Design of an Effective Heat Treatment Involving Intercritical Hardening for High Strength/High Elongation of 0.2C–3Al–(6–8.5)Mn–Fe TRIP Steels: Microstructural Evolution and Deformation Behavior
Round 1
Reviewer 1 Report
The scope of the article is very interesting, especially for mentioned by the authors' automotive industry.
However, a few issues are not clear:
1. Microhardness. Authors show a table with a microhardness of different phases (table 2), however, it is not clear is the data from the literature (where is the reference?) or results of the experiment. In the case of own data, there is a lack of methodology description – how it was measured, how many measurements for each phase were done, etc.
2. The volume fraction of phases. In Figure 4, the authors demonstrate the schematic diagram of the volume fraction of phases, however, it is not clear, how the volume of martensite was calculated/evaluated.
3. Tensile properties.
In Figure 5 the comparison of tensile properties is shown. Nevertheless, there is no information about the number of samples for each temperature and accordingly lack of statistics (standard deviation) for presented results. In conclusion, the authors stated that “the mechanical properties of the two experimental steels … are similar or superior than other medium Mn TRIP steels” (line 269-270), it would be helpful, if authors will confirm this statement showing a comparison of mechanical properties for experimental steels with data for other steels. The grade of steels with which authors compare their results will be helpful as well.4. Microstructure and phase identification.
Unclear how authors distinguish between a-ferrite and d-ferrite. The authors’ hypothesis can be confirmed by micrographs of selectively etched microstructure or EBSD analysis. It is not possible to compare the initial microstructure (Figure 2) with microstructure after tensile deformation (Figure 7) due to the different magnification.5. Conclusions.
Conclusion #1 in its current state is too obvious and brings nothing new. Conclusion #2 should be supplemented with test results - what increase in mechanical properties was obtained. In its current state, it is too general. Conclusion #3 is not confirmed by experimental data in the article and makes an impression that was done based on literature data.Author Response
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Reviewer 2 Report
The work is interesting. However, it must be improved the presentation of information. For instance, I’m not sure if this is the final version since there are several lines highlighted in yellow. The general editing should improve. Also, I recommend describing more carefully methods in section 2. It should be included a description of the analysis in Thermo-Calc.
Here are some specific points:
Line 43: What do you mean “a number of steels have been conducted on medium manganese”? Line 72 and 74: suggests Line 74: exists Line 71-75: explain in a better context that you used that model in your research along with Thermo-Calc in order to design chemical composition of steels Line 76: how were obtained those cast ingots? Line 90: diffuses Line 128: are shown Line 136-138: Is redundant Line 148: avoid to use “we” Hardness measurements are not described in Materials and Experimental Procedure.
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Reviewer 3 Report
The topic is not new and is well studied and documented in the scientific literature. For me was not possible to find what is new and unknown in this work.
The English language of this submission suffers from unclear grammatical structures and sometimes incorrect use of the terminology. I marked only part of them in the pdf.
Line 43: Unclear sentence-what mean a number of steels to be conducted?
Line 45: The austenite stability depends on the composition of the austenite. There is nothing to do with ferrite ! In the way it is presented this information is misleading.
Line 67: I expect to see the description of the “novel heat treatment” in easy understandable way-time temperature graph with clearly mentioned times and temperatures with respect to the transformation temperatures of the steel.
Line 73: "suggests"
Line 82: Is this the nominal composition (what authors charge for melting) or the real composition based on the chemical analysis of the hot rolled plate? The differences can be significant. This must be clarified. Additionally, the sum of the elements for 8.5Mn is 99.99%wt . I am almost sure that this is the nominal composition, but what is the real one …?
Line 90: Why C should diffuse from ferrite which has the lowest C content to the austenite where the C content is much higher? This is at least strange? Even if the authors have in mind the delta ferrite, based on the self-citation of the source [6] it cannot be true! I checked the cited source and there is no convincing evidence for the enrichment with C. The EPMA data are not enough convincing and the calculation for the lattice parameter of austenite was not given in this paper. Hence, most probably they have in mind martensite here. In fact the only prove of this statement is to support the assumption by APT measurements. If one accept the assumption that the delta ferrite has high carbon content it should have also a higher hardness than the alpha-ferrite. This is not the case regarding the data shown in table 2. This is a different question how correct the data in this table are. This is very weak point of this work.
