I could have swore it was the amount of nickel added to a steel that changes the cubic structure and when it's turned into Face centered cubic structure, it's non-magnetic.
Same as when you heat up steel.. It's Body centered cubic structure, until a certain temperature. Then it changes to Face centered and loses it's ability to be magnetic.
(Sorry, I'm just going off memory here... And It's not the greatest. Maybe Send me a PM and we can talk about it!)