Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-09-25
2003-03-04
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Ferrous
C148S543000, C148S612000
Reexamination Certificate
active
06527878
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a thixocast casting material, a process for preparing a thixocast semi-molten casting material, a thixocasting process, an Fe-based cast product, and a process for thermally treating an Fe-based cast product.
BACKGROUND ART
In carrying out a thixocasting process, a procedure is employed which comprises heating a casting material into a semi-molten state in which a solid phase (a substantially solid phase and this term will also be applied hereinafter) and a liquid phase coexist, filling the semi-molten casting material under a pressure into a cavity in a casting mold, and solidifying the semi-molten casting material under the pressure.
An Fe—C—Si based alloy having a eutectic crystal amount Ec set in a range of 50% by weight≦Ec≦70% by weight is conventionally known as such type of casting material (see Japanese Patent Application Laid-open No.5-43978). However, if the eutectic crystal amount Ec is set in a range of Ec≧50% by weight, an increased amount of graphite is precipitated in such alloy and hence, the mechanical properties of a cast product is substantially equivalent to those of a cast product made by a usual casting process, namely, by a melt producing process. Therefore, there is a problem that if the conventional material is used, an intrinsic purpose to enhance the mechanical properties of the cast product made by the thixocasting process cannot be achieved.
If a thixocast casting material made by utilizing a common continuous-casting process can be used, it is economically advantageous. However, a large amount of dendrite exists in the casting material made by the continuous-casting process. The dendrite phases cause a problem that the pressure of filling of the semi-molten casting material into the cavity is raised to impede the complete filling of the semi-molten casting material into the cavity. Thus, it is impossible to use such casting material in the thixocasting. Therefore, a relatively expensive casting material made by a stirred continuous-casting process is conventionally used as the casting material. However, a small amount of dendrite phases exist even in the casting material made by the stirred continuous-casting process and hence, a measure for removing the dendrite phases is essential.
In carrying out the thixocasting process, a semi-molten casting material prepared in a heating device must be transported to a pressure casting apparatus and placed in an injection sleeve of the pressure casting apparatus. To carry out the transportation of a semi-molten casting material, for example, a semi-molten Fe-based casting material, a measure is conventionally employed for forming an oxide coating layer on a surface of the material prior to the semi-melting of the Fe-based casting material, so that the oxide coating layer functions as a transporting container for the main portion of the semi-molten material (see Japanese Patent Application Laid-open No.5-44010). However, the conventional process suffers from a problem that the Fe-based casting material must be heated for a predetermined time at a high temperature in order to form the oxide coating layer and hence, a large amount of heat energy is required, resulting in a poor economy. Another problem is that even if a disadvantage may not be produced, when the oxide coating layer is pulverized during passing through a gate of the mold to remain as fine particles in the Fe-based cast product, and if the oxide coating layer is sufficiently not pulverized to remain as coalesced particles in the Fe-based casting material, the mechanical properties of the Fe-based cast product are impeded, for example, the Fe-based cast product is broken starting from the coalesced particles.
The present inventors have previously developed a technique in which the mechanical strength of an Fe-based cast product can be enhanced to the same level as of a carbon steel for a mechanical structure by finely spheroidizing carbide existing in the Fe-based cast product of an Fe—C—Si based alloy after the casting, i.e., mainly cementite, by a thermal treatment. Not only the finely spheroidized cementite phases but also graphite phases exist in the metal texture of the Fe-based cast product after the thermal treatment. The graphite phases include ones that exist before the thermal treatment, i.e., ones originally possessed by the Fe-based cast product after the casting, and ones made due to C (carbon) produced by the decomposition of a portion of the cementite phases during the thermal treatment of the Fe-based cast product. If the amount of the graphite phases exceeds a given amount, there arises a problem that the enhancement of the mechanical strength of the Fe-based cast product after the thermal treatment is hindered.
There is a conventionally known Fe-based cast product having a free-cutting property and made of a flake-formed graphite cast iron. However, the flake-formed graphite cast iron has a difficulty in that the mechanical property thereof is low, as compared with a steel. Therefore, measures for spheroidizing the graphite and increasing the hardness of a matrix have been employed to provide a mechanical strength equivalent to that of the steel. However, if such a measure is employed, there arises a problem that the cutting property of the Fe-based cast product is largely impeded. This is because the graphite phases precipitated in crystal grains is coagulated into a crystal grain boundary due to the spheroidizing treatment and hence, the graphite does not exist in the crystal grains, or even if the graphite exists, the amount thereof is extremely small, and as a result, the cutting property of a matrix surrounding the crystal grains is good, while the cutting property of the crystal grains is poor, whereby a large difference is produced in cutting property between the matrix and the crystal grains.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a thixocast casting material of the above-described type, from which a cast product having mechanical properties enhanced as compared with a cast product made by a melt casting process can be produced by setting the eutectic crystal amount at a level lower than that of a conventional material.
To achieve the above object, according to the present invention, there is provided a thixocast casting material which is formed of an Fe—C—Si based alloy in which an angled endothermic section due to the melting of a eutectic crystal exists in a latent heat distribution curve, and a eutectic crystal amount Ec is in a range of 10% by weight≦Ec≦50% by weight.
A semi-molten casting material having liquid and solid phases coexisting therein is prepared by subjecting the casting material to a heating treatment. In the semi-molten casting material, the liquid phase produced by the melting of a eutectic crystal has a large latent heat. As a result, in the course of solidification of the semi-molten casting material, the liquid phase is sufficiently supplied around the solid phase in response to the solidification and shrinkage of the solid phase and is then solidified. Therefore, the generation of air voids of micron order in the cast product is prevented. In addition, the amount of graphite phases precipitated can be reduced by setting the eutectic crystal amount Ec in the above-described range. Thus, it is possible to enhance the mechanical properties of the cast product, i.e., the tensile strength, the Young's modulus, the fatigue strength and the like.
In the casting material in which the eutectic crystal amount is in the above-described range, the casting temperature (temperature of the semi-molten casting material and this term will also be applied hereinafter) for the casting material can be lowered, thereby providing the prolongation of the life of a casting mold.
However, if the eutectic crystal amount Ec is in a range of Ec≦10% by weight, the casting temperature for the casting material approximates a liquid phase line temperature due to the small eutectic crystal amount Ec and hence,
Kikawa Kazuo
Shiina Haruo
Sugawara Takeshi
Takagi Isamu
Tsuchiya Masayuki
Combs Morillo Janelle
Lyon & Lyon LLP
Wyszomierski George
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