Effects of strain aging on the microstructure and properties of X90 high-strength pipeline steel
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摘要: X90管线钢管是一种新型高强、高韧性管材，随着强度的提高，应变时效行为的揭示与控制成为学术界研究的焦点。利用OM、SEM、TEM、应变时效敏感性试验、拉伸试验和夏比冲击试验对X90高强管线钢组织特征、冲击断口形貌、应变时效敏感性、拉伸性能、低温冲击性能和应变时效行为进行了研究。结果表明，应变时效对X90高强管线钢组织无显著影响，时效前后均为针状/块状铁素体+板条状/粒状贝氏体+M/A组元的复相组织，但对拉伸性能和拉伸曲线形态以及低温冲击韧性有显著影响。当时效温度为230 ℃时（聚乙烯层熔结温度），随时效时间延长，X90钢屈服强度、抗拉强度、屈强比和应变时效敏感性系数逐渐增大，均匀延伸率和低温冲击吸收功逐渐减小。应力-应变曲线逐渐失去连续屈服强化特征，由时效前的“Round House”拱顶型连续屈服、强化型转变成为带有明显尖峰和屈服平台的“Lüders”型曲线，15 min是X90钢应力-应变曲线形态发生转变的时效时间拐点。为了控制应变时效对X90高强管线钢的不利影响，应从减小应变量、降低防腐预热温度和缩短防腐预热高温停留时间三方面进行综合调控。建议对管材生产制备工艺进行革新，成型方面如采用柔性校平法取代刚性辊压校平法，多步渐进成型取代一步螺旋成型法；防腐方面如缩短预热高温停留时间或降低预热温度（当预热温度≥230 ℃时，高温停留时间应≤5 min；若预热温度选定<230 ℃时，高温停留时间应<15 min）。Abstract: X90 pipeline steel pipe is a new type of high-strength and high-toughness pipe. With the increase in strength, the revealing and controlling of strain aging behavior of X90 pipeline steel has become the focus of research. The microstructure characteristics, fracture morphology, strain-aging sensitivity, tensile properties, impact toughness in low temperature, and strain aging sensitivity of X90 high-strength pipeline steel before and after strain aging treatment were investigated by OM, SEM, TEM, strain-aging sensitivity test, tensile test and Charpy impact experiments. The results indicate that the strain aging has no significant effect on the microstructure of X90 high-strength pipeline steel. The microstructure of X90 high-strength pipeline steel before and after aging is a complex structure of acicular/bulk ferrite, lath/granular bainite and M/A component. However, strain aging has a significant effect on the tensile properties, tensile curve morphology and low temperature impact toughness of X90 high-strength pipeline steel. When the aging temperature is 230 ℃ (the fusion temperature of polyethylene layer), the yield strength, tensile strength, yield ratio and strain-aging sensitivity coefficient of X90 steel gradually increase with the prolongation of aging time. At the same time, the uniform elongation and low temperature impact absorption energy of X90 steel gradually decrease with the prolongation of aging time. In addition, the X90 pipeline steel gradually loses continuous yielding and strengthening characteristics, and the tensile curve changes from the round-house-type before aging to the Lüders-type yield curve after aging with obvious peaks and yield platform. 15min is the inflection point of aging time for the transformation of the stress-strain curve of X90 steel. In order to reduce the adverse effect of strain aging on X90 high-strength pipeline steel, comprehensive regulation should be carried out from three aspects: reducing strain, reducing anti-corrosion preheating temperature and shortening high temperature residence time of anti-corrosion preheating. It is recommended to innovate and revise the pipe production and preparation process, such as shortening the high temperature preheating residence time or reducing the preheating temperature, and adopting the flexible leveling method to replace the rigid roll leveling method, and the multi-step incremental forming to replace the one-step spiral forming method. It is recommended to innovate the production and preparation process of pipes. For example, in the pipe forming process, the flexible leveling method could be used to replace the rigid roll leveling method, and the multi-step progressive molding method could be used to replace the one-step spiral molding method to effectively control the prestrain. Another example, in the pipe anticorrosion process, the residence time of high temperature preheating should be shortened or the preheating temperature should be reduced (when the preheating temperature ≥ 230 ℃, the high temperature residence time should be ≤ 5 min; if the preheating temperature < 230 ℃, the high temperature residence time should be < 15 min).
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Key words:
- X90 high-strength pipeline steel /
- strain aging /
- aging time /
- impact toughness /
- acicular ferrite
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图 1 应变时效处理对X90高强管线钢拉伸性能的影响
Figure 1. Effect of strain aging treatment on tensile properties of X90 high-strength pipeline steel
图 2 应变时效处理对X90高强管线钢示波冲击曲线形态特征的影响
Figure 2. Effect of strain aging treatment on the morphological characteristics of X90 high-strength pipeline steel oscillometric shock curve
图 3 X90高强管线钢采用工艺D处理前后夏比冲击断口形貌
Figure 3. Charpy impact fracture morphology of X90 high-strength pipeline steel before and after treatment with process D
图 4 X90管线钢经工艺D处理前后的微观组织
Figure 4. Microstructures of X90 pipeline steel before and after treatment with process D
表 1 X90级管线钢的化学成分
Table 1. Chemical composition of X90 pipeline steel
% C Si Mn P S Ni Cr Cu Nb V Ti Mo Al B CEPcm Fe 0.05 0.26 1.94 0.006 0.0012 0.21 0.34 0.22 0.08 0.03 0.016 0.34 0.03 0.0004 0.21 Bal. 注：CEPcm为冷裂纹敏感系数，对于碳含量≤0.12%的高强管线钢材料更能准确反映其焊接性能。 
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表 2 应变时效前后X90级管线钢拉伸性能测试结果
Table 2. Test results of tensile properties of X90 pipeline steel before and after strain aging
工艺 Rp0.2/MPa Rm/MPa Rp0.2/Rm UEL/% 时效前 627 737 0.85 7.3 A 686 754 0.91 6.6 B 711 736 0.97 6.5 C 690 747 0.92 6.9 D 699 749 0.93 6.8
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表 3 应变时效处理前后X90高强管线钢的低温冲击韧性指标和应变时效敏感性系数
Table 3. Low-temperature impact test results and strain-aging sensitivity coefficient of X90 high-strength pipeline steel before and after strain aging treatment
工艺 T/℃ Ai/J Ap/J Ak/J 平均值/J CV/% 时效前 −20 92 306 398 401 95 310 405 93 306 399 A −20 90 300 390 390 2.7 91 295 386 92 302 394 C −20 90 283.5 373.5 376.5 6.1 89 287 376 92 288 380 D −20 91 269 360 357 10.97 89 269 358 85 269 354
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