Hi, I normalized AISI 1018 steel to improve machinability, but it ended up hardening unexpectedly, making it tough to cut. What could be causing this and how can I fix it?

I'm working on a machining project where I need the steel to be soft for easy processing. After normalizing at 900°C for an hour and air cooling, the hardness increased from about 150 HB to over 200 HB. I've verified the furnace temperature and cooling environment, but the issue remains. Any tips on diagnosing and resolving this would be appreciated.