Steel for Warm Working, Warm Working Method Using the Steel, and Steel Material and Steel Component Obtainable Therefrom

Abstract

There are provided a steel for warm working, to be subjected to warm working as various structures, components of cars, and the like, a warm working method thereof, and a steel material and a steel component obtainable from the warm working method. [Solving Means] A steel is to have a particle dispersion type fiber structure formed in the matrix by warm working. The steel is characterized in that the total amount of the dispersed second-phase particles at room temperature is 7×10 −3 or more in terms of volume fraction, and the Vickers hardness (HV) is equal to or larger than the hardness H of the following equation (2): H =(5.2−1.2×10 −4 λ)×10 2   (2) when the steel is subjected to any of annealing, tempering, and aging treatments in the as-unworked state under conditions such that a parameter λ expressed by the following equation (1): λ= T (log t +20)( T ; temperature( K ), t ; time(hr))  (1) is 1.4×10 4 or more in a prescribed temperature range of 350° C. or more and Ac1 point or less. This steel is taken as the steel for warm working.

1 - 15 . (canceled) 
   
   
       16 . A steel material having a particle dispersion fibrous grain structure, wherein average grain diameter of minor axes of fibrous ferrite crystals constituting the matrix structure is 3 μm or less, second-phase particles are finely dispersed in the matrix structure at a volume fraction of 7×10 −3  or more and 12×10 −2  or less, and Vickers hardness at room temperature is HV 3.7×10 2  or more. 
   
   
       17 . The steel material according to  claim 16 , wherein the matrix structure is composed of fibrous ferrite crystals of which average grain diameter of minor axes is 1 μm or less. 
   
   
       18 . The steel material according to  claim 16 , wherein the matrix structure is composed of fibrous ferrite crystals of which average grain diameter of the minor axes is 0.5 μm or less. 
   
   
       19 . The steel material according to  claim 16 , wherein average particle diameter of major axes of the dispersed second-phase particles is 0.1 μm or less. 
   
   
       20 . A steel for warm-working, from which the steel material according to  claim 16  is made through warm-working, containing alloy elements which disperse the second-phase particles by performing one of the heat treatments selected from annealing, tempering and aging in an as-unworked state under conditions such that a parameter λ expressed by the following equation (1):
   λ= T (log  t+ 20) ( T ; temperature( K ),  t ; time(hr))  (1)   
     is 1.4×10 4  or more in a prescribed temperature range of 350° C. or more and Ac1 point or less, wherein the steel disperses one or both of second-phase particles formed by the heat treatment and second-phase particles originally existing before the heat treatment in total amount by which the dispersed second-phase particles at room temperature after the heat treatment is 7×10 −3  or more and 12×10 −2  or less in terms of volume fraction, and the Vickers hardness (HV) after the heat treatment is equal to or larger than the hardness H of the following equation (2) at room temperature:
     H =(5.2−1.2×10 4 λ)×10 2   (2) 
 
   
   
       21 . The steel according to  claim 20 , wherein 80% by volume or more of a matrix structure consists of a single structure of martensite or bainite, or a mixed structure thereof. 
   
   
       22 . The steel according to  claim 20 , wherein the steel contains
 C: 0.70 wt % or less,   Si: 0.05 wt % or more,   Mn: 0.05 wt % or more,   Cr: 0.01 wt % or more,   Al: 0.5 wt % or less,   O: 0.3 wt % or less,   N: 0.3 wt % or less, and   
     the rest consisting essentially of Fe and unavoidable impurities. 
   
   
       23 . The steel according to  claim 22 , wherein the steel contains in place of Fe one or more of elements selected from the group consisting of
 Mo: 5.0 wt % or less,   W: 5.0 wt % or less,   V: 5.0 wt % or less,   Ti: 3.0 wt % or less,   Nb: 1.0 wt % or less, and   Ta 1.0 wt % or less.   
   
   
       24 . The steel according to  claim 22 , wherein the steel contains in place of Fe one or both of elements selected from the group consisting of
 Ni: 0.05 wt % or more, and   Cu: 2.0 wt % or less.   
   
