1 Preface
In order to meet the requirements of environmental protection and automobile exhaust emission standards, the existing ferritic stainless steels for exhaust manifolds will not meet the requirements of high temperature mechanical properties and oxidation resistance. It is necessary to design new alloy systems based on the understanding of the alloying mechanism, and deeply understand their high temperature service and degradation behavior, and develop a new generation of resource saving heat-resistant ferritic stainless steels. Supported by the key project Research on Alloying Mechanism and Key Properties of New Generation High Temperature Resistant Ferritic Stainless Steel for Automobile (Project No.: U1660205) of the National Natural Science Foundation of China & Baowu Group Joint Research Fund for Iron and Steel, RAL, coordinating with Shanghai University, has carried out relevant basic research. This paper briefly introduces the alloy design of new heat-resistant ferritic stainless steel and the research progress of Ce, W and other elements on high-temperature oxidation resistance, welding performance, mechanical properties, formability and corrosion resistance.
2 Alloy Design of High Temperature Resistant Ferritic Stainless Steel
Multi element alloying is the main idea of alloy design, which aims to improve the high-temperature comprehensive properties of ferritic stainless steel. When using above 800 ℃, increasing Cr content alone has little effect on its high temperature intensity, but it can take advantage of the high temperature solution strengthening effect of Nb, W and Mo.In terms of improving the high-temperature oxidation resistance of the alloy, the reaction element of rare earth elements has a better effect. Adding a small amount of rare earth elements (such as Y, La, Ce and Hf) can effectively reduce the oxidation reaction rate of the alloy at high temperatures and improve the adhesion of the oxide film.Therefore, it is a feasible idea to add a small amount of rare earth elements in ferritic stainless steel to improve its high-temperature oxidation and hot corrosion properties. It is an important direction to explore a new generation of heat-resistant ferritic stainless steel for high temperature components such as automobile exhaust manifolds by regulating Nb, Ti, Mo, W, rare earth (Ce) and other alloy elements and their contents, and making use of the synergistic effect of these elements on high temperature service performance. In this study, based on 444 type (19Cr-2Mo-Nb Ti) ferritic stainless steel, alloy elements Ce and W were added, and the contents of Ce, W and Mo were comprehensively adjusted. A series of new heat-resistant ferritic stainless steels for automobile exhaust manifolds were designed. The optimized composition range of ferritic stainless steels was obtained by comprehensively investigating their various properties.
3 Effect of W and Ce on High Temperature Oxidation Resistance of Ferritic Stainless Steel
The high temperature oxidation resistance property of the newly designed high temperature resistant ferritic stainless steel was tested in air and simulated automobile exhaust respectively. The results show that~0.05wt.% Ce stainless steel can significantly reduce its oxidation reaction rate, the oxides generated on its surface are more uniform, fine and show excellent adhesion, and the number of defects at the oxide film/matrix interface is greatly reduced. The sectional morphology of the oxide film of the test steel after being oxidized for 5h at a constant temperature in the synthetic tail gas environment at 1100 ℃, it can be seen that the oxide film generated in the test steel added with Ce is thinner and denser, and there are no cracks, cavities and other defects in the oxide film. Compared with the test steel without Ce, its high-temperature oxidation resistance is significantly improved. When adding Ce or composite adding Ce and 0.5wt.% W, the spinel type oxides (Mn, Cr) 3O4 formed on its surface are more dense, which can effectively reduce the volatile loss of Cr and inhibit the diffusion of reaction elements. The combined addition of Ce and W can reduce the amount of Laves phase dissolved at high temperature, and its precipitation at the intragranular prat and grain boundary can prevent the oxide film from growing inward and the diffusion of reaction elements, reducing the rate of high-temperature oxidation reaction. However, when W addition amount reaches 1.0wt.%, a large number of Laves phase will be formed at the interface of the oxide film/matrix, which significantly reduces the adhesion of the oxide film.
The precipitates near the oxide film/matrix interface will affect the diffusion of the elements forming the oxide film, and have an important impact on the growth and failure mechanism of the oxide film. The formation of the second phase with oxidation resistance near the oxide film/matrix interface will help to improve the high-temperature oxidation resistance property of the alloy. When the oxidation temperature is 1000 ℃ and 1050 ℃, Laves phase can form near the oxide film/matrix interface of ferritic stainless steel, which can be attributed to the selective oxidation induced precipitation of Cr. The selective oxidation of Cr element generates Cr2O3 oxide film, which will lead to the segregation of Nb element near the oxide film/matrix interface and reach the critical Nb concentration required for Laves phase precipitation. Therefore, the possible precipitates and their contents should also be considered in alloy design. The deep understanding of the high-temperature oxidation behavior and mechanism of multi-element alloyed high temperature resistant ferritic stainless steels will play a key role in the design and application of new high temperature resistant ferritic stainless steels.
4 Effect of W and Ce on Welding Properties of Ferritic Stainless Steel
Although ferritic stainless steel has a broad application prospect due to its excellent corrosion resistance and low cost, its application as a structural material is limited because of the reduced plasticity and toughness of its weld and surrounding heat affected zone caused by fusion welding. Therefore, it is very important to inhibit the excessive growth of grain in the welding heat affected zone and obtain high-quality welding joints. The method to inhibit the excessive growth of grains is usually to reduce the growth rate (or the migration rate of grain boundaries) in the process of grain growth. The pinning effect of precipitates (such as carbides, nitrides and intermetallics) or the dragging effect of solid solution atoms are two proven effective methods to reduce the migration rate of grain boundaries. Through alloy design, the introduction of precipitates with good stability at high temperatures into ferritic stainless steel is an effective method to inhibit the excessive growth of grain in heat affected zone during welding thermal cycle.
