For peritectic steel grades the shell formation is uneven resulting in quality problems such as surface cracks. Production of these steels calls for good control and knowledge of the critical factors for shell formation. By optimisation of process parameters, it is possible to enable production of crack sensitive slabs for heavy plate. This also gives a unique possibility to study the interaction between process parameters and shell formation. The paper presents the results from two large investigations with this ambition covering the total of 27+30 heats of 220 tons of steel at SSAB EMEA-Oxelösund and concludes how surface cracks can be significantly decreased. © 2010 Wiley-VCH Verlag GmbH & Co.