Abstract:
The present invention relates to a high-strength spring steel having excellent wire-rod rolling properties, consisting essentially of, in terms of mass %: C: 0.40% to 0.65%; Si: 1.20% to 2.80%; Mn: 0.30% to 1.20%; P: 0.020% or less; S: 0.020% or less; Cu: 0.40% or less; Ni: 0.80% or less; Cr: 0.70% or less; Ti: 0.060% to 0.140%; Al: 0.10% or less; N: 0.010% or less; and O: 0.0015% or less, and optionally: B: 0.0005% to 0.0050%, with the remainder being Fe and inevitable impurities, in which the contents in terms of mass % of the specified chemical components satisfy the following Expressions (1) to (3): 
         X 1=0.14×[Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2   Expression (1)
 
         X 2=(α−500)/β≧3.0   Expression (2)
 
       α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]
 
       β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])
 
         X 3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0   Expression (3).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a high-strength spring steel having excellent wire-rod rolling properties. 
       BACKGROUND OF THE INVENTION 
       [0002]    On the occasion of seeking weight reduction of suspension springs in answer to demands for weight reduction of vehicles (automobiles), development of springs allowing high design stress has been pursued. For the purpose of heightening the design stress of springs, it is required to engineer improvements in various characteristics of the springs, and more specifically, it is essential to add alloy elements. For example, it may be thought to add Si when improvement in settling property is intended, while it may be thought to add such an element as Cu, Ni or Cr when improvement in corrosion resistance is intended. 
         [0003]    By the way, increases in amounts of alloy elements added with the aim of improving spring characteristics tend to yield detriments such as occurrence of ferrite decarburization and formation of bainite during the cooling after wire-rod rolling. The former detriment is fatal for the springs to which shot peening is to be given, while the latter detriment may become a harmful factor at the time of secondary working, and hence it becomes important to avoid both of these detriments. As the techniques to avoid both the detriments, there have been known techniques described e.g. in the following Patent Documents 1 and 2. 
         [0004]    The following Patent document 1 has disclosed the technique of heating a steel material at a temperature of 1,170° C. or more for at least 2 minutes under hot rolling, cooling the material at an average cooling rate of 5 to 300° C./min in a temperature range from 750° C. to 600° C. after the rolling, and further adopting descaling process. The following Patent Document 2 has disclosed the technique of subjecting a steel material to hot rolling in a condition that, after heating furnace extraction, the temperature before finishing is set to less than 1,000° C., keeping the steel material in a temperature range of 1,000° C. to 1,150° C. for 5 seconds or less after finish rolling and then winding it up, thereafter cooling the wound steel material to a temperature of 750° C. or less at a cooling rate of 2 to 8° C./sec, and further gradually cooling down to 600° C. by spending 150 seconds or more after the winding-up. 
         [0005]    Patent Document 1: Japanese Patent No. 4031267 
         [0006]    Patent Document 2: Japanese Patent No. 5330181 
       SUMMARY OF THE INVENTION 
       [0007]    However, each of the techniques disclosed in Patent Documents 1 and 2 requires execution of individually specified rolling process. Accordingly, it has been desired to avoid occurrence of ferrite decarburization and formation of bainite by adjusting chemical components of a steel material instead of adopting a technique of providing a specific rolling process, thereby developing a high-strength spring steel having excellent wire-rod rolling properties. 
         [0008]    The present invention has been made against a background of the foregoing circumstances, and an object of the present invention is to provide a high-strength spring steel having excellent wire-rod rolling properties by adjusting chemical components of a steel material to avoid occurrence of ferrite decarburization and formation of bainite. 
         [0009]    Namely, the present invention relates to the following items 1 to 3. 
         [0010]    1. A high-strength spring steel having excellent wire-rod rolling properties, consisting essentially of, in terms of mass %: 
         [0011]    C: 0.40% to 0.65%; 
         [0012]    Si: 1.20% to 2.80%; 
         [0013]    Mn: 0.30% to 1.20%; 
         [0014]    P: 0.020% or less; 
         [0015]    S: 0.020% or less; 
         [0016]    Cu: 0.40% or less; 
         [0017]    Ni: 0.80% or less; 
         [0018]    Cr: 0.70% or less; 
         [0019]    Ti: 0.060% to 0.140%; 
         [0020]    Al: 0.10% or less; 
         [0021]    N: 0.010% or less; and 
         [0022]    O: 0.0015% or less, 
         [0023]    and optionally: 
         [0024]    B: 0.0005% to 0.0050%, 
         [0025]    with the remainder being Fe and inevitable impurities, 
         [0026]    in which the contents in terms of mass % of the specified chemical components satisfy the following Expressions (1) to (3): 
         [0000]        X 1=0.14×[Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2   Expression (1)
 
