An optimal multivariable looper control system based on optimal-regulator theory has been developed for hot strip finishing mills. Loopers of a hot strip finishing mill are installed between stands and are, in many cases, driven by motors. The goal of looper control is to maintain product quality at a high level by stabilizing the rolling operation. Stability is achieved by keeping the looper angle and tension applied to the strip at given reference values. The most difficult problem is mutual interaction in the simultaneous control of looper angle and strip tension. In particular, increasing use of low-temperature heating of slabs, high-carbon steels, and high-speed rolling places additional demands on the looper control system. Moreover, high quality of thickness, width, and crown are strongly requested.The optimal multivariable looper control system has the following features: 1) Controling the rotational speed of the looper motor in addition tothe existing looper motor current control has allowed a robust looper control system that can cope with changes in rolling conditions. 2 ) Based on the current rolling conditions, the system selects the control gain calculated from different weight matrices for the linear quadratic criterion.The developed control system can maintain relatively constant strip tension without operator intervention in the presence of rapid changes of hydraulic screwdown such as automatic gage control (AGC) operation. The optimal multivariable looper control system has been functioning successfully in commercial operation in the hot strip finishing mill at the Hirohata Works of Nippon Steel Corporation since January 1987.

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It is important for hot strip mill to keep finishing mill delivery temperature to it's target value because of yield strength and tensile strength of rolled strip. We propose a new method for finishing mill delivery temperature control (FDTC) of a hot strip mill using inter stand strip coolant (ISC). FDTC consists of initial setting (before threading) values calculation of ISC flow rate and finishing mill speed, feed forward FDTC (FF-FDTC) and feedback FDTC (FB-FDTC). The mathematical strip temperature model of finishing mill was identified using normal operating data and some tests. Actual mill application with 7 stands finishing mill showed good FDTC results and also it made possible to increase rolling speed which increase production rate.