Grade 91 steel forms martensite during additive manufacturing and the extent of tempering of martensite significantly affects the mechanical properties of parts.
However, the comprehensive mechanical properties of 0.062 wt.% Nb experimental steel was significantly improved at a cooling rate of 5 ☌/s. The strength of the steel was significantly improved, but the elongation of the steel was reduced. When the Nb content was increased to 0.062 wt.% and the cooling rate was increased to 5 ☌/s, the ferrite grain size was reduced from 19.5 to 7.5 μm, and the particle size of the precipitate (Nb, Ti, V)C could be effectively reduced. It was found that the main precipitates in the experimental steels were (Nb, Ti, V)C, with an average particle size of about 10–50 nm. When the cooling rate was 1 ☌/s, bainite transformation began to occur, and the amount of transformation increased with the increase in the cooling rate. Meanwhile, the ferrite phase transformation zone gradually expanded, and the pearlite phase transformation zone gradually narrowed with the increase in the cooling rate. The Nb content increased to 0.062 wt.%, and the starting temperature of the ferrite transformation decreased. The results show that when the cooling rate was within 0.3–5 ☌/s, ferrite transformation and pearlite transformation occurred in the experimental steels. The effects of niobium and composite strengthening on the phase transformation characteristics and precipitation behavior of continuous cooling transformation of high-strength rebar during thermal deformation and subsequent cooling were investigated. The study allows for future optimization of the heat-treatment temperature and time conditions of modern medium-Mn automotive sheet steels. Good symmetry and correlation between computational softwares were achieved. The computational and modelling results allowed the influence of alloying elements on equilibrium and non-equilibrium phase diagrams and microstructural and chemical composition evolutions to be studied. The influence of alloying elements, i.e., Mn and C, were studied in detail.
The simulations of the phase diagrams, the stability of the phases during simulated heat treatments, and the chemical composition evolution diagrams were made using Thermo-Calc and JMatPro material simulation softwares. The analyzed materials were lean-alloyed transformation induced plasticity (TRIP) medium manganese steels. Continuous cooling transformation (CCT) and time–temperature–transformation (TTT) diagrams were calculated to analyze the stability of phases at variable time–temperature processing parameters.
The aim of this manuscript was to study the influence of alloying elements on the phase transformation behavior in advanced high-strength multiphase steels.
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