| The traditional stainless steel can not satisfy actual demands with the developing of industrial technology and the needs of society, so the nanocrystalline stainless steel have been proposed at the historic moment, but these materials exhibit large improvements in strength when compared to their low ductility, which the higher strength originates from the nano/ultrafine-grained phase and the embedded coarse grains to restore some ductility in the materials. Currently, the techniques to produce nano/micron crystalline composite structure materials involve powder metallurgic method、thermal mechanical processing method and severe plastic deformation et.al, but these techniques are laborious, redundant, theequipment complex. Additionally, the method of small to large can lead to impurities easily. In this paper, the author utilize a novel procedure named aluminothermic reaction casting to produce the nano/micro crystalline composite structure in the pure304、316L stainless steels by one step The primary aim of this paper is to control volume fraction of microcrystalline and grains size by adjusting the isothermal process and rolling parameters, thereby improve the material mechanic performance.The results show that the casting composite structure304、316L stainless steel was constituted by micrograins/nanocrystalline austenite phase and a few of nanocrystalline delta ferrite phase. The deformation mechanism of casting304,316L steel was grain boundary sliding due to the small grain size of nanocrystalline and the small volume fraction of micrograins, the materials have both higher strength and favourable ductility. The composite structure304stainless steel has been investigated with isothermal heat treatments at800℃, the hardness and strength have increasd very fast, then with the heat treatment time increasing, the hardness and strength have increased snowly, but low plasticity; when the material was isothermal heat treatments at1000℃after0.5h, the hardness and tensile strength was reach to285HV and946MPa, respectively, then they were decreased gradually with the heat treatment time increasing, but the the ductility was improved more and more. The composite structure 316L stainless steel has been investigated with isothermal heat treatments at800℃, the hardness and tensile strength have increasd very snowly with the annealing time increasing, which is200HV and566MPa,respectively, when the heat treatment time reach to28h, but the ductility was just opposite; when the material was isothermal heat treatments at1000℃,the hardness and tensile strength were decreased gradually with annealing time increasing, but the ductility was improved, which was38.7%after annealing10h. The alloy after the heat treatments, the dominant deformation mechanism in the tensile process as follows:grain boundary sliding-grain boundary sliding and dislocation glide-dislocation glide, therefore the steels had the lower strength and hardness butthe best plasticity with the anneling time increasing. The micrograins/nanocrystalline composite structure304stainless steel was sensitive to the isothermal process, but composite structure316L stainless steel was relative just not.Nano/micro crystalline composite304stainless steel rolling at800℃, the hardness has been significantly improved, the tensile strength remain at a high level, but the plastic is poorer; rolling at1000℃after rolling, the hardness has decreasd, but the material in the tensile process has showed obvious yield,the strength is higher and the elongation increased quickly. Composite316L stainless steel rolling at800℃, with the deformation increasing, the hardness has been decreased, the tensile strength increases firstly and then decreased, the elongation improve continually. The result show taht single channel reduction has an impact on the surface quality of the rolling material. |
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