PROCESS FOR PRODUCING SEMI-SOLIDIFIED SLURRY OF IRON ALLOY

Abstract

A process for producing a semi-solidified slurry of an iron alloy including the steps of pouring a melt of an iron alloy into a semi-solidified slurry producing vessel 30 and cooling the melt therein to obtain a semi-solidified slurry having a crystallized solid phase and a residual liquid phase, wherein a hypereutectoid or hypoeutectic cast iron composition containing 0.8-4.3 wt. % C is used as a material, a melt of the composition is poured into the semi-solidified slurry producing vessel in a predetermined amount at a time, a temperature of the melt when poured into the semi-solidified slurry producing vessel is controlled to be not lower than a crystallization initiation temperature of the composition and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature, and a cooling rate of the melt poured into the semi-solidified slurry producing vessel is controlled not to exceed 20° C. per minute.

Description

This application is a Continuation-in-part of pending U.S. application Ser. No. 12/449,368, filed Aug. 5, 2009, which was a national stage of PCT/JP2007/051987, filed on Feb. 6, 2007.

TECHNICAL FIELD

The present invention relates to a process for producing a semi-solidified slurry of an iron alloy. More particularly, the present invention relates to a process for producing a semi-solidified slurry of an iron alloy such as a cast iron, by cooling the iron alloy from a molten state to obtain a semi-solidified slurry in a solid-liquid coexisting state with a solid phase developed in the melt.

BACKGROUND ART

A semi-solidified state refers to the state where a metallic material cooled from a liquid (melt) state has attained a solid-liquid coexisting state.

The metallic material in the solid-liquid coexisting state may be obtained by a semi-solid process (rheometal process) or a semi-melt process (thixometal process). For example, rheocasting is a molding process using the semi-solid process, and thixocasting is a molding process using the semi-melt process.

Processing the metal in the semi-solidified state or in the semi-molten state is generally advantageous in that finer and more homogenous crystal grains can be obtained.

Comparing the semi-solid molding process with the semi-melt molding process generally, assuming a mass production, the semi-solid molding process is more advantageous because less energy is lost. In consideration of application to small-scale lots, however, the semi-melt molding process may consume less energy because the process can be made in a necessary quantity as required. This means that it will be desirable to use both the semi-solid molding process and the semi-melt molding process depending on the circumstances.

Various processes for producing semi-solidified metallic slurries according to the semi-solid molding process have been proposed, which include: a process for producing a semi-solidified metal by applying mechanical stirring during a cooling process (Patent Document 1); rheocasting process and rheocasting apparatus using an inclined cooling plate (Patent Document 2); and an apparatus for producing a solid-liquid co-existing metallic slurry by applying electromagnetic stirring (Patent Document 3).

• Patent Document 1: Japanese Patent Application Laid-Open No. 6-297097
• Patent Document 2: Japanese Patent Application Laid-Open No. 10-34307
• Patent Document 3: Japanese Patent Application Laid-Open No. 2005-88083

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

With the process configured to apply mechanical stirring as disclosed in Patent Document 1 above, however, in the case of a material with a high melting point such as a cast iron, a stirrer 1 as shown in FIG. 7 of the present application may deteriorate easily, or its components may melt in. There is essentially no stirrer 1 that can be put into practical use. With the process using the inclined cooling plate as disclosed in Patent Document 2 above, the inclined cooling plate may deteriorate easily again if it is used for solidification of a high melting point material. Furthermore, a molten metal coming into contact with the inclined cooling plate may solidify and adhere thereto. This technique requires subtle and delicate temperature control as well as operation control of the inclined cooling plate.

Furthermore, with the process configured to apply electromagnetic stirring as disclosed in Patent Document 3 above, the viscosity of the melt needs to be kept low in order to realize substantial stirring. The resultant slurry has a low solid fraction of about 20% or less. When the slurry with such a low solid fraction is subjected to die casting or other molding process, defects including blow holes will increase.

In view of the foregoing, an object of the present invention is to solve the conventional problems as described above, and to provide a process for producing a semi-solidified slurry of an iron alloy, wherein an iron alloy, particularly a cast iron, is used to obtain a favorable semi-solidified slurry which will suffer fewer blow holes when molded by die casting or the like, without performing mechanical stirring requiring a stirrer, without using a special facility for electromagnetic stirring, and without using special contact flow-down means like the inclined cooling plate.

Means for Solving the Problems

To achieve the above object, the present inventors have diligently carried out various experiments and examinations. As a result, they have found that a semi-solidified slurry of a given solid fraction may be produced by appropriately controlling the temperature during the process where a melt is solidified, without the need of mechanical or electromagnetic stirring means, and they have finally achieved the present invention.

