Effect of compound addition of Ti–B on hardenability and hot ductility of 22MnB5 hot stamped steel

Hot stamped 22MnB5 steel has been widely used in the automotive industry due to its high strength and good toughness. At present, most hot stamped steels improve their hardenability and strength by incorporating trace amount of boron and titanium. In order to obtain excellent hardenability, the effects of boron content and austenitization temperature on the hardenability of Ti bearing 22MnB5 steel have been studied. Additionally, in order to reduce the cracking susceptibiltiy during hot stamping process, the effects of boron content and deformation rate on the hot ductility of 22MnB5 steel have also been investigated. The influence mechanism of microstructures and precipitates on both hardenability and hot ductility has also been discussed. The results suggest that the hardenability of low boron steel (8 ppm boron) is improved gradually, while that of high boron steel (35 ppm boron) is deteriorated suddenly with increasing quenching temperature from 850 °C to 1000 °C. The reason is that there should be a suitable solid solution boron content in austenite to obtain excellent hardenability performance. Too much borides dissolve in the austenite will result in excessive segregation of boron atoms at the grain boundaries and reprecipitation of new borides which deteriorate the hardenability. The low boron and high boron steels both have good hot ductility with strain rates of 0.05s⁻¹ and 0.5s⁻¹, and the section area reductions are all above 80 %, hence there are no hot cracks during hot stamp process. The addition of boron promotes the nucleation and precipitation of Ti(C,N), and boron can also occupy vacancies at grain boundaries and around precipitates to reduce the diffusion rates of C, N, and Ti, thus refine the carbides and the grains of high boron steel. The smaller the grain size, the poorer the hardenability of the high boron steel.