| Summary: | 博士 === 國立臺灣大學 === 材料科學(工程)研究所 === 82 === In this work,a series of thermomechanical processes had been
carried out in the deformation dilatometer in order to study
the effects of prior compressive deformation on the
transformation behavior of ULCB steels.It is found that prior
deformation of austenite significantly enhances the bainitic
transformation.At the same cooling rate the specimen with the
prior deformation has a higher bainite start temperature (Bs)
than the specimen without the prior deformation; in addition ,
the difference of Bs temperature becomes much larger at the
higher cooling rate.The strain energy accumulated by the
compressive deformation the driving force for bainitic
transformation.Hence,the formation of bainitic ferrite can
proceed further even until the content of carbon of the
austenite exceed To'' curve boundary,and the volume fraction of
bainitic ferrite is expected larger.A analysis has been
proposed to explain the experimental results relating to the
effect of prior deformation of austenite.On the other hand, an
isothermal experiment has been utilized to investigate the
stability of the deformed austenite.It is suggested that the
kinetics of precipitation of Nb(C,N) in deformed austenite
influences the hardenbility of bainite in local regions of the
austenite matrix. This reason can explain why the deformed
austenite grains produce finer allotriomorphic ferrite grains
in a relatively slow cooling condition.
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