CLIAMS:We Claim:
1. Hot rolled low carbon steel comprising:

C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0.01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt.;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt.preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe,
with improved ductility by way of strain hardening index of > 0.18 and % elongation in the range of 38-44.

2. Hot rolled low carbon steel as claimed in claim 1 comprising of:
Yield Strength = 240-320 MPa;
UTS = 340-410 MPa;
n value in the range of 0.18-0.22.

3. Hot rolled low carbon steel as claimed in anyone of claims 1 or 2 which is obtained in the form of coils having:
Coil Thickness in the range of: 1.6 – 3.5 mm;
Coil width in the range of : 900- 1750 mm.

4. Hot rolled low carbon steel as claimed in anyone of claims 1 to 3 adapted for higher reduction ratio of even >90% free of local necking and tearing.
5. Hot rolled low carbon steel as claimed in anyone of claims 1 to 4 comprising boron nitride involving boron tied up to any free nitrogen enabling reduced tensile strength and work hardening rate and increased ductility.

6. Hot rolled low carbon steel as claimed in anyone of claims 1 to 5 comprising microstructure involving uniformly distributed fine equiaxed grains.

7. A process for the manufacture of hot rolled low carbon steel as claimed in anyone of claims 1 to 6 comprising:
(i) selectively involving alloying elements and with Boron addition after complete de-oxidation in steel making ;
(ii)continuous slab casting;
(iii) hot charging;
(iv) re-heating followed by cooling, such as to obtain strain hardening index of > 0.18 and % elongation in the range of 38-44.

8. A process as claimed in claim 7 wherein said alloying element involved comprises :

C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0/01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt. preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe.

9. A process for the manufacture of hot rolled low carbon steel as claimed in anyone of claims 7 to 8 wherein said step of hot rolling comprises controlled hot rolling maintaining:
Furnace Temperature (Soaking) in the range of 1100-1220 oC;
Finish Rolling Temperature in the range of 860 –900 oC; and
Coiling Temperature in the range of 650 – 700 oC.

10. A process for the manufacture of hot rolled low carbon steel as claimed in anyone of claims 7 to 9 wherein small amount of boron 20 to 40 ppm preferably about 30 ppm is added to tie up any free nitrogen in the steel as boron nitride thereby reducing tensile strength and work hardening rate and increase ductility.

11. Cold rolled galvanized sheet comprising hot rolled low carbon steel having
C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0/01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt. preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe with strain hardening index of > 0.18 and % elongation in the range of 38-44.

12. Cold rolled galvanized sheet as claimed in claim 10 having uniformly distributed fine equiaxed grains of Grain size ASTM 8 or finer and a low thickness in the range of 1.6 to 3.5mm.

Dated this the 3rd day of January, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
,TagSPECI:FIELD OF THE INVENTION
The present invention relates to providing a hot rolled low carbon steel grade with high cold reducibility suitable for producing low thickness galvanised sheets and a process for producing such grade of steel. The present invention claims the advancement of achieving higher reduction ratios during subsequent cold rolling operations making thinner sheets available for galvanizing through the use of developed grade of steel having a composition comprising optimum combination of carbon, manganese and boron. Importantly, in the present invention a small amount of boron is added to pin the nitrogen and improve the strain hardening index. The purpose of boron addition is to increase strain hardening. In the present instance, the boron is added to deliberately tie up any free nitrogen in the steel as boron nitride, thereby reducing tensile strength and work hardening rate and increasing ductility. Such unique combination of properties with high strain hardening exponent (n) and high total elongation in the developed steel grade has resulted in higher percentage of cold reducibility with reduction ratio>90% of hot rolled coils. Advantageously, the developed C-Mn-B steel according to the present invention is far less expensive than alloy steels of equivalent properties favouring prospects of wide industrial application.

BACKGROUND OF THE INVENTION

Galvanized Sheets are used extensively in various applications in agriculture, automobile, construction, domestic, and furniture. The demand of thinner and thinner galvanised sheets is constantly increasing. The thickness of the sheets to be galvanised is restricted by possible reduction during cold rolling. General purpose galvanised sheets are manufactured from plain carbon steels and are subjected to reduction ratios upto 85% during cold rolling before being galvanised. If subjected to higher reduction ratios >90 % regular plain carbon steels show local necking and tearing due to excessive stretching. Also controlling the uniform sheet thickness along the entire length of the coil is very important to ensure that the product will perform consistently during the processing by the end user.

