JPS62142745A - Heat-resistant spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To obtain heat-resistant spheroidal graphite cast iron having thermal fatigue characteristics and oxidation resistance particularly higher than those of conventional heat-resistant spheroidal graphite cast iron, by providing a composition containing C, Si, Mn, P, S, Cr, Nb or/and Ta, and graphite- spheroidizing elements such as Mg, Ca, Ce, etc., each of prescribed quantity. CONSTITUTION:The heat-resistant spheroidal graphite cast iron has a composition containing, by weight ratio, 3.2-4.2% C, 3.5-4.5% Si, <=0.8% Mn, <=0.1% P, <=0.03% S, 0.2-0.8% Cr, 0.5-2.0% Nb or/and Ta, and required amounts of graphite-spheroidizing elements such as Mg, Ca, Ce, etc. mechanically, above- mentioned graphite cast iron of this invention has a composition in which the amounts of impurities such as P, S, etc., are controlled to low values and the additive quantities of C, Si, Cr, and Nb are well balanced. Accordingly, the above graphite cast iron has thermal fatigue characteristics and oxidation resistance particularly superior to those of conventional heat-resistant spheroidal graphite cast iron and combines castability with machinability.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to heat-resistant spheroidal graphite cast iron, and in particular to improvements in heat-resistant spheroidal graphite cast iron suitable for use in exhaust system parts of vehicle engines. It is something.

(Prior Art) In recent years, in vehicle engines such as gasoline engines or diesel engines, especially automobile engines,
With the increasing demand for improvements in higher output and lower fuel consumption,
Research and development is actively being carried out to improve combustion efficiency.

As a result, in automobile engines that meet such demands, the exhaust gas temperature tends to be significantly higher than that of conventional automobile engines.

In particular, exhaust system parts such as exhaust manifolds in automobile engines, turbine housings for turbochargers, and parts for exhaust gas purification systems are used under particularly high and harsh conditions, so the heat resistance of conventional high-Si spheroidal graphite cast iron is In order to further improve Cr, Ajl
! Attempts have been made to add rare earth elements such as , MO, Ce, and a.

In other words, on page 42 of Cast Iron Vol.
It is disclosed that attempts have been made to add alloying elements such as ll and Si.

Also, applicants should submit first! ! Jj L, Tokuko Sho 54-3
No. 89 (No. 38) discloses an exhaust manifold in which S I, M n , and M g are added to cast iron.

In addition, Mo, Ce, 1-
, J is disclosed, and JP-A-58-58248 discloses the inclusion of Cr and Mo.

Further, JP-A-6O-70162W also discloses the inclusion of Mo, and JP-A-48-31113 discloses spheroidal graphite cast iron containing A1.

[Problem that the invention seeks to solve]

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

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

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

In addition, the inclusion of Cr or A2 also improves oxidation resistance, but Cr precipitates network-like primary carbides and significantly reduces machinability, and Aρ reduces the fluidity of the molten metal. This significantly deteriorates castability.

Furthermore, Mo strengthens the matrix and improves thermal fatigue properties, but like Cr, it precipitates network-like primary carbides, significantly lowering machinability, and tends to increase the occurrence of shrinkage cavities.

Furthermore, although the addition of rare earth elements improves the adhesion of oxide scale and improves oxidation resistance, there is a problem that the effect of improving thermal fatigue properties is small.

Therefore, an object of the present invention is to provide a heat-resistant spheroidal graphite cast iron that has significantly superior thermal fatigue properties and oxidation resistance than conventional heat-resistant spheroidal graphite cast iron, and also has both castability and machinability. It is in.

[Means for solving problems]

Therefore, the heat-resistant spheroidal graphite cast iron according to the present invention has a weight ratio of C, 3,2 to 4.2% + s'; 3,5.
~4. 5%, Mn; 0.8% or less, Pro.

1% or less, sho, 03% or less, Cr; 0.2 to 0.8
%, Nb, at least one type of 'l'a from 0.5 to
It is characterized in that it contains graphite spheroidizing elements such as 2.0% 9Mg, Ca, and Ce in an amount necessary for spheroidizing graphite, and the remainder essentially consists of carbon and inevitable impurities.

