[1] At higher B values Fe23(CB)6 is thought to form, which promotes ferrite nucleation, and so adversely affects hardenability.
[1] Boron is effective at very low concentrations – 30 ppm B can replace an equivalent 0.4% Cr, 0.5% C, or 0.12% V.[2] 30 ppm B has also been shown to increase depth of hardening (~ +50%) in a low-alloy steel – thought to be due to its retardation of austenite decomposition to softer bainite, ferrite, or pearlite structures on cooling from an austenitization treatment.
[2] Boron steels find use in the car industry, typically as strengthening elements such as around door frames and in reclining seats.
[3][4] Flat boron steel for automotive use is hot stamped in cooled molds from the austentic state (obtained by heating to 900-950 °C).
Stamping can be done in an inert atmosphere, otherwise abrasive scale forms – alternatively a protective Al-Si coating can be used.
Boron steel flats, typically 30MnB5 modified with an addition of 0.5% chromium are used in the manufacture of fork arms for forklift trucks.