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 HIGH STRENGTH HOT ROLLED STEEL SHEETS AND A PROCESS FOR ITS MANUFACTURE.
2 APPLICANT (S)
Name : JSW STEEL LIMITED.
Nationality : An Indian Company.
Address : Dolvi Works, Geetapuram, Dolvi, Taluka Pen, Dist. Raigad,
Maharashtra-402107,India; Having the Registered Office at JSW CENTRE,BANDRA KURLA COMPLEX,BANDRA(EAST), MUMBAI-400051, STATE OF MAHARASHTRA,INDIA
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
The present invention relates to providing a low carbon high strength (YS> 650 MPa) hot rolled steel composition/grade. More particularly, the present invention is directed to developing a high strength (YS> 650 MPa) hot rolled steel sheet with good weldability and fatigue strength through thin slab caster processing route. The developed steel grade is suitable for applications in automobile components, with good formability avoiding corner/longitudinal cracks, like chassis, long and cross members for trucks, cranes, earth moving equipments and similar other applications e.g. Electrical Transformer tank body, structural components etc., aimed at weight reduction of existing component by replacement of existing low strength higher thickness steel grades with this newly developed high strength with lower section thicknesses and light weight.
BACKGROUND OF THE INVENTION
It is well known in the automobile manufacturing sector that fuel economy is related to the gross weight of a vehicle. Thus fabrication of car components with high strength but light weight steel sections help to reduce tare weight while the pay load capacity is improved. Increased fuel economy leading to environmental protection have necessitated to develop hot-rolled thin steel sheet with high tensile strength coupled with high elongation and good fatigue strength for automotive load bearing parts like long and cross members. Good weldability is a basic requirement for using the material in actual application.
EP2431491A1 discusses a high strength hot rolled steel sheet having tensile strength (UTS) greater than 540 MPa. It uses a conventional continuous casting process and the slab is heated at 1200-1300C temperature .It involves a composition that does not include 1Mb & Cr and level of Ti <0.1.
US6669789 Bl claims a process using thin slab caster and YS > 345 MPa, although it mentions steel with YS > 621 MPa, stating that further strength increase is possible with the use of Nb, V or Mo but no such trials have been made. The composition involved Boron addition. This patent discusses in detail about steel with YS ranging from 345-620 MPa without any use of Nb & V.
US5759297 claims composition and a process for achieving a steel grade with YS ranging from 620-730 MPa. The object is to obtain a hot-rolled steel sheet with high strength and

high drawability. This patent does not specify the slab thickness and it appears that process is for conventional caster. Effect of Titanium addition differs significantly in conventional & thin slab caster due to much faster cooling rates in thin slab caster. Moreover, US5759297 claims a special two or three step cooling with coiling temperature below 300 Deg C after completion of rolling. The steel grade after hot rolling and heat treatment possess a microstructure composed of at least 75% of ferrite and at least 10% of martensite. The ferrite is hardened by a precipitation of titanium carbides or carbonitrides, and also of niobium carbides or carbonitrides. The microstructure may possibly also include bainite and residual austenite.
US2004/0238080A1 discloses a high strength steel but chemical composition and process route are completely different. US2004/0238080A1 uses chemistry in higher carbon range which increases carbon equivalent and causes weldability issue. Another disadvantage of the higher carbon route is that it is very difficult to produce a defect free slab in thin slab caster where longitudinal cracks are very common problem for this chemistry. Moreover, mentioned patent uses Mo which is a costly element and Boron, which is again a difficult element in thin slab caster as it leads to transverse corner crack/edge crack.
There has been therefore a need in the art to developing a high strength hot rolled steel grade with good weldability and fatigue strength combined with good formability free of tendency of crack to suit application in automobile and other structural components.
OBJECTS OF THE INVENTION
The basic object of the present invention is directed to providing cost -effective low carbon high strength hot rolled steel sheets having mechanical properties YS> 650 Mpa & UTS > 700 MPa.
Another object of the present invention is thus directed to providing cost-effective low carbon hot rolled steel sheets having elongation > 18% and fatigue limit > 0.4 YS with good weldability while also having favourable formability free of crack development to suit application in automobile and other structural components and process for its production.

