Microstructure and Continuous Cooling Transformation of an Fe-7.1Al-0.7Mn-0.4C-0.3Nb Alloy

Round 1

Reviewer 1 Report

Authors advertise cooling dilatometry tests in abstract, they mention cooling dilatometry tests in text, and they write about the obtained curves from cooling dilatometry tests in discussion but no curves from cooling dilatometry tests are presented. Authors presented only results from heating. Curves from cooling dilatometry tests needs to be added otherwise the text needs to be significantly rewritten.

Otherwise, the article is nicely written and a detailed analysis of the resulting structures after heat treatment is done.

 

Author Response

Comment nr 1: Authors advertise cooling dilatometry tests in abstract, they mention cooling dilatometry tests in text, and they write about the obtained curves from cooling dilatometry tests in discussion but no curves from cooling dilatometry tests are presented. Authors presented only results from heating. Curves from cooling dilatometry tests needs to be added otherwise the text needs to be significantly rewritten. Otherwise, the article is nicely written and a detailed analysis of the resulting structures after heat treatment is done.

 

Answers and considerations for comment nr 1: We thank the reviewer comment. Yes, it is coherent for the enrichment of the manuscript. The cooling curves for all rates analyzed from the soaking temperature of 1250 °C applied in this study are shown in Figure 8. For the alloy under study, there were small deviations from linearity. I have highlighted those curves that were meticulously evaluated using two different analysis methods: the tangent method, where the transformation will start or end when the tangent line coincides with the values ​​of the dilatometry curve as a function of temperature; and in the derivative method, where changes occur in the curve of the second derivative are the points where the phase transformations occur. The alloy under study shows a complex phase transformation resulting in very subtle transitions, which required a lot of care and the application of these two analysis methods. ZHANG et al. (2018) [52], for alloys with similar composition (Fe-10Mn-xAl-yC (x=3, 6, 9.12%w., and y=0.2; 0.4; 0.8 and 1.2%w), describe the difficulties encountered in interpreting the dilation effect on dilatometry curves. Thus, modifications were made to the manuscript highlighted on page 9 lines 234 to 240 in addition to the addition of Figure 8.

The manuscript was carefully reviewed by all authors and implementations were added and improve the quality. Besides the revision made by Ronaldo Sérgio de Biasi as I highlighted in the acknowledgments. Ronaldo has vast experience in international academic publishing (http://lattes.cnpq.br/6179004286433720).

Author Response File: Author Response.pdf

Reviewer 2 Report

Fig. 8: How did you determine the green curve? It is related to BCT martensite, but it is also present in the area of the diffusional decay. Hardness values should be assigned to individual cooling curves. NbC is not marked in Fig. 8. The BCC martensite is expected to form just below the red curve. If you take into account partitioning of carbon between delta ferrite and austenite this M_s temperature seems to be very high. Could you document some cooling curves (dilatometric anomalies) which were used for the construction of the CCT diagram?

NbC phase in steels is usually stable below 1150 °C. In your alloy Thermocalc indicates its stability up to 1400 °C. Did you observe this phase in your alloy after heating to 1250 °C? You should characterize the size and distribution of NbC particles for individual cooling rates. How does NbC precipitation affect phase transformations in the alloy investigated?

Fig. 4, row 163: ..k-carbide was formed in high relief (smooth) regions... Is it not retained austenite? In Figs. 4a-d there are markers and in Captions are magnifications. The real magnifications of Figures do not correspond to values stated in captions.

Page 3, row 104: Are you sure that acquisition parameters for X-ray diffractograms are correct (more than 24 days)? Why diffractograms did not reveal tetragonality of BCT martensite (1.07)? 

You have to check English: Fig. 1: microconstituint, Page 2, row 57: cement (-ite), row 200: CCC structure (BCC), row 190 This choice of this cooling rate choice..., etc.

In some cases your interpretations of austenite decomposition products seem to be only intuitive. You should use more advanced techniques for unambiguous identification of decay products.

 

 

Author Response

Comment nr 1: Fig. 8: How did you determine the green curve? It is related to BCT martensite, but it is also present in the area of diffusional decay. Hardness values should be assigned to individual cooling curves. NbC is not marked in Fig. 8. The BCC martensite is expected to form just below the red curve. If you take into account the partitioning of carbon between delta ferrite and austenite this Ms temperature seems to be very high. Could you document some cooling curves (dilatometric anomalies) which were used for the construction of the CCT diagram?

