JP3332189B2 - Ferritic heat-resistant cast steel with excellent castability - Google Patents

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 the like, and in particular, has excellent durability such as thermal fatigue life, heat deformation resistance and oxidation resistance, as well as castability and workability. Excellent and can be manufactured at low cost
Related to heat-resistant cast steel.

[0002]

2. Description of the Related Art Conventional heat-resistant cast irons and heat-resistant cast steels have compositions as shown in Table 1 as comparative examples.
Exhaust system parts such as exhaust manifolds and turbine housings for automobiles are used under severe conditions at high temperatures. Therefore, high Si spheroidal graphite cast iron and niresist cast iron (Ni-Cr-Cu based austenitic cast iron) as shown in Table 1 are used. ) And heat-resistant cast irons disclosed in Japanese Patent Application Laid-Open No. 2-175841 and expensive high-alloy heat-resistant cast steels such as austenitic cast steel.

[0003]

Among such conventional heat-resistant cast irons and heat-resistant cast steels, for example, high-Si spheroidal graphite cast iron and niresist cast iron have relatively good castability, but are resistant to heat fatigue or oxidation resistance. It cannot be applied to a member having a high temperature of 900 ° C. or more because of poor durability such as resistance. The heat-resistant ferritic cast steel disclosed in Japanese Patent Application Laid-Open No. 2-175841 is excellent in heat fatigue fatigue resistance but slightly inferior in heat deformation resistance. Further, high alloy heat-resistant cast steel such as heat-resistant austenitic cast steel has high temperature strength at 900 ° C. or higher and excellent heat deformation resistance, but has a shortcoming of short thermal fatigue life due to a large thermal expansion coefficient. . In addition, due to poor castability, casting defects such as shrinkage cavities and poor runoff are likely to occur at the time of casting, and furthermore, due to poor machinability, productivity is low when manufacturing parts and the like therefrom. There were also points. In addition, there is ferritic stainless steel cast steel, but ordinary ferritic stainless steel has poor ductility at room temperature in an attempt to improve durability at high temperatures, and cannot be used for members subjected to mechanical shock and the like. There is.

Accordingly, the present invention relates to the above-mentioned conventional heat-resistant cast iron,
Solves the problems of heat-resistant cast steel, heat-resistant fatigue resistance, heat-resistant fatigue resistance,
An object of the present invention is to provide a heat-resistant cast steel which is excellent in durability such as oxidation resistance, castability, and workability and can be manufactured at low cost.

[0005]

[0006]

As a result of intensive studies in view of the above-mentioned object, the present inventors have found that W and / or Mo, Nb,
Alternatively, by adding an appropriate amount of Ni, N or the like to them, the castability is excellent, the ferrite matrix and the grain boundaries are strengthened, the transformation point is raised without impairing the ductility at room temperature, and the high-temperature strength is improved. The inventors have found that the present invention can be performed, and arrived at the present invention.

That is, the heat-resistant, ferritic cast steel of the present invention having excellent castability has a weight ratio of C: 0.15 to 1.2.
0%, C-Nb / 8: 0.1 to 0.30% , Si: 2%
Mn: 2% or less, Cr: 16.0 to 25.0%,
W and / or Mo: 1.0 to 5.0%, Nb: 0.
51 to 6.0%, Ni: 0.1 to 2.0%, N: 0.0
1 to 0.15%, balance: having a composition of Fe and unavoidable impurities, has an α ′ phase transformed from a γ phase to α + carbide in addition to a normal α phase, and has an area ratio of α ′ phase Δα '/ (Α + α')｝ is 20 to 70%.

In the heat-resistant ferritic cast steel having excellent castability according to the present invention, the transformation point from the α ′ phase to the γ phase is 900.
° C or higher.

If it is necessary to remove residual strain or to perform processing, an annealing treatment is performed after the casting at a temperature lower than the γ + α mixed region.

