JPH06256908A - Heat resistant cast steel and exhaust system parts using the same - Google Patents

Abstract

PURPOSE:To improve strength at high temp. without deteriorating other properties by incorporating specific amounts of W, Nb, and N into a Cr-Ni heat resistant cast steel, properly regulating the ratio between austenite phase and ferrite phase, and further strengthening matrix and grain boundaries. CONSTITUTION:This steel has a composition consisting of, by weight ratio, 0.20-0.60% C, <=2.0% Si, <=1.0% Mn, 4.0-6.0% Ni, 20.0-30.0% Cr, 1.0-5.0% W, 0.2-1.0% Nb, 0.05-0.2% N, and the balance Fe with inevitable impurities and also has a structure composed of a dual phase structure consisting of 20-95% austenite phase and the balance ferrite phase. Further, as occasion demands at the time of residual strain removal and working, annealing is done at a temp. in the region where the phenomenon of entering into solid solution does not occur. Annealing temp. is generally 700-900 deg.C and annealing time is 1-10hr.

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 oxidation resistance and thermal fatigue life, as well as room temperature strength, ductility, castability and machining. TECHNICAL FIELD The present invention relates to a heat-resistant cast steel which is excellent in properties 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 conventional materials. For exhaust system parts such as automobile exhaust manifolds and turbine housings, the operating conditions are severe at high temperatures, so heat resistance of high Si spheroidal graphite cast iron and Ni-resist cast iron (Ni-Cr austenitic cast iron) as shown in Table 1 Cast iron is adopted and as a two-phase heat resistant cast steel
Japanese Patent Laid-Open No. 2-4855 and Japanese Patent Laid-Open No. 4-45245 are proposed.

[0003]

Among such conventional heat-resistant cast iron and heat-resistant cast steel, for example, high Si spheroidal graphite cast iron and Niresist cast iron have relatively good castability,
Since it is inferior in durability such as heat fatigue resistance or oxidation resistance, it cannot be applied to a member having a high temperature of 900 ° C. or higher. Further, the technique disclosed in Japanese Patent Laid-Open No. 62-4855 has excellent high temperature strength and oxidation resistance, but has a problem in thermal fatigue life, and the technique disclosed in Japanese Patent Laid-Open No. 4-45245 has poor oxidation resistance. There is.

Therefore, the present invention is based on the above conventional heat-resistant cast iron,
It is an object of the present invention 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, as well as 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 the proportion of austenite phase and ferrite phase can be improved by containing an appropriate amount of C, Ni, Cr, W, Nb and N. Further, it was found that the matrix and grain boundaries are strengthened by a solid solution strengthening action, a precipitation hardening action, etc., and the high temperature strength is improved without impairing the ductility at room temperature, and the present invention was conceived.

That is, the heat-resistant cast steel of the present invention has a weight ratio of C: 0.20 to 0.60% and Si:
2.0% or less, Mn: 1.0
% Or less, Ni: 4.0 to 6.0%, Cr: 2
0.0-30.0%, W: 1.0-5.0
%, Nb: 0.2 to 1.0%, N: 0.
It is characterized by having a composition consisting of 05 to 0.2%, the balance Fe and unavoidable impurities, and having a two-phase structure in which the austenite phase is 20 to 95% and the balance is the ferrite phase.

In the above invention, when there is a need for removing residual strain or working, the annealing treatment is performed in a temperature range where the solution phenomenon does not occur.

Furthermore, the exhaust system component made of the heat-resistant cast steel of the present invention is characterized by being formed of the heat-resistant cast steel having the above composition and used for automobiles such as an exhaust manifold and a turbine housing.

[0010]

As described above, the Cr-Ni heat-resistant cast steel contains W,
By containing an appropriate amount of Nb and N, the ratio between the austenite phase and the ferrite phase is made appropriate, and by further strengthening the matrix and the grain boundaries, the ductility at room temperature is not impaired and the high temperature strength is improved.

The reasons for limiting the compositional range and the structural range of each alloying element of the heat-resistant cast steel of the present invention will be described in detail below. (1) C (carbon): 0.20 to 0.60% C is an element necessary for improving the fluidity of the molten metal, that is, the castability, and for improving the strength of the cast steel,
In order to obtain the above effect, it is necessary to contain at least 0.20% or more. However, if the content is too large, a large amount of carbide precipitates, making it hard and brittle, so 0.60% is made the upper limit. Desirably 0.30 to 0.5
It is 0%.

