NOVELTY - Combined system comprises a blast furnace (1) for producing pig iron, a converter steel mill (2) for producing crude steel, a gas line system for gases obtained during the production of pig iron and/or crude steel, a power station (3) for producing electricity, and a chemical- or biotechnology plant (11) connected to the gas line system and the power plant parallely. The power station is formed as gas turbine power plant or gas turbine- and steam turbine power plant.    USE - The combined system is useful for producing steel (claimed).    ADVANTAGE - The combined system produces steel in cost-efficient manner.    DETAILED DESCRIPTION - Combined system comprises a blast furnace (1) for producing pig iron, a converter steel mill (2) for producing crude steel, a gas line system for gases obtained during the production of pig iron and/or crude steel, a power station (3) for producing electricity, and a chemical- or biotechnology plant (11) connected to the gas line system and the power plant parallely. The power station is formed as gas turbine power plant or gas turbine- and steam turbine power plant, and is operated with a gas comprising a portion of blast furnace gas obtained during production of pig iron in the blast furnace and/or a portion of converter gas obtained in the converter steel mill. The gas line system comprises an operationally controllable gas diverter (12) for dividing the gas flow rate supplied to the power station and the chemical- or biotechnology plant. INDEPENDENT CLAIMS are also included for:    (1) operating the above combined system, comprising (a) providing portion of blast furnace gas obtained during production of pig iron in the blast furnace and/or a portion of converter gas obtained in the converter steel mill as useful gas for operating the power station and the chemical- or biotechnology plant, (b) utilizing external current and power station current to cover the electricity needs of the plant network, (c) fixing the current portion of external current relative to the total current demand of the combined system as a variable process quantity, and fixing useful gas quantity supplied to the power station as a function of the process variable, and (d) utilizing a portion of the unused useful gas after a gas conditioning as synthetic gas for producing chemical products, or supplying to the biotechnology plant after gas conditioning and utilizing for biochemical processes;    (2) use of chemical plant for coupling to a smelter having the blast furnace, the converter steel mill and the coke oven plant, with the proviso that portion of blast furnace gas obtained during production of pig iron in the blast furnace, portion of converter gas obtained in the converter steel mill, and/or portion of the coke oven gas from the coke oven plant are supplied to the chemical plant as useful gas and used for the production of chemical products; and    (3) use of biotechnology plant for coupling to smelter having blast furnace, converter steel mill and coke oven plant, with the proviso that portion of blast furnace gas obtained during production of pig iron in the blast furnace, portion of converter gas obtained in the converter steel mill, and/or portion of the coke oven gas from the coke oven plant are supplied to the biotechnology plant and used for biochemical processes.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the combined system is useful for producing steel.    Blast furnace (1)    Converter steel mill (2)    Power station (3)    Biotechnology plant (11)    Gas diverter (12)