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摘要: 為對生產進行指導，研究了DP590/DP780高強鋼焊管在液壓成形過程中的變形行為；使用場發射掃描電鏡觀察管材周向的橫截面以確定基體的組織，通過VMHT30M顯微硬度計確定管材的焊縫及熱影響區的大小，以便研究液壓成形破裂行為；采用液壓成形試驗機對兩種管件進行液壓成形研究。實驗結果表明：管材在脹形過程中的破裂壓力比理論計算公式得到的破裂壓力大，破裂位置全部位于靠近焊縫及熱影響區的母材區域；隨著管徑的增大和長徑比的增大，管材的極限膨脹率呈現下降趨勢；在自由脹形過程中，管材的焊縫區域基本上不發生減薄，最小壁厚位于管材的熱影響區和基體的過渡區域，并且壁厚的減薄率在脹形最高點所在截面最大，越靠近管材夾持區，壁厚的減薄率越小。最終得到以下結論：管材液壓成形實驗是準確獲得管材力學性能參數的途徑；提高焊接質量有助于控制失效破裂位置；合理選擇管材的長徑比有利于管材性能的充分發揮；通過合理控制各處的減薄有利于降低液壓成形件的破裂風險。Abstract: In recent years, the automotive industry has become increasingly demanding for the strength of hollow structural parts. To meet the strength and toughness requirements, major automakers have begun to use high-strength steel for the production of automotive hollow structural parts, and the hydroforming process is the most economical way to achieve this purpose. However, studies on the hydroforming process of high-strength steel in the industry are few. To guide the production of high-strength steel hydroformed parts, the deformation behavior of DP590/DP780 high-strength steel welded tube during hydroforming was investigated in this study. The cross section of the circumferential direction of the tube was observed by scanning electron microscopy to determine the microstructure of the base metal. The sizes of the weld and the heat-affected zone of the tube were determined by VMHT30M microhardness tester to study the hydroforming fracture behavior. The deformation behavior of DP590/DP780 high-strength steel welded tube during hydroforming was studied by a tube hydroforming test machine. The experimental results are as follows: the fracture pressure of the tube during the bulging process is larger than the fracture pressure obtained by the theoretical calculation formula, and the rupture position is located in the base metal area near the weld and heat-affected zone. With the increase of the tube diameter and the length-to-diameter ratio, the maximum expansion ratio of the tube exhibits a downward trend. In the process of free bulging, the weld area of the tube is basically not thinned. The position of the minimum thickness is located in the heat-affected zone of the tube and the transition zone of the base body; the wall thickness reduction rate is the largest at the highest point of the bulging region, and the closer to the tube clamping zone, the smaller the wall thickness reduction rate. Finally, the following conclusions can be drawn: the hydroforming experiment of the tube can accurately obtain the mechanical properties of the tube. Improving the welding quality could help to control the failure rupture position. A reasonable selection of the length-to-diameter ratio of the tube is beneficial to the tube overall performance. It is beneficial to reduce the risk of cracking of the hydroformed part by reasonably controlling the thickness reduction rate of each part.
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圖 2 直徑89 mm圓管截面掃描電子顯微鏡照片. （a） DP590；（b） DP780
Figure 2. SEM photograph of the cross section of a 89 mm diameter tube: (a) DP590; (b) DP780
圖 5 管材周向維氏硬度. （a） DP590-?89 mm管；（b） DP780-?89 mm管；（c） DP590-?63.5 mm管
Figure 5. Circumferential Vickers hardness of the tube: (a) DP590-?89 mm tube; (b) DP780-?89 mm tube; (c) DP590-?63.5 mm tube
圖 6 試驗管的擬合真應力–應變曲線對比
Figure 6. Comparison of fitting true stress?strain curves of the studied tubes
圖 7 DP590管脹形破裂位置. （a） ?63.5 mm；（b） ?89 mm
Figure 7. Bulging rupture position of DP590 tubes: (a) ?63.5 mm; (b) ?89 mm
圖 8 不同長徑比管材破裂壓力
Figure 8. Burst pressure of tubes with different length-to-diameter ratios
圖 9 管材極限膨脹率對比. （a） DP590管；（b） ?89 m管
Figure 9. Comparison of the ultimate expansion ratio of the tubes: (a) DP590 tubes; (b) ?89 mm tubes
圖 11 DP590管不同截面壁厚分布圖
Figure 11. Wall thickness profile of different sections of DP590 tube
表 2 兩種材料的力學性能參數
Table 2. Mechanical properties of two materials
材料 密度/（kg·m?3） 屈服強度/MPa 抗拉強度/MPa 斷后伸長率/% 彈性模量/GPa 泊松比 r00 r45 r90 n值 DP590 7850 379.7 626.4 24.2 208.0 0.33 0.84 0.85 1.06 0.19 DP780 7850 548.0 836.7 16.5 214.5 0.30 0.70 0.75 0.81 0.15 
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表 3 DP590/DP780高強鋼材料性能參數
Table 3. Material properties of DP590/DP780 high-strength steel
材料 密度/（kg·m?3） 屈服強度/MPa 抗拉強度/MPa 彈性模量/GPa 泊松比 r值 n值 K值 DP590-63.5 7850 363.9 623.1 208 0.33 0.72 0.18547 1012.94 DP590-89 7850 410.4 638.9 208 0.33 0.72 0.1421 929.68 DP780-89 7850 557.1 840.8 214.5 0.3 0.53 0.12236 1184.97
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表 4 實驗管規格
Table 4. Experimental tube specifications
材料 直徑/mm 管材壁厚/mm 長徑比 DP590 63.5 2 1.2、1.4、1.6、1.8、2.0 DP590 89 2 1.2、1.4、1.6、1.8、2.0 DP780 89 2 1.2、1.4、1.6、1.8
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表 5 管材開裂壓力
Table 5. Tube cracking pressure
材料 開裂壓力/MPa 實驗均值 理論計算值 DP590-63.5 46.2 39.3 DP590-89 33.1 28.7 DP780-89 41.1 37.8
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