The Effect of Direct Strip Casting on the Kinetics of Phase Transformation of a Dual Phase Steel

Round 1

Reviewer 1 Report

1. in Figure 2, so many figures are put together without proper labelled. so as figure 4 and 5.
2.  the scale bar of the pictures in figure 4 and 5 seems arbitrary. 
3.  in page 9 line 216. spelling mistakes "gran".
4. in Figure 7, please specify the area in b  coresponding to the area in a.
5. in figure 8， please make the overlay error bar clear to reader.

Author Response

Response to Reviewer 1

1. in Figure 2, so many figures are put together without proper labelled. so as figure 4 and 5.

Figure 2 has been updated, and the figure caption has been expanded to describe what the figure shows.

2. the scale bar of the pictures in figure 4 and 5 seems arbitrary.

All micrographs are shown at the same magnification, therefore the scale bar refers to each micrograph in the figure. This is now specified in the figure caption “All micrographs shown at the same magnification.”

3. in page 9 line 216. spelling mistakes "gran".

Thank you to the reviewer for finding this error. It has been corrected.

4. in Figure 7, please specify the area in b coresponding to the area in a.

Figure 7a is a SEM showing a low magnification image of the precipitate distribution. TEM mapping shown in (b) and (c) are from a different specimen due to the TEM requiring a thin foil for analysis. The figure caption has been updated to reflect this difference.

“Note that the SEM image shown in (a) is from a different specimen to those shown in (b) and (c).”

5. in figure 8， please make the overlay error bar clear to reader.

We are unsure what the reviewer is requesting, but we are happy to make corrections. It might be that the reviewer is referring to the points where the error bar is smaller than the datum point. We have added a note to the figure caption if this was the case.

“Note that some error bars are smaller than the height of the datum points, and cannot be seen.”

 

Reviewer 2 Report

In this manuscript, direct strip casting and thin slab casting were used to produce dual phase steels, and series heat treatment were conducted to investigate the ferrite formation. All the work is valuable to the microstructure tune of the dual phase steels. However, you write them in a laundry list rather than a logical clarity. In addition, figures and paragraph structure are arbitrary. It is suggested to rewrite it.

Author Response

Reviewers comment:

In this manuscript, direct strip casting and thin slab casting were used to produce dual phase steels, and series heat treatment were conducted to investigate the ferrite formation. All the work is valuable to the microstructure tune of the dual phase steels. However, you write them in a laundry list rather than a logical clarity. In addition, figures and paragraph structure are arbitrary. It is suggested to rewrite it.

Authors response:

Thank you to the reviewer for recognizing the significance of the work. This was a difficult manuscript to write because there were so many necessary heat treatments and comparisons to make in order to make a solid conclusion. We are happy to make corrections to the manuscript in order to improve its readability and clarity. We have followed the standard structure of Introduction, methods, results, discussion and conclusions. We therefore argue that the paragraph structure is not arbitrary. Within this structure we have added subheadings to make the manuscript easier to read. The figures are a collation of representative images, taken from several hundred that were taken within this project. Some figures have been updated in response to the other reviewers comments, so we hope that these changes also reflect your concerns. We would be more than happy to receive more specific requests in order to improve the paper.

 

 

Reviewer 3 Report

This work focuses on physical simulation of direct strip casting and thin slab casting of a dual phase steel 780.  The effect of annealing parameters on the microstructure evolution is studied and compared to that of the commercial alloy processed via conventional hot rolling. It is shown that the kinetics of polygonal ferrite formation during the intercritical annealing are retarded in the strip cast samples compared to the commercial alloy. It was related to the formation of nanoprecipitates in the material.

The article fits very well into scope of the journal. It is well structured and written in a clear and concise manner. A few comments below could help to further improve the quality of the manuscript.

1. Table 1. It should be specified if the chemical composition is in wt. % or at. %.

2. From the manuscript, it is not clear what are the Ac1 and Ac3 phase transformation temperatures of the studied steel. Could the authors provide this information?

3. Section 3.1 Microstructural characterization.

’’For samples inter-critically annealed below 550° C, inter-critical annealing allowed the transformation of only small volumes of polygonal ferrite at the austenite grain boundaries. The remainder of the microstructure transformed to bainite during the inter-critical annealing step.’’

‘’Figure 3. Typical microstructure for (a,b) benchmark processing study on commercial steel (c,d) primary processing treatment on direct strip cast and thin slab cast inter-critically annealed at 630 °C, (e,f) direct strip cast and thin slab cast inter-critically annealed at 530 °C…’’.

