Claims:We Claim:
1. A method of producing Carbon-Manganese alloyed steel hot rolled coil free of any shape defect or soft slump comprising:
Working steel composition in wt% involving Carbon= 0.068-0.67 Manganese=0.40-1.5, Silicon=0.018-0.248, Nitrogen=0.0035-0.0055, Chromium= 0.009-0.233, Sulphur= 0.008-0.004 , Phosphorous= 0.006-0.02and rest iron, by
subjecting to usual processing involving finishing mill, Run on Table (ROT) and down coiler wherein phase transformation of strip is controlled ahead of the coiler by involving modified cooling cycle in ROT such as to ensure recalescence is completed in the ROTand thereby avoid shape defects and soft slump in subsequent coiler stage.

2. A method as claimed in claim 1 wherein the modified cooling cycle in ROT includes the step of highest cooling rate in the initial cooling banks of ROT preferably in first cooling bank followed by slow cooling rate subsequently in the following banks preferably involving 2 to 15 banks.

3. A method as claimed in anyone of claims 1 or 2 comprising highest cooling rate in initial cooling bank comprises a cooling rate of 150 0C/sec to 10 0C/sec followed by slow cooling at subsequent Banks at 100 0C/sec to 5 0C/sec.

4. A method as claimed in anyone of claims 1 to 3 wherein the cooling rate in the first water box is 150 0C/sec to 10 0C/sec preferably 123 0C/sec.

5. A method as claimed in anyone of claims 1 to 4 wherein cooling rate was increased at the start of the ROT with sharper cooling rate whereby steady cooling rate is achieved within 2nd and 3rd Bank due to higher heat extraction in the coil at close to the start of the run out table.
6. A method as claimed in anyone of claims 1 to 5 wherein the transformation temperature band is shifted upwards in the range of 610 0C to 680 0C by increasing the cooling rate.
7. A method as claimed in anyone of claims 1 to 6 wherein the phase transformation is shifted from down coiler to run out table(ROT).
8. A method as claimed in anyone of claims 1 to 7 wherein time taken from the end of finishing (on ROT) to the start of coiling is 14-18 seconds, so that the cooling rate and temperature attained controls the elliptical behavior during the coiling operation.
9. A method as claimed in anyone of claims 1 to 8 wherein the coil need not be held at the mandrel for a longer duration with external water cooling and is removed immediately so that the saving in time enhances down coiler availability which enhances the productivity by 4 to 5%.

Dated the 5th day of September, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION
Present invention relates to a method of hot strip rolling to prevent the coil collapse to form elliptic shapes during coiling with simultaneous productivity improvement. More particularly, the present invention is directed to a method for hot strip rolling of carbon-manganese alloyed steel involving selective cooling pattern implemented on ROT to cool the hot rolled strips from near finish rolling temperature to coiling temperature such that phase transformation from austenite to ferrite is complete before coiling, eliminating the need for holding the coil for additional time in down coiler with water cooling, favouring avoiding the coil collapse phenomenon causing elliptic shaped coil and increased availability of down coiler leading to improved productivity.

BACKGROUND OF THE INVENTION

In a typical hot rolling process, the steel strip is made from a steel slab. The slab is usually conditioned free of surface defects by processes such as scarfing. This is followed by heating the slab in a reheating furnace to the hot rolling temperature. The slab is subjected to a high reduction in the roughing mill. This is followed by a finishing mill operation where the final strip to required thickness. The rolled strip is then made to pass through Run On Table(ROT) cooling to bring down the temperature further to coiling temperature and then coiled on coiler for subsequent end use/application.

In the manufacture of thin steel strips in the hot rolling mill, instead of the formation of a tight round shaped coil winding, there could be a tendency for the formation of a loose elliptic shape. This is commonly seen in a wide range of Carbon-Manganese alloyed steels. The formation of the elliptic shape results in improper loading and unwinding characteristics at the end user application.

