Patent Application
20200376542
The present invention relates generally to steel slab production molds. More particularly, the present invention relates to funnel molds which are cooled via a liquid spray.
In the conventional continuous steel casting method, molten steel is passed through a vertically oriented, usually curved, copper mold. As the molten steel passes through the mold its outer shell hardens. As the steel strand continues to harden, it is bent through an angle of 90 degrees so that it moves horizontally, and it is subsequently cut into individual slabs.
The temperature of molten steel is typically 2850 degrees F., although with certain grades the temperature may be as low as 2600 degrees F. In general, although most of the references herein are to steel casting, the invention contemplates the casting of any metal or metal alloy whose liquid temperature exceeds 2600 degrees F.
The mold which forms the steel strand contains the liquid steel and provides for its initial solidification, that is, hardening of the outer shell. The mold is oscillated vertically to prevent the solidifying strand from sticking to the copper mold. In addition, a high temperature powder is applied to the surface of the molten steel to reduce the coefficient of friction and form a barrier between the solidifying strand and the mold wall. The solidifying strand is extracted continuously from the bottom of the mold at a rate equal to that of the incoming liquid steel at the top, the production rate being determined by the time required for the outer shell to harden sufficiently so as to contain the inner core of liquid, solidifying steel.
The most recent slab mold design iterations in the art have focused on casting a slab that is thinner and more closely related to the final shape of the finished product, called thin slab casting. For example, when attempting to cast a slab that is 4 inches thick, or thinner, many operational and metallurgical difficulties arise. Attempting to introduce the molten steel into the narrow mold opening is very problematic. To overcome this problem a funnel shaped mold was developed whereby the center of the mold is considerably thicker than the end locations and the mold is subsequently narrowed as it descends thereby squeezing the solidifying steel shell as it descends to the exit point of the copper mold, finishing as a slab that is uniformly 4 inches thick (or thinner) for the entire width.
In all the conventional continuous casting methods the force of friction between the mold wall and the solidifying strand can result in the development of small cracks and other deformations of the solidifying slab surface. If the frictional force is great enough (greater than the ultimate tensile strength of the solidifying steel shell) the solidifying shell can actually rupture and cause the liquid steel contained within the solidifying slab to spill out and ruin the product. To alleviate this situation the casting speed of the casting machine must be slowed and casting powders (high temperature melting point compounds composed of proprietary mixtures of mineral and synthetic materials), also referred to as flux powder, must be employed to act as a lubricating film of molten slag between the copper mold wall and the solidifying steel shell. The molten slag film effectively reduces the coefficient of sliding friction, which allows for the solidifying steel shell to thicken sufficiently enough to withstand the frictional forces acting upon it.
The present invention is a funnel shaped mold for casting steel slabs in order to cast thinner slabs of steel than is practical with conventional molds. The system comprises a funnel mold with outer walls, a funnel section and a narrow face section and further comprises a liquid spray cooling system in which a plurality of spray nozzles are configured to spray the outside walls of the mold in order to provide cooling for the casting process.
The object of the present invention is to provide a steel slab mold in which the casting parameters are controlled via liquid spray cooling so as to maximize the production rate of the mold and ensure superior metallurgical properties of the produced slabs.
It is a further object of this invention to provide a steel slab mold which is relatively simple to manufacture and service by the user.
The advantages of the present invention include that it is cost effective, maximizes production from the mold and produces a predictable and high quality steel slab.
These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent in the course of the following descriptive sections.
Applicant has discovered that by controlling the coefficient of sliding friction (identified as mu (μ), where μ=f/N; f=the force of friction and N=the force normal to the surface) between the cooling copper mold wall and the solidifying steel slab results in improved surface and metallurgical conditions and production rates. In fact, by controlling the coefficient of sliding friction the casting machine operator can reduce, or eliminate for some steel grades, the necessity for high temperature casting powders or other friction reducing compositions required by the conventional casting methods.
