High strength and high ductility steel sheet plate having hyperfine crystal grain structure produced by ordinary low carbon steel to low strain working and annealing, and method for production thereof

Abstract

A high strength and high ductility low carbon steel having a tensile strength of 800 MPa or more, an uniform elongation of 5% or more, and an elongation to failure of 20% or more which is produced by a method comprising subjecting an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount being 0.01% or less and effective for accelerating martensitic transformation to processing and heat treatment to prepare a product having coarser size of austenite crystal grains and then to water-quenching, to provide a steel product having a martensite phase in an amount of 90% or more, and subjecting the steel product to a low strain processing, specifically a cold rolling at a total rolling reduction in thickness of 20% or more and less than 80%, and to a low temperature annealing at 500° C. to 600° C., and a method for producing said high strength and high ductility low carbon steel.

1 . A high strength and high ductility low carbon steel sheet having a tensile strength of 800 MPa or more, an uniform elongation of 5% or more and a elongation to failure of 20% or more, 
 which is produced by a method comprising,    carrying out a low strain processing and annealing on a steel product having a martensite phase in an amount of  90 % or more obtained by coarsening the size of an austenite crystal grain, which is existing in an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount of  0 . 01 % or less being effective for accelerating martensitic transformation, and then water-quenching.    
     
     
         2 . The high strength and high ductility low carbon steel sheet of  claim 1 , wherein said steel possesses an ultra fine crystal grain ferrite structure having an average grain diameter of 1.0 μm or less formed by carrying out a cold rolling at a total rolling reduction in thickness of 20% or more and less than 80%  
     
     
         3 . The high strength and high ductility low carbon steel of  claim 2 , wherein the annealing process. is carried out at the temperature range between 500° C. or more and less than 600° C.  
     
     
         4 . A method for producing a high strength and high ductility low carbon steel having a tensile strength of 800 MPa or more and an uniform elongation of 5% or more and a elongation to failure of 20% or more comprising, carrying out a low strain processing and annealing on a steel sheet product having a martensite phase in an amount of 90% or more obtained by coarsening the size of an austenite crystal grain, which is existing in an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount of 0.01% or less being effective for accelerating martensitic transformation, to 100 μm or more and water-quenching.  
     
     
         5 . A method for producing a high strength and high ductility low carbon steel of  claim 4 , wherein the annealing is carried out at the temperature range between 500° C. or more and less than 600° C. after carrying out a cold rolling at a total rolling reduction in thickness of 20% or more and less than 80%.