High toughness press-hardened steel part and method of manufacturing the same

Abstract

A steel sheet and press hardened steel part having a composition including, by weight percent: C 0.05-0.3%, Mn 0.5-4%, Si 0.24-1.7%, Al 0.01-0.1%, Cr 0.01-1.0%, Ti 0.01-0.1%, B 0.0005-0.08%, Cu 0.05-0.4%, P≤0.020%, S≤0.010%, N≤0.02% the remainder of the composition being iron and unavoidable impurities resulting from the production route. The press hardened steel part has a microstructure including, in surface fraction, more than 95% of martensite, the rest being optional bainite and retained austenite.

What is claimed is: 
     
         1 . A steel sheet made of a steel having a composition comprising, by weight percent:
 C: 0.05-0.3%   Mn: 0.5-4%   Si: 0.24-1.7%   Al: 0.01-0.1%   Cr: 0.01-1.0%   B: 0.0005-0.08%   Ti: 0.01-0.1%   Cu: 0.05-0.4%   P≤0.020%   S≤0.010%   N≤0.02%   and comprising optionally one or more of the following elements, by weight percent:   Sn≤0.1%   Ni≤0.4%   Mo≤0.40%   Nb≤0.08%   Ca≤0.1%   a remainder of the composition being iron and unavoidable impurities resulting from processing,   the steel sheet having a microstructure comprising, in surface fraction, 50% or more of ferrite, a rest being pearlite or cementite.   
     
     
         2 . A press hardened steel part made of steel having a composition comprising, by weight percent:
 C: 0.05-0.3%   Mn: 0.5-4%   Si: 0.24-1.7%   Al: 0.01-0.1%   Cr: 0.01-1.0%   B: 0.0005-0.08%   Ti: 0.01-0.1%   Cu: 0.05-0.4%   P≤0.020%   S≤0.010%   N≤0.02%   and comprising optionally one or more of the following elements, by weight percent:   Sn≤0.1%   Ni≤0.4%   Mo≤0.40%   Nb≤0.08%   Ca≤0.1%   a remainder of the composition being iron and unavoidable impurities resulting from processing,   the steel part having a microstructure comprising, in surface fraction, more than 95% of martensite, a rest being optional bainite and retained austenite.   
     
     
         3 . The press hardened steel part as recited in  claim 2  wherein the press hardened steel part has an average Charpy impact energy value, calculated as the average of Charpy impact energies measured at 20° C., −40° C., −60° C. and −80° C., that is above or equal to 0.90 J/mm 2 . 
     
     
         4 . The press hardened steel part as recited in  claim 2  wherein the press hardened steel part has a Charpy impact energy measured at −80° C. higher than 0.75 J/mm 2 . 
     
     
         5 . The press hardened steel part as recited in  claim 2  wherein the press hardened steel part has a loss of ductility A between the Charpy impact energy measured at 20° C. and the Charpy impact energy measured at −80° C. lower than 25%. 
     
     
         6 . A process for manufacturing a press hardened steel part comprising the following successive steps:
 providing the steel sheet as recited in  claim 1 ;   cutting the steel sheet to a predetermined shape, so as to obtain a steel blank;   heating the steel blank to a temperature T 1  from 800° C. to 980° C. and maintaining the steel blank at the temperature T 1  for a dwell time t 1  of 10s to 900s to obtain a heated steel blank;   transferring the heated steel blank to a forming press;   hot-forming the heated steel blank in the forming press to obtain a formed part; and   die-quenching the formed part.   
     
     
         7 . A process for manufacturing a martensitic steel sheet comprising the following successive steps:
 providing the steel sheet as recited in  claim 1 ;   annealing the steel sheet to a temperature T 1  from 800° C. to 980° C. and maintaining the steel sheet at the temperature T 1  for a holding time t 1  of 10s to 900s;   cooling the steel sheet below Ms; and   cooling the steel sheet to room temperature, to obtain a martensitic steel sheet having a microstructure comprising, in surface fraction, more than 95% of martensite, the rest being optional bainite and retained austenite.   
     
     
         8 . The process as recited in  claim 7  further comprising, before the annealing the steel sheet at the temperature T 1 , annealing the steel sheet to an annealing temperature T from 500° C. to 750° C. and maintaining at the annealing temperature for a holding time t of 300s to 80h. 
     
     
         9 . The process as recited in  claim 7  further comprising cold rolling the steel sheet. 
     
     
         10 . The process as recited in  claim 7  further comprising, after the cooling the steel sheet to below Ms, reheating the steel sheet to a reheating temperature from 150° C. to 270° C. and maintaining at the reheating temperature for a holding time of 1s to 600s.