JPH03257138A - Flake graphite cast iron having superior strength and damping capacity and production thereof - Google Patents

Abstract

PURPOSE:To enhance the low strength of high C and high Si flake graphite cast iron while maintaining superior damping capacity peculiar to the cast iron by austempering a casting of the flake graphite cast iron melted in a cupola and heat-treating the casting under specified conditions. CONSTITUTION:High C and high Si flake graphite cast iron contg. 3.2-4.4wt.% C and 2.2-3.4wt.% Si or further contg. 0.01-0.5wt.% Mo is melted in a cupola. A casting of the cast iron is austempered by heating at 850-950 deg.C for 0.5-2hr to form an austenite-bainite matrix structure. The casting is then quenched to 240-400 deg.C in a salt bath of sodium nitrate, potassium nitrate, etc., and isothermal transformation into bainite is caused by holding at the temp. for 0.5-2hr. The low strength of the flake graphite cast iron can be considerably enhanced while maintaining superior damping capacity peculiar to the cast iron.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the modification of flake graphite cast iron materials, and particularly relates to flake graphite cast iron having sufficient strength and large damping capacity, and a method for producing the same.

[Conventional technology] Flaky graphite cast iron (gray cast iron: FC) is made by adding C and Si to Fe.
It is a mixture of curved, streak-like (3-dimensionally, leaf-like) graphite (flake graphite) in a matrix structure (usually pearlite) containing show.

That is, it has advantages such as excellent wear resistance (abrasive wear) and machinability, and high vibration absorption ability (damping ability).

Another major feature is that flaky graphite precipitates during solidification, so there is little shrinkage overall and it is easy to cast.

On the other hand, however, since flaky graphite is brittle and weak, it tends to crack when force is applied to that part, and the presence of flaky graphite reduces the strength and toughness of the casting. These characteristics become more pronounced as the proportion of flaky graphite increases.

Spheroidal graphite cast iron (ductile cast iron: FCD) eliminates this drawback. Because graphite is spherical, its volume is small relative to its content, and its strength is greater than that of flaky graphite, so it has the advantages of flaky graphite cast iron mentioned above, but has relatively high strength and toughness. . Also, its characteristics greatly depend on the base organization. However, it has the drawback of high cost because it requires the addition of expensive Mg and Ce to make it spheroidal, requires a large riser, and is relatively difficult to cast.

Therefore, currently, spheroidal graphite cast iron is mainly used for items that require strength, such as mechanical parts and automobile parts, and flake graphite cast iron is often used for items that do not require much strength, partly because it is inexpensive.

[Problems to be Solved by the Invention] Recently, in order to eliminate the deterioration of processing accuracy due to noise and vibration, high vibration absorption capacity (damping capacity) has been developed in all consumer goods and production materials such as automobiles, home appliances, and machine tools. ) is becoming increasingly required.

However, the damping capacity and tensile strength of metal materials are generally inversely proportional, and high-damping and high-strength materials are made of expensive T1Ni alloys and Mg-
Many materials use Zn alloy or the like.

There are several types of iron-based materials, but they are all alloys of Cr or Al, and are expensive.

Incidentally, cast iron, particularly flaky graphite cast iron, has a high damping ability due to the absorption of generated energy by graphite, and is also extremely inexpensive. For example, FC-10 class or FC-15 class materials, which contain a particularly large amount of flaky graphite, exhibit excellent vibration absorption ability (damping ability) (approximately 15 to 19% as measured by the present inventors). However, the strength (tensile strength) of this class is extremely poor (approximately 10 to 15 kgf/f12). Also, in the Fe12 and FC-25 classes, which are used for boat fishing, the damping capacity becomes quite low (about 7% as well).

On the other hand, gears and automobile suspension parts (brake drums, etc.)
Spheroidal graphite cast iron used for tool holders of NC processing machines is a general FCD-45 class and has a damping capacity of 5%.
The degree is extremely low, at about the stroke % that corresponds to the normalized FCD-90 class (tensile strength: about 90 kgf/fi2). As a result, there are problems such as squealing when the brakes of automobiles are operated, and chatter caused by vibration in lathes, resulting in reduced machining accuracy and easy damage to the cutting tools.

