JP2019119926A - Aluminum alloy-made forging engine piston and a manufacturing method therefor - Google Patents

Abstract

To enable manufacturing a sound Al alloy-made forging piston of which high temperature strength of pin hole peripheral part is improved than conventional forging pistons without generating cracks during forging even with further thining a wall of a part such as a side wall.SOLUTION: There is provided an Al ally-made forging engine piston constituted by a heat treatment type eutectic Al-Si-based alloy, in which 400/mmor more of crystallization article with aspect ratio of 3 or more and longer diameter of 10 μm or more exist in an area in a range of 1 mm from a pin boss hole inside to inside and form a crystallization article network structure, and 30 to 100 circle area with diameter of 10 μm exists as an area having no crystallization article satisfying the above described condition between crystal articles satisfying the condition each other in an 320 μm×230 μm in the area of the area.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an engine piston obtained by forging using a continuous cast rod made of an aluminum alloy as a raw material, and a method of manufacturing the same.

  In recent years, in vehicles of four-wheeled vehicles (hereinafter simply referred to as "automobiles"), adoption of aluminum alloy pistons as engine pistons has been studied for the purpose of improving performance and responding to environmental problems (for example, Patent Document 1). If an aluminum alloy is used, it is possible to reduce the weight of engine driving parts such as pistons, and it is possible to reduce load during operation, improve output, and reduce fuel consumption.

  Conventionally, many cast products have been adopted as engine pistons made of aluminum alloy. However, in cast products, it is difficult to suppress internal defects that occur during casting, and an extra thickness is provided to safely design strength. Therefore, it was difficult to further reduce the weight. Therefore, in recent years, it has been considered to use a forged product for an aluminum alloy engine piston instead of the cast product. Forged products have less internal defects compared to cast products, and therefore thinner engine pistons are possible than when cast products are used, making it possible to achieve further weight reduction.

  The following method is generally used as a method of manufacturing a conventional aluminum alloy forged engine piston. That is, a molten aluminum alloy having a predetermined component composition is melted and formed by a conventional melting method, and the molten aluminum alloy is so-called continuous casting such as continuous casting, semi-continuous casting (DC casting), and hot top casting. The continuous casting and cutting by predetermined length are carried out by any of the above methods to obtain a continuous casting bar. Then, bending correction, peeling (peeling off), etc. are given to a continuous casting rod as needed, and it cut | disconnects to the length suitable for forging, and it is set as a short material (forging material). Then, it is formed into a piston rough shape by forging (generally warm forging), and further heat treatment (generally, T6 treatment (solution treatment-hardening, age hardening treatment) is performed according to the alloy type, and machining Finishing into a piston product shape according to will produce an aluminum alloy forged engine piston.

JP 2001-181769 A

In recent years, awareness of environmental and resource problems has been increased worldwide, and efforts to control CO ₂ have been advanced, and downsizing of automobile engines has been advanced for further reduction in fuel consumption than the present situation. If the automobile engine is downsized, the torque will be reduced and the engine power will also be reduced. Therefore, in order to increase the output, it is considered to increase the number of revolutions of the engine. On the other hand, if the rotational speed is increased, the acceleration of the piston is increased, so that the load applied from the connecting rod and the pictone pin to the pin boss portion supporting the piston pin is also increased, and the piston is easily broken. In order to prevent the load from increasing even if the rotational speed increases, it is effective to reduce the weight of the piston. Therefore, in recent years, a lighter weight and high mechanical strength engine piston at high temperature that can maintain high output at high speed even when the size is reduced (downsized) is required. .

 In order to reduce the weight of the piston, a portion in which a portion not affecting the function as the piston and having a relatively low strength is dug in concavely, that is, a so-called meat theft portion is formed. It is effective to finish the thickness of each part such as the periphery thin.

  However, even if a thin-walled high-temperature forging material is considered to be able to maintain sufficient strength at high temperatures even with a thin wall, the conventional manufacturing method of aluminum alloy forged engine pistons is, in fact, a severe down It has been difficult to obtain a thin, lightweight, high strength forged engine piston to the extent that the sizing requirements can be fully addressed.

  For example, in the engine piston shown in FIGS. 1 to 5 which will be described later again, it is considered to dig in the base portions on both sides of the pin boss portion 4 on the back side of the crown surface 3 to make the meat stealing portion 7 for weight reduction. Be However, in that case, the root portion of the side wall 6 connecting the skirt portion 2 and the pin boss portion 6 is thinned, and as a result, the side wall 6 is easily cracked at the time of forging.

  Here, if the temperature of the homogenization treatment (pre-heat treatment) after casting and the temperature of the preheating of the material before forging are increased, the deformation resistance of the material during forging is reduced, and the wall thickness is reduced, such as sidewalls. It is possible to forge without causing a crack in the part, but in that case, there is concern that the high temperature strength of the piston may be reduced. Accordingly, since it is necessary to avoid raising the temperature of the preheat treatment or preheating to a high temperature, it is not practical to take measures against the increase of the heat treatment temperature.

  Furthermore, when the engine pressure is increased by adding a supercharger to increase the engine speed, the crown pressure of the pin boss portion is transmitted to transmit the explosion pressure (symbol F in FIG. 6) received by the engine piston to the crankshaft. Stress may concentrate on the surface side, and fatigue failure may occur at that site. In particular, when the engine speed is increased, the load on the pin boss where the piston pin connecting the piston and the connecting rod is in contact is greater than that of the conventional one. Therefore, as an engine piston suitable for downsizing of gasoline engine vehicles, it is important to emphasize not only the crown surface where the temperature is the highest but also the strength of the pin boss, especially the high temperature strength around the pin hole. The inventors recognized.

  The present invention has been made against the background described above and, as an aluminum alloy forged piston, improves the high temperature strength and fatigue resistance characteristics of the pin boss portion, particularly the peripheral portion of the pin hole, as compared with the conventional forged piston. An object of the present invention is to make it possible to manufacture a sound piston without occurrence of cracking during forging, even if a portion such as a sidewall is further thinned than conventional.

  In order to solve the above-mentioned problems, the present inventors conducted various experiments and studies, and as a result, not only a so-called heat-treated Al-Si eutectic alloy which can be age-hardened is used as an aluminum alloy for forging. By appropriately adjusting the microstructure of the pin boss portion, particularly the portion corresponding to the surface layer of the inner surface of the pin hole, the high temperature strength and fatigue resistance characteristics of the portion are sufficiently improved, and the above problems are solved. I found that I could solve it. Further, as a method of manufacturing the engine piston, by appropriately regulating the heat history, it becomes possible to make the microstructure of the pin boss portion a structure exhibiting the high temperature strength and the fatigue resistance as described above, and at the same time other The inventors have found that it is possible to reduce the thickness of a portion, particularly a portion where it is desired to reduce the thickness for weight reduction while avoiding the occurrence of cracking during forging. Then, based on these new findings, the present invention has been made.