Steel strip is commonly produced in the form of a tight wound round coil on a mandrel. In a wide range of carbon-manganese steels, the wound coil takes a loose elliptic shape instead of tight round shape. After the final reduction to fine strip in the finishing mill, the coil is wound on a mandrel. The strip composition, thickness, temperature, and the re-calescence associated with phase transformation leads to volume changes. The lowering of strength of the coil and higher ductility of the coil at the coiling temperature promotes sagging of the coil due to the self-weight of the coil.
One technique to prevent the elliptic coil formation, the hot strip coil is held in the down coilermandrel for duration of 4 to 5 min to ensure the phase transformation associated with volume changes is controlled. This additional holding time decreases the productivity by 4 to 5%.

Kevin Bank et al. [Kevin Banks, Alison Tuling and Barrie Mintz “Influence of chemistry and runout table parameters on hot coil collapse in C- Mn steels] studied the transformation behaviour in the run out table (ROT) through dilatometer, the cooling and coiling conditions. Coil collapse was observed in low carbon steel attributed to higher N content and higher coiling temperature (650oC). Lowering Ar1 was found to increase coil collapse. Completion of transformation was before coiling required that Ar1 temperature is controlled. For low C-Mn Aluminium killed steel the Ar1 is given by
Ar1 = 706.4 - 350.4C - 118.2Mn ....................................................................[1]
The above work relates to control of Ar1 by composition and prolonging duration in ROT. The effect of cooling rate in the suppression of Ar1 was not brought out.

Hirokazu et al. [Hirokazu Sugihara, Makoto Hiramatsu, et al “Method for producing hot rolled steel sheet” Publication number WO2013137068 A1, Application number PCT/JP2013/055990, Publication date Sep 19, 2013, Filing date Mar 5, 2013, Priority date Mar 12, 2012] have patented a process to suppress coil collapse by controlling the rate of phase transformation at the intermediate temperature in the ROT in a Si steel [by mass% C: 0.05% ~ 0.3%, Mn: 1.0% ~ 2.7%, Si: 0.2% ~ 1.5%]. The phase transformation rate of different locations in the lengthwise direction reach the winder monotonically increases from the leading end to the tail end of the hot rolled steel sheet.Where as, the above patent states the decrease of phase transformation rate along the distance of the strip, the invented process attempts to use the effect of suppression of Ar1 by alloying and cooling rate. Faster cooling rate at critical process stage leads to suppression of Ar1 temperature.

In order to remedy above stated problems and limitations of prior art,the present innovation targets to resolve the issue of coil collapse and ensures that the hot strip wound on the end coiler mandrel is always cylindrical round shape in a range of steels. In addition, to the prevention of elliptic shape, there is a simultaneous enhancement of productivity demonstrated in the present invention.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide a method to prevent the coil collapse leading to formation of elliptic coil shape during coiling of hot rolled strips in a range of carbon-manganese steels.
A further object of the present invention is directed to provide a method to prevent the formation of elliptic coil shape during coiling of hot rolled strips wherein selective cooling on ROT is adopted so that phase transformation from austenite to ferrite is complete before coiling.
A still further object of the present invention is directed to provide a method to prevent the formation of elliptic coil shape during coiling of hot rolled strips wherein gradual heat extraction in the strip in ROT is replaced with sharper cooling rate where within 2nd and 3rd Bank steady cooling rate is achieved, whereby the heat extraction which conventionally done at down-coiler is replaced with higher heat extraction in the coil at close to finish rolling temperature, so that coiling temperature is lower at the down coiler.
A still further object of the present invention is directed to provide a method to prevent the formation of elliptic coil shape during coiling of hot rolled strips wherein need for holding the coil for additional time in down coiler with water cooling is avoided, saving time and cost and improving productivity of the steel coilby 4 to 5%.

