Method of producing a cold-rolled strip with a ferritic structure

Abstract

A method of making a cold strip with a ferritic structure includes casting a molten steel which forms a ferritic structure on cooling into a cast strip, wherein if necessary hot-rolling the cast strip in-line, coiling the hot-rolled strip and in one or more steps cold-rolling to form the cold strip. With such a method, cold strips can be produced, in which the risk of formation of an orange peel appearance or ridging is minimized during a cold forming process. The cast strip is cooled between the casting process and the coiling process from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C. and is then held for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.

1 . Method of making a cold strip with a ferritic structure comprising,
 casting molten steel which forms a ferritic structure on cooling into a cast strip,   coiling the cast strip   cold-rolling the cast strip to form the cold strip,   cooling the cast strip between casting and coiling from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C., and   holding the cast strip for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.   
   
   
       2 . Method according to  claim 1 , wherein the starting temperature of the cooling ranges from 1180 to 1270° C. 
   
   
       3 . Method according to  claim 1 , wherein the molten steel is composed as follows (in wt. %):
 Cr: 10.5-18%,   C: ≦0.08%,   Si: ≦1%,   Mn: ≦1.5%   Ni: ≦1.00%   P: ≦0.04%   S: ≦0.015%   Remainder iron and unavoidable impurities.   
   
   
       4 . Method according to  claim 1 , wherein the cooling rate is 150-250° C. per second. 
   
   
       5 . Method according to  claim 1 , wherein the intermediate temperature is 900-1000° C. 
   
   
       6 . Method according to  claim 5 , wherein the intermediate temperature is equal to the maintenance temperature. 
   
   
       7 . Method according to  claim 1 , further comprising hot rolling the cast strip. 
   
   
       8 . Method according to  claim 7 , wherein the cooling the cast strip and holding the cast strip are carried out after hot-rolling. 
   
   
       9 . Method according to  claim 8 , wherein cooling of the strip starts within less than three seconds, after hot-rolling. 
   
   
       10 . Method according to  claim 7 , wherein cooling the cast strip to the intermediate temperature and holding the cast strip at the maintenance temperature are carried out before hot-rolling the cast strip. 
   
   
       11 . Method according to  claim 7 , wherein cooling to the intermediate temperature is carried out before hot-rolling and holding at the maintenance temperature is carried out after hot-rolling the cast strip. 
   
   
       12 . Method according to  claim 1 , wherein the cast strip on cooling reaches an intermediate temperature of less than 900° C., and is subsequently re-heated to the maintenance temperature. 
   
   
       13 . Method according to  claim 7 , wherein hot-rolling is carried out at the intermediate temperature of less than 800° C. 
   
   
       14 . Method according to  claim 12 , wherein re-heating takes place within 1-5 seconds. 
   
   
       15 . Method according to  claim 12 , wherein casting, coiling, cooling carried out between the casting process and the coiling process to the intermediate temperature and holding at the maintenance temperature are performed in a discontinuous production sequence. 
   
   
       16 . Method according to  claim 15 , wherein on cooling, the strip reaches a temperature of less than 500° C. 
   
   
       17 . Method according to  claim 16 , wherein following casting, the strip is cooled down to ambient temperature. 
   
   
       18 . Method according to  claim 16 , wherein the strip is heated for hot-rolling within 200 seconds, to a hot-rolling temperature. 
   
   
       19 . Method according to  claim 12 , wherein the casting, coiling, cooling carried out between the casting process and the coiling process to the intermediate temperature and holding at the maintenance temperature are performed in a continuous production sequence. 
   
   
       20 . Method according to  claim 1 , further comprising casting the strip with a twin roll casting machine. 
   
   
       21 . Method according to  claim 1 , wherein the maximum thickness of the cast strip is 6 mm. 
   
   
       22 . Method according to  claim 1  further comprising hot-rolling the cast strip in-line.