Fabrique-Nationale-Herstal used Cockerill steel exclusively.
Some Browning patent very early versions of the A5 shipped to the U.S. between 1903 and 1909 are marked "Cockerill Steel" and "Special Steel" began to appear on A5 barrels in the mid-1930s. The Superposed, introduced in 1928 had "Special Steel" starting about 1948.
It would be comparable to Siemens-Martin or Krupp Fluss Stahl, possibly AISI 1040. Unfortunately I've never seen a composition analysis, which likely changed over time. Browning still uses the "Special Steel" name, but I have no idea as to the composition.

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