JP2011252208A - Heat-resistant austenitic stainless steel for metal gasket - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide austenitic stainless steel suitable for a metal gasket which is excellent in settling resistance, and maintains air tightness for a long period of time.SOLUTION: The austenitic stainless steel contains: >0.03 to 0.20 mass% C, 2.0 to 5.0 mass% Si, ≤2.5 mass% Mn, 7.0 to 17.0 mass% Ni, 13.0 to 23.0 mass% Cr and <0.05 mass% N, and the balance Fe with inevitable impurities, and an SFE value defined as 25.7+2(Ni)+410(C)-0.9(Cr)-77(N)-13(Si)-1.2(Mn) is adjusted to 2 to 20. Furthermore, the austenitic stainless steel may contain one, two or more of ≤0.80 mass% Nb, ≤2.0 mass% Mo, ≤4.0 mass% Cu, ≤0.5 mass% Ti, ≤1.0 mass% V, ≤0.2 mass% Al, ≤0.020 mass% B, ≤0.2 mass% REM (rare earth element), ≤0.2 mass% Y, ≤0.1 mass% Ca, and ≤0.1 mass% Mg.

Description

  INDUSTRIAL APPLICABILITY The present invention is used as a metal gasket that maintains excellent sag resistance even when exposed to a high temperature atmosphere such as an engine of an internal combustion engine, an exhaust gas passage member (exhaust manifold, catalytic converter), an EGR cooler, and a turbocharger. Relates to austenitic stainless steel.

  Metal gaskets have been used for engines and the like that are exposed to an atmosphere in which heating and cooling are frequently repeated. However, the exhaust gas temperature has increased due to higher performance of engines and environmental regulations, and the temperature has increased to 600-800 ° C. More gaskets are exposed to temperature. As the temperature at which the gasket is exposed is increased, the conventional gasket material has the following problems.

  Materials strengthened by SUS301-based work-induced martensite phase represented by JP-A-7-3406 and JP-A-2008-111192, and SUS431-type quenching-tempering martensite phase represented by JP-A-7-278758 In a material reinforced to a high temperature, the temperature at which it is heated corresponds to the decomposition temperature of the martensite phase, so that the softening is remarkably inferior in sag resistance.

  Precipitation strengthening materials such as NCF625, NCF718 and SUH660 specified in JISG4312 (heat-resistant steel plate) defined in JIS G4902 (corrosion-resistant heat-resistant superalloy plate) generate fine precipitates during use. Although effective for precipitation strengthening, it is expensive because it contains a large amount of expensive Ni.

  In order to solve these problems, in Japanese Patent Laid-Open No. 2003-82441, Fe—Cr—Ni austenitic stainless steel reinforced with N and Nb is disclosed in Japanese Patent Laid-Open Nos. 7-3407, 9-279315, and 11 No. -241145 discloses Fe-Cr-Mn-Ni austenitic stainless steel reinforced with N and is being applied to some heat-resistant gaskets.

  However, since these steels contain a lot of N, they have a problem that the yield stress (0.2% proof stress) becomes very high. In other words, it is desirable that the annealed material should be relatively soft to ensure the flatness required for the gasket by straightening, and that it has excellent sagability at high temperatures to ensure heat resistance during use. However, it is difficult to say that the steel having a large amount of N added satisfies the former characteristics.

  As described above, an austenitic stainless steel that does not contain a large amount of Ni has a relatively low initial proof strength, and low N steel that is excellent in heat resistance at high temperatures (sagging property) has become necessary. The component system was not necessarily clarified at present.

Japanese Unexamined Patent Publication No. 7-3406 Japanese Patent Laid-Open No. 8-111192 JP 7-278258 A JP 2003-82441 A Japanese Patent Laid-Open No. 7-3407 Japanese Patent Laid-Open No. 9-279315 JP-A-11-241145

  An object of the present invention is to provide an austenitic stainless steel suitable for a heat-resistant metal gasket that is relatively inexpensive and has improved workability at room temperature and sag resistance at high temperature at the same time.

