What type of metal is used for driver springs?

As the title suggests, I want to know exactly what type of metal is used for 
1. The `drivers spring
2. The average spring
3. The weakest spring
I think you should ask a spring company
The correct question should be what weights/spring rates the springs have. Type of metal is steel in general.
Only way I see answers is if you open up drivers and get the specs yourself. IE destroying the driver.
Closing because this thread is likely to ever answer "my" question since we have no access to TT factory specs for springs.