         [0000]        X 2=(α−500)/β≧3.0   Expression (2)
 
         [0000]      α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]
 
         [0000]      β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])
 
         [0000]        X 3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0   Expression (3).
 
         [0027]    2. The high-strength spring steel having excellent wire-rod rolling properties according to item 1, having a 400° C.-temper hardness of 53.0 HRC or more. 
         [0028]    3. The high-strength spring steel having excellent wire-rod rolling properties according to items 1 or 2, having a crystal grain size number of 9 or more. 
         [0029]    The present inventors have found that it is possible to formulate (as Expression (1)) the relation between a ferrite decarburization depth and a parameter (X1) determined by converting the degrees of contributions to the depth from respective chemical components of a steel material into numerical values, formulate (as Expression (2)) the relation between bainite formation in the case of cooling at a normal cooling rate after wire-rod rolling and a parameter (X2) determined by converting the degrees of contributions to the bainite formation from respective chemical components of a steel material into numerical values and formulate (as Expression (3)) the relation between hardness in the case of subjecting tempering treatment at 400° C. and a parameter (X3) determined by converting the degrees of contributions to the hardness from respective chemical components of a steel material into numerical values. Namely, it is possible to obtain a high-strength spring steel having excellent wire-rod rolling properties by adjusting contents of chemical components in a steel material so as to satisfy the foregoing Expressions (1) to (3). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0030]      FIG. 1  is a graph for illustrating conditions of Expression (1). 
           [0031]      FIG. 2  is a graph for illustrating conditions of Expression (2). 
           [0032]      FIG. 3  is a graph for illustrating conditions of Expression (3). 
           [0033]      FIG. 4  is a graph for showing the reason for setting the lower limit of Ti content to 0.060 mass %. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The following are descriptions of reasons and conditions for limiting individual chemical components (elements) in the composition of the present high-strength spring steel. Incidentally, the content of each component is shown in terms of mass %, and “mass %” is the same as “wt %”.
   (1) C: 0.40% to 0.65%   
 
         [0036]    C is an element essential for spring steel to secure strength. When the C content is lower than 0.40%, it is impossible to achieve the intended spring strength. On the other hand, when C is added in an amount exceeding 0.65%, degradation of tenacity and fatigue characteristics is caused, and hence the upper limit of C content is set to 0.65%. The C content is preferably from 0.45% to 0.60%.
   (2) Si: 1.20% to 2.80%   
 
         [0038]    Si is an element effective in enhancing settling resistance of spring steel. Si is therefore added in an amount of 1.20% or more. However, addition of Si in excess of 2.80% tends to cause not only degradation of settling properties but also occurrence of ferrite decarburization, and hence the upper limit of Si content is set to 2.80%. The Si content is preferably more than 1.50% and 2.50% or less, more preferably more than 2.00% and 2.50% or less.
   (3) Mn: 0.30% to 1.20%   
 
         [0040]    Mn functions as an ingredient for fixing S, which is a tenacity degrading element, in the form of MnS. Mn functions also as a quenching property improver. In order to make good use of these functions, Mn is added in an amount of 0.30% or more. However, addition of Mn in an amount exceeding 1.20% results in degradation of tenacity, and hence the upper limit of Mn content is set to 1.20%. The Mn content is preferably more than 0.50% and 1.10% or less, more preferably less than 1.00%.
   (4) P: 0.020% or less   
 