A still further object of the present invention is directed to provide a method to prevent the formation of elliptic coil shape during coiling of hot rolled strips whereby formation of a tight round shaped coil winding is ensured favouring proper loading and unwinding characteristics at the end user application.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a method of producing Carbon-Manganese alloyed steel hot rolled coil free of any shape defect or soft slump comprising:
working steel composition in wt% involving Carbon= 0.068-0.67 Manganese=0.40-1.5, Silicon=0.018-0.248, Nitrogen=0.0035-0.0055, Chromium= 0.009-0.233, Sulphur= 0.008-0.004 , Phosphorous= 0.006-0.02and rest iron, by subjecting to usual processing involving finishing mill, Run on Table (ROT) and down coiler wherein phase transformation of strip is controlled ahead of the coiler by involving modified cooling cycle in ROT such as to ensure recalescence is completed in the ROTand thereby avoid shape defects and soft slump in subsequent coiler stage.

A further aspect of the present invention is directed to said method wherein the modified cooling cycle in ROT includes the step of highest cooling rate in the initial cooling banks of ROT preferably in first cooling bank followed by slow cooling rate subsequently in the following banks preferably involving 2 to 15 banks.

A still further aspect of the present invention is directed to said method comprising highest cooling rate in initial cooling bank comprises a cooling rate of 1500C/sec to 100C/sec followed by slow cooling at subsequent Banks at 1000C/sec to 5 0C/sec.

A still further aspect of the present invention is directed to saidmethod wherein the cooling rate in the first water box is 150 0C/sec to 10 0C/sec preferably 1230C/sec.

Another aspect of the present invention is directed to said method wherein cooling rate was increased at the start of the ROT with sharper cooling rate whereby steady cooling rate is achieved within 2nd and 3rd Bank due to higher heat extraction in the coil at close to the start of the run out table.
Yet another aspect of the present invention is directed to saidmethod wherein the transformation temperature band is shifted upwards in the range of 6100C to 680 0C by increasing the cooling rate.
A further aspect of the present invention is directed to saidmethod wherein the phase transformation is shifted from down coiler to run out table(ROT).
A still further aspect of the present invention is directed to saidmethod wherein time taken from the end of finishing (on ROT) to the start of coiling is 14-18 seconds, so that the cooling rate and temperature attained controls the elliptical behavior during the coiling operation.
A still further aspect of the present invention is directed to saidmethod wherein the coil need not be held at the mandrel for a longer duration with external water cooling and is removed immediately so that the saving in time enhances the down coiler availability, i.e., reduction in production time loss from 48 hrs/month to ~3 hrs/month as shown in Fig. 6, which enhances the productivity by 4 to 5%.
The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: shows the typical layout configuration of a mill from finishing mill to run of table and down coiler.
Fig. 2:(a) shows Elliptical Coil, and (b)shows Non-elliptical Coil shapes.
Fig. 3(a): show graphically ROT cooling rate bank wise for the cooling cyclesof all 4 trials implemented.
Fig. 3(b): show graphically the Strip temperature attained bank wise in ROT for each of all 4 trials.
Fig.3(c): shows the plot of total residence time of strip on ROT.
Fig.4: The Ar3 and Ar1 temperature for the physically simulated C-Mn steel grade
Fig.5 (a): show graphically the volume changes as a function of various cooling cycles show that volume changes are lower in cycle 2, 3, 4 & A, wherein highest volume contraction is observed in modified cycle 4 which indicate that phase transformation is complete.
Fig.5(b): shows the effect of cooling cycle on phase change at ROT from Gleeble simulation.
Fig.5(c): shows the effect of cooling cycle on phase change at coiling from Gleeble simulation.
Fig.6: shows the impact of increased cooling rate in bank 1 on reduction in production time loss.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide a method to prevent the coil collapse leading to formation of elliptic coil shape during coiling of hot rolled strips in a range of carbon-manganese steels involving selective cooling pattern in ROT.

Steel strip with thickness less than 6 mm is produced by hot rolling a steel slab. The cast steel slab is reheated in a Reheating furnace for a temperature of about 1250oC. The typical processing conditions for the strip manufacture, is brought out in Table 1. The steel slab is subjected to good deformation to breakdown the cast structure in a two (RM-1 &RM-2) stand Roughing mill to a thickness of 220mm with a typical reduction ratio of 0.110, 0.180, 0.154, 0.286, 0.289 and 0.328. The steel is further processed in a finishing mill to a final required thickness varying between 1.6 and 6mm strips.

