JP2007327083A - Spheroidal graphite cast iron and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide spheroidal graphite cast iron having high strength and high elongation as-cast without performing heat treatment, and having robust mechanical properties, and to provide its production method. <P>SOLUTION: The spheroidal graphite cast iron has chemical components containing, by mass, 3.4 to 4.0% C, 2.4 to 2.8% Si, 0.2 to 0.5% Mn, 0.4 to 0.65% Cu, 1.0 to 2.5% Ni, 0.02 to 0.05% Mg and 0.005 to 0.02% S, and the balance Fe with inevitable impurities. Also, the method for producing the spheroidal graphite is disclosed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a spheroidal graphite cast iron that constitutes equipment that requires high strength and toughness, such as vehicle parts, and a method for manufacturing the same.

As a general characteristic of spheroidal graphite cast iron, it is known that the elongation decreases in inverse proportion to the increase in strength. Conventional spheroidal graphite cast iron technology that ensures high elongation at the same time with high strength requires processes represented by heat treatment after casting, such as quenching and tempering (see Patent Document 1) and austempering heat treatment. Not only is it complicated, but it is also expensive.
In addition, in order to ensure high strength and high elongation at the same time in an as-cast state without performing heat treatment, a technique of adding elements such as Cu and Ni in a cast iron molten metal component is also known (Patent Documents 2, 3, 4, See 5 and 6.). These technologies have a problem that the mechanical properties such as strength and elongation change greatly due to differences in the cooling rate of the cast molten metal depending on the thickness of the cast product, the pouring temperature, the type of mold, etc. There is.
In order to solve such problems and obtain high strength and elongation in the conventional technique, a method of controlling the cooling rate by heating after limiting the thickness, devising the mold material or releasing the mold (Patent Document 7 and 8) and a special construction method such as disposing a cooling metal on the mold (see Patent Document 9).
Japanese Patent Laid-Open No. 11-6026 JP-A 64-245 JP-A-2-290943 JP-A-7-145444 JP 2003-105484 A JP 2004-99923 A JP-A-8-176656 JP 2004-124225 A JP 2002-292453 A

  An object of the present invention is to provide a spheroidal graphite cast iron having high strength and high elongation in an as-cast state without heat treatment, and a method for producing the same, and cooling depending on the thickness of the cast product and the type of mold. The object is to provide a spheroidal graphite cast iron having a robust mechanical property and a method for producing the same by using an appropriate molten metal component that is not easily subjected to speed restrictions.

In order to achieve the above object, the present invention includes the following (1) to (3).
(1) The chemical component is mass%, C: 3.4 to 4.0%, Si: 2.4 to 2.8%, Mn: 0.2 to 0.5%, Cu: 0.4 to 0 Spheroidal graphite cast iron comprising 65%, Ni: 1.0 to 2.5%, Mg: 0.02 to 0.05%, S: 0.005 to 0.02%, balance Fe and inevitable impurities.
(2) The method for producing spheroidal graphite cast iron according to (1) above, wherein a molten cast iron having a S ratio of 0.01 to 0.02 mass% before spheroidizing graphite is spheroidized, The method, wherein the ratio is 0.005 to 0.02% by mass, inoculated, then poured into a mold and cooled.
(3) The method for producing spheroidal graphite cast iron according to (2), wherein the ratio of S after the spheroidizing treatment is 0.005 to 0.013 mass%.

  According to the present invention, it is possible to obtain spheroidal graphite cast iron that can obtain high strength and high elongation at the same time as cast (without heat treatment), and that is hardly affected by the thickness of the cast product and the cooling rate by the mold. Can do. As a result, there are fewer restrictions on the shape and the like of parts using spheroidal graphite cast iron, which increases the degree of freedom in shape design and makes it easier to respond to functional improvements of parts such as weight reduction design. The effects can be achieved.