         [0042]    Since P makes crystal grain boundaries brittle, the content thereof is required to be minimized. So long as the P content is 0.020% or less, impact of reduction in strength of the grain boundaries is slight, while extreme reduction in P content is undesirable from the industrial viewpoint because it brings about elongation of smelting process which results in an increased cost.
   (5) S: 0.020% or less   
 
         [0044]    S is inevitably present in steel and, as mentioned above, combines with Mn to form MnS inclusions which become starting points of stress concentration. Unduly high S content not only increases the amount of MnS inclusions but also causes reduction in fatigue strength. However, so long as the S content is 0.020% or less, reduction in fatigue strength is exceedingly slight.
   (6) Cu: 0.40% or less   
 
         [0046]    Cu is an element effective in improving corrosion resistance. In addition, it is also effective in preventing ferrite decarburization. The Cu content is preferably from 0.20% to 0.37%.
   (7) Ni: 0.80% or less   
 
         [0048]    Ni is an element effective in improving corrosion resistance. In addition, it is also effective in preventing ferrite decarburization. Incorporation of Ni, however, brings about an increase in cost, and hence the upper limit of Ni content is set to 0.80%. The Ni content is preferably from 0.50% to 0.75%.
   (8) Cr: 0.70% or less   
 
         [0050]    Cr is an element effective in improving corrosion resistance. In addition, it is also effective for adjustment of quenching properties. Excessive addition of Cr causes formation of sharp corrosion pits, and hence the upper limit of Cr content is set to 0.70%. The Cr content is preferably from 0.20% to 0.50%.
   (9) Ti: 0.060% to 0.140%   
 
         [0052]    Ti is an element that is apt to form carbide. Ti-based carbides contribute to fining of crystal grains and enhance a fatigue characteristic, a delayed fracture characteristic and settling resistance. For these reasons, Ti is added in an amount of 0.060% or more. When the Ti content exceeds 0.140%, however, the effects of Ti addition become saturated; on the contrary, deterioration in rolling properties is brought about. The upper limit of Ti content is therefore set to 0.140%. The Ti content is preferably from 0.080% to 0.120%. Reasons why the lower limit of Ti content is set to 0.060% will be described later.
   (10) Al: 0.10% or less   
 
         [0054]    Al is an element that acts as a deoxidizer during liquid steel treatment. However, when Al is added in an amount exceeding 0.10%, inclusions are increased, whereby lowering of fatigue strength is rather caused. The upper limit of Al content is therefore set to 0.10%.
   (11) N: 0.010% or less   
 
         [0056]    N combines with Ti to form nitride, resulting in lowering of fatigue strength. The upper limit of N content is therefore set to 0.010%.
   (12) O: 0.0015% or less   
 
         [0058]    Since O forms oxide-based inclusions, the content thereof is set to 0.0015% or less.
   (13) Remainder: Fe and inevitable impurities   
 
         [0060]    Incidentally, descriptions of Fe and inevitable impurities are omitted in Table 1.
   (14) Satisfying the following Expression (1)   
 
         [0000]        X 1=0.14×[Si]−0.11×[Mn]−0.05×[Cu]−0.11×[Ni]−0.03×[Cr]+0.02≦0.2   Expression (1)
 
         [0062]    In order to examine the adequacy of Expression (1), simulations of ferrite decarburization were conducted. In the simulations, steel samples having chemical compositions shown in Table 1, respectively, were each independently melt-formed and hot-rolled into bars having 22 mmφ. Thereafter, these samples were machined into bars having dimensions of 14 mmφ×20 mm, subjected to heat treatment with the condition that they were kept at 900° C. for 100 minutes, and then oil-cooled. Subsequently thereto, ferrite decarburization depth measurements were made on the samples after the heat treatment. Measurement results obtained are shown in Table 1 and  FIG. 1 . 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Ferrite 
                 Presence or Absence 
                 Presence or Absence 
                   
                   
               
               
                   
                 Decarbu- 
                 of Ferrite 
                 of Bainite 
                 400° C. 
               