Improvement in cold reducibility can also be measured from increase in ‘n’ value. The constant n plays a crucial role in sheet metal forming. The larger the n value, the more the material can elongate before necking. Thus, as the n value increases, the material's resistance to necking increases, and the material can be stretched farther before necking starts.

The n value is obtained by conducting a simple tensile test in which the specimen is stretched until it fractures. In tensile testing, tensile load is recorded as a function of the increase in gauge length. When load-elongation data is converted to engineering stress and strain, a curve can be plotted. If the results of tensile testing are to be used to predict how a metal will behave under other forms of loading, the data is plotted in terms of true stress and true strain. The n value is then obtained by calculating the slope of the true stress and true strain curve.

There has been therefore a need in the art to developing a steel grade which would ensure improved cold reducibility such that thinner and uniform section of steel sheets can be produced to meet the requirement of end users. It is an attempt by way of the present invention to provide a steel grade developed with selective composition and controlled processing parameters to ensure a unique combination of properties with high strain hardening exponent (n) and high total elongation resulting in higher percentage of cold reducibility of hot rolled coils.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to providing hot rolled low carbon steel grade with high cold reducibility for producing galvanized sheets and a process for its production.

A further aspect of the present invention is directed to provide a simple composition of boron added plain carbon steel for manufacturing low thickness galvanised sheets after cold rolling (reduction ratio > 90%).

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility which would be free from local necking and tearing due to excessive stretching when subjected to high reduction ratio.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility which would enable controlling the uniform sheet thickness along the entire length of the coil to ensure that the product will perform consistently during the processing by the end user.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility which would have a unique combination of properties with high strain hardening exponent (n) and high total elongation resulting in higher percentage of cold reducibility of hot rolled coils.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility which would be having optimum combination of carbon, manganese and boron such that the developed C-Mn-B steel is far less expensive than alloy steels of equivalent properties.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility such that the hot rolled steel with low carbon and low manganese in combination with boron can be used as a grade most suitable for manufacturing low thickness galvanised sheets after cold rolling.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility which would have lower yield point elongation and therefore have better dimensional control during cold rolling.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility having low values of carbon and Manganese in the steel resulted into consistent and adequate formability required by component manufacturers.

A still further object of the present invention is directed to provide hot rolled low carbon steel grade with high cold reducibility wherein due to minimum usage of various ferroalloys, the cost of production of the new grade steel is kept at minimum.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is thus directed to hot rolled low carbon steel comprising:

C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0.01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt.;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt.preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe,
with improved ductility by way of strain hardening index of > 0.18 and % elongation in the range of 38-44.

A further aspect of the present invention is directed to said hot rolled low carbon steel comprising of:
Yield Strength = 240-320 MPa;
UTS = 340-410 MPa;
n value in the range of 0.18-0.22.

A still further aspect of the present invention is directed to said hot rolled low carbon steel which is obtained in the form of coils having:
Coil Thickness in the range of: 1.6 – 3.5 mm;
Coil width in the range of : 900- 1750 mm.

A still further aspect of the present invention is directed to said hot rolled low carbon steel adapted for higher reduction ratio of even >90% free of local necking and tearing.

Yet another aspect of the present invention is directed to said hot rolled low carbon steel comprising boron nitride involving boron tied up to any free nitrogen enabling reduced tensile strength and work hardening rate and increased ductility.

A further aspect of the present invention is directed to said Hot rolled low carbon steel as comprising microstructure involving uniformly distributed fine equiaxed grains.

A still further aspect of the present invention is directed to said process for the manufacture of hot rolled low carbon steel comprising:
(i) selectively involving alloying elements and with Boron addition after complete de-oxidation in steel making ;
(ii)continuous slab casting;
(iii) hot charging;
(iv) re-heating followed by cooling, such as to obtain strain hardening index of > 0.18 and % elongation in the range of 38-44.

Yet another aspect of the present invention is directed to said process wherein said alloying element involved comprises :

C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0/01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt. preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe.

A further aspect of the present invention is directed to said process for the manufacture of hot rolled low carbon steel wherein said step of hot rolling comprises controlled hot rolling maintaining:
Furnace Temperature (Soaking) in the range of 1100-1220 oC;
Finish Rolling Temperature in the range of 860 –900 oC; and
Coiling Temperature in the range of 650 – 700 oC.

