Just a short quote from C.A. Wannamaker's article on Phil. 2:6ff.
"In this passage Paul maintains that Christ's universal sovereignty derives
from the Father and that ultimately the Son shall be subject to the Father
when he returns his present sovereignty to God. The subordinationist
character of 1 Cor. 15:24-28 demonstrates quite clearly that Paul did not
believe in Christ's absolute equality with God" (Wannamaker 187-188).
Observations from Wannamaker's article are found in my Christology and Trinity book, which can be purchased on amazon.com. See the links on this blog to obtain further details.
Cf. C.A. Wannamaker (NT Stud. Vol. 33, 1987, pp. 179-193).