High strength hot dip galvanised steel strip

Abstract

The invention relates to a high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements: 0.10-0.21% C 1.75-2.50% Mn 0.04-0.60% Si 0.20-1.40% Al 0.001-0.025% P 0.0005-0.0050% B max 0.50% Cr max 0.20% Ti max 0.004% Ca max 0.015% N the balance being Fe and inevitable impurities.

1 . A high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements:
 0.10-0.21% C   1.75-2.50% Mn   0.04-0.60 % Si   0.20-1.40% Al   0.001-0.025% P   0.0011-0.0040% B   max 0.50% Cr   max 0.20% Ti   max 0.004% Ca   max 0.015% N   
       the balance being Fe and inevitable impurities. 
     
     
         2 . The steel strip according to  claim 1 , wherein the composition meets at least one feature selected from the group consisting of:
 amounts of Al and Si are such that 0.60%<Al+Si<1.40%,   amounts of Mn and Cr are such that Mn+Cr>2.00%, and   amounts of Al and Si are such that Si≤Al.   
     
     
         3 . The steel strip according to  claim 1 , wherein element C is present in an amount of 0.13-0.18%. 
     
     
         4 . The steel strip according to  claim 1 , wherein element Si is present in an amount of 0.05-0.50%. 
     
     
         5 . The steel strip according to  claim 1 , wherein element Al is present in an amount of 0.30-1.20%. 
     
     
         6 . The steel strip according to  claim 1 , wherein element B is present in an amount of 0.0015-0.0030%. 
     
     
         7 . The steel strip according to  claim 1 , wherein the element Ti is present in an amount of max 0.10%. 
     
     
         8 . The steel strip according to  claim 1 , wherein the hot dip galvanised steel strip has an ultimate tensile strength Rm above 750 MPa and/or a 0.2% proof strength Rp of 430-700 MPa. 
     
     
         9 . The steel strip according to  claim 1 , wherein microstructure of the hot dip galvanised steel strip consists of 20-50 volume % ferrite, 10-25 volume % martensite and retained austenite, of which 5-12 volume % retained austenite, the remainder being tempered martensite, bainite and cementite. 
     
     
         10 . The method for producing a high strength hot dip galvanised dual phase steel strip according to  claim 1 , wherein
 the cast steel is hot rolled to a thickness of 2.0-4.0 mm and coiled at a Coiling Temperature CT below Bs−20° C. temperature and above Ms+60° C. temperature,   the strip is cold rolled with a reduction of 40% or more,   after which the strip is intercritically annealed at a temperature between Ac1 and Ac3 temperature, and   the annealed strip is overaged at a temperature below Bs temperature to form bainite and/or tempered martensite,   after which the strip is hot dip galvanised.   
     
     
         11 . The method according to  claim 10 , wherein microstructure of the hot rolled coil consists of 50-70 volume % ferrite, 20-50 volume % pearlite and/or bainite, and less than 10% cementite. 
     
     
         12 . The method according to  claim 10 , wherein the hot dip galvanised strip is tension rolled with a reduction of 0.2-0.8%. 
     
     
         13 . The method for producing a high strength hot dip galvanised complex phase steel strip according to  claim 1 , wherein
 the cast steel is hot rolled to a thickness of 2.0-4.0 mm and coiled at a Coiling Temperature CT below Bs−20° C. temperature and above Ms+60° C. temperature,   the strip is cold rolled with a reduction of 40% or more,   after which the strip is annealed at a temperature above Ac1 temperature plus 50° C., and   the strip is overaged at a temperature below Bs temperature to form bainite and/or tempered martensite,   after which the strip is hot dip galvanised.   
     
     
         14 . The method according to  claim 14 , wherein the hot dip galvanised strip is tension rolled with a reduction of 0.4-2.0%. 
     
     
         15 . The steel strip according to  claim 1 , wherein element C is present in an amount of 0.14-0.17 %. 
     
     
         16 . The steel strip according to  claim 1 , wherein element Si is present in an amount of 0.05-0.40%. 
     
     
         17 . The steel strip according to  claim 1 , wherein element Al is present in an amount of 0.40-1.00%. 
     
     
         18 . The steel strip according to  claim 1 , wherein the element Ti is present in an amount of max between 0.005 and 0.05%. 
     
     
         19 . The steel strip according to  claim 1 , wherein the difference between the middle and the edges of the steel strip being less than 75 MPa for both Rp and/or Rm, more preferably this difference being less than 60 MPa. 
     
     
         20 . The method according to  claim 14 , wherein the hot dip galvanised strip is tension rolled with a reduction of 0.4-1.2%.