A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit which is developed 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 sheets with good weldability and fatigue limit wherein the steel composition is having Nb and Ti as microalloying element for strengthening by grain refinement and precipitation strengthening to obtain desired fine grained ferrite microstructure with desired high strength and ductility properties.
A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit wherein the steel composition is having upper limit of carbon restricted to 0,08% 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 good weldability and fatigue limit 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 sheets with good weldability and fatigue limit wherein the product is suitable for application in automobile and other structural components with the advantage of achieving weight reduction as the developed steel grade is having good weldability and fatigue performance.
A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit wherein the product obtained through thin slab caster route due to thin section and fast cooling rates, TiN precipitates are very fine and uniformly distributed which leads to a effective utilization of TiN.
A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit wherein the product

obtained through thin slab caster route due to thin section and fast cooling, centre line segregation is not a problem in thin slab caster 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 sheets with good weldability and fatigue limit 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.
A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit wherein Ti forms TiN during solidification of steel and controls the grain size (fine and uniform microstructure is desired for good properties before hot rolling.
A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit wherein Nb forms NbN during late stages of hot rolling and helps to further refine grain size that improves mechanical properties.
A still further object of the present invention is directed to providing low carbon high strength hot rolled steel sheets with good weldability and fatigue limit wherein remaining amount of Nb & Ti forms carbonitrides during coiling operation and further helps to improve mechanical properties.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a low carbon high strength hot rolled steel sheet comprising
C: 0.04- 0.08wt%
Mn: 1.2-1.8wt%
Si: <0.5wt%
Al< 0.05wt%
N< 40-100 ppm
Nb: 0.04-0.075wt%
Cr: <0.5wt%
Ti < 0.16wt%

P < 0.018wt%
S< 0.008wt% and
rest is iron, having YS>650MPa.
A further aspect of the present invention is directed to providing a low carbon high strength hot rolled steel sheet wherein said steel sheet have slab thickness < 65mm.
A still further aspect of the present invention is directed to providing a low carbon high strength hot rolled steel sheet having microstructure of > 95% Ferrite and Ferrite grain size below 10 microns preferably in the range of 2-4 micron and substantially free of hard/brittle martensite phase.
A still further aspect of the present invention is directed to providing a low carbon high strength hot rolled steel sheet having UTS > 700 MPa and elongation > 18%.
Also said low carbon high strength hot rolled steel sheet is having good weldability and fatigue limit > 0.4 YS.
Yet another aspect of the present invention is directed to providing a process for manufacture of low carbon high strength hot rolled steel sheet as described above comprising :
(i) providing the selective low carbon composition comprising of C; 0.04-0.08wt%;Mn: 1.2-1.8wt%;Si: <0.5wt%;AI< 0.05wt%;N< 40-100 ppm;IMb: 0.04-0.075wt%Cr; <0.5wt%;Ti< 0.16wt%;P < 0.018wt%;S< 0.008wt% and rest is iron,;
(ii) casting the steel slabs in a thin slab caster;
(iii) homogenizing in a tunnel furnace at temperature >1100°C ;
(iv) controlling the desired thickness of the slab in hot rolling, and finish rolling temperature in the range of 820-900°C ;

(v) subjecting the slabs to ultra fast cooling such as to ensure high nucleation rate and fine grain structure followed by laminar cooling for facilitating coiling.
A further aspect of the present invention is directed to a process for manufacture of low carbon high strength hot rolled steel sheet wherein said ultra fast cooling followed by laminar cooling is carried out to achieve coiling temperature in the range of 500 - 600°C.
A still further aspect of the present invention is directed to a process for manufacture of low carbon high strength hot rolled steel sheet as described above wherein said hot metal is treated in a EAF and further in a ladle 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 820 to 900°C and at coiling temperature 500 - 600°C to obtain desired high strength.
A still further aspect of the present invention is directed to providing a process for manufacture of low carbon high strength hot rolled steel sheet wherein the same is controlled to produce high strength , high ductility steel sheet/strip of 3-10 mm thickness.
Yet another aspect of the present invention is directed to a process wherein said process carried out under controlled operating conditions comprising
(i) Casting Speed 4.5- 5.5 m/min
(ii) Slab Thickness 55-65 mm
(iii) Slab Cutting Temp 980-1050 Deg C
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 Deg C
(v) Homogenization Time 8-15 min
(vi) Finish Rolling Temp 820-900 Deg C
(vii) Standwise rolling reduction/time/temp comprising

