JP2002356737A - Ductile cast iron tube by centrifugal casting method and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain the improvement of quality of ductile cast iron tube by a centrifugal casting method and the simplification of the process. SOLUTION: The cast iron tube has components containing 3.0 to 4.0% C, 1.5 to 3.0% Si, 0.1 to 0.4% Mn, <=0.05% P and <=0.01% S, and the balance Fe, and further containing 0.0005 to 0.05% Bi, in particular, and, more preferably, containing 0.0001 to 0.05% Ca. The cast iron tube has a dense and tough structure in which many spheroidal graphite is crystallized into a ferrite based matrix while maintaining a spheroidization rate of >=90%, so that ferritizing annealing after casting can be performed at a lower temperature, and/or can be performed in a short time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ductile cast iron pipe,
And related to the manufacturing method, especially in ductile cast iron pipes manufactured by centrifugal casting, excellent in annealing properties,
The present invention relates to the manufacture of pipes having a dense structure and superior toughness in the past.

[0002]

2. Description of the Related Art Ductile cast iron pipes are used for fluid transportation in various fields such as water supply and sewage, agricultural water, industrial water, and the like. Since it is possible to lay pipes without the need for welding work like steel pipes, the laying work is simple and the quality is reliable, so it is most commonly used as the mainstream of pipe formation. Ductile cast iron pipes include various forms such as straight pipes and deformed pipes, but mainly straight pipes are mainly made by centrifugal casting, injecting high-temperature molten metal into a cylindrical mold that rotates at high speed. Therefore, the cooling rate is remarkably faster than that of the normal casting method, and as a result, a chill structure is generated in the as-cast state, and it cannot be used as it is in terms of ductility and strength. It is said that refining to the phase is essential.

[0003] Japanese Patent Application Laid-Open No. 2000-45011 is known as a conventional technique for solving the chill structure which occurs when casting ductile cast iron into a mold. This prior art includes C: 3.10 to 3.90%, Si: 2.50 to
4.00%, Mn: 0.45% or less, P: 0.05% or less, S: 0.008% or less, Cu: 0.5% or less, M
o: 0.3% or less, Mg: 0.05% or less, Bi + Sb
+ Ti: Spheroidal graphite cast iron used in a mold casting method containing 0.1% or less. Since the mold casting method has a higher cooling rate than ordinary sand molds, especially when pouring spheroidal graphite cast iron, it is inherently easily cooled by the spheroidal graphite crystallization mechanism that crystallizes from the molten metal, thereby chilling. Is said to be promoted. This undercooling phenomenon is interpreted as a special process in which round graphite, which is crystallized from the eutectic reaction starting point, is surrounded by austenite, and graphite exhaled from austenite grows in a stacked manner. It is described that supercooling occurs inevitably because heat is absorbed from the periphery for graphite growth as compared with flake graphite in contact with the molten metal.

In this prior art, the molten metal is subjected to spheroidal graphite treatment with a spheroidizing treatment agent, and is subjected to siliconization at least once before pouring into a mold, so that the total amount of Si is 2.50 to 50%.
It is the gist of the invention to make it 4.00%.
After casting through siliconizing in this way, it is taken out at around 900 ° C, where it can be handled sufficiently, and then left to cool in air to obtain a microstructured cast product having an ultrafine graphite structure without a chill structure. It is said that the indispensable annealing can be omitted.

According to the examples of the cited documents, when the final Si after siliconization is less than 2.50%, chill is generated although the number of graphite is large, whereas when the Si is more than 2.50%, Has no chill, and the number of graphite particles is almost 1900 /
mm ² there are, if you look at this 1900 pieces / mm ² is a region of the chill prevention, not only became no longer necessary heat treatment which was required for the conventional tempering, mechanical properties and physical properties The results are said to be accompanied by improvements.

Another prior art disclosed in Japanese Patent Application Laid-Open No. 1-309939 is to improve the elongation, especially the low-temperature impact value, and omit or facilitate the heat treatment in ferritic spheroidal graphite cast irons such as automobiles. In addition to the components of ordinary spheroidal graphite cast iron, Cr: less than 0.1%, Bi: 0.0
015-0.008%, C.I. E value of 3.9 to 4.6
By doing so, the structure is such that the number of graphite particles is 300 / mm ² or more.

