JPH06246415A - Centrifugal casting of tough ductile cast iron tube - Google Patents

Abstract

PURPOSE:To provide the centrifugal casting method capable of obtaining a ductile cast iron tube having high strength/elongation and excellent toughness even though a tube thickness is thin. CONSTITUTION:A ductile cast iron molten metal is produced by adding graphite spherodizer in the source molten metal melted by melting furnace, subjecting to inoculation on the molten metal and casting into the mold for centrifugal casting, a ductile cast iron tube is centrifugally cast. At this time, the treatment for spherodizing graphite is executed with using Mg alloy containing a rare metal element, a ductile cast iron molten metal, which contains, by weight, 2.1-2.5% Si, 0.04-0.06% Mg in the case of using the source molten metal melted by cupola (or 2.6-2.8% Si, 0.03-0.06% Mg in the case of using the source molten metal melted by electric furnace), is produced, the molten metal is subjected to inoculation to the molten metal and is cast into the mold for centrifugal casting having a powder layer of inoculant on its inside surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a centrifugal casting method for a ductile cast iron pipe having not only high strength but also toughness.

[0002]

2. Description of the Prior Art Ductile cast iron pipes are used as various pipe materials such as water supply and drainage pipes and underground pipes because they have high strength and generally excellent corrosion resistance. If further corrosion resistance is required, the inner and outer surfaces of the pipe are lined with a corrosion-resistant material such as synthetic resin.

In recent years, there has been a demand for a reduction in piping space and a buried hole, and a high-strength pipe having a thin pipe thickness has been demanded. For this reason, the mold centrifugal force cast by ductile cast iron (FCD400) having a tensile strength of 400 N / mm ² or more is conventionally used to obtain the F having a tensile strength of 600 N / mm ² or more.
Casting has started in the CD600 class. As a result, if the inner diameter is the same, the wall thickness will be
It could be reduced to about 1/2.

Conventionally, centrifugal force casting of ductile cast iron pipe is
Pure Mg in the original hot water melted by cupola or electric furnace
Graphite spheroidizing agent is added, and C: 3.2 to 3.8 by weight%
%, Si: 2.0 to 2.4%, Mg: 0.06 to 0.0
It was manufactured by smelting a ductile cast iron molten metal containing 8% and casting it into a centrifugal force casting mold.
When the pipe thickness is thin, chill (free cementite) easily enters during casting, so when pouring the molten metal, that is, pouring to receive the molten metal supplied from the pouring ladle (so-called triangular ladle). Gutter mouth (shoot)
Alternatively, at the outlet portion (spout portion) of the pouring trough, an inoculant such as Fe—Si powder is added to the molten metal to suppress chill formation.

[0005]

As described above, although chilling is suppressed by inoculation during centrifugal casting, the amount of pearlite is relatively large and the tube thickness is thin. Although a layered cementite is formed in the center of the pipe thickness and sufficient tensile strength is obtained, the elongation is low at a few percent, resulting in poor toughness and cracking when an unexpected load is applied. However, there is a problem of poor durability.

The present invention has been made in view of the above problems, and an object thereof is to provide a centrifugal casting method capable of obtaining a ductile cast iron pipe having high strength and elongation and excellent toughness even if the pipe thickness is thin. And

[0007]

[Means for Solving the Problems] The present invention is to melt a ductile cast iron melt by adding a graphite spheroidizing agent to a base melt melted in a cupola and inoculate the melt to obtain a centrifugal casting mold. A cast ductile iron pipe having toughness is subjected to a graphite spheroidizing treatment using a Mg alloy containing a rare earth element in a centrifugal casting method, and Si: 2.1 to 2.5% by weight, M
A molten ductile cast iron containing g: 0.04 to 0.06% is melted, inoculated into the molten metal, and cast into a centrifugal force casting mold having a powder layer of an inoculant formed on the inner surface.

