JP6536568B2 - Green die for casting cast steel article and method for producing the same, and method for producing cast steel article using the green die - Google Patents

Description

  The present invention relates to a green mold suitable for casting a stainless steel cast steel product particularly known as a hard-to-cut material, a method of manufacturing the green mold, and a method of manufacturing a cast steel product using the green mold.

  The foundry sand forming the casting mold of the cast steel product generally contains aggregate (sand), caking agent such as bentonite, carbon component (coal material, starch, etc.) which is a secondary additive, and water. . The proportions of aggregate, caking additive, etc. in the casting sand are appropriately set so that the molded green mold has desired physical properties [air permeability, strength, stability of cavity surface and compatibility (CB value), etc.] Be done. Carbon components such as coal powder, coke powder, graphite powder and pitch powder added to casting sand suppress adhesion (sand baking) of aggregate (sand) to the casting and cast the cast steel product in the as-cast condition Stabilize skin quality. Techniques related to coal materials are disclosed in Japanese Patent Application Laid-Open Nos. 63-177939 and 2009-291801.

  Japanese Patent Application Laid-Open No. 63-177939 discloses that 1 to 2 parts of an additive for casting sand mold containing 10 to 90% by weight of mineral oil and 90 to 10% by weight of carbonaceous raw material, 100 parts of aggregate, and bentonite (caking agent) The present invention discloses a method for producing a green casting mold by kneading 10 parts, 1 part of starch and 3 parts of water and molding the obtained casting sand. In addition, JP 2009-291801 A comprises a carbonaceous additive mainly composed of an edible vegetable oil containing glycerin, bentonite (caking agent), an additive such as starch according to need, and a certain amount of water A green casting sand is disclosed.

  However, the additives disclosed in JP-A-63-177939 and JP-A-2009-291801 all include mineral oil, carbonaceous raw material or vegetable oil, and a green mold formed of foundry sand to which these are added When casting a cast steel product having a hypoeutectoid composition containing about 0.05 to 0.60% by mass of carbon using the above, there is a risk that carburization may occur on the surface of the casting due to the carbon content contained in the green mold. If there is carburization on the surface of the cast surface, the cast steel product becomes difficult to cut. This problem is particularly serious in the case of stainless steel castings which are required to have heat resistance and corrosion resistance, for example, as exhaust members for internal combustion engines.

  Therefore, an object of the present invention is to provide a cast steel casting green mold for suppressing the carburization of the surface of the casting surface while suppressing the occurrence of sand burning and maintaining the conventional cast surface quality, and a method of manufacturing the same, and such green mold It is to provide a method of producing a cast steel product using

  As a result of keen research to make the two contradictory objectives of suppressing carburization of the surface of the casting surface compatible while maintaining the conventional casting surface quality (sand burning), as a result, (1) In the casting sand that constitutes the green mold Reduce the carbon content of carbon black to such an extent that carburization does not occur, and (2) the thickness of the coating layer of thermosetting resin formed in the hollow of the green mold until solidification of the molten metal entering the cavity starts Covering the cavity surface with the decomposition gas of thermosetting resin to prevent sand burning and setting the thickness so that the decomposition gas disappears immediately when the molten metal solidifies, maintenance of cast surface quality and carburization of cast surface layer The inventors have discovered that suppression can be achieved simultaneously and have contemplated the present invention.

That is, the green mold of the present invention for casting a cast steel article is
Foundry sand, which contains sand, caking agent, and carbon ,
A cover layer of a thermosetting resin is formed in a recess including at least a cavity for casting a cast steel product,
The carbon content is a carbon component contained in one or more selected from carbon powder, coal, graphite, coke, pitch coke, asphalt, dextrin, starch, mineral oil, and vegetable oil, and 100 parts by mass of the sand Up to 3 parts by mass (not including 0 parts by mass),
The coating layer is characterized in that it has an average hardness of 50 to 95 and a thickness of 0.5 to 2.5 mm measured using a self-hardening hardness tester NK-009 manufactured by Nakayama Co., Ltd.

It is preferable that the application quantity of the thermosetting resin which comprises the said coating layer is 100-500 g / m < ² > on solid content basis.

The amount of carbon remaining per unit volume of the coating layer after the temperature is raised to 800 ° C. at a rate of 10 ° C./min in the atmosphere is preferably 20 to 200 mg / cm ³ .

