And now it is even easier than @Kay's answer -first use ["Explode lines"](https://docs.qgis.org/latest/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#explode-lines) tool, yielding all line segments, then calculate azimuth with Field Calculator: [`main_angle($geometry)`](https://docs.qgis.org/latest/en/docs/user_manual/expressions/functions_list.html#main-angle)