Claims:We Claim:

1. Low carbon High strength line pipe steel having ductility and weldability conforming to API 5CT J55 steel standards having composition comprising wt%:C 0.26 max, Mn1.5 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, Nb 0.06 max ,Ti 0.060, Mo 0.1 max, Cr 0.25 max and rest Fe providing selective micro alloying including Nb,Ti and Cr for desired microstructure of acicular ferrite and quasi polygonal ferrite enabling said desired ductility and weldability having having YS > 400MPa, UTS > 535 MPa , %El > 19 and YS/TS ratio < 0.93 .

2. Low carbon High strength line pipe steel as claimed in claim 1 comprising (Nb+V+Ti) 0.15 max.

3. Low carbon High strength line pipe steel as claimed in claim 1 comprising:
C 0.26 max; preferably 0.055-0.065
Mn 1.50 max; preferably 1.25-1.35
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 0.25-0.30
Nb 0.06 max; preferably 0.04-0.05
Mo 0.1 max; preferably Nil
Cr 0.25 max; preferably 0.15-0.20 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.045
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm and balance Fe having said yield strength > 400MPa, preferably in the range of 460-490MPa and UTS > 535 MPa, preferably in the range of 650-580MPa, the YS/UTS ratio < 0.93, preferably in the range of 0.85-0.88, %El is >19% preferably in the range of 32-36%

4. Low carbon High strength line pipe steel as claimed in anyone of claims 1 to 3 having said high strength along with high toughness value including Impact and DWTT at subzero temperature up to -40 Deg C and lower DBTT temperature below -40 Deg C preferably
having impact property is in the range of 230-240 J at 0 Deg C, 220-230 J at -20 deg C, 200-210 J at -30 Deg C and 190-200 J at -40 Deg C.

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

6. Low carbon High strength line pipe steel as claimed in anyone of claims 1 to 5 comprising high strength high toughness steel in the thickness range of 5-10 mm as hot rolled coils.

7. A process for producing low carbon high strength line pipe steel of API 5CT J55 steel standard comprising:
i) providing selectively refined hot metal steel composition having in wt%:
C 0.26 max, Mn1.5 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, Nb 0.06 max ,Ti 0.060, Mo 0.1 max, Cr 0.25 max and rest Fe favouring selective micro alloying element provision including Nb,Ti and Cr

ii) subjecting to thin slab caster ensuring dine and distributed TiN precipitates;
iii) homogenization in the presence of TiN 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 desired YS > 400 MPa, UTS > 535 MPa , %El > 19 and YS/TS ratio < 0.93.

8. A process as claimed in claim 7 wherein the provision of said selectively refined hot metal steel composition include limiting (Nb+V+Ti) 0.15 max.

9. A process as claimed in claim 7 or 8 comprising:
• Step of carrying out said steel casting at casting speed of 4-6 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.

10. A process as claimed in anyone of claims 7 to 9 comprising:
I. involving chemical composition
C 0.26 max; preferably 0.055-0.065
Mn 1.50 max; preferably 1.25-1.35
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 0.25-0.30
Nb 0.06 max; preferably 0.04-0.05
Mo 0.1 max; preferably Nil
Cr 0.25 max; preferably 0.15-0.20 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.045
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm and balance Fe

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.
11. A process as claimed in anyone of claims 7 to 10 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 530 – 560 0C to obtain desired high strength.
12. A process as claimed in anyone of claims 7 to 11 wherein hot rolled steel manufacture is controlled to produce high strength, high ductility steel sheet/strip of 5-10 mm thickness.

13. A process as claimed in anyone of claims 7 to 12 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 530-560 Deg C,
(ix) ultrafast Cooling Rate 60-80 Deg C/sec,
(x) lamellar Cooling rate 10-25 Deg C/sec.

Dated this the 22nd July, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION:

Present invention relates to development of low carbon high strength steel for producing ERW welded oil casing tube complying to API 5CT, J55 grade and a manufacturing method thereof. More particularly, the present invention is directed to developing high strength hot rolled steel for line pipe application conforming to API 5CT J55 standard having low carbon for better weldability and excellent subzero (up to -40 Deg C) impact & DWTT properties and a method of its production through thin slab caster (TSCR) processing route. The high strength steel is characterized by low carbon (< 0.07 %), high YS 400MPa minimum, UTS 535MPa minimum, YS/TS Ratio 0.93 maximum and %El 19 % minimum at a gauze length of 50 mm. The chemical composition comprises of %C 0.26 max, %Mn1.5 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 max and rest Fe.

