Manufacturing method for very high-strength cold-rolled dual-phase steel sheets and sheets so produced

Abstract

The invention relates to a cold-rolled and annealed Dual-Phase steel sheet having a strength between 980 and 1100 MPa, and a breaking elongation greater than 9%, of which the composition comprises, the contents being expressed by weight: 0.055%≦C≦0.095%, 2%≦Mn≦2.6%, 0.005%≦Si≦0.35%, S≦0.005%, P≦0.050%, 0.1≦Al≦0.3%, 0.05%≦Mo≦0.25%, 0.2%≦Cr≦0.5%, it being understood that Cr+2Mo≦0.6%, Ni≦0.1%, 0.010≦Nb≦0.040%, 0.010≦Ti≦0.050%, 0.0005≦B≦0.0025%, and 0.002%≦N≦0.007%, the remainder of the composition consisting of iron and the inevitable impurities resulting from the smelting.

1 . A cold-rolled and annealed Dual-Phase steel sheet having a strength between 980 and 1100 MPa, and a breaking elongation greater than 9%, wherein the cold-rolled and annealed Dual-Phase steel sheet comprises iron, the following elements expressed by weight:
   055%≦C≦0.095%
     2%≦Mn≦2.6%
     0.005%≦Si≦0.35%
     S≦0.005%
     P≦0.050%
     0.1≦Al≦0.3%
     0.05%≦Mo≦0.25%
     0.2%≦Cr≦0.5%
     it being understood that Cr+2Mo≦0.6%
     Ni≦0.1%
     0.010≦Nb≦0.040%
     0.010≦Ti≦0.050%
     0.0005≦B≦0.0025%
     and 0.002%≦N≦0.007%
   
       and inevitable impurities resulting from smelting. 
     
     
         2 . The steel sheet as claimed in  claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:
   0.12%≦Al≦0.25%
   
     
     
         3 . The steel sheet as claimed in  claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:
   0.10%≦Si≦0.30%
   
     
     
         4 . The steel sheet as claimed in  claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:
     0 . 15 %≦Si≦0.28%
   
     
     
         5 . The steel sheet as claimed in  claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:
   P≦0.015%
   
     
     
         6 . The steel sheet as claimed in  claim 1 , wherein said steel sheet has a microstructure and the microstructure comprises a surface area fraction of 35 to 50% martensite. 
     
     
         7 . The steel sheet as claimed in  claim 6 , wherein the microstructure further comprises of a surface area fraction of 50 to 65% ferrite. 
     
     
         8 . The steel sheet as claimed in  claim 6 , wherein the microstructure further comprises of surface area fractions of 1 to 10% bainite and 40 to 64% ferrite. 
     
     
         9 . The steel sheet as claimed in  claim 1 , wherein its non-recrystallized ferrite surface area fraction, compared to the whole of the ferritic phase, is less than or equal to 15%. 
     
     
         10 . The steel sheet as claimed in  claim 1 , wherein the ratio of its yield strength R e  to its strength R m  is such that: 0.6 5 Re/R m ≦0.8. 
     
     
         11 . The steel sheet as claimed in  claim 1 , wherein it is continuously galvanized. 
     
     
         12 . The steel sheet as claimed in  claim 1 , wherein it includes a galvannealed coating. 
     
     
         13 . A manufacturing method for producing a cold-rolled and annealed Dual-Phase steel sheet, comprising: casting a steel having a composition as claimed in  claim 1 ,
 as a semi-finished product,   bringing said semi-finished product to a temperature 1150° C.≦T R 1250° C.,   hot-rolling said semi-finished product with an end-of-roiling temperature T FL ≧Ar3 to obtain a hot-rolled product,   coiling said hot-rolled product at a temperature T bob  such as: 500° C. T bob ≦570° C.,   descaling said hot-rolled product to obtain a descaled hot-rolled product, and   cold-rolling said descaled hot-rolled product with a reduction of between 30 and 80% to obtain a cold-rolled product,   heating said cold-rolled product at a rate 1° C./s≦V C ≦5° C./s to an annealing temperature TM such as: Ac1+40° C.≦T M ≦Ac3−30° C. at which the product is held for a time: 30 s≦t M ≦300 s so as to obtain a heated and annealed product with a structure comprising austenite, then;   cooling said heated and annealed product to a temperature less than the temperature M 5  at a rate V high enough for ail of said austenite to transform to martensite.   
     
     
         14 . A manufacturing method for producing a cold-rolled, annealed and galvanized Dual-Phase steel sheet comprising cooling a heated and annealed product with a structure comprising austenite as claimed in  claim 13   at a rate V R  high enough to prevent the transformation of said austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T Zn  is reached, then;   continuously galvanized galvanizing said product by immersing said product in a bath of zinc or Zn alloy at a temperature 450° C.≦T Zn ≦480° C. to obtain a galvanized product, then;   cooling said galvanized product to the ambient temperature at a rate V′ R  greater than 4° C./s to obtain a cold-rolled, annealed and galvanized steel sheet.   
     
     
         15 . A manufacturing method for producing a cold-rolled and galvannealed Dual-Phase steel sheet, comprising cooling a heated and annealed product with a structure comprising austenite as claimed in  claim 13   at a rate V R  high enough to prevent the transformation of said austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T Zn  is reached,   continuously galvanizing said product by immersing said product in a bath of zinc or Zn alloy at a temperature 450° C.≦T Zn ≦480° C. to obtain a galvanized product,   heating said galvanized product at a temperature TG between 490 and 550° C. for a time t G  between 10 and 40 s to obtain a galvannealed product, then;   cooling said galvannealed product to the ambient temperature at a rate V″ R  greater than 4° C./s, to obtain a cold-rolled and galvannealed steel sheet.   
     
     
         16 . The manufacturing method as claimed in  claim 13 , wherein said temperature T M  is between 760 and 830° C. 
     
     
         17 . The manufacturing method as claimed in  claim 14 , wherein said rate of cooling V R  is greater than or equal to 15° C./s. 
     
     
         18 . (canceled) 
     
     
         19 . A motor vehicle safety part comprising a steel sheet manufactured according to the method of  claim 13 . 
     
     
         20 . A cold-rolled and annealed Dual-Phase steel sheet having a strength between 980 and 1100 MPa, and a breaking elongation greater than 9%, wherein said cold-rolled and annealed Dual-Phase steel sheet consists essentially of iron, the following elements expressed by weight:
   0.055%≦C≦0.095%
     2%≦Mn≦2.6%
     0.005%≦Si≦0.35%
     S≦0.005%
     P≦0.050%
     0.1≦Al≦0.3%
     0.05%≦Mo≦0.25%
     0.2%≦Cr≦0.5%
     it being understood that Cr+2Mo≦0.6%
     Ni≦0.1%
     0.010≦Nb≦0.040%
     0.010≦Ti≦0.050%
     0.0005≦B≦0.0025%
     and 0.002%≦N≦0.007%
   
       and inevitable impurities resulting from smelting.