WO2015147069A1 - Green sand for casting of steel castings, method for producing same, and method for producing metal castings using said green sand - Google Patents

Abstract

　Provided is green sand for use in casting of steel castings, comprising a casting sand that includes sand, a binder, and a carbon component in an amount of 3 mass parts or less per 100 mass parts of sand, in which a lining layer of a thermosetting resin is formed in a recess that includes at least a cavity for casting of a steel casting, the lining layer having average hardness (measured with a self-hardening hardness tester) of 50-95, and thickness of 0.5-2.5 mm.

Description

Cast mold for casting cast steel, method for producing the mold, and method for producing cast steel using the mold

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

Foundry sand forming a casting mold for cast steel generally contains aggregate (sand), caking additive such as bentonite, carbon components (coal, starch, etc.) as secondary additives, and water. . Proportion of aggregate, caking additive, etc. in foundry sand is set appropriately so that the molded green mold has the desired physical properties such as air permeability, strength, cavity surface stability and compactability (CB value). Is done. Carbon components such as coal powder, coke powder, graphite powder, and pitch powder added to the foundry sand suppress the adhesion (sand baking) of aggregate (sand) to the casting, and cast the cast steel product in the as-cast state. Stabilize skin quality. Techniques relating to coal materials are disclosed in JP-A-63-177939 and JP-A-2009-291801.

JP-A 63-177939 discloses a molding sand mold additive 1 to 2 parts containing 10 to 90% by weight of mineral oil and 90 to 10% by weight of a carbonaceous raw material, 100 parts of aggregate, bentonite (caking binder) ) Disclose a method for producing a casting mold by kneading 10 parts, 1 part starch and 3 parts water and molding the resulting foundry sand. JP-A-2009-291801 discloses a carbonaceous additive mainly composed of edible vegetable oil containing glycerin, bentonite (binding agent), an additive such as starch as necessary, and a certain amount of water. A green foundry sand containing is disclosed.

However, the additives disclosed in JP-A-63-177939 and JP-A-2009-291801 all contain 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, there is a risk that carburization may occur on the surface of the casting due to the carbon contained in the green mold. If there is carburization on the surface of the casting surface, the cast steel product becomes difficult to cut. This problem is particularly serious in the case of stainless steel cast products that require heat resistance and corrosion resistance, for example, used as exhaust members for internal combustion engines.

Accordingly, an object of the present invention is to provide a casting mold for casting a cast steel product that suppresses carburization of the surface of the casting surface while suppressing the occurrence of sand baking and maintaining the same casting surface quality as before, and a method for manufacturing the same, and such a casting mold. It is providing the manufacturing method of the cast steel goods using this.

As a result of earnest research to achieve the two contradictory purposes of suppressing carburization of the surface of the casting surface while maintaining the same casting surface quality (sand baking), (1) (2) until the solidification of the molten metal that has entered the cavity starts. Covering the cavity surface with a thermosetting resin decomposition gas to prevent sand baking, and setting the thickness so that the decomposition gas disappears as soon as the molten metal solidifies, maintains the casting surface quality and carburizes the surface of the casting surface. It was discovered that suppression can be achieved at the same time, and the present invention has been conceived.

That is, the green mold of the present invention for casting a cast steel product is
It consists of sand, caking material, and foundry sand containing 3 parts by mass or less of carbon with respect to 100 parts by mass of sand.
A coating layer of a thermosetting resin is formed in a recess including at least a cavity for casting a cast steel product,
The coating layer has an average hardness of 50 to 95 (measured with a self-hardening hardness meter) and a thickness of 0.5 to 2.5 mm.

The coating amount of the thermosetting resin constituting the coating layer is preferably 100 to 500 g / m ² on a solid basis.

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

The method of the present invention for producing the green mold is as follows.
At least a pair of raw materials having a concave portion including a cavity for casting a cast steel product by molding sand, caking material, and molding sand containing 3 mass parts or less of carbon with respect to 100 mass parts of sand. Create mold parts (for example, upper mold and lower mold),
Applying a coating solution containing a thermosetting resin and an organic solvent to at least the recesses,
The thermosetting resin applied to the recesses is cured by heating to form a coating layer having an average hardness of 50 to 95 (measured with a self-hardening hardness meter).

