JP6793541B2 - Spheroidal graphite cast iron pipe and method for manufacturing spheroidal graphite cast iron pipe - Google Patents

Description

The present invention relates to a spheroidal graphite cast iron pipe used for a water pipe or the like, and a method for producing the spheroidal graphite cast iron pipe.

General spheroidal graphite cast iron (hereinafter referred to as ductile cast iron) includes high toughness types such as JIS standard FCD350, FCD400 and FCD450, and high strength types such as FCD600, FCD700 and FCD800. For ductile cast iron pipes mainly cast for water pipes, FCD450 (tensile strength 450 MPa or more, elongation 10% or more) having a relatively good balance between strength and elongation is selected. On the other hand, high-hardness wear-resistant materials are used for transporting slurry-like substances and hard materials with high wear resistance, and high-strength and high-bearing materials are used as materials for automobile parts and construction machinery parts. Each is selected.

For example, pearlite is the main component of the matrix (base) of the as-cast structure of ductile cast iron pipes (straight pipes) cast by centrifugal casting of dies, and since the cooling rate in this centrifugal casting of dies is high, stable graphite can be used. In addition, a mottled structure in which a large amount of semi-stable cementite is crystallized at the same time is formed. Since this cementite becomes an inhibitory factor for elongation, annealing for the purpose of decomposition of cementite and ferrite formation of the matrix is required in order to achieve both the strength and elongation required for the FCD450 type.

Annealing of ductile cast iron pipes is generally performed in a continuous annealing furnace. In this continuous annealing furnace, the ductile cast iron pipe is heated above the austenitizing temperature range (870 ° C. or higher). This completely decomposes cementite and austenites the base tissue. The decomposition of cementite depends on the treatment temperature and the treatment time, and the higher the treatment temperature, the shorter the treatment time, while the lower the treatment temperature, the longer the treatment time. In this continuous annealing furnace, it is often difficult to control the temperature uniformly in the furnace. Therefore, it is necessary to determine the treatment temperature and the treatment time in order to surely austenite the cementite.

After the austeniticization of the matrix structure is completed, in order to precipitate ferrite from this austenite, the temperature range near the eutectoid transformation point (about 680 to 750 ° C.) is maintained for a certain period of time, or the vicinity of this eutectoid transformation point is slowly cooled. Perform heat treatment. The amount of ferrite precipitated is determined by the holding time and cooling rate at this time. That is, the longer the holding time or the lower the cooling rate, the larger the ferrite precipitation amount, while the shorter the holding time or the higher the cooling rate, the smaller the ferrite precipitation amount, and the matrix is mainly composed of pearlite. Become.

When a continuous annealing furnace is used in this heat treatment, it is difficult to finely control the amount of ferrite and pearlite by strictly controlling the temperature, so basically annealing is performed under the condition that ferrite is the main component, and the toughness is increased. We are trying to secure it.

When high-strength types such as FCD600, FCD700, and FCD800 are required for ductile cast iron pipes, it is necessary to make the matrix pearlite. Since it is difficult to perform this pearlite formation only by controlling the heat treatment conditions, it is common to add trace elements such as Mn, Cr, Cu, and Sn that promote pearlite formation.

When this ductile cast iron pipe is used for transporting a slurry-like substance such as an ore slurry or a calcareous slurry, or a hard substance having high wear resistance, it has excellent wear resistance (for example, Vickers hardness of 200 Hv or more). Required. Further, in recent years, since it is expected to be used in a high temperature environment such as piping of a thermal power plant and a coal boiler device, material properties capable of maintaining sufficient hardness even at a high temperature are required.

Generally, this wear resistance is improved by increasing the hardness. As described above, in ductile cast iron pipes, special heat treatment such as tempering that requires strict temperature control is difficult. Therefore, for example, Ni is added or Ni is added as shown in Patent Documents 1 to 4. , Mo may be added to improve the matrix, and a method for improving the hardness may be adopted.

