Method for Manufacturing Bake-Hardenable High-Strength Cold-Rolled Steel Sheet

Abstract

A bake-hardenable high-strength cold-rolled steel sheet, a hot-dipped steel sheet thereof, and a method for manufacturing the same. The steel sheet comprises 0.0016˜0.01% of C; 0.1% or less of Si; 0.2˜1.5% of Mn; 0.05˜0.15% of P; 0.01% or less of S; 0.08˜0.5% of (soluble) Al; 0.0025% or less of N; 0.003˜0.1% of Nb; 0.003% or less of Ti; 0.01˜0.4% of Mo; 0.0005˜0.005% of B; and the balance of Fe and other unavoidable impurities, in terms of weight %. The method includes the steps of heating a steel slab of the above composition to a temperature of 1,200° C. or more; and then hot rolling the steel slab with finish rolling at or above the Ar 3 temperature to provide a hot rolled steel sheet; coiling the hot rolled steel sheet; cold rolling the hot rolled and coiled sheet; and continuous annealing the cold rolled sheet.

1 . A method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at or above an Ar 3  transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet;   cold rolling the hot-rolled steel sheet; and   continuous annealing the cold-rolled steel sheet.   
   
   
       2 . A method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at or above an Ar 3  transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.003% or less of Ti, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet;   cold rolling the hot-rolled steel sheet; and   continuous annealing the cold-rolled steel sheet.   
   
   
       3 . A method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet at a temperature of 600-650° C.;   cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%; and   continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.   
   
   
       4 . A method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003% or less of Ti, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet at a temperature of 600-650° C.;   cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%; and   continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.   
   
   
       5 . A method for manufacturing a hot-dipped steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at or above an Ar 3  transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet;   cold rolling the hot-rolled steel sheet;   continuous annealing the cold-rolled steel sheet;   hot dipping the annealed steel sheet; and   temper rolling the hot-dipped steel sheet.   
   
   
       6 . A method for manufacturing a hot-dipped steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at or above an Ar 3  transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.003% or less of Ti, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet;   cold rolling the hot-rolled steel sheet;   continuous annealing the cold-rolled steel sheet;   hot dipping the annealed steel sheet; and   temper rolling the hot-dipped steel sheet.   
   
   
       7 . A method for manufacturing a hot-dipped steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet at a temperature of 600-650° C.;   cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%;   continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.;   hot dipping the annealed steel sheet; and   temper rolling the hot-dipped steel sheet at a reduction rate of 12-1.5%.   
   
   
       8 . A method for manufacturing a hot-dipped steel sheet, comprising the steps of:
 hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel stab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003% or less of Ti, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;   coiling the hot-rolled steel sheet at a temperature of 600-650° C.;   cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%;   continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.;   hot dipping the annealed steel sheet; and   temper rolling the hot-dipped steel sheet at a reduction rate of 1.2-1.5%.