[Effect]

To explain in detail the present invention according to the above configuration, the heat-resistant spheroidal graphite cast iron according to the present invention has a composition with low impurities such as I7 and S, while containing C and Si.

By adding Cr and Nb in a well-balanced manner, we have created a heat-resistant spheroidal graphite cast iron that has far superior thermal fatigue properties and oxidation resistance than conventional heat-resistant spheroidal graphite cast iron, and has both castability and machinability. was able to provide it.

The reason 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 explained below.

In the following description, the amount of each alloying element added is expressed in weight percentage.

First, if C is less than 3.2%, castability deteriorates;
Exceeding this will increase the amount of graphite and make the material brittle.
It was set at 3.2 to 4.2%.

3. Si is used to improve oxidation resistance at high temperatures.
5% or more is required, but if it exceeds 4.5%, the base force will become electric and the thermal fatigue properties will deteriorate significantly, so 3.5~
It was set at 4.5%.

In addition, Mn is an effective element for fixing the poisonous S, which is unavoidably mixed in from the melted raw materials, into a harmless form as MnS, but if it exceeds 0.8%, the oxidation resistance deteriorates. %.

P is unavoidably mixed in from the melted raw material, but since it crystallizes steadite, which deteriorates machinability when mixed in a large amount, the amount was set to 0.1% or less so that its influence can be ignored.

Further, S is also inevitably mixed in from the melted raw material, but if it is mixed in a large amount, it will inhibit the spheroidization of graphite, so it was set to 0.03% or less, so that its influence can be ignored.

Cr is effective in improving oxidation resistance, but if it is less than 0.2%, the effect is not sufficient, and if it is added in excess of 0.8%, N
In relation to the fill content of b, the primary carbide of Cr becomes coarse and the machinability is significantly deteriorated, so it is 0.2 to 0.
It was set at 8%.

Nb and Ta are particularly important alloying elements in the present invention, and both have similar effects. That is, fine granular carbide is preferentially formed over Cr during casting, and the formation of Cr network primary carbide, which deteriorates machinability, is suppressed.

In addition, it is effective in improving oxidation resistance because it suppresses the precipitation of secondary carbides (CrO) at high temperatures;
If it is less than 2.0%, the effect of improving these properties will not be sufficient, while if it is added in excess of 2.0%, Nb or Ta will remain in the matrix in an undissolved state and tend to reduce the toughness.
゜5% to 2.0%.

〔Example〕

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

In order to evaluate various properties of the present invention materials, four types of the present invention materials ■ to ■ and four types of comparative materials ■ as shown in Table 1 were used.
Test materials of ~■ were manufactured by casting.

In addition, when casting and forming each test material, a 50 kg Kawakoshu melting furnace was used for melting, and the hot water was tapped at 1550°C or higher and graphite was spheroidized from l='e-3i MU gold alloy in a ladle. processed.

After that, this was inoculated with F' e -S i alloy and 14
Molten metal was poured at 40° C. or higher and cast into a Y block shape according to JIS standard A. The chemical composition of each sample material thus produced is shown in Table 1.

Table 1 Plow Using each sample material having the composition shown in Table 1,
Various evaluation tests were conducted as described below.

First, using an electro-hydraulic servo type thermal fatigue testing machine,
A thermal fatigue test was conducted on each of the above sample materials.

The thermal fatigue test used a round bar test piece with a gauge distance of 151 m and a gauge diameter of lOwφ, with the test piece mechanically restrained from elongation due to heating and contraction due to cooling. The lower limit temperature is 200℃, the upper limit temperature is 900℃,
(2) Thermal fatigue failure was caused by repeating heating and cooling cycles for 10 minutes.

Then, by keeping the heating and cooling cycle applied to each test piece constant as described above and varying the restraint rate of each test piece, we calculated the relationship between the restraint rate of each specimen and the number of repetitions (times) until failure due to thermal fatigue. The thermal fatigue properties of each sample material were evaluated based on the relationship. Here, the restraint rate (%) is a value obtained from the following 2.