Japanese Patent No. 3823347 Japanese Patent No. 5282547 Japanese Patent No. 5589646 Japanese Patent No. 5712525

In the configuration according to each patent document, when the improved matrix forms a structure such as redebrite or martensite that is prone to phase transformation at high temperature, there is a concern that the hardness may decrease due to the phase transformation. As described above, since the wear resistance and the hardness are related, the wear resistance decreases as the hardness decreases. Therefore, there is a problem that it is difficult to apply a material having a structure that may cause a phase transformation at a high temperature to a high temperature.

Therefore, an object of the present invention is to maintain the hardness of a ductile cast iron pipe in a high temperature region.

In order to solve the above problems, the present invention has C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr in% by weight. : 0.01 to 0.20%, Mn: 1.20 to 1.70%, Cu: 0.60 to 1.20%, and the balance is from Fe and unavoidable impurities. Therefore, a spheroidal graphite cast iron pipe having an area ratio of pearlite in the matrix structure after annealing of 80% or more and an area ratio of undecomposed cementite in the range of 10 to 15% was constructed.

Here, the area ratio of pearlite means the ratio (%) occupied by the area of pearlite when the area of the matrix in the field of view of a predetermined size is 100%, and the area ratio of cementite is a predetermined ratio. It refers to the ratio (%) of the area of cementite when the total area of the field of view is 100%.

Next, the reason why the content of each alloying element is limited to the above range will be described.

C was contained at least 3.20% in order to secure the amount of graphite and castability (fluidity of molten metal) required for the present invention. On the other hand, if the content is too high, graphite crystallization becomes excessive and high strength cannot be obtained. Therefore, the upper limit is set to 4.00%.

Si was contained at least 1.40% in order to ensure the action of increasing the fluidity of the molten metal and the action of promoting the crystallization of graphite. On the other hand, if the content is too high, the crystallization of graphite becomes excessive and the effect of suppressing the pearlite formation of the matrix structure becomes large, so that high strength cannot be obtained and the outer surface of the product becomes rough such as pinholes. The upper limit is set to 3.00% because it is easy to do.

Mg is an element necessary for spheroidizing graphite, and it is contained at least 0.02% in order to fully obtain the effect. On the other hand, if the content is too high, the effect is not improved so much, so the upper limit is set to 0.08%.

Cr is usually inevitably contained in an amount of 0.01% or more, but if the content is 0.20% or less, the effect is small.

Mn is an element effective for fixing S to make it harmless, allowing pearlite to exist stably, and improving the strength of pearlite, and in order to secure a predetermined hardness while sufficiently obtaining the effect. It was made to contain at least 1.20%. On the other hand, if the content is too high, the residual cementite becomes remarkable and the strength and elongation are lowered, so the upper limit is set to 1.70%.

Like Mn, Cu is an element effective for the stable presence of pearlite, and is contained at least 0.60% in order to secure a predetermined hardness while sufficiently obtaining the effect. On the other hand, even if the content is increased more than necessary, the effect is limited, so the upper limit is set to 1.20%.

In addition to the above alloying elements, unavoidable impurities such as P and S are contained, but the smaller the content, the better. For example, P is preferably 0.08% or less, and S is preferably 0.015% or less.

As described above, by containing each alloying element within the above concentration range, particularly Mn and Cu that stably present the pearlite structure within the above concentration range, a sufficient pearlite area ratio (80% or more) is obtained. At the same time, a spheroidal graphite cast iron pipe in which the area ratio of undecomposed cementite is within a predetermined area ratio (10 to 15%) can be obtained. The spheroidal graphite cast iron pipe cast by adjusting the concentration of each alloy element in this way does not require special heat treatment such as quenching and tempering, and only relatively simple annealing heat treatment is required, and only tensile strength and strength are required. Hardness that can exhibit sufficient abrasion resistance (for example, Vickers hardness of 200 Hv or more) is given. Moreover, since structures such as redebrite and martensite that are prone to phase transformation are not formed at high temperatures, the hardness can be maintained even in a high temperature environment of 400 to 500 ° C, and reliability in a high temperature environment is greatly improved. be able to.

