METHOD FOR MANUFACTURING MOLD CASTING PRODUCT OF NON-Ductile iron HAVING ULTRA-MICROSPHERICAL INK

Abstract

Provided are: a method for producing a mold-cast product of spheroidal graphite cast iron having ultra-fine spheroidal graphite cast iron, which is capable of producing ultra-fine spheroidal graphite cast iron in a simple manner with good reproducibility; and a spheroidizing agent. Provided are a sand mold manufacturing method and a spheroidizing agent with which it is possible to manufacture ultra-fine spheroidal graphite cast iron with good reproducibility even for thin-walled spheroidal graphite cast iron which is a sand mold having the same solidification and cooling conditions as a mold. A method for producing a mold cast product of spheroidal graphite cast iron, the method comprising a melting step, a spheroidizing step, an inoculation step, and a casting step, in which the spheroidizing is performed using a spheroidizing agent in which the content of C is 0.5% (mass) or more, the content of total nitrogen N is 150 ppm (mass) or less, and the content of N. The amount of nitrogen generated during melting is 15 ppm (mass) or less.

Description

[0001] This application is a divisional application of application number "201880022933.7", filed on March 29, 2018, entitled "Manufacturing method and spheroidizing agent for mold casting of ductile iron with ultrafine ductile iron". Technical Field

[0002] This invention relates to a method for manufacturing mold castings of ductile iron with ultrafine ductile iron particles and a spheroidizing agent. Background Technology

[0003] Ductile cast iron is a type of pig iron casting (also known as cast iron), also called ductile cast iron. In the case of gray cast iron, a type of cast iron, graphite has a slender, highly anisotropic flake shape. In contrast, in the case of ductile cast iron, graphite is spherical. Spheroidal graphite is achieved by adding a graphitizing agent containing magnesium or calcium to the molten metal just before casting.

[0004] Ductile iron, due to the non-strength, spherical shape of the graphite, produces castings with the same toughness and strength as steel. Ductility, or toughness, is the primary reason why spherical graphite possesses both strength and elongation. It is now widely used as a material for industrial machinery, primarily in the automotive industry.

[0005] The finer the graphite and the greater its particle count, the better it inhibits the development of cracks during impact, and the greater the impact energy. Efforts are underway to further improve the material properties by miniaturizing and uniformly dispersing spherical graphite.

[0006] In the typical metallic microstructure of conventional ductile iron, there are generally at most 400 particles per mm. 2 Typically 100 pieces / mm 2 The spherical ink on the left and right.

[0007] In contrast, the inventors have also provided a method containing far more than 400 per mm. 2 A method for manufacturing ultrafine ductile iron with a spheroidal graphite microstructure that does not chill. Specifically, a mold casting of ultrafine ductile iron and its manufacturing method are provided. The mold casting is in a cast state, without chilling, and has a microstructure of 1000 spheroidal graphite particles / mm². 2 Above, and further up to 3000 pieces / mm 2 The above describes the structure of spheroidal graphite (Patent Document 1).

[0008] Existing technical documents Patent documents Patent Document 1: PCT / JP2016 / 071036 Summary of the Invention

[0009] The problem the invention aims to solve In the technology described in Patent Document 1, the amount of nitrogen is adjusted so that the amount of nitrogen generated during the melting of the mold casting is 0.9 ppm (mass) or less, thereby achieving the aforementioned ultrafine ductile iron. Moreover, in this embodiment, by controlling the temperature of the original molten iron, nitrogen is purged from the original molten iron, and the amount of nitrogen is adjusted so that the amount of nitrogen generated during melting before casting the mold casting is 0.9 ppm (mass) or less.

[0010] However, in the subsequent spheroidizing process, since the Mg alloy used as the spheroidizing agent contains nitrogen, the amount of nitrogen generated during the melting of the original molten iron before casting may not be below 0.9 ppm (mass).

[0011] The purpose of this invention is to provide a method for manufacturing mold castings of ductile iron with ultra-fine ductile iron, which can be easily and reproducibly produced, as well as a spheroidizing agent.

[0012] means for solving problems The first invention relates to a method for manufacturing mold castings of ductile iron, which includes the following steps: The melting process of raw materials made of cast iron to obtain molten iron. The spheroidizing process involves spheroidizing. The process of gestation. The casting process of pouring material into a mold; The spheroidizing treatment is performed using a spheroidizing agent, wherein the C content is set to 0.5% (by mass) or more, the total nitrogen (N) content is set to 150 ppm (by mass) or less, and the amount of nitrogen generated during melting is set to 15 ppm (by mass) or less. The amount of nitrogen generated during melting that remains after being introduced into the molten metal through the spheroidizing treatment is set to 0.9 ppm (by mass) or less.

