FIELD OF THE INVENTION
The present invention relates to the development of microalloyed steel plates of thickness ranging from 9.5 mm to 16 mm, with excellent impact toughness at sub-zero temperature. More particularly the invention relates to development of a bendable and weldable steel grade, adaptable mainly for manufacturing pipelines, which are operating in non-sour environment.
BACKGROUND OF THE INVENTION
With the increasing demand for energy in all industrial segments, natural gas has been identified as a source of clean energy. The transportation of natural gas, or any other liquid or gaseous fuel, over a long distance, is carried out through pipelines. High pressure is maintained inside the pipelines to facilitate long distance transportation. In addition, these pipelines pass through various geographical locations and environments, some of which may be substantially adverse, such s extreme cold regions with temperature at sub-zero level.

It is therefore quite important to pay appropriate attention to the manufacturing and fabrication of these pipelines. The modern pipeline industry is driven by some important factors, such as
(a) High strength material for pipelines to withstand high fluid pressure inside
(b) Excellent fracture toughness, especially at low temperatures
(c) Excellent weldability
In order to satisfy these basic conditions, the modern pipe manufacturing requires the development of steel grades with excellent strength, toughness and weldability. The combination of these properties may be achieved through designing appropriate steel chemistry and process parameters. In general, low carbon microalloyed steels, with very fine grain microstructures, are desirable.
Microalloyed steel plates are usually produced through hot rolling mill or plate mill. In a typical hot rolling process, the cast slab is reheated at a high temperature and then subjected to rolling deformation at different stages, and finally coiled at a temperature, which is suitable for developing the desired microstructure in the steel.
The desirable mechanical properties for this steel grade, as per API 5L specification, are as follows:

Yield Strength: 483-621 MPa; Ultimate Tensile Strength: 565-758 MPa; %Elongation: 19-22.
Achieving these properties through appropriate hot rolling treatment is possible. However, it is also important to maintain a lean chemical composition, to achieve superior weldability and toughness. Therefore, properly controlled thermomechanical processing is the key to achieve all the desired properties in a line pipe grade steel. This is the driving force of the present invention.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a new chemical composition, for line pipe grade steel, suitable for achieving mechanical properties as per API 5L specification.
Another object of the invention is to conveniently process the proposed composition through hot rolling process under controlled rolling conditions.
Yet another object of the invention is to obtain a high impact toughness property, even at very low temperatures.

SUMMARY OF THE INVENTION
The present invention is based on developing a ferrite and bainite type microstructure in steel to deliver a combination of strength and toughness via thermo mechanical controlled hot rolling. The invention further describes a process of manufacturing pipelines designed for non-sour applications. The steel slab having a composition in weight % of C < 0.08, Mn :1.4 to 1.7, S < 0.006, P < 0.02, Si: 0.10 to 0.25, Al: 0.02 to 0.06, Ti: 0.01 to 0.02, Nb: 0.04 to 0.07, V: 0.03 to 0.06, Cr: 0.10 to 0.40, Ni: 0.10 to 0.20, Mo: 0.10 to 0.20, Cu: 0.01 to 0.02, N (ppm) < 80 is reheated in the range of 1150 to 1280 OC and thereafter finish rolling of the steel strip / sheet is done in the temperature range of 820 to 880 OC, above Ar3 temperature of the steel. Ar3 temperature is the temperature at which austenite begins to transform to ferrite during cooling. After finish rolling, steel sheet/strip is cooled at first step cooling stage to an intermediate holding temperature in the range of 650 to 750 OC with a cooling rate of 5-70 OC/sec. The steel sheet/strip is kept at intermediate holding temperature for 5 to 20 second and then cooled at a second stage cooling to coiling temperature with a cooling rate of 5 to 70 OC/second.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 – Schematic diagram of ROT cooling profile for two step cooling
Figure 2 – SEM micrograph of 16 mm thick material depicting polygonal ferrite and bainite

DETAIL DESCRIPTION OF THE INVENTION
In an embodiment of the current invention, a method of producing steel plate for manufacturing pipelines designed for non-sour applications is described. The invention discloses processing of a steel slab with a chemistry designed to achieve a combination of impact toughness and strength. In an embodiment of the invention, the steel slab of the composition comprises in weight percentage-C < 0.08, Mn :1.4 to 1.7, S < 0.006, P < 0.02, Si: 0.10 to 0.25, Al: 0.02 to 0.06, Ti: 0.01 to 0.02, Nb: 0.04 to 0.07, V: 0.03 to 0.06, Cr: 0.10 to 0.40, Ni: 0.10 to 0.20, Mo: 0.10 to 0.20, Cu: 0.01 to 0.02, N (ppm) < 80. The process of the current invention describes a process where steel slab is first reheated in the temperature range of 1150 to 1280 OC and then subjected to two stage cooling after finish rolling at a temperature range of 820 to 880 OC, above Ar3

temperature of the steel. Ar3 temperature is the temperature at which austenite begins to transform to ferrite during cooling. The two stage cooling after finish rolling involves first cooling at a rate of 5-70 OC/sec, to an intermediate holding temperature in the range of 650 to 750 OC for 5-20 sec and thereafter, cooling at a rate of 5-70 OC/sec, to coiling temperature. The different composition elements of the current invention have been picked to impart specific properties and play specific roles.
C – Carbon content should be optimum. It needs to be sufficient for stabilizing the austenite to the extent that on transformation it can give rise to about 10-15% bainite.
S and P – Should be as low as possible. The present invention is not a pipeline material for sour environment, and therefore, S can vary in the range of 30 to 40 ppm.
Mn – Increases solid solution strength and stabilizes austenite also. However, excessive amount of Mn is not recommended as it can deteriorate weldability of the steel, and also lead to banding in microstructure.

