JP4403033B2 - Austenitic stainless steel for exhaust manifold - Google Patents

Description

  The present invention relates to a steel for an exhaust manifold that is an exhaust gas passage member of an automobile engine, and is particularly suitable for an inner pipe of a double structure exhaust manifold that is exposed to high-temperature exhaust gas at 1000 to 1050 ° C. About.

  In recent years, automobile engines and exhaust gas treatment systems have been required to have purification performance that satisfies strict exhaust gas regulations. As an exhaust gas purification means, it is common to provide a catalytic converter in the exhaust gas path. However, immediately after starting the engine, the temperature of the purification device is low and the purification efficiency is lower than in normal operation. Is important. As countermeasures, it is effective to additionally install a purification device directly under the exhaust manifold or to raise the combustion gas temperature itself. Various studies have been made, but these have limitations.

  Thereafter, a method of making the exhaust manifold into a double structure has been proposed and has already been partially put into practical use. According to this, although the unit cost is higher than the conventional single structure pipe, the heat insulation effect of combustion gas is very high, so the purification efficiency is increased, and there is no need to add heat insulating material, heating device, further purification device, etc. Benefits of cost reduction by reducing the number of parts.

  In order to avoid thermal fatigue failure due to repeated heating and cooling, a single structure exhaust manifold uses a ferritic steel grade that has a smaller coefficient of thermal expansion than austenitic. On the other hand, in the double structure, since the outer tube (outer tube) is repeatedly restrained by heating and cooling, it is desirable to use a ferritic steel type like the single tube. However, the inner tube (inner tube) has a thickness of 1 mm or less, and therefore requires better workability than the outer tube and can be designed so that the material is not constrained. In many cases, it is more advantageous to use a steel grade.

  Since the inner pipe of the exhaust manifold is directly exposed to the exhaust gas, the material temperature reaches the same level as that of the exhaust gas (generally, generally 800 to 1000 ° C.). Although it is necessary to use a steel type having a small amount of oxidation increase in this temperature range, for example, SUS304, which is a typical austenitic stainless steel, basically has insufficient properties. In general, austenitic stainless steel is inferior in the adhesion of oxide scale to ferritic stainless steel, and therefore special attention is required for resistance to scale peeling during repeated heating and cooling.

  Further, the material for the inner pipe of the exhaust manifold is required to be excellent in high temperature strength, workability and weldability. That is, with respect to the high temperature strength, thermal fatigue failure due to repeated heating and cooling can be avoided by designing the material not to be constrained, but fatigue due to engine vibration becomes a problem. For this reason, it is desired to be excellent in high temperature and high cycle fatigue characteristics. As for workability, various processes such as bending and press forming are assumed, and characteristics suitable for each forming method are required. Regarding the weldability, a material having low weld cracking sensitivity in TIG welding, MIG welding, or the like is preferable.

  Various steel types have been developed for heat-resistant austenitic steel types (Patent Documents 1 to 12). In particular, Patent Document 12 proposes an austenitic stainless steel suitable for the inner pipe of the exhaust manifold.

Japanese Patent Laid-Open No. 50-18313 Japanese Patent Laid-Open No. 50-93219 JP 52-109420 A JP-A-53-149114 Japanese Patent Laid-Open No. 62-192562 JP 63-38558 A JP-A-5-98395 Japanese Patent Laid-Open No. 7-118810 JP 7-188869 A JP-A-8-239737 JP-A-9-87809 JP 2001-98344 A

  The austenitic stainless steel for exhaust manifold inner pipe of Patent Document 12 assumes an exhaust gas temperature of 800 to 1000 ° C., and exhibits excellent oxidation resistance in 100 cycles of intermittent heating at 1000 ° C. In addition, the component design takes into account formability and weldability. However, the maximum exhaust gas temperature of automobiles is diversified depending on the vehicle type, and an engine of a type that discharges combustion gas as high as 1000 to 1050 ° C. is also produced. The steel of Patent Document 12 cannot cope with such high temperature exhaust gas.

  On the other hand, in recent years, efforts to improve the long-term reliability of automobiles are carried out by each automobile manufacturer, and the durability against intermittent heating is not sufficient for the test of about 100 cycles performed in Patent Document 12, and 1000 cycles or more, preferably In the endurance test of 2000 cycles, excellent durability, in particular, performance showing scale peeling resistance has been desired.