A process for producing a semi-solidified slurry of an iron alloy according to the present invention has a first feature that the process includes the steps of pouring a melt of an iron alloy into a semi-solidified slurry producing vessel and cooling the melt in the vessel to obtain a semi-solidified slurry having a crystallized solid phase and a residual liquid phase, wherein a hypereutectoid or hypoeutectic cast iron composition containing 0.8-4.3 wt. % C is used as a material, a melt of the composition is poured into the semi-solidified slurry producing vessel in a predetermined amount at a time, a temperature of the melt when poured into the semi-solidified slurry producing vessel is controlled to be not lower than a crystallization initiation temperature of the composition and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature, and a cooling rate of the melt poured into the semi-solidified slurry producing vessel is controlled not to exceed 20° C. per minute.

In addition to the first feature described above, the process for producing a semi-solidified slurry of an iron alloy of the present invention has a second feature that, wherein above the semi-solidified slurry producing vessel, the melt poured from a ladle is once received in a relay and damper vessel in a predetermined amount at a time, and the melt received in the relay and damper vessel is then poured into the semi-solidified slurry producing vessel via a discharge port provided at a bottom of the relay and damper vessel, a diameter of the discharge port is not less than 10 mm, a temperature of the discharged melt at the discharge port is controlled to be not lower than a temperature that is 20° C. higher than the crystallization initiation temperature of the composition and not greater than a temperature that is 80° C. higher than the crystallization initiation temperature, a preheating temperature of the semi-solidified slurry producing vessel is not less than 400° C. lower than the crystallization initiation temperature of the composition and not greater than a temperature that is 200° C. higher than the crystallization initiation temperature, and a height of the relay and damper vessel from the semi-solidified slurry producing vessel is not less than 100 mm.

In addition to the second feature described above, the process for producing a semi-solidified slurry of an iron alloy of the present invention has a third feature that the melt is cooled by wind while the melt is falling from the relay and damper vessel down into the semi-solidified slurry producing vessel.

In addition to any of the second through forth features described above, the process for producing a semi-solidified slurry of an iron alloy of the present invention has a fifth feature that a semi-solidified slurry produced is taken out in the state where the semi-solidified slurry producing vessel is heated by high-frequency induction heating such that part of the semi-solidified slurry that is in contact with the semi-solidified slurry producing vessel is heated via the semi-solidified slurry producing vessel.

Effects of the Invention

According to the process for producing a semi-solidified slurry of an iron alloy recited in claim 1, a hypereutectoid or hypoeutectic cast iron composition containing 0.8-4.3 wt. % C is used as a material, and a melt of the composition is poured into a semi-solidified slurry producing vessel at a temperature of not lower than the crystallization initiation temperature and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature, and cooled at a cooling rate of not greater than 20° C. per minute to a temperature equal to or lower than the crystallization initiation temperature of primary crystals. As a result, it is possible to obtain a hypereutectoid or hypoeutectic cast iron semi-solidified slurry containing 0.8-4.3 wt. % C having its primary crystals in granular form, rather than in dendritic form. Using the semi-solidified cast iron slurry having the granular primary crystals for subsequent casting or other processing leads to formation of a product which has few defects, which is closely packed and excellent in structure, and which has good mechanical properties. In this case, the semi-solidified cast iron slurry does not need to be once solidified, but can suitably be used as it is for casting or other processing. Accordingly, it is possible to obtain a product which is not only excellent in mechanical properties but also advantageous in saving energy.

The cooling of the melt within the semi-solidified slurry producing vessel starts at a temperature that is higher by only 50° C. than the crystallization initiation temperature. This can reduce the time required for the cooling, and accordingly, it is possible to efficiently produce the semi-solidified cast iron slurry.

According to the process for producing a semi-solidified slurry of an iron alloy recited in claim 2, in addition to the above-described effects obtained by the configuration recited in claim 1, a ladle is used to deliver the melt from a melting furnace or the like, and a relay and damper vessel receives the melt from the ladle in a predetermined amount at a time. The melt received in the relay and damper vessel is then poured into the semi-solidified slurry producing vessel via a discharged port at a bottom of the rely and damper vessel while being cooled and homogenized in the relay and damper vessel.

Of the melt which flows from the ladle down to the relay and damper vessel, the initially discharged melt is poured into the semi-solidified slurry producing vessel at a relatively early stage and cooled within the semi-solidified slurry producing vessel. By comparison, the subsequently discharged melt remains in the relay and damper vessel for a while during which it is cooled before being poured into the semi-solidified slurry producing vessel. This can reduce the temperature difference when all the melt is poured into the semi-solidified slurry producing vessel. As the temperature difference in the melt is reduced, dendritic crystallization is prevented, and accordingly, it is possible to obtain a favorable semi-solidified slurry made up of granular crystals and the melt.

The amount of the melt acquired by the ladle from the melting furnace or the like is much larger than a predetermined amount of the semi-solidified slurry. Thus, it would be difficult to pour the melt directly from the ladle into the semi-solidified slurry producing vessel smoothly and in a stable manner. Furthermore, it would be necessary to hold the melt in the ladle in the state where the temperature of the melt is decreased to and kept at a level near the crystallization initiation temperature, while preventing initiation of solidification. According to the invention recited in claim 2, the relay and damper vessel once receives the melt from the ladle, and relays the melt to the semi-solidified slurry producing vessel. This enables a smooth melt-pouring operation with which the melt is poured in predetermined, small quantities from the ladle having large capacity. Further, as the melt is once received from the ladle into the relay and damper vessel, the melt of the predetermined amount received can be homogenized, and accordingly, the melt poured into the semi-solidified slurry producing vessel can be more homogenized. Still further, as the melt from the ladle is cooled as it passes through the relay and damper vessel, the melt can be held in the ladle at a relatively higher temperature, which facilitates the temperature control.

