Field of Technology
This disclosure relates to X-70 steels that are used in the long distance line pipes to transport natural gas and oil from the respective fields to the consumers. More specifically it relates to process for manufacturing low carbon-micro alloyed X-70 line pipe steels through thin slab casting direct rolling (TSCR) route.
Background
In a work by Mandziej et al [1] it was shown that the HDS-V40 physical simulator of Dynamic Systems Inc. allowed true melting and controlled solidification of steel samples followed by a direct simulation of multi-step hot rolling. Reliability of the melting/solidification simulation was duly verified by metallographic observations of non-metallic inclusions. The solidified columnar crystals/dendrites were comparable to those of the real industrial TSCC processes.
However, this facility is quite expensive and is available at best in two to three labs in the world, and thus, the accessibility is pretty poor. Further, the provisions for run-out-table and coiling simulation are not there in the facility.
The following works were done by developing the simulators in the respective labs.
N. Zentara and R. Kaspar [2] designed a special ceramic mold based on the melting and controlled solidification of plate shaped specimen. In the simulator specimens of thickness 25-60 mm are placed in a collapsible ceramic mold and then inductively heated above the liquidus temperature. Controlled solidification occurs through the ceramic side plates of the mold on two sides only. The cooling was done by compressed air and water. When the solidification was completed the sample was transferred to a hot deformation simulator, installed nearby to minimize the decrease in temperature during transport, which takes less than 90s. The specimen temperature at the moment of was in the range 1150-1200°C. The hot deformation simulator was a computer controlled servo-hydraulic testing machine that simulated hot rolling processes by carrying out

multistage plane strain hot compression tests. They had reported that a total strain >1.4 was required to ensure mechanical properties that were comparable or even superior to those found using the conventional cold charging process for Nb-V micro-alloyed steel. After the deformation the specimen was transferred to the cooling box where twin media nozzles can be applied, mixing water and compressed air to simulate accelerated cooling. The austenite strengthening strain (below recrystallization stop temperature) represented the dominant component of the total strain if a satisfactory toughness was to be achieved while strength properties were found to be less sensitive to the applied strain.
Further, in a research by Sobral et al, [3] a laboratory thin slab caster was built and the role of liquid core reduction on microstructure was studied in a low carbon V-Nb added steel at various speeds. The mould was made with two water cooled copper walls, and the bottom was made of a refractory material, perfectly coupled to the copper walls in order to avoid leakage.
In the work of R. Priestner and C. Zhou [4] a facility for simulating hot direct rolling was developed in which research on direct rolling processing was carried out economically using 1-kg ingots. The important feature of the work was that the steel castings of 1-kg weight were made by vacuum/argon induction melting and a ceramic split mould chat was jacketed with ceramic wool was designed in such a manner to allow rapid stripping of the ingot at any temperature up to I400°C for subsequent processing. The schematic of the process is given in Fig. 1 [4].
Li et al [5] conducted physical simulation of TSDR in laboratory. In their study the steels were melted in air as 18 kg loads and cast into three molds to produce 50 mm thick ingots. The ingots were hot stripped from the mold and transferred directly to an equalizing furnace set at one of three equalization temperatures (1050, 1100 or 1200°C) and held for 30-60 min before rolling. The schematic of the TSDR simulation is shown in Fig. 2 [5]. After equalization, the ingots were rolled on a laboratory reversing mill to 7 mm strips in five passes, which gave a total reduction of 86%, The usual inter-pass time was 6 s. After the fourth pass, the strip was held for 25- 40 s until a temperature of 870°C was reached. Finish

