How can be used the steel efficiency recipe if, when I put 100 iron bar in a furnace, it start smelting them 5 at time for a single steel bar instead of starting the efficiency recipe?

steel efficiency requires the use of a furnace that has a recipe selection window. Mods such as bob's mods, advanced furnaces mod, angel's mods (perhaps), or other mods that add a furnace-type machine with a recipe selection screen should do the trick. 0.17 update had originally intended for this to not be the case, but so far, attempts to remove this limitation have failed for one reason or another.

Thanks.
At least I can stop bonking head on the wall for being unable to use it. :P

IM SORRY!! For now, it's just a limitation with how the code works / why it works / and what the limits (or rather, the exceptions are)

Could I suggest a note on the description?
At least you warn people that wont think it's a mod bug.

I've seen it in the FAQ section, but this is not visible in the mod interface, then a warning can help a lot.

One more mod that allows recipe selection: Whistle Stop Factories.