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# Control of Upstream Austenite Grain Coarsening during the Thin-Slab Cast Direct-Rolling (TSCDR) Process

Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Metals 2019, 9(2), 158; https://doi.org/10.3390/met9020158
Received: 27 December 2018 / Revised: 27 January 2019 / Accepted: 29 January 2019 / Published: 1 February 2019

Round 1

Reviewer 1 Report

The manuscript concerns the control of austenite grain coarsening during a Thin Slab Casting Direct Rolling (TSCDR) Process. The manuscript is well written, and the topic is properly discussed.

Below the authors can find the reviewer’s comments.

ABSTRACT:

- Write “Ductile Brittle Transaction Temperature (DBTT)” instead of “Ductile Brittle Transaction (DBTT)”.

3. RESULTS AND DISCUSSIONS

- Probably “3.1. Microstructure and Model Validation” is the correct title (Replace the term “Medel”).

- Why does the temperature curve concerning the slab surface exhibit a so irregular trend (the same for fig. 9b)? Clarify in the text.

- For clarity, increase the font size of the axis titles and of the legends of Fig.2d, 3d, 4a, 4b, 5, 6, 7, 8, 9, 12, 13, 14

- The authors should provide, also as an appendix, the main formulas (e.g. to evaluate austenite and delta ferrite grain diameters, SDAS, and so on) and the values of the factors used in the numerical simulations (heat transfer coefficient, thermal conductivity, and so on). This is necessary as numerical simulations are employed. In this way, it could be possible to understand how the slab thickness affects the results obtained.

- pag. 9: L/kg means liter per kg? In this case, the unit of measure must be written as l/kg. Correct in all the text.

- Since you have discussed the manuscript by using “°C” as unit of measure of temperature. Please modify similar sentences “at 50 °C/s to 1398 K (1125 °C)” with “at 50 °C/s to 1125 °C (1398 K)”. Please also correct other similar cases if present.

Author Response

Reviewer #1:

ABSTRACT:

-Write “Ductile Brittle Transaction Temperature (DBTT)” instead of “Ductile Brittle Transaction (DBTT)”.

We are sorry for this mistake. “Ductile Brittle Transaction (DBTT)” has been changed to “Ductile Brittle Transition Temperature (DBTT)”

3. RESULTS AND DISCUSSIONS

- Probably “3.1. Microstructure and Model Validation” is the correct title (Replace the term “Medel”).

It is a typo. “Medel” is replaced by “Model”.

- Why does the temperature curve concerning the slab surface exhibit a so irregular trend (the same for fig. 9b)? Clarify in the text.

The explanation of irregular trend of cooling path is added in Page 5 line 148-151:

Finally, the cooling path T(t) at each point of the slab is also estimated by CON1D V7.0 model [22]. For example, Figure 4b shows the temperature paths at the surface, 5 mm, 10 mm, 20 mm, 30 mm below the surface, and the center of API X70 85 mm slab that was cast with 3.4 m/min casting speed. Due to the spay jet cooling and the high local heat extraction when the segment rolls contact the slab, the temperature curve on the slab surface shows irregular trend. Nonetheless, this irregular trend should not interfere with the interpretation of the grain growth with the slab position during the TSCDR process.

- For clarity, increase the font size of the axis titles and of the legends of Fig.2d, 3d, 4a, 4b, 5, 6, 7, 8, 9, 12, 13, 14.

We are sorry for this small font size of the axis tiles and of the legends of Fig.2d, 3d, 4a, 4b, 5, 6, 7, 8, 9, 12, 13, 14. All figures have been replaced with increased font size. In addition, Fig 1 has been replaced to make sure it is original.

- The authors should provide, also as an appendix, the main formulas (e.g. to evaluate austenite and delta ferrite grain diameters, SDAS, and so on) and the values of the factors used in the numerical simulations (heat transfer coefficient, thermal conductivity, and so on). This is necessary as numerical simulations are employed. In this way, it could be possible to understand how the slab thickness affects the results obtained.