Line 112: Again something unclear. The chemical compositions of the steels were already given in the table. Is this new composition that is predicted? The authors must know what they do! In this case they calculate the composition of the austenite before the transformation using Thermo-Calc. Please, correct your language and use the right terminology!
Line 113: Which data base was used? The results can differ depending on the data base.
Line 124: unclear! This is illustration for the austenite stabilization predicted by the model of Emmanuel De Moor. This is not a development of a model!
The model predicts the fraction of RA after quenching and the results are dependent of the calculated Ms. temperature. This is not mentioned in the calculations how it was taken into account. On the other hand tempering at 200°C is claimed to stabilize the austenite due to C diffusion. (see line 90). How it can be taken into account. The model of De Moor predicts higher fraction of austenite that are obtained in reality in this work.
Line 150, Fig 2: On which base is made the differentiation between the alpha and delta ferrite. Is there are parallel quantification at least via thermodynamic calculations? It is very unclear the identification of some phases. In Fig. 2d , are there additionally martensite or ferrite + carbides? If so what kind of carbides?
Line 158: Fig. 3: In general , all experimental data should be represented with error bars; I can’t see this in this figure, but at least the experimental error should be added ; In fact all experimental results are shown without any error bars. This is not acceptable for a journal paper in a scientific journal.
Line 161 : “Figure 4 shown a schematic representation of austenite, ferrite…” this is a wrong sentence.
Line 173: How the fractions shown in Fig 4 are calculated? Why the data do not have error bars?
Line 193: The figure suggest that the data for each point are taken from only one sample. No error bars are given.
Line 211: “carbon element “ is inappropriate . Just “carbon” is enough.
Line 214 ; Figure 6: On the diffraction pattern of the fractured samples is almost impossible to find any evidence for untransformed austenite. Especially on 6Mn steel.
Line 216, Fig. 7: Why the delta ferrite disappeared from the 8.5Mn sample? Or the microstructure is so heterogeneous that it simply is not present in this region? It is strange because in the previous optical image shown in Fig.2 , which covers an area of ~20x15µm it is visible but on this image that covers an area of ~=80x60µm it is not present.
Line 244 in Fig. 8: Strange symbols, looking like Cyrillic letters appear in Fig. 8 as well as on line 246?! What is this?
Line 257: Are the dark lines really cracks or simply ferrite /ferrite grain boundaries? How the authors can prove this, having in mind that the neighboring ferrite region is not cracked? See for comparison fig. 2d and 2g. The same grain boundaries exist there.
Line 264 first conclusion. First the model is proposed by De Moor et al, not by the authors of thei submission. Second, the differences between predicted data for RA (65.2% for 8.5Mn and 38% for 6Mn) are 10% and 13% respectively, with what was obtained during the experiment. (59%RA for 8.5Mn and 33% for 6.Mn). Is this a good prediction?
Line 272 the third conclusion: “3) The lamellar austenite, unlike the equiaxed microstructure, is more likely to cause stress relaxation in martensitic transformation, resulting in discontinuous TRIP effect. Furthermore, ferrite effectively relieves the internal stress caused by the volume expansion during the transformation process.” I afraid I did not find in this paper any evidence for this statement. This is more speculation and it could be but how it can be put as a conclusion without any direct prove? Hence, I can conclude that the conclusions 1 and 3 are not really supported by the results.
