Claims:We Claim:

1. Low carbon high strength low alloy(HSLA) hot-rolled steel comprising
C-0.04-0.08wt%,
Mn-0.3-1.5 wt%,
Si< 0.5 wt%,
S<0.008 wt%,
P<0.018 wt%, and
Nb 0-0.010wt% and V 0.010-0.050wt%;
for selective micro alloying and having YS > 250 MPa, UTS > 410 MPa and low YS/UTS ratio of 0.85max.

2. Low carbon high strength low alloy(HSLA) hot-rolled steel as claimed in claim 1 comprising elongation > 25% with desired weldability suitable for structural, automobile, welded tubes and pipes and cold rolling applications.

3. Low carbon high strength low alloy(HSLA) hot-rolled steel as claimed in anyone of claims 1 or 2 comprising :
C: 0.04- 0.08wt%;
Mn- 0.30-1.50 wt%;
Si: < 0.5 wt%;
Al< 0.05 wt%;
N< 40-100 ppm;
Nb: 0-0.010 wt%;
V- 0.010-0.050 wt%;
Cr: <0.5 wt%;
Ti< 0.05 wt%;
Ca 0-50 ppm;
S & P as impurities; and having Ferrite microstructure.

4. Low carbon high strength low alloy(HSLA) hot-rolled steel as claimed in anyone of claims 1 to 3 having microstructure comprising of single phase ferrite of ASTM Grain size 10.2.

5. Low carbon high strength low alloy(HSLA) hot-rolled steel as claimed in anyone of claims 1 to 4having reduced tendency of transverse corner crack sensitivity.

6. Low carbon high strength low alloy(HSLA) hot-rolled steel as claimed in anyone of claims 1 to 5having YS:400-450MPa and UTS:480-520MPa with elongation 20-24% with cold reduced thickness below 2mm and being galvanized.

7. A process for manufacturing low carbon high strength low alloy(HSLA) hot-rolled steel as claimed in anyone of claims 1 to 6 comprising involving
C-0.04-0.08wt%,
Mn-0.3-1.5 wt%,
Si< 0.5 wt%,
S<0.008 wt%,
P<0.018 wt%,and
Nb 0-0.010wt% and V 0.010-0.050wt%;
for selective micro alloying and following slab caster processing such as to yield YS > 250 MPa, UTS > 410 MPa and low YS/UTS ratio of 0.85max.

8. A process as claimed in claim 7 comprising:
carrying out casting involving a thin slab caster;
homogenizing the slab at temperature >1100C;
descaling involving descaler to remove scales;
hot rolling reduction to reduce the thickness to the required level with finish rolling temperature in the range of 820-900 DegC.

9. A process as claimed in anyone of claims 7 or 8 comprising following any combination of processes before said casting in caster such as which said steel of same chemistry.

10. A process as claimed in anyone of claims 7 to 9 comprising carrying out manufacture involving Electric Arc Furnace, Ladle Furnace, thin slab Caster, 6 stand hot rolling mill and coiling route .

11. A process as claimed in anyone of claims 7 to 10 comprising
casting said steel using a thin slab caster with slab thickness ranging from 50-65 mm;
homogenizing said slab further in a Tunnel furnace at temperature >1100°C;
Descaling of the slabs after Tunnel Furnace to remove scales;
hot rolling of slabs in 6 stand tandem rolling mill into final thickness to the required level with finish rolling temperature in the range of 820-900°C;
after finishing, carrying out an ultra fast cooling in first step followed by in combination of standard laminar cooling in second step to achieve coiling temperature 500-600°C, whereby said initial ultra fast cooling ensures a high nucleation rate thus leading to fine grain structure and subsequent laminar cooling of less severity enable maintaining good flatness of sheet produced; and
subsequently slow cooling of coils in coil yard.

12. A process as claimed in anyone of claims 7 to 11 comprising
(i) Casting Speed 5.5-6.5 m/min;
(ii) Slab Thickness 55-65 mm;
(iii) Slab Cutting Temp 950-1050 °C;
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 °C;
(v) Homogenization Time 8-15 min;
(vi) Finish Rolling Temp 820-900 °C;
(vii) Standwise reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-50 40-50 30-40 20-30 20-30 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(°C) 1050-1080 1000-1040 970-1000 900-950 870-900 850-900

(viii) Coiling Temp 500-600 °C;
(ix) Ultrafast Cooling Rate 60-80 °C/s; and
(x) Lamilar Cooling rate 10-25 °C/s.

13. A process as claimed in anyone claims 7 to 12 wherein the steps enable Vanadium to form Nitride precipitates by combining with Nitrogen and thus minimize the detrimental effect of free Nitrogen present in the steel and said Vanadium precipitating after rolling thus minimizing the detrimental effect of transverse corner crack and poor rollability.

Dated this the 28th day of April, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION

Present invention relates to Low Carbon High Strength Low Alloy (HSLA) hot rolled flat steel with low YS/UTS ratio and method of producing the same. More particularly, the present invention is directed to provide HSLA steel in the form of coil, sheet and plate etc. through thin slab caster processing route. The HSLA steel according to the present advancement is obtained by addition of micro alloying for desired microstructure, improved mechanical properties and is characterized by high UTS, low YS and thus low YS/UTS ratio less than 0.85. The HSLA steel is processed using a thin slab caster and chemistry consists of low carbon along with micro alloys such as Nb and V. By choosing the appropriate micro alloying, the developed HSLA steel help to reduce the transverse corner crack sensitivity and thus reducing internal diversion, rework cost as well as customer complain. Importantly also, the advancement ensure the rollability particularly in lower thickness (<2.0 mm) due to requirement of lower roll force and lower preheating temperature. The developed steel grade is suitable for application in structural application like PEB segments, automobile, welded tubes and pipes segments and cold rolling segments. With the new developed steel higher strength (YS and UTS) is developed after cold rolling and galvanizing as compared to previously used material of almost same mechanical properties in hot rolled stage.

BACKGROUND OF THE INVENTION

High strength low alloy (HSLA) steel are mainly used in the structural, automobile, welded tubes and pipes, cold rolling segments and many other field to reduce overall weight. This can be achieved by using lower thickness and high strength material. The advantage of HSLA steel over plain carbon steel is (i) much stronger and tougher than ordinary carbon steels, (ii) ductile, (iii) highly formable, (iv) weldable due to much lower carbon level & CE, (v) higher corrosion resistance due to lower CE - which is important since the structure may be in place for a long time.
Due to inherent characteristics of CSP process, low carbon micro alloyed steel is more preferred as compared to normal high C-Mn steel as high C-Mn steel reduces the productivity by restriction of casting speed as well as more prone to surface crack. Earlier this material was produced through low C-Mn-Nb microalloyed route. Due to this low C-Mn and Nb micro alloying, YS is much higher in CSP material as compared to conventional material produced by high C-Mn and without microalloying route. Thus for the same strength level, YS/UTS is higher in CSP material (low C-MN-Nb ) as compared to conventional material ( high C-Mn and no microalloying).
Micro alloying is generally used as strengthening element. There are various strengthening mechanisms present in micro-alloyed steels such as solid solution strengthening, grain boundary strengthening, dislocation strengthening and precipitation hardening. Nb acts first as grain boundary strengthener (by forming precipitation at the grain boundary and pinning of grain) and then precipitation strengthener. For this reason, Nb increases more of YS as compared to UTS and thus YS/UTS ratio is high in Nb micro alloyed steel as compared to other micro alloyed steel like V, Ti etc.

Also the precipitation of Nb (Niobium Carbide or Niobium carbonitrides) start before the start of hot rolling and thus roll force is higher particularly in lower thickness (<=2.0mm). To compensate the roll force, preheat temperature/ tunnel furnace temperature needs to be high which leads to surface defects like temperature mark.

