Steel Sheet for Deep Drawing Having Excellent Secondary Work Embrittlement Resistance, Fatigue Properties and Plating Properties, and Method for Manufacturing the Same

Abstract

A steel sheet for deep drawing used for automobiles, and a method for manufacturing the same are disclosed. The steel sheet comprises, by weight %, C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.10˜0.40%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0<(Ti[%]+Al[%]/16+6B[%])/3.43N[%]<4.1, and wherein the composition of Nb, Al, and C satisfies the relationship; 0.7<(Nb[%]+Al[%]/20)/7.75C[%]<3.5. The steel sheet exhibits excellent secondary work embrittlement, fatigue properties of welded joints, and an appealing plated surface as well as excellent formability.

1 . A steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties, comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.010˜0.040%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities,
 wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1, and   wherein the composition of Nb, Al, and C satisfies the relationship: 0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5.   
     
     
         2 . The steel sheet according to  claim 1 , further comprising: 0.05% or less of Mo. 
     
     
         3 . The steel sheet according to  claim 1 , wherein an average size of Nb—Ti—Al—N—C based composite precipitates is 40 nm or more, the composite precipitates comprising Ti 4 C 2 S 2  in a fraction of 50% or more, and TiC in a fraction less than 5%. 
     
     
         4 . The steel sheet according to  claim 1 , wherein the contents of Mn, P, Nb, Mo and B are controlled to provide a tensile strength in the range of 28˜50 kgf/mm 2  calculated according to a tensile strength calculation expression TS: TS=27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%]. 
     
     
         5 . A method for manufacturing a steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties, comprising:
 reheating a steel slab at a temperature of 1,100˜1,250° C., the steel slab comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.10˜0.40%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1, and wherein the composition of Nb, Al, and C satisfies the relationship: 0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5;   rough rolling the reheated steel slab;   finish rolling the rough rolled steel slab at a finish rolling temperature of 880° C. or more, followed by coiling the hot rolled steel sheet;   cold rolling the coiled steel sheet at a reduction ration of 65% or more; and continuously annealing the cold rolled steel sheet at a temperature of 780˜860° C.   
     
     
         6 . The method according to  claim 5 , wherein the steel slab further comprises 0.05% or less of Mo. 
     
     
         7 . The method according to  claim 5 , wherein an average size of Nb—Ti—Al—N—C based composite precipitates is 40 nm or more, the composite precipitates comprising Ti 4 C 2 S 2  in a fraction of 50% or more, and TiC in a fraction less than 5%. 
     
     
         8 . The method according to  claim 5 , wherein the contents of Mn, P, Nb, Mo and B are controlled to provide a tensile strength in the range of 28˜50 kgf/mm 2  calculated according to a tensile strength calculation expression TS: TS=27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%].