Author Response
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Round 2
Reviewer 1 Report
It is not possible to measure the hardness of each phase in a way described in the cover letter. Moreover, it is a lack of the number of measurements for each phase. Hardness measurement with a 50g load is a microhardness. The volume phases of a fraction. Authors did not change in text, that presented diagram is a trend (as it was mentioned in the cover letter) instead of actual measurements. In cover letter authors mentioned how many samples were used for the tensile tests. However, the manuscript doesn't have this information. There are no significant changes in the conclusions.Author Response
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Reviewer 3 Report
Thank you the authors for considering very serious my comments. They have addressed most of the reviewers comment enough clear. However, in the way the paper is submitted is very difficult for me to read it. The reason is that new text is included but it is not always clear which text it replaces. Please, prepare new draft and send it again, in which the old text is removed and only the new one remains but the new one must be clearly marked.
For example : In the current text appears:
Line 279: “This Based on previous studies [33-37], this behaviors related to discontinuous TRIP effect.”
Or:
Line 254: “By comparing the micrographs of undeformed steels with the fractured surface, we inferitis inferred that the black lath martensite….”
It is very difficult to read the submission in this form.
Additionally, some of the answers that are given by the authors must be implemented in the text as additional explanations. The fact that I raise up these questions is because I see a possibility these issue points to remain unclear for the reader.
Additional comments on the answer to reviewers that must be addressed more clear
Point 5: Line 82: Is this the nominal composition (what authors charge for melting) or the real composition based on the chemical analysis of the hot rolled plate? The differences can be significant. This must be clarified. Additionally, the sum of the elements for 8.5Mn is 99.99%wt . I am almost sure that this is the nominal composition, but what is the real one …?
Response 5: Thanks very much for your patience. We round to the nearest one decimal place in table 1. In fact, the content of Al is 3.11%, but we originally filled 3.1% in the table. We have updated the table 1 and round to two decimal places.
New comment: I still do not have an answer: Is this is the nominal composition or this is the real chemical composition of the steel. Please, provide information how it was measured in case this is the real composition!
Point 6: Line 90: Why C should diffuse from ferrite which has the lowest C content to the austenite where the C content is much higher? This is at least strange? Even if the authors have in mind the delta ferrite, based on the self-citation of the source [6] it cannot be true! I checked the cited source and there is no convincing evidence for the enrichment with C. Hence, most probably they have in mind martensite here. In fact the only prove of this statement is to support the assumption by APT measurements. If one accept the assumption that the delta ferrite has high carbon content it should have also a higher hardness than the alpha-ferrite. This is not the case regarding the data shown in table 2. This is a different question how correct the data in this table are. This is very weak point of this work.
Response 6: Thanks very much for the comments.
Ferrite is a interstitial solid solution of carbon dissolved in α-Fe. It is a body centered cubic (bcc) lattice, and its carbon dissolving ability is very low. Austenite is a interstitial solidsolution of carbon dissolved in γ-Fe. It still maintains the face centered cubic (fcc) lattice. The carbon dissolving capacity in austenite is large. It can be seen from the phase diagram of iron and carbon that the solubility range of carbon in austenite is very large. Carbon can enter the iron atom gap in the form of interstitial solid solution in austenite. However, the solubility of carbon in ferrite is very low, and only carbides can be formed with iron, with a low diffusion coefficient. Thus, C diffused from ferrite to austenite during annealing and tempering.
Forgive me for taking out the old data again. After tempering, the carbon contents of α-ferrite and δ-ferrite are almost the same (Fig. 1d, as shown in the right ordinate, the darker the color, the lower the C content).
Actually, ten measurements for each phase were done. Hardness tests were conducted using 50 g load and 5 s dwell time, with the interval of 0.5 mm between the neighboring measured points. And we added standard deviation in table 2. We have added this statement in the manuscript (Line 100-101). I think it is helpful.
Fig. 1. SEM micrographs of medium-Mn samples heat treated at 800 ℃ (a) and (c) heat treated at 800 ℃ + tempered at 200 ℃, (b) and (d) are corresponding carbon concentration by electron microprobe analysis.
New comment: I cannot accept this answer, because it does not give explanation why C should diffuse from a phase where it is in low concentration (i.e. ferrite), to phase where the concentration is high. To have a diffusion of an element from phase X to phase Y the concentration of this element before the beginning of the diffusion should be higher in phase X than in phase Y. May be the formulation in your answer is not correct. Please, try to make you explanation better using the calculated equilibrium phase diagram for your multicomponent system Fe-Mn-Al-C.