             
          
           
               
                   
                 Chemical Components (mass %) 
                   
                 rization 
                 Decarburization during 
                 Formation during 
                 Temper 
                   
               
             
          
           
               
                   
                 C 
                 Si 
                 Mn 
                   
                 Ti 
                 B 
                 Expressions (1) to (3) 
                 Depth 
                 Rod-wire Rolling 
                 Rod-wire Rolling 
                 Hardness 
                 Crystal Grain 
               
             
          
           
               
                 Steel 
                 0.40- 
                 1.20- 
                 0.30- 
                 Cu 
                 Ni 
                 Cr 
                 0.060- 
                 0.0005- 
                 X1 
                 X2 
                 α 
                 β 
                 X3 
                 (mm) 
                 0.5° C./s 
                 0.5° C./s 
                 1.5° C./s 
                 (HRC) 
                 Size Number 
               
               
                 Species 
                 0.65 
                 2.80 
                 1.20 
                 ≦0.40 
                 ≦0.80 
                 ≦0.70 
                 0.140 
                 0.0050 
                 ≦0.2 
                 ≧3.0 
                 — 
                 — 
                 ≧24.0 
                 — 
                 — 
                 — 
                 — 
                 ≧53.0 
                 ≧9.0 
               
               
                   
               
             
          
           
               
                 Ex. 1 
                 0.53 
                 2.10 
                 0.72 
                 0.27 
                 0.58 
                 0.33 
                 0.092 
                 0.0011 
                 0.148 
                 4.72 
                 816 
                 67.0 
                 25.4 
                 0.062 
                 absent 
                 absent 
                 absent 
                 53.9 
                 9.6 
               
               
                 Ex. 2 
                 0.52 
                 2.02 
                 0.65 
                 0.32 
                 0.71 
                 0.33 
                 0.090 
                 0.0015 
                 0.127 
                 4.39 
                 813 
                 71.3 
                 25.2 
                 0.061 
                 absent 
                 absent 
                 absent 
                 53.4 
                 9.5 
               
               
                 Ex. 3 
                 0.50 
                 2.15 
                 0.78 
                 0.27 
                 0.57 
                 0.33 
                 0.093 
                 0.0017 
                 0.149 
                 3.94 
                 824 
                 82.2 
                 24.7 
                 0.063 
                 absent 
                 absent 
                 absent 
                 53.5 
                 9.7 
               
               
                 Ex. 4 
                 0.50 
                 2.09 
                 0.70 
                 0.32 
                 0.72 
                 0.33 
                 0.091 
                 0.0010 
                 0.131 
                 3.69 
                 819 
                 86.3 
                 24.9 
                 0.056 
                 absent 
                 absent 
                 absent 
                 53.1 
                 9.6 
               
               
                 Ex. 5 
                 0.49 
                 2.19 
                 0.83 
                 0.27 
                 0.58 
                 0.33 
                 0.094 
                 0.0018 
                 0.148 
                 3.32 
                 826 
                 98.1 
                 24.6 
                 0.064 
                 absent 
                 absent 
                 absent 
                 53.0 
                 10.1 
               
               
                 Ex. 6 
                 0.48 
                 2.14 
                 0.75 
                 0.32 
                 0.72 
                 0.33 
                 0.090 
                 0.0017 
                 0.132 
                 3.15 
                 824 
                 102.9 
                 24.5 
                 0.060 
                 absent 
                 absent 
                 absent 
                 53.0 
                 9.5 
               
               
                 Ex. 7 
                 0.52 
                 2.09 
                 0.76 
                 0.30 
                 0.49 
                 0.25 
                 0.092 
                 0.0016 
                 0.153 
                 5.28 
                 819 
                 60.4 
                 24.7 
                 0.052 
                 absent 
                 absent 
                 absent 
                 53.6 
                 9.5 
               
               
                 Ex. 8 
                 0.52 
                 2.10 
                 0.76 
                 0.30 
                 0.50 
                 0.25 
                 0.124 
                 0.0017 
                 0.153 
                 5.20 
                 819 
                 61.3 
                 24.6 
                 0.049 
                 absent 
                 absent 
                 absent 
                 53.4 
                 11.2 
               
