SAN FRANCISCO, OCT. 21 -- Patience and a most unusual Pittsburgh turnover were the keys that enabled the San Francisco 49ers to remain one of the NFL's two unbeaten teams with a 27-7 victory today over the Steelers.

The 49ers overcame a slow start and two first-half interceptions of Joe Montana passes with two touchdowns in the second half off Pittsburgh turnovers. The scores broke open a close game and handed the Steelers their fourth defeat in seven outings.

After Mike Cofer's 20-yard field goal had given the 49ers a 13-7 lead with 3:53 to go in the third period, the Steelers gave away the game. Cofer's kickoff bounced in the general direction of rookie running back Barry Foster. Foster inexplicably let the ball bounce by him untouched and the next person to handle it was 49er Mike Wilson, who recovered the live ball at the Steelers 5. Three plays later Tom Rathman bulled over from the 1 to give the 49ers a 20-7 lead.

"I blacked out," said Foster. "In a punting situation, obviously, you can let it roll. When I finally realized it wasn't a punt, it was too late."

On the Steelers' next series, Bubby Brister fumbled when he was sacked by Charles Haley. Pierce Holt of the 49ers recovered at the Steelers 22. Rathman carried on four of the next five plays and ended the drive with his second one-yard touchdown and a 27-7 49ers lead.

The game was not a typical 49ers effort. The Steelers took away the long passing game. The Steelers. who came in yielding only 160 passing yards per game to lead the NFL, gave up only 157 today. Montana hit 20 of 30 passes.

Forty-Niners wide receivers Jerry Rice and John Taylor were not major factors. Rice caught three passes for 31 yards against double coverage most of the day. Taylor caught one pass for 14 yards before leaving with a bruised knee late in the first quarter.

Emerging as a new force was 49ers first-round draft pick Dexter Carter. With Roger Craig spending his second straight week in street clothes with a knee injury, Carter got his second start at running back. The 5-foot-9 inch, 170-pound speedster from Florida State had his best game as a professional. He rushed for 90 yards on 17 carries and caught seven passes for 57 yards. He also displayed sure-handedness, which he had not shown previously.

"Roger Craig told me to think about my best college game," said Carter. "I did and the way I played today reminded me of a Florida State game."

The 49ers held the Steelers to 200 yards. The lone Pittsburgh touchdown came on a two-yard pass to Richard Bell. An interception by Rod Woodson and a 29-yard return to the San Francisco 30 set the stage for that. Even the touchdown took an instant replay decision to determine that Bell's knee had come down inside the end zone's backline.

The Steelers dominated the first quarter. Their first possession featured the running of Merril Hoge and passes to tight ends Mike Mularkey and Eric Green. It ended when Greg Anderson was short on a 50-yard field goal attempt.

Montana ended the 49ers' ensuing possession by throwing the interception to Woodson.

San Francisco scored on a 39-yard Cofer field goal with 11:49 to play in the first half that cut its deficit to 7-3.

The 49ers survived Montana's second interception of the half and took the lead just before intermission. That drive started with a 15-yard penalty on Steeler Carnell Lake for interfering with Don Griffin's fair catch with 1:55 to play. It moved the ball to the San Francisco 44.

Montana passed to Carter, then to Rathman and moved the ball to the Steelers 21. A pass interference call on David Johnson gave the 49ers the ball on the Pittsburgh 1.

San Francisco overcame a holding penalty that nullified an apparent touchdown by Harry Sydney on the next play. Montana found Mike Sherrard three plays later near the right corner of the end zone to give the 49ers a 10-7 lead.