FIELD OF INVENTION
The present invention relates to the development of a linepipe steet sheet.
BACKGROUND OF THE INVENTION
International studies forecast that the demand for natural gas will be nearly doubled by 2030 [1]. As a consequence of this growing gas demand around the world, there has been a rising need for more economic and increased quantity of gas exploitation and transportation. However, the major reserves for natural gas are usually located quite far from their potential markets and this requires long distance transportation at high operating pressures, for which complex gas transportation pipeline networks need to be constructed. Further, these pipelines require high strength steels for such high pressure transmission of natural oil and gas.
Recent studies have demonstrated the significant economic advantages of using higher-strength linepipe steels for long-distance pipelines. Firstly, usage of higher strength linepipe grades allow significant reduction in wall thickness and consequent reduction in weight which leads to appreciable savings in construction and transportation costs.
Apart from strength, the steels used should also have excellent low temperature toughness for safe operation and at the same time very good weldability for better pipe laying efficiency and structural integrity.
For offshore applications, API X-70 with thicknesses up to 34 mm is being applied whereas for onshore applications, API X-80 with thicknesses up to 25 mm is in use [2]. In order to make long distance gas transmission more economical higher strength grades like X-100 and X-120 are also being developed for the construction of the next generation of pipelines.
Linepipe steels basically belong to the category of HSLA or microalloyed steels. The strength of these steels can be attained by grain refinement, solid solution strengthening and precipitation hardening from microalloy addition. But all the strengthening mechanisms lead to lowering the toughness, except of grain refinement. From the point of view of weldability, the amount of alloying additions especially carbon cannot be increased beyond a particular level.

Recent trend is that thermomechanical processing or TMCP is considered as the primary route for producing API linepipe grades because it results in a fine grained microstructure which is essential for obtaining an optimum high strength-toughness combination. This process route includes, essentially a multi-stage deformation process carried out both above and below the recrystallization stop temperature (Tnr). Repeated recrystallization above the Tnr during rough rolling leads to fine austenite grains which are then finish rolled below Tnr to obtain pancaked grains which are subsequently cooled to form fine ferrite grains and/or bainite or acicular ferrite.
The essential metallurgical aspects of TMCP are:
1. Controlled recrystallization,
2. Ferrite grain refinement,
3. Phase transformation and
4. Precipitation hardening.
TMCP was the only viable route to produce API grades up to X70, which are microalloyed with niobium and vanadium and with reduced carbon content [3]. However, in the eighties, an improved processing route consisting of thermomechanical rolling plus subsequent accelerated cooling emerged which made it possible to produce higher strength materials like X80 and above with a further reduced carbon content (= 0.02%C) to facilitate excellent field weldability properties.
References:
[1] Graf M, Heckmann C, Hillenbrand H, NiederhoffK. High-strength large-diameter pipe for long-distance high pressure gas pipelines. Proceedings of The Thirteenth International Offshore and Polar Engineering Conference.2003;42:482. [2] Schwinn V, Schuetz W, Fluess P, Bauer J. Prospects and state of the art of TMCP steel plates for structural and linepipe applications. Materials science forum, vol. 539: Trans Tech Publ, 2007. p.4726.
[3] Baczynski G, Jonas J, Collins L. The influence of rolling practice on notch toughness and texture development in high-strength linepipe. Metallurgical and Materials Transactions A 1999;30:3045.

SUMMARY OF THE INVENTION;
The invention provide an API linepipe steel comprising: C = 0.02-0.04, Mn = 1.6-1.75, S 
0.003, P  0.006, Si  0.26, Al - 0.03-0.05, Ti - 0.02 approx, Nb - 0.04-0.09, V  0.04, Ni 
0.24, Cr  0.2, Mo  0.24, Cu 0.2, and N (ppm) < 35 (all in wt%).
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates various steps of a process for making an API llnepipe steel.
FIGS. 2a and 2b illustrates a microstructure and stress strain curve of the API linepipe steel in
accordance with an embodiment of the invention.
FIGS. 3a-3b illustrates a microstructure texture of the API linepipe steel in accordance with
the embodiment of FIGS. 2a and 2b.
DETAILED DESCRIPTION OF THE INVENTION
Shown in FIG. 1 are various steps of process (100) to make an API linepipe steel using a composition for preparing a steel slab of an API grade at step (104) as depicted in Table. 1.


