Claims:We Claim:
1. High strength line pipe steel of API 5L X70 steel standards having steel composition comprising wt%:
C 0.12 max, Mn1.7 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, (Nb+V+Ti) 0.15 max, Mo 0.5 max, Cr 0.5 maxand rest Fe and a microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 485 MPa, UTS > 565 MPa , %El > 17 and YS/TS ratio < 0.93.

2. High strength line pipe steel as claimed in claim 1 comprising:
C 0.12 max; preferably 0.055-0.065
Mn 1.70 max; preferably 1.40-1.60
S up to 0.01 max; preferably 0.003-0.006
P up to 0.02 max; preferably 0.008-0.012
Si 0.45 max; preferably 0.20-0.30
Nb 0.06 max; preferably 0.04-0.06
Mo 0.5 max; preferably 0.1 max
Cr 0.5 max; preferably 0.25 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.040
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm and balance having
said yield strength > 485 MPa, preferably in the range of 525-540 MPa and UTS > 565 MPa, preferably in the range of 600-620 MPa, the YS/UTS ratio < 0.93, preferably in the range of 0.85-0.88, %El is >17% preferably in the range of 30-36%

3. High strength line pipe steel as claimed in anyone of claims 1 or 2 having said high strength along with high toughness value including Impact and DWTT at subzero temperature up to -40 Deg C and lower DBTT temperaturebelow -40 Deg C preferably
having impact property is in the range of 220-230 J at 0 Deg C, 200-210 J at -20 deg C, 180-190 J at -30 Deg C and 170-180 J at -40 Deg C.

4. High strength line pipe steel as claimed in anyone of claims 1 to 3 having DWTT property achieved > 90 % shear area at the temperature up to -40 Deg C

5. High strength line pipe steel as claimed in anyone of claims 1 to 4 having said high strength high toughness steel in the thickness range of 5-10 mm hot rolled coils.

6. A process for producing high strength line pipe steel ofAPI 5L X70 steel standard comprising :
i)involving selective hot metal steel composition followed by refining to have in wt%:

C 0.12 max, Mn1.7 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, (Nb+V+Ti) 0.15 max, Mo 0.5 max, Cr 0.5 maxand rest Fe
ii) casting involving thin slab caster ensuring dine and distributed TiN precipitates;
iii) controlled homogenization in the presence of TiN and ensuring uniform temperature across thickness avoiding precipitation of NbN; followed by
iv) hot rolling and cooling such as to achieve said microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 485 MPa, UTS > 565 MPa , %El > 17 and YS/TS ratio < 0.93.

7. A process as claimed in claim 6 comprising:
step of carrying out said steel casting at casting speed of 5-7 m/min;
homogenising in the temperature range of 1100 to 1700 0C ;
descaling before hot rolling;
carrying out final rolling pass below no recrystallization temperature;
involving ultra fast cooling after finish rolling in combination with standard laminar cooling;
involving coiling temperature in the range of 500-600 0C under slow cooling in coiling.

8. A process as claimed in anyone of claims 6 or 7 comprising:
I. chemical composition
C 0.12 max; preferably 0.055-0.065
Mn 1.70 max; preferably 1.40-1.60
S up to 0.01 max; preferably 0.003-0.006
P up to 0.02 max; preferably 0.008-0.012
Si 0.45 max; preferably 0.20-0.30
Nb 0.06 max; preferably 0.04-0.06
Mo 0.5 max; preferably 0.1 max
Cr 0.5 max; preferably 0.25 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.040
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm
II. Prepare the liquid steel in CONARC furnace followed by ladle refining furnace
III. Casting the steel slabs in a thin slab caster with slab thickness 55-65 mm ;
IV. Homogenizing the cast slab in a tunnel furnace at temperature 1080-1120 Deg C ;
V. Descaling the slab in a high pressure descaler followed by low pressure descaler to remove the scale formed in tunnel furnace
VI. Rolling the slab in a 6 stand tandem rolling mill to the desire final thickness and finish rolling temperature in the range of 800-880 0C ;
VII. Subjecting the hot rolled coils to ultra-fast cooling such as to ensure high nucleation rate and fine grain structure followed by laminar cooling for facilitating coiling.

9. A process as claimed in anyone of claims 6 to 8 wherein said hot metal is treated in a CONARC furnace and further in a ladle refining furnace and thereafter the liquid steel is cast in thin slab caster with casting speed of 4-6 m/min, followed by charging in tunnel furnace and rolling to sheet/strip at FRT 800 to 880 0C and at coiling temperature 500 – 580 0C to obtain desired high strength.

