JPH05287458A - Ferritic cast heat resisting steel excellent in ductility at room temperature and oxidation resistance and exhaust system parts made thereof - Google Patents

Abstract

PURPOSE:To produce a cast steel for engine exhaust system parts excellent in ductility at room temp. and oxidation resistance by adding specific small amounts of W, Nb, V, REM, and Ca or Mg to a ferritic cast stainless steel and performing, as necessary, annealing treatment. CONSTITUTION:The engine exhaust system parts are cast by using a ferritic cast stainless steel which contains, by weight, 0.15-0.45% C, <2.0% Si, <1.0% Mn, 17-22% Cr, 1-3% W, 0.01-0.45% Nb or V, 0.01-0.5% REM, such as Ce and La, and 0.005-0.03% Ca or Mg and has, besides ordinary alpha-phase, an alpha'-phase transformed from gamma-phase to (alpha + carbide) and where the area ratio [alpha'/(alpha+alpha')] of the alpha'-phase is regulated to 20-80% and also the reversal transformation point from alpha'-phase to gamma-phase is regulated to >=1000 deg.C. In the case where the removal of residual strain in the cast product and working are necessary, annealing is done at a temp. in the range where the alpha'-phase does not transform into gamma-phase. By this method, the engine exhaust system parts made of ferritic cast stainless steel excellent in heat resistance can be produced at a low cost.

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 an exhaust system component of an automobile engine, and particularly to a thermal fatigue life,
The present invention relates to a heat-resistant cast steel that is excellent in durability such as oxidation resistance, has excellent room temperature ductility as a structural component, is excellent in castability and workability, and can be manufactured at low cost, and an exhaust system component made of the same.

[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 examples. 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 object, the present inventors have found that by adding an appropriate amount of W, Nb, Nb and / or V, REM, Ca and / or Mg, the ferrite Strengthening the matrix and grain boundaries,
The inventors have found that it is possible to raise the transformation point and improve the high temperature strength without impairing the ductility at room temperature, and have conceived the present invention.

That is, the ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance of the present invention has a weight ratio of C: 0.15 to 0.45%, Si: 2.0% or less, and Mn: 1. 0% or less, Cr: 17.0 to 22.0%, W: 1.0 to 3.0%, Nb and / or V: 0.01 to 0.45%, REM: 0.01 to 0.5. %, Ca and / or Mg: 0.005 to 0.03%, the balance: Fe and inevitable impurities, and from the γ phase in addition to the usual α phase (δ-ferrite phase from high temperature). (α + carbide) and has a pearlite colony-like α'phase and α '
The area ratio of the phase {α '/ (α + α')} is 20 to 80%.

In the ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance of the above invention, the transformation point from the α'phase to the γ phase is 1000 ° C or higher.

Further, in the heat-resistant ferritic cast steel excellent in room temperature ductility and oxidation resistance of the above invention, when there is a need for residual strain removal or working, annealing is performed in a temperature range where the α'phase does not transform into the γ phase. Apply processing.

Further, the exhaust system component of the present invention, which is made of a heat-resistant ferritic cast steel having excellent room temperature ductility and oxidation resistance, is formed of heat-resistant cast steel having the above composition, and is an exhaust manifold and a turbine housing. ..

[0011]

As described above, in the ferritic heat-resistant cast steel, W is 1.0 to 3.0%, Nb and / or V is 0.01 to 0.45%, and REM is 0.01 to 0.3% by weight. 0.5
%, Ca and / or Mg: 0.005-0.03%
When added, a structure containing the α'phase is obtained, which has heat fatigue resistance and oxidation resistance superior to those of conventional high alloy steels, and castability equivalent to heat resistant cast iron without impairing ductility at room temperature, A heat-resistant cast steel that has workability and is inexpensive can be obtained. Furthermore, since the transformation point temperature is 1000 ° C. or higher, 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 excellent in room temperature ductility and oxidation resistance of the present invention will be described in detail below.

