JPS6353240A - Cast iron for exhaust manifold - Google Patents

Abstract

PURPOSE:To improve resistance to high-temp. air oxidation as well as resistance to PbO corrosion, by specifying respective contents of C, Si, P, Mn, S, Cr, Nb, and Fe and also by specifying a CE value represented by C+1/3(Si+P). CONSTITUTION:This cast iron for exhaust manifold has a composition consisting of, by weight, 1.5-2.5% C, <=1% Si, <=0.02% P, <=0.5% Mn, <=0.05% S, 4.5-8.5% Cr, 0.15-0.75% Nb, and the balance Fe. Further, the CE value represented by C+1/3(Si+P) is regulated to 1.5-2.8 in the above composition.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention provides an alloy for automobile exhaust manifolds;
In particular, it relates to cast iron for exhaust manifolds that has excellent resistance to lead oxide corrosion.

(Conventional technology) Conventionally, RCVAO (Funbakuto vermikine cast iron), etc. were used for exhaust manifolds (hereinafter referred to as EX manifolds) (JIS Handbook of Steel, New Edition Automotive Engineering), edited by Ndoptsk Society of Automotive Technology, etc. reference).

(Problems to be Solved by the Invention) In recent years, as the performance of engines has improved, new problems have increasingly occurred in areas where there were no problems in the past. Regarding the Ex manifold, -+: cannot be knocked down, and the conventional valve, valve seat or EG
Lead oxide (PbO) ic, which was a problem only with R valves
Corrosion is becoming a problem.

This problem of PbO corrosion naturally occurs in areas where paid gasoline is used, but it is especially the case where the amount of Pb is 3.5 t /
In areas with GAT or higher, the effect on the EX manifold was noticeable, and problems such as holes could occur.

The present invention was made in order to solve the above-mentioned conventional problems, and its purpose is to provide cast iron for Ex-molding that has sufficient durability even when used with leaded gasoline. It is in.

(Means to Solve the Problems) The cast iron for the Ex manifold according to the present invention to solve the above-mentioned conventional problems has a weight of 1.5 to 2.5 inches, and a Sl of 1.0 inches or less. , P 0.02% or less, Mn 0.5% or less, so, os%
Below, Cr 4.5-8.5%, Nb 0.15-0
．． 75%, remainder made up of Fs, ten shots of C (Si+P)
It is characterized in that the CE value given by is in the range of 1.5 to 2.8.

Here, C and St are important elements for improving castability, but if too much C is present, expansion, deformation, or cracking is likely to occur during use at high temperatures. This is because tissue changes tend to occur at high temperatures. -10,000, Sl is PB resistant
Since it is an element that is susceptible to corrosion, it is desirable to have as little as possible.

Therefore, C is in the range of 1.5 to 2.5 inches, and si is 1. Good results can be obtained when the value is 0 or less.

P, Mn, and S can be tolerated if mixed in at the level of impurities, but if they increase too much, they form pearlite, which impairs workability and
P, Mn, and S are each 0.0 because they reduce heat resistance.
It is necessary to set it to 2%, 0.5%, and 0.05 or less, and it is desirable that it is as small as possible.

Cr is effective in improving oxidation resistance and PbO corrosion resistance, but in the component system of the present invention, if it is less than 4.5, the effect of 7 is small, and if it exceeds 8.5, the amount of Cr carbide produced is This results in an increase in the hardness and coarsening of the hardness, and impairs workability because the hardness does not decrease even after heat treatment. C for seven
It is necessary that the r content is in the range of 4.5 to 8.5.

Nb is an element that contributes to strengthening grain boundaries and improving oxidation resistance.
If it is less than 0.15%, the effect will not be fully exhibited, while if it exceeds 0.75%, no further effect will be seen and the cost will only increase, so the preferred content of Nb is 0.
．． The content ranges from 15% to 0.75%.

CE value (Carbon Equivalent Va.
For 1.5 or less, a large number of proholes were observed.

If it is 2.8 or more, graphite precipitation will be observed, and strength and thermal fatigue will begin to deteriorate, so it is essential that it be in the range of 1.5 to 2.8.

The cast iron for Ex manifolds of the present invention can be said to be an intermediate material between conventional cast iron and steel, and when manufacturing Ex manifolds, cast iron is cast by combining materials so as to have the above-mentioned composition. , preferably heated and maintained at 910 to 945°C for 0.5 hr or more to obtain a uniform austenitic state, and then cooled to 500° C. at a cooling rate of 50 to 130 C/hr.
Cool to ℃ and then leave to cool.

