JPH05171365A - Ferritic heat resistant cast steel and exhaust system parts made of it - Google Patents

Abstract

PURPOSE:To provide ferritic heat resistant steel excellent in durability such as a heat fatigue service life and oxidation resistance as well as room temp. strength, ductility, castability and workability by adding specified amounts of W, Nb, Ni and REM or V as well to high Cr ferritic stainless steel. CONSTITUTION:High Cr ferritic stainless steel contg., by weight, 0.05 to 0.30%C, <2.0% Si, <1.0% Mn and 16.0 to 25.0% Cr is incorporated with 1.0 to 3.0% W, 0.01 to 0.45% Nb, 0.1 to 2.0% Ni and 0.01 to 0.5% rare earth metal elements such as Ce and La or 0.01 to 0.3% V as well, by that, the objective ferritic heat resistant steel in which its structure is constituted of an alpha' phase having a pearlite colony shape in which an alpha phase and a gamma phase are transformed into (X phase+carbide) as well as the area rate of {alpha'/(alpha+alpha)} is regulated to 20 to 90% in the structure and free from the generation of defects such as cracks caused by high temp. annealing at the time of executing annealing at 700 to 850 deg.C less than 950 deg.C which is the temp. at which the alpha' phase transforms into the gamma phase and removing strains in the case of casting to improve its machinability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant cast steel suitable for automobile engine exhaust system parts and the like, and particularly has excellent durability such as thermal fatigue life and oxidation resistance, as well as room temperature strength, ductility, castability, The present invention relates to a heat-resistant cast steel which is excellent in workability and can be manufactured at a low cost, and an exhaust system component made of the heat-resistant cast steel.

[0002]

2. Description of the Related Art Conventional heat-resistant cast iron and heat-resistant cast steel have, for example, the compositions shown in Table 1 as comparative materials. As exhaust system components such as automobile exhaust manifolds and turbine housings are subject to severe operating conditions at high temperatures, high Si spheroidal graphite cast iron and Ni-resist cast iron (Ni-Cr-Cu austenitic cast iron) as shown in Table 1 are used. And heat-resistant cast iron such as austenitic cast steel, which are expensive, are used.

[0003]

Among such conventional heat-resistant cast irons and heat-resistant cast steels, for example, high Si spheroidal graphite cast irons and Niresist cast irons have relatively good castability, but have heat fatigue resistance or oxidation resistance. Since it is inferior in durability such as property, it cannot be applied to a member having a high temperature of 900 ° C. or higher. Further, high alloy heat resistant cast steel such as austenitic heat resistant cast steel has a shortcoming of short thermal fatigue life due to its large coefficient of thermal expansion, although it has excellent durability at 900 ° C. or higher. In addition, since castability is poor, casting defects such as shrinkage cavities and defective molten metal rotation are likely to occur during casting, and further poor machinability results in low productivity when manufacturing parts from it. There were also points. In addition, although there is ferritic stainless cast steel, there is a problem that ordinary ferritic stainless steel has poor ductility at room temperature and cannot be used as a member to which a mechanical shock is applied when trying to improve high temperature strength.

Therefore, the present invention is directed to the above-mentioned conventional heat-resistant cast iron,
The object of the present invention is to solve the problems of heat-resistant cast steel and to provide heat-resistant cast steel that is excellent in durability such as heat fatigue resistance and oxidation resistance, and has excellent room temperature strength, ductility, castability, and workability, and that can be manufactured at low cost. To do.

Another object of the present invention is to provide an exhaust system component made of such heat-resistant cast steel.

[0006]

As a result of earnest research in view of the above-mentioned object, the present inventors have found that the ferrite matrix and the grain boundaries can be formed by adding an appropriate amount of W, Nb, Ni, REM or V to them. The inventors have found that the alloy can be strengthened, the transformation point can be increased without impairing the ductility at room temperature, and the high temperature strength can be improved, and the present invention has been made.

