Abstract:
A high-strength oil-tempered steel wire with excellent spring fabrication property that is made of spring low-alloy steel, having a decarburized layer of reduced hardness extending to a depth of not greater than 200 μm from the wire surface, a wire surface hardness in the range from an Hv (Vickers hardness) of 420 to an Hv of 50 below the Hv of the wire interior, and an Hv at the interior of the wire beyond the depth of the decarburized layer of not less than 550. The spring low-alloy steel can preferably comprise, in weight percent, 0.45-0.80% C, 1.2-2.5% Si, 0.5-1.5% Mn, 0.5-2.0% Cr and the balance of Fe and unavoidable impurities. The method for producing the foregoing steel wire comprises the steps of continuously passing and heating a starting material low-alloy steel wire fed through a furnace body through-pipe of a continuous heating furnace for oil tempering, decarburizing the low-alloy steel wire under regulation of a dew point of a decarburizing atmosphere in the pipe by introducing into the pipe from its inlet side or a desired intermediate point thereof hydrogen gas or a mixed gas of hydrogen gas and an inert gas and, to form steam by reaction therewith, oxygen gas or an oxygen-containing gas and controlling the amount of oxygen gas or oxygen-containing gas introduced, and thereafter quenching and annealing the low-alloy steel wire.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to high-strength oil-tempered steel wire with excellent spring fabrication property, high fatigue strength and low setting property that is suitable for use in vehicle internal combustion engines, suspensions systems and the like and to a method for producing the same. 
     2. Description of the Prior Art 
     Springs used in the internal combustion engines, suspension systems etc. of vehicles and the like are being reduced in size in response to the trend toward higher horse-power. This has lead to the development of more sophisticated spring materials in recent years as well as to the development of spring materials added with Mo and/or V to improve temper softening resistance. 
     While these spring materials improve spring fatigue strength, they make spring fabrication difficult. Even when minute surface defects that do not become fatigue starting points during use are present, they may cause breakage during spring fabrication, which makes it difficult to produce springs of uniform quality. 
     SUMMARY OF THE INVENTION 
     This invention was accomplished in light of the foregoing technical problems and has an object to provide high-strength oil-tempered steel wire with excellent spring fabrication property that improves spring fabricability to enable production of springs of uniform quality and exhibits high fatigue strength and high setting resistance. Another object of the invention is to provide a method for producing the high-strength oil-tempered steel wire with excellent spying fabrication property. 
     The inventors made studies toward overcoming the problems of the prior art. Through their research they discovered that the spring fabrication property of high-strength oil-tempered steel wire can be improved by regulating the dew point in a continuous heating furnace for producing the oil-tempered steel wire, subjecting low-alloy steel wire as a starting material to a combined oil-tempering and decarburizing treatment to obtain oil-tempered steel wire whose surface layer is reduced in hardness by formation of a decarburized layer to a prescribed depth, whose surface hardness is restricted to a prescribed range and whose hardness at an interior portion beyond the depth of the decarburized layer is restricted to a prescribed range. When patenting is conducted in a step prior to the oil-tempering, the decarburization can be conducted in the patenting step. 
     Through further research based on this discovery, it was found that, more specifically, high-strength oil-tempered steel wire with outstanding spring fabrication property is obtained when the oil-tempered steel wire has a decarburized layer of reduced hardness extending to a depth of not greater than 200 μm from the wire surface, a wire surface hardness in the range from an Hv (Vickers hardness) of 420 to an Hv of 50 below the Hv of the wire interior, and an Hv at the interior of the wire beyond the depth of the decarburized layer of not less than 550. 
     In accordance with this invention, high-strength oil-tempered steel wire can be imparted with stable spring fabrication property by providing a decarburized layer of reduced hardness extending to a depth of not greater than 200 μm from the wire surface and reducing the surface hardness to between an Hv of 420 and an Hv of 50 below the Hv of the wire interior, thereby lowering the fatigue notch sensitivity. 
