US20040091383A1

US20040091383A1 - Ferrite-based spheroidal graphite cast iron and exhaust system component using the same

Info

Publication number: US20040091383A1
Application number: US10/291,300
Authority: US
Inventors: Nobuaki Suzuki; Fumitaka Yamao; Toshio Yamauchi; Zhong-Zhi Zhang; Norihiro Akita
Original assignee: Aisin Takaoka Co Ltd; Suzuki Motor Corp
Current assignee: Aisin Takaoka Co Ltd; Suzuki Motor Corp
Priority date: 2001-05-16
Filing date: 2002-11-08
Publication date: 2004-05-13
Also published as: DE10252240A1; JP3936849B2; DE10252240C5; DE10252240B4; JP2002339033A

Abstract

The present invention provides a ferrite-based spheroidal graphite cast iron containing the following elements in the following contents in % by weight: C: 3.1 to 4.0%; Si: 3.6 to 4.6%; Mo: 0.3 to 1.0%; V: 0.1 to 1.0%; Mn: 0.15 to 1.6%; and Mg: 0.02 to 0.10%, and the total content of V and Mn is 0.3 to 2.0 wt %, and an exhaust system component using the spheroidal graphite cast iron. Thus, the ferrite-based spheroidal graphite cast iron having higher heat resistance than that of a conventional high Si spheroidal graphite cast iron that can be produced inexpensively by a simple method.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high heat resistant spheroidal graphite cast iron obtained by improving a high Si spheroidal graphite cast iron conventionally used only at a temperature of 800° C. or less by an alloy design so as to be used at a high temperature of 850 to 900° C. The spheroidal graphite cast iron is a ferrite-based high Si spheroidal graphite cast iron and the raw material cost is lower and the castability and the machinability are better than those of competing stainless cast steel or Ni-resist cast iron. Therefore, the spheroidal graphite cast iron can be widely used for automobile exhaust system components such as exhaust manifolds, turbo housings, turbo housing-integrated exhaust manifolds, or turbo outlet pipes.

2. Description of the Related Art

Environmental problems are becoming more and more severe, and for purposes of catalyst purification efficiency and lower fuel consumption the temperature of exhaust gas of automobiles is increasing. Under these circumstances, high heat resistant pipe exhaust manifolds employing stainless steel pipes, or sheet metal exhaust manifolds obtained by plastic working of a stainless steel sheet are starting to be used for the exhaust manifold of an engine.

In order to increase the purification efficiency of exhaust gas, it is necessary to pass a high temperature exhaust gas through a catalyst, and to dispose a heavy maniverter containing a catalyst as near the exhaust manifold as possible. In particular, a turbine housing provided with a turbine rotor is connected between the exhaust manifold and the maniverter in a turbo car, and therefore a load is increased in the exhaust manifold, requiring higher rigidity than that at high temperature.

The above-described pipe exhaust manifolds or sheet metal exhaust manifolds are easily deformed at high temperature because of linear expansion inherent to stainless steel and small thickness, they have a poor degree of freedom in the shape, and therefore cast iron still has to be used for exhaust manifolds for a turbo car at present. For conventional cast iron materials for exhaust manifolds, Mo-added high Si spheroidal graphite cast iron containing 3.6 to 4.0% of Si and 0.3 to 1.0% of Mo is only used in practice.

As a technique for improving the high temperature properties in the range of the exhaust gas temperature, for example, Japanese Patent Provisional Publication Nos. 4-218645, 5-125494, and 7-48653 disclose stainless cast steel, but there is no disclosure as to a cast iron having a content of C of 2.1 wt % or more.

Furthermore, as an example in which the properties of high Si spheroidal graphite cast iron are improved, there are techniques aiming at improving brittleness in a middle temperature range, as disclosed in Japanese Patent Provisional Publication Nos. 10-195587 and 61-73859.

However, there are the following problems. Although Japanese Patent Provisional Publication No. 61-73859 describes adjusting the composition of Mg and P, it is difficult to control the adjustment in an actual production line. In the method for producing the spheroidal graphite cast iron disclosed in Japanese Patent Provisional Publication No. 10-195587, arsenic (As), which is deadly toxic, is added, so that the work environment is very bad.

On the other hand, the conventionally used high Si spheroidal graphite cast iron has a low A _c1transformation point of about 850° C., at which a matrix of a ferrite and pearlite structure is transformed to an austenite phase by heating. Therefore, when the high Si spheroidal graphite cast iron is exposed to a high temperature exhaust gas (880 to 930° C.), the temperature of an exhaust system component itself is increased to 800 to 880° C. and exceeds the A_c1transformation point, so that the high Si spheroidal graphite cast iron is transformed readily to the austenite phase and thermal fatigue or deformation occurs because of rapid elongation increase and strength decrease. Therefore, when seeking for higher heat resistance than that of the conventional high Si spheroidal graphite cast iron, only Ni-resist cast iron and stainless cast steel are practical at present.

However, the Ni-resist cast iron and the stainless cast steel contain a large amount of Ni, Cr, W or the like for the raw material, so that the cost of the raw material is high. In addition, since the melting point of the raw material is high, production cannot be performed with a conventional cast iron production facility, and the castability, the production yield and the machinability are poor, so that the costs of components are significantly high.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above problem and to provide a ferrite-based spheroidal graphite cast iron that has a higher heat resistance than that of the conventional high Si spheroidal graphite cast iron and can be produced inexpensively by a simple method.

In order to achieve the above object, a ferrite-based spheroidal graphite cast iron of the present invention includes C, Si, Mo, V, Mn, and Mg, and the remaining portion is composed of Fe and inevitable impurities.