The thermosetting of the thermosetting resin can be performed before and / or after mold matching. In 1st embodiment, after thermosetting the said thermosetting resin, at least a pair of green mold part is united. In the second embodiment, the first curing step of heating the coating layer formed by drying the applied coating solution until the average hardness (measured with a self-hardness hardness meter) of 30 to 45 is reached, and the primary curing. The coated layer is further heated in a second curing step in which the average hardness (measured with a self-hardness hardness meter) is 50 to 95.

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

The method for producing a cast steel product according to the present invention is characterized by using the green mold.

In the green mold of the present invention, a thermosetting resin coating layer is formed in a green mold recess made of foundry sand containing 3 parts by mass or less of carbon with respect to 100 parts by mass of sand. Because it has an average hardness of 50 to 95 (measured with a self-hardening hardness meter) and a thickness of 0.5 to 2.5 mm, it should produce cast steel products that maintain the same casting surface quality while suppressing carburization of the surface of the casting surface. Can do.

It is sectional drawing which shows the structure of the green mold of this invention. FIG. 2 is an enlarged partial cross-sectional view showing part A of FIG. FIG. 2 is a cross-sectional view showing a molding process in the first example of the green mold manufacturing process of FIG. FIG. 2 is a cross-sectional view showing a coating liquid coating process in a first example of the green mold manufacturing process of FIG. FIG. 2 is a cross-sectional view showing a thermosetting resin curing process in the first example of the green mold manufacturing process of FIG. FIG. 2 is a cross-sectional view showing a mold matching process in the first example of the green mold manufacturing process of FIG. FIG. 5 is a cross-sectional view showing a molding process in a second example of the green mold manufacturing process of FIG. FIG. 4 is a cross-sectional view showing a coating liquid coating process in a second example of the green mold manufacturing process of FIG. FIG. 3 is a cross-sectional view showing a first curing step of a thermosetting resin in a second example of the production process of the green mold of FIG. FIG. 8 is a cross-sectional view showing a mold matching process in the second example of the green mold manufacturing process of FIG. FIG. 5 is a cross-sectional view showing a second curing step of the thermosetting resin in the second example of the green mold manufacturing process of FIG. It is an enlarged schematic diagram which shows the foundry sand (before coat | covering a thermosetting resin) which comprises a green mold. It is an enlarged schematic diagram which shows the state which coat | covered the thermosetting resin to the foundry sand which comprises a green mold. FIG. 2 is a cross-sectional view showing a method for producing a cast steel product using the green mold of FIG. FIG. 7 is an enlarged partial sectional view showing part B of FIG. 2 is a SEM photograph (100 times) showing casting sand constituting the green mold of Example 1. FIG. 2 is an SEM photograph (100 times) showing a state in which a molding resin constituting the green mold of Example 1 is covered with a phenol resin.

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 relating to one embodiment also applies to other embodiments unless otherwise specified.

1 shows the configuration of the green mold of the present invention, FIG. 2 shows an enlarged view of part A of FIG. 1, FIGS. 3 and 4 show the manufacturing process of the green mold of FIG. 1, and FIG. The manufacturing process of the cast steel product using a green mold is shown. Here, “cast steel product” means a sub-eutectoid composition containing 0.05 to 0.6% by mass of C and other elements (Ni, Cr, Si, W, Mo, Nb, etc.), the balance being Fe and inevitable impurities. Of course, it is not limited.

[1] Composition of green mold As shown in Fig. 1, the green mold 1 made of foundry sand that does not substantially contain carbon is composed of an upper mold 1a and a lower mold that are mold-matched by a parting surface (mold matching surface) 1e. Consists of 1b. The upper mold 1a and the lower mold 1b are combined, 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 mold 1. In addition to the product cavity 1c and the runner 1d, the green mold 1 may be provided with a hot water, a weir, a gate, and the like.

