An well annealed as possible will help you.
Are you making a lot of these or just a few?
If not many you might look at using a long anneal in hydrogen.
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Rust never sleeps
Neither should your protection
I can offer 1002(.02% max. Carbon) core material. This will provide the lowest residual magnetism. (Oe) after a simple stress relief anneal. In fact, it will out perform the 1018 without an anneal. However, a stress anneal is recommended.