(2) Si (silicon): 2.0% or less Si has the effects of improving castability, acting as a deoxidizing agent, reducing pinhole defects in castings, and further improving oxidation resistance. is there. However, if the content exceeds 2.0%, the toughness decreases. Further, Si is a ferrite stabilizing element, and since ferrite is excessively precipitated, the upper limit was set to 2.0%. It is preferably 1.75% or less.

(3) Mn (manganese): 1.0% or less 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.
However, if the content exceeds 1.0%, the toughness deteriorates, so the upper limit was made 1.0% or less. Desirably 0.7
It is 5% or less.

(4) Ni (nickel): 4.0 to 6.0% Ni is an element that stabilizes austenite,
If it is less than 4.0%, the effect is not sufficient. Also, 6.0
%, The ferrite phase decreases, so 6.0% was made the upper limit.

(5) Cr (Chromium): 20.0 to 30.0% Cr is a basic component of heat-resistant cast steel and is extremely effective for oxidation resistance, but if it is less than 20.0%, its effect is high. not enough. However, if it exceeds 30.0%, secondary carbides are excessively precipitated and it becomes easy to embrittle, so 30.0% was made the upper limit. It is preferably 20.0 to 27.0%.

(6) W (tungsten): 1.0 to 5.0% W is an element effective for improving high temperature strength. To obtain this effect, addition of 1.0% or more is necessary. However, if contained in a large amount, the oxidation resistance deteriorates, so 5.0% was made the upper limit. It is preferably 2.0 to 4.0%.

Although the same effect as W can be obtained even when Mo is contained, in this case, the Mo content is ½ of W. Mo can be contained alone or in combination with W, but in any case, the content thereof is 1.0 to 5.0% in terms of W.

However, at high temperature, W is more effective than Mo in terms of oxidation resistance, so that only W is contained in the present invention.

(7) Nb (niobium): 0.2 to 1.0% Nb combines with C to form fine carbides and improves high temperature strength. Further, the oxidation resistance is improved by suppressing the generation of Cr carbide. In order to exert these effects effectively, the content of 0.2% or more is required. However, if contained in a large amount, the toughness deteriorates, so the upper limit is made 1.0%. It is preferably 0.3 to 0.7%.

(8) N (nitrogen): 0.05 to 0.2% N is an austenite stabilizing element, and is an element mainly forming a nitride to enhance the strength at room temperature. In order to effectively exhibit these effects, it is necessary to contain 0.05% or more. However, if contained in a large amount, the nitride is excessively precipitated and embrittles, so the upper limit is made 0.2%. It is preferably 0.07 to 0.15%.

(9) Austenite phase: 20 to 95% The amount of the austenite phase is set to 20 to 95% because when the austenite phase is less than 20%, the high temperature strength becomes too low, and when it exceeds 95%, thermal fatigue occurs. 95 because the life is deteriorated
% Is the upper limit. It is preferably 30 to 90%.

The heat-resistant cast steel of the present invention may be subjected to annealing treatment in a temperature range where solid solution phenomenon does not occur after casting, if there is a need for removing residual strain and working. At this time, the annealing temperature is generally 700 to 900 ° C., and the annealing time is 1 to 10 hours.

The heat-resistant cast steel of the present invention as described above is particularly suitable for manufacturing automobile exhaust system parts. As an example of an automobile exhaust system component, FIG. 1 shows an integral structure type exhaust manifold attached to an in-line 4-cylinder engine with a supercharger. 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, 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 3.5 mm and 2.7 to
It is 4.1 mm. An exhaust manifold 1 and a turbine housing 2 made of such a thin heat-resistant cast steel
Shows excellent durability without thermal cracking even when subjected to heat cycle of heating-cooling.

[0025]

EXAMPLES The present invention will be described in detail below with reference to examples. (Examples 1 to 8 and Comparative Examples 11 to 14) Regarding the heat-resistant materials having the compositions shown in Table 1, JIS standard Y-shaped No. B test materials were produced by casting. In casting, a 100 kg high-frequency furnace was used to melt in the atmosphere, and the molten metal was immediately discharged at 1550 ° C or higher and poured at about 1500 ° C.

