Effect of Zr Additions on Non-Metallic Inclusions in X11CrNiMo12 Steel

Round 1

Reviewer 1 Report

The authors present some interesting results concerning the use of Zr as alloying element to influence the type, size and amount of non-metallic inclusions in steels. Though the microstructural results show an important modification of the microstructure, it becomes unclear why this does not affect the mechanical properties and the authors should make an effort to explain it. In general, the paper is well written (English is correct and legible) but, in my opinion, the figures should be presented in a more natural way as it is explained below. Some descriptions are missing or further explanation regarding the results should be provided in the text of the manuscript. There are other comments that need to be addressed and corrections requested by this reviewer before the paper can be considered for publication in metals.

***Line 44. Please write deoxidation correctly.

***Line 60. In the sentence “…first charge (0 ppm Zr) was remelted without Zr additions…”, introducing “(0 ppm Zr)” and “without Zr additions” seems repetitive.

***Line 62. Please write “After THE Zr addition…”.

***What atmosphere has been used in the heat treatments applied at 1180 °C and 720 °C? What type of furnace? Same question applies to the heat treatment provided in Table 1.

***Line 68. It should be “taken FOR…”

***Line 70. It should be “…by light optical MICROSCOPY USING AN Olympus DP70”.

***Line 71. Please provide the composition of the Vilella etchant or provide a reference. Also, provide the etching time employed to reveal the microstructures.

***Lines 72-73. This sentence should be re-written by something like “The non-metallic inclusion and EDS analysis were done by electron microscopy using a FEG-SEM Jeol-JSM6500F”

***I guess that this sentence should be “…the non-metallic inclusions was done using THE SAME FEG-SEM Jeol JSM-6500F, with AN Oxford HKL Channel 5 system”.

***Line 87. Please provide a reference or webpage were information about the thermodynamic software “HTCS 8 software” could be found. Please consider writing “HTCS 8 THERMODYNAMIC software”, if it is the case (or THERMODYNAMIC AND KINETIC in case it includes kinetic calculations).

***Regarding Table 2 (composition), can the authors comment on the decrease in Al as the Zr content is increased?

***Regarding Figure 1, several comments. 1) In this figure only the martensitic microstructure is shown in low magnification optical images. Not sure if it is because of the PDF compression, but the clarify/clearness of these images is quite poor, it is recommended to provide better ones. 2) Besides, concerning the text accompanying this figure, regarding the sentence “The Zr additions had decreased the grain size, as shown Figure 1”, it should be noted that the prior austenite grain boundaries are barely seen in these images. The author should revise this sentence and indicate that the martensitic microstructure has been refined by the addition of the Zr, which indirectly suggests that the prior austenite grain size has been refined by alloying with this element. 3) Finally, the authors should pinpoint by arrows or other means where they observe the oxides and nitrides in these images. I only see three yellow particles in micrograph related to the steel alloyed with 700 ppm Zr, which could be the nitrides but the authors should clarify this.

***Lines 105-108 (and the discussion regarding inclusion formation). The authors write, “The Zr additions promote the formation of numerous small homogeneously dispersed non-metallic inclusions. These results are in agreement with our previous work where Zr was added to non-Al killed steel.[19] One explanation for the formation of smaller ZrO2 non-metallic inclusions is that zirconium oxides require a lower critical radius for nucleation”. In these sentences, they talk about the formation of numerous inclusions but the type is not provided at this stage. Then they refer to the explanation why ZrO2 has been formed… still, the appearance of this inclusion was not mentioned previously or proven with any result. So, this sentence seems out of place. Wouldn’t it be more reasonable that the authors first explain what type of inclusions have been formed, provide an SEM image of each particle to show them (alumina, ZrO2, ZrN…etc) and then continue the discussion showing other information like the area fraction, size distribution, etc? Therefore, it makes much more sense that they first introduce the results concerning Figure 4, instead of Figure 2 and Figure 3. And continue showing Figures 5-7… ending up with Figures 2 and 3.

