The invention relates to a combined system for producing steel, comprising a blast furnace (1) for producing pig iron, a converter (2) for producing crude steel and a gas line system for gases, said system operating when pig iron and/or crude steel is being produced. According to the invention, said combined system also comprises a chemical or a biotechnology system (11) which is connected to the gas line system, in addition to a system (21) for generating hydrogen. Said system (21) for generating hydrogen is connected to the gas line system by means of a hydrogen-conducting line. The invention also relates to a method for operating the combined system.