JPS60230958A - Mold for centrifugal casting - Google Patents

Abstract

PURPOSE:To obtain the titled mold having superior characteristics in three points of thermal conductivity, vibration damping capacity and strength by contg. prescribed ratios of C, Si, Mn, S and >= one kind among Mg, Cs, Ce respectively and the balance Fe, and composing of vermicular cast iron having catapillar graphite after the casting. CONSTITUTION:The mold for centrifugal casting composed of the cast iron contg. by weight 2.5-4.0% C, 2.0-3.5% Si, 0.4-1.5% Mn, <=0.030% S and >=one kind among 0.010-0.02% Mg, 0.003-0.020% Ca, 0.010-0.02% Ce and the balance Fe is presented. The cast iron in this case is vermicular cast iron having caterpillar graphite after the casting. Since the mold strength of this invention is remarkably higher compared with usual common cast iron material quality, danger of mold cracking accident, etc. is decreased and service life of the mold is extended remarkably.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a centrifugal casting mold for casting cast iron pipes, rolling rolls, etc. by centrifugal casting.

Cast iron pipes and rolling rolls are generally cylindrical or cylindrical in shape, so it is most economical and efficient to cast them using the centrifugal casting method. Often applied.

Especially among rolling rolls, when manufacturing composite rolls in which the outer layer is made of a material such as high-grade grain cast iron with good wear resistance and the inner layer is made of a material such as ductile cast iron with high toughness, the centrifugal casting method is used to cast the outer layer. Since it is optimal, most of these methods have recently come to use centrifugal casting.

Conventionally, ordinary cast iron containing flake graphite has often been used as a mold for pouring molten iron in centrifugal casting. Although this ordinary cast iron has the advantage of high thermal conductivity, it does not have sufficient strength because stress concentration tends to occur at the tip of the flaky graphite, which has a sharp tip shape. ) Since the strength is only about C57f/-, the lifespan of centrifugal casting molds, which are subject to high stress due to centrifugal force due to high-speed rotation, is inevitably short, and their safety and reliability are also limited. It also had the disadvantage of being low.

On the other hand, recently, some use of spheroidal graphite cast iron or forged steel, which has much higher strength than ordinary cast iron, in centrifugal casting molds has been carried out for the purpose of extending the life of the mold. Spheroidal graphite cast iron is usually 40-50 kgf/m
The forged steel has a high strength of 80~I U Ok.
Since these materials have high strength reaching even gflvty&, by using these high-strength materials, it is possible to significantly extend the life of centrifugal casting molds and improve safety.

However, all of these high-strength materials have a problem that their thermal conductivity is much lower than that of ordinary cast iron, and a decrease in cooling efficiency is inevitable.

To explain in more detail the problem of heat conduction in centrifugal casting molds mentioned above, the solidification rate of an ingot generally depends on the heat capacity of the mold, but for molds of the same size, the thermal conductivity A mold made of a high-quality material can remove heat more quickly during the initial stage of casting, making it possible to obtain an ingot with a finer solidified structure. In particular, rolling rolls and the like are required to have a fine surface structure in order to improve surface wear resistance and hardness, and for this purpose, it is necessary that the mold has high thermal conductivity. For example, the ratio of ingot diameter to mold wall thickness is 4:l
In this case, in order to refine the solidified structure of the surface layer within 20% of the radius of the ingot, according to experiments conducted by the present inventors, the thermal conductivity of the mold should be set to 0.2 Calfs. It turns out that you need to. However, when we investigated the heat transfer coefficient at 500°C of the various centrifugal casting mold materials mentioned above, we found that ordinary cast iron had a heat transfer coefficient of 0.28 Ca1J/c.
m-wc-℃, whereas spheroidal graphite cast iron has a temperature of 0.
．． 18Ca73/crn ・wc ・0 for 'Q forged steel.
09 calA-WL-see・'c degree 1fKMgisu,
- It has been found that high-strength materials such as these are also insufficient for refining the surface layer of the ingot, and therefore these high-strength materials are not originally suitable as molds for centrifugal casting in terms of thermal conductivity.

