After some research, I think the starting material is best described as a 'ductile iron'.
References show it as a form of cast iron, but... and here's the interesting point:
It's also known as 'nodule cast iron', the main feature being Carbon nodules in the form of round congregations rather than flakes... the main result being the INHIBITION of cracks, because cracks are more likely to start at a sharp point than at a point with a radius.
This is interesting as can be. What a guy can learn with a computer...
It still isn't steel though... in steel the Carbon and Iron form an actual crystal, and there are variations thereof. A mono-steel I would expect would have to be heated above it's critical point to become homogenous, am I wrong?
Another question craigd, and I think you've done some work with steel and iron... why would not the excess Carbon 'burn out' of the iron during the welding process? It does so with steel during forging, and the forger has to limit the time carbon steel is held above a certain temperature to avoid this.
