JP2003266144A - Cold forging method for cranks - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a cold forging method capable of obtaining hardness, strength and durability required for a crank even if aging treatment after cold forging is abolished. SOLUTION: After continuously forming a billet made of carbon steel by cold forging without intermediate annealing, the temperature is adjusted from a heat generation temperature (250 to 300 ° C.) after forming to normal temperature by a fan or the like to 50 to 70 ° C./hr. Cool at a cooling rate of
By cooling in this cooling rate range, the forged product that has been hardened by cold forging is further aged, and exhibits proper characteristics as a crank.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a split crank of an engine such as a motorcycle by cold forging.

[0002]

2. Description of the Related Art Conventionally, a crankshaft to be incorporated into an engine of a motorcycle or the like has a disk-shaped split type crank with right and left shafts formed, and then pins are fitted into pin holes formed in respective disk-shaped weight portions. The left and right split cranks are connected together.

A split type crank having a large deformation rate from such a material has been conventionally manufactured by hot forging.
However, in hot forging, the die surface is easily worn, resulting in poor precision of the forged product, resulting in a large machining allowance after forging and a reduction in machining efficiency.
Since the number of machines is large due to the large cost of lace processing, the initial investment becomes huge, and scale is generated due to forging after heating, and it is also necessary to apply a release agent etc., so keep the work environment optimal. Therefore, the inventors of the present invention have proposed a method of manufacturing a crank by cold forging, as disclosed in Japanese Patent Laid-Open No. 2001-3117.

The cold forging method disclosed in Japanese Unexamined Patent Publication No. 2001-3117 contains the contents of Si, P, S and Cu, which are elements that adversely affect the deformability of the composition ratio of the billet (carbon steel) as a raw material. Component ratio, which is specifically C (carbon)
Is 0.46-0.48wt%, Si (silicon) is 0.14wt%
% Or less, Mn (manganese) is 0.55 to 0.65 wt%,
P (phosphorus) is 0.015 wt% or less, S (sulfur) is 0.0
15 wt% or less, Cu (copper) 0.15 wt% or less, Ni
(Nickel) is 0.20 wt% or less, Cr (Chromium) is 0.35 wt% or less, and the balance is Fe (iron) and impurities. A billet having such a component ratio is continuously formed without intermediate annealing. After cold forging, it is held at a temperature of 250 to 350 ° C. for 1 to 2.5 hours, and then allowed to cool to room temperature, and then subjected to an aging treatment (tempering).

Regarding the cold forging method, the inventors of the present invention disclosed in Japanese Patent Laid-Open Nos. 2001-1031 and 2001-3135.
JP-A-2001-89810 and JP-A-2001-89810 propose a billet for cold forging and a manufacturing method or processing method thereof.

[0006]

When the crank is manufactured by the cold forging method proposed by the present inventors, the greatest problem of the cold forging is that the deformability is small and cracking is likely to occur. It is possible to solve the problems of molding accuracy, working environment and initial investment while solving the above problem, but since aging treatment after cold forging is indispensable, heating energy must be supplied, and efficiency is improved. Becomes worse.

In many cases, many products after cold forging are accumulated in a pallet even if the above-mentioned aging treatment is not positively carried out. In such a case, the pallet is heated by the heat after the forging process. Since the inside is maintained at about 200 ° C., it may soften.

[0008]

In order to solve the above problems, the method for cold forging a crank according to the present invention is such that a billet made of carbon steel is continuously formed by cold forging without intermediate annealing (the die is changed). After that, the temperature was changed from the exothermic temperature after molding to room temperature at a cooling rate of 50 to 70 ° C./hr. This cooling rate is adjusted by, for example, a fan. With such a configuration, the aging treatment after cold forging (tempering:
The hardness, strength and durability required for the crank can be obtained even if the quenching and tempering) is eliminated.