A still further aspect of the present invention is directed to a process for the manufacture of hot rolled low carbon steel wherein small amount of boron 20 to 40 ppm preferably about 30 ppm is added to tie up any free nitrogen in the steel as boron nitride thereby reducing tensile strength and work hardening rate and increase ductility.

A still further aspect of the present invention is directed to cold rolled galvanized sheet comprising hot rolled low carbon steel having
C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0/01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt. preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe with strain hardening index of > 0.18 and % elongation in the range of 38-44.

Yet another aspect of the present invention is directed to said Cold rolled galvanized sheet having uniformly distributed fine equiaxed grains of Grain size ASTM 8 or finer and a low thickness in the range of 1.6 to 3.5 mm.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

Figure 1: is the flow chart showing the steps involved in the process for the manufacture of hot rolled low carbon steel with high cold reducibility according to the present invention.

Figure 2: is the graphical presentation showing the lower yield point elongation achieved due to improvement in n-value in(b) the hot rolled low carbon steel grade according to the present invention in comparison with (a)the conventional/old grade.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURE

The present invention is directed to providing a hot rolled low carbon steel grade with high cold reducibility suitable for producing low thickness galvanised sheets and a process for producing such grade of steel. The developed grade is having a selective composition comprising optimum combination of carbon, manganese and boron. Importantly, in the present invention a small amount of boron is added to pin the nitrogen and improve the strain hardening index favouring improved cold reducibility of hot rolled coil.

The advancement by way of the present invention claims the benefit of achieving higher reduction ratios during subsequent cold rolling operations making thinner sheets available for galvanizing through the use of newly developed grade of steel having optimum combination of carbon, manganese and boron. In the present invention a small amount of boron 20 to 40 ppm and preferably 30 ppm was added to pin the nitrogen and improve the strain hardening index. The purpose of boron addition is to increase strain hardening which is the strengthening of a metal by plastic deformation. In this instance, the boron is added to deliberately tie up any free nitrogen in the steel as boron nitride, thereby reducing tensile strength and work hardening rate and increasing ductility. Increase in ductility can be measured from increase in % elongation from 37-38 in similar category steel to 42-45 in the boron added steel. Improvement in cold reducibility can also be measured from increase in ‘n’ value. The constant n plays a crucial role in sheet metal forming. The larger the n value, the more the material can elongate before necking. Thus, as the n value increases, the material's resistance to necking increases, and the material can be stretched farther before necking starts. In the present work ‘n’ value increase from 0.17 in similar category steels to 0.20 in the new grade. A unique combination of properties with high strain hardening exponent (n) and high total elongation has resulted into higher percentage of cold reducibility of hot rolled coils. The developed C-Mn-B steel is far less expensive than alloy steels of equivalent properties.
The new hot rolled low carbon steel grade with high cold reducibility adapted to achieve reduction ratio > 90%, is produced according to the present invention having the selective chemical composition as given in following Table I:

Table I:
%-C %-Mn %-S max. %-P max. %-Si max %-Al %-N, max. B Balance
0.015-0.050 0.10-0.30 0.015 0.025 0.025 0.020-0.065 0.0060 10-35 ppm Fe

While deciding on the selective composition of the hot rolled low carbon steel grade to achieve high cold reducibility, desired microstructure and combination of mechanical properties, following considerations were taken into account:

C: 0.01 % or more and less than 0.06% by weight

Carbon is an element that gives the strength and hardness to steel. The carbon content needs to be 0.01% or more to ensure a required strength in rolled condition. At a C content of 0.06% or more, steel becomes hard to be rolled. Accordingly, the C content is 0.01% or more and less than 0.06% and preferably in a range of 0.015 to 0.050% by weight.

Si: less than 0.025 % by weight

Si is added as de-oxidiser and acts to reinforce steel, and a necessary amount of Si is added to steel in accordance with the intended strength of the steel. Incorporation of excess Si exceeding 0.025 % significantly deteriorates toughness. Accordingly, the Si content is 0.025 % or less and is preferably in a range of 0.015 to 0.025% by weight.

Mn: 0.1% or more and 0.45% or less by weight

Manganese is an element that improves the strength of steel material and 0.1% or more of Mn needs to be contained to ensure a required strength. In contrast, the formability is deteriorated if Mn is contained exceeding 0.045%. Accordingly, the Mn content is 0.1% or more and 0.45% or less and preferably in a range of 0.1 to 0.3% by weight.

