The one-strand slab caster at ACRONI in Jesenice was built and started up in between May and July 2009 by Siemens-VAI Metals Technologies. A wide variety of different steel grades low carbon, peritectic, middle carbon, stainless, high carbon and silicon steels will be cast with this casting machine. The present work documents and analyses the results for the casting of numerous heats during startup period. Main emphasis was placed on the heat transfer in the mould. As expected, peritectic steel grades show a distinctly lower integral heat flux compared to other steel grades. An increasing casting speed increases the heat flux. The correlation with other parameters shows no significant result due to the limited number of heats. A further point of interest was the uniformity of the center segregation: A simple method was adopted from literature in order to quantify center segregation indices from macroscopic etchings. The influence of changing soft reduction parameters and secondary cooling tables on the segregation index is clearly visible. The slabs cast after the optimization of the casting parameters show a very uniform segregation distribution and a low segregation index. The correlation between the content in tramp elements and the occurrence of star cracks confirms the well known negative influence of increasing Cu-contents. Finally the work shows the influence of casting parameters like superheat on the amount of equiaxed solidification for silicon steels.