Abstract:
Provided is a maraging steel excellent in fatigue characteristics, including, in terms of % by mass: C: ≦0.015%, Ni: from 12.0 to 20.0%, Mo: from 3.0 to 6.0%, Co: from 5.0 to 13.0%, Al: from 0.01 to 0.3%, Ti: from 0.2 to 2.0%, O: ≦0.0020%, N: ≦0.0020%, and Zr: from 0.001 to 0.02%, with the balance being Fe and unavoidable impurities.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a maraging steel. More specifically, the present invention relates to a maraging steel having fatigue characteristics improved by refining the size of a TiN inclusion. 
       BACKGROUND OF THE INVENTION 
       [0002]    A maraging steel is a steel containing a large amount of Ni, Mo, Ti, Co, etc. as strengthening elements and is a steel of a type that undergoes age hardening in a martensitic state by heat treatment, and a maraging steel is an ultrahigh-strength steel capable of achieving a very high tensile strength of around 2,000 MPa. 
         [0003]    Since a maraging steel has a high tensile strength, a maraging steel is used as a material suitable particularly for a member requiring high strength, such as aerospace or aircraft structural member, a continuously variable transmission component of an automotive engine, a high-pressure vessel, a tool material, a metal mold, etc. 
         [0004]    The strengthening mechanism of a maraging steel is attributable to precipitation hardening of an intermetallic compound such as Ni—Ti and Ni—Mo in an aging treatment, and as a representative compositional example thereof, Fe-18Ni-9Co-5Mo-0.4Ti-0.1Al steel has been conventionally known. 
         [0005]    However, a maraging steel has a problem that Ti added to steel reacts with N present in the steel to produce a coarse angular TiN inclusion and the TiN inclusion works out to a fracture origin and reduces the fatigue characteristics. Among others, the reduction in fatigue characteristics originated from a coarse TiN inclusion becomes a serious problem, for example, in thin steel sheet having a thickness of 0.5 mm or less, and it is required to solve this problem. 
         [0006]    Incidentally, as a related art of the present invention, Patent Document 1 discloses an invention of “Method for Processing and Heat-Treating Maraging Steel”, and a composition of a Zr-containing maraging steel is disclosed in claims thereof. 
         [0007]    However, Patent Document 1 does not refer to Zr at all in description as well as in Examples where Zr must have added specifically, and differs from the present invention. 
         [0008]    As another related art, Patent Document 2 discloses an invention of “Ultrahigh Tensile Strength High-Toughness Steel” and states in claim  2  that Zr can be added as a selective element. However, the steel described in Patent Document 2 has a low Ni content of 4.1 to 9.5 and in addition, Patent Document 2 describes no Example where Zr is added, and differs from the present invention. 
         [0009]    As still another related art, Patent Document 3 discloses an invention of “Ultrahigh Strength Steel”, and a composition containing Zr as one of selective elements is disclosed in claim  1  thereof. However, the steel described in Patent Document 3 has a Co content of 15.0 to 21.0, which is a high content compared with that of the present invention, and furthermore, the invention in Patent Document 3 differs from the present invention in that the reason for adding Zr is to enhance cleanliness by deoxygenation and enhance ductility by denitrogenation and prevention of grain boundary precipitation of Mo and Cr. 
         [0010]    As yet still another related art, Patent Document 4 discloses an invention of “Maraging Steel Excellent in Heat Checking Resistance”, and incorporation of Zr is disclosed in claim  1  thereof. 
         [0011]    However, the steel described in Patent Document 4 has a low Ni content of 6.0 to 11.0, and the invention in Patent Document 4 differs from the present invention. 
         [0012]    [Patent Document 1] JP-A-51-87118 
         [0013]    [Patent Document 2] JP-A-53-30916 
         [0014]    [Patent Document 3] JP-A-58-25457 
         [0015]    [Patent Document 4] JP-A-7-243003 
       SUMMARY OF THE INVENTION 
       [0016]    Under these circumstances, an object of the present invention is to provide a maraging steel having fatigue characteristics improved by refining the size of a TiN inclusion. 
         [0017]    The present invention has adopted the following constitutions [1] and [2]. 
