JPH08157945A - Spheroidal graphite cast iron having excellent vibration attenuatability and its production - Google Patents

Abstract

PURPOSE: To provide a spheroidal graphite case iron having excellent vibration attenuatability and a process for producing this cast iron. CONSTITUTION: The spheroidal graphite cast iron after casting is subjected to repetitively at least >=2 times annealing heat treatments, A1 to A4, for cooling the cast iron down to an eutectic transformation point or below after heating and to hold to and at 750 to 1100 deg.C. Base tissue adjusting heat treatments B1, B2, such as annealing and austempering, are executable after the final annealing heat treatment or in the cooling process thereof. The outer peripheral surfaces of the spheroidal graphite of the spheroidal graphite cast iron obtd. by such heat treatments are formed to rugged shapes; in addition, many fine gap parts are formed in the inner and outer peripheral parts of the graphite and the vibration attenuability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to spheroidal graphite cast iron excellent not only in strength but also in vibration damping capacity, and a method for producing the same.

[0002]

2. Description of the Related Art Spheroidal graphite cast iron is a material in which spherical graphite is crystallized in the matrix in the as-cast state, and has a higher strength than flake graphite cast iron (gray cast iron) in which graphite is crystallized into flakes. Have. This is because, in flake graphite cast iron, flake graphite has a notch action, so that mechanical properties are deteriorated.

On the other hand, regarding the vibration damping ability, since the graphite form is more flake-shaped rather than spherical, the vibration damping ability is excellent as disclosed in, for example, JP-A-63-210256. Various types of flake graphite cast iron have been proposed.

[0004]

The mounting brackets of the engine and the compressor are not only strong,
Excellent vibration damping capability is required. When such a member is made of flake graphite cast iron having excellent damping capability, it is necessary to increase the wall thickness of the member in order to satisfy the required strength, resulting in an increase in weight and bulk. This is contrary to the requirements for weight reduction and size reduction, and therefore, at present, spheroidal graphite cast iron is used while sacrificing the vibration damping ability to some extent.

The present invention has been made in view of the above problems, and an object thereof is to provide a spheroidal graphite cast iron excellent in vibration damping ability and a method for producing the same.

[0006]

The spheroidal graphite cast iron of the present invention is spheroidal graphite cast iron in which spheroidal graphite is crystallized in a matrix, and the outer peripheral surface of the spheroidal graphite is formed in an uneven shape, and A large number of minute voids are formed inside and on the outer periphery. By using bainite as the base of the spheroidal graphite cast iron, strength and vibration damping ability can be further improved.

On the other hand, according to the method for producing spheroidal graphite cast iron of the present invention, the spheroidal graphite cast iron after casting has a temperature of 750 to 1100 ° C.
To the annealing heat treatment of cooling to below the eutectoid transformation point after heating and holding.
Repeat more than once. A matrix structure adjusting heat treatment can be applied after the final annealing heat treatment or after the cooling process. By performing the austempering heat treatment as the matrix structure adjusting heat treatment, the base can be made into bainite.

[0008]

In the spheroidal graphite cast iron of the present invention, the outer peripheral surface of the spheroidal graphite is formed in an uneven shape, and a large number of voids are formed inside and outside the spheroidal graphite. Excellent in vibration absorption due to interference and graphite itself, and vibration damping capacity is increased. Furthermore, by forming the base with a bainite structure, high strength can be obtained, and the vibration damping ability can be further improved by the needle-like or feather-like structure peculiar to bainite.

According to the method for producing spheroidal graphite cast iron of the present invention, it is 750 to 1100 ° C. with respect to the spheroidal graphite cast iron after casting.
Since the annealing heat treatment held at 2 is repeatedly performed twice or more, the secondary graphite is repeatedly deposited and grown around the initial spherical graphite, and the outer peripheral surface of the spherical graphite is formed in an uneven shape. Further, the growth of cast iron causes a large number of voids to be formed in the outer peripheral portion and the inner portion of the graphite. At this time, if the annealing temperature is less than 750 ° C., austenite does not form at all in the matrix, and it becomes difficult to form secondary graphite or voids. On the other hand, 1
If the temperature exceeds 100 ° C, melting will partially occur, which is not preferable. The preferred range of annealing temperature is 850
~ 950 ° C.