The method of the present invention for producing the above green mold is
A cavity for casting a cast steel article is formed by molding a casting sand containing sand, a caking additive, and a carbon content of 3 parts by mass or less (not including 0 parts by mass) with respect to 100 parts by mass of sand. Producing at least a pair of green parts (e.g., upper and lower molds) having recesses including;
A coating liquid containing a thermosetting resin and an organic solvent is applied to at least the recess,
The thermosetting resin applied to the concave portion is heat cured to form a coating layer having an average hardness of 50 to 95 measured using a self- hardening hardness tester NK-009 manufactured by Nakayama Co. , Ltd. I assume.

The heat curing of the thermosetting resin can be performed before and / or after mold alignment. In the first embodiment, at least a pair of green parts are heat-cured after the thermosetting resin is cured. In the second embodiment, the curing of the coating layer formed by drying the applied coating solution has an average hardness of 30 to 45 as a hardness measured using a self- hardening hardness tester NK-009 manufactured by Nakayama Co. , Ltd. The first curing step of heating until the above and the second curing step of further heating the primarily cured coating layer to make the average hardness 50 to 95.

  The viscosity of the coating solution is preferably 15 to 100 mPa · s.

  The method for producing a cast steel product of the present invention is characterized by using the above-mentioned green mold.

In the green mold of the present invention, a coating layer of a thermosetting resin is formed on a green mold recess made of foundry sand containing 3 parts by mass or less of carbon (not including 0 parts by mass) with respect to 100 parts by mass of sand. Because the coating layer has an average hardness of 50 to 95 (measured with a self-hardening hardness tester) and a thickness of 0.5 to 2.5 mm, carburization of the surface layer of the cast surface is suppressed while maintaining the conventional cast surface quality Cast steel products can be manufactured.

It is sectional drawing which shows the structure of the green mold of this invention. It is an expanded fragmentary sectional view which shows the A section of FIG. It is sectional drawing which shows the molding process in the 1st example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the application | coating process of the coating liquid in the 1st example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the hardening process of the thermosetting resin in the 1st example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the type | mold alignment process in the 1st example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the molding process in the 2nd example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the application | coating process of the coating liquid in the 2nd example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the 1st hardening process of the thermosetting resin in the 2nd example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the type | mold setting process in the 2nd example of the manufacturing process of the green mold of FIG. It is sectional drawing which shows the 2nd hardening process of the thermosetting resin in the 2nd example of the manufacturing process of the green mold of FIG. It is the expansion schematic which shows the casting sand (before coating a thermosetting resin) which comprises a green mold. It is the expansion schematic which shows the state which coat | covered the thermosetting resin with the casting sand which comprises a green mold. It is sectional drawing which shows the manufacturing method of the cast steel goods using the green mold of FIG. FIG. 7 is an enlarged partial cross-sectional view showing part B of FIG. 6; It is a SEM photograph (100 times) which shows the molding sand which comprises the green mold of Example 1. FIG. It is a SEM photograph (100 times) which shows the state which covered the phenol resin on the foundry sand which comprises the green mold of Example 1. FIG.

  Embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention is not limited thereto. The description of one embodiment also applies to the other embodiments unless otherwise stated.

  1 shows the structure of the green mold of the present invention, FIG. 2 shows an enlarged part A of FIG. 1, FIG. 3 and FIG. 4 show the manufacturing process of the green mold of FIG. The manufacturing process of the cast steel goods using a green mold is shown. Here, the “cast steel product” is a hypoeutectoid composition containing 0.05 to 0.6% by mass of C and other elements (Ni, Cr, Si, W, Mo, Nb, etc.), with the balance being Fe and unavoidable impurities. Of the castings, but of course not limiting.

[1] Configuration of Green Mold As shown in FIG. 1, a green mold 1 made of foundry sand is composed of an upper mold 1a and a lower mold 1b which are mold-matched by a parting surface (mold mating surface) 1e. The upper mold 1a and the lower mold 1b are mold-matched, and the green mold 1 has a cavity (product cavity) 1c for forming a product therein and a runner 1d. The product cavity 1c and the runner 1d are formed by recesses in each of the upper mold 1a and the lower mold 1b. Of course, a mold may be arranged around the green mold 1. In addition to the product cavity 1c and the runner 1d, the green mold 1 may be provided with a pouring bath, a gutter, a sprue and the like.

(A) Foundry sand Foundry sand contains sand, caking additive and carbon content.

