PROCESS FOR PRODUCING NODULAR CAST IRON

Abstract: The process according to the present invention comprises the steps of placing a spheroidization agent and a graphitization promoting agent into a ladle; performing a spheroidization process by charging molten metal, having a composition suitable for forming nodular cast iron and comprising a graphite atomization agent, into the ladle; and pouring the molten metal into a mold.

Description: The present invention relates to a process for producing nodular cast iron of superior graphitization capability which makes it suitable for use as thin-shell cast products. For production of nodular cast iron, Fe-Si-Mg alloy or such alloy with a small amount of RE or rare earth elements added thereto is most commonly used as the spheroidization agent in industrial applications. Also, the open ladle addition process or the so-called sandwich process is most commonly used as the spheroidization process. For improving the graphitization capability of molten metal which has been temporarily reduced by addition of Mg or Mg-RE alloy, an inoculation agent such as various Si alloys or graphite-base substances has been conventionally inoculated into the ladle and/or the flow of the molten metal as it is poured into a mold. However, in industrial applications, since simply performing inoculation after a spheroidization process cannot entirely eliminate the formation of cementite, a heat treatment is required in order to decompose the cementite formation. Hence, unfavorable consequences such as the increase in the cost and time required for production are inevitable. The process of producing nodular cast iron which was the subject of a preceding patent application (Japanese patent application No. 61-144591) can produce a favorable chill prevention effect with respect to thin-shell cast iron products, but the present invention has its aim to further improve this prior invention. The efficacy of Bi addition to atomize graphite has already been reported, for instance, in AFS Internar., Cast Metals, J, 7(1982), 3, S, 22/31 and FONDERIE BELGE 52 (1982) Nr, 2, S, 5/18, and inoculating agents containing Bi such as SPHERIX (trade name) are commercially available. EP-A-0 162 194 discloses a method of producing nodular cast iron by first spheroidising the graphite using Mg and then inoculating the molten cast iron with an inoculating alloy containing Bi. The effect is to obtain optimal graphite precipitation and a high graphite/nodule count. US-A-4 432 793 also discloses that Bi may be added after the Spheroidising treatment. However, according to the present invention as given in claim 1, through the synergetic effect of processing the molten metal with a graphitization promoting agent preferably including SiC or CaC₂ as a major ingredient and of adding a graphite atomization agent, e.g. Bi, to the molten metal before carrying out the spheroidization process, the promotion of graphitization and the increase in the number of graphite nodules, which are both important for the production of high-quality thin-shell cast iron products, can be accomplished. For example, according to the results of a comparison test conducted with respect to the Y-blocks, which were cast from different kinds of nodular cast iron and are each 25 mm in thickness, by taking into account only the graphite nodules having 8 micrometers or greater in diameter, it was observed that, whereas the number of graphite nodules was 300/mm² according to the process of producing nodular cast iron disclosed in Japanese patent application No. 61-144591, and this number was no more than 300/mm² when Bi was simply added, the present invention was able to increase this number to 600/mm². Thus, a primary object of the present invention is to provide a process of producing nodular cast iron which is free from the formation of cementite when cast into thin-shell products and is provided with a sufficient deformation capability even as cast. The process of producing nodular cast iron according to the present invention comprises the steps of: incorporating a graphite atomization agent in a molten metal having a composition suitable for forming nodular cast iron; subjecting the graphite atomization agent - containing molten metal to a spheroidization process in the presence of a spheroidization agent and a graphitization promoting agent; and moulding the thus treated molten metal to form nodular cast iron. Preferably, inoculation is performed after performing the spheroidization process and before the molten metal has flowed into the cavities in the mold. The spheroidization agent may consist of Mg or material containing Mg, and the graphitization promoting agent may consist of silicon carbide, calcium carbide, silicon carbide and carbon, calcium carbide and carbon, silicon carbide, carbon and Si alloy, or calcium carbide, carbon and Si alloy. According to a most preferred embodiment of the present invention, the graphite atomization agent consists of Bi or material containing Bi. The present invention is described in the following with reference to the appended drawings, in which: Figure 1 is a perspective view of a test piece; and Figures 2 through 7 are microscopic photographs of the metallic structures of the various examples of the nodular cast iron produced by the process of the present invention at a magnification factor of 100. Various embodiments of the present invention are described in the following. - Embodiment 1 - 1) With respect to the weight of the molten metal which is to be charged into a ladle to be formed into nodular cast iron, 1.6% of Fe-Si-Mg (3.5) serving as a spheroidization agent, and 1.0% of silicon carbide and 0.5% of Fe-Si serving as a graphitization promotion agent were placed at the bottom of the ladle. 