The demand for improved product quality and increased productivity has focused attention on the need for more efficient systems of spray cooling during continuous casting. Nozzle characteristics must be investigated and test procedures developed to measure cooling patterns and heat transfer. Lechler has developed new nozzles with higher heat transfer coefficients, better water distribution performances and larger free passages.
There are also important operational benefits such as:
Billet & Bloom Casters
Continuous casting of billets and especially in the case of blooms the secondary cooling system has an important role to play in product quality and the productivity of the machine.
It is common in most billet casting machines that full cone nozzles are installed in the roll gaps below the foot rolls where one nozzle on each side covers the total strand width so that only four nozzles are needed to cover the entire strand surface at one roll gap level.
The particular nozzle flow rate depends on the location in the machine. Larger flow nozzles are used in the upper zones with decreasing nozzle flows further down in the machine. The spray angle depends on the maximum and minimum billet size, the support roll diameter, the distance between the support rolls and the design of the water pipe installation. Spray angles of 45° and 60° are most common. The nozzles are either installed on spray rings or on spray loops. In the case of spray rings, one ring carrying four nozzles encircling the billet completely. One spray ring is installed per rolle gap. In the case of a spray loop, a water supply pipe runs along each side of the billet strand and follows the casting radius. Along the pipe loop one nozzle is always installed between two support rolls.
For ordinary steel grades, such as for rebar, single fluid full cone nozzles are most common. These nozzles operate on water pressure only. For more sophisticated steel grades such as stainless steel, special spring and tool grades or steels for tire cord, air mist nozzle have to be applied. The same goes for every state of the art bloom and beam blank caster. The casting of round blooms requires air mist secondary cooling. 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 a prerequisite of a product mix covering a wide range of steel grades.
Continuous casting machines are now required to cast a wide range of steel grades, particular slab casters must cast steels ranging from ULC and low carbon grades to high carbon and high quality pipeline grades. This must be achieved while maximizing production output.
Consistent production of prime quality product requires increased operational and maintenance flexibility of the caster 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.
A good nozzle layout is paramount in fulfilling operational and production requirements.
It is essential that nozzle arrangements produce an even heat removal across the strand while maintaining a stable spray pattern. Also in continuous casting of slabs the secondary cooling system has an important influence on the quality of the casted products. There are two general cooling methods in use.
1. Conventional single fluid cooling using water nozzles only
This method is mainly applied for the first nozzle row below the mold and first strand support roll on either side. Single fluid (water only) standard flat jet nozzles, in some cases with increased spray thickness are being used. Also for narrow side cooling below the mold either single fluid flat jet or full cone nozzles are common in slab casters.
In certain thin slab casters (CSP) single fluid flat jet and full cone nozzles have become the standard; however the water pressure needs to be increased to 30bars in some cases in order to achieve the required turn down / control ration for the various operation conditions.
2. Air mist cooling (Twin fluid cooling)
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 a prerequisite of a product mix covering a wide range of steel grades.
Air mist nozzles also offer much larger internal free passage compared to single fluid nozzles of the same flow rate size. There is little doubt that air mist nozzles have become the first choice of plant designers.
In recent years, Lechler has made major advances in spray cooling system analysis. As a result, Lechler has enlarged the scope of our offering to complete audits including solidification modeling of current secondary cooling systems to identify problems and "weak links" of the existing configuration. From that we can design a new system customized around specific objectives and new requirements:
Lechler is the only spray nozzle manufacturer who offers such a wide range of capabilities.
Heat transfer coefficient measurement
The Heat Transfer Coefficient (HTC) of spray nozzles is decisive for the design of a secondary cooling system. Lechler measures the HTC by means of the “Moving Nozzle Test”, in which a steel plate is heated to 1200 °C in inert gas and then cooled down to the temperature of the sprayed water. The objective is to simulate spray cooling of a moving steel strand.
For larger nozzles with a higher cooling capacity required for HARD-HARD® INTENSIVE COOLING Lechler utilizes the liquid core HTC measurement method.
Water and impact distribution
It is the spray nozzle manufactures task to design nozzles providing the desired water distribution over the entire turn down ratio. In a slab caster the uniformity of the water distribution across the entire strand surface is essential for good quality slab. In billet- and bloom casters, over spraying and hence over cooling of corners is to be avoided. The standard method to measure water distri¬bution especially in multi nozzle arrangements is the paternator ,which measures the water density. There are factors other than the water density which can influence cooling, such as, the air-water ratio, the turn down ratio and the pattern of the spray footprint of the air-mist nozzles.
Spray patterns can be measured more accurately with a new impact measurement device. This allows measuring the spray thickness and width simultaneously with an impact sensor that scans through the entire spray.
Nozzles for every caster
From simple billet casters for rebar to sophisticated machines for tire cord grades or for casters for beam blanks and round blooms, Lechler offers the optimal nozzle solutions both for water only cooling or air mist systems. The same goes for thick or thin slab casters. Solutions for HARD-HARD® INTENSIVE COOLING and SOFT REDUCTION applications are available, too.
Modern slab continuous casting plants are in most cases fitted with specially designed and customized air mist nozzles of the Master cooler type. This applies to both conventional thick slab plants and the more compact thin slab plants.
A defect free slab, bloom, billet or beam blank and an economical operation are the objectives to be achieved by the spray nozzle cooling system. The nozzle manufacturer must have detailed knowledge of the behavior of the nozzles under operating conditions and the machine segment design.
Of particular concern are: