Claims:We Claim:

1. A low cost high strength low alloy(HSLA) hot-rolled steel comprising
C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si< 0.04 wt%
S<0.008 wt%..,
P<0.018 wt%.,
Ti 0.04-0.08 wt%,
Al 0.02-0.06 wt%,
N < 80 ppm wt%; Rest Fe and having YS > 450 MPa, UTS > 520 MPa and stretch flangability HER >100%.

2. The low cost high strength low alloy(HSLA) hot-rolled steel as claimed in claim 1 comprising elongation > 22% and HER > 100 % with desired weldability( CE 0.22-0.25) suitable for structural, automobile, welded tubes and pipes and cold rolling applications.

3. The low cost high strength low alloy(HSLA) hot-rolled steel as claimed in anyone of claims 1 or 2 comprising

C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si-0.01 to 0.04 wt%,
S-0.003 to 0.007 wt%,
P-0.008 to 0.015 wt%,
Ti- 0.04-0.08 wt%,
Al- 0.02-0.06 wt%,
N < 80 ppm wt%; Rest Fe with Ferrite microstructure.

4. The low cost 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 10.5.

5. The low cost high strength low alloy (HSLA) hot rolled steel as claimed in anyone of claims 1 to 4 having YS:460-500 MPa and UTS:525-600 MPa with elongation 24-28%of thinner gauze up to 2.5 mm thickness which can be rolled in hot rolling condition.

6. The low cost high strength low alloy (HSLA) hot rolled steel as claimed in anyone of claims 1 to 5 having coatability in hot dip galvanizing process.

7. A process for producing low cost high strength low alloy(HSLA) steel sheet as claimed in claims 1 to 6 comprising

providing for steel composition comprising:
C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si< 0.04 wt%,
S<0.008 wt%,
P<0.018 wt%,
Ti 0.04-0.08 wt%,
Al 0.02-0.06 wt%,
N < 80 ppm wt%; rest Fe;
carrying out casting involving a thin slab caster;

homogenizing the slab at temperature >1100°C preferably 1100 to 11300C;

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 850-900°C; followed by cooling and coiling.

8. The process as claimed in claim 7 comprising following Electric Arc Furnace, Ladle Furnace, thin slab Caster, 6 stand hot rolling mill and coiling route and including

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 850-900°C;

after finish rolling, carrying out an ultra fast cooling in first step followed by in combination of standard laminar cooling in second step to achieve coiling temperature 580-620°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.

9. The process as claimed in anyone of claims 7 or 8 comprising involving :
(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 850-900 °C;
(vii) Standwise reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 45-50 50-55 35-40 30-35 25-30 20-25
Stand Entry Temp(°C) 1000-1020 960-970 940-950 925-935 910-915 890-900

(viii) Coiling Temp 580-620°C;
(ix) Ultrafast Cooling Rate 60-80°C/s; and
(x) Lamilar Cooling rate 10-25°C/s.

Dated this the 24th June, 2020
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION

Present invention relates to the development of low cost hot rolled high strength low alloy (HSLA) steel sheet with improved hole expansion 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. processed through thin slab caster route. The HSLA steel according to the present advancement is obtained by addition of low carbon, low Mn and selective Ti microalloying for desired uniform fine grain ferritic microstructure, improved mechanical properties and is characterized by improved hot rollabilityto thinner gauze (2.5 mm), high UTS up to 600 MPa, with minimum hole expansion ratio of 100%. Importantly also, the advancement ensuresthe rollability to roll up to 2.5 mm thickness and improved coatability because of combination of lower Mn,l and precipitation behavior of TiC(not before rolling but after rolling and at ROT).The developed steel grade is suitable for application in structural application like automobile, PEB(pre-engineering building) segments, welded tubes and pipes segments and cold rolling segments.

