CHARACTERIZATION OF PRECIPITATES IN SEGREGATED REGIONS OF A Nb BEARING API-X70 MICROALLOYED STEEL

Despite technological advances in steelmaking equipment and processes, the formation of precipitates with sizes higher than 1 mu m is still observed in some cases of microalloyed steel plates. Even with the current controls, process variations can lead to formation of precipitates harmful to the material properties. This work aims at characterizing the nature of precipitates found in the central segregated regions of API-X70 steel. It also aims to recommend procedures that eliminate or minimize the formation of these precipitates. For the characterization were used: optical microscopy, SEM with EDS analysis and TEM in the samples obtained with FIB. With these techniques the precipitates were characterized as particles of NbC and TiN formed during solidification. These are attributed to the presence of central segregation of the plate. Thermodynamic calculations show that the concentration of carbide and nitride formers in the central segregated regions preclude its dissolution during reheating, since they are at levels above the solubility limit for the usual rolling temperatures.