1 Sheet Metal Research Institute, Shougang Research Institute of Technology, Beijing 100043 2 Technology and Quality Division, Qian'an Iron & Steel Company, Shougang Co., Ltd., Qian'an 064400
Cite this article:
Yajun HUI,Hui PAN,Na ZHOU,Ruiheng LI,Wenyuan LI,Kun LIU. STUDY ON STRENGTHENING MECHANISM OF 650 MPa GRADE V-N MICROALLOYED AUTOMOBILE BEAM STEEL. Acta Metall Sin, 2015, 51(12): 1481-1488.
Automobile beam steel with high strength is the development trend of the automotive industry. With the development of heavy-duty vehicles, low cost automobile beam steel both with high strength and high toughness need to be developed. V-N microalloying method combined with thermal mechanical controlled process has a significant grain refinement function. Therefore, the relationship of different N content and technology should be studied in detail. In this work, the microstructure and precipitates of V and V-N microalloyed steel were investigated by using OM, SEM and TEM. And their strengthening mechanism was studied. The results show that both V microalloyed steel and V-N microalloyed steel mainly consist of ferrite and little pearlite. With the increasing of coiling temperature, the strength increased first and then decreased. The optimum mechanical properties were obtained when coiling at 600 ℃, the yield strength, tensile strength and elongation reached 605 MPa, 687 MPa and 24.5%, respectively. Compared with V microalloyed steel, the ferrite in V-N microalloyed steel present finer grain size which can be refined to about 4.5 mm. The precipitates were finer and more dispersed which distribute mainly between 3~50 nm and have the average size of 8.0 nm. And the dislocation density in V-N microalloyed steel is higher. Ferrite grain refinement strengthening, precipitation strengthening and dislocation strengthening make V-N microalloyed steel possess higher yield strength. Ferrite grain refinement strengthening is the predominant mechanism and contributes 43.05% to the yield strength. And the contribution of precipitation strengthening and dislocation strengthening to the yield strength is up to 34.44%.
Table 1 Chemical compositions of V microalloyed steel and V-N microalloyed steel
Fig.1 SEM images of V microalloyed steel with Tc =600 ℃ (a) and V-N microalloyed steel with Tc =570 ℃ (b), 600 ℃ (c) and 660 ℃ (d)
Steel
Tf / ℃
Tc / ℃
ss / MPa
sb / MPa
d / %
Yield ratio
V microalloyed steel
870
600
475
546
33.5
0.87
V-N microalloyed steel
870
570
568
649
25.0
0.88
600
605 491
687
24.5
0.88
660
577
31.4
0.86
Table 2 Mechanical properties of V microalloyed steel and V-N microalloyed steel
Fig.2 TEM images of distribution of precipitates in V microalloyed steel (a) and V-N microalloyed steel (b) with Tf=870 ℃ and Tc= 600 ℃
Fig.3 TEM images of precipitates in V microalloyed steel with Tc=600 ℃ (a) and V-N microalloyed steel with Tc=570 ℃ (b), 600 ℃ (c) and 660 ℃ (d) (Inset in Fig.3c shows the SAED pattern of precipitate marked by arrow)
Fig.4 OM (a) and SEM (b) images of intragranular ferrite (IGF) in V-N microalloyed steel, EDS of precipitate (c) and nucleation schematic of IGF (d)
Steel
σ0 / MPa
Δσs / MPa
ΔσG/ MPa
ΔσDis/ MPa
ΔσOrowan/ MPa
σs/ MPa
V microalloyed steel V-N microalloyed steel
54 54
74 82
188 260
98 141
102 153
458 604
Table 3 Yield strength of V microalloyed steel and V-N microalloyed steel and their components
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