There is some info on medieval armour and its metallurgy in an appendix to Robert Hardy's "Longbow" written by Peter Jones (Hardy 1995: 232-236)
"Summarising it can be said that steels of 0.1 to 0.2% carbon were employed with a strength range from 20tons/sq in (31 kgf/mm2) to 70 tons/sq in (110 kgf/mm2) was available. Further that face hardening by hammering and carbonising was known. However, the ductilities and hence the toughness of the steels would be lower than those today."
I'm not an engineer and can't interpret that fully!
I have also read that the quality of armour varied greatly ie. bespoke armour made for the wealthiest was of a very high quality of hardened steel, whereas the livery armour issued to, or owned by ordinary men at arms was of a much lesser quality - probably iron rather than steel. This makes sense to me though I can't give you a reference for it. Most of the experiments I've seen suggest that best armour offered pretty good protection against arrows except at close range and that to penetrate the arrow has to strike the metal almost perpendicularly or it will be deflected. Even the best armour, however, was only at its strongest and heaviest around vital organs and had to be lighter around the limbs for flexibility. Again, I'm interpolating, but my guess is that you would have to shoot an awful lot of arrows for a few to get through and this is probably what lay behind the "arrow storm" tactic.
Against the quality of the steel in the armour has to be set the quality of the steel in arrowheads. Mark Stratton has done a lot of work on this - see his chapter in Hugh Soar's "Secrets of the English War Bow" (2006).
All of this applies to plate armour - the evidence is clear that mail offered very little protection against bodkin pointed arrows - I've seen that demonstrated with a relatively light bow.
Hope this helps - I'm no expert!!!! Check out the sources for yourself.
Stan