Quenching and Partitioning of Multiphase Aluminum-Added Steels

Round  1

Reviewer 1 Report

Dear Authors

I think the subject of this study to investigate the effect of Al instead of Si in Q&P steels is very interesting  but I have a few questions/comments:

 1-Firstly, although Al can retard the precipitation it is a good ferrite stabilizer, as you mentioned. So, it will accelerate bainite transformation. Although the partitioning temperature (450C) which you selected is in the range of bainite formation, you have not talked about bainite formation which I think is one of the most important questions, for the readers of this work.

-       For example, regarding Fig.4, normally the result of LePera method should show the fresh martensite/austenite as white, bainite brown and tempered martensite as blue. But surprisingly, nothing about bainite is mentioned about these figures, and no blue phases are appeared, which might be the reason of not proper etching. I think the text about this figure should improve. I think the etching time was too long or the solution was not accurately prepared.

-       As you mentioned in Table. 4 the Ms temperatures of Steel A, B and C are very different from each other. So, the partitioning condition of 450 C for 100 s, resulting in a very different phase transformation on these steels, which is not discussed in the text. Most of the discussions are mainly about the effect of quenching temperature on the subsequent mechanical properties. But I think this should be noted that 450C for steel A and C are in the range of upper bainite or even widmanstatten ferrite while it might be in the range of lower bainite for Steel B, which should be investigated.

2- As you mentioned in line 265 to 267, the volume fraction of retained austenite is much higher in XRD compared with EBSD. How much is the difference? Do the OM results confirm the XRD results because I believe that these numbers for such low carbon steel are quite high!

3- What is the result of the fitting shown in figure 2. What number did you predict for k in Eq. (1)?

4- Please explain more about the method that you have used in Figure 1 for calculation of bainite start line. What does the numbers in the figure mean? Why you have used only 10 C/s ?

 Regards,

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

thank you for the time and effort you have taken to go through our paper. Your efforts are appreciated. Please find answers to the points you raised below.

Regarding the Le Pera etching:

Le Pera works best at low fractions of martensite. For both Steel A and Steel C with a martensite fraction of approximately 25 vol-%, the contrast is excellent and ferrite shows up as the darker phase. Steel B has had a poorer response due to the high austenite fraction.

Regarding bainite formation for Steel B:

We appreciate your remarks about potential bainite development in Steel B. Possible bainite formation during partitioning is now discussed in the manuscript. Please see the revised manuscript with tracked changes.

Regarding retained austenite:

The retained austenite content was measured using standard techniques and the measured fractions are in line with the carbon concentrations calculated from the measured lattice constants. An EBSD measurement is not a reliable source of information for retained austenite fractions due to sensitivity to specimen preparation, as well as resolution limitations. The resolution limitations affect optical microscopy even more severely.

On K fitting:

The K was not predicted but fitted separately for each dilatation curve. The values of K and Ms resulting in the best fit were selected in each case. This is now emphasized in the relevant section.

On the bainite start line calculation:

The CCT curves were calculated using the standard JMATPRO software package, based on the work of Kirkaldy et al. and Lee and Bhadeshia. This information has been added to the manuscript. The numbers in Figure 1 indicate the assumed carbon concentrations of different intercritically annealed conditions, each resulting in a different bainite start line. The cooling curve 10 C/s was selected for two reasons: 1) it showcases the minimum cooling rate necessary to avoid bainite formation at the 25 vol-% condition and 2) it is a realistic cooling rate for modern continuous annealing lines. We have clarified this in the text.

We hope you will find that the above discussion and the changes made in the manuscript answer the points you have raised. Thanks again for your work.

Best regards,

Authors

Reviewer 2 Report

1.Table 2. The Authors should include the meaning of the acronyms, for instance in the caption of table 2.

2.Lines 174, 175 and 176. The authors indicate the Bragg reflections used for determining the austenite contents. At least, for one quenching temperature, the authors should present the corresponding X-Ray diffraction patterns.

3)Table 4. The authors should substitute “see text” by the estimated value of the amount of intercritical austenite.

4)Figures 5,6, 7,8, 9 and 10. Maybe the authors should check if it is a good idea to put the data corresponding to the three steels in the same plot. For instance, the retained austenite of the three steels in the same plot, joining the data of the same sample by a line. And so on with the rest of the plots: RA carbon content, Engineering stress and Engineering strain. In this way it is easier to compare them.

5)Figure 11. The authors present the EBSD images in this picture. The EBSD images allow determining the grain size. It would be nice if the authors give the average value of the grains.

