JP2011190516A - Spheroidal graphite cast iron tube and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spheroidal graphite cast iron tube that has high strength and high toughness and can be readily produced at a low cost. <P>SOLUTION: The spheroidal graphite cast iron tube has a composition containing, by weight, 3.20 to 4.00% C, 1.40 to 3.00% Si, 0.10 to 1.00% Mn, 0.02 to 0.08% Mg and 0.01 to 0.20% Cr, and further containing at least one selected from Sn and Cu in the range satisfying Sn (wt.%)+Cu (wt.%)/10<0.050, and the balance being Fe with inevitable impurities. The spheroidal graphite cast iron tube has a structure in which the area ratio of pearlite in a matrix structure is 60 to 80%, the number of graphite grains crystallized in the matrix structure is controlled to ≥500 grains/mm<SP>2</SP>and the average grain diameter thereof is controlled to ≤15 μm. The resulting spheroidal graphite cast iron tube has a tensile strength equal to that of FCD600 and an elongation above that of FCD450 and enables easy control of Sn and Cu contents, thereby realizing a low component cost. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a spheroidal graphite cast iron pipe (ductile cast iron pipe) cast by mold centrifugal casting and a method for manufacturing the same.

  Common spheroidal graphite cast irons include JIS standard FCD350, FCD400, FCD450 and other high toughness types, and FCD600, FCD700, FCD800, etc. A material corresponding to FCD450 (tensile strength: 450 MPa or more, elongation: 10% or more) having a good balance between mechanical strength and elongation is often used.

  By the way, when the spheroidal graphite cast iron pipe made of the material equivalent to the above FCD450 is manufactured by mold centrifugal casting, the cast structure is usually a mottled structure in which cementite and spheroidal graphite coexist on the base of pearlite. The following heat treatment (annealing) is performed in a continuous annealing furnace to decompose cementite and to ferritize the matrix structure so that desired mechanical properties can be obtained.

  In the heat treatment after the above-mentioned mold centrifugal casting, first, the cast tubular semi-finished product is heated to the austenite region (870 ° C. or higher), the cementite is completely decomposed, and only graphite is crystallized on the austenite base. And At this time, the cementite can be decomposed in a shorter time as the heating temperature is higher. Thereafter, ferrite is precipitated from austenite by holding for a certain time in the temperature range near the eutectoid transformation point (about 700 to 750 ° C.) or by gradually cooling the transformation zone. At this time, the amount of ferrite precipitation increases as the holding time is longer or the cooling rate is slower. A material equivalent to FCD450 can be obtained when the base structure is completely ferritized into a structure in which graphite is dispersed in the ferrite base.

  On the other hand, in recent years, there has been a strong demand from users for a spheroidal graphite cast iron pipe that has a better balance between strength and elongation than FCD450 and can be manufactured at low cost. Therefore, the present applicant has added at least one of Sn and Cu, which are pearlite stabilizing elements, to the basic chemical component of spheroidal graphite cast iron, 0.050 ≦ Sn (wt%) + Cu (wt%) / 10. ≦ 0.089 is added, pearlite is precipitated in the base structure at an area ratio of 50 to 90%, and the base structure becomes a two-phase structure composed of ferrite and pearlite, and fine graphite is contained in the base structure. A technology has been proposed that simultaneously achieves a tensile strength equal to or higher than that of FCD600 and an elongation comparable to FCD450 by allowing a large number of crystals to crystallize (Japanese Patent Application No. 2009-35314). However, in this technique, since the range of Sn and Cu contents is narrow, the management thereof is troublesome, and the cost of these components is high.

  In addition, for spheroidal graphite cast iron products manufactured by sand mold casting, the cast semi-finished product is heated to the austenite region and held for a certain period of time, and then rapidly cooled at a cooling rate of about 60 ° C./min to obtain pearlite from austenite. Technology to improve elongation while securing high strength by precipitating and heat-treating at 670-760 ° C to generate ferrite from a portion of pearlite and adjusting the properties of the remaining pearlite Has been proposed (see Patent Document 1).

  However, when the spheroidal graphite cast iron product is a pipe material such as a water pipe, the casting method is centered on mold centrifugal casting whose cooling rate is considerably higher than that of sand casting, and therefore the technique described in Patent Document 1 is applied. It is difficult. That is, in the technique of Patent Document 1, it is a precondition that cooling is performed at a cooling rate that generates pearlite in the base structure at the time of casting, but when mold centrifugal casting with a higher cooling rate is performed, Since a lot of cementite is generated in the base structure, even if the heat treatment described in Patent Document 1 is applied, desirable characteristics cannot be obtained.

JP 2003-55731 A

  An object of the present invention is to provide a spheroidal graphite cast iron pipe that has both high strength and high toughness and can be easily and inexpensively manufactured and a method for manufacturing the same.

