Abstract: A low carbon high chromium alloyed steel is made by an underpressure treatment wherein a steel melt with a high chromium and carbon content is supplied with a gaseous decarbonization agent while it is maintained at an underpressure. At the beginning of the underpressure treatment the steel flows upwardly from a ladle through an inlet into a reaction vessel and no decarbonization agent is supplied. In a second phase of the treatment a high quantity of decarbonization agent is added directly to the melt which flows upwardly from the ladle into the reaction vessel. In a third treatment phase a small quantity of decarbonization is supplied. The melt is subsequently deoxidized and alloy corrected and it is then poured off from the reaction vessel. The apparatus includes a reaction vessel which has a downwardly extending inlet connected to the ladle and an outlet spaced from the inlet extending down into the ladle within the melt thereof.