Unlike conventionally designed slab molds where the coefficient of sliding friction between the copper mold wall and the solidifying steel shell is more uniform (μ=0.40 to 0.50), it was determined that in the funnel mold design the coefficient of sliding friction between the copper mold wall and the solidifying steel shell varies significantly across the width of the caster mold. This is due to the differential ferrostatic pressure effect (the force of the liquid steel against the copper mold wall). Unlike within the conventional slab mold design, where the ferrostatic pressure is more uniform along the width of the mold, in the funnel mold design the ferrostatic pressure is significantly higher (as much as 5× greater) in the funnel section than the other locations within the mold (
Further, in addition to the beneficial surface and operational aspects of controlling the frictional forces encountered in this casting process, it was discovered that the higher frictional forces also affected the metallurgical properties of the solidifying steel. As the steel is solidifying the dendrites that are forming grow perpendicularly inward from the cooled mold wall. The higher frictional forces encountered within the funnel section of the mold cause the dendrites that are forming in the steel to separate (both horizontally and vertically) and significantly reduce further dendritic growth. This disruption in dendritic formation and growth results in irregular solidification patterns and interdendritic alloy concentrations in the solidifying steel.
Experimentation with different steel grades and with high pressure (30 psig to 150 psig) and high flow rate (6 gpm to 30 gpm) spray nozzles directed against the outside wall of the funnel mold allows for a significant reduction in, and ultimate control of, the coefficient of sliding friction across the entire width and length of the funnel mold. By controlling the coefficient of sliding friction the operator can directly influence the casting parameters and thereby ensure superior surface and metallurgical properties of the cast steel while maximizing the production rate of the casting machine.
The coefficient of sliding friction (μ) of the conventionally designed funnel mold is:
1. Narrow Face Section μ=0.60 to 0.85
2. Funnel Section μ=0.80 to 1.25
The coefficient of sliding friction (μ) of the high pressure high flow rate funnel mold is:
1. Narrow Face Section μ=0.35 to 0.55
2. Funnel Section μ=0.30 to 0.55
By employing a series of high pressure, high flow rate spray nozzles, specifically designed to conform to the irregular shape of the funnel mold and simultaneously affecting the coefficient of sliding friction along the entire width and length of the funnel mold, the metallurgical properties and surface quality of the cast steel is enhanced and can now be directly controlled by the casting machine operator. Following further experimentation, it was determined that the cooling medium (water, mixture of water and compressed air, mixture of water and iced brine, mixture of water and ethylene glycol, mixture of water and propylene glycol, for example) is concentrated at the funnel section (flow rate and pressure 3-5 times greater than at the narrow sections) and reduced at the narrow sections of the slab mold.
Further experimentation to determine what effect the spray droplet size and spray droplet density would have, at low and high pressures, upon the coefficient of friction were undertaken. Several spray nozzle inserts were fabricated and resulted in the following data:
The droplet size did not appear to appreciably influence the coefficient of sliding friction. The droplet density at 40 psi and at 120 psi did show a further reduction in the coefficient of sliding friction by μ=0.03 to μ=0.17. The data are tabulated below:
1. insert a: μ reduction 0.03
2. insert b: μ reduction 0.03
3. insert c: μ reduction 0.04
4. insert d: μ reduction 0.04
5. insert e: μ reduction 0.06
6. insert f: μ reduction 0.07
7. insert g: μ reduction 0.09
8. insert h: μ reduction 0.13
9. insert i: μ reduction 0.15
10. insert j: μ reduction 0.17
In the following drawings, which form a part of the specification and which are to be construed in conjunction therewith, and in which like reference numerals have been employed throughout wherever possible to indicate like parts in the various views:
Referring more specifically to the drawings, typical prior art systems for casting slabs are indicated generally in
Referring to the drawings
The walls 22 of the funnel mold 20 are preferably made of copper and form a complete enclosure around four sides of the interior section 21 of the mold. The walls are generally a uniform thickness, but may be thinner adjacent to the funnel section 25 of the interior as depicted in
To help reduce the sliding frictional forces encountered when casting steel slabs, high temperature melting point casting powders may be employed to form a lubricating barrier between the solidifying steel and the copper mold wall. Due to the differential ferrostatic pressures encountered within the funnel design, significant differential sliding frictional forces within the slab mold may be encountered. The result of these frictional forces also affects the metallurgical properties of the solidifying steel slab.
To overcome the detrimental effects of the frictional forces a spray cooling system 30 is utilized. The cooling system comprises a plurality of high pressure, high flow rate nozzles allocated at strategic locations conforming to the irregular shape of the funnel section of the slab mold indicated in
The invention results in superior quality of the cast steel slab, increased productivity, and reduction, or, for some grades of steel cast, elimination of, the requirement for the application of high temperature casting powders to reduce the sliding frictional forces encountered.