               
                 Ex. 9 
                 0.56 
                 2.11 
                 0.76 
                 0.30 
                 0.50 
                 0.25 
                 0.122 
                 0.0014 
                 0.154 
                 5.31 
                 810 
                 58.4 
                 25.8 
                 0.055 
                 absent 
                 absent 
                 absent 
                 53.5 
                 10.8 
               
               
                 Ex. 10 
                 0.56 
                 1.79 
                 0.39 
                 0.20 
                 0.40 
                 0.25 
                 0.089 
                 0.0016 
                 0.166 
                 22.09 
                 813 
                 14.2 
                 24.1 
                 0.056 
                 absent 
                 absent 
                 absent 
                 53.4 
                 9.7 
               
               
                 Ex. 11 
                 0.52 
                 2.09 
                 0.76 
                 0.30 
                 0.50 
                 0.25 
                 0.093 
                 0 
                 0.152 
                 5.20 
                 819 
                 61.3 
                 24.7 
                 0.051 
                 absent 
                 absent 
                 absent 
                 53.5 
                 9.4 
               
               
                 Ex. 12 
                 0.48 
                 2.11 
                 0.75 
                 0.32 
                 0.72 
                 0.33 
                 0.091 
                 0 
                 0.128 
                 3.14 
                 823 
                 102.8 
                 24.4 
                 0.048 
                 absent 
                 absent 
                 absent 
                 53.3 
                 9.5 
               
               
                 Comp. 
                 0.48 
                 2.01 
                 0.60 
                 0.37 
                 0.65 
                 0.15 
                 0.091 
                 0 
                 0.141 
                 7.17 
                 825 
                 45.3 
                 23.1 
                 0.057 
                 absent 
                 absent 
                 absent 
                 52.1 
                 9.7 
               
               
                 Ex. 1 
               
               
                 Comp. 
                 0.52 
                 2.13 
                 0.76 
                 0.30 
                 0.50 
                 0.25 
                 0.004 
                 0 
                 0.157 
                 5.21 
                 820 
                 61.4 
                 25.4 
                 0.066 
                 absent 
                 absent 
                 absent 
                 53.5 
                 7.9 
               
               
                 Ex. 2 
               
               
                 Comp. 
                 0.52 
                 2.09 
                 0.76 
                 0.30 
                 0.50 
                 0.25 
                 0.032 
                 0.0013 
                 0.152 
                 5.20 
                 819 
                 61.3 
                 25.1 
                 0.063 
                 absent 
                 absent 
                 absent 
                 53.8 
                 8.2 
               
               
                 Ex. 3 
               
               
                 Comp. 
                 0.52 
                 2.37 
                 0.35 
                 0.29 
                 0.37 
                 0.23 
                 0.081 
                 0.0020 
                 0.251 
                 23.51 
                 839 
                 14.4 
                 24.1 
                 0.163 
                 present 
                 absent 
                 absent 
                 53.4 
                 9.3 
               
               
                 Ex. 4 
               
               
                 Comp. 
                 0.56 
                 2.39 
                 0.39 
                 0.20 
                 0.43 
                 0.25 
                 0.090 
                 0.0020 
                 0.247 
                 21.89 
                 832 
                 15.2 
                 25.5 
                 0.116 
                 present 
                 absent 
                 absent 
                 54.1 
                 9.4 
               
               
                 Ex. 5 
               
               
                 Comp. 
                 0.48 
                 2.39 
                 0.40 
                 0.20 
                 0.40 
                 0.25 
                 0.089 
                 0.0019 
                 0.249 
                 21.21 
                 851 
                 16.5 
                 23.0 
                 0.158 
                 present 
                 absent 
                 absent 
                 52.0 
                 9.5 
               
               
                 Ex. 6 
               
               
                 Comp. 
                 0.61 
                 2.39 
                 0.40 
                 0.20 
                 0.40 
                 0.25 
                 0.092 
                 0.0017 
                 0.249 
                 22.78 
                 821 
                 14.1 
                 27.0 
                 0.126 
                 present 
                 absent 
                 absent 
                 55.6 
                 9.7 
               