Therefore, the aluminum alloy forged engine piston according to the basic aspect (first aspect) of the present invention is
In mass%,
Si: 10.5 to 13.5%
Fe: 0.15 to 0.90%,
Cu: 2.5 to 5.5%,
Mg: 0.3% to 1.5%
Ni: 0.8 to 5.0%,
Is made of an aluminum alloy containing Al, and the balance being Al and unavoidable impurities,
In an arbitrary cross section in the surface layer region within a range of 1 mm from the inner circumferential surface of the pin boss hole to the inside of the material, 400 pieces of crystallized materials having an aspect ratio of 3 or more and a major diameter of 10 μm or more In the presence of ² mm or more, the crystallized matter forms a network structure of the crystallized matter distributed in a network,
Moreover, in the arbitrary cross section, in any arbitrary 320 μm × 230 μm rectangular area in the cross section, an area where there is no crystallized product that satisfies the conditions satisfying the aspect ratio and the major diameter. It is characterized in that a circular area with a diameter of 10 μm is 30 or more and 100 or less.

The aluminum alloy forged engine piston according to the second aspect of the present invention is
In the aluminum alloy forged engine piston of the first aspect,
The said aluminum alloy is P0.003-0.02%, Mn 0.10-1.0%, Zr 0.04-0.30%, V 0.01-0.20% by mass% other than the said each component It is characterized in that it contains one or more of Ti of 0.01 to 0.20% and B of 0.002 to 0.04%.

Furthermore, according to a third aspect of the present invention, there is provided a method of manufacturing an aluminum alloy forged engine piston,
The continuous cast bar made of the aluminum alloy having the component composition described in the first or second aspect is subjected to a preheat treatment maintained at 200 to 500 ° C. for 2 to 7 hours as a homogenization treatment,
The material temperature of the forging material at the time of throwing the forging material into the lower die for forging in forming the short material obtained by cutting the continuous casting rod as the forging material is within the range of 380 to 460 ° C. So that the inner surface temperature of the forging lower die is within the range of 380 to 460 ° C. when the forging material before introduction is preheated and the forging material is introduced into the forging lower die Heat the lower mold so that the difference with the material temperature is within ± 10 ° C,
The forged material after forging is solutionized and quenched at a temperature in the range of 480 to 520 ° C., and then subjected to an aging treatment of holding at 170 to 230 ° C. for 1 to 10 hours,
Furthermore, it is characterized by machining to a piston shape.

Furthermore, according to a fourth aspect of the present invention, there is provided a method of manufacturing an aluminum alloy forged engine piston,
The method for manufacturing an aluminum alloy forged engine piston according to the third aspect is characterized in that the solution treatment and the quenching are omitted, and the forged material after forging is immediately subjected to the aging treatment. .

Furthermore, according to a fifth aspect of the present invention, there is provided a method of manufacturing an aluminum alloy forged engine piston according to the third or fourth aspect of the present invention.
In an arbitrary cross section in the surface layer region within a range of 1 mm from the inner circumferential surface of the pin boss hole to the inside of the material, 400 pieces of crystallized materials having an aspect ratio of 3 or more and a major diameter of 10 μm or more In the presence of ² mm or more, the crystallized matter forms a network structure of the crystallized matter distributed in a network,
Moreover, in the arbitrary cross section, in any arbitrary 320 μm × 230 μm rectangular area in the cross section, an area where there is no crystallized product that satisfies the conditions satisfying the aspect ratio and the major diameter. As a result, a forged engine piston having a circular area of 10 μm or more and 30 or more and 100 or less is obtained.

According to the present invention, as the aluminum alloy forging piston, the high temperature strength and fatigue resistance characteristics of the pin boss portion, in particular the peripheral portion of the pin hole, are improved compared to the conventional forging piston, Even if the wall thickness is further reduced, a sound piston can be obtained without the occurrence of cracking during forging.
Therefore, the engine piston of the present invention is most suitable for further downsizing of the forged piston, which aims to improve the fuel consumption by reducing the weight while maintaining the engine output.

It is a bottom view which shows the shape of an example of aluminum alloy forged engine pistons of this invention. FIG. 2 is a front view of the engine piston shown in FIG. 1; It is a vertical side view in the CC line in FIG. It is a longitudinal front view in the AA in FIG. It is a longitudinal front view by the BB line in FIG. FIG. 2 is a schematic view showing in principle an example of a situation when using an engine piston. It is a flowchart which shows an example of the manufacturing method of aluminum alloy forged engine pistons of this invention. FIG. 1 is a schematic view schematically showing a constant temperature forging apparatus as an example of a forging die apparatus used for manufacturing an aluminum alloy forged engine piston of the present invention. It is a longitudinal cross-sectional view which shows an example of the rough profile after a forge in the manufacturing process of the aluminum alloy forged engine piston of this invention. FIG. 10 is a front elevational view in the X-X line in FIG. 9; It is a schematic diagram for demonstrating a crystallized material network structure | tissue. For the forged material according to Example 1 (piston rough surface material), the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 1. For the forged material according to Example 1 (piston rough surface material), the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 2. For the forged material according to Example 1 (piston rough surface material), the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 3. For the forged material according to Example 1 (piston rough surface material), the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization photograph in 4. For the forged material according to Example 1 (piston rough surface material), the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 5. For the forged material (piston rough shape) according to Comparative Example 1, the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 1. For the forged material (piston rough shape) according to Comparative Example 1, the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 2. For the forged material (piston rough shape) according to Comparative Example 1, the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 3. For the forged material (piston rough shape) according to Comparative Example 1, the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization photograph in 4. For the forged material (piston rough shape) according to Comparative Example 1, the visual field No. 1 of the region corresponding to the periphery of the pin hole. It is an organization picture in 5. It is a structure | tissue photograph of the pin hole periphery equivalent site | part about the forged material (piston rough profile material) by Example 2. FIG. It is a structure | tissue photograph of the pin hole periphery equivalent site | part about the forged material by Example 3 (piston rough profile material). It is a structure | tissue photograph of the pin hole periphery equivalent site | part about the forged material (piston rough profile material) by the comparative example 2. FIG.

