President Trump’s decision Thursday to impose crippling tariffs on the imports of steel and aluminum took many by surprise — particularly investors, as the Dow Jones Industrial Average closed the day’s trading down more than 400 points, or 1.7 percent, at 24,608.

But one billionaire investor and former Trump adviser, Carl Icahn, was seemingly unvexed, having dumped a million shares tied to the steel industry a week before the president announced 25 percent tariffs for foreign-made steel.