Steel sheet having excellent corrosion properties after press hardening and method for manufacturing the same

Abstract

A steel sheet, coated with a metallic coating including, by weight percent, from 7.5 to 9.0% of zinc, from 1.1 to 4.0% of silicon, from 1.1 to 8.0% of magnesium, up to 3.0% of iron, optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium and unavoidable impurities up to 0.02%, a balance being aluminum, and wherein the coating weight of the metallic coating is from 50 to 500 g/m2 for the sum of both sides of the-steel sheet.

What is claimed is: 
     
         1 - 12 . (canceled) 
     
     
         13 . A coated steel sheet comprising:
 a metallic coating and a steel sheet coated with the metallic coating, the metallic coating comprising, by weight percent:
 from 7.5 to 9.0% of zinc; 
 from 1.1 to 4.0% of silicon; 
 from 1.1 to 8.0% of magnesium; 
 up to 3.0% of iron; 
   optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%; and   optionally up to 100 ppm of Calcium and unavoidable impurities up to 0.02%, a balance being aluminum; and   wherein a coating weight of the metallic coating is from 50 to 500 g/m 2  for the sum of both sides of the steel sheet.   
     
     
         14 . The coated steel sheet according to  claim 13  wherein the metallic coating includes, by weight percent,
 from 7.5 to 9.0% of zinc; 
 2.0 to 4.0% of silicon; 
 1.1 to 4.0% of magnesium; 
 up to 3.0% of iron, 
 
       and optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium, and unavoidable impurities up to 0.01%, the balance being aluminum. 
     
     
         14 . The coated steel sheet according to  claim 13  wherein the coating weight of the coating is from 80 to 150 g/m 2  for the sum of both sides of the steel sheet 
     
     
         15 . A method for manufacturing a hardened part coated with an anti-corrosion coating comprising the following steps:
 A) providing the coated steel sheet as recited in  claim 13 ;   B) cutting the coated steel sheet to obtain a blank;   C) thermally treating the blank at a temperature from 840 to 950° C. to obtain a fully austenitic microstructure in the steel sheet;   D) transferring the blank into a press tool;   E) press hardening of the blank to obtain a part; and   F) cooling of the part to obtain a press-hardened part.   
     
     
         16 . A press-hardened coated steel part obtained by press-hardening of the coated steel sheet as recited in  claim 13 , the coating being topped by a superficial oxide layer on an outer surface, the oxide layer including 20 to 60% weight of zinc on a thickness of 1.5 μm from the outer surface of the coated part. 
     
     
         17 . The press-hardened coated steel part according to  claim 16 , wherein the oxide layer includes 25 to 50% weight of zinc. 
     
     
         18 . The press-hardened coated steel part according to  claim 16  wherein the microstructure of the press-hardened part includes, in terms of volume fraction, at least 95% of martensite. 
     
     
         19 . The press-hardened coated steel part according to  claim 16  wherein the microstructure of the press-hardened part includes, in terms of volume fraction, at least 50% of martensite and less than 40% of bainite. 
     
     
         20 . The press-hardened coated steel part according to  claim 16  wherein the microstructure of the press-hardened part includes from 5 to 20% of martensite, up to 10% of bainite and at least 75% of equiaxed ferrite. 
     
     
         21 . A method for manufacturing a hardened part of an automotive vehicle comprising employing the coated steel sheet as recited in  claim 16 . 
     
     
         22 . The method as recited in  claim 21  wherein the hardened part is a wet area part of the automotive vehicle. 
     
     
         23 . The method as recited in  claim 22  wherein the hardened part is chosen from at least one of the following group: a front rail, a seat cross member, a side sill member, a dash panel cross member, a front floor reinforcement, a rear floor cross member, a rear rail, a B-pillar, a door ring, and a shotgun.