Study of Luders deformation in ultrafine low-carbon steel by the digital image correlation technique

The studied steel was a low-carbon microalloyed steel of type 08G2B, used for pipes with an outer diameter of 1420 mm for natural gas transportation with an operation pressure of 11.8 MPa with the chemical composition characterized by a low-carbon content (0.08 wt.%) with additions of Mn, Cr, Mo, Ni and microalloying elements (Sigma(Nb, V, Ti) approximate to 0.10 wt.%). Secondary refining and continuous casting followed by thermo-mechanical controlled processing (TMCP) with a strict temperature-reduction route and control of the post-deformation cooling rate are used to produce high-strength welded X80 and higher-grade pipes with an extremely ultrafine heterophase structure (d =1- 3 mu m) and high Charpy energy of more than 250 J/cm(2) at -40 degrees C. Usually sheet metal is not subjected to heat treatment. However, pipes can be heated by applying an anti-corrosion coating at 200 - 250 degrees C or by welding in a heat affected zone, which initiates the strain aging effect (SAE). The appearance of SAE can also occur upon prolonged exposure of pipes in the area of climatic temperatures, as well as in the course of installation and operation of pipelines, which inevitably leads to a change in the mechanical properties of the metal. Luders deformation in ultrafine low-carbon microalloyed steel X80 has been studied upon tensile testing by the digital image correlation technique. Two mechanisms of Luders deformation are shown to be observed depending on the strain ageing effect. It is determined that the mechanical behavior of the material at the stage of uniform deformation strongly correlates with the type of Luders deformation.