JP2001096333A - Method of cold forging for highly tough heat resisting steels - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which enables cold forging for an engine valve or the like by using a material hard to mold such as austenitic heat resisting steel. SOLUTION: A corrective process is once executed to bring a slab obtained by cutting a wire close to a form before cutting. Thereafter, in a cold forging, the material uniformly reaches the yield point in the molding part. In this way, occurrence of fogging or bite is restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold forging method using a material having poor cold forgeability, such as austenitic heat-resistant steel.

[0002]

2. Description of the Related Art Japanese Unexamined Patent Publication No. Sho 60-216944 discloses a method of manufacturing an engine valve by hot forging. This prior art has been proposed by the present applicant. First, as shown in FIG. 5 (a), a heat-resistant steel wire rod after solution treatment is cut into a predetermined length to form a slab (billet). ), And then, as shown in FIG.
At one end of the slab, a tapered surface defining an oil reservoir at the time of extrusion molding is formed, and thereafter, as shown in FIG. 2C, a large diameter portion and a small diameter portion are formed by extrusion molding. Further, as shown in FIG. 3 (d), an intermediate product in which the shape of the large diameter portion is close to the valve head portion is formed.

[0003]

SUMMARY OF THE INVENTION
When applied to martensitic steels, chromium molybdenum steels, structural steels, etc., no major problems occur.
However, high heat-resistant steel such as austenitic heat-resistant steel is required as a material for an exhaust valve that is also exposed to high temperatures even in an engine valve. When cold forging is applied for the purpose, the following problems occur.

That is, in order to cold forge an engine valve, as shown in FIG. 6, a wire 101 is cut into a predetermined size using a shear 102 to form a slab 103.
03 is cold forged. However, since the austenitic heat-resistant steel after solution treatment has higher toughness (viscosity) than other materials, the shape of the slab 103 is the same as that of cutting a martensitic steel wire. Differently, as shown in FIGS. 7A and 7B, the sheared portions at both ends are crushed to have a shape close to an elliptical shape, and sag 104 is generated in a part, and the whole is bent along the axial direction. I will.

Then, in order to form a tapered surface at one end of the slab using such a slab as in the case of hot forging disclosed in Japanese Patent Application Laid-Open No. Sho 60-216944, as shown in FIG. When the mold 103 is set in the mold 105 and pressed from above with the punch 106 to perform molding, the deformation starts first from the point a, and the deformation of the point b starts at the point a as shown in FIG. Only after entering the plastic zone after passing the elastic zone, the point b enters the fracture zone before entering the plastic zone.

As a result, as shown in FIG. 10A, a fogging phenomenon (burr phenomenon) in which the sag 104 overlaps the slab 103 occurs. As shown in FIG. 10 (b), the fog portion is stretched and foil-like burrs are generated at the shaft end. In addition, if the end of the slab is formed with a non-uniform end without forming a tapered surface at one end by shaft extrusion, as shown in FIG. 10C, thrust in the thrust direction is generated on the shaft. .

[0007]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a slab obtained by cutting a high toughness heat-resistant steel wire into a predetermined length, and bringing the shape of at least one end of the slab closer to the shape before cutting. The straightening was performed cold, and then the forging was performed cold to the desired shape.

The high toughness heat-resistant steel includes, but is not limited to, austenitic heat-resistant steel. Further, the present invention is applied to cold forging of a member requiring heat resistance, and may be an example of manufacturing an exhaust valve of an engine, but is not limited thereto.

[0009]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a cross-sectional view of a mold apparatus used for carrying out the method of the present invention, and FIGS. 2A to 2D are views for explaining cold forging incorporating the method of the present invention in the order of steps.
Although the illustrated mold apparatus is shown as being placed vertically, the present invention can also be implemented using a horizontally placed parts former. Since there is no substantial difference in the structure between the vertical and horizontal arrangements, the structure of the vertical mold apparatus will be described below.

[0010] The mold apparatus includes a reinforcing ring 2 on a base 1.
The outside of the base 1 and the reinforcing ring 2 is surrounded by a case 3, and the upper insert 4 is provided inside the reinforcing ring 2.
And the lower insert 5 are arranged separately. Each of the upper insert 4 and the lower insert 5 has a frusto-conical shape or a truncated pyramid shape, is firmly held from the outer periphery by the reinforcing ring 2, and does not rattle up and down during molding.

The upper insert 4 and the lower insert 5 are formed with a cavity 7 (forming space) into which the slab W is charged and into which the punch 6 enters. The bottom surface of the cavity 7 has a flat surface 8 and a curved surface 9. A part of the flat surface 8 is constituted by an upper end surface of the knockout pin 10, and the knockout pin 10 projects into and out of the cavity 7 by the operation of the lifting rod 11 provided in the base 1. The curved surface 9 is formed at the boundary between the bottom surface of the cavity 7 and the inner peripheral surface.

When the diameter of the slab W is D, the radius of curvature r of the curved surface 9 is preferably r = D / 4 or less, and the diameter d of the flat surface 8 is preferably d = D-2r or more. . That is, if the radius of curvature r of the curved surface 9 is too large,
Rather than returning the shape of the slab W to the state before cutting, the slab W becomes closer to the mold having a tapered surface described in the conventional example, and there is a possibility that fogging and galling may occur. Therefore, the curved surface 9
It is preferable that the radius of curvature r and the diameter d of the flat surface 8 be within the above ranges.

