CLIAMS:We Claim:

1. Hot rolled low carbon steel comprising

C:0.02 % by wt to < 0.08% by wt. preferably 0.025 to 0.055% by wt.;
Mn:0.1 % by wt. to upto 0.4 % by wt preferably 0.10 to 0.30 % by wt.;
S:upto 0.020% by wt.preferably < 0.010 % by wt.;
P:upto 0.025% by wt.preferably <0.015 % by wt.;
Al: 0.010 % by wt. to upto 0.050 % by wt. preferably 0.010 to 0.045% by wt.;
N: upto 0.0060%by wt.preferably < 0.005 % by wt.;
B:20 to 40ppm based on 0.0020 % by wt. to 0.0050 % by wt. preferably 0.0020 % by wt. to 0.0040 % by wt.; and
Balance is Fe.
having controlled yield point elongation suitable for direct drawing applications free of stretcher strain marks.
2. Hot rolled low carbon steel as claimed in claim 1 in the form of coil having
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm.
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40; and
sheet coil having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.
3. Hot rolled low carbon steel for direct drawing application as claimed in anyone of claims 1 or 2 having yield point elongation of <2% or no yield point elongation.
4. Hot rolled low carbon steel as claimed in anyone of claims 1 to 3 comprising substantially transformed soluble content of N to Boron Nitride enabling desired yield point elongation.
5. A process for the production of Hot rolled low carbon steel as claimed in anyone of claims 1 to 4 comprising:
carrying out steel making involving low carbon and low manganese in combination with boron involving a selective alloying combination of

C:0.02 % by wt to < 0.08% by wt. preferably 0.025 to 0.055% by wt.;
Mn:0.1 % by wt. to upto 0.4 % by wt preferably 0.10 to 0.30 % by wt.;
S:upto 0.020% by wt.preferably < 0.010 % by wt.;
P:upto 0.025% by wt.preferably <0.015 % by wt.;
Al: 0.010 % by wt. to upto 0.050 % by wt. preferably 0.010 to 0.045% by wt.;
N: upto 0.0060%by wt.preferably < 0.005 % by wt.;
B:20 to 40ppm based on 0.0020 % by wt. to 0.0050 % by wt. preferably 0.0020 % by wt. to 0.0040 % by wt.; and
Balance is Fe;

Wherein the said Boron is involved in the level of 20-40 ppm whereby substantially the soluble N content is controlled by conversion to precipitated boron nitride enabling desired yield point eleongation.

6. A process as claimed in claim 5 comprising
(i) selectively involving said alloying elements in said steel making comprising (a) primary steel making involving blowing in LD converter and tapping into steel ladles and (b) secondary steel making involving alloying addition, composition and temperature control;
(ii) continuous slab casting;
(iii) re-heating of slabs by hot charging directly into re-heating furnace followed by rough rolling; and
(iv) finish rolling, ROT cooling and coiling.

7. A process as claimed in anyone of claims 5 or 6 comprising controlled operating parameters involving:
Furnace Temperature (Soaking) in the range of 1180-1220 oC;
Finish Rolling Temperature in the range of 840-890 oC;
Coiling Temperature in the range of 600 – 660 oC.

8. A process as claimed in claim 7 for the production of said hot rolled low carbon steel obtained in the form of a sheet coil involving :
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm.
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40; and
said sheet coil thus obtained having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

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 low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability and a process for production of such steel grade. More particularly, the present invention is directed to providing hot rolled steel with low carbon and low manganese in combination with boron addition. Advantageously selective boron addition reduces soluble nitrogen content in steel by nitride formation and precipitation, favouring its use in direct drawing applications such as automobile components, compressor shells etc. free of stretcher strain marks/coil break defects, without the need of additional process such as tempered rolling or stretch leveling, thus favouring wide application and use of such steel grade.