That is, the ferritic heat-resistant cast steel of the first aspect of the present invention is, by weight ratio, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 16.0 to 25.0%, W: 1.0 to 3.0%, Nb: 0.01 to 0.45%, Ni: 0.1 to 2.0%, REM: 0.01 to 0. 5%, balance: Fe and inevitable impurities, and a normal α phase and a γ phase transformed from (α + carbide) into a pearlite colony-like α'phase, and an area ratio of α'phase {α '/ (Α + α')} is 2
It is characterized by 0 to 90%.

In the ferritic heat-resistant cast steel of the first invention, the transformation point from the α'phase to the γ phase is 900 ° C. or higher.

Next, the ferritic heat-resistant cast steel of the second aspect of the present invention is, by weight ratio, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 1.0% or less, Cr. : 16.0 to 25.0%, W: 1.0 to 3.0%, Nb: 0.01 to 0.45%, Ni: 0.1 to 2.0%, REM: 0.01 to 0 0.5%, V: 0.01 to 0.3%, balance: Fe and unavoidable impurities, and a pearlite colony α'phase transformed from a normal α phase and γ phase to (α + carbide). And the area ratio of the α'phase {α '/ (α + α')} is 2
It is 0 to 70%.

In the ferritic heat-resistant cast steel of the second invention, the transformation point from the α'phase to the γ phase is 950 ° C or higher.

Further, in the ferritic heat-resistant cast steels of the first and second inventions, if there is a need for residual strain removal or working, annealing treatment is performed in a temperature range where the α'phase does not transform into the γ phase. It is characterized by

Further, the exhaust system component made of the ferritic heat-resistant cast steel of the present invention is formed of the heat-resistant cast steel having the above composition, and is characterized by being an exhaust manifold and a turbine housing.

[0013]

As described above, in the heat-resistant ferritic cast steel, W is 1.0 to 3.0% and Nb is 0.01 to 3.0% by weight.
0.45%, Ni 0.1-2.0%, REM 0.0
1 to 0.5%, or V to 0.01 to 0.
When 3% is added, a structure containing an α'phase is obtained, which has heat fatigue resistance and oxidation resistance superior to those of conventional high alloy steels, and casts equivalent to heat-resistant cast iron without impairing ductility at room temperature. Heat-resistant cast steel that has high workability and workability and is inexpensive can be obtained. Further, since the transformation temperature is 900 ° C. or higher in the first invention and 950 ° C. or higher in the second invention, the thermal fatigue resistance is improved.

The reasons for limiting the compositional range and the structural range of each alloying element of the ferritic heat-resistant cast steel of the present invention will be described in detail below. (1) C (carbon): 0.05 to 0.30% C has the effects of improving the fluidity of the molten metal, that is, the castability, and generating an appropriate amount of α'phase, and further, at the transformation point or lower. It has the function of maintaining high strength at high temperatures.
In order to effectively exert such an action, C is 0.0
5% or more is required. Incidentally, in general ferritic heat-resistant cast steel, only α phase at room temperature, but by adjusting the carbon content,
In addition to the α phase that exists from high temperature to room temperature, at the high temperature, the γ phase in which C forms a solid solution is formed. This γ phase transforms into (α phase + carbide) by precipitating carbide during cooling. Such a phase is called an α'phase. On the other hand, when the C content exceeds 0.30%, the α'phase is less likely to exist, a martensite structure is formed, and the precipitation of Cr carbides, which causes deterioration of oxidation resistance, corrosion resistance and workability, is remarkable. become. Therefore, C is set to 0.05 to 0.30%. Desirably 0.10
It is 0.25%.

(2) Si (silicon): 2.0% or less Si has the effect of narrowing the range of the γ phase of the Fe-Cr alloy of the present invention, increasing the stability of the structure, and improving the oxidation resistance. ..
Further, it also has the effects of improving castability, acting as a deoxidizer, and reducing pinhole defects in the casting. However, if too much, C
And the balance (carbon equivalent) coarsen the primary carbides, reducing the workability of cast steel, and causing excessive ductility due to excessive Si content in the ferrite matrix structure. To form. Therefore, the Si content is 2.0% or less. Desirably 0.5 to 1.5
%.

(3) Mn (manganese): 0.3-1.
0% Mn, like Si, is effective as a deoxidizer for molten metal,
It also improves the productivity by improving the flowability of molten metal during casting.
In order to make such an effect effective, the Mn content is set to 0.
3 to 1.0%. It is preferably 0.4 to 0.7%.

