The world’s biggest producers of iron ore have a problem, and it lies in the steel that has already gone into China’s cars, bridges and skyscrapers. On Tuesday, BHP Billiton, the world’s biggest miner, lowered its long-run forecast for peak China steel demand to between 935 million and 985 million metric tons from one billion to 1.1 billion tons. China’s annual production is currently at about 800 million tons. This historic glut of Chinese steel, and concerns over the country’s economic prospects, have hurt prices for iron ore, the biggest ingredient in steelmaking. The commodity has fallen to roughly $50 a ton from $190 a metric ton in 2011, squeezing the profits—and share prices—of the world’s biggest mining companies. The S&P mining index has declined to 18.33 from above 70 over that stretch. China accumulated so much steel so rapidly that the total amount of steel in the economy […]