When the hot water melted in an electric furnace is used, a spheroidizing treatment of graphite is performed using a Mg alloy containing a rare earth element, and Si: 2.6 to 2.8% by weight% and Mg: 0.0
Molten ductile cast iron containing 3 to 0.06% is melted, inoculated and cast into a centrifugal casting mold having a powder layer of an inoculant formed on the inner surface.

[0009]

Since the Mg alloy containing a rare earth element is used as the graphite spheroidizing agent, the spheroidizing of graphite is promoted by the action of the rare earth element, and the amount of Mg required for spheroidizing can be reduced to 0.06% or less. Since Mg is a white pig-making element,
By reducing the Mg content, generation of free cementite can be suppressed.

When a melt of the original hot water is dissolved in a cupola, the chemical composition of the molten ductile cast iron used for casting is Si: 2.1 to 2.5% by weight and Mg: 0.0
It is adjusted to 4 to 0.06%. Si is added to secure the flowability of molten metal, accelerate graphitization, and suppress the formation of pearlite. If it is less than 2.1%, such an effect is too small and the elongation is lowered, so that the toughness is deteriorated. on the other hand,
If it exceeds 2.5%, the amount of ferrite increases and the strength decreases.

Although Mg is added for spheroidizing graphite, as described above, in the present invention, since a Mg alloy containing a rare earth element is used as a graphite spheroidizing agent, Mg is conventionally used.
The content can be reduced. If it is less than 0.03%, the spheroidizing effect of graphite is insufficient, while if it exceeds 0.06%, the amount of cementite produced increases and the elongation decreases. When melted original hot water is melted in an electric furnace, coke is not used for melting, so desulfurization treatment is not necessary, but chill is likely to occur. For this reason, it is necessary to suppress the generation of pearlite and further promote graphitization, so compared to the case of using the original hot water dissolved in cupola,
The amount of Si is increased to 2.6 to 2.8%, while the amount of Mg is decreased to 0.03 to 0.04.

Regarding C, a normal content may be used,
Typically, C: about 3.2 to 3.8%. Regarding other impurities, it is preferable to limit the impurities to the following ranges so as not to deteriorate the toughness. Mn: 0.50% or less, S: 0.012% or less, P
: 0.055% or less, Cu: 0.12% or less, Cr: 0.
12% or less, Sn: 0.015% or less, Ti: 0.050%
In the present invention below, in the casting of the ductile cast iron molten metal, as in the conventional case, the shape and number of the spheroidal graphite are adjusted, the generation of cementite is suppressed, and inoculation is performed to promote the ferritic formation, but the mold inner surface or A powder layer of the inoculant is previously formed on the inner surface of the coating mold formed on the inner surface of the mold, and after the casting, inoculation is also performed by this powder layer. The lower the temperature of the molten metal, the greater the effect of inoculation, but since the temperature of the molten metal cast in the mold is lower, it is effectively inoculated by the powder layer and has greater strength and elongation, that is, toughness after casting. Excellent ductile cast iron can be obtained.

[0013]

EXAMPLES In the present invention, rare earth elements are used as a graphite spheroidizing agent when performing a graphite spheroidizing treatment on a ductile cast iron raw water melted in a cupola or an electric furnace (after desulfurization treatment in the case of cupola melting). The Mg alloy containing is added, specifically, the Mg alloy such as pure Mg or Fe-Si-Mg and the alloy of rare earth element (RE) such as misch metal are simultaneously added, or the Mg alloy containing rare earth element is added. May be added. The content (usage) of the rare earth element may be about 25 to 50% with respect to the amount of Mg. The amount of the graphite spheroidizing agent used is about 0.2 to 1% as Mg content based on the weight of the molten metal. When the Mg alloy is used, the yield of Mg is better than when pure Mg is used.
% Is enough.

The ductile cast iron melt subjected to the spheroidization of graphite is inoculated when it is cast into a centrifugal force casting mold. The inoculation at the time of casting may be carried out at any of the pouring ladle (triangular ladle), the chute portion and the spout portion of the pouring gutter, or at two or more locations. As an inoculant, usually Fe
Although -Si powder is used, when using the hot water in which the cupola is melted, Fe-Si-Mg alloy powder containing several% of Mg may be used in the pouring ladle and the chute part. The particle size of the powder is about 35 to 150 mesh.