(1) C (carbon): 0.15 to 0.45
% C has the effect of improving the fluidity of the molten metal, that is, the castability, the function of forming an appropriate amount of the α'phase, and the function of maintaining high strength at high temperatures below the transformation point.
In order to effectively exert such an effect, C is 0.1
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 0.45%, the α'phase is less likely to exist, a martensite structure is formed, and the precipitation of Cr carbide, which causes deterioration of oxidation resistance, corrosion resistance and workability, is remarkable. become. Therefore, C is 0.15 to 0.45%. Desirably 0.20
It is 0.40%.

(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, 0.5-1.
5%.

(3) Mn (manganese): 1.0% or less Mn, like Si, is effective as a deoxidizer for molten metal,
Further, the flowability of molten metal at the time of casting is improved to improve the productivity, but if the amount is too large, the toughness decreases, so the Mn content is made 1.0% or less. Desirably, it is 0.4 to 0.7%.

(4) Cr (chromium): 17.0-2
2.0% 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 18.0 to 21.0%.

(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. Desirably, it is 1.2 to 2.5%.

It should be noted that an effect similar to that of W can be obtained even if Mo is added (however, since the amount of Mo is 2% of W in atomic%, 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) and / or V (vanadium): 0.01 to 0.45% Nb and V combine with C to form fine carbides,
Increases tensile strength at high temperature as well as thermal fatigue resistance. Further, it suppresses the formation of Cr carbides to improve oxidation resistance and machinability. For such a purpose, the content of Nb and / or V is set to 0.01% or more. However, when added in a large amount, C is consumed by forming carbides at the grain boundaries and generating Nb carbides.
The α'phase is less likely to be formed, and the strength and ductility are significantly reduced. Therefore, the content of Nb and / or V is 0.
45% or less. Desirably 0.02 to 0.40%
Is.

(7) 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. Desirably, it is 0.05 to 0.3%.

(8) Ca and / or Mg: 0.0
05-0.03% Ca and / or Mg, together with deoxidizing and desulfurizing action,
By making the non-metallic inclusion spherical, it has the effect of improving the ductility. The non-metal inclusions are composed of Si, Mn, etc., and are, for example, Mg, Ca, Si, Mn, Al.
And a compound that can be represented by a metal element such as O and an element of O or S, that is, an oxide or a sulfide. Ca and / or M
If the content of g is less than 0.005%, the sufficient effect cannot be obtained, while if it exceeds 0.03%, the material becomes brittle. When Ca and Mg are added at the same time, the contents of both are 0.
It is within the range of 005 to 0.03%. Desirably, 0.
It is 015 to 0.025%.

To summarize the above: The composition of the ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance of the present invention is as follows.

The first invention is, by weight ratio, C: 0.15 to 0.45%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 17.0 to 22.0%. , W: 1.0 to 3.0%, Nb and / or V: 0.01 to 0.45%, REM: 0.01 to 0.5%, Ca and / or Mg: 0.005 to 0. 03%, balance: Fe and inevitable impurities.

The preferred composition range of the first invention is as follows. C: 0.20 to 0.40%, Si: 0.7 to 1.5% or less, Mn: 0.4 to 0.7% or less, Cr: 18.0 to 21.0%, W: 1. 2 to 2.5%, Nb and / or V: 0.02 to 0.40%, REM: 0.05 to 0.3%, Ca and / or Mg: 0.015 to 0.025%, balance: It consists of Fe and inevitable impurities.

The ferritic heat-resistant cast steel of the present invention having the above-described composition and excellent in room temperature ductility and oxidation resistance has an α'phase transformed from γ to (α + carbide) in addition to the usual α phase.
The ordinary α phase means δ (delta) ferrite. The precipitated carbides are carbides of Fe, Cr, W, Nb and the like (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, when the area ratio of the α'phase exceeds 80%, the area becomes too hard, the ductility at room temperature decreases, and the machinability deteriorates significantly. Therefore, the area ratio {α '/ (α +
α ′)} is 20 to 80%.

