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Continuous Casting Machines and their Types


Continuous Casting Machines and their Types

Continuous casting (CC) is a process of producing an infinite solid strand from liquid steel by continuously solidifying it as it moves through a CC machine. It is the predominant process route in a modern steel plant which links steelmaking with hot rolling of steel. In the CC machine, the liquid steel is solidified into a semi-finished steel product for subsequent rolling in the hot rolling mill. The basic operation of the CC machine is to convert liquid steel of a given composition into a strand of desired shape and size through a group of operations.

Continuous casting is distinguished from other solidification processes by its ‘steady state’ appearance. That is, the liquid steel freezes against the mould walls and is withdrawn from the bottom of the mould at a rate which keeps the solid / liquid interface at a constant position with time, relative to an outside observer. Major types of continuous casting machines are given in Fig 1.

Fig 1 Major types of continuous casting machines

In the CC machine as shown in Fig 1, liquid steel flows from a ladle, through a tundish into the water cooled mould made of copper.  Liquid steel is protected from exposure to air by a slag cover over each vessel and by ceramic nozzles between vessels. Once in the mould, the liquid steel solidifies against the mould walls to form a solid shell. Drive rolls lower in the CC machine continuously withdraw the shell from the mould at a rate or ‘casting speed’ which matches the flow of incoming liquid metal, so the process ideally runs in steady state. Below mould exit, the solidifying steel shell acts as a container to support the remaining liquid. Rolls support the steel to minimize bulging due to the ferrostatic pressure. Water and air mist sprays in secondary cooling zone cool the surface of the strand between rolls to maintain its surface temperature until the liquid core is solid. After the centre is completely solid (at the ‘metallurgical length’) the strand is cut into desired length of the cast product.

Cross-sections of the cast product can be rectangular, for later rolling into plate or sheet, square for rolling into long products (rounds, rebars, angles, channels, tees, and beams etc.) , circular for wire and seamless pipes, ‘dog-bone’ or ‘beam blank’ shapes for ‘I’ or ‘H’ beams, and even thin strip, rod, and other products.

In the design of continuous casting machines the important considerations are (i) end use product which influences the quality, grade and shape of the cast product (bloom, billet, round, slab, thin slab, or beam blank), (ii) annual tonnage to be cast, (iii) availability of liquid steel and heat size, and (iv) planned operating hours.

The above factors dictate the continuous machine design parameters such as the number of cast strands and casting speed which is required to match the supply of the liquid steel to the continuous casting machine. Quality and grade of the steel to be cast are utilized in determining various design parameters of the CC machine such as its length, vertical height, curved or straight mould, water versus air mist cooling, and electromagnetic stirring etc.

The main equipments of a CC machine constitute (i) ladle turret along with turret weighing system and ladle cover manipulator, (ii) tundish and tundish car along with tundish weighing system, tundish preheater and dryer, (iii) primary cooling zone consisting of copper mould and mould oscillation along with mould level control and electromagnetic stirrer, (iv) secondary cooling zone consisting of strand cooling, strand containment and guiding, (v) withdrawal and straightening section, (vi) dummy bar, dummy bar parking and dummy bar disconnect roll unit, (vii) pinch roll and torch cut off unit or mechanical shears, (viii) product identification system, and (ix) roller , cooling bed,  and product discharge system.

The tundish, located above the mould, receives the liquid steel from steel teeming ladle and feeds it to the mould at a regulated rate. In the primary cooling zone, the solidification of liquid steel starts and a solidified outer steel shell is generated which is sufficiently strong enough to maintain the strand shape as it passes into the secondary cooling zone. In the secondary cooling zone, in association with a containment section the steel strand (still mostly liquid) passes and is sprayed with water or a mix of water and air (air mist) for further solidifying of the steel strand. In the withdrawal and straightening section, the unbending and straightening of steel strand is carried out. This section is not there in the straight vertical casting machines. Dummy bar is used for the start-up of the CC machine. The pinch roll and the cutting section are for the cutting of the solidified steel strands into desired lengths for their removal from the CC machine. The run out table is for moving the cast steel product to the cooling beds or directly to a product transfer area.

