JPS61133362A - High-si ferritic spheroidal graphite cast iron and its manufacture - Google Patents

Abstract

PURPOSE:To increase the fatigue strength of high-Si ferritic spheroidal graphite cast iron at high temp. and to make the cast iron suitable for use at high temp. by regulating the P content to the required value and varying the Si content in the required range in accordance with the service temp. CONSTITUTION:The composition of high-Si ferritic spheroidal graphite cast iron is composed of, by weight, 2.6-3.6% C, <0.5% Mn, <0.6% Mo, 0.03-0.1% P, 3.8-6% Si, <0.15% graphite spheroidizing element and the balance Fe with inevitable impurities. The Si content is made higher than Si% calculated by substituting the service temp. for the Ac1 transformation point in the equation.

Description

Detailed Description of the Invention (Field of Application of the Invention) The present invention relates to a high-Si ferritic spheroidal graphite cast iron and a method for producing the same. The present invention relates to a highly oxidation-resistant high-Si spheroidal graphite cast iron and a method for producing the same.

(Prior art and problems to be solved) High-Si ferrite spheroidal graphite cast iron is generally used for cast parts that require heat resistance and heat fatigue resistance, such as supercharger turbine casings, exhaust manifolds, and pulp diffusers for automobiles and ships. has been done. However, although this material has good oxidation resistance, it has poor thermal fatigue resistance, and it is known that there are many problems when used within the blue brittle temperature range or above or below this temperature range.

In recent years, it has been found that static elongation at break (particularly at 400° C.) is improved by refining the ferrite grains of high 5ill iron, which also improves blue brittle resistance.

However, since the grain size of cast iron products is refined mainly by controlling the cooling rate of the cast product, it is difficult to control the cooling rate of cast products with uneven wall thickness. Therefore, in practice, the current situation is to suppress the decrease in static elongation at break by controlling the Si content to about 4% or less.

According to the research of the present inventor, high S at 300-500℃
The degree of blue brittleness of cast iron can be determined by the static elongation at break in this temperature range. We found that by controlling the P content, we could stably increase the 400℃ rupture value to 8% or more, and using this, we
We have proposed a high-toughness, oxidation-resistant ferritic spheroidal graphite cast iron that suppresses blue brittleness in the 00°C temperature range (Japanese Patent Application No. 59-1
No. 96259), the main chemical composition of this spheroidal graphite cast iron is 2.6 to 3.6% C, 3 to 3.8% St, and P is 0.0.
3 or more and p+ri 0.06% or less (chemical composition is shown in weight %. The same applies hereinafter).

However, subsequent research has shown that the addition of P to Si3.
It has been found that it is also significantly effective in preventing blue brittleness in high 3i ferrite cast iron of 8% or more.

By the way, for example, in the case of a gasoline-powered vehicle whose exhaust gas temperature reaches as high as 950°C, the operating temperature of conventional high 3i ferrite cast iron significantly exceeds the material's A c 1 transformation point of 800 to 850°C, resulting in an austenitic structure. Fatigue strength decreases, or due to transformation due to repeated air cooling and heating from the operating temperature, there is a risk of cranking due to the difference in thermal expansion coefficient, making it unsuitable for use in such applications. .

(Problem to be solved by the invention) In response to the increasing usage temperature, there is a material that can be used even at high temperatures of approximately 830 to 950°C as a relatively inexpensive material, and has blue brittleness. There is a growing demand for spheroidal graphite cast iron that is strong against heat and has high high temperature fatigue strength.

The object of the present invention is to provide spheroidal graphite cast iron and a method for manufacturing the same that meet these demands.

(Means for solving the problem) The first invention has C2.6 to 3.6%, Mn 0.5% or less, and Mo 0.6.
% or less, P 0.03-0.1%, Si 3.8-6.0%, and A C1 transformation point ('l:)-49,4×Si (%)
+ 657.1 2 A S calculated by adding the operating temperature to the C1 transformation point temperature
Heat resistant high Si ferrite spherical shape with high blue brittle resistance and high temperature fatigue strength, characterized by Si% more than i%, graphite spheroidization treatment element 0.15% or less, and the remainder consisting of unavoidable elements and substantially Fe. Regarding graphite cast iron, the second invention is a method for manufacturing high-Si ferritic spheroidal graphite cast iron, comprising: 1.
C2.6~3.6%, Mn 0.5% or less, Mo0 by high frequency induction electric furnace
．． 6% or less, P 0.03-0.1%, Si 3.8-6.0%, and A c 1 transformation point (℃) ~ 49.4 x Si (%) +
657.1 Contains Si% or more determined by adding the operating temperature to the A c 1 transformation point temperature of the formula, 0.15% or less of graphite spheroidizing elements, and the remainder is unavoidable elements and substantially Fe. The present invention relates to a method for manufacturing high SL ferritic spheroidal graphite cast iron with high resistance to blue brittleness, which is characterized by melting it to a chemical composition as follows.