In order to solve the above-mentioned problems, the spheroidal graphite cast iron pipe of the present invention is C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mn: 0.10 by weight%. 1.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20%, and one or both of Sn and Cu are Sn (wt%) + Cu (weight) %) / 10 <0.050, the balance is composed of Fe and inevitable impurities, the area ratio of pearlite in the base structure is 60 to 80%, and the graphite crystallized in the base structure The number of particles was 500 / mm ² or more, and the average particle size was 15 μm or less.

  Here, the area ratio of pearlite is the ratio (%) of the area of pearlite when the area of the base tissue excluding graphite is 100% in a field of a predetermined size. Further, the number of graphite grains and the average particle diameter are values measured excluding those having a particle diameter of 3 μm or less.

  That is, in the present invention, the pearlite area ratio in the base structure is adjusted to be 60 to 80%, and a large number of fine spherical graphite is crystallized in the base structure to make the structure dense, thereby increasing the cost. High strength and high toughness can be achieved simultaneously without adding much Sn or Cu.

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

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

  Si has an effect of increasing the fluidity of the molten metal and an effect of promoting crystallization of graphite, but if the content is less than 1.40%, the effect of these actions cannot be sufficiently obtained. On the other hand, if the content exceeds 3.00%, the crystallization of graphite becomes excessive and the effect of suppressing the pearlization of the matrix structure becomes large, and high strength cannot be obtained, and pinholes or the like are formed on the outer surface of the product. Roughness is likely to occur. For this reason, the range of content was made into 1.40 to 3.00%.

  Mn is an element that fixes S and renders it harmless, and is contained in an amount of at least 0.10% in order to sufficiently obtain the effect. However, if it is excessive, the elongation is lowered, so the upper limit of the content was made 1.00%.

  Mg is an element necessary for spheroidizing graphite, and if the content is less than 0.02%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.08%, the improvement in the effect is reduced. The amount range was 0.02 to 0.08%.

  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.

  Sn and Cu are both pearlite stabilizing elements, and the effect of Cu is known to be about 1/10 of Sn. However, in order to adjust the area ratio of pearlite in the base tissue to 60 to 80%, it is not always necessary to add a large amount, and it is contained in a range where Sn (wt%) + Cu (wt%) / 10 <0.05. By suppressing the amount, the content can be easily managed and the cost can be reduced.

  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.

  Moreover, the manufacturing method of the spheroidal graphite cast iron pipe of the present invention is, in wt%, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.00% Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20%, and one or both of Sn and Cu are Sn (wt%) + Cu (wt%) / 10 A tubular semi-finished product is cast by mold centrifugal casting using a molten metal having a composition of <0.050, the balance being Fe and inevitable impurities, and 900 to 1100 for this semi-finished product. After holding at 5 ° C for 25-25 minutes, cooling to 680 ° C or less at a cooling rate of 20-30 ° C / min, and further performing heat treatment for 15-30 minutes at 700-720 ° C by heating in a continuous annealing furnace, The semi-finished product is formed into a spheroidal graphite cast iron pipe having the above-described configuration.

That is, by the combination of mold centrifugal casting using the molten metal and heat treatment under the above conditions, the pearlite area ratio is 60 to 80%, the number of graphite particles is 500 pieces / mm ² or more, and the average particle size is 15 μm or less, A high-strength and high-toughness spheroidal graphite cast iron pipe can be easily produced. At this time, if the number of graphite particles is less than 500 particles / mm ² , it takes time to decompose cementite during the heat treatment, and the graphitization of the solute carbon contained in the matrix structure is delayed. Although the area ratio is difficult to obtain, the influence can be reduced by setting the number of graphite particles to 500 pieces / mm ² or more.

  As described above, the spheroidal graphite cast iron pipe of the present invention has an area ratio of pearlite in the base structure of 60 to 80% while suppressing the Sn and Cu contents, and a large amount of fine spheroidal graphite is crystallized in the base structure. Therefore, it has characteristics of both high strength and high toughness, and the content of Sn and Cu can be easily managed and the component cost is low.

  Moreover, the manufacturing method of the spheroidal graphite cast iron pipe of the present invention is to perform heat treatment under a predetermined condition in a normal continuous annealing furnace after performing mold centrifugal casting using a molten metal with suppressed Sn and Cu contents. Therefore, it is possible to easily and inexpensively manufacture a spheroidal graphite cast iron pipe having high strength and high toughness without requiring modification of heat treatment equipment.

a to c are photomicrographs of the material structure of the spheroidal graphite cast iron pipe of the embodiment, respectively.