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 and Mn be appropriate.

The area ratio and transformation point of the α'phase 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
In addition, the area ratio of the α ′ phase is obtained by dividing the area of the structure (α ′ phase) that appears slightly gray-black inside the fudorori in the metal structure of FIG. 2 described later by the area of the entire (α + α ′). It is what I asked for.

The ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance according to 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 an exhaust gas purifying catalytic converter container 4 is connected to the turbine housing 2 via an exhaust outlet pipe 3. Further, the converter container 4 has a main catalyzer 5
Are connected. 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),
Moreover, it is designed to be inhaled 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.

The exhaust manifold 1 and the turbine housing 2 made of the ferritic heat-resistant cast steel having such a thin room temperature ductility and excellent oxidation resistance do not crack even when subjected to a heating-cooling thermal cycle. Has excellent durability.

[0033]

EXAMPLES The present invention will be described in detail below with reference to examples.

Examples 1 to 10 and Comparative Examples 1 to 6 Ferritic heat-resistant cast steels of the types shown in Table 1 were cast to produce JIS standard Y-shaped No. B test materials. 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 10, the flow of molten metal during casting was good, and no casting defects were found. Next, the cast test materials (Y block) of Examples 1 to 10 and Comparative Example 6 were heated to 800 ° C. in a heating furnace.
After that, a heat treatment of holding for 2 hours and cooling with air was performed. On the other hand, Comparative Examples 1 to 5 were all 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. The test material of Comparative Example 1 is high Si spherical 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 Casting Ins).
Title) CB-30 of the standard, and Comparative Example 4
Of the austenitic heat-resistant cast steel (JIS standard SC
(Corresponding to H12), and the test material of Comparative Example 5 is a ferritic heat-resistant cast steel disclosed in JP-A-2-175841.

Comparative Example 6 is a heat-resistant, ferritic cast steel whose main component is the same as that of the embodiment of the present invention, except that REM, Ca and Mg are not used as the deoxidizing agent and only Al is used.

As shown in Table 1, Example 1 which is the material of the present invention
Nos. 10 to 10 have a transformation temperature of 1000 ° C. or higher, and Comparative Example 1
It can be seen that it is higher than those of the above-mentioned examples.

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 Using a ribbed test piece having a gauge length of 50 mm and a gauge length of 10 mm, the test was conducted at 900 ° C.

(3) Thermal Fatigue Test A round bar test piece having a gauge length of 20 mm and a gauge diameter of 10 mm was used, under the following conditions, with the expansion and contraction due to heating and cooling being completely restrained. The heating-cooling cycle was repeated to cause thermal fatigue failure. Lower limit temperature: 100 ° C. Upper limit temperature: 900 ° C. and 1000 ° C. Each cycle: 12 minutes An electric-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
Hold in the air at 00 ° C and 1000 ° C for 200 hours, take out shot blasting after taking out to remove the oxide scale, and determine the weight change per unit area before and after the oxidation test (oxidation weight loss; mg / cm 2). The oxidation resistance was evaluated by.

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.

[0044]

[Table 1] Chemical composition (% by weight) No. C Si Mn Cr W Nb V REM Ca Mg 1 0.16 0.72 0.45 17.5 1.02 0.25-0.03 0.006-2 0.22 1.45 0.53 20.5 1.52-0.22 0.15 0.01-3 0.30 1.05 0.69 21.4 2.32 0.08 0.04 0.33 0.028-4 0.43 1.65 0.93 21.7 2.85 0.10-0.12-0.006 5 0.35 0.92 0.42 18.8 2.03-0.15 0.21-0.015 6 0.33 0.81 0.61 19.5 1.92 0.05 0.20 0.09-0.026 7 0.29 0.96 0.48 20.1 2.08 0.15-0.08 0.009 0.008 8 0.25 1.12 0.55 19.2 2.11-0.09 0.11 0.015 0.011 9 0.34 1.08 0.58 19.7 1.88 0.10 0.06 0.18 0.008 0.020 10 0.31 1.02 0.44 19.9 1.95 0.06 0.08 0.11 0.020 0.006 Comparative example Chemical composition (% by weight) No. C Si Mn Cr W Nb V REM Ca Mg 1 3.16 4.08 0.41-0.56 *-----2 1.99 4.75 0.48 1.98-----35.3 ** 3 0.27 0.99 0.52 17.2------4 0.22 1.08 0.55 18.6----- 9.9 5 0.13 1.11 0.52 18.3-1.14----6 0.31 0.95 0.50 20.1 2.05 0.06 0.05 ***--0.05 *** (Note) *: Mo, **: Ni, ***: Al