On the other hand, when ordinary cast iron, which has low strength, is used in a centrifugal casting mold, the following problems occur in relation to heat transfer. In other words, in order to improve the efficiency of heat transfer from the molten metal to the mold, it is desirable to reduce the thickness of the anti-seizure refractory coated on the inner surface of the mold, but in this case, the temperature rise on the inner surface of the mold will increase. The temperature difference between the outer and inner surfaces of the mold becomes large, and the thermal stress of the mold due to the temperature difference between the inner and outer surfaces increases significantly. The risk of breakage is high, and from this point of view, ordinary cast iron materials can be said to be insufficient in strength as molds for centrifugal casting.

By the way, in recent centrifugal casting, in order to promote the refinement of the solidified structure, the mold is rotated at a higher speed than before, and the metal is poured while rotating at a high speed with a gravitational acceleration (G) of 100 to 150. method is often used. If the mold is rotated at such a high speed, the mechanical vibration of the mold will also increase, and the imaging of the vibration may lead to component segregation in the molten metal, formation of abnormal structures, and other deterioration in product quality. This has been confirmed through experiments by researchers and others. Therefore, it is desirable that a mold for centrifugal casting has a high ability to absorb vibrations, that is, a high vibration damping ability.

It is known that ordinary cast iron is the most excellent among the mold materials mentioned above in terms of vibration damping ability. For example, if we compare the vibration life until the vibration applied to the material disappears, if normal cast iron is l, spheroidal graphite cast iron is about 1.8, and steel is about 43. It is said that it is significantly inferior. Experiments by the inventors have also shown that ordinary cast iron molds have a high vibration damping capacity of 32x10, and that the amplitude of the imaging is 5 to 7μ.
m, whereas the vibration damping capacity of the spheroidal graphite cast iron mold examined under the same conditions was as small as 5X10, and the amplitude was 15~
It was found that it was as large as 20 μm, and therefore it was difficult to obtain a sound ingot by centrifugal casting under high speed rotation using a mold made of spheroidal graphite cast iron.

As mentioned above, it is desirable that the material for centrifugal casting molds has both high thermal conductivity and vibration damping ability, as well as high strength, but ordinary cast iron is insufficient in terms of strength.
In addition, high-strength materials such as spheroidal graphite cast iron and forged steel have low thermal conductivity and vibration damping ability, so the reality is that none of these materials can satisfy all of the above requirements.

This invention was made in view of the above circumstances, and the purpose is to provide a centrifugal casting mold that has excellent properties in all three respects: thermal conductivity, vibration damping ability, and strength. be.

That is, according to the experiments conducted by the present inventors, in order to sufficiently refine the solidified layer on the surface layer of the ingot, the thermal conductivity must be 0.2 Ca.
The vibration damping capacity is preferably 8 x 10 to maintain the integrity of the ingot even at high speed rotation F where the gravitational acceleration (G) is about 100 to 150.
or more (amplitude 10 μm or less), and from the viewpoint of mold life and safety, the tensile strength is 30 k.
l? It is preferable that the material has a strength of f/nd or higher, and an object of the present invention is to provide a centrifugal casting mold made of a material that simultaneously satisfies these conditions.

The inventors of the present invention have conducted various experiments and studies to find a material for a centrifugal casting mold that can satisfy the above-mentioned required properties, and have found that vermicular cast iron having caterpillar-like graphite can satisfy the above-mentioned properties. They discovered this and came up with this invention. In other words, in general, in cast iron, the thermal conductivity of graphite, which accounts for approximately 10% of its constituent structure, is higher than that of the iron base ferrite and pearlite, and the heat transfer coefficient as a whole is high, so when used as a mold. Among these cast irons, ordinary cast iron has sufficient heat conductivity, but spheroidal graphite cast iron has a relatively low heat transfer coefficient as described above. Furthermore, normal cast iron has a high attenuation ability, but spheroidal graphite cast iron has a low ability, while spheroidal graphite cast iron has significantly higher strength. Since the heat transfer coefficient, photodamping ability, and strength are all influenced by the shape of graphite in cast iron, vermicular cast iron has a shape intermediate between flaky graphite and spheroidal graphite, that is, caterpillar-like graphite. After focusing on this and conducting repeated experiments, they discovered that vermicular cast iron satisfies the above-mentioned required characteristics not only in terms of strength and heat transfer rate, but also in terms of vibration damping ability, and completed this invention. .

