Claims:We Claim:

1. High strength high toughnesshot rolled steel angle for transmission line tower application having composition by wt%comprising:
C: 0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S: upto 0.025% max; preferably 0.005-0.015%;
P: upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto0.012% max; preferably 0.006-0.008 ppm and balance Fe; having a microstructure including polygonal ferrite + pearlite and impact toughness of more than 40 J at even at sub zero temperatures.

2. High strength high toughnesshot rolled steel as claimed in claim 1 comprising hot rolled steel angle having properties comprising YS :more than 350MPa preferably 375 to 475MPa, UTS : more than 490 MPa preferably 520 to 620MPa, %El : more than 22preferably 24 to 30 and Impact toughness: more than 40 J at upto -40 Deg C.

3. High strength high toughness hot rolled steel as claimed in anyone of claims 1 or 2which is Nb micro alloyed with galvanized coating property steel having high strength along with high impact value at subzero temperature.

4. High strength high toughness hot rolled steel as claimed in anyone of claims 1 to 3 including galvanized steel angle having yield strength>350MPa, preferably in the range of 400-430MPaand UTS>490MPa, preferably in the range of 560-570MPa.

5. High strength high toughness hot rolled steel as claimed in anyone of claims 1 to 4 including high strength steel strip wherein %El is >22% preferably in the range of 28-30%.

6. High strength high toughness hot rolled steel angle as claimed in anyone of claims 1 to 5 having high strength along with high impact property in the range of 80-100 J at -40 Deg C.

7. High strength high toughness hot rolled steel angle as claimed in anyone of claims 1 to 6 obtained as hot rolling of angles in the thickness range of 5-20mm.
8. A process for manufacture of high strength hot rolled steel as claimed in claims 1 to 7 comprising:
I. selectively providing Nb micro alloying galvanic property imparting steel composition for generating ladle steel refined composition by wt% comprising:
C:0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S:upto 0.025% max; preferably 0.005-0.015%;
P:upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto0.012% max; preferably 0.006-0.008 ppm and balance Fe;
II. casting the same as steel billets;
III. reheating the billet in a reheating furnace;
IV. Rolling the billetsto the desired final thickness followed by finish rolling;
V. Subjecting to step of cooling after hot rolling to attain desired microstructure including polygonal ferrite + pearlite and impact toughness of more than 40 J at even at sub zero temperatures.
9. A process for manufacture of high strength hot rolled steel angle for transmission line tower application as claimed in claims 1 to 8 comprising:
I. providing steel having chemical composition by wt% comprising:
C:0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S:upto 0.025% max; preferably 0.005-0.015%;
P:upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto0.012% max; preferably 0.006-0.008 ppm and balance Fe;
II. Preparing the liquid steel in CONARC furnace followed by ladle refining furnace to achieve composition as in step I and casting the same as steel billets in a continuous casting machine with dimension of either 165x165 mm or 130x130 mm;
III. Reheating the billet in a reheating furnace at 1150-1200DegC for 3-4 hrs. duration;
IV. Descaling the billets in a high pressure descaler to remove the scale formed in reheating furnace;
V. Rolling the billets into equal angles in rolling mill 8-12 passes to achieve the desired final thickness and finish rolling temperature in the range of 850-9500C;
VI. Subjecting the hot rolled angles to air cooling after hot rolling to attain desired microstructure and mechanical properties.
10. A process as claimed in anyone of claims 8 or 9 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 billet casting machine with casting speed of 2-3 m/min, followed by charging in reheating furnace and rolling to equal angle at finish rolling temperature (FRT) 850 to 9500C followed by air cooling to obtain desired high strength.
11. A process as claimed in anyone of claims 8 to 10 wherein said process carried out under controlled operating conditions comprising
(i) Casting Speed 2-3 m/min;
(ii) Billet dimension 165x165 mm2 and 130x130 mm2;
(iii) Reheating Temp (Reheating Furnace) 1150-1200Deg C;
(iv) Reheating time Time 3-4 hrs;
(v) Finish Rolling Temp 850-950Deg C.

12. A process as claimed in anyone of claims 8 to 11 wherein the composition and hot rolling process parameters are selectively maintained within the selected ranges so that higher strength alongwith high impact properties at low temperature are achieved.

