Modeling Of Spray Forming Process And Preparation Of Ferrite Based Low Density High Strength Steel

Lightweight has been pursued in automotive industry for the demand of environment and energy cost.New lightweight materials such as aluminum,magnesium,composite material,have pose a huge challenge to traditional steels.Facing this situation,steel materials are also gradually moving forward to meet the requirement of lightweight.A new method that the reduction in steel density is achieved by adding a light element such as Al has been proposed.This new type of lightweight steel,which is called low density steel,is able to maintain or even improves the mechanical properties of the material with a low density.Low-density steel is mainly based on Fe-Mn-Al-C alloy.The alloy matrix is composed of ferrite or austenite or two mixed phases.In recent years,ferrite based low-density steel(δ-TRIP)has attracted wide attention due to its good comprehensive performance and low alloying element content.Due to the addition of Al in ferrite based low-density steel,the δ ferrite produced from the liquid phase during solidification can not be completely transformed and left to room temperature.With the slow cooling condition of conventional solidification,large amount of coarse dendrites often occur in the as-cast microstructure of ferrite based low-density steels.Besides,high alloying elements are prone to segregation under slow cooling and coarse dendrites conditions.Coarse δ ferrite dendrites cannot be eliminated in subsequent thermal processing and form a severe banded structure.These defects have detrimental effects on the mechanical properties of the material.As a rapid solidification technique,spray forming has excellent advantages in eliminating dendrites,refining microstructure and inhibiting segregation.Thus,in order to overcome the defects and improve the quality of the ferrite based low-density steel,spray forming was used to prepare the steel.Aiming at the atomization solidification and deposition behavior of spray forming and the influence of process parameters,as well as the microstructure and mechanical properties of spray formed δ-TRIP steel,the following work was carried out:In order to study the thermodynamic and dynamic behaviors of the atomized droplets in the flight process of spray forming,the solidification model including,undercooling,recalescence and segregation solidification was established based on the coupling of discrete phase model.Moreover,the dynamic behavior of the droplet impact was studied by using lattice Boltzmann method(LBM).The results show that there are huge differences in the cooling and solidification behavior of droplets of different sizes during flight.As the droplet size increases,the solidification rate is gradually reduced.The droplet cooling rate is as a power function of particle size.Besides,the average solid fraction of the droplets in the atomization cone(spray solid fraction)was calculated and analyzed.The spray solid fraction increases as approximate logarithmic function of the deposition distance.The increase of atomization pressure increases the spray solid fraction.The increase in mass median diameter and distribution index decreases the spray solid fraction.By measuring the secondary dendritic arm of the atomized powder,the cooling rate model of the USGA nozzle was established and compared with the existing model for validation.The LBM model was used to simulate the two-dimensional and three-dimensional droplet impact process.The simulation properly reproduces the impact process of the droplet.With the increase of the impact velocity,the droplet spreading radius increases linearly and then reaches a certain limit value.Over this velocity threshold,the droplet begins to enters the breakup stage.Porosity is the main defect in spray forming.At present,porosity prediction during the deposition process is difficult and of great importance.The existing porosity model is lacking,and can not predict and display the number and distribution of pores inside the deposit.Based on the particle packing and solidification feeding,as well as gas diffusion theory,the porosity prediction model of deposition process was established.Besides,the porosity model was coupled with the shape model and heat transfer model to complete the simulation analysis of porosity evolution during deposition.The results of deposit growth and heat transfer simulation show that the growth of the deposited billet is determined by the combination of spray parameters.The temperature distribution in the deposit has regional characteristics.The temperature of the deposit is affected by the spray process conditions.The increase of the deposition temperature and the mass flow rate leads to an increase in the temperature of the deposit.When the substrate is preheated,the temperature of the deposit also increases,and the temperature gradient at the bottom of the deposit will decrease.In order to validate the model,the porosity amount and distribution inside the deposit were analyzed by experiments.The results show that the predicted porosity is in good agreement with the experiment data.At the same time,the porosity model was used to study the effect of droplet impact temperature and substrate preheating on the porosity distribution in the deposited billet.The increase of deposition temperature reduces the amount of interstitial porosity,but increases the number of precipitated gas porosity.The preheating of substrate reduces the porosity at the bottom of the deposited billet,thus is able to effectively improve the quality of the bottom deposit.With the optimization of spray forming processes,the ferrite based low-density steel containing 5.8 wt.% Al was successfully prepared.Compared with the conventional casting steel,the microstructure of the as-sprayed steel is obviously improved,which consists of fine equiaxed δ ferrite grains and has homogenous chemical composition.The coarse δ ferrite dendrites in the as-cast microstructure was eliminated,and the segregation of alloying elements such as Mn and Al was greatly suppressed.The ductility of the spray formed steel is significantly improved,and the product of strength and plasticity is over 35 GPa%.Continuous cooling transformation experiments show that pearlite is formed only at a very low cooling rate,while martensite structure is easy to produce.After hot rolling and annealing,the ferrite is elongated along the rolling direction.Short rods and granular carbides distributes between the ferrite bands.After cold rolling,the fine ferrite bands further forms an orderly band structure with small spacing.The fracture of hot and cold rolled sheets mainly occurs at the interface between the ferrite and carbide bands,and inside the carbide bands.The slim carbides at the ferrite grain boundaries are easy to cause crack.After annealing,the spray formed steel still maintains a high level of homogeneity.With the refinement of the microstructure,the spray formed sheet shows a higher surface quality.Experimental observation and crystal plasticity simulations show that the refinement of the δ-ferrite matrix results in an uniform stress-strain distribution of the spray formed steel during deformation,which leads to a more coordinated micro-domain deformation;moreover,the phase transformation of the austenite distributes more uniformly.These are the main reasons for the increased austenite stability and ductility of the spray formed steel.During the annealing of the spray formed ferrite based low-density steel,the retained austenite phase coarsens and the amount of retained austenite gradually increases with the increase of the annealing temperature.Annealed at 800 °C,the steel exhibits a high product of strength and plasticity and behaves a high continuous work hardening.As the annealing temperature increases,the strength of the steel increases and the plasticity decreases.The strengthening mechanism of the steel is mainly based on the TRIP effect,supplemented by the TWIP effect.The dislocation in ferrite and austenite as well as the annealed twins in the steel contribute a significant strengthening effect in the deformation.