[0010]

[0011]

As described above, W and / or Mo are added to ferritic heat-resistant cast steel having excellent castability in a weight ratio of 1.%.
0 to 5.0%, Nb is 0.51 to 6.0% , and if necessary, an appropriate amount of Ni and N is further added in combination to obtain a structure containing an α ′ phase, whereby , It has heat resistance fatigue resistance and oxidation resistance higher than conventional high alloy steel, and has castability and workability equivalent to heat resistant cast iron without impairing ductility at room temperature, and low-cost heat-resistant cast steel can be obtained . Further, since the transformation point temperature is 900 ° C. or higher, the thermal fatigue resistance is improved.

Hereinafter, the reasons for limiting the composition ranges of the alloying elements of the ferritic heat-resistant cast steel of the present invention having excellent castability and the exhaust system components will be described in detail.

(1) C (carbon): 0.15 to 1.20
% C has the effect of improving the fluidity of the molten metal, that is, the castability, and has the effect of producing an appropriate amount of the α 'phase.
It functions to maintain high strength at a high temperature of 0 ° C. or higher. Further, it has an effect of generating eutectic carbide with Nb to enhance castability. In order to exert such an effect effectively,
C needs to be 0.15% or more. It should be noted that a general heat-resistant ferritic cast steel has only an α phase at room temperature, but by adjusting the amount of carbon, there is a γ phase in which C forms a solid solution at a high temperature, in addition to an α phase existing from a high temperature to a normal temperature. The γ phase precipitates carbide during cooling and transforms into (α phase + carbide). Such a phase is called α ′ phase.

On the other hand, if the content of C exceeds 1.20%, the α 'phase becomes difficult to exist, resulting in a martensitic structure, and a Cr carbide which causes deterioration in oxidation resistance, corrosion resistance and workability. Precipitation becomes remarkable. For this reason, C is set to 0.15 to 1.20%. Desirably 0.2-1.
0%.

(2) C—Nb / 8: 0.1 to 0.30% The heat-resistant ferritic cast steel of the present invention having excellent castability is Nb
Eutectic carbides are formed to enhance castability, and an α 'phase transformed from the γ phase is generated, resulting in high strength and high ductility. Eutectic carbide (NbC) contains 8 of C and C in a weight ratio.
Although it is formed with twice as much Nb, in order to obtain an appropriate amount of α phase in addition to eutectic carbide (NbC), C that is more than C consumed for eutectic carbide formation is required. That is, in order to obtain a ferritic heat-resistant cast steel having excellent castability, high strength and high ductility in the present invention, (C-Nb / 8) needs to be 0.1% or more . However, if (C-Nb / 8) exceeds 0.30% , it becomes hard and brittle, so it is set to 0.1 to 0.30% .

(3) Si (silicon): 2% or less Si narrows the range of the γ phase of the Fe—Cr alloy of the present invention, increases the stability of the structure, and has the effect of improving oxidation resistance.
Further, there are effects such as improvement of castability, action as a deoxidizing agent, and reduction of pinhole defects in castings. But too much,
Primary carbides are coarsened by the balance with carbon (carbon equivalent) to lower the workability of cast steel, the Si content in the ferrite matrix is excessive, and the ductility is reduced, and the δ phase at high temperatures is reduced. Or to form. Therefore, the content of Si is set to 2% or less. Desirably, it is 0.3 to 1.5%.

(4) Mn (manganese): 2% or less Mn is effective as a deoxidizing agent for molten metal similarly to Si.
In addition, the flow of molten metal during casting is improved to improve productivity.
In order to make such an effect effective, the content of Mn is set to 2%
The following is assumed. Desirably, it is 0.3 to 1.5%.

(5) Cr (chromium): 16.0-2
5.0% Cr is an element which improves the oxidation resistance and stabilizes the ferrite structure. However, in order to ensure its effect, the content is set to 16.0% or more. On the other hand, a large amount of addition coarsens the primary carbides of Cr, promotes the formation of the δ phase at high temperatures, and causes significant embrittlement. Therefore, the upper limit of Cr is set to 25.0%. Desirably, it is 17.0 to 22.0%.