In this way, current cast iron is much cheaper than existing high-damping and high-strength materials, but there is no material that satisfies both strength, toughness, and damping capacity, and the development of such materials is highly recommended (unrequired). Ta.

[Means for Solving the Problems] In view of the above-mentioned current situation, the present inventors have completed the present invention as a result of research, and its characteristics are derived from cupola, that is, cupola melting. Melt in a furnace or 1
Next, melted flake graphite cast iron, high carbon ff1 (3,
2 to 4.4%) and high silica content (2.2 to 3.4%), the base structure is made into bainite by austempering treatment (mixed structure of bainite and austenite).

That is, the present invention was completed as a result of successfully increasing the strength of FC-10 and FC-15 class flake graphite cast iron by austempering the base structure into bainite.

Austempering is originally a treatment method for increasing the strength of steel.
This technology has also been applied to spheroidal graphite cast iron for the past 10 years. To explain this technology for spheroidal graphite cast iron, first, as-cast cast iron products are processed at a temperature higher than the eutectoid transformation temperature (800 to 10
00℃) for about 1 hour to change the base structure from pearlite to austenite, and then heated to 300-550℃.
The material is rapidly cooled to 100%, held in this bainite constant temperature transformation region for about 1 hour, and then allowed to cool. Since it is difficult for thick objects to be rapidly cooled to the inside, MO, Cu, Ni, etc. are added to move the pearlite region (pearlite nose) back to the right to avoid this.

Through this treatment, the strength of FCD-40 to 50 class materials is approximately doubled to around 100.

By the way, as far as the present inventors know, this treatment has not been used conventionally for flake graphite cast iron.

This is because the purpose of austempering treatment is to increase strength, and this purpose is achieved by spheroidal graphite cast iron.

Therefore, the inventor of the present invention changed his point of view and decided to maintain the excellent vibration damping ability of FC-10°15 class flake graphite cast iron while increasing the strength to a strength range that can be used for mechanical parts (at least FC
We conducted various studies on austempering treatment for flake graphite cast iron, with a view to increasing the austempering treatment to about -20, 25).

However, in the case of flake graphite cast iron having a composition commonly used in the past, desirable results were not obtained.

However, if a considerable amount of Mo is added (0.5 to 1%), it will easily become bainite, but Mo will segregate and produce a hardened phase, deteriorating the properties and causing a decrease in damping ability and deterioration of castability.

Therefore, we continued our research and tried adding a larger amount of silicon (Si) than usual, and under certain conditions M.

We succeeded in converting the base to bainite without the addition of other substances.

In general, in the case of flake graphite cast iron, if the silicon content is low, it will easily become white, and if the silicon content is high, the cost will be high, and the eutectic cells will become fine and "porosity" will occur, making it brittle or difficult to cast. It tends to suppress the
degree is usually used.

On the other hand, in the present invention, silicon is used in a larger amount than usual, at about 2.2 to 3.4%, even though flaky graphite cast iron is used. However, if the amount is too large, problems of cost and "nesting" will occur, so the more preferable range is about 2.3 to 3.0%. Carbon is also added in a large amount of 3.2 to 4.4%, which is almost the same as in the FC-10 class. A more preferable amount of carbon added is about 3.5 to 4.0%. However, flake graphite cast iron with a composition within this range has AsCa
5t), it is brittle and unusable, as it will crack even if dropped from a height of about In. By austempering this, sufficient strength and toughness are imparted.

In addition, in the case of thick materials (30+mφ, 20+++Ilt or more), it is difficult to rapidly cool the inside part, so add a small amount of MO (0.5%).
It is also conceivable to add less than a certain amount, more preferably about 0.1 to 0.3%. In this case, to prevent segregation, N
It is preferable to add a small amount of i or Cu (both about 0.8% or less).