The ferrite-based spheroidal graphite cast iron of the present invention has excellent tensile strength and yield strength in a region from room temperature to the vicinity of 800 to 900° C. Therefore, when this spheroidal graphite cast iron is applied to an exhaust system component, for example, an exhaust manifold, the component can resist high temperature exhaust gas having a temperature around 880 to 930° C. sufficiently, and therefore the temperature of the exhaust gas can be increased. Thus, efficient purification of the exhaust gas and fuel saving can be achieved, and consequently the present invention can comply with coming exhaust gas regulations such as post, post 53 or the like (legislative regulations of Japan).

Furthermore, since in the ferrite-based spheroidal graphite cast iron of the present invention, casting methods and conditions of the conventional spheroidal graphite cast iron can be used without any change, the present invention can be produced in the existing cast iron production line, and new facility investment is not required. Furthermore, the raw material costs and the processing costs are lower than those of stainless cast steel and Ni-resist, so that the production costs can be low. In addition, unlike stainless cast steel and Ni-resist, the ferrite-based spheroidal graphite cast iron has excellent machinability and castability, so that the degree of freedom in the shape of an exhaust system component can be increased.

In another embodiment of the ferrite-based spheroidal graphite cast iron of the present invention, the contents of the above elements in % by weight are as follows: C: 3.1 to 4.0%; Si: 3.6 to 4.6%; Mo: 0.3 to 1.0%; Mg: 0.02 to 0.10%, V: 0.1 to 1.0%; and Mn: 0.15 to 1.6%.

The content of the inevitable impurities are as follows: S: 0.02% or less; and P: 0.1% or less, and the total content of Cu, Sn and Cr is 0.8% or less, preferably, 0.4% or less. An amount of 0.4% or less is preferable for the following reason. These elements, Cu, Sn and Cr, promote to precipitate pearlite, and therefore when the amount of these elements mixed is increased, the amount of the pearlite precipitated in the matrix is increased. Thus, the hardness is increased, which leads to a reduction in the elongation. Furthermore, when the amount of the pearlite precipitated is increased, more of cementite (Fe ₃C) in the pearlite is dissolved at high temperature, and consequently graphite growth is facilitated, and the quality is deteriorated.

Furthermore, in still another embodiment of the ferrite-based spheroidal graphite cast iron of the present invention, the total content of V and Mn is 0.3 to 2.0 wt %.

Mn facilitates precipitation of the pearlite microstructure so as to contribute to improvement of tensile strength and yield strength, and V forms and precipitates fine carbides having a high melting point in the vicinity of the grain boundaries of eutectic cells so as to serve to improve the grain boundary potential and to prevent the pearlite microstructure from dissolving at high temperature. Therefore, the total amount of Mn and V is in the range from 0.3 to 2.0%, and the above-described effect can be larger by adding the two elements at the same time (so-called multiple addition).

Furthermore, in yet another embodiment of the ferrite-based spheroidal graphite cast iron of the present invention the Si/CE value is 0.97 or less.

This CE value is referred to as carbon equivalent and is given by the content of C+(the content of Si+the content of P)/3. When the Si/CE value is adjusted to 0.97 or less, a reduction in the elongation in the spheroidal graphite cast iron in a range from room temperature to middle temperature is suppressed, and the thermal fatigue resistance can be improved further. Mg is an element playing an important role as a graphite spheroidizing agent.

Furthermore, an exhaust system component of the present invention is produced using the ferrite-based spheroidal graphite cast iron.

In one embodiment of the exhaust system component, the exhaust system component can be an exhaust manifold, a turbo housing, a turbo housing-integrated exhaust manifold, or a turbo outlet pipe for automobiles.