         [0018]    [1] A maraging steel excellent in fatigue characteristics, comprising, in terms of % by mass: C: ≦0.015%, Ni: from 12.0 to 20.0%, Mo: from 3.0 to 6.0%, Co: from 5.0 to 13.0%, Al: from 0.01 to 0.3%, Ti: from 0.2 to 2.0%, O: ≦0.0020%, N: ≦0.0020%, and Zr: from 0.001 to 0.02%, with the balance being Fe and unavoidable impurities. 
         [0019]    [2] The maraging steel excellent in fatigue characteristics according to [1], further comprising one or more of, in terms of % by mass: B: from 0.0010 to 0.010%, Mg: ≦0.003%, and Ca: ≦0.003%. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a view showing a result of SEM observation in Example 1. 
           [0021]      FIGS. 2A and 2B  are views showing a TiN inclusion in Example 1 together with a TiN inclusion of Comparative Example 14. 
           [0022]      FIG. 3  is a view showing the standard free energies of formation of ZrN and TiN. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The gist of the present invention resides in adding a predetermined content of Zr as a component for refining the size of a TiN inclusion produced in a maraging steel. 
         [0024]    In order to refine the size of a TiN inclusion, the present inventors have focused attention on a technique of forming nuclei of TiN in a finely dispersed manner before the formation of TiN and forming TiN around the nuclei. 
         [0025]    Then, in order to explore whether an element capable of acting in such a way exists or not and if exists, what kind of an element can act so, a test of adding various elements was conducted to examine how TiN is formed. 
         [0026]    As a result, the present inventors have found out the fact that as regards the elements examined, all elements except for Zr cannot effectively Rhin a nucleus contributing to the size refinement of a TiN inclusion and only Zr can form a nucleus effective in refining the size of a TiN inclusion. 
         [0027]    The present invention has been accomplished based on the above-mentioned finding. 
         [0028]    According to the estimation of the present inventors, when Zr is added to a molten steel during primary melting (when the melting ends only by primary melting, the primary melting becomes final melting), Zr added works out to a fine Zr inclusion (here, Zr oxide) and is produced in a dispersed manner in the molten steel. 
         [0029]    When the molten steel is thereafter solidified, a TiN inclusion is crystallized around a large number of finely dispersed Zr inclusion nuclei. 
         [0030]    More specifically, the produced nuclei are finely dispersed, and TiN is crystallized around each nucleus, as a result, the size of TiN crystallized product, namely, TiN inclusion that becomes a problem in a maraging steel, is refined. 
         [0031]    In addition, at this time, Zr added to the molten steel acts to suppress TiN formation by causing N in the molten steel to be fixed as ZrN and reacting with Ti to reduce the amount of N for forming TiN. 
         [0032]    As shown in  FIG. 3  (the source of  FIG. 3  is Metal Data Book (edited by The Japan Institute of Metals and Materials), the standard free energy of formation of ZrN is smaller than the standard free energy of formation of TiN, and it is apparent from this relationship that when Zr and Ti are present together with N in a molten steel, the reaction of Zr with N can occur in preference to the reaction of Ti with N. 
         [0033]    According to the present invention, the size of an angular-shaped TiN inclusion produced in a steel resulting from combining of Ti added to the steel with N can be refined, and the fatigue characteristics of a maraging steel can be thereby effectively enhanced. 
         [0034]    The reasons for the limitations on the chemical components in the present invention are described below. 
         [0000]    C: ≦0.015% by mass 
         [0035]    C combines with Ti to form carbide or carbonitride and is caused by an aging treatment to reduce the content of Ti which forms an intermetallic compound. Also, since the fatigue strength is reduced by the formation of carbide or carbonitride, the content of C is limited to be 0.015% by mass or less. 
         [0000]    Ni: from 12.0 to 20.0% by mass 
         [0036]    Ni is caused by an aging treatment to precipitate an intermetallic compound such as Ni 3 Mo and NiAl, and enhance the tensile strength and fatigue strength. In order to obtain such an effect, the content of Ni is limited to 12.0% by mass or more. 
