Refractory aggregate for foundry sand

Abstract

To provide a refractory aggregate for molding sand made of spherical refractory sintered particles artificially produced by a sintering method, which can improve fluidity in the molding sand and can realize excellent mold strength.SOLUTION: In a refractory aggregate for molding sand made of spherical refractory sintered particles artificially produced by a sintering method, the out-of-roundness is 0.70 or more, the average particle diameter is 0.01 to 1.00 mm, a pore volume with a pore diameter of 2 to 10 μm, obtained by a mercury penetration porosimeter, lies in the range of 0.20 to 0.50 mL / g, and the amount of mixed fine powder reaches 3,000 NTU or lower in a turbidity measured by nephelometry.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a refractory aggregate for foundry sand, and more particularly to a refractory aggregate composed of spherical refractory sintered particles artificially manufactured by a sintering method, which can improve the fluidity of the foundry sand obtained by using the same and advantageously improve the strength of a mold molded from such foundry sand.

Background Art

[0002] Conventionally, as one of the molds used for casting molten metals such as cast iron, cast steel, aluminum, copper, or their alloys, a mold formed into a desired shape using foundry sand obtained by mixing or kneading refractory particles (aggregates) with a predetermined binder, hardening agent (catalyst), etc. is known. There, as a binder for binding the refractory aggregate, an organic binder mainly composed of a resin such as a phenol resin or a furan resin, or an inorganic binder mainly composed of water glass, clay, etc. is used. Further, as molding methods using such foundry sand, various methods such as a green sand method, a gas sand method, a shell mold method, and a three-dimensional lamination molding method are known, and a casting mold having a cavity (casting cavity) of a desired casting shape, particularly a self-hardening mold, is molded by any of these molding methods.

[0003] Incidentally, as refractory particles (aggregates) that are the main component of the molding sand used for molding such molds, natural sands such as silica sand, zircon sand, olivine sand, and chromite sand have been widely used. However, natural products not only have variations in quality such as physicochemical properties, but also include the problem of depletion due to resource reduction in recent years. Therefore, artificially manufactured refractory particles have been proposed and put into practical use. For example, in Japanese Patent Publication No. 3-47943 and Japanese Patent Publication No. 4-40095, as a sintering method, from a raw material composition in which alumina and silica are blended so as to have a predetermined chemical composition, after granulating into spherical shapes by a method such as a spray dryer, the obtained granulated product is sintered in a rotary kiln or the like to manufacture spherical aggregates. The method has been clarified.

[0004] However, in the case of artificial sand composed of refractory sintered particles manufactured by such a sintering method, although its sphericity is good, when sintering, unevenness occurs on the particle surface or the particle surface becomes porous. Therefore, in a mold molded from molding sand obtained by adding a predetermined amount of binder (resin) to such artificial sand, there are problems such as insufficient strength. Also, in order to avoid such surface state problems, if the addition amount of the binder is increased, there is a problem that it does not become flowing sand, in other words, molding sand with good fluidity. In particular, when molding a mold, it is not always possible to realize the target mold only with the fluidity characteristics of the sand (aggregate) itself. When considering the actual application to a mold, not only the fluidity characteristics of the sand (aggregate) itself but also the fluidity characteristics when kneaded with a resin, a hardening agent, etc. are important.

[0005] Thus, sintered sand, which is artificial sand, inherently has problems that need to be solved in terms of improving the fluidity when used as molding sand and improving the strength of the mold to be molded. In particular, when manufacturing a mold with a complex shape by a method such as hand molding, injection molding, or three-dimensional sand mold lamination molding method, it is required that the molding sand obtained by kneading with a binder or the like maintains good fluidity and the mold produced therefrom has high strength.

[0006] In addition, in Japanese Patent Laid-Open No. 2006-7319, it is clarified that for sand particles produced by the flame melting method, by defining their degree of amorphization and sphericity, it is possible to manufacture a casting mold with excellent fluidity, high strength, and low thermal expansion. However, it is difficult to think that the less the unevenness on the sand surface, the better the fluidity of the sand. On the contrary, when the unevenness on the sand surface is small, a binder (resin) layer that affects the fluidity is formed thicker, so the fluidity of the foundry sand deteriorates.

[0007] Also, in Japanese Patent Laid-Open No. 2003-311370, an aggregate for use in manufacturing a mold by blending a furan resin and a curing agent with an aggregate such as sand, kneading, and then molding is proposed. The aggregate is subjected to a fine powder removal treatment in advance or a basic ion removal treatment in advance. It is clarified that a high-strength mold can be molded by using this aggregate. However, nothing is clarified about what kind of influence the fine powder adhering to the surface of the aggregate has on the fluidity of the foundry sand formed by mixing a binder or the like. Furthermore, nothing is clarified about how the surface of the aggregate should be adjusted.