(4) Cr (chromium): 16.0-2
5.0% Cr is an element that improves the oxidation resistance and stabilizes the ferrite structure, but in order to ensure its effect, it is made 16.0% or more. On the other hand, the addition of a large amount coarsens the primary carbide of Cr, promotes the formation of the δ phase at high temperature, and causes remarkable embrittlement. Therefore, the upper limit of Cr is 25.0% or less. Desirably, it is 17.0 to 22.0%.

(5) W (tungsten): 1.0 to
3.0% W has the effect of strengthening the ferrite matrix and improving the high temperature strength without impairing the ductility at room temperature. Therefore, W is set to 1.0% or more for the purpose of improving the creep resistance and the thermal fatigue resistance by increasing the transformation temperature. However, if the W content exceeds 3.0%, coarse eutectic carbides are formed, which causes deterioration of ductility and deterioration of machinability.
Below. It is preferably 1.2 to 2.5%.

It should be noted that an effect similar to that of W can be obtained by adding Mo (however, since Mo is twice the atomic ratio of W, the addition amount is 1/2 in weight ratio). , W has a higher melting point than Mo, has a slower diffusion rate at high temperatures, contributes greatly to high temperature, especially 900 ° C. high temperature strength, and is excellent in oxidation resistance.
Contains only.

(6) Nb (niobium): 0.01-0.
45% Nb combines with C to form fine carbides, which increases the tensile strength at high temperature as well as the thermal fatigue resistance. Also, Cr
Improves oxidation resistance and machinability by suppressing the formation of carbides. For this purpose, the Nb content is 0.01% or more. However, when added in a large amount, carbides are formed at the grain boundaries, and Nb carbides are generated, so that C is consumed and the α'phase is less likely to be formed.
Strength and ductility are significantly reduced. Therefore, the Nb content is 0.45% or less. Desirably 0.02-0.3
It is 0%. . (7) Ni (nickel): 0.1 to 2.0% Ni is a γ phase forming element like C, and 0.1% or more is added to make an α ′ phase exist in an appropriate amount. Is preferred. On the other hand, if it exceeds 2.0%, the α ′ phase having excellent oxidation resistance decreases and the α ′ phase becomes martensite,
Remarkably reduces ductility. Therefore, the Ni addition amount 3 is set to 2.0% or less. It is preferably 0.3 to 1.5%.

(8) REM (rare earth metal): 0.01
~ 0.5% REM is a light rare earth element mainly composed of Ce (cerium), La (lanthanum), etc., and forms a stable oxide,
It has the functions of improving the oxidation resistance and making the grain boundaries finer. Further, the non-metallic inclusions are spheroidized to improve room temperature ductility. In order to make such an effect effective, REM is set to 0.
Add 01% or more. On the other hand, when added in a large amount, it becomes a non-metallic inclusion, which is particularly harmful to ductility. Therefore, the upper limit of REM is set to 0.5%. The addition of REM does not affect the amount of α'phase or the transformation point. It is preferably 0.05 to 0.3%.

In the ferritic heat-resistant cast steel of the second invention, the action of the above-mentioned essential elements is the same as that of the first invention, and further contains V. The reasons for limiting the amount of V added are as follows.

(9) V (vanadium): 0.01 to
0.3% V has the same action as Nb, but V is Nb at atomic%
Therefore, it is desirable that (Nb + V) does not exceed 0.5% in the case of composite addition. In the case of V alone, it is preferably 0.05 to 0.2%.

To summarize the above. The composition of the ferritic heat-resistant cast steel of the present invention is as follows. The first invention is
By weight ratio, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 16.0 to 25.0%, W: 1.0 to 3. 0%, Nb: 0.01 to 0.45%, Ni: 0.1 to 2.0%, REM: 0.01 to 0.5%, balance: Fe and inevitable impurities.

The preferred composition range of the first invention is as follows. C: 0.10 to 0.25%, Si: 0.7 to 1.5%, Mn: 0.4 to 0.7%, Cr: 17.0 to 22.0%, W: 1.2 to. 2.5%, Nb: 0.02 to 0.3%, Ni: 0.3 to 1.5%, REM: 0.05 to 0.3%, balance: Fe and inevitable impurities.

