Choosing the right steel for pressure accumulator shell

JuzerNejm101

I need to build a pressure accumulator which will be then used as a spring actually (upper side of the accumulator will be filled with nitrogen, and the lower won't be filled with oil, but will have a rod which will be holding movement of a machine part (rod) at a high frequency.) I'm having trouble choosing the right material for this purpose.
I'm not sure how much pressure I will require inside, so I took an above expected value of 60 bar. By inserting the values of pressure and a surface area in the formula p= F/A i get an value around 50 kN.
BUt then again using formulae for stresses from here (page 15-10):
www(point)navsea(point)navy(point)mil/nswc/carderock/src/mechrel/products/handbook/CHAPTER15RevB(point)pdf

I get realy low results (the stresses (after converting units) end up to be between approx 2-7 N/mm2).
So I think I'm doing something wrong. Because these results indicate I can actually use any type of steel, which can't be right because I'm worrking with some rather high pressures / forces acting on the material in there.
The other thing is, my wall thickness isn't realy constant and it's not cylindrical nor spherical as these formulae describe. It's following a curve of a membrane so it's more eliptical.
Can someone guide me to the right path? Thx.