Influence of Coiling and Annealing Temperatures on Hole Expansion Property of CP780 Hot-Rolled Ultra-High Strength Automotive Steel Sheet
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摘要:
既保证高强度又兼顾高扩孔率是高强钢应用于汽车制造的研究重点. 本文设计一种新型低碳、钛、铌微合金化的高扩孔780 MPa级热轧复相钢，采用热机械控制工艺（TMCP），探究轧制过程中不同卷取和退火温度对其组织与力学性能演变的影响，获得了高扩孔性能的机理. 结果表明：随着退火温度的升高，Ti/NbC颗粒析出强化作用加强，实验钢的晶粒逐渐细化，屈服及抗拉强度逐渐增大，而断后伸长率及扩孔率大逐渐降低；480 ℃卷取工艺下实验钢的扩孔性能主要由贝氏体/铁素体比例主导，退火温度升高导致贝氏体含量增多，不利于裂纹的偏转和分叉，因此扩孔率单调下降；在520 ℃卷取、610 ℃退火工艺下，实现了实验钢细晶组织、适宜的贝氏体/铁素体相比例与弥散细小Ti/NbC析出物的最佳匹配，能有效使扩孔裂纹偏转、分叉和产生微孔洞，从而在断裂前消耗较多能量，扩孔率达60%以上.
Abstract:Achieving a balanced combination of high strength and high hole expansion ratio is a critical research objective for the application of high-strength steel in automotive manufacturing. A novel low-carbon Ti-Nb microalloyed hot-rolled multiphase steel with a high hole expansion of 780 MPa grade was designed. The thermomechanical controlled process (TMCP) was employed to investigate the influence of different coiling and annealing temperatures on its microstructure and mechanical property evolution, as well as to elucidate the mechanisms governing the high hole expansion property. The results indicate that increasing the annealing temperature enhances the precipitation strengthening effect of Ti/NbC particles, promotes grain refinement of the experimental steel, and leads to a progressive increase in both yield and tensile strengths. However, elongation after fracture and hole expansion ratio exhibit a gradual decline with rising annealing temperature. At a coiling temperature of 480 ℃, the hole expansion property is primarily governed by the bainite/ferrite ratio. Elevated annealing temperatures result in increased bainite content, which restricts crack deflection and bifurcation, thereby causing a monotonic reduction in hole expansion ratio. At a coiling temperature of 520 ℃ and an annealing temperature of 610 ℃, an optimal matching among the fine-grained microstructure of experimental steel, an appropriate bainite/ferrite phase ratio, and dispersed fine Ti/NbC precipitates was realized. This microstructural configuration effectively promotes crack deflection, bifurcation, and microvoid formation during hole expansion, enabling greater energy consumption prior to fracture and resulting in a hole expansion ratio exceeding 60%.
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图 3 不同卷取-退火温度下实验钢组织EBSD分析（光栅：640 × 480，步长尺寸：0.1 μm）
Figure 3. EBSD analysis of experimental steel at different coiling and annealing temperatures
图 4 520 ℃卷取、不同退火温度下的实验钢第二相颗粒析出TEM分析
Figure 4. TEM analysis of second phase particle precipitation in experimental steel at coiling temperature of 520 ℃ and different annealing temperatures
图 5 不同卷取-退火温度下实验钢板扩孔边缘微裂纹形貌
Figure 5. Edge microcrack morphology of experimental steel sheets during hole expansion at different coiling and annealing temperatures
图 6 不同卷取-退火温度下实验钢板扩孔裂纹的形态
Figure 6. Crack morphology of experimental steel sheets during hole expansion at different coiling and annealing temperatures
表 1 实验钢化学成分
Table 1. Chemical compositions of experimental steel
化学成分 C Si Mn P S Als Ti + Nb Fe 质量分数/% ≤0.10 0.10～0.25 1.5～2.5 0.007 0.003 0.025 ≤0.15 Bal. 
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表 2 不同卷取-退火温度下实验钢的力学性能和扩孔特性
Table 2. Mechanical properties and hole expansion properties of experimental steel at different coiling and annealing temperatures
卷取温
度/℃退火温
度/℃屈服强
度/MPa抗拉强
度/MPa断后伸
长率/%扩孔率/% 480 580 712 756 20.5 79.83 610 733 778 17.5 63.73 640 830 860 15.0 56.50 520 580 714 767 17.8 61.67 610 721 780 17.0 67.90 640 756 802 17.5 51.20
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表 3 不同卷取-退火温度下实验钢晶粒晶界特征统计
Table 3. Grain and grain boundary characteristic statistics in experimental steel at different coiling and annealing temperatures
卷取温
度/℃加热温
度/℃晶粒个
数/个2°～15° 晶粒
含量/%＞15° 晶粒
含量/%480 580 208 49.2 50.8 610 227 45.6 54.4 640 267 43.9 56.1 520 580 250 54.2 45.8 610 343 59.3 40.7 640 200 56.6 43.4
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表 4 不同卷取-退火温度下实验钢铁素体/贝氏体体积分数
Table 4. Statistics of ferrite/bainite volume fractions in experimental steel at different coiling and annealing temperatures
卷取温度/℃ 加热温度/℃ 铁素体分数/% 贝氏体分数/% 480 580 14 86 610 12.6 87.4 640 11.5 88.5 520 580 10.6 89.4 610 12.7 87.3 640 10 90
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表 5 不同卷取-退火温度下钢板扩孔孔边缘裂纹统计
Table 5. Statistics of hole edge cracks of steel sheets during hole expansion at different coiling and annealing temperatures
卷取温度/℃ 退火温度/℃ 扩孔率/% 微小裂纹
数量/个480 580 79.83 18 610 67.90 14 640 56.50 9 520 580 61.67 10 610 63.73 12 640 51.20 8
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