2) 0.010% of metallic Bi serving as a graphite atomization agent was added to the molten metal as the latter is being poured into the ladle. The temperature of the molton metal at this point of time was 1,525 degrees C. 3) The thus obtained molten metal contained, in addition to iron and inevitable impurities, the following ingredients: 4) A stepped test piece as illustrated in Figure 1 was obtained by using this molten metal. A certain amount of Fe-Si which is equivalent to 0.1% of Si was inoculated into the flow of the molten metal as the test piece was being cast. The temperature of the molten metal at this time point was 1,410 degrees C. 5) The microscopic view of the part of the test piece which is 2 mm in thickness demonstrated a crystallization of a large number of minute graphite particles without any sign of chilling as shown in Figure 2. This test piece thus demonstrated an extremely favorable nodular graphite structure. - Embodiment 2 - 1) With respect to the weight of the molten metal which is to be charged into a ladle to be formed into nodular cast iron, 1.6% of Fe-Si-Mg (3.5) serving as a spheroidization agent, and 1.0% of silicon carbide, 0.4% of electrode powder and 0.5% of Fe-Si serving as a graphitization promotion agent were placed at the bottom of the ladle. 2) A certain amount of Fe-Si(71)-Al(0.2)-Ca(0.6)-RE(0.42)-Bi(0.5) alloy equivalent to 0.010% of metallic Bi serving as a graphite atomization agent was added to the molten metal in the furnace immediately before it was poured into the ladle. The temperature of the molten metal at this time point was 1,535 degrees C. 3) The thus obtained molten metal contained, in addition to iron and inevitable impurities, the following ingredients: 4) A stepped test piece as illustrated in Figure 1 was obtained by using this molten metal. During the casting process, Fe-Si particles formed into briquettes by a suitable binder were placed in the mold right under the sprue, or so-called in-the-mold inoculation was carried out. The amount of the inoculation agent was equivalent to 0.10% of Si. The temperature of the molten metal at this time point was 1,420 degrees C. 5) The microscopic view of the part of the test piece which is 2 mm in thickness demonstrated a crystallization of a large number of minute graphite particles without any sign of chilling as shown in Figure 3. This test piece thus demonstrated an extremely favorable nodular graphite structure. - Embodiment 3- 1) With respect to the weight of the molten metal which is to be charged into a ladle to be formed into nodular cast iron, 1.6% of Fe-Si-Mg (3.5) serving as a spheroidization agent, and 1.0% of calcium carbide and 0.5% of Fe-Si serving as a graphitization promotion agent were placed at the bottom of the ladle. 2) The molten metal which has the composition to be nodular cast iron and added with 0.010% of metallic Bi serving as a graphite atomization agent was charged into the ladle. The temperature of the molten metal at this time point was 1,530 degrees C. 3) The thus obtained molten metal contained, in addition to iron and inevitable impurities, the following ingredients: 4) A stepped test piece as illustrated in Figure 1 was obtained by using this molten metal. A certain amount of Fe-Si which is equivalent to 0.1% of Si was inoculated into the flow of the molten metal as the test piece is being cast. The temperature of the molten metal at this time point was 1,415 degrees C. 5) The microscopic view of the part of the test piece which is 2 mm in thickness demonstrated a crystallization of a large number of minute graphite particles without any sign of chilling as shown in Figure 4. This test piece thus demonstrated an extremely favorable nodular graphite structure. - Embodiment 4- 1) With respect to the weight of the molten metal which is to be charged into a ladle to be formed into nodular cast iron, 1.6% of Fe-Si-Mg (3.5) containing 1.5% of RE and serving as a spheroidization agent, and 1.0% of silicon carbide and 0.5% of Fe-Si serving as a graphitization promotion agent were placed at the bottom of the ladle. 2) The molten metal which has the composition to be nodular cast iron and added with 0.010% of metallic Bi serving as a graphite atomization agent was charged into the ladle. The temperature of the molten metal at this point of time was 1,510 degrees C. 3) The thus obtained molton metal contained, in addition to iron and inevitable impurities, the following ingredients: 4) A stepped test piece as illustrated in Figure 1 was obtained by using this molten metal. A certain amount of Fe-Si which is equivalent to 0.1% of Si was inoculated into the flow of the molten metal as the test piece is being cast. The temperature of the molten metal at this time point was 1,415 degrees C. 5) The microscopic view of the part of the test piece which is 2 mm in thickness demonstrated a crystallization of a large number of minute graphite particles without any sign of chilling as shown in Figure 5. This test piece thus demonstrated an extremely favorable nodular graphite structure. - Embodiment 5- 1) With respect to the weight of the molten metal which is to be charged into a ladle to be formed into nodular cast iron, 1.6% of Fe-Si-Mg (3.5) containing 1.5% of RE and serving as a spheroidization agent, and 1.0% of silicon carbide, 0.4% of electrode powder and 0.5% of Fe-Si serving as a graphitization promotion agent were placed at the bottom of the ladle. 