Since the exhaust system component is made of a cast material, the degree of freedom in the shape is larger than that of, for example, pipe exhaust manifolds using stainless steel pipes, or sheet metal exhaust manifolds that are processed from stainless steel sheets. Therefore, a complex shaped component can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the Si content and the transformation temperature. [0025]
FIG. 2 is a graph showing the relationship between the test temperature (20 to 900° C.) and the tensile strength in examples. [0026]
FIG. 3 is an enlarged graph of FIG. 2 showing the relationship between the test temperature (700 to 900° C.) and the tensile strength. [0027]
FIG. 4 is a graph showing the relationship between the test temperature (20 to 900° C.) and the high temperature proportional limit in examples. [0028]
FIG. 5 is an enlarged graph of FIG. 4 showing the relationship between the test temperature (700 to 900° C.) and the high temperature proportional limit. [0029]
FIG. 6 is a graph showing the amount of Ni added and the tensile strength in examples. [0030]
FIG. 7 is a graph showing the amount of Mn added and the tensile strength in examples. [0031]
FIG. 8 is a graph showing the amount of V added and the tensile strength in examples. [0032]
FIG. 9 is a graph showing the relationship between the test temperature (20 to 900° C.) and the high temperature tensile strength in examples. [0033]
FIG. 10 is an enlarged graph of FIG. 9 showing the relationship between the test temperature (750 to 900° C.) and the high temperature tensile strength. [0034]
FIG. 11 is a graph showing the relationship between the test temperature (20 to 900° C.) and the elongation in examples. [0035]
FIG. 12 is a graph showing the relationship between the Si/CE value and the elongation in examples.[0036]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, ferrite-based spheroidal graphite cast iron of this embodiment of the present invention will be described in detail. [0037]
In order to solve the above-described problems, it is most advantageous to improve the conventionally used high Si spheroidal graphite cast iron containing Mo by an alloy design. The inventors of the present invention conducted in-depth research on the following items in order to improve the heat resistance of the high Si spheroidal graphite cast iron containing Mo and thus achieved the present invention. In the following description, “%” refers to “% by weight” in any cases. [0038]
1. Increasing the A[0039] ₁transformation point
2. Improving thermal deformation resistance [0040]
3. Improving thermal fatigue resistance [0041]
4. Improving oxidation resistance [0042]
These items will be described below. [0043]
1. Increasing the A[0044] ₁Transformation Point
First, in order to improve the heat resistance of a ferrite-based spheroidal graphite cast iron, it is necessary to increase, in particular, the A[0045] _c1transformation point of the A₁transformation points. The A_c1transformation point refers to the temperature at which the matrix structure in which ferrite and pearlite are mixed is transformed to an austenite phase by heating. Therefore, when the A₁transformation point is increased, the spheroidal graphite cast iron is hardly transformed to the austenite phase, giving improved heat resistance. The A₁transformation point is increased, as the amount of Si is increased, so that Si is added in an amount more than that of a conventional cast iron material as far as practically possible, with the lower limit is set to 3.6%. However, when Si is added excessively in an amount of more than 4.6%, a significant reduction in elongation occurs in the spheroidal graphite cast iron, so that the upper limit is set to 4.6%. Therefore, the amount of Si added is 3.6% to 4.6%, preferably 4.0% to 4.5%.
Thus, the A[0046] _c1transformation point can be increased to about 890° C. by adding about 4.5% of Si, whereas the A_c1transformation point is about 850° C. in the conventional cast iron.
In general, the temperature of an exhaust system component exposed to a high temperature exhaust gas of 880° C. to 930° C. is increased to the vicinity of 800 to 880° C. Therefore, when the ferrite-based spheroidal graphite cast iron of the present invention is applied to an exhaust system component, the temperature does not exceed the A[0047] _c1transformation point even during engine operation, so that a large transformation strain involved in phase transformation can be suppressed from occurring, and the thermal fatigue life can be improved significantly.
2. Improvement of Thermal Deformation Resistance [0048]
In order to suppress the thermal deformation that is caused by heating or cooling when elongation or contraction is constrained, it is advantageous to improve the high temperature strength, in particular, the high temperature yield strength or the high temperature proportional limit. [0049]
Therefore, in order to improve the strength of the ferrite-based spheroidal graphite cast iron at high temperature, it is advantageous to add V and Mn to the Si and Mo-based spheroidal graphite cast iron. [0050]
When the ferrite-based spheroidal graphite cast iron is used, for example, for an exhaust system component, in the vicinity of the upper limit (about 850° C.) of the temperature of the exhaust system component itself during engine operation, the larger the amount of Mn and Ni added, the greater the tensile strength. On the other hand, for V, an effect of adding V can be recognized at 0.1%, and when 0.3% or more of V is added, substantially constant tensile strength can be maintained. [0051]
Herein, Mn has an important effect of facilitating precipitation of pearlite so as to improve the tensile strength and the yield strength, so that the content of Mn is 0.15% or more. Furthermore, V forms fine carbide having a high melting point and precipitates it in the vicinity of the grain boundaries of eutectic cells so as to serve to improve the grain boundary potential and to prevent pearlite from dissolving at high temperature. Therefore, the content of V is 0.1% or more. The effects of Mn and V improve the strength from room temperature to high temperature. [0052]
On the other hand, when Mn is added in an amount of more than 1.6% and V is added in an amount of more than 1.0%, the pearlite ratio in the matrix of the spheroidal graphite cast iron becomes high and elongation is reduced in a room temperature range and a middle temperature range, so that it is not preferable to add Mn and V in amounts more than those. Therefore, the upper limit of the content of Mn is 1.6%, and the upper limit of the content of V is 1.0%. [0053]
Therefore, the content of Mn is 0.15 to 1.6%, preferably 0.15 to 1.5%. The content of V is 0.1 to 1.0%, preferably 0.2 to 0.5%. [0054]
As described above, in order to improve the high temperature properties of the spheroidal graphite cast iron, it is advantageous to add V and Mn. In addition, V and Mn can provide a more desirable effect to the mechanical properties or the like, when they are added in combination than when each of them is added alone, The total amount of Mn and V is 0.3 to 2.0%, preferably 0.4% to 1.8%. [0055]
