Controlling the Content and Morphology of Phase Constituents in Nanobainitic Steel Containing 0.6%C to Obtain the Required Ratio of Strength to Plasticity

Round 1

Reviewer 1 Report

Accept

Author Response

Thank you very much for your kindly review. Please find attached file with my new version of the article. 

Kind regards

J. Marcisz

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The present work mainly focuses on the effect of isothermal temperature and time on the microstructure, properties and transformation in nanobainitic steels. It shows abundant results, including OM/SEM/EBSD/XRD/TEM, as well as tensile and dilatometric tests. The property of the tested steels present to be excellent using the designed composition and processing. Importantly, the work gives details on the evolution of retained austenite and its size effect on performance. Large blocky retained austenite caused by segregation leads to instable properties. However, the novelty of the conclusions looks insufficiently. The author should emphasize what they think the most important. Otherwise, it looks like an experimental report rather than an article. Thus, I would suggest that the author tailor the main context and the conclusions, such as “The occurrence of segregation may lead to instability of mechanical properties, especially to a decrease in toughness” can be completely deleted due to no toughness results in this work.

Other questions:

1. Line 183: “Transformation at higher temperatures also results in 183 higher content of retained austenite”, this looks inconsistent with the results, where is it from, reference or experimental data?
2. Line 285-287: The author said that higher Mo content leads to more blocky retained austenite. Please give detailed explanation.
3. Fig. 4 (a) lack of subtitle.
4. Line 475 “72-20” should be “72-120”?
5. Fig.11, it looks like that the uniform distribution of austenite just occurs at 54 hours. Does this also exist in 72, 96 or 120 hours? If not, why? According to the author, it is the segregation of alloying elements that cause the blocky austenite. At such low temperature, there is nearly no diffusion of Mo and Mn. Therefore, the author should give reasonable explanation about the evolution of austenite during isothermal and cooling process.
6. Fig. 12~17 individual title of each figure, (a)(b)(c)? It is suggested that the author give a changing regular of RA with time and temperature but not just list data.
7. Fig. 19, details about subtitles should be given.
8. The conclusions must be stressed and simplified.

Comments for author File: Comments.pdf

Author Response

Thank you very much for your kindly review and detailed suggestions and questions to our paper. We made changes to the article according to your suggestions. Please find new version of the article in the attachment and detailed answers and comments below.

 

The present work mainly focuses on the effect of isothermal temperature and time on the microstructure, properties and transformation in nanobainitic steels. It shows abundant results, including OM/SEM/EBSD/XRD/TEM, as well as tensile and dilatometric tests. The property of the tested steels present to be excellent using the designed composition and processing. Importantly, the work gives details on the evolution of retained austenite and its size effect on performance. Large blocky retained austenite caused by segregation leads to instable properties. However, the novelty of the conclusions looks insufficiently. The author should emphasize what they think the most important. Otherwise, it looks like an experimental report rather than an article. Thus, I would suggest that the author tailor the main context and the conclusions, such as “The occurrence of segregation may lead to instability of mechanical properties, especially to a decrease in toughness” can be completely deleted due to no toughness results in this work.

 

Other questions:

1. Line 183-now 203: “Transformation at higher temperatures also results in higher content of retained austenite”, this looks inconsistent with the results, where is it from, reference or experimental data?

Reference [1] – material after long-term homogenisation

In our study, different transformation temperatures and times were used and therefore a large scatter in the residual austenite content was obtained. The Fig. 8 which requires more detailed analysis and probably additional experiments was removed from the article. Our data of XRD measurements (table 4) of retained austenite content did not confirmed above theory, probably because of segregation typical for industrial material.

An example of another reference: K. Hase, C. Garcia-Mateo, H.K.D.H. Bhadeshia. Bimodal size-distribution of bainite plates. Materials Science and Engineering A 438–440 (2006) 145–148 - https://doi.org/10.1016/j.msea.2005.12.070 (Fig. 2a)

 

2. Line 285-287 now 213: The author said that higher Mo content leads to more blocky retained austenite. Please give detailed explanation.

