U.S. Steel Corp. yesterday reported that its second-quarter earnings plummeted to $111.5 million ($1.28 a share) from $150.4 million ($1.75) in the same period last year.

The nation's largest steelmaker said sales declined from $3.6 billion to $3.1 billion.

First-half net income was $237.5 million ($2.73), down from $287.7 million ($3.35). Sales dropped to $6.3 billion from $6.7 billion.

Meanwhile, Lockheed Corp. reported a loss of $26.6 million in the second quarter resulting from increased interest expense and a $70 million writeoff on the TriStar program.

Uniroyal Inc. reported a loss of $2.6 million in the second quarter, reflecting what it said was a sharp decline in the U.S. automotive markets and the sale last year of its European tire operations.

And Phillips Petroleum Co., the nation's 13th-largest oil company, said a modest 9 percent gain in its second-quarter earnings reflected a decline in its U.S. refining and marketing operations and lower chemical profits.

U.S. Steel Chairman David M. Roderick said steel orders at the end of June were almost 50 percent less than a high point reached in early March.

Despite the recession and its impact on the steel business, the chairman said "we are confident that when the economy turns upward, benefits from (productivity improvements) will become increasingly apparent."

"Steel operated at 65 percent capability compared with 93 percent in last year's second quarter," Roderick said. At the end of June, approximately 25,000 employes in steel operations were on layoff.

In addition, approximately 3,500 salaried, nonexempt employes throughout the corporation have been placed on short work weeks, the executive noted. "Most of our nonsteel businesses have also experienced some reduction in operating levels," he added.

Shipments fell to 4.2 million tons of steel in the second quarter from 6 million tons a year earlier and to 9.1 million tons in the first half from 11.5 million tons a year ago.

Turning to other business segments, Roderick said nonsteel operations in the second quarter "provided the preponderance of operating income, evidencing the effectiveness of our diversification programs."

Roderick said despite weaknesses in products serving the automotive and housing markets, the chemicals segment benefited from continued strength in agricultural and certain industrial segments.

Lockheed's $26.6 million loss contrasted with net income a year earlier of $9.3 million (55 cents a share). It occurred in spite of a rise in sales to $1.4 billion from $982 million a year ago.

Lockheed reported a first-half loss of $14.6 million on sales of $2.5 billion against a profit of $21.7 million ($1.33) a year earlier on sales of $2 billion.

The company said losses on the Tri-Star program for the first half amounted to $128.3 million compared with a loss of $68.3 million on the program in the first half of 1979.

Uniroyal said its second-quarter performance would have been worse had it not been for gains in the company's chemical operations and tire sales abroad.

In the second quarter of last year, during a 40-day strike against Uniroyal by the United Rubber Workers, the company reported a loss of $3.1 million.

The No. 3 tiremaker said second-quarter revenues were $593.2 million, sharply lower than 1979 second-quarter revenues of $726.8 million.

Uniroyal reported a loss of $14.7 million on revenues of $1.17 billion for the first half compared with earnings of $2.9 million (11 cents a share) on revenues of $1.44 billion in the 1979 first half.

Uniroyal, which has announced the closing of two tire plants, was the second of the "Big Four" tiremakers to report a loss for the half.

In May, No. 2 Firestone Tire & Rubber Co. reported it lost $65.8 million in the first half of its fiscal year compared with earnings of $67.4 million in the same period in the previous year.

The industry leader, Goodyear Tire & Rubber Co., and No. 4 B.F. Goodrich Co. have reported sharply lower earnings for both the second quarter and first half compared with the same periods a year ago.

Uniroyal, an international developer and marketer of chemical, rubber and plastic products, recently won agreement from the United Rubber Workers to reduce compensation for union employes by about 13 percent in the remaining months of this year and by 6.5 percent next year to help the company make ends meet. Nonunion employes are taking a similar cut in pay in benefits in a Uniroyal attempt to reduce expenses by a total of $54 million over the next 18 months.

Phillips earned $214.8 million ($1.41 a share) in the latest quarter, up from $214.6 million ($1.39) a year earlier. Revenues climbed 45 percent to $3.2 billion from $2.2 billion.

"Our domestic refining and marketing operations were faced with higher costs for crude oil and natural gas liquids, which we were not fully able to recover because of an industrywide surplus of gasoline and other petroleum products," said William Douce, Phillips president.

He also said "sharply lower demand" for both crude oil and petroleum products during the second quarter boosted inventory levels, which had a negative impact on earnings. Chemical profits dropped during the quarter because of higher costs for raw materials and the depressed U.S. markets for fibers, housing and autos.

First-half earnings rose 28 percent to $499.3 million ($3.26) from $391.6 million ($2.54). Revenues were up 57 percent to $6.6 billion from $4.2 billion. The first-half results were reduced by $54.2 million by higher Norwegian income taxes retroactive to Jan. 1.

Douce attributed the improvement in first-half profits to stepped-up worldwide production of crude oil and higher prices for crude and natural gas in both the United States and abroad.

Phillips was one of the last large refiners to disclose its second-quarter results.