JPH11158545A - Method for forming exhaust valve for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the creep resistance and mechanical strengths of an valve and to make machining needless or to a minimum by heating a billet having a specified compsn. essentially consisting of Cr and Ni to a specified temp., subjecting it to solution treatment, next removing scale, executing lubricating treatment and furthermore subjecting this billet to plastic working while it is held at a specified temp. SOLUTION: The billet to be used is an austenitic heat resistant steel essentially consisting of 13.5 to 19.5% Cr and 29.5 to 62.0% Ni. This billet is heated at 1050 to 1130 deg.C and is subjected to solution treatment. Scale is removed from the billet subjected to the solution treatment, which is subjected to lubricating treatment, and the treated billet is subjected to cold forging while its temp. is held to <=200 deg.C to form an exhaust valve. In this way, the big crystal grains obtd. by the solution treatment before the forging remain as they are since the quantity to be deformed is small in the valve head of the valve, and the high temp. strength and creep strength therein are made excellent, and sufficient recrystallization refining occurs by a large quantity of deformation in the axis part to improve its mechanical properties.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming an exhaust valve for an engine.

[0002]

2. Description of the Related Art An exhaust valve for an engine is a valve that faces a combustion chamber of an engine and controls the flow of high-temperature exhaust gas.
Heat resistance and corrosion resistance are required. For example, Japanese Unexamined Patent Publication No. 4-193912 discloses a method for manufacturing an exhaust valve. This manufacturing method is based on the SUH35-based austenitic system based on the flow shown in FIG. Heating temperature and forging temperature of 1
Hot forging at 150-1250 ° C, 1050-11
The solution is subjected to solution treatment at 50 ° C. and machined. By heating to the hot forging temperature and the solution treatment temperature, an exhaust valve having excellent creep resistance is manufactured. In addition, solution treatment (solution tree)
Tment) is the same as the solution treatment or solution heat treatment, which is a heat treatment method in which an alloy is heated to a solid solution range, sufficiently maintained at this temperature to form a solid solution of a component metal, and rapidly cooled.

[0003]

FIG. 16 shows a conventional SUH.
It is a graph obtained by examining the relationship between the solution temperature and the grain size number in the 35 series austenitic steel, the grain number on the vertical axis is defined by JIS-G-0551 "Austenitic grain size test method of steel", Some of them are shown below.

[0004]

[Table 1]

Therefore, when the particle size number is large, the crystal grains are small, and when the particle size number is small, the crystal grains are large. In general, it is effective to increase the crystal grains in order to improve the creep resistance, but on the other hand, if the crystal grains are large, there is an adverse effect that the mechanical strength decreases.

Further, forging is performed hot and machining is performed at room temperature. However, since the temperature is extremely different between the hot temperature (1150 ° C. or more) and the room temperature (about 25 ° C.), thermal expansion is expected. Even if forging is performed, the dimensional accuracy of the obtained forged product is not good. Therefore, machining is essential. Accordingly, an object of the present invention is to provide a molding method that is excellent in creep resistance and mechanical strength, and that can eliminate or minimize machining.

[0007]

In order to achieve the above object, the present invention provides a method for manufacturing a semiconductor device comprising the steps of: 13.5% to 19.5% of Cr;
Heat-treating a billet of austenitic heat-resistant steel containing 5% to 62.0% Ni as a main component at 1050 ° C to 1130 ° C and subjecting the solution-treated billet to scale removal and lubrication Process and processed billet 2
A cold forging process of forming an exhaust valve by plastic working while maintaining the temperature not to exceed 00 ° C.

[0008] If the Cr content is less than 13.5%, the oxidation resistance is significantly reduced. If it exceeds 19.5%, σ phase embrittlement becomes remarkable. Therefore, Cr is set in the range of 13.5% to 19.5%. Ni is a main component for stabilizing the austenite structure. If it is less than 29.5%, the high-temperature strength and fatigue strength become insufficient, and if it exceeds 62.0%, the strength is sufficient, but it is very expensive. Therefore, cost merit disappears. Therefore, Ni is set in the range of 29.5% to 62.0%.

