The Value of the Ulbrich Specialty Strip Mill (USSM)
Ulbrich’s rolling division, USSM, provides engineered strip metal and foil products to exacting specifications. USSM produces a specialized rolled coil, not readily available from other suppliers. Our ability to deliver a customized and consistent product allows us to provide our customers the material they need to create enhanced processes and products. The strip mill’s diversity of equipment and experienced staff enables us to establish robust processes capable of supplying a precise and consistent product for use in a wide variety of applications across multiple industries.
USSM converts strip metal from widely available standard specifications to meet unique customer needs including: thickness, width, specific mechanical properties, industry specifications and many other critical characteristics. The first step in the conversion process is rolling, which reduces thickness. Let’s take a more in-depth look into the process below.
Cold Rolling is the cornerstone of USSM and the heart of the conversion process. Rolling is a continuous metal forming process in which metal stock is fed between two hard work rolls which apply enough pressure to plastically deform the metal, inducing a permanent reduction in thickness uniformly across the width of the strip. Rolling at USSM is considered “cold rolling” because we do not increase the temperature of the material before we roll it. Modern rolling practices employ a combination of screw down pressure, perpendicular to the strip surface, and longitudinal tension to ensure that the reduction in thickness translates exclusively into an increase in strip length instead of a reduction in strip width.
As the strip realizes a macroscopic reduction in thickness and increase in length, the grain structure is also changed. Rolling elongates grains in the longitudinal direction and, depending on the extent of the deformation, either partially breaks down or completely transforms the individual crystal structure of each grain. This microscopic modification from equiaxed to elongated grains translates into an increase in strength and reduction in the ductility of the bulk material. Strengthening the material through plastic deformation in this way is called cold working or work hardening. The more the thickness is reduced, the more the microstructure is changed and the harder and more brittle the material becomes until it cannot be reduced any further without breaking. Each alloy work hardens at a different rate and has a specific maximum amount the thickness can safely be reduced before it must be annealed.
Understanding the work hardening rate of a given alloy or heat lot within an alloy is paramount to achieving the mechanical properties specified by the customer. Process routings for tempered items allot a specific amount of reduction in thickness during the final rolling session, which is intended to work harden the material to the mechanical properties specified by the customer. An item ordered “Quarter Hard” will require a very small amount of reduction compared to a “Full Hard” item which would be rolled a reduction approaching the maximum the alloy can withstand without breaking. Generally, appropriate reductions are determined by analyzing a combination of historic data and chemistry.
USSM currently owns 9 rolling mills of two different configurations: 3 Four High mills (H-mill) and 6 Sendzimir mills (Z-mill). H-mills are the simpler of the two, and use two large diameter work rolls which are driven and supported by a set of back up rolls situated atop and below, hence four high. The more complex Z-mills use three sets of cascaded back up rolls to focus the screw down force of the mill onto much smaller diameter work rolls. This roll cluster enables the Z-mill to apply a greater roll pressure without roll bending than an H-mill. Apart from differences in tooling, both H-mills and Z-mills operate in a similar manner. The operator can vary the screw down force applied by the work rolls, and the tension applied by the coil pay-off and take-up winders to achieve the desired amount of reduction for each pass. The equipment generally cannot provide enough force to induce the entire planned reduction at once; coils are rolled back and forth multiple times, or passes to achieve the desired reduction. USSM has been honing its rolling practices since Ulbrich purchased their first Z-mill in 1955 and first H-mill in 1957. Now, our largest H-mill can handle incoming material up to .187” thick, while our smallest Z-mill can roll foil as light as .00039″: an order of magnitude thinner than a human hair.
Written by Keith Grayeb