TISCO develops ferritic stainless steel for automotive exhaust system mufflers.
With the development of society, people have realized the need to establish stricter exhaust emission standards to protect the environment and reduce fuel consumption. Due to the excellent corrosion resistance, heat resistance, and comprehensive performance of stainless steel plates, the materials for exhaust emission systems have evolved from ordinary cast metals and aluminum coated carbon steel plates to stainless steel plates, and further developed from general characteristic stainless steel plates to high-performance special stainless steel plates.
2. Working characteristics of muffler.
The muffler is corroded by internal condensate and external melting salt, creating a harsh corrosion environment. Therefore, the requirement for material corrosion resistance in this paragraph is the main one. Especially the lower part is directly corroded by the condensate, while the upper part is corroded by the condensate vapor. The environment for etching the lower part is worse than that of the upper part.
The material has pitting corrosion and stress corrosion. The main effect of energy spectrum analysis is the erosion of solidified hydrochloric acid in organic waste gas, while the same part selected by Japan does not have pitting corrosion. The muffler is made of 436 ferritic stainless steel with no pitting corrosion.
3. The development process of ferritic stainless steel for Taigang muffler.
The development process of 3.1409L.
3.1.1 Process optimization.
Since the beginning of the last century, AOD has been used to produce 409L stainless steel plates for automotive mufflers. However, due to the high C.N and large fluctuations in the finished product, it has a significant direct impact on its characteristics; In 2004, VOD was produced and manufactured with excellent C.N control, which was improved at the beginning of the century and significantly enhanced product characteristics, gaining recognition from users.
Optimize 3.1.2 for stable elements.
To prevent stress corrosion, steel needs to add stabilizing elements to fix the C.N in the steel. The initial 409L is stabilized with Ti, which not only easily precipitates TiN and directly affects surface quality, but also causes stress corrosion during the selection process. As the understanding of Nb Ti bistability deepens, fixing N with steel and fixing C with Nb can avoid stress corrosion, as shown in Figure 2.
Develop 3.2439M.
3.2.1 Current situation of oil products.
Due to the difference in sulfur content between Chinese and foreign petroleum, using 409L abroad can meet the corrosion resistance requirements of some mufflers. However, in China, especially the lower part of the muffler, 409L cannot meet the requirement of no corrosion within a specified kilometer. Therefore, it is necessary to switch this part to higher chromium or 436L molybdenum containing materials with better corrosion resistance, greatly improving corrosion resistance and extending service life. However, there is still one issue. Due to the increasingly fierce competition in the automotive industry, choosing high-performance and low-cost materials has become a direction for car manufacturers. The 436L stainless steel plate with excellent corrosion resistance is limited due to its high price. The main material used for the car exhaust system is 409L, which needs to have a lifespan of 3 years or 60000 kilometers. Due to the high sulfur content in fuel in our country, mufflers often experience erosion damage of about 40000 kilometers during actual selection. Therefore, there is an urgent need to develop a new type of economical ferritic stainless steel suitable for China's petroleum conditions. Its corrosion resistance is better than 409L and its cost is lower than 4366L stainless steel plate.
3.2.2 Material Design.
Considering the characteristics of the corrosion environment of the muffler, increase the chromium content in the steel and add a small amount of molybdenum. At the same time, in order to achieve excellent processing characteristics and realize the dual century of Nb Ti, samples with different compositions were produced in the laboratory and subjected to etching tests in simulated condensate,
It can be seen that within the range of 11% to 17% Cr, Cr enhances the condensation corrosion resistance of ferritic stainless steel. After the content exceeds 17%, its effect is no longer significant; The effect of appropriate amount of Mo in Cr17 stainless steel plate is significant. Based on comprehensive performance comparison, 439M economical ferritic stainless steel with independent intellectual property rights has been designed.
3.2.3 Chemical composition and typical characteristics.
The chemical composition and mechanical properties of typical ferritic stainless steel are shown in Table 1.
3.2.4 Corrosion test.
(1) Stress corrosion test method: GB434.5-2000 Stainless steel plate hydrochloric acid copper sulfate etching test method. Sample state: Original state, 1200 ℃ for 5 minutes.
(2) Simulated condensation test.
① Analysis of condensate.
By removing the rust layer on the inner surface of the muffler and conducting XRD analysis, it was found that in addition to iron chromium oxide and FEO (OH), the rust layer also contains iron sulfate, indicating that sulfur in the gasoline on the inner surface of the muffler ultimately exists in the form of sulfate ions and directly participates in the corrosion process of the condensate.
② Simulate condensation design.
In the simulated exhaust system corrosion test, JFE in Japan has a reference standard for the ion composition of the solution concentration,
However, based on the actual situation of oil products in China, in the detailed analysis of corrosion damage to exhaust pipes in China, it is necessary to adjust the concentration of SO4 plasma to reflect the actual situation of corrosion solution in China as much as possible.
③ Corrosion test method.
By using the mechanism distribution function to statistically analyze the corrosion depth, predict the service life of different steel grades, and achieve the expected service life through automobile factory and road condition tests.
4. Conclusion
(1) Due to the rapid development of the automotive industry and people's recognition of the advantages of ferritic stainless steel, the development of ferritic stainless steel in automotive exhaust systems is gradually gaining momentum. The product quality has been recognized by various car models, improving the localization rate. In 2009, the total development volume exceeded 100000 tons and the market share exceeded 60%.
(2) Through continuous technological improvement, the physical quality of stainless steel plates for mufflers has reached the international advanced level, replacing imports and contributing to the revitalization of the automotive industry.
(3) By collaborating on the development of 439M with independent intellectual property rights, the research and development level of stainless steel plates for automotive exhaust system mufflers in China has been improved, creating a cooperative model between research institutions, manufacturing units, and automobile factories, and forming a perfect combination of industry, university, and research.