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

(1) Sand The sand itself as an aggregate constituting the foundry sand may be a commonly used one. For example, mountain sand, semi-synthetic sand or synthetic sand can be used. Mountain sand should only have a naturally occurring viscosity of at least 2%, such as Noma sand from Aichi prefecture, Kawachi sand from Osaka prefecture, Shima sand from Mie prefecture, Matsue sand from Shimane prefecture, Fukushima In addition to Ota sand produced in the prefecture, Enshu sand and Genkai sand are listed. Examples of the semi-synthetic sand include those obtained by appropriately blending silt sand, caking agent and additives with mountain sand. Examples of synthetic sand include those obtained by blending a binder and an additive with raw sand such as silica sand without using mountain sand at all. Examples of the raw material sand used for the synthetic sand include natural silica sand such as gyrome sand, beach sand and river sand, artificial silica sand, zircon silicic acid, oripin sand, chromite sand and the like.

(2) Binder The examples of the binder include bentonite, clay, montmorillonite, and kaolin. The amount of the binder is appropriately adjusted in consideration of green characteristics, but is generally 5 to 12 parts by mass with respect to 100 parts by mass of sand.

(3) Carbon content Examples of the carbon content include carbonaceous raw materials such as coal, graphite, coke, pitch coke, and asphalt, starch 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 binder. Carbon components may be used alone or in combination of two or more.

In order to prevent carburization of cast steel products, the carbon content in the present invention is 3 parts by mass or less with respect to 100 parts by mass of sand. When casting is performed using a green mold made of foundry sand having a carbon content of more than 3 parts by mass, carburization of the casting surface layer proceeds. The amount of carbon content is more preferably 1 part by mass, and most preferably 0.7 parts by mass or less.

(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 made of a thermosetting resin and having an average hardness of 50 to 95 (measured with a self-hardening hardness meter). Has been. For the purpose of the present invention to suppress carburization of the surface of the casting surface while maintaining the same casting surface quality as before, it is sufficient that the coating layer 1f is formed on the product cavity 1c, but the runway 1d through which the molten metal passes is also covered. Forming the layer 1f is effective in suppressing carburization. Therefore, in the present invention, the coating layer 1f is formed in the recess including at least the cavity 1c and the runner 1d. Furthermore, if the coating layer 1f is also formed on the parting surface 1e, the strength of those surfaces can be increased, and mold breakage and the like during supply of the molten metal can be suppressed.

The thermosetting resin is not particularly limited as long as it can be easily decomposed and gasified when it comes into contact with the molten steel of cast steel, and has high strength and high hardness so as not to be damaged during mold matching. 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 coating layer 1f made of the thermosetting resin is in the range of 50 to 95. The hardness of the covering layer 1f is determined using a self-hardness hardness meter (manufactured by Nakayama Co., Ltd., model: NK-009). If the average hardness of the coating layer 1f is too low, sand baking cannot be suppressed, and if it is too high, the air permeability cannot be secured and gas defects may occur.

The thermosetting resin constituting the coating layer 1f decomposes and disappears as a gas when it comes into contact with a high-temperature molten metal, but part of it is carbonized, and there is a possibility that the carbon content remains on the surface layer of the product cavity 1c. In order to effectively suppress the carburization of the surface of the casting surface, the residual amount of carbon per unit volume of the coating layer 1f when the temperature is raised from room temperature to 800 ° C at a rate of 10 ° C / min in the atmosphere is 200 mg / Preferably it is not more than cm ³ . If the residual amount of carbon is too small, there will be less generation of gas and sand baking will likely occur. Accordingly, the residual amount of carbon is preferably 20 mg / cm ³ or more. Further, since the carburization of the casting surface layer cannot be sufficiently prevented when the residual amount of carbon is too large, the upper limit of the residual amount of carbon is preferably 200 mg / cm ³ . The residual amount of carbon is more preferably 20 to 100 mg / cm ³ . The carbon residual amount can be measured by thermogravimetric analysis (TGA) of a thermosetting resin.

