LIGHTWEIGHT CAST-IRON PRODUCT

Abstract

Provided is a cast-iron product that satisfies demands for both weight reduction and strength enhancement with good balance. A cast-ion product having an inner honeycomb structure is produced by a process including: a decarburization step, in which a cast-iron product made of a hypoeutectic cast iron is heated to form a decarburized layer on the surface of the cast-iron product; an outflow-hole formation step, in which an outflow hole penetrating through the decarburized layer into an inner region is formed; and a liquation step, in which the cast-iron product is heated to a temperature lower than the melting point of the decarburized layer and higher than the melting point of the hypoeutectic cast iron remaining inside, while being held in such a manner that the outflow hole is located in the lower portion thereof.

Description

TECHNICAL FIELD

The present invention relates to a lightweight cast-iron product and a method for producing the same.

BACKGROUND ART

A number of cast-iron products are currently used for the main bodies and parts of automobiles or other mid-size to large-size mechanical systems. As compared to plastic products, cast-iron products are more suitable for such parts or portions that must have high mechanical strength or heat resistance. As compared to other kinds of metals including steels, they are more suitable for such parts which have complex shapes or which need to be inexpensively produced. Accordingly, cast-iron products are used in a large number of areas where the aforementioned properties and performance are required. However, their superiorities are gradually undermined by plastic materials and other kinds of metals which have achieved technical improvements in various functionalities. Under such a situation, similar technical improvements of cast-iron products for meeting the demands of the times are also expected.

One of the major areas where the cast-iron products are used is the automobile industry, where the reduction of weight has always been a critical problem. Needless to say, there are also other application areas in which weight reduction is regarded as an eternal problem. To address this issue, the present inventor and his associates have provided an epoch-making solution, i.e. the hollow cast-iron product (Patent Document 1).

BACKGROUND ART DOCUMENT

Patent Document

• Patent Document 1: JP-B 4099535

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

The cast-iron product described in Patent Document 1 has achieved significant progress in terms of weight reduction. As for its strength, the product is adequately stronger than plastic products. However, due to its hollow structure, the product is often not strong enough to substitute for such parts that have conventionally been made of cast iron.

Accordingly, the problem to be solved by the present invention is to provide a cast-iron product which satisfies demands for both weight reduction and strength enhancement with good balance.

Means for Solving the Problems

The lightweight cast-iron product according to the present invention aimed at solving the aforementioned problem includes a shell portion having a melting point increased by surface decarburization, and a honeycomb region inside the shell portion.

A method for producing such a cast-iron product includes:

a) a decarburization step, in which a cast-iron product made of a hypoeutectic cast iron is heated to form a decarburized layer on the surface of the cast-iron product;

b) an outflow-hole formation step, in which an outflow hole penetrating through the decarburized layer into an inner region is formed; and

c) a liquation step, in which the cast-iron product is heated to a temperature lower than the melting point of the decarburized layer and higher than the melting point of the hypoeutectic cast iron remaining inside, while being held in such a manner that the outflow hole is located in the lower portion thereof.

The aforementioned “hypoeutectic cast iron” is a cast iron having a composition whose carbon equivalent CE satisfies the following equation (I):

2.0<CE=C %+(Si %+P %)/3<4.3  (1)

where the “%” values are in weight percent.

If CE is within this range, the melting point of the hypoeutectic cast iron will be approximately 1147° C., regardless of the CE value. Accordingly, it is preferable to set the heating temperature in step c) at a value higher than that melting point (1147° C.) by up to approximately 50° C., i.e. within a range from 1147 to 1200° C. As for the carburized layer, if its carbon concentration is equal to or less than 1%, the melting point of that portion (the carburized layer) will be approximately 1350° C.

Effect of the Invention

The previously described process according to the present invention is basically similar to the process described in Patent Document 1. However, a significant difference exists: The cast-iron part according to Claim 1 of Patent Document 1 is merely described as a “cast-iron part” and there is no specific limitation on its material. By contrast, the present invention is limited to a product “made of a hypoeutectic cast iron.” It should also be noted that the cast-iron part according to Claim 3 of Patent Document 1 is characterized by “having a eutectic carbon concentration.” In this respect, the invention described in Patent Document 1 is entirely different from the method according to the present invention.

As described previously, the present invention uses a hypoeutectic east iron as the material. As can be understood from the phase diagram in FIG. 1, the hypoeutectic cast iron in the non-decarburized portion (inner region) does not completely melt in the Equation step c); a portion of the dendrite-shaped γ phase, which has a higher melting point, remains in a lattice form. As a result, the inner region (non-decarburized portion) will have a honeycomb structure, unlike the cast-iron part described in Patent Document 1 whose inner region is completely hollow. Naturally, the honeycomb structure is somewhat inferior to the complete hollow structure in terms of the weight-reducing effect. However, it is adequately stronger than the hollow structure. Thus, by the method according to the present invention, it is possible to produce a cast-iron product which satisfies demands for both weight reduction and strength enhancement with good balance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of the iron-carbon system (for Si=0%).

FIG. 2 is an external view of a cast-iron product 1 as the first example.