               
                 Ex. 7 
               
               
                 Comp. 
                 0.57 
                 2.20 
                 0.40 
                 0.20 
                 0.40 
                 0.25 
                 0.091 
                 0.0015 
                 0.223 
                 22.07 
                 824 
                 14.7 
                 25.3 
                 0.109 
                 present 
                 absent 
                 absent 
                 54.2 
                 9.8 
               
               
                 Ex. 8 
               
               
                 Comp. 
                 0.56 
                 2.78 
                 0.40 
                 0.20 
                 0.40 
                 0.25 
                 0.090 
                 0.0017 
                 0.304 
                 22.65 
                 845 
                 15.2 
                 26.3 
                 0.156 
                 present 
                 absent 
                 absent 
                 55.2 
                 9.6 
               
               
                 Ex. 9 
               
               
                 Comp. 
                 0.42 
                 2.64 
                 1.11 
                 0.31 
                 0.21 
                 0.25 
                 0.097 
                 0.0020 
                 0.221 
                 2.62 
                 857 
                 136.3 
                 22.9 
                 0.088 
                 present 
                 absent 
                 present 
                 52.2 
                 10.6 
               
               
                 Ex. 10 
               
               
                 Comp. 
                 0.41 
                 2.56 
                 0.96 
                 0.29 
                 0.21 
                 0.14 
                 0.091 
                 0.0020 
                 0.231 
                 5.27 
                 863 
                 68.7 
                 21.7 
                 0.114 
                 present 
                 absent 
                 absent 
                 51.5 
                 9.6 
               
               
                 Ex. 11 
               
               
                 Comp. 
                 0.44 
                 2.50 
                 0.98 
                 0.28 
                 0.78 
                 0.58 
                 0.087 
                 0.0016 
                 0.145 
                 0.93 
                 837 
                 362.0 
                 25.6 
                 0.048 
                 absent 
                 present 
                 present 
                 54.4 
                 9.4 
               
               
                 Ex. 12 
               
               
                 Comp. 
                 0.44 
                 2.52 
                 0.73 
                 0.30 
                 0.80 
                 0.59 
                 0.079 
                 0.0015 
                 0.172 
                 1.83 
                 841 
                 186.2 
                 25.3 
                 0.063 
                 absent 
                 present 
                 present 
                 53.2 
                 9.3 
               
               
                 Ex. 13 
               
               
                 Comp. 
                 0.41 
                 2.10 
                 1.01 
                 0.24 
                 0.24 
                 0.36 
                 0.088 
                 0 
                 0.154 
                 2.98 
                 845 
                 115.8 
                 21.6 
                 0.087 
                 absent 
                 absent 
                 present 
                 51.5 
                 9.5 
               
               
                 Ex. 14 
               
               
                 Comp. 
                 0.40 
                 2.18 
                 0.84 
                 0.26 
                 0.17 
                 0.12 
                 0.001 
                 0 
                 0.198 
                 8.60 
                 857 
                 41.5 
                 20.6 
                 0.064 
                 absent 
                 absent 
                 absent 
                 50.1 
                 7.6 
               
               
                 Ex. 15 
               
               
                 Comp. 
                 0.60 
                 1.97 
                 0.83 
                 0.10 
                 0.11 
                 0.12 
                 0.003 
                 0 
                 0.184 
                 14.05 
                 812 
                 22.2 
                 25.9 
                 0.072 
                 absent 
                 absent 
                 absent 
                 53.2 
                 7.4 
               
               
                 Ex. 16 
               
               
                 Comp. 
                 0.54 
                 1.29 
                 0.74 
                 0.09 
                 0.06 
                 0.63 
                 0.002 
                 0 
                 0.089 
                 6.79 
                 800 
                 44.1 
                 23.9 
                 0.031 
                 absent 
                 absent 
                 absent 
                 52.4 
                 7.9 
               
               
                 Ex. 17 
               
               
                   
               
             
          
         
       
     