  Further, when considering use in a high temperature region of 1000 to 1050 ° C., a ferritic steel type is more advantageous than an austenitic steel in terms of high-temperature oxidation characteristics, particularly scale peeling resistance. Recently, however, “hydroforming” has been adopted as a method of processing steel pipe members for automobiles. This is a technique for forming a steel pipe member with a desired shape by applying a liquid pressure to the inside of the steel pipe. Conventionally, a press-formed product of a steel plate could not be manufactured unless it was welded and tubular like a “monaka”. There exists an advantage which can manufacture the steel pipe member of complicated shape easily. The exhaust manifold inner pipe has been required to have ductility assuming hydroforming. It is difficult to achieve such excellent ductility in ferritic heat-resistant steel types, and component design using austenitic steel is required.

  In view of such a current situation, the present invention is suitable for an exhaust manifold used in an environment where the material temperature is 1000 to 1050 ° C., particularly high temperature strength suitable for a double structure inner pipe, and resistance to scale peeling in a long-term repetition. It is intended to develop and provide a steel having ductility that can be applied to hydroforming.

The steel provided by the present invention is, by mass%, C: 0.15% or less, Si: 1 to 3%, Mn: 2% or less, P: 0.04% or less, S: 0.01% or less, Ni : More than 12 to 20%, Cr: more than 22 to 25%, N: 0.048 to 0.15 %, Al: more than 0.5 to 2.0%, Nb: 0 to 0.5%, Ti: 0 -0.5%, Mo: 0-0.5%, Cu: 0-0.5%, total of REM, Y, Ca: 0-0.1%, V: 0-0.5%, W: It is an austenitic stainless steel for exhaust manifold that is 0 to 0.5%, Zr: 0 to 0.5%, the balance Fe and inevitable impurities, and satisfies the following formula (1).
Cr + 0.5Si + 3Al ≧ 26 (1)

Here, Nb, Ti, Mo, Cu, REM, Y, Ca, V, W, and Zr are arbitrarily added elements. The lower limit “0%” of the element content is a case where it is below the measurement limit in a normal analysis method performed in the steelmaking stage.
The content of the element expressed in mass% is substituted for the element symbol in the formula (1).

In the above steel, in particular, by mass%, Nb: 0.05 to 0.5%, Ti: 0.05 to 0.5%, Mo: 0.05 to 0.5%, Cu: 0.05 to 0 A material satisfying one or more of 0.5%, V: 0.05-0.5, W: 0.05-0.5%, Zr: 0.05-0.5% is provided.
Alternatively, a total of REM (rare earth element), Y, and Ca: 0.005 to 0.1% by mass% is provided.

  According to the present invention, a steel exhibiting excellent durability, particularly excellent scale peeling resistance when used repeatedly for a long time in a temperature range of 1000 to 1050 ° C. has been realized. This steel has excellent high-temperature strength, and has characteristics suitable for an exhaust manifold, particularly for a double-structure inner pipe. Moreover, it has ductility applicable to hydroforming. Therefore, the present invention contributes to improving the reliability of the exhaust manifold and reducing the cost in a type of automobile having a maximum exhaust gas temperature as high as 1000 to 1050 ° C.

  The steel of the present invention has the following high-temperature strength suitable for the inner pipe of a dual structure exhaust manifold exposed to a temperature of 1000 to 1050 ° C., long-term repeated scale peel resistance, and excellent ductility. The component design was performed.

  C is effective for improving the high temperature strength of austenitic stainless steel. However, if excessively contained, Cr carbides are formed during use as an exhaust manifold and the toughness deteriorates, and the amount of solid solution Cr effective in improving high-temperature oxidation resistance decreases. For this reason, C content is restrict | limited to 0.15 mass% or less. The range of preferable C content is 0.02-0.15 mass%.

  Si is very effective in improving high temperature oxidation characteristics. In the present invention, Si is added together with Cr and Al, which will be described later, to improve the scale adhesion at the 1050 ° C. level, but Si must be contained in an amount of 1% by mass or more. However, since excessive addition impairs ductility, the upper limit of Si content is limited to 3% by mass.

  Mn is an austenite stabilizing element and is added mainly for adjusting the phase balance in the present invention. However, excessive Mn addition causes a decrease in high-temperature oxidation resistance, so it is limited to 2% by mass or less.

  P is an element that impairs the hot workability of austenitic stainless steel, and is desirably reduced as much as possible. For this reason, P content is restrict | limited to 0.04 mass% or less.