[0007] Originally, in spheroidal graphite cast iron, S
It is a common theory that when a small amount of b, As, Bi, Pb, or the like is contained, spherical graphite is difficult to crystallize. However, in this conventional technique, an appropriate amount of Bi is added to reduce the number of graphite particles to 300 /
By setting it to mm ² or more, pearlite was reduced, and sufficient elongation and impact value were obtained without low-temperature heat treatment or heat treatment. However, Bi has a poor penetration rate into the molten metal and a large variation in the yield rate, and therefore, in order to maintain the residual content, it is necessary to set the addition amount in the range of 0.005 to 0.025%. 0.008% of residual Bi
It is said that the spheroidization of graphite is hindered if the spheroidization ratio exceeds 80%, and the spheroidization rate becomes 80% or less. The prior art of Japanese Patent Application Laid-Open No. Hei 2-70015 by the same applicant has almost the same gist.

[0008]

The prior art cited above is a result of research and development with a main target of a die casting for a high performance piston, and there is no room for controversy with the result. . However, although the results will be highly valued for the specific utility in question, there is no guarantee that the results will apply to other utilities and products and receive the same high rating. This principle also applies to ductile cast iron pipes made by centrifugal casting.The severe process of injecting high-temperature molten metal into a mold rotating at high speed and solidifying with the high energy of centrifugal force uses the same mold. That said,
This is because the coagulation mechanism is completely different.

When the microstructure of a ductile cast iron pipe manufactured by centrifugal force casting is observed in an as-cast state, the distribution of cementite in the structure extends over almost the entire cross section, and the proportion of the cementite phase in the visual field of the microscope is very high. 9
Exceeds 0%. Although it depends on the thickness of the casting and the coating of the mold, for example, if the minimum thickness is 2 to 10 mm and the cooling rate of a normal sand mold is about 10 ° C./s or more, the mold of the cited example is used. It is measured that cooling is performed at a rapid speed of at least 30 ° C./s or more in the centrifugal force casting of a mold, while 15 ° C./s or more is set as a requirement in casting. The cooling rate is much higher than that of a normal mold, and the degree of supercooling around the spheroidal graphite is also drastically increased in response to this. The difference in organization is too great. Also, since the cooling rate of the inner peripheral surface of the mold coated with resin coated sand is rather small,
There may be a special exception that in some cases reverses the tendency of normal chill texture to occur.

For centrifugal castings, there is no official inspection standard for the structure itself observed under a microscope, but FCD400, 45 specified in JIS G 5526 as mechanical properties.
Tensile strength required for 0 etc. 400-450 N / mm
^In order to satisfy ² or elongation of 10% or more, it cannot be achieved at all unless the internal speculum standard of 30% or more of ferrite phase (5% or less of cementite phase) is satisfied.

[0011] Further, it is needless to say that ferritization annealing for ductile cast iron pipes is intended to decompose cementite crystallized by as-cast and eventually decompose pearlite, which is a base, to form a ferrite base. However, in the first cited example, the description of the attached micrograph only includes the presence or absence of chill (white iron microstructure), but it is unlikely that the base microstructure is ferrite-based as long as the target is a high-load piston. It is reasonable to assume that it is a strong pearlite or bainite structure. In addition, the internal stress generated by the severe casting conditions of centrifugal casting (especially the tension-compression force generated between the inner and outer surfaces)
It is also one of the necessary steps to completely release the steel, and the meaning of annealing cannot be limited to merely decomposition of chill.

The second prior art cited later is also a mold stationary casting such as an automobile part. The cooling rate is completely different from that of centrifugal casting, and since it is stationary, graphite nuclei are generated and the surrounding austenite is formed. In the early solidification region, the growth of the static primary crystal structure progresses from the solid phase to the liquidus line, which collects and grows the C exhaled from the inside. The conditions are obviously different because factors that directly hit 30 to 50 times the pressure (fluid external force) and crush (stir, break) the static tissue growth overlap. Therefore, it should be understood that the requirement of the prior art for restricting the strict Bi residual amount with emphasis on the graphite Bi spheroidizing element of a trace amount of Bi does not perfectly match the centrifugal casting as in the present application. Conversely, it is possible to utilize 100% of the solidification process unique to centrifugal casting to lead to a dramatic quality improvement that cannot be achieved by static casting.