         [0063]      FIG. 1  is a graph made by plotting the coordinate data from every steel species with ferrite decarburization depth as vertical axis and X1 in Expression (1) as horizontal axis. X1 includes a polynomial formed by performing addition or subtraction of component terms each of which is obtained by multiplying each of the contents of the specified chemical components (Si, Mn, Cu, Ni and Cr) by the individually specified coefficient and, as clearly seen from  FIG. 1 , makes an almost linear correspondence relation with the ferrite decarburization depth. 
         [0064]    On the other hand, separately from the foregoing, each steel species was melt-formed, and subjected to slabbing and further to wire-rod rolling (13.5 mmφ) using a real machine at a rolling temperature of 900° C. The cooling rate in this case was set to 0.5° C./sec. And an assessment of an actual result of ferrite decarburization in each wire-rod rolled material, namely a decision as to whether ferrite decarburization occurred (ferrite decarburization was present) or not (ferrite decarburization was absent), was made. The assessment results are shown in  FIG. 1  in the form of coordinate data on every steel species with absence of ferrite decarburization as a white circle and presence of ferrite decarburization as a black circle. Additionally, in Table 1, absence of ferrite decarburization is described as “absent”, while presence of ferrite decarburization is described as “present”. 
         [0065]    As can be seen from  FIG. 1 , it is appropriate to organize the ferrite decarburization depths into the X1 in Expression (1). And considering the actual results of ferrite decarburization under the practical wire-rod rolling, it was determined that the threshold value of X1 for decision as to whether or not ferrite decarburization occurred is 0.2. In other words, by adjusting X1 to 0.2 or less, it becomes possible to obtain structure free of ferrite decarburization.
   (15) Satisfying the following Expression (2)   
 
         [0000]        X 2=(α−500)/β3 3.0   Expression (2)
 
         [0000]      α=912−231×[C]+32×[Si]−20×[Mn]−40×[Cu]−18×[Ni]−15×[Cr]
 
         [0000]      β=10̂(0.322−0.538×[C]+0.018×[Si]+1.294×[Mn]+0.693×[Cu]+0.609×[Ni]+0.847×[Cr])
 
         [0067]    In order to examine the adequacy of Expression (2), every steel species was, similarly to the above, subjected to slabbing and further to wire-rod rolling (13.5 mmφ) using a real machine at a rolling temperature of 900° C. In this case, cooling was carried out at two different rates of 1.5° C./sec and 0.5° C./sec. And an assessment of an actual result of bainite formation in each of the wire-rod rolled materials, namely a decision as to whether bainite was formed (presence of bainite formation) or not (absence of bainite formation), was made. Additionally, in Table 1 and  FIG. 2 , the unit of a cooling rate is expressed in ° C./s. 
         [0068]    The results obtained are shown in Table 1 and  FIG. 2 .  FIG. 2  is a graph made by plotting the coordinate data from every steel species with cooling rate as vertical axis and X2 in Expression (2) as horizontal axis. Although X2 includes α and β as variables, the concept of the equality itself is known (for example, see Materia, vol. 36, No. 6, 1997, pp. 603-608). α includes a polynomial formed by performing addition or subtraction of component terms each of which is obtained by multiplying each of the contents of the specified chemical components (C, Si, Mn, Cu, Ni and Cr) by the individually specified coefficient, and β is 10 to the power of such a polynomial. As shown in  FIG. 2 , considering the actual results of bainite formation under the practical wire-rod rolling, it was determined that the threshold value of X2 for decision as to whether or not bainite formation occurred is 3.0. In other words, by adjusting X2 to 3.0 or more, it becomes possible to obtain bainite formation-free structures so long as the cooling is carried out at usual execution rates.
   (16) Satisfying the following Expression (3)   
 
         [0000]        X 3=31×[C]+2.3×[Si]+2.3×[Mn]+1.25×[Cu]+2.68×[Ni]+3.57×[Cr]−6×[Ti]≧24.0   Expression (3)
 
