In Situ Observation of the Austenite Grains Growth Behavior in the Austenitizing Process of Nb–Ti Micro-Alloyed Medium Manganese Steel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is of high quality and merits publication after minor revisions. The authors are encouraged to:

• strengthen the comparative discussion with relevant literature,

• add more detail on measurement uncertainties and repeatability,

• briefly relate the results to expected mechanical properties.

Comments for author File: Comments.docx

Author Response

Ref: Coatings-3860317

Title: In-situ observation of the austenite grains growth behavior in the austenitizing process of Nb-Ti micro-alloyed medium manganese steel

Journal: Coatings

 

Limited comparative discussion: The article focuses mainly on the authors’ results, with literature comparisons present but not deeply elaborated (mostly „Effect of isothermal temperature on austenite grain growth behavior”).

 

RESPONSE: Thank you for your suggestion. I have added some literature to introduce the current status of austenite grain growth. In the discussion, some literature was also added to verify some of the findings in this article.

 

Insufficient discussion of measurement uncertainties: Grain size values are presented clearly, but there is little discussion of experimental errors and repeatability. there is also no information about the method used to measure the diameter (Table 2)

 

RESPONSE: Thank you for your reminding. I used Gaussian method to fit the average grain size and wrote the standard error in the table. I have added this part to the paper and added a discussion.

 

Lack of mechanical property validation: Although the study suggests microstructural effects on mechanical performance, no mechanical testing (strength, toughness) is provided to support this claim.

 

RESPONSE: Thank you for your suggestion. I have added an analysis of the mechanism by which precipitation phases and grain boundary energies affect grain growth in the discussion, and have also included references to relevant literature.

 

Figure 7 look copied from source material but without citation clearly marked.

 

RESPONSE: Thank you for your reminding. The use of regularly arranged spheres to express lattice and grain boundary structures is a common illustrative method. The images used in this article are mainly to illustrate the differences between the grain growth patterns discovered in this article and other viewpoints.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents an approximation of the austenization effect on Mn steels microalloyed with Nb and Ti. The results are interesting, but to enhance the scientific nature of the journal, it is necessary to clarify or conduct a more in-depth analysis of the following:

It is necessary to indicate how many zones were analyzed in these experiments and the location where the images were obtained, since the microstructure varies from the outside of the deformed bar to the center of the specimen.

It should be noted that the wettability factor is not considered in such a short heat treatment period, and that the results are from the sample surface.

Figures 12 m-p seem interesting since they show a more similar area but with different foci, and the change in grain boundaries is observed. However, it is necessary to review it with the other figures from a to l to show something similar. Please correct and add the corresponding one, as this can contribute to understanding the description in the distribution analysis of Figures 3 and 4 and Table 2.

It is necessary to indicate with microanalysis if precipitates such as Nb and Ti carbides were present and demonstrate, if they did not dissolve, how they affect the kinetics of recrystallization and grain boundary movement due to the Mn content based on equations 1 and 2, since this discussion does not seem to be completely clear.

Comments on the Quality of English Language

Some typographical and writing errors need to be revised

Author Response

Ref: Coatings-3860317

Title: In-situ observation of the austenite grains growth behavior in the austenitizing process of Nb-Ti micro-alloyed medium manganese steel

Journal: Coatings

 

1. It is necessary to indicate how many zones were analyzed in these experiments and the location where the images were obtained, since the microstructure varies from the outside of the deformed bar to the center of the specimen.

 

RESPONSE: Thank you for your suggestion. The sample used in this article is a Φ5*3 mm cylinder, taken from a large steel plate, so the microstructure of the sample is uniform. The principle of this experiment is to use laser to heat the sample in vacuum, and in-situ observe the sample continuously during the heating period. I selected photos from typical times for analysis in the article.

 

2. It should be noted that the wettability factor is not considered in such a short heat treatment period, and that the results are from the sample surface.

 

RESPONSE: Thank you for your reminding. Although the stability of grain growth in solid phases is generally linked to grain boundary energy and wettability (especially in systems involving both liquid and solid phases), studying wettability implies insights into grain boundary energy and stability. However, this study focuses exclusively on the growth behavior of austenite under consistent experimental conditions, without specifically investigating wettability effects.

 

3. Figures 12 m-p seem interesting since they show a more similar area but with different foci, and the change in grain boundaries is observed. However, it is necessary to review it with the other figures from a to l to show something similar. Please correct and add the corresponding one, as this can contribute to understanding the description in the distribution analysis of Figures 3 and 4 and Table 2.

