selection of flange material

Application? Temperature? Code requirement?

ASTM A105 is usually permitted upto 800⁰F

A515 (I presumed you have not mistyped and it is 515 and not 516) has upto 1000⁰F as per section I (though as per section VIII it is same @850^oF)

A515 is a coarse grained steel (and hence avoids graphitisation at higher temperatures)

look at application, be very sure before you try changing the specs.

application is cooling water and the temp. is max.200c.it is a low pressure fluid.

I really don't see any advantage of using SA515 instead of A105 for this type of service. May be cost ? (is 515 cheaper?)

Also you must note that 515 has a bit higher carbon (>0.30) , the PWHT may be mandatory (not sure, check up the relevant code) for this.

<...cooling water...max.200c...low pressure fluid...>

At that temperature, the saturation/boiling pressure is over 15barg. At lower pressures, it is steam, not <...cooling water...>.