Table 1: Typical parameters of strip manufacture at HSM
S. No. Parameters Values
1. Steel Composition in wt.% Carbon= 0.068-0.67 Manganese=0.40-1.5
Silicon=0.018-0.248Nitrogen=0.0035-0.0055
Chromium= 0.009-0.233 Sulphur= 0.008-0.004
Phosphorous= 0.006-0.02
2. Input slab thickness& width, mm 220x 1250 x 11000
3. Hot rolling temperature Re-Heating furnace duration = 190 minute
Exit temperature=1250°C
Descaling pressure =200 kg/cm2
4. Roughing Mill Configuration No. of stands= RM-1 & RM-2
Reduction Ratio= 0.110, 0.180,0.154,0.286,0.289,0.328
Finishing temperature = 1070°C
5. Finishing Mill Configuration No. of stands=7
Reduction Ratio= 0.368,0.436,0.328,0.353,0.372,0.178,0.054
Finishing temperature =850°C
6. Run out Table Configuration Length = 150 m No of banks = 16 no.
Upper header =16 Lower header = 8
Intensive cooling = 1 to 3 banks
Normal cooling = 4 to 8 banks
Intensive cooling = 9 to 14 banks
Fine cooling = 15 to 16 banks
7. Automation Water curtain is controlled by individual headers valves by a Level-II automation system. Based on the strip temperature measured using pyrometer located, the water flow is adjusted to control the strip temperature to the required level.
8. Down coiler No of coilers =3 no.
9. Strip thickness 1.6 to 6mm

Accompanying Fig. 1 shows the typical layout configuration of a mill from finishing mill to run of table and down coiler.

The finishing temperature at the end of rolling is 850oC.The strip undergoes phase transformation at this coiling temperature of 610oC. In addition, the strip at higher temperature has lower strength and higher ductility. Hence, due to self-weight the coiled steel strip collapses to form elliptic shape as shown in Fig. 2. Hence, it is essential to control the phase transformation. One possibility is to ensure longer residence of the coil at the ROT, which results in a loss of productivity. Another possibility is to cool the coil in the down coiler by suitable cooling medium such as with water for durations of 4 to 5 min. This is a commonly followed practice that prevents the coil from forming the elliptic shape formation. However, this results in a loss in productivity by 4 to 5%.

By a proper analysis of the cooling rate curves by physical simulations, an alternate cooling strategy was evolved by way of the present invention, which prevented the coil from forming an elliptic shape. In addition, there is a simultaneous improvement in productivity by 4 to 5%.
In the present study, the cooling rate was increased at the start of the ROT. The conventional method tries to show cooling rates 40 oC/s to a steady cooling rate of 7 oC/s spread over initial 4 banks as shown in Fig.3 (a,b,c). This gradual heat extraction in the strip was replaced with sharper cooling rate where within 2nd and 3rd Bank steady cooling rate is achieved. The heat extraction which was done at down-coiler was replaced with higher heat extraction in the coil at close to finish rolling temperature. This operation has ensured that the coiling temperature was lower at the down coiler.
The figure 3 (a,b)shows that theROT cooling pattern are actual 1st cycle, 2nd cycle, 3rd cycle and 4th cycle with respect to cooling rate and strip temperature slow to high respectively bank wise, from bank-1 to bank-16. The figure 3c shows that in the time available from the end of finishing (on ROT) to the start of coiling, 14-18 seconds, the cooling rate & temperature controls the elliptical behavior during the coiling operation.