In order to solve the above-mentioned problems, the present invention generates a larger number of graphite nuclei in an as-cast state than in the past. By comparison, fine-grained spheroidal graphite, which is finally crystallized by the much simplified ferritizing annealing, is dispersed in a ferrite-based matrix while maintaining a spheroidizing ratio of 90% or more. It is an object to provide a ductile cast iron tube.

[0014]

According to the present invention, there is provided a ductile cast iron pipe formed by centrifugal casting according to the present invention.
i: 1.5 to 3.0%, Mn: 0.1 to 0.4%, P:
Bi: 0.0005 to 0.0 with respect to the basic component consisting of 0.05% or less, S: 0.01% or less, and the remaining Fe component
The above-mentioned problem has been solved by providing a structural feature that a large number of spheroidal graphites, including 5%, are crystallized on a ferrite-based matrix while maintaining a spheroidization ratio of 90% or more.

[0015] More preferably, in addition to Bi of the above-mentioned basic constitution, Ca contains 0.0001 to 0.05%, and a larger number of spheroidal graphites crystallize on a ferrite-based matrix while maintaining a high spheroidization rate. Can be done.

Further, as a method for producing the ductile cast iron pipe, the molten metal component is C: 3.0 to 4.0%, and Si:
1.5-3.0%, Mn: 0.1-0.4%, P: 0.
After dissolving and scouring so as to be 0.05% or less, S: 0.01% or less, and the remaining Fe, a spheroidizing agent mainly composed of Mg is added to the molten metal to perform a spheroidizing treatment of graphite, and an inoculant is prepared. Inoculate into molten metal and finally Bi is 0.0005-0.05%
, More preferably Ca is added together with Bi, more preferably 0.0005 to 0.05% of Bi, and Ca: 0.0001 to 0.
A requirement is to perform a procedure of adding the yield in the calculation so that the yield can be clearly detected in the range of 05%, centrifugal casting, solidifying, and then performing a simplified ferrite annealing.

In this case, specifically, when Bi is added, the chemical components are Si: 20-80%, Bi: 0.1-25.
0%, the remaining Fe, and when Bi and Ca are added, the chemical components are Si: 20 to 80% and Bi: 0.1 to 25.0.
%, Ca: 1 to 40.0%, and an alloy powder or a mixed powder consisting of the remaining Fe is desirably used as an inoculant.

The finer the particle size of the inoculant, the higher the yield and the effect of inoculation, but if the particle size is too small, the powder is scattered during the inoculation and the effect is reduced. Therefore, the lower limit is 0.05 mm
And On the other hand, if the upper limit is more than 3 mm, the inoculant cannot be diffused uniformly in the molten metal, and the effect is not obtained. Therefore, the particle size range is 0.05 mm to 3 mm. The optimum range has a lower limit of 0.1 mm or more from the viewpoint of powder management and economy.

As for the ferrite annealing, a simplified ferrite which is treated at a lower temperature and / or with a shorter holding time than the annealing pattern (see FIG. 7) applied in the production process of a normal ductile cast iron pipe. The biggest feature of the manufacturing method is that the annealing is performed, which is also an advantage.

The components of the ductile cast iron pipe are limited by the usual JIS G 5526 or the provisions of the Japan Water Works Association (J
WWA G 113) is almost followed, but Bi is contained immediately before centrifugal casting so that 0.0005 to 0.05% can be finally detected clearly. More preferably, Ca is contained in 0.0001 to 0%. After inoculating an addition amount in consideration of the yield so that 0.05% can be detected, graphite is finely crystallized, the number of spheroidal graphite is increased, and the austenite grain size of the primary crystallization is reduced. It is characterized in that it is refined to a dense and tough structure through ferrite annealing reduced to about half of the above.

Among the above components, the most important requirement is Si: 2.50 to 4.00% in the first prior art, whereas Si: 1.5 to 3.0% in the present invention. The limitation is that as the Si content exceeds 2.5% and approaches 3.0%, roughening such as pinholes tends to occur on the outer surface of the ductile cast iron pipe. Considering the unique conditions of centrifugal casting, in which oxides of Fe and Mg adhere to the surface to form dross and are easily entrained, Si: 1.50 to 2.5
0% in principle, but only when special specifications such as durability and corrosion resistance are required, Si: 2.50-3.
The idea is to allow a range of 00%,
i: This is a fundamental difference from the prior art which has an absolute requirement of 2.50% or more.

