JPH0686642B2 - Heat resistant spheroidal graphite cast iron - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat-resistant spheroidal graphite cast iron, and in particular, an improvement of the heat-resistant spheroidal graphite cast iron suitable for use in an exhaust system component of a vehicle engine. It is about.

[Conventional technology]

In recent years, in vehicle engines such as gasoline engines or diesel engines, especially automobile engines,
With the increasing demand for higher output and lower fuel consumption,
Research and development for improving combustion efficiency are being actively implemented.

As a result, in an automobile engine that meets such a demand, the exhaust gas temperature tends to be significantly higher than that of a conventional automobile engine.

In particular, in exhaust system parts such as exhaust manifolds for automobile engines, turbine housings for turbochargers, parts for exhaust gas cleaning equipment, etc., the operating conditions become particularly severe at high temperatures, so the heat resistance of conventional high Si spheroidal graphite cast iron In order to further improve the above, it has been attempted to add rare earth elements such as Cr, Al, Mo or Ce, La.

That is, on page 42 of the casting No. 49, No. 12, as a method for improving the heat resistance of cast iron, pearlite stabilization, A ₁
It is disclosed that there is an increase in transformation temperature, solid solution strengthening of the matrix, and the like, and addition of alloying elements such as V, W, Mo, Cr, Mn, Al, and Si is attempted according to the purpose.

In addition, the applicant discloses an exhaust manifold in which Si, Mn, and Mg are added to cast iron in Japanese Patent Publication No. 54-38968 filed earlier.

Further, JP-B-60-17819 discloses the content of Mo, Ce and La, and JP-B-58-58248 discloses the content of Cr and Mo.

Furthermore, Japanese Patent Publication No. 60-70162 discloses the content of Mo,
JP-B-48-31113 discloses spheroidal graphite cast iron containing Al.

[Problems to be solved by the invention]

By the way, in order to improve the properties of the high Si spheroidal graphite cast iron, it is necessary to improve the oxidation resistance and the thermal fatigue property at the same time and not to deteriorate the castability and the workability.

However, the conventional methods for improving heat resistance have the following problems.

Increasing the Si content improves the oxidation resistance, but embrittles the ferrite matrix and deteriorates the thermal fatigue properties.

Also, the inclusion of Cr or Al improves the oxidation resistance, but Cr precipitates reticulated primary carbides and significantly reduces the machinability, and Al reduces the fluidity of the molten metal and significantly improves the castability. make worse.

Also, Mo strengthens the base and improves thermal fatigue properties,
Similar to Cr, reticulated primary carbides are precipitated to significantly reduce machinability and easily increase shrinkage cavities.

Further, the addition of the rare earth element improves the adhesion of the oxide scale and improves the oxidation resistance, but there is a problem that the effect of improving the thermal fatigue property is small.

Therefore, an object of the present invention is to provide a heat-resistant spheroidal graphite cast iron having both excellent castability and machinability, as well as having heat fatigue properties and oxidation resistance far superior to those of conventional heat-resistant spheroidal graphite cast iron. is there.

[Means for solving problems]

Therefore, the heat-resistant spheroidal graphite cast iron according to the present invention is, by weight ratio, C; 3.2 to 4.2%, Si; 3.5 to 4.5%, Mn; 0.8% or less, P;
0.1% or less, S; 0.03% or less, Cr; 0.2 to 0.8%, V; 0.5 to 1.8%,
It is characterized in that graphite spheroidizing elements such as Mg, Ca and Ce are contained in an amount necessary for spheroidizing graphite, and the balance substantially consists of Fe and inevitable impurities.

[Action]

Explaining the action of the present invention according to the above configuration, while compositionally suppressing impurities such as P, S, etc., while C, Si, Cr, V are added in a well-balanced manner, conventional heat-resistant spheroidal graphite cast iron It was possible to provide a heat-resistant spheroidal graphite cast iron having not only excellent heat fatigue characteristics and oxidation resistance, but also having castability and machinability.

Hereinafter, the reasons for limiting the range of the amount of each alloying element added to the heat-resistant spheroidal graphite cast iron of the present invention will be described.

In addition, in the following description, the addition amount of each alloying element is expressed in% by weight.

First, if C is less than 3.2%, the castability deteriorates, and if it exceeds 4.2%, the amount of graphite increases and the material becomes brittle.
It was 4.2%.

Si is 3.5% to improve oxidation resistance at high temperature.
The above is required, but if it exceeds 4.5%, the base becomes brittle and the thermal fatigue property is remarkably deteriorated, so it was set to 3.5 to 4.5%.

In addition, Mn is a harmful poison of S that is inevitably mixed from the melted raw material.
It is an element effective for fixing MnS in a harmless form, but if it exceeds 0.8%, it deteriorates the oxidation resistance, so it is set to 0.8% or less.

Although P is inevitably mixed in from the melted raw material, if it is mixed in a large amount, it crystallizes steadite which deteriorates machinability, so the effect was made negligible to 0.1% or less.

Further, S is also unavoidably mixed from the melted raw material, but if it is mixed in a large amount, it hinders the spheroidization of graphite. Therefore, the effect thereof is 0.03% or less, which is also negligible.

Cr is effective in improving the oxidation resistance, but if it is less than 0.2%, its effect is not sufficient, and if it is added in excess of 0.8%, the primary carbide of Cr is coarsened in relation to the content of V and machined. Therefore, the content is set to 0.2 to 0.8% because it significantly deteriorates the sex.

V is an alloying element which is particularly important in the present invention. That is, fine granular carbides are formed preferentially over Cr during casting, and formation of reticulated primary carbides of Cr that deteriorates machinability is suppressed.

