Provided the material is normalized there should not be much of a concern. Be sure to address the need for production impact tests as well as impact tested weld procedures. This is on the assumption that you are building to ASME Section VIII - Div 1.
In my area of the world SA 516 Gr 70 is much more common. That would permit a thinner vessel than the use of SA-516 Gr 55.
rkdpv,
You should check with ASME VIII Div.1 (with the assumption you're talking about pressure vessel).
In UCS-66 you can check, because it depends of the thickness.
For 20 kg/cm2 I think your thickness will be more than 15mm (of course it depends on the dimensions on the vessel..) so you will be below Curve C (For not normalized 516 Gr55) and below Curve D (For normalized 516 Gr55).
This means you can use both materials, but you will need impact test.