Low-expansion casting sand

Abstract

To advantageously provide casting sand that achieves low mold expansion of a casting mold and thereby gives the casting mold capable of effectively suppressing dimensional deformation during casting.SOLUTION: Casting sand mainly composed of SiO2-based refractory aggregate has a shape factor of 1.40 or more and an amount of oolitics present on a sand surface is 10 mass% or more of the total casting sand, thereby obtaining casting sand for a low expansion mold.SELECTED DRAWING: None

Description

【Technical Field】 【0001】 The present invention relates to casting sand for low-expansion molds, and particularly to casting sand that can effectively suppress the thermal expansion of molds. 【Background Art】 【0002】 Conventionally, as refractory aggregates constituting casting sand used for molding casting molds, silica sand mainly composed of silica (SiO2), various natural aggregates, and artificial sands such as synthetic mullite are well known. Among them, silica sand has been commonly used. However, SiO2-based refractory aggregates such as such silica sand generally have the property of generating large thermal expansion when exposed to high casting temperatures by molten metal poured. Therefore, a mold formed using casting sand composed of such refractory aggregates has the problem that its thermal expansion becomes large, and casting defects (for example, mold cracking, veining, squaring, welding, etc.) are likely to occur. 【0003】 Therefore, various proposals have been made to solve the problem of thermal expansion of such molds. For example, in Japanese Patent Laid-Open No. 11-188454, a refractory aggregate composed of spherical sand mainly composed of 2MgO·SiO2 obtained from molten slag of nickel ore slag, having a refractory temperature of about 1450°C and a particle size coefficient of 1.2 or less, is disclosed. In Japanese Patent Laid-Open No. 2003-251434, casting sand for molds composed of spherical objects mainly composed of synthetic mullite of 40 to 90% by weight of alumina and 60 to 10% by weight of silica, having a predetermined particle size distribution and surface area per unit volume, is proposed. Further, in Japanese Patent Laid-Open No. 2006-7319, spherical casting sand manufactured by a flame melting method, having an amorphization degree of 50 to 100%, containing Al2O3 and SiO2 as main components, and having a sphericity of 0.95 or more, is disclosed. 【0004】 However, all of the conventionally proposed low-expansion or low-thermal-expansion molding sands or foundry sands target spherical particles with a particle size coefficient (shape coefficient) or sphericity close to that of a perfect sphere. As a result, their manufacturing is time-consuming, and the refractory aggregate is inherently expensive. Moreover, the properties of these proposed molding sands and foundry sands can be achieved with artificial sand, and it has been extremely difficult to obtain spherical particles, defined by particle size coefficient and sphericity, from natural aggregates. [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Japanese Patent Application Publication No. 11-188454 [Patent Document 2] Japanese Patent Publication No. 2003-251434 [Patent Document 3] Japanese Patent Publication No. 2006-7319 [Overview of the Initiative] [Problems that the invention aims to solve] 【0006】 Herein, the present invention has been made against the background of the above circumstances, and its objective is to advantageously provide foundrying sand for low-expansion molds, and another objective is to advantageously provide foundrying sand that gives a mold in which dimensional deformation during casting can be effectively suppressed. [Means for solving the problem] 【0007】 Furthermore, in order to solve the above-mentioned problems, the present invention can be suitably implemented in various embodiments as listed below, and each embodiment described below can be adopted in any combination. It should be noted that the embodiments or technical features of the present invention are not limited in any way to those described below, and should be understood based on the inventive concept grasped from the description of the entire specification. 【0008】 Firstly, a first aspect of the present invention for solving the above-mentioned problems is a foundry sand for low-expansion molds, characterized in that it mainly consists of SiO2-based refractory aggregate, has a shape factor of 1.40 or more, and the amount of auritic present on the sand surface is 10% by mass or more of the total foundry sand. 【0009】 Furthermore, a second aspect of the present invention is characterized in that the fire-resistant aggregate is a natural aggregate. 【0010】 Furthermore, a third aspect of the present invention is characterized in that the fire-resistant aggregate mainly consists of silica sand. 【0011】 In addition, a fourth aspect of the present invention is characterized in that the fire-resistant aggregate contains SiO2 in a proportion of 75 to 90% by mass. 【0012】 Furthermore, a fifth aspect of the present invention is characterized in that the refractory aggregate is recycled from recovered sand collected during the casting process. 【0013】 Furthermore, a sixth aspect of the present invention is characterized in that the recycled sand obtained by regenerating the recovered sand contains 1 to 10% by mass of MgO and 5 to 20% by mass of Al2O3. 