The action itself of adding Bi to the molten spheroidal graphite cast iron to make graphite finer and to distribute it uniformly into the matrix is well known, and Sb, Te, and Sn are elements having the same function.
Etc. are also known. However, as specified in the present invention, the special condition of quenching by centrifugal casting and the external force of pressing (mechanically forcibly flowing) the molten metal are combined, so that the static casting is considered to be a common theory in the static casting. Is there a big difference in the factors that inhibit spheroidization? As suggested by the observation of the microstructure in Examples described later, even if the Bi content is relatively high, the spheroidization ratio of the graphite is remarkably reduced while still maintaining 90% or more. It is also estimated from the fact that tissue is obtained. Bi shows excellent graphite crystallization ability after exceeding 0.0005%,
If it is 0.05% or more, adverse effects such as reduction of graphite crystallization ability and inhibition of spheroidization appear.

Regarding the addition of Bi, in the second prior art, conical lump Bi was added, or added as wrapped paper or the like to improve the Bi yield.
At the same time, an example using an Fe-Si inoculant is reported. However, in the present invention, Bi is added by inoculation in the form of a granular material, using a mixed powder or alloy powder of Fe-Si-Bi, and desirably adding Ca together with the addition of Bi to Fe-Si-Bi-Ca. It should be particularly mentioned that the object of the present invention can be more effectively achieved by using a mixed powder or an alloy powder. Even if Ca is added in such an amount that it cannot be detected as a residual component after casting, a stabilizing effect of improving the yield of Bi by inoculation appears. That is, Ca is liquefied in the early stage of solidification of graphite nucleation, and when liquid phase Ca and Bi come into contact, an intermetallic compound of Bi-Ca is formed, which has the effect of suppressing the vaporization loss of Bi having a low vapor pressure. It is estimated that Needless to say, Ca has an action of deoxidizing and desulfurizing the molten metal, and has an action of removing (promoting) S, which is a great enemy of graphite spheroidization.
It is thought that there is a synergistic effect that contributes to normal progress of spheroidization by compensating for the spheroidization inhibitory factor. It has been clearly proved in the following Examples that this synergistic effect is even stronger when Ca is contained to the extent that it can be detected as a residual component. When Ca exceeds 0.0001%, a synergistic effect with Bi is clearly confirmed. When it exceeds 0.05%, the synergistic effect is reduced, the synergistic effect is reduced, and poor appearance is increased. In addition,
Such a function is performed by using Mg, Sr, B as elements other than Ca.
It is also expected to be an inoculant blended with a group IIA metal such as a, or a rare earth element.

As mentioned above, almost 90%
A ductile cast iron pipe manufactured by centrifugal casting that occupies the above base with cementite is subjected to ferrite annealing that is simplified as compared with the conventional method by adding only one operation of inoculating Bi or Bi, Ca. Therefore, it is the gist of the present invention to refine the structure to a finer, denser, and tougher structure than before.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Table 1 is a list of components of Examples and Comparative Examples of the present invention. Each component is a standard component applied as a normal ductile cast iron pipe. The only difference between the two is the presence or absence of Ca. Needless to say, in Comparative Example 1, both Bi and Ca are 0%. In Comparative Example 2, Bi was 0.0% in the claims.
This is the case where more than 5% is added. Example 1 uses the inoculant containing only Bi, and Example 2 shows that the detected Ca is 0%,
a is inoculated and exhausted until it cannot be detected. Examples 3 and 4 are examples in which Ca was detected by adding Ca more than Example 2.

[0026]

[Table 1]

The melting is carried out in a cupola furnace 1, the components are adjusted, deoxidation, desulfurization and the like are carried out according to the standard.
After adding pressure of Mg at 450 ° C. and pouring hot water into ladle 2, 870
Nominal diameter 100mm x 4000mm during high speed rotation at rpm
The molten metal is poured into the L centrifugal casting mold 3 (see FIG. 6). When the molten metal is dropped from the ladle 2 to the end of the trough 4 during this pouring, the chemical components are Si: 20 to 80% and Bi: 0.1.
２５25.0%, remaining Fe or Si: 20-80%,
Bi: 0.1 to 25.0%, Ca: 1 to 40.0%, alloy powder or mixed powder consisting of the remaining Fe is sprayed on the flowing molten metal to uniformly add Bi or Bi, Ca. Do. In addition, about addition of Bi or Bi and Ca, not only this pouring flow inoculation but also addition in the ladle 2 may be used, or both may be used together. Furthermore, it may be performed by tip inoculation. Pouring temperature to mold is about 1300 ℃
After solidification, the product is pulled out of the mold and allowed to cool. The inoculant used in Example 1 was Bi: 16%, Si: 5
The inoculant used in Example 2 was 8%, the remaining Fe was used, and the inoculant used in Example 2 was 16% Bi, 17% Ca, 51% Si, and the remaining Fe. The inoculant used in Examples 3 and 4 was Bi. :
15%, 23% of Ca, 47% of Si, and the balance of Fe were selected.
The amount of inoculation is empirically determined based on factors such as the amount of molten metal and product size while taking into account the yield relative to the target residual%.

