(from someone who answered my question:)
the difference between alloy steel and steel alloy is a question of context and usage.

'Alloy steel' means a steel that contains other elements
(usually in relatively small amounts) intentionally added
in controlled quantities to alter its characterics (e.g.
chromium-mobdynum steel or the various types of stainless
steel). This is essentially an engineering term, and it is
commonly used in engineering documents and technical
specifications.

'Steel alloy' is a general term used to refer to any alloy
made from steel and any other element or combination of
elements. This is noit a technical term, and it would
rarely be used in specifications. In principle, it could
even mean an alloy in which steel is not the principal
component, although it is not ordinarily used this way.

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not sure, but I think "liga de aço" would be steel alloy.


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Note added at 6 days (2007-08-06 07:33:26 GMT) Post-grading
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from Bourth (proZ member)

steel alloy

Basically the same, but depends whether one is talking about a steel that has been alloyed or an alloy that contains steel. It's possible that there is a subtle but important distinction, depending on the percentage of steel, but I think one usually deals with a steel (predominantly) that has been alloyed with tungsten, molybdenum, etc. rather than other metals that have been alloyed with steel.

I would certainly say "steel alloy" and "alloyED steel" rather than "alloy steel".