[0008] Furthermore, regarding a self-hardening flow mold using artificial sand with a sphericity of 0.8 or more produced by a melting method or a flame melting method, in Japanese Patent Application Laid-Open No. 2009-119469, in paragraph

[0006] thereof, although the artificial sand obtained by a sintering method has good sphericity, when firing, unevenness occurs on the sand surface or the sand surface becomes porous. Therefore, if the amount of resin (binder) added is not increased, there are problems such as insufficient strength. Also, due to this surface problem, if the amount of resin added is increased, there will be a problem that it will not become flowing sand. However, it is difficult to consider that the fluidity deteriorates due to an increase in the unevenness amount on the sand surface. When the amount of resin (binder) added is such that the mold strength is the same, it is considered that the fluidity will be the same regardless of the unevenness amount on the sand surface. However, it is presumed that the thickness of the binder or the like present on the sand surface contributes to the fluidity and the mold strength. If this thickness is the same, it is presumed that the fluidity and the mold strength will be the same.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0010] Here, the present invention has been made against such a background, and the problem to be solved is to provide a refractory aggregate for casting sand composed of spherical sintered particles capable of exhibiting excellent properties. Another problem is to provide a refractory aggregate for casting sand composed of spherical refractory sintered particles artificially manufactured by a sintering method, which can improve the fluidity of the casting sand and achieve excellent mold strength.

Means for Solving the Problems

[0011] The present invention can be preferably implemented in various aspects listed below in order to solve the above-mentioned problems. Moreover, each of the aspects described below can be adopted in any combination. It should be understood that the aspects or technical features of the present invention are not limited to those described below, and can be recognized based on the inventive concept grasped from the description of the entire specification.

[0012] And the first aspect of the present invention for solving the above-mentioned problems is a refractory aggregate for casting sand composed of spherical refractory sintered particles artificially manufactured by a sintering method, having a roundness of 0.70 or more, an average particle diameter of 0.01 to 1.00 mm, and a pore volume with a pore diameter of 2 to 10 μm, determined by a mercury intrusion porosimeter, in the range of 0.20 to 0.50 mL / g, and further having an amount of mixed fine powder such that the turbidity measured by a turbidimetry method is 3000 NTU or less. The gist of the refractory aggregate for casting sand is characterized by this.

[0013] Moreover, the second aspect of the refractory aggregate for casting sand according to the present invention is characterized in that the refractory sintered particles have a chemical composition containing 40 to 90% by mass of Al2O3 and 60 to 10% by mass of SiO2.

[0014] The third aspect of the present invention is characterized in that the refractory sintered particles have a chemical composition in which the content of Al2O3 is 60% by mass or more and the content of SiO2 is 40% by mass or less, and the total content of Fe2O3 and TiO2 is 3% by mass or less.

[0015] Furthermore, the fourth aspect of the refractory aggregate for casting sand according to the present invention is characterized in that the refractory sintered particles are mullite-based or mullite-cordierite-based sintered particles.

[0016] In addition, the fifth aspect of the present invention is a refractory aggregate for casting sand, characterized in that the roundness is 0.75 or more.

[0017] Furthermore, the sixth aspect of the present invention is characterized in that the pore volume of pores having a pore diameter of 2 to 10 μm is in the range of 0.25 to 0.45 mL / g.

[0018] And the seventh aspect of the refractory aggregate for casting sand according to the present invention is characterized in that the amount of mixed fine powder is configured to have a turbidity of 2000 NTU or less.

[0019] Also, the eighth aspect of the refractory aggregate for casting sand according to the present invention is characterized in that it is used as an aggregate for casting sand for molding a self-hardening mold.

[0020] Furthermore, the ninth aspect of the refractory aggregate for casting sand according to the present invention is characterized in that it is used as an aggregate for casting sand in a three-dimensional lamination molding method.

[0021] In addition, the tenth aspect of the present invention is a casting mold obtained by molding using the refractory aggregate for casting sand as described above.

Advantages of the Invention

[0022] By using the refractory aggregate for foundry sand according to the present invention, the fluidity of the foundry sand obtained by mixing or kneading a binder, a hardening agent, etc. therewith can be effectively increased, and further, the strength of the mold molded using such foundry sand can also be more advantageously improved.