BACKGROUND OF THE INVENTION
The production of API 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.API 5CT J55 grade are widely used for casing and tubing in oil and gas industry. API 5CT casing and tubing are also known as OCTG pipes and tubes. These OCTG pipes and tubes are manufactured through both seamless and ERW route.
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.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 J55 grade steel through conventional thick slab process using medium to high carbon range is well proven as it gives higher slab to finish coil thickness ratio, higher reheating temperature and time etc. However due to slower cooling rate during liquid to ? transformation, possibilities of centerline segregation defect increases. Also due to higher C range, the weldability and impact property is supposed to be poor.
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.
Currently chemistry design of J55 grade for ERW casing or tubing application is usually either high C (> 0.2 %) and Mn( 0.8-1.5 %) or low C (<0.2 %) and Nb/V/Ti addition.
CN103882301A discusses a J55 grade low-cost steel used for ERW oil casing pipe having tensile strength (UTS) greater than 550 MPa.C: 0.23% -0.27%, S1: 0.10% -0.35%, Mn: 0.85% -l.10%, P: ^ 0.015%, S: ^ 0.008%, Ti: 0.010% -0.030%, Als: 0.02% -0.06%, NS 0.008%. The mechanical properties achieved as per the requirement of J55 grade but Impact property are achieved up to -10 Deg C (67-102 J). There is no mention of DWTT property. Also due to high %C, the Carbon Equivalent (CE) value is higher resulting poor weldability as per the gravile diagram (fig 3).

CN 1884787A discloses a J 55 grade steel with low C with Nband Ti addition. The chemical composition was C in 0.15-0.22%, Si <=0.45%,Mn in 0.70-1.60%, P<=0.025%, S<=0.020%, Nb<=0.06% and Ti<=0.035% through EAF and thin slab caster route. Although mechanical properties achieved as per the requirement of J55 grade but there is no data provided on impact and DWTT results. As the %C is in the range of 0.15-0.20, the weldability will be poor as per the gravile diagram (fig 3). Also due to high %C in thin slab casting route casting speed will be lower resulting in productivity loss as well as poor surface finish.

CN 101200791A disclosed Low-cost oil casing pipe steel and method for producing the same having chemical composition of, C: 0.16 ~ 0.21; Si: 0.15 to 0.25; Mn: 1.0 to 1.2; P ? 0.020; sulfur =0.009; Ti: 0.01 to 0.02; Nb: 0.02 to 0.03. Mechanical properties achieved as per the requirement of J55 grade and Impact property mention up to -10 Deg C only. Also due to %C range weldability will be poor.
CN01126611 discloses Low-carbon low-alloy steel and produced pipe, having chemical composition of C: 0.10 to 0.16%; Si: 0.20% to 0.40%; Mn: 1.00 to 1.35%; Al: 0.02 to 0.035 %; V: 0.07 to 0.13%;Ni: 0.05 to 0.25%; B: 0.0005 to 0.0035%; P: ? 0.010%; sulfur: =0.005%; iron and trace impurities: balance.
The present invention is thus directed to develop a high strength, high toughness API 5CT J55 grade steel through CSP route using Nb-Ti-Cr as micro-alloying philosophy.

OBJECTS OF INVENTION

The basic object of the present invention is directed to provide low carbon high strength hot rolled steel for casing and tubing application conforming to API 5CT J55 standard with minimum Yield strength (YS) of 400MPa, minimum Ultimate tensile strength(UTS) of 535MPa minimum elongation of 19 % 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 high strength steel using low Carbon 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 and Cr 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 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 lower carbon and thus carbon equivalent, weldability is better as compared to conventional casting.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to low carbon High strength line pipe steel having ductility and weldability conforming to API 5CT J55 steel standards having composition comprising wt%:C 0.26 max, Mn1.5 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, Nb 0.06 max ,Ti 0.060, Mo 0.1 max, Cr 0.25 max and rest Fe providing selective micro alloying including Nb,Ti and Cr for desired microstructure of acicular ferrite and quasi polygonal ferrite enabling said desired ductility and weldability having having YS > 400MPa, UTS > 535 MPa , %El > 19 and YS/TS ratio < 0.93 .

A further aspect of the present invention is directed to said low carbon High strength line pipe steel comprising (Nb+V+Ti) 0.15 max.

A still further aspect of the present invention is directed to said low carbon High strength line pipe steel comprising:
C 0.26 max; preferably 0.055-0.065
Mn 1.50 max; preferably 1.25-1.35
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 0.25-0.30
Nb 0.06 max; preferably 0.04-0.05
Mo 0.1 max; preferably Nil
Cr 0.25 max; preferably 0.15-0.20 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.045
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm and balance Fe having said yield strength > 400MPa, preferably in the range of 460-490MPa and UTS > 535 MPa, preferably in the range of 650-580MPa, the YS/UTS ratio < 0.93, preferably in the range of 0.85-0.88, %El is >19% preferably in the range of 32-36%

Another aspect of the present invention is directed to said low carbon 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 temperature below -40 Deg C preferably
having impact property is in the range of 230-240 J at 0 Deg C, 220-230 J at -20 deg C, 200-210 J at -30 Deg C and 190-200 J at -40 Deg C.

