Effect of deep cryogenic pretreatment on microstructure and mechanical properties of warm-deformed 7 Mn steel after intercritical annealing

Microstructure and mechanical properties of 7Mn steel undertaken different intercritical annealing (IA) with or without deep-cryogenic treatment (DCT) were investigated in this work. It is found that the additional DCT process can lead to the changes in the size and composition of austenite of the steel after IA. With the decrease of the IA temperature, the improvement effect of the DCT on mechanical properties is more remarkable. The product of ultimate tensile strength (UTS) and total elongation (TE) of the pre-DCT specimen IA at 600 degrees C (DCT + IA600) is nearly twice as much as that of the non-DCT specimen. Austenite and ferrite grains exhibit different morphologies, lamellar and equiaxed, and bimodal size distribution at different IA temperatures. With the decrease of IA temperature, the occurring of serration behavior on engineering stress-strain curves is postponed, and the density of serration as well as the mean value of work hardening (WH) rate decrease, both of which are caused by the increasing stability of austenite. The volume fraction, morphology and composition of austenite can be tailored by pre-DCT and IA process to achieve the optimum combination of the retained austenite (RA) volume fraction and stability. The DCT + IA645 specimen exhibits an outstanding combination of 1167 MPa UTS and 35.4% TE.