***And, then, they should comment on aspects concerned with the inclusion formation such as:

• The area fraction and share of inclusions provided in Figure 2 and 3 correspond to the total number of inclusions or only to the type ZrO2? In complex compounds are present, these numbers also include these inclusions. This information is not provided or explained in the text.
• Have the authors observed synergies among the inclusions formed (do one serve as nucleation site for others)? This is not discussed in the text although Figure 7 shows the formation of a complex ZrO2-ZrN compounds. For which steels where these observed? Any others?

***What are the large black and white particles clearly visible in the images of Figure 2. They should be described in the text when this figure is presented.

***Regarding Table 3. How many tensile tests were carried out per steel? Could the authors provide an experimental error to these measurements?

***Table 3. It is difficult to explain that having a much finer microstructure with 300 ppm Zr, with much smaller (and fewer) inclusions compared to the base steel, results in having the same mechanical properties (the variations observed surely stay within the experimental error). Have the authors a reasonable explanation for this? Have they checked the fracture surfaces to understand the origin of the crack nucleation and propagation? Are there any other sources of failure that thy have not considered? May be pores close to inclusions because of the forming process… Is there any other alternative the authors can suggest?

***173-184. Concerning this paragraph, it seems more reasonable to start with the sentence as the introductory one: “Figure 8 shows the diagram with Gibbs free energy for the formation of ZrN, ZrO2 and Al2O3 at 1600 °C in liquid steel (X11CrNiMo12) for various zirconium contents.”

***Line 179. It should be written: “…is one of the reasons WHY so much…”

***Lines 182-183. I would rephrase the sentence as: “Furthermore, Figure 8 clearly shows that ZrN nitride non-metallic inclusions become MORE stable AS THE Zr CONTENT INCREASES”.

***Lines 183-184. The authors write: “This is why the ZrN content is low in sample with 300 ppm Zr and high in sample with 700 ppm Zr”. It is true that the formation of ZrN becomes more likely as the Zr content increases, but this is also the case for ZrO2 up to 1000 ppm, so why should ZrN form if ZrO2 is steel a much more stable inclusion and also become even more stable as the Zr increases. The authors should clarify this point further.

***In general, the reviewer is missing the use of articles “THE” and “A” throughout the text of the manuscript. Please revise.

Author Response

The authors would like to thank the reviewer for their insightful comments and suggestions that significantly improved the manuscript. We have revised the paper accordingly.

The authors present some interesting results concerning the use of Zr as alloying element to influence the type, size and amount of non-metallic inclusions in steels. Though the microstructural results show an important modification of the microstructure, it becomes unclear why this does not affect the mechanical properties and the authors should make an effort to explain it. In general, the paper is well written (English is correct and legible) but, in my opinion, the figures should be presented in a more natural way as it is explained below. Some descriptions are missing or further explanation regarding the results should be provided in the text of the manuscript. There are other comments that need to be addressed and corrections requested by this reviewer before the paper can be considered for publication in metals.

***Line 44. Please write deoxidation correctly. corrected

***Line 60. In the sentence “…first charge (0 ppm Zr) was remelted without Zr additions…”, introducing “(0 ppm Zr)” and “without Zr additions” seems repetitive. Corrected according to the reviewers suggestion. »The first charge was remelted without Zr additions, …«

***Line 62. Please write “After THE Zr addition…”. corrected

***What atmosphere has been used in the heat treatments applied at 1180 °C and 720 °C? What type of furnace? Same question applies to the heat treatment provided in Table 1. Additional information is provided in the manuscript: »Then the ingots were homogenized at 1180 °C for 6 h in a gas heating furnace without protective atmosphere. The homogenized ingots were hot forged into 30 mm diameter bars by a pneumatic forging hammer. After the hot forging the samples were annealed in an electric box furnace at 720 °C for 2 h. The forged rods were cut into 200 mm samples and heat treated for further mechanical testing, the heat treatment regime is given in table 1, the heat treatments were done in electric box furnaces without protective atmosphere.«

***Line 68. It should be “taken FOR…” corrected

***Line 70. It should be “…by light optical MICROSCOPY USING AN Olympus DP70”. corrected

***Line 71. Please provide the composition of the Vilella etchant or provide a reference. Also, provide the etching time employed to reveal the microstructures. Additional information is provided in the manuscript: »The samples for optical microscopy were etched with Vilella’s reagent (5 ml HCl + 2 g Picric acid + 100 ml Ethyl alcohol) for 10 s, the samples for electron microscopy were only polished.«