Dated this the 1st day of February, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION
The present invention relates to low cost high strength high toughness hot rolled and galvanized steel angle suitable for transmission line tower application and a manufacturing method thereof. More particularly, the present invention is directed to the development of a hot rolled steel angles conforming to IS2062-2011 E350/ EN 10025 S355 standard with excellent impact property at subzero (up to -40 Deg C) achieved through air cooling after hot rolling. The low cost, high strength steel is characterized by high yield strength (YS)350MPa minimum, ultimate tensile strength (UTS)490MPa minimum, and total El 22 % minimum at a gauze length of 5.65vA mm, where A is the cross sectional area. The invented steel having the composition with constituent elements in percentage by weight comprising:%C 0.1-0.25, %Mn 1.2-1.6, %Si 0.15-0.25, %S 0.025 maximum, %P 0.035 maximum, %Al 0.02 minimum, %N 0.012 maximum, %Ca 0.001-0.003, %Nb 0.01-0.04, % V 0.005 maximum, %Ti 0.01 maximum, %Mo 0.07maximum, %Cr 0.07maximum, %Cu 0.07 maximum, %Ni 0.07 maximum and rest Fe.
BACKGROUND OF THE INVENTION
A transmission tower is a tall structure, usually a steel lattice tower, used to support overhead electrical power transmission lines. In recent years, power grid construction in the whole world gained rapid development, especially the construction of ultra-high voltage (UHV) transmission line tower. In the transmission line tower hot-rolled steel angles is most widely used as they give strength and are economical. Although at present the grade used for making steel structure conforms to YS of 250 MPa or 350 MPa but the impact toughness property guaranteed at room temperature only which restricts the usages at areas where atmospheric temperature at winter falls below subzero temperature. The performance of this steel at subzero temperature depends mainly upon the cleanliness, impurities such as sulphur, phosphorus and chemical content such as carbon, micro-alloying elements etc.
Production of this 350 MPa strength hot rolled steel anglesthrough conventional hot rolling and air cooling processusing Vanadium as micro alloying is well established. However, as the strengthening mechanism in Vanadiaum microalloyed steel is primarily precipitation strengthening, it deteriorates the impact property particularly at subzero temperatures with the precipitates acting as nucleation sites for crack initiation.
The present invention is thus directed to develop a high strength, high toughness hot rolled and galvanized steel angle through hot rolling followed by air cooling process using Nb as microalloying for transmission tower application, particularly having adequate toughness properties at sub-zero temperature. The requirement of minimum strength, elongation and impact properties of such grades of steel for transmission line application are specified in different standards as per the following Table 1.
Table 1- Requirement as per different standards
Standard Name Grade YS (MPa) UTS (MPa) %El at GL 5.65 sqrtA Impact Temp (Deg C) Impact Energy (J)- Minimum
IS2062-2011 E350 A
BR
BO
C 350 minimum 490 minimum 22 minimum -
RT
0
-20 -
27
27
27
EN10149 S355 JR
JO
J2
K2 355 minimum 470-630 22 minimum 20
0
-20
-20 27
27
27
40
Q345 A
B
C
D
E 355 minimum 470-630 22 minimum -
20
0
-20
-40 -
34
34
34
34

Some prior arts in the related field that disclosed such grades of hot rolled steel angles with moderate strength and toughness properties are summarized below:

Patent no CN103938063A, titled “Preparation method of high-toughness Q345 grade hot-rolled equal angle steel for iron tower” discloses production of 350 MPa strength steel with chemical composition of %C 0.14-0.18, %Mn 1.30-1.70, %V 0.02-0.03and Ceq<=0.42. Mechanical properties achieved YS > 345 MPa, UTS/YS Ratio 1.20-1.35 and impact toughness of 40-80 J at -30 Deg C

Patent No CN103526120A, titled “Low-temperature impact resistant angle steel for power transmission iron tower ”discloses production of 350 MPa strength steel with chemical composition of 0.05-0.15% of C, 1.20-1.60% of Mn, 0.01-0.05% of Si, 0.02-0.10% of V, 0.02-0.06% of Ni, 0.001-0.005% of Ca, 0.01-0.20% of Cr, 0.10-0.25% of Cu, 0.01-0.03% of P, 0.20-0.35% of Mo, 0.005-0.02% of S and the balance of Fe.

