JP2003221638A - Globular graphite cast iron - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an as-cast globular graphite cast iron having a high fatigue strength by inhibiting the deposition of a ferrite structure at the surface of the mill scale to obtain a pearlitic surface structure. <P>SOLUTION: The globular graphite cast iron contains 3.2-3.8 wt.% C, 2.0-2.8 wt.% Si, ≤0.3 wt.% Mn, ≤0.04 wt.% P and 0.02-0.06 wt.% Mg. The globular graphite cast iron also contains 0.03-0.08 wt.% Sn and 0.5-1.5 wt.% Cu. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to spheroidal graphite cast iron.

[0002]

2. Description of the Related Art Generally, there are some automobile parts such as a bracket of a chassis frame and an arm of a suspension which employ spheroidal graphite cast iron which is inexpensive and has high strength. Often used in the state of the black skin surface.

[0003]

However, in the design of parts using spheroidal graphite cast iron, the design is carried out by using the fatigue strength in the black skin surface state. Compared with the internal structure, the surface structure has a lot of precipitation of ferrite structure due to decarburization, which is disadvantageous in terms of fatigue strength.Thus, thin-walled and lightweight design of parts using spheroidal graphite cast iron and cost reduction It was a major factor that prevented

Further, as a method for increasing the strength of a part made of spheroidal graphite cast iron, it has been conventionally known to subject the part to heat treatment. However, increasing the number of heat treatment steps causes a cost increase. However, even if the internal strength can be improved by such heat treatment, there is also a problem that the effect of improving the fatigue strength of the black skin surface is low, so that the as-cast spheroidal graphite cast iron having high fatigue strength Development is desired.

The present invention has been made in view of the above-mentioned circumstances, and by suppressing the precipitation of the ferrite structure on the surface of the black skin to make the surface structure pearlite, the spheroidal graphite cast iron having high fatigue strength as cast can be obtained. Is intended to provide.

[0006]

The present invention provides C: 3.2-
3.8% by weight, Si: 2.0 to 2.8% by weight, Mn:
0.3% by weight or less, P: 0.04% by weight or less, Mg:
A spheroidal graphite cast iron containing 0.02 to 0.06% by weight, wherein Sn: 0.03 to 0.08% by weight is Cu: 0.5.
It is characterized in that it is contained together with about 1.5% by weight.

[0007] Here, C is contained as an element for improving the flow of molten metal and enhancing the castability, and at least 3.2% by weight or more must be added to ensure good castability. If added in excess of 3.8% by weight, an abnormal structure such as dross will be generated, so the content ratio of C in the cast iron is specified to be in the range of 3.2 to 3.8% by weight. is doing.

Si is also contained as an element for improving the castability, but if it is less than 2.0% by weight, the yield strength is lowered, and if it is added in excess of 2.8% by weight. Since the matrix structure contains a large amount of ferrite, the content ratio of Si in the cast iron is 2.0 to 2.8.
It is specified in the range of% by weight.

Further, Mn is a component that comes in from the raw material pig iron and the return material, but if it is added in an amount exceeding 0.3% by weight, the pearlite structure becomes dense and the matrix structure becomes nonuniform. Since it begins to appear remarkably, Mn occupied in cast iron
Is suppressed to 0.3% by weight or less.

Further, P is also a component that comes in from the raw material pig iron and the return material, and it is a component that crystallizes steadite to make cast iron brittle, so that the P content in the cast iron is not to deteriorate the mechanical properties. It is specified to be 0.04% by weight or less.

Further, Mg is contained as an element that promotes spheroidization of graphite, but if it is less than 0.02% by weight, spheroidization of graphite becomes insufficient and mechanical properties deteriorate. Also, if added in excess of 0.06% by weight, Mg dross will occur, so the content ratio of Mg in cast iron should be suppressed within the range of 0.02 to 0.06% by weight. There is.