2) The molten metal which has the composition to be nodular cast iron and added with 0.010% of metallic Bi serving as a graphite atomization agent was charged into the ladle. The temperature of the molten metal at this time point was 1,510 degrees C. 3) The thus obtained molten metal contained, in addition to iron and inevitable impurities, the following ingredients: 4) A stepped test piece as illustrated in Figure 1 was obtained by using this molten metal. During the casting process, Fe-Si particles formed into briquettes by a suitable binder were placed in the mold right under the sprue, or so-called in-the-mold inoculation was carried out. The amount of the inoculation agent was equivalent to 0.10% of Si. The temperature of the molten metal at this time point was 1,410 degrees C. 5) The microscopic view of the part of the test piece which is 2 mm in thickness demonstrated a crystallization of a large number of minute graphite particles without any sign of chilling as shown in Figure 6. This test piece thus demonstrated an extremely favorable nodular graphite structure. - Embodiment 6 - 1) With respect to the weight of the molten metal which is to be charged into a ladle to be formed into nodular cast iron, 1.6% of Fe-Si-Mg (3.5) containing 1.5% of RE and serving as a spheroidization agent, and 1.0% of calcium carbide, 0.4% of electrode powder and 0.5% of Fe-Si serving as a graphitization promotion agent were placed at the bottom of the ladle. 2) The molten metal added with a certain amount of Fe-Si(71)-Al(0.2)-Ca(0.6)-RE(0.42)-Bi(0.5) alloy equivalent to 0.010% of metallic Bi serving as a graphite atomization agent was charged into the ladle. The temperature of the molten metal at this point in time was 1,525 degrees C. 3) The thus obtained molten metal contained, in addition to iron and inevitable impurities, the following ingredients: 4) A stepped test piece as illustrated in Figure 1 was obtained by using this molten metal. During the casting process, Fe-Si particles formed into briquettes by a suitable binder were placed in the mold right under the sprue, or so-called in-the-mold inoculation was carried out. The amount of the inoculation agent was equivalent to 0.10% of Si. The temperature of the molten metal at this time point was 1,415 degrees C. 5) The microscopic view of the part of the test piece which is 2 mm in thickness demonstrated a crystallization of a large number of minute graphite particles without any sign of chilling as shown in Figure 7. This test piece thus demonstrated an extremely favorable nodular graphite structure. The features of the nodular cast iron produced by the process of the present invention may be summarized as follows: With regard to a cast product of a given thickness, 1. The number of graphite particles is twice that of conventional nodular cast iron, and, hence, no chilling occurs; 2. Absence of chilling even in thin-shell products means that the products may be usable as cast or, at most, after low-temperature heat treatment whereby a saving in the cost of heat treatment can be achieved; and 3. Whereas high-temperature heat treatment of cast products having complicated shapes increases the strain in the products, the possibility of using the products as cast or after low-temperature heat treatment eliminates the need for any process of eliminating such strain. Thus, as described above, the nodular cast iron produced by the process of the present invention is highly inexpensive to produce since the production process is much simplified, and the present invention thus offers a substantial advantage in industrial applications.

Claims: A process for producing nodular cast iron, comprising the steps of: incorporating a graphite atomization agent in a molten metal having a composition suitable for forming nodular cast iron; subjecting the graphite atomization agent - containing molten metal to a spheroidization process in the presence of a spheroidization agent and a graphitization promoting agent; and moulding the thus treated molten metal to form nodular cast iron. A process as claimed in claim 2 wherein said spheroidization process comprises the steps of placing said spheroidization agent and said graphitization promoting agent into a ladle and charging said molten metal comprising said graphite atomization agent into said ladle. A process as claimed in claim 1 or claim 2 further comprising inoculation with an inoculation agent performed after said spheroidization process and before the molten metal has flowed into the cavities in the mold. A process as claimed in any one of the preceding claims, wherein said spheroidization agent consists of Mg or material containing Mg. A process as claimed in any one of the preceding claims, wherein said graphitization promoting agent consists of silicon carbide, silicon carbide and carbon, or silicon carbide, carbon and Si alloy. A process as claimed in any one of claims 1-4, wherein said graphitization promoting agent consists of calcium carbide, calcium carbide and carbon, or calcium carbide, carbon and Si alloy. A process as claimed in any one of the preceding claims, wherein said graphite atomization agent consists of Bi or material containing Bi.

Inventors: NATSUME, TAKESHI, OBATA, FUMIO, SAKAI, JUN, SUENAGA, MAKOTO, Natsume, Takeshi, c/o K.K. Honda Gijutsu, Obata, Fumio, c/o Hitachi Metals Ltd., SAKAI, JUN, C/O K.K. HONDA GIJUTSU, Suenaga, Makoto, c/o Hitachi Metals Ltd.
Applicants: HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: C21C1
IPC codes: C21C1, C22C33