JPH05179406A - Heat resistant cast steel and its production and parts for internal combustion engine - Google Patents

Abstract

PURPOSE:To provide a heat resistant cast steel showing superior heat resistance, such as thermal deformation resistance, resistance to heat check, and oxidation resistance, even in the case where exhaust gas temp. becomes high temp. and also excellent in castability, workability, and weldability. CONSTITUTION:The heat resistant cast steel can be obtained by blending at least one kind among, by weight ratio, 0.1-2% W and/or Co <=0.1% rare earth element and/or Y, 0.005-0.03% Mg and/or Ca, and 0.001-0.01% B with a composition consisting of 0.05-0.25% C, 2.5-3.5% Si, <=2% Mn, 4-8% Cr, <=0.05% N, and the balance essentially Fe with inevitable elements. This heat resistant, cast steel has a metallic matrix structure consisting substantially of ferritic phase and colony phase of pearlite state and also has superior characteristics mentioned above.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a heat-resistant cast steel which is excellent in oxidation resistance, heat crack resistance, heat deformation resistance, etc., and is also excellent in castability and workability, a method for producing the same, and a method for forming the same. Further, the present invention relates to parts such as combustion chamber parts and exhaust system parts of an internal combustion engine.

[0002]

2. Description of the Related Art Generally, the materials constituting the combustion chamber parts and exhaust system parts of internal combustion engines for automobiles, that is, gasoline engines and diesel engines, are exhaust gas temperature at full load of the engine, and further exhaust gas temperature and time. Exhaust heat quantity, component shape,
It is empirically determined in consideration of constraints on parts, heat capacity of exhaust system parts that affect the time required to reach the activation temperature of the catalytic converter for purifying exhaust gas when the engine is cold started, and the like. Therefore, in exhaust system parts such as the pre-chamber, port liner, exhaust manifold, turbocharger housing, exhaust outlet used directly below the turbocharger of the internal combustion engine for automobiles, and the catalytic converter container for exhaust gas purification, it is harsh. Since it is subject to oxidation and thermal stress under high temperature conditions, it has traditionally been relatively high heat resistant high silicon spheroidal graphite cast iron, austenitic spheroidal graphite cast iron containing a large amount of nickel, rarely austenitic heat resistant cast steel SCH1
2nd grade was used.

Particularly, when the exhaust gas temperature at full load is 900 ° C. or lower, high-silicon spheroidal graphite cast iron, FCD400, etc. are used for the exhaust manifold of a naturally aspirated engine and the exhaust gas purifying catalytic converter container connected directly below the exhaust manifold. Cast iron is mainly used, and high-silicon spheroidal graphite cast iron and austenitic spheroidal graphite cast iron are used for the exhaust manifold, turbocharger housing, and the like of engines with a supercharger because of the required function of the parts. In the latter case, high-silicon spheroidal graphite cast iron, FCD400 cast iron, etc. are mainly used for the exhaust gas purifying catalytic converter container connected directly below the turbocharger housing.

On the other hand, in the case of an ultra-high performance engine whose exhaust gas temperature at full load exceeds 900 ° C., austenite spheroidal graphite cast iron or ferritic high alloy heat-resistant cast steel is used for the exhaust manifold of the engine with a supercharger. Austenitic spheroidal graphite cast iron is also used in the exhaust manifold of some naturally aspirated high-performance engines. Further, as a turbocharger housing for such an ultra-high performance engine, those made of high alloy ferritic or austenitic heat resistant cast steel have begun to be adopted.

However, due to recent tightening of exhaust gas regulations, it has been required to further improve the exhaust gas purification efficiency at the cold start. For that purpose, an exhaust manifold is changed to an exhaust gas purification catalyst device. It is necessary to reduce the heat capacity of each component to reach the activation temperature of the catalytic converter as soon as possible after the cold start of the engine. In addition, in order to improve fuel efficiency and reduce CO ₂ emissions, it is essential to significantly reduce the weight of vehicle parts including engine parts and to improve energy efficiency through high temperature combustion.

For this purpose, SUS4 has recently been used.
BACKGROUND ART Exhaust system parts having a thin and lightweight sheet metal pipe welded structure formed by press-forming or bending a rolled plate or pipe of a ferritic stainless steel material such as 10 or SUS430 and welding have been widely used. However, such welded structural parts, such as the exhaust manifold assembly,
This results in a complicated welded structure, which requires a large manufacturing cost. Moreover, since severe thermal stress acts on this portion, it is often difficult to ensure durability (so-called heat distortion resistance, heat crack resistance, etc.).

Therefore, in order to solve the problem of the portion that is difficult to form or weld in this way, the above-mentioned so-called high-alloy heat-resistant cast steel is welded to a component cast in a complicated shape and a bent pipe. Some of the manufactured products have been put to practical use.

For example, in the case of a naturally aspirated engine, as shown in FIG. 6, a small cold start exhaust gas purifying catalyst device (auxiliary catalyst converter) 4 is provided at a position immediately connected to the exhaust manifold 1, and a downstream thereof is provided. A large-sized exhaust gas purification catalyst device (main catalytic converter) 7 is about to be provided. Sub-catalyst converter container 4
Is welded to the downstream end of the exhaust manifold 1, and the main catalytic converter container 7 is welded to the front tube welded to the downstream end of the auxiliary catalytic converter 4.
With such a structure, the heat capacity (thermal inertia) of the entire exhaust system components is reduced, whereby the exhaust gas is less likely to be deprived of heat during the process, and the exhaust gas purification performance of the catalyst during cold start is greatly improved.

On the other hand, in such an exhaust system component,
When the engine is fully loaded, high-temperature exhaust gas exceeding 900 ° C passes through, so it is strongly desired to have excellent heat resistance (oxidation resistance, heat crack resistance, heat distortion resistance).

Against this background, US Pat. No. 4,7,4
No. 90,977 is a steel alloy consisting of a ferritic phase having good oxidation resistance and creep strength at high temperature, in a weight ratio, C: about 0.01% to about 0.3%, Mn: about 2% or less, Si:
More than 2.35% to about 4%, Cr: about 3% to about 7%, Ni:
About 1% or less, N: about 0.15% or less, Al: less than 0.3%, at least one carbide and nitride forming element (Nb, Ta, V,
A steel alloy containing Ti, Zr): 1.0% or less, Mo: up to 2% and the balance substantially consisting of Fe is disclosed.

[0011]

However, it has been found that the steel alloy consisting of this ferritic phase does not necessarily exhibit satisfactory heat resistance, particularly heat distortion resistance, when exposed to a high temperature of 800 ° C. or higher.

In order to solve the above problems, the present invention provides excellent heat resistance such as heat distortion resistance, heat crack resistance and oxidation resistance when the exhaust gas temperature is 800 ° C or higher, particularly 900 ° C to 950 ° C. It is an object of the present invention to provide a heat resistant cast steel which exhibits excellent properties, is excellent in castability, workability and weldability and is inexpensive.

Another object of the present invention is to provide a method for producing such heat resistant cast steel.

Still another object of the present invention is to provide an internal combustion engine component made of such heat-resistant cast steel.

