Influence of Cooling Process Routes after Intercritical Annealing on Impact Toughness of Duplex Type Medium Mn Steel

Round 1

Reviewer 1 Report

The paper discussed the microstructures and mechanical properties of the duplex type low-C 1.5-3-5%Mn steels subjected to air cooling and isothermal transformation to obtain the different amount of reverted austenite. This study provides very valuable data showing the influence of volume fraction and stability of reverted austenite on mechanical properties, especially on the fracture morphology of the steels. The emphasis has been on the main aspects of the fracture modes. The paper has appropriate design and contains a lot of valuable data. I think the paper is suited to publication in the Metals and will be interesting for readers.

Thus I recommended this paper for publication after the following minor revisions:

• Why do the authors indicate only 5%Mn steel in the abstract while 1.5 and 3%Mn steels were also investigated?
• The dimensions of the samples for impact toughness and tensile tests should be indicated in the experiment section.
• The method of evaluation of average lath size of the studied steels wasn’t described in the text.
• Please describe in the more details the carbon-enrichment process of the reverted austenite in the steel after the air cooling (Lines 219-221).

Author Response

The paper discussed the microstructures and mechanical properties of the duplex type low-C 1.5-3-5%Mn steels subjected to air cooling and isothermal transformation to obtain the different amounts of reverted austenite. This study provides very valuable data showing the influence of volume fraction and stability of reverted austenite on mechanical properties, especially on the fracture morphology of the steels. The emphasis has been on the main aspects of the fracture modes. The paper has an appropriate design and contains a lot of valuable data. I think the paper is suited to publication in the Metals and will be interesting for readers.

 

Thus, I recommended this paper for publication after the following minor revisions:

 

[Reply] Thank you so much for your valuable comments and suggestions for us. We revised following the comments and suggestions as follows.

 

1. Why do the authors indicate only 5%Mn steel in the abstract while 1.5 and 3%Mn steels were also investigated?

 

[Reply]　This research aims is to investigate the microstructure and mechanical properties of the duplex type medium Mn steel. In general, medium Mn steel is defined as the steel containing 4 to 12 mass % Mn. Thus, we emphasize the various properties of the 5Mn steel as the medium Mn steel. The 1.5Mn and 3Mn steels were used to compare with the properties of the 5Mn steel.

 

2. The dimensions of the samples for impact toughness and tensile tests should be indicated in the experiment section.

 

[Reply] The dimensions of the samples for impact toughness and tensile tests are described at lines 86 to 88 on page 2, as follows.

 

Specimens for tensile tests (JIS-14B, 2.5 mm thick, 25 mm gauge in length, 4 mm wide) and sub-sized V-notched impact tests (JIS-5, 55 mm long, 10 mm wide, 2.5 mm thick) were machined from the plates along the rolling direction.

 

3. The method of evaluation of the average lath size of the studied steels wasn’t described in the text.

 

[Reply] The line intersecting method was used to measure the average lath size of the annealed martensite structure in SEM-EBSD images. So, “measured by line intersecting method” was added as follows.

 

The lath size of the annealed martensite structure measured by the line intersecting method is nearly the same in the cases of the AC and IT processes, although it considerably decreases with increasing Mn content. Martensite–austenite (MA) phase exists only in the 1.5Mn steel.

 

4. Please describe in more detail the carbon-enrichment process of the reverted austenite in the steel after the air cooling (Lines 219-221).

 

[Reply] I presume that “inadequate” is difficult to understand. So, I change the “inadequate” into “short time”.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper presents the work exploring the effect of cooling process routs, i.e. air-cooling and isothermal-transformation, directly after intercritical annealing on the impact toughness of a duplex type medium-Mn steel. It was found that impact value and ductile-brittle transition temperature was deteriorated by the air-cooling process compared with the isothermal-transformation process. The reasons for this variation were discussed. Generally, the experimental procedures, results and interpretation are reasonable and in-depth. But there are some places where minor revisions are needed to make it right and clear before considering it for publication.

1. Page 6 lines 249-251: “In addition, the AC process brings on nearly the same TS×TEl as the IT process in all steels although it decreases the uniform and total elongations except for the 1.5Mn steel”. Here, I think that “although it decreases ……” should be “although it increases ……” as can seen from Table 2 and Figure 7.

2. It is suggested that more explanations of Figure 13 is needed.

Author Response

Comments and Suggestions for Authors

This paper presents the work exploring the effect of cooling process routs, i.e. air-cooling and isothermal-transformation, directly after intercritical annealing on the impact toughness of a duplex type medium-Mn steel. It was found that impact value and ductile-brittle transition temperature were deteriorated by the air-cooling process compared with the isothermal-transformation process. The reasons for this variation were discussed. Generally, the experimental procedures, results and interpretation are reasonable and in-depth. But there are some places where minor revisions are needed to make it right and clear before considering it for publication.