P: 0.025% or less by weight

P in steel in an amount of larger than 0.025 % by weight will have some negative influences on the desired strength of the steel sheets. Accordingly, the P content is 0.025% or less and preferably less than 0.015% by weight.

S: 0.015% or less by weight

At a sulfur content exceeding 0.015%, strength of rolled steel is deteriorated. Accordingly, the S content is restricted to 0.015% or less and preferably less than 0.010% by weight.

N: not larger than 0.006 % by weight

More desirably, N should be as lesser as possible for better mechanical properties of the steel. N in steel in an amount of not larger than 0.006 % by weight would not have any significant negative influences on the strength and other properties of the steel. Therefore, the N content of steel is defined to be not larger than 0.006 % by weight, but preferably smaller than 0.005 % by weight.

Al: 0.020% or more and 0.065% or less by weight

Aluminium is an element needed in deoxidization during steel making and refines grain size. The Al content needs to be 0.020% or more to achieve this effect. Exceeding Al content greater than 0.065 % impacts the mechanical properties. Thus, the Al content is 0.020% or more and 0.065% or less preferably in a range of 0.02 to 0.055% by weight.

B: 0.0010% or more and 0.0035% or Less by weight

Boron is an element needed to increase the strength or specifically strain hardening index. The B content needs to be 0.0010% or more to fully bring this effect. At a B content exceeding 0.0035%, the toughness is deteriorated. Accordingly, when B is to be contained, the B content is 0.0010% or more and 0.0035% or less and preferably in a range of 0.0010 to 0.0030% by weight.
The balance is Fe and unavoidable impurities.
This new steel grade is made through converter steel making and ladle heating furnace. It is further cast into slabs through continuous casting process. These slabs are processed through re-heating furnace and hot rolling followed by controlled cooling. Samples are collected from the coils in longitudinal and transverse direction. These samples are tested in laboratory for cleanliness of steel and mechanical properties.

Thus the hot rolled low carbon steel with improved cold reducibility having above composition has been produced according to the present invention using the following process rout in sequence:
(i) Steel making by LD Converter.
(ii) Secondary steel making: Ladle Heating Furnace.
(iii) Boron addition after complete de-oxidation.
(iv) Continuous slab casting.
(v) Hot Charging.
(vi) Re-heating, Hot rolling, ROT cooling and Coiling with set optimum processing parameters.

Accompanying Figure 1 is the flow chart showing the steps involved in the process for the manufacture of hot rolled low carbon steel with high cold reducibility according to the present invention.

Thus the process for the manufacture of hot rolled low carbon steel with improved cold reducibility (Reduction ratio>90%) according to the present invention comprised the steps of:
(i) selectively involving alloying elements in said steel making to obtain the composition as given above with Boron addition after complete de-oxidation comprising (a) Primary Steel Making with Blowing in LD converter and tapping into steel ladles and (b) Secondary Steel Making with Alloying addition, composition and temperature control;
(ii) continuous slab casting;
(iii) hot charging of slabs directly into re-heating furnace;
(v) Re-heating followed by rough rolling such as to obtain uniformly distributed fine equiaxed grains under hot rolling ; and
(vi) Finish rolling, ROT cooling and coiling.
In the above process, the new hot rolled steel with low carbon and low manganese in combination with boron which can be used for manufacturing low thickness galvanised sheets after cold rolling, are produced using the hot and cold rolling parameters specified for processing are as given in following Table 2:

Table 2:
1 Furnace Temperature (Soaking) 1100 – 1220 oC
2 Finish Rolling Temperature 860 - 900 oC
3 Coiling Temperature 650 - 700 oC

The hot rolled low carbon steel with high cold reducibility is obtained in the form of coils having:
Coil Thickness in the range of: 1.6 – 3.5 mm;
Coil width in the range of: 900- 1750 mm;

As already stated, the samples are collected from the coils in longitudinal and transverse direction. These samples are tested in laboratory for cleanliness of steel and mechanical properties.

Mechanical tests and Metallography

The tensile properties (yield strength, ultimate tensile strength) are measured using test specimens with 50 mm gauge length, fitted with a class 1 extensometer on a universal testing machine. The tensile test was conducted as per JIS Z2241 with sample number 5 as per JIS Z2201. The strain rate was kept 5mm/sec up to yield point. All tests are performed at room temperature.

