JPH08176656A - Production of cast iron with high ductility - Google Patents

Abstract

PURPOSE: To attain fine graphite nucleation simultaneously with acceleration of graphitization and to produce a cast iron with high ductility by specifying the chemical components of and heat treating method for a cast iron, in the simplifying technique of heat treatment for a spheroidal graphite cast iron. CONSTITUTION: A molten cast iron, having a composition consisting of, by weight, >=2.5% C, >=2.5% Si, and the balanced Fe and satisfying 8.0%>=C+Si>=6.5, is poured, after innoculation treatment, into a metallic mold and cooled down to room temp. at >=10 deg.C/sec cooling rate. The resultant casting is reheated to >=930 deg.C, soaked and held at the reheating temp. for at least 10min, and cooled naturally. By adding large amounts of C and Si, as the components for accelerating the ferritizating of matrix, in this way and simultaneously increasing the cooling velocity at the time of solidification of a casting, the nuclei of graphite can be increased and also refined. As a result, the surface area of graphite can be increased and the formation of ferrite around the graphite can be facilitated, and practically equal heat treatment effects can be obtained in an extremely short time as compared with the conventional method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for simplifying heat treatment of spheroidal graphite cast iron, and more particularly to a method for producing highly ductile cast iron by promoting nucleation of fine graphite and ferritization.

[0002]

2. Description of the Related Art As a method for improving ductility in cast iron,
Usually, ferrite heat treatment is performed. This process is 9
After heating to about 00 ° C., the temperature is gradually cooled to a temperature of about 700 ° C. over several hours. For example, in spheroidal graphite cast iron, normalization is performed to obtain a large elongation or to improve machinability. In this case, it often contains free cementite, and in order to improve the impact value, the first-stage graphitization annealing that decomposes cementite and the second-stage graphitization annealing that precipitates ferrite by eutectoid transformation are performed. It is generally done in one heat cycle. At this time, the heat treatment time is usually 2 to 3 hours in the first-stage graphitization annealing, and the eutectoid transformation temperature section in the second-stage graphitization annealing is 20 ° C./hr. It is necessary to perform cooling at the following rate or to hold immediately below the eutectoid transformation end temperature, that is, around 700 ° C. for 3 to 6 hours. (Casting Handbook, 4th revised edition, June 1990)
Issued monthly)

As described above, at least the heat treatment requires several hours, which causes an increase in energy cost and a decrease in production efficiency. On the other hand, it is also possible to obtain a ferrite structure in the as-cast state, in which case it is necessary to slow the solidification rate, and as a result graphite becomes coarse and fatigue strength decreases. Since it is necessary to prevent the precipitation of cementite, which is a metastable phase, in order to form a ferrite structure, it is necessary to gradually cool the temperature range in which cementite may be formed. For example, in spheroidal graphite cast iron, in order to promote the formation of ferrite as cast,
Studies on ingredients and inoculations have been made. However, in the investigation from the components, for example, a method that promotes only ferritization while ensuring the strength of the matrix, or a method that facilitates spheroidization and ferritization of graphite from the study of a special inoculation method, It was difficult to produce a spheroidal graphite cast iron in a well-balanced and stable manner in strength and ductility.

Therefore, it is possible to simultaneously promote the graphitization of the cementite of the spheroidal graphite cast iron and the ferriteization of pearlite, simplifying the conventional annealing heat treatment, and further satisfying good mechanical properties and strength. And the development of the high ductility cast iron which balanced the ductility was desired.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, the object of the present invention is to examine the reduction of the ferriteization annealing time of spheroidal graphite cast iron from the chemical composition and the heat treatment pattern, and to perform heat treatment for a shorter time than before. According to the present invention, a rapid ferritization technique is achieved, and a method for manufacturing high ductile cast iron is provided. Another object of the present invention is to provide a method for producing a highly ductile cast iron, which secures the strength and ductility of spheroidal graphite cast iron by the short-time heat treatment, and makes it possible to stably obtain both of these properties in a well-balanced manner. . Further, another object of the present invention is to provide a method for producing highly ductile cast iron by optimizing the ferritic conversion of pearlite and the cementitized graphitization of spheroidal graphite cast iron.

[0006]

Means for Solving the Problems The above-mentioned objects are as follows. The chemical composition in% by weight is C ≧ 2.5%, Si ≧ 2.5%, 8.0.
% ≥ C + Si ≥ 6.5%, cast iron molten metal composed of the balance Fe and unavoidable impurities is poured into the mold after inoculation treatment 10
After cooling to room temperature at a cooling rate of ° C / sec or more, the obtained casting is reheated to a temperature of 930 ° C or more, soaked at the reheating temperature for at least 10 minutes, and then naturally cooled. It is achieved by the method for producing high ductility cast iron.

[0007]

In order to simplify the ferrite annealing treatment of the spheroidal graphite cast iron, the present invention promotes the precipitation of fine graphite and the chemical components (C, S) that promote the ferrite formation of the matrix.
i) was added in a large amount, and at the same time, the cooling rate at the time of solidification of the cast casting was increased to increase the nuclei of graphite and enable miniaturization. As a result, the surface area of the graphite is increased, and the ferrite is easily generated around the graphite, so that the substantially same heat treatment effect is exhibited in an extremely short time compared with the conventional case. The reason for limiting the constituent features of the present invention will be described below. First, the chemical component, which is the first constituent requirement, will be described. C and Si are elements that promote the precipitation of graphite and promote the ferrite formation of the matrix. further,
In order to deposit graphite in a spherical shape, its composition is preferably hypereutectic.