 

RESPONSE: Thank you for your suggestion. The laser confocal experiment used in this article is an advanced real-time laser observation technology. Observing the same location of the sample, characteristic photos taken at different times can be used for analysis. However, comparisons cannot be made between different samples, and I have added sentences to the discussion

 

4. It is necessary to indicate with microanalysis if precipitates such as Nb and Ti carbides were present and demonstrate, if they did not dissolve, how they affect the kinetics of recrystallization and grain boundary movement due to the Mn content based on equations 1 and 2, since this discussion does not seem to be completely clear.

 

RESPONSE: Thank you for your suggestion. This article uses thermodynamic methods to calculate the characteristics of precipitates, and this method is a commonly and effectively used approach for studying this field. Partial recrystallization was observed in the image. Moreover, recrystallization is mainly driven by dislocation density, so local recrystallization is mainly affected by dislocation density. The precipitates has an impact on grain growth and has been added to the discussion.

 

5. Comments on the Quality of English Language. Some typographical and writing errors need to be revised.

 

RESPONSE: Thank you for your suggestion, and I have revised the gramma of manuscript carefully.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The paper is based on carefully made light microscopy measurement of grain structure of Mn-alloyed steel at different temperature. But it seems that the chosen approach does not correspond to the formulated aim of the paper.

1. Such macroscopic observation can not be the instrument for understanding of the mechanisms of grain growth.
2. The grain size distribution data demonstrate contradictory results and must be discussed more accurately.
   • How long the procedure of structure study was and what is number of images was got for each time-temperature point?
   • At each annealing temperature the grain structure just after reaching the temperature of annealing must be added.
   • From tables 2 and 3 one could see the significant fluctuation of mean grain size. How it can be and what is the processes in the system? Otherwise, the statistical error in determination of mean grain size must be corrected.
3. Thermodynamic calculation results demonstrated on figure 5 do not correspond to the text:  3.1 one hardly could see the disapperence on FCC 296°C and any changes with cementite at 605 C.
   • It is better to indicate the temperature range of stability of the phases rather then discuss in term of the process (appearance/disappearance) because we must take into account if it is heating up or cooling down
   • what are the processes which determine such kinetics as it is demonstrated on figure 6. Both picture must be well interpreted but it seems that some mistake in modelling was made.

4. In the paper the measured effect discussed in terms of the role of different GB types and carbide particles. But unfortunately, there are no data, which characterize these factors for chosen samples, thus it is difficult to understand the real roles of the factors.

Comments on the Quality of English Language

The english language must be significantly improved.

Author Response

Ref: Coatings-3860317

Title: In-situ observation of the austenite grains growth behavior in the austenitizing process of Nb-Ti micro-alloyed medium manganese steel

Journal: Coatings

 

1. The paper is based on carefully made light microscopy measurement of grain structure of Mn-alloyed steel at different temperature. But it seems that the chosen approach does not correspond to the formulated aim of the paper.

 

RESPONSE: Thank you for this comment. The sample used in this article is a Φ5*3 mm cylinder, taken from a large steel plate, so the microstructure of the sample is uniform. The laser confocal experiment used in this article is an advanced real-time laser observation technology. Observing the same location of the sample, characteristic photos taken at different times can be used for analysis, and this method is a commonly and effectively used approach for studying this field. This article studied the grain growth behavior of medium manganese steel during the heating process before rolling. The results shows when the grain boundary is a small angle grain boundary, one grain boundary will split into several dislocations. With the extension of heating time, the lattice orien-tation difference further decreases, and the remaining dislocations may merge into new grain boundaries. When the grain boundary is a large angle grain boundary, only grain boundary movement can occur.

 

2. Such macroscopic observation can not be the instrument for understanding of the mechanisms of grain growth.

 

RESPONSE: Thank you for this comment. The principle of this experiment is to use laser to heat the sample in vacuum, and in-situ observe the sample continuously during the heating period. I selected photos from typical times for analysis in the article. Although the structure of grain boundaries cannot be observed, observing millimeter sized grains is already within the scope of microscopic observation. Other experimental methods are difficult to achieve this function, although they cannot observe more microstructures. Combining thermodynamic calculations, we can reasonably infer the form of grain growth through this experiment, and then make reasonable inferences based on existing theories. The results obtained are reasonable.

