Heat Transfer during Continuous Casting of Steel Jun19

Heat Transfer during Continuous Casting of Steel...

Heat Transfer during Continuous Casting of Steel During continuous casting (CC) of steel, careful control of the strand cooling and growth of the shell along the CC machine is of great importance. These factors play an important role on the formation of cracks and other defects which can be formed in the cast steel product. To ensure defect free cast steel products, the strand is to be cooled down according to a pattern which depends on steel grade, steel product dimensions, casting speed, and CC machine design. On the other hand, the control of the liquid pool length is a key element in optimizing the casting speed with respect to good productivity. So, the heat transfer plays a very important role in the operation of the CC machines, especially when casting crack sensitive steel grades. In fact, many diverse phenomena simultaneously control the complex sequence of events which govern heat transfer in the CC process. The temperature of liquid steel in the tundish is typically 30 deg C to 50 deg C higher than the liquidus temperature. At the beginning of the continuous casting process, the liquid steel is cooled in a water cooled mould to form a solidified shell which can support the liquid pool at the mould exit. Typical temperature at the end of the mould is1100 deg C and that in the center of the strand is 1550 deg C. Since steel does not solidify at a fixed temperature, but over a range of temperature, there is a mushy region where the steel is neither completely solid nor completely liquid. Steel grades used in continuous casting are completely solid below a temperature ranging from 1350 deg C to 1500 deg C, and the solidification starts at a temperature ranging from 1450...

Continuous Casting Machine and its Equipment Jun13

Continuous Casting Machine and its Equipment...

Continuous Casting Machine and its Equipment Continuous casting (CC) is a method 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 and hot rolling. A typical section and plan view of a CC machine is shown in Fig 1. Fig 1 Typical section and plan view of a CC machine Types of continuous casting machines CC machines have evolved from the strictly vertical type of machine to curved machines in order to limit the installation height while still using high casting speeds. 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 given below. Simple vertical CC machine with a straight mould and cutoff in the vertical position Vertical CC machine with a straight mould along with single point bending and straightening Vertical CC machine with a straight mould along with progressive bending and straightening Bow type machine with curved mould and straightening 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. The first CC machine which was built up for CC of liquid steel was a simple vertical CC machine. Then later the development led to many kinds of CC machines with various ways of bending and straightening. The main objective for these developments have...

Understanding the Process of Continuous Casting of Steel Jun07

Understanding the Process of Continuous Casting of Steel...

Understanding the Process of Continuous Casting of Steel Continuous casting (CC) of steel is a process whereby liquid steel is solidified into a semi-finished steel product (billet, bloom, beam- blank, round or slab) for subsequent rolling in the rolling mills. The basic operation of a CC machine is to convert liquid steel of a given composition into a strand of desired shape and size through a group of operations like mould operation, spray cooling zone, straightener operation, etc. For successful continuous casting, it is necessary to understand the process behaviour under different conditions for these operations. The process of continuous casting basically comprised of the following sections. A tundish, located above the mould, to receive the liquid steel from steel teeming ladle and to feed it to the mould at a regulated rate. A primary cooling zone consisting of water cooled copper mould through which the liquid steel is fed from the tundish for generating a solidified outer steel shell sufficiently strong enough to maintain the strand shape as it passes into the secondary cooling zone. A secondary cooling zone in association with a containment section positioned below the mould, through which 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. A section for the unbending and straightening of steel strand. This section is not there in the straight vertical casting machines. A cutting section consisting of cutting torches or mechanical shears for the cutting of the solidified steel strands into desired lengths for removal. A run out table to cooling beds or directly to a product transfer area. In the CC process, liquid steel flows from the steel teeming ladle, through a tundish into...

Secondary Cooling Technology in Continuous Casting Process Jul31

Secondary Cooling Technology in Continuous Casting Process...

Secondary Cooling Technology in Continuous Casting Process  A wide range of steel grades ranging from ultra low carbon (ULC) and low carbon grades to high carbon and different grades of special steels are required to be cast in continuous casting machine (CCM). The casting of these grades is to be achieved while maximizing CCM output. Consistent production of prime quality cast steel product requires increased operational and maintenance flexibility in the CCM for maintaining optimum casting parameters can be maintained. This flexibility is needed both for every element as well as control system of CCM. While the strand is continuously withdrawn at the casting speed, solidification of steel continues beneath the mould through the different zones of cooling having a series of water sprays. The secondary cooling system consists of these different zones, each responsible for a segment of controlled cooling of the solidifying strand as it progresses through the CCM. The sprayed medium is either water or a combination of air and water (mist spray cooling). Mist spray cooling provides the following advantages. Uniform cooling Less water requirement Reduced surface cracking Product quality in a CCM is considerably influenced by temperature variations during strand cooling in secondary cooling zone. Hence secondary cooling zone has a very important function for the maintenance of a correct temperature parameter and is crucial to the quality of the cast steel product. Since the quality of steel depends on the behavior of the surface temperature and the solidification of steel front in time, it is to a large extent defined by the intensity of the water sprays. Improper cooling conditions can have detrimental impact on stress distribution in solidified shell. First of all, overcooling can lead to the formation of cracks. Moreover, there must be a smooth transition...

Air mist cooling in continuous casting Apr20

Air mist cooling in continuous casting...

Air mist cooling in continuous casting  Continuous casting machines are now required to cast a wide range of steel grades which are ranging from ultra low carbon and low carbon grades to high carbon and high quality pipeline grades. Consistent production of prime quality of these products require increased operational and maintenance flexibility of a casting machine so that the optimum casting parameters can be maintained for each steel grade. This flexibility extends not only to the machine elements and control systems, but also to the secondary cooling system and demands more efficient and reliable spray cooling. Cooling by water plays an important role in extracting heat from both the mold and solidifying liquid steel during the continuous casting of steel. It is characterized by complex boiling phenomena. Heat extraction rates during water cooling, which have strong dependence on the metal surface temperature and it can rapidly change with time as the strand cools down. Hence uncontrolled cooling may cause fluctuations in the temperature gradients inside the solidifying shell of steel and generate tensile thermal stresses at the solidification front that can ultimately lead to the appearance of hot tears/cracks in the final product. The necessity of having a sound quality of cast steel product and increased productivity of continuous casting machine has focused attention on the need for more efficient systems of secondary cooling during continuous casting of liquid steel. Air mist cooling (AMC) in secondary cooling zone of continuous casting machine is a step in this direction.  Air mist nozzles utilize compressed air in combination with water pressure to atomize secondary cooling water. This provides a much wider turn down / control ratio which is necessary in case a product mix covers a wide range of steel grades. Air mist nozzles also...