How Thick is Your GI?

I just had a talk with an esteemed former colleague from National Steel Corp (Iligan City). He's now managing a drum-making plant somewhere in Northern Mindanao. Our discussion centered on his query on what could be the technical reasons why for a given GI sheets order, there is a huge and significant difference on prices between suppliers. He is buying GI sheets for their drums and he was comparing the quotations submitted.

I explained the common "technical" reasons relating to price difference. First is on the actual base metal thickness used. While the nominal thickness maybe the same, the actual maybe different and very far in comparison. The use of the old nomenclature describing thicknesses as say Gage 18 for example is misleading. For one, just what is Gage 18 really? Ordinary consumers may not really know what it is really as long as their supplier says it is Gage 18.

In the old book, The Making and Shaping of Steel, Gage 18 is actually MSG ( Manufacturers Standards Gage) Gage No. 18 which corresponds to 1.214mm. But in reality, thicknesses as low as 1.0mm to as thick as 1.4mm maybe passed off as Gage 18. A steel coil with 1.0mm actual base metal thickness will yield more sheets than a coil with 1.2mm actual thickness, thus can command a definite price advantage. This also commonly happens in GI for roofing sheets where the most popular size of Gage 26, which is supposed to be 0.4mm ( originally it is 0.455, then reduced to 0.426mm) would now be 0.35mm and below. With the Metrication system implemented a long time ago, this practice of naming sheets as Gage 18, 31, 26, 16, etc should have been stopped and instead the direct nominal thicknesses as 1.2mm, 0.2mm, 0.4mm, 1.4mm, etc should be used. In the PNS (Pilippine National Standards) and all other international standards, like ASTM, JIS, ISO, etc, the latter is specified

The second factor is on the actual mass of coating. A coating of 100gms/m2 (Z 100) would definitely be price advantageous than a coating of 140gms/m2 ( Z 140). It should be noted that in galvanizing, cold rolled steel coil (CRC) raw material and zinc comprise the bulk of production cost.To get therefore a real comparison, actual apple to apple comparison is needed.