FIGS. 1 (A), 1 (B), 1 (C), and 1 (D) are microstructure photographs (magnification: 100 ×) of each test piece in Comparative Example 1 and Examples 1, 2 and 3 in Table 1. (A) is a conventional as-cast condition, FIG. 1 (B) is Example 1 of the present invention in which only Bi is contained in the inoculant, and FIGS.
FIG. 1 (C) shows an as-cast state of Example 2 of the present invention in which only Bi is added, and FIG. 1 (D) shows an as-cast state of Example 3 of the present invention in which Bi and Ca are detected. It is a thing which showed. Examples 1, 2, and 3 correspond to FIGS. 1B and 1C, respectively, with respect to FIG.
As can be seen from (D), the number of spheroidal graphites is overwhelmingly large in all cases, and particularly when Bi and Ca are added, three times or more of the graphite is crystallized on the matrix. In addition, this photograph shows the structure without corrosion, but almost all of it is a white iron structure of a cementite phase, which confirms that almost all surfaces are chilled by the as-cast centrifugal casting method. Furthermore, it should be noted that contrary to the conventional theory that even if a small amount of Bi is contained, the spheroidization of graphite is inhibited, in the embodiment of the present invention, the shape is spherical at a high rate of 90% or more while effectively miniaturizing. This is due to the unique quenching effect of centrifugal casting and the external pressure (mechanical external force) in the opposite direction that causes primary crystal growth to crush (stir and break), and further spheroidization of graphite by Ca It is understood that the synergistic effect of the promoting action and the stabilizing action of Bi is a unique effect obtained by supporting.

FIG. 2 is a pattern diagram of ferritizing annealing applied to the examples and comparative examples of the present invention. The temperature was raised to 1,000 ° C. in a horizontal annealing furnace, and the first-stage annealing (1,0
The holding time at (00 ° C.) was reduced to almost half of the normal value of 10 minutes, and the second-stage annealing (680 to 730 ° C.) was also subjected to the two-stage annealing process of shortening to approximately half of the normal value of 15 minutes. Ferritizing annealing was performed by slow cooling operation divided into stages.

FIG. 3 is a photograph (magnification: 1) of the microstructures after ferrite annealing (FIG. 2) of the example of the present invention and the comparative example.
FIG. 3 (A) shows a comparative example 1 in which the area ratio of cementite was about 20% and the number of spheroidal graphite was 613 / mm ² , which was a standard value consisting of components for ordinary ductile cast iron pipes. In the case of the product, it was proved that the cementite could not be completely decomposed by the simple annealing in which the holding time was shortened by half, and the ferrite was not completely converted. On the other hand, in Example 1 (FIG. 3 (B)), Example 2 (FIG. 3 (C)), and Example 3 (FIG. 3 (D)), the ferrite was completely ferrite at a cementite area ratio of 0%. The number of spheroidal graphite is also 1
121, 2129, 3435 pieces / mm ^{2, which} is very large,
In comparison with the as-cast condition, the increase is about 1.31 to 1.60 times and exceeds the increase of 1.17 times of the comparative example, and the example is superior to the comparative example in the increase rate of graphite due to decomposition of cementite, The difference between the number of spheroidal graphites in the example and the comparative example clearly shows that the number is further increased by the heat treatment. The number of spheroidal graphite in the examples of the present invention is 10 magnifications.
It is a value measured in a field of view of 0 times and excluding the number of particles having a particle size of 1 μm or less.

