Cold rolled and continuously annealed high strength steel strip and method for producing said steel

Abstract

Cold rolled and continuously annealed high strength steel strip including (in wt %): 0.04-0.30% C, 1.0-3.5% Mn, 0-1.0% Si, 0-2.0% Al, 0-1.0% Cr, 0-0.02% P, 0-0.01% S, 0-0.25% V, 0-0.1% Nb, 0-0.20% Ti, 0-0.015% N, 0-0.010% B, unavoidable impurities, balance iron, provided with a hot dip galvanized or galvannealed zinc alloy coating layer, wherein the zinc alloy is 0.3-4.0% Mg and 0.05-6.0% Al; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc, and method for producing it.

1 . Cold rolled and continuously annealed high strength steel strip comprising (in wt %):
 0.04-0.30% C   1.0-3.5% Mn   0-1.0% Si   0-2.0% Al   0-1.0% Cr   0-0.02% P   0-0.01% S   0-0.25% V   0-0.1% Nb   0-0.20% Ti   0-0.015% N   0-0.010% B   unavoidable impurities   balance iron,   
       provided with a hot dip galvanized or galvannealed zinc alloy coating layer, wherein the zinc alloy consists of 0.3-4.0% Mg and 0.05-6.0% Al; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc. 
     
     
         2 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , in which the zinc alloy consists of 0.3-2.3 weight % magnesium and 0.6-2.3 weight % aluminium; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc. 
     
     
         3 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip has a microstructure which comprises between 90 and 65% of ferrite, the remainder of the microstructure being acicular ferrite, bainite, martensite or retained austenite. 
     
     
         4 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip consists of:
 0.04-0.30% C   1.0-3.5% Mn   0-1.0% Si   0-2.0% Al   0-1.0% Cr   0-0.02% P   0-0.01% S   0-0.25% V   0-0.1% Nb   0-0.20% Ti   0-0.015% N   0-0.010% B   unavoidable impurities   balance iron.   
     
     
         5 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip comprises:
 0.07-0.16% C   1.4-2.0% Mn   0.2-0.4% Si   0.5-1.5% Al   0.4-0.8% Cr   0-0.05% Ti   0-0.03% Nb   0-0.01% N   0-0.002% B   V as impurity   unavoidable impurities   balance iron.   
     
     
         6 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip comprises:
 0.07-0.20% C   1.2-3.5% Mn   0-1.5% Al   0-0.15% Ti   0-0.002% B.   
     
     
         7 . Steel strip provided with a zinc alloy coating layer according to  claim 6  wherein the steel strip comprises:
 0.07-0.12% C   1.2-2.0% Mn   0-0.4% Si   0-1.0% Al   0-0.05% Ti   0-0.07% Nb   0-0.01% N   0-0.002% B.   
     
     
         8 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip comprises:
 0.15-0.30% C   1.5-3.5% Mn   0.5-2.0% Al   0-0.05% Nb   0-0.01% N   0-0.002% B,   unavoidable impurities   balance iron.   
     
     
         9 . Steel strip provided with a hot dip galvanized zinc alloy coating layer according to  claim 8 , wherein the steel strip comprises:
 0.15-0.20% C   1.5-2.0% Mn   0.2-0.6% Si   0.5-1.5% Al   0-0.05% Nb.   
     
     
         10 . Method for producing a cold rolled and continuously annealed high strength steel strip comprising the subsequent steps of:
 providing a cold-rolled steel strip comprising:
 0.04-0.30% C 
 1.0-3.5% Mn 
 0-1.0% Si 
 0-2.0% Al 
 0-1.0% Cr 
 0-0.02% P 
 0-0.01% S 
 0-0.25% V 
 0-0.1% Nb 
 0-0.20% Ti 
 0-0.015% N 
 0-0.010% B 
 unavoidable impurities 
 balance iron 
   heating the strip to an intercritical annealing temperature IAT   holding the strip at the IAT for an annealing time sufficient to attain the desired ratio of ferrite to austenite   cooling the strip to an over-aging temperature OAT   holding the strip at the OAT for an over-aging time   heating the strip to a galvanising temperature GT   coating the strip with a zinc alloy coating layer by dipping the strip into a molten bath having a zinc bath temperature ZBT containing the zinc alloy for the zinc alloy coating layer   cooling the coated strip.   
     
