Effect of Austenite Grain Size on the Bainitic Transformation in a 690 MPa Grade High-Strength Multi-Functional Construction Steel

Round 1

Reviewer 1 Report

Remarks to paper 1598493,    08 March 2022

The subject of the paper is dependence of bainitic transformation on austenite grain size. The paper is interesting and stays in convention of studies on bainitic transformation. The results are nicely described and discussed, hovewer, their meaning is practically doubtful due to unrevealing significant part of data, which makes it hard to assess usability of the results, not to mention, reproducing or extension by other scientists, if one wanted to. The paper is generally well written, except for numerous simplifications and shorcuts. From scientific value standpoint, the work rather tends to verify general knowledge than put a new light on the subject. The subject defined as "Effect of austenite grain size on the bainitic transformation" in fact, boils down to effect of bainite start point on grain size, and effect of austenite grain size on austenization temperature.

Although, the conclusions are sound and credible, they do not grow out directly from the results on one hand, and some of the results are not explained sufficiently on the other hand.

The major remarks for consideration are:

1) The methodology and experimental procedures are well described, expect from motivation. Authors are asked to explain:

1. i) what is the reason for selection austenization temperatures as they are? Please provide explanation.
2. ii) why given chemical composition is selected? Please justify this selection by indicating target application or simulation/modification of some grade/category of steel. In addition, refer to expected properties – referring to “building steel” is vague. It’s understood that the study is meant to investigate phenomenological dependences of bainitic transformation irrespective of the grade, just on the basis of the experimental “model” material, but the title obligates supplying some more information on the material.

iii) (either) indicate accurate content of major alloying elements grouped together in the fourth column of Tab. 1 or justify why content of individual element is not significant in these studies; why there no Mn whatsoever?   The same for microalloying elements – it’s obvious that each of them has a different range of stability and action (e.g. pinning effect mentioned) and it is important for evaluation of the results what’s the amount of Ti vs. Nb/V; (Nb+V) can be grouped, if any.

2) some aspect addressed in discussion (and consequently, in conclusions) are not backed up by results or are linked indirectly with results, e.g.:

- Conclusion (4) (and corresponding paragraphs in sec. 3.3) does not really grow out of the presented results - how can we know 1000C is a threshold point is next temperature studied is 1250 C ?  One could conjecture the grain growth behaviour from the stability of precipitates in austenite, but that is not known since no precise amount of microalloy element is given

3) there are many mental shorcuts to be extended

- "The maximum length of bainite equals AGS because the growth of bainite is limited by the prior austenite grains".  Let's look on this in details:  "length" equals " size" - which is not precise and not true, because two different entities are compared. Moreover, what does "length of bainite" means? what does grain size means? (average intercept, average area, etc ?)

- conclusion (2) - what does "low range" mean?

- dragging effect --> drag effect

The following conclusion can be drawn --> conclusions

4) the references seem inadequate for the matter. There are studies mentioned concerning baintic transformation, and that alright, but few of them concern this sort of material, whereas there is a plenty of papers on accicular ferrite kinetics, pipeline steels etc. In addition to that, some of the work seem not proper for explanation findings, e.g. role of carbon in controling the kinetics is backed up by citing work on high-carbon steel. 

5) explain or revise:

1. i) “the stability of austenitization” (line 166)
2. ii) “The effect of AGS on bainitic transformation is discontinuous” (does it mean there is/are range where there’s no effect of AGS on bainitic transformation?

6) Check the text for missing spaces and punctuation. Remove [J] in ref. 19, remove " in ref. 8.

As mentioned above, the manuscript is written well, and has a potential to make an interesting work, but in the current state needs necessary supplementation.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Language

The English must be improved over the whole manuscript. Please, be careful with the use of definite article “the”. There are many cases it is being employed not correctly (see for example lines 16 and 18). Punctuation missing and or not used correctly (see for example line 57). It is recommended a complete language review.

Title

Why is this steel “multi-functional”? Is it a usual denomination? Besides that at the end of introduction, the steel is referred as “wheater-resistant”, and as “building construction”. It is necessary to look at the denomination that is more usual to homogenize that trough the text.

Abstract

Line 26 - “controlled by the volume diffusion of 26 carbon atoms are analyzed”. How was it analyzed?