10. A process as claimed in anyone of claims 6 to 9 wherein hot rolled steel manufacture is controlled to produce high strength, high ductility steel sheet/strip of 5-10 mm thickness.

11. A process as claimed in anyone of claims 6 to 10 wherein said process carried out under controlled operating conditions comprising

(i) Casting Speed 4-6 m/min,
(ii) Slab Thickness 55-65 mm,
(iii) Slab Cutting Temp 980-1030 Deg C,
(iv) Homogenization Temp (Tunnel Furnace) 1080-1120 Deg C,
(v) Homogenization Time 8-15 min,
(vi) Finish Rolling Temp 800-880 Deg C,
(vii) Stand wise rolling reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-45 45-50 25-30 20-25 20-25 15-20
Interstand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp 975-1000 925-950 875-900 860-885 850-875 830-850

(viii) Coiling Temp 500-580 Deg C,
(ix) Ultrafast Cooling Rate 60-80 Deg C/sec,
(x) Lamellar Cooling rate 10-25 Deg C/sec.

Dated this the 29th day of September, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION:

Present invention relates to development of low cost high strength line pipe steel complying with API 5L X70 standard and a manufacturing method thereof. More particularly the present invention is directed to developing low carbon high strength hot rolled steel for line pipe application conforming to APIX70 standard having excellent subzero (up to -40 Deg C) impact& DWTT properties and low DBTT temperatureand a method of its productionthrough thin slab caster (TSCR) processing route. The low cost, high strength steel is characterized by high YS 485 MPa minimum, UTS 570 MPa minimum, YS/TS Ratio 0.93 maximum and %El 17 % minimum at a gauze length of 50 mm. The chemical composition comprises of %C 0.12 max, %Mn1.7 max, %Si 0.45 max, %S 0.010 maximum, %P 0.020 maximum, %(Nb+V+Ti) 0.15 max, %Mo 0.5 max, %Cr 0.5 maxand rest Fe.

BACKGROUND OF THE INVENTION

The production of API 5L grades is of great interest because of the increasing demand of gas consumption and the forecast of new lines to be installed in the next future. The economy of gas transportation via pipeline demands for high operating pressures and large pipe diameters in order to improve transportation capacity. This requires the heavy thickness and/or high grade of the steel which has pushed the development of high strength line pipe steels aiming at superior performance and durability to operate in harsh environments. A good toughness, both in terms of high charpy V notch impact energy, low Ductile Brittle Transition (DBTT) Temperature, high DWTT value and an excellent weldability complete the requirements for the steels to be used for pipeline applications. API 5L grade production is, as a consequence, a promising as well as challenging field in the market development of steel strip producers. These new steel grades for high pressure purposes can be seen as an advanced variant of high strength low alloy (HSLA) steels. The performance of pipeline steel depends mainly upon the cleanliness, impurities such as sulphur, phosphorus and chemical content such as carbon, micro-alloying elements etc.
The technology of production of API 5L X70 grade through conventional thick slab process is matured enough as it gives flexibility of using high %C, lower casting speed, high slab thickness (200-250 mm), higher reheating temperature and time, high reduction etc. However, due to slower cooling rate during liquid to ? transformation, possibilities of centerline segregation defect increases.
Thin slab technology, on the other side allows a reduction in energy consumption (because of lower slab thickness and elimination of reheating process), with consequent benefits in terms of production costs and pollution reductions, and permits also a reduction in investment costs because of the compact layout. These features make the thin slab technology the preferred choice for new mill construction. Its processing involves thin slab (55-65 mm) continuous casting, temperature equalization (1080-1130 Deg C) at tunnel furnace, thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC). By this method the rolling mill is able to produce higher strength microalloyed steel by grain refinement using lower carbon content and thus excellent weldability.
The present invention is thus directed to develop a high strength, high toughness API 5L X70 grade steel through CSP route using Nb-Ti-Cr-Mo as micro-alloying philosophy without any V addition.

OBJECTS OF INVENTION

The basic object of the present invention is directed to provide low carbon high strength hot rolled steel for line pipe application conforming to APIX70 standard with minimum Yield strength (YS) of 485 MPa, minimum Ultimate tensile strength(UTS) of 570 MPa minimum elongation of 17 % at gauge length 50 mm and a method of its production.

