Process for diecasting graphite cast iron at solid-liquid coexisting state

Abstract: Graphite cast irons are diecast at a solid-liquid coexisting state with a mold having a gate opened at an area of not more than 1/10 of a pressurized area of a plunger chip.

Description: This invention relates to a process for diecasting graphite cast iron in a solid-liquid coexisting state. Generally, cast irons are widely used in various fields, such as for automobile parts and the like, because they have good castability and can be cast into complicated shapes. To this end, if thin-walled products can be produced by industrially diecasting the cast iron, significant weight reductions in the product can be attained. However, the melting point of cast iron is very high (not lower than 1150°C), and thus there is no mould material durable to molten cast iron. In the industrial diecasting of cast iron, it is possible only to conduct diecasting at a temperature which is lower than the melting point of the cast iron and which has less latent heat. The cast iron is in a solid-liquid coexisting state at such a temperature. Thus, it is strongly desired to industrially develop such a diecasting. Although the diecasting of cast iron has not yet been industrialized, a method of injecting molten cast iron from a diecasting machine is known. However, when molten spheroidal graphite cast iron is diecast in the diecasting machine, there is a problem as regards the heat resistance of the mould, as mentioned above. Also Ca or Mg (the graphite spheroidizing agent) easily evaporates when the spheroidal graphite cast iron is in the molten state. In the latter case, even if the molten cast iron is formed in the vicinity of the diecasting machine as far as possible, countermeasures for preventing evaporation of the graphite spheroidizing agent or adding further graphite spheroidizing agent to the melt should be taken. In the case of conducting diecasting in the solid-liquid coexisting state, a rheocasting process and a thixocasting process are known. In the rheocasting process a slurry of semi-solidified metal composition is directly supplied to a diecasting machine and is then injection moulded therefrom, while in the thixocasting process a continuously cast billet or the like is reheated to a solid-liquid coexisting state and is supplied to a diecasting machine and injection moulded therefrom. In the thixocasting process, the billet is reheated to a temperature lower than the melting point in a short time, so that there is substantially no evaporation of the graphite spheroidizing agent. In the rheocasting process, however, air can be undesirably trapped in the cast iron and there are problems in the matching of throughput capacity between the continuous production device and the working device of the semi-solidified metal composition, the handling of the semi-solidified metal composition slurry, process control and the like, so that this process is not yet industrialized. In the thixocasting process, when a statically solidified ingot of spheroidal graphite cast iron is injected in the solid-liquid coexisting state, dendritic crystals entangle with each other to form a large lump. The dendritic crystals either remain in the mould as a lump or only liquid phase is fed to fill the mould, and consequently a cast product having a uniform structure is not obtained. To prevent the product structure being inconsistent, a method is known in which an ingot of cast iron having a granular primary crystal (in the case of hypo-eutectic structure, the primary crystal is ferrite) is used. However, the ingot of granular structure is obtained by the following methods which have the following problems associated therewith. 1) A melt of the ingot is solidified with stirring. In this case, entrapment of air during stirring, entrapment of broken pieces of the agitator, fluctuation of composition and the like can be caused. 2) A statically solidified cast ingot is subjected to plastic working to impart strain and is granulated by heating. However, it is difficult to adopt this method because the cast iron has poor plastic workability. 3) A melt of the ingot is added with an inoculating agent and then cast into a given shape. In this case, eutectic cell (crystal grain consisting of iron and graphite) can be fined, but the effect of fining the primary crystal grain is small. According to the present invention, there is provided a process for diecasting graphite cast iron in a solid-liquid coexisting state, comprising: heating an ingot of graphite cast iron to a temperature at which it is in a solid-liquid coexisting state; holding the ingot at this temperature for at least 3 seconds; and then injecting the graphite cast iron through a gate into a mould by means of a plunger, wherein the area of the gate is not more than 1/10 of the pushing area of the tip of the plunger. Preferably, the graphite cast iron is a graphite cast iron of flake hypo-eutectic structure or a spheroidal graphite cast iron. The spheroidal graphite cast iron may have a structure of spheroidal graphite having a diameter of not more than 100 µm or a ledeburite structure formed by rapid solidification. It will be seen that embodiments of the invention can provide a process for diecasting graphite cast iron at a solid-liquid coexisting state to form a diecast product having a uniform structure, using not only a cast iron ingot of granular structure in the thixocasting process, but also a cast iron ingot of dendrite structure which has been statically solidified in the usual manner. In the diecasting