U.S. Steel Corp., the nation's largest steel producer, said yesterday it earned $504.5 million last year, a sharp turnaround from its $293 million loss in 1979 despite a slight dip in sales.

U.S. Steel said earnings amounted to $5.77 a share, but sales slipped to $12.5 billion in 1980 from $12.9 billion in 1979. The company's losses came to $3.41 a share in 1979.

Net income for the fourth quarter was $190 million ($2.16 a share) on sales of $3.4 billion. For the same period of 1979, the company lost $668.9 million on sales of $3 billion.

U.S. Steel shipped 17.2 million tons of steel in 1980, the lowest level since 1961 and a drop of 18 percent from the 21 million tons shipped in 1979.

U.S. Steel declared a fourth-quarter dividend of 50 cents per share of common stock, payable March 10 to shareholders of record Feb. 6.

Meanwhile, the nation's No. 8 steel producer, Wheeling-Pittsburgh Corp., reported fourth-quarter earnings of $2.1 million (31 cents a share), down from $7.9 million ($1.83) in 1979. Sales dipped to $271.4 million from $286.4 million a year earlier.

For the year, Wheeling-Pittsburgh said it earned $14.7 million ($2.85), down from $49.7 million ($12.16) in 1979. Sales totaled $1.05 billion, down 15 percent from $1.24 billion the previous year.

The company's steel shipments in 1980 totaled 2.2 million tons, a decline of 23 percent from the previous year's 2.9 million tons.

National Steel Corp. has reported lower net income for both the fourth quarter and the entire year of 1980 and has blamed the drop on sharply lower demand for sheet steel products and "inadequate prices in the marketplace."

The nation's fourth-largest steel-maker said Monday that it earned $83.8 million ($4.42 a share) last year, down from $126.5 million ($6.56) in 1979. Sales for the year totaled $3.9 billion, down 8 percent from 1979's $4.3 billion.

For the fourth quarter, National Steel earned $15.3 million (82 cents), down from 1979 earnings of $21.3 million ($1.11). Fourth-quarter sales were $1 billion in both 1980 and 1979, the company said.

Merrill Lynch & Co., the nation's largest brokerage house, said yesterday that the 1980 fourth quarter was the best in history with revenues up 44 percent from a year earlier.

Net income was up 76 percent to $52.3 million ($1.37 a share) on revenues of $853.4 million compared with $29.6 million (82 cents) a year earlier on revenues of $592.4 million.

Profits for the year were $203.3 million ($5.51) on revenues of $3.022 billion compared with $118.7 million ($3.26) in 1979 on revenues of $2.052 billion.

E.F. Hutton Group Inc. reported record annual profits of $82.6 million for 1980, a 121 percent increase from $37.3 million in 1979. Revenues rose to more than $1 billion for the first time, hitting $1.1 billion compared with $750 million in 1979.

In the last 1980 quarter, profits were $23.3 million, a 110 percent rise from $11.1 million earned in the 1979 quarter. The previous quarterly profits record was $21.3 million set in the second quarter of 1980.

Quarterly revenues rose 42 percent to $313 million from $220 million in the 1979 quarter.

Celanese Corp. had a sharp turnaround in the fourth quarter of 1980 with a 43 percent gain in per-share earnings from a year earlier after poorer earnings in the first three quarters.

Profits for the final quarter were $39 million ($2.64 a share) on sales of $913 million compared with $27 million ($1.85) a year earlier on sales of $814 million. For the year, Celanese's net income was $122 million ($8.20) on revenues of $3.348 billion compared with $141 million ($9.52) in 1979 on sales of $3.146 billion.

Singer Co. reported net income of $38.1 million ($1.92 a share) for 1980, a turnaround from a loss of $92.3 million in 1979.

Sales increased to $2.787 billion last year from $2.598 billion for the previous year.

Gains were seen in the company's aerospace and marine systems operations, meter and education divisions and its consumers sewing products and consumer durables businesses in Africa, Latin America and the Far East.

Fourth-quarter net income almost doubled to $12.9 million (68 cents) from $6.5 million (31 cents) for the corresponding period of the previous year. Sales were $722.9 million, up from $698.8 million.