JP2012250273A - Method for manufacturing aluminum die casting by cast iron die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an aluminum die casting by a low-cost cast iron die for small-lot production or trial production capable of reducing storage expenditure of old-type dies for mass production.SOLUTION: Molten metal having a composition including 3.0 to 3.7% of C, 2.0 to 3.4% of Si, 0.5 to 1.0% of Mn, 0.02 to 0.20% of P, 0.08% or less of S, 0.3 to 2.0% of Cr, and 0.2 to 0.8% of Mo in terms of mass%, and Fe and inevitable impurities as the remainder is cast into a mold larger than the finishing dimensions of a die and after rough grinding machining, a decarbonized layer including fine voids opening in the surface is formed by thermal treatment. Then, the surface of the cast iron product subjected to the rough grinding machining is further cut, an oily mold-releasing agent is applied to the inner surface of the cast iron die finished to the finishing dimensions of the die, and the cast iron die members are combined to assemble a die having a cavity in a predetermined shape and after the molten metal is injected under pressure into the cavity of the die, the die is released to take the product out of the die.

Description

  The present invention relates to a method for manufacturing an aluminum die cast product using a cast iron mold used for manufacturing a die cast product such as an automobile part.

  For consumer products such as automobiles, as described in the “Die-Casting Industry Vision” (issued by the Japan Die-Casting Association; November 2006), parts of that type are always used even after automobile production is discontinued. Having a sufficient number has too many disadvantages in terms of storage costs and asset efficiency, so the necessary number is produced when demand arises.

  For this reason, old molds used for automobile parts after production stop are stored for a certain period under the control of the manufacturing department. The current situation is that an extremely small quantity of products is produced using an old mold that has been stored according to the order of repair parts.

  On the other hand, in the manufacture of aluminum die cast products, in order to extend the life of the mold, the surface of the mold is covered with a special surface coating layer to protect it, and further, the thermal shock resistance and melting resistance described in Patent Document 1 are used. An aqueous release agent having excellent properties is applied to the inner surface of a mold.

JP 2009-208094 A

  However, the cost burden for storing old molds is increasing, and it has been pointed out that there are disadvantages of storing old molds for a long time even after production is discontinued. In order to eliminate such disadvantages, storage of old molds was stopped even after production of automobiles was stopped, and it was not as durable as prototype molds, but hundreds to thousands of times There is a demand from users to manufacture a mold for producing small quantities that can withstand shots at low cost.

  By the way, under severe conditions that are repeatedly used continuously, the aqueous mold release agent of Patent Document 1 applied to the mold is consumed with a small number of shots, and the die-cast product tends to adhere to the mold. When the adhesion occurs, it is necessary to repair the mold or replace it with a new one even if it is for small-quantity production, which decreases productivity and increases cost.

  The present invention has been made to solve the above problems, and can reduce the storage cost of an old mold for mass production, and can be used for low-cost production or prototype production of low-cost cast iron An object of the present invention is to provide a method for producing an aluminum die-cast product using a mold.

  In the method for producing an aluminum die-cast product according to the present invention, (i) the average composition is mass%, C: 3.0 to 3.7%, Si: 2.0 to 3.4%, Mn: 0.5 to 1.0%, P: 0.02 to 0.20%, S: 0.08% or less, Cr: 0.3 to 2.0%, Mo: 0.2 to 0.8%, the balance being After casting a molten metal composed of Fe and unavoidable impurities into a mold larger than the finishing dimension of the mold, the cast iron product is coarsely ground to leave a predetermined surplus dimension with respect to the finishing dimension of the mold. Then, heat treatment is performed to form a decarburized layer including fine pores that open on the surface, and the cast iron product that has been coarsely ground is further surface-cut to the finished dimensions of the mold, thereby obtaining a desired cast iron mold (Ii) applying an oil-based release agent to each inner surface of the cast iron mold member, and (iii) the cast iron mold member. In combination, a mold having a cavity with a predetermined shape is assembled, (iv) molten metal is injected under pressure into the cavity of the mold, (v) the mold is released, and a product is taken out from the mold. It is characterized by that.

  In the method for producing an aluminum die-cast product using a cast iron mold according to the present invention, an oil-based mold release agent is applied to a cast iron mold member having a decarburized layer including fine pores opening on the surface, and the applied oil-based mold release agent is removed. Since it is impregnated (held) in the pores of the coal layer, the release agent is not deficient from the inner wall of the mold surrounding the cavity, and the die-cast product does not adhere to the surface of the mold.