BACKGROUND OF THE INVENTION
High strength low alloy (HSLA) steel are mainly used in the automobile structural, 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. However, for use in automobile structural application, apart from strength and ductility, stretch flengability is also an important characteristic. Stretch flengability is measured by a testing method called hole expansion ratio(HER) test. Hole expansion ratio of any steel grade is mainly depends upon the strength level and the associated microstructure. More uniform microstructure without the absence of any hard phase will improve the HER value.
Thinner gauze High strength steel can be produced by introducing cold reduction of the hot rolled steel. However, the elongation of the cold rolled material is comparatively low because of the work hardening during cold reduction. Also the additional process of cold reduction will add to the production cost.
Low cost hot rolled high strength steel can be produced by selecting medium C (>0.14%) and high Mn but there will be loss in productivity because of restriction in casting speed with higher carbon level in thin slab caster. Also this steel is very much susceptible to surface defects such as longitudinal or transverse cracking resulted in yield loss or more rework.
High strength can also be produced by using higher Si level, but higher Si level deteriorates the surface quality of hot rolled coil by formation of sticky scale during reheating of the slab. Also the Si level >0.04 and < 0.15 have detrimental effect of coatability.
US 9238848 B2 relates to a high-strength steel sheetwhich can be preferably mainly pressed and used in theunderbody parts of automobiles and the like and structuralmaterials, and is excellent in terms of hole expansion andductility, and a method of producing the same. The chemical composition includes very high Mn (increase mill roll separating force thus low rollability), very high Si (not favorablefor coating), low level of Ti along with other microalloying such as Nb. Although high tensile strength is achieved through rich chemistry but the hole expansion ratio is very poor. This low hole expansion value is due to the presence of martensite microstructure.

There has been thus a need in the related field to develop low cost HSLA steelin the form of hot rolled coil/sheets which would have selective micro alloyinghaving high strength and favoringimproved hole expansion ratio.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide low cost High Strength Low Alloy (HSLA) hot rolled sheet steel with selective microalloying with Ti to achieve improved hole expansion ratio, hot rollability to thinner gauge and method of producing the same.

A further object of the present invention is directed to providelow cost High Strength Low Alloy (HSLA) hot rolled sheet steel with mechanical properties of YS>450Mpa& UTS >520MPa and elongation>22%, and Hole Expansion Ratio of minimum 100 %.

A still further object of the present invention is directed to provide low cost High Strength Low Alloy(HSLA) hot rolled sheet steel with improved rollabilityso that thinner gauze up to 2.5 mm thickness can be rolled during hot rolling.

A further object of the present invention is directed to provide low cost High Strength Low Alloy(HSLA) hot rolled sheet steel with improved coatability.

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

Yet another object of the present invention is directed to provide low cost High Strength Low Alloy(HSLA) hot rolled sheet steel wherein by choosing the appropriate micro alloying and low Mn & low Si, the developed HSLA steel help to reduce the chances of center line segregation of Mn and improved hot dip galvanizing property.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a low cost high strength low alloy(HSLA) hot-rolled steel comprising
C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si< 0.04 wt%,
S<0.008 wt%,
P<0.018 wt%,
Ti 0.04-0.08 wt%,
Al 0.02-0.06 wt%,
N < 80 ppm wt%; Rest Fe and having YS > 450 MPa, UTS > 520 MPa and stretch flangability HER >100%.

A further aspect of the present invention is directed to said low cost high strength low alloy(HSLA) hot-rolled steel comprising elongation > 22% and HER > 100 % with desired weldability (Carbon Equivalent 0.22-0.25) 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 cost high strength low alloy(HSLA) hot-rolled steel comprising:

C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si- 0.01 to 0.04 wt%
S-0.003 to 0.007 wt%,
P-0.008 to 0.015 wt%,
Ti -0.04-0.08 wt%,
Al 0.02-0.06 wt%,
N < 80 ppm wt%; Rest Fe with Ferrite microstructure.

A still further aspect of the present invention is directed to said low cost high strength low alloy(HSLA) hot-rolled steel having microstructure comprising of single phase ferrite of ASTM Grain size 10.5.

Another aspect of the present invention is directed to said low cost high strength low alloy (HSLA) hot rolled steel having YS:460-500 MPa and UTS:525-600 MPa with elongation 24-28% of thinner gauze up to 2.5 mm thickness which can be rolled in hot rolling condition.

