| This paper deals systematically with the microstructure and mechanical properties of hot-deformed and heat-treated AerMet100 steel. The optimal technology parameters for the hot compressive deformation and heat treatment processes of AerMet100 steel have been obtained based on the experimental data and theoretical model. An ideal hot deformation microstructure was supplied for the following heat treatment of AerMet100 steel. Effect of alloying element content on austenitic deformation resistance and aging precipitate evolution were also characterized on basis of first-principle density functional theory.The ternary-quadratic orthogonal design method was employed to arrange the hot compressive deformation parameters of AerMet100 steel. The deformation parameter ranges of hot compressive deformation temperature, strain rate and strain were selected as 878℃～1121℃, 0.061s-1～16.4s-1and 0.046～0.654, respectively. The results show that the effect of pretreatment mode on the stress-strain curves and recrystallizing grain morphology is little. The flow stress increases and the recrystallizing grain size decreases with reducing deformation temperature or increasing the strain rate and strain. For the true stress-strain curves, the serrated fluctuation caused by PLC effect becomes much more obvious with the increase of strain rate. For the experimental steel, the hot deformation activation energy Q=483kJ/mol and the constitutive equations for recrystallizing grain size d and flow stressσas a function of Zener-Hollomon parameter Z have also been obtained as follows: d = 1660.5Z-0.126 andσ= 145ε0.158 ln{( Z (/ 2.03×1019))1/8.58 + (( Z(/2.03×1019)) 2/8.58 +1)1/2}, respectively.The mathematical models of the flow stress or recrystallization grain size as a function of deformation parameters such as temperature, strain rate and strain have been established by means of BP Neural Network (BPNN). The results indicate that the BPNN models have higher precision for predicting the flow stress and recrystallization grain size under this experimental conditions compared with ternary-quadratic orthogonal regression ones.The comprehensive quality index function for evaluating the flow stress and recrystallization size has been put forward during hot compressive deformation of AerMet100 steel based on a linear processing of the double step objective function.The quality index function optimized resulting from sequential quadratic programming method is employed to predict the hot compressive deformation parameters such as final consecutive hot deformation temperature, etc.. A set of hot deformation parameter for 1100℃×1h solution-treated AerMet100 steel was given as following: the continuous hot rolling deformation temperature, strain rate and strain are in range from 1100℃to 905℃, 10s-1 and 0.6, respectively. TEM observation show that the un-dissolved carbides, twin-martensite and micro-defects in the matrix of as-received AerMet100 steel can be deleted and the final mean grain size is refined around 2.2μm using the hot rolling parameters mentioned above. The solution-treated and hot-deformed microstructure is a foundation of the following heat treatment of AerMet100 steel.Further, the microstructure evolution and mechanical properties of AerMet100 steel were investigated under different quenching and aging conditions. The effect law of the quenching temperature on hardness for the same austenizing time (60min) was obtained as follows: the hardness increases and then decreases with enhancing quenching temperature, the highest and lowest quenching hardness have been obtained to be HRC49.8 and HRC47 corresponding to 885℃and 920℃quenching of AerMet100 steel, respectively. The microstructure and mechanical properties of 885℃×60min quenched AerMet100 steel aged at 460℃and 480℃for different time were also revealed. Based on the limited strength and toughness indexes for the experimental steel, two kinds of process 460℃×300min and 480℃×60min aging were obtained, respectively. The products of strength and toughness index are 105.2 and 119.1 for 460℃×300min and 480℃×60min aging. Finally, the optimal heat treatment technology was selected as 885℃×90min quenching + 480℃×60min aging. The tensile strength, elongation, reduction of area and impact toughness of AerMet100 steel heat-treated using the selected process mentioned above are 1991MPa, 19%, 65.8% and 47.6J/cm2, respectively.TEM observations indicate that the type of carbides precipitated in martensite lath for quenched AerMet100 steel changes at different aging temperature for the same time or at same temperature for different aging time. The rod-shaped Fe3C carbides precipitate in martensite lath when the aging is conducted at 425℃for 60min. However, for 480℃×30min aging, the finer and dispersive Fe2-xCx phase with an orthogonal structure precipitate in martensite, and the Fe2-xCx carbides disappear and M2C carbides precipitate when the aging time is prolonged further to 180min. The EDS results indicate that the Mo content in the matrix decreases with increasing aging temperature or prolonging aging time. During 480℃aging, the Cr content in the matrix decreases at the beginning and then increases with prolonging aging time, it shows that the Cr element play an important role in the formation of M2C carbides. The evolution tendency of M2C carbides could be (Cr, Mo)2C→(Mo, Cr)2C→Mo2C during aging of AerMet100 steel.The hot deformation resistance of the AerMet100 steel was characterized and the effects of the alloy element M (M=Co, Ni, Cr or Mo) content and its occupying site on the Mulliken population, total energy and coherent energy of austenitic crystal were also investigated on basis of first-principle density functional theory. The results show that when Ni replaces Fe atom at the apex angle or Co, Cr and Mo replaces the Fe atom at the face-centered site, the lower the alloy element content, the less the Mulliken population, which lead to an increase in the total energy of austenite and a decrease in the coherent energy among atoms of the crystal, i.e., the deformation of austenite is easy.The electronic structure of phase (Fe, M)3C precipitated at lower aging temperature has been calculated. The results show that the alloying element Co and Ni dissolved in Fe3C decrease the formation energy of (Fe, M)3C phase, the density of state for the atom at Feg position changes little, while the peak of spin-down-state density for the atom at Fes position drifts down and is below the Fermi surface, which weaken the interaction between the atoms at Feg and Fes position. The main reason is that the incorporation of Co and Ni reduce the stability of (Fe, M)3C phase. The results from the calculation of electronic structure of (Mo, Cr)2C with different Mo content show that the crystal cell volume increases, TDOS decreases at Fermi level and the state density increases at the low energy level with increasing Mo content in (Mo, Cr)2C phase, which result in an increase in the stability of (Mo, Cr)2C carbides.The calculated results show that the formation energy of the phases Fe2C and (Fe, M)3C is higher than that of (Mo, Cr)2C phase and thus they is metastable phases, while (Mo, Cr)2C phase is a stable phase. The stability of (Mo, Cr)2C phase increases with increasing Mo content in the structure. The results mentioned above are consistent with those from TEM observation, that is, the (Fe, M)3C carbides precipitated at early aging dissolve gradually with increasing aging temperature and the (Mo, Cr)2C phases precipitated at higher temperature evolve finally into the stable Mo2C phase with prolonging aging time, which reveal the evolution process of precipitating phases during aging of AerMet100 steel based on the electronic structure calculation. |
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