page 3-3
Ionising radiations

Graphic of continuous casting
Picture 6. A continuous casting mill. The mould shapes the molten steel before it is cooled in the rollers.
Measuring steel level in a mould
Where is the mould?
A radioactive source is used to monitor the flow rate of steel in a continuous casting mould. The mould can produce long strips of steel (called slabs, blooms or billets). Molten steel, fresh from a furnace, is poured in at one end and cools and solidifies as it passes through the mould. The solid steel is cooled between rollers and cut into sections at the end. The process is, as the name suggests, continuous. Whilst the casting is in progress, the molten steel is poured into the top of the mould so as to keep a constant flow out of the bottom.

You can find out more about continuous casting in the Chemistry of steelmaking e-source.

It is important that the flow rate is kept constant to produce uniform strips. The flow rate is determined by the depth of steel in the mould. The pressure of the molten steel in the mould pushes the steel out of the bottom. The higher the level in the mould, the greater the pressure and the faster it flows out of the bottom.

This means that the steel level has to be monitored.

Preventing overflow or breakout
It is essential that the rate of flow into the mould isn't greater than the rate of flow out of it. Otherwise, there might be a build up of steel in the mould (more flowing in than out) causing an overflow. Imagine molten steel flowing out over the top - stay clear!

A breakout is even more dangerous. If the flow into the mould is too low, then the mould will run empty and the molten steel will be poured straight out of the bottom into the rollers.

Photo of continuous casting
Picture 7. Billets of steel being continuously cast. Seen from above.
Monitoring the level
The flow level can be monitored using an ionising radiation. The best radiation to use is gamma radiation. The reasons why this works well are shown in the table below.
Reason Details (roll over the word 'show' for each reason).
Try to think of the reason first.
the radiation can penetrate the walls of the mould show
gamma radiation can pass through the metal and water of the mould; beta radiation would be absorbed by the mould and would give very low readings; alpha particles, light and sound wouldn’t get through the mould at all and would give no reading, however much steel was in the mould.
the intensity is reduced by thick steel (in the mould) show
the amount of radiation getting through depends on the depth of steel in the mould; the deeper the steel the more radiation gets absorbed and the smaller the reading.
it gives an instant measurement show
the radiation is constantly being monitored so the readings are being taken all the time and will change the second that the level of steel changes.
it doesn’t damage or use up any of the steel show
The radiation has no effect on the steel and can monitor it as it continuously passes by as it is impossible to sample the steel within the mould (it would melt any probe).
the electrical signal can easily be fed into a computer show
The computer calculates the flow rate and can adjust the amount being fed in to keep everything in balance.

You can find out more about continuous casting in the Chemistry of steelmaking e-source.

Question 3-3.

The level in a mould can be monitored using an ionising radiation.

a) Which type of ionising radiation is used?   

b) Which of these are reasons for using that radiation (choose one or more):

A. it is completely absorbed by steel
B. it is a stream of particles
C. it is highly ionising
D. some of the radiation is absorbed by steel
E. it penetrates the mould walls

Summary                   Close
  • Steel can be continuously cast
  • The level in the mould has to be monitored
  • This can be done using a source of gamma radiation