Furthermore, Mo is also an element that improves the mechanical properties at high temperature, in particular, the high temperature yield strength (or high temperature proportional limit). When the content of Mo is less than 0.3%, the effect of addition is small, so that the lower limit of the content of Mo is 0.3%. On the other hand, although the Al transformation point does not depend on the content of Mo, when the content of Mo exceeds 1.0%, the pearlite ratio in the spheroidal graphite cast iron is increased and the hardness is increased, so that a significant reduction in elongation occurs. Therefore, the upper limit of the content of Mo is 1.0%. [0056]
Thus, the content of Mo is 0.3 to 1.0%, preferably 0.3% to 0.7%. [0057]
When Ni is added, the mechanical properties at high temperature can be improved by the addition, but the A[0058] ₁transformation point is reduced, so that Ni is not suitable for the spheroidal graphite cast iron of the present invention. The A₁transformation point is reduced, because Ni is an element for stabilizing austenite and therefore reduces the A₁transformation point. Furthermore, it is not confirmed that the above-described added elements prevent the spheroidal graphite cast iron from becoming spheroidal.
Summing up, the following component constitution can improve the high temperature properties of the spheroidal graphite cast iron of the present invention. [0059]
i) V and Mn are added to the ferrite-based spheroidal graphite cast containing 3.6 to 4.6% of Si and 0.3 to 1.0% of Mo that serves as the base. [0060]
ii) The amounts of V and Mn added are 0.1 to 1.0% and 0.15% to 1.6%, respectively, and the total amount of V and Mn added is 0.3 to 2.0 wt %, preferably 0.4 to 1.8%. [0061]
3. Improvement of Thermal Fatigue Resistance [0062]
In the high Si spheroidal graphite cast iron containing Mo, the following two approaches are conceivable in order to enhance the thermal fatigue resistance: an approach for canceling reduction in elongation that occurs in the vicinity of 400 to 500° C. inherent to the spheroidal graphite cast iron; and an approach for improving the tensile strength or the yield strength from room temperature to high temperature. The techniques disclosed in Japanese Patent Provisional Publication Nos. 61-73859 and 10-195587 described in the section “Description of the Related Art” employ the former approach, whereas the present invention is based on the latter approach. More specifically, the present invention focuses on suppressing plastic deformation with respect to tensile strain generated in heating and cooling cycles by enhancing the yield strength (yield point or proportional limit) so as to increase the life, which is a period up to the time an initial crack occurs. [0063]
However, in the spheroidal graphite cast iron of the present invention, in order to further improve or stabilize the thermal fatigue characteristics, an approach for ensuring elongation from room temperature to middle temperatures (in the vicinity of 400 to 500° C.) has been examined. Basically, when elongation is small, the thermal fatigue life is reduced. This is because when elongation is small, the sensitivity to cracks with respect to tensile strain involved in the compression plastic deformation at high temperature becomes large in a range from room temperature to middle temperature. Then, as a result of examination of components for ensuring elongation without reducing the total amount and the composition ratio of V and Mn added, it was clarified that the elongation of the Mo-added high Si spheroidal graphite cast iron depends significantly on the mixing ratio of C and Si, that is, the Si/CE value (or C/CE value). Herein, the “CE value” refers to carbon equivalent and is calculated with an equation: the content of C+1/3 (the content of Si+the content of P). [0064]
The elongation of the Mo-added high Si spheroidal graphite cast iron tends to be reduced drastically when the Si/CE value is 0.97 or more. Therefore, the Si/CE value is at most 0.97, preferably 0.82 to 0.96. In this lower limit (0.82) of the Si/CE, value, the content of C is 3.5%, the content of Si is 4.0%, and the content of P is 0.06%. The upper limit (0.96) is set, based on the results shown in FIG. 7. [0065]
Furthermore, according to the range of the Si/CE value that is set as above, the lower limit of a preferable content of C is 3.1%, and the upper limit of Si is 4.5%. On the other hand, as the content of Si is increased, the solid solubility of C is reduced, and graphite floatation occurs in the spheroidal graphite cast iron, thus leading to a variation in the particle size of graphite and a reduction in the graphite nodule count. Therefore, the upper limit of the content of C is 4.0%. That is to say, the content of C is 3.1 to 4.0%, preferably 3.1 to 3.7%. [0066]
Furthermore, the total amount of V and Mn is 0.3 to 2.0%, as described above. This will be described briefly. It is important to limit the pearlite ratio in the spheroidal graphite cast iron to 40% or less by setting the upper limits of V and Mn to 1.0% and 1.6%, respectively, and the upper limit of the total amount to 2.0%. The lower limit of the total content of V and Mn is 0.3% for a sufficient effect of multiple addition. [0067]
Furthermore, the total amount of Cu, Sn and Cr, which are inevitable impurities and increase the amount of pearlite and thus improve the hardness and reduce the elongation, is limited to 0.8% or less, preferably 0.4% or less. An amount of 0.4% or less is preferable for the following reason. These elements promote to precipitate pearlite, and therefore when the amount of these elements mixed is increased, the amount of the pearlite precipitated in the matrix is increased. Thus, the hardness is increased, which leads to a reduction in the elongation. Furthermore, when the amount of the pearlite precipitated is increased, more of cementite (Fe[0068] ₃C) in the pearlite is dissolved at high temperature, and consequently graphite growth is facilitated, and the quality is deteriorated.
An excessive mixture of S prevents graphite from being spheroidized and causes a reduction in the elongation, so that it is necessary that the content of S is 0.02% or less. [0069]
The content of Mg, which is a graphite spheroidizing agent, is 0.02 to 0.10%, preferably 0.02 to 0.06%. The content of P is 0.1% or less. [0070]
4. Improvement of Oxidation Resistance [0071]
The oxidation resistance of the spheroidal graphite cast iron depends on the content of Si, so that the high Si spheroidal graphite cast iron material has a better oxidation resistance than that of regular cast iron materials. The amount of an oxide film produced on the surface of an exhaust system component or the like employing the spheroidal graphite cast iron is smaller, as the content of Si is larger. Consequently, through-cracks due to cracks in the oxide film are suppressed from occurring, which contributes to improvement of the life. Therefore, the amount of Si can be determined in the range described in the items 1 to 3. [0072]
The ferrite-based spheroidal graphite cast iron of the present invention can provide a product having better heat resistance than that of the conventionally used casting material containing a high concentration of Si and Mo. Since it is a cast iron material, the ferrite-based spheroidal graphite cast iron has excellent machinability and castability, unlike stainless cast steel or Ni-resist. Therefore, according to the present invention, for example, a high performance exhaust system component can be produced at a low cost. [0073]