 

Answers and considerations for comment nr 1: We appreciate the reviewer's comment. We apologize about it, there was a misconception, and we have fixed that. Modifications were performed in Figure 9 such as the separation between the dashed and solid green line, in addition to highlighting S, F, Ms and MF markers in each curve that indicate: S=Eutectoid decomposition start temperature (a+k)e; F= Eutectoid decomposition finish temperature; Ms = Martensitic transformation start temperature; Mf = Martensitic transformation finish temperature. The solid green line corresponds to the end of the eutectoid (a+k)e microconstituent formation process. To facilitate understanding, I put an S for start referring to the beginning of the diffusional transformation, and an F for the end of the diffusional transformation that occurs at the rate of 1 and 3 °C/s. In the green dotted part, it would possibly be the region where BCT martensite began to form, but this transformation temperature needs to be verified in future works. These conclusions were obtained after comparing the results of the cooling curve treatment (Figure 8) with the microstructure analysis via EBSD. The EBSD technique, because it is very precise, allowed us to observe the presence of regions with BCC and BCT structures, which, due to the morphology of the phases, were martensite. These structures have differences between the edges, wherein the BCC structure we have a=b=c and in the BCT structure a=b¹c, which result in different diffraction patterns that can be identified via EBSD that has high resolution. This variation of the edges promotes a variation of the volume and with that, it allowed a subtle change of the linearity in the cooling curve from where there is an indication that the curve dashed in green corresponds to the formation of the BCT martensite. First, the formation of BCC martensite (a') below the red line occurs, because a lower C content in the austenite results in higher M_i values. However, even with a thorough analysis of the cooling curves, it was not possible to determine precisely whether the dotted green line is the beginning of the formation of BCT martensite (a''), but it can be inferred, due to its higher carbon content, that its M_i temperature is lower than that of BCC martensite. The blue line represents the finish of the martensitic transformation. I also added a M_s which would be the temperature at which martensite formed and M_f which would be the finish of martensite formation. The changes are highlighted in figure 9 and hope that it has improved the interpretation of the continuous cooling diagram of the alloy under study.

 

Figure 9. Continuous cooling transformation diagram (CCT) obtained for the Fe‑Mn‑Al‑C alloy, highlighting the phases present in each region (g_r = retained austenite; d = d‑ferrite; a = a‑ferrite ; a_p = pro eutectoid a‑ferrite; k = k-carbide; (a+k)_e = eutectoid microconstituent; a' = BCC martensite; a'' = BCT martensite; NbC = niobium carbide; S = Eutectoid decomposition start temperature (a+k)_e; F= Eutectoid decomposition finish temperature; M_s = Martensitc transformation start temperature; M_f = Martensitc transformation finish temperature).

As for the consideration of the high temperature of martensite formation, we must note that the alloy under study does not have a solid solution typical of conventional steels. It is a solid solution that has high levels of aluminum, Besides the addition of niobium, which promotes phase transformations and different behaviors from conventional steels. Carbon is not the only element that causes the martensite starting temperature to change. Phase stability is defined as a function of both austenite stability and the alloying elements present. Therefore, the chemical composition evaluated allows not only carbon to influence the martensite formation temperature, which promoted a high Ms temperature.

Thus, modifications were made to the manuscript highlighted on page 9 lines 234 to 240 in addition to the addition of Figure 8.

 

Figure 8. Cooling curves for all samples analyzed in the dilatometry test

No manuscripts were found in the literature that presented dilatometric cooling curves or continuous cooling diagram that presented this same behavior or had similar chemical composition to be compared. This indicates that this work will be very interesting to aid future research on this alloy system.

 

Comment nr 2: NbC phase in steels is usually stable below 1150 °C. In your alloy Thermocalc indicates its stability up to 1400 °C. Did you observe this phase in your alloy after heating to 1250 °C? You should characterize the size and distribution of NbC particles for individual cooling rates. How does NbC precipitation effect phase transformations in the alloy investigated?

 

Answers and considerations for comment nr 2: We kindly thank the reviewer for attention. The NbC phase is stable below 1150°C generally in conventional steels, but I point out that the study was done on an alloy with a different chemical composition, which has certain alloying elements that completely change the stability field of the phases present. For example, in conventional steels, one does not see such a significant amount of austenite together with ferrite at high temperatures. The precipitation of NbC can influence the phase transformation because it consumes carbon for its precipitation, which interferes with the formation of carbide k. It can also influence the homogeneity of austenite in the regions near its precipitation, which can interfere with austenite phase transformations. However, it was not the focus of this work to evaluate in detail the size, distribution, and consequences of NbC on the microstructure.

