I have an old road frame, not sure what the year is. But its somewhere within the 70's and 80's range. It's been tricky to find the year, because the frame when I got it only said Nishiki on it... no model type.
Anywho, on the top tube, it says hi-carbon steel frame. I am wondering what that means. Is this a lower grade or higher grade steel? Good? Bad?
It is probably just advertising words, unless there is enough carbon in the steel that the whole thing was post build heat treated. That would be a big deal to pull off, does the bike look worth that?
Other than that one has stuff like 4140 that is stronger than 4130, but not usually available in tubbing. The 40 or 30 are the points of carbon, and add to strength but do not require heat treating. 4130 is used in aircraft tubbing, and lots of bikes, 4140 is used in gun barrels, and shafts where greater loads are expected. So there is an example of more carbon also being better. Nothing to worry about.