of graphite cast iron in the solid-liquid coexisting state according to the invention, the molten ingot of graphite cast iron is injected into a mould having a gate opened so that the area of the gate is not more than 1/10 of the pushing area of the tip of the plunger which pushes the cast iron into the mould. Thus, even if the ingot is of a spheroidal graphite cast iron having dendritic primary crystal statically solidified in the usual manner, when the molten ingot passes through the narrow gate, dendrite crystals are finely broken to disperse equally in the mould, Thus, a diecast product having a uniform microstructure is obtained. When the ingot is heated to the temperature at which it is in the solid-liquid coexisting state, graphite in the ingot may not be completely dissolved and may form an undissolved graphite portion. If the molten ingot having an undissolved graphite portion is injected into the mould, the undissolved graphite portion becomes included in the diecast product, so that it is difficult to obtain a uniform microstructure. Therefore, it is important that the ingot is heated to the temperature at which it is in the solid-liquid coexisting state and held at this temperature for not less than 3 seconds so as to dissolve the graphite completely. If the holding time is less than 3 seconds, the iron-graphite eutectic cell in the ingot cannot be completely dissolved. Furthermore, the size of crystal grain in the ingot largely affects the size of the primary crystal in the diecast product. Therefore, in order to obtain diecast products having finer primary crystals and uniform quality, it is important to make the crystal structure of the ingot finer. For this purpose, molten iron is preferably cooled at a rate of not less than 1 °C/s in the production of the cast iron ingot. When spheroidal graphite cast iron having a diameter of not more than 100 µm is used as the ingot, the dissolution of graphite is facilitated to provide a more uniform solid-liquid coexisting state by reheating, and hence a diecast product having a more uniform microstructure is obtained. If the diameter exceeds 100 µm, the distance between graphite grains is wider and it is difficult to provide the uniform solid-liquid coexisting state when the ingot is reheated. On the other hand, when rapid solidification (e.g. not less than 1 °C/s) is carried out in the casting, ledeburite structure (eutectic structure of austenite and cementite) is produced in the microstructure of the ingot. When the ledeburite structure is reheated to a given temperature of solid-liquid coexisting state, it is easily dissolved to provide a very uniform solid-liquid coexisting state. According to the invention, the ingot of the graphite cast iron is diecast in the solid-liquid coexisting state, so that the heat-bearing capacity of the mould is mitigated as compared with diecasting molten iron, and hence the service life of the mould can largely be prolonged. The invention will now be described in the following non-limitative examples and with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic view, partly in section, of a diecasting machine used in the method of the invention; Fig. 2a is a diagrammatic front view illustrating the gate of a mould and the shape of a product; Fig. 2b is a diagrammatic side view illustrating the gate of a mould and the shape of a product; Fig. 3a is a photomicrograph showing the metallic structure of an ingot of a flake graphite cast iron; Fig. 3b is a photomicrograph showing the metallic structure of a diecast product; and Fig. 3c is a photomicrograph showing the metallic structure of a diecast product after heat treatment. Example 1 A statically-solidified ingot of spheroidal graphite cast iron containing C: 3.10 mass%, Si: 2.03 mass%, Mn: 0.82 mass%, and Mg: 0.038 mass% was diecast at a solid-liquid coexisting state under the following diecasting conditions and the structure of the resulting diecast product was investigated. For comparison, an ingot stirred in the solid-liquid coexisting state and solidified under cooling was used. Diecasting conditions: Diameter of tip of plunger: 62 mm Injection speed: 1 m/s Injection pressure: 120 MPa Temperature of ingot: 1160°C (solid fraction: 0.3) (high frequency induction heating in sleeve) Opening area of gate: 60 mm x t mm; t = 2, 5 or 6 mm Product size: 80 mm x 80 mm x 10 mm In Fig. 1 is shown a diecasting machine used in this example, and shapes of a gate in a mould and a diecast product are shown in Figs. 2a and 2b. In these figures, numeral 1 is a tip of a plunger, 2 a sleeve, 3 a high frequency heating coil, 4 a mould sleeve, 5 a spreader, 6 a gate, 7 a mould, 8 a cavity block, 9 a cavity, 10 an ingot, 11 a biscuit, 12 a runner and 13 a diecast product. The results are shown in Table 1 below. Sample No. Ingot Size of gate (mm) Gate area/area of plunger tip Structure of product Void defect Remarks 1 statically solidified 60 x 2 1/25.2 uniform absence Acceptable example 2 statically solidified 60 x 5 1/10.1 uniform absence Acceptable example 3 statically solidified 60 x 6 1/8.4 ununiform absence Comparative example 4 stirred solidification 60 x 2 1/25.2 uniform presence Comparative example As seen from Table 1, in sample Nos. 1, 2 and 4 in which the opening area of the gate was not more than 1/10 of the pressurizing area of the plunger tip, diecast products having a uniform structure were obtained, while a diecast product having a uniform structure was not obtained in sample No. 3 in which the opening area was 1/8.4 of the pushing area of the plunger tip. In sample