Continuous Casting

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.

Introduction

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.

ContiSchema.png

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.

 
obr9.gif

Methodology for cooling unit design used in the Heat Transfer and Fluid Flow Laboratory


Experiment

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.

Experimental procedure:

  • 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.

SchemaConti3.jpg

Diagram of the experiment for continuous casting secondary cooling design     

 

obr11.jpg    obr12b.jpg

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


Data Processing

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.

Picture6.jpg

The graphical representation of HTC = f(x,T=const.) with a photo of the experiment corresponding to the HTC calculation. The arrow indicates the slab’s direction of movement

                

obr16b.png
The graph shows the heat transfer coefficient HTC depending on position of a slab in the direction of movement in the secondary cooling section. The position "0" is directly under the nozzle; each curve represents one distance from the slab axis
The graph of the heat transfer coefficient HTC depending on surface temperature provides information about the homogeneity provided by a specific nozzle; each curve represents one distance from the slab axis
 
obr19.jpg

The resultant heat transfer coefficients are compared for various conditions to design continuous casting secondary cooling; the graph indicates the heat transfer coefficient for different air and water flows when a water/air nozzle is used

            



Impact Pressure Measurement

To obtain impact pressure distribution on the sprayed surface, a test bench was developed in the Heat Transfer and Fluid Flow Laboratory.

             

Picture7.jpg
 
The test bench designed for impact pressure distribution measurement - diagram and photo
           

obr22.jpg

Impact pressure distribution for two overlapping nozzles (the photo is in the figure above)

             




Water Distribution

Water distribution is graphically expressed by SimSpray, software developed in the Heat Transfer and Fluid Laboratory.

obr24.png
obr25.png
 

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


Special Cases

           

Picture9.jpg
 

The effect of support rollers on the heat transfer as well as the effect of hot slab on support rollers during continuous casting secondary cooling was investigated

            

Picture8.jpg

  

Experiments conducted for investigation of the effect of slab shape on heat transfer

 

           



Additional Information

can be found in Publications of the Heat Transfer and Fluid Laboratory.