         [0037]    On the other hand, if the content of Ni becomes excessive, the content of residual austenite is increased due to drop of the Ms temperature, whereby a sufficient martensite is not obtained. Therefore, the content of Ni is limited to 20.0% by mass or less. 
         [0000]    Mo: from 3.0 to 6.0% by mass 
         [0038]    Mo contributes to enhancement of the base metal strength by precipitating an intermetallic compound such as Ni 3 Mo. In order to obtain such an effect, the content of Mo is limited to 3.0% by mass or more. 
         [0039]    On the other hand, if the content of Mo becomes excessive, the ductility is greatly reduced. Therefore, the content of Mo is limited to 6.0% by mass or less. 
         [0000]    Co: from 5.0 to 13.0% by mass 
         [0040]    Co forms a solid solution in the matrix to thereby reduce the solid solution amount of the intermetallic compound-forming element Ni or Mo into martensite and promote the precipitation of Ni 3 Mo or NiAl. As a result, the tensile strength and fatigue strength are enhanced. For fulfilling such a function, the content of Co is limited to 5.0% by mass or more. 
         [0041]    On the other hand, in the case where the content of Co is higher than 13.0% by mass, martensite transformation is inhibited due to drop of the Ms temperature, and the amount of residual austenite after solution heat treatment is increased to cause reduction in the strength. Therefore, the content of Co is limited to 13.0% by mass or less. 
         [0000]    Al: from 0.01 to 0.3% by mass 
         [0042]    Al acts as a deoxygenation material during melting process of steel and thereby to reduce the oxygen content in the steel. Also, this element has a function of combining with Ni in an aging treatment to precipitate an NiAl intermetallic compound and in turn, enhance the tensile strength and fatigue strength. In order to obtain these effects, the content of Al is limited to 0.01% by mass or more. 
         [0043]    On the other hand, in the case where the content of Al becomes excessive, Al forms an oxide to deteriorate the cleanliness and reduce the fatigue strength. For this reason, the content of Al is limited to 0.3% by mass or less. 
         [0000]    Ti: from 0.2 to 2.0% by mass 
         [0044]    Ti forms an intermetallic compound such as Ni 3 Ti in an aging treatment, whereby the strength can be expected to be enhanced. In order to obtain this effect, the content of Ti is limited to 0.2% by mass or more. 
         [0045]    On the other hand, Ti forms a Ti-based inclusion to deteriorate the cleanliness and reduce the fatigue strength. For this reason, the content of Ti is limited to 2.0% by mass or less. 
         [0000]    O: ≦0.0020% by mass 
         [0046]    O forms an oxide such as SiO 2  and Al 2 O 3  to reduce the fatigue strength. Therefore, the content of O is preferably as small as possible. However, an excessive decrease in the content of O causes a rise of the production cost. For this reason, the content of O is limited to 0.0020% by mass. The content of O is more preferably 0.0010% by mass or less. 
         [0000]    N: ≦0.0020% by mass 
         [0047]    N forms nitride such as TiN and AlN to reduce the fatigue strength. Therefore, the content of N is preferably as small as possible. However, an excessive decrease in the content of N causes a rise of the production cost. For this reason, the content of N is limited to 0.0020% by mass. The content of N is more preferably 0.0010% by mass or less. 
         [0000]    Zr: from 0.001 to 0.02% by mass 
         [0048]    Zr forms a nucleus of nitride or carbonitride such as TiN and refines the size of a TiN inclusion. In order to obtain this effect, the content of Zr is limited to 0.001% by mass or more. 
         [0049]    On the other hand, excessive addition of Zr leads to reduction in the ductility. For this reason, the content of Zr is limited to 0.02% by mass or less. The content of Zr is preferably from 0.001 to 0.008% by mass. 
         [0000]    B: from 0.0010 to 0.010% by mass 
         [0050]    B is an element effective in enhancing the hot workability of steel and therefore, may be added. This effect starts appearing with a content of 0.0010% by mass, but excessive addition causes formation of a boride having a low melting point at the grain boundary to deteriorate the cleanliness of steel and reduce the fatigue strength. For this reason, the content of B is limited to 0.010% by mass or less. 
         [0000]    Mg: ≦0.003% by mass
 
