.flex-parent {
  display: flex;
  height: 960px;
  width: 960px;
  background: grey;  
}

.flex-child {
  display: block;
  height: 360px;
  width: 360px;
  background: #f00;
  margin: auto;
  display: flex;
}

.box {
  margin: auto;
}    
  

<div class="flex-parent">
  <div class="flex-child">
    <div class="box">
      center
    </div>
  </div>
</div>