JP5756643B2 - Low temperature casting method and low temperature casting apparatus for spheroidal graphite cast iron - Google Patents

Description

  The present invention relates to a low temperature casting method and a low temperature casting apparatus for spheroidal graphite cast iron. More specifically, the present invention relates to a low-temperature casting method and a low-temperature casting apparatus for spheroidal graphite cast iron that is high-strength and does not cause defects by precision casting using a mold and casting in a low-temperature region including a semi-solidified state.

Since cast iron contains graphite, it is excellent in castability, wear resistance, etc., but its mechanical strength is governed by the amount, shape, distribution, etc. of graphite. Conventionally, in order to refine and uniformly distribute graphite contained in cast iron, processing such as stirring and addition of an alloy has been performed to improve the mechanical strength of cast iron.
For example, stirring is performed in a semi-solid state in the cooling and solidification process of a melt made of hypoeutectic cast iron, and primary graphite and eutectic structure crystallized at that time are pulverized and dispersed. A method for producing cast iron is disclosed in which it is continuously carried out to a temperature range of -C-Si ternary eutectic or slightly higher, and then immediately cooled (see Patent Document 1). According to this manufacturing method, hypoeutectic cast iron excellent in mechanical strength and wear resistance is said to be obtained.

  In addition, cooling a molten metal such as magnesium while stirring and casting in a semi-solid state yields a more uniform microstructure than casting from a molten state, so various processing technologies including the rheocast method have been proposed. Has been. As an example of applying such a semi-solid casting method to cast iron, molten cast iron is poured onto an inclined cooling plate, cooled to a semi-solid state, cast directly by casting it into a mold, and excellent in tensile strength. A casting method for obtaining an elongated cast iron casting is disclosed (see Patent Document 2). According to this casting method, it is said that a large number of fine primary crystal crystallization nuclei are generated by the cooling by the inclined cooling plate and the hot water flow movement, and the crystal is refined. And it is said that by applying pressure immediately after casting, cooling is promoted and the crystal structure being solidified is destroyed, and further refinement of the crystal can be achieved.

  It is also known that casting cast iron into a mold in a semi-solid state can reduce the volume shrinkage due to thermal expansion during cooling and suppress the occurrence of defects such as sink marks. . However, when using a low-temperature cast iron melt, there is a problem that the flowability of the molten metal decreases during casting. Furthermore, in the case of a conventional casting method using a vanishing model, there is a problem that the hot metal flowability is impaired by the gas generated by the decomposition of the foamed resin of the vanishing model during casting. In order to solve this, a manufacturing method is disclosed in which a low temperature (1250 ° C. to 1330 ° C.) molten metal is poured into a sand mold and the mold is decompressed (see Patent Document 3). It is said that the cracked gas is sucked by the reduced pressure and the flowability of the molten metal is improved, so that the molten metal casting temperature can be lowered in the disappearance model casting method to produce a casting having no defect. In addition, it is said that the cooling rate can be increased because pressurized air flows inside the mold by pressurizing the mold after depressurizing.

Japanese Patent Laid-Open No. 2001-276862 JP 2006-122971 A JP 2006-175492 A

As described above, conventionally, many processes have been performed in which a cast iron melt is granulated by adding a metal or the like to a cast iron melt or inoculating the mixture and stirring. Moreover, keeping the cast iron melt close to the liquidus temperature close to the solidification temperature and uniformly dispersing the uniform solidification nuclei that are granulated in a semi-solid state leads to the formation of uniform axis crystals, It is known academically to increase strength.
Among cast iron, spheroidal graphite cast iron (ductile cast iron) that deposits graphite in a spherical shape in the cast iron structure is excellent in tensile strength and elongation, and can realize strength and toughness comparable to steel. It is used for such as. As is well known, this spheroidal graphite cast iron is manufactured by adding magnesium Mg or a Mg alloy into the molten cast iron (hereinafter referred to as “spheroidizing treatment”), and the added Mg, etc. This promotes the formation of spheroidal graphite.
However, adding a special metal or the like to the cast iron (alloying) for the purpose of increasing the strength of the spheroidal graphite cast iron does not save resources and costs. Moreover, in order to alloy, it is necessary to melt | dissolve at high temperature, and the energy basic unit is also high. Therefore, it is required to refine the spherical graphite and the matrix structure and obtain a uniform distribution without using a special metal or the like.

When casting spheroidal graphite cast iron, it is possible to refine and homogenize the molten metal from the melted state to the semi-solidified state (mechanical stirring or electromagnetic stirring) as in the previous example. However, in order to continuously cast as it is, it is necessary to control the structure and temperature distribution during pouring and molding. Further, even when processing using an inclined cooling plate is to be performed, practical control and management from casting to molding processing is difficult in the casting line. In order to increase the strength of spheroidal graphite cast iron, it is necessary to comprehensively control not only the molten state of cast iron but also the structure state and temperature distribution from casting to cooling.
In particular, cast iron has a narrow semi-solid temperature range (solid-liquid coexistence range) and is a treatment in a high temperature range, so it is difficult to control the temperature and the uniform dispersion of the solid phase in the semi-solid state. Therefore, it was an important issue for making stable castings to control the temperature and granulation at the time of molding processing to have a uniform distribution, including the material and structure of the mold.