In each of the above configurations, it is preferable that the graphite crystallized in the matrix structure is in a finely divided state.

By setting the size to a fine size (for example, 15.0 μm or less) in this way, it is possible to construct a spheroidal graphite cast iron pipe having high strength and high yield strength while ensuring sufficient wear resistance.

The method for producing a spheroidal graphite cast iron pipe according to the present invention is, in terms of weight%, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08. %, Cr: 0.01 to 0.20%, Mn: 1.20 to 1.70%, Cu: 0.60 to 1.20%, and the balance is Fe and unavoidable. A semi-finished product having a predetermined shape was cast at a cooling rate of 2.0 to 8.0 ° C./sec using a molten metal composed of target impurities, and the semi-finished product was held in a temperature range of 900 to 1100 ° C. for 5 to 30 minutes. After that, it is cooled at a cooling rate of 1 to 8 ° C./min to produce a spheroidal graphite cast iron tube.

As described above, by setting the content range of each alloying element as described above, it is possible to manufacture a spheroidal graphite cast iron tube having both high strength and high yield strength while ensuring sufficient wear resistance. .. Moreover, since the above heat treatment does not require strict temperature control, the heat treatment can be performed using a general continuous annealing furnace.

In this production method, when the molten metal is poured into a mold, an Fe-Si-based inoculant containing 45 to 75% by weight of Si is inoculated by pouring 0.1 to 0.5% by weight. Is preferable.

By doing so, the number of graphite grains crystallized in the matrix structure can be increased, and a higher yield strength can be obtained more reliably.

According to the present invention, by containing Mn and Cu in a molten metal of spheroidal graphite cast iron within a predetermined concentration range, hardness sufficient to exhibit wear resistance without performing special heat treatment (for example, Vickers). It is possible to construct a spheroidal graphite cast iron pipe having high strength and strength while ensuring a hardness of 200 Hv or more). Moreover, since the structure does not form a structure such as redebrite or martensite that easily undergoes phase transformation at high temperatures, its hardness can be maintained even in a high temperature environment of 400 to 500 ° C, and reliability in a high temperature environment can be maintained. Can be greatly increased.

Diagram showing the results of the high temperature Vickers test

Prior to the characteristic evaluation experiment of the ductile cast iron pipe (spheroidal graphite cast iron pipe) according to the present invention, the ductile cast iron pipe according to the embodiment of the present invention was cast. A wear-resistant steel pipe was prepared as a comparative example with respect to this example. Table 1 shows the chemical components of the molten ductile cast iron according to Examples and Comparative Examples (the balance omitted from this table is Fe and unavoidable impurities such as P and S). The chemical composition data shown in Table 1 are values obtained by analyzing a white pig iron sample prepared from each molten metal with an emission spectroscopic analyzer.

In the ductile cast iron pipe according to this embodiment, each molten metal of the chemical components shown in Table 1 is poured into a cylindrical mold of a mold centrifugal casting apparatus at 1300 ° C., and a tubular half having a wall thickness of 12.0 mm. The product (as-cast pipe) was cast. At the time of this pouring, a Fe-Si-based inoculant containing 45 to 75% by weight of Si was inoculated by pouring 0.1 to 0.5% by weight. The cooling rate during this casting was about 4.0 to 6.0 ° C./sec. This cooling rate varies depending on the shape of the mold, the amount of pouring water, and the wall thickness of the pipe, but it is often within the range of about 2.0 to 8.0 ° C./sec.

Next, this semi-finished product was annealed under the following annealing conditions to complete a ductile cast iron pipe as a product.
(Annealing condition)
・ Heating temperature: 900 to 1100 ° C
・ Heating holding time: 5 to 30 minutes ・ Cooling rate: 1 to 8 ° C / min

As a comparative example with respect to the above example, a martensitic wear-resistant steel pipe in which martensite was crystallized by a special heat treatment was adopted.