TRIAL 1: GLEEBLE SIMULATION STUDIES
Using Gleeble simulation study, the process was simulated as per details shown in Table 2. It is seen that for the steel chosen for the study the Ar1 and Ar3 temperatures follow the profile shown in Fig. 4. It was decided to simulate the conventional cooling cycle, where the coil is cooled with water in down coiler with water. The thermal regime in Banks 1-15 was simulated and the results are shown in Fig.5(a). The curve marked A brings out the actual cooling condition of the banks 1 to 15. The simulation of the cooling condition of slower cooling rate at banks 1 to 5 showed much lowest dilation [cycle -2], which implies that phase transformation is lowest at this zone. Simulation with, slow cooling rate at banks 1 to 5 and slower in rest of bank and moderate cooling just before coiling [cycle-3] showed still lower dilation. This implied that lowering cooling rates in the line may not give required phase transformation. Hence, it was decided to enhance the cooling rate in the first two cooling banks followed by slow cooling rate subsequently in the banks 2 to 15 [cycle-4]. The cooling effect in cycle-4 ensures that dilation is highest, which implies phase transformation is further enhanced. This enhanced phase transformation at the ROT ensures that recalescence is completed in ROT as shown in Fig. 5(b). There is no significant phase transformation in the coil during winding as shown in Fig. 5(c). This avoids the holding the coil in the down coiler for additional 4 to 5min under water cooling. The effect of increased cooling rate in bank 1, led to reduction in production time loss from 48 hrs/month to ~3 hrs/month as shown in Fig. 6. This saving in time enhances down coiler availability which enhances the productivity by 4 to 5%. Hence, industrial scale trials were carried out as per cycle-4.

Table 2: Gleeble parameters for Physical simulation of Carbon-Manganese alloyed steels.
S. No. Parameters Values
1. Steel Composition in wt.%
Grade: HTA1 Carbon= 0.17 Manganese=1.32
Silicon= 0.011 Nitrogen=0.0052
Chromium= 0.014 Sulphur= 0.008
Phosphorous= 0.01
2. Gleeble Machine Gleeble 3800
3. Gleeble study Dilatometric analysis using
4. Sample dimension 10 mm diameter x 85 mm length
[ strip thickness is <6mm; simulation is done in a 10mm thick sample]
6. Thermal regime Austenitizing temperature = 850°C
Cooling rate = 1 to 50°C/s
[ the actual cooling rate in bank 1 is 120 oC/s]
7. Cooling Simulation done Cooling rate in bank no.
Cycle -1 Actual cooling profile at ROT Faster cooling rate in Banks 1 to 4 followed Slower cooling rate in the Bank
Cycle-2 Conventional Slower cooling rate between finishing and coil temperature [Banks 1 to 5]
Cycle-3 Conventional slow cooling rate at bank 1,2,3,4,5 and more slower in rest of bank and moderate just before coiling
Cycle-4 Modified Highest cooling rate in Bank-1 and very slow cooling in Banks 2 to 15

The dimensional changes of sample is measured as a function of temperature. As the temperature increases, the volume of the sample increases and with decrease in temperature the volume of the specimen decreases smoothly due to thermal expansion/contraction and in other word a dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process. The dilatometric test investigations of phase transformations at heating and cooling (direct resistance heating provides rapid and precise temperature control) of steel on Gleeble as illustrated in Fig.5(a).
After rolling in the austenite region, the air and accelerated water cooling on the ROT, results in strip contraction. Once the transformation from the FCC austenitic structure to the bcc ferritic phase starts, expansion of the strip takes place. When the transformation is complete the strip again contracts due to the temperature decrease. Cooling is non-linear and another consideration is the exothermic re-calescence due to the latent heat of the pearlite transformation, which potentially reduces the subsequent cooling rate. This observation coincides with an industrially observed rise in coil temperature when transformation is still in progress during coiling. Though this temperature rise is not observed in laboratory simulation as the test temperature is controlled and forced to remain isothermal in the coiling stage. It is clear from this work that if transformation is complete before coiling, then there should be no problems with the coils collapsing.
Table 3: Industrial Scale Trialsof Carbon-Manganese alloyed steels

