In what could be one of the biggest foreign takeovers of an American company, Kaiser Steel Corp. said today it is discussing the sale of "a substantial proportion of its assets" to the giant Japanese steelmaker, Nippon Kokan K.K.

Kaiser, the ninth-largest U.S. steel producer and the only major steel company base on the West Coast, has suffered 13 consecutive quarterly losses in its steelmaking operations.

If the companies agree to terms, it would be the thrid major foreign takeover attempt in the last six weeks. The Imperial Group Ltd. of Britian wants to acquire Howard Johnson Co., the restaurant and motel company, for $630 million.

Renault, the French government's automaker, announced two weeks ago it could acquire up to 22.5 percent of American Motors Corp. for $150 million.

Kaiser did not say weather Nippon, Japan's second largest producer, was interested in buying only Kaiser steelmaking facilities or other portions of the company as well. Kaiser owns several coal mines and a 32.2 percent share of Kaiser Resources of Canada, which is involved in coal-mining and oil and gas production.

As of Sept. 30, Kaiser Steel's assets had a book value of about $500 million, roughly $71.46 a share. The stock closed on the New York Stock Exchange at $39 a share, up more than 50 percent from its Monday close of $25.50.

Even if the two companies come to terms on a sale of assets to Nippon, the deal could be blocked by either government or the state of California.

As a spokesman for the Justice Department said the Federal Trade Commission or Justice investigates every major merger to see if there are antitrust violations and this one would be no exception.

A foreign takeover of a major American steel company would be likely to cause concern in Congress and within the industry itself as well as in the antitrust office of Justice.

Steel, probably the nation's most important metal, has been almost totally a U.S.-owned industry since its beginning. Foreigners own only two U.S. steel companies, and both are minor producers of specialized steel products.

Although the company as a whole has been running at a profit, its steelmaking operations have been losing money for three years. Kaiser's steelmaking losses have been offset by the company's holding of the Canadian energy company and by an Australian mining operation it sold several months ago.

The company has been buffeted by imports -- mainly from Japan -- and by internal management difficulties. It was once part of Kaiser Industries Inc., a major family-owned holding company, but became independent in 1977 as Kaiser Industries liquidated and spun off several major companies including the steel company and Kaiser Aluminum and Chemicals.

Former Kaiser Industries chairman William Roesch, now president of the nation's largest steel producer, U.S. Steel Corp., left in 1977 after run-ins with Edgar Kaiser senior and junior.

Several months ago, Mark Anthony, who is head of the steel company, was also forced out.

Kaiser owned one major steel-making plant, the Fontana Works, about 45 miles east of Los Angeles. It has just opened two modern steel-making facilities that the company claims will make it the most efficient producer in the country.

But the company admits it has been having problems breaking in its new basic oxygen furnace and continuous casting machine.

While the basic product of most steel companies is sheet steel used in automobiles and appliances, Kaiser's primary products are tinplate and galvanized steel. Its major customers are West Coast canners as well as Boeing Co., the aerospace company.

Last year Kaiser shipped 2.35 million tons of steel. It had sales of $724.4 million and net income of $13 million.

Nippon has the ability to make 24 million tons of steel and last year had sales of $5 billion.

David Healey, an analyst for Drexel Burnham Lambert, said that part of Kaiser's problems stem from import competition, but that the company has also stretched its borrowing powers to the limit because of its new facilities. a

Most analysts expect the steel industry to show a sharp decline in coming months, which would compound Kaiser's problems.

"What I don't understand," said Healey, "is what is in if for the Japanese."

One source familiar with Kaiser's operations said, however, that once the company works out labor and production difficulties with its new operations, the acquisitioncould prove to be very valuable.

A Kaser spokesman said that technical teams from Kaiser have toured Nippon's two major Japanese facilities.The company said it would be "a number of weeks" before anything definitive comes out of the discussions.