Number of wins in placement games doesn't guarantee you a higher rank
It all depends on your previous MMR. If you were Silver 5-3, there's a possibility that you may be placed in Bronze, since your MMR is softly reset & reduced a little. So my recommendation to get placed in higher ranks such as (at least) Silver & above is to not suck, not just your teammates. Going 0/15/2 while your team helped you win a game doesn't grant you any MMR at all. You yourself have to be the part of the strength of the winning team.
Btw, this thread was meant for everyone who thinks they didn't deserve to be Bronze, despite having winning 9 out of 10 placement games.