GST rate Differential handling and Input credit

Dear All,

I have a GST input credit question i would like to clear :-

Company B buys raw material (Alloy steel blooms) from SAIL at 18% GST rate due to HSN code used for clearing material. But company B sells the material to company C at 5% GST rate due to HSN code being used to clear material. What will happen to the differential in the rate% when input credit is being requested? What credit shall be given to Company B? Company C clears the material at 5% as well. 

Also, if supposing Company C is exempt from taxes being a government company as well as using the material for self consumption, then what will happen to Input credit of Company B when they apply for the same?

Warm Regards.