Method for cleaning molten metal and apparatus therefor

An apparatus for cleaning molten metal comprising:

• a first vessel having an inlet for charging molten metal at the upper portion of the first vessel and an outlet for discharging the molten metal at the bottom of the first vessel, wherein the molten metal is pressurized by its own static weight;
• a second vessel having an inlet port for receiving the pressurized molten metal from the first vessel, at the bottom of the second vessel and an outlet port for the molten metal disposed higher than said inlet port, whereby the pressurized molten metal is reduced in pressure when the pressurized molten steel comes up from the inlet port to the outlet port;
• a communicating tube connecting the outlet of the first vessel and the inlet port of the second vessel;
• a first bubbling device, for bubbling a gas which is soluble in molten metal, disposed at the bottom of the first vessel;
• and a second bubbling device for bubbling a gas which is soluble in molten metal, said second bubbling device being disposed in said communicating tube.

A method for refining molten steel in a vacuum comprising:

• an immersion process, wherein two downwardly extending spaced apart immersion tubes are positioned at a lower portion of a vacuum vessel are immersed in molten steel in a ladle, said two immersion nozzles being a rising tube and a sinking tube;
• a dissolving process, wherein gases are dissolved in said molten steel by blowing into said molten steel a gas containing at least a gas which is soluble in said molten steel from a gas blow-in opening in said ladle;
• a first degassing process, wherein said molten steel is degassed by keeping said vacuum vessel evacuated, circulating said molten steel between said ladle and said vacuum vessel through said tubes by injecting a gas containing at least an inert gas from an intermediate portion of said rising tube into said molten steel and blowing a gas containing at least a gas which is soluble in said molten steel from a gas blow-in opening in said ladle;
• and
• a second degassing process, wherein said molten steel is degassed by keeping said vacuum vessel evacuated, stopping a gas blowing-in from said gas blow-in opening in said ladle and circulating said molten steel between said ladle and said vacuum vessel through said tubes by injecting a gas containing at least an inert gas from a middle portion of said rising tube into said molten steel.

A method for refining molten steel in a vacuum comprising:

• an immersion process, wherein two downwardly extending spaced apart immersion tubes are positioned at a lower portion of a vacuum vessel are immersed in molten steel in a ladle, said two immersion nozzles being a rising tube and a sinking tube;
• a dissolving process, wherein gases are dissolved in said molten steel by blowing into said molten steel a gas containing at least a gas which is soluble in said molten steel from a gas blow-in opening in said ladle;
• and
• a degassing process, wherein said molten steel is degassed by keeping said vacuum vessel evacuated, stopping said gas blowing-in from said gas blow-in opening in said ladle and circulating said molten steel between said ladle and said vacuum vessel through said tubes by injecting a gas containing at least an inert gas from an intermediate portion of said rising tube into said molten steel.

A method for cleaning molten metal comprising the steps of:

• keeping a pressure inside a vessel having molten metal therein at a pressure of Ps of atmospheric pressure or less;
• bubbling the molten metal in the vessel by gas soluble in the molten metal, a portion of said gas dissolving in the molten metal and the rest of said gas converting to gas bubbles;
• after bubbling said gas through said molten metal for at least five minutes, producing in the molten metal in the vessel, by pressure reduction to a pressure P.sub.E in said vessel, gas bubbles of a smaller size than the bubbles into which said gas is converted by the bubbling step and thereby trapping nonmetallic inclusions by said smaller size gas bubbles and raising them to the surface of the molten metal;
• stopping the bubbling of said gas, and
• continuing the maintenance of reduced pressure for at least five minutes after the end of bubbling and thereby removing gas dissolved in the molten metal.