Parameters Fl F2 F3 30-40
3-10 sec
970-1000 F4 20-30
3-10 sec
900-950 F5
20-30

2-10 sec
870-900 F6
Relative Reduction 40-50 40-50


15-20
Interstand time 5-15 sec 4-15 sec


2-10 sec
Stand Entry Temp 1050-1080 1000-1040


850-900

(viii) Coiling Temp 500-600 Deg C
(ix) Ultrafast Cooling Rate 60-80 Deg C
(x) Laminar Cooling rate 10-25 Deg C
A still further aspect of the present invention is directed to a process wherein for desired fatigue property, the composition and hot rolling process parameters are selectively maintained within the above ranges so that higher strength is achieved without formation of hard/brittle martensite phase.
A still further aspect of the present invention is directed to providing a process comprising maintaining desired cleanliness of steel, very fine grain size avoiding formation of hard /brittle martensite phase such as to achieve fatigue limit (endurance limit) which is 40% of yield strength in 106cycle with fatigue ratio of R= -1, suitable for automotive standard.
The objects and advantages of the present invention is described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.
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 sheet with good weldability and fatigue strength 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 2-4 microns.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURE
The present invention is directed to developing a low carbon high strength (YS> 650 MPa) hot rolled steel sheet with good weldability and fatigue strength through thin slab caster 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. Mechanical properties are YS> 650 Mpa & UTS > 700 MPa , elongation > 18% and fatigue limit > 0.4 YS 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:
C: 0.04-0.08
Mn: 1.2-1.8
Si: <0.5
Al< 0.05
N< 40-100 ppm
Nb: 0.04-0.075
Cr: <0.5
Ti< 0.16
S & P as impurities
The detailed consideration for selecting the above chemical composition are as below:
C: 0.04-0.08: Carbon is essential for solute strengthening and formation of carbide and carbonitrides of Ti & Nb but upper limit is restricted 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. Carbon level above 0.08% is particularly prone to surface defect in thin slab caster
Mn: 1.2-1.8: Mn is an important element for solid solution strengthening, but 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. Higher amount of Mn is also believed to affect fatigue performance. Centre line segregation is another major issue with increasing % Mn.
Si: <0.5: Upper limit is limited by detrimental effect on surface quality as tiger marks type scale forms.
A\< 0.05: Upper limit is limited by detrimental effect on surface quality & castability \n thin slab caster. It is only added for de-oxidation purpose.

N< 40-100 ppm: 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 of steel and strain aging.
Nb: 0.04-0.075: Nb is essential for strengthening by grain refinement & precipitation strengthening. It is one of the main sources of strengthening. Upper limit is restricted because of its effect on rolling load during hot rolling.
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. Nb alloying in combination with Ti' avoids 'Nb-N' precipitation during casting and thus make all / most of 'Nb' available in solution for maximum contribution to strengthening by grain refining / controlled amount of bainite phase / fine precipitates during & after hot rolling. 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.
Nb in combination with Nitrogen leads to transverse corner cracking during casting obtained through thin slab caster. This is another consideration for limiting their maximum contents in steel.
Ca: 0-50 ppm: Steel has to be Calcium treated to counter the harmful effect of Sulphur as well as help in casting.
Ti< 0.16: Ti has a pivotal role as TiN is essential for uniform prior austenite grain size and grain refinement. Tieff factor (Tieff = Ti-3.4*N) should be between 0.03-0.1 for effective carbide and carbo-nitride formation and its effective utilization. Tieff >0.1 is not much useful as its effect saturates at these level.
In thin slab caster, due to high rate of cooling during solidification of thin slab, Ti forms TiN which is fine is size and uniformly distributed. This restricts austenite grain size at high temperature by effectively pinning grain boundaries. This also ensures through thickness uniformity of grain size which is required for consistent mechanical properties and high toughness. The elimination of free nitrogen due to the formation of TiN is positive for the toughness and indirectly makes niobium more effective. Excess Ti' in solution is available

for further grain refining & precipitation hardening during & after hot rolling.
The process route followed was: Electric Arc Furnace→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.
Accompanying Figure 1 shows the flow chart illustrating the different steps involved in producing the high strength hot rolled steei sheet with good weldability and fatigue strength 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 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. The composition of different steel samples obtained on trials and experimental heats are given in following table 1:

