Jeong Whan Han

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: Part II. Mathematical model for liquid steel in tundishes

Fluid flow, heat transfer, and inclusion flotation have been modeled mathematically for several types of industrial tundish designs. Computations are presented to illustrate the importance of thermal natural convection currents in mixing the upper and lower layers of steel. The use of flow modification devices was shown to be reasonably effective in further reducing inclusion density levels at the intermediate to larger size ranges. Small inclusions (≦40 μm) were not readily removed with or without flow controls because of their low Stokes rising velocities.

Particle behavior in supersonic flow during the cold spray process

The cold spray process is a relatively new process that uses high velocity metallic particles for surface modifications. Metallic powder particles are injected into a converging-diverging nozzle and accelerated to supersonic velocities. In this study two-dimensional temperature and velocitiy distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperature on particle velocities and temperature inside and outside the nozzle are investigated. It was found that acceleration of the gas velocity takes place in the area of the nozzle throat, and it increases and reaches a maximum value at the nozzle exit. Due to compression shocks, irregular changes of the gas jet properties were found in the area after the nozzle and these resulted in the experience of the maximum particle velocity by the change of the particle size at a given gas pressure and temperature.

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: part III. applications—computational approach to tundish design

The attributes of various tundish designs and flow modifiers have been studied based on the computational and physical modeling efforts developed in parts I and II. The importance of thermal natural convection effects in such vessels has been demonstrated. Flow patterns and residual ratios (RR) for various tundish designs of equal volume and metal throughput showed that a conventional biflow, twin-strand, trough-type tundish fitted with flow modifiers allows for the greatest removal of inclusions, while a one-way flow, twin-strand, wedge-shaped tundish performs best in the absence of flow modifiers. More detailed analyses of the conventional trough-type tundish were performed to assess the effects of double-weir-double-dam and single-weir-double-dam arrangements. The effectiveness of steeply sloping sidewalls for enhanced removal of inclusions was demonstrated; however, this measure was associated with penalties in the form of greater metal temperature losses.

Optimization of Gating System Design for Die Casting of Thin Magnesium Alloy-Based Multi-Cavity LCD Housings

High-pressure die casting is the preferred process for manufacturing Mg-alloy components used for numerous applications. High-pressure die casting is suitable for mass production and has the advantage of also being suitable for accurately fashioning objects of complicated shapes. One disadvantage of high-speed die casting is the occurrence of defects such as shrinkage or air entrainment. Gating system design must be very effective in actual manufacturing facilities to avoid the occurrence of such defects. The objective of this study is to present a methodology for obtaining optimal designs of 4-cavity thin electronic component housings. The fluid behavior and amount of air entrainment caused by the overflows and air vent designs were analyzed using a computer fluid dynamics (CFD) simulator. The effectiveness of the proposed system was demonstrated through CFD simulations and experiments using an actual manufacturing process. Also, the effect of vacuum systems on the porosity and mechanical properties of the castings was studied. The volume of porosity in the casting was found to be significantly reduced using vacuum assistance during die casting. As a result, the tensile strength and the elongation of the die casting products are improved.

Distribution behavior of vanadium and phosphorus between slag and molten steel

The correlation of the equilibrium behaviors of phosphorus and vanadium between slag and low carbon molten steel in inert atmosphere was investigated with respect to the experimental variables of slag basicity, the (P2O5) and (V2O5) content, and the reaction temperature. The distribution ratios of phosphorus and vanadium increased with an increase in the slag basicity. The logarithms of the vanadium distribution ratio were greater by a factor of about two than those of phosphorus in the range of low slag basicity, but the difference diminished with an increase in the slag basicity. The logarithms of the vanadium distribution ratio increased linearly with an increase in the (P2O5) and (V2O5) content in the slag, while those of phosphorus remained nearly constant. The logarithms of the phosphorus and vanadium distribution ratio decreased with an increase in temperature, and the dependence on temperature was greater for the phosphorus than for the vanadium. For both the maximization of the vanadium yield and the minimization of the rephosphorization of molten steel in the steelmaking process, the ratio of N(V2O5)/N(P2O5), the slag basicity, the ratio of f[P]/f[V], and the temperature should be maximized, and the (FeO) content in the slag should be minimized.