In order to achieve the object, the heat-resistant austenitic stainless steel for metal gaskets of the present invention has C: more than 0.03 to 0.20% by mass, Si: more than 2.0 to 5.0% by mass, Mn: 2. 5 mass% or less, Ni: 7.0 to 17.0 mass%, Cr: 13.0 to 23.0 mass%, N: less than 0.05 mass%, with the balance being Fe and inevitable impurities, Furthermore, the SFE value represented by the following formula (1) is 2 to 20.
SFE = 25.7 + 2 (Ni) +410 (C) −0.9 (Cr) −77 (N) −13 (Si) −1.2 (Mn) (1)
However, each term in the formula is the content (mass%) of the alloy element.

  Further, Nb: 0.80 mass% or less, Mo: 2.0 mass% or less, Cu: 4.0 mass% or less, Ti: 0.5 mass% or less, V: 1.0 mass% or less, Al: 0.2% by mass or less, B: 0.020% by mass or less, REM (rare earth element): 0.2% by mass or less, Y: 0.2% by mass or less, Ca: 0.1% by mass or less, Mg: 0 .1% by mass or less of one kind or two or more kinds may be included.

The austenitic stainless steel of the present invention is excellent in sag resistance in a high temperature environment of 500 to 800 ° C. required for a metal gasket. On the other hand, since it has sufficient workability in the tempered state, it is easy to correct the flatness.
Due to the above two characteristics, when this austenitic stainless steel is incorporated into an automobile engine as a low-temperature metal gasket such as exhaust manifold or intech manifold, the life of peripheral equipment and the performance of the engine itself are improved. To do. In addition to engine gaskets, the present invention can also be used for elastic gaskets incorporated in ball joint parts used in automobile exhaust parts and vibration isolation joints for automobile exhaust pipes.

It is a figure which shows the shape of a gasket test piece.

  The inventors of the present invention have made various studies in order to solve the above-described problems, and have obtained the following knowledge to arrive at the present invention.

(1) Initial softening is achieved by setting N to less than 0.05%.
(2) The sagability at high temperature is achieved by strengthening the bead part (processed part) with a factor that is not easily softened by heating, that is, by being hardened mainly by stacking faults.

  The mechanism for improving sag resistance at high temperatures due to stacking faults is not always clear, but dislocations introduced by crossing stacking faults are difficult to work, and ε-martensite phase is generated by strong processing, which is heated. It is difficult to decompose by.

Hereinafter, alloy components, contents, and the like included in the austenitic stainless steel targeted by the present invention will be described.
C: more than 0.03 to 0.20% by mass
It is an alloy component effective for improving high-temperature strength, and strengthens stainless steel by solid solution strengthening and precipitation strengthening. Such an effect becomes remarkable when C content exceeds 0.03 mass%. However, when an excessive amount of C exceeding 0.20% by mass is included, huge grain boundary carbides are likely to precipitate during holding at a high temperature, and the material becomes brittle.

Si: more than 2.0 to 5.0% by mass
It is a ferrite forming element, exhibits a large solid solution strengthening ability in the austenite phase, and promotes age hardening by strain aging while maintaining a high temperature. Such an effect becomes remarkable when the Si content is 2.0% by mass or more. However, when an excessive amount of Si exceeding 5.0% by mass is added, hot cracking is induced, causing various troubles in production. The Si content is more preferably in the range of more than 3.0 to 5.0% by mass or less.

Mn: 2.5% by mass or less An austenite-forming element that is used as an alternative component of expensive Ni and can reduce the required amount of Ni. It is also effective to improve hot workability by fixing S. However, addition of an excessive amount of Mn exceeding 2.5% by mass lowers the high-temperature strength and mechanical properties.

Ni: 7.0 to 17.0% by mass
It is an essential alloy component for ensuring a stable austenite structure, and the optimum content of Ni depends on the amount of ferrite-forming elements such as Cr, Si, and Mo contained in the steel material. However, when the Ni content is less than 7.0% by mass, it becomes difficult to stabilize the austenite phase. On the other hand, when the Ni content exceeds 17.0% by mass, the steel material cost increases, which is economically disadvantageous. The Ni content is more preferably in the range of 11.0 to 15.0 mass%.

