High strength press hardened steel part and method of manufacturing the same

Abstract

A press hardened steel part having a composition including, by weight percent: C 0.2-0.34%, Mn 0.50-1.24%, Si 0.5-2%, P≤0.020%, S≤0.010%, N≤0.010%, and including optionally one or more of the following elements: Al: ≤0.2%, Cr≤0.8%, Nb≤0.06%, Ti≤0.06%, B≤0.005%, Mo≤0.35% the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The press hardened steel part has a microstructure including, in surface fraction, 95% or more of tempered martensite and 5% or less of bainite, austenite or ferrite.

What is claimed is: 
     
         1 - 8 . (canceled) 
     
     
         9 . A press hardened steel part made of a steel having a composition comprising, by weight percent:
 C: 0.2-0.34%   Mn: 0.50-1.24%   Si: 0.5-2%   P≤0.020%   S≤0.010%   N≤0.010%   and optionally one or more of the following elements, by weight percent:   Al: ≤0.2%   Cr≤0.8%   Nb≤0.06%   Ti≤0.06%   B≤0.005%   Mo≤0.35%   a remainder of the composition being iron and unavoidable impurities resulting from processing, and having a microstructure comprising, in surface fraction:   95% or more of tempered martensite, and   5% or less of the sum of bainite, austenite and ferrite.   
     
     
         10 . The press hardened steel part as recited in  claim 9  wherein the press hardened steel part has a tensile strength TS above or equal to 1000 MPa, a uniform elongation loss ΔUEI in spot welded areas below or equal to 25% and a bending angle above or equal to 55°. 
     
     
         11 . The press hardened steel part as recited in  claim 9  wherein the press hardened steel part has a fracture strain above or equal to 0.50. 
     
     
         12 . The press hardened steel part as recited in  claim 9  wherein the press hardened steel part has a yield strength YS above or equal to 980 MPa. 
     
     
         13 . A method for producing a press hardened steel part, the method comprising the following successive steps:
 providing a steel sheet having composition comprising, by weight percent:   C: 0.2-0.34%   Mn: 0.50-1.24%   Si: 0.5-2%   P≤0.020%   S≤0.010%   N≤0.010%   and optionally one or more of the following elements, by weight percent:   Al: ≤0.2%   Cr≤0.8%   Nb≤0.06%   Ti≤0.06%   B≤0.005%   Mo≤0.35%;   a remainder of the composition being iron and unavoidable impurities resulting from processing;   cutting said steel sheet to a predetermined shape, so as to obtain a steel blank;   heating the steel blank to a temperature T HF  from 810° C. to 960° C. and maintaining at said T HF  temperature for a holding time t HF  from 5 s to 1200 s to obtain a heated steel blank;   transferring the heated blank to a forming press;   hot forming the heated blank in the forming press to obtain a steel part;   die-quenching the steel part until reaching a temperature below or equal to 200° C.;   reheating the steel part to a temperature T temp  from 390° C. to 510° C., and maintaining at said T temp  temperature for a holding time t temp  from 1 s to 1000 s, to obtain a tempered steel part; and   cooling the tempered steel part to room temperature.   
     
     
         14 . The method as recited in  claim 13 , wherein the steel sheet is produced by the following successive steps:
 casting a steel to obtain a slab, the steel having the composition,   reheating the slab at a temperature T reheat  from 1100° C. to 1300° C.,   hot rolling the reheated slab at a finish hot rolling temperature from 800° C. to 950° C., to obtain a hot rolled steel sheet,   coiling the hot rolled steel sheet at a coiling temperature T coil  lower than 670° C. to obtain a coiled steel sheet,   optionally pickling the coiled steel sheet,   optionally heating the hot rolled steel sheet to a temperature T HBA  from 500° C. to 750° C., and maintaining at said T HBA  temperature for a holding time t HBA  from 300 s to 50 h,   cold rolling the steel sheet to obtain a cold rolled steel sheet,   optionally heating the cold rolled steel sheet to an annealing temperature T A  from 650° C. to 900° C. and maintaining the steel sheet at said temperature T A  for a holding time t A  from 10 s to 1200 s, to obtain an annealed steel sheet, and   cooling the steel sheet to room temperature.   
     
     
         15 . The method as recited in  claim 13  wherein the steel sheet is produced by the following successive steps:
 casting a steel to obtain a slab, said steel having the composition, 
 reheating the slab at a temperature T reheat  from 1100° C. to 1300° C., 
 hot rolling the reheated slab at a finish hot rolling temperature from 800° C. to 950° C., to obtain a hot rolled steel sheet, 
 coiling the hot rolled steel sheet at a coiling temperature T coil  lower than 670° C. to obtain a coiled steel sheet, 
 optionally pickling the coiled steel sheet, 
 optionally heating the hot rolled steel sheet to a temperature T HBA  from 500° C. to 750° C., and maintaining at said T HBA  temperature for a holding time t HBA  from 300 s to 50 h, 
 cold rolling the steel sheet to obtain a cold rolled steel sheet, 
 optionally heating the cold rolled steel sheet to an annealing temperature T A  comprised from 500° C. to 750° C. and maintaining the steel sheet at said temperature T A  for a holding time t A  comprised from 300 s to 50 h, to obtain an annealed steel sheet, and 
 cooling the steel sheet to room temperature. 
 
     
     
         16 . The method as recited in  claim 13  wherein the annealed steel sheet is coated with aluminium or with an aluminium alloy coating or with zinc or zinc alloy coating.