Continuous Casting Practices

 

STEELMAKING PROCESS

 

In general, the requirements to the steelmaking practices for continuously cast steels are higher than those to the ingot steels. Firstly, the tapping temperature should be higher and tolerance limit of temperature should be closer. Secondly, the steel must be more fully deoxidized.

 

Temperature control is more critical than in ingot production. The tapping temperature is generally higher to compensate for heat losses associated with the increased transfer time to a caster. On the other hand, the temperature must be maintained within closer limits. If the temperature is too high, there will be "breakouts"; and if the temperature is too low, premature freezing in the tundish nozzles will occur. Casting temperature can also affect the crystallization structure of the cast product. Optimum structures are developed with low superheats that should be uniform throughout the entire cast. One common practice employed to reach the uniform temperature is to stir the metal in the ladle by the injection of a small quantity of argon into the liquid steel.

 

Continuously cast steel is also required to be fully deoxidized (killed) to prevent the formation of blowholes or pinholes at or close to the surface of the cast product. Those blowholes or pinholes may cause seams in subsequent rolling operations. There are generally two practices for the deoxidation, depending on the grade of steel and product applications: (1) silicon deoxidation with a small addition of aluminum for coarse grain steels; and (2) aluminum deoxidation for fine grained steel. Silicon‑killed steels are easier to cast than aluminum killed steels. This is because deposits of alumina in the tundish nozzle, which cause nozzle blockage, are avoided. Normally, a ladle (or Ladle Metallurgical Furnace LMF) refining practice prior to casting is employed. In certain situation when still higher steel quality is expected, a vacuum degas (VDG) process is used. So the most common practice used in steel making plants is: EAF/BOF + LMF (+ VDG) + CAS.

 

LIQUID METAL FLOW CONTROL

 

Tundish application

 

In the continuous casting process, the liquid steel is transferred at first from a ladle into the tundish. Then the liquid steel is distributed into different strand of molds, through a nozzle for each strand. The tundish is essentially a rectangular box with nozzles located along the bottom. The tundish makes it possible:

 

         to reach stability of the metal streams entering the casting mold, and in turn, to achieve a constant casting speed.

         to cast a sequence of heats

         to change over the empty ladle for a full ladle without interrupting the flow of metal in the molds

         to make a mixed grade with steel from two different grade from different heats, if needed

         to provide possibility to prevent inclusions and slag from entering tundish and thus slipping into mold

 

Tundishes are usually preheated prior to casting to minimize heat losses from the liquid steel during the initial stages of casting and thus avoid metal freezing, particularly in the critical nozzle areas. Tundish covers are also used to reduce radiant heat losses throughout the casting operation. 

 

Liquid Metal Shrouding.

 

In open stream casting the liquid metal flows directly, through the air, from the ladle to the tundish or from the tundish to the mold. Under these conditions the unprotected metal stream picks up oxygen (and some nitrogen) from the air and deleterious inclusions are formed in the liquid steel. These inclusions are transferred into the casting mold where they are either retained within the cast section or float to the surface of the liquid steel. Those present on the liquid steel surface are subsequently trapped in, the solidifying shell and either result in surface defects on the product in rolling or a catastrophic break in the shell below the mold. In addition to the direct formation of inclusions in the exposed steel stream, air entrained in the stream can also react with liquid steel both in the mold and tundish.

 

To avoid these problems shrouded‑stream casting is employed. Emphasis was first placed on shrouding the metal stream between the tundish and mold because of severity of the problem. However, ladle to tundish stream shrouding is now widely employed, especially in slab casting of aluminum‑killed steels where the prevention of alumina inclusions is of paramount importance. There are two basic types of shrouding with numerous variations and combinations: (1) gas shrouding; and (2) refractory tube shrouding.

 

Gas shrouding is frequently used in casting small sections on billet machines (i.e. 4‑in. sq.) because of operating difficulties experienced with refractory tubes: there is insufficient space to introduce a tube without encountering metal freezing between the mold wall and tube.

 

There is a variety of designs including: the Pollard steel tube shroud in which gas is introduced at the mid point of the tube at low velocity and exits between the tube and nozzle, and between the tube and mold; complete enclosure between the tundish and mold using a flexible coupling; truncated pyramidal enclosures; and a liquid nitrogen curtain (Fig. 1). Nitrogen or argon is used as the protection gas. Gas shrouding alone is not commonly used for preventing oxidation of the ladle to tundish stream. However, one design in use employs a circular ring which is attached to the ladle at one end and is sealed by a sand seal at the other end when the ladle is lowered toward the tundish thus forming an enclosed box: the box is then pressurized with argon.

 

Refractory tube shrouds are commonly used for casting aluminum‑killed steel. They are used both between the ladle and tundish, and tundish and mold. One end of the tube is attached to the ladle (or tundish) with the other end immersed in the steel when the tundish for mold) is filled with metal. Refractory tubes are usually made of fused silica or alumina graphite.