Regarding the attached Fig. from the previous paper of the authors, they should compare the variations of C only in the individual map but not between the maps because they are measuring very light element (C) and this is always associate with inaccuracy of the measurement i.e. even with EPMA the measurement of C concentration remains more qualitative (semi-quantitative) than really accurate quantitative. The fact that these results were already published does not make them necessarily correct.
I also have doubts regarding the micro-hardness data. How the authors can be sure that they measure the individual phase. The phases are very dispersed with a maximum size of the phase , for example Delta ferrite of ~2-3µm. All the rest of the phases are much smaller in size often below 1 µm. How it is possible then to have a Vickers hardness print form only one phase? How small it should be? For example for µVickers hardness of 450HV0.05 the mean diagonal should be 4.5µm and for a hardness of 257HV0.05 the mean diagonal should be ~6µm. This means that the results for the hardness measurement of the individual phases may not be correct , because the side influence of the neighboring phase cannot be eliminated. The authors need to comment this.
Point 13: Line 173: How the fractions shown in Fig 4 are calculated? Why the data do not have error bars?
Response 13: Thanks very much for the comments. Figure 4 (now it is figure 5) is schematic diagram of the volume fraction of ferrite, austenite and martensite. The accurate volume fraction of austenite can be measured by XRD. But the schematic diagrams just show the trend of change on volume fractions of ferrite and martensite phases. The purpose of adding Figure 5 is to illustrate the change trend of martensite and ferrite, because the content of martensite will affect the ultimate tensile strength, and also to easier explain the change trend of ultimate tensile strength.
New comment: The question was how the fractions were calculated. Indeed the austenite can be determined via XRD , but the reader would like to know how the ferrite and martensite were differentiated –optical micrographs , color etching, something else….?
In Conclusions
Please mention explicitly in the conclusion that you use the model of De Moore . As it is written now it creates an impression that the model used in this work was proposed by the authors of this paper and this is not true.
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Round 3
Reviewer 1 Report
The information from the authors' response should be in the manuscript. It will enrich it and make presentations of results more clear.
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Reviewer 3 Report
All comments are addressed in the manuscript to the best the authors can do.
One minor comment .
On line 300: the text is: "... the maximum and change trend of volume fraction of retained austenite stabilized at room temperature calculated based on the model proposed model by E. De Moor was in good agreement with the experiment data. "
Obviously the sentence is incorrect . Please, correct it.
It is may be: "the maximum volume fraction of retained austenite stabilized at room temperature calculated based on the model proposed by E. De Moor [ref] was in good agreement with the experiment data."
I do not have any further comments and suggestions.
