Abstract: A method of producing an ultra-low-carbon steel by conducting vacuum-decarburization of a molten steel by means of a vacuum degasifier of the type having recirculation pipes and a vacuum chamber. When the carbon content of the molten steel has come down to a level of 50 ppm or less, hydrogen gas is introduced together with an inert gas into the molten steel either by directly injecting a hydrogen-containing gas into the molten steel in the vacuum chamber through a tuyere provided in the wall of the vacuum chamber or by blowing the hydrogen-containing gas onto the surface of the molten steel in the vacuum chamber through a lance provided in the vacuum chamber. In order to enhance the effect produced by the method of the present invention, it is possible to take an additional measure such as blowing of hydrogen gas through a tuyere provided in the wall of the recirculation pipe or injection of hydrogen or hydrogen-containing gas through an injection lance immersed in the molten steel held in, a ladle.

Abstract: A method of producing an ultra-low-carbon steel by using a vacuum degasser on a molten steel has the steps of conducting vacuum decarburization to attain a predetermined level of carbon content, e.g., 25 ppm or below, in the molten steel while progressively reducing the pressure in said vacuum degasser, conducting addition of hydrogen in which hydrogen is dissolved in said molten steel while said pressure is temporarily elevated to 20 Torr or above, and conducting final decarburization after reducing said pressure to 2 Torr or below. The addition of hydrogen may be conducted to meet the following conditions:[H].gtoreq.{([C]-[C]final)/5}+4wherein [H] represents the hydrogen content (ppm) in said molten steel in the state after the addition of hydrogen, [C] represents the carbon content (ppm) in the molten steel in the state after the addition of hydrogen, and [C] final represents the final carbon content (ppm) to be obtained.

Abstract: A method is disclosed for accurately detecting an outflow of a slag into a stream of a molten steel when the molten steel is poured from a first vessel such as a refining furnace or a ladle into an intermediate vessel such as a ladle or a tundish through a melt-discharging hole or a nozzle. At that time, an inert gas is fed into the stream of the molten steel in the nozzle through a side portion of the melt-discharging hole or the nozzle, and the detection is made based on a change in a flow rate of the inert gas sucked into the stream of the molten steel and/or in a back pressure.

Abstract: A process of production of high cleanness extra low carbon steel includes steps of producing low carbon rimmed steel by means of a refining furnace, supplying a deoxidization agent to a slag in a laddle for adjusting iron concentration in slag at lower than or equal to 5%, subsequently performing vacuum degassing process with blowing oxygen to lower carbon content in the steel lower than or equal to 0.006%.