JPS6070162A - Heat resistant ferritic spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To obtain the titled cast iron with high toughness in the blue brittleness temp. range by adding specified percentages of C, Si, Mn, P, S, Mo, Mg and a rare earth element to cast iron and specifying the average ferrite grain size. CONSTITUTION:Ferritic spheroidal graphite cast iron contg., by weight, 2.6-3.8% C, 3-4.2% Si, <=0.5% Mn, <=0.1% P, <=0.03% S, <=0.6% Mo and 0.02-0.15% Mg and a rare earth element and having <=25mum average ferrite grain size is prepd. The cast iron has high rupture elongation in the blue brittleness temp. range and withstands well repeated heating.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in heat-resistant spheroidal graphite ferritic cast iron, and more specifically to heat-resistant spheroidal graphite ferritic cast iron that has high toughness in the blue brittle temperature range.

For example, spheroidal graphite cast iron (equivalent to JIS-FCD40) is often used as the ferritic cast iron used for materials such as turbocharger turbine casings and exhaust manifolds, but the exhaust gas temperature has been rising to 800℃. Because of this, the heat resistance (especially oxidation resistance) of this material has become insufficient.

For this reason, attempts have been made to use high-Si cast iron, which has good oxidation resistance, but it has the disadvantage that it is more susceptible to repeated heating and more likely to crack during use than conventional spheroidal graphite cast iron. The main reason for this drawback is that the elongation at break in the so-called blue brittle temperature range of about 300 to 400°C is small;
%.

In response, high-81 cast iron with 4% Si and 1% Mo (in this specification, the chemical composition is expressed in weight percent as usual) was proposed, and has been partially adopted as a material for automobile turbocharger casings, exhaust manifolds, etc. It is also used as a material for superchargers for marine engines. However, in these cases, simply specifying the chemical composition will result in
The elongation at break at 0 to 400°C is small, and it is difficult to obtain an elongation of 5% or more.

In view of the above circumstances, the present invention aims to provide a ferritic spheroidal graphite cast iron that has a large elongation at break in the blue brittle temperature range and is resistant to repeated heating. 3-4.2%, Mn0.5
% or less, P 091% or less, 30.03% or less, Mo0
．． 6% or less, Mg + rare earth elements 0.02-0.15%
The present invention relates to a heat-resistant spheroidal graphite ferritic cast iron having a large elongation at break in the blue-brittle temperature range and having an average ferrite crystal grain size of 25 μm or less.

The chemical component composition in the present invention will be described below with reference to FIGS. 1 to 4 obtained from the research results of the present inventor.

Si is the most effective element for oxidation resistance, and an example of the inventor's research results on the relationship between the Si content and oxidation resistance of ferritic spheroidal graphite cast iron is shown in Figure 1. . In the figure, the vertical axis shows the percentage change in the thickness of the sound part of the material plate after 600 repeated heatings between room temperature and 800°C, which was examined using a microscope, compared to the original thickness. It shows the size of gender. From the figure, the amount of Si is 3.5
% or more, the plate thickness change becomes small, but if the condition of actual use is considered and the requirement is that the thickness of the healthy part be 85% or more, the required amount of St will be 3% or more.

On the other hand, according to Figure 2, which also shows the relationship between Si content and elongation at break, when the Si content is 3% or more, the
The elongation at break at 00°C begins to decrease and approaches the minimum at approximately 3.9% Si or higher (.

Therefore, as described below, the S1 content that can ensure a sound part thickness of 85% or more for oxidation resistance and the elongation at break at 400°C of 5% or more is determined by reducing the crystal grain size and Mo
Considering that the upper limit of the amount of Si can be expanded by adding an appropriate amount of Si, it is set to 3 to 4%.

Regarding MO, the elongation at break at 400°C is 5%.
According to FIG. 3, which shows the change in elongation at break when Mo is added to vF iron with a Si content of 0.
The elongation at break is greatly improved at 1 to 0.4%, and the range where the elongation at break is 5% or more is 0.05 to 0.5%.
However, this range is expanded to about 0.6 to 0.7% due to the effect of grain refinement, which will be described later. However, since cast iron has a high C content, it tends to form carbides and reduces toughness, so in the present invention, the amount of Mo is 0.
6% or less.

The other ingredients are as follows.

If C is less than 2.6%, the number of spheroidal graphites becomes insufficient, making it difficult to make the crystal grains as fine as desired, and castability also deteriorates. On the other hand, if it exceeds 3.8%, the graphite tends to become bulky, which impairs toughness and generates more dross after casting, which is undesirable, so the C content is set to 2.6 to 3.8%.

