Automobile exhaust system present situation and development of stainless steel

    1. The composition of automobile exhaust system and the materials used

   Auto exhaust system from close to the direction of the engine, the exhaust manifold, former tubes, flexible tube, catalytic converter, center pipe, main silencer and end seven parts constitute. According to difference of car, some installed several catalytic converters, some installed muffler.
   1.1 the exhaust manifold
   Exhaust manifold is close to the engine part, because of the exhaust gas temperature of 900 ℃, so the requirement materials has good oxidation resistance, high temperature strength and thermal fatigue properties. Also, for complex shape machining, can also require material should possess good formability. Adopt stainless steel exhaust manifold according to its structure is different, can be divided into two categories. Category is the stamping steel plate welding and become, after another kind is the steel pipe bend after welding and become, for the latter, and the use of double tube structure of the steel pipe.
   Used for exhaust manifold has a system of austenite stainless steel stainless steel stainless steel and iron element system, etc. System of austenite stainless steel has good high temperature strength, but because the skin peeled prone to oxidation, so on oxidation resistance than stainless iron element system. As the use of steel SUS304 (18 8 cr - ni) and SUS XM15J1 18 13 cr - ni - 4 (si). Stainless steel iron element system, on the other hand, although good oxidation resistance, but the system of austenite stainless steel is better than high temperature strength. Due to the thermal expansion coefficient is small, so the thermal fatigue properties of favor. As using steel SUH409L (11 cr - Ti - LC) and SUS430J1L (18 cr - 0.5 - Cu - Nb - LC, N), but in recent years, with the strengthening of the exhaust emission regulations, exhaust temperature assumes the trend of high temperature, it is said that some as high as 950 ℃. In this case, you can use SUS444 (19 cr mo - 2) stainless steel. In addition, as the material to reduce costs, and the use of a lower amount of Cr SUS429 (15 Cr) stainless steel. Using double tube exhaust manifold is usually use system of austenite stainless steel tube, stainless steel tube use iron element system.
   1.2 before the tube
   Top tube materials used in SHU409L, SUS436L (mo - LC 17 cr - 1, N), SUS430J1L stainless steel iron, such as system, but when using hollow double, and the use of the system of austenite stainless steel tube.
   As the future development trend of low cost, good oxidation resistance and thermal fatigue properties of iron element system will no doubt become the mainstream of stainless steel. Especially, due to the high temperature of exhaust gas temperature and thin-wall tube, and can be expected to adopt better high temperature performance of steel grade, such as SUS429 level.
   1.3 flexible pipe
   Flexible pipe by a snake belly of double tube wrapped with stainless steel wire mesh and the outer braid. From the point of using performance, requires high temperature fatigue sex, in order to make the snake shape, also requires its processability. Especially in cold area use to prevent road freeze thawing salt, so the tube outside should also has a high temperature salt corrosion.
   The materials used are mainly SUS304. However, because of in cold area use to prevent road freeze thawing salt, required tube outer should also has a high temperature salt corrosion, so using the add Si, Ni added amount of high temperature and salt corrosion resistance higher SUS XM15J1. Recently, begin to have the use of SUS316L (18 cr - ni - 2.5 No 12 - LC), its price is SUS XM15J1 cheaper, grain boundary corrosion resistance is higher than SUS304.
   1.4 the catalytic converter (catalytic carrier and the catalytic converter shell)
   To reduce the air pollution caused by automobile exhaust fumes, installation of the exhaust gas purification system called "catalytic converter". Catalytic converter installed on the exhaust manifold under or under the chassis, so to be able to withstand the test of harsh conditions, such as high temperature and vibration. For catalytic carrier, the use of iron element system made of stainless steel foil metal carrier. Stainless steel foil use iron element system of reason is that it is because of the small has a smaller thermal capacity and thermal shock properties. Metal substrate by using stainless steel foil production of honeycomb cores and shell is made of stainless steel plate production. Honeycomb core is to put the flat foil and wavy after foil roll up based on the combination of brazing and welding. Shell with high strength, resistance to high temperature and salt injury processability.
   To the requirement of foil is better oxidation resistance, thus using the Fe - Cr - Al is a ferritic stainless steel. Representative types of steel is Fe - 20 cr - 5 al alloy. In order to improve the impermeability, oxide skin membrane and add a small amount of Hf, Y, Ce and other elements of rare earth metal (REM). In the future against oxidation requirements will be further improved.