Especially according to the invention recited in claim 2, a diameter of the discharge port is less than 10 mm, a temperature of the discharged melt at the discharged port is not lower than a temperature that is 20° C. higher than the crystallization initiation temperature of the composition and not greater than a temperature that is 80° C. higher than the crystallization initiation temperature, a preheating temperature of the semi-solidified slurry producing vessel is not less than 400° C. lower than the crystallization initiation temperature of the composition and not greater than a temperature that is 200° C. higher than the crystallization initiation temperature, and a height of the relay and damper vessel from the semi-solidified slurry producing vessel is not less than 100 mm. This really enables us to obtain a favorable semi-solidified cast iron slurry with granular crystals more reliably and stably.

According to the process for producing a semi-solidified slurry of an iron alloy recited in claim 3, in addition to the above-described effects obtained by the configuration recited in claim 2, the melt is cooled by wind while it falls from the relay and damper vessel down into the semi-solidified slurry producing vessel. As the melt falling down toward the semi-solidified slurry producing vessel can forcibly be cooled, the melt which enters the semi-solidified slurry producing vessel has a temperature closer to its crystallization temperature, which can reduce the subsequent cooling time. As the melt is cooled by wind, the melt may be poured from the ladle at a higher temperature and the melt may also be held in the relay and damper vessel at a higher temperature. This facilitates the temperature control.

According to the process for producing a semi-solidified slurry of an iron alloy recited in claim 4, in addition to the above-described effects obtained by the configuration recited in claim 2, energy caused by the melt falling from the relay and damper vessel down into the semi-solidified slurry producing vessel is used to stir the melt within the semi-solidified slurry producing vessel. This means that the melt within the semi-solidified slurry producing vessel can be stirred by the stirring effect of the falling melt itself, without the need of a stirrer such as a stirring rod, or an expensive, sophisticated device such as electromagnetic stirring means. Accordingly, the temperature of the melt can be made sufficiently uniform, and thus, the nucleation is promoted, and fine and granular primary crystals are obtained.

For adjusting the falling energy so as to obtain a desired degree of stirring effect, the falling height may be determined in advance through experiments, according to the amount of the melt and the capacity of the semi-solidified slurry producing vessel.

According to the process for producing a semi-solidified slurry of an iron alloy recited in claim 5, in addition to the above-described effects obtained by the configuration recited in any of claims 2 to 4, a semi-solidified slurry produced is taken out in the state where the semi-solidified slurry producing vessel is heated by high-frequency induction heating such that part of the semi-solidified slurry in contact with the semi-solidified slurry producing vessel is heated via the semi-solidified slurry producing vessel. This allows the semi-solidified slurry to be taken out of the semi-solidified slurry producing vessel easily, without causing temperature variation in the semi-solidified slurry. That is, when the semi-solidified slurry produced is taken out of the semi-solidified slurry producing vessel to be introduced into a mold or other molding means, even if the temperature of the slurry is adjusted to the level suitable for a molding process, the part of the slurry in contact with the vessel will be low in temperature, making it difficult to take out the slurry from the vessel. On the other hand, high-frequency induction heating is suitable for quickly heating the outer periphery of the slurry. However, in the case of using the high-frequency induction heating means for a semi-solidified slurry of a cast iron, although the cast iron may be heated efficiently, the temperature will increase locally due to the poor thermal conductivity, in which case the slurry will suffer temperature variation and an unstable solid fraction. It will also be difficult to measure the temperature, hindering accurate temperature control.

According to the process recited in claim 8, the semi-solidified slurry producing vessel itself is heated quickly by high-frequency induction heating. This allows only the part of the semi-solidified slurry in contact with the vessel to increase in temperature quickly, so that the semi-solidified slurry can readily be taken out of the vessel. At the same time, the semi-solidified slurry would not suffer temperature variation, and it can be subjected to a molding process at a stable solid fraction. While there is a conventional art using high-frequency induction heating means in semi-solid die casting of an aluminum alloy, it is used for the purpose of causing the semi-solidified slurry to have a uniform temperature, not for the purpose of taking the semi-solidified slurry out of the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating an apparatus for producing a semi-solidified slurry of an iron alloy according to an embodiment of the present invention.

FIG. 2 is a photograph showing a structure of the semi-solidified slurry which is produced with the producing process and the producing apparatus according to an embodiment of the present invention.

FIG. 3 is a photograph showing, as a comparative example, a structure of a semi-solidified slurry with dendrites crystallized therein.

FIG. 4 is a table showing the conditions for producing semi-solidified slurries according to examples of the present invention.

FIG. 5 is a table showing the results of experiments in the examples of the present invention.