[0015]

As a result of earnest research to solve the above-mentioned problems, the present inventors have found that it is necessary to form a pearlite-like colony phase in the matrix in order to improve heat distortion resistance. It was found that the elements of the colony phase were not added, and the matrix of the iron alloy was substantially a ferrite phase and a pearlite-like colony phase, and (a) W and / or Co, (b) A heat-resistant cast steel capable of satisfying the above heat resistance requirements can be obtained by blending one or more of rare earth elements and / or Y, (c) Mg and / or Ca, and (d) B in combination. The present invention was discovered, and the present invention was conceived. In order to satisfy these characteristics, the area ratio of the colony phase needs to be about 15% or more.

That is, in the first heat-resistant cast steel of the present invention, the chemical composition by weight ratio is C: 0.05-0.25%, Si: 2.5-3.5.
%, Mn: 2% or less, Cr: 4 to 8%, N: 0.05% or less, W
And / or Co: 0.1 to 2%, the balance substantially consisting of Fe and an unavoidable element, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase. Here, “substantially” means that the metal structure at room temperature is M ₂₃ C
It is characterized in that it is composed of a pearlite-like colony phase, which is a co-eutectoid structure of a metal-carbon compound represented by ₆ and ferrite, and a ferrite phase. It means that things may exist. The second to tenth heat-resistant cast steels of the present invention described below have the same meaning.

In the second heat-resistant cast steel of the present invention, the chemical composition by weight ratio is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2
% Or less, Cr: 4 to 8%, N: 0.05% or less, rare earth element and / or Y: 0.1% or less, and the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure is substantially ferrite phase and pearlite. It is characterized in that it consists of a colony phase.

In the third heat-resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, Mg and / or
Ca: 0.005 to 0.03%, the balance substantially consisting of Fe and unavoidable elements, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase.

In the fourth heat-resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, B: 0.001 to
0.01%, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure is characterized by consisting essentially of a ferrite phase and a pearlite-like colony phase.

In the fifth heat resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase. Characterize.

In the sixth heat-resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, Mg and / or Ca: 0.005 to 0.03%, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase. Characterize.

In the seventh heat-resistant cast steel of the present invention, the chemical composition by weight ratio is C: 0.05 to 0.25%, Si: 2.5 to 3.5%, Mn: 2
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1-2%, B: 0.001-0.01%, balance substantially Fe
And an unavoidable element, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase.

In the eighth heat-resistant cast steel of the present invention, the chemical composition is C, 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, Mg and / or Ca: 0.005 to 0.03%, balance substantially Fe
And an unavoidable element, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase.

In the ninth heat resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr 4 to 8%, N: 0.05% or less, W and / or C
o: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, B: 0.001 to 0.01%, the balance consisting essentially of Fe and unavoidable elements, and the metallic matrix structure is substantially ferrite phase and pearlite-like And a colony phase.

In the tenth heat-resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr 4 to 8%, N: 0.05% or less, W and / or C
o: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, Mg and / or Ca: 0.005 to 0.03%, B: 0.001 to 0.
It is characterized in that 01%, the balance consists essentially of Fe and unavoidable elements, and the metal matrix structure consists essentially of a ferrite phase and a pearlite-like colony phase.

The method for producing the heat-resistant cast steel of the present invention is
The molten metal whose composition after solidification is the above composition is poured into the sand mold under reduced pressure or poured into the precision casting mold, and the temperature of the hottest part of the cast product after solidification is 900 ° C or less. It is characterized in that the mold is released by natural cooling in the mold until that time. By this method, the metallic matrix structure at room temperature becomes M ₂₃ C.
A heat-resistant cast steel having a pearlite-like colony phase, which is a eutectoid structure of a metal / carbon compound represented by _{6 and the} like, and a ferrite phase, and having excellent heat resistance can be obtained.

Further, the internal combustion engine component of the present invention has a temperature of 800 ° C.
It is exposed to the above-mentioned high temperature combustion gas or exhaust gas, and is characterized in that at least a part thereof is formed of heat resistant cast steel having the above composition.

The present invention will be described in detail below. First, the reasons for specifying the content of each element will be described below.

(A) C: 0.05 to 0.25 wt% C is an important element that influences thermal fatigue properties such as heat distortion resistance under the constraint of thermal strain. Generally, as the content increases, tensile properties and creep The characteristics are improved. On the other hand, in order to secure excellent weldability, it is required that the hardness of the weld boundary portion be as low as possible, but if it is less than 0.05% by weight, the heat distortion resistance is sharply reduced. The area ratio of colonies must be about 15% or more to ensure heat distortion resistance. For that purpose, the lower limit of the C content is 0.05% by weight. However, if the C content exceeds 0.25% by weight, not only does it form an excessive amount of carbide with Cr, W, etc. that form a solid solution in the matrix to improve the oxidation resistance, and not only reduce the important oxidation resistance of the heat-resistant material. , The as-cast structure is no longer a mixed structure of ferrite and pearlite colonies, and the A ₁ transformation point is 850 ℃
Is less than 1, which reduces the thermal fatigue life. Therefore, the C content needs to be 0.25% by weight or less.

In order to secure good heat distortion resistance, the C content is preferably about 0.12% by weight or more.
Therefore, even if the high temperature strength is sacrificed, the preferable content of C is 0.05 to 0.12.
%, And in parts where heat distortion resistance is of primary importance, the preferred C content is 0.12 to 0.18% by weight.

(B) Si: 2.5 to 3.5% by weight Si pushes up the A ₁ transformation point to the high temperature side and has a great effect of improving the oxidation resistance. Furthermore, it has the effects of improving the castability, acting as a deoxidizer, and reducing gas defects in the casting.
To obtain such effects, 2.5 wt% or more is required. On the other hand, if too much Si is dissolved in the ferrite matrix, the toughness and weldability will deteriorate.
The upper limit of the content is 3.5% by weight. The preferable Si content is 2.8 to 3.2% by weight.

(C) Mn: 2% by weight or less Mn is also effective as a deoxidizer like Si, and improves the flowability of molten metal during casting to improve productivity. However, if it exceeds 2% by weight, the material becomes brittle, which is inconvenient.
The preferable Mn content is 0.2 to 0.8% by weight.

(D) Cr: 4 to 8 wt% Cr is an important element in the present invention because it improves the oxidation resistance together with Si and raises the A ₁ transformation temperature. The oxidation resistance needs to be higher than that of the high silicon spheroidal graphite cast iron or austenite spheroidal graphite cast iron that is the substitute material, so the Cr content is set to 4% by weight or more in consideration of the Si content. On the other hand, if it exceeds 8% by weight, the flowability of the molten metal deteriorates, and the castability decreases.

(E) N: 0.05% by weight or less N is an element which improves the high temperature strength like C, but 0.05
If it is contained in the molten metal in an amount exceeding 5% by weight, it induces gas defects such as pinholes during solidification and impairs stability during production. Therefore, in order to ensure stable manufacturability, the content of N is set to 0.05% by weight or less.

The first heat-resistant cast steel of the present invention is characterized by containing W and / or Co in addition to the above basic components.

(F) W and / or Co: 0.1 to 2 wt% W and / or Co improve mechanical properties such as tensile strength at high temperature as a solid solution strengthening element, so at least 0.1 wt%. Need to be included. On the other hand, if the content is too large,
Ductility decreases on the low temperature side (less than 150 ° C) of the normal temperature and causes welding cracks. Therefore, it should not be contained in excess of 2% by weight. When both W and Co are added and contained at the same time, the total amount should be within the range of 0.1 to 2% by weight. The preferred amount of W and / or Co added is 0.2-
It is 0.8% by weight.

