Microstructure Evolution Modeling Of Hot Rolled Strip Under Ultra - Fast Cold Condition

The microstructural evolution and ultra fast cooling (UFC) process of hot rolled strip steel have been studied in this paper. The study of the stress strain curve, the continuous cooling transformation behavior, and the influence of UFC on phase transformation and precipitation further improved the hot deformation equation and the deformation resistance model of the testing steel. The static and dynamic CCT curves of the testing steel were measured, which have an good guidance on the rolling and cooling process. Combining the influence of ultra-fast cooling process on phase transformation and precipitation investigated in the laboratory, the effect of UFC process on hot rolled strip steel has been initially discussed.The influence of deformation parameters on the stress-strain curve and thermal deformation conditions on the deformation resistance were studied. The results show that stress peak accompanied with dynamic recrystallization occurs at high temperature and low strain rate (low Z value). The deformation activation energy, Z parameter, thermal deformation equation and the deformation resistance model of the testing steel under dynamic recrystallization were determined by using linear regression method.The role of Nb in the continuous cooling transformation of hot rolled strip steel was studied by comparing the static CCT curve of the Nb containing steel with the steel without Nb. The influence of thermal deformation on the continuous cooling transformation of the hot rolled strip steel was studied by comparing the dynamic CCT curve with the static CCT curve of Nb containing steel. The studies show that the alloy element of Nb can not only suppress high temperature transformation, but also refine ferrite grain size, lower the critical cooling rate and expand the cooling speed range of the acicular ferrite formation, which is beneficial to the formation of acicular ferrite. The thermal deformation accelerated the austenite to ferrite isothermal phase transformation and refined the ferrite microstructure, it also increased the beginning and ending temperatures of all of the transformation products and reduced the temperature range of martensitic transformation, however, the high and medium temperature transformations were not obviously affected. The hardness of the samples after deformation are generally higher than those without deformation at the same rate, but the cooling speed range of all samples are nearly the same under same transformation.The influence of ultra-fast cooling process on phase transformation and precipitation was discussed and the results show that by using the superposed ultra-fast cooling the time for ferrite transformation was extended, which was beneficial to refine the ferrite grain size, but had little effects on bainite and martensite microstructure. The carbonitride of Nb can quickly precipitate on grain boundary, dislocation and substrate of the testing steel deformed at850℃by using UFC process. The micro alloying particles Nb(CN) would continuously precipitate and grow up during the subsequent heat treatment and the trend became larger at higher temperatures.