This is the third phase report on this project. Previous phase reports contained information on the effects of certain. manufacturing variables on the strength and fracture toughness of line-pipe steels made in the laboratory. Since the previous reports, additional steels have been melted and evaluated. Also, additional variables have been looked at. The report presents results obtained in 1964 and 1965, and, where possible, attempts to tie together all information obtained for any one variable.