***Lines 72-73. This sentence should be re-written by something like “The non-metallic inclusion and EDS analysis were done by electron microscopy using a FEG-SEM Jeol-JSM6500F” corrected

***I guess that this sentence should be “…the non-metallic inclusions was done using THE SAME FEG-SEM Jeol JSM-6500F, with AN Oxford HKL Channel 5 system”. corrected

***Line 87. Please provide a reference or webpage were information about the thermodynamic software “HTCS 8 software” could be found. Please consider writing “HTCS 8 THERMODYNAMIC software”, if it is the case (or THERMODYNAMIC AND KINETIC in case it includes kinetic calculations). The correct name of the software is Outotec HSC Chemistry, the authors made an unfortunate typographical error, and sincerely apologise for the mistake.

***Regarding Table 2 (composition), can the authors comment on the decrease in Al as the Zr content is increased? The authors made a mistake while transcribing the values, this has been corrected. The results are now also more logical.

***Regarding Figure 1, several comments. 1) In this figure only the martensitic microstructure is shown in low magnification optical images. Not sure if it is because of the PDF compression, but the clarify/clearness of these images is quite poor, it is recommended to provide better ones. 2) Besides, concerning the text accompanying this figure, regarding the sentence “The Zr additions had decreased the grain size, as shown Figure 1”, it should be noted that the prior austenite grain boundaries are barely seen in these images. The author should revise this sentence and indicate that the martensitic microstructure has been refined by the addition of the Zr, which indirectly suggests that the prior austenite grain size has been refined by alloying with this element. 3) Finally, the authors should pinpoint by arrows or other means where they observe the oxides and nitrides in these images. I only see three yellow particles in micrograph related to the steel alloyed with 700 ppm Zr, which could be the nitrides but the authors should clarify this.

• New images of better quality were acquired for all samples. The samples were prepared again, the images were a bit lighter, while the etching method was the same.
• We would like to point out that the PAGB were visible when viewing directly in the microscope at 100x magnification, the presented images were shot at 1000x magnification to present the martensite and inclusions. The sentence »The Zr additions had refined the martensitic microstructure as shown in Figure 1.« was amended.
• The oxide and nitride inclusions are now indicated by arrows.

***Lines 105-108 (and the discussion regarding inclusion formation). The authors write, “The Zr additions promote the formation of numerous small homogeneously dispersed non-metallic inclusions. These results are in agreement with our previous work where Zr was added to non-Al killed steel.[19] One explanation for the formation of smaller ZrO2 non-metallic inclusions is that zirconium oxides require a lower critical radius for nucleation”. In these sentences, they talk about the formation of numerous inclusions but the type is not provided at this stage. Then they refer to the explanation why ZrO2 has been formed… still, the appearance of this inclusion was not mentioned previously or proven with any result. So, this sentence seems out of place. Wouldn’t it be more reasonable that the authors first explain what type of inclusions have been formed, provide an SEM image of each particle to show them (alumina, ZrO2, ZrN…etc) and then continue the discussion showing other information like the area fraction, size distribution, etc? Therefore, it makes much more sense that they first introduce the results concerning Figure 4, instead of Figure 2 and Figure 3. And continue showing Figures 5-7… ending up with Figures 2 and 3.

The authors have rearranged the text according to the reviewer’s suggestions.

***And, then, they should comment on aspects concerned with the inclusion formation such as:

• The area fraction and share of inclusions provided in Figure 2 and 3 correspond to the total number of inclusions or only to the type ZrO2? In complex compounds are present, these numbers also include these inclusions. This information is not provided or explained in the text.
• The inclusion surface area analysis includes all of the inclusions found in the samples (oxides, nitrides, and sulphides). The text has been corrected: “The SEM analysis revealed that zirconium drastically decreased the surface area of all non-metallic inclusion, as shown in Figure 6 (results of the Inca feature analysis).« The complex inclusions, nitrides on oxide nuclei are presented as ZrN. Complex inclusions such as ZrN formed on ZrO₂, were classified as ZrN, as they presented the bulk of the non-metallic inclusion as well as the matrix/inclusion phase boundary.
• Have the authors observed synergies among the inclusions formed (do one serve as nucleation site for others)? This is not discussed in the text although Figure 7 shows the formation of a complex ZrO2-ZrN compounds. For which steels where these observed? Any others? The zirconium oxides serve as nucleation sites for ZrN. »When additional Zr is added ZrN appear in larger numbers, the relative size also increases, as ZrN frequently form on previous ZrO₂ inclusions (as shown in Figure 5).«

