So what is high carbon steel and how does it fit into this line up of steels?

Materials and processes. - a Bike Culture article on Cyclorama

It doesn't appear to be mild steel according to this.

https://www.metalsupermarkets.co.uk/what-is-mild-steel/

Where does it fit in the picture of bicycle frames?

It seems to be producing quite light frames which are competitive with aluminium in weight sometimes. Is that because they have sacrificed strength?

Fuji-ta the worlds biggest bicycle manufacturer with maybe over 15 million complete bikes made a year and a huge business in selling frames to assemblers around Asia, Europe and the America's offers many high carbon steel frames even for higher end models be it racing, mountain bike etc but these seem to be models we don't really see in Europe so much where retailers favour aluminium especially mountain bikes. They claim they use an improved high carbon steel but on the site they still call it high tensile steel which is low carbon steel.

These high carbon steel frames seem to be typically lighter than the older high tensile steel frames of the past.

Looking around at tensile strength it looks like a typical old high tensile steel frame (1015 steel) had a high tensile strength of around 60,000 psi and a classic chromoly 4130 is 97,000 psi but the high carbon steels like 1065 is 92,000 psi which I suspect is after heat treatment as the high carbon steel frames are often heat treated. So would it be fair to put high carbon steel just below chromoly in pecking order or are there other factors?

The chinese seems to be promoting these high carbon steel frames as superior to older steel frames for their domestic market.

The aluminium frame materials 7005-T6 is at 51,000 psi tensile strength and 6061-T6 is at 45,000 obviously they are lighter but have no endurance limit so suffer more from fatigue so often more overbuilt. Could high carbon steel frames actually be the sweet spot in value, performance, strength and ride quality?