JP2007321176A - Titanium alloy rod wire for coil spring and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a titanium alloy rod wire for a coil spring which can simultaneously and more stably enhance three characteristics of a diameter deviation of the rod wire to be used as a base material, formability for the coil spring, and the uniformity of a distribution of cross-sectional hardness after having been subjected to aging heat treatment, and to provide a production method therefor. <P>SOLUTION: The diameter deviation of the rod wire can be improved and the deformation resistance of the rod wire can be reduced by applying a drawing rate controlled to as low as 3 to 20% to the titanium alloy rod wire of a base material for the coil spring, when cold-drawing the titanium alloy rod wire. At the same time, by setting the β transformation temperature of the β titanium alloy to 780°C or higher, the precipitation of an α phase is accelerated during aging heat treatment, and the variation of hardness distribution in a cross section can be reduced even when the rod wire is drawn at the low drawing rate and heat-treated for a short period of time. Furthermore, by controlling a metallographic structure of the rod wire before being cold-drawn to a fine unrecrystallized structure or a recrystallized structure having a grain size of 10 μm or less, the variation of the hardness distribution in the cross section after having been heat-treated can be extremely decreased. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a titanium alloy rod used for manufacturing a coil spring typified by suspension springs and engine valve springs of two-wheeled and four-wheeled vehicles, and a method for manufacturing the same.

For a coil spring that achieves a predetermined load P and a spring constant R, the mass W of the coil spring is expressed by equation (1) based on the physical properties of the material. From the formula (1), when the design conditions of the load P and the spring constant R are constant, W is determined by the shear strength τ, the lateral elastic modulus G, and the density ρ of the material, τ is large, and G and ρ are small. It becomes lighter.
W = (2G · ρ · P ² ) / (τ ² · R) (1) where W: mass of the coil spring, G: transverse elastic modulus, ρ: density of the material, P: load, τ: material shear strength, R: spring constant.

  Titanium alloys with high specific strength (strength with respect to density) and low elastic modulus compared to steel are known as materials suitable for reducing the weight of coil springs. Among them, the ability to precipitate and strengthen the α phase by aging heat treatment is known. Since a high β-type titanium alloy has a strength of 1400 MPa or more, it has many applications as a coil spring for two-wheeled and four-wheeled vehicles. In general, a β-type titanium alloy is aged from 7,8 to 20 hours at a low temperature of 400 to 600 ° C., which is an α + β two-phase region, from a state where the β phase remains after cooling from a high temperature region around the β transformation point. By performing the heat treatment, a fine α phase is precipitated in the β phase. So-called precipitation strengthening is a material that can be used very effectively.

On the other hand, the spring constant R that determines the characteristics of the coil spring is expressed by equation (2).
R = (G · d ⁴ ) / (8n · D ³ ) (2) where R: spring constant, G: transverse elastic modulus of material, d: diameter of material rod, n: coil spring , D: the outer diameter of the coil spring.

  From the equation (2), it can be seen that the spring constant R is greatly affected by the diameter d of the bar wire as the material and the outer diameter D of the coil spring. From this, the more stable the spring constant is obtained the smaller the diameter deviation of the material rod that affects d, in other words, the higher the roundness and the more stable D after molding. In order to obtain a bar wire having a smaller diameter deviation, a method of shaving or cold-drawing the bar wire after being subjected to solution heat treatment after hot working or after hot working is effective.

  Shaving is a method of cutting a predetermined amount by passing a bar wire through a circular hole die having a blade edge, and naturally, chips are lost in yield. In addition, since the β-type titanium alloy has high strength and low Young's modulus, the reaction force against the cutting edge is large, and titanium is seized and the edge tends to be lost. When continuously shaving a coil-shaped material or the like, it is difficult to obtain a stable diameter deviation when the cutting edge is damaged. Therefore, cold drawing is more often used than means for shaving as a means for improving the diameter deviation of the bar wire.

