The invention relates to a butt joint deep penetration laser welding method for joining facing end sections of flat steel products, each having a carbon content of CS < 0.02 %. In order to improve a method of this type so that improved weld seam quality can be achieved in terms of geometry and strength, at least one carbon-containing carrier material is introduced into a butt joint gap between the end sections, having a carbon content of CT ≥ 20∙CS, preferably CT ≥ 100∙CS, and/or carbon is introduced into the butt joint gap or applied to at least one end section in such a way that a volume of the carbon introduced into the butt joint gap corresponds with 1% to 20% of a volume of a melt produced by a butt joint deep penetration laser welding process.