A threaded nut of a ball-screw for a brake booster is constructed as a hydraulic piston and is produced from the group of martensitically hardening steels which are non-corroding with respect to brake fluid and which has in percent by weight: between 0.4% and 1.3% carbon (C), up to 2% silicon (Si), up to 2% manganese (Mn), between 12% and 20% chromium (Cr), and phosphorus (P) and sulphur (S) together at less than 0.015%, the balance being iron and, where applicable, melting-related impurities. Addition of the following materials to the above-described steel may be advantageous: up to 2% molybdenum (Mo), up to 0.2% vanadium (V), and up to 3% nickel (Ni).