Mold Region Research Articles

An asymptotic model of the mold region in a continuous steel caster

A model is developed to simulate the solidification of the steel shell in the mold region of the continuous casting process. Conduction-dominated temperature fields in the mold, mold flux, steel shell, and molten steel regions are determined through the development of an evolution equation for the solidifying front. This equation is derived in the limit of small aspect ratio, mold width to height, using asymptotic methods. These results are coupled with a lubrication-theory model for the mold flux region. This model assumes a temperature-dependent viscosity for the mold flux and allows for solidification of the flux at temperatures below a critical value. System response to changing casting speeds, superheat, mold wall temperatures, and mold flux properties is investigated.

Issues in Thermal-Mechanical Modeling of Casting Processes.

Synopsis Mathematical modeling of stress generation in casting processes is a difficult, complex subject that is now receiving increased attention. This paper reviews the basic equations, solution methods and important phenomena associated with casting processes that require special numerical treatment. Stress modeling begins with a coupled, transient heat transfer analysis, including solidification, shrinkage-dependent interfacial heat transfer, and fluid flow effects. Further complicating phenomena include phase transformations, temperature, stress and structure-dependent plasticcreep, interaction between the casting and the mold, hydrostatic pressure from the liquid, the effects of fluid flow, and crack formation. Computational issues include numerical methods for handling these phenomena, mesh refinement, and two-dimensional stress state. Example applications are presented for the thermal-mechanical behavior of the solidifying steel shell in the mold region of a continuous slab caster, using a finite-element model, which accounts for many of these phenomena.

CAE of Mold Cooling in Injection Molding Using a Three-Dimensional Numerical Simulation

In recent years, increased attention has been paid to the design of cooling systems in injection molding, as it became clear that cooling affects both productivity and part quality. In order to systematically improve the performance of a cooling system in terms of rapid, uniform, and even cooling, the designer needs a CAE analysis tool. For this, a computer simulation has been developed for three-dimensional mold heat transfer during the cooling stage of an injection molding process. In this simulation, mold heat transfer is considered as cyclic-steady, three-dimensional conduction; heat transfer within the melt region is treated as transient, one-dimensional conduction; heat exchange between the cooling channel surfaces and coolant is treated as steady, as is heat exchange with the ambient air and mold exterior surfaces. Numerical implementation includes the application of a hybrid scheme consisting of a modified three-dimensional, boundary-element method for the mold region and a finite-difference method with a variable mesh for the melt region. These two analyses are iteratively coupled so as to match the temperature and heat flux at the mold-melt interface. Using an example, the usefulness of the simulation developed here in the design of a cooling system for an injection molding process is amply demonstrated.

Simulation of fluid flow inside a continuous slab-casting machine

A finite element model has been developed and applied to compute the fluid flow distribution inside the shell in the mold region of a continuous, steel slab-casting machine. The model was produced with the commercial program FIDAP, which allows this nonlinear, highly turbulent problem to be simulated using the K- e turbulence model. It consists of separate two-dimensional (2-D) models of the nozzle and a section through the mold, facing the broad face. The predicted flow patterns and velocity fields show reasonable agreement with experimental observations and measurements conducted using a transparent plastic water model. The effects of nozzle angle, casting speed, mold width, and turbulence simulation parameters on the flow pattern have been investigated. The overall flow field is relatively insensitive to process parameters.

Effect of oscillation-mark formation on the surface quality of continuously cast steel slabs

In a study of early solidification during the continuous casting of steel slabs, the effect of the formation of oscillation marks on the surface quality of the slabs has been examined by metallographic in-vestigation of slab samples and by performing a set of mathematical analyses. Positive segregation of solute elements, especially phosphorus and manganese, has been observed at the bottom of the oscillation marks and has been classified into two categories. One type is observed at the end of the overflow region on subsurface hooks which originate from partial solidification of the meniscus. A heat-flow model which takes into account the shape of the oscillation marks has revealed that this type of positive segregation is caused by local delay of solidification at the bottom of the oscillation marks. The other type of positive segregation has been found in a layer on the bottom of the oscillation marks without subsurface hooks. This form of segregation cannot be explained by the heat-flow model, but is likely due to a penetration mechanism in which the negative pressure in the flux channel generated during the upward motion of the mold draws out interdendritic liquid from the semi-solidified shell. Transverse cracks are found along the bottom of oscillation marks. The surface of the transverse cracks exhibits an interdendritic appearance in the vicinity of the slab surface, which implies that the cracks are initiated as hot tears in the mold region. A heat-flow analysis predicts that deep oscillation marks cause nonuniformity of the shell in the mold, which also was observed in the metallographic in-vestigation. According to the heat-flow analysis, not only the depth but also the pitch of the oscillation marks affects the shell profile. Therefore increasing the frequency of mold oscillation effectively reduces transverse cracks, by decreasing both the depth and the pitch of oscillation marks.

Numerical models for casting solidification: Part I. The coupling of the boundary element and finite difference methods for solidification problems

An investigation on the coupling technique for boundary element and finite difference methods was carried out to simulate solidification processes. In the coupling model, solidification problems in casting and unsteady-state heat conduction problems in mold regions were analyzed by explicit finite difference and boundary element methods, respectively. A comparison was made between the coupling model and the finite difference method on the solidification of castings in metal and sand molds. The proper range of time steps for boundary elements in transient problems was presented for a simple geometry. And two types of time marching schemes were proposed for application of the boundary element method to solidification processes.

Furnace atmosphere effects on casting of eutectic superalloys

Control of furnace atmosphere is a key factor in the use of silica-bonded alumina shell molds for the directional solidification of eutectic superalloys reinforced with tantalum monocarbide whiskers. The use of a furnace atmosphere which is simultaneously oxidizing to aluminum in the eutectic alloy and reducing to silica phases in the mold results in the formation of an alumina barrier layerin situ at the metal/mold interface and an absence of silica phases in the mold region adjacent to this barrier layer. The presence of this microstructure permits castings of eutectics at metal temperatures up to 1750°C.

パウダーの性状を考慮した連鋳鋳型内伝熱モデル

パウダーの性状を考慮した連鋳鋳型内伝熱モデル

The physical and mathematical modelling of the flow field in the mold region in continuous casting systems: Part II. The mathematical representation of the turbulent flow field

A mathematical representation is developed for the turbulent flow field, temperature field, and tracer distribution in the upper region of the liquid pool in continuous casting. The problem is formulated through the statement of the two-dimensional turbulent flow equa-tions, which were then solved numerically, using the adaptation of a technique described by Spalding and coworkers. The computed results for the velocity fields were found to be in good qualitative agreement with the results of water model studies for both straight and radial flow nozzles. Furthermore, the predictions based on the model for the temperature and tracer profiles within the pool seem to be consistent with expectations.

The physical and mathematical modeling of the flow field in the mold region in continuous casting systems: Part I. model studies with aqueous systems

Experimental data are reported on velocity measurements on a water model depicting the behavior of the simulated mold region in a continuous casting system. Velocity data are reported on both “straight” and radial flow nozzles; these findings confirm earlier qualitative investigations concerning the flow patterns in these systems. The actual numerical data on the velocities indicate that the flow field in the radial flow nozzles appears to be preferable for promoting the flotation of inclusions.