Effect of Roll Material on Strip Solidification between the Rolls of a Vertical-Type High-Speed Twin-Roll Caster
Round 1
Reviewer 1 Report
The authors conducted detailed studies on the effect roll material on the parameters of solidification (i.e. The cooling rate, supercooling and the temperature) , but without investigating the effect of different roll material on microstructure and mechanical properties of the product material (aluminum alloy), which makes it seems more like a technical report rather than a research article.
Author Response
September 30, 2022
Dear Reviewer 1
Thank you for your helpful comments and suggestions.
We have revised the manuscript in accordance with your recommendations.
Please review the revised manuscript.
The authors conducted detailed studies on the effect roll material on the parameters of solidification (i.e. The cooling rate, supercooling and the temperature) , but without investigating the effect of different roll material on microstructure and mechanical properties of the product material (aluminum alloy), which makes it seems more like a technical report rather than a research article.
Answer: Thank you for your comment.
From the results of deep drawing tests on Al-Mg strips, the roll material, roll load and roll surface condition influence Mg segregation and surface cracking at the cup. We are currently performing more detailed investigations of these phenomena using methods such as tensile tests and microstructural observations and will report the results in the near future.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript reports a potential application of copper rolls in caster compared to conventional tool steel. The authors have come to the conclusion based on extensive comparisons between the two rolls, and the conclusion is sound to warrant the publication in Metals.
It will be of great help if the authors make a comment on the long-term operation of copper rolls, e.g. the quality of strips in terms of defects, surface roughness, etc.
Author Response
Dear Reviewer 2
Thank you for your review.
Thank you again.
Reviewer 3 Report
1. For the strip cast using the copper rolls, the cooling rate increases as the rolling speed increase. How can this mechanism be explained from the point of view of the physics of the process? Because the following explanation that “As the rolling speed increased, the volume of aluminum alloy between the rolls decreased rapidly and thus the alloy cooled much more quickly” is not clear.
2. Authors used rolls with a diameter of 300 mm. The solidification length can be increased by using a larger-diameter roll. The casting speed may be increased using a larger solidification length. In a CTRCA, the roll diameter is usually 600-1200 mm. Authors should provide quantitative information (maybe based on calculations) on how the diameter of the rolls and their material (mild steel or copper) affect the casting speed. Because mild steel rolls but large diameter can be as effective as copper rolls but small diameter.
3. Is there any effect of the roll material (mild steel or copper) on the surface quality and/or microstructure of the strip?
4. Results of investigation of the effect of roll material on strip solidification have shown that copper rolls are clearly superior to mild steel rolls for high-speed twin roll casting of most aluminum alloys. Mild steel rolls are still an economically attractive alternative, depending on the aluminum alloy. The results presented in the article have the practical value. But the authors should also to formulate the scientific novelty.
Author Response
September 30, 2022
Dear Reviewer 3
Thank you for your helpful comments and suggestions.
We have revised the manuscript in accordance with your recommendations.
Please review the revised manuscript.
- For the strip cast using the copper rolls, the cooling rate increases as the rolling speed increase. How can this mechanism be explained from the point of view of the physics of the process? Because the following explanation that “As the rolling speed increased, the volume of aluminum alloy between the rolls decreased rapidly and thus the alloy cooled much more quickly” is not clear.
Answer: I have revised the manuscript to explain this point. The revised text is shown in blue characters.
As the rolling speed increased, the volume of aluminum alloy between the rolls decreased rapidly because the roll gap quickly became narrow. This means that the amount of heat decreased rapidly. The distance between the roll surface and the center of the strip, which becomes heat resistant, decreased. Thus, the alloy cooled much more quickly.
- Authors used rolls with a diameter of 300 mm. The solidification length can be increased by using a larger-diameter roll. The casting speed may be increased using a larger solidification length. In a CTRCA, the roll diameter is usually 600-1200 mm. Authors should provide quantitative information (maybe based on calculations) on how the diameter of the rolls and their material (mild steel or copper) affect the casting speed. Because mild steel rolls but large diameter can be as effective as copper rolls but small diameter.
Answer: Thank you for your comment.
The difference between the coefficient of heat transfer between the roll surface and the strip surface is not clear for copper and mild steel rolls. The heat transfer coefficient is not uniform from the solidification starting point to the roll bite. In future work, I plan to determine the effect of the roll size by first performing experiments to determine the heat transfer profile by inserting thermocouples between the roll and strip surfaces, and then performing calculations to determine the heat transfer coefficient.
In a high-speed twin-roll caster, the solidification length increases as the roll diameter increases, and may be 300 mm when the roll diameter is 1000 mm. On the other hand, in a conventional twin roll caster for aluminum alloys, the setback distance is about 100 mm when the roll diameter is 1000 mm. Consequently, for a high-speed twin-roll caster, a large-diameter roll may not be essential for aluminum alloy.
- Is there any effect of the roll material (mild steel or copper) on the surface quality and/or microstructure of the strip?
Answer: From the results of deep drawing tests on Al-Mg strips, the roll material, roll load and roll surface condition influence Mg segregation and surface cracking at the cup. We are currently performing more detailed investigations of these phenomena using methods such as tensile tests and microstructural observations, and will report the results in the near future.
- Results of investigation of the effect of roll material on strip solidification have shown that copper rolls are clearly superior to mild steel rolls for high-speed twin roll casting of most aluminum alloys. Mild steel rolls are still an economically attractive alternative, depending on the aluminum alloy. The results presented in the article have the practical value. But the authors should also to formulate the scientific novelty.
Answer: I have revised the Conclusions section. The revised text is shown in “blue characters”.
Thermal conductivity is an important factor for determining the most appropriate roll material for high-speed twin-roll casters to realize rapid solidification. Due to their high thermal conductivity compared to steel, copper and its alloys are the best choices for the roll material. Even using mild steel with a very thin 6-mm-thick shell, a suitable thermal conductivity cannot be achieved.
Author Response File: Author Response.pdf