FIG. 6 is a table showing the results of experiments in the examples of the present invention.

FIG. 7 is a cross-sectional view illustrating a conventional art.

DESCRIPTION OF THE REFERENCE CHARACTERS

• • 10 ladle
  • 11 heat insulating means
  • 12 temperature measuring means
  • 20 relay and damper vessel
  • 21 discharge port
  • 22 vessel pre-heating means
  • 23 temperature measuring means
  • 30 semi-solidified slurry producing vessel
  • 31 heat insulating vessel
  • 32 preheating means
  • 33 temperature measuring means
  • 40 blower fan

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the process for producing a semi-solidified slurry of an iron alloy according to the present invention will further be described with reference to the drawings.

Referring to FIG. 1, the apparatus using for the present invention at least includes: a ladle 10 for receiving and delivering a melt from a melting furnace (not shown): a relay and damper vessel 20; and a semi-solidified slurry producing vessel 30. It may further include a blower fan 40 adapted to cool the melt while the melt is being poured down into the semi-solidified slurry producing vessel 30.

The ladle 10 and the relay and damper vessel 20 constitute melt pouring means P for pouring the melt into the semi-solidified slurry producing vessel 30.

The melt pouring means P takes charge of pouring the melt of a predetermined amount into the semi-solidified slurry producing vessel 30 under a melt pouring temperature condition of not lower than the crystallization initiation temperature of the melt and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature. This melt pouring temperature condition means that, in the case where the crystallization initiation temperature (liquidus temperature) of the melt being poured is 1300° C. for example, the temperature of the melt when entering the semi-solidified slurry producing vessel is regulated such that it is not lower than 1300° C. and not higher than 1350° C.

The ladle 10 receives the melt at the position where the melting furnace is located, and moves to the position above the semi-solidified slurry producing vessel 30. The capacity of the ladle 10 is much greater than the amount of a semi-solidified slurry to be produced in the semi-solidified slurry producing vessel 30 at one time, allowing the ladle 10 to supply the melt to the semi-solidified slurry producing vessel 30 a plurality of number of times, or to a plurality of semi-solidified slurry producing vessels 30.

The ladle 10 may be provided with heat insulating means 11. With this heat insulating means 11, the ladle 10 can keep the melt received from the melting furnace at an appropriate temperature during the melt pouring operation.

Further, the ladle 10 may be provided with temperature measuring means 12 for measuring the temperature of the melt. The temperature measuring means 12 may be an immersion thermometer, for example, which can measure the temperature of the melt within the ladle 10. The temperature of the melt may be measured, e.g., prior to pouring of the melt from the ladle 10. The melt is poured out only when the temperature of the melt measured falls within a predetermined temperature range.

With the temperature measuring means 12 and the heat insulating means 11, the temperature of the melt which is to be poured from the ladle 10 down to the relay and damper vessel 20 can be adjusted to an appropriate level.

The relay and damper vessel 20, which is placed above the semi-solidified slurry producing vessel 30, receives the melt from the ladle 10 and relays the melt to the semi-solidified slurry producing vessel 30, while dampening the impact of the falling melt as appropriate. The relay and damper vessel 20 is adapted to have the capacity at least sufficient enough to receive the melt poured at one time, which is much smaller than the capacity of the ladle 10.

The relay and damper vessel 20 may be a graphite crucible, for example.

The relay and damper vessel 20 is provided with a discharge port 21 at its bottom. The discharge port 21 may be provided, e.g., at the center of the bottom.

The diameter of the discharge port 21 is determined through experiments such that it satisfies at least the condition that the flow rate of the melt which is poured from the relay and damper vessel 20 to the semi-solidified slurry producing vessel 30 is smaller than the flow rate of the melt which is poured from the ladle 10 to the relay and damper vessel 20. In practice, the diameter of the discharge port 21 is preset to an optimum diameter through experiments, taking into consideration the capacity and the area of base of the relay and damper vessel 20, the flow rate of the melt which flows down from the ladle 10, and the amount of the melt to be poured at a time, so that the melt received from the ladle 10 is pooled and retained as appropriate within the relay and damper vessel 20, cooled as appropriate in the vessel, and mixed and homogenized as appropriate by the melt flowing down from the ladle 10, and further, poured from the discharge port 21 provided at the bottom down into the semi-solidified slurry producing vessel 30 smoothly and continuously.

Using the relay and damper vessel 20 with the discharge port 21 as described above is advantageous in that the relay and damper vessel 20 serves substantially as a funnel to introduce the melt from the ladle 10 into the semi-solidified slurry producing vessel 30 reliably and smoothly with ease, without causing turbulence. Further, it can appropriately promote and adjust the decrease in temperature of the melt while it flows out of the ladle 10 and down to the semi-solidified slurry producing vessel 30, and at the same time, it can prevent the melt from acquiring too much energy as it falls down to the semi-solidified slurry producing vessel 30.

It is noted that the discharge port 21 may be provided with open/close means such as an openable lid or an openable shutter, so that the melt can be received from the ladle 10 and discharged via the discharge port 21 at regular intervals. This enables more accurate control of homogenization as well as temperature decrease of the melt in the relay and damper vessel 20.

