JP2976777B2 - Austenitic heat-resistant cast steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic heat-resistant cast steel. The heat-resistant cast steel of the present invention is suitable for use in exhaust system parts for automobile engines and the like.

[0002]

2. Description of the Related Art In recent years, with the trend of increasing the output of automobile engines and reducing fuel consumption, there is a great demand for improving the heat resistance of exhaust system parts for automobile engines. At present, heat-resistant ferritic cast steel is mainly used for exhaust parts for automobile engines, but improvement of high-temperature strength of heat-resistant ferritic cast steel is reaching its limit. For this reason, development of an austenitic heat-resistant cast steel in which improvement in high-temperature strength can be further expected is desired.

As such an austenitic heat-resistant cast steel, for example, Japanese Patent Application Laid-Open No. 62-004444 discloses that, by weight, C: 0.3 to 2.0%, Si: 2.0% or less, Mn: 1 to 6%, P: 0.05% or less, S: 0.0
5% or less, Cr: 13 to 27%, Ni: 5 to 14%, M
o: 0.5 to 3%, Nb: 0.5 to 3%, and the balance Fe
Are disclosed.

[0004] The heat-resistant austenitic cast steel has a high temperature,
More specifically, it satisfies both requirements of strength at 700 ° C. or higher and castability, particularly hot water flowability. However, further improvement in strength at high temperatures, especially at 900 ° C. or higher, is desired. Austenitic heat-resistant cast steel is superior in high-temperature strength and oxidation resistance to ferritic heat-resistant cast steel.
Poor machinability. It is considered that the austenitic cast steel is inferior in machinability because it generally contains a large amount of Ni or the like that improves toughness.

Accordingly, the present applicant has developed a heat-resistant, austenitic cast steel with further improved high-temperature strength and improved machinability, and has filed an application for the same (Japanese Patent Application Laid-Open No. Hei 4-35015).
No. 0). This austenitic heat-resistant cast steel has a weight ratio of C: 0.2 to 0.4% and Si: 1.5 to 2.5%.
%, Mn: 0.09 to 5.0%, P: 0.05% or less,
S: 0.03 to 0.5%, Cr: 16 to 18%, Ni:
13-15%, N: 0.05-0.15%, and Nb:
Containing 0.3 to 0.7% with the balance being Fe
At a high temperature by adding a specific amount of
And the addition of specific amounts of S improve machinability.
Also disclosed is a composition in which Mo: 1.0 to 3.0% is further added to the above composition for the purpose of further improving high-temperature strength.

[0006]

However, the above-mentioned Japanese Patent Application Laid-Open No.
The austenitic heat-resistant cast steel disclosed in -350150 No., even when Mo was not added, had a tensile strength at 950 ° C. of about 90 MPa or more, and could sufficiently improve high-temperature strength. The required performance is not always satisfied in terms of machinability, and further improvement in machinability is desired.

According to the present invention, the tensile strength at 950.degree.
An object of the present invention is to provide an austenitic heat-resistant cast steel that can secure a pressure of about MPa or more and further improves machinability.

[0008]

SUMMARY OF THE INVENTION The present inventor has conducted a study to further improve the machinability of heat-resistant austenitic cast steel disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 4-350150. Carbide (NbC or MoC)
As a result, it was clarified that the carbides were precipitated in the metallographic structure, and that the massive carbides hindered the improvement of the machinability, thereby completing the present invention.

That is, the heat-resistant austenitic cast steel according to the first aspect of the present invention, which solves the above-mentioned problems, has a weight ratio of C:
0.2-0.4%, Si: 1.5-2.5%, Mn:
0.09 to 5.0%, P: 0.05% or less, S: 0.0
Less than 3%, Cr: 16-18%, Ni: 13-20%,
And N: 0.05 to 0.30%, and the balance consists of Fe and inevitable impurities.

The reason for limiting the range of the addition amount of each alloy element in the austenitic heat-resistant cast steel of the first invention will be described. In addition, in the following description, the addition amount of each alloy element is all shown by weight%. C is effective for improving the high-temperature strength and the castability. However, if the addition amount of C is less than 0.2%, the effect is not sufficient, and if it exceeds 0.4%, the oxidation resistance deteriorates.

