JPH0748659A - Production of titanium alloy bar suitable for production of engine valve - Google Patents

Abstract

PURPOSE:To produce a titanium alloy bar free from the oecurrence of crook of a shaft part in annealing after the forging of a valve head at the time of producing an engine valve in particular in a titanium alloy bar for an automotive engine valve, a two-wheel barrow or the like is made possible and capable of mass-production. CONSTITUTION:In the production of a titanium alloy bar, the stage of straightening a coiled wire rod into a linear wire rod is executed in a hot state at >=700 deg.C to produce a titanium alloy bar suitable for the production of an engine valve. Other wise its microstructure is adjusted to a temp. of Tbeta-50 or above to Tbeta+10 deg.C or below (Tbeta: beta phase transformation point), and the hot straightning is executed by the same method, by which the titanium alloy bar suitable for the producing the engine valve can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a titanium alloy rod for an engine valve, which can be mass-produced for use in engines of automobiles and motorcycles. More specifically, the present invention relates to a method for manufacturing a titanium alloy rod that does not cause bending of the shaft portion of a rough valve material during annealing after forging an umbrella portion when manufacturing an engine valve.

[0002]

2. Description of the Related Art A valve provided in an intake / exhaust hole of an engine combustion chamber of an automobile or the like comprises an umbrella portion, a shaft portion and a shaft end portion continuous with the umbrella portion. For example, a steel rod with a diameter of 7 mm is cut into a length of 250 mm, one end of which is upset forged (electrical contraction method) in a hot state while being energized and heated, and subsequently a die is forged in a hot state of the umbrella part. As a mushroom-shaped rough material, strain relief annealing is performed, then after finishing to the final shape by cutting and grinding, surface treatment such as soft nitriding treatment is given to impart abrasion resistance. Is customary. Abrasion resistance is required for the face portion, the shaft portion, and the shaft end portion of the umbrella portion of the engine valve. In addition, the valve must have high-temperature strength, corrosion resistance, and oxidation resistance because of the environment in which the valve is used, and conventional valves are generally made of heat-resistant steel.

By the way, in recent years, engines of automobiles and the like are required to be light in weight in order to improve fuel efficiency without lowering horsepower. For engine valves that repeat vertical movements at high speed, the ripple effect of fuel efficiency improvement due to weight reduction is extremely large, and therefore, the use of titanium alloy materials with high specific strength has been attempted. For example, for an intake valve of a competition vehicle, Ti-6Al, which is a typical example of α + β titanium alloy, is used.
-4V has been used frequently. However, when a titanium alloy is used as it is for an engine valve, there is a problem that abrasion occurs due to sliding between a mating member such as a valve seat and a valve guide and each part of the valve, resulting in poor durability. For this reason, the titanium alloy valve is subjected to a finishing process in almost the same manner as the heat-resistant steel valve, and is Mo-sprayed in order to impart wear resistance to the shaft portion. Therefore, the manufacturing cost is high and the product is economically neglected.

On the other hand, as proposed in JP-A-61-81505 and JP-A-62-256956,
Oxidation treatment and nitriding treatment in which heating is performed in each atmosphere are known as relatively inexpensive abrasion-resistant treatments. But the normal α
When these treatments are applied to a + β type titanium alloy valve, it is heated at a high temperature, so that thermal deformation (particularly bending of the shaft portion) occurs, and the shape and dimension accuracy required for the valve cannot be secured. Therefore, in order to use it as a valve, the titanium alloy valve subjected to the surface treatment is corrected, which makes it extremely difficult to efficiently produce the valve. This means that "titanium zirconium" vol. 35, No. 2, page 74.

The cause of dimensional change is the usual α +
Since the β type titanium alloy rod has a fine equiaxed α crystal structure,
This is because the titanium alloy valve undergoes creep deformation at the oxidation or nitriding processing temperature (700 to 900 ° C.) with a slight stress due to its own weight (about 50 g). On the other hand, as described in JP-A-5-59919, if an α + β type titanium alloy having a specific structure (for example, a needle α crystal structure) or a rod of a near α type titanium alloy is used as a valve material. , The problem of creep deformation can be solved. By the way, a rod having a fine equiaxed α crystal structure with a diameter of 5 to 10 mmφ is formed into a coil-shaped wire rod after hot rolling, and after being drawn into a perfect circle by cold drawing, it is cut by shaving and cut into a rod material. However, no specific proposal has been made for a method for producing a rod having an acicular α-crystal structure with a reduced elongation value and reduced drawing value.

