I would advocate plain steel with carbon between 0.85 and 1%, hardened to 59-60 if I had a choice.
(I guess I have that choice to advocate).
I'd say another thing for plane irons - it doesn't cost much to find a decent used one. It also (if one has the facilities to do the hardening, which isn't much) doesn't cost very much to get tool steel and make one, though that can be a tall order for an iron that should be tapered. One of the best irons I've used is one that I hardened out of 1/8th O1. It is probably in my ideal hardness range, and it was a straight up cowpat for the first 64th of an inch or so. By facilities, I mean I have a $50 torch that's normally used for weeds, and three quarts of soybean oil in an old paint can. The torch hooks to my grill bottle.
At any rate, I wouldn't cut a saw blade into an iron, just not the kind of thing I'd do because doing something proper is not very difficult.
If I didn't have proper wood around, though, I wouldn't hesitate to use plywood if I wanted to just make planes.
My opinion on this is a little further down the road than most peoples', though, and here's why. In the last 11 years, I've made a whole bunch of planes, and for the first 6 or so of that, I liked the idea of making something that was easy to make or used spare whatever is around that's free kind of stuff. What happens when you do that, though, is that you end up with a plane that works, but it doesn't work well enough that you'll keep using it very long. I have the same feeling about krenov planes, especially the type with the really little narrow irons. The little details like a substandard iron are the ones that make you put a plane down, among other things (poor design, poor construction, etc). In my opinion, someone making a tool for more than a youtube video (the youtube universe loves the "something for nothing" idea of making a plane out of junk, just like it loves videos like "newspaper log makers" or other such things that nobody would actually do in quantity off of youtube), then the idea should be to make a tool at least as good as you can buy so that you are not so willing to just cast off the tool at a later date in favor of something else.
To make a plane as good as you can buy is a pretty tall order, especially when you guys can get stuff like this postage paid:
http://www.oldschooltools.co.uk/product ... ane-wp175/
It'd be a wiser investment by someone who wanted to do serious woodworking to learn to refit these older planes, but it doesn't make those something for nothing kind of youtube videos to do it.
For various reasons, the guy who made that plane will end up casting it aside. It's possible that the iron will be one of those reasons, and certainly possible that it won't.
I guarantee if I was making plane-related videos on youtube to try to generate views, they wouldn't be about making the planes in the style of the above (which is sold out because I bought it. I don't need it, but the cost is still in the range of what it costs me just for materials to make a similar plane, even with shipping to the US).
Sometimes, when you've thought about planes as much as i have (and cast aside as many as I have - at least they were sort of fun to make), it's a pain in the ass to actually talk about those thoughts.
