Method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure and band thus obtained

Abstract

The invention relates to a method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure. The inventive method consists in hot rolling a slab having a chemical composition which comprises, by weight, 0.01%≦C≦0.1%., 0.05%≦Mn≦1%, 0.01%≦Cr≦1%, 0.01%≦Si≦0.5%, 0.001%≦P≦0.2%, 0.01%≦Al≦0.1%, N≦0.01%, the remainder being iron and impurities resulting from the preparation thereof. The method also comprises the following subsequent steps consisting in: hot winding the band obtained at a temperature of between 550 and 850° C.; cold rolling the band with a reduction ratio of between 60 and 90%; annealing the band continuously in the intercritical region; cooling said band to ambient temperature, in one or more steps, the rate of cooling between 600° C. and ambient temperature being between 100° C./s and 1500° C/s; and, optionally, tempering same at a temperature of less than 300° C. The aforementioned annealing and cooling operations are performed such that the end band comprises between 1 and 15% martensite. The invention also relates to the steel band thus formed.

1 . A process for producing a cold-rolled ferritic/martensitic dual-phase steel strip, wherein a slab, the chemical composition of which comprises, by weight: 
 0.010%≦C≦0.100%    0.050%≦Mn≦1.0%    0.010%≦Cr≦1.0%    0.010%≦Si≦0.50%    0.001%≦P≦0.20%    0.010%≦Al≦0.10%    N≦0.010%    the balance being iron and impurities resulting from the smelting, is hot rolled, said process then comprising the steps consisting in:    coiling the hot-rolled strip obtained at a temperature of between 550 and 850° C.; then    cold rolling the strip with a reduction ratio of between 60 and 90%; then    annealing the strip continuously in the intercritical range; and    cooling it down to the ambient temperature in one or more steps, the cooling rate between 600° C. and the ambient temperature being between 100° C./s and 1500° C./s; and    optionally tempering it at a temperature below 300° C.,    the annealing and cooling operations being carried out in such a way that the strip finally contains from 1 to 15% martensite.    
   
   
       2 . The process as claimed in  claim 1 , wherein the chemical composition of the steel comprises: 
 0.020%≦C≦0.060%    0.300%≦Mn≦0.500%    0.010%≦Cr≦1.0%    0.010%≦Si≦0.50%    0.010%≦P≦0.100%    0.010%≦Al≦0.10%    N≦0.010%    the balance being iron and impurities resulting from the smelting.    
   
   
       3 . The process as claimed in either of claims  1  and  2 , wherein the strip is hot rolled at a temperature above 850° C.  
   
   
       4 . The process as claimed in any one of  claims 1  to  3 , wherein the strip is hot rolled at a temperature of between 550 and 750° C.  
   
   
       5 . The process as claimed in any one of  claims 1  to  4 , wherein the strip is cold rolled with a reduction ratio of between 70 and 80%.  
   
   
       6 . The process as claimed in any one of  claims 1  to  5 , wherein the continuous annealing of the cold-rolled strip comprises a temperature rise phase followed by a soak phase at a predetermined temperature.  
   
   
       7 . The process as claimed in  claim 6 , wherein the soak temperature is between Ac 1  and 900° C.  
   
   
       8 . The process as claimed in  claim 7 , wherein the soak temperature is between 750 and 850° C.  
   
   
       9 . The process as claimed in any one of  claims 1  to  8 , wherein the cooling down to the ambient temperature comprises a first, slow cooling step between the soak temperature and 600° C., during which the cooling rate is less than 50° C./s, followed by a second cooling step at a higher rate, of between 100° C./s and 1 500° C./s, down to the ambient temperature.  
   
   
       10 . The process as claimed in  claim 9 , wherein the second cooling step is carried out by water quenching.  
   
   
       11 . The process as claimed in any one of  claims 1  to  8 , wherein the cooling is carried out in a single operation at a cooling rate of between 100° C./s and 1500° C./s.  
   
   
       12 . The process as claimed in  claim 11 , wherein the cooling is carried out by water quenching.  
   
   
       13 . A cold-rolled ferritic/martensitic dual-phase steel strip, the chemical composition of which comprises, by weight: 
 0.010%≦C≦0.100%    0.050%≦Mn≦1.0%    0.010%≦Cr≦1.0%    0.010%≦Si≦0.50%    0.001%≦P≦0.20%    0.010%≦Al≦0.10%    N≦0.010%    the balance being iron and impurities resulting from the smelting, the strip furthermore containing between 1% and 15% martensite.    
   
   
       14 . The steel strip as claimed in  claim 13 , the chemical composition of which furthermore comprises: 
 0.020%≦C≦0.060%    0.300%≦Mn≦0.500%    0.010%≦Cr≦1.0%    0.010%≦Si≦0.50%    0.010%≦P≦0.100%    0.010%≦Al≦0.10%    N≦0.010%    the balance being iron and impurities resulting from the smelting.    
   
   
       15 . The steel strip as claimed in either of claims  13  and  14 , which has a tensile strength R m  of greater than 450 MPa.  
   
   
       16 . The steel strip as claimed in  claim 15 , which has a tensile strength R m  of greater than 500 MPa.  
   
   
       17 . The steel strip as claimed in  claim 16 , further which has a tensile strength R m  of greater than 600 MPa.  
   
   
       18 . The steel strip as claimed in any one of  claims 13  to  17 , which has a mean anisotropy coefficient r of greater than 1.1.  
   
   
       19 . The steel strip as claimed in  claim 18 , further which has a mean anisotropy coefficient r of greater than 1.3.  
   
   
       20 . The steel strip as claimed in any one of  claims 13  to  19 , which furthermore contains between 1% and 10% martensite.  
   
   
       21 . The steel strip as claimed in  claim 20 , which furthermore contains between 5% and 8% martensite.  
   
   
       22 . The use of a steel strip as claimed in any one of  claims 13  to  21  for the production of automobile parts by deep drawing.