[0013] Total nitrogen content = Nitrogen content generated during melting + Nitrogen content The second embodiment relates to a method for manufacturing mold castings of ductile iron according to the first embodiment, wherein the spheroidizing agent is an Fe-Si-Mg based spheroidizing agent.

[0014] The third embodiment relates to a method for manufacturing mold castings of ductile iron according to the first or second embodiment, characterized in that the nitrogen content is adjusted so that the nitrogen content generated during the melting of the mold casting is less than 5 ppm (by mass).

[0015] The fourth embodiment relates to a method for manufacturing mold castings of ductile iron according to any one of the first to third embodiments, characterized in that: raw materials made of cast iron are heated and melted to obtain molten iron; the molten iron is heated to a specified temperature of 1500°C or higher, and then heating is stopped; oxygen is removed from the molten iron by maintaining the temperature at that temperature for a certain period of time; then, nitrogen in the molten iron is reduced by slowly cooling the molten iron; and then, spheroidization treatment, inoculation, and casting are performed.

[0016] The fifth embodiment involves an invention of a spheroidizing agent, wherein the content of C is 0.5% (by mass) or more, the content of total nitrogen N is 150 ppm (by mass) or less, and the amount of nitrogen generated during melting is 15 ppm (by mass) or less.

[0017] The invention involved in the sixth embodiment is the spheroidizing agent described in the fifth embodiment, which is a Fe-Si-Mg based spheroidizing agent.

[0018] The effects of the invention According to the present invention, ultrafine ductile iron without chilling can be manufactured with good reproducibility in a simple manner.

[0019] The inventors conducted repeated and in-depth research, resulting in the following insights: Considering that nitrogen in the spheroidizing agent, like nitrogen in the original molten iron, may have a significant impact on cold hardening, through repeated experiments, even when the nitrogen content in the original molten iron was relaxed compared to Patent Document 1, cold hardening could be prevented, and fine spheroidization could be achieved. It is speculated that this may be because the N in the spheroidizing agent, and the free N in its N form, affects cold hardening in the same way as the free N in the original molten iron.

[0020] Therefore, in this invention, even when the nitrogen content generated during melting exceeds 0.9 ppm, fine spheroidization and no cold hardening can be achieved, thus relaxing the steps for controlling the nitrogen content of the original molten iron. As a result, ultrafine ductile iron can be manufactured in a simpler and more reproducible manner. Attached Figure Description

[0021] Figure 1 These are photographs showing the metal microstructure of Example 1 and its comparative examples. A relates to Example 1, and B relates to its comparative examples.

[0022] Figure 2 This is a photograph showing the metal microstructure in Example 2.

[0023] Figure 3 These are photographs showing the metal microstructure of Example 3 and its comparative examples. A relates to Example 3, and B relates to its comparative examples. Detailed Implementation

[0024] The method for carrying out the present invention will be described for each step.

[0025] (Melting process) In the melting process, the raw molten iron material of ductile iron is melted.

[0026] As the raw material for molten iron, for example, the raw material specified in the appendix to JIS G 5502, corresponding to the "Chemical composition" of the international standard ISO 1083, can be used. Table 1 shows an example of the composition specified in "Table A.2 Example of chemical composition".

[0027] Other cast irons can also be used. Additionally, other elements can be added as needed. Furthermore, the composition range can be appropriately modified.

[0028] Examples specified in JIS G5502 include FCD400-15, FCD450-10, FCD500-7, FCD600-3, FCD700-2, and FCD800-2.

[0029] In addition, after the above-mentioned raw iron molten material is melted, Bi, Ca, Ba, Cu, Ni, Mo, and RE (rare earth elements) may be added appropriately.

[0030] In addition, the CE (carbon equivalent) can be appropriately controlled to, for example, 3.9 ~ 4.6.

[0031] In this invention, spheroidization is performed after melting.

[0032] However, in other aspects of the invention, the molten iron is further heated after melting to raise its temperature. This heating removes oxygen from the molten iron.

