Steel has surprisingly very poor thermal conductivity compared to many other metals and alloys, stainless steel especially, not sure what alloy they use for fire extinguishers, but the geometry of relatively thin-walled tanks like that means there is very little neighboring metal at any point for heat to transfer through. This means it heats particularly unevenly, wherever you're applying heat is where all the heat is going to be heavily concentrated, and takes a long time for applied heat to even out as it "soaks" through to the rest of the material. This also means it is relatively poor at transferring the heat into your target metal you're trying to melt, too. So that's even more of the heat being concentrated into the area of steel that's being directly heated, not where you're hoping for it to go. Your impatience probably just melted a hole in it.