Metals and Materials International ( IF 3.3 ) Pub Date : 2021-07-29 , DOI: 10.1007/s12540-021-00999-4 Yong Hwan Cho 1 , Jaeeun Lee 1 , Wung Yong Choo 1 , Heung Nam Han 1 , Juseok Kang 2
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The effect of separation on the fracture surface of ferrite–bainite dual-phase pipeline steels during a drop weight tear test (DWTT) was investigated via microstructural analysis and fracture surface examination. Two specimens (ST1 and ST2) were designed to have a dual-phase microstructure, with different ferrite and bainite fractions, by controlling the reduction ratio and holding time in the ferrite–austenite two-phase region during the thermomechanical process. Notably, ST2, which had a longer holding time for the retained austenite to absorb carbon, exhibited a harder bainite but softer ferrite phase compared to ST1. The greater hardness difference between those phases in ST2 induced the strain incompatibility frequently evident at the phase boundaries, resulting in lower ductility during the tensile test. Owing to the strain incompatibility, ST2 also generated more separation over a wider temperature range during DWTT. At low temperatures, where the brittle fracture was prominent, it was observed that ST2 generated separations on its fracture surface, whereas ST1 did not. These separations formed a local shear lip around themselves, thereby obstructing cleavage fracture propagation from the notch. Hence, it was confirmed that ST2 had a higher DWTT shear area than ST1 over a temperature range near the ductile–brittle transition temperature (DBTT). As a result, the DBTT of ferrite–bainite dual phase steel could be improved by increasing the amount of separation during DWTT.
Graphic Abstract
中文翻译：
落锤撕裂试验中分离对铁素体-贝氏体双相管道钢断裂面的影响
通过显微组织分析和断口检查研究了在落锤撕裂试验 (DWTT) 期间分离对铁素体-贝氏体双相管线钢断口表面的影响。通过控制热机械过程中铁素体-奥氏体两相区的压下率和保持时间，将两个试样（ST1 和 ST2）设计为具有双相显微组织，具有不同的铁素体和贝氏体比例。值得注意的是，与 ST1 相比，ST2 具有更长的残留奥氏体吸收碳的保持时间，表现出较硬的贝氏体相和较软的铁素体相。ST2 中这些相之间较大的硬度差异导致相边界处经常出现明显的应变不相容性，导致拉伸试验期间延展性较低。由于菌株不相容，ST2 还在 DWTT 期间在更宽的温度范围内产生更多的分离。在低温下，脆性断裂很明显，可以观察到 ST2 在其断裂面上产生分离，而 ST1 没有。这些分离在其自身周围形成了局部剪切唇，从而阻碍了从缺口处的解理断裂扩展。因此，证实在接近韧脆转变温度 (DBTT) 的温度范围内，ST2 具有比 ST1 更高的 DWTT 剪切面积。因此，铁素体-贝氏体双相钢的 DBTT 可以通过增加 DWTT 过程中的分离量来提高。而 ST1 没有。这些分离在其自身周围形成了局部剪切唇，从而阻碍了从缺口处的解理断裂扩展。因此，证实在接近韧脆转变温度 (DBTT) 的温度范围内，ST2 具有比 ST1 更高的 DWTT 剪切面积。因此，铁素体-贝氏体双相钢的 DBTT 可以通过增加 DWTT 过程中的分离量来提高。而 ST1 没有。这些分离在其自身周围形成了局部剪切唇，从而阻碍了从缺口处的解理断裂扩展。因此，证实在接近韧脆转变温度 (DBTT) 的温度范围内，ST2 具有比 ST1 更高的 DWTT 剪切面积。因此，铁素体-贝氏体双相钢的 DBTT 可以通过增加 DWTT 过程中的分离量来提高。
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