Would a long piece of metal work as a bow string?

Considering the bow string is intended not to be rubbery and flex back, but to pull the stick backwards would it work? No

Also how capable would it be of slicing my forearm when I shoot it? YES

I highly doubt that your piece of metal is both flexible and unable to stretch. It might be possible with a steel cable, but a simple piece of metal? Forget it.
Can it slice your arm? Depends on how thin it is. They tested it on Mythbusters with a steel cable and it just wrapped around the test pig, but did not slice it.

I really don’t think the kind of metal or wire you describe is suitable for a bow.

I’ve looked a several sets of instructions for homemade bows and they all suggest using cord or string. One of the reasons, I suppose, as to why it is generally referred to as a “bow string.” :-)

For example”

This guy used kite string

And this guy used reverse-wrap cordage

From his instructions:

Bowstrings

Bowstrings are made using the reverse-wrap method of making cordage that I described in Mother Earth News Volume 79, 1982, only you will be using sinew in most cases. For bow string, what I do is a double-reverse wrap. By reverse wrapping more than ice the length of the bow, then loading the cord in half again and reverse wrapping it, you will have a very strong and durable bow string. . Plant fibers can also be made into bow strings the same way, but they are pot as good. Plant fiber bow strings, however, will not stretch like sinew bow strings in foul weather. Velvet leaf, dogbane, and nettles make good bow strings for wet weather purposes.

I have no idea what double-reverse wrapping is, but you want to give it a try, I’m sure it’s very easily googled. :-)

It wouldn’t take many cycles of being bent to 40 degrees (at the nocking point, at full draw) and then straightened out again (at the release) before metal fatigue would set in and the “string” would break.

A piece of wire would not be flexible or “springy” enough. Releasing the arrow relies on tension in the string which wants to return to its former position. This is what “pushes” the arrow forward. Wire won’t do that: you’d let go, the wire would stay bent where it is (or return to position very slowly) and the arrow would simply drop to the floor. Also, trying to draw it would seriously hurt your fingers.

You need cord, and it needs to be non-elastic.

(I did competitive archery for 2 years when I was younger)

Whatever is used, should really be twisted, preferably the reverse wrapped cordage that @lillycoyote mentioned. The double reverse-wrapped, by the way, is made simply by taking reverse wrapped cordage and wrapping it again on itself. I like it to even out any weaker points left from splicing.

My brother has used a piece of insulated copper appliance wire. If it’s thin enough, it can work, but not super well. The bow itself is what matters, really.