1) ..the use of small amounts of nitrogen in making steels.
a) amount(s) - why it is plural?
b) steel(s) - why it is plural?
2) ..imports of China steel or China steels?
1a) Are there different types of steel? Perhaps Steel Type A requires a specific amount of nitorgen and Steel Type B requires a different amount of nitrogent.
1b) Even if there are different types of steel, I would still use the singular "steel." However,it's very possible that in the industry, they do use the plural to refer to different types, like "fish."
Having worked in the steel industry for more years than I care to count, I can say that "steels" is not the correct word in this instance, and it should simply say "steel." Steel is steel, whether it is stainless, hot rolled, cold rolled, galvanized, or whatever.
"Using small amounts of nitrogen" is correct phraseology. When steel is being smelted, there are different amounts of various elements added to the mix. And it all depends upon what type of steel you're manufacturing (most end users will specify "we need X metric tons of cold rolled steel, or X tons of stainless steel.") The most basic elements that comprise steel are Carbon, Manganese, Sulfur, Nitrogen, Phosperous and Nickel. An increase or decrease of certain elements affects the texture of the steel, that is, it can make it more brittle or more flexible. Most buyers of steel will examine the "chemistries" (a document that lists the percentage of each element in the steel being sold) of the product and determine whether or not it meets their standards. For example, sometimes a steel coil (large "roll" of steel) can be represented as suitable for a particular application, but when the chemistries are evaluated, it is noted that the Manganese is above the accepted standard and thus the steel might possibly fracture or crack when it is struck by the die.
Sorry to have prattled on, the brief answer is that when referring to the metals industry you should always say "steel" and NOT "steels."