As shown in FIG. 2, the green mold 1 composed of sand 1j and a binder (not shown) has many voids (pores) 1i between the sand 1j to ensure air permeability. Since the coating solution in which the thermosetting resin is dissolved in the organic solvent penetrates into the pores 1i existing in the surface layer of the product cavity 1c, the thermosetting resin remains on the surface of the sand 1j existing in the surface layer after the coating solution is dried. . As a result, a region where 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 referred to as a coating layer 1f. In the covering layer 1f, only the surface of the sand 1j is covered with the thermosetting resin, and the voids (pores) 1i remain. As shown in FIG. 2, since the depth of the thermosetting resin is not constant, the thickness T of the coating layer 1f is represented by an average value. The average value of the thickness T of the covering layer 1f can be obtained by measuring a plurality of (for example, three) cross-sections of the product cavity 1c on which the covering layer 1f is formed and averaging them.

If the thickness T of the coating layer 1f is too large, the thermosetting resin that has not been decomposed may be carbonized, and the cast skin surface layer may be carburized by the remaining carbon content. In order to effectively suppress carburization of the casting surface layer, the thickness T of the coating layer 1f is preferably 2.5 mm or less, more preferably 2.0 mm or less, and most preferably 1.5 mm or less. Further, if the thickness T of the coating layer 1f is too small, the coating layer 1f is easily peeled off during the casting operation. When the coating layer 1f is peeled off, the molten metal enters the peeled portion and directly contacts the green sand, so that sand baking occurs. Accordingly, the thickness T of the coating layer 1f is preferably 0.5 mm or more.

In the coating layer 1f, not only the thickness T but also the coating amount of the thermosetting resin is important. 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 application amount of the thermosetting resin is less than 100 g / m ² , sand baking cannot be suppressed. In addition, if 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 and gas defects may occur, but also the thermosetting resin that has not been decomposed. May carbonize, and the cast skin surface may be carburized by the remaining carbon. In order to effectively suppress carburization of the surface of the casting surface while maintaining good casting surface quality, the coating amount of the curable resin in the coating layer 1f is more preferably 220 to 380 g / m ² . The coating amount of the thermosetting resin can be determined by dividing the green weight increment ΔD (g) after drying the coating solution by the coating area (m ² ) of the thermosetting resin.

The air permeability of the coating layer 1f is preferably 70 to 150. If the air permeability of the coating layer 1f is too small, the generated gas is trapped in the molten metal, and defects such as pinholes are likely to occur in the resulting cast steel product. On the other hand, if the air permeability of the coating layer 1f is too high, the appearance and sand drop of the cast steel product deteriorate due to the peeling of the coating layer 1f. The air permeability can be measured by the rapid method described in Appendix 3 of JIS Z 2601.

[2] Raw mold manufacturing method
(A) First example
(1) Molding process Recesses that form product cavities 1c and runners 1d as shown in Fig. 3 (a) from casting sand prepared by kneading a predetermined amount of sand, caking additive, carbon and water An upper mold 1a and a lower mold 1b having 1g-1 and 1g-2 are formed. In order to facilitate molding and to ensure the strength of the green mold, the amount of caking additive and water added to the foundry sand is adjusted as appropriate in consideration of the characteristics of the green mold. 5 to 12 parts by mass of caking additive and 1 to 5 parts by mass of water.

The upper die 1a and the lower die 1b are, for example, cast sand into a casting frame containing a model in which a product cavity, a runner, etc. are stored, and then compressed by the squeeze squeeze method or the like, and finally the model is removed. Can be formed.