The billet component ratio is preferably in the following range. C (carbon) is 0.46-0.48 wt%,
Si (silicon) 0.14 wt% or less, Mn (manganese) 0.55-0.65 wt%, P (phosphorus) 0.015 wt%
Below, S (sulfur) is 0.015 wt% or less, Cu (copper) is 0.15 wt% or less, Ni (nickel) is 0.20 wt% or less, Cr (chromium) is 0.35 wt% or less, and the balance is Fe (iron) and impurities. The above component ratio is an improvement based on general JIS S48C (hereinafter simply referred to as S48C) carbon steel as a material for hot forging for crankshafts, and each component ratio is determined by the following standards. . First, C is the element that has the greatest effect on the cold forgeability per unit%,
It is important in terms of mechanical properties, especially material strength and hardenability. That is, the crankshaft as a whole needs to have a predetermined mechanical strength, and locally needs to have a high hardness such as a worm and a tapered portion. Thus, in order to increase the hardness by induction hardening after machining the part where high hardness is locally required, the proportion of C is set to 0.46 to 0.48 wt%. Si is present in the raw pig iron and is mostly removed during the steelmaking process, but it may be added as a deoxidizer at the end of the steelmaking process.
%, Part of which enters steel and forms a solid solution with ferrite,
Since it hinders forgeability, it is preferable to remove it as a cold forging material, and the content was 0.14 wt% or less. Further, Mn remains in the steelmaking process to some extent, but since it is added as a deoxidizer, it is contained in S48C in an amount of 0.60 to 0.90 wt%. This Mn is combined with S and dispersed in the steel as manganese sulfide, and a part is dissolved in the ferrite, but Mn that easily bonds to S becomes MnS, and this MnS easily becomes a starting point of cracks during forging. Therefore, it is desirable to reduce the amount, but Mn dissolved in ferrite makes quenching easier and suppresses the growth of crystal grains. Therefore, 0.55 to 0.
It was set to 65 wt%. Further, when P is solid-dissolved in ferrite, and when it is contained in a large amount, it is combined with part of iron to become iron phosphide, but when P is solid-dissolved in ferrite, the elongation of ferrite comes to be reduced and The impact value at is also reduced and cracks are likely to occur during processing. And this P is S4
In 8C, 0.03 wt% is allowed, and this allowable value is too high as a cold forging material. So 0.015
It was set to wt% or less. Also, S combines with a part of Mn to form MnS.
This MnS becomes the starting point of surface cracks that occur during cold forging, and up to 0.035 wt% is allowed in S48C, but the permissible value is too high as a cold forging material. Therefore, it is set to 0.015 wt% or less. Further, since Cu is less oxidized than Fe when heated at a high temperature, it is enriched on the surface and causes red heat embrittlement. Therefore, approximately equivalent amount of Ni is added to prevent red heat embrittlement. On the other hand, Cu, like P, is considered to increase the ferrite hardness and impair the cold forgeability by containing a small amount, so the content was made 0.15 wt% or less. In addition to the effects described above, Ni is added in the same amount as S48C in order to increase hardenability, prevent low temperature brittleness, and improve corrosion resistance. Further, Cr increases the hardenability and the tempering resistance, enhances the corrosion resistance, and easily forms a stable carbide, so that the same amount as that of S48C was contained.

Further, the carbide in the billet has higher moldability as the spheroidization progresses. The level of this spheroidization can be expressed by the aspect ratio (the aspect ratio of the carbide), and the aspect ratio is preferably 300% or less. If the aspect ratio is 300% or less, the critical upsetting ratio can be 90% or more.

The billet is drawn out from the heating furnace, rolled, and then rapidly cooled to form a fine martensite structure in the surface layer, and then annealed to form the martensite in the surface layer into a fine spheroidal structure composed of ferrite and cementite. Those that are preferable. By the above-mentioned annealing, the inside of the billet was a mixed phase of ferrite and pearlite,
The pearlite is divided and the spheroidization progresses. Therefore,
Both the inside and the surface layer become spherical, and the deformability during cold forging becomes extremely large.