Sample ID C% Mn % Si % N
(PP m) Nb% Cr% Ti% P% S% Al % Ca
(ppm)
Trial 1 0.103 1.711 0.29 103 0.062 0.16 0.030 0.011 0.004 0.023 18
Trial 2 0.052 1.449 0.29 55 0.057 0.18 0.108 0.012 0.004 0.025 25
Example 1 0.050 1.618 0.32 58 0.061 0.230 0.151 0.012 0.004 0.025 16
Example 2 0.049 1.608 0.34 41 0.061 0.2 0.156 0.010 0.003 0.041 23
(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 >1100C but below
1170C. 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 820-900C. Final rolling pass
below TNR (no re-crystallization temperature).The details stand wise parameters
for hot rolling are as follows:

Parameters Fl
40-50 F2 30-40 F4 F5 20-30
2-10 sec
870-900 F6
Relative Reduction
40-50
20-30
15-20
Interstand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec
2-10 sec
Stand Entry Temp 1050-1080 1000-1040 970-1000 900-950
850-900
(v) After finish rolling, an ultra fast cooling was used in combination of standard laminar cooling to achieve Coiling temperature 500-600C. 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:
The product obtained was free from any surface defects and the mechanical properties observed for different samples are presented in the following table 2:

Sample ID Width Thickness CT FT YS UTS Hardn ess %EI YS/UTS
Trial 1 1250 7.00 533 860 529 642 92 21 0.82
Trial 2 1250 6.01 501 827 620 680 90 26 0.91
Example 1 1250 5.00 555 897 676 769 95 20 0.88
Example 2 1250 4.50 532 886 691 750 95 22 0.92
Trial 1 & 2 are the experiments done to arrive at the claimed product. Chemistry of the trials are also provided in table 1. The properties during trials are less (YS<650) and chemistry was also different. Data relating to Example 1 & 2 in the table above provide the desired results.
Microstructure:
The Microstructure of samples was found to consists of >95% Ferrite and typical ferrite grain size is below 10 microns and preferably in the range of 2-4 microns. Accompanying

Figure 2a-b shows the Microstructure Images taken at 100X, having typical grain size in the range of around 2-4 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 (0.08wt%)
• %Mn(1.8% max) and % Si(<0.5%) used are less
• Low %Cr(<0.5%)
• No V , Cu, Ni used in the composition.
This leads to a computed value of favourable carbon equivalent based on the above formula which is clearly indicative of very good weldability.
Fatigue Strength:
It is known that Fatigue property of steel mainly depends upon: -Cleanliness of steel -Grain size -Absence of hard/ brittle phases.
The steel grade produced according to the present invention have following characteristics
which leads to good fatigue property
(i) Our steel is very clean due to some process involved in steel making process like
EBT( Eccentric Bottom Taping) taping for slag free taping from EAF to LF, Ca wire
injection for inclusion shape modification;
(ii) By using Nb and Ti as micro alloying, grain size is very fine;
(iii) To get good fatigue property, the composition and hot rolling process parameters
are so selected that higher strength is achieved without formation of hard/brittle
martensite phase. Martensite acts as a stress concentration point and reduces
fatigue property.

High cycle fatigue property has been tested and fatigue limit (endurance limit) achieved is 40% of yield strength in 106 cycle with fatigue ratio of R= -1. The value achieved is well accepted in automotive standards.
It is thus possible by way of the present invention to providing a low carbon high strength hot rolled steel sheet with good weldability and fatigue strength alongwith good formability to enable advantageous application in production of automobile components or similar other structural sections with the possibility of reduction of section thickness and weight and achieving fuel economy.

