What do you think Steel is, but reduced Iron (to a specific level)?
Cast Iron is also impacted by hydrogen embrittlement, albeit iron is usually already pretty brittle so folks don’t use it where that kind of failure matters as much.
If you're talking pure iron, then yes - it's very ductile and malleable. However, the low end of carbon allowed while still calling something steel is .3%, so with a few exceptions (wrought iron, which is arguably very soft steel), it's rare to run across it.
There are of course ways to treat cast iron so it's decently durable (ductile iron), but it's still pretty brittle compared to 99% of steel in the real world.
> low end of carbon allowed while still calling something steel is .3%
Incorrect. That is the maximum for low carbon steel (also called mild steel - the most common form of steel).
Wikipedia says “The carbon content of steel is between 0.002% and 2.14% by weight for plain carbon steel”. “Plain carbon-iron alloys with a higher than 2.1% carbon content are known as cast iron.”