Ca: ≦0.003% by mass
 
         [0051]    Mg and Ca are elements effective in enhancing the hot workability of steel and therefore, may be added. However, excessive addition may cause formation of an oxide to deteriorate the cleanliness of steel and reduce the fatigue strength. For this reason, the content of Mg and Ca is limited to 0.003% by mass or less, respectively. 
       EXAMPLES 
       [0052]    Working examples of the present invention are described in detail below. 
         [0053]    A 150 kg of steel having a chemical composition shown in Table 1 was melted in a high-frequency vacuum induction furnace and cast to obtain a steel ingot, and this ingot was used as an electrode for secondary melting. 
         [0054]    A 20 mm portion on the top side and a 20 mm portion on the bottom side of this electrode were removed by cutting, respectively, and the surface layer was removed by shaving to a depth of 2.5 mm. 
         [0055]    Using the thus-trimmed electrode, vacuum arc re-melting was carried out to melt the electrode, and casting was subsequently performed to obtain an ingot after secondary melting. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Chemical Composition (% by mass) 
               
             
          
           
               
                   
                 C 
                 Ni 
                 Mo 
                 Co 
                 Al 
                 Ti 
                 N 
                 O 
                 Zr 
                 B 
                 Mg 
                 Ca 
                 Fe 
               
               
                   
                   
               
             
          
           
               
                 Ex. 
                 1 
                 0.004 
                 18.7 
                 5.0 
                 9.4 
                 0.13 
                 0.46 
                 0.0005 
                 0.0005 
                 0.005 
                   
                   
                   
                 balance. 
               
               
                   
                 2 
                 0.005 
                 12.5 
                 4.9 
                 9.0 
                 0.10 
                 0.41 
                 0.0007 
                 0.0006 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 3 
                 0.004 
                 16.2 
                 4.9 
                 9.5 
                 0.11 
                 0.48 
                 0.0009 
                 0.0006 
                 0.001 
                   
                   
                   
                 balance. 
               
               
                   
                 4 
                 0.003 
                 19.4 
                 5.1 
                 9.0 
                 0.08 
                 0.47 
                 0.0005 
                 0.0005 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 5 
                 0.006 
                 18.1 
                 3.2 
                 9.1 
                 0.11 
                 0.45 
                 0.0007 
                 0.0009 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 6 
                 0.006 
                 18.5 
                 5.7 
                 9.1 
                 0.10 
                 0.44 
                 0.0011 
                 0.0005 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 7 
                 0.006 
                 18.8 
                 4.8 
                 6.1 
                 0.14 
                 0.46 
                 0.0009 
                 0.0010 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 8 
                 0.003 
                 17.2 
                 4.6 
                 10.1 
                 0.12 
                 0.43 
                 0.0012 
                 0.0006 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 9 
                 0.007 
                 17.1 
                 4.9 
                 12.7 
                 0.15 
                 0.41 
                 0.0006 
                 0.0013 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 10 
                 0.004 
                 18.9 
                 5.0 
                 9.1 
                 0.10 
                 0.48 
                 0.0014 
                 0.0006 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 11 
                 0.004 
                 17.9 
                 4.5 
                 9.4 
                 0.24 
                 0.43 
                 0.0008 
                 0.0005 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 12 
                 0.005 
                 18.5 
                 5.0 
                 9.4 
                 0.10 
                 0.45 
                 0.0005 
                 0.0005 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 13 
                 0.005 
                 18.7 
                 4.9 
                 9.3 
                 0.14 
                 1.39 
                 0.0005 
                 0.0008 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 14 
                 0.007 
                 17.5 
                 4.8 
                 9.2 
                 0.15 
                 0.44 
                 0.0006 
                 0.0012 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 15 
                 0.009 
                 18.2 
                 5.0 
                 9.4 
                 0.08 
                 0.43 
                 0.0015 
                 0.0007 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 16 
                 0.004 
                 18.4 
                 4.6 
                 9.1 
                 0.09 
                 0.45 
                 0.0006 
                 0.0005 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 17 
                 0.003 
                 18.9 
                 5.3 
                 9.5 
                 0.12 
                 0.46 
                 0.0012 
                 0.0015 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 18 
                 0.003 
                 18.4 
                 4.8 
                 9.5 
                 0.11 
                 0.46 
                 0.0010 
                 0.0006 
                 0.005 
                   
                   
                   
                 balance. 
               