[0010] With the annealing heat treatment as it is, the matrix structure is a ferrite structure, and although the strength is slightly low, the material has excellent ductility. In order to adjust the matrix structure after the final annealing heat treatment or in the cooling process thereof, matrix structure adjusting heat treatment such as austempering heat treatment or normalizing heat treatment can be performed. By applying austempering heat treatment,
The base becomes bainite, and although the elongation is slightly inferior, high strength and excellent damping capacity can be obtained. On the other hand, when the normalizing heat treatment is performed, the matrix mainly becomes a pearlite structure, which has a relatively good vibration damping ability and balances strength and elongation.

[0011]

Examples The chemical composition of the spheroidal graphite cast iron of the present invention is not particularly limited as long as it is a composition in which spheroidal graphite is crystallized.
The representative chemical composition (wt%) of the main elements and the reasons for limiting the components are as follows. C: 2.5 to 4.0%, Si:
2.0 to 3.5%, Mg: 0.02 to 0.08%, balance substantially Fe.

C: 2.5 to 4.0% C is necessary for the formation of graphite, and if it is less than 2.5%, the castability is lowered, the solidification shrinkage is increased, and the shrinkage cavity defect is increased. On the other hand, if it exceeds 4.0%, spheroidization of graphite becomes difficult and quiche graphite may be generated. A preferable range is 3.5 to 3.8%.

Si: 2.0 to 3.5% Si is necessary for promoting the formation of graphite, and is 2.0
If it is less than%, free cementite tends to crystallize, while if it exceeds 3.5%, the material becomes brittle and the impact value decreases. A preferred range is 2.6 to 2.9%. Mg: 0.02 to 0.08% Mg is necessary for spheroidizing graphite, and if it is less than 0.02%, it is difficult to spheroidize graphite, while if it exceeds 0.08%, cementite will crystallize, Further, shrinkage cavity defects and dross defects increase. The preferred range is
It is 0.025 to 0.050%.

The smaller the amount of impurities and the graphite spheroidization inhibiting element, the better. For example, although P is not a spheroidization-inhibiting element, it significantly reduces ductility and makes the material brittle, so it is preferable to keep P at 0.1% or less. Further, S is an element that significantly inhibits the spheroidization of graphite, so it is preferable to limit it to 0.03% or less. M
Since n is a pearlite-stabilizing element, it is better to reduce the amount in order to obtain a material having a large elongation, but it may be contained to some extent in the case of strengthening the matrix, and Mn: 1.0% or less (preferably 0.4 % Or less) is allowed. Also, C
u, Ni, and Mo do not have a graphite spheroidization inhibitory action, but have an action of easily transforming the matrix into a pearlite structure or a bainite structure, and are therefore effective in strengthening the matrix. Cu: 2.0% or less (preferably 1. 5% or less), Ni: 2.0% or less (preferably 1.5% or less), Mo: 0.5% or less (preferably 0.3% or less) are allowed. The thickness of the casting is 2
When the thickness is 5 mm or more, the inclusion of Cu or the like is particularly effective in order to obtain the desired structure even inside.

In order to produce the spheroidal graphite cast iron of the present invention, after casting the spheroidal graphite cast iron having the above chemical composition, as shown in FIG. 1, after heating and holding at 750 to 1100 ° C., a temperature below the eutectoid transformation point, For example, the annealing heat treatment for cooling to about 500 to 600 ° C. (furnace cooling or air cooling) is performed twice or more (A1,
4 times (A2, A3, A4), in the cooling process of the final annealing heat treatment, in the illustrated example, a normalizing heat treatment B1 or an austempering heat treatment B2 is performed as a matrix structure adjusting heat treatment. Of course, the matrix structure adjusting heat treatment may be performed after the final annealing heat treatment.

