Microstructure and mechanical properties of post-weld heat-treated joints in vanadium microalloyed Q390FRW steel
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摘要: 为提升桥梁伸缩缝用钢的综合性能，制备了一种钒微合金化的耐火耐候Q390FRW钢，研究了药芯气体保护焊（FCAW）与埋弧焊（SAW）及其焊后热处理（PWHT-525 ℃×60 min）对接头组织和性能的影响。热力学计算表明，钒的碳化物（VC）可于799 ℃析出，细化晶粒。试验结果表明：FCAW焊缝组织主要为铁素体与少量马-奥岛，而SAW焊缝则以贝氏体和铁素体为主。经热处理后，两种焊缝的马-奥岛均发生分解，晶粒显著细化，同时促进了钒的碳化物的弥散析出。热处理后，FCAW接头的抗拉强度与伸长率分别达到686 MPa与33.64%，其塑性的显著提升主要归因于钒的析出强化与组织稳定化作用。断口分析表明热处理使接头断裂机制由韧脆混合型向韧性断裂转变。Abstract: To enhance the comprehensive performance of steel for bridge expansion joints, a fire-resistant and weathering-resistant vanadium-microalloyed Q390FRW steel was prepared in this study. The effects of flux-cored arc welding (FCAW), submerged arc welding (SAW), and subsequent post-weld heat treatment (PWHT-525℃ × 60 min) on the microstructure and mechanical properties of the joints were investigated. Thermodynamic calculations indicated that vanadium carbides (VC) can precipitate at 799 ℃, contributing to grain refinement. Experimental results showed that the FCAW weld joint consisted mainly of ferrite and a small amount of martensite-austenite (M-A) islands, while the SAW weld joint was primarily composed of bainite and ferrite. After heat treatment, the M-A islands in both weld joints decomposed, grain size significantly refined, and the dispersion precipitation of vanadium carbides was promoted. The tensile strength and elongation of the FCAW joint after heat treatment reached 679 MPa and 33.64%, respectively. The remarkable improvement in plasticity was primarily attributed to the precipitation strengthening and microstructural stabilization effects induced by vanadium. Fracture analysis revealed that the heat treatment shifted the fracture mechanism of the joints from a ductile-brittle mixed mode to a ductile fracture mode.
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图 1 焊接坡口与焊缝示意（单位：mm）
(a)X型坡口—FCAW；(b)Y型坡口—SAW
Figure 1. Schematic diagram of welding groove and weld seam
图 3 Q390FRW钢热轧态母材的金相组织（F与P）
Figure 3. Microstructure of as-rolled Q390FRW steel base metal (F and P)
图 6 FCAW焊接接头显微组织
(a)焊缝中心；(b)图(a)局部放大；(c)熔合线附近区域；(d)图(c)局部放大
Figure 6. Microstructure of FCAW welded joints
图 7 SAW 焊接接头显微组织
(a)焊缝中心区域；(b)图(a)局部放大；(c)熔合线附近区域；(d)图(c)局部放大
Figure 7. Microstructure of SAW welded joints
图 8 焊后热处理（525 ℃×60 min）焊接接头显微组织
(a)FCAW焊缝中心；(b)FCAW熔合区；(c)SAW焊缝中心；(d)SAW熔合区
Figure 8. Microstructure of the welded joints after post-weld heat treatment (525 ℃×60 min)
图 9 Q390焊接接头的拉伸性能
(a)工程应力-应变曲线；(b)抗拉强度和伸长率
Figure 9. Tensile properties of Q390 welded joints
图 10 未加钒Q390钢与钒微合金化Q390FRW钢（FCAW+热处理）接头的工程应力-应变曲线对比
(a)未加钒Q390钢；(b)钒微合金化Q390FRW钢
Figure 10. Comparison of engineering stress-strain curves of joints of vanadium-free Q390 steel and vanadium microalloyed Q390FRW steel (FCAW + heat treatment)
表 1 焊接工艺参数
Table 1. Welding process parameters
Welding process Groove type Wire / Flux type Welding current /A Welding voltage
/VWelding speed/(cm·min-1) Calculate heat input/ (kJ·cm-1) FCAW X YJ711(Q) Flux-cored wire 260~280 28~30 25~30 15~20 SAW Y WH H10Mn2 Solid wire+ HJ431 Soldering flux 550~600 32~34 40~45 25~30 
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表 2 Q390FRW钢力学性能
Table 2. Mechanical properties of Q390FRW steel
Properties Yield
strength/MPaTensile
strength/MPaElongation/% Measured value 530 635 25 GB/T 1591-2018 ≥380 490~650 ≥21
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