JP6328967B2 - Spheroidal graphite cast iron pipe and manufacturing method of spheroidal graphite cast iron pipe - Google Patents

Description

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

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

  For example, the main body of the as-cast structure matrix (base) of spheroidal graphite cast iron pipe (straight pipe) cast by mold centrifugal casting is pearlite, and because the cooling rate in this mold centrifugal casting is high, stable graphite In addition to this, a plaque structure is formed in which a large amount of metastable cementite is crystallized at the same time. Since this cementite becomes an obstructive factor for elongation, annealing for the purpose of decomposition of cementite and ferrite formation of the matrix is required in order to achieve both strength and elongation required for the FCD450 type.

  In general, the spheroidal graphite cast iron pipe is annealed in a continuous annealing furnace. In this continuous annealing furnace, the spheroidal graphite cast iron pipe is heated to an austenitizing temperature range or higher (870 ° C. or higher). As a result, cementite is completely decomposed and the base structure is austenitized. This decomposition of cementite depends on the processing temperature and processing time, and the processing time can be shortened as the processing temperature is higher, while the processing time is longer as the processing temperature is lower. In this continuous annealing furnace, uniform temperature control in the furnace is often difficult. For this reason, in order to make a cementite austenite reliably, it is necessary to determine processing temperature and processing time.

  When the austenite of the base 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 time, or the vicinity of this eutectoid transformation point is gradually cooled. Heat treatment is performed. The ferrite precipitation amount is determined by the holding time and the cooling rate at this time. That is, as the holding time is longer or the cooling rate is lower, the ferrite precipitation amount is increased. On the other hand, as the holding time is shorter or the cooling rate is higher, the ferrite precipitation amount is reduced, and the matrix is mainly pearlite.

  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. Therefore, annealing is basically performed under conditions mainly composed of ferrite. We are trying to secure it.

  When the spheroidal graphite cast iron pipe is required to have a high strength type such as FCD600, FCD700, and FCD800, the matrix needs to be made pearlite. Since it is difficult to perform this pearlite formation only by controlling the heat treatment conditions, it is common to add Mn, Cr, Cu, Sn, etc. that promote pearlite formation.

  When this spheroidal graphite cast iron pipe is used for transporting slurry-like materials such as ore slurry and calcareous slurry, hard materials having high wear properties, etc., characteristics excellent in wear resistance are required. Generally, this wear resistance is improved by increasing the hardness. When the spheroidal graphite cast iron pipe is used for the purpose of transporting a highly wearable substance, it is said that the hardness evaluated by Brinell hardness is desirably 350 HB or more.

  In order to increase the hardness of the spheroidal graphite cast iron pipe, it is conceivable to perform a special heat treatment such as quenching and tempering for the purpose of forming low carbon martensite. However, as described above, heat treatment using a continuous annealing furnace is difficult to strictly control temperature, and often cannot achieve a desired hardness. For this reason, a wear-resistant steel pipe that is said to be excellent in wear resistance is adopted, or, as shown in Patent Document 1, a method of improving the matrix by adding a rare metal such as Ni to improve hardness is adopted. There are things to do.

Japanese Patent No. 3823347

  Although the above-mentioned wear-resistant steel pipes are relatively inexpensive, many of them cannot achieve the hardness required to exhibit high wear resistance (Brinell hardness of 350 HB or more). In addition, the method of adding a rare metal such as Ni can achieve a desired hardness, but has a problem of increasing the material cost because it uses an expensive rare metal.

  Accordingly, an object of the present invention is to construct a spheroidal graphite cast iron pipe having high wear resistance without using an expensive rare metal.

  In order to solve the above-described problems, the present invention is based on 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: 2.50 to 3.00%, Cu: 0.80 to 2.00%, the balance from Fe and unavoidable impurities Thus, a spheroidal graphite cast iron pipe having an area ratio of pearlite in the base structure after annealing of 80% or more and an area ratio of undecomposed cementite in the range of 10 to 15% was formed.

  Here, the area ratio of pearlite means the ratio (%) of 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 area ratio. This means the percentage (%) of the cementite area when the entire area of the size field of view is 100%.

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

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

  Si is contained in an amount of at least 1.40% in order to ensure the effect of increasing the fluidity of the molten metal and the effect 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 and high strength cannot be obtained, and roughness such as pin holes occurs on the outer surface of the product Therefore, the upper limit was made 3.00%.

  Mn is an element effective for fixing S to be detoxified and stably presenting pearlite and improving the strength of pearlite. In order to secure a predetermined hardness while sufficiently obtaining the effect. It was made to contain at least 2.50%. On the other hand, if the content is too high, cementite remains remarkably and the strength and elongation decrease, so the upper limit was made 3.00%.

