The Carter administration announced yesterday a 4.9 percent increase in minimum prices for import steel products, effective Oct. 1.

This is the second increase the administration has announced in its so-called trigger price mechanism since the program was outlined early this year. The two increases more than cover the price hikes announced by domestic steel makers since Jan. 1.

The trigger price program is designed to protect American steel producers from unfair foreign competition. If foreignors ship steel to the United States at a price below the trigger price, the Treasury will launch an expedited investigation to determine whether the steel is being dumped - sold at less than cost - in the United States.

The Treasury Department is investigating about a half dozen instances in which steel has been imported into the United States at prices apparently below the trigger prices to see if there are grounds for an anti-dumping investigation.

While Treasury officials will not name any of the country's involved, other sources said that the investigations suggests that an "expedited" anti-dumping investigation will involve Italy.

The trigger price mechanism, which took full effect for most products on April 30, has sharply reduced steel imports in May, although not as much as some steel companies had hoped.

Peter Ehrenhaft, deputy assistant secretary of the Treasury for tariff affairs, said yesterday that the new average cost per ton for Japanese-produced steel is $329.42, 4.9 percent higher than the average of $314.49 now in effect.

There are dozens of different trigger prices for the wide variety of steel products shipped to the United States and costs of shipping and insurance are added to the cost of production to devise a specific trigger price for a specific U.S. port of entry.

Ehrenhaft announced yesterday that the Treasury had decided to reduce the freight rates applied to certain products being shipped into the Great Lakes ports, in effect reducing the trigger prices there for hot and cooled rolled sheets and steel plates.

He said that the Treasury felt it had set freight rates too high for the region and that was reflected in the Great Lakes ports getting a smaller percentage of imports this year than in 1977.

Frederick Jaicks, chairman of Inland Steel Co., the only Chicago-based steel company, said that he believes the reduction in the freight rates reflects "political pressures" brought by foreign steelmakers and Great Lakes shippers because he sees no evidence that the original calculations contained any errors.

Steelmakers have been ambivalent about trigger prices since they were announced - pleased with the notion of protection but arguing that the trigger prices were too low to effectively prohibit dumping.

The Treasury changes announced yesterday reflected not only the rising value of the yen, but new calculations about Japanese production yields as well as capacity utilization. To reflect lower utilization rates, the Treasury boosted trigger prices an average of $18 a ton, but lowered them $15 (for a net $3 raise on these two scores) to reflect the Treasury's recalculation of product yield per ton of raw steel from 80 percent to 82.7 percent.