 

3. The grain size distribution data demonstrate contradictory results and must be discussed more accurately.

 

RESPONSE: Thank you for your suggestion. I used Gaussian method to fit the average grain size and wrote the standard error in the table. In the statistical data presented in this article, most of the trends are reasonable, while a small portion are due to the statistical results being unreasonable. However, this difference is very small and can be considered to be due to similar grain sizes. I have carefully described and improved this part.

 

4. How long the procedure of structure study was and what is number of images was got for each time-temperature point

 

RESPONSE: Thank you for your suggestion. In the laser confocal experiment, only one position can be observed at a time, and continuous observation is made during the heating process. The specimens were heated to 1050℃, 1100℃, 1150℃, 1200℃, and 1250℃ and iso-thermal for 600 s with the heating rate of 10℃/s, respectively. Each sample underwent approximately 2 minutes of heating and 10 minutes of insulation. In-situ observe the sample continuously during the heating period and photos from typical times were selected for analysis in the article. A sufficient number of grains were selected for analysis within the same viewfield.

 

5. At each annealing temperature the grain structure just after reaching the temperature of annealing must be added.

 

RESPONSE: Thank you for this comment. The key objective of this study is to observe grain boundary migration. At the initial stabilization of the target temperature, grain boundaries are almostly invisible within 200 s, 200 s marks the earliest time when grain boundaries become discernible. This study primarily addresses the billet heating process. In actual heating operations, the isothermal holding time is approximately half an hour, where the early stage is critical. For medium-manganese steel, prolonged heating causes grain boundary damage.

 

6. From tables 2 and 3 one could see the significant fluctuation of mean grain size. How it can be and what is the processes in the system Otherwise, the statistical error in determination of mean grain size must be corrected.

 

RESPONSE: Thank you for this comment. I used Gaussian method to fit the average grain size and wrote the standard error in the table. I have carefully checked this part, and the values in the table are consistent with the values calculated in the software. The main The main purpose of this article is to study the grain growth behavior of medium manganese steel. and to guide the rolling temperature of medium manganese steel. The most suitable heating temperature for the medium manganese steel in this paper is from 1100℃ to 1150℃. This takes into account influences such as grain size, grain boundary damage, and deformation resistance.

 

7. Thermodynamic calculation results demonstrated on figure 5 do not correspond to the text 3.1 one hardly could see the disapperence on FCC 296°C and any changes with cementite at 605 C.

 

RESPONSE: Thank you for this comment. In the composition system of medium Mn steel in this article, the content of these regions in the phase diagram is too small and the display is not clear. However, the main focus of this article is on the effect of (Nb,Ti)C precipitates on grain growth. In Fig. 5b, I have already magnified the (Nb,Ti)C precipitates curve separately through an enlarged image.

 

8. It is better to indicate the temperature range of stability of the phases rather then discuss in term of the process (appearancedisappearance) because we must take into account if it is heating up or cooling down

 

RESPONSE: Thank you for your suggestion. The phase diagram in this article adopts the thermodynamic calculation module of thermalcalc, which analyzes the equilibrium state. Although it cannot accurately correspond to the actual situation, the analysis in this article can be explained. I have carefully revised this paragraph.

 

9. what are the processes which determine such kinetics as it is demonstrated on figure 6. Both picture must be well interpreted but it seems that some mistake in modelling was made.

 

RESPONSE: Thank you for this comment. The calculation of precipitate diameter uses the thermalcalcalc precipitate phase calculation module. By inputting the heating rate, holding time, and cooling rate, the diameter curve of the precipitate can be obtained. Thermalcalcalc software is commonly used and mature in materials research, based on strong thermodynamic capabilities and rich databases.

 

10. In the paper the measured effect discussed in terms of the role of different GB types and carbide particles. But unfortunately, there are no data, which characterize these factors for chosen samples, thus it is difficult to understand the real roles of the factors.

 

RESPONSE: Thank you for this comment. The general understanding of grain growth is that the driving force for grain growth arises from the reduction of total grain boundary energy. During this process, grain boundaries migrate toward their center of curvature, causing smaller grains to be consumed by larger ones. However, effect of precipitates is the pinning effect of dispersed second-phase particles exert pinning forces that impede grain boundary motion. In practical research, the observation and theoretical calculation of grain boundary shape are mainly used, and it is difficult to observe grain boundary changes under these experimental conditions.

 

Author Response File: Author Response.pdf