【0014】 Furthermore, a seventh aspect of the present invention is characterized in that the magnetic content is 4.0% by mass or more. 【0015】 In addition, the eighth aspect of the present invention relates to low-expansion mold-coated sand, characterized in that it is made by coating the low-expansion mold-making sand described in any one of the first to seventh aspects with a binder. [Effects of the Invention] 【0016】 Thus, the low-expansion mold-making sand according to the present invention targets a shape factor of 1.40 or higher, which is significantly despheric from a perfectly spherical shape with a shape factor of 1.0. Therefore, there is no need to employ the conventional process of sphericization or perfect sphericization. As a result, while effectively avoiding an increase in sand manufacturing costs, by ensuring that the amount of auritic material on the sand surface is 10% or more by mass of the total foundry sand, the thermal expansion of the quartz component based on the SiO2 content of the refractory aggregate (foundry sand) can be effectively suppressed. This suppresses the thermal expansion of molds such as cores formed from such foundry sand, and thus advantageously suppresses dimensional deformation of such molds. Consequently, it can advantageously contribute to improving the dimensional accuracy of the castings obtained by casting, and furthermore, to preventing the occurrence of various casting defects caused by the expansion of the foundry sand. [Modes for carrying out the invention] 【0017】 First, the low-expansion casting sand according to the present invention targets SiO2-based refractory aggregates, such as silica sand, which is generally composed mainly of SiO2, and especially natural aggregates (particles). In this case, as the refractory aggregate mainly composed of silica sand, it is acceptable to use only silica sand (including that which is obtained by crushing and sieving silica ore; the same applies hereinafter), or to use mixed sand which is made by mixing silica sand with various known refractory aggregates (sands). Furthermore, in the present invention, recovered sand and recycled sand, in particular refractory aggregate (recycled sand) recycled from recovered sand recovered in the casting process, is especially advantageous to use. It is desirable that such refractory aggregates generally contain 50% by mass or more of SiO2, and particularly desirable that they contain 75-90% by mass of SiO2. By adopting such an SiO2 content, it is possible to advantageously realize the low-expansion characteristics according to the present invention while ensuring the strength of the aggregate. 【0018】 In this invention, the foundry sand mainly composed of such refractory aggregate has a shape factor of 1.40 or higher. By using sand (aggregate) with such a shape factor, it is possible to advantageously contribute to reducing the thermal expansion characteristics of the mold. However, if the shape factor exceeds 1.80, it becomes difficult to use it as foundry sand. Therefore, the foundry sand according to this invention is practically configured to have a shape factor of up to about 1.80. 【0019】 Here, the shape factor of the foundry sand (refractory aggregate) used in the present invention is also called the particle size factor or particle size index, and is generally used as a measure of the external shape of the particles. The closer the value is to 1, the closer it is to a spherical shape (perfect sphere). Therefore, foundry sand (aggregate) according to the present invention with a shape factor of 1.40 or higher can be said to be non-spherical, considerably deviating from a spherical shape. Such a shape factor can be measured by various known methods. For example, as revealed in Japanese Patent Publication No. 3253579, one method involves measuring the surface area of ​​the actual sand grains per gram using a sand surface area measuring instrument (manufactured by George Fischer), and then dividing that value by the theoretical surface area, which is the surface area assuming all sand grains are spherical, to obtain the shape factor. 【0020】 In addition, the foundry sand according to the present invention has a large amount of orichics present on its sand surface such that the amount of orichics present on the sand surface is 10% by mass or more of the total foundry sand. As a result, the thermal expansion of quartz in the refractory aggregate of SiO2 quality in the foundry sand is effectively suppressed, and thus the thermal expansion of the mold formed using such foundry sand is effectively suppressed or even prevented, improving the dimensional accuracy of the mold such as the core and, ultimately, the casting obtained by casting. Furthermore, the occurrence of casting defects (such as mold cracking, veining, squaring, welding, etc.) caused by the expansion of the foundry sand can be advantageously eliminated or suppressed. Here, if the amount of orichics present on such a sand surface becomes too large, the strength of the mold obtained by molding will decrease. Therefore, it is generally desirable to limit the upper limit to about 50% by mass of the total foundry sand. 