Heat Transfer in the Mold

In the solidifying casting

Heat transfer in the solidifying casting occurs in a complex way since the heat to be extracted originates from enthalpy changes in the steel strand both from temperature decreases and phase changes. The former is referred to as sensible heat change and the latter as latent heat. Moreover, phase changes involve not only the changes between solid phases, but also the conditions produced by the solidification of an alloy. For example, a mushy zone exists between the liquidus and solidus temperatures which depend on the carbon content of the steel. In addition, the thermal resistance increases as the shell thickness increases from the meniscus to the bottom of the mold. Heat transfer in this region is by conduction.

From steel shell surface to inner copper-lining surface

Heat transfer in this step is most complex and is the controlling step in the mold. It involves mainly two mechanisms of heat transfer: conduction and radiation. The salient feature of this heat-transfer step is the shrinkage of the solidifying steel (which is a function of steel grade and caster operating conditions), and the resulting tendency for an air gap to form between the steel shell and the mold surface.

The formation of the air gap is complex and may vary both in the transverse and longitudinal direction. Thus, it has a variable effect on the heat-transfer mechanism and the magnitude of heat flux. For example, as the air gap is formed, the heat transfer proceeds mainly from conduction to radiation with a resulting decrease in heat flux. In general, this heat-transfer step represents the largest thermal resistance of all of the four steps, especially with respect to heat transfer through the copper lining and from the latter to the mold cooling water.

The entire pattern of heat removal in the mold is dependent on the dynamics of gap formation. In general, gap width tends to increase with increasing distance from the meniscus as the steel shell solidifies and shrinks away from the mold surface. In addition, as the shell thickness increases with distance from the meniscus, it tends to withstand the opposing bulging effect of the ferrostatic pressure to reduce the gap.

Heat transfer at the copper inner surface is further complicated by the effects of mold lubrication. Another factor influencing heat transfer at this mold surface is the mold taper, which tends to increase heat transfer because it opposes the effect of gap formation.

In general, the local heat flux down the mold length reaches a maximum value at or just below the liquid steel meniscus, and decreases down the mold length. The average heat flux for the whole mold increases with increasing casting speed.

Through copper lining

Heat transfer in this step is by conduction. It is dependent on the thermal conductivity of the copper and its thickness; the greater the thickness, the higher the hot-face temperature of the copper lining.

From outer copper-lining surface to mold-cooling water

Heat transfer in this step is accomplished by forced convection. Although the bulk temperature of the cooling water, typically about 40°C (90°F), is usually below its saturation temperature at a given water pressure, boiling is still possible at local regions at the mold outer surface if the local temperature of this surface is sufficiently high for water vapor bubbles to nucleate at the surface, pass to the colder bulk cooling water, and condense. This effect increases heat transfer. Nucleate boiling can result in cycling of the temperature field through the copper mold (both at the cold face and the hot face) and can result in deleterious product quality. Boiling can be suppressed by increasing the water velocity in the cooling system or by raising the water pressure. Incipient boiling is more likely in billet molds, which have higher cold-face temperatures than slab molds because of their thinner wall thicknesses. Typical values for cold-face temperature are in the range of 150°C (302°F) for billet molds and 100°C (212°F) for slab molds.

Heat transfer from Steel, Mold to Cooling Water

Control of heat transfer in the mold is accomplished by a forced-convection cooling-water system, which must be designed to accommodate the high heat-transfer rates that result from the solidification process. In general, the cooling water enters at the mold bottom, passes vertically through a series of parallel water channels located between the outer mold wall and a steel containment jacket, and exits at the top of the mold.

The primary control parameters are:

Water Volume, temperature, pressure and quality

Typically, a pressurized recirculating closed loop system is employed. The rate of water flow should be sufficient to absorb the heat from the strand without an excessive increase in bulk water temperature. A large increase in temperature could result in a decrease in heat-transfer effectiveness and higher mold temperatures. For this same reason, the inlet water temperature to the mold should also not be excessive; a proper mold water pressure is also required. For example, as discussed previously, higher water pressures tend to suppress boiling but excessively high pressures may cause mechanical mold deformation.

Water quality is an important factor with regard to scale deposition on the mold liner. Scale deposition can be a serious problem because it causes an additional thermal resistance at the mold-cooling water interface that increases the mold-wall temperature leading to adverse effects such as vapor generation and a reduction in strength of the copper liner. The type and amount of scale formed is mainly dependent on the temperature and velocity of the cooling water, the cold-face temperature of the mold, and the type of water treatment.