   For catalytic converter shell, most is the use of SUS430 (17 cr) stainless steel, but with the increase of exhaust gas temperature, there are still are using SUS430J1L or SUS429 system characteristics such as high temperature stainless steel iron element system better.
   1.5 the center pipe
   Center tube, the materials used is to use hot dip aluminized steel sheet before, because the outside inside the tube of the moisture corrosion prevention measures, so the surge in the use of stainless steel, main is to use SUS410 (13 cr) stainless steel, SHU409L and SUS430 stainless steel.
   1.6 the silencer
   At present main muffler materials mainly is SUS436L, but in order to reduce costs, reduce Mo SUS432 stainless steel (17 cr Mo - 0.5) of the usage is increasing. For the shell and the end plate, etc., from beautiful and corrosion protection measures, and the use of aluminium coated stainless steel.
   1.7 at the end of pipe
   Due to the tube at the end of the temperature is low, so from the point of the performance of materials, using aluminized steel sheet is ok, but from a design perspective, some use SUS409L and SUS430 stainless steel of high purity iron element system, while others use SUS304 stainless steel austenite system. There are special requirements in the design, and the use of Cr plating stainless steel hot dip aluminized steel tube or stainless steel tube.
   2. The evolution of the automotive exhaust system with materials and the development trend in the future
   1 exhaust manifold
   In the past, automobile exhaust manifold with the nodular cast iron, but with the improvement of the performance of the engine, exhaust temperature will rise, and due to the need for lightweight components, so in the use of stainless steel are increasing.
   Stainless steel used in the exhaust manifold has a system of austenite stainless steel stainless steel and iron element system. System of austenite stainless steel of the high temperature strength is good, but easy to flake oxide skin, so in terms of oxidation resistance than stainless iron element system. Iron element system, on the other hand, the oxidation resistance of stainless steel is good, but the system of austenite stainless steel is better than high temperature strength. Small thermal expansion coefficient of iron element system is advantageous in terms of thermal fatigue properties of stainless steel. Therefore, stainless steel iron element system is mainly used for exhaust manifold.
   Early in order to adapt to the needs of exhaust temperature is about 800 ℃, use YUS409D cr - TI (11). Later, due to the strengthening exhaust regulations, in order to adapt to the rise of exhaust gas temperature, adopt good high temperature performance, add 19% Cr - Nb YUS180 Cr - Nb (19). Later, due to the high temperature properties and on the basis of the demand on economy and develop the optimized composition YUS450 - MS (14 cr Mo - 0.5 - Nb, Ti). In recent years, on the other hand, as the exhaust of strengthening, exhaust temperature becomes higher, if reached 950 ℃, the previous materials will not be able to adapt to, and have therefore developed to increase the Cr and Nb content, add Mo, improving oxidation resistance and high temperature strength of YUS190 - EM (19 Cr - 2 Mo, Nb, Ti).
   Nippon steel, the production of the exhaust manifold materials (YUS450 - MS and YUS190 - EM) characteristics in Nb, Ti mixed added. This can improve the high temperature properties, especially can improve the thermal fatigue properties.
   According to the alloy elements on the iron element system stainless steel at 950 ℃ 0.2% yield strength, the influence of Nb and Ta, Mo, W is helpful to improve the yield strength, especially add a small amount of Nb, effect is better. In addition, Mo and Nb in solid solution state contributes to the high temperature strength. Nb at high temperature for a long time, however, in use process will precipitation carbides and laves phase, solution Nb declines over time, high temperature strength will also decline. Therefore, in order to suppress the solid solution quantity of Nb falling, falling under the high temperature strength of Nb - Ti mixed added was studied. According to Nb alone to add steel and Nb - Ti mixed added 0.2% yield strength of steel at 900 ℃ 900 ℃ with the curve of the aging changes, Nb, Ti mixed add aging drop strength with small steel. This is due to the added Ti inhibits bulky Nb carbides precipitate (M6C).
   14 cr department Nb added individually with high strength steel and Nb - Ti mixed added steel after constraint rate change of thermal fatigue test results show that in 100% of constraint conditions, Nb, Ti mixed sometimes add steel high temperature strength, low thermal fatigue life shorter than Nb adding steel separately. But, when the constraint is 50%, Nb, Ti mixed add thermal fatigue life of steel will be greatly extended, more than Nb added individually thermal fatigue life of steel. Can think this is because when the constraint rate lower, will increase in the overall thermal fatigue strength, due to the extension of the holding time at high temperature, if it is in using the Nb - Ti mixed added steel, it can be suppressed by adding Ti Nb carbides precipitate, decreased under high temperature thermal fatigue strength decreases.

