What would happen if a 10-kg cube of iron, at a temperature close to 0 kelvin, suddenly appeared in your living room?

Nothing overly dramatic, though it would be cool to look at. The cube would very quickly become covered by a layer of nitrogen/oxygen ice as the air which came into contact with it froze. Further away, you'd see condensation of water vapor into wispy clouds, which would swirl around the block due to the air currents generated by the sudden pressure drop.

Other than that, as long as you aren't in immediate thermal contact with the block, you wouldn't notice much other than that the room cools down. Here's a video I took of a vacuum can that was just removed from a dewar of liquid helium at 4 kelvin. It's maybe 5 kg of copper, not 10 kg of lead, but I'd say that's close enough to get the idea.

You can see one of my coworkers climbing down into a pit below it; he had to be careful not to bump his head on it, which would have really ruined his day, but there was no fatal cold bomb :)

The heat capacity of iron at room temperature is 0.444 J/K per gram (it changes with temperature, but let us leave that aside). That means it will want to absorb about 1,332,000 J of heat. That is a lot, but a bathtub with 300 litres of 40 °C water (about 10 degrees above a 300 K) will have about 12,900,000 J of internal energy - if you dumped the iron into the warm water it would not become cold (ignoring losses to the environment).

The real issue is how fast it would cool. Convective heat transfer from air to the metal gives a heat flow $\approx hA(T_{hot}-T_{cool})$ where $h\approx 10$ to 100 W/(m$^2$K) and $A$ is the area (about 0.0726 square meters for the iron). So the heat flow would in theory be on the order of 217.8-2178 J/s initially: sounds fairly impressive, but you would get the same flow in the other direction from a 600 °C cube. That flow will also soon slow, since the cube would be surrounded by cold air and whatever it is sitting on.

So the sad news is that the cube would not do anything super impressive. It would sit there, making air condense like around liquid nitrogen and soon be covered with rime frost. Probably some interesting crackling and perhaps cracking as it changed volume while heating. But no explosions, just a room with cold air along the floor.

There are about 30 m3 of air in a room, and the density of air is about 1.2 kg/m3. So the mass of air in the room would be about 36 kg. The heat capacity of air is about 1 kJ/kg-K. So the mass times heat capacity of the air is about 36 kJ/K. The mass times heat capacity of the 10 kg of iron is about 4.5 kJ/K. So, if the room started out at 300 K, the final equilibrium temperature (neglecting the walls) of the room would be about $$\frac{(36\ kJ/K)(300\ K)}{36\ kJ/K + 4.5\ kJ/K}=267\ K$$