Study on Microstructure and Properties of Tailored Hot-Stamped U-shaped Parts Based on Temperature Field Control
Round 1
Reviewer 1 Report
I understood that this paper is about the change in texture and hardness due to temperature control in hot stamping.
I have 2 questions
1) We can see that there is a change in hardness due to temperature control, but the amount of change seems to be small. To what extent do you think that brittleness and ductility change with this amount of change?
2) How many points did you measure hardness? Please put an error bar if you seem to go in multiple places.
If there is only one point, there is a possibility of variation, so I think that we should measure at multiple points.
Author Response
We thanks a lot for your constructive criticisms that have helped us to improve our manuscript. The point-by-point response to the comments is given below.
Response to comment 1: We can see that there is a change in hardness due to temperature control, but the amount of change seems to be small. To what extent do you think that brittleness and ductility change with this amount of change?
Response 1: We are sorry that we failed to make us clearly.
For the quenching zone, because of the cooling effect of high-speed air flow, the sheet metal is cooled rapidly. Therefore, the hardness and toughness do not change significantly, which can be seen from the phenomenon of average elongation between 6.3 and 7.05%. For the slow cooling zone, the die temperature rises from 300 to 600°C, the cooling speed of the blank decreases, the hardness at Position H1 and H2 reduce by 47.1% and 52.8%, respectively. Thus, the toughness of slow cooling zone improves with the increase of die temperature, which can be confirmed by the increase of elongation from 8.41 to 16.09%.
Response to comment 1: How many points did you measure hardness? Please put an error bar if you seem to go in multiple places. If there is only one point, there is a possibility of variation, so I think that we should measure at multiple points.
Response 2: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments.
Chapter 2.3 of the article mentions that each data point represents an average measurement of five measurements from each region.
According to your suggestion, we have revised the micro-hardness diagram. Since another reviewer asked for scatter points to be attached to the image, we drew two images here to meet the recommendations of the two reviewers.( If the scatter points and error bar are attached to a picture at the same time, there is some confusion, so we draw separately here, please understand.)
Figure 12. Hardness of different locations at different heated die temperatures.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript contains some interesting results but their analysis is doubtful. Some described aspects need to be clarified. Moreover I suggest to make some corrections in the text of your paper - listed below.
- Title – Present title doesn’t cover the paper content. It is necessary to extend the part of microstructure analysis – relevant details in further part of my review.
Line 31 – For me is rather obvious the meaning of “CCT” abbreviation. Nevertheless I suggest to use full name when is written first time in the text – “…continuous-cooling-transformation (CCT) diagram …”.
Line 53 – The caption do not indicate the relationship between both images. I suggest to write it in this way (more less) – “Phase composition, predicted based on CCT diagram for B1500HS steel [4] (a), in examined U-shaped parts (b)”.
Line 75 –The term “metallographic structure” is not correct. It should be “The microstructure of the ….”.
Lines 112 and 138 – Please, consider to add in the captions “predicted” (maximum temperature).
Line 153 – Fig. 7 – I’m working on printed version of your manuscript. Marks on the image are hardly readable. Consider to use brighter font color.
Lines 155-160 – The text of this paragraph is written poorly. I’m afraid you don’t have to much experience in metallography. What does “The polished and polished samples …” mean? For etching process the etching reagent (or just “etchant”) is used – not “corrosive agent”! What is the reason to write about colors of martensite, bainite or ferrite? Did you use light polarized method? How can you describe the microstructure without even one its image? Such description has no sense without relevant figures. Therefore I strictly suggest to add in this part or in “Results and Discussion” some pictures of the microstructure. I can understand you use Image Pro Plus software which gives you some results but they should be verified.
Lines 177-178 – These data are unbelievable whiteout microstructure images – see previous remark. Moreover – it is more correct to write “Area fraction of …” (Y axes).
Line 192 – You determined material hardness based on five measurements. I wonder what was the scatter of values. You should add scatter bars the graph.
