Microstructural, Chemical, and Crystallographic Investigations of Dynamic Strain-Induced Ferrite in a Microalloyed QT Steel

Round 1

Reviewer 1 Report

Via LOM, SEM, APT and EBSD, displacive mechanism with C diffusion was determined for the formation of DSIT ferrite. The experiments are detailed and believable, but minor reversion may increase the rigorous of the demonstration. Please address the following questions:

1.Given the importance of NbC precipitates in the whole DSIT process, the differences between the pre-precipitates and precipitates in DSIT ferrite should be considered.

2.The DSIT ferrite and pre-eutectoid ferrite were determined via LOM and SEM (line 159-164). please address how to distinguish the two ferrites, by morphology, or process, or others?

3.Via STEM images, a bunch of small precipitates were found in DSIT ferrite, but such particles were also found in contiguous martensite. Please address the particles' number density in DSIT ferrite and martensite respectively.

4.In fig5, Nb clusters were found both along the interface and DSIT ferrite inner by APT. But EDS or HRTEM is more convincing for the conclusion (line 269-273), please address them.

5." DSIT ferrite grains can be seen along the PAGB within a martensitic matrix" (line 159-160, 265). The word "within" is ambiguous, please check it. In addition, martensite is inside the PAGs, it is dual phases (ferrite and martensite) or triple phases (ferrite, martensite, and retained austenite). please check and revise it.

Actually, minor reversion may increase the rigorous of the demonstration.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Table 2 is labelled Table 1.

Apart from this, the submission is fine.

Author Response

We changed the heading of table 2.

We would like to thank the reviewer for taking his/her precious time to help us to improve the quality our paper.

Reviewer 3 Report

only some technical things must improved,

fig 1:   Were the tests only carried out on samples where phase 1 was fully completed? There was no sample taken after only annealing at 950°C?

Row 150 -153 is not clear written

Heading on line 150 must apply to Table 2.
Text of correct heading missing, wrong repetition of table heading of table 1.

Fig. 5 and fig. 6  a) b) c) letter size to big!

Fig. 7  improve this, make a 6 bar figure with different axes and add the element name

fig. 8c   very bad to seen, what you will say or inform, improve contrast

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The author conducted lots of experiments to study the behavior of dynamic strain-induced ferrite in steel and obtained reliable results. However, the analysis of the experimental phenomenon is not deep enough, just compared with the literature, and there is no expansive analysis. There are too many summaries , and most of them are experimental results in the conclusion. The author needs to emphasize the uniqueness of his discovery, that is, to clarify the phenomena not found by others, or the differences from others' research, or to have a deeper understanding of the mechanism behind the experimental phenomena.

1. Is the material selected in this paper widely used or specially designed for the later experiments? If it is widely used, it is easy to supply the difference in mechanical properties between the conventional treatment method and the treatment method in this paper. For the later condition, the principle of material design should be briefly given.
2. Line 115, “. The T_a was chosen in a way that Nb precipitates inside the matrix dissolved within the 300 s annealing time.”. Proof should be provided, experimental or references.
3. Line 161, “Triple points are highlighted ……”. The characters of triple point should be described more carefully. Is this conclusion directly obtained from metallography reliable enough? Are there other methods to cross verify it?
4. Line 171, Fig 4. When researchers refer to the phase observed by transmission electron microscope, the selected area electron diffraction image usually needs to be given. The author shows that there are a large amount of small precipitates with a size around 5 nm in the steel. How did the author calculate the size of nano precipitates？
5. The paper shows that fine NbC precipitates (around 5 nm) were often detected in the area of the retained austenite/DSIT ferrite interface and DSIT ferrite itself, which can be seen in Figure 5 b，indicate the NbC could act as a DSIT ferrite nucleation site。This reason is not sufficient. It can also be seen from Figure 4 that there are a large number of nano precipitates in martensite. Can it also be used as a DSIT ferite nucleus site.
6. This paper analyzed the influencing factors of formation of stain-induced ferrite but did not give the proportion of ferrite. Does the ferrite content change during the three compression experiments?
7. In Fig 5-7, APT results were given to prove the conclusion about elements diffusion. However, apt is a test of a very small area in the material. It is doubtful whether the selected area is representative for the whole material. It is suggested to add the experimental results of more or larger areas to verify the reliability of the argument, like results of EDS or EELS.
8. In the 3d-apt&EBSD result ,“the fact that only C diffuses into neighboring areas during the formation of DSIT ferrite but not Mn and Cr indicates that the formation is a displacive mechanism, although accompanied by C diffusion. This assumption is supported by the results of the EBSD measurement (Figure 8 and Figure 9).” What the“displacive mechanism”? and how the change of grain boundary could affect the carbon diffusion? Please discuss it in detail.
9. What’s the method of your sample preparation, random regions or FIB? How do you confirm the region in STEM is corresponding to the 3D-APT?
10. About discussion and conclusions, the arguments are not concentrated. The author should pay more attention to the unique principle behind the experimental phenomenon, rather than repeating the test methods and experimental phenomena, like why C or Mn or other element diffuse into neighboring areas, or why not diffuse. Other than that, author pointed out that only C diffusion was found, what about B? Some researchers believe that B diffuse faster than C in alloys.
11. The author has made a detailed study on the microstructure of the material, but did not show its influence on the final properties of the product. Considering the small size of the sample used, micro tensile test or other method could be used to prove the material’s properties. If possible, the hardness could be given, at least.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

No