Research On Recrystallization And Formability And The High Temperature Oxidation Behavior Of Aluminum Contained Ultra-pure Ferritic Stainless Steel

Austenite stainless steels are gradually replaced by ferritic stainless steels and are widely usded in automobile industry and home applications due to low or no nickel content, and the ferritic stainless steels are becoming the green and environmental protective resource saving materials today. However, the mechanical properties, formability, corrosion resistance property, high temperature oxidation resistance property are inferior to those of austenitic stainless steels. So with the developments and applications of melting and refining technologies in recent years, the C and N contents in ferritic stainless steels are getting lower, hence the corrosion resistance property of ultra-pure ferritic stainless steels has been improved effectively. The addition of Al element can promote the inhomogeneous heterogeneous nucleation, increase the equixed grain ratio, and decrease anisotropy during solidification of the steels. What is more, Al can also refine the grain size after heat treatment, and improve the mechanical properties. A compact Al₂O₃ rich oxide scale can form on the steel surface when the Al content is higher than a certain value and to improve the high temperature oxidation resistance property.The effects of aluminum element on recrystallization behavior, formability and isothermal oxidation behavior at 1000 oC of ultra-pure ferritic stainless steels have been investigated. The microstructures have been studied by optical microscope, X-ray diffraction（XRD）, scanning electron microscope（SEM）, transmission electron microscopy（TEM）, electron back scattered diffracton（EBSD）, and Thermo-Calc equilibrium phase calculations. The following conclusions can be obtained:1) The addition of Al can significantly reduce the recrystallization temperature of the 429 ultra-pure ferritic stainless steels. The slightly increased average grain size after cold rolling and annealing mainly attributes to the significantly decreased recrystallization temperature and area fraction of the inclusions. The room temperature tensile strength and yield strength increase linearly with the increased Al contents, the elongation values hardly vary, but the plastic strain ratio increases first than decrases, and the steel with 0.16 wt. % Al exhibits the highest plastic strain ratio of about 1.62. The increased Al content can dramatically improve the high temperature tensile and compression mechanical properties. The inclusions evolve from single TiN particles to complex MgO·Al₂O₃-TiN particles, and the average particle size of the inclusions slightly increases with a decreased area fraction at the same time. The addition of Al can improve the pitting corrosion resistance property.2) The recrystallization texture of the surface and center layers evolves from α+γ fiber to full γ-fiber of the 429 ultra-pure ferritic stainless steels, and both the intensities of（111） [-1-12] and（111） [-2-35] increase with the addition of 0.16 wt. % Al, but the maximum intensity of γ-fiber decreases when Al content increases to 1.51 wt. %. The γ-fiber of（111） [1-21] shifts to（111） [3-52] with the increased Al content in the thickness direction, and the addition of 0.16 wt. % Al significantly increases the maximum texture intensity of γ-fiber. The γ-fiber along the thickness direction of the steel without the addition of Al disappears after room temperature tensile test, however, the γ-fiber still retains in 0.16 wt. % Al steel and the maximum intensity of <111>//ND is 4.6. What is more, the intensity of（111） [0-11] texture significantly decreases. The steel modified with 0.16 wt. % Al exhibits excellent formability due to the appearance of strong {111} γ-fiber.3) The weight gain per unit area is in parabolic relation to oxidation time at 1000 oC of the ultra-pure 429 ferritic stainless steels during pilot test. Parts of the surface regions are oxidized seriously slightly, the oxide scale exhibits a spherical morphology after oxidation for short time, and the spherical oxide scale grows together in the whole surface with the increased oxidation time. A lot of fine pores can be identified at the top of the spherical oxide scale. The surface oxide scale gets homogeneous and exhibits the granular shape with the increased Al content. The oxide scale shows the multi-layer cross-section morphology, the outer layer is in rich of Fe₂O₃ and slight Fe₃O₄, Cr₂O₃ appears under the outer layer, the middle layer is in rich of Fe₂O₃ and Cr₂O₃, and the inner layer is in rich of Fe₃O₄. The oxide scale gets thinner with the increased Al content, and improved oxidation resistance property of steels with low Al content mainly depends on the Cr₂O₃ layer. And Al₂O₃ rich outer layer forms on the sample surface when the Al content is about 1.51 wt. %, and the thickness of the oxide scale increases very slightly with the increased oxidation time. The protective, homogeneous, full and Al₂O₃ rich oxide scale contributes to the improved high temperature oxidation resistance property of ultra-pure 429 ferritic stainless steels.