Boron is a potent and economical alloying element that markedly increases hardenability when added to a fully deoxidized steel, especially low carbon grades.

Additions are small, usually between .005 and .003 percent.  It does not affect the strength of ferrite, and therefore does not sacrifice ductility, formability or machineability in the annealed state.

Steel grade designation:  XXBXX. 

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Chemical Composition Ranges
and Limits Charts.