  It is known that coil springs using β-type titanium alloy rods are generally manufactured in the order of cold drawing of rods, coiling (molded into a coil spring shape), aging heat treatment, and shot peening. (See Patent Document 1). In this case, since the cold drawing is given, the roundness of the material rod is also relatively high. Here, the cross-sectional reduction rate of cold drawing before aging heat treatment is set to 70% or more in Ti-3Al-8V-6Cr-4Mo-4Zr (hereinafter BetaC) in Patent Document 1, and α + β in Patent Document 2 An example of 30% to 95% after hot rolling or solution treatment in the two-phase region and 30% for Ti-V-Mo-Zr-Al-Fe-Cr system is shown in Patent Document 3. In Non-patent Document 1, BetaC and Ti-13V-11Cr-3Al have an example of 33 to 50%. In Nonpatent Document 2, BetaC and Ti-13V-11Cr-3Al have an example of 40, 80, and 90%. However, Non-Patent Document 3 shows an example in which Ti-13V-11Cr-3Al is 80%. Thus, in order to obtain high strength, the cross-sectional reduction rate of cold drawing is 30 to 95%, which is at least 30%. Hereinafter, the cross-sectional reduction rate of cold drawing is referred to as “drawing rate”.

JP 05-195175 A Japanese Patent Laid-Open No. 04-074856 JP 2005-154850 A R.R.Boyer et al., "Beta Titanium Alloys in The 1980's", 1983, issued by AIME, pages 295-305. M.Murakami et al., “SAE Technical Paper Series, No. 890470”, 1989 Takeshi Shigeshita et al., “Spring Papers”, No. 40, 1996, 1-5

  When manufacturing β-type titanium alloy rods for coil springs by cold drawing, if the wire drawing rate increases, the deformation resistance of the rod wire increases in addition to the low Young's modulus, so that lubrication cannot be maintained and the wire drawing die is made of titanium. May result in defects on the bar surface. Furthermore, since the suspension springs of two-wheel and four-wheel automobiles are thicker as the diameter of the rod exceeds 10 to 20 mm, the load on the wire drawing dies is large and the seizure is more likely to occur. Further, when coiling from a bar wire, if the wire drawing rate is increased, the low Young's modulus and the high deformation resistance are similarly affected as described above, and there is a problem that the moldability is easily baked on the coiling jig.

  That is, in cold wire drawing with a high wire drawing rate, a β-type titanium alloy bar wire sticks to a wire drawing die, and it is difficult to obtain a stable diameter deviation when the die is damaged, Since the β-type titanium alloy bar wire provided with the wire drawing rate has a high deformation resistance and a low formability, there is a problem that the coiling forming jig is easily seized.

  On the other hand, by reducing the wire drawing rate, an increase in deformation resistance of the β-type titanium alloy bar wire can be suppressed, and seizure to the wire drawing die or coiling jig can be suppressed. As a result, a good diameter deviation can be obtained stably. However, when the wire drawing rate is small, the strain applied in the cross section of the bar wire has a large difference between the central part and the surface layer part of the bar wire cross section, and is large in the surface layer. If this time is not long enough to exceed 10, 20 hours, there may be a large variation in the hardness distribution in the bar wire cross section at the stage of the coil spring product. That is, there is a possibility that the product is in a state where there is a difference in strength between the surface layer and the inside in the cross section. This hardness variation caused by short-term aging heat treatment is caused by strain applied by cold drawing, that is, lattice defects become α-phase precipitation sites during aging heat treatment. This is because the α phase precipitates early and is sufficiently hardened, but the α phase is deposited slowly and the hardening amount remains small in a place where the strain amount is small.

  Therefore, the present invention simultaneously and stably increases three of the following: 1) diameter deviation of the bar wire used as the material, 2) formability to the coil spring, and 3) uniformity of the cross-sectional hardness distribution after the aging heat treatment. It aims at providing the manufacturing method of the titanium alloy bar wire for coil springs. Moreover, it aims at providing the titanium alloy bar wire suitable for coil spring manufacture.

