I don't see a realism tag on this question, but realism is all I know, so I'll give it my best shot :)
What happens during re-entry? When you reenter from Earth orbit, you are going very fast, around 8500 meters every second. That is very fast. When you reenter from a higher orbit or trajectory (such as Apollo, which reentered directly from the lunar altitude, or Hayabusa or OSIRIS-REx, which have/will reenter directly from an escape trajectory (in reverse)) then you are going faster. Potentially much faster.
Worth noting, if you're reentering to a planet which is larger than earth, as your "1.5 atmospheres" indicates, then you will be reentering yet even faster.
When you are going many kilometers per second, and there is an atmosphere in the way, strange things happen. In aerodynamics there are multiple regimes--traditional aerodynamics, transonic, supersonic, and hypersonic. But Mach 26 is something else entirely. Reentry aerodynamics are considered their own regime.
The first thing is that the air in front of you is compressed, very quickly. So quickly that there is no time for heat to move. This makes it an adiabatic compression, similar to that used for ignition in a diesel engine. Remember PV=nRT? If you consider the volume in front of your reentering body as one unchanging volume, then it does not change, but the pressure does, dramatically. Your temperature will rise, dramatically. This is known as compression heating. It gets hot.
So you've got a 3000 degree C Mach 26 blowtorch pointed at your spacecraft. How do you survive it? You have to get rid of the heat so it doesn't cook/melt everything. How do you do this? There are a couple ways.
Put it in tiles: If you want to survive getting very hot, you find something that can survive getting very hot, and you ride down on that. This is how the shuttle worked. Those tiles got very very hot--after all, they just absorbed the heat, but they had a very high specific heat, so they just soaked up the heat until reentry was done. This is why the tiles had to be very thick--if they weren't thick enough, the heat would travel through the tile and melt the glue on the other side.
Put it inside your spacecraft: The X-15 survived its reentries by using the body of the vehicle as a heat sink. The frame was designed with a special alloy that could take the heat and still be strong, so they just let it get hot, and put an AC in the crew cabin. But Rene 41 is very expensive stuff, and most other metals tend to get a bit squishy at these temperatures, so you either need fancy metals or a lot of normal metals so the heat (energy) is distributed enough that the heat (temperature) in any one place is still fairly low.
Put it in water: The phase change of water from liquid to vapor requires a massive amount of energy. Dynasoar was a very serious proposal that would have also used Rene 41, but to help take the heat, it would have had a jacket of water surrounding all the essential melty bits. As the body of the vehicle heated up, the water would boil, and it would vent the steam as it descended. But water is heavy stuff, so for real-life purposes it's not the best idea. Maybe with scifi rocket engines it wouldn't matter so much.
Let it burn you up (but not all of you): Also known as ablation, this is how most heat shields work. Essentially, you find something that takes a lot of energy to burn/vaporize/whatever, and then it absorbs the heat by burning away. This doesn't even have to be particularly high-tech. Some Chinese spy sats used wood soaked in resin to do the trick of getting their film back to the ground safely. We don't tend to do that because it's heavy (seeing a theme here?) but it works.
So how do heat shields fail? Well, generally they don't. Most people are familiar with the Columbia disaster, but fewer are aware that shortly after Challenger, Atlantis lost a tile before reentry too. So why wasn't Atlantis destroyed? Because the missing tile was luckily one that was right below a steel mounting plate, so the steel just absorbed the heat. See method #2, above. Were it anywhere else, the heat would have been sufficient to melt the aluminium frame, and once that started, the frame of the vehicle would begin to distort, and, improperly oriented, the aerodynamic forces (and heat) would have torn the orbiter apart. See Columbia.
So how would your situation occur? Well, frankly, if your entire vehicle is a "giant ship made of steel", you might not even need a heat shield. While aluminium, which most space craft are made of, melts at around 700 degrees and boils well below the 3000 of reentry (source, note that space-grade Al alloys are rather different critters), steel melts at around 1500 degrees C and has a much higher specific heat. (But do note: melting isn't required for loss of structural integrity, see "jet fuel and steel beams".)
Anyways, the very massive vehicle frame might be able to just take the heat and make it down in one piece, for the same reason that metal meteors tend to get further than stony meteors do.
But let's look at that a bit further. Meteors tend to blow up (becoming many many meteorites) because where there are irregularities in the surface of a reentering body, it concentrates airflow, increasing the pressure even more, and making it VERY VERY hot in the one spot. This tends to melt/ablate it further, making the pit larger, and increasing the effect. This eventually generates a hole with so much pressure in it that it blows the whole meteor up. Sidenote: this is why heat shields need to be very very smooth, and why damaged tiles were a grave danger to the shuttle.
Now we have a possibility.
Your heat shield was severely damaged for whatever reason, and during reentry, those pits in existing tiles and missing tiles concentrated heat and pressure, burning through to the ALUMINIUM frame. The frame melted, deforming the heat shield and eroding it further. Jets of molten and boiled metal lance through the vehicle, setting fire to the interior. Luckily, this happened sufficiently late in your reentry that the entire vehicle wasn't melted by the hot plasma entering from below before it slowed down, and what fire suppression systems you had were able to mitigate the effect of the heat from suffocating your whole crew/melting propellant tanks/lighting goddamn everything on fire. Well, at least in the upper/mid decks.
The pressure differential was enough to kill most people in the lower decks, but your vehicle is huge, so the entire frame wasn't melted. Also, your magical scifi reaction systems were strong enough to maintain attitude during reentry, so it wasn't torn apart by going sideways-side-down instead of down-side-down through the atmosphere.
If it was made of steel, you'd probably actually be in fairly good shape. Such a scenario is actually rather plausible for very large, very strong structures. In fact, most spent rocket stages could conceivably survive suborbital reentry (and many, mostly stronger Russian ones, do), but they're designed for efficiency, and while they're strong going up, once there's no more pressure in tanks and they're going sideways, it's a lot harder to stay in one piece.
So there you go. Your vehicle is large enough to endure the heat load caused by heat shield failure without totally melting and (most importantly) its reaction control is strong enough that it doesn't lose attitude and disintegrate from aerodynamic forces.