The intercritical annealing is performed in the two-phase (ferrite + austenite region). In other words, the temperature of the intercritical annealing is in the range between Ac1 and Ac3 temperatures. 530 ^oC is most probably below Ac1 temperature, and it seems to correspond to the temperature range of bainite transformation. See, for example, https://www.researchgate.net/publication/228370925_Non-Equilibrium_Phase_Transformation_Diagrams_in_Engineering_Alloys

Could the authors check it? If it is the case, then this heat treatment should be referred to as isothermal bainite heat treatment, not intercritical annealing.

4. The manuscript contains typos, missing words and inaccuracies. Some of them are listed below. The text should carefully edited.

Line 127. ‘’…by heating the specimens from room temperature 1200 °C for 240 minutes…’’

Line 143. ‘’…acrylic Technovit power…’’

Line 209. ‘’This is most evident in Fig 6b were the increase in…’’

Line 285. ‘’…have a grain boundary surface area of 0.007 mm2 /mm3 , compared to 0.07 0.007 285 mm2 /mm3 in the commercial steel…’’

5. All References should be styled. References 16, 19, 21, 25, and 26 should be corrected (missing journal name, missing publisher name, names of authors, etc.).

Author Response

Reviewer Comment:

This work focuses on physical simulation of direct strip casting and thin slab casting of a dual phase steel 780.  The effect of annealing parameters on the microstructure evolution is studied and compared to that of the commercial alloy processed via conventional hot rolling. It is shown that the kinetics of polygonal ferrite formation during the intercritical annealing are retarded in the strip cast samples compared to the commercial alloy. It was related to the formation of nanoprecipitates in the material.

The article fits very well into scope of the journal. It is well structured and written in a clear and concise manner. A few comments below could help to further improve the quality of the manuscript.

1. Table 1. It should be specified if the chemical composition is in wt. % or at. %.

Response: Thank you to the reviewer for finding this omission, this has no been corrected.

2. From the manuscript, it is not clear what are the Ac1 and Ac3 phase transformation temperatures of the studied steel. Could the authors provide this information?

Response: An excellent suggestion from the reviewer. This data is vaguely provided to the reader in Figure 11, but is now stated early in the manuscript in section 2.1

“ThermoCalc predictions of the A1 and A3 temperatures for this alloy are 668°C and 825°C.”

3. Section 3.1 Microstructural characterization.

’’For samples inter-critically annealed below 550° C, inter-critical annealing allowed the transformation of only small volumes of polygonal ferrite at the austenite grain boundaries. The remainder of the microstructure transformed to bainite during the inter-critical annealing step.’’

‘’Figure 3. Typical microstructure for (a,b) benchmark processing study on commercial steel (c,d) primary processing treatment on direct strip cast and thin slab cast inter-critically annealed at 630 °C, (e,f) direct strip cast and thin slab cast inter-critically annealed at 530 °C…’’.

The intercritical annealing is performed in the two-phase (ferrite + austenite region). In other words, the temperature of the intercritical annealing is in the range between Ac1 and Ac3 temperatures. 530 oC is most probably below Ac1 temperature, and it seems to correspond to the temperature range of bainite transformation. See, for example, https://www.researchgate.net/publication/228370925_Non-Equilibrium_Phase_Transformation_Diagrams_in_Engineering_Alloys

Could the authors check it? If it is the case, then this heat treatment should be referred to as isothermal bainite heat treatment, not intercritical annealing.

Response: The reviewer is correct that the term “inter-critical” refers to heat treatments between the A1 and A3 temperatures (in the 2 phase region). We have therefore removed the term “inter-critical annealing temperature” in several places and replaced it with the expression “heat treatment temperature”.

4. The manuscript contains typos, missing words and inaccuracies. Some of them are listed below. The text should carefully edited.

Line 127. ‘’…by heating the specimens from room temperature 1200 °C for 240 minutes…’’

Corrected

Line 143. ‘’…acrylic Technovit power…’’

The word Technovit refers to the brand of the powder used.

Line 209. ‘’This is most evident in Fig 6b were the increase in…’’

Corrected

Line 285. ‘’…have a grain boundary surface area of 0.007 mm2 /mm3 , compared to 0.07 0.007 285 mm2 /mm3 in the commercial steel…’’

Corrected

5. All References should be styled. References 16, 19, 21, 25, and 26 should be corrected (missing journal name, missing publisher name, names of authors, etc.).

The references have been updated.

Reviewer 4 Report

The contribution addresses the effect of direct strip casting on the kinetics of phase transformation of a dual phase steel. The topi cis interesting both from the scienitific and industrial point of view. The methodology is properly selected and applied to the well defined scientific problem. The kinetics of polygonal ferrite formation during the intercritical anneal were quantified using the JMAK approach, and this revealed significantly retarded transformation kinetics in the strip cast samples compared to the commercial alloy that was processed through the conventional hot rolling approach.