If the solution treatment temperature is lower than 1050 ° C., the crystal grains do not become sufficiently large, and the expected creep resistance cannot be obtained. Conversely, when the temperature exceeds 1130 ° C., the crystal grains become sufficiently large, but a pear-skin-like uneven pattern is generated on the surface skin after forging, which is not preferable. Therefore, solution treatment is performed at 1050 ° C.
Performed at a temperature of １1130 ° C. Since the forging is a cold forging, precision forging can be performed, and the dimensional accuracy of the forged product is dramatically increased, so that machining can be omitted or minimized.

A second aspect of the present invention is the same as the first aspect, except that a step of heating the obtained workpiece to 1000 ° C. to 1080 ° C., performing a solution treatment again, and then performing an aging treatment is added.
The portion where the deformation stress is generated by receiving the deformation force by the cold forging is recrystallized and refined by the re-solution treatment, but the degree of the recrystallization depends on the amount of deformation. If the temperature is lower than 1000 ° C., recrystallization does not proceed sufficiently, and the expected mechanical strength cannot be obtained. On the other hand, when the temperature exceeds 1080 ° C., the growth of recrystallized grains proceeds and coarsens, and the expected mechanical strength cannot be obtained. Therefore, 1000 ° C
By performing the solution treatment again at 〜１０1080 ° C. and then performing the aging treatment, it is possible to obtain an engine exhaust valve excellent in both creep resistance and mechanical strength.

The engine exhaust valve is a valve that faces the combustion chamber of the engine and controls the flow of high-temperature exhaust gas. In particular, since the umbrella portion is exposed to high temperatures, high heat resistance and corrosion resistance are required. On the other hand, since the shaft portion must mainly consider sliding with the mating part, mechanical strength is required. As described above, although the engine exhaust valve is a single component, different characteristics are required for each part. Furthermore, since the amount of deformation of the small diameter shaft from the billet is different from the amount of deformation of the large diameter umbrella from the billet, forging is very difficult.

Therefore, the present invention is suitable for forging even if the amount of deformation between the umbrella portion and the shaft portion is significantly different, and
An engine that prepares a billet with the optimum diameter that can satisfy the characteristics required for each part, uses this billet, performs an appropriate forming process (cold forging), and then performs re-solution treatment and aging treatment It is a method of forming an exhaust valve. In other words, since the umbrella portion has a small amount of deformation, large crystal grains obtained by solution treatment before forging remain as they are, high-temperature strength,
Excellent creep resistance. In addition, the shaft portion is sufficiently recrystallized and refined by a large deformation amount, and has excellent mechanical properties.

[0013]

Embodiments of the present invention will be described below with reference to the accompanying drawings. The drawings should be viewed in the direction of reference numerals. FIG. 1 is a molding flowchart of an exhaust valve for an engine according to the present invention, in which an example and a comparative example are also described. STxx indicates a step number. ST01: Examples 1 and 2 shown in the following table were used as materials for the exhaust valve.
A few austenitic heat-resistant steel billets are prepared.

[0014]

[Table 2]

ST02: The billet is put into a furnace and subjected to a solution treatment. The processing conditions are as follows. Furnace temperature 1050 ° C to 1130 ° C Holding time 0.5 hour Post-treatment Immediately after extraction, water cooling

ST03: The oxide scale is removed. ST04: Form a film with oxalate and metal soap in preparation for cold forging.

ST05: The billet is cold forged. The conventional hot forging is now described in detail below, including the reason why the cold forging becomes possible in this embodiment. FIG. 2 (a)
(D) is a process diagram (first half, shaft portion formation) of the cold forging according to the present invention. (A): A billet 1 having a diameter D and a length L is set in a first mold 2 (having a conical surface at the tip) and set up with a first punch 3. (B): An intermediate product 4 having a conical tip (an intermediate processed product is referred to as an “intermediate product”; the same applies hereinafter) is set in a second mold 5 (forming a shaft portion having a diameter d2). Second punch 6
Upset. (C): An intermediate product 7 having a shaft portion with a diameter d2 is placed in a third mold 8
(Forming a shaft portion with a diameter d3) and upsetting with the third punch 9. (D): An intermediate product 11 having a shaft portion having a diameter d3 is placed in a fourth mold 1
2 (forming a shaft portion of diameter d4) and the fourth punch 1
Upset at 3.