By the way, as a result of the inventor's investigation of the relationship between Si content and Ac1 transformation point, it was found that there is a linear relationship as shown in FIG. Therefore, the A c of the material is higher than the operating temperature.
In order to raise the 1 transformation point, A c 1 transformation point (℃) 49.4 x Si (%) + 657.1 In the formula, S is obtained by substituting the operating temperature into the transformation point temperature.
If the 81 content is i% or more, it can be used for desired high temperature applications.

For example, in order to use the exhaust gas at 950°C, the Si content should be 5.9% or more calculated from the formula.However, if the Si content is increased like this, embrittlement at 400°C will be reduced. It becomes noticeable.

FIG. 1 shows the relationship between the P content of high-31 cast iron and the static elongation at break at 400°C, which was obtained as a result of research by the present inventor. S
In order to suppress embrittlement and secure a static elongation at break of 8% even with a high i content, the P content range is Si3.8% as in the previous application.
Although it is slightly narrower than the following cases, it is 0.03 to 0.1
It can be seen from this figure that it is sufficient to set the value to %P.

This cast iron is melted by mixing commercially available high-purity pig iron or ductile pig iron with recycled scraps and, if necessary, steel scraps, adding the required ferroalloy to this, melting in a high-frequency induction electric furnace, and finally forming graphite spherules. Adding a curing agent and casting is the same as usual.

This cast iron can also be melted in a low-frequency induction electric furnace, but the cast iron obtained is 400% melted in a high-frequency induction electric furnace.
There is a large variation in static elongation at break at °C, and it is difficult to guarantee an elongation of 5% or more unless the ferrite grain size is controlled.

Figure 3 shows an example of the relationship between P content and static elongation at break for high-frequency induction electric furnace melting and low-frequency induction electric furnace melting. It will be understood that the reliability is even higher because it is not related to the ferrite particle size.Therefore, in the present invention, melting using a high frequency induction electric furnace is adopted. The furnace lining may be basic or acidic.

The cast product is as-cast and the base has a ferrite structure, so it is best to use it after stress relief annealing.

Next, the chemical composition of cast iron produced by this method will be explained.

If C is less than 2.6%, it becomes a hypoeutectic composition, which increases the solidification start temperature and reduces castability, which is undesirable.On the other hand, if the amount of C increases, the graphite becomes bulky and bulky, impairing toughness and making it difficult to form after casting. The upper limit is set at 3.6% because it is undesirable as it causes the generation of dross.

Si is the most effective element for oxidation resistance.

Moreover, there is a linear relationship between the Si content and the A c 1 transformation point, and it is important to keep the transformation point above the service temperature in order to maintain strength. From Figure 2, the Si content is 3.8~
If it is 6.0%, the A c 1 transformation point is about 845-950
℃, which means that the usable temperature of the material can be increased accordingly.

It is said that M and o are effective in alleviating blue brittleness or improving creep bond strength, but in the present invention, blue brittleness is solved by controlling the P content, so Mo is used for the latter purpose. According to the research of the present invention, an amount of 0.6% or less is sufficient. There are many MOs (
In this case, the amount of Mo carbide in the thick part of the casting increases, and in order to reduce this, high-temperature and long-term heat treatment is required, which is economically disadvantageous.

When Mn exceeds 0.5%, pearlite tends to form, which is disadvantageous because long-term heat treatment is required to reduce it. Furthermore, it causes a decrease in static elongation at break, so the upper limit is set at 0.5%.

S tends to segregate at grain boundaries and inhibits the spheroidization of graphite, and when its amount increases, it causes a decrease in static elongation at break, so it is usually kept at 0.03% or less.