***What are the large black and white particles clearly visible in the images of Figure 2. They should be described in the text when this figure is presented. The text was corrected accordingly: »The dark inclusions are alumina and zirconium oxides, while the zirconium nitrides are seen as white inclusions.«

***Regarding Table 3. How many tensile tests were carried out per steel? Could the authors provide an experimental error to these measurements? Text was corrected: »Three round 55 mm long samples were made for each test according to EN ISO 6892-1:2019.«

***Table 3. It is difficult to explain that having a much finer microstructure with 300 ppm Zr, with much smaller (and fewer) inclusions compared to the base steel, results in having the same mechanical properties (the variations observed surely stay within the experimental error). Have the authors a reasonable explanation for this? Have they checked the fracture surfaces to understand the origin of the crack nucleation and propagation? Are there any other sources of failure that thy have not considered? May be pores close to inclusions because of the forming process… Is there any other alternative the authors can suggest?

An additional explanation has been added:  »The slight increase in mechanical properties can be attributed to the decrease in grain size. The subsequent fall in tensile strength in the 700 ppm Zr sample and the minimal increase in 300 ppm Zr can be attributed to the loss of precipitation hardening effect caused by small submicron nitride precipitates, such as V(C, N), the formation of micron sized ZrN during solidification prevented their formation during heat treatment. The precipitation hardening effect is more effective when the precipitates are a couple of nano meters in size [27]. The multitude of Zr based sharp edged inclusions, both oxide and nitride probably decrease the mechanical properties due to the notch effect and subsequent void formation. Therefore it can be concluded that the zirconium additions should be lower than 0.07 % to prevent extensive nitride formation and the weakening of the precipitation hardening effect during tempering.«

***173-184. Concerning this paragraph, it seems more reasonable to start with the sentence as the introductory one: “Figure 8 shows the diagram with Gibbs free energy for the formation of ZrN, ZrO2 and Al2O3 at 1600 °C in liquid steel (X11CrNiMo12) for various zirconium contents.” The text was modified »The diagram with Gibbs free energy for formation of Al₂O₃, ZrN, and ZrO₂ at 1600 °C in liquid X11CrNiMo12 steel for various zirconium contents is shown in Figure 8.«

***Line 179. It should be written: “…is one of the reasons WHY so much…”corrected

***Lines 182-183. I would rephrase the sentence as: “Furthermore, Figure 8 clearly shows that ZrN nitride non-metallic inclusions become MORE stable AS THE Zr CONTENT INCREASES”. corrected

***Lines 183-184. The authors write: “This is why the ZrN content is low in sample with 300 ppm Zr and high in sample with 700 ppm Zr”. It is true that the formation of ZrN becomes more likely as the Zr content increases, but this is also the case for ZrO2 up to 1000 ppm, so why should ZrN form if ZrO2 is steel a much more stable inclusion and also become even more stable as the Zr increases. The authors should clarify this point further. The oxygen reacted with Zr, so its activity decreased. The text was modified:  »This is why the ZrN content is low in sample with 300 ppm Zr and high in sample with 700 ppm Zr, in addition the active oxygen content is lowered due oxide formation, which further promotes nitride formation.«

***In general, the reviewer is missing the use of articles “THE” and “A” throughout the text of the manuscript. Please revise. corrected

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors present a manuscript where an extensive characterization of the Zr inclusions was performed. The effects of the Zr additions were evaluated over the microstructure, inclusions morphology and chemical analysis, tensile strength, and Brinell hardness on a high creep resistant martensitic steel.

However, I have to recommend that this manuscript should be rejected for publication. No novelty is detailed within this research work.

Authors have made a great effort to characterize the non-metallic inclusions, and data could be used to support the use of Zr as a deoxidation element. 