Process drawing which shows the manufacturing method of a cast iron metal mold member. The cross-sectional schematic diagram for demonstrating the aluminum molten metal erosion property of the cast iron covered with the black skin (early oxide film). The cross-sectional schematic diagram for demonstrating the molten metal erosion property of the cast iron covered with the oxide film (removal of the initial oxide film-> the film after heat processing). The microscope picture which shows the metal structure of the cast iron surface layer part of an Example. (A) is a figure which shows a male type | mold among metal mold | die parts, (b) is a figure which shows a female type | mold among metal mold | die parts. Process drawing which shows the outline | summary of the manufacturing method of the aluminum die-cast goods by a cast iron metal mold | die.

  (1) The manufacturing method of the aluminum die-cast product by the cast iron mold of this embodiment is (i) the average composition is mass%, C: 3.0-3.7%, Si: 2.0-3.4% , Mn: 0.5 to 1.0%, P: 0.02 to 0.20%, S: 0.08% or less, Cr: 0.3 to 2.0%, Mo: 0.2 to 0. A molten metal having a composition containing 8% and the balance consisting of Fe and inevitable impurities is cast into a mold larger than the finishing dimension of the mold, and the cast iron product thus cast has a predetermined surplus dimension with respect to the finishing dimension of the mold. After leaving the coarse grinding process, heat treatment is performed to form a decarburized layer containing fine pores that open on the surface, and the coarsely ground cast iron product is further subjected to surface cutting to finish the finished dimensions of the mold, Thus, each desired cast iron mold member is obtained. (Ii) An oil-based mold release agent is applied to each inner surface of the cast iron mold member. ii) Assembling a mold having a cavity with a predetermined shape by combining the cast iron mold members, (iv) injecting a molten metal into the cavity of the mold under pressure, and (v) releasing the mold The product is taken out from the mold.

  Since the surface of the cast iron product is covered with an oxide film (black skin) in the as-cast state, the graphite is not easily combined with oxygen in the atmosphere and remains in the structure as it is. On the other hand, in the present embodiment, flake graphite in the structure is exposed on the surface of the cast iron product by the rough grinding of the oxide film, and when this is heat-treated, it passes through the exposed portion in the atmosphere in the graphite. Oxygen penetrates and diffuses, the graphite reacts with oxygen in the atmosphere and is consumed, and the disappearance traces of graphite become vacancies.

  The fine pores in the decarburized layer are formed when graphite in the structure reacts with oxygen in the atmosphere by heat treatment, and the graphite consumes. In the cast iron of the component system of the present invention, the shape of graphite crystallized at the time of casting is flaky. In the metal structure of the cast iron product, the flake graphite has a network structure. When the oxide film (black skin) is removed by rough grinding and a part of the flake graphite is exposed on the surface of the cast iron product, oxygen in the atmosphere can penetrate from the exposed portion and freely diffuse in the graphite. become. For this reason, in the present invention, graphite can be consumed in an appropriate time without excessively increasing the holding time in the heat treatment. The disappearance trace of graphite becomes pores, and a decarburized layer (whitening layer) including these many pores is formed. The large number of holes formed in this way are open to the surface and communicate with each other at a deep position when viewed from a microscopic viewpoint. For this reason, when the cast iron product of the present invention is brought into contact with the molten aluminum, oxygen in the pores of the decarburized layer reacts with the molten aluminum to preferentially produce an aluminum oxide film, and a low-melting iron-aluminum alloy (Fe -Al alloy) is prevented. This preferentially produced aluminum oxide coating protects the surface of the cast iron product when the cast iron product is in contact with the molten aluminum and effectively prevents the alloying of iron and aluminum, while the molten aluminum does not After solidification, it is easily peeled off from the surface of the cast iron product together with the aluminum die cast product. By repeating the formation and desorption of the aluminum oxide protective film in this manner, the surface of the cast iron product is effectively protected from the erosion of the molten metal, and its life is greatly extended.

  In the present embodiment, an oil-based mold release agent is further applied to such a cast iron mold member, the applied oil-type mold release agent is allowed to enter into the pores opened on the surface, and the oil-based mold release agent is firmly inserted into the pores. Therefore, it becomes difficult for the die-cast product to adhere to the surface of the mold.