 

Text has been added to the article lines 314-322.

Zones with a higher Mo content are characterised by a lower Ms temperature. In these areas the transformation of austenite to bainite is likely to start and the resulting phases (bainite and retained lathy and blocky austenite) are characterised by larger sizes due to the significant difference between the actual Ms temperature and the applied IHT temperature. At the same time increase the carbon content in the austenite during transformation results in lowering Ms temperature additionally. Then carbon enriched austenite, especially in the areas with higher content of Mo is more thermodynamic stable and finally its volume fraction and grain dimensions are larger compare to the areas of lower content of Mo.

 

3. 4 (a) lack of subtitle.

OK it is clear. Thank you.

 

4. Line 475 – now 458 “72-20” should be “72-120”?

OK it is clear. Thank you.

 

5. 11, it looks like that the uniform distribution of austenite just occurs at 54 hours. Does this also exist in 72, 96 or 120 hours? If not, why? According to the author, it is the segregation of alloying elements that cause the blocky austenite. At such low temperature, there is nearly no diffusion of Mo and Mn. Therefore, the author should give reasonable explanation about the evolution of austenite during isothermal and cooling process.

 

In the article we mentioned only about primary segregation of Mo and homogenisation of ingots at 1200°C for 24 hours and as you wrote at isothermal transformation temperature the diffusion of the element practically not occurs. As you can see on the Fig. 11a, c the retained austenite content is clearly higher in the segregation bands compare to the neighbouring bands. Please see the explanation of the question 2- possible evolution of austenite during IHT.

 

6. 12~17 individual title of each figure, (a)(b)(c)? It is suggested that the author give a changing regular of RA with time and temperature but not just list data.

In this figures the authors show mainly size and distribution (homogeneity) of the blocky retained austenite of the diameter higher than about 100 nm. We did not find clear correlation of the blocky retained austenite content with IHT parameters for time in the range 72-144 hours. In the Figs. 12-17 (now Figs. 12-13) we show a representative examination result for chosen heats and heat treatment parameters.

 

In the new version of the article we leave two examples of the EBSD analysis of blocky retained austenite. Based on the results we determined typical morphology, shape, size distribution, volume fraction at selected magnification. We did not find clear correlation of the volume fraction of the austenite with IHT parameters. We also found non-uniform distribution of blocky retained austenite resulting from segregation.

 

7. 19, details about subtitles should be given.

OK – thank you.

 

 

8. The conclusions must be stressed and simplified.

We have proposed a new, shorter version of the conclusions. Thank you.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments for authors are in the attachment.

Comments for author File: Comments.pdf

Author Response

Thank you very much for your review and detailed analysis of our article and for valuable comments and suggestions. We have rewriten the article according to your suggestions. Our detailed explanation are below.

 

The article contains new scientific results that are very valuable for science and practice. There are only a few questions that will clarify the content of the article:

 

1. The relative elongation characterizes the plasticity of the material to a limited extent; for a complete analysis of these properties, it is necessary to add the values of the relative narrowing to the table. Comparison of these parameters makes it possible to diagnose the inhomogeneity of the plasticity of materials in the longitudinal and transverse directions: please look https://doi.org/10.1177/1687814016641565

I propose to add the column "relative narrowing" to table 3.

Unfortunately, for specimens with a rectangular cross-section, the relative necking is not easy to determine compare to specimens with a round cross-section. As you know this very important for rectangular specimens to precisely measure the surface of the irregularly shaped rupture area. We use the microscope to observe the shape and measure the surface area but for the nanobainitic steels we did not use this method.

 

2. The study of mechanical properties using flat specimens can lead to an underestimation of the plasticity properties of materials. This is due to the localization of deformation near the faces of the specimens. It is necessary to indicate the standard to which the tests were performed. It is also necessary to indicate the loading rate of the specimens.

