Carnegie and Bethlehem Steel were licensed to manufacture Krupp steel plate in 1897; New York Times Nov. 7th, 1897
If either produced Krupp marked shotgun barrels, or if the plate was converted post-WWI to barrel material for U.S. makers (Ithaca, Fox & Baker were still at it) is unknown.
Immediately post-WWI “Fluid Steel” barrels used by U.S. makers, including Hunter Arms, were primarily AISI 1030 or 1040 Carbon Steel, possibly rephosphorized.
This is interesting
The steel was a slightly improved form of the original medium-carbon (0.2-0.4 percent carbon) nickel-chromium steel introduced in 1894 by Krupp and later forming the basis of all high-grade armors made of steel even today by all nations. Wh n/A used some molybdenum to improve manufacturing results and was slightly tougher (crack resistant) than the original "High-Percent Nickel-Steel," also called "Krupp Soft" or "Quality 420" (Krupp's own label) steel, used through the end of WWI.