Further, although strain relief annealing is performed after the forging of the umbrella portion, when a titanium alloy is used, even if the material has a specific structure without creep deformation, the shaft portion is as large as about 100 μm during the annealing after the forging of the umbrella portion. The phenomenon of bending occurs. When the material is heat-resisting steel, the annealing causes bending of the shaft portion of about 50 μm, but this deformation does not cause a problem because grinding can be easily performed. However, in titanium alloy, the deformation of the shaft part is large, so that the deflection of the umbrella part is also large,
Although the deformation of the shaft and the umbrella will be cut, compared to heat-resistant steel, the efficiency of cutting and grinding is poor, and the material cost is high, so the method of cutting and grinding the deformation is not a problem. is there.

[0007]

SUMMARY OF THE INVENTION According to the present invention, when a valve is manufactured from a titanium alloy, by improving the method for manufacturing a titanium alloy rod, the valve finished by the subsequent oxidation treatment or nitriding treatment is not deformed. This is to reduce the amount of cutting and grinding required by reducing the bending of the shaft portion of the rough valve shaped material that occurs in the strain relief annealing after forging the umbrella portion, which is an indispensable step in order not to occur. Further, as a premise of preventing the deformation that occurs in the above strain relief annealing, it is necessary to manufacture a bar having a structure that is difficult to creep, but if the needle-shaped α crystal structure that is difficult to creep deform in the hot working step is used, the elongation value ,
Since the aperture value is lowered, the coiled wire is easily cracked when it is straightened, and a straightening method for preventing this is a problem. In addition, if a fine equiaxed α-crystal structure that easily undergoes creep deformation in the hot working process is used, when this structure is changed to a structure that is less likely to creep deformed by heat treatment, it will be greatly deformed during heat treatment, making correction difficult. The problem is a correction method to prevent this.

[0008]

According to the present invention for solving the above-mentioned problems, (1) in the production of a titanium alloy rod, at least in the final step among the steps of straightening a coiled wire into a straight rod. (2) The titanium alloy of (1) above is an α + β type titanium alloy or a near α type titanium alloy, and has a needle-like α crystal structure in the hot working step. In the case of a coiled wire adjusted to, the wire is roughly straightened or straightened by the method of (1), and (3) the titanium alloy of (1) is an α + β titanium alloy. Alternatively, in the case of a near α-type titanium alloy, which is a coiled wire rod adjusted to a fine equiaxed α crystal structure in the hot working step,
After being roughly corrected, or as it is, heated to a β transformation point of −50 ° C. or more and a β transformation point of + 100 ° C. or less, and the microscopic structure is a structure in which equiaxed α crystals are dispersed in a needle α crystal structure or a needle α crystal structure. It is characterized in that it is corrected by the method of the above (1) after the adjustment.

[0009]

The present invention will be described in detail below. A titanium alloy suitable for the present invention is an α + β type titanium alloy, Ti-6.
Al-4V, Ti-3Al-2.5V, Ti-6Al-
2Fe-0.1Si, Ti-5Al-1Fe, Ti-5
Al-2Cr-1Fe, Ti-6Al-2Sn-4Zr
-6Mo, etc., and Ti-, which is a near α-type titanium alloy
6Al-2Sn-4Zr-2Mo-0.1Si and the like. These alloys are capable of producing wire rods having a diameter of 5 to 10 mm by hot working and satisfying the usage characteristics as an engine valve. Further, these titanium alloys have a needle-shaped α crystal structure when hot-worked in the β-phase temperature range or simply heated, and a fine equiaxed α-crystal structure when hot-worked sufficiently in the α + β-phase temperature range. Becomes

When the microscopic structure at the stage of the coiled wire rod of these alloys is a structure which is unlikely to undergo creep deformation, that is, a needle-shaped α-crystal structure, it is corrected by the following method.
First, the coil is passed through a continuous drawing machine or cut as it is to an appropriate length, and then rough correction is performed to such an extent that cracking does not occur. For example, a wire having a needle-like structure has an elongation value of 7
%, The aperture value is 20%, which is about half that of a material with a normal fine equiaxed α crystal structure. Since cracking will occur, stop before rough correction. Rough straightening may be performed using the hot straightening equipment described below.