(6) W (tungsten) and / or Mo (molybdenum): 1.0 to 5.0% W has the effect of strengthening the ferrite matrix and improving the high-temperature strength without impairing the ductility at room temperature. Therefore, 1.0% or more of W is added for the purpose of improving the creep resistance and the thermal fatigue resistance by increasing the transformation point temperature. However, if the content exceeds 5.0%, coarse eutectic carbides are formed, causing a decrease in ductility and a deterioration in machinability, so that the content is made 5.0% or less.

It should be noted that almost the same effect as W can be obtained by adding Mo. Therefore, it is possible to add Mo alone or to add W and Mo in combination. Desirably 1.0 to 3.
0%.

(7) Nb (niobium): 0.51 to 0.51
6.0% Nb combines with C to form fine carbides, and increases tensile strength at high temperatures and thermal fatigue resistance. In addition, Cr
Oxidation resistance and machinability are improved by suppressing the generation of carbides. Furthermore, since eutectic carbides are generated, the castability, which is important for the production of thin castings such as exhaust system parts, is improved. For such a purpose, the Nb content is 0.51% or less.
Above . However, when added in a large amount, eutectic carbides generated at crystal grain boundaries increase, and strength and ductility are significantly reduced. Therefore, the Nb content is set to 6.0% or less. Desirably, it is 0.51 to 3.0% .

(8) Ni (nickel): 0.1 to 2.
0% Ni is a γ phase forming element like C, and it is preferable to add 0.1% or more in order to make the α ′ phase exist in an appropriate amount. On the other hand, if it exceeds 2.0%, the α 'phase having excellent oxidation resistance decreases and the α' phase becomes martensite,
Significantly reduces ductility. Therefore, the Ni content is set to 2.
0% or less. Desirably, it is 0.3 to 1.5%.

(9) N (nitrogen): 0.01 to 0.15
% N is an element that improves high-temperature strength and thermal fatigue resistance like C, and its effect appears at 0.01% or more. On the other hand, the content is set to 0.15% or less to ensure the stability of production and to avoid embrittlement due to precipitation of Cr nitride. Desirably 0.0
3 to 0.10%.

(10) Area ratio of α 'phase: 20 to 70% The ferritic heat-resistant cast steel of the present invention having the above composition and excellent castability according to the present invention can be obtained from γ to (α + carbide) in addition to the usual α phase.
Α ′ phase transformed to The α phase means δ (delta) ferrite. Further, the precipitated carbides are Fe, C
Carbides such as r, W, Nb (M23C6, M7C3, MC, etc.)
It is.

The area ratio of the α ′ phase ｛α ′ / (α +
If α ′)｝ is less than 20%, the ductility at room temperature is low,
Cast steel is extremely brittle. On the other hand, if it exceeds 70%, it becomes too hard, the ductility at room temperature decreases, and the machinability deteriorates remarkably. Therefore, the area ratio ｛α '/ (α +
α ′)｝ is set to 20 to 70%. Desirably 20 to 60
%.

The heat-resistant ferritic cast steel having excellent castability is subjected to an annealing treatment after casting at a temperature lower than the γ + α mixed temperature range. The annealing temperature at this time is generally 7
00 to 850 ° C., and the annealing time is 1 to 10 hours. This is a temperature range in which the α ′ phase does not transform into the γ phase.

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 increases, and the thermal fatigue life is shortened. for that reason,
It is necessary to have a transformation point of 900 ° C. or more. In order to have such a high transformation point, it is necessary that the balance between Cr, Si, W and / or Mo, Nb which is a ferrite forming element and C, Ni, N, Mn which is an austenite forming element is proper. It is.

The area ratio and transformation point of the heat-resistant ferritic cast steel of each embodiment having excellent castability are as follows.