The second invention is C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 16.0 to 25.0%, W: 1 0.0 to 3.0%, Nb: 0.01 to 0.45%, Ni: 0.1 to 2.0%, REM: 0.01 to 0.5%, V: 0.01 to 0.3. %, Balance: Fe and inevitable impurities.

The preferred composition range of the second invention is as follows. C: 0.08 to 0.25%, Si: 0.7 to 1.5%, Mn: 0.4 to 0.7%, Cr: 17.0 to 22.0%, W: 1.2 to. 2.5%, Nb: 0.02 to 0.3%, Ni: 0.3 to 1.5%, REM: 0.05 to 0.3%, V: 0.05 to 0.2%, balance : Fe and inevitable impurities.

The ferritic heat-resistant cast steel of the present invention having the above composition has, in addition to the usual α phase, an α'phase transformed from γ to (α + carbide). The α'phase is a slightly black texture inside the edge seen in the metallographic photograph of FIG. The ordinary α phase means δ (delta) ferrite. The precipitated carbides are Fe, Cr, W, N
Carbides such as b (M23C6, M7C3, MC, etc.).

Area ratio of this α'phase {α '/ (α +
If α ′)} is less than 20%, the ductility at room temperature is low,
Extremely brittle. On the other hand, the area ratio of the α'phase is 9 in the first invention.
0%, in the second invention, if it exceeds 70%, it becomes too hard,
The ductility at room temperature decreases and the machinability deteriorates significantly. Therefore, the area ratio {α '/ (α + α')} is 20 to 90% in the first invention and 20 to 70 in the second invention.
%.

If it is necessary to remove residual strain or to process the ferritic heat-resistant cast steel, it may be annealed in a temperature range where the α'phase does not transform into the γ phase after casting.
The annealing temperature at this time is generally 700 to 850 ° C.
And the annealing time is 1 to 10 hours.

If there is a transformation point from the α'phase to the γ phase in the operating temperature range, the thermal stress generated by the heating-cooling cycle is increased and the thermal fatigue life is shortened. for that reason,
It must have a transformation point of 900 ° C or higher. In order to have such a high transformation point, it is necessary that the balance of the ferrite-forming elements Cr, Si, W, V, and Nb and the austenite-forming elements C, Ni, and Mn be appropriate.

The area ratio and transformation point in the ferritic heat-resistant cast steel of each invention are as follows. First invention; area ratio: 20 to 90%, transformation point: 900 ° C
Above 2nd invention; Area ratio: 20-70%, transformation point: 950 degreeC
As described above, the ferritic heat-resistant cast steel of the present invention is particularly suitable for manufacturing automobile exhaust system parts. Fig. 1 shows an integral structure exhaust manifold attached to an in-line 4-cylinder engine with a supercharger as an exhaust system component of an automobile. The exhaust manifold 1 is connected to a turbine housing 2 of a turbocharger, and the turbine housing 2 is connected via an exhaust outlet pipe 3 to a catalytic converter container 4 for purifying exhaust gas.
Are connected. Further, a main catalyzer 5 is connected to the converter container 4. The outlet of the main catalyzer 5 communicates with the muffler (D). On the other hand, the turbine housing 2 communicates with the intake manifold (B) and is sucked from (C). Exhaust gas flows into the exhaust manifold 1 from (A).

Such an exhaust manifold 1
The turbine housing 2 is preferably made as thin as possible in order to reduce the heat capacity. The exhaust manifold 1 and the turbine housing 2 have wall thicknesses of, for example, 2.5 to 3.4 mm and 2.7 to, respectively.
It is 4.1 mm. Even if the exhaust manifold 1 and the turbine housing 2 made of such a thin ferritic heat-resistant cast steel undergo a heating-cooling thermal cycle,
It does not crack and has excellent durability.