  In addition, the oil-based release agent used in the present embodiment is less likely to evaporate than the aqueous release agent, and the amount consumed per shot is less than that of the aqueous release agent. Requires less than an aqueous release agent, reduces thermal shock to the mold and, as a result, extends the life of the mold.

  (2) In the method of (1), in the step (i), the surplus dimension is preferably 0.2 mm ± 0.02 mm.

  In the present embodiment, since the surplus dimension serving as a machining allowance up to the finish dimension of the mold is taken to a sufficient depth, most of the fine voids in the decarburized layer open to the surface, and the inside of these voids There is an advantage that the penetration amount (impregnation amount) of the oil-based mold release agent into the water increases. If the surplus dimension is less than 0.18 mm, the aperture ratio of the holes is insufficient. On the other hand, if the surplus dimension exceeds 0.22 mm, the aperture ratio of the holes is saturated, and therefore, further cutting processing consumes wasteful labor and time.

  The removal depth of the oxide film (black skin) by rough grinding is preferably at least 500 μm.

  (3) In the method of (1) above, the heat treatment in the step (i) may be performed in a heat treatment furnace in an air atmosphere at a temperature range of 700 to 1100 ° C. for 1 to 20 hours and then cooled in a furnace or air. preferable.

  In the present embodiment, by performing an appropriate heat treatment, carbon serving as a reducing agent for invading oxygen can be consumed, and oxygen enrichment of the base tissue can be promoted. When the holding temperature in the heat treatment furnace is lower than 700 ° C., the cost-effectiveness of graphite is insufficient. On the other hand, when the holding temperature in the heat treatment furnace exceeds 1100 ° C., the high-temperature strength is lowered and softening deformation tends to occur, and it becomes difficult to maintain the original shape of the cast iron product. More preferably, when the heat treatment temperature is in the range of 1050 ± 10 ° C., the dimensional accuracy increases while the cast iron product maintains its original shape.

  Further, if the holding time is less than 1 hour, the effect of consumption of graphite is insufficient. On the other hand, if the holding time exceeds 20 hours, it is uneconomical to continue the heat treatment for a longer time, although the consumption effect of graphite is saturated, because the energy consumption increases. Generally, it is desirable that the holding time is 10 hours or less as an economic effect. The cooling rate after the heat treatment is preferably furnace cooling or air cooling, and can be a rate at which the temperature is lowered to about 300 ° C. in 1 to 8 hours, for example.

  Hereinafter, the reason for limitation of each component of the present invention will be described.

1) C: 3.0 to 3.7%
C is an important component element for crystallization of graphite, which becomes the origin of erosion resistance together with Si. Oxygen from the outside air forms a metal oxide around the de-graphite and constitutes a barrier against molten aluminum alloy. On the other hand, if C as the reducing element is excessive, it absorbs invading oxygen and releases oxygen as CO gas. Therefore, strict management is necessary for the C content. If the C content is less than 3.0%, the melt fluidity is poor and the castability deteriorates. On the other hand, when the C content exceeds 3.7%, coarse primary crystal is crystallized due to a synergistic effect with the high Si content, and the fluidity of the molten metal is lowered even when it is excessively present. Therefore, in the present invention, it is desirable to contain 3.0% or more and 3.7% or less of C.

2) Si: 2.0-3.4%
Si is an important component for securing a graphite eutectic structure while preventing cementite eutectic, and is an extremely important component for improving the resistance to melting loss. Its content is determined by the interaction with Cr. Since the cast iron of the present invention contains 0.3 to 2.0% of Cr, if the Si content is less than 2.0%, it tends to whiten, and the amount of graphite crystallized indispensable for resistance to melting damage Decrease. Moreover, in order to prevent the quenching cementite eutectic which generate | occur | produces on a black skin surface, high Si enough to convert into a graphite eutectic is required. Cementite eutectic is an unhealthy layer that prevents oxygen from entering and impairs melt resistance. This structure not only promotes initial erosion, but also adversely affects subsequent erosion resistance. However, when the Si content exceeds 3.4%, the impact resistance deteriorates and becomes brittle. Therefore, in the present invention, it is desirable to contain 2.0% or more and 3.4% or less of Si.

3) Mn: 0.5 to 1.0%
The content of Mn is determined in the interaction with S. If the Mn content is less than 0.5%, the effect of mitigating the release of oxygen by S entering from the mold cannot be obtained except for the adverse effect of S that hinders the refinement of graphite. On the other hand, if the Mn content exceeds 1.0%, the structure becomes whitened and the amount of graphite crystallized decreases. Therefore, it is desirable to contain 0.5% or more and 1.0% or less of Mn in the present invention.