The ferritic heat-resistant cast steel having excellent castability according to the present invention is particularly suitable for manufacturing exhaust system parts for automobiles. FIG. 1 shows an integrated exhaust manifold mounted on an in-line four-cylinder engine with a supercharger as an exhaust system component of a vehicle. The exhaust manifold 1 is connected to a turbine housing 2 of a turbocharger.
An exhaust gas purifying catalytic converter container 4 is connected through an exhaust outlet pipe 3. 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 configured to be sucked in air from (C). The exhaust gas is
(A) flows into the exhaust manifold 1.

Such an exhaust manifold 1
The turbine housing 2 and the turbine housing 2 are preferably made as thin as possible in order to reduce the heat capacity. The thickness of the exhaust manifold 1 and the thickness of the turbine housing 2 are, for example, 2.5 to 3.4 mm, 2.7 to 2.7, respectively.
4.1 mm.

The thin exhaust manifold 1 and the turbine housing 2 made of the heat-resistant ferritic cast steel of the present invention having excellent castability do not crack even when subjected to a heat cycle of heating and cooling. It has high durability.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. Examples 1 to 3, 7 to 11, Reference Examples 4 to 6 , Comparative Examples 1 to 5 JIS standard Y-type B test specimens of the heat-resistant ferritic cast steels of the types shown in Table 1 having excellent castability. Was prepared. At the time of casting, the material was melted in the air using a high-frequency furnace for 100 kg, immediately poured out at 1550 ° C. or more, and poured at about 1500 ° C.

Examples 1 to 3 and 7 to 11 and the present invention
For the heat-resistant ferritic cast steels having excellent castability in Reference Examples 4 to 6, the molten metal flow during casting was good, and no generation of casting defects was observed. Next, Examples 1-3, 7-
11 and the test materials (Y blocks) of Reference Examples 4 to 6 relating to the present invention were heat-treated in a heating furnace at 800 ° C. for 2 hours and then air-cooled. On the other hand, all the comparative materials (Comparative Examples 1 to 5) were subjected to the test as cast.

In Table 1, the comparative materials (Comparative Examples 1 to 5)
5) are used for heat-resistant parts such as an automobile turbocharger housing and an exhaust manifold. The test material of Comparative Example 1 is a high Si spheroidal graphite cast iron, and the test material of Comparative Example 2. Is Niresist cast iron, and the test material of Comparative Example 3 is ACI (Alloy Cast).
IB-30 standard, and the test material of Comparative Example 4 is a kind of austenitic heat-resistant cast steel (equivalent to JIS standard SCH12), and the test material of Comparative Example 5 is a special material. Kaihei 2-1758
No. 41 discloses a heat-resistant, ferritic cast steel excellent in castability.

[Table 1] Chemical components (% by weight) Example C Si Mn Cr W Mo Nb Ni N C-Nb / 8 No.1 0.18 0.52 0.51 16.4 1.15-0.51 0.23 0.04 0.12 2 0.36 0.53 0.45 18.8 1.98-1.02 0.65 0.05 0.23 3 0.41 0.64 0.63 18.5 3.59-2.15 0.83 0.03 0.14 7 0.65 1.11 0.52 19.4 2.65-4.25 0.78 0.07 0.12 8 0.45 0.94 0.54 20.5 3.08-2.08 0.96 0.06 0.19 9 0.75 0.75 0.42 18.8 3.02-4.78 0.58 0.05 0.15 10 0.42 0.58 0.54 18.6-1.5 2.35 0.72 0.05 0.13 11 0.45 0.63 0.58 18.3 1.28 1.01 2.46 0.58 0.05 0.14 Reference example C Si Mn Cr W Mo Nb Ni N C-Nb / 8 No.4 0.45 1.63 0.88 22.0 4.58-3.08 1.25 0.08 0.05 5 1.18 1.24 0.77 24.6 4.28-5.80 1.75 0.12 0.45 6 0.38 0.92 0.70 18.8 1.75-2.52 1.30 0.05 0.06 Comparative example C Si Mn Cr W Mo Nb Ni N 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--

