and the upper limit is 0 05 by weight o and n are elements mainly introduced into the alloy from raw materials in general o may be carried in alloys in a crucible o or n introduced into alloys are present in the crucible in the form of oxides such as al 2 o 3 or nitrides such as tin or aln if these compounds are present in castings they become starting points of cracks thereby to lower creep rupture strength or to be a cause of stress strain cracks particularly o appears in the surface of castings that are surface defects to lower a yield of castings accordingly o and n should be as little as possible o and n should not exceed 0 005 by weight si is introduced into casting by raw materials in the present invention since si is not effective element it should be as little as possible even if it is contained the upper limit is 0 01 by weight mn is introduced into castings by raw materials too as same as si mn is not effective in the alloys of the present invention therefore it should be as a little as possible the upper.