As the graphite spheroidization treatment agent, a usual treatment agent containing Mg and rare earth elements is used, and the content thereof is 0.15% or less.

Since Cr carbide reduces static elongation at break, it is better to have less Cr carbide, and it should be contained as an impurity, preferably 0.
It is preferable to set it to 0.02% or less.

(Test example) Next, a test example will be described.

Foundry pig iron for spheroidal graphite cast iron (JIS-G2202, Type 3, No. 1 A equivalent), returned scraps, steel scraps, and ferroalloy are melted in a high-frequency induction electric furnace, and a Fe-31-Mg-based spheroidizing agent is applied. and P content to 0.0 with ferrophosphor.
It was adjusted to 3 to 0.10%.

The chemical composition of the melted test material is shown in Table 1, and the mechanical properties at 400°C are shown in Table 2.

Table 2 Note: Heat treatment = 750°C x 2hr, furnace cooling *Only Seed 2 melted in low frequency induction electric furnace The following can be seen from Tables 1 and 2.

(a) Nal and Arashi 2 have roughly the same St and P contents, but the static elongation at break at 400℃ differs greatly depending on the difference in the melting furnace, with high-frequency induction electric furnace melting showing a higher value. There is.

Although Tane 3 was melted in a high-frequency electric furnace and the St content was lower than Arashi 4, the reason why the static elongation at 400'e was lower than that of Arashi 4 was due to P content. ,
This is probably due to the small amount.

(C) 11h5 is made by melting high-purity pig iron and adding ferrophosphor to adjust the P content, and it shows a good static elongation at break at 400°C.

(Effects of the Invention) The first invention not only adjusts the P content to a required range in a conventional high-Si spheroidal graphite cast iron, but also adjusts the Si content to a value determined according to the operating temperature within a predetermined range. Ori, Si
Even when the content is high, the static elongation at break in the blue brittle temperature range is as high as 8% or more, and since it has a ferritic structure at the operating temperature, it has high high-temperature fatigue strength, making it a high-Si ferritic spheroidal graphite cast iron suitable for high-temperature applications. .

Further, the second invention is a method for manufacturing high-Si spheroidal graphite cast iron, in which the P content is adjusted to a required range, and the Si
In addition to keeping the content within a specified range and exceeding the value determined according to the operating temperature, Si
Even if the content is increased, the static elongation at break in the blue brittle temperature range is 8%.
With the above, it is possible to obtain high-Si ferritic spheroidal graphite cast iron that has less variation, high temperature fatigue strength, and good heat resistance, and the uses of this cast iron can be expanded, and its practical effects are extremely high. 'It's really big.

[Brief explanation of the drawing]

Figure 1 shows the 400°C static elongation at break and P of cast iron according to the present invention.
Diagram showing the relationship with content, Figure 2 is the same <S
A diagram showing the relationship between the i content and the Ac transformation point, and FIG. 3 is a diagram showing an example of the relationship between the static elongation at break at 400° C. and the 2-coupling amount for each melting furnace.

Claims (1)

[Claims] 1. C2.6-3.6%, Mn 0.5% or less, Mo0.
6% or less, P0.03-0.1%, Si3.8-6.0%, and Ac_1 transformation point (℃) = 49.4 x Si (%) + 657
．． S obtained by inserting the operating temperature into the Ac_1 transformation point temperature of the formula 1
Heat resistant high Si ferrite spherical shape with high blue heat brittle resistance and high temperature fatigue strength, characterized by Si% greater than i%, graphite spheroidization treatment element 0.15% or less, and the remainder consisting of unavoidable elements and substantially Fe. Graphite cast iron 2, a method for producing high Si ferritic spheroidal graphite cast iron, in which C2.6 to 3.6%, Mn 0.5% or less, Mo 0.6% or less, P 0.03 to 0.1%, Si3.8-6.0% and Ac_1 transformation point (℃) = 49.4 x Si (%) + 657
．． S obtained by inserting the operating temperature into the Ac_1 transformation point temperature of the formula 1
It has a high resistance to blue brittleness and is characterized by containing more than i% of Si, 0.15% or less of graphite spheroidizing elements, and the remainder consisting of unavoidable elements and substantially Fe. Manufacturing method of Si ferrite spheroidal graphite cast iron