               
                   
                 19 
                 0.004 
                 18.2 
                 5.5 
                 9.2 
                 0.08 
                 0.48 
                 0.0012 
                 0.0011 
                 0.015 
                   
                   
                   
                 balance. 
               
               
                   
                 20 
                 0.005 
                 18.6 
                 5.0 
                 9.4 
                 0.13 
                 0.48 
                 0.0005 
                 0.0008 
                 0.005 
                 0.015 
                   
                   
                 balance. 
               
               
                   
                 21 
                 0.004 
                 18.7 
                 5.0 
                 9.5 
                 0.13 
                 0.46 
                 0.0005 
                 0.0005 
                 0.005 
                   
                 0.0011 
                   
                 balance. 
               
               
                   
                 22 
                 0.005 
                 18.6 
                 5.0 
                 9.5 
                 0.13 
                 0.46 
                 0.0005 
                 0.0005 
                 0.005 
                   
                   
                 0.0025 
                 balance. 
               
               
                   
                 23 
                 0.004 
                 18.4 
                 5.0 
                 9.2 
                 0.13 
                 0.46 
                 0.0004 
                 0.0005 
                 0.006 
                   
                   
                   
                 balance. 
               
               
                   
                 24 
                 0.004 
                 18.3 
                 5.0 
                 9.3 
                 0.13 
                 0.46 
                 0.0003 
                 0.0006 
                 0.008 
                   
                   
                   
                 balance. 
               
               
                 Com. 
                 1 
                 0.030 
                 18.2 
                 5.1 
                 9.0 
                 0.05 
                 0.44 
                 0.0014 
                 0.0005 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                 Ex. 
                 2 
                 0.005 
                 8.1 
                 5.2 
                 9.4 
                 0.10 
                 0.45 
                 0.0015 
                 0.0006 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 3 
                 0.010 
                 23.1 
                 4.9 
                 9.5 
                 0.15 
                 0.49 
                 0.0009 
                 0.0005 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 4 
                 0.006 
                 18.8 
                 2.1 
                 9.3 
                 0.13 
                 0.44 
                 0.0007 
                 0.0006 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 5 
                 0.011 
                 18.3 
                 8.2 
                 8.8 
                 0.09 
                 0.43 
                 0.0009 
                 0.0006 
                 0.004 
                   
                   
                   
                 balance. 
               
               
                   
                 6 
                 0.003 
                 18.5 
                 5.1 
                 3.1 
                 0.13 
                 0.43 
                 0.0010 
                 0.0008 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 7 
                 0.006 
                 18.9 
                 5.1 
                 15.1 
                 0.14 
                 0.5 
                 0.0010 
                 0.0009 
                 0.005 
                   
                   
                   
                 balance. 
               
               
                   
                 8 
                 0.006 
                 17.8 
                 5.2 
                 9.3 
                 0.005 
                 0.49 
                 0.0008 
                 0.0013 
                 0.004 
                   
                   
                   
                 balance. 
               
               
                   
                 9 
                 0.008 
                 17.3 
                 4.8 
                 9.3 
                 0.50 
                 0.49 
                 0.0007 
                 0.0011 
                 0.004 
                   
                   
                   
                 balance. 
               
               
                   
                 10 
                 0.005 
                 18.5 
                 4.9 
                 9.6 
                 0.15 
                 0.04 
                 0.0006 
                 0.0013 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 11 
                 0.015 
                 18.8 
                 5.0 
                 9.2 
                 0.16 
                 2.11 
                 0.0007 
                 0.0009 
                 0.004 
                   
                   
                   
                 balance. 
               
               
                   
                 12 
                 0.004 
                 18.9 
                 4.9 
                 9.3 
                 0.12 
                 0.48 
                 0.0022 
                 0.0009 
                 0.003 
                   
                   
                   
                 balance. 
               