4) P: 0.02 to 0.20%
In general-purpose gray cast iron, which generally emphasizes mechanical properties, efforts are made to minimize the P content. However, in the case of the cast iron of the present invention, if it contains a trace amount of P, it has the effect of crystallizing at the crystal grain boundary of the austenite or pearlite / ferrite matrix structure to form a phosphorus eutectic and broadening the oxygen entry. However, if the P content exceeds 0.20%, mechanical properties (such as tensile strength and toughness) are impaired. On the other hand, if the P content is less than 0.02%, the above effect cannot be obtained. Therefore, in the present invention, it is desirable to contain 0.02% or more and 0.20% or less of P.

5) Cr: 0.3-2.0%
Cr is an important element that secures the necessary high-temperature strength and increases the resistance to erosion when in contact with the molten metal, and is particularly important as an element having an interaction with Si in terms of erosion resistance. . If the Cr content exceeds 2.0%, the effect of the present invention cannot be achieved because the crystallization amount of graphite tends to be insufficient. On the other hand, if the Cr content is less than 0.3%, the mechanical properties after the heat treatment are significantly lowered. Therefore, the Cr content is preferably in the range of 0.3 to 2.0%. Furthermore, if the upper limit value of the Cr content is narrowed down to 1.5%, a necessary and sufficient amount of crystallization of graphite can be ensured, and the erosion resistance is further improved. In particular, in cast iron containing 0.8% of Cr, it is recognized that the interaction between Cr and Si works effectively and shows excellent resistance to molten aluminum erosion.

6) Mo: 0.2-0.8%
Mo dissolves in the matrix structure, stabilizes fine crystal grains, and promotes oxygen grain boundary penetration. Moreover, Mo is effective as a compensation element for compensating for the decrease in impact resistance due to the high Si content specific to the cast iron of the present invention. In order for the impact resistance compensation effect to appear, it is necessary to add 0.2% or more of Mo. However, if the Mo content exceeds 0.8%, carbide tends to crystallize, and the amount of graphite crystallized decreases. Therefore, the Mo content is preferably in the range of 0.2 to 0.8%.

7) Ni: 0.5-3.0%
Ni is an element that enhances high temperature characteristics (high temperature tensile strength and high temperature corrosion resistance), which are basic characteristics of cast iron. If the Ni content exceeds 3.0%, the castability deteriorates. On the other hand, if the Ni content is less than 0.5%, the effect of improving the high temperature characteristics becomes insufficient.

8) Cu: 0.15-1.5%
Cu is an element that promotes the refinement of graphite. When Cu content exceeds 1.5%, it will become hard and workability will fall. On the other hand, if the Cu content is less than 0.15%, the effect of reducing the graphite becomes insufficient.

9) Other additive elements Ti, N, B, Sn, Pb, Al, Zn, As, and Sb can be added as other elements. Among these, Ti has an effect of refining crystal grains by adding a small amount, so it is preferable to add Ti up to 0.10% or less. In particular, it is preferable to add Ti in the form of alkali titanate because uniform dispersion in the matrix can be achieved.

10) Inevitable impurities Inevitable impurities are inevitably mixed from various elements in the production line such as scrap, and the total amount is desirably less than 0.01%. Inevitable impurities are mixed with various elements such as those harmful to aluminum melt resistance, those harmful to mechanical properties such as high-temperature strength, and those whose effects are unknown. Specific impurities include Li, Be, Na, K, Ca, La, Hf, W, Nb, Ta, Bi, Sr, V, Co, S, O, N, and H. Of these inevitable impurities, S described below is particularly important.

11) S: 0.08% or less S is not limited to those mixed into the molten metal from the melting raw material, and those entering from the mold also impair the refinement of the graphite, so it is desirable to keep it as low as possible. If the S content exceeds 0.08%, the refinement of graphite is hindered. Therefore, the S content is suppressed to 0.08% or less.

  Further, the intrusion S suppresses generation of a metal oxide by Si or the like absorbing oxygen from the atmosphere. All of these results have an adverse effect on the melt resistance of the molten aluminum. Furthermore, S is an element having an interaction with Mn, and there are many papers on the interaction between S and Mn, and it has been established, but consideration must be given to the balance of both components.