In the second heat-resistant cast steel of the present invention, the chemical composition by weight ratio is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2
% Or less, Cr: 4 to 8%, N: 0.05% or less, rare earth element and / or Y: 0.1% or less, and the balance substantially consisting of Fe and unavoidable elements, and the metal matrix structure is substantially It consists of a ferrite phase and a pearlite-like colony phase. The second heat resistant cast steel is characterized by containing a rare earth element and / or Y as described below. La, C as rare earth elements
e, Nd, Pr, Sm and the like are preferable. Especially, a misch metal mainly composed of La and Ce is preferable because of its low cost. Other components are the same as those of the first heat resistant cast steel.

(G) Rare earth element and / or Y: 0.1% by weight
Hereinafter, the rare earth element and / or Y has the effect of improving the oxidation resistance at high temperatures, but even if the content thereof exceeds 0.1% by weight, the corresponding improvement in effect cannot be obtained. Therefore, the upper limit of the rare earth element and / or Y is 0.1% by weight. The lower limit is 0.005% by weight. If it is less than these lower limits, the effect of addition is hardly obtained.

In the third heat-resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, Mg and / or
Ca: 0.005 to 0.03%, the balance substantially consisting of Fe and unavoidable elements, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase. The third heat-resistant cast steel is characterized by containing Mg and / or Ca as described below. Other components are the same as those of the first heat resistant cast steel.

(H) Mg and / or Ca: 0.005 to 0.03 wt% Mg and / or Ca has a deoxidizing and desulfurizing effect and also has a function of improving the ductility by spheroidizing the inclusions. The inclusions are composed of Si, Mn, etc., for example, Mg,
A compound of a metal element composed of Si, Mn, Al, etc. and O or S,
That is, it is an oxide or a sulfide. If the content of Mg and / or Ca is less than 0.005% by weight, sufficient effect cannot be obtained, while if it exceeds 0.03% by weight, embrittlement occurs. When Mg and Ca are added simultaneously, the total content of both is 0.005〜
It is within the range of 0.03% by weight. The preferred addition amount of Mg and / or Ca is 0.015 to 0.02% by weight.

In the fourth heat resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, B: 0.001 to
It has a composition of 0.01%, the balance being substantially Fe and unavoidable elements, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase. The fourth heat resistant cast steel is characterized by containing B as described below. Other components are the same as those of the first heat resistant cast steel.

(I) B: 0.001 to 0.01% B not only strengthens the crystal grain boundaries of cast steel, but also makes the grain boundary carbides finer and prevents their agglomeration and coarsening, improving the high temperature strength and toughness. Therefore, 0.001% by weight or more is desirable. On the other hand, a large amount of B precipitates the B compound and deteriorates the high temperature strength, so the upper limit is 0.01% by weight. The preferred B content is 0.005 to 0.01% by weight.

In the fifth heat resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, the balance substantially consisting of Fe and unavoidable elements, and the metal matrix structure is substantially a ferrite phase and a pearlite-like colony phase. Consists of. The fifth heat resistant cast steel is W and / or
Alternatively, since it contains Co and a rare earth element and / or Y at the same time, it is excellent in high temperature strength and oxidation resistance. The references regarding the contents of W and / or Co, the rare earth element and / or Y may be the same as above. Other components are the same as those of the first heat resistant cast steel.

In the sixth heat-resistant cast steel of the present invention, the chemical composition by weight ratio is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, Mg and / or Ca: 0.005 to 0.03%, the balance being substantially Fe and inevitable elements, and the metal matrix structure is substantially a ferrite phase and a pearlite-like colony phase. Consists of. The sixth heat-resistant cast steel is W and / or Co
And Mg and / or Ca at the same time, they are excellent in high temperature strength and toughness. In addition, W and / or Co, Mg and /
Alternatively, the reference regarding the content of Ca may be the same as the above. Other components are the same as those of the first heat resistant cast steel.

In the seventh heat-resistant cast steel of the present invention, the chemical composition is, by weight ratio, C: 0.05 to 0.25%, Si: 2.5 to 3.5%, Mn: 2
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1-2%, B: 0.001-0.01%, balance substantially Fe
And a metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase. Since the seventh heat-resistant cast steel contains W and / or Co and B at the same time, it is excellent in high temperature strength and toughness. The content of W and / or Co and B may be the same as above. Other components are the same as those of the first heat resistant cast steel.

In the eighth heat resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, Mg and / or Ca: 0.005 to 0.03%, balance substantially Fe
And a metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase. The eighth heat-resistant cast steel contains W and / or Co, a rare earth element and / or Y, and Mg and / or Ca at the same time, and is therefore excellent in high temperature strength, toughness and oxidation resistance.
The references regarding the contents of W and / or Co, the rare earth element and / or Y, Mg and / or Ca may be the same as above. Other components are the same as those of the first heat resistant cast steel.

In the ninth heat resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, rare earth element and / or Y: 0.1 or less,
B: 0.001 to 0.01% with the balance being substantially Fe and inevitable elements, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase. The ninth heat-resistant cast steel is W and / or Co, and rare earth elements and / or Y
And B are contained at the same time, they are excellent in high temperature strength, toughness and oxidation resistance. The references regarding the contents of W and / or Co, the rare earth element and / or Y, and B may be the same as above. Other components are the same as those of the first heat resistant cast steel.

In the tenth heat-resistant cast steel of the present invention, the chemical composition is C: 0.05-0.25%, Si: 2.5-3.5%, Mn: 2 by weight ratio.
% Or less, Cr: 4 to 8%, N: 0.05% or less, W and / or
Co: 0.1 to 2%, rare earth element and / or Y: 0.1 or less, M
g and / or Ca: 0.005-0.03%, B: 0.001-0.01
%, The balance being substantially Fe and an unavoidable element, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase. The tenth heat-resistant cast steel is W and / or
Or Co, rare earth element and / or Y, Mg and / or Ca
And B are contained at the same time, they are excellent in high temperature strength, toughness and oxidation resistance. The content of W and / or Co, the rare earth element and / or Y, Mg and / or Ca, B may be the same as above. Other components are the same as those of the first heat resistant cast steel.

The heat-resistant cast steel of the present invention having the above-described composition is poured into a sand mold under reduced pressure or a molten metal whose composition after solidification is the above composition is poured into a precision casting mold and solidified. By naturally cooling in the mold until the temperature of the hottest part of the post-casting product reaches 900 ° C or lower, and removing the mold,
It can be manufactured.

When pouring into the mold at room temperature, the mold is kept under reduced pressure and the molten metal is poured. This is because the melt having the composition that gives the cast product having the above composition has relatively low fluidity (melt flowability).
Therefore, it is an indispensable technology for sound casting of extremely thin-walled internal combustion engine parts. The reduced pressure condition is generally about 5 kPa to 40 kPa.

As for the mold releasing temperature, the maximum temperature of the cast product is 900
The temperature is below ℃. If there is a portion exceeding 900 ° C, it is rapidly cooled and the metal matrix structure becomes a sorbite-like hard structure, and desired material properties cannot be obtained.

