Method for controlling a coating weight uniformity in industrial galvanizing lines

A method for controlling and optimizing a transverse uniformity of a coating thickness on at least one side of a running metal strip in an industrial galvanization installation, the coating being deposited by hot dip coating in a pot containing a liquid metal bath, the method comprising at least the steps of:

• heating the strip substrate to a temperature higher than a pot temperature;
• passing the strip through the bath by wrapping the strip around at least a first deflector roll or sink roll followed by at least one second deflector roll, the second deflector roll being configured to improve a flatness of the strip;
• wiping excess coating thickness carried away by the strip on one or both sides of the strip by wiping nozzles blowing a gas on the strip at an exit of the liquid metal bath;
• measuring an actual distance profile between the nozzles and the strip along a direction transverse with respect to a running strip direction, and in a vicinity of the nozzles, so as to obtain an actual nozzle to strip distance profile curve;
• using a computer, to calculate a first correction on the nozzle to strip distance profile curve based on a calculation of an average slope, comprising a 1st order linear regression straight line of the nozzle to strip distance profile curve, so as to apply the first correction based on a skewness of the nozzles and to set the nozzles parallel to the strip;
• calculating a second correction on the first corrected nozzle to strip distance profile curve by subtracting from the curve a 2nd order linear regression quadratic line, a result thereof being a second corrected nozzle to strip distance profile curve, so as to apply the second correction to compensate for a crossbow by an adjustment of the deflector rolls in the pot;
• and
• acting on the nozzles' position and transverse metal strip shape by physically transposing on the industrial galvanization installation the first and second calculated corrections, as a first corresponding physical correction and a second corresponding physical correction, by modifying firstly the position of the nozzles and secondly the shape of the strip respectively, so as to obtain a coated strip which is physically corrected in position and shape.