Page 50 - MetalForming-Feb-2018-issue

The Science of Forming
By Daniel J. Schaeffler, Ph.D.
Know Your Sheetmetal Terminology—Part I
“Ineed some cold-rolled for this part!” “Get me some 1008/ 1010!”
You might think that you know what you’re asking for, but when talking with your supplier, it’s important that you both use the same language to be sure you’ll get what you need.
Sheetmetal is simply any metal alloy
that stores as a sheet, or is coiled as a
matter of convenience to ease han-
dling–some coils stretch for more than
a mile. Although no firm break exists
between categories, for most alloys,
sheet thickness ranges from 0.2 to 6 mm. Plate refers to thicker material, while foil denotes thinner (Fig. 1). Some use the term ‘shate’ for aluminum from 4 to 6 mm, too thin to be called plate.
Fig. 1—Material can be referred to as sheet, plate or foil depend- ing on thickness.
The term sheetmetal only describes thickness, and does not inform as to width, chemistry or tensile properties. Coils produced at sheet-steel mills typically measure 1000 to 2000 mm wide, with some aluminum mills capable of producing wider sheet. Rolling the thinnest-widest combination while maintaining tight tolerances on both is challenging for most
Danny Schaeffler, with 30 years of materials and applications experience, is co-founder of 4M Part- ners, LLC and founder and president of Engineering Quality Solutions (EQS). EQS provides product-applica- tions assistance to materials and manufacturing com- panies; 4M teaches fundamentals and practical details of material properties, forming technologies, processes and troubleshooting needed to form high- quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bachelor of Sci- ence degree in Materials Science and Engineering
from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
Tel. 248/66-STEEL
E-mail ds@eqsgroup.com or Danny@learning4m.com
production mills—one reason why not all products are available at every thick- ness or width. Strip mills are designed to maintain tight thickness tolerances at relatively narrow coil widths, usually less than 600 mm.
Whether steel or aluminum, all coils start off as one batch of liquid metal, with each batch referred to as a heat. Heat sizes at steel mills can measure to 300 tons, while those at some alu- minum mills approach 50 tons. Each heat has a uniform composition, so the chemistry reported on all products
from the same heat will be identical. This usually is the chemistry measurement that appears in a listing of certified metal properties.
All steel and some aluminum mills convert liquid metal into a solid form via continuous casting, with the discrete solid units referred to as slabs. One heat can produce 10 or more cast slabs. Most aluminum mills, as well as mills work- ing with other metals and alloys, cast liquid into individual units called ingots. Sometimes alloying elements segregate during solidification, especially during ingot casting. This may lead to a variation in chemistry in different locations, such as at the surface as compared with the center. However, the bulk chemistry remains the same as when the material was liquid.
Rolling comprises the next major processing step in the conversion of a slab or ingot to coiled sheet (Fig. 2). Here, thickness is reduced in a way similar to the use of rolling pins for flattening dough. Sheetmetal requires more rolling force than bread, so initial rolling occurs at a high temperature, hence the term hot rolling. Elevated temperatures deliver an easier way to make larger thickness reductions in succeeding passes. Because width does not change, the metal band elon- gates with every thickness reduction, while also increasing speed through the rolling mill. Most sheetmetal grades slow- cool in coil form, while faster cooling rates change the microstructure to increase strength. For at least some grades, hot-rolled alloys can carry the same strength and ductility as cold-rolled grades, but usually with different property ranges, wider dimensional tolerances and rougher surfaces.
Steel surfaces oxidize and create scale, a form of iron oxide that must be removed before further processing— performed by passing the material through acid in a pickling step. Rust-preventive oil added to the surface, if this is the desired end product, leads to the term “hot-rolled, pickled
Fig. 2—Rolling converts a slab or ingot to sheet.
Plate
Foil
6 mm
0.2 mm
Sheet
48 MetalForming/February 2018
www.metalformingmagazine.com