In situ observation on microstructure evolution of 22MnB5 in hot stamping process

Abstract

High temperature confocal laser scanning microscopy (CLSM) was used to investigate the microstructure evolution of high-strength boron steel 22MnB5 during hot stamping. The experimental results show that it is complete austenitized at temperatures about 810 °C during the heating process. Most of the initial austenite grain size is small and locally coarse. At 920 °C to 1000 °C, the phenomenon of B remelted and solidified was observed which played a very good pinning role at the austenite grain boundary, preventing coarsening of austenite grains. The segregation of B and the addition of Mn result in a significant reduction in both the minimum boron reverse melt content and the final solidification temperature. In the continuous cooling stage, martensitic transformation occurs at the cooling rate of 60 °C/s, and the martensite start point is 400 °C and martensite finish point is 280 °C. A large number of bursts are concentrated from 380 °C to 330 °C. There are two main forms of martensitic transformation: first, martensite begins to appear at the coarse austenite grain boundary, and grows transgranularly. Second, the new martensite laths starts from the previously formed laths and grows at a certain angle into the austenite grains. The main factor in the increase of martensite in continuous cooling is the formation of variable temperature martensite rather than the growth of martensite laths. At the cooling rate of 20 °C/s, the bainite and ferrite transformation appeared and the conversion temperature of bainite was about 600 °C. The cooling speed has a great influence on the performance of the 22MnB5 hot stamping component. The room temperature microhardness at cooling rates of 5 °C/s, 20 °C/s, and 60 °C/s was 194 HV, 243 HV, and 430 HV, respectively. Therefore, ensuring sufficient cooling rate is a key condition for obtaining ultra-high strength hot stamping components.

Localized Laser Dispersing of Titanium-Based Particles for Improving the Tribological Performance of Hot Stamping Tools

“…The blank temperature, by starting too close to both electrically powered mold halves was considered with T Close , in order to ensure uniform test conditions and the comparability of all measured temperatures profiles. In addition, the start temperature for the formation of martensite (~400 • C) [27] was indicated as T Ms . The recorded time interval between both temperatures values was taken into account by ∆t.…”

Effects of the Intermediate Precooling Parameters on the Mechanical Properties of a Tailored Press‐Hardened Component in Advanced High‐Strength Steel

“…[ 12,13 ] During this last quenching phase, the material undergoes a martensitic transformation, giving the component high mechanical strength. [ 13–15 ] This allows to adopt thinner blanks to reduce the overall weight of the vehicle.…”

Evolution of Microstructures and Mechanical Properties of Nb-V Alloyed Ultra-High Strength Hot Stamping Steel in Austenitizing Process

“…Mu et al [16] compared and analyzed the mechanical properties of 30MnB5 and 22MnB5, and the mechanical properties of the hot-formed steel after quenching were the best when the austenitizing temperature was kept at 900 • C and the holding time was 3-5 min. Kong et al [17] pointed out that the cooling rate has a significant effect on the hardness of 22MnB5 steel after quenching. Zhou et al [18] concluded that the cooling rate during quenching should exceed 40 • C/s in order to obtain higher strength 22MnB5 hot forming steel.…”