Toughness Anisotropy in Intercritically Rolled Steel Plates

Abstract

The toughness anisotropy in steel plates (0.08%C, 1.52%Mn, 0.3%Si, 0.055%Nb and 0.078%V) was studied in relation to the crystallographic texture and microstructural anisotropy of the material. The plates, with a ferrite –pearlite microstructure, were obtained by hot rolling in a laboratory reversible rolling mill to 66% reduction with the final rolling pass in the two-phase (g/a) domain followed by accelerated cooling to 570°C and subsequent slow cooling to room temperature (coiling simulation). Standard size Charpy samples with their long axis oriented at 0, 22.5, 45, 67.5 and 90° with respect to the rolling direction of the plate were tested at different temperatures varying from +20°C to –80°C. Microstructures and textures of the plates were studied by means of orientation scanning electron microscopy and XRD. A specific toughness anisotropy profile was observed which could not be correlated to the crystallographic texture of the plates, which all displayed very weak, almost random transformation type textures with a maximum intensity of approximately 2x random. Therefore, it was investigated whether the toughness anisotropy might be related to the microstructural anisotropy rather than to the crystallographic texture. The study of the grain size distribution in differently oriented sections together with the distribution of the pearlite zones in these sections revealed that the directional changes in the toughness could be successfully associated to these parameters. A significant increase in the absorbed impact energy from 140J to 270J, together with a remarkable decrease of the toughness anisotropy at room temperature, was observed after annealing the hot rolled samples at an intercritical temperature followed by an isothermal treatment in the low bainite region. The observed effect was explained by the replacement of the pearlite constituents by lower bainite in the grain boundary regions which produced a local strengthening of grain boundaries.