field, high temperature experiments in laboratory and thermodynamic calculation of FactSage software.The desulfurization processes of GOR and LF were studied. The results showed the desulfurization efficiency was improved significantly by reducing Cr2O3 content in slag as far as possible. The increase of basicity of slag can lead to the improvement of Ls in GOR process and the increase of desulfurization rate during LF refining. When the Cr2O3 content of GOR reduction slag is lower than 0.3%, Ls increase significantly and final sulfur content of steel is stability under 50ppm. When the Cr2O3 content of GOR reduction slag is lower than 0.2%, Ls increase to above 250 with the increase of basicity to 1.85. The optimal desulfurization rate is obtained when the basicity increase to about 1.9 under the condition that the Cr2O3 content of LF refining slag is lower than 0.2%.The effects of Cr2O3 in refining slag on the melting temperature, viscosity were elucidated in the temperature range from 1300℃ to 1500℃. The results showed that the melting temperature, viscosity and activation energy of viscous flow gradually increase with the increase of Cr2O3 content, the changes of the front three become more obvious under the condition of higher basicity of slag. When the basicity of slag is 1.5, melting temperature inceases by 50℃ and viscosity increase by 0.1 Pa·s with the increase of Cr2O3 content from 0 to 1.5%. When the basicity of slag is 1.7, melting temperature inceases by 100℃ and viscosity increase by 0.2 Pa·s with the increase of Cr2O3 content from 0 to 1.5%. When the basicity of slag is 1.5,1.6 and 1.7, the viscosity activation energy is 33.43 kJ·mol-1～71.75 kJ·mol-1,36.40 kJ·mol-1～137.43 kJ·moI-1 and 70.59 kJ·mol-1～143.38 kJ·mol-1, respectively.FeSi alloy used for deoxidization of stainless steel generally contains a little Al and Ca, formation of Al2O3 can occure during deoxidization process, which can lead to the formation of lamination defects on the surface of rolled sheet. The control range of Al and Ca content was obtained by the calculation of FactSage software for suppressing of the formaiton of Al2O3. The results showed that Al2O3 is easy to generate in steel during the deoxidization 304 stainless steel when the Al content is more than 1.8%, under the condition that the Ca content of FeSi is about 1%. Howerer, Al2O3 is difficult to generate when the Al content of FeSi is lower than 1.5%.The characterstic of inclusion formed in the process of GOR converter was studied. The results showed that inclusions are mainly CaO-SiO2-MgO-Al2O3-MnO-CrOx liquid spherical inclusions with different size, sizes of that in range of 1μm～22μm. The inclusions mainly divided into three categories:the first kind of inclusions is mainly from the GOR slag particles, the sizes of these inclusions are more than 10μm. The third kind of clusions generate in the deoxidization process of FeSi alloy, the sizes of these inclusions are less than 4μm. The second kind of inclusion from the collision between deoxidizing inclusions and tiny GOR slag particles, the composition is in between the first and the third kind of inclusions and the sizes is in the range of 4μm to 10μm.The evolution of inclusion composition was investigated in the process of LF refining and cooling process of continuous casting. The results showed that CaO, MgO and Al2O3 content of inclusions present a significant change, where the MgO and Al2O3 present the increase trend and the CaO content present the decrease trend. MgO content of inclusions that greater than 5μm in diameter rapidly increases first then keeps stability essentially from GOR to the end of LF, CaO content presents the opposite trend, Al2O3 content increases slowly and the other oxides content have on obvious change. The compositions of inclusions with diameter smaller than 5μm have been fluctuating in the larger range and MgO content showed a slowly increase trend. The increase of Mg plays a leading role to the change of inclusion composition in the process of GOR to LF refining. MgO and Al2O3 content of all inclusions increase rapidly and CaO content decrease rapidly from the end of LF to hot rolling. The formation of MgO·Al2O3 spinel occurres in some inclusions that smaller than 5μm in diameter. The decrease of temperature is the main cause of the change of inclusion composition.