The heat-resistant cast steel cast product which can be produced by such a method can be thinned to 3 mm or less. As described above, thinning is important for reducing the heat capacity (thermal inertia) of exhaust system components.

The cast parts made of the heat-resistant cast steel of the present invention have extremely good heat resistance. 900 for oxidation resistance
Oxidation weight loss is 0.003g / cm ² or less when kept in the air at ℃ for 200 hours continuously. Further, the material of the present invention is excellent in heat crack resistance and heat distortion resistance even under a thermal fatigue cycle in which the material temperature is repeatedly heated and cooled between 900 ° C. and room temperature, and the exhaust gas temperature is 800 ° C. or higher, particularly about 900 to 950. Even if the exhaust system components are exposed to the vibrations that they would be subjected to under normal engine operating conditions in the temperature range of 0 ° C, fatigue cracks do not occur. Moreover, the heat-resistant cast steel of the present invention does not have an A ₁ transformation point at a material temperature of 900 ° C. or lower even under the condition where the thermal strain is completely restrained, and has excellent heat distortion resistance.

[0054]

EXAMPLES The present invention will be described below with reference to examples. Examples 1 to 35, Comparative Examples 1 to 7 and Conventional Examples 8 to 12 Table 1 shows the chemical components of Examples of the heat resistant cast steel of the present invention.
Further, Tables 2 and 3 show the chemical components of the comparative example and the conventional example that were cast to compare the respective properties. The comparative materials in Table 2 were cast in order to confirm the superiority of the heat resistant cast steel of the present invention. In addition, Conventional Examples 8 to 12 shown in Table 3 are practically used for exhaust system heat-resistant parts such as an exhaust manifold for automobiles and turbocharger housings, and Conventional Example 8 is a ferrite high Si spheroidal graphite cast iron. Conventional Example 9 is austenitic spheroidal graphite cast iron, and Conventional Example 10 is JI
It is a ferritic heat-resistant cast steel corresponding to the heat-resistant cast steel SCH1 of S, and Conventional Examples 11 and 12 are ferritic heat-resistant cast steel alloys disclosed in US Pat. No. 4,790,977. (-) In Tables 1 to 3 means that the chemical components are not analyzed.

When casting each test material, the melting capacity 1
Immediately melts in the atmosphere using a 00kg high frequency melting furnace and immediately at 1550 ℃
With the above, tapping the hot water and molding it with CO ₂ sand
A test piece for test was cast by pouring it into a mold for JIS standard Y type B test material at 1500 ° C. or higher.

Then, the test material of the material of the present invention cast by the above method was placed in a heating furnace in an air atmosphere.
After holding at 00 ° C. for 2 hours, it was taken out from the heating furnace and heat-treated by natural cooling in the atmosphere.

On the other hand, the comparative materials in Table 2 were subjected to the same heat treatment as described above, if necessary. Further, Conventional Example 8 in Table 3 was subjected to the test as cast, and Conventional Example 9 was held in a heating furnace in an air atmosphere at 900 ° C. for 2 hours, then taken out from the heating furnace and naturally cooled in the air. Heat treatment was performed. Next, for Conventional Example 10, after holding in a heating furnace at 920 ° C. for 2 hours,
The furnace was cooled to 00 ° C., taken out of the heating furnace, and heat-treated by natural cooling in the atmosphere. Further, in Conventional Examples 11 and 12, a heat treatment was performed in which the furnace was held at 1120 ° C. for 2 hours, then cooled to 900 ° C., taken out of the heating furnace, and naturally cooled in the atmosphere.

Table 1 Chemical composition (wt%) Example C Si Mn P S Cr N 1 0.08 2.70 0.41 0.005 0.006 4.25 0.02 2 0.10 2.68 0.52 0.004 0.005 4.53 0.03 3 0.07 2.59 0.45 0.005 0.006 4.41 0.03 4 0.14 3.02 0.48 0.005 0.007 6.02 0.03 5 0.16 3.15 0.55 0.004 0.006 6.18 0.04 6 0.17 3.08 0.44 0.004 0.005 6.12 0.03 7 0.24 3.41 0.58 0.006 0.006 7.91 0.02 8 0.23 3.47 0.42 0.007 0.005 7.57 0.02 9 0.22 3.38 0.55 0.005 0.007 7.83 0.03 10 0.14 3.12 0.48 0.005 0.005 6.05 11 0.16 3.05 0.42 0.004 0.006 6.15 0.03 12 0.16 3.09 0.52 0.005 0.006 6.21 0.02 13 0.15 2.92 0.51 0.005 0.006 6.11 0.03 14 0.17 2.98 0.49 0.005 0.006 6.04 0.02 15 0.16 3.15 0.52 0.007 0.005 6.20 0.02 16 0.13 3.06 0.50 0.004 0.007 6.08 0.03 17 0.15 3.18 0.41 0.004 0.008 6.22 0.03 18 0.12 2.95 0.42 0.004 0.006 6.00 0.02 19 0.16 3.05 0.51 0.004 0.005 6.02 0.03 20 0.15 3.10 0.58 0.004 0.006 6.09 0.03 21 0.13 3.04 0.42 0.005 0.007 6.14 0.03 22 0.14 3.09 0.45 0.004 0.008 6.21 0.03 23 0.16 3.11 0.55 0.006 0.006 6.13 0.03 24 0.17 3.01 0.62 0.004 0.005 5.93 0.03 25 0.18 2.95 0.47 0.005 0.006 6.48 0.02

Table 1 (continued) Chemical composition (wt%) Example W Co REM ^* Y Mg Ca B 1 0.24 − − − − − − − 2 − 0.15 − − − − − 3 0.55 0.41 − − − − − 4 1.21 − − − − − − 5 − 1.03 − − − − − 6 1.12 0.78 − − − − − 7 1.95 − − − − − − 8 − 1.97 − − − − − 9 0.79 1.18 − − − − − 10 − − 0.03 − − − − 11 − − − 0.01 − − − 12 − − 0.04 0.01 − − − 13 − − − − 0.010 − − 14 − − − − − 0.013 − 15 − − − − 0.018 0.005 − 16 − − − − − − 0.003 17 − − − − − − 0.008 18 0.50 − 0.01 − − − − 19 − 0.82 − 0.007 − − − 20 0.62 0.48 0.01 0.003 − − − 21 0.72 − − − 0.008 − − 22 − 1.02 − − − 0.008 − 23 0.58 0.62 − − 0.008 0.006 − 24 0.55 − − − − − 0.004 25 − 1.02 − − − − 0.008

Table 1 (continued) Chemical composition (wt%) Example C Si Mn P S Cr N 26 0.17 2.99 0.46 0.005 0.006 6.18 0.03 27 0.14 3.14 0.42 0.004 0.005 6.33 0.03 28 0.15 3.03 0.49 0.004 0.006 6.14 0.03 29 0.16 3.18 0.44 0.005 0.006 6.08 0.03 30 0.18 3.30 0.52 0.005 0.006 6.17 0.03 31 0.16 3.09 0.58 0.004 0.006 6.02 0.03 32 0.14 3.18 0.55 0.003 0.006 6.16 0.03 33 0.19 3.28 0.61 0.004 0.006 6.28 0.02 34 0.14 3.09 0.43 0.004 0.006 6.19 0.03 35 0.16 3.01 0.40 0.004 0.006 6.08 0.03