In addition, since casting is usually performed under atmospheric pressure, there is a natural limit to nest suppression and transferability. If cast iron is poured into the mold at a low temperature, the occurrence of defects such as sink marks can be suppressed as described above. However, when trying to cast a spheroidal graphite cast iron melt at a low temperature (for example, 1300 ° C. or less), there is a problem that defects in the outside and inside of the casting occur due to poor molten metal flow and poor gas flow. there were. For this reason, in order to cast the spheroidal graphite cast iron in a low-temperature region including a semi-solid state, improvement of hot metal flowability has been an important issue.
Note that the conventional disappearance model casting method (see Patent Document 3) that uses a sand mold to perform low-temperature pouring and decompression has a problem that it takes a long time to open the frame even if forced cooling is performed by pressurization. . Further, the same solution means as in casting using a sand mold cannot be applied to precision casting using a mold aimed by the present invention. This is because the cooling rate of the molten metal poured into the mold is several tens of times faster than that of the sand mold, and the time in the semi-solid state is remarkably short.

  The present invention has been made in view of the above-described situation, and provides a low-temperature casting method and a low-temperature casting apparatus for spheroidal graphite cast iron having high strength comparable to forging and causing no defects by precision casting using a mold. The purpose is to do.

The present invention is as follows.
1. A vacuum treatment step for maintaining the molten cast iron after the graphite spheroidization treatment at a predetermined degree of vacuum for a predetermined time, and after performing the vacuum treatment step, the liquid phase temperature of the cast iron to the temperature range of 1350 ° C. A pouring step of injecting the molten metal into the mold within a predetermined time, and after the injection of the molten metal, the molten metal in the mold was filled in the cavity of the mold with reference to a temperature at which the predetermined solid fraction is reached . And a pressurizing step of pressurizing the entire cast iron melt . A low-temperature casting method for spheroidal graphite cast iron, comprising:
2. The pressurizing step starts pressurization when the temperature of the molten metal in contact with the inner surface of the cavity reaches a predetermined temperature equal to or lower than a liquidus temperature. A low-temperature casting method of the described spheroidal graphite cast iron .
3 . In the vacuum treatment step, the molten metal is maintained for 30 seconds to 150 seconds at a vacuum degree of 10 ⁻³ MPa to 10 ⁻¹ MPa. Or 2. Cold casting method of spheroidal graphite cast iron according to.
4 . 1. The inoculation step of adding 0.001 to 0.05% by mass of an inoculum to the molten metal after performing the vacuum treatment step. To 3. A low-temperature casting method for spheroidal graphite cast iron according to any one of the above.
5. In the pressurizing step, a displacement amount due to pressing of the movable part of the mold is measured, and the load of the pressing is changed in accordance with the displacement amount. To 4. A low-temperature casting method for spheroidal graphite cast iron according to any one of the above.
6). Wherein the pressurizing step, the at high speed quenching the molten metal in the cavity 20 ° C / sec or more cooling rate in a semi-solidified state 1. To 5. A low-temperature casting method for spheroidal graphite cast iron according to any one of the above.

7). A vacuum processing apparatus for maintaining the molten cast iron after performing the spheroidizing process at a predetermined degree of vacuum for a predetermined time, and the molten metal is received from the vacuum processing apparatus, and a liquid phase temperature of the cast iron to a temperature of 1350 ° C. A pouring device for injecting the molten metal in a range into the mold within a predetermined time; and after the molten metal is injected into the cavity of the mold on the basis of a temperature at which the molten metal in the mold has a predetermined solid phase ratio And a pressurizing device that pressurizes the entire molten cast iron filled, and a low-temperature casting device for spheroidal graphite cast iron.
8). 6. The pressurizing device starts pressurization when the temperature of the molten metal in contact with the inner surface of the cavity reaches a predetermined temperature equal to or lower than a liquidus temperature. The low-temperature casting apparatus for spheroidal graphite cast iron as described.
9 . The said vacuum processing apparatus hold | maintains the said molten metal for 30 second-150 second in the vacuum degree of 10 < ^-3 > MPa-10 < ^-1 > MPa. Or 8. Cold casting apparatus spheroidal graphite cast iron according to.
10. The pressure device measures the amount of displacement caused by the pressing of the movable part of the mold, and changes the pressure load according to the amount of displacement. Thru 9. The low-temperature casting apparatus for spheroidal graphite cast iron according to any one of the above.
11. 6. Copper or copper alloy is used for a portion constituting the cavity of the mold , and the molten metal in the cavity is rapidly quenched at a cooling rate of 20 ° C./second or more in a semi-solid state . To 10. The low-temperature casting apparatus for spheroidal graphite cast iron according to any one of the above.

According to the low-temperature casting method of spheroidal graphite cast iron of the present invention, the method comprises a vacuum treatment step for maintaining the molten cast iron after performing the spheroidizing treatment at a predetermined degree of vacuum for a predetermined time, so that the spherical graphite in the molten metal is uniformly distributed. It can be made finer, and the molten metal can be deoxidized and the oxide can be removed. These make it possible to increase the hot water flowability. And since it has a pouring process of pouring molten metal of spheroidal graphite cast iron that has been subjected to a vacuum treatment process into a mold within a predetermined time in a state of 1350 ° C. or lower and a liquid phase temperature or higher, in a wide temperature range in a low temperature range. Can respond. In addition, since the molten metal whose flowability is improved by vacuum treatment is instantaneously poured into the mold, the molten cast iron filled in the cavity (mold space) can be uniformly distributed with a preferable solid phase ratio. it can. Furthermore, after the injection of the molten metal, to provide a pressurizing step of pressurizing the entire cast iron molten metal filled in the cavity of the mold on the basis of the temperature at which the molten metal in the mold has a predetermined solid phase ratio , The molten metal that has become a semi-solid state in the cavity is rapidly cooled together with pressurization, and the spheroidal graphite cast iron structure can be refined by pressurization and rapid quenching effects. This low-temperature casting method combines the above vacuum processing, instantaneous pouring, and the entire pressurization process. Therefore, using low-temperature spheroidal graphite cast iron melt, it has high strength and has internal defects such as external defects and shrinkage cavities. It becomes possible to form no castings. The low temperature casting performed in accordance with the present invention is a precision casting that links cast iron casting to the technical field of forging.

