Patent Application
20110088864
1. Field of the Invention
The present invention relates to a method of casting a semi-liquid or semi-solid iron-based alloy, and a die for casting, which is used for such casting.
Priority is claimed on Japanese Patent Application No. 2008-155991, filed on Jun. 13, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
There is a die-casting technique as a technique for producing large amounts of metal parts having complicated shapes. This technique is to force molten metal under high pressure into a die to solidify the molten metal, and is effective as a method of producing metal parts having a low melting point, such as aluminum-based alloys and magnesium-based alloys. However, since iron-based alloys have a high melting point and the same iron-based alloy is frequently used as a die material, the die-casting technique has not been widely used to produce iron-based alloy parts.
In recent years, a method has been developed and put into practice which pays attention to the high strength of steel to produce metal parts from semi-liquid cast iron by using the die-casting technique. In addition to this, the development of dies having sufficient durability has been desired. Meanwhile, conventionally, various techniques have been developed to improve the durability of the dies, which are used in die-casting or injection forming, focusing on the use of nonferrous metals.
In particular, Japanese Unexamined Patent Application, First Publication No. 2004-114151 discloses a technique of performing a gas sulphonitriding treatment on the surface of a die to reduce the wettability of the surface of the die with respect to molten magnesium which is provided for die-casting or injection forming to thereby prevent seizure and improve the releasability of a cast product.
In addition, Japanese Unexamined Patent Application, First Publication No. 2001-232443 discloses a method of applying and drying a coating agent including particles such as fluorides, borides, carbides and carbonates, fats and oils, and organic metals on the surface of a die-casting die to form a coating having pores.
Japanese Unexamined Patent Application, First Publication No. 07-303933 discloses a technique of applying and drying a covering material, in which a powder such as an oxide and a fibrous material such as potassium titanate are dispersed in the water including sodium silicate, on the surface of a die for casting and performing a heating treatment for curing to conduct a covering treatment to thereby improve the durability of the die and the releasability of a cast product.
The temperature of semi-liquid cast iron is lower than the melting point of the cast iron. For example, the temperature of semi-liquid cast iron, in which the C content is 2.0%, is in the range of about 1200 to 1270° C., and is significantly higher than the melting point of an aluminum alloy or a magnesium alloy. In addition, the material which is frequently used in a die is die steel which is typified by SKD61. From these circumstances, a die is exposed to an environment, in which wear, seizure between the die and the cast product, cracks caused by thermal shock at the time of contact of the cast product, dissolution into the cast iron, and the like are significantly likely to occur. In fact, normal die-casting dies for an alloy of a low melting point withstand the production of more than ten thousand cast products. However, even when employing the techniques of Japanese Unexamined Patent Application, First Publication No. 2004-114151, No. 2001-232443, and No. 07-303933, current die-casting dies for semi-liquid cast iron have a durability limit corresponding to the production of about a thousand products. The die-casting dies for a semi-liquid cast iron have a short lifespan and an improvement in their durability is thus desired.
The present invention is contrived in view of these problems, and an object of the present invention is to provide a method of casting a semi-liquid or semi-solid iron-based alloy, in which wear at high temperatures in the inner surface of a die and seizure and corrosion caused by cast metal are prevented from occurring, and the durability is improved with good releasability in thixocasting (semi-liquid casting) and rheocasting (semi-solid casting) of an iron-based alloy (such as hypoeutectic cast iron), and a die for casting.
In order to achieve the object, the present inventor has extensively studied the coating material (release agent) which is applied to the die. Through the study, it was found that the following lubricating die-release agent has a large effect on the improvement in the durability of the die for casting an iron-based alloy in a semi-liquid or semi-solid state.
The present invention is achieved based on the above-described finding, and the main points thereof are as follows.
(1) A method of casting a semi-liquid or semi-solid iron-based alloy, the method including: applying, to a part or to the whole of an uppermost surface of an inner surface of a die, a lubricating die-release agent in which particles including at least one selected from molybdenum disulfide, graphite, tungsten disulfide, boron nitride, chrome oxide and boric oxide are dispersed in a solvent; and thereafter casting by using the die.