The relay and damper vessel 20 may be provided with vessel pre-heating means 22. The vessel pre-heating means 22 is provided to prevent the undesirable event that the melt which is initially supplied from the ladle 10 suffers a rapid temperature decrease and, hence, starts to solidify. Further, it is provided such that the temperature of the melt which flows from the relay and damper vessel 20 down to the semi-solidified slurry producing vessel 30 is stabilized in a certain temperature range which is suitably higher than the crystallization initiation temperature.

The relay and damper vessel 20 may be provided with temperature measuring means 23. The temperature measuring means 23 may be made up of a thermocouple, for example, which may be arranged in contact with the outer wall surface of the relay and damper vessel 20 to measure the temperature of the relay and damper vessel 20. This can adjust the pre-heating temperature of the vessel 20 to a predetermined level.

The semi-solidified slurry producing vessel 30 may be arranged in a heat insulating vessel 31 in such a manner that it can be freely taken in and out of the same. In the present embodiment, the heat insulating vessel 31 is provided with pre-heating means 32, to allow pre-heating of the semi-solidified slurry producing vessel 30. For the semi-solidified slurry producing vessel 30, electrically conductive ceramics may be used, for example, as a material which has a heat-resistant temperature of at least higher than the crystallization temperature of the melt and which can withstand high-frequency induction heating. Specifically, a composite material of carbon and ceramics such as silicon carbide, boron carbide and the like may be used.

In the present embodiment, the pre-heating means 32 is configured with high-frequency induction heating means. The pre-heating means 32 which is the high-frequency induction heating means heats the semi-solidified slurry producing vessel 30 itself.

The reference numeral 33 denotes temperature measuring means. For the temperature measuring means 33, a radiation thermometer may be used, for example, to measure the temperature of the semi-solidified slurry producing vessel 30. Alternatively, the temperature measuring means may be the one which can directly measure the temperature of the slurry within the semi-solidified slurry producing vessel 30, in which case the temperature of the slurry would be kept at a predetermined level more accurately.

The decrease in temperature of the slurry within the semi-solidified slurry producing vessel 30 is restricted such that it does not decrease below the level between the liquidus and the solidus for the composition of the melt, and the temperature of the semi-solidified slurry producing vessel 30 is adjusted such that the slurry is kept at that temperature level.

The blower fan 40 is for blowing the air onto the melt which is falling down from the relay and damper vessel 20 so as to cool the same.

The melt pouring means P having the ladle 10 and the relay and damper vessel 20 is provided with melt-pouring-temperature adjusting means TC for pouring the melt into the semi-solidified slurry producing vessel 30 under the melt pouring temperature condition of not lower than the crystallization initiation temperature and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature.

Specifically, the melt-pouring-temperature adjusting means TC is made up of a combination of the following elements: the heat insulating means 11 and the temperature measuring means 12 for the ladle 10, the relay and damper vessel 20, its discharge port 21, the vessel pre-heating means 22, the blower fan 40, and the height relation between the ladle 10, the relay and damper vessel 20, and the semi-solidified slurry producing vessel 30.

Of the elements of the melt-pouring-temperature adjusting means TC, the heat insulating means 11 and the temperature measuring means 12 for the ladle 10 constitute first temperature adjusting means TC1. The heat insulating means 11 and the temperature measuring means 12 for the ladle 10 constituting the first temperature adjusting means TC1 adjust the temperature of the melt which is held in the ladle 10.

Further, of the elements of the melt-pouring-temperature adjusting means TC, the material, shape, thickness, and capacity of the relay and damper vessel 20, the size and position of the discharge port 21, and pre-heating conducted by the vessel pre-heating means 22 constitute second temperature adjusting means TC2. The second temperature adjusting means TC2 adjusts the decrease in temperature of the melt which flows through the relay and damper vessel 20, and adjusts the temperature of the melt which flows out via the discharge port 21.

Still further, of the elements of the melt-pouring-temperature adjusting means TC, the blower fan 40 constitutes third temperature adjusting means TC3. This third temperature adjusting means TC3 adjusts the decrease in temperature of the melt which falls down from the relay and damper vessel 20.

The semi-solidified slurry producing vessel 30 is provided with cooling-rate adjusting means SC for cooling the melt received in the semi-solidified slurry producing vessel 30 at a cooling rate of not greater than 20° C. per minute.

Specifically, the cooling-rate adjusting means SC is made up of the material, shape, thickness, and capacity of the semi-solidified slurry producing vessel 30 itself, the heat insulating vessel 31, and the pre-heating means 32. That is, the cooling rate of the melt is adjusted by the material, shape, thickness, and capacity of the semi-solidified slurry producing vessel 30, and also by the material, shape, thickness, and capacity of the heat insulating vessel 31. Particularly, the cooling rate of the melt can be adjusted considerably freely depending on the temperature to which the semi-solidified slurry producing vessel 30 is pre-heated by the pre-heating means 32. Accordingly, the cooling-rate adjusting means SC can adjust the cooling rate of the melt within the semi-solidified slurry producing vessel 30 not to exceed 20° C. per minute.