Microalloying (Ti, Nb, V) – These microalloying elements are used for (a) maintaining strength at room temperature, (b) restrict transverse cracking, (c) raise Tnr temperature, etc.
Fine ferrite grain formation in the final microstructure requires austenite pancaking. In an embodiment of the current invention, this is achieved by substantial deformation of austenite below Tnr temperature ( austenite no-recrystallisation temperature). Niobium (Nb) is added as microalloying addition in order to meet the processing conditions and making and required amount of deformation under Tnr.
In an embodiment of the invention, Titanium (Ti) is added to restrict the transverse cracking tendency. Further, addition of Vanadium (V) leads to improved strength due to carbonitride formation. Complete dissolution of micro-alloying elements in austenite is desirable, particularly Nb. This requires a higher austenitisation temperature. In an embodiment of the current invention, the hot deformation schedule is organized in such a way that a substantial amount of deformation (about 50%) occurs under Tnr. After finish rolling, there needs to be a rapid drop of temperature, to restrict grain growth. However, the temperature should not enter the bainitic transformation region because majority of the

microstructure should be fine grained ferrite. This necessitates holding the coil for a certain length of time at a temperature, which is above bainite formation temperature. This promotes the formation of pro-eutectoid ferrite upto a volume fraction of about 85%. Finally, there should be another rapid drop of temperature, and this time the temperature profile enters the bainite zone. In this process, the remaining austenite transforms into bainite, and finally a ferrite and bainite microstructure is obtained.
The physical processing of the steel composition as per the invention involves steel making. After steel making, the cast slabs are taken to slab yard, from where these slabs are picked up and placed inside the reheating furnace, which is used for soaking the slab at a high temperature before hot rolling. In an embodiment of the invention, the residence time inside the furnace is at least 2 hr 45 min, to ensure proper soaking of the steel slabs. The slab drop out temperature is in the range of about 1150 to 1280 OC. In an embodiment of the current invention, the thickness of cast slab varies in the range of 210 to 250 mm.
The next step is roughing and may be performed in one or more ways depending on mill configuration. In an embodiment of the invention, for a single stand

roughing mill, the slab may be passed through a pair of rolls for 5-7 times. In another embodiment of the invention, for a two stand roughing mill, the slab may be passed through two pairs of rolls for 5 to 8 times. The thickness, in this process, comes down from 210 to 250 mm to around 50 to 70 mm. This is now called transfer bar. Since the final thickness of strip / plate, which is to be used for the manufacturing of pipeline, is on the higher side, a higher thickness of transfer bar is preferred, because rolling deformation at the finishing mill plays an important role in determining the final properties of the material. The roughing mill exit temperature is maintained in the range of 1010-1080 OC. After roughing operation, the transfer bar goes into finish rolling mill, where the 50 to 70 mm thick transfer bar undergoes thickness reduction in a 6-7 stand tandem mill. In an embodiment of the current invention, the final thickness varies in the range of 9.5 to 16 mm.The finish mill entry temperature is around 1000-1050 OC and the target finish mill exit temperature is in the range of 820-880 OC, to ensure a temperature higher than Ar3 temperature, but below Tnr (austenite no-recrystallisation temperature). At this range of temperature, the material undergoes hot deformation in austenite phase, but the deformed austenite does not undergo recrystallization. It is desirable that about 50% of the deformation given in finishing mill occurs below Tnr. This is very important for developing the

proper microstructure and texture of the material. After hot rolling, the steel strip is subjected to cooling on the run out table. This is the most critical stage for developing the final microstructure. A two-step cooling method was applied, as schematically shown in Figure 1. The range of cooling rate is 5-70 OC/sec, both for step cooling 1 and step cooling 2. After step cooling 1, There is an intermediate step of isothermal holding and the temperature of holding may vary in between 650 to 750 OC. The holding time may vary in between 5-20 sec. The intermediate holding is followed by step cooling 2. After step cooling 2, the temperature is brought down to 480 to 540 OC, and then it is coiled in the down coiler. This is known as coiling temperature. The ranges of physical parameters as per the current invention are:
(a) Slab drop out at around – 1150-1280 OC
(b) Roughing mill exit temperature – 1010-1080 OC
(c) Finish rolling at higher than Ar3 temperature – 820-880 OC
(d) Step cooling 1 to 650-750 OC, with cooling rate 5-70 OC/sec
(e) Isothermal holding for 5-20 sec
(f) Step cooling 2 to 480-540 OC, with cooling rate 5-70 OC/sec
(g) Coiling at 480-540 OC

Examples: The following example is for illustrating the product and process as the current invention and not to limit the invention.
Chemical composition:


The steel with table 1 composition was processed as per the processing conditions as described above and then tested for its impact toughness and strength.