In the vacuum treatment step, when the molten metal is maintained at a vacuum degree of 10 ⁻³ MPa to 10 ⁻¹ MPa for 30 seconds to 150 seconds, the cast iron molten metal is reformed (refined granular graphite) without impairing productivity. -Uniformity, deoxidation, removal of oxides, improvement of hot water flow) can be sufficiently achieved.

  After performing the vacuum treatment step, when further comprising an inoculation step of adding 0.001 to 0.05 mass% of the inoculum to the molten metal, since the molten metal is modified by the vacuum treatment, By using much less inoculum (about 1/10 to 1/100) than the current inoculation, the structure of the cast iron melt can be made finer and uniform, and the strength of the cast product is increased. Can be achieved.

The pressurizing step is preferably performed in accordance with the cooling characteristics of the molten metal in the cavity when the pressurization is started when the temperature of the molten metal in contact with the inner surface of the cavity reaches a predetermined temperature equal to or lower than the liquidus temperature. Pressurization can be performed at the timing when the solid phase ratio is reached.
In the pressurizing step, when the amount of displacement due to pressing of the movable part of the mold is measured and the load of the pressure is changed in accordance with the amount of displacement, the pressure load is increased when the molten metal in the cavity is solidified. Therefore, the load on the pressurizing device can be reduced, and a compact and low-load pressurizing device can be used.
In the pressurizing step, when the molten metal in the cavity is rapidly quenched at a cooling rate of 20 ° C./second or more in a semi-solid state, the high-strength effect comparable to the conventional semi-solid casting method due to the rapid quenching effect. It becomes possible to plan. A sufficient rapid quenching effect can be obtained by using copper or a copper alloy in the portion constituting the cavity of the mold.

  According to the low-temperature casting apparatus for spheroidal graphite cast iron of the present invention, it is possible to realize a low-temperature casting apparatus that is suitable and practical for exhibiting the effects of the low-temperature casting method.

It is a figure showing the structural example of the low temperature casting apparatus for performing the low temperature casting method of spheroidal graphite cast iron. It is a graph which shows the temperature change (cooling curve) of the cast iron in a metal mold cavity. It is a graph showing the example which measures the displacement amount by the press of the movable part of a metal mold | die in a pressurization process, and changes the load for a press according to the displacement amount. It is a longitudinal cross-sectional view explaining the structure of the principal part of a metal mold | die and a pressurization apparatus. It is a disassembled perspective view of the upper mold | type, lower mold | type, and intermediate mold | type which comprise a metal mold | die. It is sectional drawing showing the state by which the cast iron molten metal was filled into the cavity of the metal mold | die. It is an image which shows the shape of the cap component of an Example. It is an image showing the macro structure of the cross section of the cap component of an Example. It is an image showing the microstructure of the casting of an Example, (a) is distribution of the spherical graphite in a structure | tissue, (b) is a structure | tissue after an etching. It is an image showing the microstructure of the casting of a comparative example, (a) is the distribution of spheroidal graphite in the structure, (b) is the structure after etching.

  The items shown here are for illustrative purposes and exemplary embodiments of the present invention, and are the most effective and easy-to-understand explanations of the principles and conceptual features of the present invention. It is stated for the purpose of providing what seems to be. In this respect, it is not intended to illustrate the structural details of the present invention beyond what is necessary for a fundamental understanding of the present invention. It will be clear to those skilled in the art how it is actually implemented.

  The low-temperature casting method and low-temperature casting apparatus for spheroidal graphite cast iron according to the present invention include a vacuum treatment of a spheroidal graphite cast iron melt, an instantaneous pouring of a molten metal in a low temperature range after the vacuum treatment, Casting with high strength and no defects such as shrinkage cavities using a melt of low-temperature spheroidal graphite cast iron, including semi-solidified zone, through a process that combines the pressurization of the molten metal filled in the mold cavity Product.

The “spherical graphite cast iron” is one in which graphite is spherical in the cast iron structure, and the spheroidizing method is not particularly limited. For example, a known method of adding magnesium (Mg) or the like to the molten cast iron can be used. In this low-temperature casting, the cast iron melt that has been subjected to the spheroidizing treatment is subjected to vacuum treatment.
The molten metal can be modified by subjecting the cast iron melt that has been subjected to the spheroidizing treatment to a vacuum. That is, the molten metal is deoxidized and the oxide is removed, and the spherical graphite and the matrix structure are made uniform and refined. Since the melt of the spheroidal graphite cast iron that has been vacuum-treated in this way has good hot-water flow up to the vicinity of the liquidus temperature, casting in a low temperature range is possible.

The “low temperature casting” means that a molten metal having a temperature range higher than the liquid phase temperature and close to the liquid phase temperature (for example, 1350 ° C. or lower and higher than the liquid phase temperature) is poured into the mold, from the liquid phase temperature to the eutectic temperature. It is intended to mold spheroidal graphite cast iron in the temperature range. That is, the low temperature casting here includes casting in a temperature range in a semi-solid state (semi-solid casting). The semi-solid state means a coexistence state of a solid phase (solid phase) and a liquid phase.
The temperature range of spheroidal graphite cast iron for performing semi-solid casting is estimated from the liquid phase temperature to the eutectic solidification completion temperature (eutectic temperature) on the iron-carbon-silicon based metal phase diagram. In the actual measurement example, the liquid phase temperature is 1185 ° C. and the eutectic temperature is 1111 ° C., and a solid-liquid coexistence state is obtained in the temperature range of 74 ° C. therebetween. From the liquid phase temperature to the eutectic temperature, the ratio of solidified phase (solid phase ratio) increases from 0% to 100%.
In general, the ratio of the solidified phase can be approximately calculated based on the Sile equation. In the above example, the temperature at which the solid fraction is 30% is calculated as 1141 ° C. Then, in order to perform semi-solid casting with a target solid phase ratio of 30%, it is required to control to a uniform temperature in a narrow temperature range of 44 ° C. from a liquidus temperature of 1185 ° C. to 1141 ° C., Conventionally, the control has been extremely difficult.

