Method for Producing a Grain-Oriented Electrical Steel Flat Product Intended for Electrotechnical Applications

Abstract

The invention relates to a method for producing a grain-oriented steel flat product for electrotechnical applications, wherein, in a production step, “decarburising and nitriding annealing” is carried out in two stages. The first stage of the annealing process extends over a first time interval, which comprises heating the cold strip starting from a start temperature to a first target annealing temperature and holding it at this target annealing temperature, and the second stage of the annealing process extends over a second time interval, in which the cold strip is heated to a second target annealing temperature and subsequently held at this target annealing temperature.

1 . A method for producing a grain-oriented electrical steel flat product, comprising the following production steps:
 a) producing a steel melt which contains, in addition to iron and unavoidable impurities, (in % wt.):   Si: 2.5-4.0%,   C: 0.02-0.1%,   Al: 0.01-0.065%,   N: 0.003-0.015%, and   optionally:
 up to 0.30 % Mn, 
 up to 0.05% Ti, 
 up to 0.3 % P, 
 one or more elements from the group S, Se in contents of which the total is at most 0.04%, 
 one or more elements from the group As, Sn, Sb, Te, Bi with contents of up to 0.2% in each case, 
 one or more elements from the group Cu, Ni, Cr, Co, Mo with contents of up to 0.5% in each case, 
 one or more elements from the group B, V, Nb with contents of up to 0.012% in each case, 
   b) casting the melt into a strand in a continuous casting machine,   c) separating at least one thin slab from the cast strand,   d) heating the thin slab to a temperature between 1050° C. and 1300° C.,   e) hot rolling the thin slab into a hot strip having a thickness of 0.5-4.0 mm in a hot-rolling train, 2QB7015.DOC Page 6   f) cooling the hot strip,   g) coiling the hot strip into a coil,   h) cold rolling the hot strip into a cold strip having a final thickness of 0.15-0.50 mm,   i) decarburising and nitriding annealing of the cold strip obtained,   j) applying an annealing separator onto the surface of the annealed cold strip, and   k) final annealing of the cold strip provided with the annealing separator to form a Goss texture,   wherein
 the cold strip is annealed in at least two stages in the course of production step i), 
 the first stage of this annealing process extends over a first time interval and comprises heating the cold strip starting from a start temperature to a first target annealing temperature and subsequently holding the heated cold strip at the target annealing temperature, 
 the second stage of the annealing process extends over a second time interval, within which the cold strip is firstly heated to a second target annealing temperature and is subsequently held at this target annealing temperature, 
 the first target annealing temperature is 10-50° C. lower than the second target annealing temperature, and 
 the duration of the first time interval is 30-70% of an entire duration of the annealing treatment comprising the first time interval and the second time interval. 
   
     
     
         2 . The method according to  claim 1 , wherein the first target annealing temperature is 10-30° C. lower than the second target annealing temperature. 
     
     
         3 . The method according to  claim 1 , wherein the duration of the first time interval is 30-60% of the entire duration of the annealing treatment. 
     
     
         4 . The method according to  claim 1 , wherein the heating rate, at which the cold strip is heated from the start temperature to the first target annealing temperature in the first annealing stage, is 25-500° C./s. 
     
     
         5 . The method according to  claim 4 , wherein the heating rate is at least 200° C. 
     
     
         6 . The method according to  claim 5 , wherein the cold strip is inductively heated. 
     
     
         7 . The method according to  claim 1 , wherein in the production step i) the first and second annealing stages are completed following one another and a further annealing step is subsequently carried out, in which the cold strip is subjected to decarburising and nitriding annealing. 
     
     
         8 . The method according to  claim 7 , wherein the first and second annealing stages are carried out in production step i) as a pure decarburisation annealing process.