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Hot stamping components with 1500 MPa ultra-high strength are obtained by press hardening during hot stamping, and the properties depend on the microstructures. It is very important that the microstructure evolution rule is found out during hot stamping process. To characterize the microstructure evolution during hot stamping, a method combining finite element and experiment is carried out. Samples were heated to 950°C and held for 300 second at a induction heating furnace, then taken out from the furnace and stayed in the air at different time (7 s, 11 s, 13 s, 22 s), respectively, finally the specimens were formed and quenched at a die. Microstructural observation as well as surface hardness profiling of formed specimens was performed. And the numerical simulation to predict the austenite transformation into ferrite, pearlite, bainite, and martensite and the volume fraction of each phase during the hot stamping process was made with ABAQUS software. The results show that the ferrite is observed when the specimen stays in the air for 22 s, and the temperature drops to 325°C when the dwell time increases from 7 s to 22 s. The results of numerical simulation and experimental results are in good agreement. So the method finite element can be used to guide the optimization of hot stamping process parameters.

Even though a new generation advanced high strength steels with ultra-high strength and excellent formability are developed. The application of press hardening steel is still increased for the safety parts of body in white, due to the much higher demand for light weight of car body, guaranteeing the driver and passenger safety, and lower gas emission [

Nikravesh, et al. [

In this paper, the effect of the time of staying in the air during the samples being transferred on the phase transformation was analyzed, considering the influence of the sample temperature on the diffusion and diffusionless phase transformation. The current work focus on the influence of the different time of staying in the air on subsequent phase transformations. And the volume fraction of each phase during the hot stamping process was numerical simulated with ABAQUS software.

1.5 mm sheet of 22MnB5 press hardening steel with a composition of Fe-0.22C-0.25Si-1.20Mn-0.20Cr-0.002B (wt%) was used in this work. The microstructures of as-reveived steel are 76% ferrite and 24% pearlite.

The samples were heated up to 950˚C for 300 s, then taken out from the furnace and stayed in the air at different time (7 s, 11 s, 13 s, 22 s), that is sample transfer time, respectively, finally the specimens were formed and quenched at a die with water cooling system. And this process was simulated by using ABAQUS software. The microstructures were examined by light optical microscope, and the samples were inlayed in epoxy resin, ground and polished into mirror face using 180, 400, 800 and 1200 emery papering, subsequently by 2.5 μm and 1 μm polishing agent. The specimens were then etched using a 3% Natal solution for microstructure observation. And the volume fraction of every phase with different transfer time was simulated.

with different transfer time. As shown in

In the present work, the hot stamping process including forming, quenching, and transfer time was simulated by ABAQUS software. A quarter of die and steel sheet was selected to simulate the hot stamping process, because of the symmetrical die and surroundings environment.

process, the material of die is H11, and the relevant parameters are showed in

F = 1 − exp ( − b × t n ) (1)

where F is the volume fraction of phase, b is a constant of the relevant of temperature, phase and grain size, t is current time, n is a constant of phase transformation, b and n can be obtained by the isothermal transformation curve calculation.

The diffusionless phase transformation from austenite into martensite is simulated by using the finite element method, the volume fraction of martensite can be calculated by the relation given [

F m = F a × { 1 − exp [ − 0.0 11 × ( M s − T ) ] } (2)

where F_{m} is the volume fraction of martensite, F a is the volume fraction of the remaining austenite when the phase transformation from austenite into matensite is start, M_{s} is the start temperature of martensitic transformation, T is the actual time.

The simulating results show a large amount of ferrite formation when the transfer time is up to 22 s.

Heat transfer coefficient/ W・m^{−}^{1}・k^{−1} | Specific heat capacity/J・kg^{−1}・c^{−1} | Density/kg・m^{−3} | Young modulus/GPa | Poisson’s ratio |
---|---|---|---|---|

422 | 526 | 7700 | 210 | 0.3 |

The effect of transfer time on the phase transformation of hot stamping process is significant, the ferrite formation will occur when the transfer time is up to 13 s. and a considerable amount of ferrite was appeared in the microstructure with the transfer time 22 s. The results of numerical simulation and experimental results are in good agreement. So the method finite element can be used to guide the optimization of hot stamping process parameters.

The authors declare no conflicts of interest regarding the publication of this paper.

Hu, K.H., Zhou, S.Z., Han, R.D., Gao, J. and Yang, Y. (2018) Microstructure Evolution and Simulation in 22MnB5 Steel during Hot Stamping. Journal of Materials Science and Chemical Engineering, 6, 9-14. https://doi.org/10.4236/msce.2018.68002