EXAMPLES

The present invention will be described more specifically by way of example. [0074]
Various experiments were conducted, based on the following four approaches. [0075]
1. Increasing the A[0076] ₁transformation point
2. Improving the thermal deformation resistance [0077]
3. Improving the thermal fatigue resistance [0078]
4. Improving the oxidation resistance [0079]
1. Increasing the A[0080] _c1Transformation Point
Examinations were conducted to increase the A[0081] ₁transformation point at which the texture structure in which ferrite and pearlite are mixed is transformed to the austenite phase by heating.

Table 1 shows the results of measuring the Si content and the transformation temperatures of samples used for transformation temperature measurement, and FIG. 1 is a graph showing these results.

TABLE 1


Samples used for transformation
temperature measurement and measurement results

		A_c1		A_c1	A_r1		A_r1	A₁
		start	A_c1end	transformation	start	A_r1end	transformation	transformation
	Si	temperature	temperature	point	temperature	temperature	point	point
	(wt %)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)

Comparative	4.0	861.2	898.7	880	836.0	798.9	817	849
material 1
(conventional
cast
iron)
Comparative	3.6	852.3	902.5	877	831.8	789.1	810	844
material 2
(conventional
cast
iron)
Comparative	4.0	869.5	909.3	889	846.1	801.2	824	857
material 3
(conventional
cast
iron)
Test	4.4	897.4	918.6	908	864.3	820.0	842	875
material 1
Test	4.5	887.0	934.2	911	879.2	828.8	854	882
material 2

The A[0083] ₁transformation point is increased, as the Si amount is increased, as shown in FIG. 1 as well, so that it is preferable that the Si amount is larger than that of the conventionally used cast iron material and as large as possible in practice, for example, 3.6 to 4.6%. The highest A_c1transformation point of the conventional cast iron is 889° C. (comparative material 3), but when the Si amount is increased to 4.4 to 4.5%, the highest A_c1transformation point is increased to 908 to 911° C. (test materials 1 and 2). Thus, when the spheroidal graphite cast iron of the present invention is applied to an exhaust system component of automobiles, the metal temperature of the exhaust system component exposed to high temperature exhaust gas (880° C. to 930° C.) is lower than the A_c1transformation point. Therefore, since it hardly exceeds this transformation point even by heating or cooling during heavy engine operation, a large transformation strain involved in phase transformation is suppressed from occurring, and this is found to be very useful for improving the thermal fatigue life.
2. Improving the Thermal Deformation Resistance [0084]

In components combined and attached in the state where elongation and contraction is constrained, for example, in exhaust system components for automobiles, in order to suppress thermal deformation caused by heating and cooling with exhaust gas, it is advantageous to improve the high temperature strength, in particular, the high temperature yield strength or the high temperature proportional limit. As shown in Table 2, other than a comparative material 4 (conventional material) containing 0.5% of Mo, a test material 3 having a Si amount of 4.4% that is increased from that of the comparative material 4, and various cast iron samples (test materials 4 to 9) containing the test material 3 and additional elements of various kinds were produced, and the mechanical properties from 20° C. (room temperature) to 900° C. were compared and evaluated. Tables 3 to 5 and FIGS. 2 to 5 show these results.

TABLE 2


Sample No. and component composition
(Unit: wt %)

	C	Si	Mn	Cu	Sn	Cr	V	P	S	Mg	Mo	Ni	Nb	Al

Comparative	3.34	3.8	0.24	0.01	0.00	0.03	0.00	0.05	0.004	0.10	0.42	0.00	0.00	0.02
material 4
(conventional
product)
Test	3.16	4.4	0.22	0.01	0.00	0.03	0.00	0.04	0.004	0.09	0.43	0.00	0.00	0.02
material 3
(4.4Si)
Test	3.20	4.3	0.20	0.02	0.00	0.03	0.00	0.05	0.004	0.09	0.48	0.00	0.34	0.02
material 4
(0.4Nb)
Test	3.21	4.4	0.23	0.01	0.00	0.04	0.28	0.05	0.004	0.10	0.45	0.00	0.04	0.02
material 5
(0.3V)
Test	3.25	4.27	0.19					0.05	0.005	0.04	0.50	0.99
material 6
(1.0Ni)
Test	3.21	4.32	0.20	0.20				0.05	0.005	0.05	0.51
material 7
(0.2Cu)
Test	3.37	4.35	0.21					0.05	0.005	0.05	0.49			0.40
material 8
(0.4Al)
Test	3.25	4.23	0.48					0.05	0.008	0.05	0.49
material 9
(0.5Mn)

TABLE 3


Temperature and tensile strength of sample
(Unit: MPa)

No.		Test	Test	Test	Test	Test	Test	Test
Temperature	Com.	material 3	material 4	material 5	material 6	material 7	material 8	material 9
(° C.)	material 4	4.4Si	0.4% Nb	0.3% V	1.0% Ni	0.2% Cu	0.4% Al	0.5% Mn

Tensile

	20	580	640	652	681	619	655	656	641
strength	400	483	526	535	555	572	531	545	520
	500	350	379	377	399	406	384	396	383
	600	196	204	203	214	214	206	206	193
	700	92	89	92	106	89	83	90	93
	800	48	45	48	55	44	42	47	45
	850	41	35	35	44	45	34	35	46
	900	56	45		46	55			56

TABLE 4


Temperature and proportional limit of sample
(Unit: MPa)

		Test	Test	Test	Test	Test	Test	Test
No.	Com.	material 3	material 4	material 5	material 6	material 7	material 8	material 9
Temperature(° C.)	material 4	4.4Si	0.4% Nb	0.3% V	1.0% Ni	0.2% Cu	0.4% Al	0.5% Mn