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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The manuscript metals-594277-v1 has been submitted to Metals journal for peer review as a research article. The title of this manuscript is:
“Discontinuous Transformation induced Plasticity Effect in Governing High Strength High Elongation Combination of Medium Mn Steels”
Objectives of the study is a new thermal heat treatment to obtain high strength, high elongation on medium Mn steels. Motivation of this investigation is to enhance the energy absorption of steels for automotive applications. These high mechanical properties of medium Mn steels, according to authors, is not practically to be obtained at industrial scale because of the long duration but over stabilizing of retained austenite of the final heat treatment. The study is mainly carried on by experiments with various characterization techniques, including optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Microstructures and mechanical properties of two different low C (0.2 wt.%C) medium Mn (6.0 wt% Mn and 8.4 wt.% Mn) steels after thermal heat treatment were analyzed. Based on these experimental results, as well as calculation results from thermodynamic simulations, authors defined the optimum annealing temperatures to obtain high mechanical properties for each steels. The development of hardening factor during tensile tests of the steel samples is supposed to be due to the TRIP effects occurring in materials. Particularly, the discontinuity in hardening curves of the steel samples is accounted for by various stabilities of retained austenite with different morphologies. Although various results on microstructures and mechanical properties of steels have been given in this manuscript, readers could not be convinced by author’s arguments. The total content of this report is rather divergent. Studied results do not concentrate on specific issue of the report, they have been yet exhaustively analyzed. The submitted manuscript is a 12 page document, but only the last 2 paragraphs of the section 3, from line 204 to line 230, are fit to the title of the manuscript. Details comments and suggestions are given as below:
Line 2-4: The title of this manuscript clearly defines the physical phenomenon occurring in Medium-Mn steels. Nevertheless, it is not fit to the content of the report. Only in the last two paragraphs of the section 3 (line 204-230), the discontinuous transformation induced plasticity is introduced and discussed. Then readers will be confused by the mismatch between title and content of the manuscript. So what is the main focus of this report? Line 12-26: Authors need to avoid ambiguous expressions and non-standard terminologies in scientific documents, particularly in the abstract. Readers will not spend more time to read a paper which they cannot even understand the abstract. Line 14: What do author mean with “cumulative contribution of transformation induced plasticity (TRIP)”? Line 16: The term “intercritical hardening” is generally understood as plasticity of materials at elevated temperatures at which more than one phase exist. If this is not the case in this report, then authors need to find another term to describe their new experimental step. Line 31-32: This statement is in a bad format. Which have good crash worthiness, superior ductility and high strength, TRIP steels or automotive applications? Line 34: Please check the grammar of this statement “Retained austenite transformed to martensite during the TRIP effect.” Line 39-40: “While the twinning induced plasticity (TWIP) steels … “ is a bad formatted statement. Additionally, how much manganese content do authors consider as a high content? Line 43-44: “… austenite stability can be increased by distribution of manganese between austenite and ferrite …” Do authors mean the “redistribution/partition” of Mn between austenite and ferrite? Line 44: “… with long holding time.” How long does the holding time need to be called “long”? Line 45-47: “More recently, fraction of retained austenite …” is a grammatically error statement. Line 54: It is unclear for readers how “… the intercritical annealing stabilizes a high retained austenite fraction.” Line 56-57: “The heat treatments discussed above requiring cold rolling and annealing have proved to be inapplicable to the steels studied here.” References or evidences? Line 61-63: “E. De Moor et al proposed a model … in austenite” is a long, bad format and incorrect statement. Do authors have any evidence/reference for “the volume fraction of retained austenite … depends on the content of alloying elements such as … aluminum …”? Line 66: Please check the grammar of the statement “The composition of experimental steels …” Line 72: “The traditional heat treatment ...” How long has the heat treatment been applied to be called “traditional”? Line 73: If “it was proven …” then authors need to provide reference(s). Line 74: Please consider a different term for “intercritical hardening”, because it is not commonly known by readers. Line 74-79: It is unclear how and why authors chose the annealing duration of 1h and 15 min. Line 97-99: indicates simulation results by Thermo-Calc software are thermodynamic equilibria. On the other hand, authors commented in later paragraphs of the manuscript that experimental results are fit to calculation results. Then authors need to provide evidence(s) for the redistribution of Mn between ferrite and austenite during intercritical annealing. Line 99: “… equilibrium austenite was used as input to the model” Which model do authors mean? Line 113-115: It is unclear what are the differences among intercritical ferrite, δ-ferrite, retained austenite and marteniste and how authors can identify them in the SEM micrographs. Line 121-122: It is not convincible to describe “ the content of retained austenite in 8.5Mn steel was higher than that in 6Mn steel …” without any evidence or quantitative result. Line 141: What are the results shown in Figure 4? Are they calculated or measured results? Line 147: It is known that the austenite grain size increases proportionally with annealing temperature. However, there is no evidence for this phenomenon in study. Line 184: Where is the evidence for the subdivision of austenite block into thin film and granules? Line 185-187: “… the black lath martensite corresponds to the martensite …” needs evidence or reference. Line 208-209: How do authors differentiate between “granular” and “equiaxed” austenite? Line 213-214: It is not clear that ‘the fluctuations in the stage III is because stress relaxation in the martensitic transformation …” Line 236-237: Authors did not measure/provide the content of Mn in austenite of the 8.5Mn steel, then there is no point to conclude that the Mn content is responsible for high austenite content.Author Response
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Reviewer 2 Report
The authors compare two low to medium alloyed Mn steels undergoing a special annealing treatment at various temperatures with the goal of retaining a certain percentage of austenite after quenching. The two steels differ in their Mn fraction, i.e., 6% and 8.4%, respectively. The amount of retained austenite is determined and cross-correlated to the mechanical performance of the material, expressed in terms of a PSE value indicating the materials ability to absorb energy before failure. The authors conclude that the superior properties of the investigated steel grade are due to a transformation induced plasticity effect of the retained austenite, which can be quite significant given a suitable annealing temperature. They also provide metallograpical evidence and XRD results of specimens before loading and after being subjected to a tensile test.