Table 1 (continued) Chemical composition (wt%) Example W Co REM ^* Y Mg Ca B 26 0.50 0.48 − − − − 0.006 27 1.02 0.20 0.03 0.01 0.018 0.008 − 28 0.20 0.98 0.05 0.02 0.011 0.007 − 29 0.50 0.48 0.03 0.01 0.007 0.008 − 30 1.08 0.15 0.01 0.01 − − 0.005 31 0.18 1.32 0.02 0.02 − − 0.008 32 0.68 0.51 0.02 0.02 − − 0.009 33 0.22 0.22 0.01 0.01 0.006 0.007 0.003 34 0.50 0.58 0.01 0.01 0.008 0.009 0.005 35 0.58 0.49 0.02 0.01 0.010 0.011 0.008

Table 2 Chemical composition (% by weight) Comparative example C Si Mn P S Cr N 1 0.16 1.51 0.50 0.004 0.003 4.05 0.02 2 0.23 2.23 0.51 0.010 0.003 4.18 0.03 3 0.15 3.65 0.47 0.006 0.005 4.59 0.03 4 0.09 4.53 0.50 0.005 0.004 4.40 0.03 5 0.27 3.42 0.48 0.004 0.003 7.82 0.03 6 0.03 3.08 0.51 0.004 0.005 5.83 0.03 7 0.15 3.12 0.49 0.005 0.005 6.01 0.03

Table 2 (continued) Chemical composition (% by weight) Comparative example W Co REM ^* Y Mg Ca B 1 0.51 − − − − − − − 0.53 − − − − − − 3 0.48 − − − − − − 4 0.50 − − − − − − 5 0.48 − − − − − − 6 0.51 − − − − − − 7 − − − − − − − (Note) *: Rare earth element (Ce: 50%, La: 30%, Nd: 15
%, Pr: 4%, Sm: 1% misch metal)

Table 3 Chemical composition (% by weight) Conventional example C Si Mn P S Cr 8 3.25 4.11 0.46 0.018 0.010 -9 2.86 2.58 0.85 0.015 0.006 2.46 10 0.44 2.26 0.31 0.013 0.004 8.31 11 0.03 3.04 0.41 0.004 0.003 4.95 12 0.15 3.18 0.52 0.004 0.003 5.16

Table 3 (continued) Chemical composition (% by weight) Conventional example Mo Nb Ti Ni Mg 8 − − − − 0.044 9 − − − 20.9 0.085 10 − − − − − 11 − 0.29 0.15 − − 12 − 0.57 0.16 − −

Various tests as described below were carried out using each of the test materials manufactured by the above method. First,
For the purpose of analyzing the factors that govern the thermal fatigue life, the diameter of 10
Oxidation tests were carried out at 900 ° C for 200 hours in air using a solid cylindrical test piece with a length of 20 mm and a length of 20 mm. As an evaluation method of the oxidation test, the oxide film formed on the surface of the test piece was removed by sandblasting after the oxidation test, and the weight change per unit surface area before and after the oxidation test (oxidation weight loss: g / cm ² ) was evaluated.

Further, in order to examine the tensile properties, tensile tests were carried out at room temperature and high temperature. JIS Z for normal temperature tensile test
A No. 4 standard tensile test piece defined in 2201 was used. Also,
For the high temperature tensile test, a high temperature tensile test piece with a diameter of 10 mm and a gauge length of 50 mm specified by JIS G 0567 is used, and the temperature is 850 ° C.
The tests were carried out in.

Further, by using a test piece having a diameter of 3 mm and a length of 10 mm to measure the amount of thermal expansion by vacuum heating, it is known that the thermal deformation resistance is impaired when the component is in the normal temperature range. I checked the points. The impact value was measured at room temperature using a U-notched groove test piece (JIS No. 3 test piece).

Further, since exhaust system parts for internal combustion engines used in a state where thermal expansion and contraction during heating and cooling are restrained undergo severe thermal fatigue, heat-resistant materials for exhaust system parts are It is primarily important that cracks and deformation due to thermal fatigue are unlikely to occur. Therefore, the thermal fatigue life of each of the materials of the present invention was investigated by using an electric / hydraulic servo type thermal fatigue tester.

For the thermal fatigue test, a test piece having a gauge length of 20 mm and a gauge diameter of 10 mm was used, and the radio frequency coil output and cooling air injection were controlled between the gauge marks of the test specimen,
Heated and cooled. The elongation and contraction between the gauge marks caused by this heating and cooling was mechanically completely restrained by using an extensometer (in this case, the phase of temperature and strain have an inverse phase relationship). The heating / cooling conditions were a minimum temperature of 150 ° C., a temperature rising time to the upper limit temperature of 2 minutes, a holding time at the upper limit temperature of 900 ° C. of 1 minute, and a temperature decreasing time to the lower limit temperature of 4 minutes, which was one cycle of 7 minutes. .. The thermal fatigue life depends on the fracture between test piece gauge points, or the necking between test piece gauge points reduces to 75% of the tensile load at the lower limit temperature of the second cycle, whichever comes first. Defined.

Table 4 summarizes the conditions of the as-cast matrix structure, the oxidation test, the tensile test, the transformation point measurement, the Charpy impact test, and the thermal fatigue test for the examples, comparative examples and conventional examples of the material of the present invention. Show things. The materials of the present invention shown in Table 4 are colony phase area / (ferrite phase area + colony phase area).
That is, since the colony phase area ratio was 15 to 90%, it has excellent heat resistance. A typical as-cast matrix structure of the material of the present invention is shown in the micrographs (100 times) of FIGS. 1 and 2. The area ratio of the colony phase was about 30% in FIG. 1 and that of Example 7 in FIG. 2 was about 90%.

On the other hand, the as-cast matrix structure of Comparative Example 1 is shown in FIG. Almost the entire surface of the base has a sorbite-like hard structure. The reason for this is that the content of Si, which has a ferritization promoting action, is smaller than the content of C. Since the sorbite-like structure is hard, it remains brittle as it is, and it is extremely difficult to machine it. Of course, this sorbite is 85
When annealed at a temperature of 0 ° C to 900 ° C, it decomposes into a ferrite phase and granular carbides, but it requires a lot of cost for heat treatment, and it causes large heat treatment strain for extremely thin-walled parts. It is not preferable that the metal matrix structure is sorbite.

For the purpose of comparing the oxidation resistance, the test piece of the comparative example was held at 920 ° C. for 2 hours in a heating furnace and then kept at 800
The furnace was cooled to ℃, taken out from the heating furnace, and subjected to a heat treatment of natural cooling in the atmosphere. Comparative Example 1 is considerably worse than the material of the present invention, but this is because the Si content is significantly smaller than that of the material of the present invention. Further, also in the as-cast matrix structure of Comparative Example 2, almost the entire matrix exhibited a sorbite-like hard structure as in Comparative Example 1. The reason for this is also that the content of Si having a ferritization promoting effect is smaller than the content of C.

Comparative Examples 3 and 4 have a Si content of 3.5% by weight.
It was melted in order to compare each property in the case of exceeding. In both cases, the as-cast matrix structure is a mixed structure of ferrite phase and pearlite-like colonies, but the ductility decreases due to the large Si content, and as shown in Table 4, there is almost no room-temperature elongation at break and even 0.2% proof stress. It was impossible to measure.