(2) Coating process As shown in FIG. 3 (b), a coating layer is formed on the surfaces of the recesses 1g-1 and 1g-2 including the cavity 1c and the runner 1d of the upper mold 1a and the lower mold 1b and the mold mating surface 1e. A coating solution 1k containing a thermosetting resin for forming 1f and an organic solvent is applied. In the example shown in the drawing, the coating liquid 1k is applied not only to the recesses 1g-1 and 1g-2 but also to the die-matching surface 1e, but it may be applied to at least the recesses 1g-1 and 1g-2. In order to stabilize the coating amount and to make the coating layer 1f uniform, it is preferable to spray-coat the coating solution 1k with the spray nozzle 10 that moves 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 of JIS K6910) so that an appropriate amount of water penetrates into the voids 1i of the sand particles 1j from the surfaces of the recesses 1g-1 and 1g-2. 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. When the viscosity of the coating liquid 1k is too large, the coating liquid 1k hardly penetrates into the surface layers of the recesses 1g-1 and 1g-2, and the coating layer 1f is easily formed only in the vicinity of the surfaces of the recesses 1g-1 and 1g-2. For this reason, the coating layer 1f is easily peeled off, and the appearance and sand removal of the resulting cast steel product are deteriorated. On the other hand, if the viscosity of the coating solution 1k is too small, the coating solution 1k penetrates excessively and the coating layer 1f becomes too thick. The coating amount of the coating liquid 1k varies depending on the concentration of the thermosetting resin, but as described above, the amount of the thermosetting resin applied to the recesses 1g-1 and 1g-2 is 100 to 500 g / m ^{2 on a} solid basis. It is preferable to set so that.

(3) Coating layer forming step As shown in FIG. 3 (c), the coating solution applied to the recesses 1g-1 and 1g-2 of the upper mold 1a and the lower mold 1b is heated to cure the thermosetting resin. Heating may be performed while the organic solvent is evaporated or after the organic solvent is evaporated. Thereby, the coating layer 1f having an average hardness measured with a self-hardness hardness meter in the range of 50 to 95 is formed. The method for heating the coating liquid 1k is not particularly limited, and for example, as shown in FIG. 3 (c), hot air can be blown from a horizontally moving blower 11 or heated by a heater arranged on a horizontal plane.

(4) Mold matching process As shown in FIG. 3 (d), the upper mold 1a and the lower mold 1b in which the coating layer 1f is formed in the recesses 1g-1 and 1g-2 are mold-matched, and the integrated mold shown in FIG. Form 1

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

In the second example, the coating liquid 1k is applied to 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. 4 (a) by the coating process shown in FIG. 4 (b). After coating, in the first curing step shown in FIG. 4 (c), warm air is blown from the blower 11 to heat the coating liquid 1k, thereby forming the semi-cured layer 1L. After mold matching the upper mold 1a and the lower mold 1b in which the semi-cured layer 1L is formed in the mold matching process shown in FIG. 4 (d), from the runner 1d in the second curing process shown in FIG. Hot air blown from the blower 12 is blown into the cavity 1c, and the semi-cured layer 1L is heated and cured to form the coating layer 1f.

In this way, before the second curing step for forming the coating layer 1f, pre-curing in the first curing step to form a semi-cured layer 1L prevents cracking of the coating layer 1f due to rapid curing. Thus, appearance defects of cast steel products can be suppressed. From this viewpoint, the average hardness measured with a self-hardening hardness meter of the semi-cured layer 1L is preferably 30 to 45. Also in the second example, the average hardness (measured with a self-hardness hardness meter) of the covering layer 1f is preferably in the range of 50 to 95.

By 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 binder. As a result, a coating layer 1f [FIG. 5 (b)] in which the gap 1i remains in at least the recesses 1g-1 and 1g-2 of the green mold 1 is formed.

[3] Manufacturing method of cast steel product As shown in FIG. 6, sand is formed by casting molten metal through a runner 1d into a product cavity 1c of a mold 1 consisting of an upper mold 1a and a lower mold 1b on which a coating layer 1f is formed. A cast steel product is produced in which carburization of the surface of the casting surface is suppressed while maintaining the same casting surface quality in terms of seizure. The reason is not clear, but is estimated as follows. That is, (a) as shown in FIG. 7, when the coating layer 1f of the product cavity 1c touches the high-temperature molten metal M, the thermosetting resin of the coating layer 1f is almost completely gasified, so that the thermosetting resin is decomposed. Casting sand that suppresses sand baking by gas (indicated by arrows) and (b) disappears immediately after the relatively thin coating layer 1f of 0.5 to 2.5 mm touches the molten metal M, and constitutes the green mold 1 Since the carbon content of the steel is as small as 3 parts by mass or less, the time during which the molten metal M during solidification contacts the carbon is short, and carburization of the surface of the casting surface is suppressed. The cast steel product in which the formation of the carburized layer is suppressed has excellent machinability. Note that the thermosetting resin in the deep region of the coating layer 1f gasifies slightly later than the thermosetting resin in the shallow region, so it directly contacts the foundry sand of the product cavity 1c until the molten metal M solidifies. Contributes to preventing