(1) Sand The sand itself as an aggregate which comprises casting sand may be what is normally used, for example, mountain sand, semi-synthetic sand, or synthetic sand can be used. The mountain sand should have a naturally occurring clay content of at least 2%, for example, Noma sand from Aichi Prefecture, Kawachi sand from Osaka Prefecture, Shima sand from Mie Prefecture, Matsue sand from Shimane Prefecture, Fukushima Other than Ota sand from prefecture, Enshu sand, Genkai sand etc. are mentioned. Examples of semi-synthetic sand include those obtained by appropriately blending silica sand, caking agents and additives with mountain sand. Examples of synthetic sand include those obtained by blending raw material sand such as silica sand with a caking agent and an additive without using any mountain sand. As raw material sand used for synthetic sand, natural silica sand such as guillome sand, beach sand and river sand, artificial silica sand, zircon silicic acid, olipin sand, chromite sand and the like can be mentioned.

(2) Caking additive Examples of the caking additive include bentonite, clay, montmorillonite, kaolin and the like. Although the quantity of a caking additive is suitably adjusted in consideration of the characteristic of a green type, it is generally 5-12 mass parts to 100 mass parts of sand.

(3) Carbon content Carbon content includes carbonaceous raw materials such as coal, graphite, coke, pitch coke and asphalt, starchy additives such as dextrin and starch, and liquid oils such as mineral oil and vegetable oil. The carbon content does not include carbon compounds contained in sand or caking agent. The carbon content may be used alone or in combination of two or more.

In order to prevent carburization of cast steel products, in the present invention, the carbon content is 3 parts by mass or less (not including 0 parts by mass) with respect to 100 parts by mass of sand. When it casts using the green mold which consists of casting sand whose carbon content exceeds 3 mass parts, carburization of the cast surface layer progresses. As for the compounding quantity of carbon content, 1 mass part is more preferable, and 0.7 mass part or less is the most preferable.

(B) Coating layer As shown in FIGS. 1 and 2, at least the surface layer of the product cavity 1c is formed with a coating layer 1f having an average hardness of 50 to 95 (measured with a self-hardening hardness meter) made of a thermosetting resin. It is done. For the purpose of the present invention to suppress carburization of the surface layer of the cast surface while maintaining the conventional cast surface quality, it is sufficient if the coating layer 1f is formed in the product cavity 1c. Forming the layer 1 f is effective in suppressing carburization. Therefore, in the present invention, the covering layer 1 f is formed in the recess including at least the cavity 1 c and the runner 1 d. Furthermore, if the covering layer 1f is formed also on the parting surface 1e, the strength of the surface thereof can be enhanced, and mold breakage and the like at the time of supply of the molten metal can be suppressed.

  The thermosetting resin is not particularly limited as long as it is a thermosetting resin having high strength and high hardness so as to be easily decomposed and gasified when it contacts a molten metal of cast steel and not to be damaged at the time of mold alignment. Examples thereof include resins, epoxy resins, melamine resins, urea resins (urea resins), unsaturated polyester resins, alkyd resins, polyurethanes, thermosetting polyimides, and the like. The average hardness of the covering layer 1 f made of a thermosetting resin is in the range of 50 to 95. The hardness of the covering layer 1 f is determined using a self-hardening hardness tester (manufactured by Nakayama Co., Ltd., model: NK-009). If the average hardness of the covering layer 1 f is too low, sand burning can not be suppressed, and if it is too high, air permeability can not be ensured, and gas defects may occur.

The thermosetting resin constituting the covering layer 1f decomposes when it contacts the high-temperature molten metal and disappears as a gas, but there is a possibility that a part of it is carbonized and the carbon content remains on the surface layer of the product cavity 1c. In order to effectively suppress carburization of the surface layer of the casting, the residual amount of carbon per unit volume of the covering layer 1f is 200 mg / h when the temperature is raised from room temperature to 800 ° C. at a rate of 10 ° C./min. It is preferable that it is cm ³ or less. If the residual amount of carbon is too small, sand burning is likely to occur because the amount of gas generation is small. Therefore, the residual amount of carbon is preferably 20 mg / cm ³ or more. If the amount of carbon remaining is too large, carburization of the surface layer of the casting can not be sufficiently prevented, so the upper limit of the amount of carbon remaining is preferably 200 mg / cm ³ . The residual amount of carbon is more preferably 20 to 100 mg / cm ³ . The amount of carbon remaining can be measured by thermogravimetric analysis (TGA) of a thermosetting resin.