That is, the centrifugal casting mold of the present invention has, as its component composition, C2.5 to 4.0%, Si 2.0 to 3.5%,
Contains MnO, 4-1.5'4, SO, 030% or less, and Mg 0.010-0.02%, CaO
,003~0,020%,Ce 01010~0.02
%, the remainder being Fe and unavoidable impurities, and is characterized by being made of vermicular cast iron having as-cast caterpillar graphite.

Hereinafter, the centrifugal casting mold of the present invention will be explained in more detail.

The centrifugal casting mold of this invention is made of vermicular cast iron that has caterpillar-shaped graphite, which is an intermediate graphite shape between flaky graphite and spheroidal graphite, in the as-cast state, and the graphite shape can be controlled in this way. However, in order to ensure sufficient mechanical strength, thermal conductivity, and vibration damping ability, it is necessary to have the above-mentioned component composition. Here, among the above-mentioned component elements, Mg.

Ca or Ce is other component element, namely C
, Si , Mn , and S within the above range before casting the molten metal, it is added as one or more of Mg alloy, Ca alloy, or Ce alloy to control the graphite shape into a caterpillar shape. In this invention, the content of Mg, Ca or Ce is defined by the amount remaining in the ingot after solidification. That is, in manufacturing the centrifugal casting mold of this invention, C2.5 to 4.0%
, Si2.0-3.5%, Mn0, 4-1. ．． 5%, 8
A molten metal containing 0.030% Fe and unavoidable impurities is produced, and before casting the molten metal, one or more of Mg alloy, Ca alloy, or Ce alloy is added to the remaining amount in the ingot. is 0.010 to 0.020% for Mg, 0003 to 0,020% for Ca, and 0.010 to 0.0 for Ce.
Add so that it is within the range of 20%. Mg, Ca
Alternatively, if the residual amount of Ce is less than the above range, the graphite shape will not be caterpillar-like, but will become flake-like, and sufficient strength will not be obtained. On the other hand, if the residual amount exceeds the above range, the spheroidization of graphite will proceed excessively, and the ratio of spheroidal graphite mixed with caterpillar graphite will increase, resulting in characteristics similar to spheroidal graphite. in particular,
The proportion of caterpillar-like graphite cannot be ensured at 70% or more in terms of graphite number, and the number of spheroidal graphite exceeds 30%, making it impossible to obtain sufficient thermal conductivity and vibration damping ability. Therefore, the residual amounts of one or more of Mg, Ca, and Ce are o, oto~0020chi, and 0.003~0.002, respectively.
0%, limited to within the range of 0.010 to 0.020%.

Next, the reason for limiting components other than Mg, Ca, and Ce will be explained.

C: C is a major element that constitutes the structure of cast iron along with Si, and has a particularly large influence on the amount of graphite and the shape of graphite.

When the C content is less than 2.5-, the amount of graphite in the cast iron after solidification is small, the thermal conductivity decreases and it becomes similar to steel, and the graphite shape becomes almost flaky, resulting in low strength. . −
If the content exceeds 4.0%, the crystallized graphite becomes coarse and the molten metal composition becomes hypereutectic. Therefore, in order to obtain caterpillar graphite, Mg alloy, Ca alloy or Ce alloy is used.
If alloy is added, the ratio of spheroidal graphite to the total number of graphites will be 3.
It shows properties close to those of spheroidal graphite.
As a result, although the mechanical strength is improved, both the thermal conductivity and the vibration damping ability are reduced, making it undesirable as a mold for centrifugal casting. Therefore, the C content was set within the range of 2.5 to 4.0%.