Lines 197-198 – You write “… representing fully martensitic microstructure” – this calls for relevant picture confirming this observation!
Lines 213-217 – Figure 11. It is useful to show fracture surface – I agree. It would be more useful to add also scale bar (for evaluation of dimple size). You present six images using only two indicators: a) and b). Every image should be denoted and the caption should be rewrite as follow: “Fracture morphology at Location C1 under 300 (a) and 500°C (b) and Location H1 under 300 (c), 400 (d), 500 (e) and 600°C (f).”
There is no information in methodology part about SEM equipment used in experiment – it should be completed.
Lines 222-223 – This conclusion should be supported by relevant microscopic results in “Results and Discussion” section.
Author Response
We thanks a lot for your constructive criticisms that have helped us to improve our manuscript. The point-by-point response to the comments is given below.
Response to comment 1: Line 31 – For me is rather obvious the meaning of “CCT” abbreviation. Nevertheless I suggest to use full name when is written first time in the text – “…continuous-cooling-transformation (CCT) diagram …”.
Response 1: Considering the Reviewer’s suggestion, we have added the full name “Continuous Cooling Transformation (CCT) ” when CCT is written for the first time in the text.
Response to comment 2: Line 53 – The caption do not indicate the relationship between both images. I suggest to write it in this way (more less) – “Phase composition, predicted based on CCT diagram for B1500HS steel [4] (a), in examined U-shaped parts (b)”.
Response 2: Thank you for your kind suggestions. We have changed the caption of Figure 1 to " Figure 1. Phase composition: (a) Predicted based on CCT diagram for B1500HS steel [5]; (b) In examined U-shaped parts. "
Response to comment 3: Line 75 –The term “metallographic structure” is not correct. It should be “The microstructure of the ….”.
Response 3: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments. The revised sentences are as follows:
The microstructure of the original materials are ferritic-pearlitic.
Response to comment 4: Lines 112 and 138 – Please, consider to add in the captions “predicted” (maximum temperature).
Response 4: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments. The revised caption are as follows:
Figure 4. The relationship between the predicted maximum temperature of the as-quenched blank and holding time at different die temperatures.
In addition, "predicted" was added to the relevant position of the article (lines 95, 103, 105, 114, 120, 130, 132, 140).
Response to comment 5: Line 153 – Fig. 7 – I’m working on printed version of your manuscript. Marks on the image are hardly readable. Consider to use brighter font color.
Response 5: Thank you for your kind suggestions. We have changed the marks on the image to white for clear display.
Figure 7. The two regions and the sampling position.
Response to comment 6: Lines 155-160 – The text of this paragraph is written poorly. I’m afraid you don’t have to much experience in metallography. What does “The polished and polished samples …” mean? For etching process the etching reagent (or just “etchant”) is used – not “corrosive agent”! What is the reason to write about colors of martensite, bainite or ferrite? Did you use light polarized method? How can you describe the microstructure without even one its image? Such description has no sense without relevant figures. Therefore I strictly suggest to add in this part or in “Results and Discussion” some pictures of the microstructure. I can understand you use Image Pro Plus software which gives you some results but they should be verified.
Response 6: I'm very sorry for your misunderstanding because of my mistake. What we want to represent is that “The ground and polished samples” rather than “The polished and polished samples”. We have made Corrected the expression in the article.
I am very sorry that the statement of the sentence is incorrect due to my mistake. We have made correction according to the Reviewer’s comments. Replace the sentence with the following:
The ground and polished samples were treated with etching reagent for microstructure observation.
We are sorry that we failed to make us clearly. The reason to write about colors of martensite, bainite or ferrite is that after etching the samples with etching reagent, different phases present different colors, which are used to quantify the area fraction of the quenched phases. Specific explanations are as follows:
SEM images revealed the characteristic structures of the various phases, however, those images were not useful for quantification of area fractions when multiple phases were present. A two-stage colour tint etching procedure was used to reveal the various phases in different colours [1]. The etched specimens were observed using a ZEISS Axio. Scope. A1 metallographic microscope, and it was found that martensite was yellow-brown, bainite was black and ferrite was white. The ImagePro Plus 6.0 analysis software was used to manually colour the phases red (bainite), green (martensite), and blue (ferrite) and subsequently quantify the area fractions present.