[0033] The heating process continues until oxygen removal (SiO2 reduction) from the molten iron stops. Considering operational efficiency, the target heating temperature T0 is 1500℃. Heating is stopped at this temperature, and the furnace is held at T0 for a specified time. If the holding time continues, bubbles will be observed forming on the side of the crucible. This indicates that SiO2 reduction in the molten iron has stopped, and the SiO2 lining of the melting furnace is beginning to be reduced and eroded. Therefore, the holding time is stopped at this point. Typically, the holding time is between 2 and 10 minutes. After the oxygen removal process, nitrogen removal is performed. At this time, the amount of nitrogen generated during melting is set to a specified value.

[0034] The amount of nitrogen produced during melting is the amount of nitrogen gas produced during the melting and casting of the sample. Specifically, the determination is performed according to the following steps. To remove the oxide film, the surface oxide film is removed with FUJI STAR500 (Sankyori Chemicals) sandpaper until a metallic luster is visible. Then, the sample is cut into 0.5 g to 1.0 g pieces using a micro-cutting machine or rebar cutter. To remove oil, the cut sample is cleaned with acetone, dried in a desiccator for a few seconds or under vacuum before analysis.

[0035] The analysis involves connecting the device to power and introducing He gas, performing system checks and leak checks, confirming that there are no abnormalities, and starting the analysis after stabilization. During the analysis, discard analysis, blank measurement, and zero-point calibration are performed.

[0036] In the blank analysis, a crucible was first set up and approximately 0.4 g of combustion aid (graphite powder) was added (the combustion aid is to improve the nitrogen extraction rate in the alloy). He was introduced while degassing and purging were performed, replacing the sample chamber with He gas. Next, to remove oxygen and nitrogen generated from the graphite crucible through preheating, the crucible was heated at a temperature above the analytical temperature (2163°C) and held for 15 seconds to remove the gases generated from the crucible. Then, analysis was performed under elevated temperature conditions, and the resulting values ​​were used as a blank for calibration, serving as a zero-point reference.

[0037] As standard samples for preparing the standard curve, LECO 114-001-5 (nitrogen 8±2 ppm, oxygen 115±19 ppm), 502-873 (nitrogen 47±5 ppm, oxygen 34±5 ppm), 502-869 (nitrogen 414±8 ppm, oxygen 36±4 ppm), and 502-416 (nitrogen 782±14 ppm, oxygen 33±3 ppm) were used, and the standard curve was prepared based on the values ​​obtained from three measurements of each sample.

[0038] In temperature analysis, nitrogen contained in a low-melting-point substance is extracted by slowly melting it at various temperatures, resulting in waveform peaks.

[0039] The nitrogen content per unit area is calculated from the total area of ​​the waveform peaks (the sum of peak intensity values) and the nitrogen content obtained through analysis. The peak (A1) generated in the early stage of heating around 1250-1350℃ is used as the nitrogen content during melting and quantified.

[0040] Nitrogen can be removed from the molten iron by reducing its solubility. Therefore, the molten metal is cooled slowly. In rapid cooling, nitrogen may not be completely extracted from the molten iron. A cooling rate of 5°C / min or less is preferred.

[0041] Cooling is preferably performed up to T (°C) in Equation 1. If cooling is performed to a temperature below T (°C), oxygen is introduced instead. To minimize both nitrogen and oxygen, cooling to T < °C is preferred. For Equation 1 derived from equilibrium theory, from a practical point of view, cooling to (T - 15°C) ± 20 (°C) is preferred.

[0042] Equation (1) T=T k -273 (℃) log ([ Si ] / [ C ]2 )=-27,486 / T k + 15.47 During slow cooling, nitrogen is released from the original molten iron. That is, due to slow cooling, the saturated solubility of nitrogen in the original molten iron decreases, thus releasing nitrogen that has not formed compounds with other elements. Alternatively, argon bubbling can be used, for example. Through such cooling, nitrogen is removed from the original molten iron.

[0043] (Spheroidizing process) Spheroidization is performed after the melting process.

[0044] In this invention, the spheroidizing treatment is performed using a spheroidizing agent with a C content of 0.5% by mass or more, a total nitrogen (N) content of 150 ppm (by mass) or less, and a nitrogen content generated during melting of 15 ppm (by mass) or less.

[0045] From the viewpoint of ease of control, the lower limit of the amount of nitrogen (N) generated during melting is preferably 3 ppm.

[0046] Furthermore, in this invention, the spheroidizing agent contains 0.5% or more of C. By containing 5% or more, the amount of nitrogen (N) generated during melting can be controlled to below 20 ppm for the first time. Additionally, since Fe-50% by mass Si is used as a base, the upper limit for C is approximately 2.20% by mass.