rolling temperatures varied from 880 to 850°C and the total rolling times were in the range of 75-90 s. After rolling, the strip was cooled under water sprays to simulate run-out table cooling, the cooling rate being, 18 K/ s. The target for the end cool temperature of the strip was in the range 550-650°C but occasionally process difficulties were encountered which took it out of this range. Following cooling, the strips were immediately put into a furnace set at 600°C and slow cooled to simulate coiling with an average cooling rate of 35 K/ h from 600 to 400°C.
In addition to above all methods, recovery of alloying elements and surface finish was not achieved pioperly.
References
1. S.T. Mandziej, J.D. Vosburgh, R. Kawalla, H.-G. Schoss, Materials Science Forum Vols. 539-543 (2007) pp 4149-4154.
2. N. Zentara and R. Kaspar, Materials Science and Technology May 1994 Vol. 10, p. 370.
3. M. D. C. Sobral, P. R. Mei, R. G. Santos, F. C. Gentile and J. C. Bellon, Laboratory simulation of thin slab casting, Ironmaking and Steelmaking 2003 Vol. 30 No. 5, 412-416.
4. R. Priestner and C. Zhou, Ironmaking and Steelmaking 1995 Vol. 22 No.4, 327.
5. Y. u, J. A. Wilson, A. J. Craven, P. S. Mitchell, D. N. Crowther and T. N. Baker, Materials Science and Technology 2007 VOL 23 NO. 5, 509.
Objects of the invention
In view of the foregoing limitations inherent in the prior-art, it is an object of the disclosure to simulate a process for manufacturing a thin steel product via thin slab casting and direct rolling (TSCR) route that can be used for manufacturing X-70 line-pipes. The proposed process is further used in operation in plants to manufacture thin steel sheet.

SUMMARY OF THE DISCLOSURE
In one aspect, the disclosure provides a process for manufacturing a thin steel product via TSCR route, the process comprises steps of providing a molten steel containing C = 0.052 - 0.058, Mn = 1.49 -1.70, S = 0.005 - 0.007, P = 0.010 - 0.012, Si = 0.35 - 0.40, Al = 0.018 - 0.049, Ti = 0.000 - 0.019, V = 0.021 - 0.034, Nb = 0.046 - 0.050, N = 55 - 80 (in PPM), (in weight %), the remainder being Fe and unavoidable impurities. The molten steel is provided to a mold of a thin slab caster, then casted into the steel at a straightening and bending temperature of 1000-1040 deg. C, then deformed by means of a plurality of finishing stands and cooled at temperature rate of 10-15 deg./s.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 shows a flow diagram of direct rolling simulation.
FIG. 2 shows a schematic diagram for TSDR simulation.
FIG. 3 shows a flow a diagram illustrating various steps of a process for manufacturing a thin steel product via TSCR route in accordance with an embodiment of the disclosure.
FIG. 4 shows ferritic-bainitic microstructure of the thin steel product in accordance with an embodiment of the disclosure.
Detailed Description
Various embodiments of the disclosure provide a process for manufacturing a thin steel product via T5CR route, the process comprising steps of providing a molten steel containing following elements (in weight %) C = 0.052 - 0.058, Mn = 1.49 -1.70, S = 0.005 - 0.007, P = 0.010 - 0.012, Si = 0.35 - 0.40, Al = 0.018 - 0.049, Ti = 0.000 - 0.019, V = 0.021 - 0.034, Nb = 0.046 - 0.050, N = 55 - 80 (in PPM), the remainder being Fe and unavoidable impurities, providing the molten steel to a mold of a thin slab caster, casting the steel at a straightening and bending temperature of 1000-1040 deg. C, deforming the steel by means of

a plurality of finishing stands, and cooling the steel at temperature rate of 10-15
deg./s.
Shown in FIG. 3 is a flow a diagram showing a process (100) for manufacturing
a thin steel product via TSCR route. The method comprises steps of:
At step (104) a molten steel is provided containing following elements (in weight
%): C = 0.052 - 0.058, Mn = 1.49 - 1.70, S = 0.005 - 0.007, P = 0.010 - 0.012,
Si = 0.35 - 0.40, Al = 0.018 - 0.049, Ti = 0.000 - 0.019, V = 0.021 - 0.034, Nb =
0.046 - 0.050, N = 55 - 80 (in PPM), the remainder being Fe and unavoidable
impurities.
The composition of the steel is of such nature that it casts into non-peritectic grade of steel.
At step (108) the molten steel is provided to a mold of a thin slab caster.
The molded steel is further fed into an equalizing furnace. At the equalizing
furnace the steel is further cooled to a temperature 1325°C - 1150°C.
At step (112) the steel is casted at a straightening and bending temperature of
1000 - 1040 deg. C.
At step (116) the steel is deformed by means of plurality of finishing stands. Once the steel is casted, the deformation of the steel is done by using six (6) finishing stands namely F1, F2, F3, F4, F5 and F6.
The process parameters presented in Table 1 is to be followed for the finishing stands F1 to F6.