Nb- containing steels are found to be one of the steel groups most prone to transverse cracking during continuous casting for the following reasons: (1) Nb has a powerful influence in decreasing the Ar3 (dynamic transformation start on cooling), thus widening the trough at the low temperature end by increasing the temperature range in which the thin soft film of deformation induced ferrite is present surrounding the austenite (?) grains, allowing strain concentration to occur. (2) Nb precipitates out very rapidly as Nb(CN) in a fine form in the ? upon deformation in the temperature range of the straightening operation. The increased matrix strength encourages grain boundary sliding in the ? and void formation in the ferrite film, thus so increasing the depth of the trough. The precipitates at the boundaries make it easy for cracks to interlink, resulting in inter-granular failure. (3) Often there are coarser NbCN precipitates at the ? grain boundaries, which are surrounded by a soft, precipitate-free zone. These precipitate-free zones behave in a similar manner to the ferrite film so that strain concentration again occurs, resulting in poor ductility which continues for temperatures in excess of the Ac3 (equilibrium transformation start), so widening the trough at the high temperature end. (4) Nb is, of course, added for its ability to refine the ? grain size and induce precipitation hardening so that high strength and toughness can be achieved in the final product. However, the same precipitation of Nb(CN) which causes these desirable qualities also makes it more difficult to cast these steels.

There has been thus a need in the related field to develop HSLA steel in the form of hot rolled coil/sheets which would have selective microalloying favouring low YS/UTS ratio and particularly avoiding above stated problems of prior art related to Nb microalloying as well as favouring improved rollability with reduced transverse corner crack sensitivity while maintaining desired formability and strength properties to suit various structural and automobile applications.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide low Carbon High Strength Low Alloy (HSLA) hot rolled flat steel with low YS/UTS ratio and method of producing the same.

A further object of the present invention is directed to provide low Carbon High Strength Low Alloy (HSLA) hot rolled sheet steel with mechanical properties of YS> 250 Mpa & UTS > 410 MPa and elongation > 25% with low YS/UTS ratio of 0.85 max.

A still further object of the present invention is directed to provide low Carbon High Strength Low Alloy (HSLA) hot rolled sheet steel with reduced transverse corner crack sensitivity

A still further object of the present invention is directed to provide low Carbon High Strength Low Alloy (HSLA) hot rolled sheet steel with improved rollability and reduced surface defect such as temperature mark.

A further object of the present invention is directed to provide low Carbon High Strength Low Alloy (HSLA) hot rolled sheet steel through a processing route involving thin slab caster.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to low carbon high strength low alloy(HSLA) hot-rolled steel comprising
C-0.04-0.08wt%,
Mn-0.3-1.5 wt%,
Si< 0.5 wt%,
S<0.008 wt%,
P<0.018 wt%, and
Nb 0-0.010wt% and V 0.010-0.050wt%;
for selective micro alloying and having YS > 250 MPa, UTS > 410 MPa and low YS/UTS ratio of 0.85max.

A further aspect of the present invention is directed to said low carbon high strength low alloy(HSLA) hot-rolled steel comprising elongation > 25% with desired weldability suitable for structural, automobile, welded tubes and pipes and cold rolling applications.

A still further aspect of the present invention is directed to said Low carbon high strength low alloy(HSLA) hot-rolled steel comprising :
C: 0.04- 0.08wt%;
Mn- 0.30-1.50 wt%;
Si: < 0.5 wt%;
Al< 0.05 wt%;
N< 40-100 ppm;
Nb: 0-0.010 wt%;
V- 0.010-0.050 wt%;
Cr: <0.5 wt%;
Ti< 0.05 wt%;
Ca 0-50 ppm;
S & P as impurities; and having Ferrite microstructure.

A still further aspect of the present invention is directed to said Low carbon high strength low alloy(HSLA) hot-rolled steel having microstructure comprising of single phase ferrite of ASTM Grain size 10.2.

A still further aspect of the present invention is directed to said Low carbon high strength low alloy(HSLA) hot-rolled steel having reduced tendency of transverse corner crack sensitivity.