[0034]

EXAMPLES The present invention will be described in detail below with reference to examples. Examples 1 to 9 and Comparative Examples 1 to 5 With respect to the ferritic heat resistant cast steels of the types shown in Table 1, JIS standard Y-shaped No. B test materials were produced. In addition,
When casting, use a 100 kg high-frequency furnace to melt in the air and immediately release the molten metal at 1550 ° C or higher to about 1500 ° C.
I poured it in. Regarding the ferritic heat-resistant cast steels of Examples 1 to 9, the flow of molten metal during casting was good, and no casting defects were found. Next, the cast samples 1 to 9 (Y block) were heat-treated by holding them in a heating furnace at 800 ° C. for 2 hours and then air-cooling. On the other hand, all the comparative materials (Comparative Examples 1 to 5) were subjected to the test as cast. In Table 1, Comparative Examples 1 to 5 are all used for heat-resistant parts such as automobile turbocharger housings and exhaust manifolds, and the sample material of Comparative Example 1 is high Si spheroidal graphite cast iron. The test material of Comparative Example 2 was Ni-resist cast iron, and the test material of Comparative Example 3 was ACI (Alloy).
CB of Casting Institute) standard
No. 30, and the sample material of Comparative Example 4 is one of those called austenitic heat-resistant cast steel (corresponding to JIS standard SCH12), and the sample material of Comparative Example 5 is JP-A-2-
It is a ferritic heat-resistant cast steel disclosed in Japanese Patent No. 175841.

As shown in Table 1, Example 1 which is the material of the present invention
It can be seen that Nos. 9 to 9 have a transformation temperature of 900 ° C. or higher, which is higher than those of Comparative Examples 1 and 3.

Next, various evaluation tests described below were carried out using each test material. (1) Room Temperature Tensile Test A round bar test piece (JIS No. 4 test piece) having a gauge length of 50 mm and a gauge length of 14 mm was used. (2) High Temperature Tensile Test A 900 ° C. test was performed using a brim test piece having a gauge length of 50 mm and a gauge diameter of 10 mm. (3) Thermal Fatigue Test Using a round bar test piece with a gauge length of 20 mm and a gauge diameter of 10 mm, heating-cooling under the following conditions with the expansion and contraction due to heating-cooling completely restrained. The cycle was repeated to cause thermal fatigue failure. Lower limit temperature: 100 ° C. Upper limit temperature: 900 ° C. Each cycle: 12 minutes In addition, an electro-hydraulic servo type thermal fatigue tester was used as a tester.

(4) Oxidation test A round bar test piece having a diameter of 10 mm and a length of 20 mm was prepared, and 9
It is kept in the atmosphere at 00 ° C for 200 hours, and after taking out, shot blasting is applied to remove the oxide scale, and the weight change per unit area before and after the oxidation test (oxidation weight loss; mg / cm2) is determined to obtain the acid resistance. The chemical conversion was evaluated. The results of the above room temperature tensile test are shown in Table 2, and the results of the high temperature tensile test, thermal fatigue test and oxidation test are shown in Table 3.

[0038]

[Table 1] Chemical composition (wt%) Example C Si Mn Cr W Nb Ni REM No.1 0.13 0.80 0.55 16.5 1.15 0.19 0.21 0.03 2 0.15 0.93 0.48 18.6 1.95 0.33 0.74 0.04 3 0.21 1.05 0.49 20.4 2.52 0.15 0.88 0.01 4 0.25 1.43 0.82 21.8 2.72 0.04 1.33 0.03 5 0.29 0.93 0.55 24.5 2.81 0.11 1.75 0.09 6 0.17 0.88 0.60 18.6 1.25 0.42 1.26 0.1 7 0.19 1.08 0.44 18.3 2.45 0.28 0.66 0.1 8 0.24 0.95 0.61 18.0 2.93 0.10 0.95 0.3 9 0.25 0.82 0.53 17.5 2.02 0.14 0.53 0.20 Comparative Example No.1 3.33 4.04 0.35---0.62 *-2 2.01 4.82 0.45 1.91--35.3-3 0.28 1.05 0.44 17.9----4 0.21 1.24 0.50 18.8--9.1-5 0.12 1.05 0.48 18.1- 1.12--(Note) *: Mo

