Effect of Interpass Time on the Hot Rolling Behaviour of Microalloyed Steels

Abstract

The three ranges of industrial interpass time (i.e. short, intermediate and long) are described and compared. These correspond approximately to the finishing stands of rod mills, strip mills, and plate or reversing mills. The concept of the T nr or no-recrystallization temperature is reviewed. The possible types of interaction between strain-induced precipitation and softening by means of static, dynamic and metadynamic recrystallization are then characterized. With the aid of mill logs obtained from a number of rolling mills as well as of laboratory torsion tests, it is shown that when the interpass time is short, as in rod rolling, neither strain-induced precipitation nor static recrystallization is possible. Under intermediate interpass time conditions, as in strip rolling, two types of behaviour can be seen. One corresponds to the long interpass time type of response associated with plate rolling (i.e. pancaking or CCR). The other follows the rod mill pattern of strain accumulation, leading to the initiation of dynamic and then metadynamic recrystallization. The first requires the presence and the second the absence of strain-induced precipitation.