We agreed with reviewer`s comment regarding an appendix with providing the main formulas and the values of the factors used in the numerical simulations. An appendix is added at the end of the paper:

Appendix:

To calculate grain boundary mobility M(t) in Equation (2), the Turnbull mobility was used as an initial estimation:

In the above equation, w is the grain boundary thickness, DGB is the grain boundary self-diffusion coefficient, Vm is the molar volume, b is the magnitude of the Burgers vector, R is the gas constant and T is the absolute temperature. The delta-ferrite has body-centered-cubic (BCC) crystal structure, the burgers vector is b=1/2<111> and , where a is the lattice parameter of delta-ferrite, 0.286nm. The molar volume, Vm=177.75px3. The activation energy for diffusion along the grain boundary was taken to be QGB=0.68Q where Q=256kJ/mole is the activation energy for bulk diffusion in BCC. Finally, w=1nm, and . Given that Turnbull mobility does not take into account attachment kinetics, the grain boundary mobility in this way overestimates the experimental grain growth kinetics. The best fit of the experimental data was obtained using a mobility which is 1/3 of the Turnbull estimate [9]. Therefore, the delta grain mobility used in this work is:

(3)

To estimate the mobility of the austenite grain boundaries, the austenite with face-centered cubic (FCC) crystal structure has b=1/2<110>, therefore, , where a is 0.357nm. The molar volume, Vm is 171.25px3, the bulk diffusion activation energy in FCC Q=284kJ/mole, and that of grain boundary diffusion is: QGB=0.61Q. w=1nm, and. Once again, the Turnbull mobility leads to an overestimation of the austenite grain growth kinetics. The best fit of the experimental data was obtained with a mobility which is 0.96 times the Turnbull estimate [9]. Thus, the austenite grain boundary mobility used in this calculation is:

(4)

The cooling path T(t) at each point of the slab and secondary dendrite arm spacing,  thermophysical properties and spray heat transfer coefficients were calculated using CON1D V7.0 slab casting heat transfer model.  The cast speeds for different slab thickness used in the simulations are listed as the following:

85 mm slab, casting speed 3.0-3.4 m/min,

70 mm slab, casting speed 3.4-4.0 m/min,

50 mm slab, casting speed 4.5-5.5 m/min,

30 mm slab, casting speed 4.5-6.5 m/min.

- pag. 9: L/kg means liter per kg? In this case, the unit of measure must be written as l/kg. Correct in all the text.

“L/kg” means liter per kg. It has been corrected as “l/kg” in all the text.

- Since you have discussed the manuscript by using “°C” as unit of measure of temperature. Please modify similar sentences “at 50 °C/s to 1398 K (1125 °C)” with “at 50 °C/s to 1125 °C (1398 K)”. Please also correct other similar cases if present.

We are sorry for this unit inconsistency. All units have been reviewed and corrected in all the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors present some interesting suggestions regarding the importance of controlling the prior austenite grain size (PAGS) during the solidification process and prior to thermomechanical processing. While most of the results presented in the paper are theoretical in nature, to increase the validity and technological merit of their ideas they must provide actual evidence. The experimental methods used while not clearly or extensively described don't seem to be appropriate for TSCDR processing. In additon, some of their suggestions regarding the use of faster cooling rates on the solidifying slab while might result on some PAGS refinement thorugh the tickness, a problem might be the simultaneous precipitation of M-A elements in the case of HSLA steels. Fast cooling rates is used in many mini-mills and it is well-known that produces lose of M-A from solid solution. Minimizing the initial size of the as-cast slab while it might lead to PAGS refinement and lower centerline segregation, it will be conducive to less productivity and even lower ability to produce thicker products with a fine PAGS.

Author Response

The authors present some interesting suggestions regarding the importance of controlling the prior austenite grain size (PAGS) during the solidification process and prior to thermomechanical processing. While most of the results presented in the paper are theoretical in nature, to increase the validity and technological merit of their ideas they must provide actual evidence.

To demonstrate the validity and technological merit of the increasing cooling on reducing austenite grains at the center of the slab, 70mm thick slab grain size at the slab surface and the center is added on Figure 8 and the comparison is discussed on page 8 line 216-221. In addition, the limitation of increasing cooling rate was highlighted in the same paragraph:

The initial delta-ferrite grain size was, once again, taken to be SDAS, therefore, the model calculated of delta grain size as a function of position for the 30 mm, 50 mm thin slab just before the onset of the delta to gamma transformation are shown in Figure 8a, which also includes, for comparison, the results shown earlier for the 70mm and 85 mm slab. The calculated austenite grain size before entering the homogenization furnace is shown in Figure 8b. The symbols in these Figures have the same meaning as discussed previously.

 (a) (b)

Figure 8. (a) The predicted delta grain size of 30 mm, 50 mm, 70mm and 85 mm slab as a function of slab position just before the onset of the delta to gamma transformation, (b) the predicted austenite grain size of 85 mm, 70mm, 50 mm and 30 mm slab as a function of slab position when the slab is about to enter the homogenization furnace.

