Hot stamping process is widely used in the manufacture of the high strength automotive steel, mainly including the stamping and quenching process of the hot-formed steel. In the hot stamping process, the steel is heated above the critical austenitizing temperature, and then it is rapidly stamped in the mold and the quenching phase transition occurs at the same time. The quenching operation in the hot stamping process has a significant influence on the phase transition and mechanical properties of the hot-stamping steel. A proper quenching technique is quite important to control the microstructure and properties of an ultra-high strength hot-stamping steel. In this paper, considering the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and phase transition from austenite to martensite in quenching process was established for production of ultra-high strength hot-stamping steel. The temperature variation, the austenite decomposition and martensite formation during quenching process was simulated. At the same time, the microstructure and the properties of the ultra-high strength hot-stamping steel after quenching at different austenitizing temperature were experimental studied. The results show that under the conditions of low cooling rate, the final quenching microstructure of the ultra-high strength hot-stamping steel includes martensite, residual austenite, bainite and ferrite. With the increase of the cooling rate, bainite and ferrite gradually disappear. While austenitizing at 930 °C, the tensile strength, yield strength, elongation and strength-ductility product of the hot-stamping steel are 1770.1 MPa, 1128.2 MPa, 6.72% and 11.9 GPa%, respectively.

Clarifying the influence of Nb and V microalloying on the ultra-high strength hot stamping steel (UHSHSS) and exploring appropriate process parameters are the basis for effectively regulating properties of the final product. In this study, the effects of different austenitizing temperatures and holding times on the phase transitions, grain sizes and mechanical properties of 22MnB5NbV with Nb and V alloyed are studied by using JMatPro thermodynamic calculations and experiments. By comparing with 22MnB5 without Nb and V alloyed, the effects of Nb and V elements on quenching microstructures, grain sizes and mechanical properties of UHSHSS are analyzed. The suitable austenitizing process parameters of 22MnB5NbV have been obtained. The results show that the grain size of Nb-V-alloyed UHSHSS grows with the increase in the austenitizing temperature and holding time. The 22MnB5NbV steel can be completely austenitized while the austenitizing temperatures ≥870 °C and holding time ≥3 min. Combined with the actual production process, the best austenitizing temperature and holding time are 930 °C and 3 min. Under these conditions, the average grain size is 7.7 μm, the tensile strength, elongation and strength-ductility product are 1570.8 MPa, 6.6% and 10.4 GPa·%, respectively. With the addition of Nb and V elements, the nanoscale precipitates lead to the refinement of the quenched structure and grain size, and the comprehensive properties of UHSHSS have been effectively promoted, in which the elongation and strong-plastic products are increased by ~0.6% and ~1.2 GPa·%, respectively.

With the rapid development of the automobile industry and the increasing requirement of the environmental protection, ultra-high strength hot stamping steel is widely used in body parts of the car. In this paper, the precipitation changes of the second phase particles and CCT curves of 2000 MPa ultra-high strength steel under different austenitizing conditions are studied by the thermodynamic calculations and simulations. The effects of austenitizing temperature, holding time and cooling rate on the quenching transformation behaviors of the steel in the hot stamping process are explored. The microstructure differences on the surface and internal positions of the steel in the austenitizing quenching process are compared and analyzed. The austenitizing process parameters suitable for 2000 MPa ultra-high strength hot stamping steel are formulated, which provides a theoretical basis for the formulation of 2000 MPa ultra-high strength steel in the hot stamping process.

Because of its ultra-high strength, press hardening steel and hot stamped parts are widely used in the reinforcement and lightweight structure of car bodies. At present, there are hydrogen embrittlement barriers in PHS1500~PHS2000 materials with coating and PHS1800~PHS2000 materials without coating. In this paper, based on the conventional thickness coating PHS1500 without niobium and the thin thickness coating PHS1500 with niobium, the corresponding automobile hot stamped front crash beam parts are trial produced. The trial production process is completed under the dew point control condition of -20 ℃. Samples were taken from hot stamped parts made of two materials. Firstly, based on the automotive industry standard T/CSAE 155-2020, a U-shaped constant bending load test was carried out to compare the hydrogen embrittlement resistance of parts made of two materials. The results show that under the same external load conditions, the fracture time of the sample of thin coating containing niobium with austenitizing time of 220 s is higher than that of the sample of conventional coating without niobium with austenitizing time of 300 s. Further, TDS and slow strain rate tensile tests were carried out on the samples of the two materials. The results show that the thin coating containing niobium has low diffusive hydrogen content and strong resistance to hydrogen damage. The fundamental reason why the samples with thin coatings containing niobium have stronger hydrogen embrittlement resistance is the regulation effect of niobium on grain boundaries, precipitation and retained austenite, and the stronger inhibition effect of thin coatings on hydrogen induced cracks.

Bumper is one of the most important collision safety parts on the vehicle. Based on the simulation and test methods, this paper verifies the advantages of microalloying in improving the resistance to impact cracking and crack propagation of hot stamped automobile bumpers. Firstly, the two hot stamped steel plates with and without microalloyed elements are treated with the same quenching process, and then the fracture card models of the two materials are established through simulation and test. Based on the fracture card model, the drop weight impact performance of two kinds of bumper materials is simulated and compared. The results show that the sample containing microalloyed elements has better impact, intrusion resistance and energy absorption performance. The hot forming trial production of two kinds of bumper materials is carried out, and the drop weight test of the actual sample is compared. The test results are consistent with the simulation results. The results show that the crack propagation effect of the sample containing microalloyed elements is weaker than that of the sample without microalloyed elements in the same observation period.

The press hardening steel (PHS) and hot-stamping components were utilized widely on the reinforcement and lightweight structure of car body due to its ultra-high strength. There existed an obstacle of hydrogen embrittlement (HE) for coated 1.5–2.0 GPa PHS, and uncoated 1.8–2.0 GPa PHS. The typical hot-stamping components were industry-tried by slim Al-Si coated 22MnB5Nb PHS, under -20 ℃ dew points and without dew point control, and the U-shaped constant bending load test in 0.1 mol/L HCL solution of samples from components were performed based on automotive industry standard T/CSAE155-2020, to compare the hydrogen embrittlement resistance. The results were attained that, under same testing fracture stress, the fracture time is higher obviously for samples at -20 ℃ dew points comparing to the one without dew point control, and the atmosphere in austenitizing furnace affects prominently the diffusible hydrogen produced by reduction reaction in austenitizing process. A hydrogen embrittlement mechanism deduction is that the original diffusible hydrogen in steel and the diffusible hydrogen produced by reduction reaction in austenitizing process leads to initial micro cracking under higher bending stress according to HELP effect, after that microcrack passivates, and restart of microcrack by the effect of dislocation, grain boundary and applied stress, then hydrogen embrittlement fracture occurs by repeated microcrack passivation and restart.