From the grain orientation image of the HAZ structure when the peak temperature of the thermal cycle reaching 1350 ℃, it can be seen that the grain size of the HAZ of ferritic stainless steel can be refined by substituting W for Mo, while the grain refining effect of Ce on the HAZ is not obvious. The grain refining effect is closely related to the W content in the steel, and the grain refining effect is more obvious when the W content is higher. The content of Mo and W in the four groups of test steels designed are different, and their composition differences will lead to the change of precipitation behavior and chemical composition of precipitation phase. When the W content in Laves phase is high, its stability at high temperature can be improved. Due to the pinning effect, the stable existence of precipitates at the grain boundary in the heat affected zone is critical to inhibit the grain growth. The improvement of the stability of precipitates at welding temperature can strengthen the pinning effect of precipitates, inhibit the migration of grain boundaries, and further refine the grains in the heat affected zone.
5 Mechanical properties, texture and formability of ferritic stainless steels containing Ce and W
The results show that the tensile strength of all steels decreases with the increase of test temperature. However, at different test temperatures, the tensile strength increases with the increase of W content. The tensile strength of the test steel at 800-950 ℃ has little difference, and there are fine and dispersed Laves phases in the microstructure in this temperature range. At this time, the strengthening mechanism is the synergistic effect of precipitation strengthening and solution strengthening. When the test temperature is 1000 ℃, the strength of ferritic stainless steel containing W is significantly higher than that of stainless steel without W. The ferritic stainless steel added W can still precipitate Laves phase at this temperature, and its higher strength is due to the solid solution strengthening and precipitation strengthening effect of W and other elements. When the temperature reaches 1050 ℃ or above, there is no Laves phase in the steel, and the steel containing W shows higher high temperature strength mainly because of the solid solution strengthening effect of W.
Reducing the hot rolling finishing temperature can refine the micro-structure of hot rolled and cold rolled annealed plates, improve the yield strength and tensile strength of ferritic stainless steel, and also make the grain orientation uniformly distributed along the thickness direction, enhancing its anti wrinkle ability. Increasing the annealing temperature of hot rolled sheets can effectively reduce the band grains with<001>//ND orientation in hot rolled and cold rolled annealed sheets, and improve the γ The re-crystallization texture strength, which is favorable to formability. Increasing the cold rolling reduction can enhance the {223}<11-0> and {111}<1-1> texture components in the cold rolled sheet, and the γ re-crystallization texture strength. Properly increasing the cold rolling annealing temperature can reduce the banded structure in the strip, improve the uniformity of the structure and strengthen the γ re-crystallization texture strength, as well as improve the average plastic strain ratio rm of the final cold rolled and annealed sheet. If the annealing temperature is too high, the re-crystallization structure will be seriously coarsened and the structure will be uneven, and the surface roughness of the sheet after tensile deformation will be obvious. The addition of W has a positive effect on the texture development of ferritic stainless steel and can effectively inhibit the deflection of the post annealing γre-crystallization texture, and can improve the structure of the finished board and optimize the texture components. When the average plastic strain ratio rm is higher and the | △ r | value is lower, it shows good drawing performance.
6 Effect of Ce and W on Corrosion Resistance of Ferritic Stainless Steel
Ferritic stainless steel is often used in corrosive environments. For example, ferritic stainless steel for automobile exhaust system is subject to the corrosion of molten snow salt and automobile exhaust condensate, so it is required to have good corrosion resistance. The research shows that rare earth Ce can significantly reduce the number of inclusions in steel, reduce the electrode area in electrochemical reaction, improve corrosion resistance, reduce corrosion rate and improve self corrosion potential. Less number of steady-state pitting formed on the stainless steel surface with Ce addition inhibit the formation of steady-state pitting. Both Ce and W can reduce the corrosion rate of ferritic stainless steel in FeCl3 solution. After observing the pitting morphology, it can be seen that the opening of the pitting is covered by a lacy cover, showing a typical passive pitting. The addition of Ce and W does not change the self corrosion potential of ferritic stainless steel in HNO3 solution, but decreases the self corrosion current. The addition of Ce can reduce the passive current density, make the passive film more stable, and improve the polarization resistance of the passive film. The addition of W will promote the electrochemical reaction in the passivation film, prevent the stainless steel surface from entering a stable state, and improve the passivation current density. The addition of Ce and W can increase the pitting potential of stainless steel in NaCl solution, reduce the corrosion current density and improve the pitting resistance of stainless steel. Ferritic stainless steels with different Ce and W contents show a stable passivation state in neutral chlorine solution, and the addition of Ce and W improves the polarization resistance of the passive film and enhances the stability of the passive film.
7 Epilogue
Through reasonable composition design, the high temperature oxidation resistance and high temperature strength of the newly developed ferritic stainless steel have been significantly improved. At the same time, new knowledge has been gained in terms of grain size refinement in the welding heat affected zone, corrosion resistance under different corrosion media, processing and heat treatment processes, and alloying mechanism. These achievements not only lay a solid foundation for the research of the new generation of automotive exhaust system hot end materials, but also play a reference role in the research and development of other high temperature resistant materials.
The relevant research results have published 30 papers in important journals at home and abroad, such as Corrosion Science, Materials Chemistry and Physics, Oxidation of Metals, Metallurgical and Materials Transactions A, and Journal of Metals, etc., applied for 5 national invention patents, and trained more than 10 doctoral and master students.
