Effect of inclusions and microstructural characteristics on the mechanical properties and fracture behavior of a high-strength low-alloy steel

The strength and toughness properties of hot-rolled plates from three commercial heats of a high-strength low-alloy steel were investigated with respect to their intrinsic microstructural and inclusion characteristics. One heat was argon purged and contained relatively higher carbon and sulfur, whereas the other two heats, with lower carbon and sulfur levels, were sulfide shape controlled. The study revealed that although yield and tensile strengths specific to a heat were unaffected by testing direction, the anisotropy in tensile ductility was greater in steels with stringered sulfides. Despite similar grain sizes in all the steels. Charpy shelf energy and impact transition temperature were significantly affected by pearlite content and sulfide morphology and to a lesser extent by pearlite banding. The modification of stringer sulfides to tiny lenticular/globular oxysulfides resulted in considerably higher shelf energies, lowering of impact transition temperatures, and minimal anisotropy of impact properties. The macroscopic appearance of splitting on the fracture surfaces of transverse Charpy specimens associated with low impact energies confirmed failure by a low-energy mode. The presence of pancake-shaped ferrite grains and fractographic evidence of inclusion stringers inside furrows identified their role in accentuating the splitting phenomenon.