Metal founding – Control means responsive to or actuated by means sensing or... – Control of feed material enroute to shaping area
Reexamination Certificate
2000-01-13
2001-09-18
Dunn, Tom (Department: 1725)
Metal founding
Control means responsive to or actuated by means sensing or...
Control of feed material enroute to shaping area
C164S151300
Reexamination Certificate
active
06289971
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a continuous casting facility, particularly to a mold level control apparatus for controlling to maintain a molten steel level in a mold constant.
In an operation of a continuous casting facility, it is requested to realize a stable operation and to uniformly maintain cast slab at high quality. For this purpose, in the continuous casting facility, a function of maintaining a molten steel level in a mold constant plays an important role during the whole operation. The function is referred to as a function of “mold level control”.
Molten steel accumulated in a tundish is guided from an opening portion disposed at a bottom portion of the tundish, via a pipe referred to as an immersion nozzle, to a rectangle mold. Molten steel injected into the mold is deprived of heat and cooled and an interface between the molten steel and the mold is solidified, a state of the molten steel is changed into slab and the slab is discharged to a downstream side. There is installed a part referred to as a stopper or a sliding gate (hereinafter, referred to as stopper) fabricated by refractory at a bottom portion of the tundish or a portion where the tundish and the immersion nozzle are bonded to each other. The stopper is installed to provide resistance to a flow of the molten steel in the immersion nozzle. The stopper comprises a structure in which the stopper can be moved by drive force such as hydraulic pressure. By adjusting the position of the stopper, a degree of the resistance provided to the molten steel flow in the immersion nozzle is changed. As a result, a flow rate of the molten steel in the immersion nozzle can be adjusted.
Ideally, when a volume per unit time of the slab cast by the continuous casting facility is balanced with the flow rate of the molten steel injected into the mold, the molten steel level in the mold is maintained constant. However, in operating the continuous casting facility, there is a case in which the stopper is melted since the stopper is exposed to the molten steel at a high temperature and a shape thereof is changed. Further, there is a case in which a deposit having a component of the molten steel is adhered to or exfoliated from a portion for passing the molten steel and the molten steel flow rate is changed. Further, there is a case in which an amount of the molten steel accumulated in the tundish is changed and delivery pressure of the molten steel at the opening portion of the immersion nozzle is changed. Further, there is a case in which a deposit of the molten steel is adhered to or exfoliated from the inside of the immersion nozzle and a molten steel guide path of the immersion nozzle is changed.
By these unsteady-state and unpredictable causes, even when the position of the stopper is maintained constant, the molten steel flow rate injected into the mold is changed. Further, the produced slab is discharged to the downstream side in a state in which the slab has not been completely solidified and accordingly, there causes a change in the shape of the slab by bringing the slab into contact with a supporting structure such as a plurality of rolls for transferring the slab. The change constitutes a factor of varying the mold level from the downstream side by flowing back the molten steel in an unsolidified portion in the slab.
In order to maintain the level constant in injecting the molten steel into the mold under such a situation, there is generally adopted a feedback control system as a mold level control system. According to the feedback control system, the mold is installed with a sensor for detecting the molten steel level in the mold. Further, the position of the stopper is adjusted so that an output value of the sensor is made to coincide with a target level provided as an instruction value.
Currently, there has been developed a new process of directly connecting a continuous casting facility with a hot rolling facility and integrally producing hot coils from refined molten steel. This is because there are requests for downsizing a facility scale and accordingly, a reduction in a facility cost as well as high efficiency formation of operation and a reduction in energy required for production. According to a continuous casting facility for playing an important role among these, there is the following request in addition to the above-described requests. That is, with a purpose of alleviating load on a hot rolling process, there are needed properties that a thickness of produced slab is as thin as 80 through 120 (mm) and the slab stays at a comparatively high temperature. In this way, since the slab thickness is comparatively thin, in order to provide priority in production capacity in comparison with that in a conventional facility, there is requested a casting speed as high as 5 through 8 (m/minute) for a continuous casting facility.
Since the slab thickness is thinned and the sectional area of the slab in the mold is smaller than that in the conventional facility, the adjusted molten steel flow rate is reflected sensitively to the mold level. Further, since temperature of the produced slab is high, the slab is extracted to the downstream side in a state in which the slab is softer than that in the conventional facility. As a result, a variation in the shape of the slab on the downstream side is more liable to be caused owing to the contact of the slab with a supporting structure such as rolls and the degree is magnified. As a result, the degree of fluctuation of the mold level derived from the downstream side is increased. Such a variation in the shape of the flexible slab is referred to as unsteady-state bulging. An explanation will be given as follows of serious influence effected by the unsteady-state bulging.
The inventors has constituted a mold level control apparatus by applying PI(Proportional Integral) control which is frequently utilized in a chemical plant as a feedback control system. In that case, when the operation is carried out at a specific casting speed, the inventors have faced a problem that the molten steel level in the mold which has been controlled excellently, abruptly starts oscillating at a frequency of about 0.3 (Hz), the oscillation finally increases and stable operation of the facility cannot be maintained.
When a surface of the cast slab is observed, a nonuniform distribution of the surface condition of the slab which coincides with an interval of rolls can be confirmed in a direction of moving the slab. Further, the oscillation frequency of the mold level substantially coincides with a value of a casting speed divided by the interval between the rolls. The inventors have acquired the knowledge that the oscillatory state of the mold level is caused as follows, from investigation and observation on a number of operational states.
(A) When a flexible slab at a high temperature is supported by the rolls, the slab constitutes a bulged shape in correspondence with the interval between the rolls.
(B) When such slab moves to the downstream side at a constant speed, a spacial nonuniformity of a condition of the slab such as an oscillation mark operates as, for example, frictional force and the slab starts pulsating. When intervals among a plurality of rolls stay equal, degrees of pulsation caused at the respective roll intervals are amplified mutually thereby.
(C) By the above-described pulsation, unsolidified molten steel in the slab flows back to the side of the mold to thereby constitute a periodic disturbance flow which is applied on the mold level. The periodic disturbance flow produces a level fluctuation in the mold at a frequency thereof. Thereby, there is produced a nonuniformity at the above-described period in cooling operation in the mold and the like. As a result, the cast slab is provided with a spacial nonuniformity at the above-described period in surface condition, a composition and the like.
(D) When a portion of the slab having the above-described condition (C) reaches a group of supporting rolls achieving the above-described operation of (B), the
Arent Fox Kintner Plotkin & Kahn
Dunn Tom
Sumitomo Heavy Industrie's, Ltd.
Tran Len
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