[0023] And the refractory aggregate for foundry sand according to the present invention can be particularly advantageously used as an aggregate for foundry sand for molding a self-hardening mold and as an aggregate for foundry sand in the three-dimensional lamination molding method.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0025] Incidentally, the refractory aggregate for foundry sand according to the present invention is composed of spherical refractory sintered particles artificially manufactured by a sintering method. As long as they are spherical refractory particles, any known artificial sintered particles can be targeted as artificial sand. Specifically, there are sintered sands obtained by granulating by the spray drying method and then firing in a rotary kiln, sintered sands obtained by granulating by the rolling granulation method and then firing in a rotary kiln, etc. Also, in terms of material, it is possible to use materials such as sintered mullite and sintered alumina. Incidentally, the spherical refractory particles obtained by such a sintering method have fine irregularities on the particle surface, and from where a binder component or the like enters there, in order to improve the fluidity of the foundry sand and the mold strength, in the present invention, the adjustment of its surface properties is adopted as one means.

[0026] In addition, the spherical refractory sintered particles (aggregates) targeted in such an invention of the present invention preferably have a chemical composition consisting of 40% by mass or more of Al2O3 (alumina) and 60% by mass or less of SiO2 (silica). Among them, a configuration having a chemical composition containing 40 to 90% by mass of Al2O3 and 60 to 10% by mass of SiO2 will be preferably adopted. Here, if the content of such Al2O3 becomes too small, in other words, if the content of SiO2 becomes excessive, the thermal expansion of the sintered particles becomes large, and it becomes difficult to be advantageously used as the refractory sintered particles according to the present invention. In particular, in the present invention, as such refractory sintered particles, those having a chemical composition containing 60% by mass or more of Al2O3 and 40% by mass or less of SiO2, and further having a total content of Fe2O3 and TiO2 of 3% by mass or less are preferably adopted. Thus, by regulating the contents of Fe2O3 and TiO2 as impurities, sintered particles with high heat resistance can be obtained. Further, in the present invention, among the above chemical compositions, refractory sintered particles (aggregates) made of mullite or mullite-cordierite materials are preferably used. Here, the mullite-cordierite quality means a state in which the crystal structure of mullite and the crystal structure of cordierite coexist or are dispersed in the particles and is used as such.

[0027] And in the present invention, as such spherical refractory sintered particles, those having a spherical shape that can be substantially recognized as spherical, and generally, as the roundness thereof, refractory sintered particles having a roundness of 0.70 or more, preferably 0.75 or more, more preferably 0.80 or more are advantageously used. By using such spherical refractory sintered particles having such a roundness, the features of the present invention can be more advantageously exhibited.

[0028] Here, the roundness of the refractory sintered particles can be measured by known methods. For example, it can be measured by a particle shape measuring device: PartAn SI manufactured by Microtrac Bell Co., Ltd. Such a device is composed of a sample cell, a stroboscopic LED, and a high-sensitivity CCD camera. The measurement principle is that while circulating water by a pump, a sample (refractory sintered particles) is introduced, so that the sample cell arranged between the stroboscopic LED light source and the CCD camera is passed through by water in which the sample particles are mixed. By image-analyzing the projection image obtained at that time, the projected area and the maximum Feret diameter for each particle are obtained. And from the obtained values of the maximum Feret diameter and the projected area, the following formula: Roundness = [4 × projected area (mm 2 )] / [π × {maximum Feret diameter (mm)} 2 is used to calculate the roundness of each particle. Specifically, after more than 5000 sample particles are introduced and the roundness of each particle is calculated, the average value of the obtained roundness values is averaged by the number of measured particles, whereby the roundness (average value) can be obtained respectively.

[0029] Also, the refractory sintered particles (refractory aggregate) according to the present invention have an average particle diameter of 0.01 to 1.00 mm, preferably about 0.05 to 0.50 mm, more preferably about 0.07 to 0.40 mm. If the average particle diameter of the refractory sintered particles is too small, there are problems such as handling difficulties and difficulties in molding the mold. Also, if the particle diameter is too large, in addition to being difficult to fully exhibit the characteristics of the present invention, it also causes problems such as a decrease in the quality of the cast product. Here, the average particle diameter means the particle diameter (D 50 ) at the integrated value of 50% in the particle size distribution obtained by the laser diffraction / scattering method.

[0030] ​By the way, in the refractory sintered particles artificially produced by the sintering method, many small holes called micropores, mesopores, and macropores with pore diameters ranging from 0.1 nm to 1000 μm are open on the particle surface, thereby forming an uneven surface shape. And the physical properties of the sintered particles are deeply related to the properties of such pores. For this reason, conventionally, for the evaluation of the unevenness of such particle surfaces, the surface roughness (Ra) has been used to evaluate from large unevenness to small unevenness, and further to fine unevenness, or the porosity (water absorption rate) has been used to evaluate from medium-sized unevenness to small unevenness and fine unevenness. However, in these evaluation methods, it has been difficult to fully grasp the physical properties of the sintered particles, particularly the fluidity of the casting sand obtained by mixing (kneading) with a binder or the like, and the strength of the mold formed from such casting sand.