  Mg is an element necessary for spheroidizing graphite, and is contained at least 0.02% in order to sufficiently obtain the effect. On the other hand, if the content is too high, the improvement of the effect cannot be seen so much, so the upper limit was made 0.08%.

  Cu, like Mn, is an element effective for stably presenting pearlite, and is contained in an amount of at least 0.80% 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 was made 2.00%.

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

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

  Thus, each alloy element has a sufficient pearlite area ratio (80% or more) by containing Mn and Cu within the above concentration range, in particular, within the above concentration range, in which pearlite structure is stably present. At the same time, a spheroidal graphite cast iron pipe having an undecomposed cementite area ratio within a predetermined area ratio range (10 to 15%) can be obtained. Spheroidal graphite cast iron pipes cast by adjusting the concentration of each alloy element in this way do not require special heat treatments such as quenching and tempering, but only with a relatively simple annealing heat treatment, and only with tensile strength and proof stress. And a hardness capable of exhibiting sufficient wear resistance (Brinell hardness of 350 HB or more). In addition, since relatively inexpensive Mn and Cu are used instead of expensive rare metal Ni or the like, the manufacturing cost can be reduced.

  In each of the above structures, it is preferable that the graphite crystallized in the base structure is in a fine state.

  Thus, by making it a fine size (for example, 15.0 μm or less), it is possible to configure a spheroidal graphite cast iron pipe having both high strength and high proof stress while ensuring sufficient wear resistance.

  Moreover, the manufacturing method of the spheroidal graphite cast iron pipe which concerns on this invention is weight%, C: 3.20-4.00%, Si: 1.40-3.00%, Mg: 0.02-0.08. %, Cr: 0.01 to 0.20%, Mn: 2.50 to 3.00%, Cu: 0.80 to 2.00%, the balance being Fe and inevitable A semi-finished product having a predetermined shape was cast at a cooling rate of 2.0 to 8.0 ° C./second using a molten metal composed of mechanical impurities, and the semi-finished product was held in a temperature range of 900 to 1100 ° C. for 5 to 30 minutes. Thereafter, a spheroidal graphite cast iron pipe that is cooled at a cooling rate of 1 to 8 ° C./min is manufactured.

  As described above, by making the range of the content of each alloy element as described above, it is possible to produce a spheroidal graphite cast iron tube having both high strength and high yield strength while ensuring sufficient wear resistance. . In addition, 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 pouring the molten metal into a mold, an Fe-Si inoculum containing 45 to 75% by weight of Si is inoculated with 0.1 to 0.5% by weight of the molten metal. Is preferred.

  In this way, the number of graphite grains crystallized in the base structure can be increased, and high yield strength can be obtained more reliably.

  According to the present invention, the molten spheroidal graphite cast iron contains Mn and Cu within a predetermined concentration range, so that a sufficient hardness (with Brinell hardness) can be obtained without performing special heat treatment. It is possible to construct a spheroidal graphite cast iron pipe having high strength and proof stress while ensuring 350HB or more. Moreover, since relatively inexpensive Mn and Cu are used instead of expensive rare metals such as Ni, the material cost can be reduced.

The micrograph of the material structure of a spheroidal graphite cast iron pipe is shown, (a) is Example 1, (b) is Example 2, (c) is Example 3. The apparatus structure of an abrasion test is shown, (a) is a sectional view seen from the front of the entire apparatus, and (b) is a front view of the test piece. Figure showing the results of the wear test

  Prior to the characteristic evaluation experiment of the spheroidal graphite cast iron pipe according to the present invention, a spheroidal graphite cast iron pipe serving as an example of the present invention was cast. As a comparative example for this example, a wear-resistant steel pipe and a Ni-based ductile pipe were prepared. Table 1 shows the chemical components of the spheroidal graphite cast iron melts according to the examples and comparative examples (the remainder not shown in this table is inevitable impurities such as Fe, P, and S). The chemical composition data shown in Table 1 is a value obtained by analyzing a white birch sample prepared from each molten metal with an emission spectroscopic analyzer.

  In the spheroidal graphite cast iron pipe according to this example, each molten metal having the chemical composition shown in Table 1 is poured into a cylindrical mold of a mold centrifugal casting apparatus at 1300 ° C., and the tubular thickness is 12.0 mm. Semi-finished product (as-cast pipe) was cast. In the case of this pouring, 0.1-0.5 wt% pouring was inoculated with an Fe-Si inoculum containing 45 to 75 wt% Si. The cooling rate at the time of casting was about 4.0 to 6.0 ° C./second. The cooling rate varies depending on the shape of the mold, the amount of pouring, and the thickness of the pipe, but often falls within the range of about 2.0 to 8.0 ° C./second.