  In this low temperature casting, the spherical graphite and the matrix structure are made uniform and refined by the vacuum treatment, and the flowability of the molten metal is improved. Therefore, from a relatively high temperature (for example, 1350 ° C.) to the liquid phase temperature. A uniform distribution of the molten metal can be obtained in the cavity (mold space) by instantaneously pouring the mold in a wide temperature range. Then, the molten metal filled in the cavity can be pressed and molded at a predetermined temperature ranging from around the liquidus temperature to around the eutectic temperature where the solid phase ratio is high. For example, pressurization based on a state where a target solid phase ratio (for example, 30%) is used as a reference causes the axial phase and primary dendrite, which are granulated in a solid-liquid coexisting state, to be finely divided in the liquid phase of the cast iron melt. It can be crystallized in a wet state. With this semi-solid molten metal, a high-strength casting formed by the distribution of fine axons and finely divided dendrites is cast.

The cast iron filled in the mold cavity is rapidly cooled and solidified by pressurization and a mold having high thermal conductivity. By rapidly cooling (for example, at a cooling rate of 20 ° C./second or more), it is possible to increase the strength of cast iron comparable to that of a normal semi-solid casting method, and this is called a “high-speed quenching” effect. Therefore, the present low-temperature casting is a high-speed quench casting in which a low-temperature cast iron melt is used and molding is performed by pressurization and rapid cooling.
In addition, the above casting method makes it possible to cast spheroidal graphite cast iron in a much wider temperature range than conventional semi-solid casting using molten metal in the semi-solid temperature range, so the temperature control of the molten metal during casting is possible. -Management can be facilitated.

Next, the process of the low-temperature casting method of the present spheroidal graphite cast iron will be specifically described. A schematic configuration of a low temperature casting apparatus for performing the present low temperature casting method is shown in FIG. The low-temperature casting apparatus 1 includes a vacuum processing apparatus 2, a pouring apparatus 3, a mold 5 and a pressurizing apparatus 4.
(Vacuum treatment process)
A high-temperature (for example, 1400 ° C. or higher) molten metal 12 of spheroidal graphite cast iron that has already been spheroidized with graphite is placed in a ladle 21. In the vacuum processing step, the ladle 21 is accommodated in the vacuum vessel 22, and the air in the vacuum vessel 22 is discharged (A) to reduce the pressure. The vacuum processing step is a step of keeping the molten graphite 12 of the spheroidal graphite cast iron at a predetermined degree of vacuum for a predetermined time. The ultimate vacuum in the vacuum vessel 22 is preferably in the range of 10 ⁻³ MPa to 10 ⁻¹ MPa (more preferably 10 ⁻³ MPa to 10 ⁻² MPa). Setting the ultimate vacuum to a level higher than 10 ⁻³ MPa not only makes the apparatus large, but also impairs productivity. On the other hand, if the degree of vacuum is lower than 10 ⁻¹ MPa, the modification of the target molten metal cannot be sufficiently achieved. The degree of vacuum need not be particularly strict.
At the ultimate vacuum, the molten cast iron 12 is maintained for 30 seconds to 150 seconds. For example, when the degree of vacuum is 10 ⁻³ MPa, it may be maintained for 30 to 60 seconds (preferably 60 seconds), and when the degree of vacuum is 10 ⁻¹ MPa, it may be maintained for about 150 seconds.

Conventionally, molten metal used for semi-solid casting has sometimes been subjected to processing such as mechanical stirring and electromagnetic stirring in order to refine and granulate the structure. However, in this low temperature casting method, bubbling occurs in the molten metal 12 in the above vacuum processing, so that mechanical stirring or the like is not required. This bubbling is generated when the gas in the cast iron melt 12 is removed. Even without mechanical stirring, the bubbling agitates the molten metal 12, deoxidizes, removes the oxide film (SiO ₂ ), etc. Is done. In addition, the particle size of the spherical graphite in the cast iron melt 12 is reduced, and the modification suitable for low-temperature (semi-solidified) casting is promoted.
The gas that generates the bubbling is considered to be an Mg gas that vaporizes at 1100 ° C. In order to effectively generate this bubbling, it is preferable that 0.035% by mass or more of Mg is contained in the melt of spheroidal graphite cast iron introduced into the vacuum processing step. In that case, Mg contained in the molten metal after vacuum processing can be made 0.030 mass% or less by bubbling.

Conventionally, a known inoculum (FeSi, CaSi, etc.) is added to the molten metal in order to make the cast iron structure uniform and fine. Usually, when inoculating twice, about 0.1 to 0.3% by mass of the inoculum is administered for the first time (ladder inoculation), and 0.01 to 0.00 for the second time (pouring inoculation). An inoculum of about 05% by mass is administered.
Also in this low temperature casting method, the cast iron structure can be made uniform and refined by providing an inoculation step of adding an inoculum to the molten metal after the vacuum treatment. Here, in the case of this casting method, since the property of the molten metal is improved by the vacuum treatment, the amount of the inoculum used in the inoculation step can be 0.001 to 0.05% by mass. That is, the inoculation amount can be reduced to about 1/10 to 1/100 compared with the conventional case. For example, it is only necessary to use 0.01 to 0.05 mass% inoculum for ladle inoculation and 0.001 to 0.005 mass% inoculum for gate injection. In addition, the number of inoculations is not limited to two, but can be one.