Next, the above-mentioned "certain condition" is another aspect of the present invention.
There are two requirements. Surprisingly, this condition means that flaky graphite cast iron is limited to cupola-derived iron, that is, to melted or primary melted iron in a cupola melting furnace (Table 1).
1). In the experiments conducted by the present inventors, the base melted in an electric furnace did not turn into bainite at all, and therefore the strength did not increase as expected (Table 2).

The reason why this situation occurs is unknown. However, those derived from cupola have a high S content due to coke. Also Mn
, P components are also more likely to be dissolved in cupola. Therefore, we added these components separately to the same level as those derived from cupola and melted them in an electric furnace, but as expected, almost no bainite formation occurred (in Table 2, D
-13 to D-18).

Another possible reason is the difference in eutectoid transformation temperature between the two. The eutectoid transformation temperature is generally 730℃ or 723℃
It is said that However, the inventors measured (thermocouple:
When using chromel alumel), the cupola melting temperature was 708℃, while the electric furnace melting temperature was 708℃.
The temperature was 67°C, and there was a considerable difference between the two. Although the reason for this itself is unknown, it is thought that this is related to whether or not bainite formation is possible. Incidentally, even in cupola-melted materials, bainite formation does not occur if the Si content is low.

Next, conditions for the austempering treatment of the present invention will be explained. First, the cupola-derived cast product was heated to 850 to 950℃.
Heat to. Heating time is 0.5-2 depending on the thickness of the product etc.
Time time color. If the heating temperature is too low, austenitization will be insufficient, and if the heating temperature is too high, the carbon will break off and it will be difficult to rapidly cool the material into a bainite region, which is not preferable. A more preferable heating temperature is about 880°C to 900"c. Also, if the heating time is too short, the austenitization will be insufficient, and if it is too long, changes such as the disappearance of flaky graphite will occur, so cast iron An appropriate time is selected within the above range depending on the thickness of the product, component composition, etc.

Then, rapidly (within about 30 to 50 seconds), the temperature rises to 240°C, which is the bainite isothermal transformation temperature (tempering temperature).
Cool to 400°C and hold for a certain period of time (about 0° for 5 to 2 hours). If the tenba ring temperature is too low, there is a risk that the matrix structure will become martensite, and if it is too high, there is a risk that it will cross the pearlite region. A more preferable range of tempering temperature is about 250°C to 380°C. Also,
If the time is too short, the bainite formation will be insufficient, and if the time is too long, the bainite structure will become rough. Therefore, an appropriate time is selected within the above range depending on the thickness, component composition, etc. of the cast iron product.

Possible cooling methods include a salt bath, heated oil bath, metal bath, and forced hot air, but the simplest and most reliable method is a salt bath using sodium nitrate, potassium nitrate, or the like.

After the transformation is completed, it is taken out of the bath and allowed to cool (air or water cooling).

If the tempering temperature (constant temperature transformation temperature) is 300° C. or higher, the base structure tends to be upper bainite (feather-like, soft and flaky), and if it is lower than that, the base structure tends to be lower bainite (acicular, hard and brittle).

Table 1 shows Chinoku C melted or primary melted in a cupola.
A rod-shaped cast product of 30++nφ obtained from the series) was austempered, and test pieces (C series: C-1 to
C-23) was prepared and the composition and mechanical properties were measured. The test pieces were a rectangular test piece with a thickness of 51 Il# x width of 10 + s x length of 200 wna for measuring damping capacity, and a JISB No. C test piece for measuring tensile strength.

The conditions for the austempering treatment were: heating at 880-900°C for 1 hour, then tempering temperature (T, 8250°C, 350°C) in a salt bath of nitrate (NaNo3 t<N03).
and 375° C.), held for 1 hour, and then air cooled.

Table 2 shows the results except for those using cast products melted in an electric furnace.
Test piece (D series HD-1-D
-32) and show the results of measuring the same items.

In addition, in both Tables 1 and 2, the symbol in the column of "Bainitization" indicates the state of the base structure.