Next, what is to be hot-corrected is that if it is left cold-corrected, the strain introduced by the cold bending during the subsequent annealing after the forging of the umbrella portion is released and the deformation is caused. This is because it occurs. That is, 5-10 mmφ which is usually used for valves
For the thin rod of, the billet is hot-rolled and the diameter is 1-2 m.
After forming the coiled wire, the surface is refined by cold drawing and shaving, and the cross section is close to a perfect circle. Next, the coil is passed through a continuous drawing machine, cut into an appropriate length, and then cold-rolled by 2 rolls to form a straight rod, and centerless grinding is performed for finishing. During this cold straightening (including cold drawing), a large amount of strain is accumulated in the bar material, and due to subsequent heating, it returns to its original shape.

To give an example of the influence of the strain introduced in the cold during the heat treatment on the shape of the bar material, in the case of Ti-6Al-4V alloy, if the bending of 2.5 mm per 10 cm is straightened in the cold, 800 ° C. Bending of about 4% or 100 μm occurs due to the annealing. On the other hand, in the case of the valve heat resistant steel SUH11, if the same bending is straightened cold, it will be about 2 by annealing at 800 ℃.
%, Only a bend of 50 μm occurs.

In the hot straightening method, generally, the temperature of the material is raised by electric heating, and then tensile stress is applied to straighten it. Here, the temperature for straightening hot is 70
The reason why the temperature is set to 0 ° C. or higher is that the rod material straightened at a temperature lower than 700 ° C. is bent in the shaft portion by annealing after the forging of the umbrella portion. Here, it may be possible to anneal below 700 ° C so that the shaft does not bend, but the subsequent abrasion resistance treatment temperature is 700
Limited to less than ℃. As a result, even when the oxidation treatment is performed at a treatment temperature lower than that of the nitriding treatment, the treatment at a temperature of less than 700 ° C. does not sufficiently impart the hardened layer, and particularly the wear resistance of the face portion is insufficient. Although the upper limit temperature for hot straightening is not specified here, if the temperature is too high, the yield strength will decrease, so the wire rod hangs due to gravity, and the effect of straightening may not be sufficient just by applying tensile stress from both ends. . Considering this and the temperature at which the shaft portion does not bend due to annealing after forging the umbrella portion, the straightening temperature is preferably 700 to 900 ° C. When the heating time is short, it is necessary to raise the temperature somewhat. Further, cooling may be air cooling.

When the titanium alloy for a valve has a fine equiaxed α-crystal structure with a grain size of 2 to 4 μm, which easily undergoes creep deformation at the stage of coiled wire, it is corrected by the following method. First, the coil is passed through a continuous drawing machine to be cut into an appropriate length, and if necessary, roughly corrected by a roll or the like arranged in a usual birdlike shape. In addition, this wire rod
Since the elongation value is as large as 15% and the drawing value is as large as 40%, there is no problem of cracking even if it is completely straightened. Next, using a facility for hot straightening, before the hot straightening, from the β transformation point of -50 ° C or higher,
After heating to a temperature below the β transformation point, it is air-cooled to the straightening temperature and changed into a structure in which equiaxed α crystal structure has equiaxed α crystals dispersed, or before hot correction, β transformation point or more, β Transformation point +
After being heated to a temperature of 100 ° C. or lower, it is air-cooled to a straightening temperature and converted into a needle-shaped α crystal structure, and then hot straightening is performed. The reason for changing the structure in this way is to make it difficult for creep deformation during annealing and heat treatment during abrasion resistance after forging. Furthermore, the reason why a normal heat treatment furnace is not used is that if a coil-shaped wire rod having a fine equiaxed α-crystal structure that is susceptible to creep deformation is treated in a heating furnace, it is greatly deformed, and it becomes difficult to correct it later. Of course, after the heat treatment for adjusting the structure, it may be air-cooled to room temperature and then raised again to a predetermined temperature to correct it.

Here, the β transformation point of -50 ° C. or higher is
This is because below this temperature, the change from the fine equiaxed α-crystal structure is small and the effect of preventing creep deformation is small. The β transformation point + 100 ° C. or lower means that when the temperature is higher than this, the effect of preventing creep deformation can be sufficiently exerted, but oxidation becomes severe and a large grinding allowance is required in the centerless processing in the post process. Because it will be. The heating time necessary for adjusting the structure is from several minutes to 30 minutes when the temperature is below the β transformation point, and from several seconds to 1 minute when it is above the β transformation point.