Introduction

The author mentioned at lines 49-50 the importance of modelling bainitic transformation giving the impression this would be the case in this article, but there is no specific connection to the article aims reported at the end of introduction (lines 68-70).

Line 39 – ...the research and development team... What team? Maybe it is not necessary to write in this way as it is implicit the work was developed by the article authors.

Line 40 – Specify the “690 MPa strength”. Are you talking about yield strength, ultimate strength?

Line 44 – “the advantage” or better “one of the advantages”.  Is that the only advantage of using micro-alloying elements?

Line 57 – the “.” Is missing.

Line 61 – 64 – The statement is not very clear. Perhaps a better explanation referred to “This equation shows that when the global dynamics of bainite transformation is limited by slow growth rates, ……”   What are the limiting conditions? How to recognize that? The line 61 – 64 try to explain the effect of AGS for specific conditions, but need to be improved. It is confusing the reader.

Line 68 – there is a redundancy by using two times the word “study”.

Materials and Methods

The experimental procedure must be improved in order to give more details. There were replicates of the experiments carried out?

Line 83 – Correct position of the table.

Line 88 – Exchange “.” By “, “. Or write new sentence after “.”.

Lines 93 to 97 – The procedure for sample preparation, specially for the etching with 3 different etchings that were mentioned is not clear. Please, rewrite adding a more detailed explanation.

Line 99 – “with the GB6394-2002 standard”... Can the author indicate na ASTM standard.

Line 102 – Heat treating instead of “heat treatment”

Results and Discussion

At figure 4 the determined Bs temperature for the austenitizing temperature of 860 C seems to be too high. It would help a theoretical calculation based on the chemical composition for comparison.

Lines 116 a 119 – “Austenite grains grow normally due to the effect of nailing on grain boundary 116 migration of nanocomposites. The bainite transformation can only take place inside the 117 austenite grain due to the ledge growth mechanism and displacive mechanism, so the 118 austenite grain boundary in the microstructure is clear and complete ...”..

The correlation with migration of nanocomposites is not clear. Could you please rewrite this sentences and clarify, or exclude if not relevant...

Lines 153 a 155 – “The 153 fine austenite grains with high dislocation density inhibit the migration of primary crystals of bainite, which slows down the nucleation rate of bainite”... What was meant by migration of primary crystals of bainite??

Lines 161 – 163 – “Combined with Fig. 2 and Fig. 4, polygonal ferrite first appeared in the sample with the lowest austenitizing temperature (860 °C), because its Bs temperature was the highest.”  The use of “combined with” does not seems to be correct. Do you mean combining the information from...?

The explanation for the appearance of polygonal ferrite is not clear...Please improve it.

Line 174 – Better to exchange the subscribed indices “a” by “b” to avoid confusion to the readers.

Lines 183 a 186 – “The effect of AGS on bainitic transformation is discontinuous. When the austenitizing temperature is below 1000 °C, the increased AGS contributes significantly to the acceleration of the bainite transformation. However, when the austenitizing temperature is above 1000 °C, the acceleration effect is weak.”

Please rewrite this setence completely to clarify. What you meant is that when for austenitizing temperatures below 1000 C, increasing AGS a faster bainite transformation is observed. But, you have tested only one temperature below 1000C. Therefore, you are only comparing 860 C with 1000C austenitizing temperatures. You don´t have other transformation curves in between. The same happens when you refer to temperatures higher than 1000C, because only one was tested.

Lines 226 0 230 – Please review these sentences. You could also compare your results with 10.10 16/j.mat des. 2018.06.014

Conclusions

Line 252 – “low carbon bainite steel”  or “low carbon bainitic steel”, as it has or it aimed to present a bainitic microstructure.

Lines 258-259 “The Bs temperature and bainitic transformation temperature range of the specimen austenitized at 1000 °C is lower than those austenitized at 860 and 1250 °C”

The Bs temperatures could be verified by theoretical calculations from the chemical composition of the steels. The Bs for 860C seems to be too high. Can this be ferrite starting?