×...No pearlite precipitation or bainite formation.

Δ...Mixed structure of pearlite and bainite, poor bainite formation.

○...No pearlite precipitation, good bainite structure.

■・・・Good quality with a dense bainite structure.

In addition, in the table, T and S are tensile strength (kg f /5ea2
), HB indicates hardness (Brinell hardness), the column before treatment is that of the as-cast test piece, and the column after treatment is that of the test piece after austempering treatment. The damping capacity was measured by a damping capacity measurement method using free damping vibration, which will be described in detail later.

From Table 1, the following can be seen. First, good bainitic formation occurs when St is in the range of 2.2 to 3°4. )1
Tensile strength is FC-20 treated at 350℃ and 375℃.
Class, FC-25 class with 250℃ treatment, Mo
The additive shows a value of FC-30 class. Moreover,
The attenuation rate is comparable to 19% of the FC-10 class. In contrast, in FC-25, it is only 7%.

The electric furnace melted products shown in Table 2 have low tensile strength because they do not turn into bainite and are mainly pearlite even if the silicon content is within the above range. It is about FC-15 class. However, unlike the Mo additive-free one, bainite formation is seen partially and the strength is F(, -15 to 20
There are also some products with added S, etc. (D-
13-18), some bainite formation was observed and the strength was FC.
Although it shows a value of 20 class, the bainitization is unstable compared to that of cupola melting.

Next, the results of a comparative experiment with conventional flake graphite cast iron and spheroidal graphite cast iron regarding damping capacity are shown in Figures 1 to 3 and Table 3.
Shown below.

The damping capacity was measured using the above-mentioned test piece (thickness 5a + m x width 1011
+1X length 200@s) was supported in a cantilevered manner, an initial displacement (2 mm in this example) was applied to the free end, and the freely damped vibration was recorded (digital oscilloscope) when released. Obtained by calculation. FIG. 1 is a recording chart of free damping vibration of the product of the present invention shown in Table 3, which was recorded in units of 0.5 milliseconds for about 4.5 seconds after the vibration was applied. The length of each line segment represents the dimension from successive valleys to peaks and peaks to valleys of free vibration. These are X l, X 2, x3, x4, etc. in order from the largest one.
・If the slope of the straight line obtained by plotting the points xi and xi+l on the X-Y coordinates is θ, then the damping ratio ζ is ζz
l / πX l n L a n θ, logarithmic decay rate δ
(%) can be found as follows: δ=ζ×4×πX100 (%).

The value of the attenuation capacity shown in Table 3 (the same applies to Table 1) is the value of this δ. Similarly, Figure 2 shows FC-25 in Table-3.
Figure 3 shows the vibration record of FC-30 in Table 3. As is clear from the figure, the product of the present invention (Figure 1) has F
It can be seen that the vibrations are quickly attenuated compared to those of C-25 (Fig. 2) and FC-30 (Fig. 3).

As can be seen from this table, the product of the present invention has a strength of the FC-25 class and a damping capacity close to that of FC-10. It can be seen that the damping capacity is significantly superior to that of the previous model.

[Application example] Next, an application example of the present invention will be shown.

Application example 1 “Drum brake” Table 4 (a) shows the inventive product (treated at 350°C) and the conventional material (
FC-25) drum brake composition and mechanical properties (physical property values) are shown. As a result, while having the same strength,
It is extremely tough and has more than twice the damping ability that affects brake squeal, making it an ideal material for drum brakes.

In addition, a drum brake was cast with the component composition shown in Table 4(b), and the product of the present invention (bainitic temperature 250°C, 350°C
℃) and an untreated product (equivalent to FC-20), various physical properties were measured. In addition, when we conducted a cutting test on the product of the present invention (processed at 350°C), we found that the chips (ceramic 2Kyocera ■,
Wear of material 5N60) is 70% compared to untreated products.
It showed excellent machinability. In the case of the product of the present invention, the base structure is bainite (partially austenite),
It should be harder and have worse machinability than untreated products whose base structure is pearlite. However, in the present invention, there is a large amount of flaky graphite, which may have an effect, but it is also thought that the fact that there is less chatter during cutting due to the large damping capacity is also considered to be an effect.