Static tensile tests were carried out according to PN EN ISO 6892-1 (2020) standards using specimens of rectangular cross section. Applied strain rate in the quasi-static tensile tests was  0.0003 s^-1 up to 0.5% of strain and then 0.0005 s^-1 according to the above standard. We realise that applied shape of cross-section of specimens may be the reason of underestimate the elongation values. We decided to use the shape of specimens cutting from the plates to cover almost all thickness of the original plates.  

Lines 276-280.

 

3. It would be good to give data on the microhardness of steel, this will allow an assessment of its property at the micro level.

The results of hardness measurements presented in Fig. 5 were made at a load of 1kgf (HV1) matched to the width of the segregation bands. The dimensions of the imprint for this load are significantly smaller than the width of the segregation zones, in the area of which a different type of microstructure is formed. The authors of this work performed measurements of microhardness (e.g. HV0.1; HV0.01) in segregation zones but they were the subject of another article [8].

 

4. The authors did not evaluate the impact toughness of the steel, why? Is this parameter closely related to the structure and very sensitive to its change. At developing new steels or modifying their structure, this parameter is mandatory.

We have determined Charpy-V impact toughness for heat no. 1, 2 and 3 in the temperature range from  -40 to +20°C. Moreover the plates made of the steel were examined in the firing tests at 20°C and -40°C. As you know multihit firing tests are a specific for determination of resistance to cracking of the plates. I mentioned in the article about the possible application of the steel. We are preparing other article where the impact toughness data will be analyse in details.

The level of the Charpy-V impact strength of the examined steels is in the range of 13-16 J/cm² at minus 40°C and in the range of 22-26 J/cm² at room temperature depending on IHT parameters. We did not determine the Charpy-V impact toughness for all examined variants of heat treatments applied for the material analysed in the article. I have added some comments in the “discussion” – lines 564-568

 

5. Unfortunately, the article lacks data on micromechanisms of fracture. This does not allow one to assess the influence of the microstructure on the mechanisms of failure and establish a connection between it. I invite the authors to read the article: https://www.mdpi.com/1996-1944/12/3/491 in which there is a relationship "structure - strength / ductility - mechanisms of fracture."

Thank you for the link to the article. It is difficult to propose the micromechanisms of fracture for this new steel grades with microstructure compose of extremely hard bainitic matrix and different morphology of carbon enriched soft retained austenite characterized by various susceptibility to transformation, deformation and strain hardening etc. The mechanisms is quite complicated and requires more advanced investigation. In the article we did not analyse the microstructure of the specimens in the areas of plastic deformation, especially in TEM.

 

6. The authors obtained TEM structures, but carried out only their qualitative analysis, did not estimate the dislocation density in low-angle boundaries and other parameters? Why?

 

The results of microstructure examination in TEM confirm only the existence of nanolathy bainite and different morphology of retained austenite and the lack of carbides at applied magnifications. The mentioned above futures of the microstructure were confirmed on bright and dark field images and diffraction patterns.

 

7. These steels are very sensitive to hydrogenation and shock loading. It would be good if the authors clarified in what operating conditions this steel is supposed to be used.

We mentioned in the article about possible application of the steel grade as an armour plates. The operating conditions of the armour plates are rather not exposed to hydrogen. The surfaces of the plates after final heat treatment are cleaned (shot blasted) and often painted. Resistance of the plates to shock loading, especially to piercing with different type of ammunition is the subject of specific multihit firing tests according to proper standards and documents. This examination results were published elsewhere.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

The paper reports on very accurate study on the effect of nanobanitic microstructures in a series of steels. However, it cannot be accepted for publication in the present form. The authors have to be much more synthetic when presenting results: a lot of data can be summarized and reorganized to be presented clearly and shortly. At the same time a lot of results are missing while the conclusions based on them are done. Since 4 different compositions were studied, the results should be reported and compared for each of them. The authors suggest that these are carbide-free steels without giving any real experimental evidence for it. It seems that segregation and the fraction of the retained austenite were identified as the most critical parameters for mechanical properties evolution. Nevertheless, the analysis of segregation is only given for heat no.3 and no clear correlation between retained austenite fraction and tensile properties was established. The authors should provide much more clear and concise presentation of their results. The different points below should be addressed.