  Hereinafter, the aluminum alloy forged engine piston of the present invention and the method of manufacturing the same will be described in detail.

First, an outline of an example of the shape of an engine piston according to the present invention will be described with reference to FIGS.
The piston 1 according to the present embodiment is generally formed in a substantially short cylindrical shape so as to be inserted into an engine cylinder (not shown), like a general engine piston, and its outer shape is a part of the cylindrical surface It is formed of a pair of skirt portions 2 having an oval shape and a top wall portion (crown surface portion) 3 closing an end (upper end) of one side of the cylinder in the axial direction. A pair of pin boss portions 4 integrally project (hang) along the axial direction from the crown surface portion 3, and in each pin boss portion 4, a pin hole 5 into which a piston pin (not shown) is inserted is in the axial direction. It penetrates in the orthogonal direction. A plate-like sidewall 6 parallel to the axial direction is provided between the pair of skirt portions 2, and the sidewalls 6 are integrally continuous with the skirt portion 2 and the crown surface portion 3. Each pin boss 4 is integrally continuous with a plate-like sidewall 6 parallel to the axial direction. Therefore, each pin boss portion 4 is reinforced by the sidewall 6.
Furthermore, of the inner surface side of the crown surface portion 3, the corner portion formed by the pin boss portion 4 and the side wall 6 is made a carved-in (recessed) meat-slip portion 7 for weight reduction (see FIGS. 1 and 5) ).

Here, in the case of an engine piston having a piston diameter of, for example, about 50 to 100 mm, the thickness t _a (see FIG. 1) of the sidewall 6 is preferably 2 mm or less for weight reduction. As described above, even in the case of a thin side wall of 2 mm or less, in the case of the present invention, the high temperature strength is higher than that of the conventional aluminum alloy forging piston, so there is a risk of cracking in the side wall 6 Few. However, if it is thinner than 0.5 mm, there is a possibility that a crack may occur, so in practice, it is desirable to set it to 2 mm or less and 0.5 mm or more.
In addition, the thickness t _b (see FIG. 5) of the top wall 3 at the deepest part of the meat theft portion 7 is the thickness (ceiling) of the top wall 3 at the inner side (side including the axis) of the parallel pair of sidewalls 6 The thickness is preferably equal to t _c (see FIG. 4). Here, the ceiling thickness) t _c is generally 2 mm in the case of the piston of the above-mentioned dimension, and the thickness t _b of the top wall 3 at the deepest portion of the thickness theft portion 7 is also 2 mm ± 0.2 mm It is preferable to set it in the range of

For example, as shown in FIG. 6, the engine piston 1 as described above receives the explosion pressure F in the cylinder 10 by the outer surface (crown surface) 3A of the top wall portion (crown surface portion), and At this time, the force is transmitted from the pin boss portion to the piston pin 11 and the connecting rod 12 inserted in the pin hole, and converted into rotational motion by the crankshaft 13.
Here, among the portions of the piston 1, the skirt portion 2 which is rubbed against the inner wall of the cylinder 10 is required to have wear resistance and heat resistance, and the crown surface 3A receiving the explosion pressure F in the cylinder 10 High temperature strength (heat resistance) and high temperature fatigue resistance are required, and high temperature strength, high temperature fatigue resistance and wear resistance are required for the pin boss portion 4 for transmitting the explosion pressure to the piston pin 11. Further, among the pin boss portions 4, in particular, the surface layer on the inner surface of the pin hole 5 is desired to have higher high-temperature strength and fatigue resistance than the other portions. That is, when a supercharger is attached to increase the engine speed to increase the combustion pressure, etc., the rapid and large explosion pressure F received by the crown surface 3A of the engine piston is transmitted to the crankshaft 13. The stress is concentrated on the peripheral portion of the pin hole 5 of 4, particularly the portion on the crown side of the surface layer on the inner surface of the pin hole 5, so that fatigue failure may occur in that portion.

  However, in the case of the conventional aluminum alloy forged engine piston, the surface layer of the inner surface of the pin hole 5 in the pin boss 4 does not necessarily have sufficient high temperature strength and fatigue resistance.

  Therefore, in the piston of the present invention, not only the component composition of the entire material is appropriately adjusted but also the microstructure of the surface layer of the inner surface of the pin hole 5 is particularly suitable as an aluminum alloy piston manufactured by forging. It is supposed to be adjusted.

  The specific shape of the piston is not limited to the shape shown in FIGS. 1 to 5, and it is possible to apply all pistons having a pin hole for inserting a piston pin and rotatably supporting the piston pin. It is.

Basically, the aluminum alloy used in the forged engine piston of the present invention is, by mass%, Si: 10.5 to 13.5%, Fe: 0.15 to 0.90%, Cu: 2.5 to It contains 5.5%, Mg: 0.3% to 1.5%, Ni: 0.8 to 5.0%, and the balance consists of Al and unavoidable impurities.
In addition to the above components, the aluminum alloy used in the engine piston of the present invention further includes P: 0.003 to 0.02%, Mn: 0.10 to 1.0%, Zr: 0.04 to 0. Even if it contains one or more of 30%, V: 0.01 to 0.20%, Ti: 0.01 to 0.20%, B: 0.002 to 0.04% Good. Next, the reasons for limitation of these components will be described.

[Si: 10.5 to 13.5%]
Si has a composition near the Al-Si eutectic point, sufficiently crystallizes eutectic Si during continuous casting, and keeps the eutectic Si in the form of a network even after forging and heat treatment, thereby achieving wear resistance, strength, It contributes to the improvement of heat resistance. When the amount of Si is less than 10.5%, a large amount of primary α-Al crystallizes, and the strength, the wear resistance, and the heat resistance are insufficient. When the amount of Si exceeds 13.5%, coarse primary crystal Si is generated, which becomes a starting point of fatigue failure and causes a decrease in mechanical strength and fatigue strength. Therefore, the Si content is set to 10.5 to 13.5%. More preferably, the amount of Si is 11.0 to 12.0%.

[Fe: 0.15 to 0.90%]
Fe produces an intermetallic compound having a high melting point, and has an action of enhancing mechanical properties in a high temperature range. When Fe is less than 0.15%, in the temperature range of the heat treatment step after forging, the amount of crystallized products generated is small, the network structure is easily divided, and the high temperature characteristics in the high temperature range are insufficient. If Fe exceeds 0.90%, the intermetallic compound becomes coarse, which causes a decrease in formability, a decrease in mechanical strength and a decrease in fatigue strength. Therefore, the Fe content is set to 0.15 to 0.90%. Even within this range, the amount of Fe is preferably 0.20 to 0.50%.