Another object of the present invention is to develop steel with improved subzero temperature impact properties (more than 100 J at -40 Deg C) and DWTT properties (more than 85 % shear area at -40 Deg C) and low DBTT temperature (below -40 Deg C).

A further object is to develop the said low carbon high strength steel with low cost through a processing route involving thin slab caster.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel with improved toughness property wherein the steel composition is having Nb, Ti, Cr and Mo as micro-alloying element for strengthening by grain refinement predominantly to obtain desired fine grained ferrite microstructure with combination of high strength, ductility and toughness properties.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel withimproved toughness properties wherein the steel composition is having upper limit of carbon restricted to 0.07% because of poor effect on weldability and its detrimental effect on surface quality due to formation of longitudinal cracks when processed using thin slab caster.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel with sheets with improved toughness properties wherein to achieve the end properties, the microstructure of the steel grade would consist of >95% ferrite and typical ferrite grain size would be below 10 microns.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel with sheets with improved toughness properties wherein the use of Vanadium is avoided to minimize the formation of Vanadium Nitride and or Carbonitride formation which is detrimental to toughness properties particularly at subzero temperature.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel with sheets with improved toughness properties wherein the product obtained through thin slab caster route due to thin section and fast cooling, center line segregation is not a problem which is common in conventional slab caster.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel with sheets with improved toughness properties wherein due to fine grain size and effective use of micro alloying, leaner chemistry (low cost) can be used to produce desirable mechanical properties compared to conventional casting.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to aHigh strength line pipe steel ofAPI 5L X70 steel standards having steel composition comprising by wt%:
C 0.12 max, Mn1.7 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, (Nb+V+Ti) 0.15 max, Mo 0.5 max, Cr 0.5 maxand rest Fe and a microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 485 MPa, UTS > 565 MPa , %El > 17 and YS/TS ratio < 0.93.

A further aspect of the present invention is directed to said High strength line pipe steel comprising:
C 0.12 max; preferably 0.055-0.065
Mn 1.70 max; preferably 1.40-1.60
S up to 0.01 max; preferably 0.003-0.006
P up to 0.02 max; preferably 0.008-0.012
Si 0.45 max; preferably 0.20-0.30
Nb 0.06 max; preferably 0.04-0.06
Mo 0.5 max; preferably 0.1 max
Cr 0.5 max; preferably 0.25 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.040
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm and balance having
said yield strength > 485 MPa, preferably in the range of 525-540 MPa and UTS > 565 MPa, preferably in the range of 600-620 MPa, the YS/UTS ratio < 0.93, preferably in the range of 0.85-0.88, %El is >17% preferably in the range of 30-36%

A still further aspect of the present invention is directed to said High strength line pipe steel having said high strength along with high toughness value including Impact and DWTT at subzero temperature up to -40 Deg C and lower DBTT temperaturebelow -40 Deg C preferably having impact property is in the range of 220-230 J at 0 Deg C, 200-210 J at -20 deg C, 180-190 J at -30 Deg C and 170-180 J at -40 Deg C.

Another aspect of the present invention is directed to said High strength line pipe steel having DWTT property achieved > 90% shear area at the temperature up to -40 Deg C.

Yet another aspect of the present invention is directed to said High strength line pipe steel having said high strength high toughness steel in the thickness range of 5-10 mm hot rolled coils.

A further aspect of the the present invention is directed to a process for producing high strength line pipe steel ofAPI 5L X70 steel standard comprising:
i)involving selective hot metal steel composition followed by refining to have in wt%:
C 0.12 max, Mn1.7 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, (Nb+V+Ti) 0.15 max, Mo 0.5 max, Cr 0.5 maxand rest Fe;
ii) casting involving thin slab caster ensuring fine and distributed TiN precipitates;
iii) controlled homogenization in the presence of TiN and ensuring uniform temperature across thickness avoiding precipitation of NbN; followed by
iv) hot rolling and cooling such as to achieve said microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 485 MPa, UTS > 565 MPa , %El > 17 and YS/TS ratio < 0.93.

A still further aspect of the present invention is directed to said process comprising:
step of carrying out said steel casting at casting speed of 5-7 m/min;
homogenising in the temperature range of 1100 to 1700 0C ;
descaling before hot rolling;
carrying out final rolling pass below no recrystallization temperature;
involving ultra fast cooling after finish rolling in combination with standard laminar cooling;
involving coiling temperature in the range of 500-600 0C under slow cooling in coiling.