(2) The method of casting a semi-liquid or semi-solid iron-based alloy according to (1), the method further including: covering a part or the whole of a surface of a base member of the die with a film formed by at least one of spraying of a metal or a cermet, plating of a metal and deposition of a metal nitride or a metal carbonitride, wherein the lubricating die-release agent is applied to a surface of the film.
(3) The method of casting a semi-liquid or semi-solid iron-based alloy according to (1) or (2), wherein the solvent of the lubricating die-release agent is a synthetic ester oil, a silicon oil, a polyglycol, a polyacryl, an aqueous solution of a polyglycol or a polyacryl or an aqueous solution in which a surfactant is added to the aqueous solution.
(4) A die for casting which is used for semi-liquid casting or semi-solid casting of an iron-based alloy, wherein a lubricating die-release agent in which particles including at least one selected from molybdenum disulfide, graphite, tungsten disulfide, boron nitride, chrome oxide and boric oxide are dispersed in a solvent is applied to a part or to the whole of an uppermost surface of an inner surface of the die.
(5) The die for casting according to (4), wherein a part or the whole of a surface of a base member of the die for casting is covered with a film formed by at least one of spraying of a metal or a cermet, plating of a metal and deposition of a metal nitride or a metal carbonitride, and the lubricating die-release agent is applied to a surface of the film.
(6) The die for casting according to (4) or (5), wherein the solvent of the lubricating die-release agent is a synthetic ester oil, a silicon oil, a polyglycol, a polyacryl, an aqueous solution of a polyglycol or a polyacryl or an aqueous solution in which a surfactant is added to the aqueous solution.
Due to the present invention, the durability of a die is improved and roughly ten to twenty thousand cast products or more can be produced by one die without being modified or repaired. In addition, since the effects are exhibited by only applying a lubricating die-release agent between the respective shots, it is possible to reduce the care and replacement time for the die and improve production efficiency. Thus, the quality of a cast product of an iron-based alloy can be stabilized and the production cost can be reduced.
Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings in detail. In the present specification and drawings, constituent elements having substantially the same function and configuration are denoted by the same reference numerals and the redundant descriptions thereof is omitted.
In actual casting, an iron-based alloy casting material is heated to be semi-liquid or semi-solid and is then charged into the injection port 3. The cavity 5 is filled with the material by the plunger 2 via the gate 4, and immediately after that, the die 1 is opened by the separating surface 7 to remove the cast product.
As a material of the base member of the die, die steel which is typified by SKD61, high-speed tool steel, Cr heat-resistant steel, Ni-Cr heat-resistant steel, heat-resistant cast steel, a cemented carbide, a Ni alloy such as inconel 718, or a copper alloy such as copper, Be copper, and Cr—Zr copper is preferably used.
In the present invention, by applying a lubricating die-release agent, having seizure resistance with respect to an iron-based alloy, lubricity and a heat shielding property, to a part or to the whole of an inner surface constituted by the injection port 3 and the cavity 5 of the die 1, the improvement in durability of the die, the improvement in releasability and assurance of product accuracy are achieved.
Preferred materials for the lubricating die-release agent which is applied on the inner surface of the die and characteristics thereof are as follows.
a) Particles consisting of at least one selected from molybdenum disulfide, graphite, tungsten disulfide, boron nitride, chrome oxide and boric oxide of a particle diameter of 2 to 200 μm are dispersed in a solvent.
Among the components of the lubricating die-release agent, these particles act as a filler, a lubricant agent and a release agent, reduce thermal shock caused by the contact of a semi-liquid casting material to the inner surface of the die, and contribute to the reduction in friction and wear occurring by the flow of the semi-liquid material and the improvement in releasability after the completion of the casting. The above material exhibits a solid lubricating action, that is, a friction and wear reducing action without being oxidized or decomposed over short periods of time up to the temperature (in many cases, 1100 to 1400° C.) of a semi-liquid or semi-solid area of the iron-based alloy. When the particle diameter is smaller than 2 μm, the particles are not uniformly dispersed, cannot serve to form a stable film as the filler and are insufficient in thermal shock reducing action.