Regarding the austenite / ferrite dual phase heat resistant cast steels of Examples 1 to 8, the flow of molten metal during casting was good,
No casting defect was found. Next, the cast test material (Y block) of the material of the present invention (Examples 1 to 8) was held in a heating furnace at 800 ° C. for 2 hours and air-cooled. On the other hand, all the conventional examples (Comparative Examples 11 to 14) were subjected to the test as cast.

In Table 1, conventional materials (Comparative Example 11)
14 to 14) are used for heat-resistant exhaust system components such as automobile turbocharger housings and exhaust manifolds. The test material of Comparative Example 11 is Niresist cast iron D2, and Comparative Example 12 is Niresist cast iron D5S. Comparative Example 13 is a general-purpose austenitic heat-resisting cast steel, which has a JI
It is S standard SCH-12. Further, Comparative Example 4 is an austenite / ferrite dual phase heat resistant cast steel disclosed in Japanese Patent Laid-Open No. 62-4855.

[0028]

[Table 1] Chemical composition (% by weight) [Material of the present invention] C Si Mn Ni Cr W Nb N Example No. 1 0.21 1.04 0.54 4.15 21.53 1.85 0.31 0.09 2 0.33 1.46 0.58 4.89 22.12 2.85 0.48 0.11 3 0.41 1.50 0.52 4.97 22.28 2.91 0.50 0.18 4 0.57 1.47 0.53 5.94 22.75 4.86 0.92 0.09 5 0.22 1.48 0.54 4.96 24.85 2.94 0.50 0.15 6 0.40 1.49 0.58 5.30 24.98 2.93 0.49 0.08 7 0.24 1.50 0.55 4.94 27.66 2.96 0.50 0.13 8 0.40 1.49 0.51 4.81 26.78 2.88 0.48 0.12 [ Conventional material] C Si Mn Ni Cr W Nb N Comparative example No. 11 2.77 2.12 0.88 21.10 2.44---12 1.89 5.32 0.41 34.50 2.35---13 0.21 1.24 0.50 9.1 18.8---14 0.30 1.71 0.87 5.27 28.22 * 0.23 1.00 0.31 (Note) *: S

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 diameter of 14 mm was used.

(2) High Temperature Tensile Test Using a brim test piece having a gauge length of 50 mm and a gauge diameter of 10 mm, the test was conducted at 1000 ° C.

(3) Thermal Fatigue Test A round bar test piece having a gauge length of 20 mm and a gauge length 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: 150 ° C. Upper limit temperature: 1000 ° 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
After keeping in the air at 000 ° C for 200 hours, after taking out, shot blasting treatment 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. 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.

[0034]

[Table 2] Tensile properties at room temperature 0.2% Tensile strength Strength Elongation Hardness γ amount (N / mm2) (N / mm2) (%) (HB) (%) [Inventive material] Example No. 1 430 750 15.7 229 67.9 2 455 720 7.5 241 83.7 3 470 710 4.9 248 90.3 4 500 700 5.2 269 92.5 5 525 715 4.3 255 64.4 6 510 670 4.1 255 80.3 7 670 765 3.8 277 40.3 8 590 675 3.1 277 54.2 [Conventional material] Comparative example No. 11 190 455 16 179 100 12 255 485 9 163 100 13 250 560 20 170 100 14 570 650 5 262 45

[0035]

[Table 3]
Tensile properties at high temperature / thermal fatigue life / Oxidation weight loss Tensile properties at 1000 ℃ 0.2% Tensile thermal fatigue Oxidation yield strength Elongation life loss (N / mm2 ) (N / mm2) (%) (Cycle) (mg / cm2) [ Piece Inventive Material] Example No. 1 44 75 17 106 50 2 53 95 19 140 45 3 68 116 9 181 40 4 72 127 9 71 30 5 30 57 29 61 35 6 49 91 13 88 30 7 28 49 46 65 25 8 33 68 26 92 25 [Conventional material] Comparative example No. 11 29 46 22 50 765 12 24 46 22 80 55 13 22 42 30 80 85 14 35 55 40 50 30

As is clear from Tables 2 and 3, Examples 1 to 8 according to the present invention are comparative examples 11 to 1 which are conventional materials.
It can be seen that the high temperature strength of 1000 ° C. is improved as compared with the Niresist cast irons D2, D5S and SCH12 of No. 4. Further, as shown in Table 2, the material of the present invention (Example 1
~ 8) has a relatively low hardness (HB) of 229-277,
It can be seen that it has excellent machinability.

