1. Introduction
In recent years, retained austenite has been widely used in many steels, such as transformation induced plasticity (TRIP) steel, quenching and partitioning (QP) steel and so on [
1,
2,
3,
4]. When subjected to external force, the retained austenite in these steels will transfer into martensite to induce high plasticity, namely TRIP effect [
5,
6,
7,
8,
9]. At present, the retained austenite content in steel is mainly measured by the XRD method [
10,
11,
12], which is performed according to the standards YB/T 5338-2006 [
13] and ASTM E975-13 [
14]. Both standards have some limitations in the process of use: Firstly, the measured result is greatly affected by the crystallographic orientation or texture in steel [
15,
16,
17]; secondly, the shape and distribution of retained austenite cannot be characterized; thirdly, the lower detection limit is high (1% or more [
14]) and the trace retained austenite cannot be measured by the XRD method. Considering the limitations of the XRD method, EBSD method is a good solution to perform the quantitative analysis of retained austenite in steel [
18,
19,
20,
21,
22]. Firstly, the EBSD method is not influenced by the crystallographic orientation or texture in steel. Moreover, it can provide not only the content of retained austenite, but also the distribution and morphology of the austenite phase.
Many studies have shown that the EBSD method could be used to serve the purpose of microstructure characterization for both quantitative and qualitative analyses in TRIP steels and other advance high strength steels (AHSS), which mainly focused on the substructure and microstructures characterization [
23,
24,
25,
26,
27] and the subgrain and grain characterization [
28,
29,
30]. About retained austenite characterization and quantification, previous research [
3,
5,
6,
7,
8,
18,
19,
20,
26,
31,
32] showed that EBSD test parameters had a vital impact on the test results: High indexing rates, small step sizes and more field number are necessary for the analysis of retained austenite content in steel by the EBSD method. However, little research by far showed the specific quantitative results, such as the critical index rate, the relationship between step size and grain size of retained austenite, and the needed filed number for different magnifications and retained austenite contents. Furthermore, the retained austenite analysis is very complicated due to its small size, its various morphologies, its distribution and the microstructures of matrix [
16,
23,
24,
25,
26,
27]. Main parameters used to identify these complicated structures are the image quality (IQ) factor. The IQ factor represents a quantitative description of the sharpness of the bands in the EBSD pattern. A lattice distorted by crystalline defects, such as dislocations and sub-grain boundaries affect Kikuchi pattern quality leading to lower IQ values [
7,
25,
33,
34]. Moreover, when EBSD is used at high resolution, it shows the instabilities in the specimen stage and electron beam during the long periods of measurements. Poor diffraction means that the beam has to dwell for a longer period at the same position and any instability will make the results useless [
24]. That is why there are no international standards for measuring retained austenite by the EBSD method by far. Therefore, it is very necessary to further study different test parameters on the EBSD analysis of retained austenite for different steels. In this case, in future maybe we can set up a general guideline on how to select optimal test parameters in the analysis of retained austenite in steel by the EBSD method.
Thus, the objective of this paper is to first study different test parameters for three types of steels (TRIP590, TRIP780 and X90 steels) on the measured results of retained austenite content, and then obtain the optimal test parameters for the EBSD analysis of retained austenite in TRIP and pipeline steels.
Author Contributions
Conceptualization, Y.Z. and X.J.; Methodology, Y.Z.; Software, Y.Z.; Validation, Y.Z. and X.J.; Formal Analysis, P.L. and G.C.; Investigation, P.L., H.J. and G.C.; Resources, Y.Z.; Data Curation, Y.Z.; Writing—Original Draft Preparation, Y.Z.; Writing—Review and Editing, Y.Z.; Visualization, Y.Z.; Supervision, X.J.; Project Administration, X.J.
Funding
This research received no external funding.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
The morphology and distribution of retained austenite in TRIP780 steel at different indexing rates (a: 81.4%; b: 84.7%; c: 86.3%; d: 87.5%; e: 90.0%; f: 93.0%).
Figure 2.
Curves of the measured retained austenite content (a) in TRIP780 steel and its increasing rate (b) with indexing rate.
Figure 3.
The morphology and distribution of retained austenite in X90 steel at different indexing rates (a: 80.7%; b: 85.1%; c: 88.6%; d: 90.5%; e: 92.1%; f: 93.1%; g: 94.2%).
Figure 4.
Curves of the measured retained austenite content (a) in X90 steel and its increasing rate (b) with indexing rate.
Figure 5.
The morphology and distribution of retained austenite in TRIP780 steel at different step sizes (a: 1.0 μm; b: 0.5 μm; c: 0.4 μm; d: 0.3 μm; e: 0.25 μm; f: 0.20 μm; g: 0.15 μm; h: 0.10 μm; i: 0.05μm).
Figure 6.
Curves of the measured retained austenite content (a) in TRIP780 steel and its decline rate (b) with step size.
Figure 7.
The morphology and distribution of retained austenite in X90 steel at different step sizes (a: 0.3 μm; b: 0.2 μm; c: 0.15 μm; d: 0.12 μm; e: 0.10 μm; f: 0.08 μm; g: 0.06 μm; h: 0.04 μm).
Figure 8.
Curves of the measured retained austenite content (a) in X90 steel and its decline rate (b) with step size.
