U.S. steam coal exporters heard more discouraging words Friday about their sales prospects in the Japanese utility market.

Japan's steam coal imports are expected to rise significantly in the 1990s, said Hiroshi Makita, research manager for Japan's Federation of Electric Power Companies.Japan's doors are open, there are no barriers to trade, and Japanese utilities are seeking to diversify supply sources, he added.

But at an international coal conference here, the most positive message Mr. Makita could deliver to U.S. companies was to tell them that "it remains to be seen" how their market share will change.

He declined to make any kind of projection regarding the level of U.S. steam coal imports.

He was speaking at an annual conference sponsored by the Mississippi Valley Coal Exporters Council.

Mr. Makita said Japan imported 15 million tons of steam coal in the last complete fiscal year, which ended March 31, 1989. U.S. exporters accounted for just 500,000 tons, he noted. Australia, the largest supplier, shipped 10 million tons.

Mr. Makita maintained that U.S. steam coal purchases in Japan will jump substantially this fiscal year compared to last, but he did not specify how much that will be.

He said Japan's steam coal consumption is expected to climb from 24.5 million tons a year to 30 million tons in 1993, and to 47 million tons by 1998.

A fuel buyer from the Danish utility Elsam told the conference that while Denmark soon will resume diplomatic relations with South Africa, imports of coal will not be allowed until the system of apartheid is ended.

Denmark is a major importer of U.S. steam coal and also one of the most important buyers to ban South African coal from its utilities. The general manager of GKE, the Netherlands utility coal buying group, said his company's imports of U.S. steam coal will rise from 2.5 million tons in 1989 to 2.8 million tons in 1990.

A representative of Electricite de Portugal, the Portuguese power generation company, said U.S. coal imports will jump slightly this year from a level of about 1.5 million tons.

The fuel director of Finnish utility, Imatran Voima Oy, said his company, which bought no U.S. coal last year, may become a market in the 1990s for high sulfur coal from the United States.