High strength bake hardenable low alloy steel and process for manufacture thereof

Abstract

A bake hardenable steel and a process for manufacture of a bake hardenable steel. Steel alloy slabs with a predefined chemical composition are hot rolled into hot rolled strip. The hot rolled strip is cold rolled and continuously annealed on a continuous annealing line and then rapidly cooled to at least 600° C. using a cooling rate of between 20-100 K/sec, and optionally cooled to at least 140° C. using a cooling rate of between 3-20 K/sec. As such, the cold rolled and annealed sheet is not subjected to an overageing treatment and yet still exhibits excellent and well defined bake hardening values. For example BH2 values between 25 and 45 MPa are exhibited by the low alloy steel grades. In addition, the bake hardeneable steel sheet has a minimum yield strength of 220 MPa and in some instances has a minimum yield strength as high as 380 MPa.Zone

We claim: 
     
         1 . A process for producing high strength bake hardenable low alloy steel without the use of an overaging treatment, the process comprising:
 providing a steel slab having a chemical composition in weight percent within a range of 0.020-0.080 C, 0.20-1.60 Mn, 0.0-1.50 Si, 0.0-0.08 P, 0.0-0.60 Cr, 0.0-0.40 Mo, 0.0-0.50 Cu, 0.0-0.30 Ni, 0.04-0.5 Al and balance being Fe and incidental impurities;   soaking the steel slab;   hot rolling the steel slab and producing hot rolled strip;   coiling the hot rolled strip using a coiling temperature of at least 600° C.;   cold rolling and annealing the coiled hot rolled strip in a continuous annealing line at a temperature equal to or greater than 760° C.; and   cooling the annealed cold rolled sheet using a rapid cooling rate greater than 20 K/sec down to at least 600° C., the annealed and cooled cold rolled sheet having a minimum yield strength of 220 MPa and a BH2 value between 20 and 60 MPa.   
     
     
         2 . The process of  claim 1 , wherein the cooling rate is between 20 and 100 K/sec. 
     
     
         3 . The process of  claim 1 , wherein the annealed cold rolled sheet is cooled to at least 375° C. using a cooling rate between 50-120 K/sec. 
     
     
         4 . The process of  claim 3 , wherein the annealed and cooled cold rolled sheet spends less than 4 minutes in a cooling section of the continuous annealing line. 
     
     
         5 . The process of  claim 1 , wherein the BH2 value of the annealed cold rolled sheet is between 25 and 60 MPa. 
     
     
         6 . The process of  claim 5 , wherein the BH2 value is between 25 and 45 MPa, and C and Mn contents of the steel slab obey the relation:
   25≦−1300·C+118−{(−1300·C+118)·0.5·(Mn−0.2)}≦45   (1)
   
     
     
         7 . The process of  claim 1 , further including cooling the annealed cold rolled sheet using a slow cooling rate between 2-20 K/sec down to 720° C. ±20° C. before cooling the annealed cold rolled sheet using the first cooling rate. 
     
     
         8 . The process of  claim 1 , further including cooling the annealed cold rolled sheet using a less rapid cooling rate between 3-20 K/sec to a temperature of at least 140° C. after the annealed cold rolled sheet is cooled down to at least 600° C. 
     
     
         9 . The process of  claim 1 , wherein the Mn content is less than 0.40, the Si content is less than 0.1, the P content is less than 0.03 and the hot rolled strip is coiled at a temperature equal to or greater than 650° C. 
     
     
         10 . The process of  claim 1 , wherein the coiled hot rolled strip is annealed at a temperature equal to or greater than 780° C. 
     
     
         11 . The process of  claim 1 , further including hot dip coating the cooled cold rolled sheet, the hot dip coated cold rolled sheet having the minimum yield strength of 220 MPa and the BH2 value between 20 and 60 MPa. 
     
     
         12 . The process of  claim 11 , wherein the hot dip coating is hot dip galvanizing. 
     
     
         13 . The process of  claim 12 , wherein the steel slab has less than 0.04 C and the C and Mn contents obey the relation:
   25≦−1300·C+118−{(−1300·C+118)·0.5·(Mn−0.2)}≦45   (1)
   
     
     
         14 . A bake hardenable low alloy cold rolled and annealed steel sheet produced without the use of an overaging treatment, said steel sheet comprising:
 a chemical composition in weight percent within a range of 0.020-0.080 C, 0.20-1.60 Mn, 0.0-1.50 Si, 0.0-0.08 P, 0.0-0.60 Cr, 0.0-0.40 Mo, 0.0-0.50 Cu, 0.0-0.30 Ni, 0.04 -0.5 Al and balance being Fe and incidental impurities; and   a minimum yield strength of 220 MPa and a BH2 value between 20 and 60 MPa.   
     
     
         15 . The steel sheet of  claim 14 , wherein said BH2 value is between 25 and 60 MPa. 
     
     
         16 . The steel sheet of  claim 15 , wherein said BH2 value is between 25 and 45 MPa and said C and Mn contents obey the relation:
   25≦−1300·C+118−{(−1300·C+118)·0.5·(Mn−0.2)}≦45   (1)
   
     
     
         17 . The steel sheet of  claim 16 , wherein said Mn content is less than 0.40, said Si content is less than 0.1 and said P content is less than 0.03. 
     
     
         18 . The steel sheet of  claim 14 , wherein said Mn content of between 0.8 and 1.4. 
     
     
         19 . The steel sheet of  claim 14 , further comprising said chemical composition having at least one of a Ti content up to 0.025 and a B content up to 0.0070.