Si: An element related to 5iijC that has a large effect on the material quality of cast iron. If the S1 content is less than 2.0%, graphitization will be insufficient, so improvements in thermal conductivity and vibration damping ability cannot be expected. On the other hand, if the Si content exceeds 3.5 inches, the amount of graphite crystallized increases, but the shape of the graphite becomes lumpy, resulting in a decrease in vibration damping ability, making it undesirable as a mold for centrifugal casting. Therefore, the Si content is 2. It was set within the range of θ to 3.5 inches.

Mn: Mn does not have a direct effect on the graphite shape of cast iron, but it has a large effect on the strength of the matrix structure. If the Mn content is less than 0.4%, the base structure becomes ferrite, so the elongation value increases but the strength decreases, making it unsuitable as a centrifugal casting mold. Mn content is 0.4-1.5
% range, the base structure becomes a mixed structure of ferrite and pearlite (V), and as the Mn amount increases, the pearlite proportion increases, and at Mn 1.5, it becomes a completely pearlite structure, and the strength is 40 kgf/- or more. However, even if the amount of Mn is increased beyond 1.5%, the effect will not increase any further. Therefore, the Mn content was set within the range of 0.4 to 1.5%.

S: The content of S does not directly affect the properties of cast iron, but Mg is added to make the graphite shape caterpillar-like.
It is necessary to regulate the content of S because its bonding force with Ca or Ce is extremely strong and has a harmful effect of destroying the effects of these graphite shape control elements. In other words, Mg, Ca or Ce is added to molten iron containing a large amount of S.
If you add an alloy containing MgS, CaS
Alternatively, CeS is formed and the effects of these elements are lost, making it difficult to control the graphite shape. S content is 0.0
It is possible to crystallize caterpillar graphite even in a molten metal with a content exceeding 30% by adding a large amount of the above alloy.
It is preferable for the S content to exceed 0.030%, since it is uneconomical to add a large amount of alloy, and a large amount of sulfide generated remains inside the ingot and causes a decrease in strength. do not have. Therefore, the S content was limited to 0.030% or less.

Further, in the vermicular cast iron material of the centrifugal casting mold of the present invention, the content of C and Si is adjusted so that the carbon saturation degree Sc defined by C and SL is within the range of 0.80 to 1.20. This is desirable. That is, carbon saturation S
c is 5c=(0%)/(4,23-(Sin)/3.2)
It is known to indicate the degree of eutecticism of cast iron, but there is a relationship between carbon saturation and strength in vermicular cast iron, as well as the ratio of the number of caterpillar graphite grains to the total graphite grains. It has been found that there is a relationship as shown in Figure 1. In FIG. 1, curve 1 shows the relationship between carbon saturation and tensile strength, and curve 2 shows the relationship between carbon saturation and the proportion of caterpillar graphite. As is clear from Figure 1, when the carbon saturation degree Sc is less than 0.80, the proportion of caterpillar-like graphite is high, but since flaky graphite with sharp tips with a spheroidization rate of 30% or less is mixed, it is difficult to The mechanical properties, especially the tensile strength, are low, below 30 kgf/IdF, making it undesirable as a mold for centrifugal casting. On the other hand, carbon saturation is 1.00
If it becomes larger, the proportion of spheroidal graphite mixed with caterpillar graphite increases, and if it exceeds 1,200, the proportion of spherical graphite exceeds 30%, and the proportion of caterpillar graphite becomes 70% or more F. If the number of spheroidal graphites increases in this way, the mechanical properties will improve, but the vibration damping ability will decrease. In other words, according to the experiments conducted by the present inventors, there is a relationship between the proportion of caterpillar graphite and the vibration damping capacity as shown in Figure 2, and when the proportion of caterpillar graphite is 70% or less, the vibration damping capacity decreases. 5xio or less, which has been found to be insufficient for vibration isolation of centrifugal casting molds used under high rotation speeds. Therefore, in order to ensure a caterpillar graphite ratio of 70% or more in terms of vibration damping ability, it is preferable that the carbon saturation SC is 1.200 F or more.

