Excellent cold-workability exhibiting high-strength steel, wire or steel bar or high-strength shaped article, and process for producing them

Abstract

There are provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, or high-strength shaped article and a process for producing them. In particular, there is provided a process comprising carrying out hot working at 350 to 800° C. of a steel ingot, cast slab, steel slab or steel semifinished product having a C content of not greater than the solid solution limit of carbon of ferrite phase at Ae 1 point and not greater than 0.010 mass % and being free of any cementite, or having a C content of >0.01 to 0.45 mass % to thereby obtain a material whose average crystal grain diameter in a cross section perpendicular to the longitudinal direction is ≦3 μm, and thereafter carrying out cold working of the material to thereby attain formation of a ferrite structure whose average crystal grain diameter in a cross section perpendicular to the longitudinal direction is ≦500 nm.

1 - 63 . (canceled) 
     
     
         64 . An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less and being free of a cementite. 
     
     
         65 . An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ae 1  point, and having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less. 
     
     
         66 . An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a C content of 0.010 mass % or less, and having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less. 
     
     
         67 . The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in any one of  claims 64  to  66 , having a tensile strength TS of 900 MPa or more. 
     
     
         68 . The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in any one of  claims 64  to  66 , having a reduction of area RA of 60% or more. 
     
     
         69 . The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in any one of  claims 64  to  66 , having a hardness of 285 or more in terms of Vickers hardness Hv. 
     
     
         70 . A high-strength shaped article, having a ferrite structure with an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less and being free of a cementite. 
     
     
         71 . A high-strength shaped article, having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ae 1  point, and having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less. 
     
     
         72 . A high-strength shaped article, having a C content of 0.010 mass % or less, and having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less. 
     
     
         73 . The high-strength shaped article as claimed in any one of  claims 70  to  72 , having a tensile strength TS of 1000 MPa or more. 
     
     
         74 . The high-strength shaped article as claimed in any one of  claims 70  to  72 , having a hardness of 300 or more in terms of Vickers hardness Hv. 
     
     
         75 . An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less. 
     
     
         76 . The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in  claim 75 , having a reduction of area RA of 65% or more. 
     
     
         77 . The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in  claim 75  or  76 , having a tensile strength of 1000 MPa or more. 
     
     
         78 . A high-strength shaped article, having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less. 
     
     
         79 . The high-strength shaped article as claimed in  claim 78 , having a tensile strength TS of 900 MPa or more. 
     
     
         80 . The high-strength shaped article as claimed in  claim 78 , having a hardness of 285 or more in terms of Vickers hardness Hv. 
     
     
         81 . A process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, wherein a steel ingot, cast slab, steel slab or steel semifinished product having a cementite-free ferrite structure or having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ae 1  point or having a C content of 0.010 mass % or less is subjected to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and the material is then subjected to cold working to form a ferrite structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 200 nm or less. 
     
     
         82 . The process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in  claim 81 , wherein the warm working is a working such that at the working temperature in a range of from 350 to 800° C., a total reduction R represented by the following equation (1):
     R ={( S   0   −S )/ S   0 }×100  (1)   wherein
 R: total reduction (%) applied to cast slab or steel slab 
 S 0 : C directional cross section area of cast slab or steel slab just before initiation of warm working 
 S: C directional cross section area of a material obtained after completion of warm working 
   
       is 50% or more by rolling and/or forging 
       and the cold working is a working such that at the working temperature of lower than 350° C., a total reduction R′ represented by the following equation (2):
     R ′={( S   0   −S ′)/ S   0 ′}×100  (1) 
 wherein
 R′: total reduction (%) applied to a warm rolled material 
 S 0 ′: C directional cross section area of a material just before initiation of cold working 
 S′: C directional cross section area of a material obtained after completion of cold working 
 
 
       is 5% or more by rolling and/or drawing. 
     
     
         83 . A process for producing a high-strength shaped article, which comprises subjecting the excellent cold-workability exhibiting high-strength steel wire or steel bar produced by the above production process as claimed in any one of  claim 81  or  82  to cold heading, cold forging and/or machining. 
     
     
         84 . A process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, which comprises subjecting a steel ingot, cast slab, steel slab or steel semifinished product having a C content of from more than 0.01 to 0.45 mass % to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and then subjecting the material to cold working to form a ferrite main phase structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 200 nm or less. 
     
     
         85 . The process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in  claim 84 , wherein the warm working comprises working at a working temperature in a range of from 350 to 800° C., a total reduction R represented by the following equation (1):
     R ={( S   0   −S )/ S   0 }×100  (1)   wherein
 R: total reduction (%) applied to cast slab or steel slab 
 S 0 : C directional cross section area of cast slab or steel slab just before initiation of warm working 
 S: C directional cross section area of a material obtained after completion of warm working 
   
       is 50% or more is applied to the cast slab or the steel slab by rolling and/or forging 
       and the cold working is a working wherein at the working temperature of lower than 350° C., a total reduction R′ represented by the following equation (2):
     R ′={( S   0   ′−S ′)/ S   0 ′}×100  (1) 
 wherein
 R′: total reduction (%) applied to a warm rolled material 
 S 0 ′: C directional cross section area of a material just before initiation of cold working 
 S′: C directional cross section area of a material obtained after completion of cold working 
 
 
       is 5% or more is applied to the warm rolled material by rolling and/or drawing. 
     
     
         86 . A process for producing a high-strength shaped article, which comprises subjecting the excellent cold-workability exhibiting high-strength steel wire or steel bar produced by the above production process as claimed in  claim 84  or  85  to cold heading, cold forging and/or machining.