Additionally, some procedure methods are not explained in great details and characterization techniques seem not correctly used. The discussions were not sufficiently supported by literature from various authors.

To improved future manuscript submission here are some comments, I hope you find them constructive and useful:

Introduction

Line 39-41: Please incorporate the sizes of those Al2O3 inclusions to clogging the nozzles. Also, add some references from different authors.

Line 44: replace deoxdation with deoxidation

Line 46: to modify manganese sulphide inclusion in which way?. Please add a short description

Line 47-48: Separate the references according to annealing or welding process.

2. Materials and methods.

The steel fabrication process could use a Figure or diagram to improve its visualization. Readers could be focused more easily.

Standard EN ISO 6892-1:2016 was reviewed to EN ISO 6892-1:2019, please update.

No details on the creep-resistant test were given. Why? The title suggested some creep characterization. If not, consider changing the manuscript title.

3. Results

Figure 2 shows the influence of the Zr addition over the inclusion size and surface density, using the SEM micrograph. However, SEM images analysis performed at this magnification seems not a valid technique for counting the micrometric inclusion. Also, the histogram must have units of surface and must be presented in area percentage.

If the ZrO2 particles could be engulfed by the solidification front during fabrication. The thermomechanical process performed on the samples could be a more plausive explanation for the presence of inclusion homogeneous dispersed.

The obtained size of the non-metallic inclusion showed in figure 3 seems suspicious. How the authors does calculated those values? The authors use references 25,26 to supports inclusion smaller than 1 micron. However, those references used high magnification SEM images. Could the author provide those SEM images?

The EDS results presented in figure 4 must be compositional, the Y axes make no sense. Also, according to Table 2, the N content is the same for 300 and 700 Zr samples, how the authors explain the drastic increment on ZrN at sample 700 Zr.

Kikuchi pattern of Figure 5-7 must be indexed.

No details about the influences of the thermomechanical procedure over the inclusion formation process are given. More information regarding the casting process is necessary (cooling direction and rate), also a post-mechanical test is mandatory. The general aspects and the final microstructure could give important information about the mechanical behavior.

References:

A numerous of self-references are given. Self-references are ok to demonstrate continuous research work. However, references from different authors are necessary to contrast results and statements.

Author Response

The authors would like to thank the reviewer for reading and reviewing the manuscript. We have followed your suggestions whenever possible.

The authors present a manuscript where an extensive characterization of the Zr inclusions was performed. The effects of the Zr additions were evaluated over the microstructure, inclusions morphology and chemical analysis, tensile strength, and Brinell hardness on a high creep resistant martensitic steel.

However, I have to recommend that this manuscript should be rejected for publication. No novelty is detailed within this research work.

We have not found similar work on the Zr modification of creep resistant steel, the bulk of Zr modifications concern HSLA steels, and the influence of relatively high Cr and N on inclusion formation was not reported.

Authors have made a great effort to characterize the non-metallic inclusions, and data could be used to support the use of Zr as a deoxidation element. 

The authors agree. 

Additionally, some procedure methods are not explained in great details and characterization techniques seem not correctly used. The discussions were not sufficiently supported by literature from various authors.

To improved future manuscript submission here are some comments, I hope you find them constructive and useful:

Introduction

Line 39-41: Please incorporate the sizes of those Al2O3 inclusions to clogging the nozzles. Also, add some references from different authors.

Reference 9 was changed to avoid autocitations and to give more background on SEN clogging. The older literature gives alumina inclusions size from 1 to 20 µm, but we do not see the need to explore this topic at this point.

Line 44: replace deoxdation with deoxidation corrected

Line 46: to modify manganese sulphide inclusion in which way?. Please add a short description

The text has been modified »Zirconium additions are known to modify manganese sulphide inclusion [12], by forming nuclei for the formation of smaller yet more numerous MnS, and they also promote the precipitation of small nitride, carbide and carbonitride particles that cause grain refinement [12–14] reduce the grain growth during high temperature annealing [15] and in the heat affected zone during welding [16,17].«

Line 47-48: Separate the references according to annealing or welding process.

 The references were separated.

1. Materials and methods.

The steel fabrication process could use a Figure or diagram to improve its visualization. Readers could be focused more easily.