 

 

 

Fig. 1: Configurations for shrouding from ladle to mold [1].

 

The mechanical design of the refractory tube is important, especially at the exit end that is immersed in the steel. One type is a straight‑through design. Another type, generally used in the mold, has a multi‑port (opening) design, such as a bifurcated tube with the bottom of the tube closed and two side openings located near the bottom of the tube. This type of shroud avoids deep penetration of the pouring stream into the creator of the solidifying strand and modifies the flow pattern in the mold. Thus, the inclusions in the pouring stream are not entrapped in the solidifying section but rise to the surface of the liquid metal and are removed with the slag formed by the mold powder.

 

In many plants, the design of the shroud attachment includes the capability for replacing a worn shroud so that along sequence of heats can be cast without interruption.

 

At some plants, argon is introduced into the refractory tube to avoid aspiration of air through pores and joints that is caused by the venturi effect of a moving metal stream.

 

MOLD AND HEAT TRANSFER

 

The primary function of the mold system is to contain and start solidification of the liquid steel to achieve the following goals:

         shape (overall configuration and shell thickness)

         temperature distribution

         internal and surface quality (i.e. structure, chemical uniformity together with an absence of cracks, porosity and non‑metallic inclusions)

 

One of the most important features of the mold is its heat transfer ability. A mold is constructed as a box structure that contains an inner lining fabricated from a copper alloy that serves as the interface with the steel being cast. There are small water passages between the inner liner and supporting structure for the mold cooling water that absorbs heat from the solidifying steel in contact with the liner. Mold lubrication permits greater heat transfer at the upper part of the mold. Another factor influencing heat transfer at this mold surface is the mold taper, which tends to increase heat transfer because it opposes the air gap formation between the steel shell and the mold surface. Details on heat transfer between the mold and steel will be discussed later.

 

Besides the heat transfer ability, the high-temperature strength and resistant against mold wear and mold deformation is also extremely important. Although the material of construction of the inner lining is usually a high purity cold‑rolled copper, copper with small amounts of silver is commonly used to obtain increased elevated‑temperature strength. The working surface of the liner is often plated with chromium or nickel to provide a harder working surface and also to avoid copper pickup on the surface of the cast strand.

 

SECONDARY COOLING, STRAND CONTAINMENT AND WITHDRAWAL

 

In modern slab casting machines, secondary cooling, strand containment and withdrawal form a closely integrated and interlocked system that also includes strand bending and straightening. In the older designs of billet and bloom casting machines, there was a greater functional as well as physical separation of the components of this part of the casting operation. For the purposes of this discussion the concepts employed in the design and operation of modern slab casting machines will be considered.

 

Secondary cooling and the containment / withdrawal system extends from the bottom of the mold through complete solidification of the strand to the cutoff operations. The system is designed to produce a final cast section that has the proper shape, and internal and surface quality. To accomplish these results the solidifying section leaving the mold is cooled in a series of spray zones and contained and withdrawn by a series of roll assemblies until the solidified cast section reaches the cut‑off machine and horizontal runout table

 

Secondary Cooling ‑The secondary cooling system is normally divided into a series of zones to control the cooling rate as the strand progresses through the machine. This system, conventionally, consists of water sprays that are directed at the strand surface through openings between the containment rolls. Recently, air-water "mist" sprays (discussed later) have been employed which provide more uniform cooling.

 

Heat Transfer in Secondary Cooling‑The main heat‑transfer functions of the spray‑water system are to provide:

• The proper amount of water to obtain complete solidification under the constraints of the casting operation, i.e., steel grade, casting speed, etc.
• The capability to regulate the thermal conditions of the strand from below the mold to the cut‑off operation, i.e. strand surface temperature and thermal gradients in the strand.
• Auxiliary functions such as cooling of the containment rolls.

 

It is necessary to control both the temperature levels and thermal gradients in the strand to avoid the occurrence of surface and internal defects such as improper shape and cracks. At high temperature, the strength properties of the steel shell play a critical role in the ability of the shell to withstand the external and internal forces that are imposed by the casting operation. The primary forces are those exerted by the ferrostatic pressure of the liquid core and the traction of the withdrawal operation. In particular, the ductility of steel close to the solidus temperature is low and the shell is susceptible to crack formation‑ It is important to control temperature gradients because thermal strains can be caused which exceed the strength of the steel resulting in cracks. Excessive thermal strains result from changes in the heat‑extraction rate by either over‑ or under‑cooling. The latter conditions can occur by reheating, which is produced when spray cooling is terminated improperly and the strand reheats by heat transfer from the interior with an increase in temperature before decaying by radiation heat transfer to the environment. Under these conditions, excessive strains and cracks can result. This effect can be reduced by extending and varying the water‑spray cooling operation to provide a smooth transition with the radiation cooling area.