The number of annealing heat treatments is usually 2 to 4 times.
This is because it is difficult to improve the vibration absorbing ability even if it is performed five times or more, and it is disadvantageous in terms of cost. The annealing time per one time is usually set to about 0.5 to 3 hr per inch of wall thickness. Even if the matrix becomes homogeneous austenite when heated to a prescribed temperature, it will be 2 if less than 0.5 hr.
The formation and growth of subgraphite is too small. On the other hand, in the case of an inhomogeneous material in which free cementite or the like is crystallized in the matrix, it takes some time for homogenization, but 3 hr is sufficient, and it is not necessary to hold it for more than 3 hr.

As the matrix structure adjusting heat treatment, when the matrix structure is a pearlite structure, a normalizing heat treatment (B
1) should be performed. That is, from the annealing temperature to Acm + 5
Cooled to about 750-900 ° C (air-cooled or furnace-cooled)
Then, the temperature may be maintained at about 0.5 hr per inch for soaking, and then air cooling may be performed. However, depending on the composition, there may be a mixed structure in which some ferrite is generated in pearlite. Also, when the base structure is a bainite structure,
Austempering heat treatment (B2) may be performed. That is, quenching is performed in a salt bath maintained at an annealing temperature of 230 to 400 ° C., and at this temperature, 1.0 to 3.0 hr per inch.
It may be cooled by air after isothermal transformation is performed with the temperature maintained.
When the matrix has a ferrite structure, it may be cooled in the furnace as it is after the annealing heat treatment.

The spheroidal graphite cast iron of the present invention is suitable as a material for various mechanical structural members which are required to have high strength and high vibration damping ability. In addition to mounting brackets for engines and compressors, crankshafts, bearing caps, differential cases It is suitable as a material for generator brackets, transformers, installation stands for various audio equipment, and the like. Less than,
Specific examples will be given.

Flake graphite cast iron and spheroidal graphite cast iron having the following chemical composition (wt%) were cast and heat-treated under the temperature conditions shown in Table 1. In the table, Sample No. 1 is a conventional example and is as-cast flake graphite cast iron (FC250). Sample No. 2 is also a conventional example, which is as-cast spheroidal graphite cast iron (FCD450). Sample Nos. 3 to 7 are examples.・ Flake graphite cast iron composition C: 3.31%, Si: 1.85%, Mn: 0.72%, P:
0.043%, S: 0.023%, balance substantially Fe-spheroidal graphite cast iron composition C: 3.71%, Si: 2.83%, Mn: 0.28%, P:
0.031%, S: 0.008%, Mg: 0.041%, balance substantially Fe

[0020]

[Table 1]

From each sample, a tensile test piece (JIS No. 4) and a vibration test piece (cut out from φ30 mm, width 20 ×
A length of 200 mm and a thickness of 4 mm) was sampled and subjected to a tensile test and a vibration damping test. Vibration damping test, one end of the rod-shaped test piece is fixed, the other end is excited by an electromagnetic exciter, after stopping the excitation, by the optical displacement measuring machine, the vibration amplitude of the other end of the test piece is measured, The amplitude decay waveform was obtained. From the obtained amplitude decay waveform, the expected amplitude Ao immediately before the vibration stop and Ao / 3 are obtained.
The peak amplitude A n was obtained, and the damping capacity Q ⁻¹ was calculated from the following equation (1). The test results are also shown in Table 2.

Q ⁻¹ = − (1 / π) × (1 / n) × ln (An / Ao) (1)

[0023]

[Table 2]

From Table 2, the examples of Sample Nos. 3 to 7 are as cast as spheroidal graphite cast iron No. 2 although the damping ability is slightly inferior to No. 1 which is flake graphite cast iron. Compared with this, the damping capacity was significantly improved by 20% or more. Moreover, with respect to Nos. 3, 4, 5 and 7 of Examples in which the base structure adjusting heat treatment was applied,
Significantly improved strength compared to No.2. In addition, N in the example
Comparing o.4 and No.6, and No.5 and No.7, even if the number of annealing was the same, No.
It can be seen that Nos. 4 and No. 5 have higher strength and vibration damping capacity than others. In addition, although No. 6 of the example is not subjected to the matrix structure adjusting heat treatment, although the strength is slightly inferior to that of No. 2, the strength comparable to FCD400 is obtained.

