Effect of Flash Heating on Microstructure and Mechanical Properties of 2000 MPa Hot Stamping Steel
XIE Zedong, DING Cancan, WEN Pengyu, LUO Haiwen()
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
XIE Zedong, DING Cancan, WEN Pengyu, LUO Haiwen. Effect of Flash Heating on Microstructure and Mechanical Properties of 2000 MPa Hot Stamping Steel. Acta Metall Sin, 2024, 60(12): 1667-1677.
Hot stamping steels (HSSs) have been widely used in automobiles, to reduce weight and improve safety due to their ultrahigh strength and ease of synthesis at high temperatures. At present, steel sheets with high strength and good ductility are needed to further reduce the weight of manufactured products. The most popular HSS grade in use at present is 22MnB5, which has an ultimate tensile strength (UTS) of 1500 MPa, but it has a ductility of less than 7%, which is quite poor. Driven by the demand for weight reduction in the automotive industries, a 2000 MPa HSS have been developed by employing a new alloying design and an ultrafast heating process. The latter has received much less attention than the former, although it demonstrates huge potential for improving mechanical properties and production efficiency of HSSs. In this study, the effect of heating processes, including conventional and flash heating, at a ramp of 150oC/s in the temperature range of 850-950oC before tempering at 150oC on the microstructures and mechanical properties of a new type of 2000 MPa HSS were studied. Compared with the conventional heating at a relatively low ramp rate, the flash heating improved the strength and ductility of 2000 MPa HSS, simultaneously. Moreover, their best tensile properties were achieved after flash heating to 950oC: UTS was 2180 MPa and total elongation was 13%, which were approximately 200 MPa and 4% higher than those obtained using conventional heating, respectively. This is because flash heating results in the formation of a more refined hierarchical martensite structure after quenching, with a higher dislocation density and a larger fraction of retained austenite (RA). RA was formed by dissolving cementite particles containing high C/Mn concentrations, which were then inherited in the formed austenite after quenching due to insufficient time for the homogenization of solute C/Mn by diffusion during the flash heating. The volume fraction of RA increased gradually with an increase in the flash heating temperature, then, more cementite particles were dissolved. This was also confirmed by kinetic simulations that reversed the austenitization on the dissolving cementite. Finally, it was proposed that flash heating technology is a promising technology for the production of ultra-strong and ductile HSS sheets.
Fund: National Natural Science Foundation of China(51831002);National Natural Science Foundation of China(52233018);Fundamental Research Founds for the Central Universities(FRF-TP-18-002C2)
Corresponding Authors:
LUO Haiwen, professor, Tel: (010)62332911, E-mail: luohaiwen@ustb.edu.cn
Fig.1 Schematic of the manufacturing processes of hot stamping steel, including flash heating (FH) and conventional heating (CH) processes (a), and the dilatation curves of hot stamping steel during the heating and cooling process (b) (Ac1 and Ac3 are the temperatures for austenite transformation to start and finish during heating, respectively; Ms and Mf are the temperatures for martensite transformation to start and finish during cooling, respectively)
Fig.2 TEM image of the hot-rolled and annealed steel (Inset shows the EDS compositional analysis of cementite) (a) and SEM image of the cementite in the cold-rolled state (TD—transverse direction, RD—rolling direction, ND—normal direction) (b)
Fig.3 Engineering stress-strain curves for the hot stamping steel subjected to the different heating processes and the commercial 22MnB5 steel[18] for comparison
Specimen
YS / MPa
UTS / MPa
TE / %
Product of UTS and TE / (GPa·%)
22MnB5[18]
1130
1570
11.5
13.0
CH850
1540 ± 20
2025 ± 30
11.1 ± 0.2
22.4
CH900
1555 ± 22
2050 ± 35
10.6 ± 0.3
21.7
CH950
1485 ± 28
1975 ± 25
10.1 ± 0.3
19.9
FH850
1590 ± 26
2070 ± 30
13.7 ± 0.2
28.4
FH900
1655 ± 21
2155 ± 20
13.4 ± 0.2
28.9
FH950
1675 ± 25
2180 ± 25
13.7 ± 0.4
29.9
Table 1 Summaries of mechanical properties of hot stamping steel manufactured by different heating processes and commercial 22MnB5 steel[18] for comparison
Fig.4 Microstructures and precipitates formed in the hot stamping steels subjected to both the flash heating and conventional heating to 850, 900, and 950oC (a-c) SEM images of FH850 (a), FH900 (b), and FH950 (c) specimens (α and θ represent ferrite and cementite, respectively) (d-f) SEM images of CH850 (d), CH900 (e), and CH950 (f) specimens (g-i, l) TEM images (g, h) and statistical size distributions of cementite particles (i, l) in FH850 (g, i) and FH900 (h, l) specimens (j, k) EDS linear composition measurements on cementite in FH850 (j) and NbC in FH900 (k) indicated by yellow circles in Figs.4g and h, respectively
Fig.5 EBSD quality map overlapped with outlined prior austenite grain boundaries with the misorientation between 20° and 50° (yellow lines) (a, b) and images on all the high angle grain boundaries with the misorientation > 15° (d, e) in FH950 (a, d) and CH950 (b, e) specimens; the distributions of prior austenite grain size (c) and high-angle grain size (f) derived from Figs.5d and e, respectively, in which high-angle grain size was defined by all the high angle grain boundaries and consistent with the Hall-Petch strengthening mechanism
Specimen
Prior austenite grain size
High-angle grain size
CH850
3.71 ± 0.36
0.94 ± 0.12
CH900
4.88 ± 0.28
1.26 ± 0.18
CH950
6.21 ± 0.32
1.37 ± 0.16
FH850
1.42 ± 0.19
0.62 ± 0.16
FH900
1.46 ± 0.23
0.71 ± 0.15
FH950
1.52 ± 0.21
0.76 ± 0.11
Table 2 Summaries of prior austenite grain and high-angle grain sizes in the specimens subjected to the different heating processes
Fig.6 EBSD image quality map overlapped with phase distribution on the microstructures of FH950 (a) and CH950 (b) specimens; Auger mapping of C, Mn, and Cr distributions in retained austenite (RA, green regions) in FH950 specimen (c); XRD spectra (d) and RA volume fractions (e) of the specimens subjected to both the conventional and flash heating processes
Fig.7 Schematic of the microstructural evolutions of cold-rolled steel during the flash heating at 850, 900, and 950oC for 5 s
Fig.8 (200) γ diffraction peak intensities of hot stamping steel after the flash heating to the diff-erent temperatures (a) and dislocation densities of the specimens subjected to both the conven-tional and flash heating processes (b)
Fig.9 Schematic of the reverse austenitization kinetic simulation (a), the moving interfaces between cementite, ferrite, and austenite in the hot stamping steel during the flash heating to different temperatures and the isothermal holding for 5 s (b); the calculated C concentration (c) and Mn concentration (d) profiles developed near the interface boundaries time during the process of flash heating to 850oC
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