Although Si is effective in improving oxidation resistance and castability, its effect is not sufficient if the amount of Si is less than 1.5%, and if it exceeds 2.5%, the toughness decreases. M
n has the effect of stabilizing the austenitic ground, but Mn
Is less than 0.09%, their effects are not sufficient. On the other hand, if a large amount of Mn is added, it segregates at the grain boundaries and causes embrittlement, so the upper limit of the amount of Mn added is 5.0%.
And Mn combines with S to form MnS and has an effect of improving machinability. However, in the austenitic cast steel of the first invention, MnS does not precipitate because the addition amount of S is as small as less than 0.03%. .

If a large amount of P is added, thermal deterioration due to repeated heating and cooling is likely to occur, and the toughness is further reduced. Therefore, the upper limit of the amount of P added is set to 0.05%. S was considered as an impurity element and was set to less than 0.03%. Cr is effective in improving oxidation resistance and high-temperature strength, but its effect is not sufficient if the added amount of Cr is less than 16%. On the other hand, C
When a large amount of r is added, Cr has a property of stabilizing the ferrite ground, so that the austenite ground becomes unstable,
Further, embrittlement is caused by precipitation of the σ phase. Therefore, C
The upper limit of the amount of r added was 18%.

Ni contributes to the improvement of oxidation resistance and high-temperature strength by coexisting with Cr, stabilizes austenite ground, and prevents the formation of work-induced martensite generated during cutting, thereby improving machinability. , But Ni
Is less than 13%, the effect is not sufficient.
On the other hand, if a large amount of Ni is added, the castability deteriorates and the cost increases, so the upper limit of the amount of Ni added was set to 20%.

N contributes to the improvement of the high-temperature strength, stabilizes the austenite ground, and further prevents the formation of work-induced martensite generated during cutting, thereby contributing to the improvement of machinability. If it is less than 0.05%, the effect is not sufficient, and if it exceeds 0.30%, the castability is adversely affected. The heat-resistant, austenitic cast steel according to the second aspect of the present invention, which solves the above-mentioned problems, has a C: 0.2 to
0.4%, Si: 1.5 to 2.5%, Mn: 0.09 to
5.0%, P: 0.05% or less, S: 0.03 to 0.5
It contains 0%, Cr: 16 to 18%, Ni: 13 to 20%, and N: 0.05 to 0.30%, and is characterized by being composed of Fe and inevitable impurities.

The reason for limiting the range of the addition amount of each alloy element in the austenitic heat-resistant cast steel of the second invention will be described. The amounts of C, Si, Mn, P, Cr, Ni, and N are the same as the amounts added in the austenitic heat-resistant cast steel of the first invention, and the reasons for limiting the ranges are also the same. Hereinafter, the reason for limiting S will be described.

The machinability involves the MnS compound present in the cast iron by combining Mn and S. That is, when the MnS compound is present in the cast iron, the machinability is improved, and as the amount of the MnS compound present in the cast iron is larger, the machinability is improved. As described above, S is effective in improving machinability, but when the addition amount of S is less than 0.03%, it hardly combines with Mn and precipitates as MnS. Not enough, but if it exceeds 0.5%,
Thermal deterioration due to repeated heating and cooling is likely to occur, and toughness is further reduced. Therefore, the addition amount of S is set to 0.03 to 0.5%.

In the austenitic cast steel according to the second invention, from the viewpoint of improving the tensile strength, Mn
Is preferably three times or more the amount of S added. This is considered to be because if the amount of Mn added is less than three times the amount of S added, S does not become an MnS compound but dissolves in the parent phase and the toughness is reduced.

[0018]

In the austenitic heat-resistant cast steel according to the first and second aspects of the present invention having the above structure, Nb or M
Since o is not added, NbC or Mo
The massive carbide of C does not precipitate, and the machinability is not reduced by the massive carbide. Further, by adding specific amounts of Ni and N, the formation of work-induced martensite which occurs during cutting due to stabilization of the austenite phase of the matrix is prevented. Therefore, in the heat-resistant austenitic cast steels of the first and second inventions, the machinability can be significantly improved.

Further, by adding Ni and N in specific amounts, high-temperature strength and oxidation resistance can be improved, and these characteristics can be compensated for by not adding Nb or Mo. Furthermore, S is 0.03-0.5.
In the austenitic heat-resistant cast steel of the second invention to which 0% is added, MnS is precipitated by the bond between S and Mn, so that the machinability can be further improved.

Therefore, the heat-resistant austenitic cast steel according to the first and second aspects of the present invention makes it possible to reduce the thickness of automobile exhaust system parts and the like that require heat resistance, and contributes to improvement of fuel efficiency and power performance. It can contribute to higher efficiency of machining and improvement of tool durability.

