Bake-Hardenable Cold Rolled Steel Sheet With Superior Strength and Aging Resistance, Gal-Vannealed Steel Sheet Using the Cold Rolled Steel Sheet and Method For Manufacturing the Cold Rolled Steel Sheet

Abstract

A bake-hardenable cold rolled steel sheet with high strength and superior aging resistance used for outer panels of an automobile body, a galvannealed steel sheet using the cold-rolled steel sheet, and a method for manufacturing the cold-rolled steel sheet are disclosed. The steel sheet comprises, by weight %, C: 0.0016˜0.0025%, Si: 0.02% or less, Mn: 0.2˜1.2%, P: 0.05˜0.11%, S: 0.01% or less, Sol. Al: 0.08˜0.12%, N: 0.0025% or less, Ti: 0˜0.003%, Nb: 0.003˜0.011%, Mo: 0.01˜0.1%, B: 0.0005˜0.0015%, the balance of Fe and other unavoidable impurities. The steel sheet has superior bake hardenability, aging resistance at room temperature, and secondary work embrittlement resistance.

1 . A bake-hardenable cold-rolled steel sheet with high strength and superior aging resistance, comprising, by weight %: C: 0.0016˜0.0025%; Si: 0.02% or less; Mn: 0.2˜1.2%; P: 0.05˜0.11%; S: 0.01% or less; Sol. Al: 0.08˜0.12%; N: 0.0025% or less; Ti: 0˜0.003%; Nb: 0.003˜0.011%; Mo: 0.01˜0.1%; B: 0.0005˜0.0015%; and the balance of Fe and other unavoidable impurities, wherein the steel sheet satisfies Equation 1:
   C*[amount of solute carbon in grain boundaries (GB-C)+amount of solute carbon in crystal grains (G-C)]=Total C (ppm)−C in NbC=8˜15 ppm  (1)   
       [in Equation 1, GB-C (that is, the amount of solute carbon in the grain boundaries) is 5˜10 ppm, and G-C (that is, the amount of solute carbon in the crystal grains) is 3˜7 ppm], and 
       wherein the steel sheet has a grain size of ASTM No. 9 or more, the bake hardenability value (BH) of 30 MPa or more, aging index (AI) of 30 MPa or less, and a DBTT of −30° C. or less at a draw ratio of 2.0, the bake hardenability value BH) and the aging index (AI) satisfying Equations 2 and 3, respectively:
   BH=50−(885×Ti)−(1589×Nb)+(62×Al)  (2) 
   AI=44−(423×Ti)−(2119×Nb)−(125×Mo)  (3). 
 
     
     
         2 . A galvannealed bake hardenable steel sheet with high strength and superior aging resistance, comprising, by weight %: C: 0.0016˜0.0025%; Si: 0.02% or less; Mn: 0.2˜1.2%; P: 0.05˜0.11%; S: 0.01% or less; Sol. Al: 0.08˜0.12%; N: 0.0025% or less; Ti: 0˜0.003%; Nb: 0.003˜0.011%; Mo: 0.01˜0.1%; B: 0.0005˜0.0015%; and the balance of Fe and other unavoidable impurities, wherein the steel sheet satisfies Equation 1:
   C*[amount of solute carbon in grain boundaries (GB-C)+amount of solute carbon in crystal grains (G-C)]=Total C (ppm)−C in NbC=8˜15 ppm  (1)   
       [in Equation 1, GB-C (that is, the amount of solute carbon in the grain boundaries) is 5˜10 ppm, and G-C (that is, the amount of solute carbon in the crystal grains) is 3˜7 ppm], and 
       wherein the steel sheet has a grain size of ASTM No. 9 or more, the bake hardenability value (BH) of 30 MPa or more, an aging index (AI) of 30 MPa or less, and a DBTT of −30° C. or less at a draw ratio of 2.0, the bake hardenability value (BH) and the aging index (AI) satisfying Equations 2 and 3, respectively:
   BH=50−(885×Ti)−(1589×Nb)+(62×Al)  (2) 
   AI=44−(423×Ti)−(2119×Nb)−(125×Mo)  (3) 
 
     
     
         3 . A method for manufacturing a bake-hardenable cold-rolled steel sheet with high strength and superior aging resistance, comprising: performing homogenization heat treatment for an Al-killed steel slab at 1200° C. or more, the steel slab comprising, by weight %: C: 0.0016˜0.0025%, Si: 0.02% or less, Mn: 0.2˜1.2%, P: 0.05˜0.11%, S: 0.01% or less, Sol. Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0˜0.003%, Nb: 0.003˜0.011%, Mo: 0.01˜0.1%, B: 0.0005˜0.0015%, and the balance of Fe and other unavoidable impurities;
 hot rolling the steel slab with finish rolling at a finish rolling temperature of 900˜950° C. to form a hot-rolled steel sheet, followed by coiling the hot-rolled steel sheet at a temperature of 580˜630° C.;   cold rolling the hot-rolled steel sheet at a reduction ratio of 75˜80%;   continuously annealing the cold-rolled steel sheet at a temperature of 770˜830° C.; and   temper rolling the anneal steel sheet at a reduction ratio of 1.2˜1.5%.