The work is generally interesting and surely worth being presented to the scientific community, however not before addressing the following issues:
Equation 1: All symbols should be explained, i.e., N, M, R_\gamma, R_\alpha, index i From Figure 1 the authors conclude that Mn and C stabilizes the austenite. Their chain of arguments, however, is not clear. The C peak in Figure 1e seems to be at 600°C whereas the maximum amount of retained austenite is observed at around 750°C (Figure 1f). Mn drops continuously. It is unclear how austenite is stabilized if at 750°C there is comparatively little C in the austenite. The authors should also explain how C is distributed among the other phases. The lettering in Figure 1 is too small and hardly legible. Figure 7: While optical micrographs are provided to corroborate the authors’ message that austenite has undergone transformation in the fracture zone a quantitative statement is missing. What is the percentage of retained austenite in these two figures, i.e. after tensile deformation. Figure 8: The authors provide an explanation for the fluctuations in Figure 8b which may be credible. Anyhow, a convincing argument should be provided that artefacts from differentiating a somewhat wavy signal, i.e. measurement artefacts can be excluded as an explanation. Even though generally comprehensible, the language must be thoroughly checked. There is a plenitude of small grammatical mistakes and typos.
The overall recommendation is major revision.
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Reviewer 3 Report
The authors reported the impact of heat treatments, including intercritical annealing and low-temperature tempering on microstructure and mechanical properties of medium-Mn steel. It is of great interest to evaluate different heat treatment processes on microstructure and properties of medium-Mn steel. The manuscript could be considered for publishing. However, there are several issues, which have to be clarified.
The background is not well summarized. More details of the current heat treatment processes of medium-Mn steel, e.g. ART annealing, Q&P, etc. should be reported. On line 56, the authors stated that "the heat treatments discussed above requiring cold-rolling and annealing have proved to be inapplicable to the steels studied here". The authors should explain why the process did not work for the steels? The ART annealing is usually believed to be a simple annealing process for medium-Mn steel. The thesis statement is fairly weak. The objective of this study is not well presented. The aim of and mechanisms involved in the additional tempering process after intercritical annealing should be clarified. The effect of tempering on the microstructure and in particular the mechanical stability of retained austenite should be elucidated. The method for the quantification of the amount of martensite in the microstructure should be clarified. In which process step does delta-ferrite form? The formation of each phase constituents should be clarified concerning processing steps. The PLC effect in stage III is usually due to the localized deformation behavior, which results in serrations. The authors should explain more about the "discontinuous TRIP effect". Some literature is recommended: https://doi.org/10.1016/j.scriptamat.2017.01.022 DOI: 10.1016/j.actamat.2019.07.043 DOI: 10.1080/02670836.2017.1312208 DOI: 10.3390/met9070771 DOI: 10.3390/met8050357
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Round 2
Reviewer 1 Report
The manuscript metals-594277-v2 has been resubmitted to Metals journal following the revision requests from reviewer(s).
