Dynamic Phase Transformation Behavior of a Nb-microalloyed Steel during Roughing Passes at Temperatures above the Ae₃

36

 “produces   phase transformation”…  Awkward   sentence

44-46

Awkward sentence

46

……”mechanical   tests in a Gleeble”   reword

51

…backward   … Do you mean reverse?

55

…”was   attributed as resulting”….    Poor wording

63

…” the   obstacle”   … poor wording

70

“Thus the   present study…”   Be more specific what you are actually   going to do. What are these cooling conditions? I know the actual numbers are   in the experimental portion but perhaps you can explain the rationale for the   cooling numbers you have chosen.  Also,   you do NOT mention cooling in your abstract.

Introduction   Synopsis

What is the importance of DT in TMCP   processing of microalloyed steels. Perhaps you can include a few sentences   that indicate why we should care about DT?    Is it beneficial or detrimental etc.

98

“A   thermocouple was …..”  This   statement would be more appropriate when discussing the sample itself in the   previous paragraph.

98

Why was a strain of 30% chosen? This   is higher than would be expected in industrial rolling.  How does your choice of strain affect DT?

127

You mention Dynamic softening. Are you   assuming that DRX is 100% complete? Is Metadynamic recrystallization   occurring? Or given the10 second lag time between deformation perhaps static recrystallization   is occurring?

132

“…various   softening phenomena….”  This is a vague   statement

Figure 2

How  many Gleeble samples were actually tested? Figure 2 appears  (I am not sure) to show  the results from the 5 pass sample. Was this test repeated? How do the Equivalent stress values shown in  Figure 2 compare with the values obtained from other samples (1 pass, 2 pass etc)?

141-142

 “30 to   60 µm …grain size are typical…”  Do   you have a reference for this? This grain size seems small for the starting   grain size for industrial hot plate rolling.

158 -161

Do you need to put the actual %ferrite   numbers in the text.  We can see the   trends quite clearly in Figure 4. Perhaps just describe Figure 4

162

Are you saying that the amount of DT   ferrite is cumulative or that the amount produced at each rolling temperature   is independent of the previous step. It is not clear from the text. 

Figure 7

Developing a trend on the basis of 3   data points is somewhat misleading.Perhaps you can explain on a fundamental level why the fitted curve has the shape it does.

Figure 8

How do the critical strains for DRX   (Figure 8) compare with literature values and/or calculated values? Please comment.

Figure 8

Figure   8 shows the critical strain for DRX decreases with temperature (i.e., pass   number).  All else being equal, we would assume that as the Z value increases (with decreasing T) the critical          strain for DRX should increase with pass number.  How do you    explain this? Is this attributed   to prior/retained deformation.      Would a longer interpass time allow for this  effect to be minimized? Please  provide additional information in the text.

Figure 9

What   is the role of NbC precipitation as you cool from 1100 to 1020°C?  Base on the solubility constant and the   composition of the steels tested we would expect some NbC precipitation   (perhaps strain induced) to occur.    Please comment on this.

Figure 9

The   Total Energy Obstacles (Figure 9) calculation assumes very specific values   for the dilation and shear strains. In addition, you appear to have only conducted  5  tests? Given that the Driving     force   curve and the Total Obstacle curve are quite close (particularly at T <   1050°C) are you confident that the difference between the curves is real?   Please comment on this.

Conclusion 1

You should specify that this conclusion only applies  to the one set of conditions you tested.

Conclusion 2

This is not really a conclusion but just a validation of your %ferrite values. By the way, why did you mention a TTT   curve in this

conclusion?  I do not see a TTT curve in the paper.

Conclusion 3

You did not study the effect of Nb on  DT.  This conclusion is

basically a   repeat of Conclusion 1.

36

 “produces     phase transformation”…  Awkward   sentence

Response: This has been corrected in the revised manuscript. Please see below   (36-37):

“The   thermomechanical processing of steels is carried out primarily within the   austenite phase field. Previous work has shown that hot rolling produces   partial phase transformation of austenite into ferrite in the roll bite   inside the single austenite phase field [1,2].”

44-46

Awkward sentence

Response: This has been addressed in the revised manuscript. Please see below   (43-46):

“ They captured the diffraction patterns associated with   α-ferrite during deformation. Chen and Chen [6] performed experiments in 2003   by using a laser dilatometry technique and observed the reverse   transformation of dynamically transformed ferrite into austenite at   temperatures above the Ae₃.”

46

……”mechanical     tests in a Gleeble”   reword

Response: This has been reworded in the revised manuscript. Please see below   (49-50):

“The former authors deformed a 0.17%C   plain carbon steel using a Gleeble thermomechanical simulator above the Ae₃.”

51

…backward     … Do you mean reverse?

Response: This has been clarified in the revised   manuscript. Please see below (50-51):

“They confirmed the existence of both the forward and reverse   transformation at temperatures up to 115°C above the Ae₃.” 

55

…”was     attributed as resulting”….    Poor wording

Response: This has been paraphrased in the revised manuscript. Please see below   (54-56):

“ This   phenomenon was a result of dislocation pinning and solute drag of the niobium   carbonitride precipitates and Nb in solution, respectively.”