- For the ease of reading, on the one hand, the authors are advised to introduce some generalities on nanobanitic steels in the introduction section. Why is it called nanobanite (what is the difference with the banite), what steel compositions are required to form the nanobanite, what is the typical heat treatment? On the other hand, the authors are acknowledged for giving very exhaustive, clear and useful description of microstructure and deformation mechanisms features in the nanobanitic steels.

- How the time and temperature for isothermal heat treatment were chosen? It seems that several temperatures were below Ms temperature? Is it correct to produce nanobanite?

- The as-cast microstructure of heat no. 3 is only discussed? Were the similar conclusions done for other heats?

- I would suggest the authors give as recorded dilatometric curves used for analysis in Figure 8 and explain the methodology to deduce the transformation fraction from the results of dilatometry.

- In the experimental part the authors indicated they used three tensile tests per microstructure state. Therefore, the standard deviation has to be given in Table 3 together with the mean values for different mechanical properties. The tensile curves in Figure 10 should be given and discussed before showing the Table 3.

- One of the reasons for different tensile properties, as suggested by the authors, is different segregation level in different heats. Such a statement requires a special analysis of the segregation for different heats. While such an analysis is reported for heat no. 3 (Table 1), it is not given for other heats.

- I have a problem with the statement that XRD should detect both blocky and nanolathy retained austenite. Very thin austenite films (about 100 nm) could not be resolved by in lab X-ray equipment. It seems that it is mostly EBSD technique which should reveal the nanolathy austenite. However, XRD and EBSD data on the retained austenite fraction cannot be compared, since the latter analyses a very limited sample area with limited statistics and the former gives a material response from a significant volume.

Some typos/other remarks:

What is Al_t in the Table 1?

The quality of Figure 6 must be improved.

line 228: “150×60 mm” should be “150×60 mm²”

line 266: “at the casting stage” should be “in as-cast state”

Even though the English is not bad, it can be improved (especially, the syntax).

Author Response

 

Thank you very much for your detailed review of our article and for suggestions, question and comments. We have corrected the manuscript according to your suggestions. Please find new version of the article in the attachment and detailed answers and comments below.

 

The paper reports on very accurate study on the effect of nanobanitic microstructures in a series of steels. However, it cannot be accepted for publication in the present form. The authors have to be much more synthetic when presenting results: a lot of data can be summarized and reorganized to be presented clearly and shortly. At the same time a lot of results are missing while the conclusions based on them are done. Since 4 different compositions were studied, the results should be reported and compared for each of them. The authors suggest that these are carbide-free steels without giving any real experimental evidence for it. It seems that segregation and the fraction of the retained austenite were identified as the most critical parameters for mechanical properties evolution. Nevertheless, the analysis of segregation is only given for heat no.3 and no clear correlation between retained austenite fraction and tensile properties was established. The authors should provide much more clear and concise presentation of their results. The different points below should be addressed.

 

- For the ease of reading, on the one hand, the authors are advised to introduce some generalities on nanobanitic steels in the introduction section. Why is it called nanobanite (what is the difference with the banite), what steel compositions are required to form the nanobanite, what is the typical heat treatment? On the other hand, the authors are acknowledged for giving very exhaustive, clear and useful description of microstructure and deformation mechanisms features in the nanobanitic steels.

The introduction section of the article has been supplemented.

