Formation of Austenitic Structure During Heating Slabs of Pipe Steels Microalloyed with Niobium^*

The effect of heating temperature and holding duration for slabs in a furnace soaking zone on the structure of microalloyed steel austenite with a different strength category is studied. It is revealed that after heating and soaking in all cases there is possible formation of one of three types of austenitic structure: fine-grained, varying grain size, and coarse-grained. The reason for forming different types of structure is irregular growth of individual grains as a result of dissolution of fine niobium carbonitride particles. It is shown that the start of irregular grain growth depends on steel heating temperature, soaking time, and chemical composition. An effective method is proposed for calculating slab heating temperature and time in a furnace soaking zone providing maximum dissolution of niobium carbonitride inclusions without forming an inhomogeneous structure before the start of the rough rolling stage.