Effects of cold rolling reduction ratio on microstructures and tensile properties of intercritically annealed medium-Mn steels

The relationship between the cold rolling reduction ratio of medium-Mn steel before annealing, and the microstructure and tensile properties after annealing was investigated. The model alloy (Fe8Mn–0.2C3Al (wt.%) steel) was cold-rolled at different reduction ratios (between 0% and 60%) prior to intercritical annealing (IA) at 720 °C for 30 min. An increase in the cold rolling reduction ratio caused the nanolaminate morphology (α′ tempered martensite (α′temp) and retained austenite (γR) phases) to change to a nanoscale globular morphology (ferrite (α) and γR phases). However, the volume fraction of the γR phase, width of the nanolaminate phase, and the diameter of the nanoscale globular phase remained essentially unchanged, regardless of the cold rolling reduction ratio. The nanolaminate morphology exhibited a crystallographic orientation characterized by the absence of α′ recrystallization and austenite memory effect, that is preferable to the nanoscale globular morphology. The steel that was cold-rolled at a higher reduction ratio had a higher yield strength due to the increased slip resistance of the phases (consisting of a reduced area of the same orientation). In addition, with an increase in the cold rolling reduction ratio, the tensile strength and total elongation were improved owing to the more dynamic transformation- and twinning-induced plasticity in the nanoscale globular γR phase than the nanolaminate γR phase. Therefore, we have concluded that an optimized cold working prior to IA, will lead to improved mechanical properties of industrially produced medium-Mn steel.