On the other hand, when the particle diameter is larger than 200 μm, the particles are precipitated in the solvent and are not uniformly dispersed. When the particles are locally agglomerated, the thickness of the film becomes nonuniform or surface roughness increases, so the form accuracy of a cast product is reduced.
b) As the solvent for the lubricating die-release agent, a synthetic ester oil, a silicon oil, a polyglycol, a polyacryl, an aqueous solution of a polyglycol or a polyacryl or an aqueous solution in which a surfactant is added to the aqueous solution, having a kinetic viscosity of 5×10−6 to 5×10−4 m2/s, is used.
An oil-based or water-based material may be used as the material of the solvent. Among oil-based materials, mineral oil is associated with the risk of flammability to require a high casting temperature for an iron-based alloy, so a synthetic ester oil (such as a polyol ester) or a silicon oil which has a high flash point and is a material which undergoes minor evaporation is suitable to use. Among the water-based materials, a polyglycol or a polyacryl is suitable to use. A polyglycol or a polyacryl alone or in an aqueous solution exhibits an appropriate viscosity, and have the characteristic that they are evaporated stably without the occurrence of bumping when exposed to high temperatures. When the viscosity is lower than 5×10−6 m2/s, the adhesion to the inner surface of the die becomes weak, and the evaporation occurs so rapidly that the stable application cannot be performed. When the viscosity is higher than 5×10−4 m2/s, it is difficult to uniformly disperse the particles described in the paragraph a), and nonuniformity in the film thickness or sag occurs at the time of application, so a viscosity higher than 5×10−4 m2/s cannot be applied to the die. In the present invention, the kinetic viscosity is a value which is measured at 40° C. In order to secure the kinetic viscosity in the above range, it is preferable that the concentration of a synthetic ester oil, a silicon oil, a polyglycol, or a polyacryl be equal to or higher than 70% by mass in the case of employing the material in the aqueous solution. When a surfactant is added to the aqueous solution, the kind and the additive amount of the surfactant are not particularly limited. However, from the viewpoint of the increase in dispersibility of solid particles, a nonionic surfactant (such as naphthalenesulphonate) is preferably used and the additive amount of the surfactant is preferably about 0.1% by mass with respect to the water.
The lubricating die-release agent is produced by dispersing the particles described in the paragraph a) in the solvent described in the paragraph b). A mixing amount of the particles in the solvent varies depending on the particle diameter and the viscosity of the solvent, and it is possible to mix the particles in the wide range of 1 to 90% by volume. The thickness of a coating which is formed by applying the lubricating die-release agent mixed in this manner varies depending on the particle diameter, and a preferable thickness is in the range of 5 to 150 μm. The lubricating die-release agent can be applied on the inner surface of the die by being sprayed or brushed over short time periods between the respective shots of die-casting. The lubricating die-release agent may be dried after application. However, in many cases, while the lubricating die-release agent is semi-dried for several to several tens of seconds by the residual heat of the die-casting, the next casting operation can be performed without any problems. As described above, since the solvent described in the paragraph b) is stably evaporated even at high temperatures at the time of the semi-liquid casting of an iron-based alloy, there is no hindrance accompanied with the danger in the operation and the quality of the cast product does not deteriorate. Further, since bubbles are formed in the film when the solvent is evaporated at a high temperature, a further thermal shock reducing effect on the inner surface of the die can be exhibited.
The above-described lubricating die-release agent has an excellent seizure resistance with respect to an iron-based alloy used as a casting material, an excellent lubrication property and an excellent heat shielding property reducing thermal shock with respect to the die base member, and exhibits excellent characteristics and releasability. Accordingly, when any of the above materials is used, a die is obtained having a durability corresponding to the continuous production of at least about ten to twenty thousand non-defective products in the semi-liquid or semi-solid casting of an iron-based alloy.
Preferred examples in which the lubricating die-release agent is applied on the inner surface of the die according to the present invention will be described based on the cross-sectional diagram of the die for casting an iron-based alloy, shown in
Hereinafter, the present invention will be described in more detail by using examples.