Yet another aspect of the present invention is directed to said low cost high strength low alloy (HSLA) hot rolled steel having coatability in hot dip galvanizing process.

A further aspect of the present invention is directed to a process for producing low cost high strength low alloy(HSLA) steel sheet comprising

providing for steel composition comprising:
C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si< 0.04 wt%,
S<0.008 wt%,
P<0.018 wt%,
Ti 0.04-0.08 wt%,
Al 0.02-0.06 wt%,
N < 80 ppm wt%; rest Fe;
carrying out casting involving a thin slab caster;
homogenizing the slab at temperature >1100°C preferably 1100 to 11300C;
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 850-900°C; followed by cooling and coiling.

A still further aspect of the present invention is directed to said process comprising following Electric Arc Furnace, Ladle Furnace, thin slab Caster, 6 stand hot rolling mill and coiling route and including
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 850-900°C;
after finish rolling, carrying out an ultra fast cooling in first step followed by in combination of standard laminar cooling in second step to achieve coiling temperature 580-620°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.

A still further aspect of the present invention is directed to said process comprising involving :
(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 850-900 °C;
(vii) Standwise reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 45-50 50-55 35-40 30-35 25-30 20-25
Stand Entry Temp(°C) 1000-1020 960-970 940-950 925-935 910-915 890-900

(viii) Coiling Temp 580-620 °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 saidprocess wherein the steps enable Titanium to form Titanium Carbo Nitride precipitates at ROT thus improves the rollability.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1(a) & (b): shows the image of microstructure of newly developed HSLA steel according to present invention.

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

The present invention is directed to provide low cost High Strength Low Alloy (HSLA) hot rolled sheet steel with selective composition including Ti microalloying leading to YS>450MPa, UTS >520 MPa, %El >22% , HER of Min 100% with improved hot rollability to thinner gauge, improved coatability and a method of producing the same byselectively processingthrough 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 2.5-6.0 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 characterized by high YS, UTS and %El and improved hole expansion ratio of 100 % min. with judicial choice of chemical composition the steel shows improved coatability in hot dip galvanizing process. Developed HSLA steel of the invention is having uniform fine grain ferritic structure and thus have improved HER.

Thus according to present invention, a composition and method of making a low cost high strength low alloy hot rolled steel sheet is provided. Steel is processed in a thin slab caster processing route and chemistry consists of low carbon, low Si along with micro alloys such as Ti. Mechanical properties are YS>450Mpa& UTS >520 MPa and elongation >22% with improved HER.

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

C-0.04-0.08wt%,
Mn-0.50-1.50 wt%,
Si< 0.04 wt%,
S<0.008 wt%,
P<0.018 wt%,
Ti 0.04-0.08 wt%,
Al 0.02-0.06 wt%,
N < 80 ppm wt%;and rest Fe, which is selectively processed through thin slab caster route to ensure uniform fine grain ferritic microstructure, improved rollability to achieve desired end properties including HER>100%.

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 Ti but upper limit is restricted because of poor effect on weldability&castability particularly when processed through thin slab caster.Carbon level above 0.08% is particularly prone to surface defect such as longitudinal and transverse corner cracking in thin slab caster. Also high Carbon badly affected productivity by reducing the casting speed in thin slab casting process.

Mn: 0.50-1.50: Mn is an important element for solid solution strengthening, but upper values are restricted because of its poor effect on weldability. Higher Mn increases rolling load significantly restricted the rolling of thinner gauze. Centre line segregation is another major issue with increasing % Mn. Increasing Mn content increases the cost of production of steel.

Si: <0.04: Si is beneficial in terms of achieving higher tensile strength. However, it has detrimental effect on surface quality of the hot rolled strip by formation of sticky scale. Also Si level 0.04-0.15 is very detrimental to hot dip galvanizing property. So Si is restricted to 0.04 % max.

Al: 0.02-0.06: Al is a strong deoxidizer. To kill the steel fully, minimum 0.02 % is required to be maintained. Al is also a nitride forming element which combines the unwanted Nitrogen present in the steel. It also acts as a grain refiner in steel. Upper limit is limited by detrimental effect on surface quality &castabilityin thin slab caster.

