Page 32 - MetalForming February 2020

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Getting More Than You Ordered
Alower price may be reason enough for metal formers to switch sheet metal suppliers. However, the cost savings may entail a high price unless metal formers do some homework to pave the way for a smooth transition.
Consider the case of a basic steel grade, which may be specified as mild steel or 1008/1010 in a purchase order. Unfortunately, mild steel is a generic term, with no restrictions on chemistry or tensile properties. Further, as readers of this column know, there is no one grade called 1008/1010. It represents two distinct products, with only chem- istry limits serving as the control parameter that constrains sheet sup- pliers. Tensile properties influence stamping success, and grades 1008 and 1010 have no associated strength or ductility constraints.
Ordering these grades leads to the steel supplier interpreting a metal for- mer’s intentions and shipping some- thing to an appropriate specification, perhaps what ASTM defines as com- mercial steel, drawing steel (if uncoat- ed) or forming steel (if galvanized). Even within ASTM, listed tensile prop- erties are shown as typical and non-
Dr. Danny Schaeffler, with 30 years of materials and applications experi- ence, is president of Engineering Quality Solu- tions (EQS) and chief content officer of 4M Partners. EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler is the Metallurgy and Forming Technical Editor of the next release of the AHSS Applications Guidelines, available from WorldAutoSteel.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
mandatory. ASTM characterizes each of these products as Type A, B or C, with each subtype having restricted chemistry limits that allow for some generalized assumptions.
CS-Type A allows for carbon levels as high as 0.10 percent. This allows the steel supplier to ship coils containing ultra-low carbon levels associated with the most formable grades, called inter- changeably vacuum-degassed inter- stitial-free ( VDIF) steel or extra-deep- drawing steel (EDDS). These steels are found on stamped parts requiring aggressive styling with limited struc-
A metal former (working with a single supplier) can increase product quality and associated efficiencies
by tuning its process to the characteristics of the incoming metal.
tural requirements, such as automotive fenders or one-piece body-side-aper- ture door openings.
Because CS-Type A also allows a supplier to provide steel containing 0.10-percent carbon, the shipment can be a higher-strength steel, requiring the metal former to plan for challenges associated with flattening, blanking and stamping.
Working with one supplier increases the likelihood of receiving a consistent product. A metal former can increase product quality and associated effi- ciencies by tuning its process to the characteristics of the incoming metal. Changing suppliers risks increased variability, as a new supplier may pro- vide sheet metal with a different com- position that still satisfies the require-
ments of CS-Type A. Metal formers should provide current certs to the new supplier to inform them of what has been working and to minimize the risks associated with changing to a new sup- ply chain. Value-added metal suppliers also will want to review the parts to be formed to further ensure that the ordered grade is appropriate for the design and usage characteristics, there- by minimizing potential claims for poor performance.
CS-Type A, the steel grade with the fewest restrictions, usually is the least expensive. Specifying CS-Type B pre- vents the supply of ultra-low carbon steels, which narrows the typical prop- erty range to be processed and formed. CS-Type C limits the high-end carbon levels, making the supplied steels, in general, more formable.
The Cost of Getting Something For Nothing
The scheduling complexity of high- volume sheet metal production increases as more variables are con- sidered. Frequent changes in alloy or grade means more setup adjustments in the casting and rolling recipes. Mill scheduling departments maximize quality and throughput by targeting minimal changes in grade, width and thickness from one slab or band to the next.
Steel mills use fixed melt-lot sizes on the order of 100 to 300 tons, depend- ing on the location, with chemistry identical throughout each batch, known as a heat. Mills may even have a requirement targeting identical chemistry in at least three consecutive heats.
Vacuum degassing, an extra manu- facturing process performed on molten steel prior to casting it into slabs, removes much carbon and results in a very formable but soft product. The
30 MetalForming/February 2020
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