A still further aspect of the present invention is directed to said process comprising:
I. chemical composition
C 0.12 max; preferably 0.055-0.065
Mn 1.70 max; preferably 1.40-1.60
S up to 0.01 max; preferably 0.003-0.006
P up to 0.02 max; preferably 0.008-0.012
Si 0.45 max; preferably 0.20-0.30
Nb 0.06 max; preferably 0.04-0.06
Mo 0.5 max; preferably 0.1 max
Cr 0.5 max; preferably 0.25 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.040
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm
II. Prepare the liquid steel in CONARC furnace followed by ladle refining furnace
III. Casting the steel slabs in a thin slab caster with slab thickness 55-65 mm;
IV. Homogenizing the cast slab in a tunnel furnace at temperature 1080-1120 Deg C;
V. Descaling the slab in a high pressure descaler followed by low pressure descaler to remove the scale formed in tunnel furnace;
VI. Rolling the slab in a 6 stand tandem rolling mill to the desire final thickness and finish rolling temperature in the range of 800-880 0C;
VII. Subjecting the hot rolled coils to ultra-fast cooling such as to ensure high nucleation rate and fine grain structure followed by laminar cooling for facilitating coiling.
Another aspect of the present invention is directed to said process wherein said hot metal is treated in a CONARC furnace and further in a ladle refining furnace and thereafter the liquid steel is cast in thin slab caster with casting speed of 4-6 m/min, followed by charging in tunnel furnace and rolling to sheet/strip at FRT 800 to 880 0C and at coiling temperature 500 – 580 0C to obtain desired high strength.
Yet another aspect of the of the present invention is directed to said process wherein hot rolled steel manufacture is controlled to produce high strength, high ductility steel sheet/strip of 5-10 mm thickness.

A further aspect of the present invention is directed to said process wherein said process carried out under controlled operating conditions comprising
(i) Casting Speed 4-6 m/min,
(ii) Slab Thickness 55-65 mm,
(iii) Slab Cutting Temp 980-1030 Deg C,
(iv) Homogenization Temp (Tunnel Furnace) 1080-1120 Deg C,
(v) Homogenization Time 8-15 min,
(vi) Finish Rolling Temp 800-880 Deg C,
(vii) Stand wise rolling reduction/time/temp comprising

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-45 45-50 25-30 20-25 20-25 15-20
Interstand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp 975-1000 925-950 875-900 860-885 850-875 830-850

(viii) Coiling Temp 500-580 Deg C,
(ix) Ultrafast Cooling Rate 60-80 Deg C/sec,
(x) Lamellar Cooling rate 10-25 Deg C/sec.

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 and example.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURE

Figure 1: is the flow chart showing the details of different steps involved in producing the high strength hot rolled steel for line pipe application with good toughness property and good weldability according to the present invention.

Figure 2a-b: is the micrographs of the Microstructure Images taken at 100X, having typical grain size in the range of around 5-10 microns.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to provide low carbon high strengthhot rolled steel for line pipe application complying to API 5L X70 standard with improved toughness produced through CONARC furnace and thin slab casting processing route.

Thus according to the present invention, a cost effective composition of low carbon high strength low alloy hot rolled steel sheet and a method of making such steel is provided. Steel is processed using a thin slab caster and chemistry consists of low carbon along with micro alloys such as Nb,Ti, Cr and Mo addition. Mechanical properties are YS>485MPa& UTS >570 MPa, elongation > 17%, YS/UTS ratio < 0.93, Impact property 220-230 J at 0 Deg C, 200-210 J at -20 deg C, 180-190 J at -30 Deg C and 170-180 J at -40 DegC , DBTT temperature below -40 Deg C and DWTT shear area > 90 % at the temperature up to -40 Deg C with good weldability.

