High speed Casting

High speed casting of slab increases the productivity. Average casting speed in the conventional slab casters is on average 2 m/min. Now if we want to increase the casting speed we have to consider design and operational features of tundish, mold and secondary cooling zone. This is illustrated by the following example;

Consider a conventional 2 strand caster casting slab of cross section 280mmx1950mm at 2 m/min casting speed. Molten steel  is fed from  a tundish of 70 tons capacity by submerged entry nozzles  in both the molds of the continuous casting machine.

Volume of slab /minute   = 1.092 m³/minute. Assuming density of liquid steel as 7 tons/m³ steel flow rate is 7.6 tons/minute/strand.

Now if we increase casting speed to 3 m/min., the required steel flow rate would be 11.4 tons/min/strand, and   which will increase to 15.2 tons/min/strand for 4 m/min casting speed. Now let us calculate the average residence time of molten steel flowing in the tundish assuming 70 tons tundish will be used even for high speed casting

We note that the average residence time of  steel melt in the tundish decreases with the increase in the casting speed. Thus, chances of inclusion floatation will be very low at higher casting speeds. Thus, the following modifications may be considered;

In one modification tundish capacity needs to be increased, if tundish is to be used to remove inclusions during the process of continuous casting. Thus, a change in tundish design would be required. In other modification we improve the upstream steelmaking facility so that inclusion content in steel are within the tolerance limit. One may consider other alternatives depending on the available resources.

We have to consider further as to what will happen in the mold? Increase in steel flow rate will increase the steel velocity in the mould since the mold length is not being changed. One requires to consider the cross section of the submerged entry nozzle too. The higher steel flow rate in the mold would require intense cooling in the mold. The steel flow in the mold will be more turbulent. Also the mold powder consumption may increase. We would be requiring mold flux whose melting rate is relatively higher to keep pace with the casting speed.

Similarly water spray in the secondary cooling zone has to be modified in view of the increase in the casting speed.  Uniform cooling of the strand is the prerequisite for the success of the caster.