How common lorica segmentata actually was?

[Gaius Colletti]

It can also turn you carbon steel (which is a redundancy since all steel has a carbon content 0,7%-2%) into cast iron, one of the reasons metal was pretty much crap in the middle ages.

Your figures are wrong, Mild Steel alone can have up to .25% carbon, and can have significantly less as well, with high carbon having as much as 1.5% carbon content. Who said steel was crap in the Middle Ages? Some of the best metal working was done at this time? Ever see a gothic knight or some equipment used to make build cathedrals?

Are you saying Middle Age metal was less durable or less strong? Different types of steel have different properties, depends on what you intend to use it for