It is clear from these calculations that by the enhanced cooling rate, austenite grains at the centre of the thinner slabs have greatly reduced in size. When the slabs are about to enter the homogenization furnace, the austenite grain size at the center of an 85 mm thick slab is about 1058 μm. However, the grain size is 896 μm at a 70 mm thick slab, 693 μm at a 50 mm thick slab and 470 μm for a 30 mm slab. In addition, the homogeneity of the microstructure has improved by increasing the cooling rate; the ratio of largest grain size to smallest grain size is 3.8 to 1 for an 85 mm thick slab, 2.8 to 1 for a 50 mm thick slab and 2.1 to 1 for a 30 mm slab. Therefore, one can conclude that reducing the slab thickness can refine and homogenize the as-cast microstructure due to the enhanced cooling rate at the centre of the slab. Experimental measurement of 85mm (Figure 2) and 70mm (Figure 3) industrial slab austenite grain size with the distance from the slab surface to center demonstrated the validity and technological merit of the increasing cooling on reducing austenite grains at the center of the slab: The austenite grain size can be reduced from 1151 μm to 858 μm if the casting slab thickness is reduced from 85mm to 70mm. The main difficulty in applying this method is that it requires changing the layout of the TSCDR process for thinner slab such as 50mm and 30mm thick slab. In addition, the smaller slab thickness will further reduce the amount of thermomechanical processing that can be performed downstream resulting in a larger average grain size and possibly more grain size non-uniformity despite the improved initial microstructure. Thus, an optimum thickness could be determined by considering both the solidification and grain growth (as described above) as well as the subsequent thermomechanical processing.

And Section 3.4 last paragraph has been modified as the following:

To capture the effect of austenite nucleation sites on the austenite grain coarsening kinetics, the validated grain growth model can be used to calculate austenite grain evolution at different stage of TSCDR process. Figure 12d summarizes the predicted austenite grain size when the slab is about to enter the holding furnace for a different density of austenite nucleation sites. The various lines represent the grain size achieved when 12, 6, and 3 austenite grains nucleate within each delta grain. The calculation reveals that these extra nucleation sites have little effect on the final grain size at the surface of the slab, however, the austenite grain do decrease with increasing the austenite nucleation sites: the austenite grain in the 85 mm slab center can be reduced from 1058 μm to 945 μm and 856 μm respectively. The purple line in Figure 12d indicates austenite grain size trend by using the austenite nucleation density 12 and increased secondary cooling. The austenite grain size in the slab center can be refined from 1058 μm to 705 μm. The calculated results predicted the potential of increasing the number of austenite nucleation sites during delta-ferrite to austenite phase transformation to refine austenite grain size, much work still needed, however, to design new alloys and deformation schedule during casting that could take advantages of this novel approach.

The experimental methods used while not clearly or extensively described don't seem to be appropriate for TSCDR processing.

Increasing secondary cooling, liquid core reduction and control the soaking furnace temperature and holding time have been implemented at Algoma Inc. DSPC process to produce HSLA Gr50, 60, 70 and 80.

In additon, some of their suggestions regarding the use of faster cooling rates on the solidifying slab while might result on some PAGS refinement thorugh the tickness, a problem might be the simultaneous precipitation of M-A elements in the case of HSLA steels. Fast cooling rates is used in many mini-mills and it is well-known that produces lose of M-A from solid solution.

Increasing cooling is used in the slab caster secondary cooling zone. The slab surface temperature might be dropped to below 950°C, however, temperature recovers to above 1050°C once the slab entering the holding furnace. From Figure 4b, Figure 9c, it can be seen that the temperature inside the slab are far above 1000°C with slab position and most of the alloy element are still in solid solution and will precipitate at finishing mill and runout table.

Minimizing the initial size of the as-cast slab while it might lead to PAGS refinement and lower centerline segregation, it will be conducive to less productivity and even lower ability to produce thicker products with a fine PAGS.

Increasing cooling rate can refine austenite grain size and reduce the centerline segregation, thus can produce high grade HSLA with stringent DBTT and DWTT requirements. In addition, steel industry can reduce rejection rate due to product internal quality issues especially for pipe and tube applications with ultra sonic requirement.

Author Response File: Author Response.pdf

Reviewer 3 Report

N/A

Author Response

Dear Reviewer:

We sincerely thank you for your valuable feedback.

Sincerely yours,

Tihe (Tom) Zhou Ph.D, P.Eng.

January 20th 2019.

Round 2

Reviewer 2 Report

The authors have made appropriate additions and corrections to the manuscript. There is a small typo on line 148 that shoud be corrected.

Author Response

Dear Reviewer: We are sorry for this typo. "spay" has been replaced by "spray" on line 148.

we sincerely thank you for your valuable feedback.

Best regards,

Tihe Zhou, Ph.D, P.Eng.