The as-cast matrix structure of Comparative Example 5 is shown in FIG.
Similar to Comparative Examples 1 and 2, almost all the surface of the matrix has a sorbite-like hard structure. This is also Comparative Examples 1 and 2
Similarly to the above, the content of Si having a ferrite formation promoting action is C
Due to the small amount relative to the content.

Comparative Example 6 was prepared for the purpose of comparing the respective properties when the C content was less than 0.05% by weight. The as-cast matrix structure is a mixed structure of ferrite phase and pearlite-like colonies, but since the C content is 0.03% by weight, the colony area ratio is 10% or less, and the thermal fatigue life is compared with the material of the present invention. Then it is inferior. Further, the tensile properties, which are important for improving the heat distortion resistance, are lower than those of the material of the present invention.

Comparative Example 7 is prepared by melting for the purpose of comparing the respective properties when W, Co, B and the like, which improve the tensile properties, are not contained. Although the as-cast matrix structure is a mixed structure of ferrite phase and pearlite-like colonies, W,
Compared with the material of the present invention containing Co, B, etc., the tensile strength is low and therefore the thermal fatigue life is short.

Next, in Conventional Examples 8 and 9, both the oxidation resistance and the thermal fatigue property are so low as to be incomparable with the material of the present invention. Therefore, it cannot be applied to the parts to which the present invention material is applied.

Further, the conventional example 10 has a high C and low Si content as compared with the material of the present invention, so that the as-cast matrix structure is sorbite-like, the transformation point is low, and the required material properties are important for exhaust system parts. Both the oxidation resistance and the thermal fatigue property are lower than those of the material of the present invention. Furthermore, Conventional Examples 11 and 12 contain Nb and Ti that inhibit the formation of pearlite-like colonies, so that the entire cast-cast matrix structure becomes almost a ferrite phase, and tensile properties and thermal fatigue properties are compared with the material of the present invention. Low.

The materials of the present invention shown in Table 4 have a colony area ratio defined by colony phase / (ferrite phase + colony phase) of about
It is 15 to 90%, and it is understood that it is comprehensively excellent in various characteristics that should be provided as an exhaust system member for an internal combustion engine. Since the materials of the present invention have thermal fatigue properties superior to any of the comparative materials, it is understood that they are excellent materials in terms of heat distortion resistance and heat crack resistance, which are important required characteristics particularly for exhaust system parts.

Table 4 Charpy thermal fatigue life Oxidation weight loss Transformation point Impact value Life rupture service Example Structure ⁽¹⁾ (g / cm ² ) (° C) (J / cm ² ) Number of icles 1 ○ 0.0022 1000 or more 3.0 251 2 ○ 0.0019 1000 or more 3.0 265 3 ○ 0.0020 1000 or more 4.2 304 4 ○ 0.0016 1000 or more 5.0 454 5 ○ 0.0017 1000 or more 4.2 488 6 ○ 0.0015 1000 or more 3.0 495 7 ○ 0.0013 1000 or more 2.7 502 8 ○ 0.0015 1000 or more 2.7 551 9 ○ 0.0014 1,000 or more 2.7 563 10 ○ 0.0012 952 6.3 354 11 ○ 0.0011 965 5.0 316 12 ○ 0.0010 970 3.7 385 13 ○ 0.0017 960 4.2 337 14 ○ 0.0017 970 3.7 349 15 ○ 0.0016 950 3.2 311 16 ○ 0.0018 965 4.2 325 17 ○ 0.0017 955 3.5 385 18 ○ 0.0011 1000 or more 2.7 465 19 ○ 0.0012 975 3.0 428 20 ○ 0.0010 1000 or more 2.7 486 21 ○ 0.0019 1000 or more 2.7 506 22 ○ 0.0020 985 2.7 498 23 ○ 0.0017 1000 or more 3.0 457 24 ○ 0.0017 985 3.0 425

Table 4 (continued) Charpy thermal fatigue life Oxidation weight loss Transformation point Impact value Life rupture Example Example Structure ⁽¹⁾ (g / cm ² ) (℃) (J / cm ² ) Number of icles 25 ○ 0.0017 1000 or more 2.7 592 26 ○ 0.0018 1000 or more 4.2 558 27 ○ 0.0012 980 2.7 540 28 ○ 0.0012 1000 or more 3.0 497 29 ○ 0.0010 1000 or more 4.2 614 30 ○ 0.0011 978 6.3 588 31 ○ 0.0010 1000 or more 2.7 582 32 ○ 0.0010 1000 or more 3.0 601 33 ○ 0.0009 965 2.7 657 34 ○ 0.0011 985 3.0 602 35 ○ 0.0012 980 2.7 624 Comparative Example 1 × 0.238 − − − 2 × 0.0044 − − − 3 ○ 0.0014 − 2.3 355 4 ○ 0.0018 − 2.1 201 5 × 0.0021 − − − 6 △ 0.0014-3.5 183 7 ○ 0.0021-3.0 191

Table 4 (continued) Charpy thermal fatigue life Oxidation weight loss Transformation point Impact value Life rupture Conventional structure Microstructure ⁽¹⁾ (g / cm ² ) (° C) (J / cm ² ) Number of ukule 8 * ⁽²⁾ 0.185 805 2.0 34 9 ** ⁽³⁾ 0.137 None 15.0 15 10 × 0.0054 885 4.3 157 11 △ 0.0015 1000 or more 4.5 163 12 △ 0.0018 1000 or more 4.0 172

[0084] Table 4 (continued) Tensile Properties ⁽⁴⁾ Room temperature 850 ° C. 0.2% tensile 0.2% Tensile Example Strength Strength Elongation Hardness strength strength elongation 1 420 450 1.8 207 20 29 79 2 445 470 1.5 217 22 32 102 3 460 490 2.0 207 24 30 78 4 4 440 505 2.2 217 27 35 50 5 5 460 510 1.3 217 26 36 48 6 490 520 1.3 223 28 39 70 7 535 545 1.0 241 30 39 48 8 520 545 1.5 241 30 41 55 9 520 550 1.6 241 31 43 54 10 430 505 3.1 212 17 26 73 11 435 510 2.5 217 17 26 92 12 420 500 2.0 223 18 26 70 13 445 485 1.2 212 18 27 81 14 435 495 1.3 223 20 29 79 15 425 505 2.0 217 21 29 98 16 450 505 2.2 217 22 29 67 17 455 490 1.9 229 22 31 86 18 435 510 1.3 223 27 36 50 19 425 485 2.0 217 28 40 68 20 450 495 1.5 241 30 41 52 21 490 520 1.3 223 29 38 55 22 480 510 1.4 229 28 39 65 23 505 530 1.8 241 31 40 48

[0085] Table 4 (continued) Tensile Properties ⁽⁴⁾ Room temperature 850 ° C. 0.2% tensile 0.2% Tensile Example Strength Strength Elongation Hardness strength strength elongation 24 520 540 2.0 241 28 40 60 25 535 550 1.8 241 27 39 66 26 540 535 2.2 232 29 41 58 27 495 520 1.3 223 31 41 70 28 480 525 2.0 223 31 39 52 29 490 515 2.2 223 30 42 80 30 455 505 2.8 212 31 43 62 31 470 520 1.8 217 30 41 58 32 485 535 2.3 223 31 43 70 33 505 530 1.3 241 32 41 55 34 500 520 1.8 241 30 39 65 35 495 505 1.2 241 31 43 72 Comparative example 1 − − − 321 − − − 2 − − − 285 − − − − 3 − ⁽⁵⁾ 450 0.2 248 19 29 67 4 − ⁽⁵⁾ 430 0 255 17 28 55 5 − − − 311 − − − 6 385 465 3.3 187 14 25 84 7 415 440 2.1 212 17 26 78