[Table 1] (continued) {α ′ / (α + α ′)} Transformation point (%) (° C.) Example No. 1 50 940 2 45 960 3 40 1000 7 55 960 8 60 950 9 30 960 10 35 950 11 40 980 Reference example No. 4 35 1030 5 30 1050 6 40 930 Comparative example No. 1-800 to 850 2--3 93 910 4--5 0> 1100

As shown in Table 1, Example 1 which is the material of the present invention
~ 3, 7 ~ 11, and Reference Examples 4 ~ 6 relating to the present invention ,
It can be seen that the transformation point temperature is 900 ° C. or higher, which is higher than Comparative Examples 1 and 3.

Next, various evaluation tests described below were performed using the test materials. (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 diameter of 14 mm was used.

(2) High-Temperature Tensile Test The test was conducted at 900 ° C. using a flanged test piece having a gauge length of 50 mm and a gauge diameter of 10 mm.

(3) Thermal Fatigue Test A round bar test piece having a distance between gauge points of 20 mm and a diameter between gauge points of 10 mm was used under the following conditions under the condition that expansion and contraction due to heating and cooling were completely restrained. The heating-cooling cycle was repeated to cause thermal fatigue failure. Lower limit temperature: 100 ° C. Upper limit temperature: 900 ° C. Each cycle: 12 minutes As the tester, a thermal fatigue tester of an electro-hydraulic servo system was used.

(4) Oxidation test A round bar test piece having a diameter of 10 mm and a length of 20 mm was prepared.
It is kept in the atmosphere at 00 ° C. for 200 hours, shot blasted after being taken out to remove the oxide scale, and the change in weight per unit area before and after the oxidation test (oxidation weight loss: mg / cm 2) is obtained. The sex 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.

[Table 2] Room temperature 0.2% yield strength Tensile strength Elongation Hardness (MPa) (MPa) (%) (HB) Example No. 1 380 515 8 197 2 370 470 6 201 3 355 450 4 197 7 370 490 3 217 8 350 470 5 192 9 340 450 5 197 10 330 495 3 197 11 350 500 5 197 Reference Example No. 4 360 480 5 201 5 330 440 3 192 6 360 500 5 201 Comparative Example No. 1 510 640 11 215 2 245 510 19 139 3 540 760 4 240 4 250 560 20 170 5 300 370 1 149

[Table 3] 900 ° C 0.2% proof stress Tensile strength Elongation Thermal fatigue life Oxidation weight loss (MPa) (MPa) (%) ( cycle ) (mg / cm2) Example No. 1 20 35 48 186 3 2 25 40 52 232 3 3 27 42 48 390 2 7 25 50 50 205 1 8 28 58 56 334 1 9 26 55 42 280 1 10 24 45 52 294 2 11 26 55 56 284 2 Reference example No. 4 27 44 42 162 1 5 25 38 44 338 1 6 26 52 52 220 2 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 apparent from Tables 2 and 3 , Examples 1 to 3 and 7 to 11 according to the present invention and the present invention are described.
It can be seen that in Reference Examples 4 to 6 , the high-temperature strength, the oxidation resistance and the thermal fatigue life were remarkably improved as compared with the test materials of Comparative Examples 1 to 5, which are conventional materials. This is because the ferrite matrix was strengthened by containing an appropriate amount of W and / or Mo, Nb, Ni and N, and the transformation point was raised to 900 ° C. or more without impairing the ductility at room temperature.

As shown in Table 2, the materials of the present invention (Examples)
1-3, 7-11), and Reference Examples 4-6 relating to the present invention.
Has a relatively low hardness (HB) of 192 to 217, and is excellent in machinability.

FIGS. 2 and 3 show micrographs (× 100) of Example 3 and Comparative Example 5, respectively. The gray-white part in FIG. 2 is a normal α-phase called δ-ferrite, and the slightly gray-black part inside the frame is an α ′ phase transformed from the γ phase. In Example 3, the area ratio of α ′ is ｛α ′. / (Α + α ′)} is 40%.