Another aspect of the present invention is directed to said Low carbon high strength low alloy(HSLA) hot-rolled steel having YS:400-450MPa and UTS:480-520MPa with elongation 20-24% with cold reduced thickness below 2mm and being galvanized.

Yet another aspect of the present invention is directed to a process for manufacturing low carbon high strength low alloy(HSLA) hot-rolled steel comprising involving
C-0.04-0.08wt%,
Mn-0.3-1.5 wt%,
Si< 0.5 wt%,
S<0.008 wt%,
P<0.018 wt%,and
Nb 0-0.010wt% and V 0.010-0.050wt%;
for selective micro alloying and following slab caster processing such as to yield YS > 250 MPa, UTS > 410 MPa and low YS/UTS ratio of 0.85max.

A further aspect of the present invention is directed to said process comprising:
carrying out casting involving a thin slab caster;
homogenizing the slab at temperature >1100C;
descaling involving descaler to remove scales;
hot rolling reduction to reduce the thickness to the required level with finish rolling temperature in the range of 820-900 DegC.

A still further aspect of the present invention is directed to said process comprising following any combination of processes before said casting in caster such as which said steel of same chemistry.

A still further aspect of the present invention is directed to said process comprising carrying our manufacture involving Electric Arc Furnace, Ladle Furnace, thin slab Caster, 6 stand hot rolling mill and coiling route .

Another aspect of the present invention is directed to said process comprising:

casting said steel using a thin slab caster with slab thickness ranging from 50-65 mm;
homogenizing said slab further in a Tunnel furnace at temperature >1100°C;
Descaling of the slabs after Tunnel Furnace to remove scales;
hot rolling of slabs in 6 stand tandem rolling mill into final thickness to the required level with finish rolling temperature in the range of 820-900°C;
after finishing, carrying out an ultra fast cooling in first step followed by in combination of standard laminar cooling in second step to achieve coiling temperature 500-600°C, whereby said initial ultra fast cooling ensures a high nucleation rate thus leading to fine grain structure and subsequent laminar cooling of less severity enable maintaining good flatness of sheet produced; and
subsequently slow cooling of coils in coil yard.

Yet another aspect of the present invention is directed to said process comprising
(i) Casting Speed 5.5-6.5 m/min;
(ii) Slab Thickness 55-65 mm;
(iii) Slab Cutting Temp 950-1050 °C;
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 °C;
(v) Homogenization Time 8-15 min;
(vi) Finish Rolling Temp 820-900 °C;
(vii) Standwise reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-50 40-50 30-40 20-30 20-30 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(°C) 1050-1080 1000-1040 970-1000 900-950 870-900 850-900

(viii) Coiling Temp 500-600 °C;
(ix) Ultrafast Cooling Rate 60-80 °C/s; and
(x) Lamilar Cooling rate 10-25 °C/s.

A further aspect of the present invention is directed to said process wherein the steps enable Vanadium to form Nitride precipitates by combining with Nitrogen and thus minimize the detrimental effect of free Nitrogen present in the steel and said Vanadium precipitating after rolling thus minimizing the detrimental effect of transverse corner crack and poor rollability.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1(a) & (b): shows the image of microstructure of (a) newly developed HSLA steel according to present invention with 100X magnification as compared to microstructure of (b)conventional HSLA steel.

Figure 2(a) & (b): shows the image of microstructure of (a)newly developed HSLA steel with 500X magnification as compared to microstructure of (b) conventional HSLA steel.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING FIGURES AND EXAMPLE

The present invention is directed to provide low Carbon High Strength Low Alloy (HSLA) hot rolled flat steel with low YS/UTS ratio and method of producing the same through thin slab caster route.