BACKGROUND OF THE INVENTION

It is well known in the related field that low carbon steels are generally one of the important categories of steels used for automotive sheet applications in cold rolled condition due to excellent formability. These Simpler C-Mn steels in the cold rolled and annealed condition have moderately high strength levels with excellent formability. However, these low carbon grades are susceptible to yield point elongation or Luder Bands or stretcher strains. It is important to avoid strain ageing, as this causes discontinuous yielding during subsequent deformation in the forming of exposed components. Such discontinuous yielding results in the formation of Luders Bands (also called stretcher-strain marks) with an unacceptable ripple in the surface finish of the formed part. Strain ageing is caused by interstitial carbon and nitrogen atoms migrating to dislocations and pinning them, thus increasing strength and reducing ductility. This ageing can take place at room temperature or may take several months for the strain ageing effect to occur. It is possible to prevent strain ageing in subsequent cold rolling process by the application of a small (typically 0.8-1.8%) cold rolling reduction (called temper rolling or skin pass) after annealing to introduce mobile dislocations that will enable a continuous yield to occur.
It is also a common experience that for certain drawing applications particularly thicker sections, hot rolled sheets are preferred in terms of cost savings. However thicker hot rolled coils are typically not subjected to skin pass or temper rolling. Hence for direct drawing applications, the same grade material frequently fails by yield point phenomenon resulting in coil break defects. Coil Breaks are creases or ridges in sheet that appear as parallel lines across the direction of rolling, and that generally extend the full width of the sheet or strip. Strain ageing can be avoided by ensuring that there is no free interstitial carbon or nitrogen, by reducing the total C and N content to very low levels (0.003%) through modern secondary steelmaking practices and through the addition of strong carbide and nitride forming elements. The existing known methods of hot rolling low carbon steel do not provide consistence suitable microstructure with improved surface properties for direct drawing.
There has been therefore a continuing need in the art to developing a cost effective low alloy hot rolled steel grade suitable for direct drawing application free of stretching strain marks or coil break defects without the need for temper rolling or skin pass to suit a variety of industrial applications.

OBJECTS OF THE INVENTION

The basic object of the present invention is thus directed to a low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability and a process for production of such steel grade.

A further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability having the benefit of reduction in coil break defects during subsequent direct drawing operations having optimum combination of carbon, manganese and boron.

A still further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability wherein the new grade can be used for direct drawing without the need of additional process such as tempered rolling or stretch levelling.

A still further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability wherein selective low carbon based C-Mn-B steel composition is far less expensive than other alloy steels of equivalent properties.

A still further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability wherein hot rolling of new low carbon steel grade would ensure consistent suitable microstructure with improved surface properties for direct drawing.

A still further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application with reduced yield point elongation and increased formability wherein Born is electively added to pin the nitrogen and improve the strain hardening index without any segregation in the austenite grain boundaries.

A still further object of the present invention is directed to a hot rolled steel grade with low carbon and low manganese in combination with boron for direct drawing application wherein a simple and cost effective composition with reduced yield point phenomenon and increased formability is proposed which is suitable for use by automobile component manufacturers for direct drawing application such as compressor shells.

A still further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application wherein said Boron-added steel exhibits very less or no yield point elongation, thus can eliminate the formation of strain marking (coil break) on the strip surface without the need of additional process such as tempered rolling or stretch levelling.

A still further object of the present invention is directed to a low carbon and low manganese hot rolled steel grade for direct drawing application wherein due to minimum usage of various ferroalloys, the cost of production of the new grade steel is kept minimum

SUMMARY OF THE INVENTION

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

C:0.02 % by wt to < 0.08% by wt. preferably 0.025 to 0.055% by wt.;
Mn:0.1 % by wt. to upto 0.4 % by wt preferably 0.10 to 0.30 % by wt.;
S:upto 0.020% by wt.preferably < 0.010 % by wt.;
P:upto 0.025% by wt.preferably <0.015 % by wt.;
Al: 0.010 % by wt. to upto 0.050 % by wt. preferably 0.010 to 0.045% by wt.;
N: upto 0.0060%by wt.preferably < 0.005 % by wt.;
B:20 to 40ppm based on 0.0020 % by wt. to 0.0050 % by wt. preferably 0.0020 % by wt. to 0.0040 % by wt.; and
Balance is Fe.
having controlled yield point elongation suitable for direct drawing applications free of stretcher strain marks.

A further aspect of the present invention is directed to said hot rolled low carbon steel as stated above in the form of coil having
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm;
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40; and
sheet coil having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

A still further aspect of the present invention is directed to said hot rolled low carbon steel for direct drawing application having yield point elongation of <2% or no yield point elongation.

Yet another aspect of the present invention is directed to said hot rolled low carbon steel comprising substantially transformed soluble content of N to Boron Nitride enabling desired yield point elongation.