Thank you for your kind suggestions. We add some images of microstructures here.
The typical microstructures observed from C1, C2, H1 and H2 locations are shown in Figure 9. It can be seen from Figure 9a and 9b that the microstructures for C1 and C2 locations exhibit the packets of parallel lath crystals which are characteristic of martensite [10]. From Figure 9c to 9f, you can see that the characteristic feature of bainite is a ferrite matrix with dispersed cementite particles [11]. As depicted in Figure 9c and 9d, there is a little difference in the organization of H1 and H2 positions when the heated die temperature is 400°C.
Figure 9. The SEM image at: (a) Location C1 with a 300℃ heated die temperature; (b) Location C2 with a 500℃ heated die temperature; (c) Location H1 with a 400℃ heated die temperature; (d) Location H2 with a 400℃ heated die temperature; (e) Location H1 with a 500℃ heated die temperature; and (f) Location H1 with a 600℃ heated die temperature.
Two-stage colour tint etched optical micrographs and the manually generated microstructure images are shown in Figure 10. As shown in Figure 10a, the microstructures for C1 location are found to be predominantly martensitic with some bainite. The formation of abundant martensite is because of the higher cooling rate which is the result of low die temperature. The formation of bainite and ferrite is due to the longer transfer time and the left shift of the CCT which is caused by the plastic deformation[5]. It can be concluded from Figure 10b and 10c, both heated die conditions result in a mixed martensite, bainite and ferrite microstructure with visibly larger amounts of the softer bainite phase for the 600°C die condition.
Figure 10. Various micrographs showing (from top to bottom) two-stage colour tint etched optical micrographs, and the manually generated microstructure images at: (a) Location C1 with a 300℃ heated die temperature; (b) Location H1 with a 300℃ heated die temperature; and (c) Location H1 with a 600 ℃ heated die temperature.
[10] Naylor, J.P. Influence of the Lath Morphology on the Yield Stress and Transition-Temperature of Martensitic-Bainitic Steels. Metall and Mater Transactions 1979, 10, 861–873.
[11] Feng, C.; Fang, H.S.; Zheng, Y.K.; Bai, B.Z. Mn-series low-carbon air-cooled bainitic steel containing niobium of 0.02%. Iron and Steel Research, Int 2010, 17, 53-58.
Response to comment 7: Lines 177-178 – These data are unbelievable whiteout microstructure images – see previous remark. Moreover – it is more correct to write “Area fraction of …” (Y axes).
Response 7: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments.
Figure 11. The area fraction under different tool temperatures of: (a) Martensite; (b) Bainite; and (c) Ferrite.
Response to comment 8: Line 192 – You determined material hardness based on five measurements. I wonder what was the scatter of values. You should add scatter bars the graph.
Response 8: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments.
Since another reviewer asked for error bar to be attached to the drawings, we drew two images here to meet the recommendations of the two reviewers. ( If the scatter points and error bar are attached to a picture at the same time, there is some confusion, so we draw separately here, please understand.)
Figure 12. Hardness of different locations at different heated die temperatures.
Response to comment 9: Lines 197-198 – You write “… representing fully martensitic microstructure” – this calls for relevant picture confirming this observation!
Response 9: I'm very sorry that my previous mistake have made this statement inadequate. Although the sentence, “hot stamping parts with fully martensitic microstructure refer to components with high tensile strengths of approximately 1500 MPa and Vickers hardness values in excess of 425 HV”, is mentioned in the literature[2], I think it may be more convincing to describe "martensite-dominated microstructures" than "fully martensite structure", so I changed it in this article. The revised sentence are as follows:
This indicates that hardened section is in a hardened state due to its high cooling rate, as shown by its high UTS and hardness, representing the martensite-dominated microstructures (figure 9a, 9b and 10a).