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

  Test specimens were collected from each of the cast pipes obtained in this way, and each was subjected to structure observation after corrosion treatment with 5% nital liquid and measurement of mechanical properties (tensile strength, yield strength, hardness). did. FIG. 1 shows the result of tissue observation with a microscope, Table 2 shows the result of image analysis of the tissue surface, and Table 3 shows the result of measurement of mechanical properties. Image analysis is performed at the center of the tube in the thickness direction.

  As shown in Table 2, by setting the contents of Mn and Cu within a predetermined range (Mn: 2.50 to 3.00%, Cu: 0.8 to 2.0%), a high pearlite area ratio (95% or more) and a cementite area ratio within a predetermined range (10 to 15%) could be secured. If the pearlite area ratio is 80% or more, sufficient strength can be ensured.

  Here, the area ratio of pearlite means the ratio (%) of 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 or graphite is The ratio (%) of the area of cementite or graphite when the total area of the field of view of a predetermined size is 100%. In addition, regarding graphite, measurement is performed except for particles having a particle size of 3 μm or less.

  In addition, as shown in Table 3, by setting the contents of Mn and Cu within the above predetermined range, a high tensile strength (equivalent to FCD700 or higher) of 700 MPa or higher and a high yield strength (equivalent to FCD450 or higher) are ensured. However, it was confirmed that a high hardness of 350 HB or more can be achieved in the Brinell hardness (see Examples 1 to 3). It can be said that the tensile strength and proof stress are superior to those of wear-resistant steel pipes and Ni-based ductile pipes, or are inferior to each other. Further, the Brinell hardness is significantly higher than that of the wear-resistant steel pipe, and it can be said that the Brinell hardness is comparable to that of a Ni-based ductile pipe using an expensive rare metal.

  Next, an abrasion test was performed using these samples. The configuration of the test apparatus used for this wear test is shown in FIG. In this test apparatus, a mixture 2 in which water and silica sand are mixed is placed in a test container 1, and a rod-like shape (length L 70 mm, diameter D 8.0 mm) is provided in the vicinity of the outer periphery of the mixture 2 as shown in FIG. The disk 4 to which the processed test piece 3 is attached is horizontally embedded, and the disk 4 is rotated around the rotation axis 5. In this test, the disk was rotated at a peripheral speed r = 4 m / second (speed near the outer periphery of the disk) for 360 hours.

  The results of this wear test are shown in FIG. The wear weight (%) on the vertical axis of the graph means the ratio of the decrease in the weight after the test to the weight before the test of each test piece. Although the wear weight of the test pieces according to Examples 1 to 3 is slightly higher than that of the Ni-based ductile pipe, it can be confirmed that the wear weight is superior to that of a commonly used wear-resistant steel pipe. It was.

  As described above, the content of each additive element in the molten metal, particularly the contents of Mn and Cu are within a predetermined range (Mn: 2.50 to 3.00%, Cu: 0.80 to 2.00%). No special heat treatment is performed on the as-cast product 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%. In addition, a spheroidal graphite cast iron pipe having high hardness and excellent wear resistance can be formed without using an expensive rare metal such as Ni.

  In the above embodiment, the Fe-Si type inoculum was used as the inoculum, but Bi type inoculum containing 0.5 to 5.0% by weight of Bi and 45 to 75% of Si, respectively. Can also be used. In addition, these inoculants are used to crystallize more graphite, but it is acceptable to omit the use of the inoculants as long as necessary proof stress is ensured.

1 Test container 2 Mixture 3 Test piece 4 Disk 5 Rotating shaft

Claims (4)

In mass %, 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: 2.50~3.00%, Cu: contains at 0.80 to 2.00% scope, balance of Fe and unavoidable impurities, the area ratio of pearlite in groups ground weave 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 graphite crystallized in the matrix structure is refined to 15.0 µm or less . In mass %, 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: 2.50 to 3.00%, Cu: 0.80 to 2.00% contained in the range, the balance using the molten metal consisting of Fe and inevitable impurities, cooling rate 2.0 ~ After casting a semi-finished product of a predetermined shape at 8.0 ° C./second, holding the semi-finished product within a temperature range of 900 to 1100 ° C. for 5 to 30 minutes, and then cooling at a cooling rate of 1 to 8 ° C./min .
A method for producing a spheroidal graphite cast iron pipe in which the area ratio of pearlite in the base structure is 80% or more and the area ratio of undecomposed cementite is in the range of 5 to 15% . The spherical shape according to claim 3, wherein when pouring the molten metal into a mold, 0.1 to 0.5% by mass of a Fe-Si inoculum containing 45 to 75% by mass of Si is inoculated. A method for producing a graphite cast iron pipe.