Next, test pieces for observing the structure were sampled from No. 2, No. 5 and 6 and the metal structure was observed with an optical microscope and an electron microscope. The results are shown in FIGS. Figure 2
Is a structure photograph by a No. 2 optical microscope (500 times), and FIGS. 3 and 4 are structure photographs by a No. 4 optical microscope (500 times).
2) and a structure photograph by an electron microscope (1000 times), and FIGS. 5 and 6 are the same structure photograph of No. 6.

As shown in FIG. 2, the graphite before the heat treatment of the present invention has a substantially spherical shape. However, the heat treatment of the present invention causes the outer peripheral surface of the graphite to become uneven, and a large number of fine voids (see FIG. 4 and (6, white part in graphite and its peripheral part)
Is recognized.

[0027]

As described above, according to the spheroidal graphite cast iron of the present invention, since it does not have flake graphite, it has high strength and has excellent vibration damping ability due to the form of graphite and the voids. Furthermore, by forming the base with a bainite structure, the strength can be improved and the vibration damping ability can be further improved by the layered structure peculiar to bainite. Further, according to the manufacturing method of the present invention, it is possible to easily manufacture a spheroidal graphite cast iron having excellent vibration damping ability. Further, by performing the matrix structure adjusting heat treatment after the final annealing heat treatment or in the cooling process thereof, the matrix can be made into pearlite, bainite or the like, and the strength and the vibration damping ability can be improved.

[Brief description of drawings]

FIG. 1 is a heat treatment diagram according to the present invention.

FIG. 2 is a photograph (500 times) as a substitute for a metallographic drawing of a conventional example of spheroidal graphite cast iron by an optical microscope.

FIG. 3 is a photograph (500 times) as a substitute for a metallographic drawing of spheroidal graphite cast iron of the example by an optical microscope.

FIG. 4 is a photograph (1000 times) as a substitute for a metallographic structure drawing of spheroidal graphite cast iron of an example by an electron microscope.

FIG. 5 is a photograph (500 times) as a substitute for a metallographic drawing of spheroidal graphite cast iron of another example by an optical microscope.

FIG. 6 is a photograph (1000 times) as a substitute for a metallographic structure drawing of spheroidal graphite cast iron of another example by an electron microscope.

Front page continuation (72) Inventor Satoshi Ando 5-33-8 Shiba, Minato-ku, Tokyo Within Mitsubishi Motors Corporation (72) Inventor Kenichi Asano 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation In the company

Claims (5)

[Claims] 1. A spheroidal graphite cast iron in which spheroidal graphite is crystallized in a matrix, wherein the outer peripheral surface of the spheroidal graphite is formed in a concavo-convex shape, and a large number of fine voids are formed inside and outside the spheroidal graphite. Formed spheroidal graphite cast iron with excellent vibration damping capability. 2. The spheroidal graphite cast iron according to claim 1, wherein the matrix is formed of ferrite, pearlite or bainite. 3. From 750 to spheroidal graphite cast iron after casting
A method for producing spheroidal graphite cast iron having excellent vibration damping ability, in which an annealing heat treatment of heating and holding at 1100 ° C. and then cooling to a temperature below the eutectoid transformation point is repeated twice or more. 4. The method for producing spheroidal graphite cast iron according to claim 3, wherein the matrix structure adjusting heat treatment is performed after the final annealing heat treatment or during the cooling process. 5. The method for producing spheroidal graphite cast iron according to claim 4, wherein austempering heat treatment is performed as the matrix structure adjusting heat treatment.