高强钢经历焊接热循环过程后，将在粗晶热影响区(CGHAZ)产生脆性组织[11]，造成粗晶脆化现象，导致其冲击韧性恶化[12]。为改善高强钢CGHAZ的冲击韧性，其微观组织与冲击韧性之间的关系得到了广泛的关注与研究[13~19]。CGHAZ中的微观组织通常包括板条贝氏体(LB)[20]和粒状贝氏体(GB)[21]。对于LB而言，原奥氏体晶粒(PAG)可由一个或多个贝氏体块(packet)组成[22,23]，而一个packet又可以重新划分为多个贝氏体束(block)[24]。目前关于PAG、packet和block等微结构对冲击韧性的影响尚无定论。有研究发现，微裂纹穿过PAG边界，发生拐折后继续扩展[25]，在packet边界处停止扩展[26]，贝氏体packet可以作为有效控制微裂纹扩展的结构单元[27]；而Zhang等[28]认为贝氏体block是控制CGHAZ冲击韧性的微结构单元。因此，贝氏体微结构与冲击性能之间的调控关系及其内在机理仍存在争议。
本工作采用Gleeble 3500热模拟试验机模拟了Q500qE钢在不同焊接热输入下CGHAZ的焊接热循环过程，借助光学显微镜(OM)、扫描电镜(SEM)和电子背散射衍射(EBSD)等手段研究焊接热输入、微观组织与冲击韧性之间的关系及内在机理，分析Q500qE钢中控制CGHAZ冲击韧性的最有效贝氏体微结构单元。
1 实验方法
实验材料选用Q500qE钢板，板厚20 mm，其化学成分(质量分数，%)为：C 0.06，Si 0.20，Mn 1.55，P 0.006，S 0.001，Cr 0.30，Ni 0.19，Cu 0.22，Mo 0.177，Ti 0.010，Nb 0.038，Fe余量。在钢的粗轧阶段，开轧温度为1150℃，终轧温度为1080℃；精轧阶段，开轧温度为900℃，终轧温度为820℃；开冷温度为740℃，返红温度为580℃。从轧态钢板上取样，采用Axion-200mat OM进行组织观察，其原始组织为GB和铁素体(F)，如图1所示。
图1
参照GB/T 714-2015《桥梁用结构钢》，从Q500qE钢板上沿平行于轧制方向(纵向)取样，加工成直径6 mm、长80 mm的棒状试样和10.5 mm × 10.5 mm × 80 mm的立方体试样若干，分别用于微观组织和断裂行为表征。采用Gleeble-3500热模拟试验机模拟Q500qE钢气体保护焊的CGHAZ焊接热循环过程，其热输入(Ej)分别为15、20、25以及30 kJ/cm。焊接热循环曲线采用HAZ软件包Rykalin-3D传热模型生成，加热速率为100℃/s，试样加热到1350℃保温1 s，使其微观组织完全奥氏体化，而后冷却到200℃，相变完全结束，如图2所示。
图2
图2
不同热输入下粗晶热影响区(CGHAZ)的焊接热循环曲线
Fig.2
Welding thermal cycles of CGHAZs with different heat inputs (CGHAZ—coarse grained heat affected zone)
将焊接热模拟后的棒状试样沿热电偶所在截面切开，经磨制、机械抛光和4% (体积分数)硝酸酒精溶液腐蚀后，在Axion-200mat OM和S-3400N SEM下观察CGHAZ的微观组织。借助Image Pro Plus软件，随机统计20个SEM视场下至少50个PAG的等效圆直径，取其直径的平均值作为PAG的平均尺寸(Dγ )；统计20个SEM视场下至少50个贝氏体packet的平均长度(L1)与宽度(L2)，packet的平均尺寸(DP)为L = (L1·L2)1/2的算术平均值。然后，使用10%HClO4 + 90%CH3OH (体积分数)溶液对试样进行电解抛光，利用S-3400N SEM搭载的EBSD组件定量表征CGHAZ中板条贝氏体的block宽度，借助Image Pro Plus软件，采用线截距法统计5个EBSD图中至少50个贝氏体block的宽度，求其宽度的算术平均值作为block的平均宽度(WB)。
将经焊接热模拟后的立方体试样加工成标准Charpy V型缺口冲击试样，尺寸为10 mm × 10 mm × 55 mm，V型缺口加工在热电偶所在截面上，在JB-300B型半自动冲击试验机上测试不同热输入下CGHAZ的冲击功KV2 (-40℃)。并且，利用S-3400N SEM观察不同热输入下CGHAZ的冲击断口形貌和二次裂纹形貌，分析微观组织对Q500qE钢冲击韧性的影响规律及其作用机理。其中，解理面尺寸(Df)通过Image Pro Plus软件随机统计每个CGHAZ样品在20个SEM视场下至少50个解理面的等效圆直径，求其等效圆直径的算术平均值获得。
2 实验结果
2.1 CGHAZ的微观组织
Q500qE钢不同热输入下CGHAZ显微组织的OM像如图3所示。各热输入下的微观组织主要是LB和GB。当焊接热输入为30 kJ/cm时，CGHAZ组织主要由LB和少量GB构成；随着焊接热输入的降低，LB增多，GB减少，组织逐渐细化。
图3
图3
不同热输入下CGHAZ显微组织的OM像
Fig.3
OM images of CGHAZs with different heat inputs (LB—lath-like bainite, GB—granular-like bainite)
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm
图4
图4
不同热输入下CGHAZ的典型SEM像
Fig.4
Typical SEM images of CGHAZs with different heat inputs (PAGB—prior austenite grain boundary)