12) Melting method The cast iron of the present invention is melted using a high frequency induction furnace. The reason is that the dissolution rate is high and the component adjustment is easy. The cast iron of the present invention requires adjustment of various alloy components, and particularly for S inevitably mixed, the amount thereof needs to be adjusted to a narrow range. Therefore, generally it melts using an induction electric furnace (high frequency induction furnace). In the present invention, the oxygen intrusion in the heat treatment process of the cast product is regarded as important.

13) Processing method (coarse grinding + heat treatment to remove oxide film)
In the present invention, decarburization and oxidation of graphite by heat treatment and oxygen diffusion of the matrix structure are essential. Generally, general-purpose cast iron products are heat-treated in an as-cast state, but only when high resistance to erosion is required as in the case of the cast iron of the present invention, the cast iron products are roughly ground before heat treatment. Remove the oxide film (black skin). The removal depth is preferably at least 500 μm. Rough grinding removes the oxide film (black skin) from the surface of the cast iron product, which causes the cross section of the graphite to be widely exposed on the surface of the cast iron product, and oxygen in the atmosphere enters the graphite in the next heat treatment. Easily spread. As a result, graphite is consumed from the surface to a relatively deep location, and a large number of holes are formed in the surface.

  In the present invention, the surface of the cast iron product is roughly ground, and the surface layer of the cast iron product is removed until a surplus dimension, which will be described later, remains with respect to the finished dimensions of the mold. A machining center or the like can be used for this roughing process.

  The surface-finished cast iron product is kept in a temperature range of 700 to 1100 ° C. in an air atmosphere in a heat treatment furnace for 1 to 20 hours, and then furnace-cooled or air-cooled. The heat treatment consumes carbon as a reducing agent for invading oxygen and promotes oxygen enrichment of the base tissue. Most of these phenomena occur in the heat treatment furnace.

  When the holding temperature in the heat treatment furnace is lower than 700 ° C., the cost-effectiveness of graphite is insufficient. On the other hand, when the holding temperature in the heat treatment furnace exceeds 1100 ° C., the high-temperature strength is lowered and softening deformation tends to occur, and it becomes difficult to maintain the original shape of the cast iron product. More preferably, when the heat treatment temperature is in the range of 1050 ± 10 ° C., the dimensional accuracy increases while the cast iron product maintains its original shape.

  Further, if the holding time is less than 1 hour, the effect of consumption of graphite is insufficient. On the other hand, if the holding time exceeds 20 hours, it is uneconomical to continue the heat treatment even though the consumption effect of graphite is saturated and the energy consumption increases. Generally, it is desirable that the holding time is 10 hours or less as an economic effect. The cooling rate after the heat treatment is preferably furnace cooling or air cooling, and can be a rate at which the temperature is lowered to about 300 ° C. in 1 to 8 hours, for example.

14) Thickness of decarburized layer: 300 μm or more and 1000 μm or less The thickness of the decarburized layer is preferably 300 μm or more and 1000 μm or less. When the thickness of the decarburized layer is less than 300 μm, the aluminum melt resistance is not sufficient, and a desired life cannot be obtained. In order to adjust the thickness of the decarburized layer to a range of 300 μm or more and 1000 μm or less, it is necessary to control the heat treatment conditions within a desired range. Under the above heat treatment conditions, a decarburized layer having a thickness exceeding 1000 μm cannot be generated. Further, even if a decarburized layer having a thickness exceeding 1000 μm is formed, the effect of extending the life may be substantially saturated, which may be uneconomical. The thickness of the decarburized layer is more preferably 400 μm or more and 800 μm or less, and most preferably 500 μm or more and 700 μm or less.

15) Extra dimension: 0.2 mm ± 0.02 mm
In the present invention, the surplus dimension is 0.2 mm ± 0.02 mm. The surplus dimension serves as a cutting allowance up to the finish dimension of the mold during rough grinding before heat treatment. That is, in the final cutting finishing process, in order to remove the oxide film newly generated by the heat treatment from the surface, and to reliably open the pores formed on the surface after the graphite in the cast iron structure is consumed by the heat treatment The surplus dimension is defined as a cutting allowance up to the finish dimension of the mold.

