Microstructural evolution of two-way cold rolled 12Cr ODS steel under 1073–1373 K annealing

Abstract

Microstructural evolution and hardness variations of cold rolled 12Cr ODS steel with 60%–90% thickness reductions after 1073–1373 K annealing were systematically studied by electron backscatter diffraction technique, transmission electron microscopy, and Vickers hardness measurements. Recovery took place with heating up to 1273 K in specimens with 60% thickness reduction accompanying with gradual hardness decrease, and followed by recrystallization at 1373 K with an evident hardness reduction. By contrast, regarding specimens with 75% and 90% thickness reductions, recrystallization was observed at 1273 K which is 100 K lower than that of 60% thickness reduction, and 400 K lower than that of as-fabricated specimen without further cold rolling, indicating that a higher thickness reduction stimulated the onset of recrystallization at a lower temperature. Furthermore, partial recrystallization was characterized at 1073 K and 1173 K in the specimen with 90% thickness reduction, which produced equiaxed grain bands along the final rolling direction with none/lower density of oxide particles, and full recrystallization occurred at 1273 K and above, forming coarse elongated grains with high density of oxide particles and a smaller fraction of equiaxed grains. These bimodal grains were probably ascribed to heterogeneous distribution of oxide particles in the matrix, which were partially dissolved during cold rolling process, and re-precipitated after the thermal annealing.

Keywords

• ODS steel
• Two-way cold rolling
• Recrystallization
• Bimodal grains
• Oxide particles