     
         11 . Method according to  claim 10 , wherein the OAT is at most 150° C. lower than the GT and/or wherein the OAT is at least 10° C. lower than the GT. 
     
     
         12 . Method according to  claim 10 , wherein the temperature increase from the OAT to the GT is achieved by an induction heating step. 
     
     
         13 . Method according to  claim 10 , wherein the ZBT is at most 25° C. lower than the GT. 
     
     
         14 . Method according to  claim 10 , wherein the OAT is between 350 and 450° C. and/or wherein the ZBT is between 430 and 490° C. and/or wherein the AIT is between 750 and 850° C. 
     
     
         15 . Method for producing a cold rolled and continuously annealed high strength steel strip according  claim 10 , wherein the steel strip comprises, in wt %:
 0.07-0.16% C   1.4-2.0% Mn   0.2-0.4% Si   0.5-1.5% Al   0.4-0.8% Cr   0-0.05% Ti   0-0.03% Nb   0-0.01% N   0-0.002% B   V as impurity unavoidable impurities balance iron.   
     
     
         16 . Method according to  claim 15 , wherein the OAT is between 380 and 430° C. for an over-aging time of between 40 seconds and 150 seconds. 
     
     
         17 . Method according to  claim 10 , wherein the coated steel substrate is subjected to a galvannealing step after the coating step and prior to the cooling step. 
     
     
         18 . Method according to  claim 10 , wherein
 the zinc alloy consists of 0.3-4.0% Mg and 0.05-6.0% Al; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc.   
     
     
         19 . Method according to  claim 18 , in which
 the zinc alloy consists of 1.5-2.3 weight % magnesium and 1.5-2.3 weight % aluminium; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc; or   the zinc alloy bath contains 0.6-1.3 weight % aluminium; or   the zinc alloy bath contains 0.3-1.3 weight % magnesium.   
     
     
         20 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the zinc alloy consists of 1.6-2.3 weight % magnesium and 1.6-2.3 weight % aluminium; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc. 
     
     
         21 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip comprises:
 0.08-0.12% C   1.5-1.8% Mn   0.25-0.4% Si   0.5-1.0% Al   0.4-0.8% Cr   0-0.05% Ti   0-0.03% Nb   0-0.01% N   0-0.002% B   V as impurity   unavoidable impurities   balance iron.   
     
     
         22 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip comprises:
 0.07-0.17% C   1.2-2.5% Mn   0-1.0% Al   0-0.15% Ti   0-0.002% B.   
     
     
         23 . Steel strip provided with a zinc alloy coating layer according to  claim 1 , wherein the steel strip comprises:
 0.15-0.24% C   1.5-2.0% Mn   0.5-1.5% Al   0-0.05% Nb   0-0.01% N   0-0.002% B,   unavoidable impurities   balance iron.   
     
     
         24 . Method according to  claim 10 , wherein the AIT is between 780 and 830° C. 
     
     
         25 . Method for producing a cold rolled and continuously annealed high strength steel strip according  claim 10 , wherein the steel strip comprises, in wt %:
 0.08-0.12% C   1.5-1.8% Mn   0.25-0.4% Si   0.5-1.5% Al   0.4-0.8% Cr   0-0.05% Ti   0-0.03% Nb   0-0.01% N   0-0.002% B   V as impurity   unavoidable impurities   balance iron.   
     
     
         26 . Method according to  claim 10 , wherein the strip being heated to the IAT has a substantially ferritic steel matrix upon commencement of the heating to the IAT and the heating to the IAT transforms the strip having the substantially ferritic steel matrix partly to austenite, wherein the volume fraction of austenite content prior to cooling to the OAT is at most 50%. 
     
     
         27 . Method according to  claim 18 , in which
 the zinc alloy bath contains 0.7-1.2 weight % aluminium; or   the zinc alloy bath contains 0.7-1.2 weight % magnesium.