Table 1 (continued) α '/ (α + α') transformation point Example (%) (℃) No. 1 65 920 2 50 970 3 40 1000 4 35 1020 5 30 1070 6 65 920 7 50 990 8 75 990 9 85 930 Comparative example No. 1-800 to 850 2--3 93 910 4--5 0> 1100

[0040]

[Table 2]

[0041]

[Table 3] 900 ℃ 0.2% Tensile Thermal Fatigue Oxidation Strength Strength Elongation Life Reduction Weight Example (MPa) (MPa) (%) (Cycle) (mg / cm2) No. 1 22 38 44 86 2 2 24 41 54 280 1 3 23 42 54 495 1 4 24 45 48 385 2 5 20 40 56 500 2 6 24 50 54 365 1 7 24 48 50 330 1 8 27 52 41 535 1 9 29 58 42 485 1 Comparative example No. 1 20 40 33 9 200 2 40 90 44 23 20 3 25 42 58 18 1 4 65 128 31 35 2 5 15 28 93 185 2

As is clear from Tables 2 and 3, Examples 1 to 9 according to the present invention are remarkably superior in high temperature strength, oxidation resistance and thermal fatigue life to the test materials of Comparative Examples 1 to 5. You can see that it has been improved. This is a proper amount of W, Nb,
By containing Ni and REM, the ferrite matrix is strengthened, and the transformation point is 900 without damaging the ductility at room temperature.
This is because the temperature rises above ℃. When the material properties such as 0.2% proof stress, tensile strength, elongation, thermal fatigue life, and weight loss due to oxidation are compared individually, there are comparative examples that are superior to the material of the present invention. The invention material has a good balance of various properties and is extremely excellent as a material for exhaust system parts.

Further, as shown in Table 2, the materials of the present invention (Examples 1 to 9) have a relatively low hardness (HB) of 170 to 235 and are excellent in machinability.

A micrograph (100 times) of the heat-resistant cast steel of Example 8 is shown in FIG. The white part in FIG.
Example 8 is a normal α phase called ferrite, and the slightly black portion inside the trim is an α ′ phase transformed from the γ phase.
Has an area ratio of α '{α' / (α + α ')} of 75%.

A photomicrograph (100 times) of the heat-resistant material of Comparative Example 5 is shown in FIG. The white portion in FIG. 3 is a normal α phase called δ-ferrite, and Nb of Nb carbide is present at grain boundaries.
bC is observed. The area ratio of the α ′ phase in Comparative Example 5 is 0%.

Examples 10 to 19 For the ferritic heat-resistant cast steels of the types shown in Table 4, JIS standard Y-shaped No. B test materials were prepared in the same manner as in Example 1.

Regarding the ferritic heat-resistant cast steels of Examples 10 to 19, the flow of molten metal during casting was good, and no casting defects were found. Next, the cast material of the present invention (Example 10)
〜 19) Test material (Y block) in a heating furnace at 800 ℃
After that, a heat treatment of holding for 2 hours and cooling with air was performed.

As shown in Table 1, Example 1 which is the material of the present invention
0 to 19 have a transformation point temperature of 950 ° C. or higher, and Comparative Example 1
It turns out that it is higher than ~ 3.

Next, a room temperature tensile test, a high temperature tensile test, a thermal fatigue test, and an oxidation test were performed on each of the test materials after casting under the same conditions as in Examples 1-9.

The results of the above room temperature tensile test are shown in Table 5, and the results of the high temperature tensile test, thermal fatigue test and oxidation test are shown in Table 6.

[0051]

[Table 4] Chemical composition (wt%) Example C Si Mn Cr W Nb Ni REM V No.10 0.12 0.88 0.48 15.6 1.48 0.02 0.07 0.12 0.20 11 0.14 1.00 0.65 18.8 2.05 0.42 0.50 0.08 0.08 12 0.23 1.50 0.82 21.8 1.52 0.10 1.50 0.05 0.42 13 0.27 1.20 0.48 23.0 2.92 0.07 0.62 0.29 0.13 14 0.15 0.75 0.78 18.1 2.65 0.21 0.15 0.24 0.04 15 0.17 0.92 0.45 20.3 1.94 0.05 1.02 0.03 0.18 16 0.09 1.05 0.54 18.6 2.24 0.08 1.92 0.13 0.06 17 0.41 1.11 0.49 18.3 2.25 0.09 0.15 0.006 0.25 18 0.30 0.89 0.54 17.7 1.88 0.08 0.11 0.09 0.16 19 0.13 1.32 0.91 18.9 2.12 0.13 0.09 0.4 0.10

