Effect of thermomechanical processing on anisotropy of cleavage fracture stress in microalloyed linepipe steel

The anisotropy of cleavage fracture stress and its relationship to thermome chanical processing (TMP) and microstructure has been determined by extensi ve study on a Nb, Ti microalloyed line pipe steel. 127 mm-thick pieces cut from a commercially cast slab were reduced to a 25 mm-thick plate by three different rolling schedules. The final microstructures of the rolled plate were various combinations of polygonal ferrite, bainite and deformed ferrit e, having a range of grain sizes. Four-point bend tests (L-T, L-S, T-L orie ntations), compact tension tests (S-L orientation) and tensile tests (L, T, S orientations) were carried out at -196 degrees C for each rolling schedu le. An elastic-plastic finite element method was used to calculate the stre ss and strain distributions ahead of the notches for the various orientatio ns. By measuring the distance of the cleavage fracture initiation site from the root of the notch, the cleavage fracture stress was determined for eac h orientation. The results show a significant variation in cleavage fractur e stress with orientation and TMP treatment. The anisotropy of cleavage fra cture stress increases with decreasing finish-rolling temperature (FRT), th e S-L orientation generally giving the minimum value. The TMP treatment whi ch produces unrecrystallized austenite at the FRT has the best combination of high cleavage fracture stress and low anisotropy. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.