In a filing with the World Trade Organization (WTO), the EU said the tariff is essentially a safeguard measure. And as a safeguard measure, it was improperly imposed, the EU argues. 

“Notwithstanding the United States' characterization of these measures as security measures, they are in essence safeguard measures," the EU wrote in the filing. 

China earlier this month filed a complaint with the WTO over the tariffs. Both are seeking to begin discussions with the U.S. under the WTO’s dispute resolution process.