(Function) By having the above-mentioned composition, the pboX corrosion resistance of the cast iron is significantly improved.

At the same time, resistance to high temperature air oxidation is also improved.

(Example) Examples of the present invention will be described below in comparison with comparative examples.

Example 1 Atmospheric melting was performed using a 20 Kf high-frequency melting furnace, and various casting test pieces (20
A rectangular parallelepiped of gourd
It was subjected to a corrosion test. In addition to the present invention material and the comparative material, a conventional high-St spheroidal graphite cast iron melted to factory standards was also included as test materials for comparison. Their composition is the first
The results are as shown in the table (the remaining Fe is not shown, the same applies to Table 2 later), and the numerical values in the table represent the content of each element in the %F6 base (parts by weight).

Table 1 The PbO corrosion test was conducted by immersing the test piece with a thickness of 5 mm in PbO powder and holding it at 800° C. for 100 hours. The evaluation was based on the method of measuring the amount of reduction in plate thickness after the test.

The results of Section 7 are shown in Figure 1. According to this, the thickness reduction of conventionally used high St spheroidal graphite cast iron is 5■ (residual thickness 0-), whereas the thickness reduction of 1-C of the present invention is 7. The amount is less than 0.9 proverbs (average 0.7 a
m), and it can be seen that it exhibits extremely excellent PBO corrosion resistance.

Comparative material 1 is a material in which Si is added in an amount exceeding the upper limit of 81 according to the present invention, and as can be seen by comparing the amount of reduction in the plate thickness with that of the present invention material, the increase in the Si content improves the PbO corrosion resistance. It has been shown to worsen sexuality.

Comparative material 2 is a material to which Cr is added in an amount higher than the upper limit of Crf according to the present invention. The PbO corrosion resistance is the same as that of the material of the present invention and there is no problem, but even after annealing, the hardness is high and is about 350 H (test load 209), so the workability is significantly reduced.

Comparative material 3 has a lower Cr content than the range of component content according to the present invention.
As with Comparative Material 1, the thickness of the wood was reduced by about 3 times compared to the material of the present invention. This is because the effect of Cr is small, and it shows that sufficient PbO corrosion resistance cannot be obtained even if the amount of Nb is slightly increased.

Comparative material 4 had a CE value lower than the range according to the present invention, and therefore had many blowholes, shrinkage cavities, etc., and was not in a condition to be tested, so the test was discontinued.

Example 2 Inventive materials D-Fk having the composition shown in Table 2 were obtained by the same procedure as in Example 1, and these were subjected to an atmospheric oxidation test. For comparison, a conventional material of high St spheroidal graphite cast iron having the composition shown in Table 2 was also obtained and subjected to the same test.

The test in Table 2 used a test piece having the same shape as that used in Example 1, and was carried out by holding it at 950°C I/C for 100 hours.

The results are shown in Figure 2. From this, it is clear that the oxidation weight of the materials D to F of the present invention is reduced to about 1/8 compared to the conventional material, high-St spheroidal graphite cast iron, and their resistance to high-temperature air oxidation is significantly superior. be.

Note that FIGS. 3(a) and 3(b) show the metal structure of the material of the present invention, and it is clear that it does not contain graphite and carbides are finely distributed.

(Effects of the Invention) As explained in detail above, the cast iron for exhaust manifold according to the present invention has significantly improved PbO corrosion resistance, and is suitable for use in regions where leaded gasoline is used, such as Europe and the Middle East. If used in the exhaust manifolds of automobiles that drive locally, their useful life can be significantly extended.

It also has excellent high-temperature oxidation resistance in the atmosphere, making it suitable for exhaust manifolds used in high-speed engines and engines that lean the air-fuel ratio (A/F) to increase combustion efficiency. .

[Brief explanation of the drawing]

Fig. 1 is a graph showing the PBO corrosion test results of the embodiment of the cast iron for exhaust manifold of the present invention in comparison with comparative materials and conventional materials. Fig. 2 shows the atmospheric oxidation test results of other embodiments of the conventional material. Graphs shown in comparison with FIGS. 3(&) and 3(b) are micrographs showing the metal structure of the example. Figure 1 Figure 2 Figure 3 xloo (magnification) x 400 (magnification)

Claims (1)

[Claims] By weight, C1.5-2.5%, Si1.0 or less, P0
．． 0.02% or less, Mn 0.5% or less, S 0.05% or less,
It consists of 4.5 to 8.5% Cr, 0.15 to 0.75% Nb, and the balance is Fe, and is characterized by a CE value given by C+1/3 (Si+P) in the range of 1.5 to 2.8. Cast iron for exhaust manifold.