In order to solve the above problems, the gist of the present invention is as follows.
(1) β-type titanium alloy having a β transformation point of 780 ° C. or higher, 0.2% proof stress is less than 1300 MPa, 1 mm depth from the surface, the difference in cross-section Vickers hardness is 20 or more, and the rod diameter deviation is A titanium alloy bar wire for coil springs, characterized in that it is ± 0.08 mm or less.
(2) The titanium alloy bar wire for coil springs according to (1) above, wherein the β-type titanium alloy contains 2 to 8 mass% of Fe and V is 0.1 mass% or less.
(3) Using a rod wire obtained by hot-working a β-type titanium alloy having a β transformation point of 780 ° C. or higher, or by further solution heat treatment after hot working, a cold drawing of 3 to 20% in terms of the cross-sectional reduction rate The manufacturing method of the titanium alloy bar wire for coil springs characterized by implementing.
(4) Production of titanium alloy rod for coil spring according to (3) above, wherein the metal structure before cold drawing is an unrecrystallized structure or a recrystallized structure having a crystal grain size of 10 μm or less. Method.
(5) The titanium alloy rod for coil spring according to (3) or (4) above, wherein the β-type titanium alloy contains 2 to 8 mass% of Fe and V is 0.1 mass% or less. Wire manufacturing method.
(6) The method for producing a titanium alloy rod for a coil spring according to any one of (3) to (5) above, wherein a hole die is used in cold wire drawing.

  In the present invention, when the maximum diameter and the minimum diameter of the same cross section of the bar wire are measured and the maximum diameter-target diameter is "+ difference" and the target diameter-minimum diameter is "-difference", Both are called the rod diameter deviation.

  According to the present invention, three of the titanium alloy bar wire for coil springs 1) diameter deviation, 2) formability to coil springs, and 3) uniformity of cross-sectional hardness distribution after aging heat treatment are simultaneously and more stably enhanced. A manufacturing method and a titanium alloy bar wire for a coil spring can be provided. In the present invention, the wire drawing rate can be made relatively small compared to 30% or more of the prior art, so that it is also effective in terms of manufacturing cost.

  The inventors of the present invention have simultaneously stabilized three of the titanium alloy rods for coil springs: 1) diameter deviation, 2) formability into coil springs, and 3) uniformity of cross-sectional hardness distribution after aging heat treatment. As a result of earnest research on how to improve the results, we found the following. The deformation resistance of the bar wire can be kept low by keeping the cold wire drawing rate applied to the titanium alloy rod used as the material of the coil spring as low as 3 to 20%, preferably 3 to 10%. Further, since seizure with the wire drawing die is prevented, a stable diameter deviation can be obtained, and the moldability to the coil spring can be sufficiently secured. In addition, by applying a β-type titanium alloy having a β transformation point of 780 ° C. or higher, preferably 790 ° C. or higher, the precipitation of the α phase during aging heat treatment becomes relatively fast. The influence of the strain distribution given by the lines is reduced, and the variation in the hardness distribution in the cross section can be kept small even with a short aging heat treatment. Furthermore, by making the bar wire before cold drawing into a non-recrystallized structure which is a fine metal structure or a recrystallized structure having a crystal grain size of 10 μm or less, variation in the hardness distribution in the cross section after aging heat treatment is extremely small. can do.

  The basis for setting each element of the present invention will be described below.

  First, the cross-section reduction rate (drawing rate) of cold drawing will be described.

  Since the deviation of the diameter of the bar wire subjected to hot rolling and further deske pickled is about 0.2 to 0.5 mm, the wire drawing ratio is required to be at least 3% for the diameter of the bar wire of about 10 to 20 mm. It becomes. FIG. 5 shows the relationship between the deviation of the actual diameter of the bar wire from the target diameter and the wire drawing rate. It can be seen that the deviation is ± 0.08 mm or less at a drawing rate of 3% or more. Therefore, the lower limit of the wire drawing rate is 3%.

  The upper limit of the wire drawing rate was limited by the rate of increase in 0.2% yield strength, which is an index of deformation resistance after cold drawing. FIG. 1 shows the relationship between the wire drawing rate, the 0.2% proof stress as cold drawing and the rate of increase. As can be seen from FIG. 1, the rate of increase in 0.2% proof stress increases almost in proportion to the wire drawing rate. On the other hand, if the 0.2% proof stress of the bar wire exceeds 1300 MPa, surface lubrication cannot be maintained, and seizure with the wire drawing die tends to occur significantly during cold wire drawing. Further, when the strength exceeds 1300 MPa, the frequency of seizure with the forming jig and the amount of spring back tend to increase during coiling, and these problems in spring formability may become apparent. FIG. 1 shows a case of a relatively high-strength material in which the drawing rate is zero, that is, the 0.2% proof stress in the initial state exceeds 1069 MPa and 1000 MPa. In order to prevent the 0.2% yield strength from exceeding 1300 MPa after cold drawing even for a high-strength material, the upper limit of the wire drawing rate was set to 20%, which is about a 20% increase in 0.2% yield strength. Preferably, 0.2% proof stress is 10% which increases by about 10%. In FIG. 1, when the wire drawing rate is 10%, the 0.2% proof stress is less than 1200 MPa.

