SURFACE SOLIDIFICATION OF MICRO-ALLOYED STEELS UNDER CONTINUOUS CASTING CONDITIONS

摘要

The structure of continuous cast steel slabs shows in many cases irregularities of surface solidification. In the present paper early solidification of micro-alloyed steels was studied to investigate both, dendritic structure and subsurface precipitations of nitrides. In a laboratory rig surface solidification was experimentally investigated. The CC copper mould was simulated by a copper sampler as a substrate for solidification. The contact time of copper substrate with melt was in the range of 2 s. Two cases of heat transfer were determined: direct contact between melt and substrate, and solidification of steel on a liquid slag film of mould casting powder. Low carbon steel grades (ULC, ELC, LC) and a high carbon melt as well react significantly on the direct and indirect contact by developing different amounts of secondary dendrite arm spacings. The effect of heat resistance of the slag layer is obvious: decreased heat flux into the copper substrate in comparison with direct contact deminishes cooling rate and increases local solidification time. In the case of solidification on mould flux the dendrites develop mostly in relatively large packages of ten or more main dendrites (2-dimensional) growing in parallel. During solidification directly on copper primary dendrites mostly grow in the shape of fans starting from one nucleation point. The last phenomenon is well known by investigations on strip casting, and there are efforts to control regularity of solidification e.g. by coating or roughness control of the casting rollers. The influence of precipitations like TiN in the surface and subsurface area on primary solidification of a slab should be discussed by case splitting: Calculations on the mentioned steel grades show that those precipitations can be only created in micro-segregated areas, i.e. not before nucleation of the steel melt. Their extentions are lower than secondary dendrite arm spacings; therefore they can't influence the solidification structure. But on the other hand inclusions of alumina and titanium nitride can be formed e.g. by entrapped air by reactions of oxygen and nitrogen with dissoluted [Al] and [Ti] when solubility products are reached. Those inclusions have an extension of 10 μm and more, so that it can not be excluded that an interaction between such an impurity and the solidifying metal exists. Investigations in that field will be continued.