The Effects And Mechanism Of Rolling And Annealing On Microstructure And Mechanical Properties Of Nanocrystalline/Microcrystalline Composite 304 Austenite Stainless Steel

This paper studied the effect and mechanism of rolling and annealing temperature on the micro structure and tensile properties of 304 austenite stainless steel is prepared by aluminothermic reaction casting methods with adding superfluous Fe₂O₃. By analysis the grain size of nanocrystalline austenite, submicrocrystalline austenite and the volume fraction of ferrite. we conclude the following research results.1. With the superfluous Fe₂O₃ from 2.5% to 7.5%, the Al content in steel decreased firstly and then increased, at the same time the other concent of Cr and Ni decreased by the decreasing of adiabatic temperature. And we can also get the nanocrystalline\microcrystalline 304 austenite stainless steel. The major phase of the steel was austenite and a few ferrite still existed. And when the superfluous Fe₂O₃ was 5%,the XRD pattern shew no ferrite and all element content can meet the requirement of the 304 stainless steel.2. When the superfluous Fe₂O₃ was 5%, the steel rolled in different temperatures, the results shew no phase transformation during rolling process, and the phase also consisted of few ferrite and austenite. The volume fraction had no obvious change with different temperares, but the nanocrysytalline austenite grains transformed to sub microcrystalline/microcrystalline grains, which lead to the volume fraction of nanocrystalline decreased. At the same time, nanocrystalline grains are not thermally stable at elevated temperature due to the large amount of grain boundary enthalpy stored in the large grain boundary area, and rolling force and affect the smash of submicrocrystalline/microcrystalline. The steel strength and ductility were much improved after rolling compared with the original cast steel. When the steel rolled in 1000℃, the steel had the maximum tensile strength and yield trength but low ductility which affected by the grain size of microcrystalline/nanocrystalline composite structure.3. After the steel of superfluous 5% Fe₂O₃ rolling at 1000℃-40%-700℃-60% then annealed at 800℃ with different times, the grain size firstly increased dramatically and then increased gently, which grain growth depend on temperature. This was no new phase formation during annealing. The strength of the steel decreased after annealing but the ductility increased. When the steel anneaing at 800℃ with 8h had the best combination of strength and ductility resulted from the optimized nanocrystalliine/microcrystalline composite structure.