Vlastnosti oceli L485MB pouzìvane pro výrobu plynovodnìch trub (2. cást)

Our research into the mechanical and fracture mechanics properties of the L485MB steel followed up our previous work on L415MB and L450MB steels, of which spirally welded DN 500 pipes are manufactured at Mittal Steel Ostrava, a.s. The investigated L485MB steel ranks in the highest-strength class of steels used for the manufacture of gas pipelines in this country, and the results of static tensile tests reflect this fact. For the all tensile test bars the yield stress R t0,5 exceeded the minimum standard value of 485 MPa and the ultimate strength R m ranged between 625 MPa (overloaded pipe) and 645 MPa (a rolled-up strip). On the other hand, the results of fracture mechanics tests have shown that the fracture toughness of the rolled-up strip as well as of the overloaded and non-overloaded pipe is only average, being less than e.g. for the L415MB steel. The magnitude of the J-integral for the beginning of a stable subcritical crack growth for the overloaded pipe was J in = 377 N/mm, that of J 0,2 was 583 N/mm, and that of J m was 663 N/mm. The overloading of the finished pipe did not yield the expected result in improving its fracture toughness, especially the values of J in and J 0,2 .