Abstract
The role of microalloying elements in IF steels is quite complex and very important. In this paper, the distribution profiles of Ti, Nb, V and C and impurities through the thickness of an IF steel slab are evaluated and compared in two localities, one with embrittlement and one with high toughness, in the slab surface zone. The occurrence of high concentration peaks of elements within the slab subsurface zone in contrast to the average composition was confirmed in both localities, with much higher concentration peaks especially in the embrittled locality, mainly for As, Sb, P, C, Nb, Al and N, but not for Ti, for which the concentration peaks are low, often below its mean content in steel. It was found that the place where the maximum peaks occur corresponds approximately with the boundary between the surface fine grain zone and the columnar dendrite zone. Precipitation of particles was also confirmed, such as nitrides of Al and Ti, or complex particles based on Al and Ti, sometimes in combination with MnS. The presence of high concentration peaks of some elements in the embrittled locality reveals significant local segregation, which could arise after solidification, e.g. as either stress induced or quench induced segregation during local cooling changes. The most probable reason for the sample embrittlement from cut no. 2 is a combination of factors. There was non-equilibrium impurity segregation after solidification, mostly with As and P, which was apparently intensified due to more coarse grained original austenite in this locality, together with precipitation of fine cementite particles and line distributed particles based on Al and Ti.
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