     The surface hardness range is selected in light of the ratio of mean coil diameter to wire diameter (D/d) in spring fabrication. 
     Although reducing the surface hardness of a spring ordinarily lowers the spring&#39;s fatigue strength, the oil-tempered steel wire whose surface layer hardness has been reduced in accordance with this invention recovers or more than recover its surface hardness upon nitriding and/or hard shot peening treatment after spring fabrication. This enables production of high-strength springs with high fatigue strength and excellent setting resistance property. 
     The inventors further made studies regarding control of the heating furnace atmosphere for enabling stable production of the high-strength oil-tempered steel wire with excellent spring fabrication property according to the invention. 
     In closed furnaces, atmosphere control is a common practice, for example when carrying out nitriding and carburizing treatments. However, in the case of the continuous heating furnace (a heating furnace that effects in-line quenching and tempering of continuously fed steel wire) used to produce the oil-tempered steel wire of this invention, complete blocking of atmospheric air inflow through the inlet and outlet of the heating furnace is hard to achieve. Stable control of the atmosphere inside the furnace is therefore difficult. 
     Research was therefore pursued regarding a method for controlling the internal atmosphere of the continuous heating furnace during continuous passing and heating of a starting material low-alloy steel wire fed through the furnace body through-pipe. It was discovered that the dew point of the atmosphere in the pipe can be regulated for decarburizing the low-alloy steel wire by introducing into the pipe from its inlet side or a desired intermediate point thereof hydrogen gas or a mixed gas of hydrogen gas and an inert gas and, to form steam by reaction therewith, oxygen gas or an oxygen-containing gas, and controlling the amount of oxygen gas or oxygen-containing gas introduced. It was further ascertained that a stable decarburizing atmosphere can be secured when an inert gas such as Ar gas or nitrogen gas is introduced into the pipe from a point more toward the upstream side of the furnace than the point of the pipe where the hydrogen gas or the mixed gas of hydrogen gas and inert gas and the oxygen gas or oxygen-containing gas are introduced, so as to continuously push the steam atmosphere generated in the pipe toward the downstream side of the heating furnace. It was additionally learned that the hardness of the low-alloy steel wire surface can be regulated by changing the point of the pipe where the hydrogen gas or the mixed gas of hydrogen gas and inert gas and the oxygen gas or oxygen-containing gas are introduced, so as to change the duration of the exposure of the low-alloy steel wire under treatment to the decarburizing atmosphere. 
     This invention was accomplished based on these various discoveries. Its essential features are set out below. 
     One aspect of the invention provides a high-strength oil-tempered steel wire with excellent spring fabrication properly that is made of spring low-alloy steel, has a decarburized layer of reduced hardness extending to a depth of not greater than 200 μm from the wire surface, has a wire surface hardness in the range from an Hv (Vickers hardness) of 420 to an Hv of 50 below the Hv of the wire interior, and has an Hv at the interior of the wire beyond the depth of the decarburized layer of not less than 550. 
     The spring low-alloy steel can preferably comprise, in weight percent, 0.45-0.80% C, 1.2-2.5% Si, 0.5-1.5% Mn, 0.5-2.0% Cr and the balance of Fe and unavoidable impurities. 
     The spring low-alloy steel can preferably further comprise, in weight percent, one or more of 0.1-0.7% Mo, 0.2-2.0% Ni, 0.05-0.60% V and 0.01-0.20% Nb. 
     Another aspect of the invention provides a method for producing any of the foregoing high-strength oil-tempered steel wires with excellent spring fabrication property comprising the steps of continuously passing and heating a starting material low-alloy steel wire fed through a furnace body through-pipe of a continuous heating furnace for oil tempering, decarburizing the low-alloy steel wire under regulation of a dew point of a decarburizing atmosphere in the pipe by introducing into the pipe from its inlet side or a desired intermediate point thereof hydrogen gas or a mixed gas of hydrogen gas and an inert gas and, to form steam by reaction therewith, oxygen gas or an oxygen-containing gas and controlling the amount of oxygen gas or oxygen-containing gas introduced, and thereafter quenching and annealing the low-alloy steel wire. 