  As shown in FIG. 2, the green mold 1 made of sand 1j and a caking additive (not shown) has many voids (pores) 1i between the sand 1j in order to secure air permeability. The coating solution in which the thermosetting resin is dissolved in the organic solvent penetrates into the pores 1i present in the surface layer of the product cavity 1c, so the thermosetting resin remains on the surface of the sand 1j present in the surface layer after drying of the coating solution. . As a result, an area in which the surface of the sand 1j is covered with the thermosetting resin is formed on the surface layer of the product cavity 1c. This region is called a covering layer 1 f. In the covering layer 1f, only the surface of the sand 1j is covered with the thermosetting resin, and the void (void) 1i remains. Since the depth of the thermosetting resin is not constant as shown in FIG. 2, the thickness T of the covering layer 1 f is represented by an average value. The average value of the thickness T of the covering layer 1 f can be determined by measuring a plurality of (for example, three) cross sections of the product cavity 1 c in which the covering layer 1 f is formed, and averaging the measured values.

  If the thickness T of the covering layer 1 f is too large, the thermosetting resin which has not been decomposed may be carbonized, and the remaining carbon content may cause the surface layer of the casting to be carburized. In order to effectively suppress carburization of the cast surface layer, the thickness T of the coating layer 1 f is preferably 2.5 mm or less, more preferably 2.0 mm or less, and most preferably 1.5 mm or less. In addition, when the thickness T of the covering layer 1 f is too small, the covering layer 1 f is easily peeled off during the casting operation. When the covering layer 1f is peeled off, the molten metal intrudes into the peeled portion to be in direct contact with the green sand, thereby causing sand burning. Therefore, the thickness T of the covering layer 1 f is preferably 0.5 mm or more.

Not only the thickness T but also the coating amount of the thermosetting resin is important for the covering layer 1 f. The coating amount of the thermosetting resin is represented by the dry weight (g / m ² ) of the thermosetting resin per unit area. The coating amount of the thermosetting resin is preferably 100 to 500 g / m ² . If the coating amount of the thermosetting resin is less than 100 g / m ^2, it is not possible to suppress sand burning. In addition, when the coating amount of the thermosetting resin is more than 500 g / m ² , not only the air permeability of the green mold becomes too small to cause gas defects but also the thermosetting resin that is not decomposed However, there is a possibility that the surface layer of the casting is carburized due to carbonization and the remaining carbon content. In order to effectively suppress carburization of the cast surface layer while maintaining good cast surface quality, the coating amount of the curable resin in the coating layer 1 f is more preferably 220 to 380 g / m ² . The coating amount of the thermosetting resin can be determined by dividing the weight increment ΔD (g) of the green mold after drying of the coating liquid by the coating area (m ² ) of the thermosetting resin.

  The air permeability of the covering layer 1 f is preferably 70 to 150. If the permeability of the covering layer 1 f is too small, the generated gas is trapped in the molten metal, and defects such as pinholes are likely to occur in the obtained cast steel product. Further, if the air permeability of the covering layer 1 f is too large, the appearance of the cast steel product and the sand fall deteriorate due to the peeling of the covering layer 1 f. The air permeability can be measured by the rapid method described in Annex 3 of JIS Z 2601.

[2] Production method of green mold
(A) First example
(1) Forming process As shown in FIG. 3 (a), a recess for forming a product cavity 1c and a runner 1d as shown in FIG. 3 (a) from a casting sand prepared by mixing predetermined amounts of sand, a caking additive, carbon and water. The upper mold 1a and the lower mold 1b having 1g-1, 1g-2 are molded. The amount of caking additive and water to be added to the foundry sand is appropriately adjusted in consideration of the characteristics of the green mold in order to facilitate molding and secure the green mold strength, but generally it is based on 100 parts by mass of sand. 5 to 12 parts by mass of caking agent and 1 to 5 parts by mass of water.

  The upper mold 1a and the lower mold 1b are, for example, cast sand in a flask containing a model in which a product cavity, a runner, etc. are formed, and then squeeze them by the Jört squeeze method etc. and finally remove the model. It can be formed by

(2) Coating step As shown in FIG. 3 (b), the covering layer is formed on the surfaces of the recesses 1g-1 and 1g-2 including the cavities 1c and runners 1d of the upper and lower molds 1a and 1b and the mold mating surface 1e. A coating liquid 1k containing a thermosetting resin for forming 1f and an organic solvent is applied. Although the coating liquid 1k is applied not only to the concave portions 1g-1 and 1g-2 but also to the mold mating surface 1e in the illustrated example, the coating liquid 1k may be applied to at least the concave portions 1g-1 and 1g-2. In order to stabilize the coating amount and to make the film thickness of the coating layer 1f uniform, it is preferable to spray-coat the coating solution 1k by means of a spray nozzle 10 moving horizontally as shown in FIG. 3 (b).

