12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Nowadays, automotive steel customers are more and more strict on the coating quality, especially for the exposed parts that should not have any aspect defect.
To satisfy this requirement at the exit of galvanising line, one important parameter is the cleanliness of the substrate surface after the cleaning section at the entry of the line in terms of carbon pollution and iron fines.
Indeed, if carbon (as component of the mill oil) is still present at the surface when entering the furnace, furnace pollution will occur in the long term by production of soot that will not only stay on the furnace walls but will eventually lead to dirt falling on the rolls and/or on the strip.
Besides, the iron fines can also lead to roll pick-up defects inside the furnace, to an increase of dross in the bath by combination with zinc and aluminium and also to a drift of the bath composition.
These phenomena will eventually result in aspect defects on the final product.

A direct on-line measurement is so mandatory to fully evaluate and control the surface cleanliness all along the coil and is more and more required by customers as an assurance of final product quality.
Using this cleanliness measurement, the cleaning section can be optimised by adjusting relevant parameters, such as the brushes pressure, the solution composition, the current applied if electrolytic cleaning is used.
On the market, no on-line methods are able to separately measure the mentioned pollutants. CRM has developed an innovating method based on LIBS principle which succeeds in an on-line independent measurement of both pollutants. This will be realised by using a groundbreaking approach that will allow reaching the measurement sensitivity required after the cleaning section sensitivity (i.e. 10-15 mg/m² for carbon, 20 mg/m² for iron fines) thanks to atmosphere conditioning around the plasma and use of adequate spectral lines for carbon and iron fines levels measurement.
This system can be used at a later stage of development in a control loop for the optimisation of the final cleaning line and of the production speed.