[Cu: 2.5 to 5.5%]
When Cu is contained in the above-mentioned range, there is an effect of improving the strength by precipitating Al ₂ Cu by the aging treatment in the post heat treatment step after forging. If the amount of Cu is less than 2.5%, even if Cu forms a solid solution in the matrix at the solution treatment temperature after forging, the amount of the solid solution is small, so the precipitation of Al ₂ Cu in the subsequent aging treatment is small Therefore, the strength improvement by aging treatment is not enough, and there is a possibility that product strength may run short. On the other hand, if the amount of Cu exceeds 5.5%, a decrease in fatigue strength due to the generation of coarse crystallized substances, a decrease in corrosion resistance and the like are caused. Therefore, the amount of Cu is set to 2.5 to 5.5%. Even within this range, 3.5 to 4.5% is preferable.

[Mg: 0.3 to 1.5%]
When Mg is also contained in the above range, the strength is improved by precipitating Mg ₂ Si by the aging treatment in the post heat treatment step after forging. If the amount of Mg is less than 0.3%, even if Mg forms a solid solution in the matrix at the solution treatment temperature after forging, the amount of the solid solution is small, so the amount of precipitation of Mg ₂ Si in the subsequent aging treatment is There is a possibility that the strength improvement due to the aging treatment is not sufficient and the product strength is insufficient. On the other hand, if the Mg content exceeds 1.5%, the elongation decreases and the processability decreases. Therefore, the amount of Mg is set to 0.3 to 1.5%. The amount of Mg is preferably 0.3 to 1.0% even within this range.

[Ni: 0.8 to 5.0%]
The addition of NI within this range produces an intermetallic compound having a high melting point, and has the effect of enhancing the mechanical properties in the high temperature range. Below this range, the amount of crystallized products formed is small in the temperature range of the post heat treatment step, the network structure is easily divided, and the high temperature characteristics in the high temperature range are insufficient. If this is exceeded, the intermetallic compound becomes coarse and causes a decrease in formability, a decrease in mechanical strength and a decrease in fatigue strength.

[P: 0.003 to 0.02%]
P has the effect of refining primary Si. If the amount of P is less than 0.003%, the effect is small, and if it exceeds 0.02%, oxides of P are mixed in the molten metal, and inclusions adversely affecting fatigue strength increase. Therefore, the amount of P is preferably 0.003 to 0.02%. The amount of P is more preferably 0.003 to 0.01%.

[Mn: 0.10 to 1.0%]
Mn crystallizes as an Al-Mn compound and has an effect of improving heat resistance and wear resistance. If the amount is less than 0.10%, the effect is small, and if it exceeds 1.0%, coarse compounds are crystallized to cause a decrease in fatigue strength and a decrease in plastic formability. Therefore, the amount of Mn is preferably 0.10 to 1.0%. The amount of Mn is more preferably 0.10 to 0.30%.

[Zr: 0.04 to 0.30%]
If Zr is added, the high temperature strength improvement by solid solution strengthening is expected, but the effect is small at 0.04%, while if it exceeds 0.30%, coarse compounds crystallize, the fatigue strength decreases, and the plasticity It causes the deterioration of processability. Therefore, the amount of Zr in the case of adding Zr is preferably 0.04 to 0.30%. The amount of Zr is more preferably 0.04 to 0.15%.

[V: 0.01 to 0.20%]
If V is added, high temperature strength improvement by solid solution strengthening is expected, but if less than 0.01%, the effect is small, while if over 0.20%, coarse compounds are generated, and workability, strength, toughness May decrease. Therefore, the amount of Zr in the case of adding V is preferably 0.01 to 0.20%. The amount of V is more preferably 0.01 to 0.10%.

[Ti: 0.01 to 0.20%]
The addition of Ti is expected to improve the high temperature strength by solid solution strengthening, and works effectively to refine the solidified structure of the aluminum solid solution. If the content is less than 0.01% Ti, the effect is small. If the content exceeds 0.20%, coarse compounds may be generated, and the processability, the strength and the toughness may be reduced. Therefore, the amount of Ti when adding Ti is preferably 0.01 to 0.20%. The amount of Ti is more preferably 0.01 to 0.05%.

[B: 0.002 to 0.04%]
If B is added, it works effectively to refine the solidified structure of the aluminum solid solution. If the B content is less than 0.002, the effect is small. If the B content is more than 0.04%, the Ti-B particles with high hardness may reduce the processability. Therefore, the amount of Ti in the case of adding B is preferably 0.002 to 0.04%.

<Microstructure>
In the piston of the present invention, the tissue in the surface layer area in the range of 1 mm from the inner circumferential surface of the pin boss hole to the inside of the material is defined as follows.

That is, first of all, in an arbitrary cross section in the surface layer region in the vicinity of the pin boss holes, crystallized materials having an aspect ratio of 3 or more and a major diameter of 10 μm or more are 400 pieces / mm ^{2 in} number density. It is necessary for the above-mentioned crystallized matter to form a network structure of the crystallized matter distributed as above.

  Second, in any cross section in the surface layer area in the vicinity of the pin boss holes, a crystallized product satisfying the aspect ratio and the major diameter in any 320 μm × 230 μm square area in the cross section There are 30 or more and 100 or less circular regions with a diameter of 10 μm as a region where there is no crystallized material satisfying an aspect ratio and a long diameter condition among crystallized materials having an aspect ratio of 3 or more and a long diameter of 10 μm or more). It is necessary to.

  Here, the circular region with a diameter of 10 μm under the above second condition is the diameter when the circle is virtually drawn in a region where a spectral ratio is 3 or more and a crystallized material with a major axis of 10 μm or more does not exist. It means an area in which a 10 μm circle can be drawn.

  For example, FIG. 11 shows a virtually circular area of the crystallized network structure. As shown in FIG. 11, when the crystallized product 40 having an aspect ratio of 3 or more and a long diameter of 10 μm or more is present in a network, the crystallized material 40 having an aspect ratio of 3 or more and a long diameter of 10 μm or more in the network. In a region where there is not, a circular region 42 with a diameter of 10 μm is virtually drawn. Reference numeral 43 denotes primary crystal Si.