3 (a) to 3 (c) are process drawings of cold forging according to the present invention (second half, formation of an umbrella portion). (A): An intermediate product 14 having a shaft portion with a diameter d4 and having an umbrella portion with a diameter D1 is set in a fifth mold 15 (forming an umbrella portion with a diameter D2) and upset with a fifth punch 16. . (B): An intermediate product 17 having an umbrella portion having a diameter D2 is formed by using a sixth mold 1
8 (forming an umbrella portion of diameter D3) and setting the sixth punch 1
Upset at 9. (C): The engine exhaust valve 20 completed through the above six steps.

FIGS. 4A and 4B are explanatory diagrams of the upsetting ratio in the exhaust valve. The exhaust valve 20 in (b) includes a shaft portion 21 and an umbrella portion 22, and the shaft portion 21 is a straight portion having a diameter d6 and a length 16. The length (height) of the umbrella portion 22 is L6. The volume v6 of the shaft portion 21 is (π / 4) × (d6) ² ×
It is determined by l6. In (a), the billet 1 is
a and an umbrella portion 1b. Specifically, the shaft portion 1a is set at the height corresponding to v6 {v6} (π / 4) × D ² }.
The rest may be the height L0 of the umbrella portion 1b. Since the upsetting ratio is a value obtained by dividing the upsetting length by the length before the upsetting, in the present invention, the upsetting ratio is ｛(L0−L6) /
L0｝ × 100 (%) is defined.

By the way, it was found that the work (intermediate product, finished product) became hot in the cold forging process shown in FIGS. Then, the temperature was measured. FIG. 5 is a graph showing the relationship between the upsetting ratio and the surface temperature when the exhaust valve is cold forged. The horizontal axis represents the upsetting ratio, and the vertical axis represents the surface temperature. In order to plastically deform a material that is difficult to be deformed by cold forging, the surface temperature increased in proportion to the upsetting rate, and reached about 250 degrees at an upsetting rate of 70% and about 300 degrees at an upsetting rate of 80%.

Therefore, the present inventors examined the relationship between temperature and cracks in heat-resistant steel. FIG. 6 is a graph showing the relationship between the temperature of the austenitic heat-resistant steel and the upsetting limit. The horizontal axis shows the test temperature, and the vertical axis shows the upsetting ratio when a crack occurs.

Referring to FIG. 6, both of the first, second and third embodiments have -5.
In the range of 0 ° C. to 600 ° C., the upsetting limit becomes smaller as the temperature rises. That is, it is easily broken. This is shown in FIG.
It is considered that the work was broken due to the temperature rise accompanying the processing described in (1). The details are described in Example 1 (30Ni-15
In the case of (Cr material), the upsetting limit is less than 70% at 200 ° C. In Example 2 (40Ni-15Cr material), the upsetting limit at 200 ° C. was 73%, and in Example 3 (60Ni-18Cr material), the upsetting limit at 200 ° C. was 75%. It is more advantageous. From this, in order to make an austenitic heat-resistant steel billet an exhaust valve with a 70% upsetting rate and a cold forging method, cracking can be prevented by setting the temperature of the billet or intermediate product to 200 ° C. or less. I understand. By controlling billets (including intermediate products) not to exceed 200 ° C., complete cold forging of the exhaust valve was realized. Since it is a cold forging method, there is no need to worry about scaling,
No heating equipment is required, and the finishing accuracy is improved.