  In the present invention, since the surplus dimensions are taken to a sufficient depth, most of the fine pores in the decarburized layer open to the surface, and the amount of the oil-based mold release agent penetrating into these pores (impregnation amount) ) Has the advantage of increasing. If the surplus dimension is less than 0.18 mm, the aperture ratio of the holes is insufficient. On the other hand, if the surplus dimension exceeds 0.22 mm, the aperture ratio of the holes is saturated, and therefore, further cutting processing consumes wasteful labor and time.

  Incidentally, when a cast iron product is heat-treated at 1050 ° C. for 8 hours, a decarburized layer of about 700 μm is formed. The surplus dimension is preferably kept to a minimum, but the surplus dimension is preferably 0.2 mm ± 0.02 mm in order to compensate for surface oxidation and thermal deformation due to heat treatment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

  First, a method for manufacturing an aluminum die casting mold of this embodiment will be described with reference to FIG.

  Iron scrap is charged into a high-frequency induction furnace with a capacity of 1 ton, and is heated by induction heating and melted. In addition, electrode scraps from the raw material shooter and alloy iron such as ferrosilicon, ferromanganese, and ferrochrome are added. Are put into the furnace to melt them, and the molten metal adjusted to the desired components is melted (step K1). In-furnace analysis is performed, and the molten metal is adjusted to a desired component based on the result. The average composition of the molten metal whose components were adjusted was, by mass, C: 3.0 to 3.7%, Si: 2.0 to 3.4%, Mn: 0.5 to 1.0%, P: 0 0.02 to 0.20%, S: 0.08% or less, Cr: 0.3 to 2.0%, Mo: 0.2 to 0.8%, with the balance being Fe and inevitable impurities is there. However, the amount of Si is reduced by the amount increased by inoculation.

  After adjusting the molten metal to a desired component, a part of the molten metal is poured from a high-frequency induction furnace into a ladle at a tapping temperature of about 1500 ° C. When the hot water was poured into the ladle, the Fe-Si-based inoculant was placed and added. The ladle is conveyed to the casting operation position, and the molten metal is cast from the ladle into the mold (step K2). The mold was a furan sand mold, and a normal alcohol solvent coating mold was applied. The molten metal is poured from a ladle into a mold spout, distributed through a runner to a plurality of cavities, and then solidified into a primary product of a predetermined shape. Note that the dimension of the cavity of the mold is made larger than the finished dimension of the mold in consideration of the shrinkage allowance due to the solidification shrinkage of the molten metal and the allowance due to surface cutting. The cast iron product (test material) as the primary product is covered with an oxide film (black skin) 8 as shown in FIG. 2, and contains fine flake graphite 91 in the structure.

  Next, the cast iron product is roughly ground (step K3). Coarse grinding is performed to a dimension that is larger by a predetermined surplus dimension than the finish dimension of the mold. In this embodiment, the surplus dimension is 0.2 ± 0.02 mm. This is because the cross section of graphite in the structure can be exposed wide by removing the black skin, and oxygen in the atmosphere can easily enter and diffuse into the graphite through the cross section exposed on the surface.

  The coarsely ground cast iron product is charged into a heat treatment furnace and heat treatment is performed (step K4). As the heat treatment specification, the furnace is cooled according to the planned holding temperature and time. Specifically, it is held at a temperature of 700 ° C. or higher and 1100 ° C. or lower for 1 to 20 hours in an air atmosphere in a heat treatment furnace. By such heat treatment, a decarburized layer having a predetermined depth is formed, and graphite existing in the surface layer portion of the cast iron product is consumed, and the disappearance trace becomes a void or a gap. These holes or gaps open to the surface and communicate with each other inside.

  Finally, a predetermined finishing process for removing the surface scale generated by the heat treatment and finishing the cast iron product is performed to finish a cast iron mold member having a predetermined finishing dimension (step K5).

  The cast iron mold members 6a and 6b thus obtained are covered with a decarburized layer 8A as shown in FIG. The decarburized layer 8A includes a large number of pores (or gaps) 81 formed by consuming the graphite 91 in the metal structure 9 by heat treatment. The cast iron mold members 6a and 6b having the decarburized layer 8A including the voids 81 are excellent in resistance to erosion against molten aluminum alloy.

  Next, an embodiment of the present invention will be described in comparison with a comparative example with reference to Table 1 and FIGS.