Although in this new version authors have implemented corrections and modifications as suggested by reviewer(s), it is still difficult for readers to follow the manuscript. The report still has a lot of language and grammatical errors. The scientific content in the report is rather low. Uncertainties and ambiguities remain existence but have not been clearly addressed. A major and critical revision on this manuscript is required. The manuscript has not been ready to be published.
Comments and suggestions to the second version of the manuscript are given as following:
Line 15-18: “In this regard, we elucidate here via critical experimental analysis that the cumulative contribution of transformation-induced plasticity (TRIP) and microstructural constituents that governed high strength-high elongation in 0.2C-3Al-(6-8.5)Mn-Fe steels.” What do authors mean with this statement? Please carefully check grammar and expression of this sentence, as well as the whole manuscript. Line 37-39: “The steels … is considered as conventional low alloyed steels.” Check grammar please. Line 39-40: “The low-alloyed TRIP steels consist of several kinds of phases. The dominant phase is ferrite, and other constituents are retained austenite, bainite and martensite.” Do authors consider ferrite, martensite and bainite different kinds of phases? Then according to authors what is the definition of phase? Line 41-42: “While the high manganese twinning induced plasticity (TWIP) steels with PSE of ~60±10 GPa% are generally completely austenite.” How comes this is only half of a sentence? Line 43-55: are completely missing on the manuscript. Line 46: As they indicated in the cover letter, authors define “long holding time means enough holding time. For the hot-rolled sheets with 4 mm thickness, 1 hour is enough.” Could authors please confirm their definition? And do they consider this is a good definition for scientific studies? Line 60-62: “More recently, …was obtained by Merwin” Please check the grammar! Line 66-78: This paragraph is so confused. First, authors describe the heat treatment for “conventional TRIP steel”. Then they mention “the Q&P process”. Finally, they address the “austenite reverted transformation (ART) is the process …”. So readers would not understand what authors talking about. Line 80-83: “The model suggest …” contains simple but really annoying grammatical error. Line 81-82: Could authors confirm that “austenite is stabilized by aluminum enriched in this phase”? Line 117-118: How could authors investigate “the TEM samples with ~40 μm thickness …”? Line 124-126: What are the differences among Iα, Iγ, Rγ and Rα? They are all called integrated intensity in the manuscript. Line 159-160: “The two kinds of ferrite, austenite and martensite differed in morphology, but also in microhardness …” How do authors differentiate α-ferrite, δ-ferrite, austenite and martensite in microstructures by Vickers micro-hardness values? Do they consider the size of indentation with the size of the microstructure features? Line 165-167: Authors conclude “… the theoretical predictions are in a good agreement with the experimental results.” However, they also mentioned that Thermo-Calc and equilibrium thermodynamics were used to calculate volume fraction and chemical composition of austenite. Additionally, they admitted the redistribution/partitioning of Mn between ferrite and austenite during intercritical annealing is necessary to enhance the stability of austenite at room temperature. So the questions are how long the annealing time and how high the annealing temperature need to be for the equilibrium condition of austenite, as indicated by simulation results. If authors cannot confirm the equilibrium condition of intercritical austenite, then it will be meaningless to compare and draw the conclusion on “the good agreement” between simulation and experimental results. Line 181-182: The results shown in Figure 4 are not well explained, although it was requested for the revised version of the manuscript. On the cover letter, authors mentioned these are XRD measurement results. However, this explanation brings more uncertainties to the study. How could authors quantitatively measure the volume fraction of austenite, ferrite and martensite on XRD patterns, particularly the last two microstructure features? Line226-233: In this paragraph, authors argue the differences between quenched martensite and deformed martensite on optical micrographs. They correlate these two different martensite microstructures to the gray level of the features, which in turn links to the carbon content of the martensite/austenite. Nevertheless, the gray level of features in optical micrographs is mostly dependent on the topology meaning the flatness, the height and inclination to direction of light. Unless authors can show the link between topology of phases/microstructure features and their chemical composition, it would be not convinced to conclude “carbon element can effectively improve the stability of austenite”. Line 262-263: “… the fluctuations in the stage III is …” has grammatical error.