Cr: 13.0 to 23.0% by mass
It is an alloy component necessary for corrosion resistance and oxidation resistance, and considering the use of a metal gasket exposed to a severe high temperature corrosion atmosphere, an amount of Cr of at least 13.0% by mass is necessary. However, if an excessive amount of Cr exceeding 23.0% by mass is contained, δ ferrite is formed and a stable austenite phase cannot be maintained.

N: Less than 0.05% by mass An alloy component effective for increasing the high-temperature strength of austenitic stainless steel, but when 0.05% by mass or more of N is contained, C in M ₂₃ C ₆ transitions to C + N And reduce the deposition rate. Therefore, the transformation to precipitation strengthening is delayed during high temperature holding, and the high temperature strength is lowered.

Nb: 0.80% by mass or less By forming a precipitate in a high temperature atmosphere to which the metal gasket is exposed or by dissolving in austenite matrix, the hardness is increased and the softening resistance is improved. However, the excessive Nb content exceeding 0.80% by mass reduces the hot workability due to the improvement of the high temperature strength.

Mo: 2.0% by mass or less Mo is an alloy component added as necessary, and is effective for improving corrosion resistance, and becomes carbonitride during fine temperature holding to finely disperse and increase high temperature strength. At the time of aging treatment, the strength is improved by the formation of precipitates. Therefore, even if the metal gasket is exposed to a severe high temperature atmosphere, the strength is prevented from being reduced by the addition of Mo. However, excessive addition of Mo exceeding 2.0% by mass promotes the formation of δ ferrite at high temperatures.

Cu: 4.0% by mass or less Cu is an alloy component added as necessary, and generates Cu-based precipitates as the temperature of the atmosphere in which the metal gasket is used to improve high temperature strength and softening resistance. . However, excessive addition of Cu exceeding 3.0% by mass reduces hot workability and causes cracking.

Ti: 0.5% by mass or less Ti is an alloy component added as necessary, and forms a precipitate effective in increasing hardness and improving resistance to settling in a high temperature atmosphere. However, excessive addition of Ti exceeding 0.5% by mass causes surface defects.

V: 1.00% by mass or less An alloy component added as necessary, and forms a precipitate effective in increasing the hardness and improving the anti-sagging property in a high-temperature atmosphere. However, excessive addition of V exceeding 1.0% by mass causes a reduction in surface workability and toughness.

Al: 0.2% by mass or less An alloy component that is added as necessary. It is added as a deoxidizer during steelmaking, and has an adverse effect on punchability when a steel sheet is punched into a gasket shape. Has the effect of drastically reducing. Such an effect is saturated at an Al content of 0.2% by mass, and even if Al is increased further, surface defects are increased.

B: 0.020% by mass or less An alloy component that is added as necessary, promotes fine precipitation of carbonitrides effective in increasing high-temperature strength, and causes grain boundary segregation such as S in the hot rolling temperature range. Suppresses and prevents the occurrence of edge cracks. However, when an excessive amount of B exceeding 0.020% by mass is added, a low-melting boride tends to be generated, and hot workability is deteriorated.

REM (rare earth element): 0.2 mass% or less Y: 0.2 mass% or less Ca: 0.1 mass% or less Mg: 0.1 mass% or less It is effective in improving hot workability and improving oxidation resistance. The effects on hot workability and oxidation resistance both become more pronounced as the amount added increases, but saturates at 0.20% by mass for REM and Y and 0.10% by mass for Ca and Mg.

SFE value: 2-20
The SFE value is an index of stacking fault energy defined by the following formula, and is limited to 2 to 20 in the present invention.
SFE = 25.7 + 2 (Ni) +410 (C) −0.9 (Cr) −77 (N) −13 (Si) −1.2 (Mn) (1)
However, each term in the formula is the content (mass%) of the alloy element.
In the present invention, the SFE value is limited to the above range in order to ensure sag resistance. By maintaining this range, good workability and softening resistance after heating and holding are ensured. If the SFE is less than 2, the work hardening becomes too large, so that when the product is processed, generation of wrinkles and cracks occur. Moreover, although it becomes easy to process when SFE exceeds 20, the production | generation of (epsilon) martensite etc. does not occur, but the amount of bead sag by 600-800 degreeC heating becomes large.