 

Comment nr 3: Fig. 4, row 163: ..k-carbide was formed in high relief (smooth) regions... Is it not retained austenite? In Figs. 4a-d there are markers and in Captions are magnifications. The real magnifications of Figures do not correspond to the values stated in the captions.

 

Answers and considerations for comment nr 3: For a better understanding of the passage of text highlighted in your comment, it has been rewritten as presented on lines 164 and 165.

As for the magnifications, they have been checked and are correct. It can be seen that Figure 4(b) actually corresponds to the magnification highlighted with the red square in Figure 4(a). The lower edge of the red square in Figure 4(a) equals 50 mm and matches the scale of Figure 4(b), which has a width of approximately 5 times the scale bar of 10 mm. The SEM equipment used goes through constant calibrations to avoid equivocal values in the image scales. We apologize for it. We have improved and modified the figures.

 

Comment nr 4: Page 3, row 104: Are you sure that acquisition parameters for X-ray diffractograms are correct (more than 24 days)? Why diffractograms did not reveal the tetragonality of BCT martensite (1.07)?

 

Answers and considerations for comment nr 4: We appreciate the reviewer attention. We appreciate the reviewer's attention. We apologize for the mistake, there was a typo and the correct time value per step is 2.4 s.

The interplanar distance in similar structures such as BCC and BCT is very next and when using wavelength from cobalt, and copper radiation, there is not enough resolution in conventional X-ray diffraction equipment to distinguish the diffraction peaks of these structures. To differentiate these similar structures or phases with the same structure, it is necessary to apply the X-ray diffraction technique using synchrotron accelerator with particular configurations. BAÊTA JÚNIOR (2020) describes that it was possible to verify through high-resolution diffraction with synchrotron accelerator, that there are differences between the lattice parameters of ferrites a and d, with ferrite d having a higher lattice parameter, even though both have BCC structure. However, due to the diffraction angles of these structures differing in centesimal places, there is possibly an overlap of the diffraction peaks of these phases (BCC and BCT) which does not allow a clear view of the tetragonal martensite peaks in the configurations in which the analysis was performed. analysis (Conventional diffractometer and cobalt tube).

 

Comment nr 5: You have to check English: Fig. 1: microconstituent, Page 2, row 57: cement (-ite), row 200: CCC structure (BCC), row 190 This choice of this cooling rate choice..., etc.

 

Answers and considerations for comment nr 5: We kindly thank the reviewer for attention. The authors have revised the manuscript and improved the quality.

 

Comment nr 6: In some cases, your interpretations of austenite decomposition products seem to be only intuitive. You should use more advanced techniques for the unambiguous identification of decay products.

 

Answers and considerations for comment nr 6: We thank the reviewer comment. All analyses, results and conclusions are substantiated and compared with the literature for alloys of the Fe-Mn-Al-C system. An extensive literature review was carried out with comparisons regarding the characterization and morphology of the present phases. Techniques such as EBSD were used to avoid doubts in certain phases. This technique confirmed the presence of the BCC and BCT structure in addition to the other phases present in this new chemical composition, since it is a precise and high-resolution technique.

 

The manuscript was carefully reviewed by all authors and implementations were added and improve the quality. Besides the revision made by Ronaldo Sérgio de Biasi as I highlighted in the acknowledgments. Ronaldo has vast experience in international academic publishing (http://lattes.cnpq.br/6179004286433720).

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper deals with the evaluation of an alloy belonging to the Fe-Mn-Al-C family. A number of dilatometric tests covering a large domain of cooling rates (from -1°C/s to -50°C/s) have been carried out using a Gleeble machine. A Continuous Cooling Transformation diagram is established which will be of interest for the analyses of mechanical behavior of structures subjected to high temperature operations like welding and heat treatments. 

The paper is well organised with a good description of the adopted methods leading to a clear presentation of the results.

Just one detail: improve certain illustrations in order to be better readable  

Author Response

Comment nr 1: The paper deals with the evaluation of an alloy belonging to the Fe-Mn-Al-C family. A number of dilatometric tests covering a large domain of cooling rates (from -1°C/s to -50°C/s) have been carried out using a Gleeble machine. A Continuous Cooling Transformation diagram is established which will be of interest for the analyses of mechanical behavior of structures subjected to high temperature operations like welding and heat treatments.  The paper is well organised with a good description of the adopted methods leading to a clear presentation of the results. Just one detail: improve certain illustrations in order to be better readable

 

Answers and considerations for comment nr 1: We thank the reviewer comment. We have made some changes to the Figures. I hope it was enough to make them more readable.