 

[Reply] Thank you so much for your valuable comments and suggestions for us. We revised following the comments and suggestions as follows.

 

1. Page 6 lines 249-251: “In addition, the AC process brings on nearly the same TS×TEl as the IT process in all steels although it decreases the uniform and total elongations except for the 1.5Mn steel”. Here, I think that “although it decreases ……” should be “although it increases ……” as can see from Table 2 and Figure 7.

 

[Reply] I think that “decreases” is right.

 

2. It is suggested that more explanation of Figure 13 is needed.

 

[Reply] The explanation in Figure 13 was revised as follows,

 

In the IT process, the mechanical stability of reverted austenite considerably increases at the testing temperatures between 100 °C and 200 °C [36] (Figure 13).

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Sugimoto et al. [36] investigated the testing temperature dependence of the mechanical stability of the reverted austenite in the 1.5Mn, 3Mn and 5Mn steels subjected to the IT process and found that the mechanical stability of the 3Mn and 5Mn steels is considerably influenced by the testing temperature compared to that of the 1.5Mn steel as shown in Figure 13. Namely, the mechanical stability at the temperatures lower than 25 °C is lower than that of the 1.5Mn steel but it at the temperatures between 100 °C and 200 °C is higher than that of 1.5Mn steel.  

Reviewer 3 Report

The authors have studied the influence of two different cooling process routes on the mechanical properties and impact toughness of three hot rolled C-Mn-Si TRIP steels with Mn contents between 1.5 and 5%. This paper follows on from a previous paper published by the same authors and it should be noted that five of the figures are reprinted from other work. A large amount of useful data is provided and this is a subject area of considerable interest. The paper is fairly well written and clear. On the experimental side, the techniques used are appropriate although the use of sub-sized tensile and Charpy-V samples needs to be justified. Some of the analysis needs to be reviewed (see below).   

• Please change “Routs” to “Routes” everywhere.
• Please provide indications for the measurement errors in phase fractions, compositions, lath sizes etc.
• Figure 1: The A,F and P domains are not given for the 3Mn and 5Mn alloys – please correct this as it is an important omission for understanding the cooling behaviour.
• Table 1: Can you say how these values compare with calculated Ae1 and Ae3 temperatures?
• If hot rolling was done with 100kg slabs, why were sub-sized tensile specimens used ?
• Similarly, what was the reason for using sub-size 2.5 mm Charpy bars when (presumably) thicker plates could have been produced? Have all of the Charpy energies given been converted to standard dimension values?
• Figure 2: Are the air cooling profiles drawn accurately? The initial cooling rates should depend upon the start temperature.
• Was any cementite detected in the AC samples? Was bainite present in the IT samples?
• What was the step size used in the EBSD images? The EBSD austenite phase fractions in Figure 3 appear to be much smaller than the XRD values in Table 2? Can the authors comment?
• Why is there more retained austenite in the 1.5% Mn AC sample than in the 1.5% Mn IT sample? This does not make sense to me as (from Figures 1 and 2) you should hit the pearlite nose in the AC sample.
• The authors often make statements like “the IT process increases the volume fraction of the reverted austenite”. This is confusing – the cooling process changes the decomposition kinetics of the reverted austenite fraction present at the end of intercritical annealing. It is more correct to say that the IT process increases the volume fraction of retained austenite (after reversion).
• The authors appear to assume that austenite stability is determined by carbon content alone. This ignores the contribution of Mn partitioning, which is known to be critical for medium Mn alloys. This should be discussed in the text.
• Line 237 – I do not know of any case where austenite decomposes to bainite following an increase in temperature?
• Figure 10: Are all the data points for 2.5 mm Charpy samples?
• Figure 12: Please define “non-quasi cleavage fracture”. Is it intergranular, cleavage or mixed?

Author Response

The authors have studied the influence of two different cooling process routes on the mechanical properties and impact toughness of three hot rolled C-Mn-Si TRIP steels with Mn contents between 1.5 and 5%. This paper follows on from a previous paper published by the same authors and it should be noted that five of the figures are reprinted from other work. A large amount of useful data is provided and this is a subject area of considerable interest. The paper is fairly well written and clear. On the experimental side, the techniques used are appropriate although the use of sub-sized tensile and Charpy-V samples needs to be justified. Some of the analysis needs to be reviewed (see below).  

 

[Reply] Thank you so much for your valuable comments and suggestions for us. We revised following the comments and suggestions as follows.

 

1. Please change “Routs” to “Routes” everywhere.

 

[Reply] Thank you so much. We changed “Routs” to “Routes”.

 

2. Please provide indications for the measurement errors in phase fractions, compositions, lath sizes etc.

 

[Reply] The measurement errors are very important to understand the properties of phase fractions, compositions, lath sizes, etc. Unfortunately, I have not this full data because significant differences in these properties are clear.

