Cold-rolled steel sheet having excellent spot weldability, and manufacturing method therefor

Abstract

A cold-rolled steel sheet having excellent spot weldability suitable for use in vehicles, electric appliances, etc. is provided. The cold-rolled steel sheet has a steel composition containing, in mass%: C: 0.05% to 0.13%; Si: 0.05% to 2.0%; Mn: 1.5% to 4.0%; P: 0.05% or less; S: 0.005% or less; Al: 0.01% to 0.10%; Cr: 0.05% to 1.0%; Nb: 0.010% to 0.070%; Ti: 0.005% to 0.040%; and N: 0.0005% to 0.0065%, with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, wherein 70 mass % or more of Ti in steel exists as a precipitate, and 15 mass % or more of Nb in the steel exists as solute Nb, and a tensile strength is 980 MPa or more.

1 . A cold-rolled steel sheet having excellent spot weldability, the cold-rolled steel sheet having a steel composition containing, in mass %:
 C: 0.05% to 0.13%;   Si: 0.05% to 2.0%;   Mn: 1.5% to 4.0%;   P: 0.05% or less;   S: 0.005% or less;   Al: 0.01% to 0.10%;   Cr: 0.05% to 1.0%;   Nb: 0.010% to 0.070%;   Ti: 0.005% to 0.040%; and   N: 0.0005% to 0.0065%,   with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, p 1  wherein 70 mass % or more of Ti in steel exists as a precipitate, and 15 mass % or more of Nb in the steel exists as solute Nb, and   a tensile strength is 980 MPa or more.   
     
     
         2 . The cold-rolled steel sheet having excellent spot weldability according to  claim 1 ,
 wherein the steel composition further contains one or more selected from, in mass %:   Mo: 0.01% to 1.0%;   Cu: 1.0% or less;   Ni: 1.0% or less; and   V: 0.1% or less.   
     
     
         3 . A manufacturing method for a cold-rolled steel sheet having excellent spot weldability, the manufacturing method comprising:
 heating a steel material having the steel composition according to  claim 1  to a temperature range of (Ts−50)° C. or more and (Ts+200)° C. or less where Ts is a temperature defined by the following Formula (1), hot rolling the steel material with a finisher delivery temperature of 850° C. or more to obtain a hot-rolled steel sheet, and then coiling the hot-rolled steel sheet at a temperature of 650° C. or less;   cold rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and   continuously annealing the cold-rolled steel sheet by: heating the cold-rolled steel sheet to a temperature range of 700° C. or more and 900° C. or less; and, in a subsequent cooling process, cooling the cold-rolled steel sheet to a temperature range of 200° C. or more and 450° C. or less with an average cooling rate of 12° C./s or more and 100° C./s or less, and holding the cold-rolled steel sheet in the temperature range of 200° C. or more and 450° C. or less for a time of 30 s or more and 600 s or less,
   Ts (° C.)=6770/[2.26−log  10 {[% Nb]×([% C]+0.86[% N])}]−273   (1)
 
   
       where [% Nb], [% C], and [% N] respectively denote Nb, C, and N contents in steel in mass %. 
     
     
         4 . A manufacturing method for a cold-rolled steel sheet having excellent spot weldability, the manufacturing method comprising:
 heating a steel material having the steel composition according to  claim 2  to a temperature range of (Ts−50)° C. or more and (Ts+200)° C. or less where Ts is a temperature defined by the following Formula (1), hot rolling the steel material with a finisher delivery temperature of 850° C. or more to obtain a hot-rolled steel sheet, and then coiling the hot-rolled steel sheet at a temperature of 650° C. or less;   cold rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and   continuously annealing the cold-rolled steel sheet by: heating the cold-rolled steel sheet to a temperature range of 700° C. or more and 900° C. or less; and, in a subsequent cooling process, cooling the cold-rolled steel sheet to a temperature range of 200° C. or more and 450° C. or less with an average cooling rate of 12° C./s or more and 100° C./s or less, and holding the cold-rolled steel sheet in the temperature range of 200° C. or more and 450° C. or less for a time of 30 s or more and 600 s or less,
   Ts (° C.)=6770/[2.26−log  10 {[% Nb]×([% C]+0.86[% N])}]−273   (1)
 
   
       where [% Nb], [% C], and [% N] respectively denote Nb, C, and N contents in steel in mass %.