It has long been recognized that the shape and crown of rolled strips are important in terms not only of product quality but also of material yield and rolling operability. Consequently, there has arisen a strong demand for the development of methods to control these factors. Over a considerable period, a great variety of control methods for this purpose have been tried. In recent years, attention has focused on ways of improving the control functions of the rolling mill itself as a means of tackling this issue. One of these improvements involves 6-high mills in which the intermediate rolls are shifted in order to obtain control. This method has achieved big improvements in mill performance, and has already been brought into practical use. However, since 6-high mills use more rolls than conventional 4-high mills, revamping 4 mills into 6 mills is a complex operation especially in the case of hot-rolling mills, where revamping to 6-high mills requires remodelling of large amounts of equipment. This difficulty has limited the range of application of 6-high mills. Furthermore, recent years have seen a trend toward hot charge rolling (HCR) and hot direct rolling (CC-HDR), which have received much attention as productivity raising and energy saving technologies, and toward small-lot multi kind production of high quality strips. However, conventional "coffin" type rolling schedule, in which slabs are processed in the descending order of their width, requires long waiting times between casting and rolling, which pose problems for methods such as HCR and HDR. In other words, what is required is "schedule free rolling" (SFR), which can free rolling from schedule constraints. Two techniques are indispensable for the realization of SFR. (See Fig. 1.) They are: ~R the "roll curve free" technique, which permits rolling of products for any desired application without changing the rolls, and (2) the "roll wear free" technique, which permits rolling of large quantities of strips of the same width, or of narrower strips followed by broader strips. For the realization of roll curve free rolling, it must be possible to obtain, as required, optimum roll curves that match rolling conditions such as properties and temperature of the rolled material, rolling load, strip width, etc., and in addition there must be easily-implementable compensation for variations in work roll initial crown (wear crown, etc.). These requirements in turn necessitate rolling mills with a high degree of control over shape and crown. For the realization of roll wear free rolling, height differences (steps) in the strip path must be eliminated, in order to ease the wear on the work rolls. To meet the requirements for roll curve free rolling, pair cross (PC) mills, which have ample control ability over shape and crown, have been developed. To meet the requirements for wear free rolling, online roll grinders (ORG) have been developed. We will leave reporting of ORGs until another occasion; in this paper, we report on the basic characteristics of PC mills, and their employment as hot rolling mills. In addition we discuss the broadening of the application of PC mills to include employment as plate-rolling and cold-rolling mills.
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