Hi J-V,
Good to know that I am not the only one that likes to combine blacksmithing with knotting. But I would never attempt to work with something as heavy as 20 mm rod. I have made some poker handles using a figure-8 knot in 12 mm, and some gatepost stays in 16mm and I know how the amount of effort needed goes up exponentially with every additional millimetre of thickness.
I have been doing some smaller work lately in 6mm stainless steel for keyrings. The Carrick bend is fairly challenging in stainless (I have a bucket full of failures) and is one knot that I think is easier the more you scale it up (but not as big as 20mm).