There are several references that will have S-N data for HSLA (high strength low alloy) structural steels such as those that meet the requirements of ASTM A 242 (CORTEN).  I suggest you consult one or more of these so that you can make your own judgement on what properties to use:

ASM HANDBOOK Volume 19 Fatigue and Fracture

Atlas of Fatigue Curves edited by Howard E. Boyer, ISBN: 0-87170-214-2

Both of these books are available from ASM International via their website at http//:www.asminternational.org .  A good university library should also have them in the engineering/science reference section.

Have you tried to contact anyone at US Steel directly?  I guarantee that they have some amount of fatigue data on the CORTEN steels, so you may be able to obtain what you are looking for from them.

As for Rockwool, do you understand what this product is?  My limited understanding of this product leads me to the following conclusions:

1. The creators/manufacturers of this product do not understand what fatigue is, and therefore

2. Fatigue data does not exist for this product, especially since

3. Conventional fatigue mechanisms that apply to metals, polymers, and their composites, do not apply to this product.

If you can send me the DIN standard name of it, maybe i can help you! Up to this just an approximation:
   Fatigue limit= UTS/2
   Cycles=2000000
You can calculate the inclination from 2 points. One is exist the other is: Cycles=1000, Stress=UTS

Best regards,

Irwin