| Based on the research platform for steel grade design of the state key Laboratory of Rolling and Automation (Northeastern University) and Puyang Steel Corporation, this paper mainly studied the effect of the controlled rolling and controlled cooling process parameters of Q420 steel on the products' mechanical properties. Therefore, with the thermo-mechanical simulation and experimental rolling, the effect of the different deformation process variables on the microstructure and mechanical properties has been researched. On the basis of the hot rolling trials, the industry trials of Q420 steel were carried out and the expected goals were reached. In addition, FEM has been used for simulating rolling force in rolling and temperature field in cooling. The main contents of the article include the following:(1)The effect of deforming conditions on the stress of the steel is researched by single pass and the mathematical model of resistance of deformation was established. The critical strain was determined by two methods. The results revealed some facts as follows. The activation energy of the dynamic recrystallization is 278.964KJ/mol. The mathematical model of hot deformation is Z=εexp(278964/RT). The relation on the peak strain and critical strain with lnZ areεp=0.0365lnZ-0.7151 andεc= 0.0182lnZ-0.3599.(2) The softening behavior of hot deformed austenite of the tested steel was studied by double-pass compression method, and the influence of temperature and interval time was analyzed. The softening percentage of static recrystallization was defined by the 0.2% stress offset method. The static recrystallization activation energy of the tested steel was determined and the static recrystallization kinetics model to predict the evolution of softening was developed.(3) The static and the dynamic CCT curves of the tested steel have been determined on thermo-mechanical simulation to simulate the hot rolling processes of the tested steel. The two kinds of CCT curves have been analyzed. The effect of microstructures varying rules under different hot deformation processes has been investigated. According to the CCT curves, conclusions are drawn that the transformation point of deformed austenite is higher than the transformation point of undeformed austenite, the ferrite area is extended and the bainite area is narrowed after deformation and between, which will provide basis for working out a reasonable process.(4) The effect of deformation and cooling on the microstructure and mechanical proper-ties of tested steels were studied for the industry trials. The results showed that with the incr-ease of the starting rolling temperature of the controlled rolling pricess, the yied strength, te-nsile strength and the impact ductility in low temperature decrease but the elongation increa-se.(5) The rolling force using Sims formula and finite element method can come to a better close to the measured rolling force. The stress-strain field distribution in the rolling process was analyzed. The change rule in temperature rise phenomenon of the tested steel was obtain-ned by temperature field simulation in the rolling process.The results indicate that both the duration and the value of temperature increase with the increment of temperature difference between center and surface caused by the increase of thickness and cooling rate.(6) Through the industrial rolling trial of Q420 steel, the reason of generating defects in the steel was analyzed. The abnormalities that exist in its microstructure (bainite and mart-ensite) and wismanstaten structure in the direction of thickness, inclusions is the main reason for the low elongation. It is reasonable for the development of a rolling process to improve the method. |
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