Patent No CN101509099A, titled “High-strength Q420C level steel tower corner iron and production process” discloses production of 420 MPa strength steel with chemical composition of C: 0.01-0.16%, Mn: 1.15-1.45%, Si: 0.30-0.50%, S, P: less than or equal to 0.025%, V: 0.070-0.100%, Ti: 0.004-0.010%, and the balance of Fe and Ceq: less than or equal to 0.42% with mechanical properties of yield strength > 420 MPa Tensile strength 530~720 MPa Elongation > 24% and impact toughness > 34 J at 0Deg C.
There has been thus a need in the related filed to develop hot rolled steel angles with adequate strength and low temperature toughness suitable for transmission tower application in areas subjected to ambient temperatures much below zero degrees.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide Nb micro alloyed high strength high toughness hot rolled steel angles for transmission tower application with minimum YS of 350MPa, minimum UTS of 490MPa, minimum elongation of 22% at gauge length 5.65 vA mm and a method of its production.

Another object of the present invention is directed to develop the Nb micro alloyed high strength hot rolled steel angle with high impact properties at subzero temperature (-40 Deg C).

Yet another object of the present invention is directed to develop the Nb micro alloyed high strength hot rolled steel angle having good galvanizing property after hot rolling.

A further object of the present invention is directed to develop said Nb micro alloyed high strength hot rolled steel anglewith low cost through a processing route involving hot rolling and air cooling.

A still further object of the present invention is directed to providing said Nb micro alloyed high strength hot rolled steel angle with improved toughness property wherein the steel composition is having Nb, as micro-alloying element for strengthening by grain refinement predominantly to obtain desired fine grained quasi polygonal ferriteplus pearlite microstructure with high strength, ductility and toughness properties.

A still further object of the present invention is directed to providing said Nb micro alloyed high strength hot rolled steel withimproved toughness properties wherein the steel composition is having close limit of Si (0.15-0.25%),to avoid poor effect on galvanizing property outside this range of Si.

A still further object of the present invention is directed to providing the Nb micro alloyed high strength hot rolled steel assheets with improved impact toughness and galvanizing properties wherein to achieve the end properties, the microstructure of the steel grade would consist of quasi polygonal ferrite +pearlite.

A still further object of the present invention is directed to providing the Nb micro alloyed high strength hot rolled steel angle with improved toughness properties wherein the use of Vanadium (V) 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 the Nb micro alloyed high strength hot rolled steel angle with improved toughness properties wherein the product obtained through conventional hot rolling and air cooling after hot rolling process.

A still further object of the present invention is directed to providing the Nb micro alloyed high strength hot rolled equal angle steel with improved toughness properties wherein due to 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 high strength high toughness hot rolled steel angle for transmission line tower application having composition by wt%comprising:
C: 0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S: upto 0.025% max; preferably 0.005-0.015%;
P: upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil;
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto 0.012% max; preferably 0.006-0.008 ppm and balance Fe; having a microstructure including polygonal ferrite + pearlite and impact toughness of more than 40 J at even at sub zero temperatures.

A further aspect of the present invention is directed to high strength high toughness hot rolled steel comprising hot rolled steel angle having properties comprising YS :more than 350MPa preferably 375 to 475MPa, UTS : more than 490 MPa preferably 520 to 620MPa, %El : more than 22 preferably 24 to 30 and Impact toughness: more than 40 J at up to -40 Deg C preferably in the range of 80-100 J at -40 Deg C.

A still further aspect of the present invention is directed to high strength high toughnesshot rolled steel which is Nb micro alloyed with galvanized coating property steel having high strength along with high impact value at subzero temperature.

Another aspect of the present invention is directed to high strength high toughness hot rolled steel including galvanized steel angle having yield strength >350MPa, preferably in the range of 375 to 475MPa and UTS>490MPa, preferably in the range of 520 to 620 MPa.

Yet another aspect of the present invention is directed to high strength high toughness hot rolled steel including high strength steel strip wherein %El is >22% preferably in the range of 24-30%.

A further aspect of the present invention is directed to high strength high toughness hot rolled steel angle having high strength along with high impact property in the range of 80-100 J at -40 Deg C.

A still further aspect of the present invention is directed to high strength high toughness hot rolled steel angle obtained as hot rolling of angles in the thickness range of 5-20mm.