FIG. 3 is a table showing the major chemical composition of the cast-iron product 1 of the first example.

FIG. 4 is a sectional view of the cast-iron product 1 after a honeycomb-structure formation process.

FIG. 5 is a table showing the major chemical composition of a cast-iron product 2 as the second example.

BEST MODE FOR CARRYING OUT THE INVENTION

Two examples of the cast-iron product according to the present invention will be hereinafter described.

FIRST EXAMPLE

The first example is a bearing cap as shown in FIG. 2. Its chemical composition was as shown in FIG. 3. As indicated in the last field of FIG. 3, the material of the cast-iron product of the present example (which is hereinafter called the “cast-iron product 1”) has a CE value of 4.03, and therefore, is hypoeutectic.

A decarburization process was performed as follows: The cast-iron product 1 was heated to 1060° C. in an electric furnace and maintained at that temperature for 12 hours in an ambience of a converted gas prepared from a city gas by altering its composition to CO/(CO+CO₂)*100-75%. Following the decarburization process, the ambient gas was replaced with N₂ gas and the furnace was cooled to 500° C., after which the product was cooled in atmospheric air.

After the cast-iron product 1 was cooled to room temperature, a hole of 3 mm in depth (outflow hole) was bored in one portion of the product (indicated by “A” in FIG. 2) with a drill of 4.5 mm in diameter.

Then, the cast-iron product 1 was placed in the same electric furnace, with the outflow hole directed downward. After the inner space of the furnace was filled with N₂ gas, the temperature was increased to 1185° C. in 50 minutes and maintained at that temperature for eight minutes. When the cast-iron product 1 reached that temperature, the molten metal began to flow out of the outflow hole. A few minutes later, the outflow of the molten metal began to slow down, which was almost stopped at the end of the eight-minute period. Then, the product in the furnace was cooled to 500° C. in the same N₂ ambience, after which it was cooled in atmospheric air.

After being cooled to room temperature, the cast-iron part 1 was cut at a plane in the middle of its thickness. FIG. 4 shows the cross section. As shown, a honeycomb structure was formed inside the decarburized peripheral layer which had a thickness of approximately 3 mm.

The weight of the cast-iron part 1, which was 370 gram before the previously described process, decreased to 315 gram after the process. The weight reduction rate was 15%.

SECOND EXAMPLE

The second example is an oil pump cover. The cast-iron product in the present example (which is hereinafter called the “cast-iron product 2”) is a disc-shaped part of 185 mm in outer diameter having a 45-mm-diameter hole at its center. The central hole was surrounded by a circumferential rim with a thickness of 28 mm. The outer circumferential portion of the disc was 21 mm in thickness. Before undergoing the process, this cast-iron product 2 weighed 3.45 kg, which was approximately nine times the weight of the cast-iron product 1. The chemical composition of the used material was as shown in FIG. 5. Its CE value was 4.11 (hypoeutectic).

The decarburization of the cast-iron product 2 was performed in the same manner as the previous example. That is to say, it was heated to 1060° C. in an electric furnace and maintained at that temperature for 12 hours in an ambience of a converted gas prepared from a city gas by altering its composition to CO/(CO+CO₂)*100=75%. Following the decarburization process, the ambient gas was replaced with to N₂ gas and the furnace was cooled to 500° C., after which the product was cooled in atmospheric air. In the obtained cast-iron product 2, two outflow holes were bored in the circumferential portion with a drill of 8 mm in diameter.

Then, the cast-iron product 2 was placed in the same electric furnace, with the outflow holes directed downward. After the inner space of the furnace was filled with N₂ gas, the heating temperature was increased to 1190° C. in 60 minutes and maintained at that temperature for 12 minutes. Similar to the previous example, when the cast-iron product 2 reached that temperature, the molten metal began to flow out of the outflow holes. The outflow was almost stopped at the end of the 12-minute period. Then, the product in the furnace was cooled to 500° C. in the same N₂ ambience, after which it was cooled in atmospheric air.

After being cooled to room temperature, the cast-iron part 2 was cut in a radial direction, and its inner condition was visually checked. This checking confirmed that a honeycomb structure was formed both inside the rim surrounding the central hole and inside the outer circumferential portion having the increased thickness. After the hollow-structure formation process, the weight of the cast-iron part 2 was 2.31 kg. The weight reduction rate was 33%.

Claims

1. A lightweight cast-iron product comprising a shell portion having a melting point increased by surface decarburization and a honeycomb region inside the shell portion.

2. The cast-iron product according to claim 1, wherein a a carbon equivalent of a composition of a cast-iron before the surface decarburization is hypoeutectic.

3. A method for producing a lightweight cast-iron product, comprising: a) a decarburization step, in which a cast-iron product made of a hypoeutectic cast iron is heated to form a decarburized layer on a surface of the cast-iron product;b) an outflow-hole formation step, in which an outflow hole penetrating through the decarburized layer into an inner region is formed; andc) a liquation step, in which the cast-iron product is heated to a temperature lower than a melting point of the decarburized layer and higher than a melting point of the hypoeutectic cast iron remaining inside, while being held in such a manner that the outflow hole is located in a lower portion thereof.