【0021】 Here, the orichics present on the sand surface is a coating layer forming component composed of calcined clay, ash, etc., formed on the sand particles by minerals such as feldspar other than quartz contained in the refractory aggregate of SiO2 quality such as silica sand. Generally, it is determined in accordance with the silica program test method for green sand (Japan Foundry Engineering Society, Tokai Branch, Sand Mold Research Group, Test Method: TJFS-210). That is, the amount of orichics (O: %) is calculated by the following formula based on the mass of the sample after hydrofluoric acid treatment (FW: g) and the mass of the sample after hydrochloric acid washing (HW: g) obtained by subjecting the sample after hydrochloric acid washing to a dissolution treatment with a 30% hydrofluoric acid solution. O = [HW × (5 - FW)] / 1.25 【0022】 In particular, in the case of the foundry sand according to the present invention, when it is composed of recycled sand obtained by recycling the recovered sand recovered in the casting process, it is desirable to adjust the MgO content to be 1 - 10% by mass and the Al2O3 content to be 5 - 20% by mass. This makes it easier to more advantageously exhibit the characteristics of the present invention, particularly the effect of low expansion. 【0023】 Furthermore, by setting the magnetic content in such foundry sand to 4.0% by mass or more, it becomes possible to effectively suppress or prevent an increase in the coefficient of thermal expansion. Generally, the upper limit for this magnetic content is set at around 20.0% by mass. This is because if the magnetic content becomes too high, problems such as difficulty in ensuring sufficient strength of the mold may arise. 【0024】 Furthermore, it is desirable that the amount of igloss in the foundry sand be 1.0 mass% or less, which will advantageously help avoid problems such as gas defects in casting operations using molds obtained with such foundry sand. 【0025】 Incidentally, since the auritic material present on the sand surface as described above is formed when the foundry sand (refractory aggregate) undergoes a thermal history such as roasting, the foundry sand according to the present invention is advantageously made from recycled sand obtained by regenerating waste silica sand or recovered silica sand mainly from the casting process, in the same manner as before, according to a regeneration process that includes magnetic separation, roasting, and polishing. However, in the regeneration process to obtain such recycled sand, by shortening, weakening, or omitting the polishing process, the recycled sand can be kept in an angular shape, resulting in non-spherical particles with a shape coefficient of 1.40 or more, while retaining sufficient auritic material on the particle surface, thereby advantageously obtaining recycled sand with an auritic material content of 10% by mass or more. Furthermore, regarding the magnetically attached components, it is also easy to ensure that the magnetically attached components in the recycled sand are at least 4.0% by mass by adjusting the conditions of the magnetic separation process used in such recycling processes, such as shortening, weakening, or omitting them. 【0026】 Thus, the foundry sand according to the present invention can be easily obtained from recovered sand collected in the casting process by subjecting it to a predetermined regeneration treatment, for example, by performing a roasting treatment and a magnetic separation treatment of 1000G or more (~5000G) once or more. However, it goes without saying that the present invention is not limited to such recycled sand, and that natural new sand or artificial sand can also be used as a target of the present invention as long as they satisfy the shape coefficient and auricular content specified in the present invention. Furthermore, in cases where the auricular content is low, the present invention can also be used to increase the auricular content by adding auricular-forming components such as bentonite to refractory aggregate and subjecting it to heat treatment such as roasting, thereby achieving an auricular content within the range specified in the present invention. 【0027】 Furthermore, the foundry sand (refractory aggregate) described above generally has an AFS index of 30 to 90, preferably 35 to 80, and more preferably 40 to 70. If the AFS index is less than 30, the particle size of the foundry sand becomes too large, which may reduce its properties as a mold and may also cause problems such as deterioration of the casting surface. On the other hand, if the AFS index exceeds 90, the particle size of the foundry sand becomes too small, which can lead to a large amount of clumps (aggregates of foundry sand) being generated when mixed with the mold binder, and may also make it difficult to fully realize the features of the present invention. 