S.No. Parameter Trial-1
Trial-2 Trial-3
Trial-4
Trial-5
Grade HTA1 HTA1 HTA1 CM41A TR11
Coil shape Elliptical Coil Non-elliptical Non-elliptical Non-elliptical Non-elliptical
1. Steel Composition in wt.% C=0.17 ; Mn=1.32; Si=0.011; S=0.008; P=0.01; Cr=0.014; N=0.0052; C=0.17 ; Mn=1.32; Si=0.011; S=0.008; P=0.01; Cr=0.014; N=0.0052; C=0.17 ; Mn=1.32; Si=0.011; S=0.008; P=0.01; Cr=0.014; N=0.0052; C=0.425; Mn=1.49; Si=0.19; S=0.005; P=0.018; Cr=0.04; N=0.0043; C=0.068; Mn=0.67; Si=0.016; S=0.008; P=0.012; Cr=0.015; N=0.0042;
2. Critical transformation points* Ar1=490.81
Ar3= 708.10 Ar1=490.81
Ar3=708.10 Ar1=490.81
Ar3=708.10 Ar1=381.36
Ar3=574.33 Ar1=603.38
Ar3=804.19
4. Cast Slab dimension, mm 220x 1250 x 10500 220x 1250 x 10500 220x 1750 x 10500 220x 1250 x 10500 220x 1310 x 10500
5. Hot rolling temperature, oC 1210 1210 1210 1220 1230
6. Roughing Reduction ratio; Exit Temperature 0.110,0.180,0.154,
0.286,0.289,0.328

1070°C 0.110,0.180,0.154,
0.286,0.289,0.328

1070°C 0.150,0.193,0.114,
0.295,0.345,0.371

1070°C 0.153,0.188,0.115,
0.284,0.325,0.344

1060°C 0.135,0.191,0.122,
0.310,0.369,0.416

1055°C
7. Finishing
Reduction ratio; Exit Temperature 0.368,0.436,0.328,
0.353,0.372,0.178,
0.054
850°C 0.368,0.436,0.328,
0.353,0.372,0.178,
0.054
850°C 0.412,0.364,0.313,
0.285,0.250,0.177,
0.078
860°C 0.632,0.532,0.588,
0.408,0.685,0.218,
0.0440
870°C 0.530,0.406,0.331,
0.300,0.307,0.208,
0.098
880°C
8. Strip thickness, mm 3.5 3.5 4 3.5 2.2
8. ROT Cooling rate (°C/sec) 1st,2nd&3rd cycle in Fig. 3(a) 4th cycle in Fig.3(a) 4th cycle in Fig.3(a) 4th cycle in Fig.3(a) 4th cycle in Fig.3(a)
9. Coiling temperature 610 °C 610 °C 610 °C 630°C 650°C
10. Down coiler residence time Zero minutes (4-5 Minutes no elliptical) Zero minutes Zero minutes Zero minutes Zero minutes
* Ar1 = 706.4 – 350.4C -118.2Mn
* Ar3 = 879.4 – 516.1C – 65.7Mn + 38Si + 274.7P

Industrial scale trials of Carbon-Manganese alloyed steels are taken and the process parameters of all the trial are shown in table 3. The trial 1 is taken with cooling rate 1st, 2nd & 3rd cycle (Fig. 3a) and coil shape observe elliptical after the removal of the mandrel, and in same cooling rate coil is hold on the down coiler mandrel for 5 additional minutes and cooled with water then no elliptical shape found. On the basis of ROT cooling and coiling pattern, the 4th cycle (as shown in Figure 3a) of cooling pattern for the Carbon-Manganese alloyed steels are the best cycle and no coil collapsing was observed, even with zero holding time on mandrel and no target coiling temperature varies.
It is thus possible by way of the present invention to provide a method for hot strip rolling of carbon manganese steel by involving controlled cooling rate maintained selectively higher in 1st and 2nd bank and relatively lower rate in rest of the banks thereby ensuring complete phase transformation before coiling so as to eliminate coil collapse or elliptic shape of coils after removal of mandrel and also simultaneously reducing total time on ROT and avoiding additional holding time at down coiler that helps enhancing the productivity of coils.