Although lubricating oil is applied to the inner peripheral surface of the cavity 7, if the lubricating oil accumulates at the bottom of the cavity 7 during the correction, the correction will be hindered. Therefore, in the present embodiment, the discharge path 12 for the lubricating oil is formed in the mold device. Then, a gap 13 formed between the divided surfaces of the upper insert 4 and the lower insert 5 is formed in the discharge path 1.
2 and the divided surface is provided near the bottom surface of the cavity 7 so that lubricating oil does not remain at the bottom of the cavity 7.

Further, the clearance 13 has a structure in which the cross-sectional area of the flow passage gradually increases toward the outside in the radial direction, so that the lubricating oil is easily discharged.

A procedure for cold forging an exhaust valve of an engine using the mold apparatus having the above configuration will be described with reference to FIG. First, a wire rod of a heat-treated austenitic heat-resistant steel is cut into a slab W having a predetermined size.
And After spraying (applying) a lubricant to the slab W through a nozzle (not shown), as shown in FIG. 2A, the lubricant is set in a cavity 7 defined by the upper insert 4 and the lower insert 5, Straightening (cold forging) with a punch 6. Thus, the corrected slab W shown in FIG. 2B is obtained.

FIG. 3 is a graph comparing the amount of deformation of the shearing slab and the amount of deformation of the straightening slab. The step, longitudinal strain and radial strain of the slab W corrected by the present invention are extremely small as compared with the conventional slab. Has become. It should be noted that FIG. 7 shows where the step, longitudinal distortion and radial distortion indicate.

As described above, a slab W having very little step, longitudinal distortion and radial distortion as compared with the state before cutting is shown in FIG.
As shown in FIG. 2C, the slab W is set in another mold apparatus and is extruded using a punch to obtain a slab W having a tapered portion shown in FIG. 2D. Thereafter, similarly to the conventional example, the intermediate member having the large-diameter portion and the shaft portion is cold-forged using another mold, and the intermediate member is further cold-forged to achieve the purpose. To obtain an exhaust valve.

By the way, when forming a tapered portion in the slab W with the mold apparatus shown in FIG. 2C, as apparent from comparison with FIG. 8, the points a and b of the corrected slab W are simultaneously set. It hits the molding surface of the mold. Accordingly, the compressive stress generated by the punch acts equally on the slab W, and in FIG. 9, the line segment representing the strain at the point a and the line segment representing the strain at the point b overlap or come very close to each other, The entire end will reach the yield point almost simultaneously.

As a result, it is possible to prevent fogging and galling which have become problems when cold forging a high toughness heat resistant steel such as an austenitic heat resistant steel.

FIG. 4 shows another embodiment in which the flat surface 8 of the bottom surface of the cavity 7 is formed by the upper end surface of the knockout pin 10, and the boundary between the bottom surface of the cavity 7 and the inner peripheral surface. Is formed on the lower insert 5 only.

[0021]

According to the present invention as described above,
In cold forging using a material having extremely high toughness (viscosity) after solution treatment, such as austenitic heat-resistant steel, a wire (austenitic heat-resistant steel) is cut into slabs, and then the slab is cut. Since the cold forging is started after performing the straightening process to make at least the shape on one end side close to the shape before cutting, the probability of occurrence of defective products such as fogging and galling is greatly reduced.

Further, according to the present invention, it was conventionally impossible to form a target object by cold forging using a difficult-to-form material such as austenitic heat-resistant steel. Even a difficult-to-form material such as heat-resistant steel can be used to form the target object by cold forging.

[Brief description of the drawings]

FIG. 1 is a sectional view of a mold apparatus used for carrying out the method of the present invention.

FIGS. 2 (a) to 2 (d) are diagrams illustrating a cold forging incorporating the method of the present invention in the order of steps.

FIG. 3 is a graph comparing the amount of deformation of a shearing slab and the amount of deformation of a correction slab.

FIG. 4 is an enlarged sectional view showing another embodiment of the mold apparatus.

FIG. 5 is a diagram illustrating a prior art.

FIG. 6 is a diagram showing a state in which a wire is being cut;

FIG. 7 (a) is a side view of a slab cut from a wire rod,
(B) is an end view of the slab.

FIG. 8 is a view showing a state where the slab shown in FIG. 7 is set in a conventional mold;

9 is a graph showing the relationship between stress and strain at points a and b of the slab shown in FIG.

FIGS. 10A to 10C are diagrams for explaining the drawbacks of the conventional method.

[Description of Signs] 1 ... Base, 2 ... Reinforcement ring, 3 ... Case, 4 ... Upper insert, 5 ... Lower insert, 6 ... Punch, 7 ... Cavity, 8 ... Flat surface of cavity, 9 ... Curved surface of cavity, 10 … Knockout pin, 11… lift rod, 12
... Lubricating oil discharge path, 13... Gap formed between divided surfaces of upper insert and lower insert, W.

Claims (3)

[Claims] 1. A wire made of a high-toughness heat-resistant steel is cut into a predetermined length to form a slab, and at least one end of the slab is subjected to a straightening process close to a shape before cutting, and thereafter, the straightened slab is processed. Cold forging of a high toughness heat-resistant steel by cold forging to the desired shape. 2. The cold forging method for a high toughness heat-resistant steel according to claim 1, wherein the high toughness heat-resistant steel is an austenitic heat-resistant steel. 3. The cold forging method for a high toughness heat resistant steel according to claim 1, wherein the cold forging method is applied to manufacture of an exhaust valve of an engine. Forging method.