PURPOSE: To control the carbon concn. at the time when decarburization ends with high accuracy by determining the decarburization end point timing from the relation between the CO concn. and the decarburization time at the time of smelting a low- carbon steel by vacuum refining.

CONSTITUTION: The correlations between the CO concn. and the decarburization time in exhaust gases are determined by each decarburization speed to determine the timing for the end point of the decarburization by simultaneously holding equations I, II, III, IV at the time of smelting the low-carbon steel of ≤50ppm carbon concn. by vacuum refining. The end point carbon concn. corresponding to the fluctuation in the decarburization speed is controlled in accordance with this timing. In the equations I to IV, [C] denotes the carbon concn. (ppm) in steel; (t) denotes the decarburization time (min), Kc denotes a coefft. (1/min)) [C]0 denotes the initial carbon concn. in the steel; [C] denotes the decarburization quantity per minute; (x) denotes the amt. of the molten steel to be treated (ton); (CO) denotes the amt. of the CO generated (kg/hr); %CO denotes the CO concn. in the exhaust gases; Y denotes the amt. of the gas exclusive of the CO to be introduced into a vacuum system (kg/hr). The excessive decarburization treatment is averted in this way.