     An inert gas is preferably further introduced into the pipe from a point more toward the upstream side of the furnace than the point of the pipe where the hydrogen gas or the mixed gas of hydrogen gas and inert gas and the oxygen gas or oxygen-containing gas are introduced, thereby stabilizing the decarburizing atmosphere by continuously pushing the steam atmosphere generated in the pipe toward the downstream side of the heating furnace. 
     Another aspect of the invention provides a method for producing any of the foregoing high-strength oil-tempered steel wires with excellent spring fabrication property comprising the steps of continuously passing and heating a starting material low-alloy steel wire fed though a furnace body through-pipe of a continuous heating furnace for oil tempering, decarburizing the low-alloy steel wire under regulation of a dew point of a decarburizing atmosphere in the pipe by introducing into the pipe from its inlet side or a desired intermediate point thereof hydrogen gas or a mixed gas of hydrogen gas and an inert gas and, to form steam by reaction therewith, oxygen gas or an oxygen-containing gas, introducing an inert gas into the pipe from a point more toward the upstream side of the furnace than the point of the pipe where said gases are introduced, and controlling the amount of inert gas introduced, and thereafter quenching and annealing the low-alloy steel wire. 
     The hardness of the low-alloy steel wire surface can be preferably regulated by changing the point of the pipe where the hydrogen gas or the mixed gas of hydrogen gas and inert gas and the oxygen gas or oxygen-containing gas are introduced, so as to change the duration of the exposure of the low-alloy steel wire under treatment to the decarburizing atmosphere. 
     The production methods constituted in the foregoing manner according to the invention enable manufacture of high-strength oil-tempered steel wire that has a uniform decarburized layer and, as such, reduces occurrence of breakage during spring fabrication, even when minute surface defects that do not become a problem during spring operation are present, thus enabling fabrication of springs of uniform quality. 
     Oil-tempered steel wires to which the invention applies are not particularly limited by chemical composition and encompass such oil-tempered steel wires as the chromium-vanadium steel oil-tempered steel wire for valve springs, silicon-chromium steel oil-tempered steel wire for valve springs and silicon-manganese steel oil-tempered steel wire for springs standardized under JIS G 3565, 3566 and 3567. The advantageous effects of the invention are, however, particularly pronounced when the invention is applied to the low-alloy steel wires of the compositions set out above. Application of the invention is, however, in no way limited to the specific low-alloy steel materials mentioned. 
     The above and other features of the present invention will become apparent from the following description made with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a continuous heating furnace for oil tempering used to produce the oil-tempered steel wire of this invention. 
     FIG. 2 is a graph showing the surface hardness distribution of a Comparative Material A. 
     FIG. 3 is a graph showing the surface hardness distribution of a Comparative Material B. 
     FIG. 4 is a graph showing the surface hardness distribution of an Invention Material C. 
     FIG. 5 is a graph showing how the dew point of the decarburizing atmosphere in the oil-tempered steel wire treatment pipe 2 of FIG. 1 varied as a function of the amount of air introduced into the pipe and as a function of the amount of inert gas introduced thereinto. 
     FIG. 6 is a graph showing how oil-tempered steel wire surface hardness varied as a function of the dew point of the decarburizing atmosphere. 
     FIG. 7 is a graph showing how the result of a coiling test (number of breaks per 100 winds) varied as a function of the value of the difference between the internal hardness and the surface hardness of the oil-tempered steel wire. 
     FIG. 8 is a graph showing how fatigue strength varied as a function of the surface hardness of oil-tempered steel wires used to manufacture springs. 
     FIG. 9 is a graph showing how the surface hardness of oil-tempered steel wires used to manufacture springs varied as a function of decarburization depth. 
     FIG. 10 is a graph showing how the fatigue strength of oil-tempered steel wires used to manufacture springs varied as a function of internal hardness. 
