Grain growth and phase transformation in the welded joint HAZ of TiNbV microalloyed steel
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摘要: 采用激光共聚焦显微镜原位观察方法，研究了大热输入用TiNbV微合金钢在模拟焊接热循环作用下焊接热影响区(HAZ)晶粒长大过程及相变的规律. 热循环过程中加热温度升高至860 ~ 980 ℃时，发生由铁素体和珠光体向奥氏体的转变，1 100 ℃时，奥氏体晶粒开始有明显长大的趋势，1 300 ~ 1 400 ℃时，晶粒以合并长大方式迅速长大；冷却过程中温度降低至1 400 ~ 1 350 ℃时，晶粒以晶界迁移方式缓慢长大，660 ~ 580 ℃时，发生奥氏体迅速向贝氏体转变，焊接HAZ主要由贝氏体与铁素体组成，贝氏体的尺寸是由奥氏体晶粒大小决定的. 热循环高温停留时间延长，奥氏体与贝氏体的形成、终了、转变温度区间均有下降. 结果表明，组织中先共析铁素体含量先降低后增加，贝氏体含量降低，多边形铁素体消失，先共析铁素体含量增加，冷却组织趋于均匀粗大. 焊接过程中，选择合适的高温停留时间可提高组织中IAF的含量，提高力学性能.Abstract: The grain growth and phase transformation in HAZ of TiNbV microalloyed steel under simulated welding thermal cycling were studied by in situ observation method using laser confocal microscope. The results show that the transformation from ferrite and pearlite to austenite is occurred while the heating temperature rises to 860 ~ 980 ℃ during thermal cycling. The austenite grain begins to grow obviously while the temperature reaches 1 100 ℃. At the temperature range of 1 300 ~ 1 400 ℃, the grain grows rapidly with the form of combining. During cooling process, when the temperature declines to 1 400 ~ 1 350 ℃, the grain grows slowly with the form of grain boundary migrating. When the temperature decreases to 660 ~ 580 ℃, the austenite transforms rapidly to bainite. The content of HAZ is mainly composed of bainite and ferrite. The size of austenite grain determines the maximum size of bainite. The formation, transformation and end temperature range of austenite and bainite decreases with the prolongation of the high temperature residence time. The content of proeutectoid ferrite decreases first and then increases, the content of bainite decreases, the polygonal ferrite disappears, and the cooling structure tends to be uniform and coarse. During the welding process, the choosing of appropriate high temperature residence time can increase the content of IAF in HAZ and improve the mechanical properties of the joints.
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图 1 热循环升温与降温阶段的奥氏体晶粒形貌
Figure 1. Austenite grain morphology during heating and cooling stages of thermal cycle. (a) 1 100 ℃; (b) 1 200 ℃; (c) 1 350 ℃; (d) 1 400 ℃; (e) 1 300 ℃; (f) 1 100 ℃
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图 2 热循环升温与降温阶段晶粒尺寸
Figure 2. Grain size during heating and cooling stages of thermal cycle
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图 3 晶粒合并长大方式
Figure 3. Several small grains transform into a big grain. (a) before merging; (b) after merging
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图 4 晶界移动长大方式
Figure 4. Movement of grain boundary. (a) before migration; (b) after migration
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图 5 相变过程
Figure 5. Phase transformation process. (a) 860 ℃; (b) 960 ℃;(c) 650 ℃; (d) 600 ℃
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图 6 IAF形成温度、终了随高温停留时间变化趋势
Figure 6. Trend of the formation and ending temperature of IAF with the change of high-temperature dwell time
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图 7 贝氏体形成温度、终了温度随高温停留时间变化趋势
Figure 7. Trend of the formation and ending temperature of bainite with the change of high-temperature dwell time
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图 8 不同高温停留时间下的金相组织
Figure 8. Metallographic structure under different holding time. (a) 5 s of holding time; (b) 100 s of holding time; (c) 300 s of holding time
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图 9 热循环过程中晶粒长大及相变随温度变化的模型
Figure 9. Modeling of the growth and phase transformation of the grains in HAZ with the heating temperature during thermal cycling. (a) original structure; (b) austenitic transition begins at 860 °C; (c) austenitic transition ends at 980 °C; (d) rapid growth of austenite at 1 300 °C; (e) grains merge at 1 400 °C; (f) grains migrate at 1 300 °C; (g) bainite transition begins at 660 °C; (h) bainite transition ends at 580 °C
表 1 试验用钢化学成分(质量分数，%)
Table 1 Chemical composition of the steel plate
C Si Mn P S Ni Nb Al Ti N 0.079 0.2 1.45 0.003 6 0.001 5 0.16 0.021 0.018 0.016 0.005 6 
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表 2 试验用钢力学性能
Table 2 Mechanical properties of the steel plate
屈服强度ReH/MPa 抗拉强度Rm/MPa 断后伸长率A(%) −40 ℃冲击吸收能量 AKV/J 435 522 28 310
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