Molten steel is used to merge lootboxes with the 
Anvil
You can smelt other lootboxes with the command and merge them with the command
Normal lootbox Gives 100 Molten Steel when smelted and you can merge it for 300 Molten Steel
Uncommon Gives 250 Molten steel when smelted and you can merge it for 750 Molten Steel
Rare Gives 450 Molten Steel when smelted and you can merge it for 1250 Molten Steel
Legendary Gives 1000 Molten Steel when smelted and you can merge it for 2500 Molten Steel
Ultimate Gives 2500 Molten Steel when smelted and you can merge it for 7500 Molten Steel
Mythic Cannot be smelted nor merged but it can be bought from the 
Traveling Merchant for 25,000 Molten Steel And the Ultimate can be bought from the Traveling Merchant for 6000 Molten Steel