  Hereinafter, the experiment conducted in order to confirm the characteristic of the spheroidal graphite cast iron pipe of the embodiment of the present invention will be described. In the experiment, first, the spheroidal graphite cast iron pipe (Examples 1 to 3) of the embodiment was manufactured. Table 1 shows the chemical components of the molten metal used in the experiment. Here, the remainder which abbreviate | omitted description consists of Fe and an unavoidable impurity (P: 0.04-0.06%, S: 0.002-0.005% included). In addition, the data of a chemical component are the values which analyzed the birch sample produced from each molten metal with the emission-spectral-analysis apparatus.

  First, molten metal having the composition shown in Table 1 was poured into a cylindrical mold of a centrifugal casting apparatus at about 1300 ° C. to cast a tubular semi-finished product (as cast pipe) having a thickness of about 7.0 mm. At the time of casting, the poured molten metal was solidified by mold water cooling, and then the solidified tubular body was pulled out of the mold and air-cooled.

Then, each as-cast pipe was heat-treated (two-stage annealing) under the following conditions in a continuous annealing furnace to finish a spheroidal graphite cast iron pipe as a product.
(First stage annealing condition)
Heating and holding: 900 to 1100 ° C. × 5 to 25 minutes Cooling rate: 25 to 30 ° C./min (Examples 1 and 2), 20 to 25 ° C./min (Example 3)
・ Cooling temperature: about 650 ° C (680 ° C or less)
(Second stage annealing conditions)
・ Heat holding: 700 to 720 ° C. × 15 to 30 minutes • Cooling rate: 1 to 8 ° C./minute • Cooling temperature: about 600 ° C.

  Here, the cooling rate of the first stage is set so that most of the base structure after cooling becomes pearlite and ferrite does not precipitate. For this reason, the content of Sn and Cu, which are pearlite stabilizing elements, is larger than those in Examples 1 and 2 (the value of Sn (wt%) + Cu (wt%) / 10 is high). The eye cooling rate is slower than those in Examples 1 and 2. The second stage of heating and holding precipitates some ferrite from the pearlite and adjusts the pearlite properties to improve the elongation.

  Test pieces were collected from the cast iron pipes of the respective examples thus produced, and the properties and mechanical properties of the material structures were investigated for each. The survey results are shown in Tables 2 and 3. Moreover, FIG. 1 shows the microscope picture of the material structure | tissue of each Example. Here, the data on the properties of the material structure in Table 2 are all measured by image analysis of the central portion in the thickness direction of the tube, and the pearlite area ratio of the data is the area of the base tissue in a visual field of a predetermined size. It is the ratio of the area of pearlite when it is 100%, and the graphite area ratio is the ratio of the area of graphite when the area of the entire visual field of a predetermined size is 100%. In addition, graphite is measured except for particles having a particle size of 3 μm or less.

1 (a) to 1 (c), it can be seen that the base structure of each example has a two-phase structure composed of pearlite and ferrite, and a large number of fine graphite is crystallized. As apparent from Tables 2 and 3, the pearlite area ratio of each example is 60 to 80%, the number of graphite particles is 600 particles / mm ² or more, and the average particle size is 13 μm or less. However, a tensile strength of about 600 MPa and an elongation of 13% or more are secured.

  From the above results, the spheroidal graphite cast iron pipe of each example adjusts the pearlite area ratio in the base structure to 60 to 80%, and combines mold centrifugal casting with a large cooling rate and heat treatment under appropriate conditions, By crystallizing a lot of fine graphite in the base structure, even if the content of Sn and Cu is kept low, it has high tensile strength and high toughness with tensile strength equivalent to FCD600 and higher elongation than FCD450. It was confirmed that it can be manufactured easily and inexpensively.

Claims (2)

By weight, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.00%, Mg: 0.02 to 0.08%, Cr: It contains 0.01 to 0.20%, and further contains one or both of Sn and Cu in a range of Sn (wt%) + Cu (wt%) / 10 <0.05, with the balance being Fe. And the area ratio of pearlite in the base structure is 60 to 80%, the number of graphite grains crystallized in the base structure is 500 / mm ² or more, and the average particle diameter is 15 μm or less. Spheroidal graphite cast iron pipe.   By weight, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.00%, Mg: 0.02 to 0.08%, Cr: It contains 0.01 to 0.20%, and further contains one or both of Sn and Cu in a range of Sn (wt%) + Cu (wt%) / 10 <0.05, with the balance being Fe. A tubular semi-finished product is cast by mold centrifugal casting using a molten metal having a composition composed of unavoidable impurities, and the semi-finished product is held at 900 to 1100 ° C. for 5 to 25 minutes, and then 20 to 30 ° C. 2. The spheroidal graphite cast iron pipe according to claim 1, wherein the semi-finished product is cooled to 680 ° C. or less at a cooling rate of 1 min / min and further subjected to heat treatment at 700 to 720 ° C. for 15 to 30 minutes in a continuous annealing furnace Manufacturing method of spheroidal graphite cast iron pipe.