Cold rolled steel sheet and galvanized steel sheet having strain aging hardening property and method for producing the same

Abstract

An object of the present invention is to provide a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet in which tensile strength is effectively increased by press forming and heat treatment while maintaining excellent deep drawability in press forming. Specifically, a steel composition contains less than 0.01% of C, 0.005 to 1.0% of Si, 0.01 to 1.0% of Mn, 0.005 to 0.050% of Nb, 0.005 to 0.030% of Al, 0.005 to 0.040% of N, 0.0005 to 0.0015% of B, 0.05% or less of P, and 0.01% or less of S, the balance substantially composed of Fe, in which the following equations (1) and (2) are satisfied: N %≧0.0015+14/93·Nb %+14/27·Al %+14/11·B %  ( 1 ) C %≦(12/93)·Nb %  (2)

What is claimed is:  
     
         1 . A cold-rolled steel sheet having excellent strain age hardenability comprising a composition, by mass %, comprising: 
 C: 0.15% or less;    Si: 1.0% or less;    Mn: 2.0% or less;    P: 0.1% or less;    S: 0.01% or less;    Al: 0.005 to 0.030%; and    N: 0.0050 to 0.0400%;    wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, and the balance is composed of Fe and inevitable impurities.    
     
     
         2 . A cold-rolled steel sheet having excellent strain age hardenability according to  claim 1 , comprising a composition, by mass %, further comprising at least one of the following groups a to d: 
 Group a: at least one of Cu, Ni, Cr and Mo in a total of 1.0% or less;    Group b: at least one of Nb, Ti and V in a total of 0.1% or less;    Group c: 0.0030% or less of B; and    Group d: one or both of Ca and REM in a total of 0.0010 to 0.010%.    
     
     
         3 . A cold-rolled steel sheet having excellent strain age hardenability comprising a composition, by mass %, comprising: 
 C: less than 0.01%;    Si: 0.005 to 1.0%;    Mn: 0.01 to 1.5%;    P: 0.1% or less;    S: 0.01% or less;    Al: 0.005 to 0.030%; and    N: 0.005 to 0.040%;    wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, and the balance is composed of Fe and inevitable impurities.    
     
     
         4 . A cold-rolled steel sheet having excellent strain age hardenability according to  claim 3 , wherein the composition, by mass %, preferably further comprises: 
 B: 0.0001 to 0.0030%; and    Nb: 0.005 to 0.050%;    wherein the ranges of B and Nb satisfy the following equations (1) and (2), and the balance is substantially composed of Fe:    N %≧0.0015+14/93·Nb %+14/27·Al %+14/11·B %  (1) C≦0.5·(12/93)·Nb %  (2)    
     
     
         5 . A cold-rolled steel sheet having excellent strain age hardenability according to  claim 3  or  4 , wherein the composition, by mass %, further comprises at least one of Cu, Ni and Mo in a total of 1.0% or less.  
     
     
         6 . A cold-rolled steel sheet having excellent strain age hardenability according to  claims 1  to  5 , wherein the steel sheet has a crystal grain diameter of 20 μm or less.  
     
     
         7 . A cold-rolled steel sheet having excellent strain age hardenability according to claims  1  to 6, wherein strength after forming is increased by 60 MPa or more by heat treatment in a low temperature region of 120 to 200° C.  
     
     
         8 . An electro-galvanized, hot-dip galvanized or alloyed hot-dip galvanized steel sheet having excellent strain age hardenability comprising a coated layer formed on the surface of a cold-rolled steel sheet according to  claims 1  to  7  by electro-galvanization, hot-dip galvanization, or alloying hot-dip galvanization.  
     
     
         9 . A method of producing a cold-rolled steel sheet having excellent strain age hardenability comprising hot-rolling a steel slab under conditions in which the steel slab is cooled immediately after the end of finish rolling and coiled at a coiling temperature of 400 to 800° C., cold-rolling the hot-rolled sheet with a rolling reduction ratio of 60 to 95%, and then recrystallizing and annealing the cold-rolled sheet at a temperature of 650 to 900° C., wherein the steel slab has a composition, by mass %, comprising: 
 C: less than 0.01%;  
 Si: 0.005 to 1.0%;  
 Mn: 0.01 to 1.5%;  
 P: 0.1% or less;  
 S: 0.01% or less;  
 Al: 0.005 to 0.030%; and  
 N: 0.005 to 0.040%;  
 wherein N/Al is 0.30 or more, and the balance is substantially composed of Fe.  
 