The above-described first and second conditions are satisfied by filling the texture of the surface layer area within the range of 1 mm from the inner circumferential surface of the pin boss hole toward the inside of the material, thereby achieving a high temperature at the pin boss portion, in particular, around the pin hole. The strength can be significantly improved.
That is, in such a network structure of crystallized products having a large aspect ratio and a long crystallized product, when used as a piston, these crystallized products function as a resistance to the deformation of the aluminum matrix under high temperature. As a result, excellent mechanical strength (high temperature strength) and fatigue resistance can be obtained even at high temperatures exceeding 250 ° C. at the pin hole peripheral portion.

Here, under the above first condition, if the number density of crystallized materials having an aspect ratio of 3 or more and a major axis of 10 μm or more is less than 400 pieces / mm ² , the effect of improving high temperature strength can not be sufficiently obtained. . With regard to the second condition, even if the circular area with a diameter of 10 μm is less than 30, similarly, the effect of improving the high temperature strength can not be sufficiently obtained. Further, with regard to the second condition, if the circular area with a diameter of 10 μm exceeds 100, the effect of improving the high temperature strength can not be sufficiently obtained.

  Such a microstructural condition is desired if the pin boss portion is satisfied with an arbitrary cross section in the surface layer area within the range of 1 mm from the inner peripheral surface of the pin boss hole toward the inside of the material. Although this can be achieved, it goes without saying that the above condition may be satisfied from the inner peripheral surface of the pin hole to the inside of the material to a region exceeding 1 mm.

<Overview of the entire piston manufacturing process>
FIG. 7 shows an outline of an example of a manufacturing process for manufacturing an aluminum alloy forged engine piston, and an example of a method for manufacturing an aluminum alloy forged engine piston of the present invention will be described according to this.

  First, a molten aluminum alloy having the above composition is melted (S1), continuously cast in a rod shape (S2), and the resulting ingot (continuous casting ingot) is cut according to a conventional method, and continuous casting of a predetermined length is performed. Let's be a stick. Here, as a continuous casting method, any of a known hot top continuous casting method, a vertical continuous casting method, a horizontal continuous casting method, and a DC casting method (semi-continuous casting method) can be used.

  Subsequently, pre-heat treatment as a homogenization treatment is performed on the continuous cast bar of a predetermined length (S3). Further, the continuously cast bar after the homogenization treatment is subjected to bending correction (S4) and peeling (S5) as necessary, and then cut into a short material of a predetermined length (raw material for forging) (S6). Further, the surface of the short material is lubricated for lubrication between the die for forging and the forging material (S7). The short material (material for forging) after the lubrication treatment is preheated to a temperature in the range of 380 to 460 ° C. as a temperature suitable for obtaining the engine piston of the present invention by forging as described later. (S8).

  Subsequently, the preheated short material (raw material for forging) is put into a forging device, and forged and formed into a rough piston shape by die forging (S10). Here, in forging, as will be described later, the mold (at least the lower mold) is heated in advance (S9). In this case, it is desirable that the mold heating be performed so that the inner surface temperature of the lower mold (the surface temperature of the molding cavity) becomes equivalent to the surface temperature of the forging material introduced through the preheating. Specifically, it is desirable to set the preheating temperature of the forging material to ± 10 ° C.

A post heat treatment is applied to the forged material (forged material) (S11). The post-heat treatment comprises the steps of heating to a solution treatment temperature (S11A), quenching (S11B), and further aging treatment (S11C).
The post-heat-treated forged material is subjected to machining such as cutting and polishing (S12) to form a piston shape of a product.

  Further, details of each step in the method for producing the forged piston of the present invention will be described.

<Smelting (S1) to Continuous Casting (S2)>
In the aluminum alloy in the vicinity of the Al-Si eutectic composition used in the present invention, when the molten metal solidifies, a large amount of Al-Fe, Cu, and Ni compounds crystallize in addition to eutectic Si eutectic as a crystallized product . In particular, when continuous casting is applied, a cast structure (crystal network structure) in which eutectic Si is crystallized in a network (network shape) is obtained because the solidification speed is high. And, as will be described later, according to the thermal history in the manufacturing method of the present invention, the crystallized product network structure in the continuous casting structure, even in the state after forging and post heat treatment, in the pin boss portion, especially in the peripheral portion of the pin hole. At least a portion thereof remains and contributes to the further improvement of the high temperature strength and fatigue resistance characteristics of the pin hole peripheral portion.

  According to the heat history in the manufacturing method of the present invention, the portions requiring thinning, except the pin boss portion, for example, the side wall, the skirt portion, the crown surface portion, etc. It is possible to reduce the thickness without causing flow and causing cracking during forging. In this case, it is usual that the cast structure is broken due to plastic flow at the time of forging at these portions, but the post heat treatment after forging ensures that the high temperature strength to such an extent required for these portions is secured. It becomes possible.

As a continuous casting method, any of a known hot top continuous casting method, a vertical continuous casting method, a horizontal continuous casting method, and a DC casting method (semi-continuous casting method) may be used.
The ingot drawn from the continuous casting mold is immediately cut by a cutting machine provided in the continuous casting facility and discharged from the continuous casting facility as a continuous casting bar of a predetermined length.

<Pre-heat treatment (homogenization treatment) (S3)>
The continuous cast bar is subjected to pre-heat treatment (S3) as a homogenization treatment. In this pre-heat treatment, the continuous casting bar is heated and held at a temperature in the range of 200 to 500 ° C. for 2 to 7 hours. If the heating and holding temperature is less than 200 ° C., a sufficient homogenization effect can not be obtained, while if it exceeds 500 ° C., eutectic melting may occur. Even if the heating and holding time is less than 2 hours, the effect of homogenization can not be sufficiently obtained. On the other hand, when the heating and holding time is longer than 7 hours, the effect of homogenization saturates, which only impairs productivity and economy. Among the temperatures in the above range, the temperature of the preheat treatment is preferably in the range of 460 to 480 ° C., and the time of the preheat treatment is in particular 4 to 6 hours of the time in the above range. Temperatures in the range of are preferred.

<Curly correction (S4) to peeling (S5)>
The continuous cast bar after the pre-heat treatment is usually subjected to a treatment for correcting the curvature produced by the pre-heat treatment and further removing the surface defects such as irregularities on the surface of the continuous cast bar. These bending corrections and peelings may be performed according to a conventional method.

<Short cutting (S6)>
The continuously cast bar after bending correction to peeling is cut into short pieces (raw materials for forging) of a predetermined length. This short cutting may be performed using a conventional cutting machine.