The microstructure, taken from 16 mm thick material, using a Scanning Electron Microscope, depicts a typical polygonal ferrite and bainite type microstructure, which is shown in Figure 2.
The crystallographic texture is depicted by 2=450 section of ODF plot (Bunge notation) as shown in Figure 3. Orientation Distribution Functions, or ODFs in brief, are constructed by standard series expansion method, using raw X-ray diffraction data of three or four incomplete texture pole figures [8, 9]. In this case, pole figures using {110}, {200}, {112} and {222} reflections were used. The 2=450 ODF section has a special advantage that most of the important texture components of BCC iron are present in this section. The different contour lines indicate the intensity levels. This plot indicates that a moderate γ fibre texture was developed in this steel, which is desirable in general. There is some weak rotated cube component, but the overall texture is comparable with other good quality material of same grade.
As herein narrated with an exemplary embodiment should not be read and constructed in a restrictive manner as various modifications, alterations and adaptations are possible within the scope and ambit of the invention as defined in the appended claims.

References :
1. Z. Zhang, X. Zuo, Y. Hu, R. Li and Z. Zhang, Materials Science Forum, Vols. 155-156, pp. 527-548, 1994.
2. Yu. D. Morozov,1 M. Yu. Matrosov, S. Y. Nastich and A. B. Arabei, Metallurgist, Vol. 52, Nos. 7–8, pp. 450-456, 2008.
3. T. Tanaka, International Metals Reviews, No. 4, pp. 185-212, 1981.
4. A. J. DeArdo, Ironmaking and Steelmaking, Vol. 28 No. 2, pp. 138-144, 2001.
5. B. Buchmayr, Steel Research Int. Vol. 88, No. 10, 2017.
6. K. M. Banks, A. S. Tuling, B. Mintz, International Journal of Metallurgical Engg. Vol. 2(2), pp. 188-197, 2013.
7. R. Lagneborg, T. Siwecki, S. Zajac and B. Hutchinson, “The Role Of Vanadium In Microalloyed Steels” Scandanavian Journal of Metallurgy, October 1999.
8. P. Van Houtte: Manual of MTM-FHM, ed. MTM-KULeuven, Belgium, 1995.
9. H.J. Bunge: Texture Analysis in Materials Science, Butterworths, London, 1982.

WE CLAIM :
1. A line pipe steel comprising in weight % of C < 0.08, Mn :1.4 to 1.7, S < 0.006, P < 0.02, Si: 0.10 to 0.25, Al: 0.02 to 0.06, Ti: 0.01 to 0.02, Nb: 0.04 to 0.07, V: 0.03 to 0.06, Cr: 0.10 to 0.40, Ni: 0.10 to 0.20, Mo: 0.10 to 0.20, Cu: 0.01 to 0.02, N (ppm) < 80 wherein the steel exhibit YS in the range of 483 to 621 MPa, UTS in the range of 565 to 758 MPa and %Elongation in the range of 19 to 22.
2. The steel as claimed in claim 1, wherein the steel has an impact toughness ranging from about 300J to about 324J at minus 20 OC temperature.
3. The steel as claimed in claim 1, wherein the steel has an impact toughness ranging from about 282J to about 287J at minus 40 OC temperature.
4. The steel as claimed in claim 1, wherein the steel is employed for non-sour applications.

5. The steel as claimed in claim 1, wherein the steel conforms to API 5L X70 specification.
6. The steel as claimed in claim 1, wherein the steel is used for manufacturing steel plates with thickness in the range of 9.5 mm to 16 mm.
7. The steel as claimed in claim 1, wherein the steel has a fine grain microstructure comprising polygonal ferrite and about 10-15% bainite.
8. A method of producing line pipe steel designed for non-sour applications, comprising the steps of:
a. preparing a steel slab of having a composition in weight % of C <
0.08, Mn :1.4 to 1.7, S < 0.006, P < 0.02, Si: 0.10 to 0.25, Al: 0.02 to
0.06, Ti: 0.01 to 0.02, Nb: 0.04 to 0.07, V: 0.03 to 0.06, Cr: 0.10 to
0.40, Ni: 0.10 to 0.20, Mo: 0.10 to 0.20, Cu: 0.01 to 0.02, N (ppm) <
80;
b. reheating the slab in the range of 1150 to 1280 OC;

c. finish rolling the steel strip / sheet at a temperature range of 820 to
880 OC, above Ar3 temperature of the steel;
d. cooling at a rate of 5-70 OC/sec, to an intermediate holding
temperature in the range of 650 to 750 OC for 5-20 sec;
e. cooling at a rate of 5-70 OC/sec, to coiling temperature;
f.coiling the plate at 480-540 OC.