| Titanium resources are very abundant in Chin and its price is relatively cheap as compared to niobium and vanadium. Ti addition can produce precipitation strengthening and grain refinement in steels, which has the potential to develop high strength and toughness steels. The test steels are based on the ordinary low carbon steel and microalloyed with appropriate amount of Ti element. Referring to the production process of titanium micro-alloyed steels at Pearl River Steel and combing with thermo-mechanical control process, the austenite recrystallization behavior and microstructure evolution were simulated under different deformation parameters and different cooling process through gleeble-3800 simulation experiments in laboratory, respectively. Also, the precipitation in ferrite for isothermal treatment was simulated. With the help of optical microscope, micro-hardness tester, transmission electron microscopy(TEM) and Matlab, this paper studied and analysed the characters of recrystallization, microstructure revolution and precipitation mechanism of carbonitride in ferrite of titanium micro-alloyed steels under different process. The experimental results show that:(1)The austenite reasonable heating temperature of titanium micro-alloyed steels is about1200? with a 5-minute holding time. Stress- strain curves were presented in two forms, namely, dynamic recovery and dynamic crystallization by 50% single- pass compressive deformation when deformation temperature is 1100 to 850?and strain rate is in the range of 0.025 to 1.0s-1. At low temperature and low strain rate, the strain rate has little effect on the deformation resistance, and the deformation temperature plays a decisive role. Dynamic recrystallization grains were refined with the increase of the degree of deformation and the refinement effect was weakened after reaching the steady state strain. The grains are reduced from 99.5 um to 17.4 um when the deformation temperature is 1050? and the strain rate is 0.1s-1.(2)The deformation activation energy of titanium micro-alloyed steel is Q=427.85 k J mol-1. The peak stress and peak strain are both in linear relationship with lnZ and increase with increasing of the lnZ. The model of critical parameter of dynamic recrystallization and thermal deformation resista nce are established by nonlinear regression according to the degree of deformation, deformation temperature and strain rate. The thermal deformation resistance models are in good agreement with the experiment results.(3)Whether deformation or not, bainite could be obtained by increasing the cooling rate(V > 3?/s), which indicates that the Ti micro-alloyed steel is easy to obtain bainite transformation. With the increase of the cooling rate, the phase transformation microstructures are changed from the granular bainite to the slab bainite. Deformation promotes the nucleation of bainite, which refines the bainite structures.(4)Deformation makes the dynamic continuous cooling transformation curve moving upward, which improves the starting temperature and accelerates the transformation speed of the phase transformation; It enlarges the region of proeutectoid ferrite phase and induces the pearlite transformation. When the cooling rate is less than 1?/s, the microstructure of deformation austenite is proeutectoid ferrite and pearlite. However, transformed microstructure is ferrite with a small amount of granular bainite without deformation.(5)The test steel was interphase precipitation and the maximum nucleation rate occurs at about 5 to 10 min when isothermally holding at 600?. Two different morphologies precipitations were present, which the particles diameter were 10 to 20 nm and about 5nm.(6)The maximum precipitation strengthening was obtained after isothermal treatment at 600? for 1h. If heat insulation continues, precipitation strengthening effect was weakened. |
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