The manuscript was carefully reviewed by all authors and implementations were added and improve the quality. Besides the revision made by Ronaldo Sérgio de Biasi as I highlighted in the acknowledgments. Ronaldo has vast experience in international academic publishing (http://lattes.cnpq.br/6179004286433720).

Author Response File: Author Response.pdf

Reviewer 4 Report

This paper focused on the possible phase transformations and a transformation diagram of Fe-0.77Mn-7.10Al-0.45C-0.31Nb low-density steel during continuous cooling at different cooling rates. A Gleeble thermomechanical simulator with different cooling rates (1, 3, 5, 10, 15, 20, 30, and 50 °C/s) was performed to test the dilatometry of the low-density steel. However, the quality of the paper is poor, which conducted to the suggestion of rejection. The main comments are given below:

1. The authors must have this paper reviewed by someone with expertise in technical English editing before submission. Some sentences contain grammatical mistakes, and some sentences are too long to be understood.

2. The description in Introduction lacks of logic. The subject of the article is about the continuous phase transformation behavior of low-density steel at different cooling rates, but the authors did not discuss the research progress on this phase transformation behavior. Moreover, the purpose of conducting continuous phase transition studies at this temperature is not clear.

3. Lines 238-240, “ it can be inferred that proeutectoid a-ferrite (a_p) formation occurs at about 1080 °C”, how to find it? Please explain clearly.

4. Line 215, “niobium carbide (NbC) is also observed”, the corresponding EDS information needs to be provided. In addition, according to Fig. 9 (b), the NbC began to precipitate at about 1400 °C and could be present stably up to room temperature. However, they were not observed in the sample tested at cooling rates of 1 °C/s and 3 °C/s, what was the reason?

5. Lines 258-262, the BCC martensite and BCT martensite formed in different temperature ranges. How to reach this conclusion, please explain in detail.

6. Line 281, “The high Al content of the alloy may be the main reason for shifting the austenite single-phase field for higher C content.” The alloy composition of the low-density steel in this work was Fe-0.77Mn-7.10Al-0.45C-0.31Nb without any change, and Fig. (b) was an equilibrium volume fraction-temperature diagram. So how to come to this conclusion? Line 286, “The single-phase field from austenite is shifted to the right…” What was the right? This expression was very inaccurate.

7. The discussion in Section 4.2 was illogical and contained too many descriptions of experimental phenomena that should have appeared in the Results section. This section discussed the effect of alloy elements on the microstructure, but this did not seem to be the focus of the article. The authors should analyze the effect of different cooling rates on the microstructure in detail.

8. Line 402, “The authors pointed out that there was no Nb presence in the k-carbide.” How did the authors determine this phenomenon?

9. Line 455, the effect of Nb on the phase transformation of low-density steels was not obvious in this work, because all the samples have the same Nb content. It is recommended to set Nb-free samples for comparative experiments.

Author Response

Comment nr 1: The authors must have this paper reviewed by someone with expertise in technical English editing before submission. Some sentences contain grammatical mistakes, and some sentences are too long to be understood.

 

Answers and considerations for comment nr 1: We thank the reviewer comment. The manuscript was carefully reviewed by all authors and implementations were added and improve the quality. Besides the revision made by Ronaldo Sérgio de Biasi as I highlighted in the acknowledgments. Ronaldo has vast experience in international academic publishing (http://lattes.cnpq.br/6179004286433720).

 

 

Comment nr 2: The description in Introduction lacks of logic. The subject of the article is about the continuous phase transformation behavior of low-density steel at different cooling rates, but the authors did not discuss the research progress on this phase transformation behavior. Moreover, the purpose of conducting continuous phase transition studies at this temperature is not clear.

 

Answers and considerations for comment nr 2: We apologize about it. The research on this new chemical composition is still in its early stages and it is necessary to understand the behavior of the alloy regarding phase transformations. This Fe-Mn-Al-C system has a distinct behavior from conventional steels because the addition of these elements promotes more complex underlying metallurgical issues, such as the formation of the k carbide. Because it is a distinct composition from those found in the literature for contains high aluminum content. Besides the addition of niobium, we sought to evaluate which phases would be present after the application of different thermal cycles. This information will be important to help other works that are being developed by the same research group. This will allow the definition of parameters for processing that will make it possible to obtain a microstructure that will bring interesting results for future applications of the alloy. To aid the understanding of the manuscript the following explanation has been added on page 2, lines 96 and 97: 'Since the information obtained from the thermal cycles applied will serve as a basis for the analysis of the hot behavior of this new composition in future work'.