No. 4, void defect existed in the product. This was due to the fact that a void defect existing in the stirred solidification ingot was retained in the diecast product. On the other hand, the diecast products had a microstructure in which iron as the primary crystal was distributed in the form of grains, and the structure between the grains was a ledeburite structure (eutectic structure of iron and cementite) due to the rapid cooling in the diecasting. When the diecast product was subjected to a heat treatment for graphitizing the ledeburite structure, the ledeburite could be graphitized by heating to a temperature of 800-900 °C in a very short time. In sample Nos. 1 and 2 according to the invention, therefore, products having an excellent quality without void defect in which fine graphite was uniformly dispersed therein were obtained. Example 2 A cast iron of hypo-eutectic structure containing C: 3.10 mass%, Si: 2.03 mass%, and Mn: 0.82 mass% (liquidus temperature: 1230°C, solidus temperature: 1135°C) was used as an ingot. In this example were used a statically-solidified ingot of flake graphite structure having dendritic primary crystal (ferrite) (cooling rate is varied from molten iron) and, for comparison, a stirred solidification ingot of granular structure solidified under cooling while stirring to a solid fraction of 0.2. Each ingot was diecast in the solid-liquid coexisting state under the same diecasting conditions as in Example 1 in the same manner as in Example 1 and the uniformity of the structure and presence or absence of voids was investigated in the resulting diecast products. The results are shown in Table 2. Sample No. Ingot Holding time at heating Size of gate (mm) Gate area/area of plunger tip Structure of product Void 1 statically solidified 3 60 x 2 1/25.2 uniform absence 2 statically solidified 3 60 x 5 1/10.1 uniform absence 3 statically solidified 3 60 x 6 1/8.4 ununiform absence 4 stirred solidification 3 60 x 2 1/25.2 uniform presence 5 statically solidified 1 60 x 2 1/25.2 coarse structure of graphite in the ingot locally remains absence As seen from Table 2, in sample Nos. 1, 2, 4 and 5, diecast products having a uniform structure were obtained, while a diecast product having a uniform structure was not obtained in sample No. 3 in which the opening area of the gate was more than 1/10 of the pushing area of the plunger tip. In sample No. 4, void defect existed in the product. This was due to the fact that a void defect existing in the stirred solidification ingot was retained in the diecast product. In sample No. 5, the structure of the product locally became coarse when the diecasting was conducted immediately after heating of the ingot. In view of the product quality, it is favourable that a statically-solidified ingot is used as the starting ingot, that the cooling rate in the casting step is not less than 1 °C/s, and that the holding time after the ingot is reheated is not less than 3 seconds. The metallic structures of the ingot, diecast product and heat-treated diecast product (temperature: 900 °C, holding time: 10 minutes) in sample No. 2 are shown in Figs. 3a-3c, respectively. In the metallic structure of Fig. 3a, flake graphite is equally dispersed in the ingot, while the diecast product shown in Fig. 3b has a metallic structure in which ferrite is distributed in the form of grains and the structure between the grains is a ledeburite (eutectic structure of cementite and iron) due to the rapid cooling. In the metallic structure of Fig. 3c after the heat treatment for the graphitization of ledeburite, fine graphites are uniformly distributed in the product. As mentioned above, according to the invention, the diecasting of graphite cast iron at the solid-liquid coexisting state is carried out by restricting the opening area of the mould gate to not more than 1/10 of the pushing area of the plunger tip, whereby diecast products of complicated shapes having a uniform microstructure without void defect can be obtained, even if flake graphite cast iron or spheroidal graphite cast iron are used as the starting material. Furthermore, the service life of the mould can be prolonged as compared with the case of diecasting molten iron. Therefore, the invention considerably contributes to the industrialisation of the diecasting of graphite cast iron.

Claims: A process for diecasting graphite cast iron in a solid-liquid coexisting state, comprising: heating an ingot of graphite cast iron to a temperature at which it is in a solid-liquid coexisting state; holding the ingot at this temperature for at least 3 seconds; and then injecting the graphite cast iron through a gate into a mould by means of a plunger, wherein the area of the gate is not more than 1/10 of the pushing area of the tip of the plunger. A process as claimed in claim 1, wherein the graphite cast iron is a graphite cast iron of flake hypo-eutectic structure or a spheroidal graphite cast iron. A process as claimed in claim 2, wherein the spheroidal graphite cast iron has a structure of spheroidal graphite having a diameter of not more than 100 µm or a ledeburite structure formed by rapid solidification.

Inventors: ANDO YUICHI, KITAMURA KUNIO, YAHATA SEIRO, YOSHIDA CHISATO, ANDO, YUICHI, KITAMURA, KUNIO, YAHATA, SEIRO, YOSHIDA, CHISATO, ANDO, YUICHI, C/O RHEO-TECHNOLOGY, LTD., KITAMURA, KUNIO, C/O RHEO-TECHNOLOGY, LTD., YAHATA, SEIRO, C/O RHEO-TECHNOLOGY, LTD., YOSHIDA, CHISATO, C/O RHEO-TECHNOLOGY, LTD.
Applicants: RHEO TECHNOLOGY LTD, RHEO-TECHNOLOGY, LTD
Application year: 1995
Main classification: B22D17
IPC codes: B22D17