A further aspect of the present invention is directed to a process for the production of said hot rolled low carbon steel comprising:
carrying out steel making involving low carbon and low manganese in combination with boron involving a selective alloying combination of

C:0.02 % by wt to < 0.08% by wt. preferably 0.025 to 0.055% by wt.;
Mn:0.1 % by wt. to upto 0.4 % by wt preferably 0.10 to 0.30 % by wt.;
S:upto 0.020% by wt.preferably < 0.010 % by wt.;
P:upto 0.025% by wt.preferably <0.015 % by wt.;
Al: 0.010 % by wt. to upto 0.050 % by wt. preferably 0.010 to 0.045% by wt.;
N: upto 0.0060%by wt.preferably < 0.005 % by wt.;
B:20 to 40ppm based on 0.0020 % by wt. to 0.0050 % by wt. preferably 0.0020 % by wt. to 0.0040 % by wt.; and
Balance is Fe.
Wherein the said Boron is involved in the level of 20-40 ppm whereby substantially the soluble N content is controlled by conversion to precipitated boron nitride enabling desired yield point eleongation.

A still further aspect of the present invention is directed to said process comprising
(i) selectively involving said alloying elements in said steel making comprising (a) primary steel making involving blowing in LD converter and tapping into steel ladles and (b) secondary steel making involving alloying addition, composition and temperature control;
(ii) continuous slab casting;
(iii) re-heating of slabs by hot charging directly into re-heating furnace followed by rough rolling; and
(iv) finish rolling, ROT cooling and coiling.

Yet another aspect of the present invention is directed to said process comprising controlled operating parameters involving:
Furnace Temperature (Soaking) in the range of 1180-1220 oC;
Finish Rolling Temperature in the range of 840-890 oC;
Coiling Temperature in the range of 600 – 660 oC.
A further aspect of the present invention is directed to said process for the production of said hot rolled low carbon steel obtained in the form of a sheet coil involving :
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm.
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40; and
Said sheet coil thus obtained having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1: is the flow chart of the process for production of hot rolled low carbon low manganese boron added steel according to the present invention showing the steps involved in sequence.
Figure 2: is the graphical plot of load vs axial strain showing improvement in yield point elongation(< 2%) (b) in the new grade with low carbon low manganese in combination with boron in comparison with(a) old grade (3 – 5 %) wherein Boron-added steel exhibits very less or no yield point elongation.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide a hot rolled steel grade with low carbon and low manganese in combination with Boron for direct drawing application with reduced yield point elongation and increased formability and a cost effective process for production of such steel grade which is suitable for application by automobile component manufacturers for direct drawing application with consistent and adequate formability required by component manufacturers with a favourable combination of mechanical properties and surface quality suitable for high-end products made through drawing and forming.

The present invention claims the benefit of reduction in coil break defects during subsequent direct drawing operations 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. If an excessive amount of boron (>40 ppm) is present, the boron constituents become segregated in the austenite grain boundaries, which not only lowers hardenability, but also may decrease toughness, cause embitterment and produce hot shortness. The effect of boron on hardenability also depends on the amount of carbon in the steel. The existing known methods of hot rolling low carbon steel do not provide consistent suitable microstructure with improved surface properties for direct drawing. The new grade can be used for direct drawing without the need of additional process such as tempered rolling or stretch levelling. The developed C-Mn-B steel is far less expensive than alloy steels of equivalent properties.

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.

The selective chemical composition of the new hot rolling steel grade for direct drawing application with minimized yield point elongation according to the present invention is as given in the following table 1.

Table 1:

%-C %-Mn %-S max. %-P max. %-Si max %-Al %-N, max. B ppm
0.025-0.055 0.10-0.30 0.020 0.025 0.030 0.010-0.045 0.0060 20-40

While deciding on the selective composition of the hot rolled steel grade with low carbon and low manganese in combination with Boron for direct drawing application with minimized yield point eleongation, desired microstructure and combination of mechanical properties, following considerations were taken into account:

C: 0.02 % or more and less than 0.08% by weight

Carbon is an element that gives the strength and hardness to steel. The carbon content needs to be 0.02% or more to ensure a required strength in rolled condition. At a C content of 0.08% or more, steel becomes hard to be rolled. Accordingly, the C content is 0.02% or more and less than 0.08% and preferably in a range of 0.025 to 0.055% by weight.

Si: less than 0.03 % 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 0.03 % significantly deteriorates toughness. Accordingly, the Si content is 0.03 % or less and is preferably in a range of 0.015 to 0.025% by weight.

Mn: 0.1% or more and 0.4% 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 0.04%. Accordingly, the Mn content is 0.1% or more and 0.4% 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: 0.020% or less by weight

At a sulfur content exceeding 0.020%, strength of rolled steel is deteriorated. Accordingly, the S content is restricted to 0.020% 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: 0.010% or more and 0.050% or less

Aluminium is an element needed in deoxidization during steel making and refines grain size. The Al content needs to be 0.01% or more to achieve this effect. Exceeding Al content greater than 0.050 % impacts the mechanical properties. Thus, the Al content is 0.010% or more and and 0.050% or less preferably in a range of 0.02 to 0.045% by weight

B: 0.0020% or more and 0.0050% or Less

Boron is an element needed to increase the strength or specifically strain hardening index. The B content needs to be 0.0020% or more to fully bring this effect. At a B content exceeding 0.0050%, the toughness is deteriorated. Accordingly, when B is to be contained, the B content is 0.0020% or more and 0.0050% or less and preferably in a range of 0.0020 to 0.0040% by weight.

