Claims:We Claim:
1. Low carbon micro-alloyed high strength hot rolled steel coils having composition in weight percent comprising:
C : 0.08% or less
?n : 1.00% - 1.60%;
Si : 0.100% - 0.350%;
Al : 0.02-0.06%;
P : 0.015 % or less;
S : 0.010 % or less;
? : 60 ppm or less;
Nb : 0.020% - 0.06%;
Ti : 0.01- 0.020%
V : 0.020% - 0.06%;
Cr : 0.10% - 0.20%;
Mo : 0.10% or less; and
the balance being Fe, whereinNb+V+Ti is maintained in the range of 0.05-0.15 % with minimum YS of 500MPa, minimum UTS of 580 MPa, and minimum elongation of 25 % at gauge length 50 mm.

2. The Low carbon micro-alloyed high strength hot rolled steel coils as clamed in claim 1 having properties comprising
YS/ UTS ratio of 0.92 or less,
Total elongation of 25% or more,
Impact Energy of 150 J at –40oC,
Drop Weight tear test shear fraction area of 90% or more at 0oC, and having
Low Ductile to Brittle Transition Temperature(DBTT) below -40oC.

3. The Low carbon micro-alloyed high strength hot rolled steel coils as clamed in anyone of claim 1 or 2 comprising micro-alloying elements contributing to fine carbide precipitation, substitutional and interstitial strengthening, and grain-size refinement favouring attaining high strength.

4. The Low carbon micro-alloyed high strength hot rolled steel coils as clamed in anyone of claims 1 to 3 having microstructure of the steel after hot rolling comprising predominantly ferrite with acicular ferrite and pearlite wherein ferrite grain size is in the order of ASTM number 10 or finer.

5. A process of manufacturing low carbon micro-alloyed high strength hot rolled steel coils as clamed in claims 1 to 4 comprising

(a) providing steel slab having composition in wt % comprising
C : 0.08% or less
?n : 1.00% - 1.60%;
Si : 0.100% - 0.350%;
Al : 0.02-0.06%;
P : 0.015 % or less;
S : 0.010 % or less;
? : 60 ppm or less;
Nb : 0.020% - 0.06%;
Ti : 0.01- 0.020%
V : 0.020% - 0.06%;
Cr : 0.10% - 0.20%;
Mo : 0.10% or less; and
the balance being Fe, whereinNb+V+Ti is maintained in the range of 0.05-0.15 %.

- (b)carrying out reheating, rolling and coiling steps on said slab under controlled operating conditions at hot strip mill comprising:
(i)Charging of slabs to reheat furnace to have
- Slab soaking Temperature: 1200°C-1250oC for min 30 minutes;
- Slab drop out Temperature: 1200°C-1250oC;
(ii)Rough Rolling finish Temperature: 1000oC -1050oC;
(iii)Finish rolling Temperature: 850-900oC;
(iv) Cooling the finish rolled steel with average run out table cooling rate of 10°C/second or more;
(v)Low Coiling Temperature: 500-570°C after finishing mill with standard laminar cooling.

6. The process as claimed in claim 5 wherein reduction draft schedule of roughing mill and finishing mill (as per Figure 1), alongwith processing parameter and temperature profile at hot rolling have been selectively maintained so as to achieve the desired strength and microstructure.

7. The process as claimed in any one of claims 5 or 6 wherein uniformly fine grain of the microstructure has been achieved with the appropriate process parameters and chemistry of the steel, without sacrificing the rolling mill productivity.

Dated this the 17th day of March, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
LOW CARBON MICRO-ALLOYED HOT ROLLED STEEL COIL WITH HIGH STRENGTH AND HIGH TOUGHNESS SUITABLE FOR LINE PIPE APPLICATION AND METHOD OF MANUFACTURING THE SAME.



2 APPLICANT (S)

Name : STEEL AUTHORITY OF INDIA LIMITED.

Nationality : Indian.

Address : Research & Development Centre for Iron & Steel,
Doranda, Ranchi, Jharkhand, India. PIN-834002.






3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
Present invention relates to development of high strength hot rolled steel coils for line-pipe application and a manufacturing method thereof. More particularly, the present invention is directed to developing low carbon micro-alloyed high strength hot rolled steel processed through thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC) and low temperature coiling achieving high Yield Strength(YS) 500 MPa minimum, Ultimate Tensile Strength (UTS) 580 MPa minimum, Yield to tensile Ratio (YS/UTS) 0.92 maximum and Elongation (El) 25% minimum at a gauge length of 50 mm, excellent sub-zero(upto -40oC) impact toughness and Drop Weight Teat Test(DWTT) properties at 0oC and a method of hot rolling into coil form upto 10 mm thickness.

