President-elect Donald Trump's threat to slap punitive tariffs on Chinese goods is a worry for firms that trade with the country in the US Midwest, a decisive region in the Republican's election triumph. Strong support across America's "Rust Belt," and frustration at lost industrial jobs blamed on globalization, carried Trump to victory last month in key battleground states, including Michigan and Ohio. But some companies in the region that benefit from global trade are worried about early signs the president-elect plans to take a hardline stance with China. "We export a lot of products to China," said David Shogren, president…