Method for estimating the temperature and the oxide thickness of a steel strip

A method of thermally treating a steel strip, comprising:

• providing a furnace comprising burners with adjustable power along the width of a steel strip passing through the furnace;
• heating the steel strip by passing the steel strip through the furnace at a furnace temperature from 100° C to 1100° C;
• measuring at least two radiation intensities emitted from the heated steel strip in the furnace at different wavelengths, in a range from 1 to 5 μm;
• estimating the temperature of the heated steel strip, TESTIMATED, based on
• the at least two measured radiation intensities and
• at least two reference radiation intensities and at least two reference emissivities, at different wavelengths, of a reference steel strip having a known temperature for at least N oxide layer thickness from 0 to 200 nm;
• estimating the emissivity coefficient of the heated steel strip, εESTIMATED, using at least one of the measured radiation intensities and the estimated temperature, TESTIMATED;
• and
• estimating the oxide thickness, OxESTIMATED, of said heated steel strip using said estimated emissivity, εESTIMATED;
• controlling the power of the burners along said heated steel strip based on OxESTIMATED, thereby homogenizing the oxide thickness along the width of the heated steel strip.