(Pouring process)
The molten graphite 13 of the spheroidal graphite cast iron after the vacuum treatment is received by a small ladle 31. The pouring step is a step of instantaneously pouring the cast iron melt 13 after the vacuum treatment into the cavity of the mold 5 at a low temperature close to the liquidus temperature. Since the cast iron melt 13 has been subjected to a vacuum treatment, an oxide film seen in a normal spheroidal graphite cast iron melt is reduced and the molten metal flowability is improved. By improving the hot water flowability, the temperature of the molten metal when pouring into the mold can be lowered (for example, 1350 ° C. or lower). In addition, the pouring time to the mold in the pouring process can be shortened, a uniform distribution of the cast iron melt can be obtained in the cavity, and the occurrence of external defects and internal defects in the cast product can be prevented. Become.

After the vacuum treatment, the cast iron melt 13 is set to a predetermined temperature within a temperature range of 1350 ° C. or lower and a liquidus temperature (eg, 1185 ° C.) or higher. This temperature range is a wider temperature range than the semi-solidification temperature range.
Specifically, for example, in a small ladle 31 preheated to a high temperature (about 1000 ° C.), the molten metal for one frame of the mold is measured and received from the ladle 21 accommodated in the vacuum vessel 22, The temperature of the cast iron melt 13 is measured with a thermometer. The predetermined temperature can be appropriately set according to the amount of molten metal, the mold material and preheating temperature, the casting method, the casting conditions, etc. (for example, 1300 ± 10 ° C.).

  The cast iron melt 13 in the small ladle 31 is poured into the mold 5 when the predetermined temperature is reached. For this reason, the pouring apparatus 3 can be made into the structure which can tilt the small ladle 31 toward the pouring gate of the pouring gate type | mold 54, for example. The cast iron melt 13 is injected into the mold 5 within a predetermined time. This injection time is as short as possible, and so-called instantaneous pouring is preferable. The cast iron melt 13 is suitable for instantaneous pouring because the molten metal flow is improved by the previous vacuum treatment even at a low temperature. The injection time can be set to be the shortest according to casting conditions such as the amount of molten metal and the mold structure. For example, when the amount of hot water is about 2 kg, the predetermined time can be about 0.5 to 1 second (preferably 0.5 to 0.6 second).

FIG. 1 schematically shows a mold 5. The mold 5 includes an upper mold 51 and a lower mold 52 that can be moved toward and away from each other by a driving device (not shown) via a connecting portion 41, and a cavity C is formed therebetween. An intermediate die 53 that is movable in the vertical direction between the upper die 51 and the lower die 52 is provided so that the entire cast iron melt filled in the cavity C can be pressurized.
In order to prevent a temperature drop of the molten metal at the time of pouring, the gate mold 54 of the mold 5 is preferably composed of a heat insulating sand mold or the like. Further, in order to enhance the rapid cooling effect at the time of pressurization, the part constituting the cavity of the mold 5 is preferably formed using a metal material having a particularly high thermal conductivity (for example, copper or copper alloy). .

  It is preferable that the temperature and structure distribution of the molten cast iron injected from the gate and filled into the cavity C are nearly uniform. The melt of spheroidal graphite cast iron injected in this pouring step has been refined and homogenized with spheroidal graphite and structure by the vacuum treatment, and the flowability of the hot metal is improved. Then, since the molten metal is poured at a predetermined temperature in a short time, the temperature and structure distribution of the molten metal filled in the cavity C can be made closer to each other, and a uniform graphite particle size and a uniform microstructure can be obtained. Can be obtained.

(Pressure process)
The pressurizing step is a step of pressurizing the entire cast iron melt filled in the cavity C of the mold 5 after the molten graphite iron is injected into the mold 5 by the pouring step.
Conventionally, a method such as pressing a gate with a pin has been used in order to prevent the occurrence of defects such as a shrinkage cavity of a cast product. However, even if the gate having a relatively high temperature is pressed, the pressure acts only in a shallow range (about several mm) from the pressing surface. In addition, when the solid phase ratio is high (exceeding about 30%), the molten metal does not move even if a part is pressed. As a result, there is a problem that the pressurization becomes uneven and a nest is formed in the casting.
In the present casting method, the cast iron in the cavity C has a highly uniform semi-solid structure because a low-temperature cast iron melt with good melt flow is injected by the pouring step. In this pressurizing step, pressure is applied to the entire surface of the cavity C, that is, the entire semi-solid cast iron filled in the cavity C. By this pressurization, the base structure of the spheroidal graphite cast iron is further refined and cooled by the mold. Normally, the solidified phase of cast iron forms long and directional dendrites, but as mentioned above, the dendrites are destroyed and shortened by the pressurization and rapid quenching effects in the semi-solid state. As a result, a cast product having high strength and no defects such as nests can be formed.

  Needless to say, cast iron flows according to pressure when it is higher than the liquidus temperature (for example, 1185 ° C.), but does not flow when it falls to the eutectic temperature (for example, 1111 ° C.). For this reason, for example, after a molten cast iron of 1300 ° C. is injected into the cavity C, the temperature range in which the filled cast iron melt becomes lower than the liquidus temperature, that is, a semi-solid state (eg, 1185 to 1111 ° C.) When the predetermined temperature is reached, pressurization of the entire cavity is started. Here, the predetermined temperature can be determined based on, for example, a casting method or actual measurement with reference to a temperature at which the solid phase ratio becomes 30%. The temperature of the molten metal in the cavity C can be measured by providing a temperature sensor at a site where the molten metal contacts the cavity surface.

