David Grayson scored on a 28-yard fumble return and 14-yard interception return as the revamped defense of the new Cleveland coach, Bud Carson, scored three touchdowns today in a 51-0 rout of Pittsburgh.

It was the worst loss in the Steelers' 58-year history. The Steelers' worst previous defeat was a 54-7 drubbing by Green Bay in 1951.

The Browns' defense forced eight turnovers and sacked Bubby Brister five times.

''It was the worst I've ever seen as a coach,'' said Chuck Noll of the Steelers.

Tim Worley, the Steelers' first-round draft pick, lost three fumbles, including two in a 17-point Browns first quarter in which the defense created all of the scoring. The lead was 30-0 by halftime.