As for the mechanical properties of the vermi-keeled cast iron which is the material for the centrifugal casting mold of the present invention as described above, a tensile strength value of 30 to 45 and 9 f/- can be obtained.

Although this strength is slightly inferior to that of spheroidal graphite cast iron, it is more than twice the strength of ordinary cast iron, which has been widely used in the past, and is sufficiently satisfactory as a mold for centrifugal casting.

Regarding thermal conductivity, as shown in FIG. 3, vermi-keeled cast iron 3, which is the centrifugal casting mold material of the present invention, is slightly inferior to normal cast iron 4, but about 1. 3 times the value, 0.22 C at 5oooC
The value of a176m・formula・℃ is obtained. Such a thermal conductivity sufficiently satisfies the required value for a centrifugal casting mold.

On the other hand, the vibration damping ability, which is the ability to absorb vibration, has a value measured by the internal friction method of 9
It is inferior to ordinary cast iron by about IA, but it is about 14 times better than spheroidal graphite cast iron, and 40% longer than ordinary cast iron when compared in vibration damping time by stress relief method. However, it has been confirmed that it is approximately 30 inches shorter than spheroidal graphite cast iron, and the vibration damping ability of vermicular cast iron has been found to be sufficient for centrifugal casting molds used under high-speed rotation. ing.

Examples and comparative examples in which the present invention is applied to a centrifugal casting mold for hot rolling rolls will be described below.

The dimensions of the manufactured mold were an outer diameter of 12,601 flll and an inner diameter of 71 mm.
Qw, a hollow cylindrical shape with a length of 250011111-ID, and a weight of approximately 16) tons. Molds made of vermi-keeled cast iron with compositions within the range of the present invention shown in sample codes 1 to 6 in Table 1, and molds with sample code 7 made of conventional ordinary cast iron as a comparative example, also as comparative examples. A mold with sample code 8 made of spheroidal graphite cast iron was created. Here, when creating the molds of the examples of the present invention (sample symbols 1 to 6), Mg, Ca
Alternatively, melt a molten metal containing components other than Ce, and add 1 to the molten metal.
At 400℃, for sample code 1>3, Fe-8i
-25% Ca alloy is 1 yen by weight, and for sample code 4, Fe-8i -15%Mg alloy is 0.5% by weight.
%, in sample code 5 the Fe-43%Ce alloy was 0.05% by weight, and in sample code 6 the Fe-43%Ce alloy was 0.05% by weight.
10%Mg-4%Ca alloy 0.5% by weight
, were added and then cast, thereby controlling the graphite shape to be caterpillar-like. In addition, when creating a comparative example mold of sample code 7 made of ordinary cast iron material, the first
Pour the molten metal with the composition shown in the table as it is, and sample code 8
In order to create a comparative mold made of spheroidal graphite cast iron material, a molten metal with the composition shown in Table 1 excluding Mg was melted and a Fe-Si-15%Mg alloy was melted by weight before casting. The shape of the graphite was controlled to be spherical by adding 1.2 g.

Among the molds of the examples of the present invention, the microstructure (graphite structure) of the mold with sample code 1 is shown in FIG. 4 under a 100x magnification.

As represented in FIG. 4, each sample symbol 1-
According to the observation results of the structure of the mold in No. 6, the caterpillar-like graphite, which occupies 30 to 60 inches in terms of the area ratio of graphite to the circumscribed circle of each individual graphite grain, accounts for more than 80% of the total number of graphite grains. It was confirmed that there is.

Table 2 summarizes the mechanical properties, vibration damping ability, and thermal conductivity of the molds with sample numbers 1 to 8 above.