Table 4 (continued) α '/ (α + α') Transformation point Example (%) (℃) No.10 60 970 11 30 1045 12 28 1080 13 28 1080 14 35 1020 15 25 1080 16 40 1010 17 35 1010 18 50 960 19 40 1020

[0053]

[Table 5]

[0054]

[Table 6] 900 ℃ 0.2% Tensile Thermal Fatigue Oxidation Strength Strength Elongation Life Reduction Weight Example (MPa) (MPa) (%) (cycle) (mg / cm2) No. 10 22 42 48 215 3 11 24 46 54 180 2 12 20 44 45 200 1 13 25 52 52 240 2 14 23 48 54 260 2 15 27 52 60 270 1 16 22 47 51 200 1 17 24 45 48 245 1 18 26 50 57 320 2 19 25 48 50 255 1

As is clear from Tables 5 and 6, Examples 10 to 19 according to the present invention are significantly higher in high temperature strength, oxidation resistance and thermal fatigue life than the test materials of Comparative Examples 1 to 5. You can see that it has been improved. This is a proper amount of W, N
By including b, Ni, REM, and V, the ferrite matrix is strengthened, and the transformation point is 9 without impairing the ductility at room temperature.
This is because the temperature rose to 50 ° C or higher. When the material properties such as 0.2% proof stress, tensile strength, elongation, thermal fatigue life, and weight loss due to oxidation are compared individually, there are comparative examples that are superior to the material of the present invention. The invention material has a good balance of various properties and is extremely excellent as a material for exhaust system parts.

Further, as shown in Table 5, the materials of the present invention (Examples 10 to 19) have relatively low hardness (HB) of 174 to 217 and are excellent in machinability.

A micrograph (100 times) of the heat-resistant cast steel of Example 18 is shown in FIG. The white part in Fig. 4 is δ
-In a normal α phase called ferrite, the slightly black part inside the trim is an α'phase transformed from the γ phase, and in Example 18, the area ratio of α '{α' / (α + α ')} is 50.
%.

Next, using the ferritic heat-resistant cast steels of Examples 5 and 15, the exhaust manifold (pipe wall thickness: 2.5 to 3.4 mm) and turbine housing (wall thickness: 2.7) shown in FIG. 1 were used. .About.4.1 mm) was cast. The heat-resistant cast steel parts obtained were all sound.

Further, these cast steel parts were subjected to machining to evaluate their machinability, but no problems occurred in any of them.

Next, as shown in FIG. 1, an endurance test was conducted with an exhaust simulator that emits exhaust gas equivalent to a high-performance gasoline engine with a displacement of 2000 cc in an in-line four-cylinder in which an exhaust manifold and a turbine housing are assembled. As a test condition, a heat-cooling (GO-STOP) cycle having 1 cycle of full load operation (continuous 14 minutes) -idling (1 minute) -complete stop (14 minutes) -idling (1 minute) corresponding to 6000 rpm was performed. , 500 cycles. The exhaust gas temperature at full load was 930 ° C., which was the inlet temperature of the turbine housing. Under this condition, the surface temperature of the exhaust manifold is about 870 ° C. at the exhaust manifold collecting portion, and the surface temperature of the turbine housing is about 890 at the waste gate portion.
It was ℃. As a result of the evaluation test, it was confirmed that gas leakage and thermal cracking due to thermal deformation did not occur, and that it had excellent durability and reliability.