     
     
         10 . A method of producing a cold-rolled steel sheet having excellent strain age hardenability according to  claim 9 , wherein the composition, by mass %, further comprises: 
 B: 0.0001 to 0.0030%; and    Nb: 0.005 to 0.050%;    wherein the ranges of B and Nb satisfy the following equations (1) and (2), and the balance is substantially composed of Fe:    N %≧0.0015+14/93·Nb %+14/27·Al %+14/11·B %  (1) C %≦0.5·(12/93)·Nb %  (2)    
     
     
         11 . A method of producing a cold-rolled steel sheet having excellent strain age hardenability according to  claim 9  or  10 , wherein in the heating-up step in the recrystallization annealing, the temperature is increased at a rate of 1 to 20° C./s in the temperature region from 500° C. to the recrystallization temperature.  
     
     
         12 . A method of producing an galvannealed steel sheet having excellent strain age hardenability comprising hot-dip galvanization and then heat alloying after recrystallization and annealing according to  claims 9  to  11 .  
     
     
         13 . A cold-rolled deep drawing steel sheet having excellent strain age hardenability comprising a composition, by mass %, comprising: 
 C: 0.01% or less;    Si: 1.0% or less;    Mn: 0.01 to 1.5%;    P: 0.1% or less;    S: 0.01% or less;    Al: 0.005 to 0.020%; and    N: 0.0050 to 0.040%;    wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, and Ts x r value is 750 MPa or more.    
     
     
         14 . A cold-rolled deep drawing steel sheet having excellent strain age hardenability according to  claim 13 , wherein the composition, by mass %, further comprises: 
 B: 0.0001 to 0.0030%; and    Nb: 0.005 to 0.050%;    wherein the balance is composed of Fe and inevitable impurities, TS×r value is 750 MPa or more, and the ranges of B and Nb satisfy the following equations (1) and (2),:    N %≧0.0015+14/93·Nb %+14/27·Al %+14/11·B %  (1) C %≦0.5·(12/93)·Nb %  (2)    
     
     
         15 . A cold-rolled deep drawing steel sheet having excellent strain age hardenability according to  claim 13 , wherein the composition, by mass %, further comprises at least one of the following: 
 B: 0.0001 to 0.0030%;    Nb: 0.005 to 0.050%;    Ti: 0.005 to 0.070%; and    V: 0.005 to 0.10%;    wherein N/(Al+Nb+Ti+V+B) is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, and TS x r value is 750 MPa or more.    
     
     
         16 . A method of producing a cold-rolled deep drawing steel sheet having excellent strain age hardenability comprising heating a steel raw material to 950° C. or more, roughly rolling the raw material so that the finisher delivery temperature is Ar 3  to 1000° C., finish-rolling the material while lubricating it in the temperature region of 600° C. to Ar 3 , coiling the rolled sheet in which the total reduction by rolling starting from rough rolling to finish rolling is 80% or more, recrystallizing and annealing the hot-rolled sheet, cold-rolling the rolled sheet with a rolling reduction of 60 to 95%, and then recrystallizing and annealing the resultant cold-rolled sheet, wherein the steel raw material has a composition, by mass %, comprising: 
 C: less than 0.01%;  
 Si: 0.005 to 1.0%;  
 Mn: 0.01 to 1.5%;  
 P: 0.1% or less;  
 S: 0.01% or less;  
 Al: 0.005 to 0.030%;  
 N: 0.005 to 0.040%; and  
 at least one of the following: 
 B: 0.0003 to 0.0030%;  
 Nb: 0.005 to 0.050%;  
 Ti: 0.005 to 0.070%; and  
 V: 0.005 to 0.10%;  
 wherein N/(Al+Nb+Ti+V+B) is 0.30 or more.  
 
 
     
     
         17 . A high-tensile-strength cold-rolled steel sheet having excellent moldability, strain age hardenability and natural aging resistance comprising a composition, by mass %, comprising: 
 C: 0.0015 to 0.025%;    Si: 1.0% or less;    Mn: 2.0% or less;    P: 0.1% or less;    S: 0.02% or less;    Al: 0.02% or less;    N: 0.0050 to 0.0250%; and    one or both of the following: 
 B: 0.0001 to 0.0050%; and  
 Nb: 0.002 to 0.050%;  
 wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, the structure is composed of an acicular ferrite phase at an area ratio of 5% or more and a ferrite phase having an average crystal grain diameter of 20 μm or less, and the r value is 1.2 or more.  
   
     
     
         18 . A cold-rolled steel sheet according to  claim 17 , wherein the composition further comprises at least one of the following groups a to c: 
 Group a: at least one of Cu, Ni, Cr and Mo in a total of 1.0% or less;    Group b: one or both of Ti and V in a total of 0.1% or less; and    Group c: one or both of Ca and REM in a total of 0.0010 to 0.010%.    
     
