[Geezer (OP)]

It's probably worth pointing out that the reason big lumps of iron and stel don't (usually) burst into flames is because the large mass of metal disperses the heat quite rapidly and only a small amount of the iron is exposed to the air, so the supply of oxygen is very limited.
 
However, you can burn a hole in a really thick piece of steel if you supply enough oxygen. That's what oxyacetylene cutting torches do. Once the steel starts to melt, the acetylene is shut off and the burning steel generates all the heat required to keep cutting as long as oxygen is supplied.
 
Steel and iron particles can even burn so rapidly that they can produce an explosion in a confined space. Fortunately, the particles are dense, so they fall out of the atmosphere quite quickly, but if you put fine iron filings into a sieve and shake it over a naked flame, the filings will burn in a quite spectacular fashion.
 
This is a very dangerous experiment. Do not attempt it in your kitchen.