 

Comment nr 3: Lines 238-240, “ it can be inferred that proeutectoid a-ferrite (ap) formation occurs at about 1080 °C”, how to find it? Please explain clearly.

 

Answers and considerations for comment nr 3: Our apologies for not being clear. We kindly thank the reviewer attention. For the alloy under study, a thermal cycle was applied with a soaking temperature of 1250 °C. After this soaking, cooling was performed at a rate of 1 °C/s until a temperature of 850 °C, and from this temperature on, the different cooling rates were applied. This thermal cycle was defined under these conditions in order to help analyze the hot behavior in future work. By thoroughly evaluating the cooling curves obtained in the dilatometry test (Figure 8) from the soaking temperature of 1250 °C to the temperature of 850 °C, an inflection in all curves was observed near the temperature of 1080 °C. When confronting the presence of this inflection with the Thermo-Calc results obtained and the final microstructures, it is inferred that the chemical composition of the alloy and the thermal cycle conditions evaluated allowed the formation of a proeutectoid ferrite at this temperature. This is evident by the presence of low relief regions amidst the martensite regions in the samples where high cooling rates were applied, highlighted as a_p in Figure 7 (page 8 - line 220). These low relief regions are present amidst previous grains of austenite. Thus, cooling applied between 1250 °C to 850 °C has allowed the formation of proeutectoid ferrite, before the onset of martensite formation. The formation of these low relief areas among the previous austenite grains has been observed by Jeong et al. [37] for hot-rolled samples of a Fe 9Mn 6Al 0.15C alloy. Jeong et al. [37] conclude that the low relief regions among martensite and retained austenite are composed of proeutectoid ferrite formed before the martensitic transformation onset (page 13 - lines 347 to 341).

 

Comment nr 4: Line 215, “niobium carbide (NbC) is also observed”, the corresponding EDS information needs to be provided. In addition, according to Fig. 9 (b), the NbC began to precipitate at about 1400 °C and could be present stably up to room temperature. However, they were not observed in the sample tested at cooling rates of 1 °C/s and 3 °C/s, what was the reason?

Answers and considerations for comment nr 4: We kindly thank the reviewer attention. This work was performed in conjunction with a research group on Fe‑Mn‑Al‑C system steels in which another researcher cited throughout the manuscript performed the EDS analyses. The information on EDS is described by BAÊTA JÚNIOR (2020) [20] who evaluated samples of the same chemical composition of the alloy under study and confirms that regions with the same morphology and characteristics are NbC.

The alloy was not subjected to temperatures above 1250 °C so there was no total dissolution of precipitates that occurs at 1400 °C so these precipitates are also present at rates of 1 and 3 °C. At the 1 and 3 °C rates, because they are slower, they allowed this precipitate to be present in a more refined form. This may be below the detection limit for the X-ray diffraction technique and thus no peaks referring to this precipitate were observed in the diffractograms in Figure 5. In the micrographs, there was a mistake, but I have highlighted in Figure 4 and in the legend the NbC precipitates. Very well observed you are completely right, the necessary highlights were made.

 

Comment nr 5: Lines 258-262, the BCC martensite and BCT martensite formed in different temperature ranges. How to reach this conclusion, please explain in detail.

 

Answers and considerations for comment nr 5: Our apologies for not being clear. After the dilatometry test was performed, the cooling curves were analyzed, and with this, variations of the slope were subtly observed, which indicates that some phase transformation occurred in that region. By observing the micrographs obtained by optical microscopy (Figure 2), lenticular regions with intermediate and high cooling rates are observed. These regions are probably martensitic due to the conditions applied in the dilatometry test. However, this phase may exhibit a BCC or BCT structure. Since the peaks of these structures overlap in the X-ray diffraction analysis, and EBSD/SEM analysis was performed (Figure 6) to verify the nature of the phases and their arrangement in the microstructure. The EBSD/SEM was performed on the sample obtained after the dilatometry test for a cooling rate of 50 °C/s. The choice of this cooling rate was based on the higher expected martensite fraction at the expense of the other phases that could also be present. In this way, martensite can be much more evident at this 50 °C/s rate, allowing for better identification by EBSD analysis and thereby correlating with the other results obtained (page 6 lines 85 to 95). When the EBSD analysis is performed using only the BCC plug, it was observed the presence of large areas without any phase indexation.  Besides that, adding the plug referring to the BCT structure is identified in the previously non-indexed regions the presence of this structure as shown in figure 6. Thus, when comparing all the results it was concluded that the formation of martensite with BCC and BCT structure would occur in the alloy under study.