Fabrication of Magnesium Alloy (AZ31) Sheet by Extrusion

In order to investigate the extrusion characteristics of magnesium alloy (AZ31), a computer simulation was attempted. Tensile properties of as-cast billet with different strain rates were incorporated into the simulation. The results showed a great change in distribution of stress and strain at near die region by the ram travel. It was found that the average gain size at longitudinal edge of the extruded sheet was smaller than that at center region, which might be attributed to recrystallization caused by severe plastic deformation.

Fluid flow analysis in horizontal continuous casting tundish

Water model studies were carried out to analyze fluid flow characteristics in a horizontal continuous casting tundish. The effects of LSN (ladle submerged nozzle) and dams on the flow pattems in a tundish were investigated and casting speeds were also changed as experimental parameters. Dye injection experiments were made to observe qualitative flow patterns and mixing characteristics in a continuous flowing tundish. And quantitative analyses of RTD (residence time distribution) curves using tracer pulse injection were also made to calculate the dead volume fraction in the water model tundish under consideration. From the results it was found that flow modification devices with an open stream have little effect on the flow patterns in the tundish, whether the dam is installed or not. This was confirmed by the flow visualization method showing upward flow near falling stream driven by the buoyancy force caused by air entrapment during liquid falling. However when LSN was installed between a ladle and a tundish there was a great change in flow pattern and a bypassing flow that existed in bare tundish was decreased with the use of dams. Especially, dead volume fraction was decreased with the increase of dam height.

Precipitate prediction model of Mg-xAl(x=3,6,9) alloys

A model was developed to predict the precipitation of Mg17Al12 in Mg-(3,6,9)wt%Al alloys. The phase equilibrium of the Mg-Al alloy system was calculated in detail by utilizing a phase diagram calculation commercial package and other thermodynamic data for Mg alloys. The precipitation kinetics of Al in Mg-(3,6,9)wt%Al alloys after different thermal treatments were studied by measuring electrical resistivity and hardness. The results obtained were explained by a model based on a simple nucleation and growth model and were compared with the measured precipitates volume fraction. And, the experimental results agreed on the thermodynamic data with the conditions of 12 hr for Mg-3Al, 30 min for Mg-6Al, 15 min for Mg-9Al alloys. Therefore, the model was applied to predict the effects of the precipitation of Mg17Al12.

Effect of Viscosity on the Separation of Copper from Al2O3-CaO-SiO2-10wt%MgO Slag System

In order to explore the possibility to extract copper from electronic scraps, a high temperature melting process was adopted to investigate the influence of slag viscosity on the gravity separation of copper from the liquid slag. For the purpose of optimizing the separation condition of the copper from slag by gravity, quaternary Al 2 O 3 -CaO-SiO 2 -10wt%MgO slag systems were chosen as basic slag compositions. Especially, two slag compositions, 15wt%Al 2 O 3 -29wt%CaO-46wt% SiO 2 -10wt%MgO and 20wt%Al 2 O 3 -35wt%CaO-35wt%SiO 2 -10wt%MgO, were chosen for the comparisons since they have been known to have low slag viscosity in the temperature range of 1300 and 1500°C. From the experiments viscosities of 15wt%Al 2 O 3 -29wt%CaO-46wt%SiO 2 -10wt%MgO slag were measured to be 7.2, 16.5 and 52 poise at temperatures of 1500, 1400, and 1300°C, whereas those of 20wt%Al 2 O 3 -35wt%CaO-35wt%SiO 2 -10wtMgO were 3.06 and 6.92 poise at temperatures of 1500 and 1400°C, respectively. It was found that the slag viscosity made an influence on the gravity separation of copper from the slag and that less than 5 poise were enough for nearly 99% extraction of copper from the slag in 30 minutes by gravity separation.

Improvement in Mechanical Properties of Magnesium Alloy (AZ31) Sheet Fabricated by Casting and Subsequent Plastic Working

In order to enhance the mechanical properties of magnesium alloy (AZ31) sheet, an integrated sheet-making process including billet casting, extrusion and rolling was attempted. Microstructural analysis was carried out and the mechanical properties at each processing step were also investigated by tensile tests. By an extrusion process of ingot-processed billet, average grain size of billet was reduced from 91.2 to 28.2 μm, and a further reduction in grain size to 16.9 μm was obtained by subsequent rolling of extruded 5 mm thin slab. In the final step, a rolled sheet of 1mm showed a remarkable refinement in grain size down to 8.8 μm. Regarding mechanical properties, tensile strength and elongation for as-cast billet, as-extruded thin slab and as-rolled sheet increased from 189, 258 to 234 MPa, and from 13, 16.5 to 23 %, respectively.