From the base material according to Examples (Samples 1 to 3), test pieces used for strength test and hardness test were prepared. Table 2 shows the results of each test of tensile strength, proof stress, and Brinell hardness performed at room temperature (30 ° C.) for each test piece. Table 2 also shows the respective characteristic ranges of the tensile strength, proof stress, and Brinell hardness of the wear-resistant steel pipe as a comparative example. From this result, it was confirmed that each of the samples 1 to 3 according to the examples was equal to or higher than the comparative example in all the results of tensile strength, proof stress, and Brinell hardness.

Next, using these samples, the Vickers hardness was measured under the following test conditions.
(Test condition)
-Test temperature: Room temperature (30 ° C), 200 ° C, 300 ° C, 400 ° C, 500 ° C (5 levels)
・ Load: 20 kg
・ Time: 30 seconds ・ Indenter: Diamond ・ Atmospheric gas: Argon

The results of the Vickers test at each temperature are shown in FIG. The Vickers hardness of Samples 1 to 3 according to Examples gradually decreased due to the softening of the material as the test temperature increased, but maintained a hardness of 200 Hv or more even in a high temperature region of 400 to 500 ° C. This is because the matrix structure of Samples 1 to 3 is pearlite, so that phase transformation does not occur even at a high temperature of about 500 ° C., and the structure does not soften due to this phase transformation. By ensuring sufficient hardness in the high temperature region in this way, reliability in a high temperature environment can be significantly improved.

On the other hand, the Vickers hardness of the sample according to the comparative example gradually decreased in the temperature range of 300 ° C. or lower, as in the samples 1 to 3 according to the example. However, when the temperature exceeds 300 ° C, the Vickers hardness drops sharply (see the inclination of the graph of the wear-resistant steel pipe in FIG. 1), and it falls below 200 Hv at a high temperature of 500 ° C, which is a large hardness as compared with the examples. There was a decline. It is considered that this is because the martensite layer crystallized in the structure of the wear-resistant steel pipe begins to soften in a temperature range of about 300 ° C. or higher.

As described above, the content of each additive element in the molten metal, particularly the content of Mn and Cu, is within a predetermined range (Mn: 1.20 to 1.70%, Cu: 0.60 to 1.20%). By setting the area ratio of pearlite in the base structure after annealing to 80% or more and the area ratio of undecomposed cementite to 10 to 15%, no special heat treatment is performed on the as-cast product. In addition, a spheroidal graphite cast iron tube having sufficient hardness in a high temperature region of 400 to 500 ° C. can be constructed.

In the above embodiment, Fe-Si-based inoculation agent was used, but Bi-based inoculation containing 0.5 to 5.0% by weight of Bi and 45 to 75% by weight of Si, respectively. Agents can also be used. In addition, although these inoculants are used to crystallize more graphite, it is permissible to omit the use of the inoculants as long as the required yield strength is ensured.

Claims (4)

By weight%, it contains C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20%. Further, Mn: 1.20 to 1.70% and Cu: 0.60 to 1.20% are contained, and the balance is composed of Fe and unavoidable impurities, and the area ratio of pearlite in the matrix structure after annealing. Is 80% or more, and the area ratio of undecomposed cementite is in the range of 10 to 15%. The spheroidal graphite cast iron pipe according to claim 1, wherein the graphite crystallized in the matrix structure is refined to 15.0 μm or less . By weight%, it contains C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20%. Further, using a molten metal containing Mn: 1.20 to 1.70% and Cu: 0.60 to 1.20% and the balance being Fe and unavoidable impurities, the cooling rate is 2.0 to 2. A spherical shape in which a semi-finished product having a predetermined shape is cast at 8.0 ° C./sec, the semi-finished product is held in a temperature range of 900 to 1100 ° C. for 5 to 30 minutes, and then cooled at a cooling rate of 1 to 8 ° C./min. A method for manufacturing a ductile iron tube. The spherical shape according to claim 3, wherein when the molten metal is poured into a mold, an Fe-Si-based inoculant containing 45 to 75% by weight of Si is inoculated by pouring 0.1 to 0.5% by weight of the molten metal. A method for manufacturing a ductile iron tube.

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