performing arc furnace smelting, ladle refining and Vacuum Oxygen Decarburization (VOD) refining, alternatively performing arc furnace smelting, ladle refining and Ruhstahl Hausen (RH) refining;

when performing at least one of the arc furnace smelting, the ladle refining, the VOD refining, and the RH refining, using a mixture of hydrogen rich gas and argon gas to generate hydrogen plasma for denitrification of molten steel;

wherein the arc furnace smelting comprises: spraying argon gas into the furnace through a hollow graphite electrode, with a gas flow rate 10-30 NL/min/t, an inlet gas pressure 0.1-1.0 MPa, and a bottom blown argon gas flow rate of 1-5 NL/min/t; before performing power transmission and forming a molten pool, spraying the mixture of hydrogen rich gas and argon gas into the molten steel through the hollow graphite electrode, with a proportion of hydrogen rich gas of 5%-80%, a flow rate of 10-30 NL/min/t, an inlet gas pressure 0.1-1.0 MPa, and the bottom blown argon flow rate of 1-5 NL/min/t; from forming the molten pool to completely melting, spraying the mixture of hydrogen rich gas and argon gas into the molten steel through the hollow graphite electrode, with a proportion of hydrogen rich gas of 5%-80%, a flow rate of 20-30 NL/min/t, an inlet gas pressure 0.1-1.0 MPa, and the bottom blown argon flow rate of 2-6 NL/min/t; from completely melting to steelmaking, spraying the mixture of hydrogen rich gas and argon gas into the molten steel through the hollow graphite electrode, with a proportion of hydrogen rich gas of 10%-80%, a flow rate of 10-20 NL/min/t, an inlet gas pressure 0.1-1.0 MPa, and the bottom blown argon gas flow rate of 2-6 NL/min/t; during steelmaking, stopping the hollow graphite electrode from blowing gas;

the ladle refining comprises: adding refining slag after the arc furnace smelting to cover a molten steel surface, preventing the molten steel from absorbing nitrogen; after moving a ladle car to a refining station, descending the hollow graphite electrode to 30-50 cm above a molten steel level, and introducing pure argon gas with a gas flow rate 10-20 NL/min/t, and an inlet gas pressure 0.1-0.6 MPa; during desulfurization of the molten steel, introducing pure argon gas blowing from a bottom nozzle to cause stirring of the molten steel with a gas flow rate 25-30 NL/min/t and an inlet gas pressure 0.1-0.6 MPa; after desulfurization under stirring, adding slag material, deoxidizer, and alloy to the ladle, introducing pure argon gas into the hollow graphite electrode, with a gas flow rate 5-10 NL/min/t and an inlet gas pressure 0.1-0.6 MPa; when performing power transmission for slagging, introducing the mixture of hydrogen rich gas and argon gas into the hollow graphite electrode, with a proportion of hydrogen rich gas of 5-80%, a total gas flow rate of 5-10 NL/min/t, and an inlet gas pressure 0.1-0.6 MPa; after slagging, the slag covers the arc as foam, the proportion of hydrogen rich gas is 5-80%, with a total gas flow 20-30 NL/min/t, and the inlet gas pressure is 0.4-0.5 MPa; after power transmission, lifting the hollow graphite electrode 30-50 cm above the molten steel, introducing pure argon gas, with a gas flow rate 10-20 NL/min/t and an inlet gas pressure 0.1-0.6 MPa; after refining the ladle, stopping the hollow graphite electrode from blowing gas, lifting the electrode to a highest limit, and moving the ladle car to a hanging position;

the VOD refining process comprises: adjusting the bottom blown gas flow rate to 200 NL/min when vacuuming to 20-25 kPa, and simultaneously turning on a plasma torch to spray the mixture of hydrogen rich gas and argon gas, with a total flow rate of the mixture of 1-8 NL/min/t, a proportion of the hydrogen rich gas of 5%-80%, a total gas source pressure of 0.1-0.5 MPa, and a spraying time of 2-3 minutes; vacuuming to 3-10 kPa, stirring by bottom blowing, increasing the bottom blown gas flow rate to 500 NL/min, and keeping parameters of the plasma torch for spraying the mixture of hydrogen rich gas and argon gas; vacuuming to below 67 Pa, increasing the bottom blown gas flow rate to 800 NL/min, increasing the mixture flow rate of hydrogen rich gas and argon sprayed by the plasma torch to 2-10 NL/min/t, increasing the proportion of hydrogen rich gas to 20%-80%, and increasing the total gas source pressure to 0.1-0.5 MPa, and spraying for 10-15 minutes; stopping the plasma torch from spraying, keeping the bottom blown gas flow rate unchanged, keeping the vacuum degree below 67 Pa, keeping the pressure for 5-10 minutes, and removing hydrogen from the molten steel;

the RH refining process comprises: vacuuming to 7-10 kPa, turning on the plasma torch to blow the mixture of hydrogen rich gas and argon gas for pre-denitrification, with a total flow rate of 1-8 NL/min/t of the mixture, with the proportion of hydrogen rich gas of 5%-80%, a total gas source pressure 0.1-0.5 MPa, and processing for 2-3 minutes; vacuuming to below 3 kPa, spraying the mixture of hydrogen rich gas and argon gas by the plasma torch, keeping the spraying parameters unchanged, and performing initial denitrification for 3-5 minutes; vacuuming to below 67 Pa, spraying the mixture of hydrogen rich gas and argon gas into the plasma torch, increasing the total gas flow rate to 2-10 NL/min/t, and increasing the proportion of hydrogen rich gas to 20%-80%, performing deep denitrification while keeping the total gas source pressure unchanged, and processing for 10-15 minutes; stopping the plasma torch from spraying, keeping the vacuum degree below 67 Pa, further processing for 5-10 minutes, and removing hydrogen from the molten steel.