A method for the manufacture of a coated steel sheet

Abstract

Method for the manufacture of a coated steel sheet including the following successive steps A. the provision of an annealed steel sheet having the following chemical composition in weight: 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<2.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0%, and on a purely optional basis, one or more elements such as Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.50%, Ni≤1.0%, Ti≤0.5%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, such steel sheet being annealed at a temperature between 600 to 1200° C., B. the coating of the steel sheet obtained in step A) with a first coating including nickel, such first coating not including iron, and having a thickness equal or above 0.5 μm and C. the coating of the steel sheet obtained in step B) with a second coating based on zinc. A steel sheet, a spot welded joint and further uses of the steel sheet are also provided.

What is claimed is: 
     
         1 - 27 . (canceled) 
     
     
         28 . A method for the manufacture of a coated steel sheet comprising the following successive steps:
 providing an annealed steel substrate having the following chemical composition in weight:
 0.10<C<0.40%, 
 1.5<Mn<3.0%, 
 0.7<Si<2.0%, 
 0.05<Al<1.0%, 
 0.75<(Si+Al)<3.0%, 
   and on a purely optional basis, one or more elements such as
 Nb≤0.5%, 
 B≤0.005%, 
 Cr≤1.0%, 
 Mo≤0.50%, 
 Ni≤1.0%, 
 Ti≤0.5%, 
   remainder of the composition making up of iron and inevitable impurities resulting from processing, the steel sheet being annealed at a temperature between 600 to 1200° C.;   coating of the annealed steel substrate with a first coating including nickel but not including iron, and having a thickness equal or above 0.5 μm; and   coating, with a second coating based on zinc, the annealed steel substrate coated with the first coating.   
     
     
         29 . The method as recited in  claim 28  wherein the steel substrate is annealed in a continuous annealing. 
     
     
         30 . The method as recited in  claim 28  wherein the annealing is performed in an atmosphere comprising from 1 to 30% of H 2  at a dew point between −10 and −60° C. 
     
     
         31 . The method as recited in  claim 28  wherein the first coating includes above 80% by weight of nickel. 
     
     
         32 . The method as recited in  claim 31  wherein the first coating includes above 90% by weight of nickel. 
     
     
         33 . The method as recited in  claim 32  wherein the first coating consists of nickel. 
     
     
         34 . The method as recited in  claim 28  wherein the first coating does not include phosphorus, nickel hydroxide or sulfur compounds. 
     
     
         35 . The method as recited in  claim 28  wherein the first coating has a thickness equal or above 1.0 μm. 
     
     
         36 . The method as recited in  claim 35  wherein the first coating has a thickness equal or above 1.6 μm. 
     
     
         37 . The method as recited in  claim 36  wherein the first coating has a thickness between 1.8 to 7.0 μm. 
     
     
         38 . The method as recited in  claim 28  wherein the second coating includes above 50% of zinc. 
     
     
         39 . The method as recited in  claim 38  wherein the second coating includes above 75% of zinc. 
     
     
         40 . The method as recited in  claim 39  wherein the second coating includes above 90% of zinc. 
     
     
         41 . The method as recited in  claim 28  wherein the second coating does not comprise nickel. 
     
     
         42 . The method as recited in  claim 40  wherein the second coating consists of zinc. 
     
     
         43 . A steel sheet obtainable from the method as recited in  claim 28 , the steel sheet comprising a steel substrate coated with a first coating comprising nickel and having a thickness equal or above 0.5 μm, the first coating being directly topped by a zinc based layer. 
     
     
         44 . The steel sheet as recited in  claim 43  wherein a steel microstructure of the steel substrate includes from 1 to 50% of residual austenite, from 1 to 60% of martensite and optionally at least one element chosen from: bainite, ferrite, cementite and pearlite. 
     
     
         45 . The steel sheet as recited in  claim 44  wherein the microstructure comprises from 5 to 25% of residual austenite. 
     
     
         46 . The steel sheet as recited in  claim 44  wherein the microstructure comprises from 1 to 60% of tempered martensite. 
     
     
         47 . The steel sheet as recited in  claim 44  wherein the microstructure comprises from 10 to 40% of bainite. 
     
     
         48 . The steel sheet as recited in  claim 44  wherein the microstructure comprises from 1 to 25% of ferrite. 
     
     
         49 . The steel sheet as recited in  claim 44  wherein the microstructure comprises from 1 to 15% of untempered martensite. 
     
     
         50 . A spot welded joint comprising:
 at least two metal sheets including a first metal sheet being the steel sheet as recited in  claim 43 , the joint containing less than 3 cracks having a size above 100 μm and wherein the longest crack has a length below 300 μm.   
     
     
         51 . The spot welded joint as recited in  claim 50  wherein a second metal sheet of the metal sheets is a steel sheet or an aluminum sheet. 
     
     
         52 . The spot welded joint as recited in  claim 50  wherein a second metal sheet of the metal sheets is a second steel sheet including an annealed second steel substrate with a first coating including nickel but not including iron, and having a thickness equal or above 0.5 μm; and with a second coating based on zinc. 
     
     
         53 . The spot welded joint as recited in  claim 50  wherein the at least two metal sheets include a third metal sheet being a steel sheet or an aluminum sheet. 
     
     
         54 . A spot welded joint comprising:
 at least two metal sheets including a first metal sheet being a steel sheet obtained by the method as recited in  claim 28 , the joint containing less than 3 cracks having a size above 100 μm and wherein the longest crack has a length below 300 μm.   
     
     
         55 . An automotive vehicle part comprising the steel sheet as recited in  claim 43 . 
     
     
         56 . An automotive vehicle part comprising a steel sheet obtained by the method as recited in  claim 28 .