"Basic steel is made from iron whose normal carbon content of 4 percent or so by weight is reduced to usually less than 1 percent and whose properties are extremely sensitive to precise differences in composition. Special alloys depend on specific additions of other elements such as chromium for stainless steel, or chromium, tungsten, and vanadium for particularly hard tool steels."
(National Institute of Standards)

Stainless contains Chromium. The major benefit of stainless in modern armor is that it is low maintenance. Stainless is nigh-universally believed to be not a period material, but some people like it because of the low maintenance and wil argue that it is visibly indistinguishable from other steels.

Springsteel is a high-carbon steel and can be heat-treated to be very strong and dent-n-scratch resistant. Since it is stronger than mild steel, armor made of springsteel can be a fair bit thinner and thus lighter than mild steel. There are also stainless springsteels.

For more info, try a few searches on www.google.com

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Actually both chromium and nickle enhance the corrosion resistance of steel but chromium is the primary alloying element in the stainless steels. Nickle helps the chromium out but it is usually listed as a strength and toughnes enhancer rather than a corrosion resistance enhancer. (sic. Metalurgy Fundementals, by Brandt and Warner) If memory serves a steel must contain over 11% chromium to be cosidered stainless and look at a steel catalog handy here shows most of the 3- series stainless steels to have 17%-21%.

As an aside, I once ran into a batch of stainless that had been nickle plated for extra rust resistence- you couldn't tell untill the pieces were formed and we started buffing at which point it looked like makume-gane. Wasn't a very happy experience.

What Is Stainless Steel and Why Is it Stainless?

In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain 'less' than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. (Iron, on the other hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather than tightly-packed layer and flakes away.)
http://chemistry.about.com/library/weekly/aa071201a.htm

There's also a nice little table with the the trace elements here:
http://pw1.netcom.com/~dwelding/stainless.htm

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