Furthermore, when we conducted a wear resistance test (using a Suga wear tester) on the product of the present invention (treated at 350°C), we found that the wear-resistant cast iron material treated with wear-resistant cast iron (FC-28+
p, cr + tuft treatment, and FCD-45 + tuft treatment) and quenched materials (SPC carburized and quenched and tempered materials, and other materials), which is very good. Therefore, it can be suitably used for sliding wear parts.

Application example 2 “Bite holder” Invention product (tempering temperature 350°C), conventional invention product (tempering temperature 350°C), and conventional product bite holder (all dimensions are 150sa x 200mm x 1
50+m) strength, hardness and damping capacity were measured. Table-5
shows the component composition and various physical properties.

In the product of the present invention, most of the matrix structure is bainite (the rest is austenite), so it has a large damping capacity and, as shown in Application Example 1, also has excellent wear resistance. On the other hand, the conventional product is made of spheroidal graphite cast iron and has a pearlite base, and as a result, its strength is considerably high (approximately equivalent to FCD-50 to 60).

However, the performance required of a tool holder is high damping capacity, high rigidity (hardness), and wear resistance, and the strength is FC-20 ~
About 25 is sufficient. In particular, damping capacity is an extremely important element from the standpoint of suppressing chatter of the cutting tool and improving machining accuracy. Note that the compliance (vibration displacement/excitation crab unit μ/g) representing the degree of chatter is approximately 20% lower in the product of the present invention than in the conventional product. Therefore, the product of the present invention that has both of these features can be said to be an ideal material for a tool holder.

[Effects of the Invention] As detailed above, the flaky graphite cast iron of the present invention is obtained by austempering high-carbon, high-silicon flake graphite cast iron that has been melted or primary melted in a cupola melting furnace. It is something. Therefore, it has excellent vibration damping ability that takes full advantage of the properties of flaky graphite, and since the base structure is a mixed structure of bainite and austenite, it has high tensile strength and toughness, and has excellent machinability and wear resistance. It can be said to be an extremely ideal material for mechanical parts used in areas where vibrations occur. Further, by adding a small amount of molybdenum, copper, etc., the strength is further increased, and it becomes possible to sufficiently cope with the formation of thick materials into bainite.

In addition, the method of the present invention produces flake graphite cast iron that has both damping ability and tensile strength, and since it is basically flake graphite cast iron, it is easy to manufacture, and since only austempering is required, special technology and equipment are not required. It is possible to easily and reliably modify flake graphite cast iron without need.

[Brief explanation of drawings]

Figures 1 to 3 are recording charts of free damped vibration, where Figure 1 is the product of the present invention, Figure 2 is the FC-25 material, and Figure 3 is the FC-25 material.
-30 material is shown respectively. FIG. 4 is a graph showing the results of a reciprocating motion wear test.

Claims (1)

[Claims] 1. Carbon melted or primary melted in a cupola melting furnace 3
．． Flaky graphite cast iron with excellent strength and damping ability, characterized by being made by austempering high-carbon, high-silicon flake graphite cast iron containing 2 to 4.4% silicon and 2.2 to 3.4% silicon. . 2. The flake graphite cast iron having excellent strength and damping ability according to claim 1, containing 0.01 to 0.5% molybdenum. 3. Carbon melted or primary melted in a cupola melting furnace 3
．． High carbon high silicon flake graphite cast iron containing 2 to 4.4% silicon and 2.2 to 3.4% silicon was heated to zero at a temperature of 850℃ to 950℃.
．． Heat for 5-2 hours to austenitize, then 24 hours
A method for producing flaky graphite cast iron having excellent strength and damping ability, characterized by rapidly cooling it to 0°C to 400°C and maintaining it at this temperature for 0.5 to 2 hours to effect isothermal transformation.