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

A further aspect of the present invention is directed to said low carbon High strength line pipe steel comprising high strength high toughness steel in the thickness range of 5-10 mm as hot rolled coils.

A still further aspect of the present invention is directed to a process for producing low carbon high strength line pipe steel of API 5CT J55 steel standard comprising:
i) providing selectively refined hot metal steel composition having in wt%:
C 0.26 max, Mn1.5 max, Si 0.45 max, S 0.010 maximum, P 0.020 maximum, Nb 0.06 max ,Ti 0.060, Mo 0.1 max, Cr 0.25 max and rest Fe favouring selective micro alloying element provision including Nb,Ti and Cr
ii) subjecting to thin slab caster ensuring dine and distributed TiN precipitates;
iii) homogenization in the presence of TiN 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 desired YS > 400 MPa, UTS > 535 MPa , %El > 19 and YS/TS ratio < 0.93.

A still further aspect of the present invention is directed to said process wherein the provision of said selectively refined hot metal steel composition include limiting (Nb+V+Ti) 0.15 max.

A still further aspect of the present invention is directed to process comprising:
• Step of carrying out said steel casting at casting speed of 4-6 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 process comprising:
I. chemical composition
C 0.26 max; preferably 0.055-0.065
Mn 1.50 max; preferably 1.25-1.35
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 0.25-0.30
Nb 0.06 max; preferably 0.04-0.05
Mo 0.1 max; preferably Nil
Cr 0.25 max; preferably 0.15-0.20 max
Ti 0.060 max; preferably 0.010-0.020
Al 0.020-0.070; preferably 0.030-0.045
B 5 ppm max
N up to 120 ppm max; preferably 60-80 ppm and balance Fe
I. Prepare the liquid steel in CONARC furnace followed by ladle refining furnace
II. Casting the steel slabs in a thin slab caster with slab thickness 55-65 mm ;
III. Homogenizing the cast slab in a tunnel furnace at temperature 1080-1120 Deg C;
IV. Descaling the slab in a high pressure descaler followed by low pressure descaler to remove the scale formed in tunnel furnace
V. 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 ;
VI. 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.
Yet 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 530 – 560 0C to obtain desired high strength.
Yet another aspect 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 still 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 530-560 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.

Figure 3: Gravile diagram for determination of susceptibility of welding crack

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to provide high strength hot rolled steel for line pipe application complying to API 5CT J55 standard having low carbon and 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>405MPa& UTS >535MPa, elongation > 19%, YS/UTS ratio < 0.93, Impact property230-240 J at 0 Deg C, 220-230 J at -20 deg C, 200-210 J at -30 Deg C and 190-200 J at -40 Deg C. 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 0.26 max; preferably 0.055-0.065
II. Mn 1.50 max; preferably 1.25-1.35
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 0.25-0.30
VI. Nb 0.06 max; preferably 0.04-0.05
VII. Mo 0.1 max; preferably Nil
VIII. Cr 0.25 max; preferably 0.15-0.20 max
IX. Ti 0.060 max; preferably 0.010-0.020
X. Al 0.020-0.070; preferably 0.030-0.045
XI. B 5 ppm max
XII. N up to 120 ppm max; preferably 60-80 ppm and balance Fe

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 1.25-1.35 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 0.25-0.30 %

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 recrystallization) temperature to a great extent and thus prohibiting any recrystallization& grain coarsening during final phases of hot reduction.
It also reduces the rate of recrystallization 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 bainitictransformation. 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 %

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 andCr. 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) CE (IIW)
Trial 1 0.053-0.062
1.26-1.31 0.24-0.28 55-70 0.040-0.043 0.031-0.035 Nil Nil Nil 0.029-0.037 18-25 0.34-0.38
Trial 2 0.060-0.065 1.26-1.31 0.24-0.30 60-75 0.045-0.050 Nil 0.15-0.17 Nil 0.015-0.020 0.031-0.044 25-30 0.35-0.38

(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
Interstandtime 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 530-560°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
Trial 1 1380 6.5 590-610 840-870 470-500 560-590 32-35 0.85-0.88
Trial 2 1380 6.7-7.9 535-555 850-880 460-490 550-580 32-36 0.84-0.87

Sample ID Impact (J) DWTT ( % Shear Area)
0 °C -20 °C -30 °C -40 °C 0 °C -20 °C -30 °C -40 °C
Trial 1 184 175 152 147 99 98 90 86
Trial 2 237 225 209 195 99 98 98 95

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(1.5% max) and % Si(<0.3%) used are less
• Low %Cr(<0.2%), no Mo addition
• No V , Cu, Ni used in the composition.

This leads to a computed value of carbon equivalent based on the above formula to CE < 0.4, which is clearly indicative of very good weldability as shown in Fig 3.

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 5CT J55 standard suitable for casing and tubing application.