The pickling process involves unwinding the steel and running it through a series of hydrochloric acid baths and rinse tanks. Once the steel is free of scale, it is typically coated with rust preventive oil and recoiled. Pickled steel can be sold to market or sent to a cold mill for further processing. The steel also can be trimmed to a customer’s width specification.
Pickled and oiled steel products are used for automotive frames, electrical cabinets, or painted, unexposed structural parts.
A typical pickle line consists of the following:
Uncoiler – Coils are received from the hot strip mill and unwrapped for processing;
Processor (Breaker Roll) – Minimizes tendency for coil breaks;
Shear & Welder – Provide square coil ends for welding;
Temper Mill/Tension Leveling – Increase pickling efficiency by breaking the surface oxide and provide a degree of strip flatness;
Pickle Tanks – Chemical reaction with hydrochloric acid removes black oxide scale;
Rinse Tanks – Clean residual acid from surface;
Dryer – Removes excess water to prevent rust and/or stain;
Shear – Cuts rolls to desired weight;
Oiler – Adds a thin layer of oil for rust protection;
Side Trimmer – Cuts each edge for width control;
Recoiler – Steel is recoiled and banded ready for transport.
Alignment of the rolling elements in the pickle line is crucial. As you can see, misalignment of any of these processes to each other or the whole will result in poor quality, scrap, and wasted time and money.
Misaligned rolls can cause strip tracking and differential sheet tension, which will affect line control and quality. Other common issues include: Payoff entry tension rolls misalignment affecting thread-up and entry steering; wringer roll misalignment affecting tracking, roll wear and acid contamination; Recoiler and steering control misalignment affecting even and straight windups.
Secondary misalignments create excess vibration, which in turn causes excessive impacting that will result in poorly finished products and scrap. The damage to the bearings that support the rolls can be severe, causing premature failure and unscheduled downtime, not to mention the cost of the bearings and the cost of replacement.
There are other issues specific to the pickle line, typically occurring in the pickle and rinse sections. These issues are usually generated at the payoff end of the line and show up in pickle and rinse.
Telescoping at the rewind mandrel is a common result of misalignment. The eye coil or the first several wraps do not wind true to the coil. Telescoping makes coils difficult to handle and susceptible to damage.
If a line is a continuous pickle line, there may be problems with the looper cars that allow for continuous operation. The looper cars store or accumulate steel, allowing different sections to keep running, usually the pickle and rinse, while one section is stopped. This is to allow a new coil to be welded or stitched to the end of the coil in process, or to allow a coil to be sheared and unloaded from the delivery end. Historically, as little as 45% to 50% storage capacity is used because of tracking issues, which seriously reduces throughput.
The issue: Uncontrolled tracking to one side of the line at the delivery (exit).
Operators had to feed the strip in off-center to get it to track at the delivery end. This was a consistent trend for all strip widths, but especially for wide, heavy gauge steel. Correcting the strip feed during operation required stopping the line, adding unnecessary additional time to the process.
The first step in improving the strip tracking was measuring the process tank rolls for square and level. The data was reviewed and the decision was made to correct eight roll stands. Priority was given to the stands that would improve strip steering in the tank section and reduce the bias of the strip on the operator side. Significant corrections were made to the roll stands.
The entry equipment, bridle rolls, tension leveler and flattener were measured for level and square. Opportunities for improving alignment were also found in this section. The flattener was found to be out of alignment and in the opposite direction of the lead-in pinch rolls.
Once the root cause of the misalignment had been identified, corrections were made to the roll stands (corrected for square, rolls leveled), flattener, bridle rolls (level and square), and the entry payoff stubs were rebuilt and replaced.
The first benefit was improved runnability. The strip was easier to control and did not need to be offset at the entry of the line. The second benefit was speed through the tanks. Throughput increased since it was no longer necessary to aggressively steer the strip as it ran consistently down the process centerline. Edge damage was reduced, increasing end product yield. Improvements in the control of speed and steering produced more control in the pickling tanks and better quality pickling.
The measurement and alignment in this case study was conducted with lasers. This makes the data repeatable at unrelated time intervals over the course of the entire measurement and alignment. Because laser measurements offer live data, moves can be made in real time, eliminating the need to move and recheck and then repeat until the desired alignment is achieved, and saving time and money.
All of the measurements and corrections were made without disassembling the pickle tanks by using a patented enclosed roll process. This significantly reduced the downtime required.
Proper alignment of all components to a single baseline is critical to the operations and maintenance of all sections in a pickle line. The laser measurement techniques presented here can be applied to all sections and/or rolls, whether measuring an entire section or just replacing individual rolls.