Study On Selective Oxidation Model Of Advanced High Strength Steel During Annealing Treatments

With high strength and excellent elongation,advanced high-strength steel(AHSS)can achieve the purpose of the weight loss,energy saving and the improvement of collision performance in the automotive industry.Strength,formability,weldability and corrosion resistance are the required properties of advanced high-strength steels(AHSS)for automotive applications,with the alloying elements adding to these steels to enhance the strength while maintaining sufficient ductility.As a result of the inevitable presence of water vapour in the industrial annealing lines,the alloying elements such as Mn,Si will diffuse to the surface of the steel and be oxidized during annealing.Since alloying element oxides affect the wettability of zinc liquid on steel sheet and generate leakage point during the galvanizability processes,which is the biggest difficulty for the production of high-strength galvanized steel.The concentration distribution of the alloying elements can be predicted theoretically through the research on mathematical model of selective oxidation.Obviously,this study can provide theoretical basis for the design of alloying elements and the development of annealing process,and it has important theoretical and practical significance to improve the properties of the advanced high strength steel.Firstly,we solve the isothermal annealing of Fe-Al binary alloy with the Wagner model,and obtain the concentration distribution of Al.Because the solubility product of AL2O3 is low,A1 element diffuse to the surface of steel plate and generate constant AL203 in the internal oxidation zone,and the concentration of Al deep in the steel is the core concentration.Then considering the oxide solubility product,establish the extended model of Wagner model,and solve the isothermal annealing of Fe-Mn binary alloy.Because the solubility product of MnO is relatively large,the concentration value of the Mn element decreases dramatically and then increases gradually to equal to the core concentration of Mn along the depth direction in the internal oxidation zone.This paper presents a new model for simultaneous difusion and precipitation of alloying elements in the matrix,and the model basically stands as an extension of the classical Wagner model for internal oxidation of steels.This model allows for dependence of the diffusion coefficients and solubility products on temperature,allowing for the possibility of redissolution of the precipitate formed.The numerical scheme is based on explicit 1D finite differences,in spite of its rusticity and the restrictions on the time-step,is more efficient than the strategies based on the finite-element method because of its convergence in the iterative process.The diffusion equation discretized in time using an explicit Euler scheme of order 1 and in space using finite differences and combined with the initial conditions and boundary conditions,resulting a numerical model to simulate concentration depth profiles.The numerical results agree with the analytical solution well,which indicates that the numerical method is feasible and correct.Finally,modeling internal oxidation of the TWIP steel under different annealing conditions with the numerical model of internal oxidation which has already been established before.The greater the partial pressure of oxygen,the greater the oxidation zone of Al elements,and the smaller the concentration of the alloying element Al in the inner oxidation zone;on the contrary,the greater the maximum value of Mn element concentration.