Many studies propose thermomechanical routes to improve HSLA (high strength low alloy) steels by microstructural modification but ignore the study of occurrence of CSL boundaries. The occurrence of these special boundaries is influenced by thermomechanical treatment in which the modification of misorientation profile is related with the increase of Σ3 and Σ11 boundaries and inhibition of Σ5 Σ7 and Σ9 ones. The formation of Σ3 boundaries, beneficial to improve strength, is highly stimulated by this treatment in which deformation plays an important role in the development of CSL boundaries.

Phase transformations in API 5L X80 steel were studied in different thermomechanical sequences with a view to increasing the fracture toughness of this steel. Dilatometry tests performed on the quenched steel detected a phase transformation occurred, during heating, in the temperature range 593-618 K. This phase transformation was identified as the dissolution of M-A islands. Based on preliminary dilatometric tests, ten thermal and thermomechanical treatments were performed on X80 steel samples. Initially, the material was hot rolled and quenched and only quenched. On the material without deformation, aging was also performed at temperatures of 603, 673, 723, 773, 823 and 873 K. These treatments resulted in the formation of the acicular ferrite constituent, among others. Tensile tests showed that the aging treatments produced reductions in yield strength and increases in the elongation and toughness of X80 steel. All the treatments resulted in an increase in the tensile strength of steel.