Guanghua Wen

Behavior of Mold Slag Used for 20Mn23Al Nonmagnetic Steel During Casting

Because of the addition of significant quantities of aluminum and manganese, the great challenge of casting 20Mn23Al nonmagnetic steel is to design a mold slag that is chemically compatible with this steel composition. A new mold slag with low melting point and low basicity was proposed. The initial slag (the base mold slag) and two corresponding mold slags sampled after 8 min and 15 min were investigated, and the changes of mold slag composition and properties of molten slag were discussed. The results showed that the slag quickly changed in chemical composition to a slag that contained a small quantity of silica and a larger quantity of alumina, and the basicity increased obviously; the heat flux density through the slag film decreased slightly compared with the slag film at 0 min, 8 min and 15 min; the mold slag used for nonmagnetic steel was precipitated as CaF2 crystals. The casting experiment shows that the slab has good surface quality without longitudinal face cracks and slag inclusions; the Al2O3 content in the molten slag pool increased with time at the beginning stage, and it can approach equilibrium state after about 600 s.

CCT and TTT Diagrams to Characterize Crystallization Behavior of Mold Fluxes

The isothermal and non-isothermal experiments were performed to construct the continuous cooling transformation (CCT) and temperature time transformation (TTT) diagrams of four industrial mold fluxes through visual observations in an experimental apparatus based on the single hot thermocouple technique (SHTT). The results of the CCT diagrams indicate that ➀ the crystallization temperature of mold fluxes lowers as the cooling rate increases, ➁ the mold fluxes have larger critical cooling rate, higher crystallization temperature, and less onset time of crystallization when the basicity increases or the viscosity decreases, ➂ the influences of the melting points of the mold fluxes on their crystallization tendency are not significant. Isothermal tests show that the onset time of crystallization decreases at first, and then increases, and finally represents a “C” shape with increasing isothermal temperature. The TTT diagrams of four industrial mold fluxes were divided into two separate “C” shape regions. The crystal phase of C20A selected was analyzed by X-ray diffraction, which is cuspidine (Ca4 Si2 O7 F2) over 1100 °C and calcium silicon oxide fluoride (Ca2 SiO2 F2) below 1100 °C. When compared with the TTT diagram, the CCT diagram can provide a more realistic estimate of the critical cooling rate of the mold fluxes. Thus, both the CCT and TTT diagrams can unambiguously describe the crystallization phenomena of the mold fluxes.

Viscosity and Viscosity Estimate Model of Fluoride-Free and Titanium-Bearing Mold Fluxes

The relationship between the binary basicity (CaO/SiO2), TiO2, Na2O, Li2O, MgO, MnO, B2O3 and viscosity for fluoride-free and titanium-bearing mold fluxes were systematically researched. The rotating cylinder method was employed in the experiment to measure the viscosity of the slag. The results indicate that Li2O, B2O3 and Na2O play major roles in decreasing viscosity, especially Li2O, which is the most effective flux, while MgO and MnO exert little influence on viscosity. Meanwhile, it can be concluded that with increasing TiO2 content, the viscosity of fluoride-free and titanium-bearing mold fluxes increases at first but then falls when the amount of TiO2 is greater than 6.0%. Based on large amounts of experimental statistics of the viscosity of fluoride-free and titanium-bearing mold fluxes, an available model in literature for predicting the mold-slag viscosity was modified. This modified model can be used to predict the viscosity of fluoride-free and titanium-bearing mold fluxes. In fact, the predicted values approximate the observed values with a ±10.6% average deviation. Compared with the classical models, the average deviation is higher and it was found that the modified model can be used to estimate the viscosity of fluoride-free and titanium-bearing mold fluxes.

Analysis of Thermal Behavior for Beam Blank Continuous Casting Mold

Finite element models of steady heat conduction for cross section of beam blank mold were developed by using ABAQUS software. The effect of mold grinding thickness, cooling water velocity, diameter of restrictor rods and water channel design on hot face temperature was analyzed in detail. Attention was focused on the peak temperature and temperature uniformity along hot face. The results showed that the peak temperature of existing mold, about 337.2 °C, is located in the fillet, and two valleys of hot face temperature are found in flange corner and junction of wide face and narrow face, respectively. Decreasing mold thickness, increasing cooling water velocity and increasing diameter of restrictor rods can all reduce peak temperature and improve temperature uniformity along hot race at the expense of lower overall temperature. Redesigning the water channel can decrease peak temperature and thermal gradient of mold without lowering overall temperature of hot face. In particular, the small hole design can improve temperature uniformity across hot face and obtain the best advantage.

Heat flux through slag film and its crystallization behavior

An experimental apparatus for simulating copper mold is used to quantify the heat flux through the slag film and to obtain a solid slag for further determining its crystallization behavior. The result indicates that both the chemical composition of the mold powder and the cooling rate have an important influence on the heat flux through the slag film. With increasing the binary basicity, the heat flux of slag film decreases at first, reaches the minimum at the basicity of 1.4, and then increases, indicating that the maximum binary basicity is about 1.4 for selecting “mild cooling” mold powder. The heat transfer through the slag film can be specified in terms of the crystalline ratio and the thickness of the slag film. Recrystallization of the solid slag occurs and must be considered as an important factor that may influence the heat transfer through the solid slag layer.

Characteristics of Heat Flux Through Slag Film of Mold Slag Used for High Al-TRIP Steel Casting

Due to the high content of the Al-TRIP steel, Al reacts with SiO2 in the mold slag during the casting process, which results in the increase of

Crystallization Behaviors of Mold Fluxes Containing Li2O Using Single Hot Thermocouple Technique

w_{Al_2O_3}/w_{SiO_2}

Special-shaped billet casting machine continuous casting weathering steel crystallizer protection slag

ratio and the Al2O3 content, respectively; the characteristic of heat transfer through the slag film is then changed, which influences the smooth operation of the continuous casting process. The heat flux simulator of mold slag film was used to study the change tendency of the heat flux density through slag film and the results were discussed; at the same time, the characteristics of the crystals in the No. 15 mold slag were determined by BSE and XRD. The results obtained show that heat flux density decreases with the increase of