POSCO is trying to figure out why the United States slapped the highest anti-subsidy tariff on it among a host of steelmakers including those from Brazil, Japan, Turkey, the Netherlands and Australia.

Last week, the U.S. Department of Commerce (DOC) imposed 57.04 percent countervailing duties on hot-rolled steel plates from POSCO while locking in rates of 3.9 percent to 11.3 percent on the others.

The punitive import taxes are levied when an exporter gets unfair subsidies from its government.

"A few theories have been raised such as relatively low electricity fees here or the fact that POSCO was once a state-owned corporation. But such explanations don't add up," said a professor in Seoul familiar with the issue.

"POSCO was fully privatized in 2000 and the Korean government currently has no stake. If the electricity fees are the main reason, why don't POSCO's local rivals face a similar level of duties?"

The DOC set countervailing duties for products of Hyundai Steel, Korea's No. 2 steel producer, at a mere 3.89 percent. In addition, Hyundai consumes more power because it operates energy-hungry electricity furnaces while POSCO runs only blast furnaces.

The Korea Electric Power Corp., the nation's monopolistic power supplier, also rebuffs the hypothesis because its electricity rates are far higher than costs thanks to low procurement costs of petroleum.

Against this backdrop, an industry source said that the U.S. targets POSCO to simply protect its own steel manufacturers, which are struggling to survive amid a global supply glut caused by overproduction in China.

"Do you know why the DOC set so high anti-subsidy tariff for POSCO? I think it is because POSCO sends the largest amount among those subject to import taxes this time," said the source who asked not to be named.

"The U.S. strives to safeguard its steelmakers, which have shed so many jobs since early last year, and POSCO's substantial shipment volume appeared on its radar screen. That's it."

Last year, the world's largest economy saw hot-rolled steel imports from the above-mentioned nations more than double to almost $2 billion from a year ago, with up to a quarter coming from POSCO.

In a statement released after the DOC decision, U.S. Steel President Mario Longhi praised the department's determinations.

"The DOC's thorough investigations have resulted in findings confirming what we knew to be true ― that these countries have subsidized and dumped steel products on our shores," he said.

Meanwhile, POSCO said that it is considering various options against the stringent measure including bringing the case to the World Trade Organization.

Theoretically, the DOC ruling could still be overturned by the U.S. International Trade Commission but domestic observers said that has hardly ever happened in the past.

Hot-rolled steel plates are used in automobiles, construction and heavy machinery.