 

3. Figure 1: The A, F and P domains are not given for the 3Mn and 5Mn alloys – please correct this as it is an important omission for understanding the cooling behaviour.

 

[Reply] In the 3Mn and 5Mn steels, ferrite and pearlite transformations did not take place. In this case, “A” was omitted to avoid complications.

 

4. Table 1: Can you say how these values compare with calculated Ae1 and Ae3 temperatures?

 

[Reply] The measured Ae1 and Ae3 are not compared with the calculated temperatures because these temperatures are shown for reference. The most important temperatures are intercritical annealing temperatures which were chosen as the temperatures at which the ferrite and austenite volume fractions are 50% and 50%, respectively.

 

 

5. If hot rolling was done with 100kg slabs, why were sub-sized tensile specimens used?

Similarly, what was the reason for using sub-size 2.5 mm Charpy bars when (presumably) thicker plates could have been produced? Have all of the Charpy energies given been converted to standard dimension values?

 

[Reply] Our research group compares the impact properties of the first and third AHSSs, as well as tensile properties. These AHSSs have different hardenability. So, small size specimens like sub-sized 2.5 mm Charpy bars and the sub-sized tensile specimens are needed to obtain the ideal uniform microstructure. 

 

6. Figure 2: Are the air-cooling profiles drawn accurately? The initial cooling rates should depend upon the start temperature.

 

[Reply] The air-cooling profiles are not drawn accurately. We correct slightly the cooling profile of this figure. Please understand that this diagram is a rough profile.

 

7. Was any cementite detected in the AC samples? Was bainite present in the IT samples?

 

[Reply] There are no cementite and bainite in the AC samples. This is well known in the D-MMn steel.

 

8. What was the step size used in the EBSD images? The EBSD austenite phase fractions in Figure 3 appear to be much smaller than the XRD values in Table 2? Can the authors comment?

 

[Reply] The step size used in the EBSD images is 0.1 μm. This was added to the manuscript.

In general, the reverted austenite fraction measured by X-ray is larger than that measured EBSD. So, we use the volume fraction measured by X-ray.

 

9. Why is there more retained austenite in the 1.5% Mn AC sample than in the 1.5% Mn IT sample? This does not make sense to me as (from Figures 1 and 2) you should hit the pearlite nose in the AC sample.

 

[Reply] High reverted austenite fraction of 1.5Mn AC sample is caused by its low carbon and manganese concentrations. Such a result is also reported by many researchers. This result does not relate to the pearlite nose because the pearlite does not present in all samples.

 

10. The authors often make statements like “the IT process increases the volume fraction of the reverted austenite”. This is confusing – the cooling process changes the decomposition kinetics of the reverted austenite fraction present at the end of intercritical annealing. It is more correct to say that the IT process increases the volume fraction of retained austenite (after reversion).

 

[Reply] The reviewer’s opinion is right. However, we cannot use “the retained austenite” in this paper because the reverted austenite fraction means the volume fraction decomposed during cooling.

 

11. The authors appear to assume that austenite stability is determined by carbon content alone. This ignores the contribution of Mn partitioning, which is known to be critical for medium Mn alloys. This should be discussed in the text.

 

[Reply] In this research, the mechanical stability of the reverted austenite is evaluated by k-value. The carbon concentration is shown for reference because the mechanical stability is also controlled by Mn concentration as you think.

 

12. Line 237 – I do not know of any case where austenite decomposes to bainite following an increase in temperature?

 

[Reply] It is well known that austenite decomposes to bainite or ferrite plus carbide following an increase in temperature by many researchers including us. Please confirm the fact in a paper of Ref. 35.

 

13. Figure 10: Are all the data points for 2.5 mm Charpy samples?

 

[Reply] Yes.

 

14. Figure 12: Please define “non-quasi cleavage fracture”. Is it intergranular, cleavage or mixed?

 

[Reply] ”non-quasi-cleavage fracture means any fracture other than quasi-cleavage fracture. In this paper, we expect intergranular fracture as non-quasi-cleavage.

 

 

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

My apologies but I cannot review this paper without having a clear idea of the errors present in the microstructural characterisation experiments.

Author Response

My apologies but I cannot review this paper without having a clear idea of the errors present in the microstructural characterization experiments.

 

[Reply] I think that the above comment equivalents to the following comment of Round 1.

“Please provide indications for the measurement errors in phase fractions, compositions, lath sizes, etc.”

Your opinion is correct. Like the reply to your comment of round 1, we have not the measurement errors in phase fraction, composition, lath size, etc., in the same way as those of other mechanical properties. However, we obtain the average values measured at three locations to increase the reliability. Such a method is accepted by many researchers. So, we added the following sentence on page 3.

 

In this research, the average values of volume fractions and carbon concentrations of reverted austenite measured at three locations were adopted, as well as the lath size of annealed martensite structure.

Author Response File: Author Response.pdf