Nanobainitic steels are characterized by a unique combination of ultra-high strength and high ductility: their tensile strength can attain the value of 2.5 GPa and total tensile elongation is in the range of 12÷25%. Such properties of nanobainitic steels arise from their microstructure, produced at low temperature directly above M_S, which for most chemical compositions of nanobainitic steels is below 300–250℃. The nanobainite consists of carbide–free carbon-supersaturated laths of bainitic ferrite with a thickness of a few tens of nanometers and interlath films or platelets of retained austenite. In addition to film-like retained austenite, nanobainitic steels contain another morphological type of retained austenite in the form of polyhedral grains and thick plates of sizes between a few tenths of μm and several μm. In total, nanobainitic steels typically contain 20–40% of retained austenite, depending on their chemical composition and temperature and the duration of transformation. Except for the size, the important difference between the two morphologies of retained austenite is the amount of carbon remaining in solid solution; the carbon content is higher in film-like than in blocky retained austenite, with crucial consequences for its mechanical stability.

Chemical composition of an ultra-strength nanobainitic steel should fulfil several condition, the most important of which are: respectively low Ms temperature to proceed bainite transformation in the temperature range 210-275°C to obtain nanometer-size laths; addition of silicon to suppress precipitation of cementite and high enough hardenability to avoid diffusion-type transformation during cooling from austenitising temperature to the temperature of IHT [8].

 

- How the time and temperature for isothermal heat treatment were chosen? It seems that several temperatures were below Ms temperature? Is it correct to produce nanobanite?

Ms temperature determined from dilatometric measurements is close to 200°C at cooling rate of 1°C/s for all of the examined nanobainitic steel grades. We used a minimum IHT temperature of 200°C. Based on our examination results so far, and industrial trials of plates heat treatment, the difference between the Ms temperature and the IHT temperature should not exceed about +30°C. Furthermore, the Ms temperature changes (decreases) during the progress of the transformation due to the diffusion of carbon into the austenite. Taking this phenomenon into account, it can be assumed that the applied IHT temperature close to Ms determined on the basis of dilatometric studies was not lower than it. We did not find any martensite in our specimens but as you know sometimes it is difficult to observe laths of martensite between packets of lathy bainite or find individual martensitic laths in the TEM (on the thin foil).

The optimum transformation time range was determined mainly based on the results of mechanical properties in static tensile tests to achieve required values of UTS, YS_0.2 and TE.

 

- The as-cast microstructure of heat no. 3 is only discussed? Were the similar conclusions done for other heats?

Heats no. 1, 2 and 3 were produced using a similar technology in the industrial conditions. All the industrial heats were homogenised directly before forging at 1200°C for 24 hours in the same way. We explain in the text similarity of the applied industrial heats. We decided to remove heat no. 4 of different production method in a laboratory scale from the article.

 

- I would suggest the authors give as recorded dilatometric curves used for analysis in Figure 8 and explain the methodology to deduce the transformation fraction from the results of dilatometry.

We have decided to remove Fig. 8 which requires detailed analysis.

 

- In the experimental part the authors indicated they used three tensile tests per microstructure state. Therefore, the standard deviation has to be given in Table 3 together with the mean values for different mechanical properties. The tensile curves in Figure 10 should be given and discussed before showing the Table 3.

We have added more details to the applied tensile tests methodology according to your and other reviewers suggestions.

Tensile tests were carried out according to PN-EN ISO 6892-1 (2020) standards at strain rate of 0.0003 s^-1 up to 0.5% of strain and then at 0.0005 s^-1. For each of the heat treatment variants, minimum three tensile samples were tested, and obtained results were within the range of +/- 20 MPa for YS_0.2 and UTS and +/-2% for TE from the arithmetic mean values.

Correct determination of standard deviation requires rather more than three measurements.

The tensile curves were moved and discussed before the Table 3.

 

- One of the reasons for different tensile properties, as suggested by the authors, is different segregation level in different heats. Such a statement requires a special analysis of the segregation for different heats. While such an analysis is reported for heat no. 3 (Table 1), it is not given for other heats.