  In addition, since the 0.2% proof stress of the bar wire that has already been cold-worked before cold drawing has increased by 0.2% due to work hardening, the present invention has been described in order to keep the 0.2% proof stress low. Then, it is set as the state which carried out the solution heat treatment after the hot processing of the rod wire before cold drawing, or hot processing. In addition, you may add suitably the descaling process which removes the scale of hot processing or heat processing, and the lubrication process for cold drawing before cold drawing.

  Second, the β transformation point of the β-type titanium alloy will be described.

  In (1), (2), and (3) of FIG. 2, BetaC, Ti-15V-3Cr-3Sn-3Al (hereinafter referred to as Ti-15-3), Ti-1.5Al-, which are β-type titanium alloys, respectively. The change (age hardening curve) of the cross-sectional Vickers hardness by the aging heat processing time of 6.8Mo-4.5Fe (henceforth, LCB) is shown. Here, before the aging heat treatment, each of the materials used was a bar wire that had been cold-drawn from a diameter of 14 mm to 13.5 mm at a drawing rate of 7%. The aging heat treatment temperatures were 490, 510, and 530 ° C. for BetaC, Ti-15-3, and LCB, respectively, so that the hardness reached by the aging heat treatment was about 410 to 430 in terms of Vickers hardness. In order to evaluate the difference in hardness between the surface layer in the cross section and the internal hardness, the Vickers hardness at 1 mm depth from the surface layer and the center at two locations were measured in the T cross section. In FIG. 2, when the surface layer 1 mm depth and the curing behavior at the center are compared, the BetaC of (1) and Ti-15-3 of (2) are age hardened because the surface layer is given more strain than the inside. Is occurring quickly, and on the short time side of 3 to 8 hours, the difference in hardness between the surface layer of 1 mm and the center is relatively large. The difference between the surface layer and the center is almost eliminated by aging heat treatment of BetaC for 48 hours and Ti-15-3 for 14 hours. On the other hand, the LCB of (3) has a very small difference between the age hardening behavior of the surface layer of 1 mm and the center, and the hardness difference is almost eliminated by a relatively short aging heat treatment for 3 hours. In the case of productivity and atmospheric heat treatment, an aging heat treatment is preferable for a short time from the viewpoint of scale generation.

  These β-type titanium alloys are greatly different in β-transformation point and are 730, 760, and 805 ° C. for BetaC, Ti-15-3, and LCB, respectively. In order to precipitate the same α phase in β-type titanium alloys having different β transformation points, it is necessary to lower the aging temperature as the β transformation point is lower. The lower the aging heat treatment temperature, the smaller the diffusion rate and the slower the precipitation of the α phase. As a result, the curing is slow and the aging heat time for reaching the maximum hardness is prolonged. On the contrary, the higher the β transformation point, the higher the aging heat treatment temperature, and the faster age hardening occurs.

  The strain imparted by cold drawing, that is, lattice defects, acts as α-phase precipitation sites. Therefore, the greater the amount of strain, the more α-phase precipitation sites. As a result, the α-phase precipitates faster and hardens. Get faster. When the wire drawing rate is small, strain distribution occurs between the surface layer and the center, and age hardening becomes faster as the surface layer has a larger amount of strain. In BetaC and Ti-15-3, which have a low β transformation point and a slow age hardening, the effect of strain due to cold drawing appears significantly on the short time side, as shown in (1) and (2) of FIG. It is considered that a difference in hardness occurs in the cross section. On the other hand, LCB with a high β transformation point has an early age hardening even if there is no strain, so the effect of the strain is small, and the hardness in the cross section can be reduced even with a short aging heat treatment as shown in (3) of FIG. There seems to be almost no difference.

