It was piece of galvanized iron from who knows what--my Dad had picked it up more than 30 years ago. I'm like a lot of people who never throw anything away. Since it was soft iron, I heated it with charcoal and dropped it in a bucket of water to give it some casehardening.
The previous trigger I made using knifemaker's steel seemed to toughen up just from grinding on it, so I didn't heat treat it.