     FIG. 11 is a graph showing how oil-tempered low-alloy steel wire surface hardness varied as a function of the point at which an H 2  +N 2  mixed gas and air were introduced into the oil-tempered steel wire treatment pipe 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The low-alloy steels exemplified by this invention have chemical compositions of, in weight percent, 0.45-0.80% C, 1.2-2.5% Si, 0.5-1.5% Mn, 0.5-2.0% Cr and, as required, one or more of 0.1-0.7% Mo, 0.2-2.0% Ni, 0.05-0.60% V and 0.01-0.20% Nb, the balance being Fe and unavoidable impurities. 
     The reasons for the restrictions on the chemical composition of the low-alloy steel are as follows: 
     C: Although carbon is an element that effectively increases the steel strength, it does not provide the desired strength at a content below 0.45% and produces little additional strength enhancement when added to more than 0.80%. The range of C content is therefore specified as 0.45-0.80%. 
     Si: Silicon enters solid solution in ferrite. By this it increases the strength of the steel and delays tempering to their heighten temper softening resistance. However, since it has no effect at a content below 1.2% and provides no additional effect at a content above 2.5%, its content range is specified as 1.2-2.5%. 
     Mn: Although manganese is an element that effectively enhances quenching property, it has little effect at a content below 0.5% and produces no additional effect when added to more than 1.5%. The range of Mn content is therefore specified as 0.5-1.5%. 
     Cr: Although chromium is an element that effectively enhances quenching properly, it has little effect at a content below 0.5% and lowers strength by carbide formation at a content of more than 2.0%. The range of Cr content is therefore specified as 0.5-2.0%. 
     Mo: Molybdenum effectively enhances temper softening resistance and imparts strength and toughness. However, its effect does not appear at a content below 0.1% and saturates at a content above 0.7%. Since it also degrades toughness by carbide formation at a content above 0.7%, the range of Mo content is specified as 0.1-0.7%. 
     Ni: Although nickel is an element that effectively enhances toughness, it has little effect at a content below 0.2% and produces no additional effect when added to more than 2.0%. The range of Ni content is therefore specified as 0.2-2.0%. 
     V: Although vanadium is an element that effectively enhances crystal grain refinement and improves strength by precipitation of vanadium carbide, it has no effect at a content below 0.05% and produces no additional effect when added to more than 0.60%. The range of V content is therefore specified as 0.05-0.60%. 
     Nb: Although, like vanadium, niobium is also an element that effectively enhances crystal grain refinement, it has little effect at a content below 0.01% and degrades toughness by carbide formation when added to more than 0.20%. The range of Nb content is therefore specified as 0.01-0.20%. 
     EXAMPLES 
     The invention will now be explained with reference to specific examples. 
     Table 1 shows the chemical compositions of the test materials (low-alloy steels) used in the examples. 
     
                       TABLE 1______________________________________Test  (wt %)Material C      Si     Mn   Cr   Mo   Ni  V    Nb   Fe______________________________________No. 1 0.66   1.50   0.75 1.02 --   --  --   --   BalanceNo. 2 0.73   2.01   0.75 1.02 0.22 --  0.365                                       0.02 BalanceNo. 3 0.75   2.01   0.75 1.02 0.22 1.0 0.365                                       0.02 Balance______________________________________ 
    
     FIG. 1 is a schematic diagram showing a continuous heating furnace for oil tempering and the locations of gas introduction points. A 5-meter-long electric furnace was used as the continuous heating furnace. 
     In FIG. 1, reference numeral 1 designates the electric furnace, 2 a furnace boy through-pipe (the oil-tempered steel wire treatment pipe) and 3 a low-alloy steel wire under treatment The numerals (1) to (4) indicate gas introduction points. 
     An oil tempering means installed on the outlet side of the electric furnace 1 is omitted from the drawing. 
     Example 1 
     The low-alloy steel wire material shown as Test Material No. 1 in Table 1 was drawn to a wire diameter of 3.4 mm and the drawn wire was oil-tempered using the continuous heating furnace 1 to obtain oil-tempered steel wires as Comparative Material A and Comparative Material B. Table 2 shows the decarburizing atmosphere conditions and the oil-tempered steel wire property values for these comparative materials. 