   
       25 . A steel plate formed by warm-working the steel according to  claim 20 , wherein a particle dispersion fibrous grain structure is formed in at least a surface layer of the steel plate. 
   
   
       26 . A steel wire rod formed by warm-working the steel according to  claim 20 , wherein a particle dispersion fibrous grain structure is formed in at least a surface layer of the steel wire rod. 
   
   
       27 . A steel bolt formed by warm-working the steel according to  claim 20 , wherein a particle dispersion fibrous grain structure is formed in at least a surface layer of a screw portion. 
   
   
       28 . A method of making the steel material having a particle dispersion fibrous grain structure, wherein average grain diameter of minor axes of fibrous ferrite crystals constituting the matrix structure is 3 μm or less, second-phase particles are finely dispersed in the matrix structure at a volume fraction of 7×10 −3  or more and 12×10 −2  or less, and Vickers hardness at room temperature is HV 3.7×10 2  or more, wherein the steel according to  claim 20  is warm-worked in a prescribed shape under conditions such that a parameter λ expressed by the following equation (1):
   λ= T (log  t+ 20) ( T ; temperature( K ),  t ; time(hr))  (1)   
     is 1.4×10 4  or more in a prescribed temperature range of 350° C. or more and Ac1 point or less. 
   
   
       29 . A method of making a steel plate from a steel material having a particle dispersion fibrous grain structure, wherein average grain diameter of minor axes of fibrous ferrite crystals constituting the matrix structure is 3 μm or less, second-phase particles are finely dispersed in the matrix structure at a volume fraction of 7×10 −3  or more and 12×10 −2  or less, and Vickers hardness at room temperature is HV 3.7×10 2  or more, wherein the steel according to  claim 20  is warm worked in a prescribed shape under conditions such that a parameter λ expressed by the following equation (1):
   λ= T (log  t+ 20) ( T ; temperature( K ),  t ; time(hr))  (1)   
     is 1.4×10 4  or more in a prescribed temperature range of 350° C. or more and Ac1 point or less, and wherein a particle dispersion fibrous grain structure is formed in at least a surface layer of the steel plate. 
   
   
       30 . A method of making a steel wire rod from a steel material having a particle dispersion fibrous grain structure, wherein average grain diameter of minor axes of fibrous ferrite crystals constituting the matrix structure is 3 μm or less, second-phase particles are finely dispersed in the matrix structure at a volume fraction of 7×10 −3  or more and 12×10 −2  or less, and Vickers hardness at room temperature is HV 3.7×10 2  or more, wherein the steel according to  claim 20  is warm worked in a prescribed shape under conditions such that a parameter λ expressed by the following equation (1):
   λ= T (log  t+ 20) ( T ; temperature( K ),  t ; time(hr))  (1)   
     is 1.4×10 4  or more in a prescribed temperature range of 350° C. or more and Ac1 point or less and wherein a particle dispersion fibrous grain structure is formed in at least a surface layer of the steel wire rod. 
   
   
       31 . A method of making a steel bolt from the steel material having a particle dispersion fibrous grain structure, wherein average grain diameter of minor axes of fibrous ferrite crystals constituting the matrix structure is 3 μm or less, second-phase particles are finely dispersed in the matrix structure at a volume fraction of 7×10 −3  or more and 12×10 −2  or less, and Vickers hardness at room temperature is HV 3.7×10 2  or more, wherein the steel according to  claim 20  is warm worked in a prescribed shape under conditions such that a parameter λ expressed by the following equation (1):
   λ= T (log  t+ 20) ( T ; temperature( K ),  t ; time(hr))  (1)   
     is 1.4×10 4  or more in a prescribed temperature range of 350° C. or more and Ac1 point or less and wherein a particle dispersion of fibrous grain structure is formed in at least a surface layer of a screw portion. 
   
   
       32 . A steel product produced by cut-machining the steel material according to  claim 16 . 
   
   
       33 . A steel product produced by cut-machining the steel plate according to  claim 25 . 
   
   
       34 . A steel product produced by cut-machining the steel wire rod according to  claim 26 . 
   
   
       35 . A steel product produced by cut-machining the steel bolt according to  claim 27 .