FIG. 7 is a sectional view of a forging die according to the present invention. The forging die is the fifth die 15 shown in FIG. 3A, and the fifth die 15 is a cemented carbide. The die 15a is fitted to a base 15b of alloy tool steel (for example, SKD-JIS), and the triangular cross section passage 24 is formed by largely shaving the bottom edge of the cylindrical die 15a. .

FIG. 8 is a plan view of a forging die according to the present invention, and a coolant supply passage 25 and a discharge passage 26 are provided in a base 15b.
The cooling water is supplied to the triangular cross section passage 24 through the supply passage 26. The cooling water forcibly cools the die 15 a while passing through the triangular cross section passage 24, and then discharges it through the discharge passage 26. 27 is a water supply hose and 28 is a drain hose. By forcibly cooling the die 15a, the work
The temperature should not exceed 00 ° C.

As described above, the triangular cross section passage 24 can be formed extremely easily as long as it is a mold having a structure in which the die 15a is fitted to the base 15b, and a rise in the mold cost can be suppressed. . However, since it is important to flow a cooling agent such as water, the cross-sectional shape, the location to be formed, and the number are arbitrary.

In this embodiment, the fifth mold 15 and the sixth mold 18 are provided with the triangular section passage 24, and
The fourth molds 2, 5, 8, and 12 were not provided with the triangular section passage 24. The reason is as follows. FIG.
(D) is substantially a shaft portion forming step. The shaft portion is obtained by reducing the diameter of the billet 1. Austenitic heat-resistant steel and martensitic heat-resistant steel are poor in ductility but strong in compression. Since the diameter reduction accompanying the formation of the shaft portion compresses the surface, even if it exceeds 200 ° C., there is no fear of cracking. Therefore, the first to fourth molds 2, 5, 8, and 12 were not cooled. On the other hand, FIGS. 3A and 3B show an umbrella portion forming step, and the umbrella portion is obtained by expanding the diameter. Austenitic heat-resistant steel and martensitic heat-resistant steel are strong in compression but weak in tension. The diameter expansion accompanying the formation of the umbrella causes a tension to act on the surface. Therefore, when the temperature exceeds 200 ° C., there is a concern about cracking. Therefore, the fifth mold 15 and the sixth mold 18 are forcibly cooled.

By using only the fifth mold 15 and the sixth mold 18 as cooling molds, it is possible to suppress a rise in various costs. However, the first to fourth dies 2, 5, 8, and 12 may be used as cooling dies. Further, it is desirable that the coolant is sufficiently cooled. In addition to inexpensive water, an antifreeze and a refrigerant for a refrigerator may be used.

FIG. 9 is a view showing another embodiment of the cooling system according to the present invention, in which a cooling gas is injected from the cooling nozzles 31, 31 to the intermediate product 14 between the fifth die 15 and the fifth punch 16. For forced cooling. The cooling gas is preferably air sufficiently cooled by a refrigerator. In this example, since the cooling gas is blown to the umbrella portion of the intermediate product 14, it has an immediate effect, and the die cost can be reduced. However, the cooling gas has a much smaller heat capacity than the cooling liquid, and is not suitable for the large intermediate product 14.

Therefore, it is also possible to combine both the mold cooling shown in FIG. 7 and the direct cooling by gas shown in FIG. 9 to bring out the advantages of each other.

Returning to FIG. 1, the one after ST05
The primary exhaust valve may be a completed exhaust valve. It is useful to perform the solution treatment again for the purpose of improving the mechanical strength of the primary exhaust valve. ST06: Solution treatment of the primary exhaust valve is performed again in the following manner. Furnace temperature 1000 ° C. to 1080 ° C. Holding time 0.5 hour Post-treatment Immediately after extraction, water-cooling is followed by aging treatment. Furnace temperature 700 ° C to 800 ° C Holding time 4 hours Post-treatment Air cooling This increases the mechanical strength.

