High strength cold rolled steel sheet excellent in weldability and method for manufacturing the same

Abstract

A high strength cold rolled steel sheet excellent in weldability and has a TS of 440 MPa or more includes a composition including C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to 2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, and Ni: 0.03 to 0.5% by mass with the balance Fe and incidental impurities; and a microstructure constituted by a ferrite single phase.

1 . A high strength cold rolled steel sheet excellent in weldability and has a tensile strength of 440 MPa or more, comprising a composition including C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to 2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, and Ni: 0.03 to 0.5% by mass with the balance Fe and incidental impurities; and a microstructure constituted of a ferrite single phase. 
     
     
         2 . The high strength cold rolled steel sheet according to  claim 1 , further comprising O: 0.0025 to 0.0080% by mass. 
     
     
         3 . The high strength cold rolled steel sheet according to  claim 1 , comprising at least one of Se: 0.0005 to 0.01% and Te: 0.0005 to 0.01% by mass. 
     
     
         4 . A method for manufacturing a high strength cold rolled steel sheet excellent in weldability and has a tensile strength of 440 MPa or more, comprising hot-rolling a slab having the composition of  claim 1 , coiling at a coiling temperature of 680° C. or less, pickling, cold-rolling at a reduction ratio of 40% or more, and performing recrystallization annealing at 700 to 850° C. 
     
     
         5 . The high strength cold rolled steel sheet according to  claim 2 , further comprising at least one of Se: 0.0005 to 0.01% and Te: 0.0005 to 0.01% by mass. 
     
     
         6 . A method for manufacturing a high strength cold rolled steel sheet excellent in weldability and has a tensile strength of 440 MPa or more, comprising hot-rolling a slab having the composition of  claim 2 , coiling at a coiling temperature of 680° C. or less, pickling, cold-rolling at a reduction ratio of 40% or more, and performing recrystallization annealing at 700 to 850° C. 
     
     
         7 . A method for manufacturing a high strength cold rolled steel sheet excellent in weldability and has a tensile strength of 440 MPa or more, comprising hot-rolling a slab having the composition of  claim 3 , coiling at a coiling temperature of 680° C. or less, pickling, cold-rolling at a reduction ratio of 40% or more, and performing recrystallization annealing at 700 to 850° C. 
     
     
         8 . A method for manufacturing a high strength cold rolled steel sheet excellent in weldability and has a tensile strength of 440 MPa or more, comprising hot-rolling a slab having the composition of  claim 5 , coiling at a coiling temperature of 680° C. or less, pickling, cold-rolling at a reduction ratio of 40% or more, and performing recrystallization annealing at 700 to 850° C.