  The cast iron product specimens A3 and A4 of Examples 1 and 2 shown in Table 1 were prepared using the methods of the above steps K1 to K6, respectively. In Example 1, after removing the black skin of the specimen A3 to a place where the surplus dimension was 0.2 mm ± 0.02 mm by rough grinding, a heat treatment was carried out at 1050 ° C. for 8 hours in an air atmosphere. In Example 2, after removing the black skin of the specimen A4 to the extent that the surplus dimension was 0.2 mm ± 0.02 mm by rough grinding, a heat treatment was performed at 1050 ° C. for 8 hours in an air atmosphere.

  Moreover, the cast iron article test materials A1 and A2 of Comparative Examples 1 and 2 shown in Table 1 were produced using a method in which the above-described step K4 (heat treatment after surface processing) was not performed. In Comparative Examples 1 and 2, the specimens A1 and A2 are not heat-treated.

  In addition, C of the chemical components in Table 1 indicates the total amount of carbon obtained by adding up the carbon solid-dissolved as an alloy component in the base structure and the carbon contained in the graphite. Further, since Comparative Example 1 is a JIS FC250 equivalent product that does not contain Cr and Mo, these components are not shown in Table 1.

<Metal structure of mold>
Next, the metal structure of the example sample of the present invention will be described with reference to FIGS. These metallographic structures were representative of those obtained by observing the sample with three or more fields of view with an optical microscope.

  FIG. 4 is a photomicrograph (magnification: 50 ×) showing the metal structure of an example sample that was heat-treated at 1050 ° C. for 8 hours after removing the black skin.

  When heated in a heat treatment furnace in an atmospheric atmosphere, the growth of graphite causes a crack in the base structure from its tip, and oxygen in the furnace atmosphere enters from here. At the same time, oxygen enters from the boundary between graphite and the matrix structure. Since oxygen absorbs graphite carbon and is diffused as CO gas, graphite is consumed and disappears in the cast iron structure. Holes are formed in the disappearance trace of the graphite. The vacancies further promote the penetration of oxygen into the structure, and an oxide mainly composed of Si is generated in the surrounding area to form a crushed structure. From the figure, it can be seen that the graphite 91 is consumed and disappears or thins and voids (or gaps) 92 are formed. These pores (or gaps) 92 tend to be generated more easily as the graphite 91 is finer.

  On the other hand, the invading oxygen in the matrix structure mainly diffuses along the grain boundaries, but this phenomenon is facilitated by the fine graphite eutectic structure and the matrix microstructure. Si contributes to refinement of the eutectic structure of graphite, and Mo contributes to refinement of crystal grains. Further, P segregates at the crystal grain boundaries to widen the oxygen entrance.

  When heat-treated at a high temperature exceeding the eutectoid transformation point, pearlite is transformed into austenite, and carbon movement is activated. This carbon reduces oxygen that enters from the furnace atmosphere and releases it as CO gas. As a result, a decarburized layer is generated. The invading oxygen after the disappearance of the carbon as the reducing agent generates a metal oxide such as Si, and the intrusion of the molten aluminum resistant metal forms an alumina barrier layer. Note that the austenite structure returns to a low carbon ferrite structure as shown in FIG. 4 at room temperature. In the illustrated sample, a decarburized layer of about 200 μm is formed on a pearlite phase of about 800 μm.

<Manufactured mold>
FIG. 5 shows an example of a small lot automobile part mold.

  FIG. 5A shows the inner surface of a male mold as a cast iron mold member, and FIG. 5B shows the inner surface of a female mold as a cast iron mold member. The male mold 6a is put on the female mold 6b, and the mold is assembled by fastening both using a clamp fitting or a bolt / nut. A cavity having a desired shape and size is formed inside the assembled mold. When a molten aluminum melt is injected into this cavity under pressure by an injection device, an aluminum die-cast product having a desired shape can be obtained.

  In the case of a small lot mold, the mold of the present invention can reduce the manufacturing cost to about 1/3 compared with the old mold (SKD61). Here, the small lot means a product having up to about 3000 products.

<Manufacture of aluminum die-cast products>
Next, the case where an aluminum die-cast product is manufactured using the above-described cast iron mold will be described with reference to FIG.

  The oil-based mold release agent was apply | coated to each inner surface of the cast iron metal mold | die member (a male type | mold and a female type | mold) shown to Fig.5 (a) (b) (process S1). In this example, WFR-33 and WFR-53S manufactured by Aoki Science Laboratory Co., Ltd. were used as oil-based mold release agents. WFR-33 was applied at the initial stage when the shot was started, and WFR-53S was applied from the middle stage to the latter stage when the mold temperature increased.

