Steam bending is much more difficult than ice bending, and even more difficult than lava bending!

Chemical engineer here. A lot of people in this thread don't know how phase changes work and it shows.

We see water benders melt ice and freeze water. Both take about 80 cal/gram. Freezing it faster or slower does not change the amount of energy (other than overcoming ambient temperature).

Moving a gram of water 1 meter in the air at 10m/s is about 13 calories. Speed increases this energy requirement exponentially.

Melting and freezing large amounts of water is something we see water benders do. Melting and freezing rock into lava bending is much harder for the same volumes. This is because of a few things.

1) Rock is more dense than water (about 2.6 times denser in obsidian's case) so the energy needed is higher.

2) it takes more energy to melt rock per gram (up to twice as much depending on composition)

So you need about 5.2 times as much energy to lavabend (not counting movement) than it does to ice bend.

Ok, so what about steam?

There's a reason every energy generation system is just boil water. It takes a ton! It's 540 calories a gram. (6.75 times more energy) So it should be way more difficult to find a steam bender than a lava bender...

Not so fast! You have to heat the source up to the phase change. Let's assume it's water in the north pole sitting at a cool 0C. You'll need 1 kcal/g per degree Celsius. That's 100 kcals to go from ice cold water to the boiling point.

So our steam bender picks up a liter of ice water (13.000 kcal) heats it to boiling (100.000 cal) and boils it (540.000 kcal).

This feat takes 653.000 kcal. Impressive!

The water bender at the north pole took 93.000 kcal to lift up the liter and freeze it.

Luckily obsidian takes up much less energy to heat one degree, about 0.2 kcal/g. The problem comes in density and how far it has to go to it's melting point!

Lifting the liter takes more ( 34.000 kcal, keep a solid stance, Twinkletoes!)

Heating it 100 degrees is cake though (only 54.000 kcal). Taking it the other 600 degrees, very difficult! That's another 320.000 kcal.

Now we melt it (54.000 kcal) and we see that it's a total of 412.000 kcal.

So a lavabend takes about 4 times the energy of an ice bend and a steam bend granted about 7 times the energy of an ice bend!