6)Figure 13. The symbol gamma is not properly written.

7) In line 15 in the abstract. The authors say: “The steels assumed ether a ferrite/martensite/retained austenite microstructure or a multiphase structure with ferritic, bainitic and martensitic constituents”. It is strange that the authors speak about a bainitic phase when it is never mentioned in the manuscript except in the abstract and in the introduction. In the manuscript it is not said anywhere that a bainitic phase is present. The authors should write again the abstract. In the abstract, the authors must give a small resume about the goal of their work, the results and the conclusions.

Author Response

Dear reviewer,

we appreciate your work in reviewing this paper. Please find answers to the points you raised below.

1.Table 2. The Authors should include the meaning of the acronyms, for instance in the caption of table 2.

We have added the meanings of the acronyms.

2.Lines 174, 175 and 176. The authors indicate the Bragg reflections used for determining the austenite contents. At least, for one quenching temperature, the authors should present the corresponding X-Ray diffraction patterns.

We feel that this would serve only to make the manuscript longer, without any concrete benefit for the reader and have therefore decided to omit the patterns.

3)Table 4. The authors should substitute “see text” by the estimated value of the amount of intercritical austenite.

We have made the substitution.

4)Figures 5,6, 7,8, 9 and 10. Maybe the authors should check if it is a good idea to put the data corresponding to the three steels in the same plot. For instance, the retained austenite of the three steels in the same plot, joining the data of the same sample by a line. And so on with the rest of the plots: RA carbon content, Engineering stress and Engineering strain. In this way it is easier to compare them.

We feel that the differences between the studied steels are such that direct comparison between them is not beneficial and would only result in a cluttered figure. As such, the Figures highlight the effect of austenite starting condition on the final microstructure and the resulting properties.

5)Figure 11. The authors present the EBSD images in this picture. The EBSD images allow determining the grain size. It would be nice if the authors give the average value of the grains.

The amount of grains in the EBSD measurements is very small for a statistically reliable grain size measurement and we have therefore decided to forego a quantitative analysis.

6)Figure 13. The symbol gamma is not properly written.

The symbol is the gamma as given by the font Arial, which is used elsewhere in the Figure.

7) In line 15 in the abstract. The authors say: “The steels assumed ether a ferrite/martensite/retained austenite microstructure or a multiphase structure with ferritic, bainitic and martensitic constituents”. It is strange that the authors speak about a bainitic phase when it is never mentioned in the manuscript except in the abstract and in the introduction. In the manuscript it is not said anywhere that a bainitic phase is present. The authors should write again the abstract. In the abstract, the authors must give a small resume about the goal of their work, the results and the conclusions.

We appreciate this suggestion and regret that the discussion on bainite formation was omitted. We have added discussion of potential and observed bainite formation in the main text.

We hope that the above discussion satisfies you. Thank you for taking the time to review.

BR, Authors

Reviewer 3 Report

Line 200: Haven´t you used an extensometer in your tensile tests?

Line 204: Provide the values of the proportionality ratios, k1 and k2 you have used in expression (3).

Fig. 4: A better explanation is required. Two phases appear in Steel C (martensite and ferrite), but 3 phases seem to appear in Steel A, or are there also two phases in this case?

It is finally suggested to include a Table with the mechanical properties (strength, elongation and the product of both properties) obtained with the three steels under the most suitable Q+P treatment in order to make a final comparison of the steels

Author Response

Dear reviewer,

thanks for taking the time to go over our manuscript. Please find replies below to the points you have raised.

Line 200: Haven´t you used an extensometer in your tensile tests?

We have indeed used a miniature axial extensometer, which is now mentioned in the text.

Line 204: Provide the values of the proportionality ratios, k1 and k2 you have used in expression (3).

We have added the equation for calculating the proportionality ratios to the main text (Equation 4).

Fig. 4: A better explanation is required. Two phases appear in Steel C (martensite and ferrite), but 3 phases seem to appear in Steel A, or are there also two phases in this case?

In Steel A, the light shade is austenite and the two darker shades are both ferrite. The difference in hue is related to a difference in crystallographic orientation between grains.

It is finally suggested to include a Table with the mechanical properties (strength, elongation and the product of both properties) obtained with the three steels under the most suitable Q+P treatment in order to make a final comparison of the steels

The steels are different enough from each other that we felt that it is not profitable to compare them between each other directly. As it is, the discussion is on the effect of austenite conditioning during intercritical annealing on the final microstructure and resulting properties.

Once again, we thank you for the review and the discussion.

Best regards, Authors