Inhomogeneous deformation behavior in compressive deformation process at room temperature of graphitized carbon steel
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摘要: 將0. 46%含碳量(質量分數) 的石墨化碳素鋼在萬能材料試驗機上進行室溫壓縮變形, 試驗鋼表現出良好的壓縮變形性能.根據載荷-位移曲線的變化特點, 試驗鋼的壓縮變形過程以位移7. 0 mm (對應相對壓下量為58. 3%) 為節點分為兩個階段: 在位移≤7. 0 mm的壓縮階段, 載荷呈線性增加, 壓縮試樣的鼓度值逐漸增加而達到一個極大值(14. 6%), 壓縮試樣中心位置的維氏硬度增幅最大, 為38. 1 HV, 至位移7. 0 mm時試樣端面徑向伸長率的增幅為34%;而在位移 > 7. 0 mm的壓縮階段, 載荷呈指數增加, 壓縮試樣的鼓度值從極大值開始逐漸減小, 至位移為10. 72 mm時(相對壓下量為89. 3%), 試樣端面的徑向伸長率相比于位移7. 0 mm時增加了83. 1%, 壓縮試樣的中心位置的維氏硬度增幅最小, 為32. 7 HV.上述試驗數據表明, 在位移≤7. 0 mm的壓縮過程中, 壓縮試樣內的三個不均勻變形區的位置與傳統壓縮模型一致, 但是當壓縮變形進入位移 > 7. 0 mm的壓縮過程中, 試樣中心位置已不再是傳統壓縮模中變形程度最大的變形區了, 即在這個階段試樣中的3個不均勻變形區的變形程度發生了改變.正因這種不均勻變形區變形程度的改變導致了變形過程中載荷的急劇增加和鼓度值的減低.另外, 在壓縮變形過程中, 三個不均勻變形區中石墨粒子的微觀變形量總是高于鐵素體基體, 其原因之一可以歸結為石墨粒子中層與層之間容易于滑動的結果.Abstract: Based on the development trends, graphitized carbon steel has been proposed as a low-sulfur and Pb-free free-cutting steel. This steel has attracted considerable attention because of its excellent cutting performance and good cold forging performance.This study investigates graphitized carbon steel containing 0. 46% C with ferrite and graphite. In particular, its compression deformation at room temperature was studied using a universal testing machine. The load-displacement curve was fitted, the drum shape and radial elongation of the end face of the compression specimens were calculated, the surface quality and microstructure of the compression specimens were observed using optical microscopy and field-emission scanning electron microscopy, and the micro-deformation of graphite particles and the ferritic matrix in the compression specimens was statistically analyzed using Image-Pro 6. 0. The results show that the tested steel exhibits good compression deformation performance. According to the varying characteristics of the load with respect to displacement, the compression deformation process of the tested steel is divided into two stages with a displacement of 7 mm (corresponding to 58. 3% reduction) : at the compression stage with displacement ≤7. 0 mm, the load increases linearly with displacement.The value of the drum shape increases with increasing displacement, reaching a maximum value of 14. 6%, the radial elongation of the end face of the compression sample increases 34%, and the Vickers hardness at the center of the compression sample reaches its maximum value of 38. 1 HV. At the compression stage with displacement > 7. 0 mm, the load increases exponentially, the value of the drum shape gradually decreases from its maximum value, the radial elongation of the end face of compression sample increases by 83. 1%compared with that at 7. 0 mm displacement, and the Vickers hardness at the center of the compression sample reaches its minimum value of 32. 7 HV. The aforementioned experimental data show that, in the compression process with displacement ≤7. 0 mm, the three non-uniform deformation zones within the compression sample are consistent with the traditional compression model; however, in the compression process with displacement > 7. 0 mm, the center of the sample is no longer the deformation zone with the largest deformation degree in the traditional compression model. That is, the deformation degree of the three nonuniform deformation zones changes at this stage. This change leads to sharp increase in the load and to a decrease in the drum shape. In addition, during the compression deformation process, the micro-deformation degree of the graphite particles is greater than that of the ferritic matrix in the three inhomogeneous deformation zones. This is attributed to the crystal structure of graphite. In particular, graphite has a layered, planar structure in which bonding between layers occurs via weak van der Waals interactions, which enables layers of graphite to be easily separated or to slide past each other.
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圖 1 試樣用鋼.(a) 金相組織; (b) 圓柱形壓縮試樣及其上的應變網格(單位: mm)
Figure 1. Tested steel: (a) microstructure; (b) cylinder compression sample with the square grid (unit: mm)
圖 2 不同相對壓下量下壓縮試樣的軸向應變與周向應變之間的對應關系
Figure 2. Corresponding relation between axial and circumferential strains of the compression sample under different deformation degree
圖 4 不同變形程度下壓縮試樣的宏觀形貌.(a) 20%; (b) 40%; (c) 58.3%; (d) 75%; (e) 89.3%
Figure 4. Macro profile of samples under different deformation degree: (a) 20%; (b) 40%; (c) 58.3%; (d) 75%; (e) 89.3%
圖 5 壓縮試樣鼓形程度、端面徑向伸長率與相對壓下量之間的關系
Figure 5. Relation between value of drum shape, radial elongation of end face, and deformation degree
圖 6 壓縮試樣內三個不均勻變形區及其維氏硬度與相對壓下量之間的關系.(a) 壓縮試樣內的三個不均勻變形區; (b) 維氏硬度與相對壓下量之間的關系
Figure 6. Nonuniform deformation zones inside compression specimen and the relation between their hardness and deformation degree: (a) three nonuni-form deformation zones; (b) the relation between Vickers hardness and deformation degree
圖 7 不同相對壓下量時壓縮試樣中心區域的金相組織.(a) 40%; (b) 58.3%; (c) 75%; (d) 89.3%
Figure 7. Metallographic structure of cold compression for graphitized carbon steel: (a) 40%; (b) 58.3%; (c) 75%; (d) 89.3%
圖 8 89.3%相對壓下量試樣內部大變形區石墨粒子變形形態的精細觀察
Figure 8. Fine observation on the deformation morphology of graphite particles in the large deformation zone in sample with 89.3%relative reduction using FESEM
圖 9 石墨粒子和鐵素體基體的微觀變形曲線.(a) 不均勻變形Ⅱ區; (b) 不均勻變形Ⅲ區; (c) 不均勻變形Ⅰ區
Figure 9. Micro-deformation curve between graphite particles and ferritic matrix: (a) non-uniform deformation zoneⅡ; (b) non-uniform deformation zoneⅢ; (c) non-uniform deformation zoneⅠ
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