U.S. patent application number 10/291300 was filed with the patent office on 2004-05-13 for ferrite-based spheroidal graphite cast iron and exhaust system component using the same.
This patent application is currently assigned to Suzuki Motor Corporation. Invention is credited to Akita, Norihiro, Suzuki, Nobuaki, Yamao, Fumitaka, Yamauchi, Toshio, Zhang, Zhong-Zhi.
Application Number | 20040091383 10/291300 |
Document ID | / |
Family ID | 32830167 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040091383 |
Kind Code |
A1 |
Suzuki, Nobuaki ; et
al. |
May 13, 2004 |
Ferrite-based spheroidal graphite cast iron and exhaust system
component using the same
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.
Inventors: |
Suzuki, Nobuaki;
(Hamamatsu-shi, JP) ; Yamao, Fumitaka;
(Hamamatsu-shi, JP) ; Yamauchi, Toshio;
(Hamamatsu-shi, JP) ; Zhang, Zhong-Zhi;
(Toyota-shi, JP) ; Akita, Norihiro; (Toyota-shi,
JP) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
Suzuki Motor Corporation
Aisin Takaoka Co., Ltd.
|
Family ID: |
32830167 |
Appl. No.: |
10/291300 |
Filed: |
November 8, 2002 |
Current U.S.
Class: |
420/27 |
Current CPC
Class: |
C22C 33/08 20130101;
C22C 37/04 20130101; C22C 37/10 20130101 |
Class at
Publication: |
420/027 |
International
Class: |
C22C 037/04 |
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.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.degree. C. or less by an alloy design so as to
be used at a high temperature of 850 to 900.degree. 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.
[0003] 2. Description of the Related Art
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] On the other hand, the conventionally used high Si
spheroidal graphite cast iron has a low A.sub.c1 transformation
point of about 850.degree. 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.degree. C.),
the temperature of an exhaust system component itself is increased
to 800 to 880.degree. C. and exceeds the A.sub.c1 transformation
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.
[0011] 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
[0012] 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.
[0013] 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.
[0014] 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.degree. 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.degree. 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).
[0015] 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.
[0016] 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%.
[0017] 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.sub.3C) in the
pearlite is dissolved at high temperature, and consequently
graphite growth is facilitated, and the quality is
deteriorated.
[0018] 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 %.
[0019] 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).
[0020] 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.
[0021] 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.
[0022] Furthermore, an exhaust system component of the present
invention is produced using the ferrite-based spheroidal graphite
cast iron.
[0023] 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.
[0024] 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
[0025] FIG. 1 is a graph showing the relationship between the Si
content and the transformation temperature.
[0026] FIG. 2 is a graph showing the relationship between the test
temperature (20 to 900.degree. C.) and the tensile strength in
examples.
[0027] FIG. 3 is an enlarged graph of FIG. 2 showing the
relationship between the test temperature (700 to 900.degree. C.)
and the tensile strength.
[0028] FIG. 4 is a graph showing the relationship between the test
temperature (20 to 900.degree. C.) and the high temperature
proportional limit in examples.
[0029] FIG. 5 is an enlarged graph of FIG. 4 showing the
relationship between the test temperature (700 to 900.degree. C.)
and the high temperature proportional limit.
[0030] FIG. 6 is a graph showing the amount of Ni added and the
tensile strength in examples.
[0031] FIG. 7 is a graph showing the amount of Mn added and the
tensile strength in examples.
[0032] FIG. 8 is a graph showing the amount of V added and the
tensile strength in examples.
[0033] FIG. 9 is a graph showing the relationship between the test
temperature (20 to 900.degree. C.) and the high temperature tensile
strength in examples.
[0034] FIG. 10 is an enlarged graph of FIG. 9 showing the
relationship between the test temperature (750 to 900.degree. C.)
and the high temperature tensile strength.
[0035] FIG. 11 is a graph showing the relationship between the test
temperature (20 to 900.degree. C.) and the elongation in
examples.
[0036] FIG. 12 is a graph showing the relationship between the
Si/CE value and the elongation in examples.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] Hereinafter, ferrite-based spheroidal graphite cast iron of
this embodiment of the present invention will be described in
detail.
[0038] 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.
[0039] 1. Increasing the A.sub.1 transformation point
[0040] 2. Improving thermal deformation resistance
[0041] 3. Improving thermal fatigue resistance
[0042] 4. Improving oxidation resistance
[0043] These items will be described below.
[0044] 1. Increasing the A.sub.1 Transformation Point
[0045] 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.sub.c1 transformation point of the
A.sub.1 transformation points. The A.sub.c1 transformation 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 A.sub.1 transformation point is
increased, the spheroidal graphite cast iron is hardly transformed
to the austenite phase, giving improved heat resistance. The
A.sub.1 transformation 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%.
[0046] Thus, the A.sub.c1 transformation point can be increased to
about 890.degree. C. by adding about 4.5% of Si, whereas the
A.sub.c1 transformation point is about 850.degree. C. in the
conventional cast iron.
[0047] In general, the temperature of an exhaust system component
exposed to a high temperature exhaust gas of 880.degree. C. to
930.degree. C. is increased to the vicinity of 800 to 880.degree.
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.sub.c1 transformation 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.
[0048] 2. Improvement of Thermal Deformation Resistance
[0049] 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.
[0050] 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.
[0051] 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.degree. 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.
[0052] 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.
[0053] 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%.
[0054] 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%.
[0055] 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%.
[0056] 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%.
[0057] Thus, the content of Mo is 0.3 to 1.0%, preferably 0.3% to
0.7%.
