High strength and high ductility steel sheet having ultrafine crystalline grain structure obtained by low strain processing and annealing of ordinary low carbon steel, and a method for producing 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 - 3 . (canceled)  
   
   
       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%.  
   
   
       6 . A method for producing a high strength and high ductility low carbon steel of  claim 4 , wherein an ultra fine crystalline grain ferrite structure having an average grain diameter of 1.0 μm or less is formed.  
   
   
       7 . A method for producing a high strength and high ductility low carbon steel of  claim 4 , wherein said steel sheet product having a martensite phase in an amount of 90% or more obtained by coarsening the size of an austenite crystal grain steel, is obtained by using a hot rolled plate having 2 mm thickness, and is comprised of C 0.13%, Si; 0.01%, Mn; 0.37%, P; 0.02%, S; 0.004%, sol. Al; 0.04%, wherein an austenitization step is carried out on said steel sheet product at 1000° C. for 15 minutes so as to make coarse the size of an austenite crystal grain to 100-200 μm size, then water-quenched.