The liquid metal from steel melting shop (SMS) are cast in thin slab continuous caster directly enters into tunnel furnace for temperature homogenization. The heated thin slabs are then roll in 6 stand tandem rolling mill into final thickness of 1.6-20 mm as per customer requirement. The rolled steel then passes through run out table (ROT) where water cooling takes place and finally take the form of coil in coiler. The HSLA steel in the present invention is conformed to the international standard of IS 2062:2011 E 250 and is characterized by high UTS, low YS and thus low YS/UTS. YS achieved is 250 Mpa minimum, UTS 410 Mpa minimum and YS/UTS ratio less than 0.85. Developed HSLA steel of the invention is having uniform ferritic structure and thus have low YS/UTS ratio.

Thus according to present invention, a composition and method of making a high strength low alloy hot rolled steel sheet is provided. Steel is processed using a thin slab caster and chemistry consists of low carbon along with micro alloys such as Nb and V. Mechanical properties are YS> 250 Mpa & UTS > 410 MPa and elongation > 25% with good weldability.

The HSLA steel according to present invention is having chemical composition by weight percent as follows:

C: 0.04- 0.08wt%;
Mn- 0.30-1.50wt%;
Si: < 0.5wt%;
Al< 0.05wt%;
N< 40-100 ppm;
Nb: 0-0.010wt%;
V- 0.010-0.050wt%;
Cr: <0.5wt%;
Ti< 0.05wt%;
Ca 0-50 ppm;
S & P as impurities, and having microstructure consisting of Ferrite.

The details of technical reasoning to ascertain the weight percent ranges of constituents as in the above stated chemical composition to achieve the intended properties of the resulting steel are as below:

C: 0.04- 0.08: Carbon is essential for solute strengthening and formation of carbide and carbonitrides of Nb & V 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. Also high Carbon badly affected productivity by reducing the casting speed in thin slab casting process.

Mn: 0.30-1.50: 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

Al< 0.05: Upper limit is limited by detrimental effect on surface quality & castability in 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: 0-0.010: Nb is essential for strengthening by grain refinement and precipitation strengthening. It is one of the main sources of strengthening. Upper limit is restricted because of its detrimental effect on high YS/UTS ratio, transverse corner crack sensitivity particularly in thin slab caster and poor rollablity in low thickness rolling.

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

V 0.010-0.050: V used as precipitation strengthener. V forms Nitride precipitates by combining with N and thus minimize the detrimental effect of free N present in the steel. V precipitates after rolling thus minimize the detrimental of transverse corner crack and poor rollability.

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

Process used for making the product is as below:

Hot metal was refined with the help of Electric Arc Furnace and final chemistry adjustments were done in a ladle refining furnace. Steel was cast using a thin slab caster with slab thickness ranging from 50-65 mm. Slab was further homogenized in a Tunnel furnace at temperature >1100C. Descaler was used after Tunnel Furnace to remove scales. 6 stand reductions was used to reduce the thickness to the required level with finish rolling temperature in the range of 820-900 DegC. After finishing, an ultra fast cooling was used in combination of standard laminar cooling to achieve Coiling temperature 500-600 Deg C. Initial ultra fast cooling ensures a high nucleation rate thus leading to fine grain structure and subsequent laminar cooling of less severity is used to maintain good flatness. Coil was subsequently slow cooled in coil yard.

In the above process the Critical Process Parameters used are as follows:

I. Casting Speed 5.5-6.5 m/min;
II. Slab Thickness 55-65 mm;
III. Slab Cutting Temp 950-1050 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 reduction/time/temp:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-50 40-50 30-40 20-30 20-30 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. Laminar Cooling rate 10-25 Deg C.