[0031] Therefore, the present inventors evaluated the unevenness of the sintered particle surface in terms of the pore volume obtained by a mercury porosimeter or a mercury intrusion porosimeter based on the mercury intrusion method at an arbitrary pore diameter. As a result, regarding the absorption of the binder that affects the fluidity of the casting sand and its coating thickness, the presence of large unevenness and fine unevenness is hardly related, and the influence of those with a pore diameter in the range of 2 to 10 μm, which is mainly related to medium-sized unevenness and small unevenness, is large. Moreover, it has become clear that for those with such a pore diameter of 2 to 10 μm, the pore volume measured by the above-described mercury intrusion method needs to be within the range of 0.20 to 0.50 mL / g.

[0032] Here, the mercury intrusion method (mercury intrusion porosimeter) applies pressure to mercury to intrude it into the pores or gaps on the particle surface, and calculates the pore diameter from the pressure applied at that time. Since the amount of mercury intrusion is the volume of the pores, by changing the pressure, the pore volume at each pore diameter can be calculated. Therefore, the integrated pore volume with pore diameters ranging from 2 to 10 μm is defined as the pore volume with pore diameters of 2 to 10 μm. Note that if the pore volume with pore diameters of 2 to 10 μm is lower (smaller) than 0.20 mL / g or higher (larger) than 0.50 mL / g, as will be discussed later, it will have an adverse effect on the fluidity when used as foundry sand and the strength of the mold to be formed. In particular, in the present invention, sintered particles with a pore volume of the pores with pore diameters of 2 to 10 μm within the range of 0.25 to 0.45 mL / g are advantageously used.

[0033] In addition, spherical refractory sintered particles artificially manufactured by a sintering method constituting a refractory aggregate for foundry sand inevitably contain fine powder. When a binder or the like is mixed (kneaded) to obtain foundry sand, it adheres to the fine powder, which will have an adverse effect on the resulting mold strength. Therefore, in the present invention, the amount of fine powder mixed in the refractory sintered particles (refractory aggregate) is adjusted so that the turbidity measured by a known nephelometry method is 3000 NTU or less, preferably 2000 NTU or less, more preferably 1000 NTU or less. Using refractory sintered particles with a fine powder amount such that the turbidity exceeds 3000 NTU will cause problems such as difficulty in achieving high mold strength.

[0034] Here, the turbidity measured by the nephelometry method is obtained by uniformly mixing the sample (refractory sintered particles) with water, allowing it to stand, collecting the supernatant liquid, and performing a comparative calculation of the scattered light intensity and the transmitted light intensity, and is calculated as the NTU (nephelometer turbidity unit) value. For example, it can be measured using a portable turbidimeter 2100Q manufactured by HACH Co., Ltd. (sold by Toa DKK Corporation, Japan). Also, the obtained turbidity can be calibrated using a formazin standard solution so that NTU = FTU[℃(formazin)].

[0035] When manufacturing the spherical refractory sintered particles (refractory aggregate for casting sand) according to the present invention having the above-described characteristics, an Al2O3 raw material and an SiO2 raw material are used to form a raw material composition blended so as to provide the above chemical composition, and a method of artificially manufacturing sintered particles having the desired roundness and average particle diameter is adopted according to a conventionally known sintering method. At that time, in order to make the pore volume with a pore diameter of 2 to 10 μm according to the present invention fall within a predetermined range, it is necessary to control the unevenness on the particle surface.

[0036] Incidentally, the surface unevenness in the sintered particles is derived from crystallization. For example, in refractory particles mainly composed of mullite (3Al2O3·2SiO2), needle-like crystals are formed on the particle surface, and since these are randomly oriented, selecting the manufacturing conditions so as not to grow the needle-like crystals from the places where convex portions are formed will lead to suppression of the convex portions. Therefore, when manufacturing the above-described mullite-based refractory sintered particles, for example, when firing a spherical granulated product of the raw material composition in a rotary kiln, a method of lowering the firing temperature to suppress or prevent the precipitation of mullite crystals is advantageously adopted. In addition, after the firing process or the sizing process, surface polishing (mechanical polishing) is performed, or surface coating is performed using an inorganic substance such as water glass to fill the recesses, whereby the pore volume of the pores providing the desired pore diameter is adjusted.

[0037] Also, even in the amount of fine powder mixed in such refractory aggregates for foundry sand (refractory sintered particles), in the sizing process carried out after firing of the granulated product, a dust collection operation is carried out, and by changing the dust collection power, the removal amount of the fine powder mixed in the sintered particles is controlled to prepare sintered particles that satisfy the turbidity defined in the present invention.

