What it does
Steel Beam is a Steel-type special move with very high Base Power. On use, the user takes damage equal to half their max HP, rounded up. This cost is unconditional: it fires through Protect, through a substitute, and through a miss.
When the conditions are right, though, the power is genuine. There are matchups where Lucario's other moves won't secure a KO through the opponent's remaining HP or bulk, and Steel Beam bridges that gap.
Who runs it
Lucario runs Steel Beam at 25% usage — lower than Meteor Mash's 45%, which tells you it's a deliberate slot choice rather than a default. Lucario sometimes runs both physical and special moves; Steel Beam is the special-attack nuke option, while Meteor Mash is the physical STAB. Most sets run one or the other unless the EV spread is deliberately mixed.
How to use it
Steel Beam is not an every-turn move. You use it when you need a guaranteed KO and nothing else on your set can reach the target's remaining HP. Before clicking it, confirm: can you survive at half HP afterward?
The cleanest timing is when the opponent's key Pokémon is at low HP, your other moves fall just short of the KO, and you might not get another free turn to finish it. Trading half your HP for a confirmed elimination is the right call there.
In doubles, be aware that after the self-damage you're sitting in KO range for most priority moves and spread attacks. Either have your partner ready to cover you, or plan to Protect the following turn.
Tips & strategy
Don't open with Steel Beam when you don't need it. The HP cost leaves you fragile for the rest of the game, and a Pokémon at half HP gets cleaned up by chip damage faster than you'd expect.
If your partner has Helping Hand — which would be unusual but possible — the damage output becomes extreme. More practically, a Friend Guard partner makes Lucario slightly less vulnerable after the self-damage, which improves the math on whether clicking it is worth it.