Most of the sound getting through doors is the result of small gaps. If you have taken care of that, FOR SURE, then you are most of the way there. One large often overlooked aspect however is the door seal on the bottom against the threshold, because the door needs to open over carpet or something. So to facilitate a good soundproof seal, you have to use an automatic door bottom seal, a type that raises up when you open the door, but seals tight against the floor or threshold when closed.
If you have already done that, and you are confident that the steel door is acting as a sounding board, you can deaden that effect by filling the interior with foam. but this can be very tricky if you don't know what the interior is like, it pretty much has to be completely hollow for that to work. If there is any kind of honeycomb like structure inside, you are stuck.
Don't waste your money on the foam egg crate material, it doesn't BLOCK sound coming through a solid wall or door, it would ABSORB sound emanating from INSIDE of your apartment. As mentioned there are heavy pads that can be hung / attached to the inside of the door that can help deaden the sound coming through. The best kind have foam on one side that goes against the door, then a rubber coated lead sheet on the side facing into your room. The foam absorbs the vibrations from the door, but the lead stops them from re-transmitting.
By the way, FOAM weather proofing strips to NOT block sound transmission. You need the more solid type. If foam blocked sound, why would they put in on the front of speakers or use it as covers for ear pieces?