It is known that the kinetics of martensitic transformation is influenced by the chemical composition and the homogeneity of the alloying element, especially C, in the austenitic grains. In addition, the influence of the cooling rate and the austenite grain size can affect the martensite formation temperature. This martensite with a BCT structure is formed when the alloy has high C concentrations, generally above the solubility limit of this element in the BCC structure [39,43-46]. When confronting the observations cited with the results of the alloy under study, there are indications that martensite formation with BCC (a') and BCT (a'') structures come from a compositional heterogeneity in the volume of the austenite grains. The time interval of 300 s at the soaking temperature of 1250 °C was insufficient for the complete homogenization of the alloying elements in the austenite grains. since the alloy has a high content of Al and Nb and a medium C content. According to Da Silva De Souza, Moreira, and De Faria [44] and Van Bohemen [46], temperature and soaking time do not directly affect the martensite transformation kinetics. However, the changes produced in soaking, either in the austenite chemical composition (phase precipitation/dissolution) or in the austenite grain size, influence martensite formation and must be considered (page 13 - lines 394 to 408).

Thus, firstly, martensite formation BCC (a') takes place below the red line, because a lower C content in the austenite results in higher Mi values. However, even with a thorough analysis of the cooling curves, it was not possible to determine precisely, if the dotted green line is the beginning of martensite formation BCT (a''), but it can be inferred from its higher carbon content that its Mi temperature is lower than that of BCC martensite. The blue line represents the end of the martensitic transformation.  Some modifications have been made in Figure 9 and I hope that I have been clear as to your doubts and any other information you wish I am at your disposal by the e-mail described in the manuscript.

 

Comment nr 6: Line 281, “The high Al content of the alloy may be the main reason for shifting the austenite single-phase field for higher C content.” The alloy composition of the low-density steel in this work was Fe-0.77Mn-7.10Al-0.45C-0.31Nb without any change, and Fig. (b) was an equilibrium volume fraction-temperature diagram. So how to come to this conclusion? Line 286, “The single-phase field from austenite is shifted to the right…” What was the right? This expression was very inaccurate?

 

Answers and considerations for comment nr 6: Our apologies for not being clear. The material under study has high aluminum content and it is known from the literature that aluminum is a ferrite stabilizer. Aluminum expands both the ferrite and ferrite region, and restricts the austenite region in the phase diagram, thus suppressing austenite formation. It also contributes to the increase in the onset temperature of martensite formation. The manuscript by Chen [1], shows the effects of this addition of Al to the Fe-C system that has a large effect on the phase fields and phase constituents. The addition of aluminum promotes a shift of the austenite field to the right. The two-phase field region (d+g) is expanded and is present between intermediate contents of 0.1 to 0.7 wt.% C. The single-phase field from austenite is shifted to the right as the temperature and the C concentration are increased [1,24,25] (page 11 - lines 299 to 303). All these conclusions were based on and supported by an extensive literature search.

 

Comment nr 7: The discussion in Section 4.2 was illogical and contained too many descriptions of experimental phenomena that should have appeared in the Results section. This section discussed the effect of alloy elements on the microstructure, but this did not seem to be the focus of the article. The authors should analyze the effect of different cooling rates on the microstructure in detail.

 

Answers and considerations for comment nr 7: Our apologies for not being clear. As the steel under study presents high contents of alloying elements promoting different transformations from conventional steels, it is necessary to evaluate the dissolution of alloying elements in the phases present, in order to understand the effect of cooling rates. The additions of these elements promote more complex underlying metallurgical issues, such as the formation of k carbide. There are major consequences of these elements on the thermodynamic stabilities of the iron allotropes. Therefore, understanding the influence of the alloying elements is critical, because from this knowledge it is possible to determine phases that may be present in the alloy. These phases can directly influence the production, processing, and use of these steels. As described in the literature and highlighted between lines 394 and 408, the kinetics of martensite transformation is influenced by the chemical composition of the alloy and the compositional homogeneity of the austenite, in addition to the cooling rate and grain size of the austenite. Because of the high content of alloying elements, the soaking temperature of 1250 °C for 300 s was not enough to homogenize the austenite and thus the formation of distinct martensite structures was observed. Another factor that influences this austenite homogeneity is the NbC precipitation, which promotes a concentration gradient near the precipitate interface. This gradient can reach a larger or smaller region depending on the stage of carbide formation or dissolution. Therefore, it is very important to understand the effects of alloying elements to understand the phase transformations at the different cooling rates applied.

