I think the term is fuzzy, so you have to fall back on the linguist's interpretation: if it's commonly called 'steel', it's steel. Note that you need to pay attention to different language groups, though -- if there's a firm population of technical types that consider anything less than 0.2% to be "iron" and not "steel", then more power to them -- but don't let that keep you from calling your steel supplier and ordering
Ordinary steels max at around 1%, but, for example, music wire can run up over 1.2%
In theory, you get maximum hardness from martensite conversion (the basic hardening mechanism for carbon steel) at 0.7%. But that assumes perfect mixtures of iron and carbon. In fact, you can get greater hardness up to around 1.2%, or even a little more -- up to close to 2% -- if your only hardening occurs by drawing. For example, music wire.
There are some other specialty steels that run up to 2%.
There are graded carbon steels, such as 1008, that run as low as 0.08%. Before we had HSLA steels, that was auto-body sheet metal. Below that, and you have soft iron. Commerically, if it has a lot of silicon added, it's "magnetic iron," which was used for motor and transformer laminations. It probably still is.
Above 2%, and you have some very cranky steel that's better classed as iron. If it has impurities in it, it's pig iron or cast iron (same thing).
I'm not official, but I was Materials Editor for _American Machinist_. I'm no expert and I know little chemistry, but I did have to study the hell out of it at one time.
Plain-carbon straight razors. Sorry I don't know of anything more recent. That was in a book I read about steel, and it was printed some time in the '30s.
IIRC, early surgical scalpels also were made of carbon steel of very high carbon content, well above the amount in AISI/SAE graded steels.
Music wire traditionally has been made from proprietary, small-crucible melts, and can run well over 1.2%. It's some version of electrode-remelt today, either vacuum-arc remelt (VAR) or electroslag remelt.
You're testing my memory. I haven't read or written about most of this for close to 30 years.
That's interesting. When I lived in Massachusetts for a while I'd go into supply places and I'd say "Back in Oregon we called this a flognozzle*, but I don't know what you guys call this here". They'd grin and say "Oh yea, that's a blogflorgle** -- I got one right here".
They call a standard stamped steel outlet box a 'Handibox'. I needed some TFFN wire to replace a damaged wiring harness for a TV transmitter in the Florida panhandle. At first the local distributor refused to deal with me, even though I had an account at a different branch. I smiled and told him that was fine, that I would wait till Saturday when I would be in Orlando and pick it up, and have the manager contact corporate to file my written complaint. He nearly filled his pants and looked like he was going to faint. Then I found that they only stocked Blue, Brown and White and only 14 AWG.
Other times I had to take the OEM catalog with pictures to show them what I needed. When I lived up north, you asked for EMT. Down here it's a 'Stick of pipe'. They thought Romex was a brand name, so they had several other names for it.