               
                   
                 13 
                 0.005 
                 18.9 
                 4.9 
                 9.4 
                 0.11 
                 0.41 
                 0.0011 
                 0.0021 
                 0.002 
                   
                   
                   
                 balance. 
               
               
                   
                 14 
                 0.005 
                 18.2 
                 5.0 
                 9.5 
                 0.14 
                 0.45 
                 0.0009 
                 0.0008 
                 &lt;0.001 
                   
                   
                   
                 balance. 
               
               
                   
                 15 
                 0.004 
                 18.2 
                 5.0 
                 9.4 
                 0.14 
                 0.44 
                 0.0012 
                 0.0005 
                 0.023 
                   
                   
                   
                 balance. 
               
               
                   
               
               
                 Ex.: Example, Com. Ex.: Comparative Example 
               
             
          
         
       
     
         [0056]    The ingot was forged, further hot-rolled to a thickness of 3 mm (3 mmT), then annealed under the condition of 650° C.×8 hr, subsequently cold-rolled to 0.32 mmT, and subjected to solid solution formation/heat treatment at 900° C. and aging treatment under the condition of 480° C.×3 hr. 
         [0057]    For this steel subjected to aging treatment, the following tensile test, hardness test, fatigue test and chemical extraction test were performed. Also, for the steel after hot rolling, microscopic observation was performed. 
         [0058]    Each of the microscopic observation, tensile test, hardness test, fatigue test and chemical extraction test was performed as follows. 
       [Microscopic Observation] 
       [0059]    A test piece was sampled from the material after hot rolling and observed for an inclusion on the longitudinal cross-section by SEM (scanning electron microscope). Also, identification of the inclusion was performed by EDX (energy dispersive X-ray analysis). 
       [Tensile Test] 
       [0060]    The tensile test was performed in accordance with the metal tensile test method of JIS Z 2241 (2011). As the test piece, a No. 13B test piece by JIS Z 2201 (2005) was employed. The test temperature was set to room temperature. 
       [Hardness Test] 
       [0061]    The test was performed in accordance with the Vickers hardness test method provided in JIS Z 2244 (2009). The sample was measured under a load of 4.9 N, and the measurement site was set to a position of ½ the sample thickness. An average value of 5 points was employed as the measured value. 
       [Fatigue Test] 
       [0062]    The fatigue characteristics were examined in accordance with general rules for the fatigue test method of metal materials of JIS Z 2273 (2010). Specifically, vibration was applied to the test piece under the conditions of an amplitude stress of 850 N/mm 2  in terms of alternating oscillation and a vibration rate of 1,200 rpm to repeatedly cause bending deformation of the test piece, and the number of repeated vibrations (deformations) until reaching a fracture was measured. 
         [0063]    The evaluation of fatigue characteristics was rated “A” when the number of repetitions was 10 7  or more, and rated “B” when less than 10 7 . Incidentally, the test specimen had a profile of 0.32 mmT×10 mmW×100 mmL. 
       [Chemical Extraction Test] 
       [0064]    A plurality of 0.32 mmT test pieces of 15 mm×15 mm were sampled, and attachments and the like on the surface layer were removed by pickling. The test piece was subjected to chemical dissolution with 5 g in total of bromine and methanol, and extraction of inclusions was performed by an extraction filter having a pore size φ of 5 μm. The extracted residue was observed by SEM, and the shape and size of the inclusion were measured. Also, the identification of the inclusion was performed by EDX. 
         [0065]    The long side a and short side b of nitride or carbonitride were measured, and the size of the carbonitride-based inclusion was evaluated by the maximum size of the long side a. 
         [0066]    These results are shown in Table 2. 
         [0067]    Furthermore,  FIG. 1  shows the microscopic observation results of Example 1 as a representative of Examples 1 to 24.  FIG. 2A  shows the results of chemical extraction test, and in addition,  FIG. 2B  shows the results of chemical extraction test (results of observation by SEM) of Comparative Example 14. 
         [0068]    Incidentally, in Table 2, the carbonitride-based inclusion is a Ti carbonitride-based inclusion and in all planar views, its shape is a square or substantially square angular shape. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Tensile Test 
                   
                 Maximum Length of 
               
             
          
           
               