  The forging material obtained by cutting into a short length may be subjected to the next lubricant coating step (S7) as it is, but in some cases, it is pre-set to forge into a piston rough profile It is good also as a shape suitable for shaping | molding in the forge process (S10) for. The setting in this case may be performed cold or may be performed warmly by heating the forging material to about 380 to 460 ° C.

<Lubrication process (S7)>
The short material (forging material) is subjected to a lubricating treatment for forming a lubricating material layer or a lubricating film on the surface in order to lubricate the die and the forging material at the time of forging. In this lubrication process, as in the case of ordinary die forging, a water-soluble lubricant such as a graphite-based lubricant is applied to the surface of the short material or dipped in a lubricant crucible such as a graphite-based lubricant. The surface may be coated with a lubricant, or a chemical conversion film for lubrication may be formed on the surface by bonding treatment (phosphate treatment) or the like. In addition, when covering with a lubricant, you may heat a short material as needed. The heating temperature in this case is preferably 100 ° C. or less.

<Preheating (S8)>
The short-length material (raw material for forging) after the lubrication treatment is preheated so as to have a material temperature suitable for the subsequent forging. The heating at this time is performed such that the material temperature of the forging material when the forging material is introduced into the mold for subsequent forging is in the range of 380 to 460 ° C. Although the heating time in preheating is not specifically limited, Usually, what is necessary is just about 15 to 60 minutes. That is, it is preferable to set the temperature for 15 minutes or more so that the entire forging material uniformly becomes a temperature within the range of 380 to 460 ° C. On the other hand, heating for more than 60 minutes only impairs economic efficiency and productivity. Therefore, the heating time of the preheating is preferably about 15 to 60 minutes.

<Mold heating (S9)>
Prior to forging, it is desirable to heat the mold, in particular the lower mold into which the forging material is introduced, and it is desirable to heat not only the lower mold but also the upper mold in advance. By preheating the mold in this manner, when the forging material is put into the mold, the temperature (material temperature) of the forging material is prevented from lowering due to contact with the mold surface, Forging at a substantially constant temperature, that is, so-called constant temperature forging is possible.

An example of such a forging apparatus (isolating die apparatus) for constant temperature forging is schematically shown in FIG.
In the mold apparatus 20 shown in FIG. 8, the high frequency induction coil 22A is wound around the lower die 21 in which the molding cavity (forming recess) 21A is formed, and around the upper die 23. The high frequency induction coil 22B is also wound. A copper shielding plate 24 is disposed at an appropriate position of the lower mold 21 and the upper mold 23. In such a mold apparatus, the lower mold 21 and the upper mold 23 are inductively heated by supplying a high frequency current to the high frequency induction coils 22A and 22B from a high frequency power supply (not shown).

Here, the heating temperature of the lower mold is such that the surface temperature (inner surface temperature of the forging cavity) in the lower mold becomes equal to the material temperature of the forging material when the forging material is introduced into the lower mold. It is desirable to set. Specifically, the surface temperature of the lower mold when the forging material is put into the lower mold is in the range of 380 to 460 ° C., and the forging material when the forging material is put into the lower mold It is desirable to heat so that the difference with the material temperature is within ± 10 ° C. When the upper mold is also heated together with the lower mold, the upper mold is heated so that the surface in contact with the forging material in the upper mold (the lower end surface) has the same temperature as the lower mold surface temperature. It is desirable to do.
In the example of FIG. 8, the mold is heated by high frequency induction heating, but the heating means is not limited to high frequency induction heating, and may be heated by a heat exchange method using a heat medium, for example.

<Forging (S10)>
While heating the mold as described above, in particular the lower mold, the preheated forging material is introduced into the lower forging cavity, and forged by closed forging or semi-closed forging, A rough section (forged material) close to the shape of the product (engine piston) to be finally obtained.
When the forging material is put into the mold, as described above, the forging material is preheated to be in the range of 380 to 460 ° C. Also, the mold also has a surface temperature of at least the lower mold of 380 to It is preheated to be within the range of 460 ° C., and the difference between the surface temperature of the lower mold and the material temperature of the forging material is within ± 10 ° C. Therefore, the material temperature is maintained at a substantially constant temperature within the range of 380 ° C. to 460 ° C. from the introduction of the forging material into the mold to the start of forging and the end of forging. Thus, by forging while holding the material temperature in a relatively high temperature range of 380 to 460 ° C., forging is performed in a state where the deformability of the forging material is high (state in which the deformation resistance is small). Become.

  Here, in the state where the deformability of the material is high and the deformation resistance is low, the surface layer portion of the material flows easily plastically at the time of forging, so it is possible to easily form the outer contour shape of the roughly shaped material. This in turn means that little plastic flow is required to occur in the region inside the material.

  An example of the rough-shaped material after forging is shown in FIG. 9 and FIG. As can be understood from FIG. 9 and FIG. 10, in the as-forged shaped material 30, each portion to be the skirt portion 2, the crown portion 3 and the sidewall 6 in the final piston product is any surface region of the material. These portions can be easily formed without causing the occurrence of cracks even if the portion is thin, due to the high deformability as described above. .

  On the other hand, in the as-forged shaped material 30, a portion 4 'corresponding to the pin boss portion 4 to be the pin boss 4 in the final piston product, in particular a portion 5' to be the periphery of the pin hole 5 (a portion surrounded by a dotted line in FIG. The portion corresponding to the periphery of the pin hole) is an internal region in the forged up rough section 30, and the portion 5 'corresponding to the periphery of the pin hole is formed with almost no plastic flow of the material at the time of forging. That is, the deformation amount of the material due to forging is small at the portion 5 'corresponding to the peripheral area of the pin hole. Therefore, in the region 5 'corresponding to the peripheral area of the pin hole, the structure before forging is hardly broken and the ratio of remaining as it is increased. That is, the structure in which eutectic Si derived from continuous casting is crystallized in the form of a network (network) is maintained at a considerable rate even during forging, and a portion corresponding to the pin hole periphery equivalent portion 5 ' , The network structure of eutectic Si will remain.

  If the material temperature during forging is less than 380 ° C., the deformability of the material is small, and cracking tends to occur in thin portions such as the sidewall equivalent portion 6 'during forging, and at the same time the amount of deformation inside the forging material also becomes large. As a result, the internal structure is likely to be destroyed, and it becomes difficult to maintain the eutectic Si network structure of the equivalent region 5 ′ of the pin hole. Therefore, the lower limit of the preheating temperature for the forging material is set to 380.degree. Further, even if the heating temperature of the lower mold is less than 380 ° C., the temperature of the forging material put into the lower mold at the time of forging drops to a temperature lower than 380 ° C., causing the same problem as described above.

