Steel for plastic molds and process for their heat treatment

Maraging steel with improved machinability, good weldability, and high corrosion resistance, a process for the heat treatment of such a steel, as well as its use. According to the invention this steel contains (in % by weight) 0.02-0.075% carbon; 0.1-0.6% silicon; 0.5-0.9% manganese; 0.08-0.25% sulfur; maximum 0.04%; phosphorus; 12.4-15.2% chromium; 0.05-1.0% molybdenum; 0.2-1.8% nickel; maximum 0.15% vanadium; 0.1-0.45% copper; maximum 0.03% aluminum; 0.02-0.08% nitrogen; as well as optionally one or more additional alloying elements up to maximum 2.0%, residual iron, and impurities caused in manufacturing, and a ferrite percentage in the structure of less than 28% by volume. A process is also provided for the heat treatment of a maraging steel with improved machinability, which process makes an object that is through-hardened even with a large cross-section, lies essentially in that a steel block with the above composition is subjected in a first step to an annealing treatment for the formation and adjustment of a ferrite percentage in the structure and in a second step a hot forming of the same takes place, after which in a third step a soft annealing and then a thermal tempering are performed.