If you have not taken any course in microecon or econometrics, I would say your chance is kind of slim at top 20. The finance application now is different from what it was 10 years ago, when lots of students with physics phd degree got admitted at decent programs. But remember, they are PhD, not Master, it is totally different, a PhD degree in any field can prove one's potential. Students with solid background in Economics are prefered now.