Method for galvannealing steel materials

Abstract

Method for use when galvannealing a steel material ( 1 ), in which the material ( 1 ), in a first step, is preheated to a first process temperature and is coated with a layer of a liquid alloying metal ( 3 ), in a second step is further heated to a second, higher process temperature, and in a third step is kept at the second process temperature during a predetermined time period so that the alloying metal coating at least partially is caused to alloy with the steel material ( 1 ). The heating in the second step is caused to be carried out by one or several DFI burners ( 5 ).

1 - 13 . (canceled) 
     
     
         14 . Method for use when galvannealing a steel material ( 1 ) in the form of an elongated steel product which is transported along a process line, in which the material ( 1 ), in a first step, is preheated to a first process temperature of between 350 and 450° C. and is coated with a layer of a liquid alloying metal ( 3 ), in a second step is further heated to a second process temperature which is caused to be between 50 and 200° C. warmer than the first process temperature, and in a third step is kept at the second process temperature during a predetermined time period of at least a number of seconds so that the alloying metal coating at least partially is caused to alloy with the steel material ( 1 ), characterised in that the heating in the second step is caused to be carried out by several DFI burners ( 5 ), arranged one after another along the process line at such a distance from the steel material ( 1 ) so that their respective flames strike the surface of the material ( 1 ), in that at least one DFI burner ( 5 ) is caused to be driven with an oxidant which to at least 80 percentages by weight is comprised of oxygen, in that the steel material ( 1 ) is held in continuous motion in relation to each DFI burner ( 5 ), and in that the DFI burners ( 5 ) are arranged at such a distance from each other so that the surface of the steel material ( 1 ) has time to cool down between two consecutive DFI burners to such an extent so that when passing the next consecutive DFI burner it will not be heated above a certain predetermined temperature at which the risk for material deterioration is unacceptably high. 
     
     
         15 . The method according to  claim 14 , characterised in that the certain predetermined temperature is not more than 565° C. 
     
     
         16 . The method according to  claim 14 , characterised in that the steel material ( 1 ) is a steel strip. 
     
     
         17 . The method according to  claim 14 , characterised in that the additional heating is caused to be so intense that it goes on only a few seconds before the second process temperature is reached. 
     
     
         18 . The method according to  claim 14 , characterised in that the flame from at least one DFI burner ( 5 ) is caused to remove any surplus alloying metal from the surface of the material ( 1 ). 
     
     
         19 . The method according to  claim 14 , characterised in that the alloying metal ( 3 ) comprises zinc. 
     
     
         20 . The method according to  claim 14 , characterised in that the alloying metal ( 3 ) comprises aluminum.