As shown in Table 2, the mold of the comparative example made of ordinary cast iron (
Sample code 7) has a heat transfer coefficient of 0.31 at 500°C.
Cal/lb'TrL・sw・℃, vibration damping rate is 30
×IQ' both show high values, but on the other hand, the tensile strength is 12 kg fd1 Young's modulus 10500 f/mrl
The comparative mold (sample code 8) made of spheroidal graphite cast iron had a tensile strength of 50 kgt/n.
d. It has excellent mechanical properties with a Young's modulus of 17,000 kgf/m-, but the heat transfer coefficient at 500°C is 0.1
6 Ca17 eclipse, crown, °C, and vibration damping ability are all inferior at 5 x 10. On the other hand, the molds of the present invention examples (sample codes 1 to 6) have a tensile strength of 33 to 44 ur/md.
, Young's modulus is 13,700-15,300 klf/mis, vibration reduction capacity is 8-10 x 10.5008C, heat transfer coefficient is 0.20-0.26 Cal/CIrL-ytc'
In terms of strength, C is close to spheroidal graphite cast iron, its vibration damping capacity is about twice that of spheroidal graphite cast iron, and its heat transfer coefficient is also higher than that of spheroidal graphite cast iron. It is clear that the desired characteristic values for a mold are satisfied.

Furthermore, the results of actually casting a cast iron roll for hot rolling by centrifugal casting using the above-mentioned valley metal 41J will be described below.

The object was a hot strip mill finish rolling work roll with an as-cast body diameter of 700 fi and a body length of 2300 m, the casting weight of the molten metal was 11 tons, and the casting temperature was 1350°C. Further, the mold rotation speed was 550 rpm, and the gravitational acceleration to which the molten metal was subjected was 140 G, and casting was performed under high-speed rotation. The vibration of the mold was then measured using a cradle on the rotating shaft. As a result, the vibration amplitude after pouring the molten metal was a small value of 5 to 7 μm in the case of the comparative mold made of ordinary cast iron (sample code 7), but in the comparative example made of spheroidal graphite cast iron, which is a high-strength material. In the case of the mold (sample code 8), it showed a large value of 15 to 20 μm, exceeding the vibration control limit of 10 μm, and
It was found that the integrity of the product was adversely affected. On the other hand, a mold made of vermicelli cast iron according to the present invention (sample code 1
In the case of ~6), the vibration amplitude is about 8 to IOμm, which is not as high as that of ordinary cast iron, but spheroidal graphite cast iron has a smaller vibration amplitude and can actually suppress mold vibration and produce sound castings. It turned out that it can be effective.

As is clear from the above explanation, if the mold of this invention is used to cast cylindrical or cylindrical castings such as rolling rolls and cast iron pipes by the centrifugal casting method in which the mold rotates at high speed, the mold strength will be improved. Compared to conventional ordinary cast iron materials, the risk of mold breakage is significantly reduced, and the life of the mold is greatly extended. Since it has higher thermal conductivity and vibration damping ability than molds made of other materials, it has excellent cooling efficiency and can suppress mold vibrations, making it possible to obtain high-quality and healthy cast products.

[Brief explanation of the drawing]

Figure 1 is a correlation diagram showing the relationship between carbon saturation, tensile strength, and caterpillar graphite ratio in cast iron materials; Figure 2 is a correlation diagram showing the relationship between caterpillar graphite ratio and vibration damping capacity; The figure is a correlation diagram showing the relationship between temperature and thermal conductivity in various cast iron materials, and FIG. 4 is a microscopic structure photograph showing the graphite structure of the mold of this invention. Applicant Kawasaki Steel Co., Ltd. Agent Patent attorney Takehito Toyota (and 1 other person) Figure 1 Figure 2 Imo Shishiga! 9. Force side number (%) Fig. 3 Deflection (0C) Fig. 4

Claims (1)

[Claims] C2.5 to 4.0% (weight %, same hereinafter), Si2.0
~3.511Mn 0.4~1.5%, SO,03
Contains 0'llr or less, and further contains Mg 0.010 to 0
．． 02%, Ca0.003-0.020%, CeO,
Gold for centrifugal casting, characterized in that it is made of vermicular cast iron containing one or more of 0.010 to 0.020%, the remainder consisting of Fe and unavoidable impurities, and having caterpillar graphite as cast. Type.