[0047] Spheroidizing is generally performed by adding Mg. Other methods can also be used (e.g., spheroidizing using a treatment agent containing Ce). However, compared to Ce, Mg provides overwhelmingly superior refinement and a significantly higher number of carbon particles per unit size. Furthermore, excessive Ce content is a major cause of cold hardening and is therefore not preferred.

[0048] The Mg-containing treatment agent is preferably Fe-Si-Mg. In particular, a treatment agent with a Fe:Si:Mg ratio of 50:50:(1 to 10) (mass ratio) is preferred. When the Mg ratio is less than 1, sufficient spheroidization cannot be achieved. Furthermore, if it exceeds 10, the vapor pressure of Mg as an alloy becomes high, and bubbling during spheroidization becomes more intense, thus causing nitrogen absorption. From this viewpoint, 1 to 10 is preferred, and 1 to 5 is more preferable.

[0049] The spheroidizing treatment is preferably performed when the oxygen content of the original molten iron is below 20 ppm (by mass). By setting it to below 20 ppm, fine spheroidal graphite can be obtained.

[0050] (Pregnancy / Development Process) Immediately after spheroidization, an inoculation treatment is performed. This inoculation is carried out by adding an Fe-Si based inoculant, for example, containing a small amount of an element with a strong affinity for nitrogen (Ca, Ba, Al, etc.), to the molten metal. For example, an Fe-75Si (mass ratio) system is preferred.

[0051] (Casting process) Casting is performed after adding the inoculant Fe-Si. Casting is preferably performed when the inoculant has not yet diffused and homogenized. Considering equipment factors, a short casting time, such as less than 10 minutes, less than 5 minutes, less than 1 minute, or less than 5 seconds, is preferred.

[0052] Casting is preferred at T p The test was conducted at ±20 (℃).

[0053] Where T p =1350-60M (℃) M = V / S V is the volume of the product (cm²) 3 S is the surface area of ​​the product (cm²) 2 ) The mold temperature is preferably set to T. d ±20 (℃).

[0054] T d =470-520M (℃) M = V / S V is the volume of the product (cm²) 3 S is the surface area of ​​the product (cm²) 2 ) The mold temperature is preferably controlled according to the volume of the product. By controlling the mold temperature, spherical graphite can be formed more finely and uniformly.

[0055] However, since incomplete casting may occur depending on the conditions, the minimum temperature of the mold is preferably set to 100°C.

[0056] (Pregnancy) The purpose of incubation is to render cold hardening harmless by converting free N into nitrides and to increase the number of graphite crystallization sites by maintaining Si concentration spots.

[0057] The inoculation treatment is preferably carried out by adding an Fe-Si inoculant containing a small amount of elements with strong affinity for N (Ca, Ba, Al, Sr, Zr, etc.).

[0058] Casting is preferably carried out as quickly as possible after the addition of Fe-Si. The shorter the incubation time, the lower the risk of chilling and the greater the number of finer spheroidal graphites per unit area. The shorter the time, the less Fe-Si diffuses into the molten metal, and consequently, the higher the density of the spheroidal graphites.

[0059] While it also depends on the equipment, it is preferable to perform the above-mentioned casting within 10 minutes, more preferably within 5 minutes, and even more preferably within 30 seconds or 5 seconds. When casting is performed with Fe-Si inoculants in a state before diffusion after melting, the number of spherical graphite particles increases dramatically compared to when uniformly melted. For molten metals containing small amounts of free nitrogen from elements such as Ca, Ba, and Al, the risk of cold hardening can be suppressed by minimizing the newly absorbed free nitrogen during the time up to casting. To further promote this state, it is preferable to perform casting without stirring.

[0060] By carefully selecting the smelting method for the original molten iron, as well as the spheroidizing agent and inoculant, the amount of nitrogen (N) generated during surface melting can be kept low. However, due to factors such as foaming during the Mg reaction, increased surface area in contact with the atmosphere due to the thinning of the flow during molten iron / casting, and the presence of nitrogen in the coating binder, nitrogen generated during melting is absorbed. Although free N can be removed through inoculation, the amount of nitrogen (N) carried into the molded product during melting is preferably set to 5 ppm (mass) or less, more preferably 3 ppm (mass) or less, and even more preferably as low as 1 ppm (mass) or less.

[0061] A molding compound is preferably applied to the mold. A heat-insulating molding compound is particularly preferred, especially one with a thermal conductivity of 0.42 W / (m·K) or less. Specifically, a heat-insulating molding compound with a thickness of 0.4 mm or more is preferred.