BACKGROUND OF THE INVENTION
The production of high strength low alloy hot rolled steel coils are of great interest because of the increasing demand of new line-pipes for high operating pressures and large pipe diameters. API X70 is a grade which is used for linepipe application and belongs to the category of HSLA steels. A good toughness, both in terms of high impact energy, low Ductile Brittle Transition Temperature(DBTT), high DWTT value and an excellent weldability complete the requirements for the steels to be used for linepipe applications.
Indian Patent No. 286760 (5444/DELNP/2009) disclosed high strength hot rolled steel plate for spiral line pipe superior in low temperature toughness and method of production of same. The steel contains chemical compositions by the following weight percentages: C: 0.01 to 0.1%, Si: 0.05 to 0.5%, Mn: 1 to 2%, Nb: 0.005 to 0.08%, Ti: 0.005 to 0.02%, V: 0.01 to 0.04%, Ni: 0.01 to 0.30%, Mo: < 0.1%, Cr: 0.01 to 0.3%, and Cu: <0.3%. According to inventors, the steel plate is hot rolled in a recrystallization temperature region at a reduction rate in each individual reduction pass of 10% to less than 25%, cooled at a cooling rate of 5oC/Sec or more from the start of cooling to coiling and then coiled at a temperature region of 500oC to 600oC.
Indian Patent No. 385995 (9815/DELNP/2013) disclosed Steel plate with low yield ratio, high toughness and manufacturing method thereof. . The steel contains chemical compositions by the following weight percentages:C: 0.02-0.08%, Si=0.40%, Mn:1.2-2.0%, Cu = 0.40%, Ni= 0.30%, Mo:0.10-0.30%, Nb: 0.03-0.08%, Ti: 0.005-0.03%,. According to inventors, the steel plate is hot rolled and cooled at a cooling rate of 15-30?/sand coiled at a temperature region of 250-450?. The microstructure of invented steel have Ferrite 20-40%, Bainite and 1-3& Martensite and achieved YS: 530-630MPa, TS: 660-800MPa, EL:=10%, and YS/UTS: =0.80.
No prior information is available for co-relation of Strength with low temperature properties like toughness, DBTT and DWTT for linepipe steel.
The present invention targets to solve the problems and limitations of prior art by developing a low carbon higher strength and high toughness hot rolled steel by grain refinement using lower carbon micro-alloyed steel in coil form through thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC) achieving fine grained microstructure of fine ferrite grains and bainite or acicular ferrite which is essential for an optimum high strength-toughness combination suitable for line pipe application.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide a low carbon micro-alloyed steel in hot rolled coil form with higher strength and high toughness and a process for its manufacturing by grain refinement through thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC).
A further objective of the present invention is to manufacture low carbon micro-alloyed high strength hot rolled steel for line pipe application with minimum YS of 500MPa, minimum UTS of 580 MPa, minimum elongation of 25 % at gauge length 50 mm and a method of its production.
Another objective of the present invention is to provide said low carbon micro-alloyed hot rolled steel with improved subzero temperature impact energy (more than 150 J at -40oC) and DWTT properties (more than 90 % shear area at 0oC) and low DBTT temperature (below -40oC).
A still further objective of the present invention is to develop cost economic steel composition having Nb, V, Ti, Mo 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.
Yet another objective of the present invention is directed to production of said high strength low carbon micro-alloyed steel with optimum cost at any hot strip mill having facilities to achieve cooling rate 10oC and more and low coiling temperature in the range of 500-570oC.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to low carbon micro-alloyed high strength hot rolled steel coils having composition in weight percent comprising:
C: 0.08% or less
?n: 1.00% - 1.60%;
Si: 0.100% - 0.350%;
Al: 0.02-0.06%;
P: 0.015 % or less;
S: 0.010 % or less;
?: 60 ppm or less;
Nb: 0.020% - 0.06%;
Ti: 0.01- 0.020%
V: 0.020% - 0.06%;
Cr: 0.10% - 0.20%;
Mo: 0.10% or less; and
the balance being Fe, wherein Nb+V+Ti is maintained in the range of 0.05-0.15 % with minimum YS of 500MPa, minimum UTS of 580 MPa, and minimum elongation of 25 % at gauge length 50 mm.