The present invention will be described in more detail with reference to experimental examples below, but the present invention is not limited thereto.

Example 1
(1) Sand preparation step For 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 foundry sand.

(2) Molding process Cast sand was put into a casting frame on which a casting model model was set and then compressed by the Jolt skies method to form an upper mold and a lower mold. The average hardness of the concave portions of the upper mold and the lower mold was 20 measured with a self-hardness hardness meter (NK-009 manufactured by Nakayama Co., Ltd.). Fig. 8 (a) shows an SEM photograph (100x) of the surface of the molded concave part. As is clear from FIG. 8 (a), there were many voids between the sand covered with the binder.

(3) Application process As shown in Table 2, an application liquid (viscosity: 20 mPa · s) consisting of 40% by mass of phenol resin and 60% by mass of ethanol was applied to the recesses and mold-matching surfaces of the upper and lower molds. . The coating amount of the coating solution was 300 g / m ^{2 based} on the solid content.

(4) Coating layer forming step The coating solution applied to the concave portions of the upper mold and the lower mold and the mold mating surface was heated and cured with an incandescent lamp to form a coating layer. FIG. 8 (b) shows a SEM photograph (100 times) of the concave surface of the green mold on which the coating layer was formed. As is apparent from FIG. 8 (b), it can be seen that sufficient voids remain between the sand covered with the coating layer, which is sufficient to exhaust the decomposition gas of the thermosetting resin.

(a) Measurement of thickness T Cut out 5 blocks of length x width x depth 3 cm x 3 cm x 3 cm with a spoon from the surface of the recesses of the upper and lower molds on which the coating layer was formed, and remove the coating layer The foundry sand was removed from the block with a brush without breaking, and the thickness of the sample consisting only of the cured coating layer was measured with calipers. 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 carbon remaining amount One sample in which the thickness of the coating layer was measured [the surface area of the coating layer: 3 × 3 cm ² , the thickness of the coating layer: T, the volume of the sample: 3 × 3 × T cm ³ ] Was subjected to thermogravimetric analysis (TGA) in which the temperature was increased from room temperature to 800 ° C. at a rate of 10 ° C./min in the air, and the carbon residue per unit volume was measured. As a result, the carbon residue in the coating layer was 100 mg / cm ³ .

(c) Measurement of hardness The hardness of the coating layer was determined by measuring five points with a self-hardness hardness meter (NK-009 manufactured by Nakayama Corporation) and averaging. As a result, the hardness of the coating layer in the concave portion was 67.

(5) Mold Matching Process The upper mold and the lower mold with the coating layer formed on the recesses and the mold mating surface were mold-matched by a normal method to obtain a green mold.

0.45 wt% C, 1.30 wt% Si, 1.02 wt% Mn, 10.1 wt% Ni, 19.9 wt% Cr, 10.0 wt% Nb, 0.15 wt% S, and A molten metal containing 0.18% by mass of N and having the balance of Fe and inevitable impurities was poured at 1620 to 1630 ° C. After the molten metal solidified, the cast steel product was taken out by mold breaking and shot blasting using a steel ball having an average diameter of 2.4 mm was performed for 15 minutes to remove foundry sand adhering to the casting surface. Similarly, a total of 100 cast steel products were produced.

(a) Measurement of the rate of sand baking After visually observing the sand baking on the casting surface after shot blasting, divide the number of cast steel products where sand baking occurred by the total number of cast steel products (100), and the rate of sand baking (%) Was calculated. As a result, the sand baking rate was 1%.