In the present invention, especially C, Si ≧ 2.5
%, C + Si ≧ 6.5%, because if this condition is not satisfied, a ferrite structure cannot be obtained by natural cooling, and it takes a long time to decompose free cementite. The reason why C + Si ≦ 8.0 is set is that if it exceeds 8%, the amount of coarse graphite crystallized in the primary crystal increases and the ductility decreases. Further, the cooling condition at the time of solidification, which is the second constituent feature of the present invention, will be described. In the present invention,
Increasing the cooling rate during solidification makes graphite finer and increases the number of grains. As a result, the surface area of graphite is increased, which facilitates the formation of ferrite around graphite. That is, solidification cooling rate ≧
The reason for setting the temperature to 10 ° C./sec is that if this condition is not satisfied, the ferrite structure cannot be obtained by the natural cooling treatment and that it takes a long time to decompose the free cementite. The solidification cooling rate is an average cooling rate from 1300 ° C to 1200 ° C.

In the present invention, the reheating heat treatment after casting is set to 930 ° C. or higher in order to release graphite in a short time. Cementite almost disappears and graphite is formed. This is explained as follows. That is, austenite as a matrix increases the solubility of cementite with increasing temperature. Further, when the temperature is maintained at a constant temperature for a certain period of time, it approaches a saturated austenite solid solution. On the other hand, since cementite is a metastable product, when the degree of metastable state increases at this temperature (that is, due to the increase in solidification cooling rate and reheating), the solid-solution difference in austenite is further emphasized. It is considered that the state of accelerating is promoted to the precipitation of fine graphite, and the graphitization as a whole is promoted, and as a result, the ferritization is promoted. Hereinafter, the present invention will be further described with reference to embodiment examples and comparative examples of the present invention.

[0010]

EXAMPLES As examples of the present invention, the present invention examples No. 1 to 1
No. 1 was manufactured into an ingot with the combination of the C and Si contents of the components shown in Table 1 and the solidification cooling rate.

[0011]

[Table 1]

In this example, the raw material of the molten metal was melted by high-frequency melting, and the Fe-Si-Mg alloy was added immediately before casting,
After that, further inoculation with Fe—Si alloy was performed. The ingot obtained by casting was soaked and kept at 950 ° C. for 30 minutes, and then naturally cooled. Then, a test piece was cut out from the obtained ingot for microstructure observation and mechanical test, and subjected to a tensile test to measure strength and elongation at break. In this example, three types of molds were used as the casting mold to control the cooling rate. The cooling rate of each mold is
It was 9.5, 29 and 60 ° C./sec. In Table 1 above, the cooling rate, the tensile strength and the elongation value are shown together for each chemical component. From the results shown in Table 1, in the inventive examples, the microstructures are all ferrite and pearlite. Further, the tensile strength is in the range of 628 to 782 MPa and the elongation is 9.3 to 21.8%, which shows that the ductility is excellent. Further, in the case of the same component, as the cooling rate increases to 9.5, 29 and 60 ° C./sec, the tensile strength slightly decreases, and the elongation tends to be good in inverse proportion to that of the structure. It can be seen that the ferrite ratio increases and the ferritic conversion is further promoted.

(Comparative Example) On the other hand, as a comparative example, C and Si as chemical components were changed as shown in Table 1 above and cast,
For some, the solidification cooling rate is 2.2 ° C by sand casting.
Ingot production, structure observation, and tensile test were performed under the same conditions as the above-mentioned examples of the present invention except that the time was set to / sec. For the present invention example and the comparative examples Nos. 1 to 6, the chemical components C, Si,
The case where C + Si and the cooling rate are the same for each constitution of the present invention is shown as a mark, and the ones where the cooling rate is out are shown as a mark for reference.

1 and 2, the invention sample No.
Representative metallographic photographs of the cast products after heat treatment are shown for No. 9 and Comparative Example No. 5, respectively. From these figures,
Despite having the same composition, the matrix of the present invention is mostly ferrite while the matrix of the comparative example is pearlite. Further, in the example of the present invention shown in FIG. 1, it can be seen that the spheroidization rate of graphite is about 85% or more, and the pearlite rate of the matrix is about 30% or less.

[0015]

As is apparent from the above examples, the present invention makes it possible to promote the precipitation of fine graphite and the ferrite formation of the matrix at the same time by improving the chemical composition and the heat treatment method. By shortening the chemical annealing treatment time and further by conventional ferritic heat treatment, the strength is significantly reduced when a material with high elongation is used, whereas in the present invention, the strength is improved by the solid solution strengthening of Si. We have achieved the production of highly ductile cast iron with a comparatively small reduction in tensile strength and elongation characteristics of mechanical properties.

[Brief description of drawings]

FIG. 1 is a metallographic photograph by an optical microscope showing a microstructure of Inventive Example No. 9 according to an example of the present invention.

FIG. 2 is a metallographic photograph by an optical microscope showing a microstructure of Comparative Example No. 5 according to an example of the present invention.

Claims (1)

[Claims] 1. The chemical composition is C ≧ 2.5% by weight,
A cast iron melt consisting of Si ≧ 2.5%, 8.0% ≧ C + Si ≧ 6.5%, the balance Fe and unavoidable impurities is poured into a mold after inoculation treatment and cooled at a cooling rate of 10 ° C./second or more. After cooling to room temperature, the obtained casting is reheated to a temperature of 930 ° C. or higher, soaked and held at the reheating temperature for at least 10 minutes, and then naturally cooled, thereby producing a high ductile cast iron.