| In view of limited material resources and energy supply together with serious environmental impact concerns at the present time, the development of high strength steels has attracted a great deal of attentions from many countries in the world. Among all the strengthening mechanisms of steels, refining the grain size to a much finer extent is more difficult and the increment of grain refinement hardening is much harder to improve when the grain size reaches about 2-3 μm, The efficiency of improving the strength by solid solution hardening is quite low. Although dislocation hardening and/or phase transformation hardening are more feasible to increase the strength of steel, they have a significantly harmful effect on the toughness and ductility. Precipitation hardening has a minimum damage on the ductility except grain refinement hardening, which is an important research direction to improve the strength of high strength steel. However, precipitation hardening has a huge potential to be explored. Conventionally, the contribution of precipitation hardening to yield strength of high strength steel is lower than 200 MPa. Improving precipitation hardening is an economic and effective way of increasing strength. Adding appropriate complex microalloyed elements into steel has two benefits:on the one hand, increasing the volume fraction of precipitates; on the other hand, refining the size of precipitates. Therefore, complex microalloyed technology becomes attractive in terms of improving the strength of hot rolled steel. The combination of the complex microalloyed elements and the thermal mechanical control processing in the steel could develop a novel high strength hot rolled steel with a large precipitation hardening which has a significant theoretical and practical value in industrial application.In this study, the Ti-V-Mo complex microalloying hot rolled steel with large increment of precipitation hardening was expected to be developed by the appropriate thermo mechanical controlled processing and through reasonable control MC precipitates of austensite and ferrite. The microstructure of Ti-V-Mo steel was characterized by means of transmission electron microscopy (TEM)、 scanning electron microscope (SEM) 、 electron backscatter diffraction (EBSD) physics-chemical phase analysis, etc. The main research contents and results are as followed.An analytical model for describing the precipitation thermodynamic of quadruple-element precipitation was built and applied in the Ti-V-Mo steel. Ti steel, V steel, Ti-V steel, Ti-Mo steel and Ti-V-Mo steel were compared in terms of the thermodynamics, kinetics and the coarsening behaviors at low temperature. Results showed that Ti-V-Mo complex microalloying design doesn’t increase the solid solution temperature of MC precipitates. More importantly, it potentially reduce the content of MC precipitated from austenite, and thus increase the MC content precipitated from ferrite. Furthermore, complex MC carbides have a strong ability to resist coarsening. Therefore, Ti-V-Mo complex microalloyed steel is the optimium microalloying system to obtain the maximum increment of precipitation hardening. The formulas of interfacial energy between MoC and austenite or ferrite as a function of temperature have been calculated. It can provide the basic data for the thermodynamics calculation of Mo containing precipitates.The influence of deformation stored energy on the kinetics of (Ti, V, Mo)C in austenite and the effect of the amount of strain induced precipitation on the kinetics of (Ti, V, Mo)C in ferrite were analyzed. The results showed that the increase of deformation stored energy of austenite could promote the precipitation of (Ti, V, Mo)C at high temperature and prevent growth of the austenite grain. In addition, raising the amount of deformation induced (Ti, V, Mo)C precipitation suitably, which could decrease the maximum nucleation rate of temperature of (Ti, V, Mo)C carbides and increase the nucleation rate of (Ti, V, Mo)C. As a result, a great density of (Ti, V, Mo)C carbides and large increment of precipitation hardening will be obtained. Moreover, the nose temperature of nucleation-temperature and precipitation-time-temperature curves were calculated to be-630-650 ℃ and 720-740 ℃ resprectively, which provided a theoretical guidance for obtaining finer ferrite grain size and large increment of precipitation hardening.The effect of finishing rolling temperature, cooling rate, coiling temperature on the microstructure and mechanical properties were analyzed. Meanwhile, the specific processing parameters for obtaining best mechanical properties have been gained. The revolution of (Ti, V, Mo)C at different stages was characterized. The results showed that under the condition that finish rolling temperature is below 800~850 ℃, cooling rate is larger than 20 ℃/s and coiling temperature is around 600~625 ℃, the comprehensive mechanical properties of high Ti-V-Mo steel is excellent. V-riched MC carbides were precipitated at 600 ℃ and 650 ℃ while rich Ti-riched MC carbide were precipitated at 500 ℃ and 550 ℃.The best combination of mechanical properties of high Ti-V-Mo steel with ultimate tensile strength of 1134 MPa, yield strength of 1080 MPa, elongation of 13.2% and uniform elongation of 6.8% was obtained at coiling temperature of 600 ℃ in laboratory by optimizing thermal mechanical control processing. The increment of precipitation hardening reached up to about 444-487 MPa, which is beyond the toplimit of precipitation hardening of conventional hot rolled high strength steel. The effect of coiling temperatures on yield strength and the influence of (Ti, V, Mo) C particles on uniform elongation of high Ti-V-Mo steel were discussed. |
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