Research On The Preparation Technology And Deformation Mechanism Of Automobile Light-weight Fe-Mn-Al High Strength Steel

With the intensification of global energy crisis and increasingly serious environmental pollution problems, the automobile industry has been promoting the development of light-weight design patterns step by step. As a new kind of high strength steel, Fe-Mn-Al steel has been put forward to satisfy the higher requirement for automobile material with excellent mechanical performance, lower density, and corrosion resistance. So the preparation technology, namely, the composition design, hot deformation behavior, hot and cold rolling with followed heat treatment, were investigated systematically as well as the tensile deformation behavior at room temperature.Based on the designed Fe-27Mn-11.5Al-0.95C-0.59Si composition, the steel consists of stable austenite matrix and small amount of δ-ferrite. The measured density is6.55g/cm3, an apparent reduction of16.6%in comparison with pure iron. The dynamic recrystallization (DRX) behavior of experimental steel was investigated by hot compression at the temperatures of900-1150℃and strain rates of0.01-10s-1using Glebble-1500thermal-mechanical simulator. Typical dynamic recrystallization curves were observed with chosen deformation conditions:the yield-point-elongation-like effect caused by DRX of8-ferrite at the early stage and austenite DRX at high strain. On the basis of hyperbolic sine function and fitting calculation, the calculated thermal activation energy for the experimental steel was294.204kJ/mol. The occurrence of DRX for both austenite and δ-ferrite was estimated and plotted by related Zener-Hollomon equations. The δ-ferrite morphogy was sensitive to the strain rate with banded structure at high strain rates and island-like at low strain rates. The internal δ-ferrite DRX and external austenite grain growth contribute the evolution of ferrite morphology, from banded to island-like structure.Effects of solid solution treatment on the mechanical properties and microstructure of hot rolled steel were studied. The results show that coiling temperature can affect the ductility of hot rolled Fe-Mn-Al steel by controlling the carbide precipitation at austenite boundary. Solid solution treatment contributes to austenite grain growth and δ-ferrite crashing, and increases the ductility. Whereas higher temperature and longer time promote δ-ferrite grain growth, and decrease the strength and ductility with a high weight percent. Solution treated at1050℃for1h, the steel exhibits excellent combination of strength and ductility with a product of tensile strength and total elongation of46.48GPa-%.Effects of annealing temperature within the range of850-1050℃on the cold rolled Fe-Mn-Al steel were investigated, including the microstructure, mechanical properties, and fracture behavior. Eutectoid transition of austenite at lower annealing temperature makes the steel higher κ carbides content and poor elongation during tensile deformation. Annealed at1000℃, the steel exhibits a high tensile strength of1003MPa, elongation of41.3%, and the product of41.4GPa%. Hollomon equation was utilized to analyze the multistage strain hardening behavior of cold rolled Fe-Mn-Al steel to study the relationship between annealing temperature and mechanical performance. Determination coefficient with the modified Hollomon equation was increased from0.9468to0.9995.In situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis with various strains were performed on the Fe-Mn-Al high-strength steel at room temperature to examine the microstructure evolution. In situ SEM revealed that slip bands appeared in the austenite grain at the early deformation stage, and different slip systems intersected with higher density as the displacement increased. Due to the existence of ordered phase, the δ-ferrite exhibits higher hardness and less deformation ability than austenite phase, which results into the ferrite crack propagation at the later stage. Considering the composition difference between the dual phases, the calculated stack fault energy of austenite is～80mJ/m2. The evolution of dislocation substructure with increasing strain shows typical planar glide characteristics, namely, dislocation pile-ups, Taylor lattice, high density dislocation walls, domain boundary and intersected microbands at high strains. Grain subdivision by microband intersection results in stable work hardening rate and continuous strain hardening behavior.