We appreciate the comment, and have considered the idea even before submission, but in the end found it redundant.

Standard EN ISO 6892-1:2016 was reviewed to EN ISO 6892-1:2019, please update.corrected

No details on the creep-resistant test were given. Why? The title suggested some creep characterization. If not, consider changing the manuscript title. The creep tests are still in progress due to the long experimental times. That is why we focused on the inclusions.

1. Results

Figure 2 shows the influence of the Zr addition over the inclusion size and surface density, using the SEM micrograph. However, SEM images analysis performed at this magnification seems not a valid technique for counting the micrometric inclusion. Also, the histogram must have units of surface and must be presented in area percentage.

The authors disagree, the magnifications are appropriate for counting and analysing inclusions larger than 0.79 µm ECD, which amounts to 5 pixels in the image, since the analysis is done on a large number of fields, it is relevant and gives a good statistical description of steel cleanliness in a reasonable time frame.

If the ZrO2 particles could be engulfed by the solidification front during fabrication. The thermomechanical process performed on the samples could be a more plausive explanation for the presence of inclusion homogeneous dispersed.

The reviewer’s theory is valid, but this is more true for the elongation of clusters, which were sometimes observed, but were not the dominant distribution of inclusions, while this was more true for the non-modified steel.

The obtained size of the non-metallic inclusion showed in figure 3 seems suspicious. How the authors does calculated those values? The authors use references 25,26 to supports inclusion smaller than 1 micron. However, those references used high magnification SEM images. Could the author provide those SEM images?

An explanation is given in the materials and methods section: The automatic non-metallic inclusion analysis was done by INCA Feature, the total analysis area for each sample was 8.42 mm². The values are simply histograms from the obtained data. High magnification images of representative inclusions are given infigures 5-7 (now 2-4). The reference 25 and 26 are used to present that they obtained similar results.

The EDS results presented in figure 4 must be compositional, the Y axes make no sense. Also, according to Table 2, the N content is the same for 300 and 700 Zr samples, how the authors explain the drastic increment on ZrN at sample 700 Zr.

The graph in figure 4 (now 5) presents different types of inclusions, so it does indeed make sense, it is however a refined view of the composition, not just a raw EDS analysis. The nitride content increased because there was more Zr leftover after the ZrO2 formation, not because of different nitrogen contents.

Kikuchi pattern of Figure 5-7 must be indexed.

The Kikuchi patterns have been indexed to verify the phase and orientation, but we would prefer to give non-indexed patterns for better visibility, as the patterns are clear and not blurry, so no additional post processing has been performed.

No details about the influences of the thermomechanical procedure over the inclusion formation process are given. More information regarding the casting process is necessary (cooling direction and rate), also a post-mechanical test is mandatory. The general aspects and the final microstructure could give important information about the mechanical behavior.

While we agree this would be a good idea, it is not in the scope of this manuscript to focus on failure analysis. The steel was cast into ingots, which were elongated by hot forging, the samples for non-metallic inclusion analysis were taken in the longitudinal direction.

References:

A numerous of self-references are given. Self-references are ok to demonstrate continuous research work. However, references from different authors are necessary to contrast results and statements.

The authors have exchanged one self-reference and only three self-references remain.

Reviewer 3 Report

102:    

Can the inclusions be seen in the images of Figure 1 (I couldn't see any)? If so, then they should be marked.

“material properties [1].” instead of “material properties. [1]”

95:

Table 2

The proportion of aluminum between charge 2 and charge 3 is very different. Line 59 states that the melt was deoxidized with Al, afterwards, Zr was added. Why is the Al proportion so different?

116:

Can you mention the black and white inclusions in Figure 2? This makes reading comprehension easier.

135 and 136:

It would be nice to add "Figure 6" after "300 ppm Zr" and "Figure 7" after "700 ppm" to the sentence

161:

It is not clear between Tables 3 and 4 whether the sample was cooled to room temperature after the heat treatment and then heated again to 500 ° C or not.

Furthermore, a reference to Table after "…..at room temperature after heat treatment" would be appropriate.

182:

et al.. [28].      One point is too many.

Point setting and references

It would be more appropriate if the sentence ended with a point after the references and not before the reference. as an example, the sentence in line 29.