On the other hand, if the material temperature at the time of forging exceeds 460 ° C., not only the material may be embrittled and a crack may easily occur at the time of forging, but also the spheroidization of the crystallized material proceeds during forging It becomes difficult to maintain the network structure of eutectic Si in the peripheral equivalent region 5 '. When the heating temperature of the lower mold exceeds 460 ° C., the temperature of the forging material put into the lower mold at the time of forging rises to a temperature higher than 460 ° C., causing the same problem as described above.
Therefore, the preheating temperature of the forging material before forging and the heating temperature of the lower mold are both in the range of 380 to 460 ° C. In addition, the inside of a range is especially preferable in the range of 380-420 degreeC.

  Furthermore, if the difference between the surface temperature of the lower mold and the material temperature of the forging material exceeds ± 10 ° C., the surface temperature of the forging material in contact with the lower mold changes significantly when the forging material is introduced into the lower mold. It will be. At this time, since the surface temperature of the material varies depending on the contact condition with the lower mold, the shape of the contact portion, etc., there is a possibility that the optimum target forging temperature determined according to the component composition of the material can not be secured. . If the above temperature difference is within ± 10 ° C., there is little possibility that such a variation will occur, and it becomes possible to easily secure the optimum target forging temperature.

<Post heat treatment (S11)>
The post forging material (rough shape material) is subjected to solution treatment (S11A), hardening (S11B), and aging treatment (S11C) as post heat treatment (S11).

  The solution treatment temperature is a temperature in the range of 480 to 520 ° C. If the solution treatment temperature is less than 480 ° C., Cu, Mg, etc. contributing to age hardening can not be sufficiently dissolved in solid solution, and as a result, the strength after the aging treatment may not be sufficient. On the other hand, if it exceeds 520 ° C., there is a possibility of local melting. The heating and holding time in the temperature range of 480 to 520 ° C. in the solution treatment is preferably 0.5 to 3 hours. If it is less than 0.5 hours, Cu, Mg and the like which contribute to age hardening can not be sufficiently dissolved, and as a result, the strength after the aging treatment may not be sufficient. On the other hand, if it exceeds 3 hours, not only the effect of solution treatment will be saturated, but also spheroidization of eutectic Si will progress, and it may be difficult to maintain the network structure of eutectic Si derived from continuous casting There is.

  The quenching subsequent to solution treatment is not particularly limited, but it is preferable to rapidly quench at a cooling rate of 10 ° C./sec or more to about 100 ° C. or less (usually from normal temperature to 60 ° C.).

  After quenching, aging treatment is applied. This aging treatment is carried out at 170 to 230 ° C. for 1 to 10 hours. If the aging temperature is less than 170 ° C., the effect of improving the strength by age hardening can not be sufficiently obtained. If the temperature exceeds 230 ° C., over-aging may occur to lower the strength. Further, even if the heat holding time of the aging treatment is less than 1 hour, the effect of improving the strength by age hardening can not be sufficiently obtained. On the other hand, if it exceeds 10 hours, the age hardening does not progress further and impairs productivity and economy. It is only. The conditions for the aging treatment are preferably held at 190 to 220 ° C. for 1 to 10 hours.

  Examples of the present invention will be described below. The following examples are for clarifying the operation and effects of the present invention, and it goes without saying that the conditions described in the examples do not limit the technical scope of the present invention.

Example 1
A molten aluminum alloy having the composition shown in Table 1 is melted according to a conventional method, continuously cast into a round bar having an outer diameter of 84 mm by a gas pressure type hot top continuous casting method, and cut to a length of 5000 m for continuous casting bar did. The continuous cast bar was subjected to homogenization treatment (pre-heat treatment) held at 490 ° C. × 7 hours, and then bending correction and peeling were performed according to a conventional method. Subsequently, it was cut into short pieces of 30 mm in length, and used as a forging material. Then, as preheating before forging, the plate was heated to 400 ° C. for 30 minutes, and forged into a shape of an engine piston rough shape by a constant temperature forging apparatus as shown in FIG. Here, the lower mold and the upper mold in the constant temperature forging apparatus were preheated so that the inner surface temperature was 400 to 450 ° C., and heating was continued even during forging. The temperature of the material during forging was 400 to 450 ° C.
After forging, the forged material (piston rough shape) was subjected to solution heat treatment at 495 ° C. × 3 Hr as post heat treatment, then quenched by water cooling, and 180 ° C. × 6 Hr as aging treatment.

Comparative Example 1
A molten aluminum alloy having the composition shown in Table 1 is melted according to a conventional method, and continuous casting, cutting, pre-heat treatment (homogenization treatment), bending correction, peeling, and short cutting are performed in the same manner as in Example 1. The material for forging was preheated and forged into a shape of a rough piston by a general warm forging device which is not a constant temperature forging device. The die temperature during forging in this case was 250 to 350 ° C., which was much lower than the material temperature (400 to 450 ° C.). The post-heat treatment similar to Example 1 was performed to the obtained forged material (piston rough profile material).

Evaluation Results of Example 1 and Comparative Example 1
The microstructure of the cross section of the piston rough section (forged product) obtained in Example 1 and the piston rough section (forged product) obtained in Comparative Example 1 was observed for each portion after the post heat treatment. In particular, for an area of 200 μm × 230 μm at a portion corresponding to the area from the inner surface of the pin hole to 1 mm in the product piston, the average number density of crystallized materials having an aspect ratio of 3 or more and a major axis of 10 μm or more is examined. The number of circular areas not including 3 or more and crystallized substances having a major axis of 10 μm or more was counted in 5 different visual fields (visual field No. 1 to visual field No. 5), and the presence or absence of a network structure of crystallized substances was determined. . The results are shown in Table 2. Further, each visual field No. 1 to No. The tissue photographs of No. 5 are shown in FIG. 12 to FIG. 1 to No. The structure | tissue photograph of 5 is shown in FIGS. 17-21. Among the regions in the structure picture where the aspect ratio is 3 or more and the crystallized material having a major axis of 10 μm or more does not exist, the circular region is shown by being circled at a place where a circular region of 10 mm in diameter can be drawn.

  From Table 2, in Example 1 according to the present invention, it was confirmed that a network structure of crystallized material was present, and that 30 or more of the above-mentioned circular regions could be drawn for any of the visual fields. On the other hand, in Comparative Example 1, it was confirmed that no network structure of crystallized material was present, and the above-mentioned circular region could not be drawn at 30 or more.

