United States Corp., the nations's largest steel producer reported yesterday that it lost $293 million last year after taxes because it was forced to close 15 of its plants across the country.

The giant steel maker said income for 1979 -- excluding provisions for the recently announced shutdowns -- was $273.5 million, compared with $250 million in 1978. But after all charges, U.S. Steel reported a 1979 loss after taxes in the amount of $293 million ($3.41 per share) on sales of $12.9 billion.

For 1978, income was $242 million ($2.85) on sales of $11 billion.

"Operating income for the nonsteel business segments improved significantly with chemicals showing most improvement," said Chairman David M. Roderick. "However, steel manufacturing reported an operating loss of $94.9 million for the year, compared with operating income of $25.5 million in 1978."

Roderick said most of the 15 permanent shutdowns were loss operations and "their "elimination is expected to result in improved profitability and a net favorable cash flow in future years."

Fourth quarter results were an after-tax loss of $561.7 million ($6.50) on sales of $3.03 billion, compared with income of $94.6 million ($1.11) on sales of $2.95 billion in the fourth quarter of 1978.

Xerox Corp. reported a 15 percent gain in earnings last year on a 16.5 percent rise in sales. Except for a capital gain in the 1978 earnings, the 1979 profit rise would have been 18 percent.

Net income was $563.1 million ($6.69), up from $488.5 million ($5.81) in 1978. The 1978 profit included 14 cents a share on a capital gain.

Sales rose to $7.027 billion from $6.033 billion.

Final quarter profit was up 14 percent from a year to $127.8 million ($1.52) from $111.8 million ($1.33) a share as sales rose to $1.877 billion from $1.596 billion a year earlier.

Two major brokerage firms, Merrill Lynch and E.F. Hutton, reported sharpley higher earnings for the fourth quarter, reflecting high stock trading volume.

Merrill Lynch & Co., the parent firm of Merill Lynch, Pierce, Fenner & Smith Inc., said it earned $29.6 million (82 cents) in the quarter, up 225 percent from $9.1 million (26 cents) in the same quarter of 1978.Revenues rose 46 percent to $592.4 million from $406.4 million.

E.F. Hutton Group Inc., said its profits were $11.1 million ($1.35), up 200 percent from $3.7 million (43 cents) in the 1978 quarter. Revenues climbed 58 percent to $220 million from $139 million the year before. Operating expenses rose to $2.77 billion last year, far above the $2.28 billion of the previous year.

Another airline, Southwest Airlines of Texas, reported reduced earnings for the final quarter.

Southwest said it earned $4.4 million in the fourth quarter (97) cents) compared to $4.5 million in 1978 ($1.10).Yearly earnings were $16.7 million ($3.70) compared to $16.3 million ($3.62). But earnings for the last quarter of 1978 and for all of 1978 included nonrecurring gains from the sale of aircraft.

Allis Chambers Corp. increased its earnings last year to $6.23 a share from $6.18 in 1978 in spite of a 55-cents-a-share slash in profit caused by an accounting change.

The change was the extensive adoption of LIFO inventory accounting. Net income rose to 81.4 million from $76 million as sales rose to $1,973 billion from $1.762 billion.

Fourth quarter profit was $21.7 million ($1.66) on sales of $529 million compared with $17 million ($1.34) a year earlier on sales of $493 million.