     
                                           TABLE 2__________________________________________________________________________Decarburizing atmosphere conditions and oil-tempered steel wireproperties                Comparative                      Comparative                              Invention                Material A                      Material B                              Material C__________________________________________________________________________Test material        No. 1 No. 1   No. 2Inert gas            None  None    ArInert gas feed rate (l/min)                --    --      6Inert gas introduction point                --    --      (1)Decarburizing gas    Air   H.sub.2 + N.sub.2 + Air                              H.sub.2 + N.sub.2 + AirH.sub.2 + N.sub.2 feed rate (l/min)                --    2       2Air feed rate (l/min)                4     0.25    0.25Decarburizing gas introduction point                (1)   (1)     (2)Dew point (° C.)                +10   +8      +10Oil-tempered steel wire surface hardness (Hv)                590   Max 618 Min 540                              540Oil-tempered steel wire internal hardness (Hv)                625   625     625Difference between internal hardness and surface                35    Max 7 Min 85                              85hardness of oil-tempered steel wire (Hv)Tensile strength (kgf/mm.sup.2)                230   231     231Reduction of area (%)                45            43Amount of residual austenite (%)                7     7       7__________________________________________________________________________ 
    
     FIG. 2 shows the surface hardness distribution of Comparative Material A. Comparative Material A is an oil-tempered steel wire obtained with only air introduced into the oil-tempered steel wire treatment pipe 2. Although decarburization occurred owing to the oxygen content of the introduced air, it was of low level and the decarburizing effect by this oxygen alone was insufficient. In addition, the oxygen produced a scale reaction and the surface scale peeled locally. 
     FIG. 3 shows the surface hardness distribution of Comparative Material B. Comparative Material B is an oil-tempered steel wire obtained by effecting oil tempering treatment with an H 2  +N 2  mixed gas and air introduced into the oil-tempered steel wire treatment pipe 2 from point (1). Although a decarburized layer was formed owing to a rise in the dew point of the heating atmosphere, the hardness in the lengthwise direction of the oil-tempered steel wire was not constant because the high-dew-point atmosphere stagnated in the pipe. 
     The low-alloy steel wire material shown as Test Material No. 2 in Table 1 was drawn to awire diameter of 3.4 mm and the drawn wire was oil-tempered using the continuous heating furnace 1 to obtain the oil-tempered steel wire shown as Invention Material C in Table 2. 
     FIG. 4 shows the surface hardness distribution of Invention Material C. Invention Material C is an oil-tempered steel wire obtained by effecting oil tempering treatment with an H 2  +N 2  mixed gas and air introduced into the oil-tempered steel wire treatment pipe 2 from point (2) and an inert gas (Ar gas) introduced from point (1) thereof. 
     In this oil tempering treatment, the introduction of the inert gas prevented stagnation of the furnace atmosphere by discharging it from the downstream side of the furnace. Since the degree of decarburization was therefore constant in the lengthwise direction of the oil-tempered steel wire, a uniform decarburized layer was formed. Moreover, compared with the case of introducing only oxygen, the decarburizing reaction proceeded more rapidly and no peeling of wire surface scale occurred. 
     These results show that when, in accordance with the invention, an H 2  +N 2  mixed gas and air are introduced and an inert gas is further introduced from a point more toward the upstream side of the furnace than the point where the H 2  +N 2  mixed gas is introduced, the furnace atmosphere is discharged from the downstream side of the continuous heating furnace, thereby preventing stagnation of the high-dew-point atmosphere in the furnace, enabling stable atmosphere control, and enabling the decarburization reaction to be effected uniformly and efficiently in the lengthwise direction of the oil-tempered steel wire. 
     Example 2 
     The low-alloy steel wire material shown as Test Material No. 2 in Table 1 was drawn to awire diameter of 3.4 mm and the drawn wire was oil-tempered under different decarburizing atmosphere conditions using the continuous heating furnace 1 to obtain oil-tempered steel wires as Invention Materials D, E, F, G and H. 