[0016]

Example 1 A Ti-6Al-4V billet was hot-worked in a β-phase temperature range to obtain a wire having a diameter of 9 mm. The winding diameter of the coil is about 1 m, and the microstructure of this material is needle-like α.
It has a crystal structure.

This wire rod was subjected to cold drawing and shaving to refine the surface and make a true circle having a diameter of 7 mm. This part is straightened at a temperature of 200 ° C to prevent cracking with a roll arranged in a normal dust-like shape, and the other parts are roughly straightened, and then hot drawn by a method of pulling while energizing at each temperature of 600 to 900 ° C. I corrected it. Each of them was made centerless ground into a bar material of 6.7 mmφ without bending, and the umbrella part was forged by a predetermined method, and the umbrella diameter was 36 mm and the valve length was 11 mm.
After making a 5mm valve rough material, 1 at 600-900 ℃
Annealing was performed with the time valve lying sideways. Table 1 shows the results of measuring the bending generated in the shaft portion of the rough valve material after the above steps, that is, the required amount of centerless grinding per one surface.

For the bending of the shaft portion, the valve was rotated while supporting both ends of the shaft portion, and the minimum and maximum values of the shake of the central portion were measured with a dial gauge, and the difference was set to half the difference. According to this, by straightening at a temperature of 700 ° C. or higher, it is possible to remove 94% of the bending generated by the subsequent heating, so that the required amount of centerless grinding in valve processing can be drastically reduced. On the other hand, 6
Straightening at 00 ° C is only 50% removal. 85
When annealed at 0 to 900 ° C, some deformation due to creep occurs. Next, after finishing the final valve shape with umbrella diameter 35mm, valve length 110mm, and shaft diameter 6.68mm, 700 ~
Table 2 shows the results of the oxidation treatment at 900 ° C. for 1 hour with the valve lying sideways and in a standing state. The titanium alloy rod manufactured according to the present invention has little bending of the shaft during annealing after the forging of the umbrella, so that the amount of centerless grinding required for valve processing is extremely small, and an appropriate temperature and valve can be installed by oxidation treatment. By selecting the state, it can be seen that a valve with high dimensional accuracy can be manufactured. Here, the dimensional accuracy required for the valve is within 10 μm. Also, for those with little bending after annealing, since the role of annealing has been completed in the hot straightening step at least for the shaft portion, the annealing step can be omitted if umbrella forging is performed at high temperature so that no strain remains. . That is, the oxidation treatment may be performed directly after the forging and finishing.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

Example 2 A Ti-6Al-4V billet was hot worked into a wire rod having a diameter of 9 mm. The coil winding diameter is about 1
m, and the microstructure of this material is an acicular α-crystal structure.
In step 1, this wire rod was subjected to cold drawing and shaving to remove surface flaws and form a perfect circle having a diameter of 7.9 mm, and a part of this was drawn continuously to obtain a 7.6 mmφ rod. When two-roll straightening was performed with a roll offset amount of 10 mm, the bending of the bar could be completely removed. After that, a 7.3 mmφ polishing rod was processed by centerless grinding and a penetrant flaw detection test was carried out. As a result, 30% of C-direction flaws shown in FIG. Next, when the roll offset amount was set to 7 mm, the bending defect was 10% and the C-direction flaw was 15%. Further, when the roll offset amount was set to 5 mm, the bending defect was 20% and the C-direction flaw was 10%. The results are shown in Table 3.
Shown in. Here, a bent rod having a bending length of more than 0.25 mm per 1 m was regarded as a defective bending.

In step 2, using the above-mentioned perfect circular material having a diameter of 7.9 mmφ, continuous drawing is carried out to obtain a bar material having a diameter of 7.6 mmφ.
After performing hot straightening by pulling while electrically heating at each temperature of 700 to 1000 ° C. to remove bending of the rod, centerless grinding was performed to obtain a polished rod of 7.3 mmφ. When a penetrant flaw detection test was performed on this polishing rod, no external flaw was generated. In step 3, the rolled material is subjected to cold drawing and shaving to form a 7.6 mmφ round material, and the wire material is 700 to 10 as it is.
Hot-rectification was performed by pulling while heating by energization at each temperature of 00 ° C. to obtain a rod without bending, and then centerless grinding was performed to obtain a polished rod of 7.3 mmφ. When a penetrant flaw detection test was performed on this polishing rod, no external flaw was generated.

