In the manufacture of molten hot metal to steel in the Q-BOP process, oxygen is blown into the bottom of the converter or vessel during the refining of the molten hot metal to steel. When a flux, such as finely divided lime or the like, is required in the refining process, it is injected along with the oxygen through a plurality of tuyeres located in the bottom of the converter into the molten metal bath. Whenever such flux is injected into the converter, a back pressure results which decreases the oxygen flow rate. When the flux injection ceases, the back pressure is removed and the oxygen flow rate increases back to its normal or prior flow.
In the conventional operation of a Q-BOP converter, the total source oxygen pressure is applied to the tuyeres, thereby resulting in sonic flow or flow velocities greater than Mach 1.0 to the tuyere outlet. Manufacturers of such conventional process claim that this results in a better oxygen distribution in the molten metal bath and requires the minimum number of tuyeres in the bottom of the converter. The maximum oxygen flow rate attainable in the conventional system is hence fixed by the oxygen source pressure and the number and size of the tuyeres employed in the Q-BOP converter. Whenever flux is injected into the molten metal bath, the increased back pressure results in decreased oxygen flow through the tuyeres and since the oxygen source pressure cannot be increased (because full source pressure has been applied) to overcome this back pressure, the oxygen flow rate remains lower during the flux injection. However, in most cases of conventional operation, the oxygen pressure in the tuyere during the flux injection is still high enough to maintain the sonic flow of the oxygen through the tuyere outlet and to prevent the molten steel in the bath from moving downwardly through the tuyeres. Under sonic flow conditions, the oxygen flow rate varies directly as the oxygen pressure so that a drop in tuyere oxygen pressure would result in a directly proportional drop in the oxygen flow rate through the tuyeres.
An advantage of this conventional system is that the oxygen pressure at the tuyeres is maintained higher than is necessary at all times, thereby eliminating any possibility of the passage of hot molten metal from the bath into the tuyeres.
One disadvantage of the conventional system is that during the flux injection (which occurs during about 50% of the blowing time) the oxygen flow rate through the tuyeres is decreased, thereby increasing the length of blowing time required to refine the hot molten metal into steel. In order to keep the oxygen blowing rate constant during the entire blowing time, the oxygen supply pressure must be increased thereby requiring the addition of costly equipment which is not economically feasible.
We have found that the Q-BOP converter can be operated with larger and a greater number of tuyeres with oxygen flow through such tuyeres below sonic flow velocities without adverse affect on the oxygen distribution in the hot molten bath. During the flux injection period, the increased back pressure would also result in lower pressure at and decreased oxygen flow through the tuyeres. However, since the oxygen flow rate at subsonic velocities varies as the square root of the oxygen pressure changes, the oxygen flow rate per unit change in oxygen pressure is considerably less. In addition, when the Q-BOP converter is operated at subsonic oxygen velocities and at lower oxygen pressure through the tuyeres, the margin between the operating source oxygen pressure and the minimum oxygen pressure required to keep the hot molten metal out of the tuyeres during flux injection, is much less, and therefore is more critical. Since all of the source oxygen pressure has not been applied to the tuyeres, there is a reserve oxygen pressure at the oxygen pressure source which is available to preserve the margin between the required operating pressures to maintain the oxygen flow rate through the tuyeres and the minimum allowable oxygen operating pressure on the tuyeres which will keep the hot molten metal out of the tuyeres.