63

…” the   obstacle”   …   poor wording

Response: This has been modified in the revised manuscript. Please see below   (62-64):

“The free energy barrier against the driving force for DT   consists of the Gibbs free energy difference between the phases as well as   the lattice dilatation work and shear accommodation work.”

70

“Thus the     present study…”   Be more specific what you are actually   going to   do. What are these cooling conditions? I know the actual numbers are     in the experimental portion but perhaps you can explain the rationale for the     cooling numbers you have chosen.  Also,   you do NOT mention   cooling in your abstract.

Response: Thanks for the suggestions. This has been addressed in the modified   manuscript. Please see below (20-21 and 73-74):

“A five-pass torsion simulation of the roughing passes applied   during hot plate rolling was performed in the single-phase austenite region   of a Nb-microalloyed steel under continuous cooling conditions.”

“Thus the present study represents an advance over the previous   investigations in that the DT behavior of a Nb-microalloyed steel is   investigated under continuous cooling condition of 2°C/s, comparable to   industrial roughing rolling schedules.”

Introduction   Synopsis

What is the   importance of DT in TMCP   processing of microalloyed steels. Perhaps   you can include a few sentences   that indicate why we should care about   DT?    Is it beneficial or detrimental etc.

Response: Thanks for the feedback. We certainly agree with this comment.   Additional sentences were added to address this concern. Please see below (70-75):

“The occurrence   of DT above the Ae₃ temperature during thermomechanical processing   is known to generate lower rolling loads and mean flow stresses (MFS) [16].   Moreover, the volume flow rate (as the bar passes through a rolling mill)   increases due to formation of less dense ferrite. This type of transformation   involves carbon partitioning, which can generate undesirable volume fractions   of martensite. Thus, an accurate account of phases during high temperature   deformation leads to better mechanical properties of the material.”

98

“A     thermocouple was …..”  This   statement would be more appropriate when discussing   the sample itself in the   previous paragraph.

Response: The sentence has been moved to the appropriate section. Please see   below (97-98):

“A thermocouple was welded to the sample to accurately track   the deformation temperatures at every pass.”

98

Why was a strain   of 30% chosen? This   is higher than would be expected in industrial   rolling.  How does your choice of strain affect DT?

Response: Thanks for clarification. More information has been added. Please see   below (108-110):

“Note that the samples were strained to 0.3 during each pass   applied at a strain rate of 1 s^-1. A strain higher than the   critical stains for the onset of DT [9-15] was selected so as to allow for DT   to take place.”

127

You mention   Dynamic softening. Are you   assuming that DRX is 100% complete? Is   Metadynamic recrystallization   occurring? Or given the10 second lag   time between deformation perhaps static recrystallization is occurring?

Response: We certainly agree with the reviewer that other types of   recrystallization should be accounted, thus, the statement has been modified.   Please see below (160-165):

“Note that after the five-pass simulation   (see Figure 3d), the grain sizes decreased to less than 10 μm, which suggests   the occurrence of either static recrystallization (SRX), metadynamic   recrystallization (MDRX), dynamic transformation (DT), or combination of   these softening mechanisms. Although a long interpass time of 10s was   employed, it is important to note that the presence of Nb can significantly   delay SRX in-between passes.”  

132

“…various     softening phenomena….”  This is a vague   statement

Response: Thanks doe pointing this out. The statement has   been modified. Please see below (145-147):

“This is an indication of softening by combination of   recrystallization and phase transformation occurring in the material during   deformation.”

Figure 2

How  many Gleeble samples were actually   tested? Figure 2 appears  (I am not sure) to show  the results from   the 5 pass sample. Was this test repeated? How do the Equivalent stress   values shown in  Figure 2 compare with the values obtained from other   samples (1 pass, 2 pass etc)?

Response: The authors certainly agree to include this information to give the   readers an idea about the accuracy of the experiments. More information has   been added. Please see below (113-114):

“Additionally, the experiments were repeated three times to   validate the results. In general, less than 3% difference in the level flow   curves was observed.”

141-142

 “30 to     60 µm …grain size are typical…”  Do   you have a   reference for this? This grain size seems small for the starting   grain   size for industrial hot plate rolling.

Response: Thank you for clarification. We agree with the reviewer that the   initial grain size are quite small compared to the industrial hot plate   rolling. The authors also measured the actual grains for the readers to have   a better idea about the measured sizes. Please see below (157-160):

“ Here the grain sizes before the initial deformation are quite   large, which was measured to be around 54 ± 15 μm (see Figure 3a). These   grain sizes are slightly smaller than the typical sizes of austenite phase in   industrial plate rolling before applying the roughing passes.”

158 -161

Do you need to   put the actual %ferrite   numbers in the text.  We can see the     trends quite clearly in Figure 4. Perhaps just describe Figure 4

Response: This has been modified. Please see below (179-180):

“Even though the reverse   transformation of ferrite back into autenite can take place during the pass intervals,   the amount of ferrite continously increases with applied strain.”

162

Are you saying   that the amount of DT   ferrite is cumulative or that the amount   produced at each rolling temperature   is independent of the previous   step. It is not clear from the text. 