         [0070]    In order to examine the adequacy of Expression (3), steel samples prepared by melt-forming individual steel species, hot-forging them into their respective bars having 22 mmφ and then machining them into their respective bars having dimensions of 20 mmφ×10 mm were kept at 950° C. for 60 minutes, subjected to oil quenching, then kept at 400° C. for 30 minutes, and further tempered under air cooling. Hardness (HRC) measurements were conducted on the thus-treated steel samples. 
         [0071]    The measurement results obtained are shown in Table 1 and  FIG. 3 .  FIG. 3  is a graph made by plotting the coordinate data from every steel species with hardness as vertical axis and X3 in Expression (3) as horizontal axis. X3 includes a polynomial formed by performing addition or subtraction of component terms each of which is obtained by multiplying each of the contents of the specified chemical components (C, Si, Mn, Cu, Ni, Cr and Ti) by the individually specified coefficient. 
         [0072]    As can be seen from  FIG. 3 , it is appropriate to organize the hardness into the X3 in Expression (3). And in order for high-strength spring steel according to the invention to secure hardness (HRC) of at least 53.0 in the case of setting the tempering temperature at 400° C., it was determined that the threshold value of X3 is 24.0. In other words, by adjusting X3 to 24.0 or more, it becomes possible to obtain a high-strength structure with hardness (HRC) of 53.0 or more in the case of setting the tempering temperature to 400° C.
   (17) B: 0.0005% to 0.0050%   
 
         [0074]    B is an element effective in improving a tenacity of spring steel by preventing P and S from segregating to crystal grain boundaries. Therefore, the B content is preferably 0.0005% or more. On the other hand, excessive addition of B causes formation of nitride of B, thereby resulting in degradation of the tenacity. Therefore, the B content is preferably 0.0050% or less.
   (Others) Reasons for setting the lower limit of Ti content to 0.060%   
 
         [0076]    On the samples having undergone hot forging, subsequent quenching at 950° C. and further tempering at 400° C., crystal grain size (austenitic crystal grain size) measurements were made in accordance with the austenitic crystal grain size testing method (JIS G 0551:2005). The measurement results (crystal grain size numbers) obtained are shown in Table 1 and  FIG. 4 .  FIG. 4  is a graph made by plotting the coordinate data from every steel species with crystal grain size number as vertical axis and Ti content as horizontal axis. 
         [0077]    The austenitic crystal grain size influences various characteristics (a fatigue characteristic, a delayed fracture characteristic, a settling property), and it is generally possible to improve these characteristics through the fining of crystal grains. In the high-strength steel of the present invention, the lower limit of Ti content is set to 0.060 based on  FIG. 4  so that the crystal grain size after quenching-and-tempering becomes No. 9 or more. In other words, by adjusting the Ti content to 0.060% or more, it becomes possible to obtain fine structure which is No. 9 or more in crystal grain size number. 
         [0078]    Calculation results, measurement results and assessment results of Expressions (1) to (3) corresponding to each steel species (in each of Examples 1 to 12 and Comparative Examples 1 to 17) are shown in Table 1. As shown in Examples 1 to 12, high-strength spring steels having excellent wire-rod rolling properties, and more specifically, steels causing neither ferrite decarburization nor bainite formation during the wire-rod rolling and having 400° C.-temper hardness of 53.0 or more and a crystal grain size number of 9 or more, can be obtained by adjusting each of chemical components to fall within the individually specified content range and satisfying Expressions (1) to (3). 
         [0079]    On the other hand, in each of Comparative Examples 1, 6, 10, 11, 14, 15 and 17, Expression (3) was not satisfied; as a result, the 400° C.-temper hardness was below 53.0 HRC. Additionally, in each of Comparative Examples 4 to 11, Expression (1) was not satisfied; as a result, ferrite decarburization occurred during the rod-wire rolling. 
         [0080]    Further, in each of Comparative Examples 2, 3 and 15 to 17, the Ti content was below 0.060 mass %; as a result, the crystal grain size number thereof became below No. 9. Furthermore, in each of Comparative Examples 10 and 12 to 14, Expression (2) was not satisfied; as a result, bainite formation occurred during the wire-rod rolling. 
         [0081]    As can be clearly seen from the above descriptions, according to the present invention, it is possible to obtain a high-strength spring steel having excellent wire-rod rolling properties. Incidentally, the present invention should not be construed as being limited to the foregoing Examples, but can be carried out in modes undergone various changes and modification so long as they do not depart from the gist of the invention. 
         [0082]    The present application is based on Japanese Patent Application No. 2014-206311 filed on Oct. 7, 2014, and the contents are incorporated herein by reference.