A photomicrograph (100 times) of the heat-resistant cast steel of Example 3 is shown in FIG. Moreover, the micrograph (100 times) of the heat-resistant cast steel of Example 7 is shown in FIG. Furthermore, the heat-resistant material of Comparative Example 14 was micrographed (100
Times) is shown in FIG.

Next, using the austenite-ferrite two-phase heat-resistant cast steel of Examples 3 and 7, an exhaust manifold (wall thickness: 2.5 to 3.4) for automobile exhaust system parts was used.
mm) and turbine housing (wall thickness: 2.7-4.
1 mm) was cast. The heat-resistant cast steel parts obtained were all sound.

Further, these exhaust system parts were subjected to machining to evaluate the machinability, but no problems occurred in any of them.

Next, as shown in FIG. 4, 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 1050 ° C. at the inlet temperature of the turbine housing. Under this condition, the surface temperature of the exhaust manifold is about 980 ° C at the exhaust manifold converging portion, and the surface temperature of the turbine housing is about 102 at the wastegate portion.
It was 0 ° 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, the Niresist D5S turbine housing of Comparative Example 12 had a large wastegate crack at 100 cycles, and the alloy turbine housing of Comparative Example 14 disclosed in Japanese Patent Laid-Open No. 62-4855 was used. After the evaluation test, the waist gate is deformed by 0.62 mm
It was 0.5 mm or more, which is the acceptance criterion.

[0042]

As described above, the austenite / ferrite dual phase heat resistant cast steel of the present invention has a high temperature, particularly 100%.
In the high temperature region of 0 ° C., the strength is excellent, the room temperature ductility is not impaired, and the castability and workability are excellent, so that the manufacturing can be performed at low cost. The austenite / ferrite dual-phase heat-resistant cast steel of the present invention is suitable for automobile exhaust system parts such as an exhaust manifold and a turbine housing. The exhaust system component of the present invention has excellent high-temperature strength and extremely excellent durability.

[Brief description of drawings]

FIG. 1 is a microscopic metallographic photograph (1) of an austenite / ferrite dual phase heat resistant cast steel of Example 3 of the material of the present invention.
It is a figure showing (00 times).

FIG. 2 is a microscopic metallographic photograph (1) of the austenite / ferrite dual phase heat resistant cast steel of Example 7 of the material of the present invention.
It is a figure showing (00 times).

FIG. 3 is a view showing a microscope metallographic photograph (100 times) of a heat-resistant material of Comparative Example 14.

FIG. 4 is a diagram showing an embodiment in which an austenite / ferrite two-phase heat-resistant cast steel of the present invention and an exhaust manifold which is an exhaust system component and a turbine housing are assembled.

[Explanation of symbols]

1: Exhaust manifold, 2: Turbine housing, 3: Exhaust outlet, 4: Converter container, 5: Main catalyst

Claims (6)

[Claims] 1. A weight ratio of C: 0.20 to 0.
60%, Si: 2.0% or less, Mn:
1.0% or less, Ni: 4.0-
6.0%, Cr: 20.0 to 30.0%, W:
1.0 to 5.0%, Nb: 0.2 to 1.
0%, N: 0.05 to 0.2%, a composition having a balance of Fe and unavoidable impurities, an austenite phase of 20 to 95%, and a balance of a two-phase structure of a ferrite phase. Cast steel. 2. The heat-resistant cast steel according to claim 1, wherein when there is a need for residual strain removal or working, the heat-resistant cast steel is subjected to an annealing treatment in a temperature range where a solution phenomenon does not occur. 3. An exhaust system component made of the heat-resistant cast steel according to any one of claims 1 and 2. 4. An exhaust system component according to claim 3, which is for an automobile. 5. An exhaust system component according to claim 4, wherein the exhaust system component is an exhaust manifold. 6. An exhaust system component according to claim 4, which is a turbine housing.