Germany was heavily reliant on Swedish iron ore imports during WWII. While Germany had large iron ore deposits, these were largely low quality, resulting in low yields of steel. Scrap metal could supply some of the required high-quality steel, but this was a more limited resource. As such, imports made up the majority of Germany's iron ore supply, both in peacetime and war. In 1939, Germany imported 10.5 million tons of iron ore, compared to a domestic production of 3.9 million. About half of the imported iron ore came from Sweden alone. The start of the war brought a drop in imports, as a result of the Allied blockade. In 1940, imports dropped to 6.9 million tons, while domestic production rose to 5 million. 83% of the imported ore came from Sweden, which had unobstructed sea routes to Germany. The Fall of France and Operation Barbarossa, however, loosened German reliance on Sweden to some extent. These offensives gave Germany unfettered access to the iron ore mines of Alsace-Lorraine and the Donets Basin. This meant that, while iron ore imports soared in 1942-43 (13.6 million tons in 1942, 15.2 million tons in 1943), the proportion of these imports that came from Sweden dropped. Still, Swedish iron ore was highly important to Germany in the early part of the war.

Source:

Wages of Destruction: The Making and Breaking of the Nazi Economy, Adam Tooze, Allen Lane, 2006