[0058] When Ni is added, the mechanical properties at high
temperature can be improved by the addition, but the A.sub.1
transformation point is reduced, so that Ni is not suitable for the
spheroidal graphite cast iron of the present invention. The A.sub.1
transformation point is reduced, because Ni is an element for
stabilizing austenite and therefore reduces the A.sub.1
transformation point. Furthermore, it is not confirmed that the
above-described added elements prevent the spheroidal graphite cast
iron from becoming spheroidal.
[0059] Summing up, the following component constitution can improve
the high temperature properties of the spheroidal graphite cast
iron of the present invention.
[0060] 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.
[0061] 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%.
[0062] 3. Improvement of Thermal Fatigue Resistance
[0063] 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.degree. 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.
[0064] 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.degree. 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).
[0065] 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.
[0066] 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%.
[0067] 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.
[0068] 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.sub.3C) in the pearlite is
dissolved at high temperature, and consequently graphite growth is
facilitated, and the quality is deteriorated.
[0069] 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.
[0070] 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.
[0071] 4. Improvement of Oxidation Resistance
[0072] 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.
[0073] 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.
EXAMPLES
[0074] The present invention will be described more specifically by
way of example.
[0075] Various experiments were conducted, based on the following
four approaches.
[0076] 1. Increasing the A.sub.1 transformation point
[0077] 2. Improving the thermal deformation resistance
[0078] 3. Improving the thermal fatigue resistance
[0079] 4. Improving the oxidation resistance
[0080] 1. Increasing the A.sub.c1 Transformation Point
[0081] Examinations were conducted to increase the A.sub.1
transformation point at which the texture structure in which
ferrite and pearlite are mixed is transformed to the austenite
phase by heating.
[0082] 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.
1TABLE 1 Samples used for transformation temperature measurement
and measurement results A.sub.c1 A.sub.c1 A.sub.r1 A.sub.r1 A.sub.1
start A.sub.c1 end transformation start A.sub.r1 end transformation
transformation Si temperature temperature point temperature
temperature point point (wt %) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (.degree.
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
[0083] The A.sub.1 transformation 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.sub.c1
transformation point of the conventional cast iron is 889.degree.
C. (comparative material 3), but when the Si amount is increased to
4.4 to 4.5%, the highest A.sub.c1 transformation point is increased
to 908 to 911.degree. 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.degree. C. to 930.degree. C.) is lower
than the A.sub.c1 transformation 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.
[0084] 2. Improving the Thermal Deformation Resistance
[0085] 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.degree. C. (room temperature) to
900.degree. C. were compared and evaluated. Tables 3 to 5 and FIGS.
2 to 5 show these results.
2TABLE 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) Test materials 4 to 9
are based on test material 3.
[0086]
3TABLE 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
(.degree. 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
[0087]
4TABLE 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(.degree. 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
[0088]
5TABLE 5 Temperature and elongation of sample (Unit: %) Test Test
Test Test Test Test Test No. Com. material 3 material 4 material 5
material 6 material 7 material 8 material 9 Temperature(.degree.
C.) material 4 4.4Si 0.4% Nb 0.3% V 1.0% Ni 0.2% Cu 0.4% Al 0.5% Mn
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
[0089] 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.
[0090] Then, a change in the mechanical properties between room
temperature (20.degree. C.) to 900.degree. C. depending on the
composition ratio of the additional elements was investigated.
Tables 6 to 9 and FIGS. 6 to 8 show these results.
6TABLE 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
[0091]
7TABLE 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
[0092]
8TABLE 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
[0093]
9TABLE 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
[0094] 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.degree. 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.
[0095] 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.
[0096] 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.
10TABLE 10 Transformation temperature of major samples (Unit:
.degree. C.) A1(A.sub.c1, A.sub.r1 A.sub.c1 A.sub.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
[0097] Thus, in the test material 6 to which Ni was added, a
reduction of the A.sub.1 transformation 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.degree. 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.
[0098] 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. 9 to 11 show the results.
11TABLE 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
[0099]
12TABLE 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]
13 TABLE 13 Hardness average Pearlite ratio Graphite area ratio
Test material 16 235 HV 10% 13% Test material 17 247 HV 40% 11%
[0101] 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.
[0102] 3. Improving Thermal Fatigue Resistance
[0103] 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.degree. C.) inherent to this material; and an approach
for improving the tensile strength or the yield strength from room
temperature to high temperature.
[0104] 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.
[0105] The thermal fatigue tests were conducted between 200 to
850.degree. C. at a constraint ratio of 50%. Table 14 shows the
results.
14TABLE 14 Thermal fatigue life and oxidation resistance of test
product thermal oxidation resistance fatigue 850.degree. C. .times.
100 h life oxidation oxidation thickness 200-850.degree. C.
increase decrease reduction constraint amount amount ratio ratio:
50% (mg/cm.sup.2) (mg/cm.sup.2) (%) 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
[0106] 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 comparative
material 4, which is a conventional material.
[0107] 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).
[0108] 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.
15TABLE 15 Relationship between Si/CE value and elongation CE
Elonga- c Si Mn Cu Sn P S Mg Mo value Si/CE tion (wt %) (wt %) (wt
%) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (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 note: V:
0.001 wt % or less
[0109] 4. Improving the Oxidation Resistance
[0110] As shown in Table 14, the results of an oxidation test in
which samples were held in air at 850.degree. 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.
[0111] 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.
[0112] 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.
[0113] The ferrite-based spheroidal graphite cast iron of the
present invention has excellent tensile strength and yield strength
in a range from 20.degree. C., which is room temperature, to high
temperature (around 800 to 900.degree. 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.
* * * * *