The composition, process parameters and the properties achieved in resulting steel under various trials carried out according to present invention including some comparative trials designated as “Old” are illustrated with the following example and Table I to III:

Example I:

Chemistry and Mechanical properties of Hot rolled steel

Table 1: Steel Composition

Sample ID C % Mn % Si % N (ppm) Nb % V % P % S % Al % Ca (ppm)
Newly developed- 1 0.050-0.065 0.75-0.95 0.10-0.30 65-85 0.005-0.010 0.020-0.035 0.008-0.012 0.004-0.007 0.020-0.035 15-25
Newly developed- 2 0.050-0.065 0.60-0.80 0.10-0.30 65-85 Nil 0.020-0.035 0.008-0.012 0.004-0.007 0.020-0.035 15-25
Newly developed -3 0.050-0.065 0.4-0.6 0 -0.20 65-85 0.004-0.006 0.015-0.030 0.008-0.012 0.004-0.007 0.020-0.035 15-25
Old-1 0.050-0.065 0.75-0.95 0.10-0.30 50-75 0.015-0.025 Nil 0.008-0.012 0.004-0.007 0.020-0.035 15-25
Old -2 0.050-0.065 0.60-0.80 0.10-0.30 50-75 0.012-0.020 Nil 0.008-0.012 0.004-0.007 0.020-0.035 15-25
Old-3 0.050-0.065 0.40-0.60 0 -0.20 50-75 0.010-0.015 Nil 0.008-0.012 0.004-0.007 0.020-0.035 15-25

Table 2: Process parameters and mechanical properties

Sample ID Width Thickness CT FT YS UTS % El
YS/UTS
Newly developed- 1 1250 8.01-20.00 570-600 840-870 350-380 430-460 31-36 0.80-0.83
Newly developed- 2 1250 3.50 - 8.0 560-580 850-880 360-380 435-460 32-36 0.81-0.83
Newly developed -3 1250 1.6 - 3.49 540-570 860-900 370-390 435-460 32-36 0.81-0.83
Old-1 1250 8.01-20.00 570-600 840-870 370-390 440-460 31-36 0.84-0.87
Old -2 1250 3.50 - 8.00 560-580 850-880 380-400 440-465 32-36 0.84-0.87
Old-3 1250 1.6 - 3.49 540-570 860-900 380-400 440-465 32-36 0.84-0.87

Transverse corner crack sensitivity:

Transverse corner crack sensitivity has been drastically reduced in newly developed steel:

Newly developed steel Old Steel
% of Transverse corner crack < 2% ~ 6.5%

Microstructure of Hot rolled Steel:

Microstructure of both newly developed steel and old steel composed of ferrite but the newly developed steel is having little coarse grain(ASTM Grain size 10.2) as compared to old one with finer grains(ASTM Grain size 10.4). This is one of the reasons of low YS and YS/UTS ratio in newly developed grade steel. Accompanying Figure 1(a) shows the image of microstructure of newly developed HSLA steel with 100X magnification as compared to microstructure of old steel shown in figure 1(b). Similarly, Figure 2(a) and (b) shows the corresponding images of microstructures of new and old steels respectively with 500X magnification.

Importantly, also with the newly developed HSLA HR sheet steel, higher strength (YS and UTS) is developed after cold rolling to thickness less than 2mm and galvanizing as compared to previously used material of almost same mechanical properties in hot rolled stage. The results are shown in the following table 3.

Table 3: Mechanical Properties after cold rolling and galvanizing of newly developed steel:

HR
Thk CR
Thk % cold reduction HR
YS HR
UTS HR
%El CR YS CR UTS CR %El
Newly Developed 2.4-3.49 1.3-1.8 45-50 360-380 430-450 32-35 400-450 480-520 20-24
Old 2.4-3.49 1.3-1.8 45-50 370-390 430-445 32-35 370-420 400-440 25-30

It is thus possible by way of the present invention to provide low carbon HSLA hot rolled steel sheets with selective microalloying having low yield ratio and ferritic microstructure. The steel product in HSLA category according to present invention for catering to structural, automobile, welded tubes & pipes, cold rolling and other segments as per international specification of IS 2062: 2011 E250. Newly developed HSLA steel is having low YS/UTS ratio suitable for different advantageous applications having reduced transverse corner crack sensitivity and reduced internal diversion, rework cost as well as customer complain. Importantly also, further high strength is achieved by cold reduction and galvanizing of the hot rolled product obtained by the process. A product with such unique property combinations is achieved through thin slab caster/CSP route