     
         19 . A method of producing a high-tensile-strength cold-rolled steel sheet having a r value of 1.2 or more, and excellent moldability, strain age hardenability and natural aging resistance comprising: 
 the hot-rolling step of roughly rolling a steel slab by heating to a slab heating temperature of 1000° C. or more to form a sheet bar, finish-rolling the sheet bar so that the finisher delivery temperature is 800° C. or more, and coiling the finish-rolled sheet at a coiling temperature of 800° C. or less to form a hot-rolled sheet;    the cold rolling step of pickling and cold-rolling the hot-rolled sheet to form a cold-rolled sheet; and    the cold-rolled sheet annealing step of continuously annealing the cold-rolled sheet at a temperature in the ferrite-austenite two-phase region, and cooling the annealed sheet to the temperature region of 500° C. or less at a cooling rate of 10 to 300° C./s;    wherein the steel slab has a composition, by mass %, comprising at least one of: 
 C: 0.0015 to 0.025%;  
 Si: 1.0% or less;  
 Mn: 2.0% or less;  
 P: 0.1% or less;  
 S: 0.02% or less;  
 Al: 0.02% or less;  
 N: 0.0050 to 0.0250%; and  
 at least one of the following: 
 B: 0.0001 to 0.0050%; and  
 Nb: 0.002 to 0.050%;  
 wherein N/Al is 0.30 or more.  
 
   
     
     
         20 . A method of producing a cold-rolled steel sheet according to  claim 19 , wherein the composition further comprises, by mass %, at least one of the following groups a to c: 
 Group a: at least one of Cu, Ni, Cr and Mo in a total of 1.0% or less;    Group b: one or both of Ti and V in a total of 0.1% or less; and    Group c: one or both of Ca and REM in a total of 0.0010 to 0.010%.    
     
     
         21 . A high-tensile-strength cold-rolled steel sheet having a high r value and excellent strain age hardenability and natural aging resistance comprising a composition, by mass %, comprising: 
 C: 0.025 to 0.15%;    Si: 1.0% or less;    Mn: 2.0% or less;    P: 0.08% or less;    S: 0.02% or less;    Al: 0.02% or less; and    N: 0.0050 to 0.0250%;    wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, the structure is composed of a ferrite phase having an average crystal grain diameter of 10 μm or less at an area ratio of 80% or more and a martensite phase as a second phase at an area ratio of 2% or more, and the r value is 1.2 or more.    
     
     
         22 . A high-tensile-strength cold-rolled steel sheet according to  claim 21 , wherein the composition further comprises at least one of the following groups d to g: 
 Group d: at least one of Cu, Ni, Cr and Mo in a total of 1.0% or less;    Group e: at least one of Nb, Ti and V in a total of 0.1% or less;    Group f: 0.0030% or less of B; and    Group g: one or both of Ca and REM in a total of 0.0010 to 0.010%.    
     
     
         23 . A method of producing a high-tensile-strength cold-rolled steel sheet having a r value of as high as 1.2 or more, and excellent strain age hardenability and natural aging resistance comprising: 
 the hot-rolling step of roughly rolling a steel slab by heating to a slab heating temperature of 1000° C. or more to form a sheet bar, finish-rolling the sheet bar so that: the finisher delivery temperature is 800° C. or more, and coiling the finish-rolled sheet at a coiling temperature of 800° C. or less to form a hot-rolled sheet;    the cold rolling step of pickling and cold-rolling the hot-rolled sheet to form a cold-rolled sheet; and    the cold-rolled sheet annealing step of box-annealing the cold-rolled sheet at an annealing temperature of the recrystallization temperature to 800° C., then continuously annealing the annealed sheet at an annealing temperature of Ac 1  transformation point to Ac 3  transformation point—20° C.), and then cooling the sheet to the temperature region of 500° C. or less at a cooling rate of 10 to 300° C./s;    wherein the steel slab has a composition, by mass %, comprising at least one of: 
 C: 0.025 to 0.15%;  
 Si: 1.0% or less;  
 Mn: 2.0% or less;  
 P: 0.08% or less;  
 S: 0.02% or less;  
 Al: 0.02% or less; and  
 N: 0.0050 to 0.0250%;  
 wherein N/Al is 0.30 or more.  
   
     
     
         24 . A method of producing a high-tensile-strength cold-rolled steel sheet according to  claim 23 , further comprising performing over aging in a temperature region of 350° C. to the cooling step temperature for a residence tine of 20 seconds or more subsequent to cooling after the continuous annealing.  
     
     
         25 . A method of producing a high-tensile-strength cold-rolled steel sheet according to  claim 23  or 24, wherein the composition further comprises, by mass %, at least one of the following groups d to g: 
 Group d: at least one of Cu, Ni, Cr and Mo in a total of 1.0% or less;  
 Group e: at least one of Nb, Ti and V in a total of 0.1% or less;  
 Group f: 0.0030% or less of B; and  
 Group g: one or both of Ca and REM in a total of 0.0010 to 0.010%.