Proportional	20	392	475	465	504	483	483	492	483
limit	400	312	345	359	370	363	344	363	331
	500	277	311	318	342	319	312	318	312
	600	144	179	168	179	178	182	182	166
	700	67	64	70	77	67	61	67	70
	800	31	30	32	42	34	34	40	36
	850	30	25	25	33	31	25	24	31
	900	41	31		33	50			50

TABLE 5


Temperature and elongation of sample
(Unit: %)

Elongation

20	17	14.5	11	9.3	1.8	13.3	12.3	13.7
400	16.6	14	13.4	14.6	8.1	10.8	10.3	11.8
500	23.7	17.4	26.9	20.1	14.8	15.4	14.5	15.4
600	16.6	27.1	37.8	26.4	23.3	26.1	25.6	26.7
700	25.3	22.9	27.9	34	34.6	38.4	32.8	33.4
800	34.3	30.3	26.3	24.2	48.7	49.3	50	51.4
850	60	36	44	30	42.5	82.5	78	54
900	75.7	49.9		50.5	78.1			76.2

These tables and graphs indicate that it is advantageous to add V, Mn, and Ni to the high Si and Mo-based cast iron material in order to improve the high temperature strength. [0089]

Then, a change in the mechanical properties between room temperature (20° C.) to 900° C. depending on the composition ratio of the additional elements was investigated. Tables 6 to 9 and FIGS. 6 to 8 show these results.

TABLE 6


Samples subjected to additional addition tests
(Unit: wt %)

	C	Si	Mn	P	S	Mg	Mo	Ni	V

Test	3.25	4.28	0.22	0.048	0.005	0.048	0.49	1.58
material
10
Test	3.30	4.27	0.20	0.045	0.005	0.047	0.49	2.12
material
11
Test	3.26	4.25	0.97	0.047	0.006	0.044	0.49
material
12
Test	3.34	4.31	1.46	0.099	0.007	0.043	0.48
material
13
Test	3.25	4.33	0.20	0.050	0.006	0.046	0.48		0.67
material
14
Test	3.28	4.17	0.19	0.048	0.005	0.086	0.56		0.96
material
15

TABLE 7


Change of mechanical properties by addition of Ni

				Test	Test					Test	Test
		Com.	Test	material	material			Com.	Test	material	material
		material
4	material 6	10	11			material 4	material 6	10	11
		0.0%	0.99%	1.58%	2.12%			0.0%	0.99%	1.58%	2.12%

Tensile

	20	640	619	627	653	elongation	20	14.5	1.8	2.0	1.2
strength	400	526	572			(%)	400	14.0	8.1
(MPa)	500	379	406				500	17.4	14.8
	600	204	214				600	27.1	23.3
	700	89	89	102	92		700	22.9	34.6	27.9	34.0
	750			63	68		750			38.8	36.0
	800	45	44	45	46		800	30.3	48.7	46.4	44.4
	850	35	45	50	56		850	36.0	42.5	48.3	50.8
	900	45	55	55	57		900	75.7	78.1	65.6	59.4

TABLE 8


Change of mechanical properties by addition of Mn

				Test	Test					Test	Test
		Com.	Test	material	material			Com.	Test	material	material
		material
4	material 9	12	13			material 4	material 9	12	13
		0.0%	0.48%	0.97%	1.46%			0.0%	0.48%	0.97%	1.46%

Tensile

	20	641	625	615	elongation	20	14.5	13.7	1.7	0.9
strength	400	520			(%)	400	14.0	11.8
(MPa)	500	383				500	17.4	15.4
	600	193				600	27.1	26.7
	700	93	105	120		700	22.9	33.4	30.8	27.0
	750		65	72		750			39.1	36.6
	800	45	49	51		800	30.3	51.4	48.4	75.4
	850	46	48	61		850	36.0	54.0	57.4	55.4
	900	56	57	61		900	75.7	76.2	81.1	63.0

TABLE 9


Change of mechanical properties by addition of V

				Test	Test					Test	Test
		Com.	Test	material	material			Com.	Test	material	material
		material
4	material 5	14	15			material 4	material 5	14	15
		0.0%	0.27%	0.67%	0.96%			0.0%	0.27%	0.67%	0.96%

Tensile

	20	640	681			elongation	20	14.5	9.3
strength	400	526	555			(%)	400	14.0	14.6
(MPa)	500	379	399				500	17.4	20.1
	600	204	214				600	27.1	26.4
	700	89	106	105	109		700	22.9	34.0	20	23.0
	750			75	75		750			24.6	26
	800	45	55	56	58		800	30.3	24.2	25.5	26.3
	850	35	44	46	48		850	36.0	30.0	41.7	46
	900	45	46	53	55		900	75.7	50.5	68.8	76.4

These results indicate that as the amount of Mn or Ni is larger, the tensile strength tends to be larger even in the vicinity of about 850° C., which is the upper limit of the metal temperature to which an exhaust system component is subjected. On the other hand, the addition of 0.1% of V can provide an effect, but even if V is added in an amount of 0.3% or more, there is no large change. [0094]
However, in the test material 13 (1.46% of Mn) and the test material 15 (0.96% of V), the pearlite ratio in the texture is increased, which leads to the assumption that elongation is reduced in a low temperature range and a middle temperature range. Therefore, it is found that addition of Mn and V in amounts of more than those is not appropriate. [0095]

The transformation temperatures were measured for the comparative material 4 and the materials 3, 5, 6 and 9 of the present invention. Table 10 shows the results.

TABLE 10


Transformation temperature of major samples
(Unit: ° C.)