The present invention will be described in detail below.
The spheroidal graphite cast iron of the present invention has C: 3.4 to 4.0 mass%, Si: 2.4 to 2.8 mass%, Mn: 0.2 to 0.5 mass%, Cu: 0.4 to 0.65% by mass, Ni: 1.0 to 2.5% by mass, Mg: 0.02 to 0.05% by mass, S: 0.005 to 0.02% by mass, balance Fe and inevitable impurities It is possible to easily realize high strength and elongation by optimizing the balance of chemical components.
If C is less than 3.4% by mass, not only the castability is deteriorated, but also the formation of graphite grains is reduced, which hinders good graphite spheroidization. On the other hand, if it exceeds 4.0% by mass, quiche-like graphite and flotation graphite are easily produced, and strength and elongation are lowered.
If Si is less than 2.4% by mass, carbides are likely to be generated and the elongation is lowered. Moreover, when it exceeds 2.8 mass%, elongation will fall under the influence of a silico ferrite.
If Mn is less than 0.2% by mass, the effect of Mn is not obtained and the strength cannot be obtained. Moreover, when it exceeds 0.5 mass%, pearlite will increase and elongation will fall.
If Cu is less than 0.4% by mass, the effect of Cu is not obtained and strength cannot be obtained. Moreover, when it exceeds 0.65 mass%, pearlite will increase and elongation will fall.
If Ni is less than 1.0% by mass, the effect of Ni is not obtained and strength cannot be obtained. Moreover, when it exceeds 2.5 mass%, pearlite will increase and elongation will fall.
If Mg is less than 0.02% by mass, the spheroidization of graphite is not sufficient, and the tensile strength and elongation are lowered. On the other hand, if it exceeds 0.05 mass%, abnormal graphite and flotation graphite are generated, and the tensile strength and elongation are lowered. In addition, non-metallic inclusions are likely to occur, causing defective products.
If S is less than 0.005% by mass, the amount of graphite that crystallizes decreases, making it difficult to form a good graphite spheroid and lowering the strength. On the other hand, if it exceeds 0.02% by mass, it reacts with Mg during the spheroidizing treatment to produce Mg sulfide, consuming Mg necessary for graphite spheroidization, and a good graphite spheroidization rate is obtained. Loss, strength and elongation decrease. Further, when the amount of C crystallized as graphite during solidification increases, ferritization of the matrix structure in contact with graphite proceeds and the strength decreases.
Specific examples of the inevitable impurities include P, and the content thereof is preferably 0.1% by mass or less.

Next, an example of the method for producing the spheroidal graphite cast iron of the present invention will be specifically described.
First, as a raw material, various iron alloy materials such as steel scrap and pig iron are blended in consideration of the amount of blended components, and are melted in a cast iron melt using an electric furnace (low frequency furnace or high frequency furnace) or cupola. The molten metal produced according to the target composition is subjected to a molten metal treatment in a ladle using a graphite spheroidizing agent. The graphite spheroidizing agent is preferably an Mg alloy, and more preferably an Fe—Si—Mg—RE alloy. At this time, if necessary, the inoculum is added into the pan or added to the tapping stream. In the present invention, a cast iron melt having a S ratio of 0.01 to 0.02 mass% before spheroidizing graphite is spheroidized to make the S ratio 0.005 to 0.02 mass%. However, it is preferable to set it as 0.005-0.013 mass%.
After the molten metal treatment is performed, the molten metal is poured from a ladle into a mold formed by a molding machine, poured, and solidified and cooled as it is in the mold. In addition, at this time, in order to prevent the formation of carbides in the thin-walled portion and to suppress the appearance of the pearlite phase unevenly by refining the graphite particle size, the inoculum is added to the pouring flow during casting into the mold It is preferable to perform secondary inoculation (pouring flow inoculation).
When the article in the mold is cooled, it is separated into an article and molding sand by a drum cooler, and then the sand adhering to the surface of the article is removed by shot blasting to perform casting finish. In this casting, a cast iron casting as a product can be obtained by finishing dams, deburring and the like.

