In-Situ Observation of Dislocation Motions in Strain-Based-Design Steels Under Combined Environment of the Bauschinger Effect and Strain Aging

Abstract

Many researchers have developed various design concepts to fabricate high-performance line-pipe steels by combining effective fabrication methods with the addition of alloying elements to achieve the proper volume fraction of microstructural phases and desired mechanical properties. A new concept for fabricating line-pipe steels is the strain-based-design approach, which focuses on obtaining a large, uniform elongation. One of the most serious concerns with steel is the loss of their large, uniform elongation after the anti-corrosion coating process, which is the final step in the fabrication process. In the present study, the effects of deformation history and dislocation behaviors, which are associated with uniform elongation, were investigated through direct observation using an in-situ heating and straining stage in a transmission electron microscope. A thick steel plate showed variations in the grain sizes and dislocation structures along the thickness direction owing to the cooling rate and plastic-deformation history. It is confirmed that, during the collective U-ing, O-ing, and expansion pipe-shaping process, followed by the anti-corrosion coating process, dislocation motion is affected by the Bauschinger effect and strain-aging phenomenon. The Bauschinger effect could reduce the stress for dislocation motions with the aid of back stress, while strain aging could hinder dislocation motions by forming a Cottrell atmosphere.