Steel sheet hot-dip plated with zinc based layer with superior bake hardenability and aging resistance, and manufacturing method thereof

Abstract

Provided are a cold-rolled steel sheet having excellent bake hardenability and aging resistance, and manufacturing method thereof. The cold-rolled steel sheet comprises, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprises one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises a remainder of iron (Fe) and unavoidable impurities, and comprises, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure.

1 . A steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, comprising a cold-rolled steel sheet and a zinc based plating layer formed on a surface of the cold-rolled steel sheet,
 wherein the cold-rolled steel sheet comprises, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprises one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises a remainder of iron (Fe) and unavoidable impurities, and comprises, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure,   wherein a ratio (a/b) of an average carbon concentration a in the martensite and an average carbon concentration b in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of ¼ t of a sheet thickness of the cold-rolled steel sheet is 1.4 or less, and   wherein a ratio (d/c) of an average manganese concentration c in the martensite and an average manganese concentration d in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of ¼ t of a sheet thickness of the cold-rolled steel sheet is 0.9 or less.   
     
     
         2 . The steel sheet hot-dip plated with zinc based layer according to  claim 1 ,
 wherein an occupancy ratio (M) of martensite having an average circle equivalent diameter of 5 μm or less (excluding 0 μm) present at ferrite grain boundaries (including grain boundary triple points) defined by the following Relationship 1, in the cold-rolled steel sheet, is 90% or more:
   [Relationship 1]  M={M   gb /( M   gb + M   in )}×100
 
   (Where M gb  refers to the number of martensite having an average circle equivalent diameter of 5 μm or less (excluding 0 μm) present at ferrite grain boundaries, and M in  refers to the number of martensite having an average circle equivalent diameter of 5 μm or less (excluding 0 μm) present in ferrite crystal grains)   
     
     
         3 . The steel sheet hot-dip plated with zinc based layer according to  claim 1 ,
 wherein the cold-rolled steel sheet further comprises bainite as a microstructure, and an area ratio (B) of the bainite defined by the following Relationship 2 is 3 or less:
   [Relationship 2]  B={A   B  /( A   F + A   M + A   B )}×100
 
   (Where A F  refers to an area ratio of ferrite, A M  refers to an area ratio of martensite, and A B  refers to an area ratio of bainite)   
     
     
         4 . The steel sheet hot-dip plated with zinc based layer according to  claim 1 ,
 wherein the zinc based plating layer is a galva-annealed layer.   
     
     
         5 . The steel sheet hot-dip plated with zinc based layer according to  claim 1 ,
 wherein the steel sheet hot-dip plated with zinc based layer has the bake hardenability (BH) of 35 MPa or more.   
     
     
         6 . The steel sheet hot-dip plated with zinc based layer according to  claim 1 ,
 wherein the steel sheet hot-dip plated with zinc based layer has a yield ratio of 0.57 or less and an elongation of 33% or less.   
     
     
         7 . A method of manufacturing a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, comprising:
 reheating a steel slab comprising, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprising one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprising a remainder of iron (Fe) and unavoidable impurities;   hot-rolling the reheated steel slab in a single phase temperature region of austenite to obtain a hot-rolled steel sheet;   coiling the hot-rolled steel sheet;   cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;   continuously annealing the cold-rolled steel sheet at a temperature in a range of 760 to 850° C.;   firstly cooling the continuously annealed cold-rolled steel sheet to a temperature in a range of 630 to 670° C. at an average cooling rate of 2 to 14° C./sec;   secondly cooling the firstly cooled cold-rolled steel sheet to a temperature in a range of (Ms+20) to (Ms+50)° C. at an average cooling rate of 3 to 12° C./sec;   thirdly cooling the secondly cold-rolled steel sheet to a temperature in a range of 440 to 480° C. at a rate of 4 to 8° C./sec;   immersing the thirdly cooled cold-rolled steel sheet in a zinc based hot bath to obtain a steel sheet hot-dip plated with zinc based layer; and   finally cooling the steel sheet hot-dip plated with zinc based layer to a temperature in a range of (Ms−100)° C. or lower at an average cooling rate of 3° C./sec or higher.   
     
     
         8 . The method according to  claim 7 , wherein the reheating temperature is within a range of 1100 to 1300° C. at the time of reheating the slab. 
     
     
         9 . The method according to  claim 7 , wherein a finish rolling temperature at the time of the hot-rolling is within a range of (Ar3+50) to 950° C. 
     
     
         10 . The method according to  claim 7 , wherein the coiling temperature at the time of the coiling is within a range of 450 to 700° C. 
     
     
         11 . The method according to  claim 7 , wherein a cold-reduction ratio at the time of the cold-rolling is 40 to 80%. 
     
     
         12 . The method according to  claim 7 , wherein the annealing temperature at the time of the continuously annealing is within a range of 770 to 810° C. 
     
     
         13 . The method according to  claim 7 , wherein a temperature of the zinc based hot bath is within a range of 440 to 480° C. 
     
     
         14 . The method according to  claim 7 , further comprising subjecting the steel sheet hot-dip plated with zinc based layer to an alloying heat treatment at a temperature in a range of 480 to 600° C., before the final cooling. 
     
     
         15 . The method according to  claim 7 , further comprising temper rolling at a reduction ratio of 0.3 to 1.6%, after the final cooling.