Also, Sn promotes perlite formation and C
It is contained as an element that suppresses graphitization of at least 0.03% in order to obtain an effective effect of promoting pearlite
Although it is necessary to add more than 0.08% by weight, if added in excess of 0.08% by weight, graphitization is excessively suppressed and stable crystallization of spheroidal graphite is significantly hindered. Content rate is specified in the range of 0.03 to 0.08% by weight.

Here, the suppression of graphitization of C by the addition of Sn means that C remains in the matrix structure and the ferrite structure is easily transformed into pearlite, and furthermore, promotion of pearlite formation is promoted. Since the effect also acts synergistically, the precipitation of the ferrite structure on the surface of the black skin is suppressed and the surface structure becomes pearlite.

Further, Cu is contained as an element that promotes pearlite formation, and acts synergistically with the graphitization suppression effect of Sn described above to suppress the precipitation of the ferrite structure, but has an effective pearlite formation promoting effect. At least 0.5% by weight or more is required to obtain Cu, while Cu also has the effect of increasing the strength of the matrix structure of cast iron.
If added in excess of 5% by weight, the hardness will increase too much and the machinability and toughness will be significantly reduced. Therefore, the Cu content in cast iron should be in the range of 0.5 to 1.5% by weight. Stipulated in.

Thus, in this way, the graphitization of C is suppressed by the addition of Sn and C remains in the matrix structure, whereby the ferrite structure is easily made pearlite, and moreover,
Since the Sn and Cu promote the pearlite formation, it is possible to achieve the pearlite formation of the surface texture in which the precipitation of the ferrite texture due to the decarburization has been frequently observed in the related art.

That is, in the spheroidal graphite cast iron obtained by such a composition, the precipitation of the ferrite structure on the surface of the black skin is suppressed and the surface structure is made pearlite. The spheroidal graphite cast iron has a high fatigue strength in which a pearlite structure is formed.

Further, in the present invention, Ni: 0.5-
2.0 wt% may be contained. By doing so, it is possible to increase the yield strength and improve the mechanical properties, but if it is less than 0.5 wt%, the yield strength improves. Does not appear sufficiently, and even if it exceeds 2.0% by weight,
Since the cost is higher than the proof stress level required for practical use, the Ni content in cast iron should be 0.5%.
Stipulated to be 2.0% by weight.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

In the present invention, C (carbon): 3.2 to 3.8% by weight, Si (silicon): 2.0 to 2.8% by weight, Mn (manganese): 0.3% by weight or less, P (Phosphorus): 0.04% by weight or less, spheroidal graphite cast iron containing Mg (magnesium): 0.02 to 0.06% by weight, Sn (tin):
0.03 to 0.08% by weight of Cu (copper): 0.5 to 1.
The present invention relates to spheroidal graphite cast iron having a chemical composition of 5% by weight, and Table 1 below shows Example A of spheroidal graphite cast iron according to the present invention and Comparative Example B which is a conventional product (FCD500). Each is shown. All numerical values of each element in Table 1 are% by weight, and the balance excluding these elements is basically Fe (iron).

[0020]

[Table 1]

FIG. 1 is a graph evaluating fatigue strength of Example A and Comparative Example B in Table 1. In this graph, the number of folds is plotted on the horizontal axis and the stress amplitude (MP
The vertical axis is a).

The fatigue strength test whose results are shown in the graph of FIG. 1 is produced by processing a flat plate 4-point bending test piece from the Y-type test material No. B, and using a 5t servo pulsar type fatigue tester at a frequency of 10 Hz. A double swing 4-point bending fatigue test was performed.

Further, as shown in the graph of FIG. 1, the fatigue strength of the conventional comparative example B was about 186 MPa, but in the example A of the present invention, it was about 300 MPa.
It was found that the fatigue strength of a was significantly improved.

This is because an element that promotes pearlite formation and suppresses graphitization of C, Sn, and an element that promotes pearlite formation, Cu, can form a pearlite base from the surface to the inside. It is believed that this is a major factor, and more specifically, the addition of Sn causes C
Since the graphitization of S is suppressed and C remains in the matrix structure, the ferrite structure is likely to become pearlite.
Since n and Cu promote pearlite formation, it is considered that pearlite formation of the surface structure where precipitation of ferrite structure was often observed due to decarburization was achieved in the past.