  Stainless steel having the composition shown in Table 1 is melted in a 300 kg vacuum melting furnace, and then the slab is subjected to hot forging, cutting, hot rolling, annealing, pickling, cold rolling, annealing, and pickling steps to obtain a thickness of 0. Stainless steel strips of 0.5 to 0.2 mm were produced.

  Table 2 shows the 0.2% proof stress of each stainless steel cold-rolled annealed material. Invented steels all show low proof stress, and it can be seen that processing into gaskets is easy.

  A 150 mm x 150 mm square test piece is cut out from each stainless steel strip, a circular opening with an inner diameter of 75 mm is formed in the center of the test piece, and a bead with a width of 2.5 mm, a height of 0.25 mm, and a protrusion 2R is pressed around the opening. A metal gasket (FIG. 1) was produced by molding. Next, the bead portion was pressed with a jig so as to be flat, held at 700 ° C. for 48 hours, and then gradually cooled to room temperature. About the test piece after slow cooling, the residual bead height at room temperature was measured. In addition, the focus bead microscope was used for the measurement of residual bead height, and it computed as an average value of 3 points | pieces.

The results are shown in Table 3. As seen in the measurement results, the steel type No. 1 to 10 (examples of the present invention) had a residual bead height of 0.210 mm or more at room temperature after heating at 700 ° C. for 48 hours, and had a residual bead height required for a metal gasket. On the other hand, Comparative Steel No. In 11 to 15 (comparative examples), the residual bead height was less than 0.200 mm, and there was a problem in performance as a metal gasket. The reason why the remaining bead height is low can be considered as follows.
In comparative steel No. 11, since the amount of Si is insufficient and the amount of N is too large, the age hardening ability during heating and holding is lowered, so the residual bead height is not sufficient. Comparative steel No. Nos. 12 and 13 were remarkably softened because the heating temperature corresponded to the decomposition temperature of the martensite phase, and the residual bead height was lowered. Comparative steel No. Since 14 and 15 contained an excessive amount of N, the SFE value was low, the processing-induced martensite was generated, and tempered during holding at a high temperature, so the residual bead height was low.

  As is clear from this comparison, even with austenitic stainless steel with a reduced amount of N, it is possible to obtain a metal gasket that is excellent in shape freezing property and maintains hermeticity for a long period of time by appropriate management of the SFE value. confirmed.

The austenitic stainless steel of the present invention is excellent in sag resistance in a high temperature environment of 500 to 800 ° C. required for a metal gasket. In addition, when this austenitic stainless steel is incorporated in an automobile engine as a low-temperature metal gasket such as an exhaust manifold or intech manifold, the life of peripheral devices and the performance of the engine itself are improved. In addition to engine gaskets, the present invention can also be used for elastic gaskets incorporated in ball joint parts used in automobile exhaust parts and vibration isolation joints for automobile exhaust pipes.

Claims (3)

C: more than 0.03 to 0.20% by mass, Si: more than 2.0 to 5.0% by mass, Mn: 2.5% by mass or less, Ni: 7.0 to 17.0% by mass, Cr: 13 0.0-23.0% by mass, N: less than 0.05% by mass, the balance being Fe and unavoidable impurities, and the SFE value represented by the following formula (1) being 2-20 Heat resistant austenitic stainless steel for metal gaskets.
SFE = 25.7 + 2 (Ni) +410 (C) −0.9 (Cr) −77 (N) −13 (Si) −1.2 (Mn) (1)
However, each term in the formula is the content (mass%) of the alloy element.   Furthermore, the heat-resistant austenitic stainless steel for metal gaskets of Claim 1 containing Nb: 0.8 mass% or less.   Furthermore, Mo: 2.0 mass% or less, Cu: 4.0 mass% or less, Ti: 0.5 mass% or less, V: 1.0 mass% or less, Al: 0.2 mass% or less, B: 0.0 mass%. One or two of 020 mass% or less, REM (rare earth element): 0.2 mass% or less, Y: 0.2 mass% or less, Ca: 0.1 mass% or less, Mg: 0.1 mass% or less The heat-resistant austenitic stainless steel for metal gaskets according to claim 1 including the above.

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