The comparative example consists of ST101 to ST105, but hot forging at 1150 ° C. to 1250 ° C.
The solution treatment at a temperature of 1 to 1150 ° C and machining are the basic steps. On the other hand, in the example, 1050 ° C.
The basic steps are a solution treatment at 1130 ° C., a cold forging at 200 ° C. or lower, and a re-solution treatment at 1000 ° C. to 1080 ° C. which is performed as necessary. That is,
In the present embodiment, the billet was softened by a solution treatment (1050 ° C. to 1130 ° C.) in order to smoothly perform cold forging. The mechanical strength is improved by correcting internal strain and the like due to cold forging again by solution treatment (1000 ° C. to 1080 ° C.).

[0032]

Embodiments of the present invention will be described below. Billet material: Austenitic heat-resistant steel (equivalent to the above Examples 1, 2 and 3) Solution treatment conditions: Furnace temperature 1120 ° C Holding time 0.5 hour Post-treatment Immediately after extraction, water cooling

The treated product was descaled, lubricated, and cold forged at 200 ° C. or lower to obtain a first exhaust valve. FIG. 10 is a schematic cross-sectional view of the primary exhaust valve.
Most of the cross section of the secondary exhaust valve 41 had the crystal grain size shown in the following figure. FIG. 11 is a photomicrograph (magnification: 100) of a cross section of the primary exhaust valve. From this photo, it was found that the crystal grain size was about “3.5”, although it appeared to be crushed by plastic deformation.

The first exhaust valve 41 was subjected to solution treatment again to form a second exhaust valve. Solution treatment condition again: Furnace temperature 1050 ° C Holding time 0.5 hour Post-treatment Immediately after extraction, water cooling

FIG. 12 is a schematic sectional view of the secondary exhaust valve. The umbrella portion 43 of the secondary exhaust valve 42 has a core 44, an intermediate layer 45, and a surface layer 46 that are clearly different in crystal grain size and a shaft portion. 4
7 was almost uniform. The degree of recrystallization changes depending on the amount of deformation in forging, and the larger the deformed part, the more recrystallization progresses to a finer grain size.

FIGS. 13A and 13B are micrographs (magnification: 100) of the cross section of the core and the intermediate layer of the secondary exhaust valve. The crystal grain size of the core 44 was about “3.5”. The crystal grain size of the intermediate layer 45 was about “4.5”. FIG.
(A) and (b) are micrographs (magnification: 100) of a cross section of a surface layer and a shaft portion of the secondary exhaust valve. The crystal grain size of the surface layer 46 was about “7.0”. The crystal grain size of the shaft portion 47 was about “7.0”.

When the crystal grain size is 7.0, the crystal grain size is sufficiently small and the mechanical strength is high. On the other hand, the crystal grain size is 3.5
If it is ~ 4.5, the crystal grain size is sufficiently large and the high temperature creep strength is high. Since the surface layer 46 of the umbrella portion hits the valve seat, the higher the mechanical strength, the better. Since the shaft portion 47 also receives a compressive axial force, the higher the mechanical strength, the better. In this regard, surface layer 4
6 and the shaft portion 47 have a crystal grain size of 7.0 and a sufficiently high mechanical strength. Since the core 44 and the intermediate layer 45 have a crystal grain size of 3.5 to 4.5 and have a high high-temperature creep strength, an excellent exhaust valve is obtained.

FIG. 15 is a graph showing the relationship between the solution temperature and the grain size number in the austenitic steel of the present invention (Examples 1, 2, and 3). In Examples 1, 2, and 3, when the solution treatment is performed at 1050 ° C. to 1130 ° C., crystal grain size numbers of 3 to 5.5 are obtained. However, 1130 ° C
If it exceeds 300, the crystal becomes coarse, and after forging, a satin-like uneven pattern is generated on the surface. At 1050 ° C. or lower, the crystal grains do not become large, and the softening is insufficient, so that cold forging becomes difficult, and the high-temperature creep characteristics required for the exhaust valve cannot be obtained. Therefore, the solution treatment is performed at 1050C to 1130C.

In Examples 1, 2 and 3, 100
When the solution treatment is performed again at 0 ° C. to 1080 ° C., the surface layer and the shaft portion have a crystal grain size number of 4.5 to 7.5, and the mechanical strength increases due to the refined crystal grains.