Free elongation - restrained elongation restraint ratio (%') = X100 free elongation The results of the thermal fatigue test conducted in this manner are shown in FIG.

As is clear from FIG. 1, it can be seen that the thermal fatigue life of the inventive materials (1) to (2) is significantly shorter than that of the four comparative materials (1) to (2), which are conventional materials.

Next, in order to evaluate the oxidation resistance, each test material having the above-mentioned composition was cast, and 3Q am
A plate-shaped test piece with a size of 20 am x 5 am was prepared, and an oxidation test was conducted by holding it in the air at 900° C. for 100 hours.

The test results of this oxidation test were determined by performing shot blasting after the oxidation test to remove oxide scale, and calculating the weight change per unit area (oxidation ftl;
The evaluation was made by determining the ratio (■/cm").

The results of the oxidation test performed by the above method are shown in FIG.

As is clear from Figure 2, the oxidation loss of all of the inventive materials ■ to ■ is at least about half that of the conventional materials comparative materials ■ to ■, and their oxidation resistance is significantly superior. I understand.

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

As a result, molding can be performed without causing casting defects such as "sinkage cavities,""pinholes,""blowholes,""sandholes,""poor hot water circulation," and "hot water gaps," resulting in good casting. '4.i LF, 2 was able to have sex.

When this material is then subjected to machining processes such as drilling and milling, there is no chipping or abnormal wear of the cutting tools, and it has machinability equivalent to that of conventional high-Si spheroidal graphite cast iron. This was confirmed.

Next, in order to evaluate heat resistance and durability, an exhaust manifold for a 2.81 gasoline engine was manufactured using the present invention material and a comparative material, and was actually mounted on the engine and subjected to a severe on-board engine durability test. .

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

For comparison, the conventional high-Si spheroidal graphite cast iron is used.

The test conditions in Table 1 are as follows: Maximum rotation speed is 560 Orpm, and cold and heat cycle durability is carried out under full load conditions up to 900 cycles.
The durability of the exhaust manifold was evaluated based on the presence or absence of thermal cracks.

As a result, the product of the present invention showed no thermal cracking until the end of the 900-cycle test, whereas the comparative product showed no thermal cracking until the end of the 900-cycle test.
Thermal cracks penetrating the wall thickness were observed to occur after 30 cycles.

From the above two results, it has been revealed that the exhaust manifold manufactured using the material of the present invention has sufficient thermal cracking durability under severe heat load conditions.

Although the present invention has been described above with reference to specific embodiments, it is understood that the present invention is not limited thereto. Conceivable.

For example, in the above embodiment, an example was explained in which only Nb was contained, but since Nb and i'a are difficult to separate in the refining process, Nb and '1'a are actually contained at the same time. .

〔Effect of the invention〕

As explained above, the heat-resistant spheroidal graphite cast iron according to the present invention suppresses impurities such as P and S to a low level in terms of composition, while
By adding C, Si, Cr, and Nb in a well-balanced manner, it has thermal fatigue properties and oxidation resistance that are significantly better than conventional heat-resistant spheroidal graphite cast iron, and it also has both castability and machinability. did it.

This provides an excellent effect in that it can be used as a material for exhaust system parts in automobile engines, such as exhaust manifolds.

[Brief explanation of drawings]

FIG. 1 is a graph showing the results of a thermal fatigue test of the present invention material and the comparative material, and FIG. 2 is a graph showing the oxidation test results of the present invention material and the comparative material. Applicant Toyota Motor Corporation Collection 1 Biji

Claims (1)

[Claims] In terms of weight ratio, C: 3.2-4.2%, Si: 3.5-4
．． 5%, Mn; 0.8% or less, P; 0.1% or less, S;
0.03% or less, Cr; 0.2-0.8%, Nb, Ta
0.5 to 2.0% of at least one of these, Mg, C
A heat-resistant spheroidal graphite cast iron, characterized in that it contains a graphite spheroidizing element such as a, Ce, etc. in an amount necessary for spheroidizing graphite, and the remainder substantially consists of Fe and unavoidable impurities.