     Invention Materials D, E, F, G and H are oil-tempered steel wires obtained by effecting oil tempering treatment with an inert gas introduced into the oil-tempered steel wire treatment pipe 2 from point (1) and an H 2  +N 2  mixed gas and air introduced from point (2) thereof. 
     Table 3 shows the decarburizing atmosphere conditions and the property values for Invention Materials D, E, F, G and H. 
     
                                           TABLE 3__________________________________________________________________________Decarburizing atmosphere conditions and oil-tempered steel wireproperties                  Invention                          Invention                                  Invention                                          Invention                                                  Invention                  Material D                          Material E                                  Material F                                          Material                                                  Material__________________________________________________________________________                                                  HTest material          No. 2   No. 2   No. 2   No. 2   No. 2Inert gas              Ar      Ar      Ar      Ar      ArInert gas feed rate (l/min)                  6       6       6       8       4Inert gas introduction point                  (1)     (1)     (1)     (1)     (1)Decarburizing gas      H.sub.2 + N.sub.2 + Air                          H.sub.2 + N.sub.2 + Air                                  H.sub.2 + N.sub.2                                          H.sub.2 + N.sub.2 +                                                  H.sub.2 + N.sub.2                                                  + AirH.sub.2 + N.sub.2 feed rate (l/min)                  2       2       2       2       2Air feed rate (l/min)  0.58    0.32    0.25    0.32    0.32Decarburizing gas introduction point                  (2)     (2)     (2)     (1)     (1)Dew point (° C.)                  +20     +12     +10     +10     +14Oil-tempered steel wire surface hardness (Hv)                  450     500     540     540     470Oil-tempered steel wire internal hardness (Hv)                  625     625     625     625     625Difference between internal hardness and surface                  175     125     85      85      155hardness of oil-tempered steel wire (Hv)Tensile strength (kgf/mm.sup.2)                  233     233     230     230     230Reduction of area (%)  43      40      45      45      43Amount of residual austenite (%)                  10      10      10      10      10__________________________________________________________________________ 
    
     FIG. 5 shows how the dew point varied as a function of the amount of introduced air and as a function of the amount of introduced inert gas. FIG. 6 shows how oil-tempered steel wire surface hardness varied as a function of the dew point. 
     The H 2  reacts with oxygen in the air to generate steam and raise the dew point. The rise in the dew point lowers the hardness of the oil-tempered steel wire surface and can be controlled by varying the amount of air introduced. It can also be controlled by varying the amount of inert gas introduced from the upstream side of the furnace. 
     In other words, the invention enables control of decarburization (surface hardness) by varying the amount of air introduced into the oil-tempered steel wire treatment pipe 2 and the amount of inert gas introduced from the upstream side of the furnace so as to control the dew point of the decarburizing atmosphere. 
     Example 3 
     The low-alloy steel wire material shown as Test Material No. 2 in Table 1 was drawn to a wire diameter of 3.4 mm and the drawn wire was oil-tempered under different decarburizing atmosphere conditions using the continuous heating furnace 1 to obtain oil-tempered steel wires as Invention Materials L and M and Comparative Materials I, J and K. 
     Table 4 shows the decarburizing atmosphere conditions and the property values for the materials. 