Another aspect of the present invention is directed to a process for manufacture of high strength hot rolled steel as described above comprising:
I. selectively providing Nb micro alloying galvanic property imparting steel composition for generating ladle steel refined composition by wt% comprising:
C:0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S:upto 0.025% max; preferably 0.005-0.015%;
P:upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto0.012% max; preferably 0.006-0.008 ppm and balance Fe;

II. casting the same as steel billets;
III. reheating the billet in a reheating furnace;
IV. Rolling the billetsto the desired final thickness followed by finish rolling;
V. Subjecting to step of cooling after hot rolling to attain desired microstructure including polygonal ferrite + pearlite and impact toughness of more than 40 J even at sub zero temperatures.
Yet another aspect of the present invention is directed to a process for manufacture of high strength hot rolled steel angle for transmission line tower application comprising:
I. providing steel having chemical composition by wt% comprising:
C:0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S:upto 0.025% max; preferably 0.005-0.015%;
P:upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto0.012% max; preferably 0.006-0.008 ppm and balance Fe;

II. Preparing the liquid steel in CONARC furnace followed by ladle refining furnace to achieve composition as in step I and casting the same as steel billets in a continuous casting machine with dimension of either 165x165 mm or 130x130 mm;
III. Reheating the billet in a reheating furnace at 1150-1200DegC for 3-4 hrs. duration;
IV. Descaling the billets in a high pressure descaler to remove the scale formed in reheating furnace;
V. Rolling the billets into equal angles in rolling mill 8-12 passes to achieve the desired final thickness and finish rolling temperature in the range of 850-9500C;
VI. Subjecting the hot rolled angles to air cooling after hot rolling to attain desired microstructure and mechanical properties.
A further 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 billet casting machine with casting speed of 2-3 m/min, followed by charging in reheating furnace and rolling to equal angle at finish rolling temperature (FRT) 850 to 9500C followed by air cooling to obtain desired high strength.
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 2-3 m/min;
(ii) Billet dimension 165x165 mm2 and 130x130 mm2;
(iii) Reheating Temp (Reheating Furnace) 1150-1200Deg C;
(iv) Reheating time Time 3-4 hrs;
(v) Finish Rolling Temp 850-950Deg C.

A still further aspect of the present invention is directed to saidprocess wherein the composition and hot rolling process parameters are selectively maintained within the selected ranges so that higher strength alongwith high impact properties at low temperature are achieved.

The objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative figures.

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 equal angle steel for transmission line tower application with good toughness property and good galvanizing property according to the present invention.

Figure 2a-b: are the micrographs of the Microstructure Images taken at 100X and 200X magnification, having typical microstructure of quasi polygonal ferrite and pearlite.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING FIGURES
The present invention is directed to provide high strength high toughness hot rolled Niobium(Nb) micro alloyed steel angles for transmission line tower application complying to IS2062 -2011- E350 / EN10025 S355 standard through CONARC, continuous billet caster and hot rolling followed by air cooling processing route.

Thus according to the present invention, a cost effective composition for high strength low alloy hot rolled equal angle steel and a method of making such steel is provided. Steel is processed using a CONARC- Billet caster- Hot rolling- Air cooling and chemistry consists of Nb micro alloy addition. Mechanical properties are YS>350MPa & UTS >490MPa, elongation >22%, Impact property 80-100 J at -40 Deg C.

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:
C:0.1-0.25%; preferably 0.15-0.20%;
Mn: 1.20-1.60%; preferably 1.30-1.40%;
S:upto 0.025% max; preferably 0.005-0.015%;
P:upto 0.035% max; preferably 0.010-0.020%;
Si:0.15-0.25%;preferably0.16-0.22%;
Nb:0.01-0.04% max; preferably 0.02-0.025%;
V: 0.005% max; preferably nil;
Mo: 0.07% max; preferably nil;
Cr: 0.07% max; preferably nil
Ni: 0.07% max; preferably nil;
Ti: 0.010% max; preferably nil;
Al: 0.020% minimum; preferably 0.030-0.045%;
Ca: 0.001-0.003%; preferably 0.0015-0.0025%;
N:upto0.012% max; preferably 0.006-0.008 ppm and balance Fe;

