The comment made earlier on Friday before the team's departure for match venue Lviv caused a stir in Germany due to its military connotations.
The German word for steel helmet (Stahlhelm) not only refers to the actual military helmet used by German soldiers in both World Wars but also to a post-World War I paramilitary organisation.
"I am sorry if my unfortunate comment created irritations," Flick said in a statement. "It was a verbal error that should not create wrong impressions. It is not my style to use military vocabulary for sporting issues."
"I would like to apologise for my expression at the news conference and I am angry with myself because I know how sensitive we are with these issues," he added.
A delegation from the Germany team visited the former Nazi death camp at Auschwitz in Poland on June 1.
Ronaldo packs a powerful strike from dead-ball situations and the Germans have been told to minimise the risk of committing fouls outside the penalty area in one-on-one situations, Flick had earlier told reporters.
"I think just steel helmets and to make themselves big," the assistant coach had said when asked how the Germans planned to deal with any potential free kicks from the Portuguese forward.
"At 20, 25 or even 30 metres we need to be clever in the one-on-one situations. Ronaldo has an exceptional free-kick quality."
Germany also play Netherlands and Denmark in their group.
