Oberflächenqualität peritektisch erstarrender Kohlenstoffstähle : Vergleich zwischen Strangguss und Bandguss

Peritectic carbon steels, produced by conventional continuous casting, show slab surface defects as a consequence of the shrinkage caused by the delta-gamma transformation. Unsteady heat flux conditions between strand shell and mould plate leads to an irregular solidification.By following the hypothesis, that the cooling rate has an effect to the peritectic reaction, twin roll strip cast trials were done at a pilot strip caster at RWTH Aachen University. Local cooling rates of -1000 K/s in combination with an eightfold higher heat flux compared to continuous casting are typical for strip casting process. The indication for effects from the strip casting process to the peritectic reaction was on the one hand the steady heat flux and on the other hand the strip surface quality.Trials were performed in 3 campaigns with the chemical analyses of peritectic carbon steels known from continuous casting. The campaigns just distinguish between the casting roll surfaces structure and the range of carbon content. All other process parameters were constant. The casting roll surface was developed from a fine, hexagonal open structure to a coarse hexagonal structure. The structure is regular and was produced by knurling. The carbon range was 0.06 to 0.17 wt %.The result of the 1st campaign was that nearly all strips were attack by many cracks. The number of cracks rises to higher carbon content up to 0.17 wt %. The maximum of defects around 0.1 wt % of carbon, known from continuous casting, was not found. By proofing the strip surfaces, it was found out that the steel melt didn’t wet the roll surface intensive. An unsteady solidification process was the result.The results of the 2nd campaign were different. Just very few cracks were remaining at the strip surfaces. No dependence between carbon content and number of crack were found. Once again the critical range around 0.1 wt % carbon wasn’t noticeable by strips produced with the twin roll casting process.The important different in comparison to the first campaign was an excellent wetting behaviour between steel melt and roll surface. An imprint of the roll surface structure was found at all strip surfaces. A homogenous heat flux and solidification condition exists.The results of the 3rd series, performed with a further regular roll surface structure, confirm the above explained facts. The geometric shape and adjustment were again diversifying to coarser values.The work could demonstrate that the twin roll process is suitable for the production of peritectic carbon steel grades. The key factor for solidification and heat flux is the wetting condition at the roll surface. Steel specific roll surface structures are required