Cold rolled steel sheet and method of manufacturing same

Abstract

The present disclosure relates to a cold rolled steel sheet having excellent weldability, strength and formability and a method of manufacturing the same.An aspect of the present disclosure is to provide a cold-rolled steel sheet having excellent weldability, strength, and formability, and a method of manufacturing the same.As set forth above, according to an aspect of the present disclosure, a cold-rolled steel sheet having excellent weldability, strength, and formability and a method of manufacturing the same may be provided.

1 . A cold-rolled steel sheet, comprising by weight:
 C: 0.14 to 0.16%, Si: 0.3 to 0.6%, Al: 0.01 to 0.3%, Mn: 2.6 to 3.0%, Cr: 0.01 to 0.25%, Mo: 0.15 to 0.4%, B: 0.0001 to 0.005%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities, wherein the following Relational Expressions 1 and 2 are satisfied,   wherein a microstructure consists of retained austenite: more than 1% and 5% or less, fresh martensite:   10% or more and less than 25%, bainite: less than 20% (excluding 0%), tempered martensite: 55% or more and less than 80%, and ferrite: 5% or less (including 0%),
   234×[C]−29×[Si]−128×[Al]+29×[Mn]+10×[Cr]−17×[Mo]−37×[Nb]−49×[Ti]+100×[B]≥80  [Relational Expression 1]
 
   5×[C]+[Si]+0.5×[Al]≤1.5  [Relational Expression 2]
 
   where a content of each element in the above Relational Expressions 1 and 2 refers to % by weight.   
     
     
         2 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet further comprises one or two of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%). 
     
     
         3 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet further comprises V: 0.05% or less (excluding 0%). 
     
     
         4 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet has a tensile strength of 1180 to 1350 MPa, a yield strength of 740 to 980 MPa, an elongation of 8% or more, hole expandability of 20% or more, and an AE value of 0 kA or more,
 where the AE value refers to a difference obtained by subtracting a minimum current value at which expulsion occurs from a minimum current value at which LME occurs.   
     
     
         5 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet has a hot-dip galvanized layer or alloyed hot-dip galvanized layer formed on at least one surface of the cold-rolled steel sheet. 
     
     
         6 . A method for manufacturing a cold-rolled steel sheet, comprising:
 heating a slab including by weight: C: 0.14 to 0.16%, Si: 0.3 to 0.6%, Al: 0.01 to 0.3%, Mn: 2.6 to 3.0%, Cr: 0.01 to 0.25%, Mo: 0.15 to 0.4%, B: 0.0001 to 0.005%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities, wherein the following Relational Expressions 1 and 2 are satisfied;   finish hot rolling the heated slab at a temperature within a range of 830 to 980° C. to obtain a hot-rolled steel sheet;   coiling the hot-rolled steel sheet at a temperature within a range of 450 to 700° C.;   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 within a range of 800 to 840° C.;   primarily cooling the continuously annealed steel sheet at an average cooling rate of less than 10° C./s to a primary cooling end temperature of 550 to 650° C.;   secondarily cooling the primarily cooled steel sheet at an average cooling rate of 10° C./s or more to a secondary cooling end temperature of 320 to 360° C.; and   reheating the secondarily cooled steel sheet to a temperature within a range of 380 to 480° C.,
   234×[C]−29×[Si]−128×[Al]+29×[Mn]+10×[Cr]−17×[Mo]−37×[Nb]−49×[Ti]+100×[B]≥80  [Relational Expression 1]
 
   5×[C]+[Si]+0.5×[Al]≤1.5  [Relational Expression 2]
 
   where a content of each element in the above Relational Expressions 1 and 2 refers to % by weight.   
     
     
         7 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein the slab further comprises one or two of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%). 
     
     
         8 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein the slab further comprises V: 0.05% or less (excluding 0%). 
     
     
         9 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein when the slab is heated, a heating temperature is 1150 to 1250° C. 
     
     
         10 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein after the finish hot rolling is performed, cooling is performed at an average cooling rate of 10 to 100° C./s to a coiling temperature. 
     
     
         11 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein during the cold rolling, a cold rolling reduction rate is 30 to 60%. 
     
     
         12 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein during the continuous annealing, an atmosphere in a continuous annealing furnace is controlled with a gas consisting of by volume, 95% or more of nitrogen and a remainder of hydrogen. 
     
     
         13 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein during the secondary cooling, a gas consisting of by volume, 5 to 80% of hydrogen and a remainder of nitrogen is used. 
     
     
         14 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , wherein during the reheating, an average temperature increase rate is 0.5 to 2.5° C./s. 
     
     
         15 . The method for manufacturing a cold-rolled steel sheet of  claim 6 , further comprising, after the reheating:
 hot-dip galvanizing the cold-rolled steel sheet in a plating bath at a temperature within a range of 450 to 470° C.   
     
     
         16 . The method for manufacturing a cold-rolled steel sheet of  claim 15 , further comprising, after the hot-dip galvanizing:
 performing an alloying heat treatment of the cold-rolled steel sheet at a temperature within a range of 470 to 550° C.   
     
     
         17 . The method for manufacturing a cold-rolled steel sheet of  claim 16 , further comprising, after performing the alloying heat treatment:
 in which the cold-rolled steel sheet is cooled to room 10 temperature, and then temper rolled at a reduction rate of less than 1%.