【0028】 Furthermore, the foundry sand according to the present invention can be used alone to form a target mold, thereby exhibiting the characteristics of the present invention. It can also be mixed with other foundry sands to form a foundry sand composition, which can then be used to form a target mold. In this case, the proportion of the foundry sand according to the present invention in the foundry sand composition should be at least 5% by mass of the total. Other foundry sands that can be mixed with the foundry sand according to the present invention include various known foundry sands (refractory aggregates), but in particular, various known low-expansion sands such as olivine sand, zircon sand, alumina sand, ferroalloy residues such as ferrochrome-based and ferronickel-based sands, synthetic mullite particles, foresterite particles, etc. Furthermore, the shape of these other foundry sands is not limited to the non-spherical shape as in the present invention; spherical particles are also acceptable. 【0029】 Furthermore, when forming a mold for a target core or the like using foundry sand or a foundry sand composition containing the same according to the present invention, first, such foundry sand or foundry sand composition is mixed with a conventionally known mold binder, and the surface of the foundry sand (aggregate or particles) is coated with the mold binder, thereby forming coated sand (CS) that exhibits excellent characteristics. 【0030】 Furthermore, as a mold binder used to manufacture such CS, inorganic binders such as water glass and resin binders such as phenolic resins can be appropriately used, but in particular, resin binders are preferably used in the present invention. As for the resin binder, various conventionally known types can be listed, for example, phenolic resins, furan resins, urethane resins, amine polyol resins, unsaturated polyester resins, diallyl phthalate resins, polyether polyol resins, etc., and can be appropriately selected and used, but among these, phenolic resins are advantageously used. 【0031】 Furthermore, phenolic resins, which are suitably used as resin binders, are, as is well known, solid or liquid (including varnish and emulsion forms) condensation products obtained by reacting phenols and aldehydes in the presence of an acidic or basic catalyst. Depending on the type of catalyst used, they are referred to as novolac type or resol type, and are phenolic resins that exhibit thermosetting properties by heating in or without a predetermined curing agent or curing catalyst. 【0032】 Novolac-type phenolic resins are formed by a condensation reaction using phenols and aldehydes with an acidic catalyst, as is well known. Resol-type phenolic resins are formed by a condensation reaction using phenols and aldehydes with a basic catalyst, as in the conventional method. These novolac-type and resol-type phenolic resins can be used individually or mixed in appropriate proportions without any problem. Furthermore, as is well known, modified phenolic resins obtained by changing some of the phenol to components such as bisphenol A and naphthol can also be used, and they can even be used as benzylic ether-type phenolic resins. 【0033】 Furthermore, when mixing a resin binder such as the phenolic resin mentioned above into the foundry sand, the amount of such resin binder to be added is determined appropriately considering the type of resin used and the required strength of the mold, and cannot be uniquely defined. However, generally, it is in the range of 0.2 to 10 parts by mass per 100 parts by mass of foundry sand, preferably in the range of 0.5 to 8 parts by mass, and more preferably in the range of 1 to 5 parts by mass. 【0034】 Incidentally, when manufacturing CS for forming a target mold using the foundry sand according to the present invention, the foundry sand or foundry sand composition containing it is mixed with the above-mentioned mold binder and other conventional compounding components in a conventional manner. The manufacturing method used is not particularly limited, and any conventionally known method such as the dry hot coat method, semi-hot coat method, cold coat method, or powder solvent method can be used. However, in the present invention, it is particularly recommended to use the so-called dry hot coat method, in which the preheated foundry sand and mold binder are mixed in a kneader such as a whirl mixer or speed mixer, an aqueous solution of a predetermined hardening agent or hardening accelerator such as hexamethylenetetramine, and other compounding components are added, and the lump contents are separated into granules by forced air cooling, and then a lubricant such as calcium stearate is added. The timing for mixing mold binders, hardeners / hardening accelerators, etc., with the foundry sand is to be appropriately selected based on the knowledge of those skilled in the art. They can be added and mixed individually, sequentially, or in combination as appropriate. 【0035】 Furthermore, when forming a predetermined mold, such as a shell mold, using the CS obtained as described above, the mold is formed under heating in order to heat-harden the CS. However, there are no particular limitations on the heating method used, and any conventionally known method can be used to its advantage. For example, the CS described above can be filled into a mold preheated to about 150-300°C, which has a desired shape space to give the desired mold, by gravity drop or blowing, and after hardening, the hardened mold can be removed from the mold to obtain the desired casting mold. 