(b) Measurement of surface defect occurrence rate Surface defects of cast steel products such as pinholes generated due to defective gas escape and burrs generated due to cracks and breakage of the concave coating layer were visually observed, and surface defects occurred. The number of cast steel products was divided by the total number of cast steel products (100) to determine the surface defect occurrence rate (%). As a result, the surface defect occurrence rate was 2%.

(c) Machinability evaluation In order to evaluate the machinability of the cast steel product's casting surface, a carbide insert coated with TiAlN and PVD was used, and the surface layer of the cast steel product (depth including the casting surface) 1.0 mm range) Milled.
Cutting speed: 150 m / min Cutting depth: 1.0 mm
Feed per tooth: 0.2 mm / tooth Feed speed: 381 mm / min Rotation speed: 76 rpm
Cutting fluid: None (dry type)

The tool life was judged to be reached when the wear amount on the flank surface of the carbide insert reached 0.2 mm or more, and the cutting time until the tool life was reached was used as a machinability parameter. When the tool life (machinability) of Comparative Example 1 was 100, the machinability of Example 1 was 126.

Example 2
(a) The proportion of phenol in the coating solution was 30% by mass, (b) the viscosity and coating amount of the coating solution were 17 mPa · s and 100 g / m ² respectively, and (c) the coating layer formation process conditions were changed Thus, 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 carbon residual amount of 22 mg / cm ³ was formed in the recess. The machinability, sand baking rate, and surface defect rate were measured in the same manner as in Example 1. As a result, the machinability was 133, the sand baking rate was 3%, and the surface defect rate was 3%.

Example 3
(a) The proportion of phenol in the coating solution is 20% by mass, (b) the viscosity of the coating solution is 13 mPa · s, and (c) the coating layer is cured in two stages, the hardness is 50, the thickness T 100 cast steel articles were produced in the same manner as in Example 1 except that a coating layer having a thickness of 1.7 mm and a carbon residual amount of 50 mg / cm ³ was formed in the recesses. In the two-stage curing, a semi-cured layer having a hardness of 36 was formed in the first curing process, and after mold matching, the semi-cured layer was further heated and completely cured in the second curing process. The machinability, sand baking rate, and surface defect rate were measured in the same manner as in Example 1. As a result, the machinability was 130, the sand baking rate was 2%, and the surface defect rate was 4%.

Examples 4-6
100 cast steel products were manufactured in the same manner as in Example 1 except that the ratio of phenol 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, sand baking rate, and surface defect rate of each cast steel product were measured. In Example 4, the machinability was 113, the sand baking rate was 1%, and the surface defect rate was 4%. In Example 5, the machinability was 109, the sand baking rate was 1%, and the surface defect rate was 3%. In Example 6, the machinability was 118, the sand baking rate was 2%, and the surface defect 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. The machinability, sand baking rate and surface defect rate were measured in the same manner as in Example 1. As a result, the machinability was 100, the sand baking rate was 3%, and the surface defect rate was 11%.

Comparative Example 2
100 cast steel products were produced in the same manner as in Comparative Example 1 except that the ratio of phenol in the coating solution and the coating amount of the coating solution were changed as shown in Table 2. The machinability, sand baking rate, and surface defect rate were measured in the same manner as in Example 1. As a result, the machinability was 92, the sand baking rate was 1%, and the surface defect rate was 35%.

Comparative Example 3
100 cast steel products were manufactured in the same manner as in Example 1 except that the ratio of phenol in the coating solution and the coating amount of the coating solution were changed as shown in Table 2. The machinability, sand baking rate and surface defect rate were measured in the same manner as in Example 1. As a result, the machinability was 72, the sand baking rate was 23%, and the surface defect rate was 10%. The deterioration of machinability is thought to be due to sand baking of the casting surface.

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 solution applied to the green mold, and the hardness, thickness and carbon residual amount of the coating layer are shown. Table 2 shows. Table 3 shows the machinability, sand baking rate, surface defect rate, and comprehensive judgment of the cast steel products of Examples 1 to 6 and Comparative Examples 1 to 3 in a three-step evaluation as follows.