The low content of carbon improves the strength by forming micro alloyed carbides and carbonitrides precipitate. Further the composftion also improves weldability and also provides resistance to MIC and ductile crack
Mn: (1.6-1.75 wt.%)
The said composition of manganese helps in solid solution strengthening. Ferrite grains are also refined. The composition also helps in suppressing  - ɑ transformation temperature. Further, it promotes the formation of acicular ferrite or bainitic transformation.
Si: ( 0.26 wt.%)
Low silicon acts as a deoxidiser in molten steel and also acts as a strong solid solution strengthening agent. Further, high content of Si deteriorates weldability and toughness in HAZ. Therefore the content has been kept low,
At: (0.03-0.05 wt. %)
Aluminum acts as de-oxidant and helps in grain refinement thereby improving toughness properties. Further high Al content produces inclusions.
P: ( 0.006 wt. %)
It is an inevitable impurity element of steel and very low content is desirable to produce clean steel. Other degrade effect of phosphorus is that it forms segregation, inclusions which affects the toughness and HIC properties. Hence it has been kept as low as possible.
S: (<0.003 wt. %)
The lower content of S is essential for sour applications and requires Ca treatment. It further supports the formation of inclusions, segregation and other defects. It also deteriorates welding performance, HIC and impact toughness properties.
N: (<35 ppm)
The N content is mainly effective for improving low temperature fracture toughness. It also helps in suppressing y - a transformation temperature. It also improves corrosion property.

Nb: (0.04-0.09 wt. %)
It is a Ferrite strengthener (NbCN precipitation) and imparts good combination of strength and toughness. It also helps in grain refinement by retardation of austenite recrystallization. It helps in suppressing y - a transformation temperature. It also promotes the formation of bainite - Nb+V or Nb+Ti Is effective for high strength grades of API,
Mo: ( 0.24 wt.%)
It is added to promote the formation of lower temperature phases (i.e. bainite or acicular ferrite). It has more potential in increasing hardenability than other alloying elements and suppresses pearlite transformation.
Cr: ( 0.2 wt. %);
Chromium imparts strength in atmospheric corrosive conditions (when Cu is also added) and promotes formation of lower temperature transformation products.
Copper ( 0.2 wt. %)
Copper promotes the formation of a (Fe Cu)S protective surface film on the steel, which retards the hydrogen generation reaction. It also reduces corrosion rate under severe sour conditions and suppresses y - a transformation temperature. Copper also acts as Ferrite strengthener.
Titanium: (0.02 wt% approx.)
The titanium acts to control grain size (TiN formation). In an embodiment T/N ratio is 3.42. It also acts to control grain growth during welding and is Ferrite Strengthener.
Vanadium: (0.04 wt.%)
Vanadium is more soluble in austenite and acts as Ferrite strengthener by precipitation strengthening by forming carbonitrides. It suppresses bainite/pearlite transformations.

Nickle: (0.24 wt.%)
Nickle Is effective in improving low temperature fracture toughness and suppresses y - a transformation temperature and improves corrosion property.
At step (108) the steel slab Is reheated at a temperature 1100-1250°C in a furnace for soaking. In an embodiment a residence time inside the furnace is at least 120 mins for every 100mm of thickness so as to ensure proper soaking of the steel slabs. The steel slab is further transformed into a steel plate for finish rolling.
The next step is roughing, which is actually the first stage of hot rolling. The slab undergoes a massive thickness reduction. The slab is passed through a pair of rolls for generally 4 times, and the thickness comes down about 50%. The numbers of passes depends on the thickness the steel slab requirement pre and post the roughing pass. The roughing mill exit temperature is in the range of 1025-1040°C.
After roughing operation, the slab goes into finish rolling mill, in which the slab undergoes 70-90% reduction in 6 passes in same rolling mill in forward and reverse operation. The final thickness is anywhere in between 5 - 6 mm, as per requirement.
The finish mill entry temperature is around 1025°C. At step (112) the steel plate is finish rolled at a temperature 820-890°C, which is above Ar3 temperature of the steel. Again the numbers of passes depends on the thickness the steel slab required pre and post the finishing mill.
At this range of temperature, the material undergoes hot deformation in austenite phase, but the deformed austenite does not undergo recrystallization because finish rolling temperature Is below No-recrystallization temperature (Tnr). This step leads the development of the appropriate microstructure and grain size of the material.

