Steel Bloons

After much research by the Bloon Empire, steel has been found to be MUCH tougher than lead! Guess what they decided to do with this advantage... If you guessed "new bloons made of steel", you're correct. Also, there are three different types of steel bloon, being the Stainless Steel Bloon, the Carbon Steel Bloon, and the Low-Background Steel Bloon. Oh, and did we mention that they are IMMUNE TO ALL THINGS THAT LEADS ARE IMMUNE TO? Oh, and speaking of which, all three types contain a single lead bloon.

Stainless Steel Bloons
These steel bloons have 3 HP and reflect away energy-based attacks, like Mirror Bloons. Reflected attacks have half the pierce they would normally have.

Carbon Steel Bloon
These steel bloons have 10 HP and an AD of 1 (0_o), because that's the toughest metal that we know of. However, both Water Monkeys rust the steel, removing the AD and removing the immunity to sharp objects, lasers, and boomerangs (because carbon steel rusts very easily, even though it's the strongest metal XD).

Low-Background Steel
Constructed in a low-radiation environment, these steel bloons have 6 HP (but no AD). However, Bloontonium-based attacks do double damage to these bloons (because low-background steel is really sensitive to radiation, hence its use in Geiger counters and such).

Trivia

 * An X-4 Danmaku Monkey can easily tear apart swarms of Low-Background Steel Bloons, due to it being a nuclear powered crow. XD