It is noted that the temperature of the melt within the semi-solidified slurry producing vessel 30 is decreased to and kept at a predetermined level between the liquidus and the solidus of the melt, so that the ratio between the solid phase and the liquid phase becomes a predetermined ratio.

For taking the semi-solidified slurry out of the semi-solidified slurry producing vessel 30, the pre-heating means 32 constituted by the high-frequency induction heating means may be used to quickly heat the semi-solidified slurry producing vessel 30 to thereby heat only the part of the semi-solidified slurry that is in contact with the semi-solidified slurry producing vessel 30. This facilitates taking out the semi-solidified slurry, and further prevents occurrence of temperature variation in the semi-solidified slurry. Accordingly, the slurry can reliably be taken out at a desired solid fraction.

An embodiment of the process for producing a semi-solidified slurry of an iron alloy using the apparatus as described above will now be described.

As a raw material for producing a semi-solidified slurry, a material of a hypereutectoid or hypoeutectic cast iron composition containing 0.8-4.3 wt. % C is used. The material is melt in a melting furnace to obtain a melt of a given hypereutectoid or hypoeutectic cast iron composition.

The melt melted in the melting furnace is received by the ladle 10 in an appropriate amount each time, which is moved to the position above the relay and damper vessel 20.

The ladle 10, while keeping the melt within a predetermined temperature range, discharges the melt down to the relay and damper vessel 20 in a predetermined amount at a time (which amount corresponds to the amount of the semi-solidified slurry to be produced at a time).

The melt flowing down from the ladle 10 once enters the relay and damper vessel 20, and further flows down via the discharge port 21 provided at the bottom into the semi-solidified slurry producing vessel 30. The melt poured into the semi-solidified slurry producing vessel 30 is cooled therein, whereby a semi-solidified slurry composed of a primary crystal solid phase and a residual liquid phase is produced. The semi-solidified slurry in this state is taken out of the semi-solidified slurry producing vessel 30, which is then subjected to rheocasting or other molding process.

The temperature of the melt of the hypereutectoid or hypoeutectic cast iron which is poured into the semi-solidified slurry producing vessel 30 is regulated to the temperature range of not lower than the crystallization initiation temperature for the component composition and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature.

For controlling this melt pouring temperature, the temperature of the melt which comes out of the ladle 10 may be adjusted (adjustment by the first temperature adjusting means TC1), and further, the amount of temperature decrease may be adjusted by the shape, thickness, and capacity of the relay and damper vessel 20, the diameter of the discharge port 21, presence/absence of pre-heating, and the pre-heating temperature (adjustment by the second temperature adjusting means TC2), and still further, the amount of temperature decrease of the melt flowing down from the relay and damper vessel 20 may be adjusted by the blower fan 40 (adjustment by the third temperature adjusting means TC3).

The amount of temperature decrease of the melt from when it comes out of the ladle 10 until when it reaches the semi-solidified slurry producing vessel 30 may of course be adjusted through adjustment of the height relation between the ladle 10, the relay and damper vessel 20, and the semi-solidified slurry producing vessel 30, or put more simply, by adjusting the time (drop) during which the melt is exposed in the air while it falls from the ladle 10 down to the semi-solidified slurry producing vessel 30. It is assumed that the melt-pouring-temperature adjusting means TC includes adjustment of this drop.

For the adjustment and control of the melt pouring temperature as described above, it may be predetermined through experiments how much the temperature of the melt will decrease from when it is discharged from the ladle 10 until when it reaches the semi-solidified slurry producing vessel 30, or how much temperature decrease will be needed for the melt from when it flows out of the ladle 10 until it reaches the semi-solidified slurry producing vessel 30. Then, it is only necessary to control the temperature of the melt that is discharged from the ladle 10 to fall within a predetermined temperature range, so that the temperature of the melt when it is poured into the semi-solidified slurry producing vessel 30 is adjusted and controlled to a predetermined temperature (of not lower than the crystallization initiation temperature and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature).

In the above configuration, increasing the drop from the ladle 10 to the semi-solidified slurry producing vessel 30 can increase the amount of temperature decrease, which is advantageous in that the melt can be held at a higher temperature within the ladle 10. However, it may cause excessive energy by the falling melt and, hence, turbulence in the poured melt. In view of the foregoing, according to the present invention, the relay and damper vessel 20 is provided in the middle, for appropriately dampening the energy caused by the falling melt due to the increased drop. As a result, the melt can be poured into the semi-solidified slurry producing vessel 30 with accuracy, involving not too much energy (so that excessive stirring and spattering of the melt are both suppressed).