FIG. 2 shows an example in which a temperature change (cooling curve) is measured when a molten metal of about 1200 ° C. is injected into a cavity C formed using copper. In the graph, the horizontal axis represents time, and the vertical axis represents the temperature at the center of cast iron in the cavity. As is apparent from this cooling curve, the temperature of the cast iron melt that is instantaneously injected into the mold cavity (injection time 0.6 seconds) drops in a very short time. For Figure 2, the liquidus temperature _{T L} is 1162 ° C., eutectic temperature _{T E} is 1096 ° C., the time _{t h} in semi-solid state is much shorter and 2.7 seconds.
Therefore, it is necessary to pressurize at a timing according to the semi-solid state. The optimum timing for uniformly pressurizing in this semi-solidifying temperature range can be controlled based on the time from the start of instantaneous pouring in the pouring step. The optimum pressurization timing varies depending on the amount of molten metal, the material of the mold, the preheating temperature, the casting method, the casting conditions, etc., but can be determined by analyzing the defects and structure of the prototype casting. For example, the pressurization can be controlled to start at a predetermined timing within a range of 0.3 to 2.5 seconds after the start of pouring.
When the sand mold is used, the time (t _h ) in the semi-solidified state is significantly longer than when the mold is used (in the actual measurement example, about 70 to 160 seconds).

Pressurization of the entire semi-solid cast iron filled in the cavity C can be performed, for example, by pressing (P) the upper mold 51 shown in FIG. 1 toward the lower mold 52. The applied pressure can be less than 18 MPa. The structure for pressing the upper mold 51 for this pressurization is not particularly limited. For example, as will be described later, there is a structure in which the gate mold 54 is pressed and the upper mold 51 is pressed in the direction of the lower mold 52 through the gate mold 54. The optimal load for this pressing can be determined by the casting conditions and the like, as with the above-mentioned pressing timing. For example, in the example described later, the load is 45 t.
The load applied for the pressing is measured by measuring the amount of displacement due to the load of the movable part of the mold (for example, the amount of downward displacement of the upper mold 51), and changing it according to the amount of displacement or the displacement speed. Also good. FIG. 3 shows an example in which the pressing load (vertical axis) is changed with respect to the displacement amount (horizontal axis) of the movable part of the mold. The amount of displacement can be measured based on the position before the load is applied. By controlling the pressing load in this way, the maximum load can be applied in a state where the solid phase rate of the cast iron in the cavity is low, and the load can be reduced as the solid phase rate increases. It can be used and effectively pressurized with a small maximum load.

Moreover, since the cavity part is formed with the metal raw material excellent in thermal conductivity, the semi-solid cast iron is rapidly cooled by pressurizing the entire surface of the cavity, and a high-speed quenching effect can be obtained. In the cooling curve shown in FIG. 2, the cooling rate is about 20 ° C./second. The solidified phase of semi-solid cast iron forms a long directional dendrite, but the dendrite is destroyed and shortened by pressurization and rapid quenching effects, and the spheroidal graphite cast iron structure is further refined. The refinement of the structure leads to an increase in strength of the spheroidal graphite cast iron casting.
In addition, pressurization causes mass feeding (mass replenishment) action for massi (solid) solidification peculiar to spheroidal graphite cast iron, and suppresses the occurrence of internal defects in the casting with its shrinkage prevention effect. Can do.
As described above, as an effect of pressurization and rapid cooling in the pressurization step, it is possible to produce a cast iron of high strength and no defects.

(Low temperature casting equipment)
The low-temperature casting apparatus of the present invention realizes a suitable and practical apparatus for performing the low-temperature casting method described above. For example, as shown in FIG. 1, the low-temperature casting apparatus includes a vacuum processing apparatus 2 that maintains a molten spheroidal graphite cast iron at a predetermined degree of vacuum for a predetermined time, and a cast iron molten metal received from the vacuum processing apparatus 2 at 1350 ° C. or less. In addition, a pouring device 3 for pouring into the mold 5 within a predetermined time in a temperature range equal to or higher than the liquid phase temperature, and a pressurizing device 4 for pressurizing the entire cavity C of the mold 5 after pouring the cast iron melt are provided.

The vacuum processing apparatus 2 is configured to accommodate a ladle 21 having a heat insulating structure in a vacuum vessel 22. The vacuum processing apparatus 2 is controlled to keep the molten metal for 30 seconds to 150 seconds at a vacuum degree of preferably 10 ⁻³ MPa to 10 ⁻¹ MPa (more preferably 10 ⁻³ MPa to 10 ⁻² MPa). . The vacuum processing step is performed by the vacuum processing apparatus 2.
The pouring device 3 measures the molten metal for one frame of the mold, for example, receives the molten metal from the vacuum processing ladle 21 to the small ladle 31, and when it is measured that the molten metal 13 has reached a predetermined temperature. In addition, it is configured to inject (instantaneous pouring) into the cavity C of the mold 5 within a predetermined time. For this reason, the pouring device 3 can be configured to tilt the small ladle 31 toward the pouring gate of the mold so that the molten metal is poured into the mold within a predetermined time. The pouring process is performed by the pouring device 3.
The pressurizing device 4 is configured to apply a predetermined pressure to the entire cast iron melt filled in the cavity of the mold 5 based on the time from the start of the pouring. The pressurizing step is performed by the pressurizing device 4.

  The said metal mold | die and a pressurization apparatus can be made into a structure as shown to FIGS. FIG. 4 is a cross-sectional view illustrating the structure of the main part of the mold and the pressure device, and FIG. 5 is an exploded perspective view illustrating the structure of the mold. The mold 50 includes an upper mold 510, a lower mold 520, and an intermediate mold 530, and a cavity C is formed therebetween. The upper die 510 is connected to a driving mechanism (not shown) (for example, a fluid pressure cylinder) via the connecting portion 41 so that the upper die 510 can approach and separate from the lower die 520 by the driving mechanism. In addition, a floating cylinder 44 that floats the intermediate mold 530 relative to the lower mold 520 is provided. In addition, a heater or the like (not shown) for preheating the mold can be provided.