 

Thus, in the design of a secondary cooling system, the thermal conditions along the strand must be established which satisfy the product integrity and quality. For example, the surface temperatures along the strand are specified. They are generally in the range of 1200 to 700C (2190 to 1290F). Based on this information the cooling rates along the strand are determined from heat‑transfer equations. Important parameters in these calculations include the convection heat transfer coefficient of the water sprays and the water flux (the amount of water per unit area of surface contact). The type of spray nozzle, nozzle position with respect to the strand surface, number of nozzles and water pressure are selected to provide the required water flux and distribution throughout the secondary cooling sector. Multiple nozzles are typically used at each level along the strand that has an overlapping pattern.

 

Generally a series of cooling zones is established along the strand, each of which has the same nozzle configurations and heat‑transfer characteristics. Since the required cooling rates decrease along the length of the strand, its water flux in successive zones decreases.

 

During operation, changes in the water flux are made to compensate for changes in casting conditions such as casting speed, strand surface temperature, cooling‑water temperature and steel grade.

 

The spray‑water system is typically a recirculating system.

 

Strand Containment ‑The strand is contained by a series of retaining rolls which extend across its two opposite faces of the cast sections in a horizontal direction: edge rolls may also be positioned across the other pair of faces in a direction perpendicular to the casting direction to further enhance containment. The basic functions of the mechanical strand containment and withdrawal equipment, which forms an integral part of its secondary cooling system, are: (1) to support and guide the strand from the mold exit to the cut‑off operations; and (2) to drive the strand at a controlled speed through the caster. In both of these functions, the final objective is to minimize the mechanical stress and strains incurred during the process.

 

For illustrative purposes, a casting machine design that consists of a vertical discharge from the mold to horizontal delivery prior to the cut‑off operations is discussed. In this typical case there is a series of rolls or guides arranged vertically below the mold, followed by a series of rolls arranged in a curve (which provides a transition to the horizontal) and a series of rolls in a horizontal plane before the cut‑off equipment. Each series of rolls may be segmented and contain different diameter rolls and roll spacings to meet the conditions existing at that location.

 

Strand support involves the restraint of the solidifying steel shape that consists of a solid steel shell with a liquid core. The ferrostatic pressure, created by the height of liquid steel present, tends to bulge the steel especially in the upper levels just below the mold where the solidified shell thickness is small (Fig. 2). Bulging at this location would not only cause product defects such as internal cracks but also cause a skin rupture and a breakout. Bulging is controlled by an appropriate roll spacing that, in general, is closest just below the mold and progressively increases in the lower levels of the machine as the skin thickness increases. All four faces of the strand are usually supported below the mold with only two faces supported at the lower levels. In addition to ferrostatic pressure and skin thickness, roll spacing is also based on strand surface temperature and the grade of steel cast.

 

 

Fig. 2. Stresses in the solidifying skin due to ferrostatic pressure [1].

 

Strand Bending and Straightening‑In addition to contain the strand, the series of rolls that guide the strand through a prescribed arc from the vertical to the horizontal plane must be strong enough to withstand the bending reaction forces. During bending, the outer radius of the solid shell is placed in tension and the inner radius in compression. The resulting strain, which is a function of the radius of the arc and the strength of the particular grade of steel being cast, can be critical; excessive strain in the outer radius will result in metal failure and surface defects (cracks). To minimize the occurrence of surface defects but, at the same time, maintain a minimum effective arc radius, triple‑point bending has recently been introduced (i.e., three arcs, with progressively smaller radii).

 

A multi‑roll straightner is installed following the completion of bending which, as the name implies, straightens the strand and completes the transition from the vertical to horizontal phase. During straightening the strand is "unbent" which reverses the tension and compression forces in the horizontal faces of the strand.

 

Strand Withdrawal ‑The strand is drawn through the different parts of the casting machine by drive rolls which can be located in the vertical, curved and horizontal roll sections. This multiple drive‑roll system is designed, wherever possible, to produce compression forces in the surface of the strand to enhance the surface quality. Thus, the objective is to "push" the strand through the casting machine, as opposed to "pilling" the strand with the attendant tensile stresses that tend to produce surface defects. In addition, the use of multiple sets of drive rolls distributes the required traction force along the length of the strand and consequently reduces the deleterious effects of tensile forces. The proper placing of drive rolls can also reduce adverse bending and straightening strains by exerting an offsetting compression force, i.e., by placing drive rolls before a set of bending rolls. In all cases, the pressure exerted by the drive rolls to grip the strand must not be excessive; excessive pressure will deform the shape of the section being cast.

Following straightening, the strand is conveyed on roller tables to a cut off machine where the section is cut to the desired length. There are two types of cut‑off machines: oxygen torches and mechanical shears. Oxygen torches are employed for large sections such as slabs and blooms. Billets are either cut by torches or shears. The cast product is then either grouped or transported directly to the finishing mills or, in the case of billets, to cooling beds which are predominantly of the walking beam type to maintain product straightness.

 

 

References

[1] The making, Shaping and Treating of Steel, 1985, US Steel.

[2] The making, Shaping and Treating of Steel, 2002, AISE Steel Foundation.