In fact, the surface texture of Example A is as enlarged and shown in the photograph of FIG. 2, whereas the surface texture of Comparative Example B is as enlarged and shown in the photograph of FIG. ,
In the photograph of Comparative Example B in FIG. 3, many ferrite textures (white portions in the photograph) are deposited around the spherical graphite (black spots in the photograph) crystallized in the surface texture of Comparative Example B. However, in the photograph of Example A in FIG. 2, it was confirmed that this type of ferrite structure was well pearlized and significantly reduced.

According to the verification test conducted by the present inventors, Sn needs to be at least 0.03% by weight or more in order to obtain an effective effect of promoting pearlite.
If added in excess of wt%, graphitization will be excessively suppressed and stable crystallization of spheroidal graphite will be significantly impaired. Therefore, the Sn content in cast iron is 0.03 to 0.0.
It is necessary to specify in the range of 8% by weight.

Further, Cu needs to be at least 0.5% by weight or more in order to obtain an effective promotion effect of pearlite, while Cu also has the effect of enhancing the strength of the matrix structure of cast iron, so that 1.5 If added in excess of wt%, the hardness will increase too much and the machinability and toughness will drop significantly, so the Cu content in cast iron should be 0.5.
It is necessary to stipulate in the range of up to 1.5% by weight.

Further, when an additional test was conducted on the influence of the addition of Sn and Cu on the static mechanical properties and the structure,
It has been confirmed that, in the range of the above-mentioned content rate, the tensile strength and the proof stress increase, the hardness increases, and the elongation decrease due to the increase of Cu, but such a static mechanical property increases Sn. It was not particularly confirmed that the change was caused by. However, regarding the continuous ferrite structure on the surface of the black skin, it was confirmed that the increase of Sn significantly improved the reduction effect, and that the increase of Sn and Cu also tended to reduce the graphite particle size.

Further, for such spheroidal graphitized cast iron,
If necessary, Ni: 0.5 to 2.0% by weight may be contained. By doing so, it is possible to increase the proof stress value and improve the mechanical properties. If it is less than%, the yield strength will not be sufficiently improved, and if it is added in excess of 2.0% by weight, it will exceed the yield strength level required for practical use and the cost will increase. It is necessary to regulate the Ni content to be 0.5 to 2.0% by weight.

The spheroidal graphite cast iron of the present invention is not limited to the above-mentioned embodiment, and it goes without saying that various modifications can be made without departing from the gist of the present invention.

[0031]

As is apparent from what has been described above, according to the spheroidal graphite cast iron of the present invention described above, the precipitation of the ferrite structure on the surface of the black skin can be suppressed and the surface structure can be made pearlite. Therefore, it is possible to obtain as-cast spheroidal graphite cast iron having high fatigue strength.
It is possible to achieve an excellent effect that it is possible to realize a thin and lightweight design and cost reduction of a part using spheroidal graphite cast iron.

[Brief description of drawings]

FIG. 1 is a graph in which fatigue strength is evaluated in Examples of the present invention and Comparative Examples.

FIG. 2 is an enlarged photograph of the surface texture of the example of the present invention.

FIG. 3 is an enlarged photograph of a surface texture of a conventional product which is a comparative example.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yasuhiro Nemoto             Fukushima City, Fukushima Prefecture             Shima Steel Co., Ltd.

Claims (2)

[Claims] 1. C: 3.2 to 3.8% by weight, Si: 2.
0-2.8 wt%, Mn: 0.3 wt% or less, P: 0.
A spheroidal graphite cast iron containing 0.4 wt% or less and Mg: 0.02 to 0.06 wt%, Sn: 0.03 to 0.0
Spheroidal graphite cast iron characterized by containing 8 wt% together with Cu: 0.5 to 1.5 wt%. 2. The spheroidal graphite cast iron according to claim 1, wherein Ni: 0.5 to 2.0 wt% is contained.