Author Response

The authors would like to thank the reviewer for giving many helpful comments and spotting mistakes.

102:    

Can the inclusions be seen in the images of Figure 1 (I couldn't see any)? If so, then they should be marked.

Figure 1 has been changed accordingly, the samples were prepared again and the new microstructural were taken. The inclusions are better visible and marked.

“material properties [1].” instead of “material properties. [1]” corrected

95:

Table 2

The proportion of aluminum between charge 2 and charge 3 is very different. Line 59 states that the melt was deoxidized with Al, afterwards, Zr was added. Why is the Al proportion so different?

 Thank you for spotting the inconsistency, it was a typo, the values are now more reasonable.

116:

Can you mention the black and white inclusions in Figure 2? This makes reading comprehension easier.

We have added an explanation : »The dark inclusions are alumina and zirconium oxides, while the zirconium nitrides are seen as white inclusions.«

135 and 136:

It would be nice to add "Figure 6" after "300 ppm Zr" and "Figure 7" after "700 ppm" to the sentence

The text was modified: »Fine zirconium oxide (ZrO₂) inclusions are dominant in the sample with 300 ppm Zr (Figure 3). The sample with 700 ppm Zr (Figure 4)…«

161:

It is not clear between Tables 3 and 4 whether the sample was cooled to room temperature after the heat treatment and then heated again to 500 ° C or not.

 All the samples were heat treated (and cooled to RT), and then tested, this includes the 500 °C test. “The after heat treatment in table 1” table captions 3 and 4 were added to avoid confusion, also the text was modified: »This trend continues when the samples are tensile tested at 500 °C.«

Furthermore, a reference to Table after "…..at room temperature after heat treatment" would be appropriate. corrected

182:

et al.. [28].      One point is too many. corrected

Point setting and references

It would be more appropriate if the sentence ended with a point after the references and not before the reference. as an example, the sentence in line 29. corrected

Round 2

Reviewer 1 Report

The authors have replied to all comments and made changes to the original manuscript as requested by the reviewer. Thanks.

I am only concerned about the images presented in Figure 1. The authors have made the effort to prepare the samples again for new micrographs, but they are now much lighter than the original version. It would be nice that the martensitic microstructure could be better unveil. May be etching for longer time... or might be just a problem of adjusting properly the light in the microscope to have a bit darker images or a better contrast. Could the authors try to realize if they can improve the contrast/Brightness of these images.

Nothing else to note about the revised version of the manuscript.

Author Response

The samples were re polished and re etched for 15s, both 10 s and 15 s are now mentioned in the paper.

"The samples for optical microscopy were etched with Vilella’s reagent (5 ml HCl + 2 g Picric acid + 100 ml Ethyl alcohol) for 10 to 15 s, the samples for electron microscopy were only polished."

We did have some trouble with the lighting, but this is the best we can do now.

Reviewer 2 Report

The authors have provided a new version of the manuscript. The details and recommendations were addressed correctly. Overall, the quality of the manuscript has been improved. However, a few minor details still need to be amended.

a. Line 39-41:
Add the size of the Al2O3 inclusions and references to justify the problem authors mentioned.

b. If the creep tests are in progress, please remove the term creep from the title, this could lead to misinterpretations.

c. Regarding Figure 6, the reviewer still considers that: 1) SEM images at higher magnification must be provided, 2) Bar-chart must be changed to area percentage.

Author Response

a) Al₂O₃ inclusions frequently cluster in the liquid state, leading to submerged entry nozzle clogging during casting (inclusions that are between 1 and 20 µm in diameter are the most detrimental for clogging), and an uneven distribution of hard brittle inclusions that will typically break during deformation, furthermore the combination of low bonding strength to the matrix during deformation leads to void creation and separation (debonding) [9-12].

11. Long, M.; Zuo, X.; Zhang, L.; Chen, D. Kinetic modeling on nozzle clogging during steel billet continuous casting. ISIJ Int. 2010, 50, 712–720.

12. Singh, S. N. Mechanism of Alumina Buildup in Tundish Nozzles During Continuous Casting of Aluminum-Killed Steels. Trans. 1974, 5, 2165–2178.

b) the “creep resistant” was removed from the title

c) the figure has been changed accordingly