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Eighty percent heavy cold thickness reduction and reversion transformation in the temperature range 700-950 • C for 60 s were performed to obtain the reverted ultrafine-grained (UFG) structure in 304 austenitic stainless steel. Through mechanical property experiments and transmission electron microscopy (TEM) of micro deformation of the UFG austenite structure, the tensile fractographs showed that for specimens annealed at 700-950 • C, the most frequent dimple sizes were approximately 0.1-0.3 µm and 1-1.5 µm. With the increase in annealing temperature, the dimple size distribution of nano-sized grains turned to micron-size. TEM micro deformation experiments showed that specimens annealed at 700 • C tended to crack quickly. In the grain annealed at 870 • C, partial dislocations were irregularly separated in the crystal or piled up normal to the grain boundaries; stacking faults were blocked by grain boundaries of small grains; twins held back the glide of the dislocations. In the grain annealed at 950 • C, the deformation twins were perpendicular to ε martensite. Fine grain was considered a strengthening phase in the UFG structure and difficult to break.

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The effect of precipitated phases on the pitting corrosion of a Z3CN20.09M cast duplex stainless steel (CDSS) which has been widely used in primary coolant pipes of nuclear power plants was investigated by using isothermal aging treatment and potentiodynamic anodic polarization methods. It was found that M 23 C 6 carbide and · phase precipitated at the ferrite/austenite boundaries and in ferrite of the aged steel. The pitting potentials of the specimens aged at 700°C decreased with increasing of the precipitates content. The experimental results indicated that even a few percent of precipitates (about 1.0 vol%) in this steel could also worsen its pitting resistance. The pitting of the aged Z3CN20.09M specimens developed at the ferrite/austenite phase boundaries where precipitates formed often. These effects could be directly attributed to the presence of secondary austenite at the ferrite/austenite phase boundaries, which was found to be poor in Cr by energy dispersive X-ray (EDX).

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This paper presents our latest studies on the effects of coiling temperature on the microstructure, precipitation behavior, and mechanical properties of an Nb-Ti microalloyed steel produced by endless strip processing (ESP) and coiled at different temperatures. The amounts of soluble elements were measured using inductively coupled plasma optical emission spectrometry (ICP-OES). The microstructure and precipitates were analyzed using SEM, EBSD, TEM, and electrolytic dissolution and filtration tests. The results revealed that large amounts of microalloying elements were still in solution before coiling. As the coiling temperature decreased from 600°C to 560°C, the content of acicular ferrites (AF) increased and the average ferritic grain size was refined from 2.01 μm to 1.29 μm, the yield strength and tensile strength of the tested steel increased by 22 MPa and 20 MPa, respectively, under the effect of microstructural strengthening. As the coiling temperature increased from 600°C to 640°C, the mass fraction of precipitates increased from 0.083% to 0.110% and the percentage of fine precipitates (smaller than 18 nm) increased from 12.2% to 14.7%; the intense precipitation strengthening effect increased the yield strength and tensile strength by 35 MPa and 42 MPa, respectively. Therefore, as the coiling temperature decreased from 640°C to 560°C, the strength of the tested steel decreased first and then increased while the elongation decreased steadily from 18.9% to 14.1% due to the increasing content of AF.