[0038] Thus, the refractory aggregate for foundry sand according to the present invention is advantageously formed in the form of new sand. However, as long as the roundness, average particle diameter, pore volume with pore diameters of 2 to 10 μm, and turbidity defined in the present invention are satisfied, it is also possible to target recycled sand obtained by subjecting recycled sand recovered from a mold making process or a casting process of a mold to a predetermined recycling treatment, and it should be understood that the features of the present invention can be exhibited.

[0039] And the refractory aggregate for foundry sand according to the present invention thus obtained is mixed or kneaded in the same manner as in the prior art using a known suitable binder such as an inorganic binder or an organic binder and a curing agent (catalyst), etc., and after being prepared as foundry sand, according to a normal molding method, for example, a green sand molding method, a gas molding method, a shell mold method, etc., a target casting mold, particularly an inorganic or organic self-hardening mold, is formed. And in that case, the features of the present invention can be advantageously exhibited. In addition, the features of the present invention can be more advantageously exhibited even in the use as a refractory aggregate for foundry sand in a three-dimensional laminated molding method.

Examples

[0040] Examples according to the present invention are shown below together with comparative examples to clarify the present invention more specifically. Needless to say, the present invention is not subject to any restrictions by such descriptions of the examples. Also, it should be understood that the present invention can be subjected to various changes, modifications, improvements, etc. based on the knowledge of those skilled in the art without departing from the gist of the present invention in addition to the following examples and further beyond the above specific descriptions.

[0041] [Example 1] First, as refractory aggregates for foundry sand, natural sand, fused sand, and sintered sands A to H having the chemical compositions and average particle diameters shown in Table 1 below were each prepared. Among them, as the natural sand, Wedron silica sand (produced in the United States) was obtained from Hyoya Co., Ltd., and as the fused sand, GREEN BEADS (green beads: manufactured by Kinsai Matech Co., Ltd.) was obtained. Further, for sintered sands A to H, according to a known sintering method, using a raw material composition giving the chemical composition shown in Table 1 below, spherical granulated products were formed by a spray dryer, and then sintered in a rotary kiln, and then sized to obtain each sintered sand. In the production of these sintered sands A to H, the firing temperature was set as shown in Table 1, and in the sizing process, the presence or absence of dust collection was selected, and further, the dust collection force was changed for each sintered sand to give a difference in the amount of fine powder removed in each sintered sand.

[0042]

Table 1

[0043] Next, for each of these various refractory aggregates, the roundness, pore volume, mold strength, and fluidity were measured according to the following methods. And the results of the obtained roundness, pore volume, mold strength, and fluidity are shown in Table 2 below.

[0044] -Measurement of roundness- Using a particle shape measuring device: PartAn SI manufactured by Microtrac Bell Co., for each refractory aggregate (particle), the projected area and the maximum Feret diameter of each particle were obtained, and from the obtained values of the maximum Feret diameter and the projected area, based on the roundness calculation formula in the text, the roundness of each particle was calculated. After calculating the roundness of each of 5000 or more particles, the average value of the obtained roundness values was averaged by the number of measured particles to obtain the roundness (average value) for each, and this was taken as the roundness of each refractory aggregate.

[0045] -Measurement of pore volume- Measuring device: AutoPore IV9520 (manufactured by Shimadzu Corporation) Measuring pressure range: atmospheric pressure to 50,000 psia (345 MPa) (Low pressure port: ~30 psia, high pressure port: 30 - 50,000 psia) Sample weight: approximately 1.5 g After filling approximately 1.5 g of the sample into a predetermined container, mercury is pressed into the container. The pressure of mercury is gradually increased from atmospheric pressure, and the pressure at the time of increase and the amount of mercury pressed in are measured. Then, from the pressure of the mercury, the pore diameter of the sample particles is calculated using the following formula. d = -4σ(cosθ) / P [However, assuming the pore shape is a cylindrical shape with a diameter of d (m), the surface tension of mercury is σ (N / m), the contact angle between mercury and sample particles is θ (°), and the pressure applied to mercury is P (Pa)] Also, since the amount of mercury pressed in is the volume of the pores, by changing the pressure, the pore volume at each pore diameter is calculated. Then, the integrated pore volume with pore diameters of 2 - 10 μm is defined as the pore volume with diameters of 2 - 10 μm, while the integrated pore volume with pore diameters exceeding 10 μm is defined as the pore volume with diameters exceeding 10 μm.

[0046] -Measurement of mold strength- First, for 100 parts by mass of refractory aggregate, a first component (ISOCURE Part I 330T, manufactured by ASK CHEMICALS) consisting of a phenol - formaldehyde resin which is a cold box resin and a second component (ISOCURE Part II 630T, manufactured by ASK CHEMICALS) which is a polyisocyanate are blended at a ratio of 0.6 parts by mass each, and the kneaded sand (casting sand) obtained by kneading is used. After filling it into a mold of 30 mm × 10 mm × 85 mm by air - blowing, triethylamine which is a tertiary amine catalyst is blown into the mold to cure the filled casting sand, and the target mold (test piece) is produced.

