Formation of micro-structural banding in hot-rolled medium-carbon steel

Two-dimensional symmetric finite element (FE) models were employed to simulate the behaviour of the dendritic and inter-dendritic regions respectively during hotrolling with a view to determining the microstructural changes leading to microchemical banding. In this modelling study the equivalent plastic strain and von-Mises equivalent stress distributions were determined as a function of rolling reduction. During hot rolling, the secondary arms experienced larger elongations than the segregated regions within strain bands due to the lower yield point of the dendrite arms. The primary dendrite arms rotated about the rolling direction and align with the rolling direction at rolling reductions of more than 70% while secondary dendrite arms located between two (harder) segregated regions experience significant thinning during rolling until the segregated regions latch onto each other at a rolling reduction of about 70%. At rolling reductions between 65 and 75%, the primary and secondary dendrite arms as well as the inter-dendritic regions (roughly spherical) pre-existing in the as-cast structure, were destroyed and transformed into distinctive elongated microchemical bands that align parallel to the rolling direction. In order to further explore how banding originates, an attempt was made to directly observe the formation of micro-structural bands by the use of high-temperature laserscanning confocal microscopy. These observations have been made in real time and at temperatures typically used in industrial hot-rolling practice. During cooling from the austenite phase field, ferrite nucleates on austenite grain boundaries and grows progressively into the austenite grains. Pearlite/ferrite band formation could clearly be observed in-situ when care was taken to prevent surface decarburisation. The banding observed in-situ in the high-temperature microscope was similar to that detected by optical microscopy following the same heat treatment that was used in the microscope. This study provided new insights with respect to the origins and alignment of bands of different chemical composition, which eventually results in the formation of microstructural banding.