The wear resistance and the seizure resistance of surfaces, on which the various lubricating die-release agents have been applied, with respect to an iron-based material in a hot condition were evaluated by using an evaluation device shown in
As the test conditions, the number of rotations was 500 r/m, a sliding rate of the pin and the disk was 0.92 m/s, a pressing load of the pin was 980 N, and an atmosphere temperature was 400° C.
Table 1 shows the results obtained by comparing the wear resistance and the seizure resistance of the various lubricating die-release agents according to the present invention with those of other materials using the above method and evaluating the wear resistance and the seizure resistance. Table 2 shows the components of the lubricating die-release agents used in the evaluation. The wear resistance was evaluated by detaching the disk tested for 30 minutes, observing the cross-section of a sliding surface between the disk and the pin and measuring the amount of thickness loss of the most worn part. The seizure resistance was evaluated by visually checking the presence or absence of the transfer of the material of the tip of the pin in the surface of the disk after the test and performing the cross-sectional observation. As a result of the evaluation, it was confirmed that all the dies of the present invention have an excellent wear resistance and seizure resistance. “Slight wear” in Table 1 indicates the surface property of the disk and expresses that no recognizable recesses, scratches or adhered matters (protrusions) are detected. In Table 2, a formaldehyde condensate of naphthalenesulfonate was used as a surfactant and 0.1% by mass of the surfactant was added with respect to water.
Next, a lubricating die-release agent was applied on an actual test die for die-casting to obtain an aspect of the die of the present invention, and then a semi-liquid iron-based alloy was cast and formed. The shape of a trial cast product is shown in
Table 3 shows the results obtained by casting a semi-liquid iron-based alloy using the die according to the present invention under various conditions and evaluating the shape formability and the durability of the die. The material of the used iron-based alloy is cast iron including C of 2.4% by mass, Si of 1% by mass, and impurities. The shape of the material was a cylindrical shape having a diameter of 50 mm and a height of 50 mm and the preheating temperature for the casting material was 1250° C. The temperature was increased from room temperature to the preheating temperature within 15 minutes and the holding time was 3 to 5 minutes. A hardened and tempered product of SKD61 having a hardness of an HRC of 45 to 47 was used as the material of the base member of the die. The preheating and the heat retention of the die are performed using an electric heater and the temperature of the die before casting was controlled so as to be in the range of 250 to 300° C. in the inner surface of a cavity.
Plural devices for preheating the material were installed and the casting was then started. Actual 1-shot die-casting was completed in 1 to 2 seconds, and the idle time from when a cast product was taken out to when the next material was charged into the die was in the range of about 30 seconds to 5 minutes. The lubricating die-release agent was applied to the inside of the die by being air-sprayed for about 15 to 30 seconds before the start of the next shot. The thickness of the film formed by applying the lubricating die-release agent was in the range of 20 to 150 μm at the time of drying.
The shape formability was evaluated by the inflow thickness of the stair-like part of the cast product. Regarding the durability of the die, the worn state of the inside of the die after casting of a certain number of shots was visually observed. As a result of the evaluation, it was confirmed that all the dies of the present invention, on which the lubricating die-release agent was applied, had excellent wear resistance and seizure resistance, could perform the casting of ten to twenty thousands times without being replaced and had improved shape formability due to the reduction of the wear between the die and the casting material.
Results of Casting and Forming of Iron-Based Alloy
As described above, the preferred embodiments of the present invention have been described with reference to the accompanying drawings. However, needless to say, the present invention is not limited only to such examples. It is evident that those skilled in the technique can conceive various changed or modified examples in the category described in the claims and it is understood that the examples belong to the technical scope of the present invention definitely.
As described above, a die for semi-liquid or semi-solid casting of an iron-based alloy according to the present invention can be widely applied to die-casting from a semi-liquid or semi-solid state of the iron-based alloy. The present invention improves the durability life of the die, prevents the seizure between the material and the die to thereby obtain an effect of promoting releasability, and contributes to the improvement in the casting operation, such as the reduction in production cost, an improvement in the productivity and an improvement in the quality and form accuracy of a cast product.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-155991 | Jun 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/060878 | 6/15/2009 | WO | 00 | 12/9/2010 |