Table 1 (continued) α '/ (α + α') Transformation point (%) (℃) Example No.1 70 1020 2 55 1060 3 30 ＞ 1100 4 50 ＞ 1100 5 78 1050 6 40 ＞ 1100 7 50 1060 8 65 1040 9 70 1030 10 60 1080 Comparative example No. 1-800 to 850 2--3 90 900 4--5 0 ＞ 1100 6 50 1080

[0046]

[Table 2]

[0047]

[Table 3] 900 ° C 0.2% Tensile thermal fatigue Oxidation yield strength Strength elongation Life loss (MPa) (MPa) (%) (cycle) (mg / cm2) Example No. 1 20 35 70 190 2 2 22 38 65 200 2 3 23 40 60 205 1 4 25 42 50 305 1 5 27 45 45 365 1 6 29 43 40 450 2 7 26 40 55 385 1 8 28 41 60 425 1 9 25 39 50 335 1 10 26 42 65 315 1 Compare Example No. 1 19 38 35 10 205 2 40 95 45 25 22 3 26 42 60 20 3 4 68 130 35 35 2 5 14 29 90 190 1 6 22 35 40 180 2

[0048]

[Table 4] 1000 ° C 0.2% Tensile Thermal Fatigue Oxidation Strength Strength Elongation Life Loss (MPa) (MPa) (%) (Cycle) (mg / cm2) Example No. 1 15 22 95 90 30 2 16 25 84 170 30 3 17 25 90 210 15 4 21 30 70 250 17 5 19 27 100 340 20 6 18 26 120 285 18 7 17 26 95 265 16 8 16 25 105 205 19 9 18 27 80 310 14 10 17 27 100 290 14 Compare Example No. 6 16 25 80 155 18

As is clear from Tables 2, 3, and 4, Examples 1 to 10 according to the present invention are compared with the test materials of Comparative Examples 1 to 5 and Al deoxidized Comparative Example 6, respectively. It can be seen that room temperature ductility, oxidation resistance and thermal fatigue life are remarkably improved. This is because the ferrite matrix was strengthened by containing an appropriate amount of W, Nb and / or V, REM, Ca and / or Mg, and the transformation point was increased to 1000 ° C. or higher without impairing the ductility at room temperature. ..
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, it is understood that the materials of the present invention (Examples 1 to 10) have a relatively low hardness (HB) of 170 to 223 and are excellent in machinability.

Regarding Example 7 and Comparative Example 5,
The micrographs (100 times) are shown in FIGS. 2 and 3, respectively.
Shown in. The white part in FIG. 2 is a normal α phase called δ-ferrite, and the slightly gray-black part inside the trim is an α ′ phase transformed from the γ phase to (α phase + carbide).
Has an area ratio of α '{α' / (α + α ')} of 60%.

The white part in FIG. 3 is a normal α phase called δ-ferrite, and NbC of Nb carbide is observed at grain boundaries. The area ratio of the α ′ phase in Comparative Example 5 is 0%.

Further, with respect to the heat-resistant cast steel of Example 10, a micrograph (400 times) of nonmetallic inclusions is shown in FIG. R
By the addition of EM, Ca and Mg, the non-metallic inclusions become spherical, which enhances the room temperature ductility.

