High strength hot dip galvanised steel strip

Abstract

A high strength hot dip galvanised steel strip having, in mass percent, the following elements: 0.13-0.19% C; 1.70-2.50% Mn; max 0.15% Si; 0.40-1.00% Al; 0.05-0.25% Cr; 0.01-0.05% Nb; max 0.10% P; max 0.004% Ca; max 0.05% S; max 0.007% N; and optionally at least one of the following elements: max 0.50% Ti, max 0.40% V, max 0.50% Mo, max 0.50% Ni, max 0.50% Cu, max 0.005% B, the balance being Fe and inevitable impurities; wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%. This steel offers improved formability at a high strength, has a good weldability, and surface quality together with a good producability and coatability.

1 . A high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements:
 0.13-0.19% C   1.70-2.50% Mn   max 0.15% Si   0.40-1.00% Al   0.05-0.25% Cr   0.01-0.05% Nb   max 0.10% P   max 0.004% Ca   max 0.05% S   max 0.007% N   
       and optionally at least one of the following elements:
 max 0.50% Ti 
 max 0.40% V 
 max 0.50% Mo 
 max 0.50% Ni 
 max 0.50% Cu 
 max 0.005% B 
 
       the balance being Fe and inevitable impurities, 
       wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%, 
       wherein the hot dip galvanised steel strip has a microstructure containing 8-12% retained austenite, 10-20% martensite, the remainder being a mixture of ferrite and bainite, the hot dip galvanised steel strip containing not more than 10% bainite, and wherein the hot dip galvanised steel strip has an ultimate tensile strength Rm of at least 700 MPa, a 0.2% proof strength Rp of at least 400 MPa and a total elongation of at least 18%. 
     
     
         2 . The steel strip according to  claim 1 , wherein element C is present in an amount of 0.13-0.16%. 
     
     
         3 . The steel strip according to  claim 1 , wherein element Mn is present in an amount of 1.95-2.40%. 
     
     
         4 . The steel strip according to  claim 1 , wherein element Si is present in an amount of 0.05-0.15%. 
     
     
         5 . The steel strip according to  claim 1 , wherein element Al is present in an amount of 0.60-0.80%. 
     
     
         6 . The steel strip according to  claim 1 , wherein element Cr is present in an amount of 0.10-0.25%. 
     
     
         7 . The steel strip according to  claim 1 , wherein the element Nb is present in an amount of 0.01-0.04%. 
     
     
         8 . The steel strip according to  claim 1 , wherein the hot dip galvanised steel strip has an ultimate tensile strength Rm of at least 750 MPa. 
     
     
         9 . The steel strip according to  claim 1 , wherein the hot dip galvanised steel strip has an 0.2% proof strength Rp of at least 450 MPa. 
     
     
         10 . The steel strip according to  claim 1 , wherein the hot dip galvanised steel strip has a hole expansion coefficient of at least 35% when Rm is 750 MPa and Rp is 450 MPa. 
     
     
         11 . The steel strip according to  claim 1 , wherein the hot dip galvanised steel strip has an Erichsen cupping index of more than 10.5 mm when Rm is 750 MPa and Rp is 450 MPa. 
     
     
         12 . The steel strip according to  claim 1 , wherein the hot dip galvanised steel strip has an average grain size of at most 5 μm. 
     
     
         13 . A method for producing a high strength hot dip galvanised steel strip according to  claim 1 , comprising:
 hot rolling and cold rolling the cast steel to a strip having a desired thickness,   after said rolling to the desired thickness reheating the strip in an annealing line to a temperature above the Ac1 temperature of the steel, and   fast cooling the reheated strip at a cooling rate to avoid retransformation to ferrite, after which isothermal overaging is applied to form bainite, and the strip is hot dip galvanised.   
     
     
         14 . The method according to  claim 13 , wherein the annealing is applied at a temperature between 750° C. and 850° C. 
     
     
         15 . The method according to  claim 13 , wherein the overaging is applied at a temperature between 360° C. and 480° C. 
     
     
         16 . The steel strip according to  claim 1 , wherein element Mn is present in an amount of 1.95-2.30%. 
     
     
         17 . The steel strip according to  claim 1 , wherein element Mn is present in an amount of 2.00-2.20%. 
     
     
         18 . The method according to  claim 14 , wherein the annealing is applied at a temperature between 780° C. and 820° C. 
     
     
         19 . The method according to  claim 14 , wherein the strip is reheated in the annealing line to a temperature between the Ac1 and the Ac3 temperature of the steel type.