Figure 9.
The average value of retained austenite content (a) and the relative accuracy of 95% confidence interval (b) for different fields.
Figure 10.
The variation curves of the average relative accuracy of 95% confidence interval (a) and its decline rate (b) with different fields for TRIP590 steel.
Figure 11.
The variation curves of the average relative accuracy of 95% confidence interval (a) and its decline rate (b) with different fields for TRIP780 steel.
Figure 12.
The variation curves of the average relative accuracy of 95% confidence interval (a) and its decline rate (b) with different fields for X90 steel.
Figure 13.
The morphology and distribution of retained austenite in three steels (a: TRIP590; b: TRIP780; c: X90).
Figure 14.
The measured XRD patterns for different steels (α, ferrite or other phases with bcc structures; γ, retained austenite phase).
Table 1.
The chemical composition of test steels (wt%).
Steel | C | Si | Mn | P | S | Al | Cu | Cr | Mo + Nb + Ti |
---|
TRIP590 | 0.11–0.13 | 1.1–1.3 | 1.4–1.6 | 0.009 | 0.003 | 0.03–0.06 | - | - | - |
TRIP780 | 0.15–0.20 | 1.2–1.4 | 1.5–1.7 | 0.006 | 0.004 | 0.05–0.08 | - | - | - |
X90 | 0.05–0.06 | 0.2–0.4 | 1.8–2.0 | 0.010 | 0.003 | 0.03–0.05 | 0.1–0.2 | 0.2–0.3 | 0.15–0.35 |
Table 2.
The parameters used in this study for Electron Backscattered Diffraction (EBSD) analysis.
Items | Value |
---|
Accelerating voltage(kV) | 15 |
Beam current (μA) | 10~15 |
Working distance (mm) | 15~18 |
Hough resolution | 80 |
Number of bands detected | 6~10 |
Time per frame (ms) | 5~15 |
Binning | 2 × 2 or 4 × 4 |
Gain | 10~12 |
Table 3.
The optimal parameters for analyzing retained austenite in TRIP590 steel (reproduced from [
18], with permission from Springer, 2019).
Item | Indexing Rate, % | Step Size, μm | Field Number |
---|
Optimal value | ≥88.9 | ≤0.12 | ≥5 |
Table 4.
The proportion of retained austenite (P) with different grain sizes (d).
TRIP590 | TRIP780 | X90 |
---|
d, μm | P, % | d, μm | P, % | d, μm | P, % |
---|
<0.5 | 66.91 | <0.5 | 51.48 | <0.3 | 77.88 |
0.5~1.1 | 26.10 | 0.5~2.5 | 43.11 | 0.3~0.5 | 18.23 |
>1.1 | 6.99 | >2.5 | 5.41 | >0.5 | 3.89 |
Table 5.
The statistical results of average grain size and maximum grain size.
Steel | Average Grain Size (Standard Deviation), μm | Maximum Grain Size (Standard Deviation), μm |
---|
TRIP590 | 0.524 (±0.049) | 3.780 (±0.285) |
TRIP780 | 1.195 (±0.094) | 7.987 (±0.652) |
X90 | 0.297 (±0.019) | 1.097 (±0.089) |
Table 6.
The relationship between the maximum step size and the average grain size of retained austenite.
Steel | | Maximum Step Size SZm, μm | |
---|
TRIP590 | 0.524 | 0.12 | 0.229 |
TRIP780 | 1.195 | 0.25 | 0.209 |
X90 | 0.297 | 0.06 | 0.202 |
Table 7.
The needed field number and scanning area for different steels.
Steel | Magnification | Needed Field Number | Scanning Area for Each Field, mm2 | Total Scanning Area, mm2 |
---|
TRIP590 | 1000× | ≥5 | 0.011264 | ≥0.05632 |
2000× | ≥17 | 0.002816 | ≥0.047872 |
TRIP780 | 1000× | ≥4 | 0.011264 | ≥0.045056 |
X90 | 1000× | ≥6 | 0.011264 | ≥0.067584 |
Table 8.
The test parameters for analyzing retained austenite in three steels.
Steel | Indexing Rate, % | Step Size, μm | Field Number | Total Scanning Area, mm2 |
---|
TRIP590 | 94.2 | 0.10 | 6 | 0.068 |
TRIP780 | 91.5 | 0.12 | 6 | 0.068 |
X90 | 92.8 | 0.05 | 6 | 0.068 |
Table 9.
The measured retained austenite content in three steels by the EBSD method.
Steel | Average Retained Austenite, % | 95% CI, % | %RA, % | |
---|
TRIP590 | 5.32 | 0.36 | 6.6 | 5.32 ± 0.36 |
TRIP780 | 20.02 | 1.37 | 7.0 | 20.02 ± 1.37 |
X90 | 0.42 | 0.06 | 13.7 | 0.42 ± 0.06 |
Table 10.
The measured retained austenite content in three steels by XRD.
Steel | Average Retained Austenite, % | 95% CI, % | %RA, % | |
---|
TRIP590 | 5.49 | 0.28 | 5.8 | 5.49 ± 0.28 |
TRIP780 | 19.55 | 0.39 | 2.0 | 19.55 ± 0.39 |
X90 | 0 | 0 | 0 | 0 |
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