Lines 262 to 264 – “For the novel high strength multi-functional structural steel, there is a critical austenite grain size corresponding to the austenitizing temperature of 1000 °C. Below the critical austenite grain size, there is a significant grain size effect, and above which it is not significant”

Only three austenitizing temperatures were tested. How is possible to be sure that 1000 C is the mentioned critical temperature? It can be higher of lower than 1000 C.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The authors studied the influence of austenite grain size on the kinetics and resulting microstructure of bainitic transformation. A low-carbon microalloyed steel was chosen as the experimental material, which, thanks to its high strength and weather resistance, is a modern multi-functional building structural steel. Therefore, this manuscript can be considered very useful and up-to-date.

The austenite grain size (AGS) is strongly dependent on austenitization temperature. The kinetic curves of bainite transformation show that for achieving a certain volume fraction of transformed phase, a lower cooling temperature is required for the austenite formed at 1000 °C than at 860 °C and at 1250 °C, thus, this dependence is not monotonous. Also, the transformation rate is highest for temperature of 1000 °C and together with the temperature of 1250 °C another peak in the transformation rate curves occurs at the end stage of the transformation. The authors cite Gupta’s explanation of the occurrence of this peak.

It would be very interesting to study the kinetic curves for more austenitization temperatures. However, this represents a lot of experimental work and we may see it in another article of these authors.

The manuscript is written very clearly and concisely, its English (not being reviewer’s native tongue) is good and fully understandable (some my comments below are rather my opinion than the inaccuracy of the authors). Only two self-citation of Zhao X. (if it is not a match of names) are fully acceptable. The citations are very new, most from recent years, a fifth from this year and last year. All of them are closely tied to the studied topic. The reviewer found no text reused from another source without appropriate citation.

The manuscript fully meets the requirements of a scientific article. Modern methods of materials engineering are correctly used. The obtained results are interesting (the review really likes the images of microstructures) and the conclusions follow from them logically. The reviewer has no significant objections or comments. Some formal shortcomings will certainly be eliminated during the editorial process.

Specific comments:

Line 89/90: “Each sample was held at the peak temperature for 5 seconds to obtain full austenitize.” “… full austenitization” seems more appropriate.

Line 183: “The effect of AGS on bainitic transformation is discontinuous.” The reviewer means that “contradictory” or “ambivalent” (from a general point of view) or “non-monotonic” (from a mathematical point of view) would be more appropriate.

Line 207: “It was observed that the effect of AGS on transformation rate is nonlinear.” Here also “non-monotonic” would be more appropriate (nonlinear function can be in addition to the non-monotonic also monotonic, which is not the case here).

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Revision of the manuscript (formerly) entitled "Effect of austenite grain size on the bainitic transformation in a high 2 strength multi-functional steel for building construction" was reviewed. The paper exhibit significant improvement in quality of both the text and the scientific content, which results in easier readability and understanding the discussion, addressed remarks are taken into consideration in fair degree,  enough to mitigate the former errors and sufficient for the work to be clear and consistent. tweaking introduction and redefinition of the steel with providing more details on the composition allowed justification of the research plan and put some new light on motivation of authors in this research. 

However, there are still some issues to improve, but they involve little edition plus one suggestion which came up having read the complete discussion of the results. These are:

1) line 88: how Ae3 was calculated  - commercial code/software (if so - what code?) or equation (cite)

2) conclusion 3 still might sound better. Without several readings it's not clear what is more important than what for determining what. This implication must be revised.   In addition to this, two minor notices can be amended:

• "multi-functional structural steel" - analyse one (!) because this notion is too general, and may include hardening grades, plain carbon steels etc.
•  why not used "microalloyed" and, as in the title, 960MPa ?

3) the results' discussion is extensive and it's advisable to highlight the compared parameters. Thus, please make a visual summary of the dependence of austenization temperature on major parameters appearing in the discussion (AGS and other microstructure parameters, Bs point, transfromation kinetcs parameters, ...) It's optional, but it will make it easier to to follow evaluation.

4) Arrows in Fig. 6 should be numbered and, in addition to text, described in caption.

5) small editing corrections are needed:

-intercept method - add " linear "

• line 208: remove "obviously" and "apparently"
• line 242: "believed" - better use present tense (it's likely cited authors still maintain what's cited)
• Ae3 - 3 should be lower-case

Have the listed remarks been implemented, I will recommend the paper to be published in Metals journal. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have addressed the suggested changes. The paper can be published in the present form.

Author Response

Thanks for your time and effort.