  As described above, it is effective to raise the β transformation point in order to suppress the variation in hardness in the cross section even in a short time aging heat treatment when the wire drawing rate is small. Therefore, the relationship between the β transformation point of the β-type titanium alloy and the hardness variation ΔHV (ΔHV is the difference between the surface layer of 1 mm and the center Vickers hardness) was arranged using the results of FIG. The result is shown in FIG. FIG. 3 shows two conditions of 3 hours and 8 hours in order to compare the shorter aging heat treatment time. From FIG. 3, the β transformation point is 790 ° C. or more in the case of aging for 3 hours, the β transformation point is 780 ° C. or more in the case of aging for 8 hours, and ΔHV is sufficiently 10 or less. Further, from FIG. 3, even when the aging time is 3 hours when the β transformation point is 780 ° C., ΔHV is relatively small at about 15. Here, it can be seen that ΔHV of 10 to 15 corresponds to less than 3 to 5% with respect to Vickers hardness of 410 to 430 and is small.

  Furthermore, only the wire drawing rate was changed from 7% to 3%, and the same treatment as above was performed. As a result, as in the case of the drawing rate of 7%, the condition that ΔHV is 10 or less is that the β transformation point is 780 ° C. or more when the aging time is 8 hours, and the β transformation point when the aging time is 3 hours. Was 790 ° C. or higher.

  From the above, in claim 1 of the present invention, the β transformation point of the β-type titanium alloy used is 780 ° C. or higher. Moreover, since ΔHV is reduced even on the shorter aging time side, the β transformation point is preferably 790 ° C. or higher.

  Overall, a titanium alloy bar wire suitable for coil spring production will be described.

  As shown in FIG. 4, the titanium alloy rod for coil spring of the present invention is in a state of cold drawing when the drawing rate is 3 to 20%, and therefore ΔHV is 20 or more. The upper limit of ΔHV is substantially about 70-80. As can be seen from FIG. 4, when the drawing rate is small or too large, ΔHV is rather small and less than 20.

  In FIG. 5, the horizontal axis is the wire drawing rate, and the vertical axis is the deviation of the actual diameter from the target diameter. Regarding the deviation on the vertical axis, the maximum diameter and the minimum diameter of the same cross section of the bar wire were measured, and the maximum diameter−target diameter was “+ difference” and the target diameter−minimum diameter was “−difference”. As shown in FIG. 5, by performing cold drawing with a drawing rate of 3% or more, the deviation of the actual diameter with respect to the target diameter becomes at least ± 0.08 mm or less for both the + difference and the − difference. A product of ± 0.05 mm or less is obtained. Here, it is the result of the cold wire drawing using the hole die. Based on the formula (2), the spring constant is ± 1.6 to 3.2% with a deviation of ± 0.08 mm and ± 1 to 2% with a deviation of ± 0.05 mm, assuming a rod with a diameter of 10 to 20 mm. It will be in the following range. Of course, a deviation of ± 0.05 mm is preferable. When the wire drawing rate exceeds 20%, the 0.2% proof stress exceeds 1300 MPa, and seizure occurs during wire drawing. In such a case, cold drawing was completed by relubrication in the middle of drawing. However, in the actual production of the bar wire, if re-lubrication is performed by cold drawing, there is a problem that the manufacturing cost correspondingly increases. Further, as described above, when the 0.2% proof stress exceeds 1300 MPa, there arises a problem of seizing with the forming jig during coiling.

  Thus, the bar wire of claim 1 of the present invention which has been cold drawn has a ΔHV of 20 or more and a diameter deviation of ± 0.08 mm or less, preferably ± 0.05 mm or less.

  Therefore, in the invention according to claim 1 of the present invention, as a titanium alloy for a coil spring capable of producing a coil spring having a stable spring constant and a uniform material, a β-type titanium alloy having a β transformation point of 780 ° C. or higher. The 0.2% proof stress was less than 1300 MPa, the difference between the depth of 1 mm from the surface and the central cross-section Vickers hardness (ΔHV) was 20 or more, and the diameter deviation of the bar wire was ± 0.08 mm or less. Preferably, as described above, each of the β-type titanium alloy has a β transformation point of 790 ° C. or higher, a 0.2% proof stress of less than 1200 MPa, and a rod wire diameter deviation of ± 0.05 mm or less. In addition, the minimum of 0.2% yield strength as cold drawing is at least about 900 MPa.