When Mn exceeds 0.5%, pearlite tends to form,
It also lowers the elongation at break, making it difficult to obtain a value of 5% or more, so the upper limit is set at 0.5%.

If P is contained in an amount of 0.1% or more, it tends to segregate at grain boundaries, lowering the elongation at break, and making it difficult to obtain an elongation at break of 5% or more.

S tends to segregate at grain boundaries and inhibits graphite spheroidization, and if its amount increases, it becomes difficult to obtain an elongation at break of 5% or more, so it is usually kept at 0.03% or less.

The remaining amount of Mg + rare earth elements is 0.02 to 0.15%,
If this is less than 0.02%, the graphite will not become spheroidized, and if it is more than 0.15%, the effect will be almost constant, and
Since a large amount of Mg oxide is produced, the content is set to 0.15% or less. It is important not to use Mg alone as a spheroidizing agent, but to use one containing a rare earth element to finely disperse a large number of spheroidal graphites.

Regarding the relationship between ferrite grain size and elongation at break,
Figure 4 shows 2.6-3.8%C, 3-4.2%StSMo
It shows the relationship between the average ferrite grain size determined by the cutting method specified in the JIS-GO552 test method for spheroidal graphite cast iron with 1% or less and the elongation at break of 300 to 40 [t:, but the average ferrite grain size becomes smaller. It can be seen that the elongation at break increases as the value increases. From this result, in order to reliably obtain an elongation at break of 5% or more within the above-mentioned chemical composition range, it is necessary to set the average ferrite grain size to 25 μm or less.

The ferrite particle size can be reduced by a known heat treatment method or by appropriately using a spheroidizing agent (for example, a spheroidizing agent containing a rare earth element) to finely disperse a large number of spheroidal graphites.

Next, various test results conducted on cast iron according to the present invention and comparison materials will be described. Table 1 shows the chemical composition of typical test materials, Table 2 shows the mechanical properties, and Table 3 shows the oxidation resistance, ferrite crystal grain size, and heat treatment.

Samples Nos. 1 to 3 in the table are spheroidal graphite cast irons according to the present invention with low, medium, and high Si contents, and Mg+rare earth element (RE) was used as a graphite nodularizing treatment agent.

Samples NCL4 to NCL6 are comparative materials, where grade 4 has a high Mo%, grade 5 has a high St% and large grains, and grade 6 has a J
IS spheroidal graphite cast iron equivalent product with the lowest St% among the samples,
Although the elongation at break at 400°C is high, the oxidation resistance is inferior to the product of the present invention, and Mg is used as the spheroidizing agent.

Table 1 (wt%) Note: N[11-3: Po, 04-0.06. S.O.
, 01-0.02 4-6: Po, 01-0.06.
50.01~0.02 Table 2 Note, numbers in parentheses are MPa Table 3 Note, *800℃X2h, FC, 920℃X10h, F
C As can be seen from the above table, spheroidal graphite cast irons Nos. 1 to 3 according to the present invention have fine ferrite crystal grains, excellent oxidation resistance, and the highest elongation at break at 400°C.

Comparative materials Nos. 4 and 5 satisfy the chemical composition of the present invention, but their elongation at break at 400° C. is extremely small due to their large crystal grains.

As mentioned above, the JIS-FCD40 equivalent product of 11h6 has a high elongation at break at 400°C, but its oxidation resistance is inferior to the product of the present invention.

As mentioned above, the spheroidal graphite cast iron according to the present invention has a higher St content than JIS spheroidal graphite cast iron, but the so-called high Sl
It has excellent oxidation resistance even though it is less than cast iron, and has improved toughness by making the crystal grains finer.
Since it has a large elongation at break in the blue brittle temperature range, it can be used as a material for mechanical parts that are repeatedly heated to high temperatures, such as turbochargers or manifolds, to prevent oxidation and cracking, thereby extending life. Its practical effects are extremely large.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between St content and oxidation resistance of ferritic spheroidal graphite cast iron, Figure 2 is the same (Graph showing the relationship between St content and elongation at break, Figure 3 is the same < M
A graph showing the relationship between o content and elongation at break, and FIG. 4 is a graph showing the relationship between ferrite crystal grain size and elongation at break. Applicant's agent Patent attorney Kamoshi1) Second son (11) (x), Vλ4aYshokamaγ (Xλ, /2

Claims (1)

[Claims] C2, 6-3.8%, st 3-4.2%, Mn 0.5
% or less, P O31% or less, 30.03% or less, Mo0
．． 6% or less, Mg + rare earth elements 0.02-0.15%
A heat-resistant spheroidal graphite ferritic cast iron having a large elongation at break in the blue brittle temperature range, and having an average ferrite crystal grain size of 25 μm or less.