Abstract
Heavy cold rolling is generally required for efficient grain size refinement in the martensitic reversion process, which is, however, not desirable in practical processing. In the present work, the influence of cold rolling reductions of 32%, 45% and 63% on the microstructure evolution and mechanical properties of a metastable austenitic AISI 301LN type steel were investigated in detail adopting scanning electron microscopy with the electron backscatter diffraction method and mechanical testing. A completely austenitic microstructure and a partially reversed counterpart were created. It was found that the fraction of grains with a size of 3 µm or larger, called medium-sized grains, increased with decreasing the prior cold rolling reduction. These grains are formed mainly from the shear-reversed austenite, transformed from slightly-deformed martensite, by gradual evolution of subgrains to grains. However, in spite of significant amounts of medium-sized grains, the tensile properties after the 32% or 45% cold rolling reductions were practically equal to those after the 63% reduction. The austenite stability against the formation of deformation-induced martensite in subsequent straining was reduced by lowering the cold rolling reduction, due to the larger grain size of medium-sized grains and the shift of their orientation towards {211} <uvw>.
Highlights
Grain-refined austenitic stainless steels are an interesting group of materials from a scientific, and commercial, point of view, due to their excellent combination of high strength and good ductility achievable in annealed condition
In the metastable 301LN steel, the austenite phase only transformed partially to martensite in the cold rolling stage
The deformation-induced martensite (DIM) fractions are plotted as a function of the cold rolling (CR) reduction under the present conditions in Figure 1, determined by a Feritscope and X-ray diffraction (XRD)
Summary
Grain-refined austenitic stainless steels are an interesting group of materials from a scientific, and commercial, point of view, due to their excellent combination of high strength and good ductility achievable in annealed condition. Martensitic reversion has been shown to be an effective method for refining the grain size of metastable austenitic stainless steels, including commercial grades, such as AISI types 304, 301, 301LN, 201LN and 204Cu [1,2,3,4,5]. Submicron-sized grains are formed during short annealing, while deformation-induced martensite (DIM) transforms into austenite. In studies targeting efficient grain size refinement, high cold rolling (CR) thickness reductions in the range of 60–90% have commonly been applied, e.g., [10,11,12,13,14], or cryorolling at subzero temperatures [1,2,13,15,16], multi-step rolling schedules [12,17], or very unstable high-Mn low-Cr alloys have been designed, such as those shown in [18,19]
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