Path scheduling method and system for rolling mills

Details Path scheduling method and system for rolling mills Path scheduling method and system for rolling mills

1999-12-06

2001-06-05

Tolan, Ed (Department: 3725)

Metal deforming

With use of control means energized in response to activator...

Metal deforming by use of roller or roller-like tool element

C072S010300, C072S010400, C072S011800, C072S365200

Reexamination Certificate

active

06240756

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a path scheduling method and system. More specifically, the invention relates to a path re-scheduling method for rolling mills and a path re-scheduling system for rolling mills, and in particular, to an optimum path schedule determining method for a rolling mill that rolls a coil to be coiled and the like (hereafter collectively referred to “coil”), as well as to an optimum path schedule determining system for such a rolling mill.
2. Description of the Related Art
In a rolling mill having N (N≦2) stands for rolling a coil, the determination of a schedule covering an optimum exit thickness of the coil at each stand is important from the standpoint of achieving stable mill operation and maintaining high quality of a finished product.
In a conventional approach to determine an optimum path schedule, a basic path schedule is determined, covering e.g. rolling reductions at respective stands to be distributed as specified in value, and employed for calculation of values of associated parameters at each stand, such as rolling force, bite angle, linear force, neutral point position, torque, power, and rolling speed, and when a calculated value exceeds a specified mechanical limit or conditional limit for stable operation, an optimization is made by changing distribution of rolling forces such as to the offending stand, thereby preparing an optimized path schedule.
With recent advances in production technology and diversifying demands for product quality, however, the actual operation of rolling mills has become extremely complex. For stable mill operation to be still maintained, necessary factors to be considered have increased in number for determination of a path schedule to be optimized yet better, with increased importance to a precise prediction by calculation.
Conventionally employed limits are as follows:
(1) Rolling force. To provide mechanical protection for mill elements such as load cells, a limit is imposed on the withstanding force. Typically, in order to prevent fatigue failures after long periods of operation, a safety factor is multiplied to an actual specified value to be smaller.
(2) Rolling torque. A limit on rolling torque is established so as to protect the drive system elements such as the mill spindle.
(3) Motor power. This limit is established to provide electrical protection for the main motor of the mill.
(4) Bite angle. With hot rolling in particular using a hot strip mill, the bite angle at the end of a coil is a particularly important factor in achieving stable operation. If the rolling reduction of a stand is excessive, so that the bite angle limit is exceeded, the bite at the next stand is adversely affected, thereby risking accidents. This limit is provided to prevent such occurrences.
(5) Unit force per width. In a tandem cold mill that cold rolls a coil, if the unit force per width exceeds a certain value, the condition for lubrication between the coil surface and the roll surface worsens, leading sometimes to surface damages known as heat scratches. Setting this limit is done to prevent such damages.
(6) Neutral point. This limit is also set in a tandem cold mill. If conditions are set so that the neutral point is deviated near the exit or entrance side of the roll bite, or so that it slips out of the roll bite, slipping can occur within the roll bite, this being a direct cause of vibration of the mill. If this slipping is excessive, it can even lead to breakage of the coil, and this limit is set to prevent such problems.
(7) Rolling speed. In order to protect the main motor, it is necessary to check the speed control at each stand of the mill.
It will be understood that checking criteria other than those noted above are generally set, in accordance with running conditions, and that the more limit items there are, the better must be the optimum path schedule.
A conventional method is disclosed, in Japanese Patent Application Laid-Open Publication No. 1-233003, whereby if the predicted power of the motor at a particular stand exceeded a limit value, based on the difference between the predicted motor power and the limit value, an influence factor, which is the calculated amount of power change at other stands for a motor power change that causes a minute variation in entrance and exit thickenesses of coil at each stand, and a standard power distribution ratio are used to distribute the power of the limit-exceeding stand among other stands, so as to correct the exit thicknesses at each stand in the basic path schedule, thereby maintaining the power balance between the stands.
In another method disclosed in Japanese Patent Application Laid-Open Publication No. 5-269514, a number of rolling conditions required for normal operation at each stand are checked and, with regard to a stand at which any limit value is exceeded, based on influence factors of entrance and exit thicknesses of coil for that condition, the basic schedule for that stand is changed so that the limit value is not exceeded.
SUMMARY OF THE INVENTION
In the above-noted methods, a plurality of rolling conditions that must be satisfied in order to achieve normal operation of each stand of a rolling mill are checked and, if any limit value is exceeded, the optimum path schedule is adjusted so as to correct the exit thickness of the offending stand. In a multistand rolling mill, because adjustment of speed is important, when the exit thickness is corrected, it is necessary to simultaneously adjust the speed of other stands, in order to satisfy the principle of constant mass flow. In the methods of the past, however, an influence factor is used to determine only the amount of exit thickness correction, the calculation method being used not taking into account the amount of speed correction. That is, when the exit thickness at each stand is changed during actual operation, in order to keep constant mass flow, it is necessary to simultaneously determine the speed (or more precisely the work roll peripheral speed) at each stand. This is because, with a change in the exit thickness, there is a change in speed to maintain constant mass flow, causing a change in speed of deformation of the material, and an accompanying change in deformation resistance at each stand, resulting in a change in quantities such as rolling force, rolling torque, and motor power, which are related to force. Because the amount of exit thickness correction determined without considering the change in speed that accompanies the change in exit thickness either does not strictly satisfy the requirement for constant mass flow, or does not take into consideration the change in force characteristics that are dependent upon speed, such as change in deformation resistance that accompanies a change in speed, there exists a problem with calculating an incorrect balance of rolling forces, by using the speed before the correction.
Using the methods of the past, it is possible to determine a path schedule so that limit values are not exceeded, by correcting the exit thickness from a stand for which a limit value is exceeded, and to maintain a balance of various quantities at all the other stands by means of basic path schedule, because the amount of speed correction required to maintain constant mass flow is not calculated when the exit thickness correction is determined, the results of the corrected path schedule does not necessarily followed the prescribed force, thereby hindering the achievement of the desired balance between various parameters. Additionally, using a path schedule that is corrected for exit thickness without consideration given to the speed required to satisfy the condition of constant mass flow, if the amount of exit thickness correction is particularly large, the passage of the coil itself can become unstable, leading to a worsening of flatness and crown quality problems. In extreme cases, serious accidents such as breakage of the coil can even occur.
The present invention has been made with such points in view. It therefore i

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