We have resigned from this analysis because not enough data. Moreover for the other reasons we removed heat no. 4 from the article. The examination of the segregation degree of the heat no. 4 require more analysis and additional experiments. Heats no. 1, 2 and 3 were produced in a similar way/technology (melting, casting, hot forging and hot rolling) in the industrial conditions.

 

- I have a problem with the statement that XRD should detect both blocky and nanolathy retained austenite. Very thin austenite films (about 100 nm) could not be resolved by in lab X-ray equipment. It seems that it is mostly EBSD technique which should reveal the nanolathy austenite. However, XRD and EBSD data on the retained austenite fraction cannot be compared, since the latter analyses a very limited sample area with limited statistics and the former gives a material response from a significant volume.

 

The XRD quantitative phase analysis method is capable to measure the total amount of retained austenite in nanobainitic steel, including the film-like morphology [doi:10.1098/rspa.2011.0212]. As you know the method is also widely used to identification of precipitations of similar or lower dimension and often lower volume fraction than lathy retained austenite. Shape and dimensions of retained austenite film and also high carbon content in the bainite nanolaths and internal stress level result in widening of XRD lines. Moreover the nanobainitic ferrite - which is detectable in XRD has a width close to the laths (film) of retained austenite – of course the volume fraction of the bainite is significantly higher than retained austenite, especially film of RA.

On the other hand, EBSD technique can detect the areas (grains) of retained austenite of the size not smaller than about 0.1μm, that is the blocky morphology. Because of small area of analysis in a single EBSD measurement, sufficiently large number of measurements should be done to get statistically representative results.

Measurements of the blocky retained austenite content were made at two or three representative locations of the investigated surface at selected magnification (from our article).

Proc. R. Soc. A (2011) 467, 3141–3156 ; doi:10.1098/rspa.2011.0212

 

Some typos/other remarks:

What is Al_t in the Table 1?

t – means total content. As you know in case of the Al, the content of the element in the solid solution (not in form of oxides and mixed nonmetallic inclusions) is also important (Al_s) but we determined only total content, so the „t” should be deleted.

Thank you.

The quality of Figure 6 must be improved.

1.  

line 228: “150×60 mm” should be “150×60 mm²”

Thank you – I have added number 2 in the lines 242 and 243. Heat no. 4 (150x60 mm²) was removed from the article. 

line 266: “at the casting stage” should be “in as-cast state”

In the original sentence I would like to indicate the casting stage of the manufacturing process, not state of the material. In my opinion the expressions „in as-cast state” and „material produced at the casting stage of the manufacturing process” have the similar meaning.  

Even though the English is not bad, it can be improved (especially, the syntax).

I sent my English Certificate to the Editor.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The work is much better now. Thanks.

Author Response

Thank you once again for your review and valuable comments and suggestions. In the new version of the manuscript we have added std. dev. values for the mechanical properties according to reviewier suggestion. 

Best regards 

Reviewer 3 Report

Accept.

Comments for author File: Comments.docx

Author Response

Thank you once again for your review and valuable comments and suggestions. In the new version of the manuscript we have added std. dev. values for the mechanical properties according to reviewier suggestion. 

Thank you also for the suggestion about the oryginal research published by prof. Hutsaylyuk. We are keen to learn more about its findings and research methods. 

Best regards

Reviewer 4 Report

The authors have carefully reponsed to all points raised in my review. I would suggest two last minor modifications before the acceptance:

• The authors should add their response on the choice of time and temperature of isothermal heat treatment to the text of the paper.
• Even for three tests the standard deviation should be added for the results reported in Table 3. Otherwise, three tests carried out per microstructure state are senseless.

Author Response

Thank you once again for your review and valuable comments and suggestions. In the new version of the manuscript we have added std. dev. values for the mechanical properties according to your suggestion. 

Best regards