A further aspect of the present invention is directed to said low carbon micro-alloyed high strength hot rolled steel coils having properties comprising
YS/ UTS ratio of 0.92 or less,
Total elongation of 25% or more,
Impact Energy of 150 J at –40oC,
Drop Weight tear test shear fraction area of 90% or more at 0oC, and having
Low Ductile to Brittle Transition Temperature(DBTT) below -40oC.

A still further aspect of the present invention is directed to said low carbon micro-alloyed high strength hot rolled steel coils comprising micro-alloying elements contributing to fine carbide precipitation, substitutional and interstitial strengthening, and grain-size refinement favouring attaining high strength.

A still further aspect of the present invention is directed to said low carbon micro-alloyed high strength hot rolled steel coils having microstructure of the steel after hot rolling comprising predominantly ferrite with acicular ferrite and pearlite wherein ferrite grain size is in the order of ASTM number 10 or finer.

Another aspect of the present invention is directed to a process of manufacturing low carbon micro-alloyed high strength hot rolled steel coils comprising

(a) providing steel slab having composition in wt % comprising
C : 0.08% or less
?n : 1.00% - 1.60%;
Si : 0.100% - 0.350%;
Al : 0.02-0.06%;
P : 0.015 % or less;
S : 0.010 % or less;
? : 60 ppm or less;
Nb : 0.020% - 0.06%;
Ti : 0.01- 0.020%
V : 0.020% - 0.06%;
Cr : 0.10% - 0.20%;
Mo : 0.10% or less; and
the balance being Fe, wherein Nb+V+Ti is maintained in the range of 0.05-0.15 %.

- (b)carrying out reheating, rolling and coiling steps on said slab under controlled operating conditions at hot strip mill comprising:
(i)Charging of slabs to reheat furnace to have
- Slab soaking Temperature: 1200°C-1250oC for min 30 minutes;
- Slab drop out Temperature: 1200°C-1250oC;
(ii)Rough Rolling finish Temperature: 1000oC -1050oC;
(iii)Finish rolling Temperature: 850-900oC;
(iv) Cooling the finish rolled steel with average run out table cooling rate of 10°C/second or more;
(v)Low Coiling Temperature: 500-570°C after finishing mill with standard laminar cooling.

A further aspect of the present invention is directed to said process wherein reduction draft schedule of roughing mill and finishing mill (as per Figure 1), alongwith processing parameter and temperature profile at hot rolling have been selectively maintained so as to achieve the desired strength and microstructure.

A still further aspect of the present invention is directed to said process wherein uniformly fine grain of the microstructure has been achieved with the appropriate process parameters and chemistry of the steel, without sacrificing the rolling mill productivity.

The above aspect and other advantages of the invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows the reduction schedule of roughing mill and finishing mill indicating the stage wise percent reduction involved in producing the high strength hot rolled steel for line pipe application leading to good toughness property and strength according to the present invention.
Figure 2: shows the DBTT curve for the steel of present invention and ductile to brittle transformation temperature is below -40oC.
Figure 3: shows the optical microstructure of the hot rolled steel produced according to present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
The present invention is directed to provide a lower carbon micro-alloyed steel in hot rolled coil form with higher strength and high toughness and a process for its manufacturing by grain refinement through thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC).

The chemical composition of the low carbon micro-alloyed steel according to present invention comprises by wt % of C:0.08% max,Mn:1.00-1.60%, S: 0.010% max, P: 0.020%max, Si: 0.100-0.400%, Al: 0.02%-0.06%, Nb:0.02- 0.06%, V: 0.02- 0.06%, Ti: 0.01-0.03%, Mo: 0.10% max, Cr:0.10-0.20% wherein (Nb+V+Ti):0.05- 0.15%,andrest is Fe.

The various aspects of the present invention are described hereunder with reference to a preferred embodiment as per the following example:

Example:
A cost effective composition of low carbon and low alloy high strength steel slabs are processed through hot strip mill. Steel chemistry consists of low carbon along with micro alloys such as Nb, V, Ti, Cr and Mo addition. The developed steels possess high strength primarily by micro-alloying elements contributing to fine carbide precipitation, substitutional and interstitial strengthening, and grain-size refinement.
The steel composition of various heats under the trials conducted for the purpose of this invention is shown in following Table 1:

Table-1: Heat Chemistry(wt%) of steels
Heat No. C Mn P S Si Al Nb V Ti Cr Mo [N]
ppm
10003 0.05 1.45 0.008 0.004 0.207 0.031 0.047 0.028 0.018 0.18 0.073 32
10007 0.05 1.45 0.008 0.004 0.209 0.030 0.046 0.027 0.018 0.20 0.073 38
10826 0.06 1.57 0.008 0.003 0.350 0.030 0.047 0.050 0.020 0.20 - 35
10823 0.05 1.46 0.010 0.004 0.200 0.030 0.050 0.050 0.016 0.18 - 36

Two heats were made with Mo and two heats were made without Mo. Also no Ni & Cu was used for heat making to make a cost economic.
The reduction draft schedule of roughing mill and finishing mill, processing parameter and temperature profile at hot rolling have been determined so as to achieve the desired strength and microstructure.

