Being aware of all requirements the Heat Transfer and Fluid Flow Laboratory developed experimentally based methodology to design secondary cooling units for continuous casting. Secondary cooling during continuous casting has a major influence on the quality of the cast product. Secondary cooling which is too intense hinders internal layer shrinking and the conditions for crack initiation arise. A basic demand for the secondary cooling system is an appropriate cooling intensity with regard to casting speed and sufficient homogeneity across the slab width.
Continuous casting is the most effective method to mass-produce of high-quality metal products (steel, aluminum, copper, nickel …) in a variety of sizes and shapes. The molten metal solidifies when passing a water-cooled crystallizer (primary cooling) and secondary cooling section where the metal is leaded by support rollers toward to tertiary cooling section. After complete solidification, the metal is rolled.
Diagrams of secondary cooling section of continuous casting
To design secondary cooling unit, the heat transfer coefficient at the slab surface in dependence on position and surface temperature needs to be known for variable conditions. There is no analytical solution; the problem has to be solved experimentally. In the Heat Transfer and Fluid Flow the sets of experiments are conducted to investigate influence of parameters as nozzle type, water/air pressure, geometrical configuration and casting speed on heat transfer coefficient. The heat transfer coefficient is then used in numerical model (e.g. ProCast). The resultant slab temperature history belonging to the cooling unit is possible to provide together with impact forces caused by spray impingement. When required, the radiation and natural convection is subtracted.
The slab in the secondary cooling section moves through the areas without cooling (support roller placement) and alternately through the areas covered by the nozzles. This process is simulated in the laboratory by the relative movement of the tested specimen and the spraying nozzles. The specimen for the test is made of an austenitic plate (600x320x30 mm) with a set of thermocouples inset.
- An electric furnace heats the test plate to the initial temperature for the experiment (1200°C).
- The water/air pressures in the nozzles covered by deflector are adjusted.
- When the furnace is put aside, the deflector is opened and the spraying nozzles start to move under the test plate according to requirements.
- The temperatures are recorded by thermocouples and saved to a data logger located outside of the spray box in a control room.
The test plate, with insert thermocouples, is shown in the left picture; the initial stage of the experiment (the furnace is moving to the right, and pressure has been set) is shown in the right picture; the red arrow indicates the direction of nozzle movement
The heat transfer coefficient (as well as the heat flux) is calculated using the IHCP (inverse heat conduction problem) from temperatures measured by thermocouples. The IHCP solution does not give the heat transfer coefficient as a continuous function but as a set of discrete values. The objective of the final data processing is to obtain the heat transfer coefficient as a function of position x at a slab and of surface temperature T; HTC = f(x,T). The function approximation used is given by a formula that contains a Gaussian function and coefficients. The coefficients are calculated from measured values of HTC.
To obtain impact pressure distribution on the sprayed surface, a test bench was developed in the Heat Transfer and Fluid Flow Laboratory.
Water distribution is graphically expressed by SimSpray, software developed in the Heat Transfer and Fluid Laboratory.
A graphical representation of water distribution on the slab. The red color is for the highest water flow [l/min∙m2], and the blue color is for the lowest water flow