The Knight and the Blast Furnace by Alan Williams explores the metallurgy of medieval armor in great detail. While he covers armor metallurgy through the early 17th c., I was most interested in his coverage of the 14th c.
He provided a metallurgical analysis of some 48 pieces of 14th c. armor, as well as three lames from the battle of Visby. 14, or 29%, are simple wrought iron. Most of these are from Northern Europe, which seems to have lagged Italy in metallurgic development: only two of the iron pieces are from Italy. In Northern Europe iron seems to have been still in use even for armor for men at arms: well shaped and finished visored bascinets like the c.1370 bascinet in the Museum de Valere at Sion in Switzerland or Veste Coburg cat. no. 50 c. 1380 were made from iron rather than steel.
These iron pieces had VPH (Vickers Pyramidal Hardness) of 130-175, and the lowest quality iron armor was, according to Williams, perhaps half as efficient at resisting penetration as modern mild steel.
The next step up in quality was low carbon steel: 17 pieces, or 35%. These had from .1% to .3% carbon, and VPH ranging from 108 to 233. Modern mild steel might have carbon content of .15% carbon and VPH of 152. Medieval steel, however, had a much higher level of slag inclusion than modern mild steel. Even high quality 14th c. Italian harness had slag content between .8% and 1.9%. Modern steel is virtually slag free. Williams rated this grade’s resistance to penetration at about 75% of modern mild steel.
Next came medium carbon steel, with .4% carbon or more but without full hardening: 11 pieces or 23%, with VPH ranging from 193 to 276. Williams rated this grade of armor about 10% superior to modern mild steel.
The highest grade was medium carbon steel with successful hardening by heat treatment. According to Williams only six pieces from the 14th c, at least four of them from Italy, fell into this category: with VPH ranging from 366 to 374, or 12% of the total. These had resistance to penetration about 50% better than modern mild steel.