  Third, a preferred component system of the β-type titanium alloy will be described.

  Since it is preferable to use a cheaper alloying element from the viewpoint of cost performance, it is preferable not to add relatively expensive V as a β-stabilizing element but to add relatively inexpensive Fe. Similarly, there is a method in which Mo or Cr, which is a β stabilization element, is added with ferromolybdenum (Fe—Mo) or ferrochromium (Fe—Cr) containing Fe, in which case Fe is also added. The Fe needs to be at least 2 mass% in order to stabilize the β phase. On the other hand, it is preferably at most 8 mass% because it tends to segregate during dissolution and solidification. From the above, according to claim 2 of the present invention, since V is not added, V is inevitably contained in an amount of 0.1 mass% or less, and a β-type titanium alloy containing 2 to 8 mass% of Fe. The addition of Fe includes a case where iron is added alone, or a raw material containing Fe such as ferromolybdenum, ferrochrome, steel scrap, or iron oxide.

  Next, the manufacturing method of the titanium alloy bar wire for coil springs of this invention is demonstrated.

  In the manufacturing method according to claim 3 of the present invention, a β-type titanium alloy having a β transformation point of 780 ° C. or higher is used, and cold drawing is performed at a cross-section reduction rate of 3 to 20%. It is as follows. In addition, as described above, since the 0.2% yield strength of the bar wire that has already been cold-worked before cold drawing has increased by 0.2% due to work hardening, to keep the 0.2% yield strength low. The rod wire before cold drawing is in a hot-worked state or a solution heat treatment state after hot working. In addition, you may add suitably the descaling process which removes the scale of hot processing or heat processing, and the lubrication process for cold drawing before cold drawing.

  By applying the titanium alloy bar wire manufactured by the manufacturing method according to claim 3 of the present invention, a coil spring having a stable spring constant and a uniform material is ensured by ensuring the formability to the coil spring. Can do.

  According to claim 4 of the present invention, in the invention of claim 3, in order to advance the α phase precipitation more uniformly by aging heat treatment, a fine structure is preferable. Alternatively, a recrystallized structure having a crystal grain size of 10 μm or less was used. In the present invention, even in the non-recrystallized structure, it is not in the state of cold working, but as described above, since the solution treatment is performed in the hot working state or after that, a recovery phenomenon occurs, and to some extent Distortion is removed and softened.

  According to claim 5 of the present invention, Fe is contained in an amount of 2 to 8 mass% and V is 0.1 mass% or less. The reason is as described above.

  As a die used for cold wire drawing, so-called wire drawing die, there are generally a roller die and a hole die. In order to further improve the diameter deviation after the cold wire drawing, a hole die that passes through a circular hole is more preferable. Therefore, in the invention according to claim 6 of the present invention, a hole die is used for cold drawing.

  The invention is explained in more detail using the following examples. In Tables 1 to 5, numerical values that deviate from the scope of the present invention are underlined.

  Table 1 shows component compositions and β transformation points of symbols A, B, C, and D as test materials. The β transformation points of symbols A, B, C, and D are 730, 760, 787, and 805 ° C., respectively. The V concentrations of Symbol C and Symbol D to which V is not added are both 0.02 mass%. In addition, all the test materials inevitably contain Fe, O, C, N, H and the like in addition to the elements described. Symbols C and D correspond to the component systems according to claims 3 and 6 of the present invention in which V is 0.1 mass% or less and Fe is added in an amount of 2-8 mass%.

  Bars obtained by hot rolling billets having the respective compositions shown in Table 1 were prepared and used as materials. A hot-rolled rod or a solution-treated rod was descaled and subjected to surface lubrication treatment and subjected to cold drawing. The rod after cold drawing corresponds to the titanium alloy rod for coil spring of the present invention. In addition, all the cold drawing used the hole type | mold die. If seizure occurred during cold drawing, re-lubrication was performed at the time of occurrence. As a result, it was possible to complete cold drawing even at the level where seizure occurred. Further, aging heat treatment was performed after cold drawing. The heat treatment performed after coiling is assumed.