The annealing conditions include, for example, holding the material at about 740 ° C. for 6 hours and then increasing the temperature to about 680 ° C. to 20 ° C.
At a cooling rate of / hr, the furnace is cooled after cooling, or the material is held at about 750 ° C for 4 hours and then at about 735 ° C for 3.5 hours, then to about 680 ° C at 15 ° C / hr. It is conceivable to cool down the furnace at the cooling rate of.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a view for explaining a cold forging process of a crank according to the present invention, in which a spheroidized billet is used as a starting material, and drawing forming, upsetting forming, rough forming, finish forming, peripheral cutting and pin punching. The series of cold forging of the crank is cold forged without intermediate softening treatment.

FIG. 2 is a diagram showing an example of a method for producing a billet as a starting material. A bar material (carbon steel) is pickled and then subjected to a first spheroidizing annealing to obtain cementite. The spheres are made spherical to improve the workability of the entire material so that distortion can be given to the inside as well, and the pearlite is miniaturized. This first spheroidizing annealing is
After holding at 740 ° C. for 6 hours, the temperature was lowered to 680 ° C. at 20 ° C./h, and then the furnace was cooled.

Next, pickling and bonder treatment are carried out to carry out drawing to improve the critical upsetting rate. It is presumed that the reason why the critical upsetting rate is increased by performing the drawing is that by performing the drawing, the austenite grains at the time of annealing are refined and the spheroidizing speed can be increased. In the present example, the cold drawing ratio (reduction of area) was about 20% so that the maximum critical upsetting ratio could be obtained. By the way,
The cold drawing rate (area reduction rate) is a value represented by (D−d) / D × 100 when the diameter before processing is D and the diameter after processing is d.

Next, this bar material was cut into a desired size to form a billet, which was pickled and then subjected to a second spheroidizing annealing to disperse the carbide and increase the spheroidizing rate.
In this example, the second spheroidizing annealing was 75
After the temperature was maintained at 0 ° C. for 2 hours, the temperature was lowered to 730 ° C. at 20 ° C./h, the temperature lowering rate was then lowered to 15 ° C./h to 680 ° C., and then the furnace was cooled.

After completion of the second spheroidizing annealing, shot blasting and bonder treatment were carried out to adjust the surface, to obtain a cold forging billet.

FIG. 3 is a diagram showing another example of a method for manufacturing a billet as a starting material. In this example, first, a material (carbon steel) led out from a heating furnace is rolled by a rolling mill and a cutting shear 4 is predetermined. After being cut into a size and then rapidly cooled through a cooling device, the billet and the wire are separated, the billet is sent to a cooling floor, and the wire is wound.

The billet has a high hardness martensite structure due to its rapid cooling. The billet whose surface layer has a martensite structure is cut, pickled, and then spheroidized.

The annealing condition is that the billet is held at about 740 ° C. for 6 hours and then cooled to about 680 ° C. at a cooling rate of 20 ° C./hr.
The pattern of cooling down the furnace and cooling the billet at about 750 ° C
After keeping the temperature for about 3.5 hours, the temperature was kept at about 735 ° C. for 3.5 hours, and then the temperature was lowered to about 680 ° C. at a cooling rate of 15 ° C./hr. In any of the patterns, there was no difference in the metallographic structure, and the surface layer had a fine spheroidized structure in which the martensite phase was a mixed phase of ferrite and cementite.

FIG. 4 shows that the cooling rate after forging is 25% immediately after forging.
It is a graph which shows the hardness (HRB) change with a temperature fall at the time of 0-300 degreeC to 50-70 degreeC / hr, and the case of natural cooling. As described above, the surface hardness can be increased by controlling the cooling rate within an appropriate range using a fan or the like. In addition, 50 to 70 ° C./hr was set to 50
If it is less than ℃ / hr, the hardness becomes close to that in the air cooling shown in FIG. 4, and if it is less than this, it is not efficient because it takes time, and if it exceeds 70 ° C./hr, the cooling rate is further accelerated. This is because the number of man-hours for aligning workpieces and adjusting the air pressure increases, and efficiency deteriorates.