Accompanying Figure 1 shows the reduction schedule of roughing mill and finishing mill indicating the stage wise percent reduction starting with a slab involved in producing the high strength hot rolled steel for line pipe application leading to good toughness property and strength according to the present invention.

The process is carried out under controlled operating conditions at hot strip mill and comprises:
? Charging of slabs to reheat furnace
- Slab soaking Temperature: 1200°C-1250oC for min 30 minutes,
- Slab drop out Temperature: 1200°C-1250oC,
? Rough Rolling finish Temperature: 1000oC -1050oC,
? Finish rolling Temperature: 850-900oC,
? Coiling Temperature: 500-570°C.

The product obtained was free from any surface defects and the mechanical properties observed for different samples are presented in the following Table 2:

Table-2: Tensile properties of steel
Coil No Transverse Direction Diagonal Direction
YS UTS El YS/UTS YS UTS El YS/UTS
N728889 521 588 36 0.89 530 583 32 0.91
N728891 572 637 30 0.90 537 586 30 0.92
N728892 536 581 30 0.92 526 579 36 0.91
N728893 545 613 30 0.89 550 609 36 0.90
N728894 544 592 40 0.92 538 589 36 0.91
N728895 564 629 36 0.90 522 587 32 0.89
N875363 523 614 32 0.852 -- -- -- --
N875364 532 615 34 0.865 -- -- -- --
N875365 539 615 34 0.876 -- -- -- --
N875366 541 615 32 0.880 -- -- -- --
N875369 537 608 34 0.883 532 596 34 0.89
N875370 542 607 32 0.893 -- -- -- --
N875372 531 604 34 0.879 -- -- -- --
N875378 527 608 32 0.867 -- -- -- --

The coils (N728891, N728893, N728894 & N728895) of heats made of 10003 & 10007 i.e. having Mo in the chemistry shows higher yield strength than tensile strength compared with heats which doesn’t have Mo.

The Impact toughness testing and Drop Weight Tear Test at 0oC, -20oC& -40oC were performed for different samples results of which are presented in the following Table 3.
DBTT Curve is shown in accompanying Figure 2 which shows DBT temperature below -40oC. Drop Weight Tear Test was performed as per recommended practice as per ASTM standard E-436.

Table-3: Impact Energy and %Sheared Area (SA) of DWTT
Coil No Impact Energy(J)
at temp. DWTT, % SA
0oC -20oC -40oC 0oC -20oC -40oC
N728889 264 175 172 99% 98% 96%
N728891 209 280 175 100% 100% 100%
N728892 293 183 261 --- --- ---
N728893 290 175 186 100% 100% 100%
N728894 201 204 186 --- --- ---
N728895 256 217 199 100% 100% 100%
N875363 --- --- 196 --- --- ---
N875365 --- --- 196 100 --- ---
N875367 --- --- 181 99 --- ---
N875369 --- --- 186 100 --- ---

Microstructure was studied under optical microscope and the developed steel showed very fine ferrite with acicular ferrite and pearlite as per the optical microstructure shown in accompanying Figure 3 and the ferrite grain size have ASTM No 10 or finer.

ADVANTAGES OF THE INVENTION
- Optimum chemistry and process parameters of the steel resulted in consistent and favorable mechanical properties suitable for linepipe steel products.
- Minimum usage of micro-alloying and other elements in the steel resulted into excellent balance of strength and impact toughness required by linepipe manufacturer.
- Uniformly fine scale of the microstructure has been achieved with the appropriate process parameters and chemistry of the steel, without sacrificing the rolling mill productivity.

INDUSTRIAL APPLICABILITY
Present invention is applicable for production of high strength low carbon micro-alloyed steel with optimum cost at any hot strip mill by grain refinement through thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC), having facilities to achieve cooling rate 10oC and more and coiling temperature 500-570oC.