     
                                           TABLE 4__________________________________________________________________________Decarburizing atmosphere conditions and oil-tempered steel wireproperties                  Comparative                          Comparative                                  Comparative                                          Invention                                                  Invention                  Material I                          Material J                                  Material K                                          Material                                                  Material__________________________________________________________________________                                                  MTest material          No. 2   No. 2   No. 2   No. 2   No. 2Inert gas              Ar      Ar      Ar      Ar      ArInert gas feed rate (l/min)                  6       6       6       6       6Inert gas introduction point                  (1)     (1)     (1)     (1)     (1)Decarburizing gas      H.sub.2 + N.sub.2 + Air                          H.sub.2 + N.sub.2 + Air                                  H.sub.2 + N.sub.2                                          H.sub.2 + N.sub.2 +                                                  H.sub.2 + N.sub.2                                                  + AirH.sub.2 + N.sub.2 feed rate (l/min)                  2       2       2       2       2Air feed rate (l/min)  0.05    0.10    0.75    0.21    0.58Decarburizing gas introduction point                  (2)     (2)     (3)     (4)     (4)Dew point (° C.)                  -20     -10     +25     +7      +20Oil-tempered steel wire surface hardness (Hv)                  620     600     380     575     450Oil-tempered steel wire internal hardness (Hv)                  625     625     625     625     630Difference between internal hardness and surface                  5       25      245     50      180hardness of oil-tempered steel wire (Hv)Tensile strength (kgf/mm.sup.2)                  233     233     232     230     233Reduction of area (%)  41      43      45      44      40Amount of residual austenite (%)                  10      10      10      10      10__________________________________________________________________________ 
    
     The spring fabrication properties of Invention Materials L and M and Comparative Materials I, J and K were evaluated by a coiling test. In spring fabrication of ordinary valve springs, the ratio of mean coil diameter to wire diameter (D/d) is around 5. In this Example, fabrication was conducted under the more severe conditions of D/d=4 and D/d=2 (self-diameter coiling). 
     FIG. 7 shows how the results of the coiling test varied with the oil-tempered steel wire surface hardness. The results are expressed in terms of number of breaks per 100 winds. When D/d was 2, almost no breaks occurred when the difference between the surface hardness and the internal hardness at a depth of greater than 200 μm from the wire surface (i.e., internal hardness minus surface hardness) was 50 or greater (Hv). When D/d was 4, almost no breaks occurred when the difference between the surface hardness and the internal hardness at a depth of greater than 200 μm from the wire surface (i.e., internal hardness minus surface hardness) was 25 or greater (Hv). 
     On the other hand, materials with reduced surface hardness exhibit low fatigue strength. Springs manufactured with Comparative Materials J and K and Invention Materials L and M were therefore examined for fatigue strength. After fabrication, the springs were subjected to nitriding and/or hard shot peening treatment 
     FIG. 8 shows how fatigue strength varied as a function of the surface hardness of the oil-tempered steel wires used to manufacture the springs. Fatigue strength degradation arose when the surface hardness (Hv) was below 420. 
     FIG. 9 shows how the surface hardness of the oil-tempered steel wires varied as a function of decarburization depth. The decarburization depth increased with decreasing hardness of the wire surface and the decarburization depth was 200 μm when the surface hardness (Hv) was 420. Based on these results, this invention, in consideration of spring fabrication property and fatigue strength, decarburized the wire surface to a depth of not greater than 200 μm from the oil-tempered steel wire surface and in this case defines the wire surface hardness as falling between an Hv of 420 and an Hv that is 50 below the Hv of the wire interior. 
     Example 4 
     The low-alloy steel wire material shown as Test Material No. 3 in Table 1 was drawn to a wire diameter of 3.4 mm and the drawn wire was oil-tempered under different decarburizing atmosphere conditions using the continuous heating furnace 1 to obtain oil-tempered steel wires as Invention Materials N and O and Comparative Material P. 
     Table 5 shows the decarburizing atmosphere conditions and the property values for the materials. 
