There are a few options you can take, but to better understand you problem, I am guessing you cannot set 1 sheet = 50kg because the sheet thickness varies . if that is the case you can:
Define each Sheet thickness as one Product and use the “Weight Per Unit” field in “Item” to define the weight for that Sheet, you are basically converting a collective amount to a Unit.
You will basically, end up with multiple products for the different type of metal sheets(based on thickness) you have.
You can actually use dual UOM via the conversion factor, for example, have applied a case where a company would buy in the Cubic Meter and sell in Tonnage. In this case we just needed to define what the conversion factor was, albeit an estimate due to the nature of the product.
In your case however, You have a more exact option by using the above method.