부트스트랩 버튼 가운데정렬

<div class="col-sm-3 text-center">
	<button class="btn btn-primary btn-block" type="button">제출</button>
</div>

또는

<div class="col-sm-3" style="text-align:center;">
	<button class="btn btn-primary btn-block" type="button">제출</button>
</div>