[0047] Next, using the prepared mold (test piece), it was held for 24 hours in an environment of temperature: 25°C, humidity: 55%RH, and then using an autograph (manufactured by Shimadzu Corporation, AUTOGRAPH AG-X plus 20kN), under the conditions of span: 50mm, descent speed: 5mm / min, the flexural strength (kg / cm 2 ) was measured.

[0048] -Evaluation of fluidity- Using the kneaded sand prepared by adding water at a ratio of 0.1% by mass to each refractory aggregate and kneading it, a fluidity (tap flow) evaluation test is carried out in accordance with JIS-R-2521 (1995). Specifically, after filling a cylindrical flow cone with upper diameter: 37mm, lower diameter: 88mm, and height: 74mm with the kneaded sand to be measured, the flow cone is pulled out, and after applying the falling motion from a height of 12mm 5 times using a flow table, the maximum diameter of the collapsed kneaded sand and the diameter in the direction perpendicular to it are measured, and the average value is used as the flow value, which is used as an index of the fluidity of such kneaded sand. The larger this flow value is, the better the fluidity is shown.

[0049]

Table 2

[0050] And, in the results of Table 2, when plotting the relationship between the roundness, fluidity, and mold strength of each refractory aggregate, it becomes as shown in Figure 1. There, it can be seen that the smaller the roundness value (the more non-circular), in other words, the more non-spherical the particles are, the better the fluidity tends to be (refer to the area surrounded by the solid-line long ellipse in the figure). This is considered to be because as the roundness decreases, the amount of water affecting the flow (the water thickness on the sand surface) decreases. Sintered sands A to H have a smaller roundness and show good fluidity compared to fused sand. On the other hand, natural sand is found to have low fluidity. This is considered to be due to the angular shape of natural sand and its small roundness.

[0051] Also, regarding the mold strength, it is recognized that the mold strength tends to decrease as the roundness of the refractory aggregate decreases (refer to the area surrounded by the long ellipse of the broken line in Fig. 1). Similar to the influence on fluidity described above, when the roundness of the refractory aggregate decreases, in other words, when it becomes non-circular, the surface area of the refractory aggregate increases, and the amount of binder effective for the manifestation of mold strength decreases. Therefore, it is considered that this is the reason.

[0052] From the above results, it is recognized that in order to obtain good fluidity and high mold strength, it is necessary to use a refractory aggregate with a roundness of 0.70 or more.

[0053] Fig. 2 shows the relationship between the pore volume of pores with a diameter of 2 to 10 μm in each refractory aggregate in Table 1 above, fluidity, and mold strength. In this figure, it is recognized that the better the fluidity is shown as the pore volume of pores with a diameter of 2 to ~10 μm increases (becomes larger) (refer to the solid line in the figure). This is considered to be because when such a pore volume increases, the amount of water entering the pores increases, so that the amount of water affecting fluidity (the water thickness on the sand surface) decreases. Although all of the sintered sands A to H used as refractory aggregates show good fluidity, the fused sand gives a low fluidity value. This is considered to be because the pore volume of the fused sand is very small, so that the water thickness on the sand surface is thick.

[0054] Therefore, in order to obtain good fluidity in foundry sand, it is considered necessary that the total volume (pore volume) of pores with a diameter of 2 to 10 μm in the refractory aggregate is 0.20 mL / g or more.

[0055] Regarding the relationship with the mold strength shown in Fig. 2, as the pore volume with a diameter of 2 to 10 μm increases, as shown by the broken line, the mold strength tends to decrease. In particular, it is recognized that the strength of the mold obtained using sintered sand A is low. It is considered that the reason for this decrease in mold strength is that when the pore volume increases, the binder penetrates into the unevenness on the surface of the refractory aggregate, and the amount of binder effective for the development of mold strength decreases.

[0056] Therefore, in order to obtain a high mold strength, it is considered effective that the total volume of pores with a diameter of 2 to 10 μm in the sintered sand is 0.50 mL / g or less.

[0057] In addition, in the previous Table 2, the measurement results are also shown for the total volume of pores with a diameter larger than 10 μm. When examining the relationship between fluidity and mold strength with respect to the pore volume with a diameter exceeding 10 μm, it is clear that, unlike the relationship between fluidity and mold strength with respect to the pore volume with a diameter of 2 to 10 μm, the pore volume with a diameter exceeding 10 μm hardly contributes to fluidity and mold strength.