【0036】 When casting a predetermined molten metal, such as molten aluminum, using a mold formed in this manner to produce a target casting, even if high heat from the molten metal acts on the mold, since such a mold is formed using low-expansion mold-making sand according to the present invention, the thermal expansion of the mold can be effectively suppressed, and dimensional deformation of the mold, especially the core, can be effectively suppressed. As a result, the dimensional accuracy of the resulting casting can be advantageously improved, and the occurrence of casting defects can also be advantageously suppressed or prevented. [Examples] 【0037】 The present invention will be further clarified by showing some embodiments below, but it goes without saying that the present invention is not limited in any way by the description of such embodiments. It should be understood that, in addition to the embodiments below and the specific descriptions above, various changes, modifications, and improvements can be made to the present invention based on the knowledge of those skilled in the art, as long as they do not depart from the spirit of the present invention. 【0038】 Furthermore, in the following description, parts and % refer to parts by mass and mass%, respectively, unless otherwise specified. In addition, the magnetic deposition content, spheroidity coefficient, and auritic content of the foundry sand produced below, as well as the fusion point of the resin-coated sand (RCS) obtained using it, and the flexural strength and thermal expansion coefficient of the molds obtained from each RCS, were evaluated according to the following method. 【0039】 -Measurement of magnetic deposition- An electronic balance (sensitivity 0.1g) and a magnet (1500G) are used as measuring instruments. First, approximately 10g of foundry sand is accurately weighed, and then the weighed foundry sand is placed on a piece of paper for measurement. Next, the magnet is moved lightly over the foundry sand on the paper to allow the magnetic particles (sand) to adhere to the magnet. This process of removing the magnetic particles from the magnet is repeated until no more sand adheres to the magnet. The magnetic particles (sand) that have adhered to the magnet are then collected, and their mass is weighed using the electronic balance. The magnetic attachment percentage (%) is calculated by dividing the total amount of magnetic particles attached to the magnet by the mass of the foundry sand before the magnet treatment and expressing the result as a percentage. 【0040】 -Measurement of shape factor- For each type of foundry sand, a sand surface area meter (manufactured by George Fischer) is used to measure the actual surface area of ​​the sand grains per gram. This value is then divided by the theoretical surface area, which is the surface area assuming all sand grains are spherical, and this value is defined as the shape factor. 【0041】 -Measurement of Auriticus amount- In accordance with "4.7 Determination of Auriculars" in the Silica Program Test Method for Green Sand (Japan Foundry Engineering Society, Tokai Branch, Sand Mold Research Committee, Test Method: TJFS-210), 5g of the sample after hydrochloric acid washing is used, 30mL of 30% hydrofluoric acid solution is added to it, and the mixture is stirred for 1 minute. Then, after standing for 1 minute, the mixture is stirred again for 1 minute, and after repeating the water washing process, the mixture is dried, and the weight of the sample after the hydrofluoric acid treatment is determined. The amount of auriculars (0%) is then calculated according to the formula shown earlier in the main text of this specification. 【0042】 -Measurement of RCS fusion point- In accordance with the "Fusion Point Test Method" of JACT Test Method C-1, the fusion point (°C) of each RCS manufactured from each type of foundry sand as described below will be determined. 【0043】 -Measurement of bending strength- In accordance with the "Bending Strength Test Method" of JACT Test Method SM-1, the fracture load of a test specimen measuring 10 mm wide x 10 mm thick x 60 mm long (molding temperature: 250°C), obtained using each RCS described below, is measured using a measuring instrument (Takachiho Seiki Co., Ltd.: Digital Foundry Sand Strength Tester). Then, using this measured fracture load, the flexural strength is calculated using the following formula to obtain the flexural strength (N / cm²). 2 ) Furthermore, its bending strength is assumed to be kgf / cm². 2 Convert to and display. Transverse bending strength (N / cm 2 ) = 1.5 × LW / ab 2 [However, L: distance between supports (cm), W: breaking load (N), a: width of the test specimen (cm), b: thickness of the test specimen (cm)] 【0044】 -Measurement of thermal expansion coefficient- The test was conducted according to the rapid thermal expansion coefficient measurement method described in the "Thermal Expansion Coefficient Measurement Test Method" of JACT Test Method M-2. First, a test piece (28.3 mmφ × 51 mmL, approximately 1 / 4 of the circumference cut) prepared at a firing temperature of 280°C for 120 seconds was placed in a high-temperature casting sand tester adjusted to a furnace temperature of 1000°C. The test piece was held for 0.0 minutes (before exposure to heat) and then in 0.5-minute increments up to 4.0 minutes, and the test piece was removed after each holding time. Then, the thermal expansion coefficient (%) after each holding time was calculated from the length of the test piece before