[0086] Table 4 (continued) Tensile Properties ⁽⁴⁾ Room temperature 850 ° C. 0.2% tensile 0.2% tensile conventional Strength Strength Elongation Hardness strength strength elongation 8 510 635 11.0 217 21 32 45 9 230 455 16.3 170 75 105 20 10 400 726 14.1 211 36 50 64 11 435 500 3.1 170 14 25 100 12 450 530 1.5 192 16 27 85

(Note) (1): The state of the as-cast matrix structure is evaluated as follows. ○: Ferrite phase + colony phase △: Ferrite phase + colony phase (almost all ferrite phase) ×: Almost all sorbite phase (2): Ferrite phase + pearlite phase + graphite (3): Austenite phase + graphite (4) : The unit of each tensile property test is as follows. (a) 0.2% proof stress: N / mm ² (b) Tensile strength: N / mm ² (c) Elongation:% (d) Hardness: H _B (5): Not measurable

Under the conditions shown in Table 5, the general wall thickness shown in FIG.
A cast integral structure exhaust manifold 1 (heat-resistant cast steel product A) for in-line 4-cylinder engines with a supercharger of 2.5 mm to 3.4 mm and a turbocharger for the same in-line 4-cylinder engine with a general wall thickness of 2.7 mm to 4.1 mm A housing 2 (heat-resistant cast steel product B) and an exhaust outlet part 3 (heat-resistant cast steel product C) having a general wall thickness of 2.3 mm to 2.8 mm,
A flange part 5 (heat resistant cast steel product D) having a general wall thickness of 2.3 mm to 2.8 mm was cast respectively. Also, as shown in FIG. 6, a general wall thickness of 2.3 mm to 2.8 mm for other in-line 4-cylinder engines.
Assembly part 10 (heat-resistant cast steel product E) for welded exhaust manifold 1 with mm and general wall thickness 2.3 to 2.8 mm
And a flange part 5 (heat-resistant cast steel product F) for a welded structure each having the above.

In FIG. 5, 4 indicates an auxiliary catalytic converter container made of a rolled plate of SUS430, 6 indicates an exhaust pipe, 7 indicates a main catalytic converter container, 8
Indicates a center housing, and 9 indicates an oxygen sensor. Further, W in the drawing represents the auxiliary catalytic converter container itself and a welded portion thereof with the exhaust outlet part 3 and the flange part 5.

Further, in FIG. 6, 11 is a SUS430 pipe welded to the assembly part 10 of the exhaust manifold 1, 4 is a SUS430 auxiliary catalytic converter container,
6 is an exhaust pipe, 7 is a main catalytic converter container, 9 is an oxygen sensor, and 12 is a SUS410 flange. W indicates a welded portion.

Table 5 Casting conditions Casting chemical composition (% by weight) Product casting number Mold depressurization condition C Si Mn A, C, D 6 each sand type 28kPa 0.11 2.95 0.47 B 5 sand type 15kPa 0.16 3.08 0.51 E, F 10 each Sand type 35kPa 0.08 2.99 0.48

[0092] Table 5 (continued) Casting Chemical Composition (wt%) Product P S Cr N W Co B A , C, D 0.006 0.005 6.14 0.02 0.41 - - B 0.005 0.005 6.81 0.03 0.55 0.34 0.002 E, F 0.004 0.005 4.89 0.03 0.28 − −

The productivity of these cast products was evaluated. As a result, a sound cast component could be obtained under any of the conditions. Further, the cast material products were subjected to machining to confirm their machinability, but no problems occurred in any of them.

As to the products C and D, the exhaust outlet with auxiliary catalytic converter having the integral welding structure is welded to the auxiliary catalytic converter container 4 formed by forming a rolled plate of SUS430 having a thickness of 2 mm (see FIG. 5). Was produced. Also,
For products E and F, SUS430 wall thickness 2 mm, inner diameter 40
The pipe 11 of mm and the auxiliary catalytic converter container 4 formed by forming a rolled plate of SUS430 having a thickness of 2 mm were welded to manufacture an exhaust manifold with an auxiliary catalytic converter (see FIG. 6) having an integrally welded structure. The welding machine used was a MIG type and argon gas was used as the seal gas.
The wire has a diameter equivalent to SUS430 with a C content of 0.01% by weight or less.
A 0.8 mm one was used. As a result, the welded parts of the SUS430 pipe and the rolled plate were welded soundly in all of the four welded structural parts of the product of the present invention, and sufficient welding reliability was obtained as a heat resistant exhaust system part with integrated welding structure. I confirmed that I had it.

Next, the exhaust manifold 1 of the product A, the turbocharger housing 2 of the product B, the in-line 4 cylinder in which the exhaust outlet 3 with the auxiliary catalytic converter in which the products C and D are welded are assembled, and the displacement is 20
An actual machine durability test was conducted using a 00cc high-performance gasoline engine. The test conditions were as follows: continuous operation under full load at 6000 rpm for 14 minutes-idling for 1 minute-complete stop for 14 minutes-idling for 1 minute was set as one cycle, and this heat cooling (GO-ST
OP) cycle was carried out up to 500 cycles. The exhaust gas temperature at full load was 930 ° C at the inlet of the turbocharger housing 2. Under this condition, the maximum surface temperature of the exhaust manifold 1 is about 870 ° C at the collecting part,
The maximum surface temperature of the turbocharger housing 2 was about 890 ° C at the wastegate. As a result, it was confirmed that these parts had excellent durability and reliability without gas leakage or thermal cracking due to thermal deformation.

On the other hand, with the chemical composition shown in Table 6, an exhaust manifold made of high Si spheroidal graphite cast iron having the same shape as product A and NI-RESIST D2 (trade name of INCO) having the same shape as product B
The following austenitic spheroidal graphite cast iron turbocharger housing was manufactured and tested using the same engine under the same conditions as above. Also in this test, the exhaust outlet with the auxiliary catalytic converter, in which the products C and D were welded, was attached to the rear end of the turbocharger housing. As a result, the exhaust manifold made of high Si spheroidal graphite cast iron became unusable after 98 cycles due to thermal cracking in the vicinity of the assembly due to oxidation and thermal fatigue. After that, when the exhaust manifold was replaced with the product A and the test was continued, the NI-RESIST D2 turbocharger housing generated a crack penetrating the scroll portion at the 2183th cycle. On the other hand, there was no problem with the exhaust outlet with auxiliary catalytic converter.

Table 6 Casting conditions Casting chemical composition (% by weight) Number of castings Mold depressurization condition C Si Mn Exhaust manifold ⁽¹⁾ 2 sand type 15kPa 3.15 3.95 0.47 Turbocharger housing ⁽²⁾ 2 sand type 10kPa 2.91 2.61 0.81

[0098] Table 6 (continued) Casting Chemical Composition (wt%) P S Cr Ni Mo Mg exhaust manifold ⁽¹⁾ 0.024 0.008 0.03 - 0.55 0.048 turbocharger housing ⁽²⁾ 0.018 0.010 2.57 21.5 - 0.084 (Note) (1) : Exhaust manifold made of high Si spheroidal graphite cast iron with the same shape as product A. (2): Austenite spheroidal graphite cast iron turbocharger housing with the same shape as product B.