[2] Bardelcik, A.; Salisbury, C.P.; Winkler, Sooky.; Wells, M.A.; Worswick, M.j. Effect of cooling rate on the high strain rate properties of boron steel. 2010, 37, 694-702.
Response to comment 10: Lines 213-217 – Figure 11. It is useful to show fracture surface – I agree. It would be more useful to add also scale bar (for evaluation of dimple size). You present six images using only two indicators: a) and b). Every image should be denoted and the caption should be rewrite as follow: “Fracture morphology at Location C1 under 300 (a) and 500°C (b) and Location H1 under 300 (c), 400 (d), 500 (e) and 600°C (f).”
Response 10: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments.
Figure 14. Fracture morphology at: (a) Location C1 under 300°C; and (b) 500°C; and (c) Location H1 under 300°C; (d) 400°C; (e) 500°C; and (f) 600°C.
Response to comment 11: There is no information in methodology part about SEM equipment used in experiment – it should be completed.
Response 11: Thank you for your kind suggestions. We have made correction according to the Reviewer’s comments. The revised sentences are as follows:
TESCAN VEGA3 scanning electron microscope (SEM) was used to make metallographic observation of quenched phases.
Response to comment 12: Lines 222-223 – This conclusion should be supported by relevant microscopic results in “Results and Discussion” section.
Response 12: Thank you for your kind suggestions. We have revised our analysis and conclusions about the microstructure. The results and discussion sections are revised as follows:
The area fractions of quenched phase are shown in Figure 11. With the increase of die temperature, the trend of area fraction of quenched phase at C1 position is consistent with that at C2 position. The area fraction of martensite at C1 position is above 74%, and that of C2 position is above 64%. While the proportion of ferrite and bainite at both positions is small. For the H1 and H2 specimen locations, there is a strong downward trend of martensite and a strong increase in bainite and the ferrite has a slightly change, which is owing to increasing die temperatures. As the tool temperature increases from 300 to 600°C, the area fraction of bainite at position H2 increases from 38% to 74%, and that of position H1 increases from 32% to 68%. While the area fraction of martensite at H1 position drops by 80%, and that at H2 position decreases by 80.7%. The proportion of ferrite at both positions is small. The martensite content on the bottom of the part (C1 and H1) is higher than that on the side (C2 and H2) because C1 and H1 contact with the tool first.
The amendments to the conclusions are as follows:
(1) The area fraction of martensite at C1 position is above 74%, and that of C2 position is above 64%. While the proportion of ferrite and bainite at both positions is small. As the tool temperature increased from 300 to 600°C, the area fraction of bainite at position H2 rose from 38% to 74%, and that of position H1 increased from 32% to 68%. While the area fraction of martensite at H1 position drops by 80%, and that at H2 position decreases by 80.7%. The proportion of ferrite and bainite at both positions is small.
Author Response File: Author Response.pdf
Reviewer 3 Report
The topic is interesting and enhancing the mechanical properties
during thermo-mechanical process represents a chanllenge and an usefull
contribution both to industry and accademy. Even if the topic is current
and important, the paper is not well conducted.
The experimental procedure is not properly reported and i did not catch the details of some experiments. The discussion is poor and the metallographic part (that is missing) must be deeply studied and reported. Are the author sure that C1 and C2 (as well as) H1 and H2 show the quite the same properties? undergoing different treatements the must be differtn or, maybe, i did not catch the job.
Author Response
Report (Reviewer 3)
We thanks a lot for your constructive criticisms that have helped us to improve our manuscript. The point-by-point response to the comments is given below.
Response to comment 1: The experimental procedure is not properly reported and i did not catch the details of some experiments.
Response 1: Thank you for your kind suggestions. We have revised the experimental steps of microstructure and performance.
The dimension of specimens for the microstructure observation and hardness tests were 10*10*1 mm3.
