Microstructure Characterization Of TRIP590Steel After Rapid Heat-treatment

With the development of automobile industry, three urgent issues must be facing including the fuel consumption, safety and environment. The most effective measure is using lightweight materials with high strength to reduce fuel consumption and emissions of the automobiles, without losing the safety regulations of automobile. The TRIP (Transformation induced plasticity) steel has high strength as well as good elongation, which provides direction for the trade-off between strength and plasticity. There is a potential to solve the aforementioned questions when the TRIP steel is used as the plate material of automobile.This study mainly focuses on the characterizations of the microstructures for TRIP590steel before and after tensile tests, which has been treated by continuous annealing and salt-bath. Microstructures have been observed by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The contents of retained austenite have been investigated by X-ray diffraction (XRD) and Vibrating sample magnetometer (VSM). Mechanical properties of different phases in the heat-treated samples have been characterized by micro-indentation and nano-indentation, respectively. The main results are presented as follows:(1) After salt-bath treatment, the microstructures mainly consist of the equi-axed ferrite, bainite and retained austenite. The size of retained austenite is smallest and regularly distributed among all the phases. The blocky retained austenite around the grain boundaries of ferrite is unstable, easily transformed into the martensite phase under the stress. On the other hand, the granular retained austenite located at the interior of the ferrite grains is very stable, which is beneficial for inducing the TRIP effect during deformation. Under stress, retained austenite transformed into ε martensite and al martensite. And the dislocation density is higher in the ferrite grains in comparison with other phases. The misorientation angles of most grain boundaries exhibit as the small angles. (2) For the sample treated by continuous annealing, the microstructures mainly consist of ferrite, bainite and martensite-austenite (M-A) complex phases. The M-A irregularly distributed in the ferrite matrix. Keeping the annealing time constant at60s, the size of M-A block increases with increasing annealing temperature while the content of ferrite decreases. SEM observations indicate that nucleation site of micro-voids should be inclusions and the crack may be inactivated by retained austenite.(3) The macro-hardness is related to the content of each phase in the sample. The high hardness of the sample treated by continuous annealing can be due to the martensite resulted from the subsequently quench. A highest hardness is3.54GPa for the ferrite in the sample treated at780℃for60s while a highest hardness is10.94GPa for martensite phase in the sample treated at850℃for60s.(4) The content of the retained austenite is as high as16.5%and the product of strength and ductility is26448MPa·%for the sample annealed at800℃for120s plus keeping at400℃for300s. The highest product of strength and ductility is31798MPa·%for the sample annealed at850℃for120s plus keeping at450℃for300s, resulting in a volume fraction of14.8%for retained austenite.(5) The content of retained austenite for the sample annealed at850℃for120s plus keeping at420℃for300s is determined as7.7%,13.6%and16.0%by using electron-backscatter-diffraction (EBSD), XRD and VSM techniques, respectively. The data from EBSD measurement are not reliable because of some factors including the preparation of specimen and the selection of experimental parameters as well as observation area. There is a small difference between the results from XRD and magnetization measurements, due to the fact that the relatively large areas are selected to observe during analysis. Consequently, the results can represent the sample’s real state and XRD and VSM are more reliable and sensitive to detect the content of retained austenite in the TRIP steel after heat-treatment.