We Claim:
1. A low carbon high strength hot rolled steel sheet comprising
C: 0.04-0.08wt%
Mn: 1.2-1.8wt%
Si: <0.5wt%
Al< 0.05wt%
N< 40-100 ppm
Nb: 0.04-0.075wt%
Cr: <0.5wt%
Ti< 0.16wt%
P < 0.018wt%
S< 0.008wt% and
rest is iron, having YS>650MPa.
2. A low carbon high strength hot rolled steel sheet as claimed in claim 1 wherein
said steel sheet have slab thickness < 65mm.
3. A low carbon high strength hot rolled steel sheet as claimed in anyone of claims
1 to 2 having microstructure of > 95% Ferrite and Ferrite grain size below 10
microns preferably in the range of 2-4 micron and substantially free of
hard/brittle martensite phase.
4. A low carbon high strength hot rolled steel sheet as claimed in anyone of claims 1 to 3 having UTS > 700 MPa and elongation > 18%.
5. A low carbon high strength hot rolled steel sheet as claimed in anyone of claims 1 to 4 having good weldability and fatigue limit > 0.4 YS.
6. A process for manufacture of low carbon high strength hot rolled steel sheet as claimed in anyone of claims 1 to 5 comprising :
(i) providing the selective low carbon composition comprising of C: 0.04-0.08wt%;Mn: 1.2-1.8wt%;Si: <0.5wt%;AI< 0.05wt%;N< 40-100 ppm;Nb:

0.04-0.075wt%Cr: <0.5wt%;Ti< 0.16wt%;P < 0.018wt%;S< 0.008wt% and rest is iron,;
(ii) casting the steel slabs in a thin slab caster;
(iii) homogenizing in a tunnel furnace at temperature >1100°C ;
(iv) controlling the desired thickness of the slab in hot rolling and finish rolling temperature in the range of 820-900°C ;
(v) subjecting the slabs to ultra fast cooling such as to ensure high nucleation rate and fine grain structure followed by laminar cooling for facilitating coiling.
7. A process for manufacture of low carbon high strength hot rolled steel sheet as claimed in claim 6 wherein said ultra fast cooling followed by laminar cooling is carried out to achieve coiling temperature in the range of 500 - 600°C.
8. A process for manufacture of low carbon high strength hot rolled steel sheet as claimed in anyone of claims 6 or 7 wherein said hot metal is treated in a EAF and further in a ladle furnace and therafter 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 820 to 900°C and at coiling temperature 500 - 600°C to obtain desired high strength.
9. A process for manufacture of low carbon high strength hot rolled steel sheet as claimed in anyone of claims 6 to 8 wherein the same is controlled to produce high strength , high ductility steel sheet/strip of 3-10 mm thickness.
10. A process as claimed in anyone of claims 6 to 9 , wherein said process carried out under controlled operating conditions comprising
(i) Casting Speed 4.5- 5.5 m/min
(ii) Slab Thickness 55-65 mm
(iii) Slab Cutting Temp 980-1050 Deg C
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 Deg C
(v) Homogenization Time 8-15 min

(vi) Finish Rolling Temp 820-900 Deg C
(vii) Standwise rolling reduction/time/temp comprising

Parameters Fl F2 F3
30-40 F4 20-30 F5 20-30 F6
Relative Reduction 40-50 40-50



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 1050-1080 1000-1040 970-1000 900-950 870-900 850-900
(viii) Coiling Temp 500-600 Deg C
(ix) Ultrafast Cooling Rate 60-80 Deg C
(x) Lamilar Cooling rate 10-25 Deg C
11. A process as claimed in anyone of claims 6 to 10, wherein for desired fatigue
property, the composition and hot rolling process parameters are selectively
maintained within the above ranges so that higher strength is achieved without
formation of hard/brittle martensite phase.
12. A process as claimed in anyone of claims 4 to 10, comprising maintaining
desired cleanliness of steel, very fine grain size avoiding formation of hard /brittle
martensite phase such as to achieve fatigue limit (endurance limit) which is 40% of
yield strength in 106 cycle with fatigue ratio of R= -1, suitable for automotive
standard.