| Understanding the hot deformation behaviors of metals and alloys is very important for designing metal forming process(rolling. forging and extrusion). During hot deformation dynamic recovery and dynamic recrystallization occur, which have important effect on the performance of the final microstructure and properties. Microalloyed steel is a kind of important common steel, in order to save energy and protect environment, it is also urgent to develop high strength steel. Nowadays, C-Mn-Al-Si or C-Mn-Al based high strength steels which reduce or remove the harmful effects of Si on galvanizability have attracted more and more attention. Studying the hot deformation behaviors of microalloyed steel and high strength steel plays an important role in the control of microstructure and properties after hot deformation.The constitutive equation plays an important role in designing and optimization of the hot forming process parameters. Current researches mostly focus on the apparent constitutive equation, which is used widely, but lack of certain physical basis. In recent years some studies have presented a physical model based on the theory of creep, but further in-depth studies are still rare for such physical model.In order to play a guiding role in the control of hot rolling process of microalloyed steels, this paper studied the hot deformation behaviors of low/medium carbon vanadium and niobium microalloyed steels and clarified the effects of carbon and micro alloying elements (V,Nb.Ti) on the hot deformation behaviors of steels. The results showed that the effect of carbon content on the hot deformation behaviors of vanadium microalloyed steel was strain rate sensitive, that at low strain rates the increase of carbon content had softening effect, and at higher strain rate the increase of carbon content had hardening effect. Different elements had different effects on apparent activation energy Q values:the increase of carbon content in V microalloyed steel and Nb microalloyed steel decreased the activation energy Q value, V micro alloying had little effect on Q value, but V-Ti micro alloying can increase Q value obviously, Nb micro alloying increased the value of Q obviously. Different elements in steels had different effects on dynamic recrystallization behaviors:the increase of carbon content in V microalloyed steel decreased critical strain and accelerated the process of dynamic recrystallization, the increase of carbon content in Nb microalloyed steel promoted the occurrence of dynamic recrystallization, Nb delayed the occurrence of dynamic recrystallization.The study of constitutive equation showed that the fitting accuracy of the physically based approach was improved through modification, the physically based constitutive model incorporating the strain effect can accurately predict the hot deformation flow stress of C-Mn steel and C-Mn-V steel,but the prediction accuracy of the physically based model for C-Mn steel was higher than that for C-Mn-V steel, which may be due to the data for y-iron was used in the present study, and C-Mn steel contained less alloying elements and was more similar to y-iron than C-Mn-V steel. Comprehensive analysis showed that through modification, the physically based constitutive model can be an alternative method to predict the flow stress in hot working, which was not only effective and simple, but also had physical and metallurgical backgrounds.The compressive deformation behaviors of a0.23C-1.50Mn-1.79Al based high strength steel were investigated. Results showed that the dynamic recrystallization behavior was dependent sensitively on the deformation temperature and strain rate of the experimental steel, increasing deformation temperature and/or decreasing strain rate would lead to more adequate proceeding of dynamic recrystallization. The hyperbolic sine equation with strain dependent constants was developed. The predicted flow stress curves by the developed equations well agreed with the experimental results, indicating that the developed equations can give an accurate estimate of the flow stress for the experimental steel. According to processing map analysis, the dynamic recrystallization zone was determined as1000-1100℃,0.01-1s-1of this steel. Microstructure observation showed that dynamic recrystallization occurred in the dynamic recrystallization zone, and in the instability zone, flow localization and necklace structure was observed. Finally, by combining the constitutive equation with processing map, we provided reference of investigating the hot deformation behavior of the experimental steel under different conditions.The compressive deformation behaviors of two0.26C-1.56Mn-1.72Si based and0.23C-1.50Mn-1.79Al based high strength steels were investigated and compared with each other in order to investigate the effect of total substitution of Si by Al on hot deformation behaviors within high-strength steels. Results showed that the flow stress of the C-Mn-Al steel is higher than that of the C-Mn-Si steel at the same hot deformation condition due to the higher solute strengthening parameters (SSP) of Al than that of Si. The flow stress constitutive equations of hot deformation for both steels were developed. The activation energy Q about 396kJ/mol for the C-Mn-Si steel was higher than the310kJ/mol for the C-Mn-Al steel, inferred that Si addition had a more dominant effect than Al on hot deformation activation energy Q, which can distinctly increase the Q value greatly. The peak strain of the C-Mn-Al steel was higher than that of the C-Mn-Si steel under the same hot deformation conditions, indicating that Al alloying delayed distinctly the onset of dynamic recrystallization. The dynamic recrystallization kinetics of the C-Mn-Al steel was faster than that of the C-Mn-Si steel due to the higher hot deformation activation energy Q of the C-Mn-Si steel than the C-Mn-Al steel. |
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