At step (116) the steel plate is cooled in ROT at 20-80°C/s. The temperature is brought down to about 620°C and then it is kept in furnace at temperature of 600- 620°C for 1 hour for coiling step (120). This is also known as coiling temperature, which influences the precipitation coarsening and formation of bainitic phase in linepipe steels. Maintaining the said coiling temperature ensures formation of Bainlte phase in correct fraction so as to achieve the desired mechanical properties.
Following are the properties obtained of the API line pipe and can be compared with standard reference value stated in Table 2. YS and UTS are 630-690 MPa and UTS 710-770 MPa respectively. The elongation is 25%. The hardness values obtained is 245-270 Hv and the impact toughness at subzero temperature (-40°C) of 110-160 3 in longitudinal and 80-102 J in
transverse direction. The API linepipe steel has y fiber crystallographic texture. The API linepipe steel has ferrite and Bainite microstructure. The API linepipe steel has a grain size of 5-6 urn.

Experimental Analysis
The steel slab is prepared as per the preferred composition disclosed in Table 1. The cast slabs are being cut In pieces for next stage hot rolling. The slab pieces are placed inside the reheating furnace, which is used for soaking the slab at 1200°C before roughing and transforming the steel slab into a steel plate.

The steel plate is finish rolled temperature 870°C.
The steel plate is cooled in ROT at 80°C/s and coiled at 620°C.
Following are the properties o;8btained shown in FIG. 2a and 2b
YS - 633 MPa, UTS - 701 MPa, Elongation 25%, Hardness is 245-253 Hv, Impact toughness at -
40°C 160 L-CVN (J) and 102 T-CVN (J), at 0°C 160 L-CVN (J) and 110 T-CVN (J),
Crystallographic texture ɣ fiber, microstructure ferrite and Bainite, Grain size 5-6 m.
The texture is determined using X-ray Diffraction technique for the rolled samples on rolling surface and also mid-section of the rolling plane. The textures are represented in FIGS. 3a & 3b. The ODF sections presented in this chart depict that the mid-section has an excellent texture, whereas the rolling surface showing completely different texture components.

WE CLAIM:
1. An API linepipe steel comprising:
C = 0.02-0.04, Mn = 1.6-1.75, S  0.003, P  0.006, Si  0.26, Al - 0.03-0.05, Ti -approx. 0.02, Nb - 0.04-0.09, V  0.04, Ni  0.24, Cr  0.2, Mo  0,24, Cu 0.2 (all in wt%) N (ppm) < 35.
2. The API linepipe steel as claimed in claim 1, wherein the API linepipe steel has Y5 -630-690 MPa.
3. The API linepipe steel as claimed in claim 1, wherein the API linepipe steel has UTS 710-770 MPa.
4. The API linepipe steel as claimed in claim 1, wherein an elongation of the API linepipe steel has 25% max.
5. The API linepipe steel as claimed in claim 1, wherein a hardness values Is 245-270 Hv.
6. The API linepipe steel as claimed in claim 1, wherein the API linepipe steel has impact toughness at subzero temperature (-40°C) is 110-160 J in longitudinal and 80-102 J in transverse direction.
7. The API linepipe steel as claimed in claim 1, wherein the API linepipe steel has fiber crystallographic texture.
8. The API linepipe steel as claimed in claim 1, wherein the API linepipe steel has ferrite and Bainite microstructure.
9. The API linepipe steel as claimed in claim 1, wherein the API linepipe steel has a grain size of 5-6 n.

10. A method of producing API linepipe steel comprising:
- preparing a steel slab of API grade having a composition in weight % of C = 0.02-0.04, Mn = 1.6-1.75, S  0.003, P  0.006, Si  0.26, Al - 0.03-0.05, Ti - 0.02 approx, Nb - 0.04-0.09, V  0.04, Ni  0.24, Cr  0.2, Mo  0.24, Cu 0.2 (all in wt%) N (ppm) < 35;
- reheating the steel stab at 1100-1250°C, and transforming the steel slab into a steel plate;
- finish rolling the steel plate at a temperature 820-890°C, the temperature being above Ar3 temperature of the steel;
- cooling the steel plate in ROT at 20-80°C/s; and
- coiling the steel plate at 600-620°C.