By adjusting the height (drop) from the relay and damper vessel 20 to the semi-solidified slurry producing vessel 30, the melt can surely be poured smoothly and without spattering. Furthermore, by virtue of the suitable stirring effect achieved by appropriate energy of the falling melt, the melt within the semi-solidified slurry producing vessel 30 can be homogenized. Specifically, if the height of the relay and damper vessel 20 with respect to the semi-solidified slurry producing vessel 30 is small, the stirring effect by the falling melt cannot be obtained, in which case the melt poured into the semi-solidified slurry producing vessel 30 may not be homogenized sufficiently. As the height of the relay and damper vessel 20 with respect to the semi-solidified slurry producing vessel increases, the melt may be homogenized more sufficiently by the stirring effect as described above. If the height is too great, however, the melt may spatter when it is poured into the semi-solidified slurry producing vessel 30, leading to an excessively stirred state. Accordingly, the desired height of the relay and damper vessel 20 is predetermined through experiments, taking into consideration the size of the discharge port 21 provided in the relay and damper vessel 20, the shape and capacity of the semi-solidified slurry producing vessel 30, and others.

The function of the relay and damper vessel 20 is to once receive the melt from the ladle 10 and hold the same in the vessel 20, for homogenizing the melt and also for cooling the melt within the vessel 20.

Another function of the relay and damper vessel 20 is to continuously discharge the melt through the discharge port 21 provided at the bottom down to the semi-solidified slurry producing vessel 30, to ensure smooth and stable pouring of the melt.

It is configured such that the melt which has been poured into the semi-solidified slurry producing vessel 30 is cooled at a cooling rate of not greater than 20° C. per minute. The cooling rate is controlled primarily by pre-heating the semi-solidified slurry producing vessel 30. The cooling rate will of course vary depending on the cooling properties specific to the semi-solidified slurry producing vessel 30, according to its shape, thickness, capacity, and the like. Therefore, it is predetermined through experiments how much the semi-solidified slurry producing vessel 30 will have to be pre-heated in relation to the temperature of the melt being poured therein, so as to obtain a semi-solidified slurry composed of favorable granular primary crystals and the residual liquid phase within the prescribed cooling rate range.

The semi-solidified slurry producing vessel 30 may be pre-heated by high-frequency induction heating. This can pre-heat the vessel 30 as necessary, more quickly than in the case of using an electric heater, thereby enabling finer temperature control. Accordingly, the cooling rate can be controlled with accuracy, so that a favorable semi-solidified slurry can be obtained, with crystallization of dendrites being suppressed therein.

Furthermore, as the semi-solidified slurry producing vessel 30 is heated quickly and with accurate temperature control by high-frequency induction heating, it is possible to quickly heat only part of the semi-solidified slurry that is in contact with the semi-solidified slurry producing vessel 30 when taking the semi-solidified slurry out of the vessel 30. Accordingly, the semi-solidified slurry can readily be taken out of the vessel 30, as it is only necessary to turn the vessel 30 over to take the semi-solidified slurry out of the vessel. Furthermore, the semi-solidified slurry can be taken out of the vessel 30 without changing the solid-liquid ratio of the semi-solidified slurry and without causing temperature variation therein.

EXAMPLES

A raw material of a hypoeutectic cast iron composition containing 2.6 wt % of C (carbon) and 1.5 wt % of Si (silicon) as a component composition was melted in a melting furnace to obtain a melt. The hypoeutectic cast iron of this composition has a liquidus temperature of about 1300° C. and a solidus temperature of 1150° C. Accordingly, a semi-solidified slurry can be obtained by cooling the melt to a temperature between 1300° C. and 1150° C. and keeping it at the temperature.

The melt in the melting furnace was received by the ladle 10 and poured therefrom. Specifically, the melt in the ladle 10 was poured in a predetermined amount at a time, down to the relay and damper vessel 20 made up of a graphite crucible which was preheated. Further, the melt was discharged from the discharge port 21 provided at the bottom of the relay and damper vessel 20, down to the semi-solidified slurry producing vessel 30 which was preheated. The melt was cooled within the semi-solidified slurry producing vessel 30, whereby the semi-solidified slurry was produced.

The conditions for producing semi-solidified slurries were as shown in FIG. 4.

For the semi-solidified slurry producing vessel 30, a composite material of carbon and ceramics (silicon carbide and boron carbide) was used as a material that can withstand high-frequency induction heating. The slurry accumulated in the semi-solidified slurry producing vessel 30 was cooled to 1200° C., and then the vessel 30 was subjected to high-frequency induction heating. When the vessel 30 was heated to 1300° C., the vessel 30 was reversed to take out the semi-solidified slurry, which was water-cooled and subjected to structure observation.

The results of experiments are shown in FIGS. 5 and 6.

According to the results of experiments shown in FIGS. 5 and 6, under the conditions shown in FIG. 4 adopted this time, it was possible to pour the melt from the ladle 10 via the relay and damper vessel 20 down into the semi-solidified slurry producing vessel 30 in the case where the temperature of the melt flowing out of the ladle 10 was not lower than 1350° C., the pre-heating temperature of the relay and damper vessel 20 was not lower than 600° C., and the discharge port 21 at the relay and damper vessel 20 had a diameter of not less than 10 mm.

In the case where the temperature of the melt flowing out of the ladle 10 was 1300° C. (Sample 1), the melt remained and solidified in the relay and damper vessel 20.