[0058] From the above results, it is recognized that in order to obtain good fluidity and high mold strength, it is necessary to use a refractory aggregate in which the total volume of pores with a diameter of 2 to 10 μm is in the range of 0.20 to 0.50 mL / g.

[0059] Here, regarding sintered sands A to H used as refractory aggregates, in order to know the amount of remaining fine powder for each, the turbidity was measured according to the following nephelometry method, and the results are shown in Table 3 below together with the fluidity and mold strength shown in Table 2 obtained previously.

[0060] -Measurement of turbidity- Twenty grams of refractory aggregate to be measured and 50 mL of water were stirred and mixed with a stirrer for 30 minutes, and then allowed to stand for 30 minutes. The supernatant obtained was used to determine the turbidity (NTU) with a turbidimeter using the 90° scattered light / transmitted light ratio measurement method (transmission scattering method) (manufactured by HACH, USA, portable turbidimeter 2100Q, sold by Toa DKK Corporation). Note that the smaller the value of this turbidity (NTU), the smaller the amount of fine powder mixed in.

[0061]

Table 3

[0062] And, regarding the turbidity of each sintered sand shown in Table 3, the relationship with fluidity and mold strength is shown in Figure 3. Therein, as shown by the solid line, it is recognized that the sintered sands A to H exhibit better fluidity as the turbidity (NTU) increases. This is because as the turbidity increases, the amount of fine powder remaining in the sintered sand increases, and the amount of water adhering to the increased fine powder also increases, so that the amount of water (the water thickness on the sand surface) affecting fluidity decreases.

[0063] On the other hand, the mold strength decreased as the turbidity increased, and in sintered sand A, it showed a particularly low value. The reason for this is that when the turbidity is high and the amount of residual fine powder increases, the binder adheres to the fine powder on the surface of the sintered sand, so that the amount of binder effective for the expression of mold strength decreases.

[0064] Then, it can be said that it is important to configure the turbidity corresponding to the amount of fine powder mixed in the sintered sand so that such turbidity is 3000 NTU or less in order to achieve good fluidity and high mold strength.

[0065] [Example 2] Similar to Example 1 described above, sintered sands I - O and fused sand having the chemical compositions and average particle diameters shown in Table 4 below were prepared as refractory aggregates for foundry sand, respectively. Among them, the fused sand is the same as that prepared in Example 1. For sintered sands I - O, finer sintered sands were formed according to the same sintering method as in Example 1. In the sintering process, the firing temperature was set as shown in Table 4. In the sizing process, the presence or absence of dust collection was selected, and further, the dust collection force was varied for each sintered sand to create a difference in the amount of fine powder removed in each sintered sand.

[0066]

Table 4

[0067] Next, for each of these various refractory aggregates, the roundness, pore volume, mold strength, and fluidity were measured in the same manner as in Example 1. The results of the obtained roundness, pore volume, mold strength, and fluidity are shown in Table 5 below.

[0068] Note that the test piece (mold) for measuring the mold strength was prepared using furan resin as a binder. Specifically, 0.2 mass% of a curing agent (manufactured by Kao Quaker, C - 17) was added to the refractory aggregate and kneaded. Then, 1.0 mass% of furan resin (manufactured by Kao Quaker, EF - 5301) was added and further kneaded to obtain kneaded sand. The obtained kneaded sand was filled into a wooden frame of 50 mmφ×50 mmH and cured. After being kept in an environment of temperature: 30°C, humidity: 80%RH for 24 hours, the compressive strength was measured using an autograph in the same manner as in Example 1.

[0069] Regarding the evaluation of fluidity, the tap - flow value was obtained in the same manner as in Example 1 using the kneaded sand obtained by adding 0.2 mass% of a curing agent (manufactured by Kao Quaker, C - 17) to the refractory aggregate and kneading.

[0070]

Table 5

[0071] As is clear from the results in Table 5 above, in the case of fine-grained sintered sand, the more surface irregularities (pore volume with a diameter of 2 to 10 μm), the higher the fluidity (tap flow value). Also, in the mold strength using furan resin, the more surface irregularities, the lower the furan strength tended to be. In particular, sintered sand I showed high fluidity because of its large pore volume (0.61 mL / g), but showed a low value for mold strength (furan).

[0072] From these facts, it became clear that even when using furan resin as the mold strength, it shows a similar tendency to that in Example 1. Note that furan resin is widely used not only in the elaborate molding using a wooden mold or the like in the production of molds, but also in the three-dimensional sand mold lamination molding method using a 3D molding machine. And in the three-dimensional sand mold lamination molding method, for example, a kneaded sand obtained by kneading a refractory aggregate and a curing agent is recoated by vibration or the like, and the furan resin is dropped only on the target molding part, so that the mold is produced. Also, after the dropping of the furan resin, unlike the elaborate molding method in which the molded mold is tamped, the mold strength tends to be low. Therefore, when taking out the mold produced by lamination molding, an appropriate mold strength that can withstand the vibration and handling during the taking out is required. Generally, it is required to have a strength of 10 kg / cm 2 as described above. In addition, when producing a mold having a complex shape, from the viewpoint of the mold surface roughness, casting sand of fine grains with a center particle size of about 0.11 mm to 0.13 mm is widely used. Therefore, in the three-dimensional sand mold lamination molding method, higher fluidity and higher mold strength are required for the fine-grained refractory aggregate.