Machinability (expressed as a relative value with Comparative Example 1 as 100)
A: 120 or more.
○: More than 100 and less than 120.
X: 100 or less.

Sand baking rate ◎: 2% or less.
○: More than 2% and less than 10%.
X: 10% or more.

Surface defect occurrence rate A: 2% or less.
○: More than 2% and less than 10%.
X: 10% or more.

Comprehensive judgment ◎: When the evaluation of machinability, sand baking rate and surface defect rate is all ◎.
○: When any one of machinability, sand baking rate, and surface defect rate is evaluated.
X: When evaluation of any of machinability, sand baking rate, and surface defect rate is x.

注：(1) 造型した上型及び下型の凹部（被覆層を形成していない）の平均硬度。

Note: (1) Average hardness of the upper and lower mold recesses (no coating layer formed).

注：(1) 凹部における表面硬度。
　　(2) フェノール。
　　(3) 熱硬化性樹脂の加熱硬化を第一及び第二の硬化工程により行い、第一の硬化工程後の凹部表面硬度は36であった。

Note: (1) Surface hardness in the recess.
(2) Phenol.
(3) 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, the machinability was excellent as judged by ○ or ◎ as shown in Table 3, sand. It was possible to obtain a cast steel product in which the occurrence of seizure and surface defects was suppressed as ◯ or ◎. On the other hand, in Comparative Examples 1 to 3 in which the proportion of carbon contained in the green mold and the surface hardness of the coating layer are outside the range defined by the present invention, any one of machinability, sand baking and surface defects, Or all became X judgment.

1: Raw mold 1a: Upper mold 1b: Lower mold 1c: Product cavity 1d: Runway 1e: Parting surface 1f: Cover layer 1g-1, 1g-2: Recess 1i: Cavity 1j: Sand 1k: Coating liquid 1L: Half Hardened layer M: Molten metal

Claims (9)

A mold for casting cast steel products,
It consists of sand, caking material, and foundry sand containing 3 parts by mass or less of carbon with respect to 100 parts by mass of sand.
A coating layer of a thermosetting resin is formed in a recess including at least a cavity for casting a cast steel product,
A green mold characterized in that the coating layer has an average hardness of 50 to 95 (measured with a self-hardness hardness meter) and a thickness of 0.5 to 2.5 mm. 2. The 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 solid content basis. The green mold according to claim 1 or 2, wherein the amount of carbon remaining per unit volume of the coating layer after heating up to 800 ° C at a rate of 10 ° C / min in the atmosphere is 20 to 200 mg / cm ³ . A life model characterized by being. A method for producing a green mold according to any one of claims 1 to 3,
At least a pair of raw materials having a concave portion including a cavity for casting a cast steel product by molding sand, caking material, and molding sand containing 3 mass parts or less of carbon with respect to 100 mass parts of sand. Make the mold part,
Applying a coating solution containing a thermosetting resin and an organic solvent to at least the recesses,
A method of forming a coating layer having an average hardness of 50 to 95 (measured with a self-hardening hardness meter) by heat-curing the thermosetting resin applied to the recess. 5. The method for producing a green mold according to claim 4, wherein the thermosetting resin is heat-cured before and / or after mold matching. 6. The method for producing a green mold according to claim 5, wherein at least a pair of green mold parts is matched after the thermosetting resin is heat-cured. 6. The green mold manufacturing method according to claim 5, wherein the coating layer obtained by drying the applied coating solution is heated until the average hardness (measured with a self-hardness hardness meter) of 30 to 45 is reached. A method comprising: a curing step; and a second curing step in which the primary cured coating layer is further heated to have an average hardness (measured with a self-hardness hardness meter) of 50 to 95. The method for producing a green mold according to any one of claims 5 to 7, wherein the coating solution has a viscosity of 15 to 100 mPa · s. A method of producing a cast steel product using the green mold according to any one of claims 1 to 3.

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