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm
表1 不同热输入下CGHAZ的微结构尺寸测量结果和解理面尺寸统计
Table 1
| Ej / (kJ·cm-1) | Microstructure | Dγ / μm | DP / μm | WB / μm | Df / μm |
|---|---|---|---|---|---|
| 15 | 92%LB + 8%GB | 41.2 ± 4 | 18.9 ± 2 | 4.0 ± 0.5 | 15 ± 2 |
| 20 | 84%LB + 16%GB | 68.7 ± 5 | 48.2 ± 5 | 4.5 ± 0.5 | 28 ± 2 |
| 25 | 78%LB + 22%GB | 81.7 ± 8 | 64.3 ± 6 | 5.2 ± 0.6 | 32 ± 3 |
| 30 | 71%LB + 29%GB | 99.3 ± 9 | 78.8 ± 8 | 5.6 ± 0.7 | 37 ± 3 |
图5
图5
不同热输入下CGHAZ的EBSD像
Fig.5
EBSD images of CGHAZs with different heat inputs
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm
2.2 CGHAZ的冲击性能
不同热输入下CGHAZ的-40℃冲击功如图6所示。当焊接热输入在15~30 kJ/cm范围内，CGHAZ的KV2随热输入的降低而升高。当热输入为30 kJ/cm时，KV2 = 52 J，低温韧性较差；随着焊接热输入的降低，冲击功在20 kJ/cm时缓慢升至74 J；当热输入降低到15 kJ/cm，冲击功显著升高到153 J，低温韧性明显改善。
图6
图6
不同热输入下CGHAZ的Charpy冲击功
Fig.6
Charpy impact energies at -40oC for CGHAZs with different heat inputs
图7
图7
不同热输入下CGHAZ的冲击断口形貌
Fig.7
Impact fracture morphologies of CGHAZs with different heat inputs
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm
将典型热输入为15和25 kJ/cm冲击断裂的试样沿垂直于断裂表面的平面中心取样，通过SEM观察典型热输入下CGHAZ的二次裂纹形貌，如图8所示。2种热输入下CGHAZ的二次裂纹都在一个贝氏体packet中沿直线传播，笔直穿过贝氏体block，在贝氏体packet边界处止裂。由此可见，贝氏体packet能够阻碍裂纹扩展，而block不能阻碍裂纹的扩展。此外，随着焊接热输入的降低，二次裂纹扩展的单位距离变短，表明微裂纹扩展的阻力明显增大。
图8
图8
典型热输入下CGHAZ的二次裂纹形貌
Fig.8
Secondary crack morphologies of CGHAZs with typical heat inputs of 15 kJ/cm (a) and 25 kJ/cm (b)
3 分析讨论
3.1 热输入对CGHAZ微观组织的影响
图9为不同热输入下焊接热循环中冷却过程的热膨胀曲线，采用切线法标示出不同热输入下冷却过程中奥氏体向铁素体转变开始温度(Ar3)。可以看出，随着热输入的降低，Ar3逐渐降低。
图9
图9
不同热输入下CGHAZ的热膨胀曲线以及对应的奥氏体向铁素体转变开始温度
Fig.9
Expansion curves and Ar3 of CGHAZs with different heat inputs (Ar3—the starting temperature for γ→α transformation)
3.2 晶粒尺寸对CGHAZ冲击行为的影响
由于不同热输入下冲击断口表面都表现出明显的解理断裂特征，所以将CGHAZ试样断口的Df与Dγ 、DP、WB进行拟合比较，如图10所示，其对应的拟合方程如式(
图10
图10
解理面尺寸与PAG尺寸、贝氏体packet尺寸和block宽度的拟合关系
Fig.10
Cleavage facet size as functions of PAG size, bainitic packet size, and bainitic block width
拟合方程(1)~(3)的线性相关系数分别为0.96、0.99和0.87，可见Dγ 和DP与Df之间的相关性明显优于WB，表明PAG和packet是控制CGHAZ冲击韧性的有效微结构单元。由图8可知，当热输入为15和25 kJ/cm时，CGHAZ的二次裂纹都在一个贝氏体packet中直线传播，并在该packet边界处终止，再结合其与Df良好的线性关系，相较于PAG，贝氏体packet应该是最有效控制CGHAZ冲击韧性的微结构单元。
4 结论
(1) Q500qE钢粗晶热影响区的微观组织主要是板条贝氏体和粒状贝氏体；随着焊接热输入的降低，板条贝氏体增多，粒状贝氏体减少，Ar3逐渐降低，原奥氏体晶粒、贝氏体packet和block微结构的平均尺寸减小。
(2) 随着焊接热输入的降低，粗晶热影响区的冲击功KV2 (-40℃)显著提高；不同热输入的冲击断口都呈现出明显的解理断裂特征，其解理面尺寸随着热输入的降低而减小。
(3) 对于Q500qE钢而言，贝氏体packet是控制粗晶热影响区冲击韧性的最有效微观结构单元，packet边界能够有效阻止二次裂纹的扩展。
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