Response: This has now been clarified in the revised manuscript. Please see   below (178-179):

“ The volume fraction of ferrite   was measured based on the cumulative strain.”

Figure 7

Developing a   trend on the basis of 3  data points is somewhat misleading.Perhaps you   can explain on a fundamental level why the fitted curve has the shape it   does. 

Response: Thank you for this comment. Please note that there are 5 data points   that were used in Figures 7 and 8. The minima of the curves in Figure 7 is   associated with softening mechanisms of the material. This approach is   commonly used to calculate for the onset of various softening mechanisms such   as recrystallization and phase transformation. We agree that these data might   not be enough to generate a smooth curve in Figure 8, thus, we opt to just   display data points. Please refer to 247-248.

Figure 8

How do the   critical strains for DRX   (Figure 8) compare with literature values   and/or calculated values?

Response: More detail has been added to address this concern. Please see below   (234-237):

“These values are consistent with previous   investigations of the present authors in the present material [21]. Moreover,   the critical strains for DRX falls within the range of values shown in the   literature [22] using the same method employed in the current work [19].”

Figure 8

Figure   8 shows the critical strain for DRX   decreases with temperature (i.e., pass   number).  All else being   equal, we would assume that as the Z value increases (with decreasing T) the   critical strain for DRX should increase with pass number.  How do   you    explain this? Is this attributed   to prior/retained   deformation.  Would a longer interpass time allow for this  effect   to be minimized? Please  provide additional information in the   text. 

Response: Thank you for this comment. We certainly agree with the reviewer.   Additional sentences have been inserted to include this point. Please see   below (237-245):

“The critical strains for both DT and DRX   display a slight decrease with increasing pass number. Note that for DRX, the   critical strains should increase during cooling from one pass to another. The   discrepancy in the trend of critical strains can be attributed to retained   work hardening from previous pass. This is quite noticeable in the flow   curves displayed in Fig. 2. Since it is expected that NbC precipitates can   form and pin down the dislocations, there is less recovery in-between passes.   This leads to higher retained work hardening, which provides additional   energy on top of the applied stress. A lower retained work hardening is   expected if the material is low-alloyed steel or if the interpass time is increased   [16].”

Figure 9

What   is the role of NbC precipitation as   you cool from 1100 to 1020°C?  Base on the solubility constant and the     composition of the steels tested we would expect some NbC   precipitation   (perhaps strain induced) to occur.    Please   comment on this.

Response: Thank you for pointing this out. This question   has been covered by the answer above (237-245).

Figure 9

The   Total Energy Obstacles (Figure 9)   calculation assumes very specific values   for the dilation and shear   strains. In addition, you appear to have only conducted  5  tests?   Given that the Driving     force   curve and the   Total Obstacle curve are quite close (particularly at T <   1050°C)   are you confident that the difference between the curves is real?     Please comment on this.

Response: We certainly   agree that there are assumptions made when the driving forces and energy   obstacles were calculated. Thus, more details and error bars have been added   to account for this. Please see below (277-280 and 290-297):

“ Since the dilatation and shear accommodation   strains are difficult to measure experimentally, the present work assumed   values of 0.03 and 0.36, respectively [24]. These values are based on the   theoretical required deformation strains to transform austenite into   ferrite.”

“Although the calculated   values may not represent the exact driving force and total energy obstacles   due to the assumptions specified above, the difference between the curves   look reasonable. This is supported by the microstructures which show that   austenite transforms into ferrite. The kinetics of transformation may   possibly be hindered by niobium which can delay the progression of   transformation due to pinning and/or solute drag effects [16]. For this   reason, the present material only obtained less than 10 % ferrite at total   applied torsional strain of 1.5. This may also be the reason for lower   difference between the calculated driving forces and total energy obstacle.”

Conclusion 1

You should   specify that this conclusion only applies  to the one set of conditions   you tested.

Response: Conclusion 1 has been modified. Please see below (311-314):

“Dynamic transformation of austenite to ferrite can take place   during roughing passes of the plate rolling process and its volume fractions   formed and retained in the simulations increase as the pass number increases.   The present work showed that this can take place on a Nb-microalloyed steel   subjected to roughing passes at temperature range 1020 to 1100 °C.”

Conclusion 2

This is not   really a conclusion but just a validation of your %ferrite values. By the   way, why did you mention a TTT   curve in this 

conclusion?    I do not see a TTT curve in the paper.

Response: The authors apologize for this mistake.   Conclusion 2 has been revised. Please see below (315-317):

“The calculated CCT diagrams confirmed that the   measured cooling rate of 1200 °C/s is enough to prevent the formation of   ferrite by cooling. Therefore, the volume fraction of ferrite measured in   present work is only attributed to the applied deformation.”

Conclusion 3

You did not study   the effect of Nb on  DT.  This conclusion is

basically a     repeat of Conclusion 1.

Response: Conclusion 3 has been modified to avoid confusion. Please see below   (318-320):

“The thermodynamic calculations show that the driving force   for DT is higher than the total barrier in the present investigation.   Assumptions were made on the values of dilatation and shear accommodation   strains.”