In order to produce the low carbon hot rolled steel grade with above stated properties, the selective steel composition used for processing through thin slab caster is as follows:
I. C 0.12 max; preferably 0.055-0.065
II. Mn 1.70 max; preferably 1.40-1.60
III. S up to 0.01 max; preferably 0.003-0.006
IV. P up to 0.02 max; preferably 0.008-0.012
V. Si 0.45 max; preferably 0.20-0.30
VI. Nb 0.06 max; preferably 0.04-0.06
VII. Mo 0.5 max; preferably 0.1 max
VIII. Cr 0.5 max; preferably 0.25 max
IX. Ti 0.060 max; preferably 0.010-0.020
X. Al 0.020-0.070; preferably 0.030-0.040
XI. B 5 ppm max
XII. N up to 120 ppm max; preferably 60-80 ppm

The process route followed was: Electric Arc Furnace (CONARC) ?Ladle Furnace? thin slab Caster? 6 stand hot rolling mill? coiling; however, any other combination of processes (before caster) which gives steel of same chemistry can also be used.

The detailed consideration for selecting the above chemical composition are as below:

Carbon:Carbon is the most effective and cheap strengthening element in steel by solute strengthening and formation of carbide and /or carbonitrides of Nb. However it has adverse effect on toughness and weldability. Also it has detrimental effect on surface quality due to formation of longitudinal cracks when processed using thin slab caster with carbon level above 0.08% being particularly prone to surface defects. Present invention is selected %C 0.04-0.07

Manganese: Mn is an important element for solid solution strengthening. Usually in low C steel, with increase in Mn content, toughness of the steel increase and DBTT temperature decreases. Mn also decrease the YS/TS ratio. However upper values are restricted because of its poor effect on weldability, castability in thin slab caster and load during hot rolling apart from cost implication. Centre line segregation is another major issue with increasing % Mn. Range of %Mn 1.4-1.6 is selected in this invention.

Silicon: Si is generally used as a deoxidizer as well as an alloying element. Silicon significantly increases the tensile strength and a lesser extent yield strength and thus lowers the YS/UTS ratio. Upper limit is limited by detrimental effect on surface quality as sticky scale is form on the surface with high Si. In this present invention, Si range is selected 0.20-0.30 %

Niobium: Nb is essential for strengthening by grain refinement & precipitation strengthening. It is one of the main sources of strengthening. Nb increases TNR(temp of no recrystallisation) temperature to a great extent and thus prohibiting any recrystallisation& grain coarsening during final phases of hot reduction.
It also reduces the rate of recrystallisation of austenite during controlled rolling of HSLA Steel to improve grain refinement. Other outstanding effects of niobium, is to lowering the austenite / ferrite transformation temperature by a solute drag effect and thus used as the effective precipitation strengthening element also.

Upper limit is restricted because of its effect on rolling load during hot rolling. Also because of the limitation of higher temperature and retention time at tunnel furnace, maximum Nb is restricted. In this invention Nb range selected is (0.04-0.05)

Titanium: Titanium is used to fix the N at higher temperature by forming TiN precipitates. Ti should be added in stoichiometric ratio i.e 3.4 *N or a little lesser. Here the selected range is 0.01-0.020

Chromium: Cris a powerful alloying element in steel. It increases the hardenability of steel and thus helps in reducing the critical cooling rate required to avoid pearlite transformation and helps in bainitic transformation. However its presence in excess amount cause significant hardness increase and cracking in and adjacent to weld. The selected range in this invention is 0.15- 0.18 %

Molybdenum:Mo is a strong carbide former and also increases the hardenability of steel. Maximum limit is restricted because of the cost implication. The range selected for this invention is 0.06-0.08 %

Nitrogen: Nitrogen is a key element with significant role in formation of nitride and conbonitride precipitates; however upper limit is restricted because of its poor effect on formability and toughness of steel. In our case, N is selected in the range of 60-80 ppm.

Calcium: 0-50 ppm: Steel has to be Calcium treated to counter the harmful effect of Sulphur as well as help in casting.

Accompanying Figure 1 shows the flow chart illustrating the different steps involved in producing the high strength hot rolled steel with improved toughness according to the present invention.

The process used for making the product according to an embodiment of the present invention is described in details with the help of following example:

EXAMPLE:

(i) Hot metal from blast furnace was refined with the help of Electric Arc Furnace (CONARC) and final chemistry adjustments were done in a ladle refining furnace to obtain a selective composition as given above involving micro alloying with Nb,Ti, Cr and Mo. The composition of different steel samples obtained on trials heats are given in following table 1:
Table 1:
Sample ID C % Mn % Si % N (ppm) Nb % V% Cr % Mo% Ti% Al % Ca (ppm)
Experiment1 0.055-0.060
1.46-1.51
0.28-0.30 60-70 0.043-0.045 0.040-0.045 Nil Nil 0.015-0.020 0.024-0.025 20-25
Experiment 2 0.060-0.065 1.45-1.50 0.29-0.30 60-70 0.045-0.050 0.050-0.055 Nil Nil 0.015-0.020 0.020-0.025 22-28
Trial 1 0.055-0.060
1.40-1.45 0.25-0.28 60-70 0.045-0.050 0.040-0.045 0.15-0.18 0.06-0.08 0.015-0.020 0.030-0.035 25-30
Trial 2 0.060-0.065 1.45-1.50 0.28-0.30 60-70 0.045-0.050 Nil 0.17-0.19 0.06-0.08 0.015-0.020 0.030-0.035 25-30

