Study On Hot Deformation And κ Carbide Control Of Austenitic Low Density Steel

Fe-Mn-Al-C low-density steel is expected to become the main direction of future automotive lightweight research due to its high strength,good plasticity and low density.In this paper,a series of experiments were carried out on austenitic low-density steel to study thermal deformation andκcarbide control under different heat treatment regimes.In this paper,austenitic single-phase low-density steel was obtained through composition design.The effects of different deformation temperatures（850～1100℃）and different deformation rates(0.01～10 s^-1)on the true stress-strain curves and thermal deformation microstructures were studied by using the Gleeble 3800 thermal simulation testing machine.The microstructure,precipitation morphology and mechanical properties of the experimental steel after rolling,solid solution,different cooling rate and aging treatment were characterized and analyzed,and the following results were obtained.The following research results were obtained:Due to the large addition of Mn element,the microstructure of the experimental steel after hot deformation and heat treatment is a single-phase austenite structure with annealing twins,the stacking fault energy is 117.7 m J/m²,and the deformation mechanism is mainly plane slip.The true stress-strain curves of the experimental steels show dynamic recovery and recrystallization curves.The peak stress increases with decreasing deformation temperature and increasing strain rate.The dynamic recrystallization and grain growth of austenite are facilitated by increasing the deformation temperature or decreasing the strain rate.The minimum average grain size of the experimental steel in the rolling state is about 12μm,the highest tensile strength is 1188 MPa,and the elongation is 33.0%;at 1100℃,the average grain size increases to 163μm,and the tensile strength is the lowest at this time.was 854 MPa,and the elongation increased to 77.9%.Intragranular nano-κcarbides exist in the austenite of the experimental steel both in the rolled state and after solid solution.As the cooling rate slows down,the size and volume fraction ofκcarbides dispersed in the grains increase continuously.Intergranularκcarbides began to precipitate under the cold oil condition.The increase of aging temperature promotes the precipitation of intragranular and intergranular kappa carbides.Aging at 700℃,the intragranularκcarbides are transformed from the dispersed granular shape to the rectangular domain shape through the secondary ordering.At 600℃,the intergranularκcarbides are gradually precipitated from the widened austenite grain boundaries in a granular form,and gradually become serrated and continuously distributed and extend into the austenite grains with the increase of the aging temperature.Intragranularκcarbides and austenite matrix have a coherent relationship between（100）_κ//（100）_γand[010]_κ//[010]_γ.And the effect of intergranularκcarbide on the strength of the material has two sides:when the intragranularκcarbides are finely dispersed,the hardness of the material can be improved with the increase of the size and volume fraction,but the strengthening effect will be weakened when a rectangular domain with an excessively large size is formed.Intergranularκcarbides contribute little to the hardness of the steel,but tend to cause stress concentration at the grain boundary,promote the propagation of crack and lead to intergranular brittle fracture,which reduces the impact property at low temperature.