[0040]

According to the present invention, the following effects are exhibited by the above configuration. According to claim 1, since the forging is a cold forging, precision forging is possible, and the dimensional accuracy of the forged product is dramatically increased.
Machining can be omitted or minimized.

A second aspect of the present invention is the same as the first aspect, except that the obtained work is heated to 1000 ° C. to 1080 ° C., subjected to a solution treatment again, and then subjected to an aging treatment. By the aging treatment after the solution treatment again, the grain size of the forged product is adjusted, and an exhaust valve having both an umbrella part having high high-temperature creep strength and a shaft part having high mechanical strength can be obtained.

According to the molding method of the present invention, even for other materials having different component ranges, after solution treatment, cold forging and processing, and then solution treatment again and aging treatment are performed. , Requirements for one product are different for each part, for example, creep resistance is required,
The amount of deformation (strain) of the umbrella is small and the amount of deformation (strain) of the shaft
It is possible to produce products having a large size, in particular, automobile engine parts such as engine valves.

[Brief description of the drawings]

FIG. 1 is a flow chart of forming an exhaust valve for an engine according to the present invention.

FIG. 2 is a process diagram of the cold forging according to the present invention (first half, shaft portion formation).

FIG. 3 is a process diagram of cold forging according to the present invention (second half, umbrella portion formation)

FIG. 4 is an explanatory diagram of an upsetting ratio in an exhaust valve.

FIG. 5 is a graph showing the relationship between the upsetting ratio and the surface temperature when the exhaust valve is cold forged.

FIG. 6 is a graph showing the relationship between the temperature of an austenitic heat-resistant steel and the upsetting limit.

FIG. 7 is a sectional view of a forging die according to the present invention.

FIG. 8 is a plan view of a forging die according to the present invention.

FIG. 9 is a diagram showing another embodiment of the cooling system according to the present invention.

FIG. 10 is a schematic cross-sectional view of a primary exhaust valve.

FIG. 11 is a micrograph (100 magnification) of a cross section of a primary exhaust valve.

FIG. 12 is a schematic cross-sectional view of a secondary exhaust valve.

FIG. 13 is a micrograph (magnification: 100) of a cross section of the core and the intermediate layer of the secondary exhaust valve.

FIG. 14 is a micrograph (100 magnification) of a cross section of a surface layer and a shaft portion of a secondary exhaust valve.

FIG. 15 shows an austenitic steel according to the present invention (Examples 1, 2, 2).
Graph showing the relationship between solution temperature and grain size number in 3)

FIG. 16 is a graph showing the relationship between the solution temperature and the grain size number of a conventional SUH35 austenitic steel.

[Explanation of symbols]

1. Billet, 2, 5, 8, 12, 15, 18 ... Mold,
20 ... engine exhaust valve, 41 ... primary exhaust valve, 42 ...
Secondary exhaust valve.

Claims (2)

[Claims] 1. 13.5% to 19.5% of Cr and 29.
Heat-treating a billet of austenitic heat-resistant steel mainly containing 5% to 62.0% Ni to 1050 ° C to 1130 ° C to perform solution treatment; scale-removing the solution-formed billet and performing lubrication treatment A method for forming an exhaust valve for an engine, comprising: a step; and a cold forging step of forming an exhaust valve by plastically processing the treated billet at a temperature not exceeding 200 ° C. 2. 13.5% to 19.5% of Cr and 29.
Heat-treating a billet of austenitic heat-resistant steel mainly containing 5% to 62.0% Ni to 1050 ° C to 1130 ° C to perform solution treatment; scale-removing the solution-formed billet and performing lubrication treatment A cold forging step of plastically processing the processed billet into the shape of an exhaust valve while maintaining the temperature of the treated billet at a temperature not exceeding 200 ° C., and heating the obtained work piece to 1000 ° C. to 1080 ° C. to perform solution treatment again. Performing an aging treatment, and then performing an aging treatment.