The Steelers hogged the first period but only a Jonathan Phillips power-play effort (5.52) separated the sides at the first interval.
When Doug Sheppard made it 2-0 to the Steelers three minutes into the second session, it looked all over.
But Cardiff perked up with a fine strike from Brad Voth (25.20) and when Matt Elich equalised barely a minute later, it was game on again.
The Devils then lost Wes Jarvis to the penalty box on the half-hour mark, and Jeff Legue made them pay with a third for the Steelers.
Young Rob Dowd's power-play effort made it 4-2 in the third period (49.55) and the Devils wasted a full two minutes of 5-on-3 shortly afterwards.
The win was completed when Randy Dagenais flipped the puck into the empty net as Devils pulled goalie Peter Aubry.
In tomorrow's final (3pm), Steelers will play either Nottingham Panthers or Coventry Blaze, who clash at 5pm this afternoon.