  Tables 2 and 3 show the types of β alloys used, the state before cold drawing, the conditions of cold drawing and aging heat treatment, 0.2% proof stress after cold drawing, and T after aging heat treatment. The Vickers hardness of each part in a cross section and its ΔHV are shown. Table 2 shows examples of β-type titanium alloys of symbols A and B, and Table 3 of symbols C and D. The Vickers hardness was measured with a load of 1 kgf.

  From Table 2, when β-type titanium alloys with symbols A and B having β transformation points of 730 ° C. and 760 ° C., which are lower than those of the present invention, are used, the drawing rate is as small as 7 and 13.8%. No. is as short as 3 hours. In A1, A2, A5, A6, B1, and B2, ΔHV after the aging heat treatment exceeds 19 to 54 and 10. On the other hand, when the drawing rate increases to 43.8%, ΔHV decreases. ΔHV exceeds 10 as in A3 and A7, or about 10 as in B3, and although there is an improvement effect, the wire drawing cost naturally increases. No. A7 has a 0.2% proof stress after cold drawing exceeding 1300 MPa, which increases the possibility of seizing with a die or jig during cold drawing or coil spring molding. Although the wire drawing rate is as small as 7%, if the aging heat treatment time is increased to 48 hours or 14 hours, no. As in A4 and B4, ΔHV is as small as 5 or less. However, the aging heat treatment time exceeds 10 hours, the productivity is low, and in the case of the atmospheric heat treatment, there is a concern of oxidation progress such as generation of scale. Thus, in the case where the β transformation point is as low as 730 ° C. and 760 ° C. below 780 ° C., if the wire drawing rate is smaller than 7% or 13.8% and 20%, the heat treatment is performed for a short time of 8 hours or less. However, it does not reach ΔHV of 10 or less.

  From Table 3, when β-type titanium alloys having high C transformation points of 787 ° C. and 805 ° C. and symbols C and D corresponding to 780 ° C. or higher of the present invention are used, the upper limit of the drawing rate of the present invention is 20 No. exceeding 50% In C11, D11, and D12, ΔHV is very small at 1 or less, but the 0.2% yield strength after cold drawing exceeds 1300 MPa, and it is possible to seize with dies and jigs when forming cold drawing or coil springs. Will increase. On the other hand, No. with a wire drawing rate of 20% or less. In C1 to C10 and D1 to D10, the 0.2% yield strength after cold drawing is suppressed to 1280 MPa or less, and ΔHV is also as small as 10 or less. Furthermore, the wire drawing rate is as small as 10% or less. C1, C2, C3, C5, C6, C8, C9, D1, D2, D4, D5, D7, and D8 have a 0.2% proof stress after cold drawing of 1053 to 1152 MPa, which is suppressed to less than 1200 MPa. . In Table 3, all of the aging heat treatment times are short. C1 is 8 hours and all others are 3 hours.

  Comparing symbol C and symbol D under the same wire drawing rate and aging heat treatment conditions, it can be seen that symbol HV, which has a higher β transformation point of 790 ° C. or higher, has a smaller and smaller ΔHV. For example, no. C2-C4 and No. When D1 to D3 are compared, the β transformation point is as high as 805 ° C. For D1 to D3, ΔHV is as small as 5 or less.

  Furthermore, when the influence of the metal structure before cold drawing is compared, No. 1 is a fine structure with a crystal grain size of 10 μm or less or an unrecrystallized structure. C5 to C10 and D1 to D10 are No. when compared with the same wire drawing rate and aging heat treatment conditions. ΔHV is stably smaller than C1 to C4 and D1 to D3.

  Tables 4 and 5 show the types of β alloys used, the state before cold drawing, the cold drawing rate, the presence or absence of re-lubrication during cold drawing, the diameter deviation after cold drawing, and cold drawing. The Vickers hardness and ΔHV of each part in the T cross-section of 0.2% proof stress after cold drawing and cold drawing are shown. No. in Table 4 A0, B0, C0, and D0 correspond to the state without cold drawing. Other than that, it was cold-drawn. Is in common. The Vickers hardness was measured with a load of 1 kgf. As shown in Table 1, the symbols C and D correspond to a component system in which V of claim 7 of the present invention is 0.1 mass% or less and Fe is added in an amount of 2-8 mass%. As for the level at which cold drawing was performed, the diameter deviation of the rod after cold drawing was good in both Examples and Comparative Examples.