The n value is obtained by conducting a simple tensile test in which the specimen is stretched until it fractures. In tensile testing, tensile load is recorded as a function of the increase in gauge length. When load-elongation data is converted to engineering stress and strain, a curve can be plotted. If the results of tensile testing are to be used to predict how a metal will behave under other forms of loading, the data is plotted in terms of true stress and true strain. The ‘n’ value is then obtained by calculating the slope of the true stress and true strain curve.
The improvement in ‘n’ value achieved is as follows:

Conventional Grade New Grade
‘n’ value 0.16-0.18 0.18-0.22

New grade has lower yield point elongation and therefore have better dimensional control during cold rolling. Improvement in yield point elongation in the new grade in comparison to old grade is as shown in accompanying Figure 2.

The new hot rolling grade steel with boron can be subjected to higher reduction ratios (> 90%) during cold reduction. The superior ‘n’ - value of the material allows easy rolling to low thicknesses.
The improvement in strain hardening exponent (n-value) in the new hot rolled steel grade with low carbon and low manganese in combination with boron as compared to the comparable old grades are illustrated in the following table 3:

Table 3:

%-C %-Mn %-S max. %-P max. %-Si max %-Al %-N, max. B n-value Remarks
0.03 0.21 0.008 0.007 0.012 0.04 0.005 25 0.22 Invention
0.034 0.19 0.006 0.01 0.01 0.042 0.0045 22 0.21 Invention
0.034 0.2 0.007 0.009 0.02 0.041 0.004 1 0.17 Comparison
0.032 0.19 0.008 0.008 0.011 0.055 0.0044 1 0.16 Comparison
0.033 0.17 0.005 0.01 0.02 0.054 0.0037 1 0.16 Comparison

Metallographic analysis is carried to rate the cleanliness of steel. Metallographic samples prepared are polished and etched with 5% nital. A simple light optical microscope is used to record the size of the grains comprising the material. Microstructure of the newly developed steel helped achieve a favourable combination of mechanical properties. Microstructure is found to be uniformly distributed fine equiaxed grains having Grain size ASTM 8 or finer.

The hot rolled low carbon low manganese boron added steel with improved cold reducibility produced following the process according to the present invention found to have unique combination of properties with high strain hardening exponent (n) and high total elongation in the developed steel grade has resulted in higher percentage of cold reducibility with reduction ratio >90% of hot rolled coils, having following properties:
Yield Strength = 240-320 MPa;
UTS = 340-410 MPa;
% Elongation = 38-44;
Strain hardening Index (n-value) > 0.18.
It is thus possible by way of the present invention to providing hot rolled low carbon steel grade with high cold reducibility suitable for producing low thickness galvanized sheets and a process for its production, wherein optimum combination of carbon, manganese and boron in said steel grade favoured achieving a unique combination of properties with high strain hardening exponent (n) and high total elongation has resulted into higher percentage of cold reducibility of hot rolled coils. The new steel grade with improved cold reducibility which is most suitable for manufacturing of low thickness galvanised sheets after cold rolling free of local necking and tearing due to excessive stretching, offers the following advantageous features ensuring wide industrial applicability:
1. The new hot rolled steel with low carbon and low manganese in combination with boron can be used for manufacturing low thickness galvanised sheets after cold rolling.
2. New grade has lower yield point elongation and therefore have better dimensional control during cold rolling.
3. Improvement in yield point elongation with higher n-value in the new grade in comparison to old grade is effectively achieved.
4. The new hot rolling grade steel with boron can be subjected to higher reduction ratios (> 90%) during cold reduction.
5. The superior n - value of the material allows easy rolling to low thicknesses.
6. Addition of boron helped in pinning the dissolved nitrogen primarily responsible for reduction ratios.
7. Boron addition also helps in controlling the uniform sheet thickness along the entire length of the coil.
8. Optimum chemistry of the steel resulted in consistent and favourable mechanical properties suitable for galvanised sheets.
9. Judicious selection of alloying elements in combination with boron helped achieve high reduction ratios during cold rolling without compromising the strength.
10. Low values of carbon and Manganese in the steel resulted into consistent and adequate formability required by component manufacturers.
11. Microstructure of the newly developed steel helped achieve a favourable combination of mechanical properties.
12. The new grade of steel possesses a favourable combination of mechanical properties and surface quality suitable for galvanised sheets.
13. Due to minimum usage of various ferroalloys, the cost of production of the new grade steel is kept minimum.
We Claim:
1. Hot rolled low carbon steel comprising:

C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0.01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt.;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt.preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe,
with improved ductility by way of strain hardening index of > 0.18 and % elongation in the range of 38-44.