Even in the case where the temperature of the melt flowing out of the ladle 10 was 1400° C., when the diameter of the discharge port 21 at the relay and damper vessel 20 was 5 mm (Sample 26), the melt remained and solidified in the relay and damper vessel 20.

Further, even in the case where the temperature of the melt flowing out of the ladle 10 was 1400° C., when the pre-heating temperature of the relay and damper vessel 20 was as low as 300° C. (Sample 27), the melt remained and solidified in the relay and damper vessel 20.

In the case where the temperature of the melt being poured into the semi-solidified slurry producing vessel 30 exceeded 1350° C. (Samples 29, 31, 32, 33, 34, 35, and 42), primary crystals were in dendritic form, as shown in FIG. 3.

Further, in the case where the cooling rate in the semi-solidified slurry producing vessel 30 exceeded 20° C./min (Samples 40 and 49), even if the melt pouring temperature was not lower than 1300° C. and not higher than 1350° C., primary crystals became dendrites, as shown in FIG. 3.

On the other hand, in the case where the cooling rate in the semi-solidified slurry producing vessel 30 was not greater than 20° C./min, primary crystals were in granular form, as shown in FIG. 2.

However, even in the case where the cooling rate in the semi-solidified slurry producing vessel 30 was not greater than 20° C./min, when the height of the relay and damper vessel 20 from the semi-solidified slurry producing vessel 30 was less than 100 mm (50 mm in the examples) (Samples 2 to 9, 28, and 30), dendrites were found near the part coming into contact with the semi-solidified slurry producing vessel 30, although the granular crystals were found inside. This is presumably because, even though the cooling rate was not greater than 20° C./min at the temperature-measuring position in this experiment (near the central portion of the vessel 30), the cooling rate would have exceeded 20° C./min near the above-described part contacting the vessel at the outer periphery.

In the case where the cooling rate in the semi-solidified slurry producing vessel 30 was not greater than 20° C./min and the height of the relay and damper vessel 20 from the semi-solidified slurry producing vessel 30 was not less than 100 mm (100 mm and 200 mm in the examples), the stirring effect by the energy of the falling melt was obtained, and accordingly, the resultant semi-solidified slurry had granular crystals suitable for a molding process, not only inside the slurry, but also at the outer periphery thereof (coming into contact with the vessel 30).

INDUSTRIAL APPLICABILITY

The process and the apparatus for producing a semi-solidified slurry of an iron alloy according to the present invention are favorably applicable to a rheocasting process and a rheocasting apparatus, and also to other processes and apparatuses using the semi-solidified slurries.

Claims

1. A process for producing a semi-solidified slurry of an iron alloy comprising the steps of: pouring a melt of an iron alloy into a semi-solidified slurry producing vessel; and cooling the melt in the vessel to obtain a semi-solidified slurry having a crystallized solid phase and a residual liquid phase; wherein

a hypereutectoid or hypoeutectic cast iron composition containing 0.8-4.3 wt. % C is used as a material, a melt of the composition is poured into the semi-solidified slurry producing vessel in a predetermined amount at a time, a temperature of the melt when poured into the semi-solidified slurry producing vessel is controlled to be not lower than a crystallization initiation temperature of the composition and not greater than a temperature that is 50° C. higher than the crystallization initiation temperature, and a cooling rate of the melt poured into the semi-solidified slurry producing vessel is controlled not to exceed 20° C. per minute.

2. The process for producing a semi-solidified slurry of an iron alloy according to claim 1, wherein above the semi-solidified slurry producing vessel, the melt poured from a ladle is once received in a relay and damper vessel in a predetermined amount at a time, and the melt received in the relay and damper vessel is then poured into the semi-solidified slurry producing vessel via a discharge port provided at a bottom of the relay and damper vessel, a diameter of the discharge port is not less than 10 mm, a temperature of the discharged melt at the discharge port is controlled to be not lower than a temperature that is 20° C. higher than the crystallization initiation temperature of the composition and not greater than a temperature that is 80° C. higher than the crystallization initiation temperature, a preheating temperature of the semi-solidified slurry producing vessel is not less than 400° C. lower than the crystallization initiation temperature of the composition and not greater than a temperature that is 200° C. higher than the crystallization initiation temperature, and a height of the relay and damper vessel from the semi-solidified slurry producing vessel is not less than 100 mm.

3. The process for producing a semi-solidified slurry of an iron alloy according to claim 2, wherein the melt is cooled by wind while the melt is falling from the relay and damper vessel down into the semi-solidified slurry producing vessel.

4. The process for producing a semi-solidified slurry of an iron alloy according to claim 2, wherein energy caused by the melt falling from the reply and damper vessel down into the semi-solidified slurry producing vessel is used to stir the melt within the semi-solidified slurry producing vessel.

5. The process for producing a semi-solidified slurry of an iron alloy according to claim 2-4, wherein a semi-solidified slurry produced is taken out in the state where the semi-solidified slurry producing vessel is heated by high-frequency induction heating such that part of the semi-solidified slurry that is in contact with the semi-solidified slurry producing vessel is heated via the semi-solidified slurry producing vessel.