The ground and polished samples were treated with etching reagent for microstructure observation. TESCAN VEGA3 scanning electron microscope (SEM) was used to make metallographic observation of quenched phases. SEM images revealed the characteristic structures of the various phases, however, those images were not useful for quantification of area fractions when multiple phases were present. A two-stage colour tint etching procedure was used to reveal the various phases in different colours [1]. The etched specimens were observed using a ZEISS Axio. Scope. A1 metallographic microscope, and it was found that martensite was yellow-brown, bainite was black and ferrite was white. The ImagePro Plus 6.0 analysis software was used to manually colour the phases red (bainite), green (martensite), and blue (ferrite) and subsequently quantify the area fractions present.
After the hot forming experiments were completed at the various heated die temperatures, samples were cut from the parts and ground and polished for micro-hardness testing. The Vickers hardness was measured by HVS-1000ZDT micro-hardness tester. The experiments were carried out at load of 1.96 N with a holding time of 10 s. Each data point represents an average measurement of five measurements from each region.
Uniaxial tension tests were conducted on miniature dogbone specimens using a WDW-300 universal testing machine from the parts that were hot stamped at different die temperature. The specimen size are shown in Figure 8.
Response to comment 2: The discussion is poor and the metallographic part (that is missing) must be deeply studied and reported.
Response 2: Thank you for your kind suggestions. We are sorry that we failed to make us clearly. We add some images of microstructures here.
The typical microstructures observed from C1, C2, H1 and H2 locations are shown in Figure 9. It can be seen from Figure 9a and 9b that the microstructures for C1 and C2 locations exhibit the packets of parallel lath crystals which are characteristic of martensite [10]. From Figure 9c to 9f, you can see that the characteristic feature of bainite is a ferrite matrix with dispersed cementite particles [11]. As depicted in Figure 9c and 9d, there is a little difference in the organization of H1 and H2 positions when the heated die temperature is 400°C.
Figure 9. The SEM image at: (a) Location C1 with a 300℃ heated die temperature; (b) Location C2 with a 500℃ heated die temperature; (c) Location H1 with a 400℃ heated die temperature; (d) Location H2 with a 400℃ heated die temperature; (e) Location H1 with a 500℃ heated die temperature; and (f) Location H1 with a 600℃ heated die temperature.
Two-stage colour tint etched optical micrographs and the manually generated microstructure images are shown in Figure 10. As shown in Figure 10a, the microstructures for C1 location are found to be predominantly martensitic with some bainite. The formation of abundant martensite is because of the higher cooling rate which is the result of low die temperature. The formation of bainite and ferrite is due to the longer transfer time and the left shift of the CCT which is caused by the plastic deformation[5]. It can be concluded from Figure 10b and 10c, both heated die conditions result in a mixed martensite, bainite and ferrite microstructure with visibly larger amounts of the softer bainite phase for the 600°C die condition.
Figure 10. Various micrographs showing (from top to bottom) two-stage colour tint etched optical micrographs, and the manually generated microstructure images at: (a) Location C1 with a 300℃ heated die temperature; (b) Location H1 with a 300℃ heated die temperature; and (c) Location H1 with a 600 ℃ heated die temperature.
[10] Naylor, J.P. Influence of the Lath Morphology on the Yield Stress and Transition-Temperature of Martensitic-Bainitic Steels. Metall and Mater Transactions 1979, 10, 861–873.
[11] Feng, C.; Fang, H.S.; Zheng, Y.K.; Bai, B.Z. Mn-series low-carbon air-cooled bainitic steel containing niobium of 0.02%. Iron and Steel Research, Int 2010, 17, 53-58.
Response to comment 3: Are the author sure that C1 and C2 (as well as) H1 and H2 show the quite the same properties? undergoing different treatements the must be differtn or, maybe, i did not catch the job.
Response 3: We are sorry that we failed to make us clearly. We further elaborate on the microstructures and performance of four different positions. (The differences between C1 and C2 (and) H1 and H2 are marked in yellow, and the corrected text in the article is marked in red.)