The amount of the inoculant used in the case of using the cupola-dissolved hot water is 0.
It may be about 2 to 0.5%. For example, in the pouring ladle, the Si content is 1% or less, and in the chute part, 50% Si-3% Mg-Fe.
Alloy powder 0.04 to 0.08%, 75 in spout
% Si—Fe alloy powder is used in an amount of 0.1 to 0.15%. When melted in an electric furnace is used as the hot water, the tendency of chilling is larger than that in melted cupola, so the amount of inoculation is increased, and the total amount is preferably 1.1 to 1.4% in terms of Si content. For example, if a 75% Si-Fe alloy is used, 1.
Use about 5 to 1.8%.

A coating mold is usually formed on the inner surface of the centrifugal casting mold to be used for casting in order to protect the inner surface of the mold and to prevent rapid cooling of the casting. A powder layer is formed directly on the inner surface of the mold by an inoculant such as Fe-Si powder. The role of the powder layer is to effectively inoculate the low-temperature molten metal, improve the heat retaining property, and, in some cases, substitute the coating type. The powder layer is easily formed by rotating the mold and spraying the powder on its inner surface. The particle size of the powder is slightly smaller than that of the inoculant (65 to 150 mesh) so that it can be easily dissolved in the molten metal whose temperature has been lowered. Two
00 g / m ² or more is preferable. However, if it is too much, it will be difficult to melt it, and the casting surface will be deteriorated, so it is preferable to keep it at about 400 g / m ² or less.

Next, specific examples will be given. Example A (1) After performing desulfurization treatment on the hot water melted by cupola, graphite spheroidizing agent containing RE was added in Examples and Comparative Examples, and pure Mg was added in the conventional example to make graphite spheroidized. Treatment is carried out to produce a molten ductile cast iron having the chemical composition shown in Table 1, and the inoculation conditions described in the same table using this molten metal,
Diameter 800 mm, tube thickness 10 mm under mold coating conditions
This ductile cast iron pipe was cast by centrifugal force. Casting temperature is 13
The mold rotation speed is GNo. 35 at 00 to 1320 ° C. In the table, Nos. 1 and 2 are Examples, Nos. 3 to 5 are Comparative Examples, and Nos.
6 and 7 are conventional examples.

The graphite spheroidizing agent containing RE used was 5
0% Si-3.5% Mg-1.7% RE-Fe alloy, the inoculant is 50% Si-Fe alloy in the triangular ladle, 50% Si-3% Mg-Fe alloy in the chute part, spout 75% Si-Fe alloy powder (grain size 65-150)
Mesh). The coating mold was formed on the inner surface of the mold by wet spray coating using diatomaceous earth slurry to a thickness of 1.0 to 1.2 mm. Powder layer is 50% Si
-Fe alloy powder was sprinkled at 300 g / m < ² >.

[0019]

[Table 1]

(2) After solidification of the molten metal, rotation of the die was stopped, the tube was cooled in the die, and then the tube was taken out of the die. A tensile test piece was sampled from this as-cast tube and a tensile test was performed. The results are also shown in Table 1. (3) From the test results, in Nos. 1 and 2 of the examples, the tensile strength is as high as 660 N / mm ² or more and the elongation is 11
% Or more, which shows that the toughness is excellent. On the other hand, in Comparative Examples Nos. 4 and 5 in which the Mg content was within the present invention and the powder layer was not formed on the inner surface of the mold even though the graphite-containing spheroidizing agent containing RE was used. Although it has high strength, the elongation is only 8% and the toughness is insufficient.