The upper mold 510 can be made of hot tool steel (eg, made of SDK61) having excellent rigidity.
The lower mold 520 includes a main body mold 520a made of hot tool steel (for example, made of SDK61) disposed on the pedestal 43, and a plurality of nested molds 520b provided on the main body mold 520a. . Further, as shown in FIG. 5, the insert mold 520 b is made of a copper alloy (for example, chromium steel) inner divided mold 521 and a hot tool steel fitted in the inner divided mold 521 in the vertical direction. The outer divided type 522 (for example, made of SDK61) is used. A gap (not shown) of about 0.2 mm can be provided for degassing between the fitting portions of the inner split mold 521 and the outer split mold 522.
The intermediate mold 530 can be made of hot tool steel (for example, made of SDK61). As shown in FIGS. 4 and 5, the intermediate die 530 has a hole portion 531 that is fitted in the nested die 520 b of the lower die 520 in the vertical direction. A gap for degassing can be provided in a fitting portion between the intermediate mold 530 and the nested mold 520b of the lower mold 520. Further, the intermediate die 530 is movable in the vertical direction between the upper die 510 and the lower die 520 so that the cast iron melt filled in the cavity C can be pressurized. The intermediate die 530 is levitated about 0.3 to 2 mm from the lower die 520 by the levitating cylinder 44 supported by the pedestal 43. A gap for degassing can be provided in a fitting portion between the intermediate mold 530 and the nested mold 520b of the lower mold 520.

The operation of the mold and the pressure device will be described below with a focus on the pressure step. In the mold open state shown in FIG. 4, the upper mold 510, the lower mold 520, and the intermediate mold 530 are preheated by the heater. Next, the upper die 510 is brought close to the lower die 520, and the cavity C is formed between the upper die 510, the lower die 520, and the floating cylinder 44 between the floating intermediate die 530. In this state, as shown in FIG. 6, the gate mold 540 is placed on the intermediate mold 530 through the center hole of the upper mold 510, and the gate mold 540 is pressed against the upper mold 510 by the pressing cylinder 45 to float. To prevent. The pressing cylinder 45 is provided on the fixed side (for example, a frame body) of the pressurizing device 4 and applies a load so as to press the gate mold 540 downward.
Subsequently, the said pouring process which pours cast iron molten metal in the cavity C within predetermined time (for example, within 0.6 second) using a small ladle (31) is performed. FIG. 6 shows a state in which the molten cast iron is filled in the cavity C.

Thereafter, as shown in FIG. 6, a load (for example, 45 t) is applied by the pressing cylinder 45 at a predetermined timing (for example, 0.5 seconds after the start of pouring) after the start of pouring or after the completion of pouring, The upper die 510 is brought closer to the lower die 520 against the levitation force of the levitation cylinder 44. Then, the intermediate mold 530 moves downward together with the upper mold 510, the entire cast iron melt in the cavity C is pressurized with a predetermined pressure, and a plurality of cast products D are molded. As described above, the displacement amount of the upper mold 510 may be measured, and control may be performed so that the pressing load is changed according to the displacement amount or the displacement speed.
Thereafter, after the pressing of the gate mold 540 by the pressing cylinder 45 is released, the upper mold 510 is separated from the lower mold 520 to open the mold, and the casting D attached to the upper mold 510 is removed. The time from the pressurization to the removal of the cast product can be about 20 to 60 seconds.
When the upper mold 510 and the lower mold 520 are in the mold open state, the intermediate mold 530 is lifted again by the action of the flying cylinder 44. Thereafter, the above-described operation is repeated, and the cast product D is mass-produced.

Examples of the present invention will be described below. The entire low-temperature casting apparatus of this example is configured as shown in FIG. 1, and the mold and pressure device shown in FIGS. In the casting of the example, two cap parts having an appearance as shown in FIG. 7 are taken. One cap component has a bottom surface (downward in FIG. 7) of about 73 mm × 73 mm and a height of about 46 mm, and is provided with a recess on the bottom surface. The mass is about 1.0 kg.
The molten spheroidal graphite cast iron used was preliminarily subjected to graphite spheroidization treatment by adding Mg at 1510 ° C. In the vacuum treatment, about 110 kg of molten cast iron having a spheroidizing temperature of 1400 ° C. or higher was received in a ladle 21 and stored in a vacuum vessel 22. And the inside of the vacuum vessel 22 was pressure-reduced with the vacuum pump, and it hold | maintained for 90 second in the degree of vacuum 9.6 * 10 < ^-3 > MPa.
Table 1 shows the components of the cast iron after the vacuum treatment. From the analysis by a CE meter, it was estimated that the liquid phase temperature (primary crystal temperature) of the cast iron was 1185 ° C. and the eutectic temperature was 1111 ° C.

Next, 1 frame (about 2 kg) of the cast iron melt that has been vacuum-treated is received in a small ladle 31 that is kept at about 1000 ° C., and inoculated when the melt reaches 1300 ° C. (0.05 mass). %) And immediately injected into a mold preheated to about 200 ° C. The pouring time is 0.5 seconds.
0.6 seconds after the start of pouring, the mold was pressed with a load of 45 t, and the entire cast iron melt filled in the cavity was pressurized. And the upper mold | type was raised after 25-35 second progress from pressurization start.

  FIG. 8 shows a macro structure image of a cross section of the cap part cast by the above process. As is clear from this, no defects are found inside and outside the casting. Improvements in the flow of molten metal by vacuum treatment, and refinement and homogenization of the structure by vacuum treatment, instantaneous pouring, and pressure molding have prevented the occurrence of defects such as defects in appearance, cracks, and internal nests of cast products. .