The area fractions of quenched phase are shown in Figure 11. With the increase of die temperature, the trend of area fraction of quenched phase at C1 position is consistent with that at C2 position. The area fraction of martensite at C1 position is above 74%, and that of C2 position is above 64%. While the proportion of ferrite and bainite at both positions is small. For the H1 and H2 specimen locations, there is a strong downward trend of martensite and a strong increase in bainite and the ferrite has a slightly change, which is owing to increasing die temperatures. As the tool temperature increases from 300 to 600°C, the area fraction of bainite at position H2 increases from 38% to 74%, and that of position H1 increases from 32% to 68%. While the area fraction of martensite at H1 position drops by 80%, and that at H2 position decreases by 80.7%. The proportion of ferrite at both positions is small. The martensite content on the bottom of the part (C1 and H1) is higher than that on the side (C2 and H2) because C1 and H1 contact with the tool first.
Figure 11. The area fraction under different tool temperatures of: (a) Martensite; (b) Bainite; and (c) Ferrite.
Figure 12 displays the hardness measurements as the heated die temperature increases. The hardness of C1 and C2 can reach 445 and 428 HV, respectively. The hardness at positions H1 and H2 decreases from 397 and 382 HV to 210 and 180 HV respectively when the die temperature rises from 300 to 600°C, which is a result of the effect of the introduction of some volume fraction of bainite into the as-quenched microstructure. The hardness of the bottom (C1 and H1) of the part is higher than that of the side wall(C2 and H2). This is because of the case that the bottom of the part first contacts the die as discussed earlier which causes a high martensite content and high hardness. There is an anomalous behavior at position C1 and C2 which shows an overall decreasing trend when the die temperature is higher than 400°C. The reason is that a step exists at the interface between the tool segments (at the gap) due to thermal expansion. As a result, the contact between the die and the blank in the quenching section is not tight when the die is closed. Thus, more bainite was formed and the hardness falls because of the small quenching rate.
Figure 12. Hardness of different locations at different heated die temperatures.
The ultimate tensile strength (UTS) and elongation for two regions at different heated tool temperatures are shown in Figure 13. As can be seen from the figure, the tensile strength of C1 is slightly greater than that of C2, and that of H1 is slightly greater than that of H2. The elongation of C1 is less than C2, and that of H1 is less than H2. However, the tensile strength and elongation of C1 and C2 are very close, so are H1 and H2. Therefore, only average tensile strength and elongation are analyzed here. The average UTS of C1 and C2 can reach 1452 MPa, and the average elongation remains at a relatively low level, with an average of 6.65%. This indicates that hardened section is in a hardened state due to its high cooling rate, as shown by its high UTS and hardness, representing the martensite-dominated microstructure (figure 9a, 9b, 10a). As the die temperatures rise, the average UTS of H1 and H2 drops by 47.8%, while the average elongation increases from 8.41 to 16.09%. The reason is that the appearance of bainite, resulting in lower UTS and hardness and higher toughness.
Figure 13. The mechanical properties for two regions at different heated tool temperatures: (a) The average UTS; and (b) Average elongation.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear Authors
I'm glad you have taken into consideration all my remarks. Your paper has higher quality level presently. There is still important mistake in fig. 9b - the chemical formula of cementite is Fe3C (not FeC3!).
Author Response
We thanks a lot for your constructive criticisms that have helped us to improve our manuscript.
I'm glad you have taken into consideration all my remarks. Your paper has higher quality level presently. There is still important mistake in fig. 9b - the chemical formula of cementite is Fe3C (not FeC3!).
I'm sorry to bring you so much trouble. We have corrected the error in Figure 9.
Figure 9. The SEM image at: (a) Location C1 with a 300℃ heated die temperature; (b) Location C2 with a 500℃ heated die temperature; (c) Location H1 with a 400℃ heated die temperature; (d) Location H2 with a 400℃ heated die temperature; (e) Location H1 with a 500℃ heated die temperature; and (f) Location H1 with a 600℃ heated die temperature.
Author Response File: Author Response.pdf
