Microstructural refinement by Cu addition and its effect on strengthening and toughening of sour service line pipe steels

Abstract

The Mn content of sour service line pipe steels is desirable to be low because Mn forms the elongated MnS which acts as nucleation sites of hydrogen induced blister cracking, and the segregation streak which provides the propagation path of the cracking. On the other hand, Mn is one of the most effective and economical alloying elements to strengthen and toughen. This study investigates the effect of Cu as an alternative to Mn to strengthen and toughen the line pipe steels containing low Mn. The Cu addition up to 1.7 mass% simultaneously increases yield strength and decreases Charpy transition temperature, which effect of Cu is attributable to grain-refinement. However, the Cu addition over 2.0 mass% substantially raises the transition temperature with increasing the yield strength, which adverse effect is attributable to the precipitation of Cu during cooling after hot rolling. Analyzing the results of the isothermal holding after hot deformation and the continuous cooling after hot deformation provides the mechanisms of the grain-refinement by solute Cu as follows. The solute Cu in austenite retards the recrystallization of hot deformed austenite, which retains more non-recrystallized austenite enhancing ferrite nucleation. Moreover, the solute Cu in austenite suppresses the austenite-ferrite transformation, which decreases the transformation temperature and thereby prevents formed ferrite grains from coarsening.