Table 4 shows the manufacturing process, and Table 5 shows the results of the penetrant inspection test. As described above, by carrying out hot straightening, it is possible to manufacture a straight rod having no bending defect and no external flaw with a high yield.

[0024]

[Table 3]

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

Example 3 The prepared titanium alloy is a coil-shaped wire, and the types thereof are Ti-6Al-4V and Ti-3Al-2.5V having an equiaxed α crystal structure of 2 to 4 μm grains, and 2 to Ti-6Al-4V, Ti-3Al-2.5V, and Ti-6Al-2Fe-0.
1Si, Ti-5Al-1Fe, Ti-5Al-2Cr
-1Fe, Ti-6Al-2Sn-4Zr-6Mo, T
i-6Al-2Sn-4Zr-2Mo-0.1Si.

The method of the present invention was applied to these. For example, the one having an equiaxed α-crystal structure of 2 to 4 μm grains was heated by energization using the same equipment to change the microstructure before hot correction, and then air-cooled to the correction temperature for correction. These rods were processed into valves after centerless grinding. That is, the umbrella portion was forged, annealed, finished, and oxidized.

Table 6 shows the processing steps and the bending of the shaft after each step. In the annealing after the forging of the umbrella portion, the rough valve material was laid sideways and heated in an atmospheric furnace for 1 hour. For the oxidation treatment, the atmosphere furnace was heated with the valve standing. According to the method of the present invention, it was found that the bending of the shaft portion after annealing was extremely small within 13 μm, and the required grinding amount could be reduced.

[0030]

[Table 6]

[0031]

EFFECTS OF THE INVENTION By using the present invention, it is possible to efficiently manufacture a bar material having a small elongation value and drawing value and being resistant to creep deformation. Further, in the valve manufacturing process, since the bending of the shaft portion after die forging is extremely small, the cutting allowance of the shaft portion can be reduced. As a result, Table 7 shows an example in which the manufacturing cost can be reduced.
Shown in.

[0032]

[Table 7]

As a result of applying the present invention, about 10% of the material cost for the valve shaft portion and 90% or more of the cutting cost can be reduced. Further, the bending of the shaft portion also affects the deflection of the umbrella portion, and it is necessary to increase the grinding allowance of that portion, but the present invention can greatly improve this.

[Brief description of drawings]

FIG. 1 is a side view of a valve.

FIG. 2 is a diagram showing an example of a state in which a shaft portion of a rough valve material is deformed.

FIG. 3 is a view showing a state in which the valve is laid sideways.

FIG. 4 is a view showing a state in which the valve is stood.

FIG. 5 is a diagram showing a C-direction flaw in a valve material.

[Explanation of symbols]

 1: Umbrella part 2: Shaft part 3: Shaft end part 4: Face part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Satoshi Yamamoto Satoshi Yamaguchi 3434 Shimada, Hikari City Yamaguchi Prefecture Inside the Nippon Steel Co., Ltd. Hikari Steel Works (72) Inventor Satano Toyoguchi 3434 Shimada, Hikari City Shimada Prefecture New Japan (90) Haruo Oguro Inventor Haruo Oguro No. 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Works Hikari Steel Co., Ltd.

Claims (3)

[Claims] 1. In the production of a titanium alloy rod, in the step of straightening a coil-shaped wire into a straight rod, at least in the final step, straightening is performed by heat of 700 ° C. or higher. A method for manufacturing a titanium alloy rod suitable for manufacturing. 2. When the titanium alloy according to claim 1 is an α + β type titanium alloy or a near α type titanium alloy and is a coiled wire rod adjusted to an acicular α crystal structure in the hot working step, A method for manufacturing a titanium alloy rod suitable for manufacturing an engine valve, which comprises roughening or straightening by the method according to claim 1. 3. When the titanium alloy according to claim 1 is an α + β type titanium alloy or a near α type titanium alloy, which is a coiled wire rod adjusted to a fine equiaxed α crystal structure in the hot working step. After rough correction, or after being directly heated to a β transformation point of −50 ° C. or higher and a β transformation point of + 100 ° C. or lower, the microscopic structure is a structure in which equiaxed α crystals are dispersed in equiaxed α crystals or needle α crystals. A method for manufacturing a titanium alloy rod suitable for manufacturing an engine valve, which comprises adjusting the structure and then straightening it by the method according to claim 1.