Method for improving a metal coating on a steel strip

Abstract

A method for improving a metal coating on a steel strip or a steel sheet or plate. The coating is melted to a maximum temperature above the melting temperature of the material of the coating by inductive heating performed by at least one induction coil and subsequently cooled to a quenching temperature, below the melting temperature, in a cooling device. In order to improve the corrosion stability of the coating, even in the case of thin coating layers, the coating is kept at a temperature above the melting temperature during a holding time and the holding time is adapted to the maximum temperature and the thickness of the coating by moving at least one of the induction coils with respect to the cooling device, in order to melt the coating completely over its entire thickness to the boundary layer with the steel strip.

1 . Method for improving a metal coating on a steel strip or steel sheet, wherein the coating is melted by inductive heating, with at least one induction coil, to a maximum temperature above the melting temperature of the material of the coating, and is subsequently cooled, in a cooling device, to a quenching temperature below the melting temperature, wherein the coating is held during a holding time at a temperature above the melting temperature and that the holding time is adapted to the maximum temperature and the thickness of the coating by moving at least one of the induction coils relative to the cooling device so as to completely melt the coating over its entire thickness down to the boundary layer with the steel strip. 
     
     
         2 . Method according to the preamble of  claim 1 , wherein the maximum temperature is higher than 310° C. and that the coating is completely melted over its entire thickness down to the boundary layer with the steel strip. 
     
     
         3 . Method according to  claim 1 , wherein the maximum temperature is between 310° C. and 360° C., and preferably between 320° C. and 350° C. 
     
     
         4 . Method according to  claim 1 , wherein the heating rate of the inductive heating is between 600 K/s and 1300 K/s, and preferably between 900 K/s and 1100 K/s. 
     
     
         5 . Method according to  claim 1 , wherein the coated steel strip is moved at a strip speed relative to the induction coil. 
     
     
         6 . Method according to  claim 1 , wherein the distance of the induction coil to the cooling device can be adjusted continuously, so as to set the holding time at a desired value. 
     
     
         7 . Method according to  claim 1 , wherein the holding time is between 0.1 s and 1.0 s, and preferably between 0.2 s and 0.3 s. 
     
     
         8 . Method according to  claim 1 , wherein a thin alloy layer, which essentially consists of iron atoms and atoms of the coating material, is formed on the boundary layer between the coating and the steel strip. 
     
     
         9 . Method according to  claim 7 , wherein the alloy layer is thinner than 1.3 g/m 2 , and preferably thinner than 1.0 g/m 2 . 
     
     
         10 . Apparatus for the application of a metal coating on a steel strip, in particular, a strip tin-plating unit, in which a continuous steel strip is moved at a strip speed in a movement direction of the strip and is electrolytically provided by a coating device with a metal coating, wherein a melting device follows the coating device in the movement direction of the strip, and in the melting device, the coating is melted by inductive heating at a maximum temperature above the melting temperature of the material of the coating, and a cooling device follows the melting device, and in the cooling device, the coated steel strip is quenched to a quenching temperature that is below the melting temperature, wherein the melting device can be moved relative to the cooling device so as to set the distance between the melting device and the cooling device in the movement direction of the strip. 
     
     
         11 . Apparatus according to  claim 10 , wherein the melting device contains at least one induction coil arranged so it can move in the movement direction of the strip. 
     
     
         12 . Apparatus according to  claim 11 , wherein the melting device contains a plurality of induction coils arranged one behind the other in the movement direction of the strip, wherein at least the last induction coil, which is closest to the cooling device, can move relative to the cooling device. 
     
     
         13 . Apparatus according to  claim 10 , wherein the cooling device comprises a quenching tank filled with a cooling liquid.