N<80 ppm: Nitrogen is a key element with significant role in formation of nitride and conbonitride precipitates. But to get the advantage of Ti in strengthening, N content should be as low as possible as with higher N, Ti effectiveness will be reduced by forming coarse TiN precipitates.

Ti: 0.04-0.08: Ti is essential for strengthening by precipitation TiC. Ti at higher temperature during continuous casting form TiN precipitates. The remaining left out Ti(#Ti eff) combines with C to form TiC precipitates at run out table[ROT]. This TiC actually contributes to strengthening of steel. Higher limit of Ti is restricted as it will increase the cost and also mechanical properties will be much higher which will restrict the thinner gauze rolling.

[#Ti eff = Ti total- 4.3 *N]

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

Above composition of HSLA steel as targeted leads to carbon equivalent(CE) in the range of 0.22 to 0.25 ensuring improved weldability.

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.

Stretch flange formability was evaluated by the Hole Expansion Test as per the ISO 16630:2009. A 100 mm square sheet having a 10 mm diameter hole drilled in the center were used. The hole was expanded by a cylindrical punch until crack appeared at the hole edge. The hole expansion ratio was evaluated using the following formula

%HER = (Df _ Di)/Di*100

Where Df is the final diameter and Di is the initial diameter.

Details of Process steps adopted for making the product according to present invention are as follows:

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 >1100DegC. Descaler was used after Tunnel Furnace to remove scales. 6 stand reductions were used to reduce the thickness to the required level with finish rolling temperature in the range of 850-900 DegC. After finishing, an ultra fast cooling was used in combination of standard laminar cooling to achieve Coiling temperature 580-620 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 850-900 Deg C;
VII. Standwise reduction/time/temp:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 45-50 50-55 35-40 30-35 25-30 20-25
Stand Entry Temp(°C) 1000-1020 960-970 940-950 925-935 910-915 890-900

VIII. Coiling Temp 580-620 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 are illustrated with the following example and Table I to III:

Comparative data are given in Example 4 and 5. In Example 4, Ti was low, below 0.04 and mechanical properties (YS and UTS does not meet the requirement), whereas Example 5, the steel was made with Nb microalloying, where although mechanical properties were achieved but minimum 3 mm thickness able to roll.

Example I:

Chemistry and Mechanicalproperties of Hot rolled steel:

Table 1:Steel Composition

Sample ID C % Mn % Si % N (ppm) Ti tot % Ti eff % % Nb P % S % Al % Ca (ppm) CE (IIW)
Example 1 0.054 0.95 0.03 60 0.05 0.024 0.013 0.004 0.03 27 0.217
Example 2 0.046 1.15 0.025 65 0.060 0.032 0.010 0.005 0.032 24 0.242
Example 3 0.048 1.03 0.022 68 0.073 0.044 0.010 0.005 0.035 29 0.223
Example 4 0.057 1.17 0.029 62 0.037 0.014 0.016 0.004 0.034 30 0.257
Example 5 0.055 1.15 0.22 72 - - 0.032 0.012 0.006 0.032 26 0.283

Table 2: Process parameters and mechanical properties

Sample ID Thickness CT FT YS UTS % El
%HER
Example 1 2.5 581 900 460 525 28 150
Example 2 2.5 585 895 480 540 26 130
Example 3 2.5 582 890 530 600 24 105
Example 4 2.5 586 892 420 480 28 120
Example 5 3.0 550 910 475 540 27 95

Microstructure of Hot rolled Steel:

Microstructure of newly developed steel composed of fine ferrite having ASTM grain size number of 10.5(average grain diameter is approx. 10 micron) are shown in Fig 1 (a) and 1(b).Due to this fine and almost single ferrite grain, the steel exhibit high strength & elongation along with high HER.

Accompanying Figure 1(a) shows the image of microstructure of newly developed HSLA steel in optical microscope and 1(b)shows the corresponding images of microstructures in scanning electron microscope.

It is thus possible by way of the present invention to provide low carbon HSLA hot rolled steel sheets with selective microalloying having high strength & elongation along with improved HER 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.