Next, using the heat-resistant ferritic cast steel of Example 3 having excellent castability, the exhaust manifold shown in FIG. 1 (wall thickness of the pipe portion: 2.5 to 3.4 mm) and the turbine housing (wall thickness: (2.7-4.1 mm). The resulting heat-resistant cast steel parts were all sound.

Further, these cast steel parts were machined to evaluate the machinability, but no problem occurred in any of them.

Next, as shown in FIG. 1, an endurance test was carried out using an exhaust simulator that emits exhaust gas equivalent to a high-performance gasoline engine with a displacement of 2,000 cc in an in-line four-cylinder assembly of an exhaust manifold and a turbine housing. As a test condition, a heat-cooling (GO-STOP) cycle with one cycle of full-load operation at 6000 revolutions (continuous 14 minutes) -idling (1 minute) -complete stop (14 minutes) -idling (1 minute) is performed. , Up to 500 cycles. The exhaust gas temperature at full load was 930 ° C. at the inlet temperature of the turbine housing. Under these conditions, the surface temperature of the exhaust manifold is about 870 ° C. at the assembly of the exhaust manifold, and the surface temperature of the turbine housing is about 890 at the wastegate.
° C. 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 high-Si spheroidal graphite cast iron having the chemical components shown in Table 4, and NI-RESIST D2 (INC
A turbine housing was made of austenitic spheroidal graphite cast iron (trade name of Company O). 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 assembly at 98 cycles. Thereafter, the exhaust manifold was replaced with the one in Example 3, and the test was continued. As a result, a crack was generated in the scroll portion of the austenitic spheroidal graphite cast iron turbine housing at the 324th cycle through the wall thickness. As a result, it was clarified that the exhaust manifold and the turbine housing of the present invention have excellent durability.

[0052]

[Table 4] Chemical composition (% by weight) 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

[0053]

As described above, according to the present invention, the addition of W and / or Mo, Nb, Ni, and N in an appropriate amount enhances the ferrite matrix and the crystal grain boundaries, respectively, and increases the room temperature. It raises the transformation point without impairing ductility, and shows properties that are particularly important, such as high-temperature tensile strength, thermal fatigue resistance, and oxidation resistance, over conventional heat-resistant cast steel. Also,
Since it is excellent in castability and workability, it can be manufactured at low cost. Such a ferritic heat-resistant cast steel excellent in castability of the present invention is particularly suitable for engine exhaust system parts.
The exhaust system component made of the heat-resistant ferritic cast steel of the present invention having excellent castability exhibits extremely excellent durability without causing thermal cracks.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an exhaust manifold and a turbine housing which can be made of a heat-resistant ferritic cast steel having excellent castability of the present invention.

FIG. 2 is a micrograph (× 100) showing the metal structure of a heat-resistant ferritic cast steel having excellent castability in Example 3.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Exhaust manifold 2 Turbine housing 3 Exhaust outlet 4 Converter container 5 Main catalyzer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-218645 (JP, A) JP-A-3-257143 (JP, A) JP-A-3-271345 (JP, A) 127320 (JP, A) JP-A-52-126612 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (1)

(57) [Claims] 1. C: 0.15 to 1.20 by weight ratio
%, C-Nb / 8: 0.1 to 0.30% , Si: 2% or less, Mn: 2% or less, Cr: 16.0 to 25.0%, W
And / or Mo: 1.0 to 5.0%, Nb: 0.5
1 to 6.0%, Ni: 0.1 to 2.0%, N: 0.01
0.15%, balance: Fe and composition inevitable impurities, in addition to a normal α phase, a phase transformed from γ phase to α + carbide (hereinafter referred to as “α ′ phase”), and α ′ Phase area ratio {α ′ / (α + α ′)} is 20 to 7
Ferritic heat-resistant cast steel with excellent castability characterized by being 0%.