Actual auto exhaust system components materials by the influence of the surrounding parts, and under the constraints condition under repeated thermal lag, so its thermal fatigue properties is very important, but in practice, cannot be bound rate (100%), so in the case of constraints, low rate Nb - Ti mixed added to improve the thermal fatigue properties is very effective.
As is the development trend of future, exhaust temperature will continue to rise, so you need to high temperature and better thermal fatigue properties of materials. In addition, because of the exhaust manifold with complex shape, so hope to improve the formability, make the process easier.

   2 the main muffler

   All kinds of stainless steel condensate corrosion resistance of the comparison results show that in saline content under the condition of coexistence, prone to corrosion. For inhibiting corrosion in this environment, effective solution is to add Cr and Mo, and based on this consideration, is to accelerate the development of materials.
   As in severe corrosive environment in and outside of the main muffler with material, ever use hot dip aluminized steel sheets and SUS410L and YUS409D, but is now higher corrosion resistance of stainless steel shall be used for substitution. First USES a high Cr, corrosion resistance, good YUS180 YUS180S, raise the workability and reduce the Si content.
   Followed, as the actual muffler with the development of materials in corrosive environment evaluation, put forward various corresponding evaluation test method of stainless steel. Nippon steel is put forward half an immersion heating cycle test method (hereinafter referred to as the NSC test method). In this way to close to the actual environment of silencer with materials to evaluate the corrosion resistance. Result shows that adding Cr and Mo can effectively inhibit local corrosion occurred in stainless steel in the cooling liquid, so the developed YUS436S (17 Cr Mo - 1.2 - Ti). Characteristics of the steel is by optimizing the addition ratio of Cr and Mo, inhibits decided silencer life the occurrence of pitting corrosion and expansion, in addition to fixed carbide, chose not to add the Nb, but Ti is added, so the efficiency of ordinary steel production processes (in tandem mill cold rolling and continuous annealing, pickling) can produce. At present, the YUS436S is widely used for exhaust system with spare parts.
   Recently, due to the need to reduce costs, is to look at the actual muffler corrosion environment, with the liquid under the corresponding material selection, some have begun to use YUS432 (17 cr Mo - 0.5 - Ti).
   For aesthetic and anticorrosion, and some shells or end plate adopt aluminum plating processing. Can use YUS409D, YUS436S and YUS432 aluminized steel sheet (NSAIYUS409D NSAIYUS436S NSAIYUS432).
   conclusion
   In this paper, the automobile exhaust system with the development of stainless steel parts is introduced. Can be predicted is referred to as a new generation of car revolution of the popularization of fuel cell car still needs a period of time, so the current internal combustion motors still dominate. In this case, the purifying exhaust gases from the cars, reduce weight, reduce fuel, protect the environment is undoubtedly auto makers should shoulder social mission. To accomplish this mission, auto makers to produce cars with raw material properties will become more and more demanding. Although arguably automobile exhaust system is stainless steel has a basic implementation, but the future will use higher performance and lower price of materials. In addition, two rounds of motorcycle exhaust system of stainless steel will also further development in the future.