[0062] (Sand-cast thin-walled ductile iron) The above describes molded ductile iron. However, the spheroidizing agent of the present invention can also be applied to sand-cast thin-walled ductile iron with a wall thickness of 30 mm or less, where the solidification and cooling conditions are the same as those for molds. Since resin is used to bind the sand particles, it cannot be preheated to above 200°C as in molds. This is because the resin decomposes above 200°C, losing the binding properties of the sand particles. In practice, preheating to around 60°C is performed to remove moisture. From the viewpoint of ensuring the fluidity of the melt, the casting temperature is set at 1400~1500°C. On the other hand, in wet sand casting, since moisture is imparted to the viscosity as a binder for the sand particles, casting at 1400~1500°C without preheating is achieved. Therefore, in sand casting, although the cooling rate is slightly slower than in mold casting, it becomes a casting condition that easily produces chilling hardening. The spheroidizing agent of the present invention can also be applied to sand-cast thin-walled ductile iron.

[0063] Example (Example 1) By combining raw materials such as pig iron, steel scrap, and Fe-Si, the following target chemical composition can be achieved.

[0064] (quality% ) C: 3.60, Si: 2.60, Mn: 0.10, P: 0.025, S: 0.005, balance Fe These materials are heated in a high-frequency induction furnace to melt them.

[0065] After melting, the material continues to be heated, reaching 1425°C. At temperatures above 1425°C, oxygen is removed through the generation of CO.

[0066] The temperature was further increased, and CO was confirmed to be generated from the heat-resistant material in the furnace at temperatures exceeding 1510°C. Therefore, the heating was stopped at 1510°C and held at that temperature for 5 minutes. During this period, oxygen was efficiently removed from the original molten iron.

[0067] After holding at 1510℃ for 5 minutes, slowly cool to 1425℃ (=T℃) at a rate of approximately 10℃ / minute. Temporarily lower the temperature to 1440℃ midway, then raise it to 1460℃, and then cool at a rate of 10℃ / minute.

[0068] Mg treatment was performed at 1425 °C. The Mg treatment was based on Fe-50% Si-3% Mg (by mass), with the addition of a spheroidizing agent containing total nitrogen N: 87 ppm, nitrogen generated during melting N: 4.5 ppm, and C: 1.5% (by mass).

[0069] Inoculation was performed after Mg treatment. Surface inoculation was carried out with stirring using a 0.6 wt% Fe-75 wt% Si-based inoculant. The finished product was a coin with a diameter of 34 mm and a thickness (t) of 5.4 mm. The casting temperature and mold temperature were set as follows.

[0070] In addition, a 0.4 mm thick heat-insulating molding coating was applied to the mold. The thermal conductivity of the coating was 0.42 W / (m·K).

[0071] Casting temperature T p , T p =1350-60M=1320℃ M = V / S = 0.34 V is the volume of the product (cm²) 3 S is the surface area of ​​the product (cm²) 2 ) mold temperature T d , T d =470-520M=293 (℃) At the set casting and mold temperatures, 10 seconds after incubation, casting was performed into the mold. The following results were obtained after casting.

[0072] The chemical composition of the product is as follows.

[0073] C: 3.61%, Si: 3.11%, Mn: 0.10%, P: 0.024%, S: 0.008%, Mg: 0.018% (mass%) The amount of nitrogen generated during the melting of the mold casting is 3 ppm (by mass).

[0074] For the cast specimens, the microstructure was observed using microscopic photographs. The microstructure is illustrated in [image description missing]. Figure 1 (A)

[0075] The spheroidal graphite is extremely fine and evenly distributed. The number of spheroidal graphite particles was counted, yielding 1963 particles / mm². 2 No hardening was observed.

[0076] (Comparative Example) In this example, the Mg treatment was performed by adding Fe-Si-7.5% Mg (N: 250 ppm). All other aspects were the same as in Example 1.

[0077] The results are shown in Figure 1 (B)

[0078] The spheroidal graphite is extremely fine and evenly distributed. The number of spheroidal graphite particles was counted, resulting in 760 particles / mm². 2 In addition, it was confirmed that a lot of cold hardening occurred.

[0079] (Example 2) In this example, after holding at 1510°C for 5 minutes, it was slowly cooled to 1425°C (=T°C) at a rate of approximately 5°C / minute. The temperature was temporarily lowered to 1440°C midway through the process, then raised to 1460°C, followed by cooling at a rate of 5°C / minute. Spheroidization was then performed at 1425°C.

[0080] The amount of nitrogen generated during the melting of sand castings is 0.7 ppm (by mass).