Author Response
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Reviewer 2 Report
The authors satisfactorily responded to the concerns listed in the first review report. Recommendation for publication of the manuscript can thus now be given.
Author Response
The viewer recommend acceptance and publication.
Reviewer 3 Report
The manuscript was significantly improved. It could be considered to publish.
Author Response
The viewer recommend acceptance and publication.
Round 3
Reviewer 1 Report
The third version of the manuscript metals-594277 has been submitted to Metals journal following revision requests on the first and the second ones. As compared to previous versions, the third one is rather better to follow. However, grammar and language errors are still visible. Ambiguous expressions and subjective explanations make the study inconvincible. Although the report was requested for major revision twice, its content is not significantly improved. All corrections and modifications applied to the manuscript up to now are mainly for typo and grammar errors. There is very little enhancement on the scientific content of the report. Authors replied to questions and suggestions from reviewers by cover letters, but did not tend to improve the manuscript effectively for general readers. The report still misses key factors for scientific reports to be fully accessible and reproducible. Additionally, it seems could not be further improved, even after the second revisions. Therefore, I could not agree with the suggestion that the manuscript is highly qualified for publication. Detailed comments on the third version of the manuscript are given as following:
Line 40-42: “While the high manganese twinning induced plasticity (TWIP) steels with PSE of ~ 60±10 GPa% are generally completely austenite.” This statement was suggested to be grammatically checked. Authors confirmed there was no error or grammatical mistake. So what is the purpose for the conjunction “While” at the beginning of this statement? Line 54-66: What do authors mean with this paragraph? They should clarify their purposes with this paragraph to readers, rather than arguing to reviewer(s). Line 63: “The heat treatments discussed above requiring cold rolling …” Authors did not mention anything on cold rolling previously, then there is no point to comment on this process. Line 71: “… it further suggest that a temperature exist …” Is this statement grammatically correct? Line 75-77: “Finally, the billets … cooled to room temperature in air.” If authors consider the subsequent intercritical annealing at 650-800℃ is the key step to produce their steels, then finish rolling temperature and the cooling rate after the hot rolling are also important parameters. Anyhow, they need to mention these parameters here to complete the manufacturing procedure. Line 87-88: “Carbon diffused from ferrite to austenite during tempering, which enhanced the stability of austenite, leading to superior ductility.” The materials were intercritically annealed at 600-850℃ during 1h before this tempering. So why do C atoms diffuse from ferrite to austenite during tempering, instead of during the intercritical annealing? Additionally, the C concentration in ferrite, particularly at low temperatures, is negligibly small. Then how could the carbon diffusion from ferrite to austenite during tempering stabilize the austenite? Line 96: “The TEM samples with ~40μm thickness …” could be understood as the initial thickness of the sample. However, could authors justify which is more important between the initial and the final thickness of the TEM samples? Line 104-105: “Rγ and Rα are the standardization constant of austenite and ferrite, respectively” What are these standardization constants specifically? Authors should not use ambiguous arguments and subjective statements for scientific documents. Line 136-138: “The two kinds of ferrite, austenite and martensite differed in morphology, but also in microhardness” It remains unclear how authors could differentiate ferrite, austenite and martensite by microhardness measurements on the microstructures as shown in Figure 2, for instance. Moreover, do they really consider the sizes of microstructure features with the size of Vicker indentation? Line 157-158: Results of X-ray diffraction experiments only show diffraction peaks for the α- and γ-Fe crystallographic structures. If authors could not differentiate ferrite and martensite on the same results, then there will be no point to show the volume fractions of ferrite and martensite separately as given in Figure 4. Line 202-209: Comments on this paragraph of the previous version are still valid. It is inconvincible to conclude that “carbon element can effectively improve the stability of austenite” just only by different light contrast of martensite. Line 237-238: “The volume fraction of retain austenite in 6Mn-700 sample was similar to a few previously studied steels.” Could not be the reason for “the fluctuations in the stage III …” Line 246: “… and second because was larger amount of austenite …” is grammatically error.