When Japan's steel companies saw their core business rusting away some years ago, they wasted no time in searching for other moneymaking opportunities. One started raising pigs. Some plunged into biotechnology. Another built an amusement park.

Today, Nippon Steel Corp. will announce another novel twist to this diversification strategy: The company plans to make lightweight laptop computers in California.

The company, whose $18 billion in annual sales makes it the world's largest steelmaker, will create a new subsidiary in San Jose that will begin selling its Librex computer line this fall.

The manufacturing of the 6.7-pound, notebook-size computers will first be contracted to other firms. But, if sales warrant, Nippon will make its own computers in a new factory in California.

Japanese steel companies, which long ago stepped in to bail out struggling U.S. steelmakers, are no strangers to the U.S. high-technology industry.

Several U.S. semiconductor makers, such as Texas Instruments Inc. and LSI Logic Corp., for example, have steel companies as partners in Japan. Also, some Japanese heavy equipment and steel manufacturers have invested in U.S. computer and computer-equipment companies, among them MIPS Computer Systems Inc. and Komag Inc. in California.

Japanese steel firms also have emerged as buyers or partners for several U.S. makers of computer-chip materials and equipment.

But it is rare that a Japanese steel company would stray so far from its roots -- both technologically and geographically -- without a partner.

"There is no other steel company that has set up a wholly owned subsidiary {in} anything outside the steel business in this country," said Susan MacKnight, chief economist with the Japan Economic Institute in Washington, a research group of the Japanese government.

To date, the diversification efforts by Japanese steel companies, begun in earnest about 1986, have been slow to bear fruit.

Diversification in Japan "is working slower than expected" but is seen as an "investment for the future," said John Jacobson, a steel industry specialist with AUS Consultants near Philadelphia. Steel sales still account for the vast majority of the companies' revenues, and the expansions have become less pressing recently amid a rebound in the demand for steel.

Still, Nippon Steel remains committed to far-flung diversification -- a strategy that requires massive retraining. Some employees who had spent their lives amid blast furnaces were reassigned to sew uniforms. Other workers were deployed to a recently opened amusement part that features an outer-space theme. About 4,000 people are involved in the biggest undertaking -- electronics and large computers.

But small laptop computers are a new and particularly risky business for Nippon Steel. With its planned $3,000 to $4,000 machines designed to fit in a briefcase, Nippon will face a field crowded with competitors and expected to attract even more.

Said William Lempesis, publisher of a computer-industry newsletter: "They'll probably have a difficult time."