Water flow velocity

To achieve the proper flow velocity, the cooling system is designed such that the velocity is high enough to produce an effective heat-transfer coefficient at the mold-cooling water interface. Too low a flow velocity will produce a higher thermal resistance at this interface, which may lead to boiling and its adverse effects. In general, the higher the cooling-water velocity, the lower is the mold temperature. The cooling system should also be designed to maintain the required flow velocity distribution uniformly around the mold and to maximize the area of the faces that are directly water-cooled. Uniform flow distribution can be achieved by the proper geometrical design of the water passages with the use of headers and bale plates.

Monitoring the operating parameters of the mold cooling system provides an assessment of the casting process. For example, with a constant cooling-water flow rate, the heat removed from a mold face will be directly related to the difference between the inlet and outlet water temperature, AT. Thus an excessively large DT may indicate an abnormally low flow rate for one or more mold faces, whereas an excessively small DT may indicate an abnormally large scale buildup for one or more mold faces. An unequal DT for opposite faces may result from an unsymmetrical pouring stream mold distortion, or from strand misalignment.

Mold Oscillation

During casting as the strand moves down the mold, tensile forces are developed in the solidifying skin due to high friction and sticking of the casting skin to the working face of the mold. The friction and sticking can be further enhanced by increasing ferrostatic pressure. If these tensile forces exceed the cohesive forces of the solidifying steel, the skin will tear and a breakout may occur. Sticking can be exacerbated by local rough areas in the mold such as gouges.

To reduce the mold-strand adhesion and the risk of breakouts, in which liquid steel breaks through the thin solidified shell either in or below the mold, the mold is oscillated and lubricated. Oscillation may be accomplished by:

Motor-driven cams, which support and reciprocate the mold, are used primarily.

Mold oscillating cycles are many and varied with respect to frequency, amplitude and form. Many oscillation systems are designed so that the cycle can be changed when different section sizes on steel grades are cast on the same machine. However, there is one feature that has been adopted, almost without exception, which applies a negative strip to the solidifying shell. Negative strip is obtained by designing the down stroke of the cycle such that the mold moves faster than the withdrawal speed of the section being cast. Under these conditions, compressive stresses are developed in the solidifying shell which tend to seal surface fissures and porosity and thus enhance the strength of the shell. During the up stroke portion of the cycle, the mold is very rapidly returned to the starting position and the cycle then repeated. Thus the shape of the oscillating cycle is non-symmetrical with respect to time.

Mold Lubricants

Mold oscillation alone is insufficient to prevent skin ruptures and the use of mold lubricants is essential. Mold lubricants can be divided into two categories:

Oil lubricants (used with open pouring) tend to wet the copper mold and permit greater heat transfer at the upper part of the mold. Liquid oil lubricants include those of mineral, vegetable, animal and synthetic origin. Rapeseed oil was commonly used but is being replaced by semi-refined vegetable oils. Because of the casting environment, the oil lubricants require high-temperature properties, such as a high flash point, so that they can effectively lubricate the mold surface in contact with the steel. The oil is continually injected through a series of small holes or slots in the upper portion of the mold above the steel meniscus to form a thin continuous film over the surface of the mold walls. Oils are principally used in billet or bloom machines casting silicon-killed steels.

Solid lubricants (mold fluxes or mold powders) are widely used with submerged refractory tube shrouds in casting aluminum-killed steels on slab and bloom casters. Both mold fluxes (used with refractory shroud pouring) and mold powders result in greater heat transfer.

The powders serve not only as lubricants but also provide other functions:

Mold powder is added to the surface of the liquid steel shortly after the start of casting either manually by rakes or by mechanical feeders. Powder in contact with the liquid steel melts forming a liquid slag that then infiltrates between the mold wall and surface of the solidifying steel. Additional powder is added continually to replace that removed on the surface of the cast section. Lubrication by mold powders is a complex phenomenon and depends not only on flux properties such as viscosity, but also on the operating conditions, such as steel grade, casting speed, and oscillation condition.

In addition to viscosity, which is dependent on the silica and alumina content of the powder, the melting point or crystallization temperature characteristics of the powder are also important. Very fluid slugs with low viscosities and low crystallization temperatures tend to provide the most effective heat transfer in the mold.

Additional characteristics affecting the other functional requirements of powders include: a minimal iron oxide content, for example, to protect the liquid steel surface from reoxidation; and a low density which, together with graphitic carbon to retard sintering, fusibility and melting, enhances the thermal insulation capabilities.

Mold powders consist of a mixture of materials of which Si0₂-CaO-AI₂0₃-Na₂O-CaF₂ is the basic component with varying amounts of carbon and other coinpounds. They can be broadly divided into:

References

1. The making, Shaping and Treating of Steel, 1985, US Steel.
2. The making, Shaping and Treating of Steel, 2002, AISE Steel Foundation.