Hey shump
Two reasons your back is rounding. 1) you're blending Step 4™ and Step 5™ and not really getting your back set before you pull. 2) you're pulling around your knees. Your first move is to open the chest, so the bar goes out and around the knees, causing the back to have to do more work than it should. Set the back until you get all of the slack out of the bar, then set it some more. Then, push the floor away with your feet so your quads do their job of breaking the bar off the ground and the bar comes straight up.
Make sure you can set your back without the belt so you know what that feels like.