FIG. 5 is a graph comparing the billet hardness with the hardness (HRB) of the crankshaft obtained by cold forging according to the present invention. From this graph, three positions (φ25.
It can be seen that, in all of No. 3, φ21.3, and φ12.3), the hardness falls within the target hardness required for the crank.

FIG. 6 is a graph comparing the billet hardness with the hardness (HRB) of the disk-shaped weight portion (internal portion, center line and peripheral portion of the pin hole) of the crank obtained by cold forging according to the present invention. From the graph, it can be seen that the inside of the weight portion, the center line, and the peripheral portion of the pin hole are all within the target hardness required for the crank.

[0024]

As described above, according to the present invention,
The temperature of the molded product after cold forging is 250 to 300 due to heat generation.
℃, and from this temperature to room temperature 50-70 ℃ /
By cooling at a cooling rate of hr, the forged product work-hardened by cold forging is further aged continuously and exhibits appropriate characteristics as a crank. Further, according to the present invention, the heat treatment (aging treatment) after the end forming, which is indispensable in the conventional cold forging of the crank, can be omitted, and the grinding process after the forging can be simplified.

Further, by using a billet having a fine spheroidized structure as a material for cold forging, cold forging can be continuously performed to the end without performing softening treatment in the middle. The cost required for the work can be significantly reduced and the working environment can be improved.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a cold forging process of a crank according to the present invention.

FIG. 2 is a diagram showing an example of a method for manufacturing a billet used for cold forging of a crank according to the present invention.

FIG. 3 is a diagram showing another example of a method for manufacturing a billet used for cold forging of a crank according to the present invention.

[FIG. 4] Hardness (HR) accompanying a temperature decrease when the cooling rate after forging is set to 50 to 70 ° C./hr and when naturally cooled.
B) Graph showing changes

FIG. 5 is a graph comparing the billet hardness with the hardness of the crankshaft obtained by cold forging according to the present invention.

FIG. 6 is a graph comparing the billet hardness with the hardness of a disc-shaped weight portion of a crank obtained by cold forging according to the present invention.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/42 C22C 38/42 (72) Inventor Fumio Takeshima 1-10 Sayama, Sayama-shi, Saitama Honda Engineering (72) Inventor Toyoshige Nakata 1500 Hirakawa, Otsu-cho, Kikuchi-gun, Kumamoto Honda Motor Co., Ltd.Kumamoto Manufacturing Co., Ltd. (72) Inventor Hideki Matsuura 1500, Hirakawa, Otsu-machi, Kumamoto-ken Honda Kumamoto Co., Ltd. F term in the factory (reference) 4E087 AA05 AA08 BA02 BA14 CA11 CA22 CA31 CB03 CC01 DA05 DB16 DB24 GA09 HA32 HA36 HA83 HB13 4K042 AA16 BA03 BA05 BA13 CA05 CA06 CA10 DA03 DE02

Claims (4)

[Claims] 1. A billet made of carbon steel is continuously formed by cold forging without intermediate annealing, and then cooled at a cooling rate of 50 to 70 ° C./hr from the exothermic temperature after forming to normal temperature. Crank cold forging method. 2. The method for cold forging a crank according to claim 1, wherein the billet has a C (carbon) content of 0.4.
6 to 0.48 wt%, Si (silicon) is 0.14 wt% or less,
Mn (manganese) 0.55 to 0.65 wt%, P (phosphorus) 0.015 wt% or less, S (sulfur) 0.015 wt%
%, Cu (copper) 0.15 wt% or less, Ni (nickel) 0.20 wt% or less, Cr (chrome) 0.35 wt%
% Or less, with the balance being Fe (iron) and impurities, a method of cold forging a crank. 3. The method for cold forging a crank according to claim 1, wherein the carbide has an aspect ratio of 300% or less in the billet. 4. The method for cold forging a crank according to claim 1 or 2, wherein the billet is drawn from a heating furnace, rolled, and then rapidly cooled to form a fine martensite structure in the surface layer, and then annealed. A method for cold forging a crank characterized in that the martensite in the surface layer has a fine spheroidal structure composed of ferrite and cementite.