Abstract
A series of experiments were carried out on the effects of rolling variables on the occurrence of fire cracks on the roll surfaces of a universal type slabbing mill.
Types and arrangements of the nozzles for the roll cooling water were changed in succession, while observing the roll surfaces after rolling. It was established from both experiments and theoretical consideration that cooling of roll surface should be done at a location as far from the delivery point of the slabbs as possible, in order to prevent thermal shocking, causing severe fire cracks.Yet, cooling should be performed in such a way that the rolls may be cooled sufficiently to minimize the rise in the overall temperature of the roll.
The maximum temperature of the roll surface at contact with slabbs was determined by inspecting the hardness and microscopic structure of the quenched steel pieces plugged in the roll surfaces before rolling.The surface temperature was found to be from 470°C to 600°C, where the temperature of the bottom rolls was higher than that of the top rolls by about 30 degrees. The depth of heat-affected zone was found to be approximately 2mm from the surface, beyond which an instantaneous rise in temperature was less than 200°C.
The mode of machining and the roughness of the finished roll surface had no apparent effect on the occurrence of randomly oriented fire cracks, while they had definite influence on the occurrence of circumferential cracks.Every sharp notch on roll surfaces created by machining did not fail to lead to severe circumferential cracks.Apparently smooth surface provided by a flat-tipped bit also induced circumferential cracks.On the other hand, considerably rough surfaces provided by a sharp-tipped bit did not lead to such cracks.
Wear profiles of the top and bottom rolls were obtained by micrometer.The top rolls were worn 35% more in average than the bottom rolls.
Roll speed was recorded by means of 16mm motion picture.The top roll was found to revolve 2 degrees more than the bottom roll during a pass, which was an indication of the existence of slip between the top roll and the slabs, causing more wear on the top roll than the bottom roll. The greater wear of the top roll than the bottom roll was considered to be the main cause of the fact that less severe fire cracking was observed on the top roll than on the bottom roll.
Thanks to the two years study reported here, the rolling efficiency twice that of the initial state has been attained.