The coating solution 1k has a viscosity of 15 to 100 mPa · s (measured with a Brookfield viscometer according to JIS K 6910) so that a suitable amount of water penetrates from the surface of the recess 1g-1 or 1g-2 into the void 1i of the sand particle 1j. Is preferred. As a result, a coating layer 1f having a thickness T of 0.5 to 2.5 mm is formed on the surface layer of the recesses 1g-1 and 1g-2. If the viscosity of the coating solution 1k is too high, the coating solution 1k does not easily penetrate into the surface layer of the recesses 1g-1 and 1g-2, and the covering layer 1f is easily formed only in the vicinity of the surface of the recesses 1g-1 and 1g-2. For this reason, the coating layer 1 f is easily peeled off, and the appearance and the sand drop of the cast steel product obtained are deteriorated. On the other hand, when the viscosity of the coating solution 1k is too small, the coating solution 1k permeates excessively and the coating layer 1f becomes too thick. The amount of the coating solution 1k applied varies depending on the concentration of the thermosetting resin, but as described above, the amount of the thermosetting resin applied to the concave portions 1g-1 and 1g-2 is 100 to 500 g / m ^{2 based} on solid content. It is preferable to set so that

(3) Coating Layer Forming Step As shown in FIG. 3C, the coating liquid applied to the concave portions 1g-1 and 1g-2 of the upper mold 1a and the lower mold 1b is heated to cure the thermosetting resin. The heating may be performed while evaporating the organic solvent, or may be performed after the evaporation of the organic solvent. Thereby, the coating layer 1 f having an average hardness of 50 to 95 measured by a self-hardening hardness tester is formed. The heating method of the coating solution 1k is not particularly limited. For example, as shown in FIG. 3C, warm air can be blown from a horizontally moving blower 11 or heating can be performed by a heater arranged in a horizontal plane.

(4) Mold alignment process As shown in FIG. 3 (d), upper mold 1a and lower mold 1b having cover layers 1f formed in recesses 1g-1 and 1g-2 are mold-aligned, and integrally shown in FIG. Form a green mold 1

(B) Second Example A second example of the method of manufacturing the green mold 1 will be described with reference to FIG. The same reference numerals as in FIG. 3 denote the same parts in FIG. 4 and a detailed description thereof will be omitted. The second example of the method of manufacturing the green mold 1 is the same as the first example except that it has the first and second curing steps shown in FIGS. 4 (c) and 4 (e).

  In the second example, the recesses 1g-1 and 1g-2 of the upper mold 1a and the lower mold 1b formed in the molding process shown in FIG. 4A are applied with the coating liquid 1k in the coating process shown in FIG. 4B. After coating, warm air is blown from the blower 11 in the first curing step shown in FIG. 4C to heat the coating solution 1k, thereby forming the semi-cured layer 1L. After the upper mold 1a and the lower mold 1b on which the semi-hardened layer 1L is formed in the mold alignment step shown in FIG. 4 (d) are aligned, in the second curing step shown in FIG. 4 (e) Hot air blown from the blower 12 is blown into the cavity 1c, and the semi-hardened layer 1L is heated and cured to form the covering layer 1f.

  Thus, prior to the second curing step of forming the covering layer 1f, the semi-hardening layer 1L is pre-cured in the first curing step to prevent cracking and the like of the covering layer 1f due to rapid curing. Thus, appearance defects of cast steel products can be suppressed. From this viewpoint, the average hardness measured by the self-hardening hardness tester of the semi-hardened layer 1L is preferably 30 to 45. Also in the second example, the average hardness (measured with a self-hardening hardness tester) of the coating layer 1 f is preferably in the range of 50 to 95.

  According to the method of the present invention, a thin thermosetting resin layer is formed on the surface [FIG. 5 (a)] of the sand 1j bonded with the caking agent. As a result, a covering layer 1 f [FIG. 5 (b)] is formed in which the air gaps 1 i remain at least in the recesses 1 g-1 and 1 g-2 of the green mold 1.

[3] Production method of cast steel article As shown in FIG. 6, sand is produced by casting a molten metal through a runner 1d into a product cavity 1c of a green mold 1 consisting of an upper mold 1a and a lower mold 1b on which a cover layer 1f is formed. A cast steel product is produced in which carburization of the surface layer of the cast surface is suppressed while maintaining the cast surface quality as conventional in terms of baking. Although the reason is not clear, it is estimated as follows. (A) As shown in FIG. 7, when the covering layer 1f of the product cavity 1c touches the molten metal M at a high temperature, the thermosetting resin of the covering layer 1f is almost completely gasified, so the decomposition of the thermosetting resin While the sand burning is suppressed by the gas (indicated by the arrow), the coating layer 1f relatively thin as (b) 0.5 to 2.5 mm disappears immediately after touching the molten metal M, and casting sand constituting the green mold 1 Since the amount of carbon content of carbon is also as small as 3 parts by mass or less, the time for which the molten metal M in solidification is in contact with carbon is short, and carburization of the surface layer of the casting is suppressed. Cast steel products in which the formation of a carburized layer is suppressed have excellent machinability. Since the thermosetting resin in the deep region of the covering layer 1f gasifies slightly later than the thermosetting resin in the shallow region, it directly contacts the casting sand of the product cavity 1c until the molten metal M solidifies. Contribute to the prevention of

