Chemical element used in hardening steel and glassmaking; symbol B (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
boron
(Other definitions for boron that I've seen before include "Chemical element, symbol B" , "element needed for steel" , "component of steel" , "Metalloid element, B" , "Non-metallic element, symbol B" .)