FIG. 9 is an image of the microstructure of the cap part cast by the above process. FIG. 4A shows spheroidal graphite distributed in the structure, and FIG. 4B shows the structure after the etching process. The spheroidization rate of this structure is about 90%, the chill structure is about 20%, and the dendrite length is about 100 μm, and a uniform and fine structure is realized.
As a comparative example, FIG. 10 shows a microstructure of a cast product when no vacuum treatment is performed. As in the previous figure, (a) represents spheroidal graphite distributed in the structure, and (b) represents the structure after the etching treatment. Compared to FIG. 9, the spherical graphite and the matrix structure are not refined and are not uniform. From the results shown in FIGS. 9 and 10, it can be seen that the effect of refining the spheroidal graphite cast iron structure mainly by vacuum treatment is large.

The results of measuring the strength of the cast product are as shown in Table 2. As a comparative example, a strength measurement example of conventional spheroidal graphite cast iron cast using a sand mold will be given. All show the strength after austempering treatment (constant temperature transformation treatment). Compared with the conventional spheroidal graphite cast iron, it can be seen that the cast product of the example has an elongation of about 1.7 times and is excellent in toughness. Further, the hardness is about 7% lower than that of the comparative example, and the processability is excellent. From these, it can be said that the cast product obtained by the low-temperature casting method of the present spheroidal graphite cast iron is superior in toughness compared to conventional spheroidal graphite cast iron and has the same high toughness as low alloy steel. Moreover, although it is equivalent in hardness to low alloy steel, it can be said that it is a high-strength material excellent in machinability utilizing the characteristics of cast iron.
As described above, high-strength castings with a fine structure without defects are produced by the overall effects of vacuum treatment, instantaneous pouring using low-temperature molten metal, and pressurization and rapid quenching in this low-temperature casting method. We were able to.

  In the present invention, the present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention depending on the purpose and application.

  DESCRIPTION OF SYMBOLS 1; Low temperature casting apparatus, 2; Vacuum processing apparatus, 21; Ladle, 22: Vacuum container, 3; Pouring apparatus, 31; Small ladle, 4; Pressurization apparatus, 5; Mold, 52, 520; Lower mold, 53, 530; Intermediate mold, 54;

Claims (11)

A vacuum treatment step of maintaining the molten cast iron after performing the spheroidizing treatment at a predetermined degree of vacuum for a predetermined time;
After performing the vacuum processing step, a pouring step of pouring the molten metal in a temperature range of the cast iron from a liquid phase temperature to 1350 ° C into a mold within a predetermined time;
A pressurizing step of pressurizing the entire cast iron melt filled in the cavity of the mold with reference to a temperature at which the melt in the mold has a predetermined solid phase ratio after the injection of the melt;
A low-temperature casting method for spheroidal graphite cast iron, comprising:   2. The low-temperature casting method for spheroidal graphite cast iron according to claim 1, wherein the pressurizing step starts pressurization when the temperature of the molten metal in contact with the inner surface of the cavity reaches a predetermined temperature equal to or lower than a liquidus temperature. The said vacuum treatment process is a low-temperature casting method of the spheroidal graphite cast iron according to claim 1 or 2 , wherein the molten metal is maintained for 30 seconds to 150 seconds at a vacuum degree of 10 ^-3 MPa to 10 ^-1 MPa. The low-temperature casting method of spheroidal graphite cast iron according to any one of claims 1 to 3, further comprising an inoculation step of adding 0.001 to 0.05 mass% of an inoculum to the molten metal after the vacuum treatment step.   The low temperature of the spheroidal graphite cast iron according to any one of claims 1 to 4, wherein a displacement amount due to pressing of the movable part of the mold is measured in the pressurizing step, and a load of the pressing is changed in accordance with the displacement amount. Casting method. In the pressurizing step, the low-temperature casting method of spheroidal graphite cast iron according to any one of claims 1 to 5 for high speed quenching the molten metal in the cavity 20 ° C / sec or more cooling rate in a semi-solidified state. A vacuum processing apparatus for maintaining the molten cast iron after performing the spheroidizing treatment at a predetermined degree of vacuum for a predetermined time;
A molten metal pouring apparatus that receives the molten metal from the vacuum processing apparatus and injects the molten metal in a temperature range from the liquid phase temperature of the cast iron to 1350 ° C into a mold within a predetermined time;
A pressure device that pressurizes the entire cast iron melt filled in the cavity of the mold with reference to a temperature at which the melt in the mold has a predetermined solid phase ratio after the injection of the melt;
A low-temperature casting apparatus for spheroidal graphite cast iron, comprising:   The low-pressure casting apparatus for spheroidal graphite cast iron according to claim 7, wherein the pressurizing device starts pressurization when the temperature of the molten metal in contact with the inner surface of the cavity reaches a predetermined temperature equal to or lower than a liquidus temperature. The low-temperature casting apparatus for spheroidal graphite cast iron according to claim 7 or 8, wherein the vacuum processing apparatus maintains the molten metal for 30 seconds to 150 seconds at a degree of vacuum of 10 ^-3 MPa to 10 ^-1 MPa.   10. The spheroidal graphite cast iron according to claim 7, wherein the pressing device measures a displacement amount due to pressing of the movable portion of the mold and changes a load of the pressing in accordance with the displacement amount. Low temperature casting equipment. 11. The method according to claim 7 , wherein copper or a copper alloy is used for a portion constituting the cavity of the mold , and the molten metal in the cavity is rapidly quenched at a cooling rate of 20 ° C./second or more in a semi-solid state. The low-temperature casting apparatus for spheroidal graphite cast iron as described.