     
                                           TABLE 5__________________________________________________________________________Decarburizing atmosphere conditions and oil-tempered steel wireproperties                Invention                       Invention                              Comparative                Material N                       Material O                              Material P__________________________________________________________________________Test material        No. 3  No. 3  No. 3Inert gas            Ar     Ar     ArInert gas feed rate (l/min)                6      6      6Inert gas introduction point                (1)    (1)    (1)Decarburizing gas    H.sub.2 + N.sub.2 + Air                       H.sub.2 + N.sub.2 + Air                              H.sub.2 + N.sub.2 + AirH.sub.2 + N.sub.2 feed rate (l/min)                2      2      2Air feed rate (l/min)                0.25   0.25   0.25Decarburizing gas introduction point                (4)    (4)    (2)Dew point (° C.)                +21    +15    +13Oil-tempered steel wire surface hardness (Hv)                455    460    450Oil-tempered steel wire internal hardness (Hv)                630    550    500Difference between internal hardness and surface                175    90     50hardness of oil-tempered steel wire (Hv)Tensile strength (kgf/mm.sup.2)                232    190    171Reduction of area (%)                40     46     50Amount of residual austenite (%)                10     7      3__________________________________________________________________________ 
    
     Invention Materials N and O and Comparative Material P are oil-tempered steel wires whose internal hardnesses were changed by changing the tempering temperature. 
     FIG. 10 shows how fatigue strength varied with internal hardness. Fatigue strength degradation arose when the internal hardness (Hv) was below 550. 
     In light of this, the invention defines the hardness (Hv) at the interior of the wire beyond the depth of the decarburized layer as not less than 550. 
     Example 5 
     The low-alloy steel wire material shown as Test Material No. 2 in Table 1 was drawn to awire diameter of 3.4 mm and the drawn wire was oil-tempered under different decarburizing atmosphere conditions using the continuous heating furnace 1 to obtain oil-tempered steel wires as Invention Materials Q, R and S. 
     Table 6 shows the decarburizing atmosphere conditions and the property values for the materials. 
     
                                           TABLE 6__________________________________________________________________________Decarburizing atmosphere conditions and oil-tempered steel wireproperties                Invention                       Invention                              Invention                Material Q                       Material R                              Material S__________________________________________________________________________Test material        No. 2  No. 2  No. 2Inert gas            Ar     Ar     ArInert gas feed rate (l/min)                6      6      6Inert gas introduction point                (1)    (1)    (1)Decarburizing gas    H.sub.2 + N.sub.2 + Air                       H.sub.2 + N.sub.2 + Air                              H.sub.2 + N.sub.2 + AirH.sub.2 + N.sub.2 feed rate (l/min)                2      2      2Air feed rate (l/min)                0.70   0.70   0.70Decarburizing gas introduction point                (2)    (3)    (4)Dew point (° C.)                +20    +20    +20Oil-tempered steel wire surface hardness (Hv)                450    510    560Oil-tempered steel wire internal hardness (Hv)                625    625    625Difference between internal hardness and surface                175    115    65hardness of oil-tempered steel wire (Hv)Tensile strength (kgf/mm.sup.2)                232    233    233Reduction of area (%)                43     44     42Amount of residual austenite (%)                10     10     10__________________________________________________________________________ 
    
     The surface hardnesses of Invention Materials Q, R and S were examined. The results are shown in FIG. 11. 
     In this Example, the introduction point at which the H 2  +N 2  gas and air for generating steam was introduce was changed among (2), (3) and (4). 
     From the results of this &amp;ample, it was ascertained that the surface hardness of the oil-tempered steel wire can be controlled by valuing the point at which the H 2  +N 2  mixed gas and air are introduced. 
     Table 7 shows specifications and nitriding conditions of the springs used in the fatigue tests whose results are shown in FIGS. 8 and 10. 
     
                       TABLE 7______________________________________Specification of Test SpringsWire diameter         3.4 mmCoil mean diameter    19.4 mmEffective no. of winds                 4.76 mmTotal no. of winds    6.76 mmFree height           44.6 mmSpring constant       97 kgf/mmNitriding ConditionsNitriding temperature 500° C.Nitriding period      120 min______________________________________ 
    
     The high-strength oil-tempered steel wire of this invention exhibits excellent spring fabrication property enabling stable spring fabrication with no breakage during fabrication, even when minute surface defects that do not become fatigue starting points during use are present. 
     Further, springs manufactured using the invention oil-tempered steel wire can be imparted with high fatigue strength by nitriding and/or hard shot peening treatment. 
     Moreover, the production method of the invention enables manufacture of oil-tempered steel wire with outstanding fabrication property and uniform excellent quality.