Bethlehem Steel Corp. said yesterday that earnings for the final quarter of 1980 were 46 percent above those for the same period a year earlier but that a slumping automobile industry and high interest rates reduced profits for the full year to less than half the 1979 total.

Also reporting 1980 results yesterday were Republic Steel, Philip Morris, Weyerhaeuser and PPG.

"The economic recession hurt us badly, especially in the second and third quarters," said Donald Trautlein, chairman of Bethlehem, the nation's second-largest steelmaker. "The low shipment and production levels in 1980, combined with higher costs and failure to obtain adequate price relief, reduced Bethlehem's [1980] net income by about 56 percent."

Trautlein said Bethlehem Steel earned $121 million ($2.77 a share) in 1980 on sales of $6.7 billion compared with 1979 earnings of $275.7 million ($6.31) on sales of $7.1 billion.

Fourth-quarter earnings were $55.8 million ($1.28) on sales of $1.7 billion compared with 1979 fourth-quarter earnings of $38.6 million (88 cents) on sales of $1.8 billion.

Bethlehem shipped 2.83 million tons of steel products during the last quarter, compared with 3.01 million tons in the fourth quarter of 1979, and produced 3.72 million tons of raw steel during the latest quarter, compared with 4.65 million tons a year earlier.

For the full year, Bethlehem shipped 11.08 million tons of steel products and produced 15 million tons of raw steel. In 1979, steel shipments totaled 13.44 millions tons and raw steel production 19.4 millions tons.

Republic Steel Corp. reported yesterday that its 1980 earnings dropped about 59 percent from $121.2 million in 1979 to $50.9 million.

Fourth-quarter earnings were $35.9 million compared with $6 million for the last three months of 1979.

Total sales for the year did not decline as steeply, falling from $3.98 billion to $3.76 billion. Fourth-quarter sales went from $917 million to $1 billion.

Republic, which is the nation's fifth-largest steelmaker, manufactured 8.5 million tons of steel in 1980 compared with about 10 million tons in 1979. Shipments went from 7.3 million tons to 6.3 million.

Philip Morris Inc. cited gains from sales of cigarettes and beer yesterday in announcing record earnings and revenues for the final quarter of 1980 and the full year.

Fourth-quarter earnings were $135.4 million ($1.09 a share) on sales of $2.4 billion compared with earnings of $123 million (99 cents) on sales of $2.2 billion in the same 1979 period.

For all of 1980, earnings reached $576.8 million ($4.63) on sales of $9.8 billion compared with 1979 earnings of $507.9 million ($4.08) on sales of $8.3 billion.

Last year's earnings of Weyerhaeuser Co. dropped 46 percent below those for 1979 despite a sales increase of 3 percent.

The wood and wood fiber products manufacturer earned $277.98 million ($2.12 a share) for 1980 after extraordinary charges compared with $511.62 million ($4.01) a year earlier. Sales were $4.5 billion versus $4.4 billion.

Extraordinary charges, including the impact of the eruption of Mount St. Helens and settlement of certain carton antitrust civil suits which were not included in the general 1978 settlement, totaled 35 cents a share, or $43.5 million after taxes.

Fourth-quarter earnings fell 66 percent to $36.62 million (27 cents) after the extraordinary charges from $108.88 million (84 cents) for the 1979 fourth quarter. Sales were $1.10 billion, a decrease of 1 percent from $1.11 billion.

PPG Industries Inc., one of the nation's largest producers of glass and a major manufacturer of paints and industrial coatings, said yesterday that earnings declined 4.6 percent last year as domestic sales suffered from the slump in the automotive and construction industries.

The company reported profits of $209 million ($6.34 a share) for 1980, down from $219 million ($6.78) the previous year. PPG's worldwide sales dipped to $3.2 billion from $3.1 billion a year earlier.

Fourth-quarter earnings rose to $67 million ($2.02) from $55 million ($1.69) in the same 1979 period.