Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-09-11
2002-04-23
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S443000, C164S414000, C164S455000
Reexamination Certificate
active
06374903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to continuous casting of metals, particularly steel. More specifically, this invention pertains to an improved continuous casting mold and processes for operating and retrofitting continuous casting molds that provide enhanced cooling during the solidification process.
2. Description of the Related Technology
Several different types of continuous casting molds are used in the metal casting industry today. The main differences between molds relate to the size and shape of the products being cast. Billet production, i.e. small cross-sections generally used for manufacturing so-called “long products” such as structural steel shapes (angles and channels), rails, rod and wire, are generally cast through a copper tube mold. The inside of the copper tube serves as the casting surface, forming a product that is equal in size and shape to the inside of the copper tube itself. The outside of the copper tube is water cooled, generally by fast flowing water, but sometimes by spray water.
Most billet casting machines used for making long products have multiple molds and produce multiple strands of steel simultaneously as they are fed from a single tundish. The tundish in a continuous casting operation is a refractory-lined vessel used to feed the mold or multiple molds in this case.
Another type of mold commonly used in continuous casting forms a slightly larger cross-section called a bloom. A bloom can be round and formed in a round copper tube mold, but it is more generally a rectangular shape used to make long products as well as seamless plates in tubes. A mold of this type typically includes a number of liner plates, usually made of copper, and water jackets surrounding the liner plates. The liner plates are often referred to as “coppers,” and define a portion of the mold that contacts the molten metal during the casting process. Parallel vertically extending water circulation slots or passageways are provided between the water jackets and the liner plates to cool the liner plates. During operation, water is introduced to these slots, almost always from the bottom end of the mold, from a water supply via an inlet plenum that is in communication with all of the slots in a liner plate. The cooling effect so achieved causes an outer skin of the molten metal to solidify as it passes through the mold. The solidification is then completed after the semi-solidified casting leaves the mold by spraying additional coolant, typically water, directly onto the casting. This method of metal production is highly efficient, and is in wide use in the United States and throughout the world.
In the case of a rectangular shaped bloom mold, four plates (i.e. two widefaces and two narrowfaces) generally form the mold cavity. These four separate copper mold liners generally fit together to form a nonadjustable rectangular box that serves as the casting chamber. Commonly, a four piece bloom mold will have chamfered comers as opposed to the square comers found in a four piece slab mold.
Slabs are also rectangular in shape, but are generally much wider than they are thick. Slab casting accounts for the major share of the nearly 800 million tons of continuously cast steel product produced worldwide per annum. Most slab molds and bloom molds have four copper plates that serve as the inner casting surface of the mold. Typically, these mold liners are slotted on the back side to form cooling passages through which cooling water can flow. In some cases, the cooling passages are formed by drilling a series of vertical round holes, but this method has cost implications and performance limitations that are generally not found in the slotted copper design.
Another mold type that is called the “beam blank mold” is used to cast a strand of metal in the shape of an H-beam that can be further reduced in section to a size that is commonly used or structural purposes, such as the construction of buildings and bridges. Beam blank production is commonly referred to as a form of “near net shape” casting because the continuously cast shape is very near to the final size and shape of the product.
Smaller H-beam product sizes are being made in beam-shaped copper tube molds while larger product sizes are made in four plate molds. The wideface coppers of a four plate beam blank mold are generally produced from very thick pieces of copper. In this case, drilled holes are the normal method used for cooling passages since slotting such a thick piece of copper would be impractical. The cooling passages of all molds are positioned such that they surround the perimeter of the cast product to remove heat from the liquid metal being poured into the mold. Thus the cooling passages surrounding the perimeter of a beam blank mold are very complex when compared to those of flat plate molds such as those that are used for blooms and slabs.
The thermal/mechanical dynamics of continuous casting molds, particularly near net shape molds, grow to be more complex with the shape of the mold cavity. Funnel molds are another type of near net shape casting mold with its own set of unique dynamics. Funnel molds have an enlarged pouring region and are generally four plate molds used for casting thin slabs. Thin slab molds need this funnel because the widefaces are brought very close together to form a thin slab measuring only two to three inches in thickness, as opposed to more conventional slabs that generally measure 6 to 12 inches in thickness. Since steel is generally poured into a continuous casting mold through a refractory tube called a submerged entry nozzle or SEN, the enlarged pouring region or funnel provides space for the SEN and the steel to enter the mold.
Thin slab casting has grown to be more widely used today because of the economics of rolling a thin slab into a coil of steel. The thin slab process also lends itself well to hot charging or going directly from the caster into the rolling mill without having to totally reheat the product. It further lends itself well to the mini-mill environment of electric arc furnace production as opposed to the iron-based oxygen furnace methods of the integrated steel producers. Thus, thin slab casting reduces energy consumption and is better for the environment, two important factors in today's world. In the United States, thin slab casting through funnel molds accounts for nearly 20 percent of the hot band coil production and is expected to continue growing into the future.
Funnel molds have very complex thermal/mechanical dynamics. Since the product being cast is thin, for example ⅕ the thickness of a normal slab, casting speed has to be increased by a factor of 5 to match the production tonnage capability of the thicker slab casting process. Along with this increase in casting speed comes an increase in the mold copper surface temperatures, which are very detrimental to the service life of the mold. This increase in temperature brings about a large amount of thermal expansion and deformation of the mold coppers, which limit their life as well. As a result of all of this, the maintenance cost of funnel molds is much higher than that of conventional, thick-slab casting molds.
To better understand the thermal profiles of a mold in continuous casting, researchers and machine operators have monitored the temperatures of the copper liners by instrumenting them with a series of thermocouples. They learned that the area just below the top of the liquid metal, and what is known in the industry as the meniscus area, is generally the hottest.
In continuous casting, molten metal comes into contact with the upper surface of the water-cooled mold in the meniscus area where it first surrenders heat. This transfer of heat begins the solidification process, forming the shell or outer skin of the cast product. As the solidifying shell travels downward through the mold and eventually through the containment area below the mold, it continues to relinquish heat and grows in thickness. This occurs at a rate equal to the condu
AG Industries, Inc.
Elve M. Alexandra
Knoble & Yoshida LLC
Tran Len
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