Molten steel refining method

A molten steel refining method using an RH vacuum degassing apparatus, wherein an immersion depth l of an immersion tube into molten steel inside a vacuum tank or a circulating gas flow rate G is determined such that a stirring power energy density ε for the molten steel expressed by the following Formulae (1) to (4) meets the following Formula (5):

• ε=[371GT×ln{1+(ρLgH0/P)}]/WV (1)
• WV=(π·DV2/4)×hv×ρL/1000 (2)
• H0=hV+L−hG (3)
• hV=(P0−P)/(ρLg)+1−L (4)
• 1.35×105×DU/WV<ε<2.1×104 (5)
• where the symbols represent the following:
• ε:
• the stirring power energy density (watt/ton) for the molten steel inside the vacuum tank,
• G:
• the circulating gas flow rate (Nm3/sec),
• T:
• a temperature (K) of the molten steel,
• ρL:
• a density (kg/m3) of the molten steel,
• g:
• a gravitational acceleration of 9.8 m/sec2,
• WV:
• a mass (ton) of the molten steel inside the vacuum tank,
• DV:
• an inside diameter (m) of the vacuum tank,
• H0:
• a height (m) from a position of a circulating gas blow-in nozzle to a bath surface of the molten steel inside the vacuum tank in a stationary state,
• P:
• a pressure (Pa) inside the vacuum tank,
• P0:
• an atmospheric pressure of 101325 Pa,
• hV:
• a height (m) from the bath surface of the molten steel inside the vacuum tank in the stationary state to a bed,
• L:
• a height (m) from a lower end of the immersion tube to the bed,
• hG:
• a height (m) from the lower end of the immersion tube to the position of the circulating gas blow-in nozzle,
• l:
• the immersion depth (m) of the immersion tube into the molten steel, and
• DU:
• an inside diameter (m) of a rising tube,
• the method comprising:
• controlling the stirring power energy density so as to meet Formula (5) by adjusting the immersion depth of the immersion tube or the circulating gas flow rate.