Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheets

Abstract

Disclosed herein are 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% of 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 steel sheet has fine AlN precipitates, and a grain size (ASTM No.) of 9 or more. The AlN precipitates have a grain size, which can suppress grain growth. The steel sheet has enhanced strength, bake hardenability, aging resistance, and secondary work embrittlement resistance.

1 . A bake-hardenable high-strength cold-rolled steel sheet manufactured through hot rolling, cold rolling and continuous annealing of a steel, 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, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.    
   
   
       2 . The steel sheet according to  claim 1 , wherein Al content is greater than 0.1% and equal to or less than 0.5%.  
   
   
       3 . The steel sheet according to  claim 1 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       4 . The steel sheet according to  claim 1 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       5 . The steel sheet according to  claim 3 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       6 . The steel sheet according to  claim 1 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       7 . The steel sheet according to  claim 3 , wherein the steel sheet further comprises NbC precipitates having an average grain size of 30 nm or less.  
   
   
       8 . The steel sheet according to  claim 4 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       9 . The steel sheet according to  claim 5 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       10 . A bake-hardenable high-strength cold-rolled steel sheet manufactured through hot rolling, cold rolling and continuous annealing of a steel, 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, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.    
   
   
       11 . The steel sheet according to  claim 10 , wherein Al content is greater than 0.1% and equal to or less than 0.5%.  
   
   
       12 . The steel sheet according to  claim 10 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       13 . The steel sheet according to  claim 10 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       14 . The steel sheet according to  claim 12 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       15 . The steel sheet according to  claim 10 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       16 . The steel sheet according to  claim 12 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       17 . The steel sheet according to  claim 13 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       18 . The steel sheet according to  claim 14 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       19 . A bake-hardenable high-strength cold-rolled steel sheet, 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, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (Al) of 30 Mpa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.    
   
   
       20 . The steel sheet according to  claim 19 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       21 . A bake-hardenable high-strength cold-rolled steel sheet, 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, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (AI) of 30 MPa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.    
   
   
       22 . The steel sheet according to  claim 21 , wherein the steel sheet comprises greater amounts of solute carbon in a grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       23 . A hot-dipped steel sheet manufactured through hot rolling, cold rolling, continuous annealing, hot dipping, and temper rolling of a steel, 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, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.    
   
   
       24 . The steel sheet according to  claim 23 , wherein Al content is greater than 0.1% and equal to or less than 0.5%.  
   
   
       25 . The steel sheet according to  claim 23 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       26 . The steel sheet according to  claim 23 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       27 . The steel sheet according to  claim 25 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       28 . The steel sheet according to  claim 23 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       29 . The steel sheet according to  claim 25 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       30 . The steel sheet according to  claim 26 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       31 . The steel sheet according to  claim 27 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       32 . A hot-dipped steel sheet manufactured through hot rolling, cold rolling, continuous annealing, hot dipping, and temper rolling of a steel, 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, wherein the steel sheet has fine AN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.    
   
   
       33 . The steel sheet according to  claim 32 , wherein Al content is greater than 0.1% and equal to or less than 0.5%.  
   
   
       34 . The steel sheet according to  claim 32 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       35 . The steel sheet according to  claim 32 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       36 . The steel sheet according to  claim 34 , wherein the AlN precipitates have an average size of 20 μm or less.  
   
   
       37 . The steel sheet according to  claim 32 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       38 . The steel sheet according to  claim 34 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       39 . The steel sheet according to  claim 35 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       40 . The steel sheet according to  claim 36 , wherein the steel sheet further comprises NbC precipitates having an average size of 30 nm or less.  
   
   
       41 . A hot-dipped steel sheet, 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, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (Al) of 30 Mpa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.    
   
   
       42 . The steel sheet according to  claim 41 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       43 . A hot-dipped steel sheet, 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, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (AI) of 30 MPa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.    
   
   
       44 . The steel sheet according to  claim 43 , wherein the steel sheet comprises greater amounts of solute carbon in the grain boundary than in grain, and the amount of solute carbon in grain is in the range of 3-6 ppm.  
   
   
       45 . 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.    
   
   
       46 . 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.    
   
   
       47 . A method of 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.    
   
   
       48 . A method of 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.    
   
   
       49 . 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.    
   
   
       50 . 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.    
   
   
       51 . 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 1.2-1.5%.    
   
   
       52 . 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% 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%.