[0021]

Embodiments of the present invention will be described below. (Example 1) As shown in Table 1, each test material of heat-resistant cast steel having a composition according to claim 1 of the present invention was obtained by casting. This casting was performed by melting in the air using a 50 kg high frequency melting furnace, deoxidizing with Fe-Si (75% by weight), and pouring the mold at 1550 ° C.

[0022]

[Table 1] (Example 2) As shown in Table 2, each test piece of heat-resistant cast steel having a composition according to claim 2 of the present invention was obtained in the same manner as in Example 1.

[0023]

[Table 2] (Comparative Example) Test samples of heat-resistant cast steel as comparative examples having the compositions shown in Table 3 were obtained in the same manner as in Example 1 above. The comparative material 11 is a general material (SCH1; JI) made of heat-resistant ferritic cast steel.
SG 5122), and the comparative material 12 is a general material (HK40; ASTM A2) of austenitic heat-resistant cast steel.
97 standard).

[0024]

[Table 3] (Evaluation of Machinability) The machinability of each of the test materials according to Examples 1 and 2 and Comparative Example was evaluated. This means that for each test material, cutting speed: 100 m / min, feed rate: 0.2 m
A machinability (continuous turning) test was performed under cutting conditions of m / rev, a depth of cut: 1.5 mm, and a total cutting length: 1000 m, and the wear amount of the cutting tool was examined. The result is shown in FIG.

As is apparent from FIG. Comparative materials according to Comparative Examples 1 to 4 were only added with less than 0.03% of S as a free-cutting element, but contained 0.03 to 0.10% of S as a free-cutting element. No. It has machinability equivalent to 8-10. Here, the material No. of the present invention according to the first embodiment. Optical micrograph (× 400) showing the metal structure of Comparative Example No. 1 and Comparative Material No. 1 according to Comparative Example. Optical micrographs (× 400) showing the metallographic structures of No. 10 are shown in FIGS. 2 and 3, respectively. As is clear from these figures, the comparative material No. In No. 10, NbC and MoC were precipitated as hard massive carbides. In 1,
Only Cr ₇ C ₃ as granular carbide was observed. Thereby, it became clear that the form of the carbide greatly contributed to the improvement of the machinability of the material of the present invention. That is, since the material of the present invention did not contain the carbide-forming elements Mo and Nb, hard massive carbides did not precipitate, thereby significantly improving machinability.

Further, according to the present invention material No. 1
Inventive material No. 1 according to Example 2 in which S was added as a free-cutting element to Nos. Nos. 5 to 7 are materials of the present invention. 1 to
4 and Comparative Material No. Machinability was remarkably improved as compared with 8 to 10. Here, the material No. 1 of the present invention according to the second embodiment. 6
FIG. 4 shows an optical micrograph (× 400) showing the metal structure of FIG.
Shown in As is clear from FIG. 6
In the above, Cr ₇ C ₃ as granular carbide and Mn
Precipitation of S was observed. Thereby, the effect of improving machinability by MnS was confirmed.

(Evaluation of High Temperature Strength) 1 to
7 and Comparative Material No. A high-temperature tensile test was performed on each of the test materials 8 to 12. This test is based on JIS Z 2241
The test was performed at a temperature of 950 ° C. The result is shown in FIG. As is clear from FIG.
The tensile strength at 50 ° C. is 90 MPa or more, and has the same high-temperature strength as the comparative material.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of a continuous turning test of a material of the present invention and a comparative material.

FIG. 1 is an optical micrograph (× 400) showing a metal structure of Example 1.

FIG. 10 is an optical micrograph (× 400) showing a metal structure of No. 10.

FIG. 6 is an optical micrograph (× 400) showing a metal structure of No. 6.

FIG. 5 is a graph showing the results of a high-temperature tensile test of a material of the present invention and a comparative material.

Claims (2)

(57) [Claims] C: 0.2 to 0.4% by weight,
Si: 1.5 to 2.5%, Mn: 0.09 to 5.0%,
P: 0.05% or less, S: less than 0.03%, Cr: 16
-18%, Ni: 13-20%, and N: 0.05-
An austenitic heat-resistant cast steel containing 0.30%, the balance being unavoidable impurities and Fe. 2. C: 0.2 to 0.4% by weight,
Si: 1.5 to 2.5%, Mn: 0.09 to 5.0%,
P: 0.05% or less, S: 0.03 to 0.50%, C
r: 16-18%, Ni: 13-20%, and N: 0.
An austenitic heat-resistant cast steel containing 0.05 to 0.30%, the balance being unavoidable impurities and Fe.