  S, like P, is an element that impairs the hot workability of austenitic stainless steel, and it is desirable to reduce it as much as possible in order not to reduce the production yield of the steel. For this reason, S content is restrict | limited to 0.01 mass% or less.

  Ni is an austenite stabilizing element. In the steel of the present invention having a high Cr content, Ni content exceeding 12% by mass is required for adjusting the austenite balance. However, addition of a large amount of Ni lowers ductility and impairs economy, so the upper limit of Ni content is limited to 20% by mass. A more preferable Ni content is 15 to 19% by mass.

  Cr is a basic element that suppresses scale formation at high temperatures, and in the present invention, Cr content exceeding 22% by mass is required in order to impart high-temperature oxidation resistance at the 1050 ° C. level. However, if Cr is excessively contained, σ embrittlement is likely to occur due to heating, and the raw material cost increases. For this reason, Cr content is restrict | limited to 25 mass% or less.

N is effective for improving the high-temperature strength by solid solution strengthening, but excessive addition reduces the toughness and ductility of the steel by forming Cr nitride. For this reason, the lower limit of the N content is limited to 0.048% by mass, and the upper limit is limited to 0.15% by mass.

  Al is effective in improving the high-temperature oxidation resistance. In particular, Al oxide is formed in the inner layer of the Cr and Si oxides to improve the scale adhesion. In order to fully exhibit this effect, Al content exceeding 0.5 mass% is required. However, if contained in a large amount, the steel becomes hard and the ductility is impaired, and the raw material cost is also increased. For this reason, the upper limit of the Al content is limited to 2.0% by mass.

In addition to the above elements, the following elements can be selectively contained in the present invention.
Nb, Ti, V and Zr have the effect of improving the high temperature strength by precipitation strengthening. Any of these elements can effectively exhibit the above-mentioned action by containing 0.05% by mass or more. However, since addition of a large amount may cause a decrease in ductility, when adding these elements, it is necessary to carry out each element within a range of 0.5% by mass or less. These elements may be added alone or in combination of two or more.

  Mo, Cu, and W have the effect of improving the high temperature strength by solid solution strengthening. Any of these elements can effectively exhibit the above-mentioned action by containing 0.05% by mass or more. However, since addition of a large amount may cause a decrease in ductility, when adding these elements, it is necessary to carry out each element within a range of 0.5% by mass or less. These elements may be added alone or in combination of two or more.

  REM, Y, and Ca are effective elements for ensuring a higher level of scale peel resistance at 1000 to 1050 ° C. In order to fully exhibit the effect, it is desirable that the total content is 0.005 mass% or more by adding one or more of these elements. However, if contained in a large amount, the steel becomes hard and ductility is hindered. For this reason, the total content of these elements is limited to 0.1% by mass or less.

In the present invention, it is an important subject to impart excellent scale peel resistance that can withstand repeated use at a temperature of 1050 ° C. for a long period of time. Specifically, in a high-temperature oxidation test at 1050 ° C. and 2000 cycles, which will be described later in the examples, an excellent characteristic is obtained such that the thickness reduction rate is less than 20% with a material having a thickness of 0.8 mm. As the method, Cr, Si and Al are contained in a composite so as to satisfy the following formula (1).
Cr + 0.5Si + 3Al ≧ 26 (1)

  Expression (1) is a relational expression found as a result of detailed studies by the inventors. When the above-mentioned Cr, Si, Al content range satisfies the formula (1), the “wedge type” in which Si and Al are concentrated inside the Cr oxide on the steel surface by heating at 1000 to 1050 ° C. It is considered that the inner layer scale is formed, and this exerts a keying effect to strengthen the scale adhesion. When at least one of REM, Y, and Ca is added within the above-described content range, the scale adhesion is further enhanced.

FIG. 1 shows the influence of alloying elements on the scale peel resistance of Cr-10 to 20% Ni austenitic stainless steel having a thickness of 0.8 mm. The horizontal axis shows the value of Cr + 0.5Si + 3Al as an index of the alloy element, and the vertical axis shows the amount of weight change in the 1050 ° C., 2000 cycle test according to the method shown in the examples described later, as an index of scale peeling resistance.
From FIG. 1, it can be seen that by setting Cr + 0.5Si + 3Al ≧ 26, the scale peel resistance at 1050 ° C. and 2000 cycles is remarkably improved.

