Distribution feature of specific misorientation angle in a bainitic steel
Introduction
Grain refinement is the most popular way to improve the properties of steels, because it can raise strength, improve ductility and restrain cleavage fracture at the same time [1]. However, the definition of “grain refinement” is not so straightforward for bainitic/martensitic steels due to their multi-level hierarchy in morphology and crystallography [[2], [3], [4], [5]]. From the perspective of variants, the substructures of martensite and bainite are different. According to the study by Takayama et al. [6], the content of V1/V2 in lath bainite was higher than martensite, and the situation of V1/V4 is opposite. Stormvinter et al. [7] also found that carbon in martensite steel helps to form more V1/V2 variant pairs. Coherent transformation, which refers to the transformation process in which the product phase retains a specific orientation relationship with the parent phase, also reserves prior austenite grain boundaries (PAGBs) and forms finer crystallographic units within each prior austenite grain. Based on the orientation relationship, these finer crystallographic units could be characterized as packets, blocks/sub-blocks or variants. Some studies have showed that packet is the effective crystallographic unit, whose size has stronger influence on toughness of bainitic/martensitic steels [8,9], and the strength is related to both of the packet size and the block size [10].
With the further understanding of the crystallographic orientation relationship of variants, more studies [[11], [12], [13], [14], [15], [16]] also proposed that high angle grain boundaries (HAGBs) have a strong influence on toughness improvement by arresting the cleavage crack. However, it may be just the tip of the iceberg, the relation between coherent transformation and mechanical properties is still not well understood. As Guo et al. pointed out in their study [5], two adjacent sub-volumes that have different orientation relationships do not necessarily mean two effective grains. And actually, different mechanical behaviors are related to the orientation relationship of different crystallographic directions/planes more essentially, rather then the overall misorientation of neighboring crystallographic units. For example, as the principle of HAGBs hindering crack propagation is the misorientation of cleavage planes of neighboring grains [4,5,17], then the misorientation of {100}-cleavage plane is more relevant to the cleavage fracture than overall misorientation. Similarly, the crystallographic feature governing dislocation plasticity is normally the {110}-slip plane, but not the overall misorientation or the misorientation of {100}-plane [5]. Therefore, the investigation of the effective grain size in bainitic/martensitic steels should not only characterize boundaries with overall misorientation angle (OMA), but also has to consider the misorientation angle between specific crystallographic planes, i.e. specific misorientation angle (SMA) [18].
In this study, the boundaries of a bainitic steel microstructure were characterized using the electron back-scattered diffraction (EBSD) technique and statistically analyzed with the criteria of OMA, SMA of {100}-cleavage plane ({100}-SMA) and SMA of {110}-slip plane ({110}-SMA). The practical measured results and the calculated results with the ideal Kurdjumov-Sachs (K-S) orientation relationship [19] are also compared.
Section snippets
Experimental procedures
The chemical composition (weight percentage) of the experimental steel used in this study was 0.07%C, 0.23%Si, 1.06%Mn, 1.25%Cu, 1.74%Ni, 1.04%Cr, 0.49%Mo, 0.049%Nb, and balance Fe. This low alloyed steel is used to produce high-strength high-ductility thick plates for offshore structures and bulk shipbuilding [20]. In order to make the observation of microstructure more clear, larger prior austenite grain size is needed, thus heat treatment was carried out again using a Gleeble 3800
Results and discussion
Fig. 1 shows the interpreted result of EBSD measurement. Fig. 1(a) is the image quality map with boundaries whose OMAs are larger than 5o. Obviously, it is a lath structure, therefore it could be lath bainite or lath martensite. Considering the chemical composition, heat treatment processing and high quality of Kikuchi line for general EBSD measurement, the microstructure is more suitable to be seen as lath bainite. Fig. 1(b) is the frequency distribution of boundary OMAs. It could be found
Conclusions
- 1.
For the steel in this study, most boundaries have OMAs lower than 15o or higher than 45o, {100}-SMAs lower than 10o or higher than 30o and {110}-SMAs lower than 10o.
- 2.
PAGBs always have high OMA, meanwhile have high {100}-SMA and relatively high {110}-SMA, thus the refinement of prior austenite grain is an effective grain refinement way with respect to both cleavage fracture and slip behavior.
- 3.
Not all variant boundaries have high {100}-SMAs or {110}-SMAs at the same time. Four combinations of high
Declaration of Competing Interest
There is no conflict of interest to declare.
Acknowledgements
Xiucheng Li and Jingxiao Zhao contributed equally to this article.
This work was supported by National Key Research and Development Program of China (2017YFB0304900) and Key Research and Development Program of Shandong Province, China (2019JZZY020238). The authors are grateful to Dr. Dongsheng Liu for experimental results sharing and discussions.
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