Daniel SchaefflerGetting More Than You Ordered
January 30, 2020Comments
A lower 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 chemistry limits serving as the control parameter that constrains sheet suppliers. 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 former’s intentions and shipping something to an appropriate specification, perhaps what ASTM defines as commercial steel, drawing steel (if uncoated) or forming steel (if galvanized). Even within ASTM, listed tensile properties are shown as typical and nonmandatory. 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 interchangeably vacuum-degassed interstitial-free (VDIF) steel or extra-deep-drawing steel (EDDS). These steels are found on stamped parts requiring aggressive styling with limited structural requirements, such as automotive fenders or one-piece body-side-aperture 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 efficiencies by tuning its process to the characteristics of the incoming metal. Changing suppliers risks increased variability, as a new supplier may provide sheet metal with a different composition that still satisfies the requirements 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 supply 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, thereby 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 prevents the supply of ultra-low carbon steels, which narrows the typical property 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 considered. 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, depending 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 manufacturing process performed on molten steel prior to casting it into slabs, removes much carbon and results in a very formable but soft product. The extra processing also results in a steel grade that has reduced property variability compared with other grades, which should improve productivity and quality at the metal stamping company. A metal former specifying EDDS will be charged accordingly for this extra step required to achieve the EDDS properties.
