Grain Boundary Character Distribution And Its Effect On Ductile-brittle Transition Temperature Of Interstitial Free Steel

In recent years, as the development of metallurgical and automobile industry, interstitial free steel(IF steel) has been widely used in industrial manufacturing, and it is known as the third generation of deep drawing steel after the “rimmed steel” and “killed steel”. The IF steel, with a perfect deep drawing performance, achieves its high strength by solid solution strengthening. At the same time, however, the solute atoms tend to segregate to grain boundaries thus causing a sharp increase in brittleness. Low angle grain boundaries and low Σ value CSL grain boundaries both have strong grain boundary binding force for their low grain boundary energy, which is benenfit to its ductility. This article aims to study the influence of grain boundary character distribution, texture and phosphorus’ s segregation at grain boundaries on interstitial free steels by means such as Electron Back-Scattered Diffraction technique, Scanning Electron Microscope and Auger Electron Spectroscopy.In our study the samples were firstly annealed at 700℃ for different time. We found as the time went on, the percentage of low angle grain boundaries significantly decreased. CSL grain boundaries had the highest percentage of 16.1% when annealed for 2 hours; as the time went on, a mutual conversion between-<110>//RD texture and-<111>//ND texture was found. The segregation of phosphorus increased at first to reach the maximum at the annealing time of 2 hours then decreased, which conformed to the regular pattern of non-equilibrium grain boundary segregation of phosphorus. The segregation of phosphorus coincided with the percentage of random large grain boundaries, which verified that phosphorus is not capable to segregate at low angle grain boundaries and CSL grain boundaries. We also found that steel’s hardness changed opposite to the pattern of grain size. After 200 hours of aging treatment under 550℃, we found certain grain orientations grew constantly, while the percentage of low angle grain boundaries and CSL grain boundaries increased slightly. That the texture in IF steel was closely related to low angle grain boundaries and CSL grain boundaries was also found. The DBTT of aged samples coincided with the percentage of random grain boundaries, while the difference of grain size and hardness lead to diffenrent DBTT of samples with the same grain boundary character.The samples were treated at 700℃, 750℃, 810℃ and 830℃. We found that as the tempereture went on, the percentage of low angle grain boundaries decreased. Also we learned that the percentage of CSL grain boundaries is related to the grain size and uniform, and high annealing temperature benefits to improving the uniform of γ texture. The segregation of phosphorus at grain boundaries related to the percentage of random high angle grain boundaries which indicated that the segregation of phosphorus and grain boundary character distribution are the main factors of DBTT, while the grain size and hardness are the secondary factor.