As can be seen from the above, the addition of Bi alone increases the number of graphite particles in the as-cast state. However, when Ca is added together with Bi, the number of graphite particles increases remarkably, and Ca is detected. It can be seen that if they coexist to the extent possible, the increase in the number of graphite particles further increases due to the synergistic action, and the improvement effect of densification of the structure further increases. This effect is further promoted by simplified annealing, and the result that the difference in the increase in the number of graphite particles due to the decomposition of cementite is further increased is remarkably shown.

FIG. 4 is a pattern diagram of the ferrite annealing in Examples and Comparative Examples of the present invention. In FIG. 2, the first-stage annealing temperature is lowered to 950 ° C. Equivalent 20 minutes, 2nd stage annealing 3
The time was set to 0 minutes, and ferrite annealing by two-step annealing was performed.

FIG. 5 is a photograph (magnification: magnification) of the microstructures after ferritizing annealing (FIG. 4) of the example of the present invention and the comparative example.
100 times). In the comparative example (FIG. 5 (A)), the decomposition of cementite remained incompletely with the lowering of the temperature.
Example 1 (FIG. 5B) and Example 2 (FIG.
(C)), the cementite of Example 3 (FIG. 5 (D))
All are decomposed to show a complete ferrite structure. In addition, the number of spheroidal graphite particles is very large, and when Bi and Ca are added, the value is three times or more that of a standard product, and excellent mechanical properties can be expected.

Looking at the crystal grain size of the base ferrite phase, there is a clear difference between (A) and (B), (C) and (D) in FIGS. Small examples, large and evenly distributed fine spherical graphite on the crystal structure of the dense matrix, surpassing all properties desirable for metal materials such as strength, toughness, corrosion resistance, wear resistance, metal fatigue, etc. I foresee that. That is, despite the remarkable miniaturization of graphite due to the addition of Bi, or Bi or Ca, graphite was maintained as a spheroidal graphite cast iron in any of the examples, while maintaining a high degree of spheroidization comparable to that of the comparative example. The retention of the spheroidization rate of 90% or more suggests that it has extremely excellent mechanical properties (see Table 2).

[0035]

[Table 2]

[0036]

As described above, the present invention does not aim to prevent chilling in the as-cast state after the centrifugal casting in the production of ductile cast iron pipes. Based on the compositional basis, the crystallization of fine spheroidal graphite and the maintenance of fine primary austenite grain size, cementite is decomposed by annealing that has been shortened to about half that of conventional and widely used annealing based on this base. In addition, the increased number of spheroidal graphite and the base of fine secondary crystallized ferrite crystals have the effect of constructing a structure that surpasses the conventional technology in all aspects such as strength, toughness, corrosion resistance, wear resistance, and fatigue strength. This not only improves the quality of the manufactured ductile cast iron pipe itself to an ideal state, but also enables the shortening, simplification, and significant cost reduction of heat treatment-related processes. The effect on the world is immeasurable.

[Brief description of the drawings]

FIG. 1 (A) is a photograph of a microstructure in an as-cast state in Comparative Example 1 of the present invention.

FIG. 1 (B) is a photograph of a microstructure in an as-cast state in Example 1 of the present invention.

FIG. 1 (C) is a photograph of a microstructure in an as-cast state in Example 2 of the present invention.

FIG. 1 (D) is a photograph of a microstructure in an as-cast state in Example 3 of the present invention.

FIG. 2 is a heat treatment chart when the normal annealing time applied to the examples and the comparative examples of the present invention is reduced by half.

FIG. 3 (A) is a photograph of a microstructure after ferritizing annealing in Comparative Example 1 of the present invention.

FIG. 3 (B) is a photograph of a microstructure after ferritizing annealing in Example 1 of the present invention.

FIG. 3 (C) is a photograph of a microscope structure after ferritizing annealing in Example 2 of the present invention.

FIG. 3 (D) is a photograph of a microstructure after ferritizing annealing in Example 3 of the present invention.

FIG. 4 is a heat treatment chart when the first-stage annealing temperature applied to Examples and Comparative Examples of the present invention is lowered to 950 ° C.

FIG. 5 (A) is a photograph of a microstructure after ferritizing annealing in Comparative Example 1 of the present invention.

FIG. 5 (B) is a photograph of a microscope structure after ferritizing annealing in Example 1 of the present invention.

FIG. 5 (C) is a photograph of a microstructure after ferritizing annealing in Example 2 of the present invention.

