Willie Parker had his third consecutive 100-yard rushing game this season, gaining 133 yards on 24 carries, to pace a rushing attack that gained 205 yards on the ground as the Steelers improved to 3-0 for the first time since 1992, Bill Cowher's first season with the team.
"It's great to be 3-0," said first-year head coach Mike Tomlin. "We started to show some personality things and that's what I was excited about for the football team. We ran the ball effectively; we played great ball on defense; we scored on defense. Special teams was a big contributor."
The Steelers played nearly error-free for the third week in a row, with the lone mistake being a fumble by quarterback Ben Roethlisberger on the team's second play from scrimmage.
San Francisco (2-1) opened the game with a drive to the Pittsburgh 14, but was forced to settle for a 3-0 lead on Joe Nedney's 32-yard field goal, the first points allowed by the Steelers in the first half of a game this season.
But Pittsburgh erased that lead quickly as return specialist Allen Rossum took the ensuing kickoff 98 yards untouched up the middle of the field for a touchdown and a 7-3 lead, quickly changing the momentum of the game.
"Allen started it off," said cornerback Bryant McFadden, who returned an interception 50 yards for a touchdown with 4:01 remaining in the fourth quarter.
"Whenever he gets a little crease, then he's able to take it (for a touchdown). That kind of set the tone at the beginning of the game."
After forcing a San Francisco punt, Roethlisberger, who finished 13 of 20 for 160 yards and a touchdown, was sacked by Hannibal Navies on the Steelers' second play from scrimmage, with Navies also recovering the loose ball at the Pittsburgh 22.
But the defense once again rose to the occasion, stopping San Francisco at the 4. A 22- yard field goal from Nedney cut Pittsburgh's lead to 7-6, but the tone for the game had been set.
It clearly did that to the 49ers, who didn't challenge the Steelers offensively again in the first half. And when Roethlisberger tossed a nine-yard TD pass to tight end Jerame Tuman with 42 seconds remaining in the first half to give the Steelers a 14-6 lead, there seemed to be little doubt of the game's outcome.
But the 49ers weren't finished.
The Steelers tacked on a 36-yard field goal by Jeff Reed on the opening possession of the second half to go up 17-6, but San Francisco put together its best drive since the start of the game, moving the ball from its own 22 to the Pittsburgh 29.
Quarterback Alex Smith threw a pass deep over the middle to tight end Vernon Davis, who was nailed by strong safety Troy Polamalu as he caught the ball. Davis got one foot on the ground before Polamalu flipped him onto his head, jarring the ball loose. The play was initially called a fumble, with the Steelers' Ryan Clark recovering it and returning it to midfield, but San Francisco head coach Mike Nolan threw a challenge flag.
Referee Gerry Austin ruled the pass an incompletion instead and the 49ers were forced to settle for yet another Nedney field goal, this one from 49 yards out to cut Pittsburgh's lead to 17-9.
The Steelers then added another Reed field goal on their next possession to go up 20-9 on the opening play of the fourth quarter and another on their next possession to stretch the lead to 23-9.
McFadden then picked off Smith to increase the lead to 30-9, with Smith recovering on San Francisco's next possession to thrown a 21-yard TD pass to Taylor Jacobs with 2:30 remaining in the game.
Najeh Davenport then finished things off for the Steelers, bursting loose for a 39-yard touchdown run on the first play after the two-minute warning.