2. Hot rolled low carbon steel as claimed in claim 1 comprising of:
Yield Strength = 240-320 MPa;
UTS = 340-410 MPa;
n value in the range of 0.18-0.22.

3. Hot rolled low carbon steel as claimed in anyone of claims 1 or 2 which is obtained in the form of coils having:
Coil Thickness in the range of: 1.6 – 3.5 mm;
Coil width in the range of : 900- 1750 mm.

4. Hot rolled low carbon steel as claimed in anyone of claims 1 to 3 adapted for higher reduction ratio of even >90% free of local necking and tearing.
5. Hot rolled low carbon steel as claimed in anyone of claims 1 to 4 comprising boron nitride involving boron tied up to any free nitrogen enabling reduced tensile strength and work hardening rate and increased ductility.

6. Hot rolled low carbon steel as claimed in anyone of claims 1 to 5 comprising microstructure involving uniformly distributed fine equiaxed grains.

7. A process for the manufacture of hot rolled low carbon steel as claimed in anyone of claims 1 to 6 comprising:
(i) selectively involving alloying elements and with Boron addition after complete de-oxidation in steel making ;
(ii)continuous slab casting;
(iii) hot charging;
(iv) re-heating followed by cooling, such as to obtain strain hardening index of > 0.18 and % elongation in the range of 38-44.

8. A process as claimed in claim 7 wherein said alloying element involved comprises :

C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0/01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt. preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe.

9. A process for the manufacture of hot rolled low carbon steel as claimed in anyone of claims 7 to 8 wherein said step of hot rolling comprises controlled hot rolling maintaining:
Furnace Temperature (Soaking) in the range of 1100-1220 oC;
Finish Rolling Temperature in the range of 860 –900 oC; and
Coiling Temperature in the range of 650 – 700 oC.

10. A process for the manufacture of hot rolled low carbon steel as claimed in anyone of claims 7 to 9 wherein small amount of boron 20 to 40 ppm preferably about 30 ppm is added to tie up any free nitrogen in the steel as boron nitride thereby reducing tensile strength and work hardening rate and increase ductility.

11. Cold rolled galvanized sheet comprising hot rolled low carbon steel having
C- 0.010-0.06 % by wt. preferably 0.015-0.050% by wt.;
Mn-0/01- 0.45 % by wt. preferably 0.10-0.30% by wt.;
S- Upto 0.015% by wt. preferably < 0.010 % by wt;
P- Upto 0.025 % by wt. preferably < 0.015 % by wt.;
Si-Upto 0.025 % by wt preferably 0.015- 0.025% by wt.;
Al-0.020-0.065% by wt.preferably in the range of 0.02- 0.055 % by wt;
N- Upto 0.0060% by wt.preferably upto 0.0050 % by wt.;
B- 10-40 ppm preferably 10-35 ppm in the range of 0.0010 to 0.0035% by wt. preferably in the range of 0.0010-0.0030% by wt.: and
balance being Fe with strain hardening index of > 0.18 and % elongation in the range of 38-44.

12. Cold rolled galvanized sheet as claimed in claim 10 having uniformly distributed fine equiaxed grains of Grain size ASTM 8 or finer and a low thickness in the range of 1.6 to 3.5mm.

Dated this the 3rd day of January, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT
TITLE: HOT ROLLED LOW CARBON STEEL WITH HIGH COLD REDUCIBILITY AND A PROCESS FOR ITS PRODUCTION.
The present invention relates to providing a hot rolled low carbon steel grade with high cold reducibility suitable for producing low thickness galvanised sheets and a process for producing such grade of steel. The present invention claims the advancement of achieving higher reduction ratios during subsequent cold rolling operations making thinner sheets available for galvanizing through the use of developed low cost grade of steel having a composition comprising optimum combination of carbon, manganese and boron. In the hot rolled low carbon steel grade, a small amount of boron is added to pin the nitrogen and improve the strain hardening index. The boron is added to deliberately tie up any free nitrogen in the steel as boron nitride, thereby reducing tensile strength and work hardening rate and increasing ductility. Such unique combination of properties with high strain hardening exponent (n) and high total elongation resulted in higher percentage of cold reducibility with reduction ratio>90% of hot rolled coils.
(Figure 1)