			A1(A_c1, A_r1
	A_c1	A_r1	average)

						No.				No.		No.
						1, 2				1, 2		1, 2
	Material	No.	Start	End	average	average	Start	End	average	average		average

Com.	Conventional	1	848.0	895.2	872	872	838.8	789.8	814	814	843	843
material 4	product	2	847.2	899.1	873		834.8	790.8	813		843
Test	4.4Si	1	890.8	929.7	910	907	861.7	819.6	841	841	875	874
material 3		2	882.9	925.3	904		864.1	818.5	841		873
Test	0.3V	1	888.3	921.5	905	902	858.7	814.8	837	836	871	869
material 5		2	876.6	921.5	899		858.4	813.5	836		868
Test	1.0Ni	1	871.3	914.5	893	892	842.3	795.7	819	818	856	855
material 6		2	868.8	911.9	890		838.0	797.8	818		854
Test	0.5Mn	1	877.3	917.1	897	893	847.9	806.3	827	826	862	860
material 9		2	860.1	919.1	890		845.3	805.4	825		857

Thus, in the test material 6 to which Ni was added, a reduction of the A[0097] ₁transformation point was seen, and it was found that the test material 6 was not suitable for a material of an exhaust system component. This is because Ni is an element for stabilizing austenite and reduces the Al transformation point to the low temperature side. Therefore, it was judged that an increase in the strength that was seen from the vicinity of 850° C. in the test material 6 was due to the strength of the texture that already had been transformed to austenite, and thereafter evaluation of the materials containing Ni as an additional element was omitted.
The evaluation indicates that it is advantageous to add V and Mn in order to improve the high temperature properties. Then, an effect of multiple addition of V and Mn was investigated. Tables 11 to 13 and FIGS. [0098] 9 to 11 show the results.

TABLE 11

Chemical component of samples

with which multiple addition effect

(Unit: wt %)

C Si Mn P S Mg Mo V

Test 3.30 4.32 0.48 0.046 0.007 0.050 0.48 0.30

material 16

Test 3.34 4.31 1.46 0.048 0.007 0.051 0.50 0.31

material 17

TABLE 12


Mechanical properties of samples
to which V + Mn are added

	Com.
	mate-	Test	Test	Test
	rial
4	mate-	mate-	mate-	Test	Test
	conven-	rial	rial	rial	material	material
	tional	3	5	9	16	17
	product	4.4 Si	0.3 V	0.5 Mn	0.3% V + 0.5% Mn	0.3% V + 1.5% Mn

Tensile
strength
(MPa)
20	580	640	681	641	631	610
400	483	526	555	520	551	642
500	350	379	399	383	394	503
600	196	204	214	193	219	289
700	92	89	106	93	112	157
800	48	45	55	45	55	62
850	41	35	44	46	48	66
900	56	45	46	56	56	65
Elongation
(%)
20	17.0	14.5	9.3	13.7	2.7	0.3
400	16.6	14.0	14.6	11.8	9.9	3.3
500	23.7	17.4	20.1	15.4	16.5	9.7
600	16.6	27.1	26.4	26.7	20.8	16.3
700	25.3	22.9	34.0	33.4	26.8	21.7
800	34.3	30.3	24.2	51.4	53.2	46.4
850	60.0	36.0	30.0	54.0	51.5	63.3
900	75.7	49.9	50.5	76.2	88.7	67.3

[0100]

TABLE 13

Hardness average Pearlite ratio Graphite area ratio

Test material 16 235 HV 10% 13%

Test material 17 247 HV 40% 11%
The results indicate that a larger effect is provided when V and Mn are added at the same time in combination than is added alone. The test material 17 has high hardness and a high pearlite ratio, so that elongation is low at room temperature. However, since a reduction in the elongation in a middle temperature range of interest cannot be seen, the test material 17 can be used for an exhaust system component. Furthermore, if further elongation is necessary, annealing heat treatment can be performed to degrade pearlite. In all the examples described above, there was no observation that spheroidization was inhibited by additional elements. [0101]
3. Improving Thermal Fatigue Resistance [0102]
In the high Si spheroidal graphite cast iron containing Mo, the following two approaches are conceivable in order to enhance the thermal fatigue resistance: an approach for canceling reduction in elongation that occurs in the vicinity of middle temperature (400 to 500° C.) inherent to this material; and an approach for improving the tensile strength or the yield strength from room temperature to high temperature. [0103]
The present invention is based on the latter approach. More specifically, the present invention focuses on suppressing plastic deformation with respect to tensile strain generated in heating and cooling cycles by enhancing the yield strength (yield point or proportional limit) so as to increase the life, which is a period up to the time an initial crack occurs. [0104]

The thermal fatigue tests were conducted between 200 to 850° C. at a constraint ratio of 50%. Table 14 shows the results.

TABLE 14


Thermal fatigue life and oxidation resistance of test product

	thermal	oxidation resistance
	fatigue
	850° C. × 100 h

life	oxidation	oxidation	thickness
200-850° C.	increase	decrease	reduction
constraint	amount	amount	ratio
ratio: 50%	(mg/cm²)	(mg/cm²)	(%)

com.	conventional	263	−3.99	62.44	3.4
material	material
4
test	High Si +	237	−3.17	72.67	3.4
material	Mo
3
test	(test	293	−4.56	65.22	3.8
material	material 3) +
4	0.3 V
test	(test	277	−3.5	55.33	3.2
material	material 3) +
9	0.5 Mn
test	(test
material	material 3) +
13	1.5 Mn
test	(test
material	material 3) +
16	0.3 V + 0.5
	Mn
test	(test	384
material	material 3) +
17	0.3 V + 1.5
	Mn

Table 14 clearly indicates that the material containing V, the material containing Mn, and the material containing V and Mn in which the tensile strength and the proportional limit are improved have a better thermal fatigue life than that of the [0106] comparative material 4, which is a conventional material.
Furthermore, in order to improve or stabilize the thermal fatigue characteristics of the spheroidal graphite cast iron of the present invention, in-depth research was conducted to ensure elongation from room temperature to middle temperature. When elongation is small, the thermal fatigue life is reduced, as described above. Then, as a result of examining means for ensuring elongation without reducing the total amount or the composition ratio of V and Mn, it was found that elongation of the high Si spheroidal graphite cast iron containing Mo depends significantly on the mixing ratio of C and Si, more specifically, the Si/CE value (or C/CE value). [0107]

In other words, as shown in Table 15 and FIG. 12, the elongation of the high Si spheroidal graphite cast iron was reduced drastically when the Si/CE value was 0.97 or more.