[0081] The results are shown in Figure 2 In this example, the number of graphite particles is 2605 / mm, which is more than in Example 1. 2 spherical graphite.

[0082] (Example 3) An example of sand-cast thin-walled ductile iron is shown. The product is a box-shaped casting with an average wall thickness of 6.5 mm and a weight of 125 kg.

[0083] It is combined with raw materials such as pig iron, steel scrap, and Fe-Si to achieve the following target composition.

[0084] (quality% ) C: 3.70, Si: 2.60, Mn: 0.50, P: 0.025, S: 0.035, balance Fe These raw materials are heated to melt using a high-frequency induction furnace. After melting, the temperature is increased to approximately 1500°C, exceeding the temperature at which the oxygen reduction reaction begins (1425°C), and held for 5 minutes.

[0085] Mg treatment was performed at approximately 1500°C. The Mg treatment was based on Fe-50% Si-3% Mg (by mass), with the addition of a spheroidizing agent containing N: 87 ppm and C: 1.5% (by mass).

[0086] As a covering material to adjust the reaction of the spheroidizing agent, Fe-75 mass% Si with 1 wt% composition adjustment is used.

[0087] No inoculation treatment was performed after Mg treatment.

[0088] The sand mold is shaped using a furan self-hardening process and coated with a general MgO-based mold coating. The casting temperature and sand mold temperature are set as follows.

[0089] Casting temperature: 1420℃ Mold temperature; no preheating / room temperature At the set casting and sand mold temperatures, casting was performed in the sand mold 88 seconds after the Mg treatment. The casting time was approximately 8 seconds. The following results were obtained after casting.

[0090] The chemical composition of the product is as follows.

[0091] C: 3.69%, Si: 3.65%, Mn: 0.53%, P: 0.047%, S: 0.017%, Mg: 0.043% (mass%) For the cast specimens, the microstructure was observed using microscopic photographs. The microstructure is illustrated in [image description missing]. Figure 3 (A)

[0092] The spheroidal graphite particles are finely and uniformly distributed. The number of spheroidal graphite particles was counted, yielding 853 particles / mm. 2 No hardening was observed.

[0093] (Comparative Example) In this example, the Mg treatment was performed by adding Fe-Si-7.5% Mg (N: 250 ppm). All other aspects were the same as in Example 3.

[0094] The results are shown in Figure 3 (B)

[0095] Although spheroidal graphite is fine, its spheroidization rate is low. The number of spheroidal graphite particles was counted, resulting in 178 particles / mm². 2 In addition, it was confirmed that a lot of cold hardening occurred.

Claims

1. A method for manufacturing a mold casting of ductile iron, comprising the following steps: The melting process of raw materials made of cast iron to obtain molten iron. The spheroidizing process involves spheroidizing. The process of gestation. The casting process of pouring material into a mold; in, The spheroidizing treatment is performed using a spheroidizing agent in which the content of C is set to 0.5% (by mass) or more, the content of total nitrogen (N) is set to 150 ppm (by mass) or less, and the amount of nitrogen generated during melting is set to 15 ppm (by mass) or less.

2. The method for manufacturing mold castings of ductile iron according to claim 1, wherein, The spheroidizing agent is a Fe-Si-Mg based spheroidizing agent.

3. The method for manufacturing mold castings of ductile iron according to claim 1 or 2, characterized in that, The nitrogen content is adjusted so that the amount of nitrogen generated during the melting of the mold casting is less than 5 ppm (by mass).

4. The method for manufacturing mold castings of ductile iron according to any one of claims 1 to 3, characterized in that, The raw material, made of cast iron, is heated and melted to obtain molten iron. After heating the molten iron to a specified temperature of 1500°C or higher, heating is stopped, and the temperature is maintained for a certain period of time to remove oxygen from the molten iron. Then, the nitrogen in the molten iron is reduced by slowly cooling it. Next, spheroidizing treatment, inoculation, and casting are performed.

5. A spheroidizing agent, wherein, Set the C content to 0.5% (by mass) or higher, the total nitrogen N content to 150 ppm (by mass) or lower, and the amount of nitrogen generated during melting to 15 ppm (by mass) or lower.

6. The spheroidizing agent according to claim 5, wherein it is a Fe-Si-Mg based spheroidizing agent.

7. The spheroidizing agent according to claim 5 is a Fe-Si-Mg based spheroidizing agent applicable to thin-walled ductile iron in sand molds, wherein the solidification and cooling conditions of the sand mold are the same as those of the die.

Images