(ii) Steel was cast using a thin slab caster with slab thickness ranging from 50-65 mm.
(iii) Slab was further homogenized in a Tunnel furnace at temperature >1100 °C but below 1120 °C. Descaler was used after Tunnel Furnace to remove scales.
(iv) Hot rolling with 6 stand reductions was used to reduce the thickness to the required level with finish rolling temperature in the range of 800-880°C.Final rolling pass below TNR (no re-crystallization temperature).The details stand wise parameters for hot rolling are presented in table 2 as follows:
Table 2:
Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-45 45-50 25-30 20-25 20-25 15-20
Interstand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp 975-1000 925-950 875-900 860-885 850-875 830-850

(v) After finish rolling, an ultra-fast cooling was used in combination of standard laminar cooling to achieve Coiling temperature 500-550°C. Initial ultra-fast cooling ensures a high nucleation rate thus leading to obtain fine and uniform grain size/structure across the thickness and subsequent laminar cooling of less severity is used to maintain good flatness of sheet.
(vi) Coil was subsequently slow cooled in coil yard.
(vii) The resulting steel grade is subjected to testing and inspection to ascertain attainment of desired properties.

Mechanical properties:

Tensile test and Impact toughness testing was done by following ASTM standards E-8 and E-23 respectively. Drop Weight Tear Test (DWTT) was performed as per recommended practice for API grades under ASTM standard E-436.

The product obtained was free from any surface defects and the mechanical properties observed for different samples are presented in the following table 3:

Table 3:
Sample ID Width Thickness CT FT YS UTS % El
YS/UTS
Experiment1 1025 7.20 528 835 490-503 567-583 32-36 0.85-0.87
Experiment 2 1025 7.20 519 840 492-506 565-586 34-36 0.85-0.87
Trial 1 1025 7.20 525 835 521-536 596-616 28-34 0.87-0.88
Trial 2 1025 7.20 527 838 525-539 605-620 32-34 0.86-0.88

Sample ID Impact (J) DWTT ( % Shear Area)
0 °C -20 °C -30 °C -40 °C 0 °C -20 °C -30 °C -40 °C
Experiment1 227 219 200 182 95 95 93 90
Experiment 2 189 188 178 165 95 95 93 87
Trial 1 223 197 184 169 95 95 93 90
Trial 2 205 191 184 168 90 90 90 90

High YS (>525 MPa) and UTS (>600 MPa) is targeted because during ERW pipe formation of higher diameter, there is always drop in mechanical properties. Pipe making trial was also done and hardly 10-15 MPa drop was observed after pipe formation.

Microstructure:
The Microstructure of samples was found to consist of acicular ferrite and quasi polygonal ferrite and typical grain size is below 10 microns and preferably in the range of 5-10 microns. Accompanying Figure 2a-b shows the Microstructure Images taken at 100X, having typical grain size in the range of around 5-10 microns.

Weldability:
As we know that Carbon Equivalent (CE which is a measure of weldability) depends on following empirical relation:
CE= %C + (%Mn+%Si)/6 + (% Cr+%Mo+%V)/5 + (% Cu+%Ni)/15

The grade of steel according to present invention found to have better weldability for the following reasons
• Low %C (<0.07%)
• %Mn(1.6 % max) and % Si(<0.3%) used are less
• Low %Cr(<0.2%), %Mo (< 0.1 %)
• No V , Cu, Ni used in the composition.

This leads to a computed value of carbon equivalent based on the above formula to CE < 0.45, which is clearly indicative of very good weldability.

It is thus possible by way of the present invention to providing a low cost low carbon high strength hot rolled steel sheet processed through thin slab caster route having fine ferrite microstructure to ensure excellent low temperature impact toughness properties with good ductility and weldability conforming to API 5L X70 standard suitable for line pipe application.