[Example 2, Example 3]
About the aluminum alloy of the component composition shown in Example 2 and Example 3 of Table 3, the process conditions similar to Example 1 were applied, and it was set as the piston rough-formed material (forgings).

Comparative Example 2
About the aluminum alloy of the component composition shown to the comparative example 2 of Table 3, the process conditions similar to the comparative example 1 were applied, and it was set as the piston rough-profile material (forgings).

As in the case of Example 1 and Comparative Example 1, the microstructure of the cross section of the piston rough profile (forged product) was observed for Example 2, Example 3, and Comparative Example 2. In particular, in an area of 320 μm × 230 μm at a portion corresponding to the area from the inner surface of the pin hole to 1 mm in the product piston, the average number density of crystallized materials having an aspect ratio of 3 or more and a major axis of 10 μm or more is examined. The number of circular regions not including 3 or more and having a long diameter of 10 μm or more was found. The results are shown in Table 4.
In addition, for piston rough profiles (forged products) according to Example 2 and Example 3 and Comparative Example 2, a structure photograph of a 200 μm × 230 μm square area of a portion corresponding to the area from the inner surface of the pin hole to 1 mm in the product piston is shown. 22 to 24.

[High temperature tensile test]
Furthermore, for Example 2, Example 3, and Comparative Example 2, a high temperature tensile test specimen conforming to JIS G 0567 is cut out from a portion corresponding to the inner surface of the pin hole in the product piston from the piston rough profile (forged product) A high temperature tensile test at 250 ° C. was performed. The results are shown in Table 4.

As is clear from Table 4, in Examples 2 and 3 according to the present invention, it was confirmed that a network structure of crystallized material was present, and that 30 or more of the above-mentioned circular regions could be drawn. On the other hand, in Comparative Example 2, it was confirmed that no network structure of crystallized material was present, and the above-mentioned circular area could not be drawn at 30 or more.
Further, from the results of the high temperature tensile test, it was confirmed that the high temperature tensile strength at 250 ° C. is clearly higher in Example 2 and Example 3 of the present invention as compared with Comparative Example 2.

  From the above results, if the network structure of crystallized material exists in the pin boss portion, the deformation resistance in the high temperature region is increased at that portion, the high temperature strength is improved, and as a result, the reciprocating motion due to the smaller diameter of the piston pin It is clear that the weight reduction of the department can be realized. In addition, it has been confirmed by a separately conducted test that high-temperature strength at the crown surface can also be higher than that of conventional materials. Therefore, the engine piston of the present invention is required to improve fuel efficiency by weight reduction while maintaining engine power, and is suitable for an aluminum alloy forged piston for downsizing.

  In each of the above embodiments, the side wall is thin (2 mm), but it could be forged as designed without cracking. On the other hand, in each comparative example which made the forging conditions the conventional general condition, the crack generate | occur | produced in the side wall.

  DESCRIPTION OF SYMBOLS 1 ... Piston, 2 ... Skirt part, 3 ... Crown surface part, 4 ... Pin boss part, 5 ... Pin hole, 6 ... Side wall, 7 ... Meat stealing part, 20 ... Forging die apparatus, 21 ... Lower mold, 30 ... Forged material (piston rough material), 40 ... crystallized product 42 ... circular area

Claims (5)

In mass%,
Si: 10.5 to 13.5%,
Fe: 0.15 to 0.90%,
Cu: 2.5 to 5.5%,
Mg: 0.3% to 1.5%
Ni: 0.8 to 5.0%
Is made of an aluminum alloy containing Al, and the balance being Al and unavoidable impurities,
In an arbitrary cross section in the surface layer region within a range of 1 mm from the inner circumferential surface of the pin boss hole to the inside of the material, 400 pieces of crystallized materials having an aspect ratio of 3 or more and a major diameter of 10 μm or more In the presence of ² mm or more, the crystallized matter forms a network structure of the crystallized matter distributed in a network,
Moreover, in the arbitrary cross section, in any arbitrary 320 μm × 230 μm rectangular area in the cross section, an area where there is no crystallized product that satisfies the conditions satisfying the aspect ratio and the major diameter. An aluminum alloy forged engine piston characterized in that a circular area with a diameter of 10 μm is 30 or more and 100 or less.   P: 0.003 to 0.02%, Mn: 0.10 to 1.0%, Zr: 0.04 to 0.30%, V: One or more selected from 0.01 to 0.20%, Ti: 0.01 to 0.20%, and B: 0.002 to 0.04%. Aluminum alloy forged engine piston as described in. A continuous casting rod made of an aluminum alloy having the component composition according to any one of claims 1 and 2 is subjected to a preheat treatment maintained at 200 to 500 ° C for 2 to 7 hours as a homogenization treatment,
The material temperature of the forging material at the time of throwing the forging material into the lower die for forging in forming the short material obtained by cutting the continuous casting rod as the forging material is within the range of 380 to 460 ° C. So that the inner surface temperature of the forging lower die is within the range of 380 to 460 ° C. when the forging material before introduction is preheated and the forging material is introduced into the forging lower die Heat the lower mold so that the difference with the material temperature is within ± 10 ° C,
The forged material after forging is solutionized and quenched at a temperature in the range of 480 to 520 ° C., and then subjected to an aging treatment of holding at 170 to 230 ° C. for 1 to 10 hours,
A method of manufacturing an aluminum alloy forged engine piston characterized by further machining into a piston shape.   The method for manufacturing an aluminum alloy forged engine piston according to claim 3, wherein the solution treatment and the quenching are omitted, and the forged material after forging is immediately subjected to the aging treatment. Method of manufacturing forged engine pistons. A method of manufacturing an aluminum alloy forged engine piston according to any one of claims 3 and 4
In an arbitrary cross section in the surface layer region within a range of 1 mm from the inner circumferential surface of the pin boss hole to the inside of the material, 400 pieces of crystallized materials having an aspect ratio of 3 or more and a major diameter of 10 μm or more In the presence of ² mm or more, the crystallized product forms a network-like dispersed network-like structure,
Moreover, in the arbitrary cross section, in any arbitrary 320 μm × 230 μm rectangular area in the cross section, an area where there is no crystallized product that satisfies the conditions satisfying the aspect ratio and the major diameter. A method of manufacturing an aluminum alloy forged engine piston characterized in that a forged engine piston having a diameter of 10 μm and a circular area of 30 or more and 100 or less is obtained.