                   
                 Proof Stress 
                 Strength 
                 Elongation 
                 Hardness 
                 Fatigue 
                 Carbonitride-Based 
               
               
                   
                 (MPa) 
                 (MPa) 
                 (%) 
                 (HV) 
                 Characteristics 
                 Inclusion (μm) 
               
               
                   
                   
               
             
          
           
               
                 Ex. 
                 1 
                 1819 
                 1860 
                 11 
                 498 
                 A 
                 4.4 
               
               
                   
                 2 
                 1780 
                 1821 
                 12 
                 471 
                 A 
                 5.2 
               
               
                   
                 3 
                 1797 
                 1845 
                 9 
                 471 
                 A 
                 5.4 
               
               
                   
                 4 
                 1861 
                 1891 
                 10 
                 529 
                 A 
                 4.8 
               
               
                   
                 5 
                 1766 
                 1805 
                 9 
                 484 
                 A 
                 5.0 
               
               
                   
                 6 
                 1880 
                 1930 
                 9 
                 545 
                 A 
                 5.1 
               
               
                   
                 7 
                 1778 
                 1812 
                 10 
                 458 
                 A 
                 4.7 
               
               
                   
                 8 
                 1856 
                 1883 
                 9 
                 471 
                 A 
                 5.5 
               
               
                   
                 9 
                 1880 
                 1935 
                 8 
                 545 
                 A 
                 4.8 
               
               
                   
                 10 
                 1809 
                 1848 
                 12 
                 484 
                 A 
                 5.7 
               
               
                   
                 11 
                 1873 
                 1905 
                 8 
                 545 
                 A 
                 5.0 
               
               
                   
                 12 
                 1816 
                 1857 
                 9 
                 471 
                 A 
                 4.2 
               
               
                   
                 13 
                 2084 
                 2139 
                 8 
                 615 
                 A 
                 4.5 
               
               
                   
                 14 
                 1838 
                 1865 
                 10 
                 484 
                 A 
                 5.0 
               
               
                   
                 15 
                 1837 
                 1880 
                 10 
                 484 
                 A 
                 6.1 
               
               
                   
                 16 
                 1832 
                 1867 
                 10 
                 498 
                 A 
                 4.6 
               
               
                   
                 17 
                 1796 
                 1835 
                 9 
                 513 
                 A 
                 5.4 
               
               
                   
                 18 
                 1819 
                 1867 
                 10 
                 513 
                 A 
                 5.4 
               
               
                   
                 19 
                 1829 
                 1854 
                 10 
                 484 
                 A 
                 5.7 
               
               
                   
                 20 
                 1825 
                 1868 
                 8 
                 498 
                 A 
                 5.5 
               
               
                   
                 21 
                 1843 
                 1886 
                 10 
                 498 
                 A 
                 4.5 
               
               
                   
                 22 
                 1803 
                 1851 
                 12 
                 498 
                 A 
                 5.8 
               
               
                   
                 23 
                 1820 
                 1869 
                 11 
                 499 
                 A 
                 4.8 
               
               
                   
                 24 
                 1824 
                 1860 
                 12 
                 500 
                 A 
                 4.7 
               
               
                 Com. 
                 1 
                 1858 
                 1892 
                 7 
                 513 
                 B 
                 5.5 
               
               
                 Ex 
                 2 
                 1663 
                 1710 
                 9 
                 440 
                 B 
                 5.2 
               
               
                   
                 3 
                 1635 
                 1690 
                 7 
                 484 
                 B 
                 4.8 
               
               
                   
                 4 
                 1542 
                 1570 
                 13 
                 402 
                 B 
                 4.5 
               
               
                   
                 5 
                 1789 
                 1830 
                 4 
                 485 
                 A 
                 5.0 
               
               
                   
                 6 
                 1629 
                 1650 
                 12 
                 422 
                 B 
                 5.5 
               
               
                   
                 7 
                 1710 
                 1734 
                 10 
                 470 
                 B 
                 5.3 
               
               
                   
                 8 
                 1696 
                 1730 
                 10 
                 458 
                 B 
                 4.7 
               
               
                   