FIG. 5 (D) is a photograph of a microstructure after ferritizing annealing in Example 3 of the present invention.

FIG. 6 is a partial cross-sectional front view illustrating a centrifugal casting method used in Examples of the present invention.

FIG. 7 is a heat treatment chart of an annealing pattern applied in a process of manufacturing a normal ductile cast iron pipe.

[Explanation of symbols]

 1 cupola furnace 2 ladle 3 mold 4 trough

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshisada Michiura 1-12-19 Kitahorie, Nishi-ku, Osaka-shi, Osaka Inside Kurimoto Iron Works Co., Ltd. (72) Inventor Mayoshi Kitagawa 1 Kitahorie, Nishi-ku, Osaka-shi, Osaka No. 12-19, Kurimoto Iron Works Co., Ltd. No. 1-12-19 Kitahorie Inside Kurimoto Iron Works Co., Ltd. (72) Inventor Akira Horie 10-67 Kitayuganasecho, Morioka-shi, Iwate F-term (reference) 4K014 BA03 BA13 BA16 BB06 BC12 BC13

Claims (10)

[Claims] 1. C: 3.0-4.0%, Si: 1.5-
3.0%, Mn: 0.1 to 0.4%, P: 0.05% or less, S: 0.01% or less, Bi: 0.0005 to 0 .05
%, And a large number of spheroidal graphites are crystallized on a ferrite-based matrix while maintaining a spheroidization ratio of 90% or more. 2. The method according to claim 1, wherein Ca:
A ductile cast iron tube formed by a centrifugal casting method, comprising 0.0001 to 0.05%, wherein a larger number of spheroidal graphites are crystallized on a ferrite-based matrix while maintaining a spheroidization ratio of 90% or more. 3. The method according to claim 1, wherein at least 1,000 spherical graphite / mm ² or more are crystallized on the ferrite-based matrix, and the spheroidization ratio is maintained at 90% or more. Ductile cast iron tube by centrifugal casting. 4. The spheroidal graphite according to claim 2, wherein the spheroidal graphite crystallized on the ferrite-based matrix is at least 2,000 / mm ² or more, preferably 3,000 / mm ² or more. Is maintained by 90% or more. 5. The molten metal component is C: 3.0-4.0%, S
i: 1.5 to 3.0%, Mn: 0.1 to 0.4%, P:
After melting and refining so that 0.05% or less, S: 0.01% or less, and the remaining Fe, graphite is spheroidized by adding a sphering agent mainly composed of Mg to the molten metal, and inoculation is performed. Inoculant is inoculated into molten metal and finally Bi is 0.0005-0.0
A method for producing a ductile cast iron pipe, characterized by adding a component in a range of 5% so that it can be clearly detected, centrifugally casting, solidifying, and then performing simplified ferritization annealing. 6. The molten metal component is C: 3.0-4.0%, S
i: 1.5 to 3.0%, Mn: 0.1 to 0.4%, P:
After melting and refining so that 0.05% or less, S: 0.01% or less, and the remaining Fe, graphite is spheroidized by adding a sphering agent mainly composed of Mg to the molten metal, and inoculation is performed. Inoculant is inoculated into molten metal and finally Bi is 0.0005-0.0
5%, Ca: A method for producing a ductile cast iron tube, characterized in that it is added so as to be clearly detected in the range of 0.0001 to 0.05%, centrifugally cast, solidified, and then subjected to simplified ferrite annealing. . 7. The method according to claim 5, wherein the addition of Bi includes chemical components of Si: 20 to 80%, Bi: 0.1 to 25.0%,
A method for producing a ductile cast iron pipe, comprising using an alloy powder or a mixed powder of the remaining Fe as an inoculant. 8. The method according to claim 5, wherein Bi, Ca
Is added when the chemical components are Si: 20-80%, Bi: 0.1
A method for producing a ductile cast iron pipe, comprising using an alloy powder or a mixed powder consisting of 〜25.0%, Ca: 1-40% and the balance of Fe as an inoculant. 9. The method according to claim 5, wherein the ferritizing annealing is performed at a lower temperature and / or with a shorter holding time than is applied in a normal ductile iron pipe manufacturing process. Manufacturing method of ductile cast iron tube. 10. The method for producing a ductile cast iron pipe according to claim 5, wherein the particle size of the inoculant when adding Bi and Ca is 0.05 mm to 3 mm.