On the other hand, an exhaust manifold was prepared from the high Si spheroidal graphite cast iron having the chemical composition shown in Table 7, and the NI-RESIST D2 (INC of the chemical composition shown in the same table was prepared.
A turbine housing was made of austenitic spheroidal graphite cast iron (trademark of O company). As a result, the exhaust manifold made of high Si spheroidal graphite cast iron became unusable due to thermal cracking due to oxidation in the vicinity of the aggregated portion in 98 cycles. After that, when the exhaust manifold was replaced with that of Example 7 and the test was continued, a crack penetrating the wall thickness was generated in the scroll portion of the turbine housing of the austenitic spheroidal graphite cast iron at the 324th cycle. As a result of the above, it became clear that the exhaust manifold and the turbine housing, which are the products of the present invention, have excellent durability.

[0062]

[Table 7] Grade C Si Mn P S Cr Ni Mo Mg High Si spheroidal graphite cast iron 3.15 3.95 0.47 0.024 0.008 0.03-0.55 0.048 Austenitic spheroidal graphite cast iron 2.91 2.61 0.81 0.018 0.010 2.57 21.5-0.084

[0063]

As described above, according to the present invention, in particular, W, Nb, Ni, REM or V is added thereto in an appropriate amount to strengthen the ferrite matrix and the grain boundaries, respectively, and to improve the ductility at room temperature. It raises the transformation point without deteriorating the properties of the steel, and exhibits particularly important properties at high temperature tensile strength, heat fatigue resistance, and oxidation resistance that exceed those of conventional heat-resistant cast steel. Moreover, since it is excellent in castability and workability, it can be manufactured at low cost. Such a ferritic heat-resistant cast steel of the present invention is particularly suitable for engine exhaust system parts. The exhaust system component made of the ferritic heat-resistant cast steel according to the present invention exhibits extremely excellent durability without causing thermal cracks.

[Brief description of drawings]

FIG. 1 is a schematic view showing an exhaust manifold and a turbine housing that can be manufactured from the ferritic heat-resistant cast steel of the present invention.

FIG. 2 is a micrograph (× 100) showing the metal structure of the ferritic heat-resistant cast steel of Example 8.

FIG. 3 is a micrograph (100 ×) showing a metal structure of a heat-resistant material of Comparative Example 5.

FIG. 4 is a micrograph (100 times) showing a metal structure of a ferritic heat-resistant cast steel of Example 18.

[Explanation of symbols]

 1 Exhaust Manifold 2 Turbine Housing 3 Exhaust Outlet 4 Converter Vessel 5 Main Catalyzer

Claims (8)

[Claims] 1. By weight ratio, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 16.0 to 25.0%, W: 1 0.0 to 3.0%, Nb: 0.01 to 0.45%, Ni: 0.1 to 2.0%, REM: 0.01 to 0.5%, balance: Fe and inevitable impurities And has a pearlite colony-like phase (hereinafter referred to as α'phase) transformed from a normal α phase and γ phase to (α + carbide), and has an area ratio of α'phase {α '/ (α + α')} Is 20-9
Ferritic heat-resistant cast steel characterized by being 0%. 2. The ferritic heat-resistant cast steel according to claim 1, wherein the transformation point from the α ′ phase to the γ phase is 900 ° C. or higher. 3. By weight ratio, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 16.0 to 25.0%, W: 1 0.0 to 3.0%, Nb: 0.01 to 0.45%, Ni: 0.1 to 2.0%, REM: 0.01 to 0.5%, V: 0.01 to 0.3. %, Balance: Fe and a composition consisting of unavoidable impurities, and an ordinary α phase and a pearlite colony-like α ′ phase transformed from the γ phase to (α + carbide), and an area ratio of the α ′ phase {α ′ / (Α + α ')} is 20 to 70%, a ferritic heat-resistant cast steel characterized by the above-mentioned. 4. The ferritic heat-resistant cast steel according to claim 3, wherein the transformation point from the α ′ phase to the γ phase is 950 ° C. or higher. 5. The ferritic heat-resistant cast steel according to any one of claims 1 to 4, wherein when it is necessary to remove residual strain or to process, the annealing treatment is performed in a temperature range where the α'phase does not transform into the γ phase. Ferritic heat-resistant cast steel. 6. An exhaust system component made of the ferritic heat resistant cast steel according to any one of claims 1 to 5. 7. The exhaust system component according to claim 6, wherein the exhaust system component is an exhaust manifold. 8. The exhaust system component according to claim 6, which is a turbine housing.