 

Comment nr 8: Line 402, “The authors pointed out that there was no Nb presence in the k-carbide.” How did the authors determine this phenomenon?

 

Answers and considerations for comment nr 8: Our apologies for not being clear. This analysis was not done by the authors of this manuscript. It was described that in the reference by Khaple et al. [32], who evaluated a steel with similar chemical composition (Fe-0.35C-7Al (wt.%)) with different Nb concentrations (0.2; 0.4; 0.7 and 1 (wt.%). Khaple et al. [32] observed that there was no niobium present in the carbide k, as described on page 13 - lines 416 to 419. Changes have been made in the text to make it clear that this conclusion is not an analysis made by the authors of this manuscript.

 

Comment nr 9: Line 455, the effect of Nb on the phase transformation of low-density steels was not obvious in this work, because all the samples have the same Nb content. It is recommended to set Nb-free samples for comparative experiments.

 

Answers and considerations for comment nr 9: The authors thank the reviewer for comments. Manufacture a specimen amount with Nb-free in the short and medium term for comparative experiments it would be unfeasible However, the objective of this work was not to observe the effects of niobium, although it is an alloy with added niobium. The focus of the work was to understand the possible phase transformations from this new chemical composition. The results allowed us to offer to the academic community something new, which is a continuous cooling diagram for the Fe Mn Al C system, contributing to future work on similar compositions. Alloys with different niobium contents are already being developed to be analyzed with a focus on the niobium effect in future works.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Answer to comment nr. 1: Dilatometric curves prove that dilatometric anomalies related to individual products of austenite decay are very subtle. It is very difficult to accurately determine temperatures of transformations.

In captions to Fig. 9 there is NbC which is not marked in any field of the CCT diagram. I do recommend to add to Fig. 9 Table with hardness results for individual cooling rates - it is very common and useful for users of CCT diagrams.

I am aware that carbon is not the only element affecting Ms temperature, but the effect of substitutional elements is much weaker.

Answer to comment Nr. 5: please check again English: Captions of Fig. 1: eutetoid, Fig. 9 martensitc (twice) etc.

Answer to comment Nr. 3: "As for the magnifications, they have been checked and are correct." I am sure that you microscope is calibrated properly. However, when you change the size of the image you will change magnification. It is correct that you crossed out magnifications 500x and 3000x in the captions. Now it is OK.

Answer to comment Nr. 2: I accept that characterization of NbC was not included in your paper. However it is important to know how niobium affects phase transformations in your alloy. Without this knowledge you cannot justify the addition of this element to the alloy.

Author Response

 Reply to reviewers’ comments and queries
Manuscript ID: metals-1808334
Title: Microstructure and continuous cooling transformation of a
Fe7.1Al0.7Mn0.4C0.3Nb alloy
Dear Reviewer,
We would like to acknowledge the valuable comments made by kindly reviewing our manuscript. The quality of the manuscript has been significantly improved with appropriate suggestions/modifications suggested in the reviewers' reports. We are satisfied that the modifications made to the manuscript are sufficiently clear and concise for acceptance.
The manuscript was modified accordingly to the reviewer’s suggestions as following:
Reviewer #2
Answer to comment nr. 1: Dilatometric curves prove that dilatometric anomalies related to individual products of austenite decay are very subtle. It is very difficult to accurately determine temperatures of transformations.Answers and considerations for comment nr 1: We thank the reviewer comment.
In captions to Fig. 9 there is NbC which is not marked in any field of the CCT diagram. I do recommend adding to Fig. 9 Table with hardness results for individual cooling rates - it is very common and useful for users of CCT diagrams. Answers and considerations for comment nr 1: We thank you for your very pertinent comment and apologize that there was a misunderstanding about the NbC in the legend in Figure 9. The correction was made. As for the hardness analysis, microhardness measurements were made with a maximum load of 500 gf with a square diamond penetrator and a permanence time of a maximum of 15 seconds. Ten measurements were randomly collected at each cooling rate, respecting the requirement of distance from the indentations, due to the limited size of the analysis surface with dimensions of 5 mm x 5
mm, in order to estimate mean values of Vickers hardness and their respective standard deviation.
The results obtained for microhardness show a high value of standard deviation. This may be related to factors such as (i) the size of the sample analyzed, since in microhardness the deformation zone can influence up to 7 times the diagonal of the indentation; (ii) the banded structure, because as the analyses were performed randomly the indentation can be performed on the ferritic matrix (soft), between the matrix and the second phase and on the second phase, which can significantly vary the values and result in high standard
deviation values. This fact becomes evident during the analyses when observing
irregularities in the shape of the indentations.

