Tandem Hot Strip Rolling Research Articles

Integrated AGC Approach for Balancing the Thickness Dynamic Response and Shape Condition of a Hot Strip Rolling Control System

Thickness and shape are two significant indices in the tandem hot strip rolling process. Excessive thickness correction can cause rolling force imbalance, leading to deteriorated shape conditions. Aiming at this problem, we propose a new monitor automatic gauge control (M-AGC) with consideration of the rolling force distribution to not only keep the shape condition but also improve the robustness of the rolling process. First, the M-AGC with the Smith predictor control structure was studied. Second, based on the rolling force and thickness correction redistribution algorithm, shape balance automatic gauge control (SB-AGC), the phenomenon of rolling force imbalance was improved. Third, the transient response was improved by adding a command prefilter to the upstream stands, which is asynchronous shape balance automatic gauge control (ASB-AGC). The proposed algorithms can be applied to all steel grades and are easy to implement. Finally, cross-validation was conducted through numerical simulations and application results, confirming the alignment between the theoretical derivation and practical implementation. The simulation results show improvements in both the rolling force balance and the thickness response. The practical application results also confirm that better flatness conditions and thickness response can be achieved, significantly reducing the amount of head-end cutoff losses.

A novel mass flow controller for tandem hot rolling mills

In tandem hot strip rolling mills, the roll speed of the mill stands is synchronized to ensure a uniform mass flow. A uniform mass flow is desirable to keep the strip tension constant and thus avoid large movements of the loopers which are located between the mill stands in order to control the strip tension. This also improves the general thickness quality and prevents the strip from necking in case of too excessive strip tension. In typical state-of-the-art implementations, the roll speed is synchronized in a cascade master–slave control structure where the very last downstream mill stand serves as speed master and the upstream mill stands operate in a slave mode to make sure that the mass flow is kept constant. While the relation between the roll speeds in the classical synchronization control concept is typically constant, a thorough analysis of the mass flow reveals that the slip between the work rolls and the strip is time varying. Therefore, in order to ensure a uniform mass flow, a novel adaptive speed synchronization controller is proposed, which can be easily added to the existing control structure and is computationally undemanding. To this end, a mathematical model of the whole tandem mill including the complete control structure is derived and validated by measurements. Simulation results for this validated model prove that the proposed mass flow controller clearly outperforms the existing solution.

Asymmetric hydrodynamic roll gap model and its experimental validation

In tandem hot strip rolling mills, different friction between the rolls and the strip material on the upper and lower strip surface can occur due to asymmetric surface temperatures or different conditions of oil lubrication. To capture these effects, this paper presents a hydrodynamic roll gap model with asymmetric friction. Based on similarities between the rolled material and viscous fluids, fluid mechanics theory is used to derive this model. Due to the nature of this model, the influence of the rolling speed is inherently taken into account, which allows an accurate prediction of the rolling force and the forward slip. As an analytic solution for the hydrodynamic roll gap model is available, it is well suited for online applications in rolling plants. For validation of the proposed model, an experiment with asymmetric work roll roughness was performed. A specimen of steel strip with copper pins inserted was repeatedly rolled to visualize the material flow inside the roll gap for multiple passes. The resulting deformed copper pins were cut out of the strip and show good agreement with the deformation profiles calculated by the developed model.

A Useful Control Model for Tandem Hot Metal Strip Rolling

The tandem hot metal strip rolling process is a highly complex nonlinear system that presents a difficult control challenge. This challenge is exacerbated by the hostile hot metal rolling environment, which precludes the location of sensors to measure variables that are important for control. In addition, the controller must have a structure that offers simplicity of tuning during commissioning by personnel who usually are unfamiliar with advanced process control techniques. Based on our previous work using a state-dependent Riccati equation technique for control of the tandem cold metal rolling process, it is considered that a similar method might also be useful for control of tandem hot strip rolling. For the hot rolling process, the development of a process model in a form that is suitable for control development is a significant and challenging task. This paper describes our work to expand on an initial portion of this model to develop a comprehensive nonlinear model of this process. Based on simulation results, it is determined that the complete model has the potential to be useful in the development of a viable nonlinear control method for significant improvement in the control of the tandem hot rolling process.

An Initial Model for Control of a Tandem Hot Metal Strip Rolling Process

The tandem rolling of hot metal strip presents a significant control challenge because of nonlinearities and process complexities. The challenge is heightened by an extremely hostile environment that precludes the location of sensors to measure process variables that are important for control. In addition, it is essential that the controller structure allows for a high degree of physical intuition in the design process and provides for simplicity of tuning during commissioning. Based on our previous work using a state-dependent Riccati equation (SDRE) technique for control of the tandem cold rolling process, it is considered that a similar method might also be useful as a basis for the development of a control technique for tandem hot strip rolling. For the hot rolling process, the development of a process model in a form that is suitable for use with a SDRE-based method is a significant and challenging task. This paper describes our work to develop an initial model for this process. Based on simulation results, it is determined that this initial model has the potential to be the basis for the development of the complete nonlinear model that can be used for development of a viable control method which offers the likelihood for improvement in the control of the hot metal rolling process.

Control Scheme Using Forward Slip for a Multi-stand Hot Strip Rolling Mill

Forward slip is an important parameter often used in rolling-speed control models for tandem hot strip rolling mills. In a hot strip mill, on-line measurement of strip speed is inherently very difficult. Therefore, for the set-up of the finishing mill, a forward slip model is used to calculate the strip speed from roll circumferential velocity at each mill stand. Due to its complexity, most previous researches have used semi-empirical methods in determining values for the forward slip. Although these investigations may be useful in process design and control, they do not have a theoretical basis. In the present study, a better forward slip model has been developed, which provides for a better set-up and more precise control of the mill. Factors such as neutral point, friction coefficient, width spread, shape of deformation zone in the roll bite are incorporated into the model. Implementation of the new forward slip model for the control of a 7-stand hot strip tandem rolling mill shows significant improvement in roll speed set-up accuracy.

Mathematical modelling for rolling force and microstructure evolution and microstructure controlling with heavy reduction in tandem hot strip rolling

en

Optimal Control of Looperless Tandem Hot Strip Rolling Mills

A looperless tandem hot strip rolling mill system must be treated as a single multivariable system due to the strong interaction between stands. In this paper we develop a model of such a looperless system which consists of some 100 dynamic state equations and several hundred algebraic equations. The optimal control feedback matrix is calculated, and its relation to conventional control methods is discussed. Next, as a more realistic method, a decentralized optimal control technique is developed by using parameter optimization technique which greatly simplifies the controller hardware. Finally a novel method utilizing statistical methods is developed for obtaining an appropriate weighting matrix which forms the performance index for the optimal control.