FIG. 5 shows a photomicrograph (400 times) of non-metallic inclusions in the heat-resistant cast steel of Comparative Example 6. Al
Since only the deoxidizing agent is used, the nonmetallic inclusions are clustered, which lowers the room temperature ductility.

Next, using the ferritic heat-resistant cast steel of Example 7, the exhaust manifold (wall thickness of pipe part: 2.5 to 3.4 mm) and turbine housing (wall thickness: 2.7 to 2.7 shown in FIG. 1 were used. 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 the 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 5, and the NI-RESIST D2 (INC of the chemical composition shown in the 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.

[0059]

[Table 5] 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

[0060]

As described above, according to the present invention, by adding an appropriate amount of W, Nb and / or V, REM, the ferrite matrix and the grain boundaries are strengthened, respectively, and the ductility at room temperature is impaired. Without increasing the transformation point, especially important high temperature tensile strength, heat fatigue resistance, oxidation resistance,
Shows properties superior to conventional heat-resistant cast steel. Moreover, since it is excellent in castability and workability, it can be manufactured at low cost. The ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance of the present invention is particularly suitable for engine exhaust system parts. The exhaust system component made of the ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance of the present invention exhibits extremely excellent durability without causing thermal cracking.

[Brief description of drawings]

FIG. 1 is a schematic view showing an exhaust manifold and a turbine housing which can be manufactured from a ferritic heat-resistant cast steel having excellent room temperature ductility and oxidation resistance of the present invention.

FIG. 2 is a micrograph (100 ×) showing a metal structure of a ferritic heat-resistant cast steel of Example 12. The white part in the figure is the α phase (δ-ferrite phase from high temperature), and the slightly gray-black part inside the rim is the α'phase (from the γ phase (α phase + carbide)
Phase transformed into.

FIG. 3 is a micrograph (100 ×) showing a metal structure of a heat-resistant material of Comparative Example 5. The white part in the figure is α (δ-ferrite phase from high temperature), and Nb carbide (NbC) is present at the grain boundaries.
Is observed.

FIG. 4 is a micrograph (400 of the metal structure of Example 10).
Times). The spherical gray-black part in the figure is a non-metallic inclusion.

5 is a photomicrograph (400 ×) of the metal structure of Comparative Example 6. FIG.
Is. The clustered gray-black areas in the figure are non-metallic inclusions.

[Explanation of symbols]

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

Claims (6)

[Claims] 1. By weight ratio, C: 0.15 to 0.45%, Si: 2.0% or less, Mn: 1.0% or less, Cr: 17.0 to 22.0%, W: 1 0.0 to 3.0%, Nb and / or V: 0.01 to 0.45%, REM: 0.01 to 0.5%, Ca and / or Mg: 0.005 to 0.03%, balance : A pearlite colony-like phase (hereinafter referred to as α'phase) that has a composition consisting of Fe and unavoidable impurities and is transformed from the γ phase to (α + carbide) in addition to the usual α phase (δ-ferrite phase from high temperature). And an area ratio of the α ′ phase {α ′ / (α + α ′)} of 20 to 80%, and a ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance. 2. The ferritic heat-resistant cast steel according to claim 1, wherein the transformation point from the α'phase to the γ-phase is 1000 ° C. or higher, and the ferritic heat-resistant cast steel has excellent room temperature ductility and oxidation resistance. .. 3. The ferritic heat-resistant cast steel according to claim 1 or 2, characterized in that when residual strain must be removed or processing is required, annealing treatment is performed in a temperature range in which the α'phase does not transform into the γ phase. Ferritic heat-resistant cast steel with excellent room temperature ductility and oxidation resistance. 4. An exhaust system component made of a ferritic heat-resistant cast steel excellent in room temperature ductility and oxidation resistance according to claim 1. 5. The exhaust system component according to claim 4, wherein the exhaust system component is an exhaust manifold. 6. The exhaust system component according to claim 4, wherein the exhaust system component is a turbine housing.