Decarburization refining method for molten steel under reduced pressure

A decarburization refining method for molten steel under reduced pressure, the method comprising:

• an oxygen-blowing decarburization treatment of blowing an oxidizing gas onto molten steel under reduced pressure to perform a first decarburization treatment;
• and
• a rimmed decarburization treatment of (i) stopping feeding of oxygen sources including the oxidizing gas to the molten steel after the oxygen-blowing decarburization treatment has been performed and (ii) performing a second decarburization treatment under reduced pressure until a carbon concentration in the molten steel becomes equal to or lower than a target value,
• wherein, in a heat for which the decarburization refining method is performed, by using operation data taken at a time when the oxygen-blowing decarburization treatment is started and at a time when the oxygen-blowing decarburization treatment is ended, an amount of carbon removed while the oxygen-blowing decarburization treatment is performed is estimated,
• based on the estimated amount of carbon removed while the oxygen-blowing decarburization treatment is performed, the carbon concentration in the molten steel at a time when the rimmed decarburization treatment is started is estimated,
• by using the estimated value as the carbon concentration in the molten steel at the time when the rimmed decarburization treatment is started, a change over time in the carbon concentration in the molten steel while the rimmed decarburization treatment is performed in the relevant heat is calculated, and
• based on the calculated change over time in the carbon concentration in the molten steel while the rimmed decarburization treatment is performed, a determination is made about a time when the rimmed decarburization treatment is ended.