exposure to heat (0.0 minute holding) and after exposure to heat (after holding at 0.5-minute intervals) according to the following formula. Thermal expansion coefficient (%) = [(Test piece length after exposure - Test piece length before exposure)] × 100 / (Test piece length before exposure) 【0045】 - Manufacturing of foundry sand A to H - Using recovered sand, primarily used silica sand (recovered silica sand) that has been recovered and accumulated from various casting processes, a regeneration process including magnetic separation, roasting, and polishing was carried out. By either omitting the polishing process or changing the processing conditions, the amount of uric acid present on the surface of the recovered sand and the shape coefficient were adjusted, thereby producing various types of foundry sand A to H as shown in Table 1 below. 【0046】 Furthermore, for the foundry sands A to H obtained, the magnetic content, shape factor, and auritic content were measured according to the method described above, and the results are shown in Table 1 below. 【0047】 [Table 1] 【0048】 -RCS fabrication- Using the various foundry sands A to H shown in Table 1 above, resin-coated sands (RCS) according to Examples 1 to 5 and Comparative Examples 1 to 4 were prepared as follows. 【0049】 Specifically, a selection was made from the various foundry sands shown in Table 1. 100 parts of the foundry sand, heated to 150°C, were mixed with the amounts of commercially available resin binders (binders X and Y) shown in Tables 2 and 3 below. The mixture was then kneaded in a speed mixer for 50 seconds. A solution of 0.35 parts of hexamethylenetetramine, a hardening agent, dissolved in 1.5 parts of water was added, and the mixture was kneaded until the sand separated into individual particles. Finally, 0.1 parts of calcium stearate was added and mixed for 15 seconds. The mixture was then removed from the mixer, thereby obtaining the desired RCS, in which the surface of the foundry sand was coated with the binder resin. For the resin binder, binder X was SP6384, a novolac-type phenolic resin manufactured by Asahi Organic Chemicals Co., Ltd., and for binder Y, BP399, a bisphenol A-modified novolac-type phenolic resin manufactured by Asahi Organic Chemicals Co., Ltd., was used. 【0050】 -Evaluation of mold characteristics- Then, from the RCS obtained in Examples 1 to 5 and Comparative Examples 1 to 4, test pieces for evaluating flexural strength and test pieces for evaluating thermal expansion coefficient were fabricated, respectively. The flexural strength and thermal expansion coefficient were measured according to the previously described procedure, and the results obtained are shown in Tables 2 and 3 below. 【0051】 [Table 2] 【0052】 [Table 3] 【0053】 As is clear from the comparison of the results in Tables 1 to 3 above, the foundry sand used in the RCS of Examples 1 to 5 all have the characteristics of a shape factor of 1.40 or higher and a sand surface auritic content of 10% or higher. Therefore, by using the RCS of Examples 1 to 5, a mold with a low coefficient of thermal expansion is realized while ensuring sufficient mold strength (flexural strength), thereby suppressing mold deformation and thus advantageously improving dimensional accuracy. 【0054】 In contrast, in the molds using RCS shown in Comparative Examples 1 to 4, the foundry sand is sufficiently polished, resulting in a shape factor of less than 1.40, making it nearly spherical, or the amount of auritic material is less than 10%, which increases the coefficient of thermal expansion. Consequently, when using RCS according to Comparative Examples 1 to 4, mold deformation may occur, potentially leading to a decrease in the dimensional accuracy of the resulting casting or the occurrence of casting defects.

Claims

[Claim 1] SiO 2 A low-expansion molding sand characterized by being primarily composed of high-quality refractory aggregate, having a shape coefficient of 1.40 or more, and the amount of auritic present on the sand surface being 10% by mass or more of the total molding sand. [Claim 2] The low-expansion molding sand according to claim 1, characterized in that the refractory aggregate is a natural aggregate. [Claim 3] The low-expansion molding sand according to claim 1, characterized in that the refractory aggregate mainly consists of silica sand. [Claim 4] The aforementioned fire-resistant aggregate is SiO 2 The low-expansion molding sand according to claim 1, characterized in that it contains 75 to 90% by mass of the above. [Claim 5] The low-expansion molding sand according to claim 1, characterized in that the refractory aggregate is recycled from recovered sand collected during the casting process. [Claim 6] The recycled sand obtained by recycling the aforementioned recovered sand contains 1 to 10% by mass of MgO and 5 to 20% by mass of Al 2 O 3 The low-expansion molding sand according to claim 5, characterized in that it contains the following. [Claim 7] The low-expansion molding sand according to claim 5, characterized in that the magnetic content is 4.0% by mass or more. [Claim 8] A coated sand for low-expansion molds, characterized in that it is made by coating the low-expansion molding sand described in any one of claims 1 to 7 with a binder.

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