3D-SPD Rolling Method of 45 Steel Ultrafine Grained Bar with Bulk Size
LIN Pengcheng1,2, PANG Yuhua1,2(), SUN Qi1,2, WANG Hangduo1,2, LIU Dong3, ZHANG Zhe3
1.School of Metallurgical Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China 2.Shaanxi Metallurgical Engineering Technology Research Center, Xi'an University of Architecture and Technology, Xi'an 710055, China 3.School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
Cite this article:
LIN Pengcheng, PANG Yuhua, SUN Qi, WANG Hangduo, LIU Dong, ZHANG Zhe. 3D-SPD Rolling Method of 45 Steel Ultrafine Grained Bar with Bulk Size. Acta Metall Sin, 2021, 57(5): 605-612.
Based on the limitations of severe plastic deformation (SPD) technology, such as a small effective deformation area and huge forming load, in the preparation of ultrafine grained/nanocrystalline materials, preparing industrial grade bulk ultrafine grained materials is difficult. In this work, a new SPD method, titled 3D-SPD, for preparing bulk ultrafine grained bars is proposed based on the cross-rolling principle. A severe torsion and compression deformation region was constructed using specially-curved conical rolls and guide plates, super large feed angles, and diameter reduction ratios. During the 3D-SPD process, a billet entered the deformation region from the large diameter side of the roll, where the deformation pressure was MPa grade and able to realize the SPD process with effective strains greater than 6.5. A crack control model based on Oyane criteria was established. Through the optimization analysis of damage factors under different deformation conditions, the crack induced by the Mannesmann effect was effectively restrained. Based on theoretical and experimental results, optimal parameters were determined as follows: cone angle 5°, feed angle 24°, diameter reduction ratio 50%, temperature 700oC, ovality coefficient 1.02, and roll speed 40 r/min. A 25 mm-diameter ultrafine grained bar of 45 steel was obtained by the single pass deformation. The average grain size was refined from 46 μm to 1 μm, and the yield and tensile strengths were increased by 46% and 42%, respectively.
Fig.1 Schematics of 3D severe plastic deformation (3D-SPD) finite element model from views of front (a), left (b), and top (c) (n—roll speed, α—cone angle, β—feed angle, γ—cross angle, Vx—feeding direction)
Fig.2 Comparisons of deformation zones between 3D-SPD (a) and traditional cross-rolling (b)
Process
α / (°)
β / (°)
Reduction rate / %
Ovality factor
3D-SPD
4-6
20-24
35-55
1.02-1.05
Traditional cross-rolling[26]
1-3
8-15
±5-10
1.10-1.20
Table 1 Comparisons of parameters between 3D-SPD and traditional cross-rolling[26]
Fig.3 Calculated damage values (upper) and longitudinal section morphologies (lower) of samples fabricated by traditional cross rolling (a) and 3D-SPD (b)
Fig.4 Comparisons of equivalent strain between traditional longitudinal rolling (a) and 3D-SPD (b)
Fig.5 Comparisons of torsion angles between traditional cross rolling (a) and 3D-SPD (b)
Fig.6 Comparisons of forces (a) and torques (b) between traditional longitudinal rolling and 3D-SPD
Fig.7 Equivalent strains in 3D-SPD process (a) and changes of equivalent strain with time (b) (r—distance between tracking point and rolling line, R—bar radius)
Fig.8 Distribution of temperature in 3D-SPD process
Fig.9 Homemade 3D-SPD testing machine (a) and 45 steel bars after rolling (b)
Fig.10 Microstructures of 45 steel before (a) and after (b) rolling
Fig.11 Stress-strain curves of 45 steel before and after rolling (The inset shows the specific fracture position of the sample before and after rolling)
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