  The present invention will be described in more detail by the following experimental examples, but the present invention is not limited thereto.

Example 1
(1) Sand Preparation Step With respect to 100 parts by mass of silica sand, 8.1 parts by mass of bentonite, 3.0 parts by mass of water, and 3 parts by mass of carbon powder were mixed to prepare a foundry sand.

(2) Molding process Casting sand was introduced into a flask set with a casting plan model, and then compressed by the Joert Squeez method to form an upper mold and a lower mold. The average hardness of the recesses of the upper and lower molds measured at five points with a self-hardening hardness tester (NK-009 manufactured by Nakayama Co., Ltd.) was 20. The SEM photograph (100 times) of the surface of the concave part of the molded green mold is shown in FIG. 8 (a). As evident from FIG. 8 (a), there were many voids between the cinder-covered sand.

(3) Coating step As shown in Table 2, a coating solution (viscosity: 20 mPa · s) consisting of 40% by mass of phenol resin and 60% by mass of ethanol was applied to the recesses and the mating surfaces of the upper and lower molds. . The coating amount of the coating solution was 300 g / m ² on a solids basis.

(4) Coating layer formation process The coating liquid apply | coated to the recessed part and type | mold matching surface of upper die and lower die was heat-hardened with the incandescent lamp, and the coating layer was formed. An SEM photograph (100 ×) of the surface of the concave portion of the green mold on which the covering layer is formed is shown in FIG. As is clear from FIG. 8 (b), it can be seen that sufficient voids remain even between the sand covered with the covering layer, which is sufficient to exhaust the decomposition gas of the thermosetting resin.

(a) Measurement of thickness T From the surface of the upper and lower mold recesses on which the covering layer is formed, five blocks of length x width x depth 3 cm x 3 cm x 3 cm are cut out with a spoon, and the covering layer is The cast sand was removed from the block by brushing without breaking and the thickness of the sample consisting only of the hardened coating layer was measured with a caliper. This measurement was performed on all the blocks, and the obtained measurement values were averaged to obtain the thickness T of the coating layer. As a result, the thickness T of the cured coating layer was 1.1 mm.

(b) Measurement of residual carbon content One sample for which the thickness of the coating layer was measured [surface area of coating layer: 3 × 3 cm ² , thickness of coating layer: T, volume of sample: 3 × 3 × T cm ³ The thermogravimetric analysis (TGA) was carried out at room temperature to 800.degree. C. at a rate of 10.degree. C./minute in the atmosphere to measure the amount of carbon remaining per unit volume. As a result, the carbon remaining amount of the coating layer was 100 mg / cm ³ .

(c) Measurement of Hardness The hardness of the coating layer was measured at five points using a self-hardening hardness tester (NK-009 manufactured by Nakayama Co., Ltd.) and determined by averaging. As a result, the hardness of the coating layer in the recess was 67.

(5) Die Alignment Step The upper die and the lower die having the covering layer formed on the recess and the die mating surface were die-aligned by a conventional method to obtain a green die.

  In the green mold cavity, 0.45% by mass C, 1.30% by mass Si, 1.02% by mass Mn, 10.1% by mass Ni, 19.9% by mass Cr, 10.0% by mass Nb, 0.15% by mass S, and A melt containing 0.18% by mass of N and the balance being Fe and unavoidable impurities was poured at 1620 to 1630 ° C. After the molten metal solidified, mold disintegration was performed to take out the cast steel product, and shot blasting using steel balls having an average diameter of 2.4 mm was performed for 15 minutes to remove casting sand attached to the cast surface. A total of 100 cast steel articles were produced in the same manner.

(a) Measurement of sand burning incidence rate Sand burning on the cast surface after shot blasting was visually observed, and the number of cast steel products in which sand burning occurred was divided by the total number (100) of cast steel articles, sand burning incidence rate It asked for (%). As a result, the incidence of sand burning was 1%.

