Formation of nanoprecipitates in microalloyed steels subjected to thermal fluctuations

Microalloyed steels are nowadays a class of materials very diffused on the steel market and, thanks to their characteristics and mechanical properties, find a wide application in many areas of engineering. The properties of these steels are strictly bounded with the alloying elements that characterize this type of materials. Despite these elements never exceed the limit of 0.1% in weight, their presence strongly influences the material behavior, due to the fact that they tend to form stable precipitates that modify the iron matrix in which are dispersed increasing the mechanical properties. The purpose of this work is to expand the knowledge, to date very limited, about the mechanism of precipitation during the continuous casting process proper of this material. According to the requests of Dillinger Hütte (one of the major steel working factory) an innovative instrument has been developed for the simulation of the contact between the steel skin and the rollers of the continuous casting machine. This contact deeply influences the cooling behavior of the slab and consequently the precipitation one. The device allows studying different parameter of casting in order to analysis their effects on precipitations formation, their size, density number and the volume fraction. Observation, on brittle fracture surfaces, performed by FEG-SEM allows to statistically prove that the elements precipitation is strongly favored by the thermal fluctuations due to the rollers contact; also it is presented the influence of the casting speed and the number of contacts. Also interesting results obtained from TEM analysis show that some precipitations are formed in different layers. Chemical analysis proved that each layer has different chemical composition.