To start the process of the continuous casting, a dummy bar (which is connected to an external mechanical withdrawal system) is inserted into the mould and positioned so that the top of the dummy bar closes the bottom of the mould. Liquid steel in a steel teeming ladle is delivered to the casting floor where it is poured at a controlled rate into the tundish. Liquid steel flows through nozzles in the bottom of the tundish and fills the mould. When the liquid steel level in the mould reaches a predetermined position, withdrawal of the dummy bar is initiated. Once in the mould, the liquid steel solidifies against the walls of the water cooled copper mould to form a solid shell. Drive rolls lower in the CC machine continuously withdraw the dummy bar from the mould. The withdrawal speed of the dummy bar is preset based on the casting speed required or the liquid steel flow rate from the tundish. When the dummy bar head, which is now attached to the solidified shape being cast, reaches a certain position in the withdrawal system, it is mechanically disconnected and the dummy bar is removed. The solidified cast shape continues through the withdrawal system to the cutting equipment at a rate (casting speed) which matches the flow of incoming liquid steel, so the process ideally runs in steady state.

CC machines are identified by the shape of the product they casts. Accordingly, the CC machines can be a billet casting machine, a bloom casting machine, a near net shape, beam blank, or dog bone casting machine, a round casting machine, a slab casting machine, a thin slab casting machine, or a strip casting machine.

CC machines can be a single strand machine or a multi-strands machine. In case of multi-strand CC machines, the number of maximum strands depends on the type of machines. In case of billet casting machines, the number of maximum strands can be upto 8 numbers or can be even higher while in case of bloom casting machines number of maximum strands can be upto 6 numbers. In case of near net shape, beam blank, or dog bone casting machine and round casting machine normally number of maximum strands are 4 in numbers. In case of flat product casting machines (slab CC machine or thin slab CC machine), the casting machines are either single strand or twin strands. Strip casting machines are usually of single strand.

In the course of the development of the continuous casting machines, several types of casters have been realized with significant difference in design heights (Fig 1). These types are (i) vertical casting machines, (ii) vertical straight mould, bent run out with straightening of the solid products, (iii) vertical curved mould, bent run out with straightening of partial liquid product, (iv) curved mould with horizontal run out, (v) curved mould with gradual straightening, and (vi) horizontal CC machines. Some of these types were having restricted caster productivity either due to limited support length (vertical casters) or due to casting speed limited by high mould friction (horizontal casters). In case of strip casting machines, the casting is done in twin rolls. Besides, there were also characteristics differences with respect to the product quality.

Fig 1 Types of continuous casting machines

The first CC machine which was built up for CC of liquid steel was a simple vertical CC machine. The vertical casting machine has the natural machine design, casting with gravity and also assuring a symmetric macrostructure. However, the productivity vertical CC machine is severely limited by machine height. The evolution of CC machines from the strictly vertical type of machine to curved machines has taken place in order to limit the installation height while still using high casting speeds.

This later development has led to many kinds of CC machines with various ways of bending and straightening. The main objective for these developments have been to construct lower and simpler CC machines with smaller need for space, lower investment costs, and high flexibility in production and maintenance. One of the main issues with a vertical CC machine is that the distance between the mould and the point of cutting is limited. Due to this the casting speed is low and low speed means low production rate. The advantage of the vertical CC machine is that there is no bending or straightening of the strand. In the case of large strand sizes, the stress caused by the ferrostatic pressure of the liquid steel inside the strand can lead to bulging of the solidified strand shell.

In recent years, CC machines of more sophisticated mechanical design are being constructed. These machines apply several techniques for achieving higher casting speeds and higher outputs and are with progressive straightening or progressive bending over a liquid core. The main types of the CC machines which are in operation these days are (i) simple vertical CC machine with a straight mould and cutoff in the vertical position, (ii) vertical CC machine with a straight mould along with single point bending and straightening, (iii) vertical CC machine with a straight mould along with progressive bending and straightening, (iv) bow type machine with curved mould and straightening, and (v) bow type machine with curved mould and progressive straightening.

In all cases, the bending and straightening is usually carried out in one or several steps. Multistep bending and straightening reduces the mechanical stresses and reduces the risk of strand cracking.

 

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