(b) Measurement of surface defect occurrence rate Surface defects were visually observed by visually observing surface defects of cast steel products such as pinholes generated due to gas leakage failure and burrs generated due to cracking or breakage of the recess covering layer. The number of cast steel products was divided by the total number (100) of cast steel products to determine the surface defect incidence rate (%). As a result, the surface defect incidence was 2%.

(c) Evaluation of machinability In order to evaluate the machinability of the cast surface of the cast steel article, a cemented carbide insert coated with TiAlN using PVD is used, and the surface layer of the cast steel article (depth including the cast surface) under the following conditions 1.0 mm range) milling cut.
Cutting speed: 150 m / min Depth of cut: 1.0 mm
Feed per blade: 0.2 mm / blade Feed rate: 381 mm / min Rotational speed: 76 rpm
Cutting fluid: None (dry type)

  It was judged that the tool life was reached when the wear amount of the flank of the carbide insert became 0.2 mm or more, and the cutting time until the tool life was used as the machinability parameter. When the tool life (the machinability) of Comparative Example 1 is 100, the machinability of Example 1 is 126.

Example 2
(a) The proportion of the phenolic resin in the coating solution is 30% by mass, (b) the viscosity and coating amount of the coating solution are 17 mPa · s and 100 g / m ² respectively, and (c) the conditions for the coating layer forming step 100 cast steel articles were produced in the same manner as in Example 1 except that a coating layer having a hardness of 50, a thickness T of 2.3 mm and a residual carbon content of 22 mg / cm ³ was formed in the recess by changing it. . As a result of measuring the machinability, the sand burning incidence rate and the surface defect incidence rate in the same manner as in Example 1, the machinability was 133, the sand burning incidence rate was 3%, and the surface defect incidence rate was 3%.

Example 3
(a) The proportion of the phenol resin in the coating solution is 20% by mass, (b) the viscosity of the coating solution is 13 mPa · s, and (c) the curing of the coating layer is performed in two steps. 100 cast steel articles were produced in the same manner as in Example 1 except that a coating layer having a T of 1.7 mm and a residual carbon content of 50 mg / cm ³ was formed in the recess. In the two-step curing, a semi-cured layer having a hardness of 36 was formed in the first curing step, and after mold alignment, the semi-cured layer was further heated and completely cured in the second curing step. As a result of measuring the machinability, incidence of sand burning and occurrence of surface defects in the same manner as in Example 1, the machinability was 130, the incidence of sand burning was 2%, and the incidence of surface defects was 4%.

Examples 4 to 6
100 cast steel articles were produced in the same manner as in Example 1 except that the ratio of the phenol resin in the coating solution and the coating amount of the coating solution were changed as shown in Table 2. In the same manner as in Example 1, the machinability, the incidence of sand burning and the rate of occurrence of surface defects of the cast steel products of the respective examples were measured. In Example 4, the machinability was 113, the sand burning incidence rate was 1%, and the surface defect incidence rate was 4%. In Example 5, the machinability was 109, the sand burning incidence rate was 1%, and the surface defect incidence rate was 3%. In Example 6, the machinability was 118, the sand burning incidence rate was 2%, and the surface defect incidence rate was 2%.

Comparative Example 1
100 cast steel articles were produced in the same manner as in Example 1 except that the proportion of carbon powder in the foundry sand was 4.0 parts by mass. As a result of measuring the machinability, the incidence of sand burning and the incidence of surface defects in the same manner as in Example 1, the machinability was 100, the incidence of sand burning was 3%, and the incidence of surface defects was 11%.

Comparative example 2
One hundred cast steel articles were produced in the same manner as in Comparative Example 1 except that the ratio of the phenolic resin in the coating solution and the coating amount of the coating solution were changed as shown in Table 2. As a result of measuring the machinability, the sand burning incidence rate and the surface defect incidence rate in the same manner as in Example 1, the machinability was 92, the sand burning incidence rate was 1%, and the surface defect incidence rate was 35%.

Comparative example 3
100 cast steel articles were produced in the same manner as in Example 1 except that the ratio of the phenol resin in the coating solution and the coating amount of the coating solution were changed as shown in Table 2. As a result of measuring the machinability, the incidence of sand burning and the incidence of surface defects in the same manner as in Example 1, the machinability was 72, the incidence of sand burning was 23%, and the incidence of surface defects was 10%. The deterioration of the machinability is considered to be due to sand-baking of the casting surface.
The plane defect incidence rate was 35%.