Further, it is effective in improving the oxidation resistance because it suppresses the precipitation of secondary carbides of Cr at high temperatures, but if it is less than 0.5%, the effect of improving those properties is not sufficient, while if it is added in excess of 1.8%. V is undissolved and remains in the matrix to easily lower the toughness, so 0.5 to 1.8% was set.

〔Example〕

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In order to evaluate various properties of the material of the present invention, four types of test materials of the present invention to 4 types of comparative materials and 4 types of comparative materials shown in Table 1 were produced by casting.

In casting and molding each test material, a 50 kg high-frequency melting furnace was used to melt in the air, the molten steel was tapped at 1550 ° C. or higher, and the graphite was spheroidized with Fe—Si—Mg alloy in a ladle.

Then, this was inoculated with a Fe-Si alloy and then poured at 1440 ° C or higher, and cast into a Y block shape according to JIS standard No. A. Table 1 shows the chemical composition of each test material thus manufactured.

Using each test material having the composition shown in Table 1,
Various evaluation tests as described below were carried out.

First, using an electro-hydraulic servo type thermal fatigue tester,
A thermal fatigue test was carried out on each of the above test materials.

In the thermal fatigue test, the gauge length is 15 mm and the gauge diameter is 10 mm.
Using a round bar test piece with φ, mechanically restraining the elongation of the test piece due to heating and the contraction due to cooling,
The lower limit temperature is 200 ℃ and the upper limit temperature is 900 ℃.
As a result, thermal fatigue failure was caused by repeating heating and cooling cycles.

Then, the heating and cooling cycle to be applied to each test piece is made constant as described above, and the restraint rate of each test piece is changed, and the restraint rate of each test material and the number of repetitions (times) until failure due to thermal fatigue The thermal fatigue property of each test material was evaluated by the relationship. Here, the restraint rate (%) is a value obtained by the following equation.

The result of the thermal fatigue test is shown in FIG.

As is clear from FIG. 1, when compared with four types of comparative materials, all of which are conventional materials, the materials of the present invention have significantly improved thermal fatigue life.

Next, in order to evaluate the oxidation resistance, using each test material having the above-mentioned composition molded by casting, 30 mm × 20 mm × 5 mm
A plate-shaped test piece having the above shape was manufactured, and an oxidation test was carried out at 900 ° C. for 100 hours in the atmosphere.

The test result in this oxidation test was that shot blasting was applied after the oxidation test to remove the oxide scale, and the weight change per unit area before and after the oxidation test (oxidation weight loss; mg / mg /
It was evaluated by determining cm ² ).

The result of the oxidation test by the above method is shown in FIG.

As is clear from FIG. 2, when compared with the comparative material, which is a conventional material, all of the materials of the present invention have an oxidation loss of at least about half or less, and it is understood that the oxidation resistance is remarkably excellent.

Next, an exhaust manifold was manufactured by casting using the material of the present invention in order to evaluate the castability and machinability.

As a result, it is possible to form without defective casting such as "shock cavity", "pinhole", "blowhole", "sand bite", "pouring of hot water", "water boundary", and good castability. I was able to confirm that it is doing.

Then, when this is subjected to machining such as drilling and milling, chipping or abnormal wear of the cutting tool does not occur, and it has machinability equivalent to that of conventional high Si spheroidal graphite cast iron. I was able to confirm that.

Next, in order to evaluate the heat resistance durability, an exhaust manifold for a 2.8 gasoline engine was manufactured by using the material of the present invention and the comparative material, and the exhaust manifold was actually mounted on the engine and subjected to an engine bench severe durability test.

Table 2 shows the chemical composition of each sample of the exhaust manifold used for the engine bench severe durability test.

The comparative product is a conventional high Si spheroidal graphite cast iron.

The test conditions were such that the maximum rotation speed was 5600 rpm, and the thermal cycle durability under full load conditions was carried out up to 900 cycles, and the durability of the exhaust manifold was evaluated by the presence or absence of thermal cracking.

As a result, in the product of the present invention, the occurrence of thermal cracking was not observed until the end of the test of 900 cycles, whereas the comparative product was 630.
The occurrence of thermal cracks that penetrated through the wall thickness was observed during cycling.

From the above results, it became clear that the exhaust manifold manufactured by the material of the present invention has sufficient thermal crack durability under the condition of severe heat load.

Although the present invention has been described above based on the specific embodiments, the present invention is not limited to the embodiments, and various other modes that can be implemented by those skilled in the art are conceivable within the scope of the claims.

〔The invention's effect〕

As described above, the heat-resistant spheroidal graphite cast iron according to the present invention has a composition that suppresses impurities such as P and S to be low, while C and S
By adding i, Cr, and V in a well-balanced manner, it was possible to have both excellent heat fatigue properties and oxidation resistance as compared to conventional heat-resistant spheroidal graphite cast iron, as well as castability and machinability.

As a result, there is an excellent effect that it can be applied to a material for exhaust system parts in an automobile engine, for example, an exhaust manifold.

[Brief description of drawings]

FIG. 1 is a graph showing the results of thermal fatigue tests of the present invention material and comparative material, and FIG. 2 is a graph showing the results of oxidation test of the present invention material and comparative material.

Claims (1)

[Claims] 1. A weight ratio of C; 3.2 to 4.2%, Si; 3.5 to 4.5
%, Mn; 0.8% or less, P; 0.1% or less, S; 0.03% or less, Cr; 0.2 ~
0.8%, V; 0.5 to 1.8%, Mg, Ca, Ce and other graphite spheroidizing elements are contained in an amount necessary for spheroidizing graphite, and the balance is essentially Fe and unavoidable impurities. Heat resistant spheroidal graphite cast iron.