Hi,
Why do I need a comma here?
Expecting the strike, the company bought large amount of steel.
The pause that the comma represents helps the reader to understand the meaning more easily.
Note that the same thing is true of the longer pause that is represented by the period in your example. Consider how it would be to read a book that had no commas or periods.
In your sentence, you also need to say either 'a large amount of steel' or 'large amounts of steel'.
Best wishes, Clive