  The production conditions of the green molds of Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Table 1, and the composition, viscosity and coating amount of the coating liquid applied to the green mold, and hardness, thickness and residual carbon amount of the coating layer It shows in Table 2. The machinability, incidence of sand burning and occurrence of surface defects, and the overall judgment of the cast steel products of Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Table 3 in the following three-step evaluation.

Machinability (comparative example 1 is expressed as a relative value as 100)
◎: 120 or more.
○: over 100 and under 120.
X: 100 or less.

Sand burning incidence rate ◎: 2% or less.
○: more than 2% and less than 10%.
X: 10% or more.

Surface defect incidence rate ◎: 2% or less.
○: more than 2% and less than 10%.
X: 10% or more.

Overall judgment ◎: When the machinability, sand burning incidence rate and surface defect incidence rate were all ◎.
○: When the evaluation of any of machinability, sand burning incidence rate and surface defect incidence rate is ○.
X: When the evaluation of any of machinability, sand burning incidence rate and surface defect incidence rate is x.

Note: (1) Average hardness of upper and lower mold recesses (not coated).

Note: (1) Surface hardness in the recess.
(2) Phenolic resin .
(3) The heat curing of the thermosetting resin was performed by the first and second curing steps, and the concave surface hardness after the first curing step was 36.

  In Examples 1 to 6, by adjusting the proportion of carbon contained in the green mold and the surface hardness of the coating layer as described above, as shown in Table 3, the machinability is excellent with the judgment of ◎ or 、, sand It was possible to obtain a cast steel product in which the occurrence of seizure and surface defects was suppressed to be ○ or 判定. On the other hand, in Comparative Examples 1 to 3 in which the ratio of the carbon content contained in the green mold and the surface hardness of the coating layer deviate from the ranges specified by the present invention, any of machinability, sand burning and surface defects, Or all became x judgment.

1: Raw type
1a: Upper type
1b: Lower type
1c: Product cavity
1d: runner
1e: Face-off side
1f: Coating layer
1g-1, 1g-2: Recess
1i: Air gap
1j: Sand
1k: Coating solution
1L: Semi-hardened layer
M: Molten metal

Claims (9)

It is a green mold for casting cast steel products,
Foundry sand, which contains sand, caking agent, and carbon ,
A cover layer of a thermosetting resin is formed in a recess including at least a cavity for casting a cast steel product,
The carbon content is a carbon component contained in one or more selected from carbon powder, coal, graphite, coke, pitch coke, asphalt, dextrin, starch, mineral oil, and vegetable oil, and 100 parts by mass of the sand Up to 3 parts by mass (not including 0 parts by mass),
A green mold characterized in that the coating layer has an average hardness of 50 to 95 and a thickness of 0.5 to 2.5 mm measured using a self-hardening hardness tester NK-009 manufactured by Nakayama Co., Ltd. The green mold according to claim 1, wherein the coating amount of the thermosetting resin constituting the coating layer is 100 to 500 g / m ² on a solids basis. In the green form according to claim 1 or 2, the carbon remaining amount per unit volume of the coating layer is 20 to 200 mg / cm ³ after the temperature is raised to 800 ° C. at a rate of 10 ° C./min in the atmosphere. A raw pattern characterized by a certain thing. A method of producing a green form according to any one of claims 1 to 3, wherein
A cavity for casting a cast steel article is formed by molding a casting sand containing sand, a caking additive, and a carbon content of 3 parts by mass or less (not including 0 parts by mass) with respect to 100 parts by mass of sand. Producing at least a pair of green parts having recesses including
A coating liquid containing a thermosetting resin and an organic solvent is applied to at least the recess,
The thermosetting resin applied to the concave portion is heat cured to form a coating layer having an average hardness of 50 to 95 measured using a self- hardening hardness tester NK-009 manufactured by Nakayama Co. , Ltd. And how to.   The method for producing a green mold according to claim 4, wherein the thermosetting of the thermosetting resin is performed before and / or after the mold alignment.   The method of manufacturing a green mold according to claim 5, wherein at least a pair of green mold portions are mold-matched after the thermosetting resin is heat-cured. In the method for producing a green mold according to claim 5, the curing of the coating layer formed by drying the applied coating solution is a hardness measured using a self- hardening hardness tester NK-009 manufactured by Nakayama Co. , Ltd. A method comprising: a first curing step of heating to an average hardness of -45; and a second curing step of further heating the primary-cured coating layer to set an average hardness of 50 to 95.   The method for producing a green mold according to any one of claims 4 to 7, wherein the viscosity of the coating liquid is 15 to 100 mPa · s. A method for producing a cast steel product using the green mold according to any one of claims 1 to 3.