As a result of the above, it was revealed that the exhaust manifold and the turbocharger housing, which are the products of the present invention, have excellent heat resistance.

Further, product E and product F which are parts of the present invention
An exhaust manifold with a welded structure with a sub-catalyst converter was installed in an in-line 4-cylinder 1800cc naturally aspirated gasoline engine, and the same thermal cycle actual machine durability test as described above was performed. As a test condition, a cycle of 6400 rpm full load operation for 14 continuous minutes-idling for 1 minute-complete stop for 14 minutes-idling for 1 minute was set as one cycle, and this heat cooling (GO
-STOP) cycle was performed up to 500 times. The exhaust gas temperature at full load is 910 ° C at the exhaust manifold outlet.
Met. The maximum product surface temperature of the exhaust manifold was 840 ° C at the assembly where Product E was used. In this test as well, no problem occurred in the exhaust manifold with the auxiliary catalytic converter and the welded structure.

[0101]

As described above, the heat-resistant cast steel of the present invention is superior to conventional heat-resistant cast iron and heat-resistant cast steel in oxidation resistance, heat crack resistance and heat deformation resistance, which are particularly important in exhaust system parts of internal combustion engines. Since it has characteristics, it has an excellent effect on parts exposed to combustion gas or exhaust gas of an internal combustion engine. Further, since the heat-resistant cast steel of the present invention is equivalent to the conventional ferritic heat-resistant cast steel in castability, machinability, and welding reliability, it can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a micrograph (100 times) showing a metal structure of a heat-resistant cast steel of Example 2 in Table 1 in an as-cast state.

FIG. 2 is a micrograph (100 times) showing the metal structure of the heat-resistant cast steel of Example 7 in Table 1 in the as-cast state.

FIG. 3 is a micrograph (100 times) showing the metal structure of the heat-resistant cast steel of Comparative Example 1 in Table 2 in the as-cast state.

FIG. 4 is a micrograph (100 times) showing the metal structure of the heat-resistant cast steel of Comparative Example 5 in Table 2 in the as-cast state.

FIG. 5 is a schematic diagram showing an example of an exhaust system device with an auxiliary catalytic converter that is integrally welded by using an integrally formed exhaust manifold, a turbocharger housing, an exhaust outlet and a flange formed by using the heat-resistant cast steel of the present invention. It is a perspective view shown.

FIG. 6 is an exhaust with a sub-catalyst converter formed by welding a collective part for a welded structure exhaust manifold and a welded structure flange part formed by using the heat-resistant cast steel of the present invention to an exhaust manifold pipe and a sub-catalyst converter container in an integrated structure. It is a perspective view which shows an example of a system apparatus schematically.

[Explanation of symbols]

 1 ... Exhaust manifold 2 ... Turbocharger housing 3 ... Exhaust outlet 4 ... Flange component 5 ... Auxiliary catalytic converter container 6 ... Exhaust pipe 7 ... Main catalytic converter container 8 ...・ Center housing 9 ・ ・ ・ Oxygen sensor 10 ・ ・ ・ Exhaust manifold collecting section 11 ・ ・ ・ Exhaust manifold pipe 12 ・ ・ ・ Flange W ・ ・ ・ Welding section

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location F01N 7/16 7114-3G

Claims (17)

[Claims] 1. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase Heat resistant cast steel. 2. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, rare earth element and / or Y: 0.1% or less, balance consisting essentially of Fe and unavoidable elements, and metal matrix structure consisting essentially of ferrite phase and pearlite-like colony phase Heat resistant cast steel. 3. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, Mg and / or Ca: 0.005-0.03%, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase Heat resistant cast steel. 4. The weight ratio of chemical components is C: 0.05 to 0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, B: 0.001 to 0.01%, the balance substantially Fe
And a heat resistant cast steel comprising an unavoidable element and having a metallic matrix structure substantially consisting of a ferrite phase and a pearlite-like colony phase. 5. The weight ratio of chemical components is C: 0.05 to 0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, the balance being substantially Fe and unavoidable elements, and the metal matrix structure being substantially a ferrite phase. A heat-resistant cast steel characterized by comprising a pearlite-like colony phase. 6. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, Mg and / or Ca: 0.005 to 0.03%, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure is substantially a ferrite phase. A heat-resistant cast steel characterized by comprising a pearlite-like colony phase. 7. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, B:
A heat-resistant cast steel comprising 0.001 to 0.01%, the balance being substantially Fe and unavoidable elements, and the metal matrix structure being substantially composed of a ferrite phase and a pearlite-like colony phase. 8. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, Mg and / or Ca: 0.0
A heat-resistant cast steel, characterized in that the balance is composed of 05 to 0.03%, the balance being essentially Fe and inevitable elements, and the metal matrix structure is substantially composed of a ferrite phase and a pearlite-like colony phase. 9. The weight ratio of chemical components is C: 0.05-0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, B: 0.001 to 0.01%,
A heat-resistant cast steel, characterized in that the balance consists essentially of Fe and unavoidable elements, and the metal matrix structure consists essentially of a ferrite phase and a pearlite-like colony phase. 10. The chemical composition, by weight ratio, is C: 0.05 to 0.25.
%, Si: 2.5 to 3.5%, Mn: 2% or less, Cr: 4 to 8%,
N: 0.05% or less, W and / or Co: 0.1 to 2%, rare earth element and / or Y: 0.1% or less, Mg and / or Ca: 0.0
05-0.03%, B: 0.001-0.01%, the balance consisting essentially of Fe and unavoidable elements, and the metal matrix structure consisting essentially of a ferrite phase and a pearlite-like colony phase, a heat-resistant cast steel. 11. The heat-resistant cast steel according to claim 1, wherein the pearlite-like colony phase has a eutectoid structure of a metal-carbon compound and ferrite. 12. The method for producing heat-resistant cast steel according to claim 1, wherein a molten metal having the composition after solidification is poured into a sand mold under reduced pressure, or A method comprising pouring molten metal into a casting mold, cooling the mold naturally until the temperature of the hottest part of the cast product reaches 900 ° C. or lower, and then removing the mold. 13. The method according to claim 12, wherein the as-cast steel is reheated to 800 to 900 ° C., held for 30 to 600 minutes, and then naturally cooled. 14. A component for an internal combustion engine, which is exposed to high-temperature combustion gas or exhaust gas, which is integrally formed of the heat-resistant cast steel according to any one of claims 1 to 11. 15. A component for an internal combustion engine exposed to high-temperature combustion gas or exhaust gas, at least a part of which is formed of the heat-resistant cast steel according to any one of claims 1 to 11. Parts. 16. The internal combustion engine component according to claim 15, wherein the portion made of the heat-resistant cast steel and the portion made of a stainless thin plate material are integrally joined by welding. parts. 17. The internal combustion engine component according to claim 15 or 16, which is a heat-resistant cast steel exhaust manifold welded to a catalyst container made of a stainless thin plate material.