JMJones, I'm sorry I didn't follow up on your question. I never got a message.....
I have not tried any more 15n20 in a while, but hope to soon. I'm not sure why the residual curve stayed. I tempered the blades while c-clamped to angle iron at 375F, but no luck there. Even over-shimmed to compensate. I have another in the works though, and we'll see how it goes.