On the other hand, in the conventional examples Nos. 6 and 7, the Mg content was higher than that of the present invention, the graphite spheroidizing agent did not contain RE, and the powder layer was not formed, so that the strength was very high. Indicates an elongation of 4% or less, which is inferior in toughness. Example B (1) Graphite spheroidizing agent containing RE was added to the original hot water melted by an electric furnace (cupola for the conventional example) for the examples and comparative examples, and pure Mg was added for the conventional example to form the graphite spheroid. Chemical treatment to melt the ductile cast iron melt having the chemical composition shown in Table 2, and use this melt under the inoculation conditions and mold coating conditions described in the same table, the bore diameter φ450 mm, and the pipe thickness 7
A mm-sized ductile iron pipe was centrifugally cast. Casting temperature is 1
The mold rotation number is GNo. 32 at 300 to 1320 ° C.
In the table, Nos. 11 and 12 are examples, Nos. 14 to 15 are comparative examples, and Nos. 16 and 17 are conventional examples. The inoculant used was only 75% Si-Fe alloy powder. Graphite spheroidizing agent,
The powder layer and coating type are the same as in Example A.

[0022]

[Table 2]

(2) In the same manner as in Example A, a tensile test piece was taken from the as-cast pipe and a tensile test was carried out. The results are also shown in Table 2. (3) From the test results, it is understood that in Nos. 11 and 12 of the examples, the tensile strength is as high as 600 N / mm ² or more, and the elongation is 13% or more, which is excellent in toughness. On the other hand, even though the graphite spheroidizing agent containing RE was used and the powder layer was used on the inner surface of the mold, in Comparative Examples Nos. 13 to 15 in which the Mg or Si content was out of the range of the present invention. Some have good elongation, but generally have low strength and insufficient toughness. On the other hand, in Nos. 16 and 17 of the conventional example, similar to Example A, the strength is very high, but the elongation is 4.5% or less, and it is understood that the toughness is poor.

[0024]

As described above, according to the centrifugal force casting method for a tough ductile cast iron pipe of the present invention, the graphite spheroidizing treatment is performed using the Mg alloy containing the rare earth element, and the cupola melted original hot water is used. If it is, Si: 2.1 to 2.5%,
In the case of using the original hot water melted in an electric furnace by melting molten ductile cast iron containing Mg: 0.04 to 0.06%, Si:
Centrifugal casting in which a melt of ductile cast iron containing 2.6 to 2.8% and Mg: 0.03 to 0.04% is melted, and the melt is inoculated to form a powder layer of an inoculant on the inner surface. Since it is cast into a mold for casting, the production of pearlite and the production of free cementite are suppressed even when the pipe thickness is thin, and FCD6
It is possible to easily obtain an as-cast iron pipe having a strength of 00 or more and a large elongation and excellent toughness.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 37/04 E

Claims (2)

[Claims] 1. A graphite spheroidizing agent is added to a base metal melted in a cupola to melt a ductile cast iron melt, the melt is inoculated and cast into a centrifugal casting mold, and the toughness is as-cast. In a centrifugal casting method to obtain a ductile cast iron pipe having: a graphite spheroidizing treatment is performed using a Mg alloy containing a rare earth element, and Si: 2.1 to 2.5% by weight% and Mg: 0.04
Tough ductile, characterized in that a molten ductile cast iron containing 0.06% to 0.06% is melted, and the molten metal is inoculated and cast into a centrifugal force casting mold having a powder layer of an inoculant formed on the inner surface. Centrifugal casting method for cast iron pipe. 2. A ductile cast iron molten metal is melted by adding a graphite spheroidizing agent to the original molten metal melted in an electric furnace, and the molten metal is inoculated and cast into a centrifugal force casting mold, which is tough in the as-cast state. In a centrifugal casting method for obtaining a ductile cast iron pipe having properties, graphite spheroidizing treatment is performed using a Mg alloy containing a rare earth element, and Si: 2.6 to 2.8% by weight% and Mg: 0.03
Tough ductile, characterized in that a molten ductile cast iron containing 0.04% to 0.04% is melted, and the molten metal is inoculated to be cast into a centrifugal force casting mold having a powder layer of an inoculant formed on the inner surface. Centrifugal casting method for cast iron pipe.