  The steel whose components have been adjusted as described above can be made into a steel plate having a thickness of, for example, about 0.8 mm using a normal stainless steel plate manufacturing facility, and can be made into a steel tube for an exhaust manifold by welding pipe making. The steel pipe is bent into a predetermined shape, or formed into a complex shape by combining bending and crushing and hydroforming, and used as a member of an exhaust manifold (for example, an inner pipe).

  The steel shown in Table 1 was melted, and a steel plate having a thickness of 2.0 mm was obtained by the steps of hot rolling → anneal pickling → cold rolling → anneal pickling according to normal stainless steel plate manufacturing conditions. Further, cold rolling and annealing pickling were performed to obtain a steel plate having a thickness of 0.8 mm.

  A tensile test piece (JIS 13B) parallel to the rolling direction was cut out from each steel plate having a thickness of 0.8 mm, a tensile test was performed at room temperature in accordance with JIS Z 2241, and the elongation was measured to evaluate ductility. did. Further, a high-temperature tensile test piece perpendicular to the rolling direction was cut out from each steel plate having a thickness of 2.0 mm, and a high-temperature tensile test was conducted at 1050 ° C. in accordance with JIS G 0567. 2% yield strength was determined.

In addition, a 25 × 35 mm high-temperature oxidation test piece was cut out from each steel plate having a thickness of 0.8 mm, and 2000 cycles in which “air cooling at 1050 ° C. in the atmosphere × 5 minutes → 5 minutes” in one cycle in accordance with JIS Z 2282 Subjected to repeated high temperature oxidation tests. The weight change before and after the high-temperature oxidation test and the thickness reduction rate of the portion where the plate thickness decreased most after the test were determined. The thickness reduction rate is calculated by the following equation.
Thinning rate = (plate thickness before test−plate thickness after test) / plate thickness before test × 100
The results are shown in Table 2.

As can be seen from Table 2, the material satisfying the specified chemical composition in the present invention has a thickness reduction rate of less than 20% after a high-temperature oxidation test of 2000 cycles, and excellent scale peeling resistance in repeated heating at 1050 ° C. Was presented. The change in weight before and after the high temperature oxidation test was as small as 10 kg / m ² or less. That is, it was confirmed that the steel according to the present invention stably exhibits excellent durability when exposed to repeated heating in the range of 1000 to 1050 ° C. The high temperature strength (0.2% proof stress at 1050 ° C.) was also sufficient, 20 N / mm ² or more. Regarding ductility, the room temperature elongation is 45% or more, and it can be said that it has performance applicable to hydroforming.

  On the other hand, steel Nos. 12, 15, and 21 do not satisfy the above formula (1), and steel Nos. 13, 17, and 19 have too low a content of Cr, Si, or Al. Was inferior in scale peeling resistance (thinning rate). Steel Nos. 14, 16, 18, 20, 22, and 23 had low ductility (room temperature elongation) because the content of any of Si, Mn, Ni, Cr, N, and Al was too high.

The graph which shows the influence of the alloy element which acts on scale peeling resistance about Cr-10-20% Ni austenitic stainless steel.

Claims (3)

In mass%, C: 0.15% or less, Si: 1 to 3%, Mn: 2% or less, P: 0.04% or less, S: 0.01% or less, Ni: more than 12 to 20%, Cr : More than 22 to 25%, N: 0.048 to 0.15 %, Al: more than 0.5 to 2.0%, Nb: 0 to 0.5%, Ti: 0 to 0.5%, Mo: 0 to 0.5%, Cu: 0 to 0.5%, REM, Y, Ca total: 0 to 0.1%, V: 0 to 0.5%, W: 0 to 0.5%, Zr : Austenitic stainless steel for exhaust manifold that is 0 to 0.5%, balance Fe and inevitable impurities, and satisfies the following formula (1).
Cr + 0.5Si + 3Al ≧ 26 (1)   Nb: 0.05-0.5%, Ti: 0.05-0.5%, Mo: 0.05-0.5%, Cu: 0.05-0.5%, V: The austenitic stainless steel for an exhaust manifold according to claim 1, satisfying one or more of 0.05 to 0.5, W: 0.05 to 0.5%, and Zr: 0.05 to 0.5%.   The austenitic stainless steel for an exhaust manifold according to claim 1, wherein the mass ratio is REM, Y, and Ca: 0.005 to 0.1%.

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