Any steel that has had carbon arc gouging done on it will have a HAZ regardless of the size of the electrode used. Using good technique will have more to do with the size/extent of the HAZ than the size of the electrode. You would not have any operating problems using up to even 5/8" diameter electrodes on your 1/2" plate. I doubt that you want a gouge that wide though. I am not sure what EN355N plate is, but if it is a 355MPa normalized carbon steel (just a guess on my part based on the nomenclature), proper carbon-arc gouging will not have a detrimental affect on the material. Just make sure you remove all the carbon residue before welding if done. If it is a more hardenable steel, then preheating should be used since the HAZ for proper carbon-arcing cools off faster than the HAZ for welding, it has a propensity to harden more than welding. However, if subsequent welding is done, it eliminates the HAZ from arcing.