                 9 
                 1759 
                 1808 
                 10 
                 480 
                 B 
                 5.0 
               
               
                   
                 10 
                 1706 
                 1734 
                 10 
                 484 
                 B 
                 4.4 
               
               
                   
                 11 
                 2376 
                 2410 
                 3 
                 675 
                 B 
                 4.6 
               
               
                   
                 12 
                 1742 
                 1770 
                 8 
                 484 
                 B 
                 8.8 
               
               
                   
                 13 
                 1767 
                 1790 
                 8 
                 484 
                 B 
                 6.0 
               
               
                   
                 14 
                 1815 
                 1855 
                 11 
                 458 
                 B 
                 7.9 
               
               
                   
                 15 
                 1815 
                 1850 
                 5 
                 471 
                 B 
                 6.2 
               
               
                   
               
               
                 Ex.: Example, Com. Ex.: Comparative Example 
               
             
          
         
       
     
         [0069]    In  FIG. 1 , it is seen that a Zr inclusion (ZrO 2 ) is present in the center part of the inclusion TiN, that is, the inclusion TiN is thinned around ZrO 2  serving as a nucleus. 
         [0070]    Also, in  FIGS. 2A and 2B , it is seen that in Example 1 where Zr is added, the size of the TiN inclusion is small owing to the addition of Zr ( FIG. 2A ) but in Comparative Example 14 where Zr is not added, a large-size TiN inclusion ( FIG. 2B ) is produced. 
         [0071]    Here, in  FIGS. 2A and 2B , the rounded black-looking portion is a pore of the extraction filter, and the portion looking black as the ground color is the extraction filter itself. 
         [0072]    As regards the results of Table 2, in Comparative Example 1, formation of carbide or carbonitride is thought to be promoted due to a large C amount and in turn, the fatigue characteristics are poor. 
         [0073]    In Comparative Examples 2, 4, 6, 8 and 10 where respectively the amounts of Ni, Mo, Co, Al and Ti are small, respectively, a sufficient intermetallic compound is not precipitated by an aging treatment and in turn, the tensile strength and fatigue characteristics are poor. 
         [0074]    In Comparative Examples 3 and 7 where the amounts of Ni and Co are large, respectively, it is thought that an austenite phase was stabilized and a sufficient martensite texture was not obtained. In turn, the tensile strength and fatigue characteristics are poor. 
         [0075]    In Comparative Example 5 where the content of Mo is large, the tensile strength and fatigue characteristics are good owing to aging/hardening, but reduction in the ductility is great. 
         [0076]    In Comparative Example 9 where the content of Al is large, an oxide is thought to be readily formed, leading to decrease in the cleanliness. As a result, an inclusion works out to a fracture origin and in turn, the fatigue characteristics are poor. 
         [0077]    In Comparative Example 12 where the content of N is large, the formed nitride and carbonitride grow into a coarse diameter, and a fatigue is generated from a carbonitride origin, as a result, the fatigue characteristics are deteriorated. 
         [0078]    In Comparative Example 13 where the content of O is large, a O-containing nonmetallic inclusion is readily formed, and the fatigue characteristics are deteriorated. 
         [0079]    In Comparative Example 14 where the content of Zr is small, the TiN diameter becomes coarse and in turn, the fatigue characteristics are deteriorated. In Comparative Example 15 where the content of Zr is large, the ductility is deteriorated. 
         [0080]    On the other hand, in Examples 1 to 24 where the content of Zr is adjusted to fall in the range of 0.001 to 0.02% and each component of C, Ni, Mo, Co, Al, Ti, N and O is incorporated in a predetermined proper content, since a TiN inclusion is produced using a Zr-based oxide as a nucleus, the size of the TiN inclusion is refined and the fatigue characteristics and other properties are excellent. 
         [0081]    While the mode for carrying out the present invention has been described in detail above, the present invention is not limited to these embodiments, and various changes and modifications can be made therein without departing from the purport of the present invention. 
         [0082]    Incidentally, this application is based on Japanese patent application No. 2013-173761 filed Aug. 23, 2013 and Japanese patent application No. 2014-106152 filed May 22, 2014, and the entire contents thereof being hereby incorporated by reference.