After applying the release agent, the male mold is placed on the female mold and fixed with a clamp fitting, and the mold is assembled (step S2). Al molten metal is pressurized and injected into the assembled mold at a pressure of about 5.5 to 6.5 MPa (550 to 650 kg / cm ² ) (step S3).

  The clamp fitting is removed, the male mold is detached from the female mold, and the mold is released (step S4). The solidified aluminum die-cast product is taken out from the released mold (step S5).

  An aluminum die-cast product can be manufactured as described above.

<Durability evaluation>
Table 1 shows the results of evaluating the durability (life) in terms of the number of shots (times) that the molds of Examples 1 and 2 and Comparative Examples 1 and 2 were able to withstand use. While the number of shots in Comparative Examples 1 and 2 was 150 and 180, respectively, the number of shots in Examples 1 and 2 increased significantly to 2000 and 3000. That is, the life of the mold of Example 1 was 11 to 13 times that of the comparative example, and the life of the mold of Example 2 was 16 to 20 times that of the comparative example.

From these results, it can be seen that the manufacturing method of the aluminum die cast product using the cast iron mold of this example is extremely superior to the conventional method, and it is confirmed that the life of the mold is greatly extended. I was able to.

  The reason why the durability of the mold of the present invention is greatly improved will be considered.

  The metal mold | die of this invention consists of cast iron in which the decarburization layer was formed in the surface layer part. First, it can be mentioned that the decarburized layer itself has excellent resistance to molten aluminum erosion. As described above, the decarburized layer is exposed on the surface of the mold by removing the black skin (oxide film) on the surface of the cast iron product by rough grinding in the final finishing step before the heat treatment. This decarburized layer is composed of a combination of a specific composition and structure, and is particularly excellent in various characteristics such as thermal shock resistance, molten metal erosion resistance, and adhesion resistance.

  Second, the oil release agent penetrates into the pores formed by the disappearance of graphite by heat treatment, and the oil release agent is stably held in the pores in these decarburized layers, so that the temperature exceeds 600 ° C. It can be mentioned that the surface of the mold is not easily dried even under severe use in contact with a high temperature Al molten metal. In this way, the retention performance of the oil-based mold release agent by the decarburized layer is excellent, making it difficult for the mold release agent to be insufficient or disappearing on the mold surface, effectively preventing the product from sticking to the mold. Is done. As a result, the life of the mold is extended.

  The present invention is suitably used for manufacturing a small-lot aluminum die-cast product, particularly when the old mold is not preserved in a car model that has been discontinued, and there is a demand for repair parts for auto parts of that car model. In addition, the demand can be fully met.

6a ... cast iron mold member (male), 6b ... cast iron mold member (female),
7 ... molten metal (aluminum melt), 8 ... black skin (initial oxide film),
8A ... oxide film (removal of black skin → oxidized film after heat treatment, decarburized layer), 81 ... voids or gaps,
91 ... graphite, 92 ... hole or gap.

Claims (3)

(I) The average composition is% by mass, C: 3.0 to 3.7%, Si: 2.0 to 3.4%, Mn: 0.5 to 1.0%, P: 0.02 to 0 20%, S: 0.08% or less, Cr: 0.3-2.0%, Mo: 0.2-0.8%, the balance is composed of Fe and inevitable impurities and the molten metal having the composition After casting into a mold larger than the finished dimensions of the mold, the cast iron product is coarsely ground leaving a predetermined surplus dimension with respect to the finished dimensions of the mold, and then subjected to heat treatment to form fine voids that open on the surface. Forming a decarburized layer including holes, further grinding the cast iron product that has been coarsely ground to finish the finished dimensions of the mold, thereby obtaining each desired cast iron mold member,
(Ii) applying an oil-based mold release agent to each inner surface of the cast iron mold member;
(Iii) Assembling a mold having a cavity of a predetermined shape by combining the cast iron mold members,
(Iv) Injecting a molten metal into the cavity of the mold under pressure,
(V) A method for producing an aluminum die-cast product using a cast iron mold, wherein the mold is released and a product is taken out from the mold.   The manufacturing method according to claim 1, wherein in the step (i), the surplus dimension is 0.2 mm ± 0.02 mm.   The manufacturing method according to claim 1, wherein the heat treatment in the step (i) is performed by furnace cooling or air cooling after being held in a temperature range of 700 to 1100 ° C. for 1 to 20 hours in a heat treatment furnace in an air atmosphere.

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