The Iron Chest is an upgrade from the standard Chest and the Copper Chest. It has 54 slots, which is 27 more than a standard Chest and 9 more than the Copper Chest. In addition to crafting it, you can also right click on a Copper Chest with a Copper to Iron Chest Upgrade to upgrade it to the Iron Chest.
There is a bug that causes the game to crash if you attempt to make double chests so please don't try it.
Refined Iron can also be used, however it provides no bonus and is slightly more costly hence why you should use non-refined iron instead.