EXPERIENCE IN THE BETTER UTILIZATION OF ROLLS

N.

S. P. O r e l , S h i f t m a n a g e r 500 m i l l

U . P a u k o v , S e n i o r f o r e m a n 500 m i l l

Magnitogorsk Steel Works

The 500 mill consists of nine two-high stands. There is one pass on each stand. The varied batches pro- duced on this mill require an enormous stock of rolls. Together with ff~ systematic improvement in the quality of the rolls and the introduction of high-strength rolls with east grooves a system of combined dressing of the roils was employed providing a considerable improvement in their utilization.

For example, the channel iron No. 12-18 roll was designed on fine girder method. Since the slope of the outer edges of the flanges of the finished profile of the angle iron is not accounted for in the OST Soviet spec- ifications, the f+aishing passes are made with a tolerance of 0.5%. Worn rolls were restored only by employing

the width tolerance.

In the majority of eases the roils of the ninth and seventh stands were written off after re-turning once (taking off a metal thickness 20-25 mm) since on rolling with them after subsequent renewal the profile would be outside the tolerance limits in width. The design features of the mill do not provide for the arrangement of channel iron roiling with increased draw-off in the finishing pass and subsequent removal of the opening-up of the flanges on roll-straightening machines.

The following system of dressing the top (open) roll of the finishing stand was adopted at the plant. Two passes are cut in a new roll for rolling channel iron No. 14 with minimum width tolerance. After working both passes (strength of pass 700-800 tons) the rollisre-turned for channel iron No. 16 and further for No. 18. This re- turningdoes not reduce its strength. In a similar way re -turning is carried out on the closed roll of the seventh stand for rolling the same channel irons. This method of re-turningrolls allows for: double the length of utilization of the rolls of the seventh find ninth stands for rolling No, 14-18 channel iron ; re-turning without reducing the diameter of the roU ; and an improvement in the quality of straightening of the channel irons through the con-

stant width.

The system of overhauling rolls without reducing the diameter makes it possible to re-turn all the closed rolls of the seventh and ninth stands after wear through rolling channel irons to barrels and to use them again for

rolling strip.

For rolling strip use is made of the closed rolls of the eighth stand (control pass), after exceeding the width tolerances subsequent to rolling channel iron. Edging passes are turned on them for the fifth and eighth stands. The preparation in this way of edging passes considerably reduces the amount of lathe work as compared with cutting them on a fresh roll.

Out of the worn closed rolls of the finishing stand designed for rolltngR-18 rails, the open rolls are made for the sixth stand of the same section.

Interest is also provided by the practice of utilizing the rolls for roiling batch sections of reinforcement

bars.

168

The miU rolls No. 50-80 reinforcement round bars on a universal pass, attaining a high productivity. The durability of all the passes with lfie exception of the finishing pass provides for the rolling of very large batches of metal. The mean strength of the roll of the finishing stand is 650-700 tons.

Attempts to increase the strength of the passes by using harder roils remlted in a higher level of reject rolls in turning the grooves for the spiral projections.

In the majority of cases the cause for changing the pass was the inadequam depth of the spiral projections on account of wear of the roll (see diagram). In this case there is wear on one side of the miUed groove of the rell. Wear occurs on the edge first coming into contact during rotation of the roll and, as it were, impeding egress of the metal from the seat of deformation. The degree of wear at the opposite edges is negligible. This is explained by the sharply developed lead of the metal which amounted on average for No. 80 rounds to appro• imately 14%o, No. 70-12% No. 60-I0%o and No. 50 approximately 8%. After working all the passes the tells of the finishing stand were changed over, that is to say the roll occupying the top position in the previous campaign was replaced by the bottom roll and vice versa. The results were completely satisfactory since in operation the rolls were employing the edges of the milled grooves which were not worn.

IUeemplete projections (1) and wear of the finishing pass groove (2)

In this way without re-turning and milling the rolls it was possible m roll in each pass double the quantity of reinforcement steel of ba~h section No. 50-80.

In order to reduce the amount of metal taken off the diameter for this group of sections the wom ~r011s for rolling batch sections were turned in the following man- ner : from passes No. 50 No. 60 were made, from No. 60- No. 70, and so on.

Experience has shown that combined re-turning of roils provides a considerable increase in the degree of utilization of the rolls.

169