[0073] From the results of Table 5 above, it is clear that in the mold strength using furan resin for sintered sand with fine grains, since there is a high correlation with the pore volume, not only in the hand-packed molding method but also in the three-dimensional sand mold lamination molding method, by using sintered sand with a limited pore volume (0.20 to 0.50 mL / g), a good mold with high mold strength can be produced while maintaining good fluidity. In particular, in sintered sand J, while maintaining high fluidity, the furan strength is 13.1 kg / cm 2 and it also has sufficient mold strength as a three-dimensional sand mold lamination molding method. On the other hand, the molten sand has low surface irregularities, and the fluidity is significantly lower even compared to the coarse-grained one in Example 1. From this, it became clear that the molten sand has poor fluidity in the three-dimensional sand mold lamination molding method and is not suitable for sand mold molding.

[0074] Also, for sintered sands I to O and molten sand, in order to know the amount of remaining fine powder respectively, in the same manner as in Example 1, the turbidity was measured according to the nephelometry method, and the results are shown in Table 6 below together with the fluidity and mold strength obtained previously.

[0075]

Table 6

[0076] As is clear from the results of Table 6 above, in sintered sands I to O, the higher the turbidity, the higher the fluidity is shown. On the other hand, the mold strength of the mold using furan resin as the binder resin decreases, showing the same results as in Example 1. In particular, in sintered sand J, since the turbidity is 2780 NTU, while maintaining high fluidity, the furan strength is 13.1 kg / cm 2Moreover, it was revealed that even in the three-dimensional sand mold lamination forming method, it has sufficient mold strength. In addition, although sintered sand O has a low turbidity, it showed fluidity that can be adopted in the three-dimensional sand mold lamination forming method, while it was revealed that molten sand has significantly lower fluidity compared to sintered sand. This is because sintered sand has an appropriate pore volume on the sand surface, and the amount of hardening agent that enters the pore volume increases, so the amount of hardening agent that affects fluidity (the thickness of the hardening agent present on the sand surface) decreases, showing good fluidity. On the other hand, molten sand has less pore volume on the sand surface, so the amount of hardening agent that affects fluidity is larger than that of sintered sand, resulting in a significant decrease in fluidity.

Claims

1. A refractory aggregate for casting sand, which has a chemical composition containing 40 to 90% by mass of Al₂O₃ and 60 to 10% by mass of SiO₂, and is composed of spherical refractory sintered particles artificially manufactured by a sintering method, having a circularity of 0.70 or more, an average particle diameter of 0.01 to 1.00 mm, and a pore volume with a pore diameter of 2 to 10 μm, determined by a mercury intrusion porosimeter, being in the range of 0.20 to 0.50 mL / g, and further configured such that the amount of mixed fine powder is 3000 NTU or less in turbidity measured by a turbidimetry method. The refractory aggregate for casting sand is characterized by this.

2. The refractory sintered particles contain 60% by mass or more of Al 2 O 3 and 40% by mass or less of SiO 2 , and further have a chemical composition in which the total content of Fe 2 O 3 and TiO 2 is 3% by mass or less. The refractory aggregate for casting sand according to claim 1, characterized in that it has such a chemical composition.

3. The refractory aggregate for casting sand according to claim 1, wherein the refractory sintered particles are mullite-based or mullite-cordierite-based sintered particles.

4. The refractory aggregate for casting sand according to claim 1, wherein the circularity is 0.75 or more.

5. The refractory aggregate for casting sand according to claim 1, wherein the pore volume with a pore diameter of 2 to 10 μm is in the range of 0.25 to 0.45 mL / g.

6. The refractory aggregate for casting sand according to claim 1, wherein the amount of mixed fine powder is configured to have a turbidity of 2000 NTU or less.

7. The refractory aggregate for casting sand according to any one of claims 1 to 6, characterized by being used as an aggregate for casting sand for molding a self-hardening mold.

8. The refractory aggregate for casting sand according to any one of claims 1 to 6, characterized by being used as an aggregate for casting sand in a three-dimensional lamination molding method.

9. A method for manufacturing a casting mold, characterized by obtaining a target casting mold by molding using the refractory aggregate for casting sand according to any one of claims 1 to 6.

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