Thus, the idea way to proceed with macrohardness measurements or a much larger number of microhardness measurements with equal or more significant load (respecting the 1 kgf for the technique). However, the size of the dilatometry specimens from Gleeble's tests prevents them from following this procedure option. Therefore, the authors of the paper thought it was essential to suppress this data. But if the reviewer thinks it is important to indicate them as they are and with the justifications, he provides in the answer, the authors will also agree to include them in the article.
I am aware that carbon is not the only element affecting Ms temperature, but the effect of substitutional elements is much weaker.
Answers and considerations for comment nr 1: We thank the reviewer comment.
Answer to comment Nr. 5: please check again English: Captions of Fig. 1: eutetoid, Fig.9 martensitc (twice) etc.
Answers and considerations for comment nr 5: We kindly thank the reviewer attention.
The authors have revised the manuscript and improved the quality.
Answer to comment Nr. 3: "As for the magnifications, they have been checked and are correct." I am sure that you microscope is calibrated properly. However, when you change the size of the image you will change magnification. It is correct that you crossed out magnifications 500x and 3000x in the captions. Now it is OK.
Answers and considerations for comment nr 3: We thank the reviewer comment.
Answer to comment Nr. 2: I accept that characterization of NbC was not included in your paper. However it is important to know how niobium affects phase transformations in your alloy. Without this knowledge you cannot justify the addition of this element to the alloy.
Answers and considerations for comment nr 2: We thank the reviewer comment. For the fusion of this alloy, a thermodynamic study was performed by BAÊTA JÚNIOR (2020) [20], regarding the niobium content. Particularly, due to the promising results indicated in the doctoral thesis of BAÊTA JÚNIOR (2020) [20], we chose the chemical composition that was evaluated in this manuscript. From the thermodynamic analyses, it was found that in this chemical composition it would be very interesting to understand the phase transformations from dilatometry tests and thereby obtain a continuous cooling diagram. These results would help the hot processing of the alloy under study. However,
besides these results being interesting for the IME research group, it would also help the Uff researcher, co-author of this article, who explores materials with different chemical compositions (mentioning with and without the addition of Nb, besides variations in the contents C and Al ) in a similar way in Gleeble. We agree with the reviewer that it is still necessary to do further work focusing on the effects of Nb to justify and understand the addition of this alloying element. 

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors improved the manuscript in a satisfactory way by taking into account all the comments in the reviewer reports. It is suggested to accept the paper for publication. 

Author Response

Dear Reviewer,

We would like to acknowledge the valuable comments made by kindly reviewing our manuscript. The quality of the manuscript has been significantly improved with appropriate suggestions/modifications suggested in the reviewers' reports. We are satisfied that the modifications made to the manuscript are sufficiently clear and concise for acceptance.

Thank you.

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

This manuscript presented the results of experimental tests in a Gleeble thermomechanical simulator with different cooling rates (1, 3, 30 5, 10, 15, 20, 30, and 50 °C/s) of a Fe-Mn-Al-C alloys system with high Al and low Mn contents and addition Nb. The authors have done a valued research work, but to meet the standards of Metals the paper should be major improved. In the paper I found some inaccuracies that should be explained and corrected:

1. The used literature is sufficient for the paper's issues clarification. However each one (two) of the quoted references should be discussed individually and demonstrate their significance to the work. It is not necessary used even ten references in one bracket: [1-10], [6,11–13], [1–3,5,16,17], [1,2,10,15,18], [1,2,8,15,19,21,22].
2. In the sentences we can write: “The k-carbide has a significant influence on mechanical properties.” and “Also, it is necessary to evaluate and understand the phase transformations to obtain a suitable microstructure and, thereby, the desired mechanical properties [23].” That’s why authors should explain which how does the cooling rate effect on the mechanical properties. There is no information about it.
3. The sentence: “The samples were taken from the condition of a hot rolling mill draft.” There are no information about rolling parameters, type of rolling mill, initial dimension of the feedstock, etc.
4. The sentence: “The temperature of 1250 °C refers to the slab reheating temperature, and 850 °C is the exit temperature of the finishing rolling mill.” For what type of rolling mill, plate or bar. For the continuous rolling mill (bar) the initial temperature of feedstock usually is 1150°C and the end rolling temperature is above 900°C. Add some explanation.