The present invention will be described in more detail with reference to examples, but the present invention should not be construed as being limited thereto.
Examples 1-4
Using a high-frequency induction furnace with a capacity of 100 kg and using cast iron return scraps and steel scraps as raw materials, a molten cast iron of about 1500 ° C. is melted, and additive elements C, Si, Mn, S, Cu, Ni Was added as appropriate, and a graphite Fe spheroidizing process was performed on a commercially available Fe-Si-Mg-Ca-RE alloy by the sandwich method. This molten metal was cast into JIS G 5502 Y-shaped specimens A and B, which are self-hardening molds, while adding 0.1% pouring inoculation with an Fe-Si-based inoculant to a temperature below the eutectoid transformation point. After cooling and releasing, a Y-shaped specimen was cast.
A JIS Z 2201 No. 4 test piece was processed from this Y-shaped specimen, and its tensile strength, elongation, and the like were measured.

Comparative Examples 1-4
The same high-frequency induction furnace as in the above example and the same kind of raw materials and additive elements are used for melting, casting the same test material as in the example, and processing into a test piece in the same manner as in the example, The tensile strength, elongation, etc. were measured.

  Table 1 shows the composition of the spheroidal graphite cast iron obtained in the examples and comparative examples, and Table 2 shows the results of measuring the tensile strength and elongation of the test pieces of the spheroidal graphite cast iron obtained in the examples and comparative examples. Shown in

  FIG. 1 is a graph showing the relationship between tensile strength and elongation when the thickness of the test material shown in Table 2 is 25 mm, and FIG. 2 is a graph showing the relationship between tensile strength and elongation when the thickness of the test material is 12 mm. .

The test pieces of spheroidal graphite cast iron obtained in Examples 1 to 4 have high tensile strength and elongation at any wall thickness, and the difference in tensile strength and elongation due to the wall thickness is small. On the other hand, in the spheroidal graphite cast iron test pieces obtained in Comparative Examples 1 to 3, a high value can be secured for the tensile strength, but the elongation becomes low. In the test piece of spheroidal graphite cast iron obtained in Comparative Example 4, the elongation was close to that of the test piece of spheroidal graphite cast iron obtained in Examples 1 to 4, but high tensile strength was not obtained. Further, in the examples, the tensile strength and the elongation are not significantly changed depending on the thickness, but in the comparative example, the tensile strength and the elongation are different depending on the thickness.
That is, the test piece of spheroidal graphite cast iron obtained in the present invention is a robust spheroidal graphite cast iron material that is not easily affected by the cooling rate of the cast iron melt due to external factors such as product thickness differences and mold types. Yes.

It is drawing which showed the relationship between the tensile strength in the case of the thickness of 25 mm in the test piece of the spheroidal graphite cast iron obtained by the Example and the comparative example, and elongation. It is drawing which showed the relationship between the tensile strength and elongation at the time of thickness 12mm in the test piece of the spheroidal graphite cast iron obtained by the Example and the comparative example by the graph. It is a metal structure photograph by the optical microscope which shows the microstructure of the test piece of the spheroidal graphite cast iron obtained in the Example of this invention.

Claims (3)

  Chemical component is mass%, C: 3.4 to 4.0%, Si: 2.4 to 2.8%, Mn: 0.2 to 0.5%, Cu: 0.4 to 0.65% Spheroidal graphite cast iron composed of Ni: 1.0 to 2.5%, Mg: 0.02 to 0.05%, S: 0.005 to 0.02%, balance Fe and inevitable impurities. A method for producing the spheroidal graphite cast iron according to claim 1,
A cast iron melt having a S ratio of 0.01 to 0.02 mass% before spheroidizing graphite is spheroidized to make the S ratio 0.005 to 0.02 mass%, and then inoculated. And then pouring and cooling the mold.   The method for producing spheroidal graphite cast iron according to claim 2, wherein the ratio of S after the spheroidizing treatment is 0.005 to 0.013 mass%.

Images