A new high-speed rebar mill in South America struggled to roll consistently. Production was experiencing a high cobble rate when rolling 8 mm rebar at speeds greater than 32 m/s.  Russula was contacted by the mill when they just started to roll small product but were really struggling to roll high-speed rebar through blocks to a rotary channel high-speed delivery. The mill equipment was supplied by another manufacturer, which also supplied the pass design to slit the section to the two blocks.

Process know-how

The slitting process in the finishing mill is fundamental to stable rolling. Using modern diagnostic tools, Russula was able to monitor how the control system was reacting to the process. These insights can help determine the root cause of problems in the rolling mill with a slitting process.

Efficient modern day rolling requires a deep understanding of how the rolling process and control system are intertwined. Slitting gives us a unique opportunity to understand the relationship between the two because any variances in the division of material between the two lines can be detected and regulated outside the process by skilled operators / electrical engineers. One way to study the slitting process is to monitor how the control system is reacting to the material in each of the loopers in the rolling line. In modern rolling mills, the loop regulation is controlled by altering the stand elongations, to ensure that the loops stay at their programmed set point, and this data can be used directly to gauge process stability.

By watching how the elongations (reduction factors) react to the material, from the head of the billet to the tail, as well as from line to line after the slit has taken place, a significant amount of information can be gathered on the overall stability of the rolling process.  Russula utilizes this information to help improve the rolling process together with the customer.

Solution

At this customer site, a high amount of variation was seen in the finishing mill elongations between billets.  This suggested too much movement in the guiding or possibly an unrobust pass design. Russula worked together with the customer to evaluate their guide set up as well as alignment procedures in the mill. After a full review on the mill floor, Russula determined that while the design and shape of the guide rolls used in the mill did contribute to some variability, the main source of the problem was the slitting pass design itself.

After proposing a more modern approach to slitting pass design, the customer contracted Russula to redesign the sequence. Russula optimized their approach to slitting by changing the fill ratio of the passes to eliminate the constant chasing of a perfect division of area by guide alignment, which is the most difficult part of the slitting process for the operators. Russula also designed the slit sections to prevent the section tear from aligning itself into the gap of the downstream oval, which causes a visible seam on the up and down of sections rolled from two and four stands in the block.

Results

After redesigning the grooves, Russula assisted with the commissioning of the new design and was able to dramatically improve the overall system stability, allowing the mill to successfully slit all products.