Information employed in fuel-conserving control of metal heating

Abstract

In rolling, 15‐25% of the basic equipment costs are accounted for by the measurement and centralized monitoring systems and by software for automatic control of the technological processes. In furnaces of chamber and pass-through type, considerable energy resources are consumed, and hence the costs of furnace automation should be quickly recouped through fuel conservation. Pass-through furnaces are complex inertial systems with distributed parameters. In each heating zone, there are different heat-transfer conditions between the combustion products (hot gas), the protective refractory lining of the working space, and the metal surface. Analysis of the operation of the furnaces in the 2000 mill at OAO MMK shows that, in the case of nonuniform productivity (250‐1000 t/h), with simultaneous heating of 3‐7 batches of billet of different steel, in very different initial thermal states (100‐700 ° C), ongoing fuel-conserving control is difficult. In these circumstances, individual (differential) control of the heating of each billet batch is expedient. However, this requires correct selection of the method of temperature monitoring for each billet as it moves through the working space of the furnace. Currently, the temperature of the working space t ws ( τ ) is used as the basic source of information in the temperature control of furnaces. It is measured by a thermocouple or an optical pyrometer pointed at the bottom of the carborundum vessel. The measurements characterize the rate of heat transfer between the surface of the metal being heated and the working space, and reliably represent the current state of the metal only in stable (steady) operating conditions of the mill and furnace. In nonsteady conditions, t ws ( τ ) is not objective and does not uniquely characterize the current thermal state in the heating zones of the furnace. To confirm this, the variation in natural-gas flow rate V g ( τ ), working-space temperature t ws ( τ ) (measured by a radiant pyrometer in the furnace roof), and the temperature of the rolled metal t rm ( τ ) after the roughing group (measured by a partial-radiation optical pyrometer) is plotted in Fig. 1a for the soaking zone of continuous furnace 1 of the OAO MMK 2500 mill. The temperature of the metal t rm ( τ ) is regarded as an objective measure of the quality of metal heating in the hot-rolling mill. The temperature conditions of the soaking and other zones of furnace 1 are controlled automatically by stabilizing the specified workingspace temperature ( τ ) on the basis of adjustment of the gas flow rate. At high rolling temperatures, with motion of the unheated metal in the soaking zone for 120 min (Fig. 1), it is established that ( τ ) = 1305 ° C, tws