Has a good cold deformation capacity, you can save the cold deformation before the annealing;
Quenching brittle tendencies are low, you can use water quenching;
The addition of trace boron can partially replace the addition of expensive alloying elements;
The latent fracture sensitivity of low carbon steels is relatively low.
The basic principle of boron steel composition design is to reduce the carbon content and improve the cold deformation capacity of steel. Add the trace amount of 0.0005% ~ 0.003 5% boron to compensate for the loss of strength and hardenability caused by carbon reduction. In addition, according to the need to join Cr, Mn, TI and other alloying elements, to further improve the hardenability.
Normally 8.8 bolts are made of low-carbon manganese-containing boron steel similar to SAE10B23. Type 9.8 and 10.9 bolts are made of medium-carbon manganese-containing boron steel equivalent to SAE10B35 or ML35MnB, ML370B boron equivalent to ML40Cr, ML35CrMo Steel manufacturing. However, due to the anti-tempering softening ability of boron steel, the tempering temperature is lower than SCM435, ML40Cr 60 ~ 80, so the use of boron steel 10.9 high strength bolts delayed fracture sensitivity.