To consider values from Table 1, if one value prior to forward slash is taken, all the values prior to forward slash should be considered. Similarly, if one value after forward slash is taken, all the other values after forward slash should be considered. For example in F1, if roll force of 19310N is taken then speed should

be considered between (1.20-1.22), strain (0.77-0.78) and %reduction (54-55)
should be considered.
The thin steel product produced is further cooled at the temperature rate of 10-
15 deg./sec, step (120).
The thin steel product produced is majorly ferrite-bainite in nature which imparts
the strength and impact toughness.
The thin steel product which is produced at step (120) is further coiled at
temperature 570-600°C. The thin steel product produced has the microstructures
of majorly ferritic-bainitic shown in FIG. 4 out of which ferrite grain size is of
2.5-4 µm.
The properties of the thin steel product produced have the following properties
Yield Strength (YS): 510-530 MPa
Ultimate Tensile Strength (UTS): 688-739 MPa
Yield Ration (YR): 0.72 - 0.77
Vickers Hardness no. (VHN): 215-260 The thin steel product can be used for manufacturing the X-70 steels to be used for line-pipe. Experiments:
The above mentioned process for manufacturing the thin steel product via TSCR route can be validated by the following examples. The following examples should not be construed to limit the scope of disclosure. Experiment 1:
Molten steel containing following elements (in weight %): C=0.054, Mn=1.60, S=0.005, P=0.011, Si=0.37, AM0.030, Ti=0.010, V=0.027, Nb=0.046, N=65 (in PPM), the remainder being Fe and unavoidable impurities was provided in the thin slab caster. The steel was casted at a straightening and bending temperature of 1010 deg. C. The steel was deformed by means of 6 finishing stands as mentioned in Table 2


The thin strip of steel is cooled at temperature rate of 12 deg./s. The thin strip of
steel is further coiled at temperature 590 deg. C. The thin strip of steel was
tested for the following properties:
Yield Strength (YS): 513 MPa
Ultimate Tensile Strength (UTS): 691 MPa
Yield Ration (YR): 0.73
Vickers Hardness no. (VHN): 218
Advantages:
Improvements in yield strength, yield ratio and toughness can be achieved. In addition recovery of alloying elements and surface finish can also be improved.

We claim:
1. A process for manufacturing a thin steel product via thin slab casting
and direct rolling, the process comprising steps of:
providing a molten steel containing following elements (in weight %): C=0.052 - 0.058, Mn=1.49 -1.70, S=0.005 - 0.007, P=0.010 - 0.012, Si=0.35 - 0.40, Al = 0.018 - 0.049, Ti=0.000 - 0.019, V=0.021 - 0.034, Nb=0.046-0.050, N=55-80 (in PPM), the remainder being Fe and unavoidable impurities;
providing the molten steel to a mold of a thin slab caster;
casting the steel at a straightening and bending temperature of 1000-1040 deg. C;
deforming the steel by means of a plurality of finishing stands; and
cooling the steel at temperature rate of 10-15 deg./s.
2. The process for manufacturing the thin steel product as claimed in claim 1, wherein coiling temperature of the steel product is 570-600 deg. C.
3. The process for manufacturing the thin steel product as claimed in claim 1, wherein a first stand of the plurality of stands have following parameters


4. The process for manufacturing the thin steel product as claimed in
claim 1, wherein a second stand of the plurality of stands have
following parameters

5. The process for manufacturing the thin steel product as claimed in
claim 1, wherein a third stand of the plurality of stands have following
parameters

6. The process for manufacturing the thin steel product as claimed in
claim 1, wherein a fourth stand of the plurality of stands have
following parameters

7. The process for manufacturing the thin steel product as claimed in
claim 1, wherein a fifth stand of the plurality of stands have following
parameters


8. The process for manufacturing the thin steel product as claimed in
claim 1, wherein a sixth stand of the plurality of stands have following
parameters

9. The process for manufacturing the thin steel product as claimed in
claim 1, wherein microstructure of the thin steel product comprises
majorly ferrite-bainite.
10. The process for manufacturing the thin steel product as claimed in
claim 9, wherein size of the ferrite grain is of 2.5-4 urn.
11. The process for manufacturing the thin steel product as claimed in
claim 1, wherein properties of the thin steel product is following:
Yield Strength (YS): 510-530 MPa Ultimate Tensile Strength (UTS): 688-739 MPa Yield Ration (YR): 0.72 - 0.77 Vickers Hardness no. (VHN): 215-260