  From Tables 4 and 5, the examples manufactured in claims 1, 2 and 3 of the present invention (see the remarks in Table 4) are made from β-type titanium alloys having a β transformation point of 780 ° C. or higher. Since the 0.2% proof stress is less than 1300 MPa, the difference between the 1 mm depth from the surface and the center cross-section Vickers hardness (ΔHV) is 20 or more, and seizure did not occur during cold drawing, re-lubrication is unnecessary. There is a titanium alloy bar wire having a diameter deviation of ± 0.08 mm or less. Therefore, by using these, a coil spring having a stable spring constant and a uniform material can be manufactured.

  As a coil spring more suitable for manufacturing, the crystal grain size of 10 μm or less is a fine structure or an unrecrystallized structure No. 4 in Table 4. C5-C10 and D4-D10 correspond. Preferably, No. 4 in Table 4 in which the β transformation point is 790 ° C. or higher. D1-D10, No. of Table 4 whose 0.2% yield strength is less than 1200 MPa. C1, C3 to C6, C8, C9, D1, D2, D4, D5, D7, D8, No. in Table 4 in which the diameter deviation of the bar is ± 0.05 mm or less. C3-C10, D2-D6, D8-D10.

  On the other hand, no. A0, B0, C0, and D0 naturally have a small ΔHV and a low 0.2% proof stress, but have a large diameter deviation of ± 0.1 mm or more.

  In addition, No. with a high wire drawing rate exceeding 20%. C11, D11, and D12 have a 0.2% proof stress exceeding 1300 MPa.

It is a figure which shows the relationship between the wire drawing rate in 0.2 type titanium alloy, 0.2% proof stress as it is cold wire drawing, and its increase rate. It is a figure which shows the change of a cross-section Vickers hardness by the aging heat processing time after cold drawing in three types of (beta) type titanium alloys from which (beta) transformation point differs. It is a figure which shows the change of a cross-section Vickers hardness by the aging heat processing time after cold drawing in three types of (beta) type titanium alloys from which (beta) transformation point differs. It is a figure which shows the relationship of (beta) transformation point of beta type titanium alloy, and dispersion | variation (DELTA) HV of the hardness in the cross section after an aging heat processing (difference of surface layer 1mm and center Vickers hardness). It is a figure which shows the relationship between a drawing rate, the cross-section Vickers hardness with a cold drawing, and (DELTA) HV. It is a figure which shows the relationship of the deviation with respect to a drawing rate and a target diameter.

Claims (6)

  It is made of a β-type titanium alloy having a β transformation point of 780 ° C. or higher, a 0.2% proof stress is less than 1300 MPa, a 1 mm depth from the surface and a difference in cross-section Vickers hardness of 20 or more, and a rod wire diameter deviation of ± 0. A titanium alloy bar wire for coil springs, characterized in that it is not more than 08 mm.   2. The titanium alloy rod for coil spring according to claim 1, wherein the β-type titanium alloy contains 2 to 8 mass% of Fe and V is 0.1 mass% or less.   Use a wire rod that has been hot worked on a β-type titanium alloy with a β transformation point of 780 ° C. or higher, or that has undergone solution heat treatment after hot working, and cold-drawing at a cross-section reduction rate of 3 to 20%. A method for producing a titanium alloy bar wire for a coil spring.   4. The method for producing a titanium alloy rod for coil spring according to claim 3, wherein the metal structure before cold drawing is an unrecrystallized structure or a recrystallized structure having a crystal grain size of 10 [mu] m or less.   5. The method for producing a titanium alloy rod for a coil spring according to claim 3, wherein the β-type titanium alloy contains 2 to 8 mass% of Fe and V is 0.1 mass% or less.   The method for producing a titanium alloy rod for coil spring according to any one of claims 3 to 5, wherein a hole die is used in cold wire drawing.

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