Remarks:

1/ The temperature control when preparing samples from the melt seems to be not sufficient. The explanation and estimation is required.

2/ The intercritical temperature in a range from 630 to 850°C has a very wide range. What is an optimal temperature ? What i san annealing time ?

3/ There was no plastic deformation. Hence, the phase transformation from the austenite to ferrite took place from the huge austenite grains. How does it affect the phase transformation kinetics ?

4/ Chapter 3.1. Anealing at 550°C is not done in the intercritical region as the authors stated. What is Ac1 temperature of the steel ?

5/ Summarizing discussion should be improved, especially concerning comparing the results to other authors. If there are no data for DP steels the discussion should based on steels of semilar chemical composition.

Author Response

Reviewer 4 comments:

The contribution addresses the effect of direct strip casting on the kinetics of phase transformation of a dual phase steel. The topi cis interesting both from the scienitific and industrial point of view. The methodology is properly selected and applied to the well defined scientific problem. The kinetics of polygonal ferrite formation during the intercritical anneal were quantified using the JMAK approach, and this revealed significantly retarded transformation kinetics in the strip cast samples compared to the commercial alloy that was processed through the conventional hot rolling approach.

1/ The temperature control when preparing samples from the melt seems to be not sufficient. The explanation and estimation is required.

Response: We agree with the reviewer that temperature control was difficult. For this reason we measured the specimen temperature using an optical pyrometer, and this data is shown in Fig 1c.

2/ The intercritical temperature in a range from 630 to 850°C has a very wide range. What is an optimal temperature ? What i san annealing time ?

Response: The two phase region has a predicted temperature range of 668°C and 825°C. This has now been added to the text in section 2.1. The inter-critical temperatures were chosen to cover this entire range, and overlap slightly at the upper and lower temperatures, to ensure that we examined the full range of temperatures where transformation could take place. We did not study an optimal temperature, simply quantified how fast the transformation occurred at the different temperatures to determine what effect strip casting had on the transformation.

3/ There was no plastic deformation. Hence, the phase transformation from the austenite to ferrite took place from the huge austenite grains. How does it affect the phase transformation kinetics ?

Response: We agree with the reviewer, that the large austenite grain size was the dominant effect in these specimens. For this reason we examine both austinite coarsened and refined specimens to test the hypothesis that the coarse grain size is the main reason that the kinetics of transformation are restricted. We can then conclude:

“The effect of prior austenite grain size was interrogated by producing specimens with both coarsened and refined austenite grain sizes. Results show that the large prior austenite grain is the main factor for slow transformation kinetics in DSC and TSC samples.”

 

4/ Chapter 3.1. Anealing at 550°C is not done in the intercritical region as the authors stated. What is Ac1 temperature of the steel ?

Response: This reviewer and reviewer 2 have both picked up on the same error. We have replaced the term “inter-critical annealing” with “heat treatment” in several places to address this concern. We have also calculated the A1 and A3 temperatures, and these are now stated in the manuscript text in section 2.1.

5/ Summarizing discussion should be improved, especially concerning comparing the results to other authors. If there are no data for DP steels the discussion should based on steels of semilar chemical composition.

An entire new section (Section 4.2) has been added to the revised manuscript that compares our data to the data publishing in literature.

Round 2

Reviewer 1 Report

The author has addressed most of the comments raised by the reviewer. I recommend the acceptance of this manuscript.

Author Response

No response text required

Reviewer 3 Report

The authors have revised the document, though some inaccuracies still are present in the text. They can be confusing for the readers. In some parts of the text, the authors refer to the annealing at 450 oC and 550 oC as intercritical annealing. The authors should correct it. For instance,

‘’Figure 4. Microstructure of samples after secondary processing at different inter-critical annealing temperatures as indicated. Heat treatment time was 5 minutes. All micrographs shown at the same magnification.’’

’’Figure 6. (a, b) Volume fraction and (c, d) grain size of ferrite with respect to inter-critical annealing conditions (legend applies to all graphs).’’

Author Response

We thank the reviewer for their thorough reading of the manuscript. The two figure captions have ben corrected. Two additional corrections in the main text have also been made.

Reviewer 4 Report

The authors addressed all of my comments and incorporated the required changes / modifications. They added one chapter covering the discussion with published earlier literature data. In my opinion the manuscript can be published in its current form.

 

Author Response

Thanks to the reviewer for their comments. We agree that the additional section has improved the manuscript.