JPS6250546B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in heat-resistant spheroidal graphite ferrite cast iron, and more specifically, to heat-resistant spheroidal graphite ferrite cast iron that has a large elongation at break in the blue-brittle temperature range. For example, the ferritic cast iron used for the exhaust manifold of the turbine casing of a turbocharger is spheroidal graphite cast iron (JIS/FCD40
However, as the temperature of exhaust gas is rising, reaching nearly 800℃, this material has begun to lack heat resistance (particularly oxidation resistance). 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 than conventional spheroidal graphite cast iron, and is more likely to crack during use. The main cause of this shortcoming is approximately
The elongation at break in the so-called blue brittle temperature range of 300 to 400°C is small, being about 2 to 3%. In response, high-Si cast iron with 4% Si and 1% Mo (in this specification, the chemical composition is expressed in weight percent as usual) has been 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 by specifying the chemical composition, the elongation at break at 300 to 400°C is small, and it is difficult to obtain an elongation of 5% or more. In view of the above-mentioned 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. Less than 3.5%, Mn 0.5% or less, P 0.1% or less, S 0.03% or less, Mo 0.1 to 0.4%, graphite nodularizing element 0.02 to 0.15%, the remainder substantially consisting of accompanying elements and Fe, average ferrite crystal grain size The present invention relates to a heat-resistant spheroidal graphite ferrite cast iron characterized by having a diameter of 21 μm or less and an elongation at break in a blue brittle temperature range of 7% or more. 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 board after 600 repeated heatings between room temperature and 800°C using a microscope. It shows the size of gender. As shown in the figure, when the amount of Si increases to 3.5% or more, the change in plate thickness decreases, but if the condition of actual use is taken into consideration and the requirement is that the thickness of the healthy part be 85% or more, the required amount of Si is 3%. That's all. 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 elongation at break at 300-400℃ begins to decrease, and
It approaches the lowest at Si3.9% or higher. Therefore, the Si content is set to be 3% or more and less than 3.5% so that a sound part thickness of 85% or more can be ensured as oxidation resistance and the elongation at break is 7% or more at 300°C or higher. Regarding Mo, for example, cast iron with Si content has a fracture elongation of 7% or less at 300 to 400℃.
The third part shows the change in elongation at break when Mo is added.
According to the figure, the elongation at break is greatly improved when the Mo content is 0.1 to 0.4%, and the elongation at break becomes 7% or more. Therefore, the amount of Mo should be 0.1 to 0.4%. 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 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%. If Mn exceeds 0.5%, pearlite tends to form and the elongation at break decreases, 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, so it is usually kept at 0.1% or less. 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 graphite spheroidizing elements, which is a combination of Mg and rare earth elements, should be 0.02 to 0.15%. If it is less than 0.02%, graphite spheroidizing will be insufficient, and if it is more than 0.15%, the effect will be almost constant. Since the formation of Mg oxide increases, the content should be 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 particle size and elongation at break, Figure 4 shows 2.6 to 3.8% C, 3 to 4.2% Si,
JIS/G0552 for spheroidal graphite cast iron with Mo0.6% or less
The graph shows the relationship between the average ferrite particle size determined by the cutting method specified in the test method and the elongation at break at 300 to 400°C. In order to reliably obtain 7% or more, it is necessary to make the average ferrite particle size approximately 21 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. can. Next, various test results conducted on the cast iron according to the present invention and a comparative material 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. Sample No. 1 in the table is spheroidal graphite cast iron according to the present invention, and No. 2 and No. 3 are
The amount of Si was increased, and the graphite spheroidizing treatment elements used included Mg and rare earth elements (RE). Samples No. 4 to 6 are comparative materials, with No. 4 having a high Mo% and No. 5 having a high Si% and large crystal grains.
No.6 is equivalent to JIS spheroidal graphite cast iron and has the highest Si in the sample.
% and a large elongation at break at 400°C, but the oxidation resistance is inferior to the product of the present invention, and Mg is used as a spheroidizing agent.

【table】

【table】

【table】

[Table] As can be seen from the above table, spheroidal graphite cast iron No. 1 according to the present invention has fine ferrite crystal grains, excellent oxidation resistance, and high elongation at break at 400°C. sample
Samples Nos. 2 and 3 also had fine grains, and showed the same results as sample No. 1. However, No. 3 has less oxidation due to its high Si content and is excellent in terms of oxidation properties, but its elongation at break at 400°C is slightly lower than Nos. 1 and 2. Samples Nos. 4 and 5 have similar chemical compositions to the products of the present invention, but their crystal grains are large and their elongation at break at 400°C is extremely low. As mentioned above, the No. 6 JIS/FCD40 equivalent product
Although the elongation at break at 400°C is high, the oxidation resistance is inferior to the product of the present invention. As mentioned above, the spheroidal graphite cast iron according to the present invention is
Although the Si content is higher than JIS spheroidal graphite cast iron, but less than so-called high-Si cast iron, it has excellent oxidation resistance, and by making the crystal grains finer, it has a higher elongation at break in the blue brittle temperature range. Therefore, if it is used as a material for mechanical parts that are repeatedly heated to high temperatures, such as turbochargers or manifolds, it can prevent oxidation and cracking, and has extremely large practical effects such as extending its life.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between Si content and oxidation resistance of ferritic spheroidal graphite cast iron, Figure 2 is a graph showing the relationship between Si content and elongation at break, and Figure 3 is also a graph showing the relationship between Si content and elongation at break. FIG. 4 is a graph showing the relationship between ferrite crystal grain size and elongation at break.

Claims (1)

[Claims] 1 C 2.6 to 3.8%, Si 3% to less than 3.5%, Mn 0.5% or less, P 0.1% or less, S 0.03% or less, Mo 0.1 to 0.4%, graphite nodularizing element 0.02 to 0.15% A heat-resistant spheroidal graphite ferritic cast iron characterized in that the remainder substantially consists of accompanying elements and Fe, has an average ferrite crystal grain size of 21 μm or less, and has an elongation at break in the blue brittle temperature range of 7% or more.