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

Carbon(C):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. Present invention selected %C is0.15-0.20.
Manganese(Mn): Mn is an important element for solid solution strengthening. Usually, 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 andcastability. Centre line segregation is another major issue with increasing % Mn. Range of wt% of Mn 1.3-1.4 is selected in this invention.
Silicon(Si): 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. Si also affects the galvanizing property of hot rolled sheets angles. There are 2 safe range for galvanizing: (i) Si<=0.04 or (ii) Si 0.15-0.25. Later one is choose because of cost implication as it reduces Aluminum consumption. Si range is selected in this invention is 0.16-0.22%.
Aluminum(Al): Al is added for de oxidation purpose. Aluminum also acts as grain refiner by forming AlN precipitates. Upper limit is limited by detrimental effect on surface quality & cast ability. Al range in this present invention is 0.03-0.045%.
Niobium(Nb): 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 Nb, 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 as well as cost. In this invention Nb range selected is 0.02-0.025%.
Nitrogen(N): 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 0.006-0.008 %.
Calcium(Ca): Steel has to be Calcium treated to counter the harmful effect of Sulphur as well as help in casting. Calcium range selected in this invention is 0.0015-0.0025%

The process route followed was: Electric Arc Furnace (CONARC) ?Ladle Furnace?Continuous billet caster ?8-12 pass hot rolling ?Air coiling; however, any other combination of processes (before caster) which gives steel of same chemistry can also be used.

Accompanying Figure 1 shows the flow chart illustrating the different steps along with relevant parameters, involved in producing the high strength hot rolled equal angle 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. The composition of different steel samples obtained on trials heats are given in following table 2:

Table 2: Chemical composition of the trial
Sample ID C % Mn % Si % N (ppm) Nb % S% P% Al % %V %Ti %Cr %Ni %Mo Ca (ppm)
Exp1 0.15-0.20 1.30-1.40
0.15-0.25 60-80 0.02-0.025 0.005-0.015 0.010-0.020 0.030-0.045 0-0.005 0-0.010 0-0.07 0-0.07 0-0.07 15-25
Exp 2 0.15-0.20
1.30-1.40
0.15-0.25 60-80 0.02-0.025 0.005-0.015 0.010-0.020 0.030-0.045 0-0.005 0-0.010 0-0.07 0-0.07 0-0.07 15-25
Example 1 0.156 1.308 0.221 63 0.02 0.005 0.016 0.037 0.003 0.001 0.007 0.004 0.007 20
Example 2 0.158 1.315 0.213 67 0.021 0.003 0.017 0.042 .001 0.001 0.005 0.006 0.005 23

(ii) Steel was cast using a continuous billet caster with dimension of 165x165 mm2 and 130x130 mm2.
(iii) Slab was reheated in a furnace at temperature 1170 °C. Descaler was used after reheating furnace to remove scales.
(iv) Hot rolling of 8-12 passes was used to make the final dimension of the angle with finish rolling temperature in the range of 880-900 °C.
(v) After finish rolling, normal air cooling is done to cool the angle and to get the desired mechanical properties.
(vi) The resulting steel grade is subjected to testing and inspection to ascertain attainment of desired properties.
Mechanical properties:
To evaluate the tensile strength, elongation and impact properties of the steel samples obtained according to the process of the present invention, following test protocols were adopted:
Tensile Testing: Tensile testing done as per ASTM standards E-8. Samples are taken parallel to rolling direction.

Impact Testing: Impact testing done as per standard ASTM standards E-23. Samples are taken parallel to rolling direction.

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: Mechanical properties of the trial
Sample ID Size YS (MPa)
>355 UTS(MPa)
>490 % El
>22 YS/UTS
Example 1 110X110X08 mm 400-410 565-570 30 1.39-1.40
Example 2 110X110X10 mm 420-430 570-575 28-29 1.34-1.36

Table 4: Impact properties of the trial
Sample ID Size Impact (J)
>40 J
0 °C -20 °C -30 °C -40 °C
Example 1 110X110X08 mm 155 152 133 93
Example 2 110X110X10 mm 140 132 110 80

Microstructure:
The Microstructure of samples was found to bequasi polygonal ferrite and pearlite. Accompanying Figure 2a-b shows the Microstructure Images taken at 100X and 200X magnification.

It is thus possible by way of the present invention to provide high strength high toughness hot rolled and galvanized steel angle conforming to IS2062-2011 E350/ EN 10025 S355 standard suitable for transmission line tower application and a manufacturing method thereof. Such hot rolled steel angles having excellent impact property at subzero (up to -40 Deg C) achieved through selective processing including air cooling after hot rolling. The low cost, high strength steel is characterized by high yield strength (YS)350MPa minimum, ultimate tensile strength (UTS)490MPa minimum, and total El 22 % minimumwherein the composition comprises Nb as micro-alloying element for strengthening by grain refinement predominantly to obtain desired fine grained quasi polygonal ferrite plus pearlite microstructure.