The Sun-Cutter uses a small glass ball lens. My idea is also to use a large ball lens made out of plastic and water. It could be used to boil water or drive Stirling engine.

The big problem with solar energy in most climates is that it is undependable.  Yeah, if you live in the sub-sahara you can rely on the sun being out most of the day, but temperate climates have clouds part or most of the day.  Then there is a problem with losing your energy source every time night rolls around.  Can you imagine getting all set up to cast a big part out of steel to be melted in a solar furnace only to have a storm put you out of business for 2-3 days?  Frustrating, to say the least.

And now for a practical look at recycling steel...

Generally, for any high-quality steel making process, it will typically go like this:

The scrap steel is melted down to molten, and then fired in a reducing environment (with excess carbon & limited oxygen or air injection, to burn off the additives, including the carbon that make it steel vs. raw iron).

The molten iron is poured off, as it is much easier to separate from the slag which will be everything else that didn't get burnt off, and the iron is poured into ingot molds for later use, or poured into other crucibles for mixing & making steel.

On a small scale, it might not be poured into a new container, but instead, the alloy materials may be added to the original crucible, and it is then mixed to form the alloy desired, before pouring into the molds for use as steel.  The lances that are used for injection of oxygen (or plain air for the old-fashioned Bessemer process) can be cooled with a water jacket to keep them from burning up (too quickly).

Once the iron has enough carbon in it to be proper for the mixture, adding air or oxygen will make the reaction exothermic, so one wouldn't need to keep heating it further, but a straight induction furnace simple won't have the chemical properties needed to recycle scrap steel & iron to high quality steel, without a great deal of external control of the environment, because at the temperatures involved, it is nearly impossible not to just make a lot of oxides (slag & rust) if one doesn't control it.

Face it, there's much more to making steel than even just getting an induction furnace, and right now, coke is still one of the best raw materials to use for making iron from steel.  An induction furnace can be useful for melting many metals and do a good job of it, but process control to make specific kinds of steel from other kinds of steel or scrap of unknown alloy will be a crap-shoot with no way to make the process repeatable.  Chemically, one will still have to start with scrap and reduce it to iron, and then start over, or have little control over the mixture in the alloy.  Lots of nasty chemistry at high temps involved there, with lots of horrid smells and potential air pollutants to deal with along the way, without some major prep work for scrubbing the exhaust.

The induction furnace looks really good in concept, but even if OSE can get there, they are still a long way from using it in the fashion they've described in the goals, as presented.

I still wish OSE luck with it, but I don't have inflated hopes that I could use this for anything in the next 5+ years or more.