High strength cold rolled and galvannealed steel sheet and manufacturing process thereof

Abstract

A cold rolled and galvannealed steel sheet having a composition including, by weight percent: C 0.15-0.25%, Mn 2.4-3.5%, Si 0.30-0.90%, Cr 0.30-0.70%, Mo 0.05-0.35%, Al 0.001-0.09%, Ti 0.01-0.06, B 0.0010-0.0040%, Nb 0.01-0.05%, P≤0.020%, S≤0.010% and N≤0.008%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction, between 80% and 90% of martensite, the balance being ferrite and bainite.

1 - 8 . (canceled) 
     
     
         9 . A cold rolled and galvannealed steel sheet having a chemical composition comprising, in weight %:
 C: 0.15-0.25%   Mn: 2.4-3.5%   Si: 0.30-0.90%   Cr: 0.30-0.70%   Mo: 0.05-0.35%   Al: 0.001-0.09%   Ti: 0.01-0.06%   B: 0.0010-0.0040%   Nb 0.01-0.05%   P≤0.020%   S≤0.010% and   N≤0.008%;   a remainder of the composition being iron and unavoidable impurities resulting from processing, the steel sheet having a microstructure consisting of, in surface fraction: martensite from 80% to 90%, ferrite and bainite.   
     
     
         10 . The cold rolled and galvannealed steel sheet as recited in  claim 9  wherein the ferrite is above or equal to 5%. 
     
     
         11 . The cold rolled and galvannealed steel sheet as recited in  claim 9  wherein the bainite is above or equal to 5%. 
     
     
         12 . The cold rolled and galvannealed steel sheet as recited in  claim 9  wherein the silicon content is comprised between 0.30% and 0.70%. 
     
     
         13 . The cold rolled and galvannealed steel sheet as recited in  claim 9  wherein the tensile strength is above or equal to 1450 MPA. 
     
     
         14 . A process for manufacturing a cold rolled and galvannealed steel sheet comprising the following and successive steps:
 casting a steel to obtain a semi-product, the semi product having a composition comprising, in weight %:   C: 0.15-0.25%   Mn: 2.4-3.5%   Si: 0.30-0.90%   Cr: 0.30-0.70%   Mo: 0.05-0.35%   Al: 0.001-0.09%   Ti: 0.01-0.06%   B: 0.0010-0.0040%   Nb 0.01-0.05%   P≤0.020%   S≤0.010% and   N≤0.008%;   a remainder of the composition being iron and unavoidable impurities resulting from processing;   reheating the slab to a temperature Treheat comprised from 1150° C. to 1300° C.;   hot rolling the reheated slab with a final rolling temperature comprised from 850° C. to 950° C., so to obtain a hot rolled steel sheet; then   cooling said steel sheet to a coiling temperature Tcoil comprised from 250° C. to 650° C.; then   coiling the steel sheet at said temperature Tcoil so to obtain a coiled steel sheet; then   pickling the steel sheet;   annealing the steel sheet to an annealing temperature T A  comprised from 500° C. to 650° C. and maintaining the steel sheet at said temperature T A  for a holding time t A ;   optionally further pickling the steel sheet;   cold rolling the hot-rolled steel sheet with a reduction rate between 20% and 80%, to obtain a cold rolled steel sheet;   heating the cold rolled steel sheet to a soaking temperature T soak  comprised from Ac1 to Ac3 and maintaining the steel sheet at said temperature T soak  for a holding time tsoak comprised from 30 s to 200 s, in order to obtain between 85% and 95% of austenite and between 5% and 15% of ferrite;   cooling the steel sheet to a temperature comprised from 440° C. to 480° C.;   coating the steel sheet by continuous dipping in a zinc bath at a temperature TZn comprised from 450° C. to 480° C.;   reheating the steel sheet to a galvannealed temperature T GA  comprised from 510° C. and 550° C., and maintaining the steel sheet at the temperature T GA  for a holding time t GA  comprised from 10 s and 30 s; and   cooling the reheated steel sheet to room temperature to obtain a cold rolled and galvannealed steel sheet.   
     
     
         15 . The process for manufacturing a cold rolled and galvannealed steel sheet as recited in  claim 14  wherein the annealing of the steel sheet is performed by batch in an inert atmosphere, the holding time t A  being comprised from 1800 s to 36000 s. 
     
     
         16 . The process for manufacturing a cold rolled and galvannealed steel sheet as recited in  claim 14  wherein the annealing of the steel sheet is performed by continuous annealing, the holding time t A  being comprised from 30 s to 100 s.