Research On The Microstructure And Mechanical Properties Of Automobile High Strength Fe-Mn-Al Light-weight Steels

Four high-Mn light-weight steels of Fe-19Mn-(3.5, 6, 8, 11)Al-0.3Si-0.8C with different Al contents were fabricated by smelting, ingot casting, hot rolling, pickling, cold rolling and annealing, etc. The hot rolled and cold rolled high-Mn light-weight steels with tensile strength higher than 800 MPa, elongation higher than 30%, and weight loss higher than 5% were exploited successfully. The microstructure, mechanical properties, deformation mode, strain hardening behaviors and fracture behaviors of the steels were tested and analyzed by tensile test, XRD, metallography observation, TEM, and SEM fracture scanning, etc. The results are as follows:For the four experimental steels after hot rolling, with the increase of Al content, ferrite content increases, grain size of austenite decreases, yield strength and tensile strength increase, ductility and strain hardening capacity decrease. The steels after cold rolling and annealing at 850 ℃ show the similar behavior. The microstructure of the hot rolled or annealed 3.5A1 steel and annealed 6Al steel are fully austenitic. The microstructure of the other steels are constituted with primary austenite and some ferrite, but Fe2 MnAl precipitates appeare in the 11 Al steel after annealing at 850 ℃. In addition to the hot rolled 11 Al steel exhibiting a mixed ductile and quasi-cleavage fracture, the other steels exhibit ductile fractures. Some fracture dimples of the steels contain AlN inclusions. Compared to water cooling, furnace cooling tends to induce an increase of grain sizes of austenite of the hot rolled steels while the ferrite keeps the banded shape, increase yield strength and tensile strength, and decrease ductility and strain hardening capacity.The hot rolled 3.5Al, 6Al, 8Al, and 11 Al steels exhibit a three-stage strain hardening behavior. In the initial stage, all four steels show high initial strain hardening capacity. The initial strain hardening rate increases with the increase of Al content. In the second stage, the strain hardening rate decreases steadily with the increase of strain. Continuous strain hardening effect become poor with the increase of Al content. In the third stage, the strain hardening rate decreases rapidly, necking appears and causes specimen fracture.The calculated stacking fault energy increases with the increase of Al content. The stacking fault energy of the 3.5Al, 6Al, 8Al and 11 Al steels are 42 mJ/m2, 53 mJ/m2, 64 mJ/m2 and 79 mJ/m2, respectively. The main deformation mechanisms of the 3.5Al and 6Al steels are dislocation slip and TWIP effect. The main deformation mechanism of the 8Al and 11 Al steels is dislocation slip.The microstructures of the 8Al steels after annealing at 850 ℃, 820 ℃, 790 ℃ and 760 ℃ are constituted with primary austenite and some ferrite. With the decrease of annealing temperature, grain size of austenite decreases, ferrite content increases, yield strength and tensile strength increase, ductility and strain hardening capacity decrease. All steels exhibit ductile fractures. Some fracture dimples of the steels contain AlN inclusions.