President Trump on Thursday launched a trade probe against China and other exporters of cheap steel into the US market, raising the possibility of new tariffs and sending shares of some US steel makers up over 8%. Trump cited concerns about national security as reasons for the probe. “Steel is critical to both our economy and our military,” said Trump. “This is not an area where we can afford to become dependent on foreign countries.” The unusual step of launching an investigation comes as Trump is pressuring China to do more to rein in an increasingly belligerent North Korea, Reuters reports. When Chinese President Xi Jinping visited Trump in Florida earlier this month, Trump raised the possibility of using trade as a lever to coax China to do more.