TABLE 15


Relationship between Si/CE value and elongation

CE

Elonga-

c

Si

Mn

Cu

Sn

P

S

Mg

Mo

value

Si/CE

tion

(wt %)

value

(%)

Test	3.59	4.0	0.28	0.20	0.00	0.02	0.005	0.02	0.48	4.92	0.81	15.3
mate-
rial
18
Test	3.24	4.0	0.36	0.07	0.00	0.02	0.007	0.02	0.53	4.58	0.88	16.4
mate-
rial
19
Test	3.45	3.3	0.23	0.02	0.00	0.04	0.009	0.03	0.43	4.55	0.73	19.4
mate-
rial
20
Test	3.21	4.1	0.3	0.20	0.01	0.02	0.008	0.05	0.50	4.58	0.90	15.1
mate-
rial
21
Test	3.7	4.1	0.27	0.22	0.01	0.02	0.004	0.02	0.51	5.07	0.81	15.9
mate-
rial
22
Test	3.05	4.7	0.28	0.20	0.01	0.02	0.004	0.03	0.54	4.62	1.02	4.3
mate-
rial
23
Test	3.13	4.5	0.28	0.20	0.01	0.03	0.004	0.02	0.75	4.63	0.97	5.3
mate-
rial
24
Test	3.33	4.0	0.28	0.22	0.00	0.02	0.005	0.04	0.32	4.66	0.86	17.7
mate-
rial
25
Test	3.21	4.5	0.27	0.20	0.00	0.02	0.005	0.03	0.65	4.71	0.96	14.2
mate-
rial
26
Test	3.20	4.2	0.29	0.19	0.02	0.02	0.04	0.03	0.54	4.60	0.91	14.4
mate-
rial
27

4. Improving the Oxidation Resistance [0109]
As shown in Table 14, the results of an oxidation test in which samples were held in air at 850° C. for 100 hours indicate that the material containing V, the material containing Mn and the material containing V and Mn were substantially equal to the conventional material (comparative material 4) in the decrease amount of oxidation, the increase amount of oxidation, and the reduction ratio of thickness, and it was found that the oxidation resistance rather depends on the Si content. [0110]
The present invention is not limited to the above-described embodiments of the present invention, and can be changed or modified based on the technical idea of the present invention. Hereinafter, variations of the application in which the spheroidal graphite cast iron is used for an exhaust system component of the present invention will be described briefly. [0111]
Even one exhaust system component has a large thermal load in some places and a small thermal load in other places, and in some places, thermal expansion cannot be allowed, and in other places, thermal expansion can be allowed. Therefore, when the spheroidal graphite cast iron of the present invention is used in places having a large thermal load because welding or mechanical connection is performed or places where thermal expansion cannot be allowed, the heat resistance of the exhaust system component can be improved. Furthermore, an exhaust manifold and a turbo housing or a turbo housing-integrated exhaust manifold are cast-molded to improve the heat resistance and reduce the component cost. Furthermore, the spheroidal graphite cast iron of the present invention has not only a better heat resistance, but also compression strength that is inherent to cast iron, so that the present invention also can be applied to a substalk material by aluminum-low pressure cast that requires oxidation resistance and compression deformation resistance at high temperature. [0112]
The ferrite-based spheroidal graphite cast iron of the present invention has excellent tensile strength and yield strength in a range from 20° C., which is room temperature, to high temperature (around 800 to 900° C.). Therefore, when this spheroidal graphite cast iron is applied to an exhaust system component, for example, an exhaust manifold, the component can withstand high temperature exhaust gas sufficiently, and therefore the temperature of the exhaust gas can be increased, and efficient purification of the exhaust gas and fuel saving can be achieved. [0113]

Claims

What is claimed is:

1. A ferrite-based spheroidal graphite cast iron comprising C, Si, Mo, V, Mn, and Mg, wherein a remaining portion is composed of Fe and inevitable impurities.

2. The ferrite-based spheroidal graphite cast iron according to claim 1, wherein

contents of the elements in % by weight are as follows:

C: 3.1 to 4.0%;

Si: 3.6 to 4.6%;

Mo: 0.3 to 1.0%;

V: 0.1 to 1.0%;

Mn: 0.15 to 1.6%; and

Mg: 0.02 to 0.10%.

3. The ferrite-based spheroidal graphite cast iron according to claim 1 or 2, wherein a total content of V and Mn is 0.3 to 2.0 wt %.

4. The ferrite-based spheroidal graphite cast iron according to any one of claims 1 to 3, wherein a Si/CE value is 0.97 or less.

5. An exhaust system component produced using the ferrite-based spheroidal graphite cast iron according to any one of claims 1 to 4.

6. The exhaust system component according to claim 5, wherein the exhaust system component is an exhaust manifold, a turbo housing, a turbo housing-integrated exhaust manifold, or a turbo outlet pipe.