The balance is Fe and unavoidable impurities.

Thus the hot rolled steel grade having above selective composition for direct drawing application has been produced 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) Re-heating, Hot rolling, ROT cooling and Coiling with set optimum processing parameters.

Accompanying Figure 1 is the flow chart of the process for production of hot rolled low carbon low manganese boron added steel for directed drawing application according to the present invention showing the steps involved in sequence.

The process for the manufacture of hot rolled steel sheet for direct drawing application according to the present invention comprised the steps of:
(i) selectively involving said alloying elements in said steel making comprising (a) primary steel making involving blowing in LD converter and tapping into steel ladles and (b) secondary steel making involving alloying addition, composition and temperature control;
(ii) continuous slab casting;
(iii) re-heating of slabs by hot charging directly into re-heating furnace followed by rough rolling; and
(iv) finish rolling, ROT cooling and coiling.

In the above process, hot rolling parameters specified for processing are as given in following Table 2:
Table 2:
1 Furnace Temperature (Soaking) 1180-1220 oC
2 Finish Rolling Temperature 840-890 oC
3 Coiling Temperature 600 – 660 oC

In the above process the hot rolled sheet coil material specification in supply condition is having:
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm.

The hot rolled coils are inspected manually. 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.

Metallographic analysis is carried out 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.

The hot rolled coils with minimized yield point elongation produced following the above process are having the following mechanical properties:
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40;
Grain Size: ASTM 8 or finer.

The microstructure of the sheet coils finally obtained is having uniformly distributed fine eqi-axed grains.

Reduction in yield point elengation in the new grade in comparison to similar category old grades as observed through various trials are as shown in the following table 3.
Table 3:
%-C %-Mn %-S max. %-P max. %-Si max %-Al %-N, max. B ppm YPE Remarks
0.035 0.21 0.009 0.01 0.01 0.029 0.005 32 <2% Invention
0.038 0.24 0.01 0.008 0.01 0.032 0.004 29 <2% Invention
0.034 0.22 0.014 0.009 0.01 0.058 0.0038 1 5% Comparison
0.034 0.25 0.014 0.016 0.006 0.051 0.0038 1 4% Comparison
0.033 0.17 0.012 0.013 0.014 0.06 0.0032 1 6% Comparison

It is apparent from the above data that improvement in yield point elongation in the new grade (< 2%) in comparison to old grade (3 – 5 %). Boron-added steel exhibits very less or no yield point elongation.

Accompanying Figure 2 illustrates the graphical plot of load vs axial strain showing improvement in yield point elongation of the order of < 2% in the new hot rolled steel grade with low carbon low manganese in combination with boron, as compared to the similar old grade having 3 – 5 % yield point elongation and thus Boron-added steel according to the present invention exhibits very less or no yield point elongation.

It is thus possible by way of the present invention to provide a hot rolled steel grade with low carbon and low manganese in combination with selective boron addition for direct drawing application wherein said Boron-added steel exhibits very less or no yield point elongation, thus can eliminate the formation of strain marking (coil break) on the strip surface without the need of additional process such as tempered rolling or stretch levelling. The new grade of steel is thus suitable for application and use by automobile component manufacturers requiring consistent and adequate formability such as for producing compressor shells without stretcher strain marks. The hot rolled steel grade and process for its production according to the present invention thus ensure the following advantageous features:

1. The new hot rolled steel with low carbon and low manganese in combination with boron can be directly used for direct drawing applications such as compressor shells without stretcher strain marks.
2. Improvement in yield point elongation in the new grade (< 2%) in comparison to old grade (3 – 5 %) is as shown below. Boron-added steel exhibits very less or no yield point elongation.
3. This result suggests that soluble N content, that causes the yield point elongation, is lower in the boron-added steel than that in the boron-free steel. Lower soluble content of N is caused by the tendency of boron that readily forms nitride and precipitates in steel.
4. Boron-added steel that exhibits no yield point elongation can eliminate the formation of strain marking (coil break) on the strip surface without the need of additional process such as tempered rolling or stretch levelling.
5. The new hot rolling grade steel with boron reduced the coil break defects.
6. Addition of boron helped in pinning the dissolved nitrogen primarily responsible for coil breaks
7. Optimum chemistry of the steel resulted in consistent and favourable mechanical properties suitable for products made through drawing.
8. Judicious selection of alloying elements in combination with boron helped achieve high drawability of the steel without compromising the strength.
9. Low values of carbon and Manganese in the steel resulted into consistent and adequate formability required by component manufacturers.
10. Microstructure of the newly developed steel helped achieve a favourable combination of mechanical properties.
11. The new grade of steel possesses a favourable combination of mechanical properties and surface quality suitable for high-end products made through drawing and forming.
12. 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:0.02 % by wt to < 0.08% by wt. preferably 0.025 to 0.055% by wt.;
Mn:0.1 % by wt. to upto 0.4 % by wt preferably 0.10 to 0.30 % by wt.;
S:upto 0.020% by wt.preferably < 0.010 % by wt.;
P:upto 0.025% by wt.preferably <0.015 % by wt.;
Al: 0.010 % by wt. to upto 0.050 % by wt. preferably 0.010 to 0.045% by wt.;
N: upto 0.0060%by wt.preferably < 0.005 % by wt.;
B:20 to 40ppm based on 0.0020 % by wt. to 0.0050 % by wt. preferably 0.0020 % by wt. to 0.0040 % by wt.; and
Balance is Fe.
having controlled yield point elongation suitable for direct drawing applications free of stretcher strain marks.
2. Hot rolled low carbon steel as claimed in claim 1 in the form of coil having
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm.
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40; and
sheet coil having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.
3. Hot rolled low carbon steel for direct drawing application as claimed in anyone of claims 1 or 2 having yield point elongation of <2% or no yield point elongation.
4. Hot rolled low carbon steel as claimed in anyone of claims 1 to 3 comprising substantially transformed soluble content of N to Boron Nitride enabling desired yield point elongation.
5. A process for the production of Hot rolled low carbon steel as claimed in anyone of claims 1 to 4 comprising:
carrying out steel making involving low carbon and low manganese in combination with boron involving a selective alloying combination of

C:0.02 % by wt to < 0.08% by wt. preferably 0.025 to 0.055% by wt.;
Mn:0.1 % by wt. to upto 0.4 % by wt preferably 0.10 to 0.30 % by wt.;
S:upto 0.020% by wt.preferably < 0.010 % by wt.;
P:upto 0.025% by wt.preferably <0.015 % by wt.;
Al: 0.010 % by wt. to upto 0.050 % by wt. preferably 0.010 to 0.045% by wt.;
N: upto 0.0060%by wt.preferably < 0.005 % by wt.;
B:20 to 40ppm based on 0.0020 % by wt. to 0.0050 % by wt. preferably 0.0020 % by wt. to 0.0040 % by wt.; and
Balance is Fe;

Wherein the said Boron is involved in the level of 20-40 ppm whereby substantially the soluble N content is controlled by conversion to precipitated boron nitride enabling desired yield point eleongation.

6. A process as claimed in claim 5 comprising
(i) selectively involving said alloying elements in said steel making comprising (a) primary steel making involving blowing in LD converter and tapping into steel ladles and (b) secondary steel making involving alloying addition, composition and temperature control;
(ii) continuous slab casting;
(iii) re-heating of slabs by hot charging directly into re-heating furnace followed by rough rolling; and
(iv) finish rolling, ROT cooling and coiling.

7. A process as claimed in anyone of claims 5 or 6 comprising controlled operating parameters involving:
Furnace Temperature (Soaking) in the range of 1180-1220 oC;
Finish Rolling Temperature in the range of 840-890 oC;
Coiling Temperature in the range of 600 – 660 oC.

8. A process as claimed in claim 7 for the production of said hot rolled low carbon steel obtained in the form of a sheet coil involving :
Coil Thickness: 1.6 – 6.0 mm;
Coil width: 900- 1600 mm.
Yield Strength: 260 - 350 MPa;
UTS: 340 – 430 MPa;
% Elongation: 32 – 40; and
said sheet coil thus obtained having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

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

ABSTRACT

TITLE: HOT ROLLED LOW CARBON STEEL SHEETS FOR DIRECT DRAWING APPLICATION.

The present invention relates to hot rolled low carbon steel grade for direct drawing application with reduced yield point elongation and increased formability and a process for production of such steel grade. Importantly, the invention provides a hot rolled steel having selective composition comprising low carbon and low manganese in combination with boron addition and processed through controlled rolling parameters to produce sheet coils with desired properties including low yield point elongation. Advantageously, selective boron addition reduces soluble nitrogen content in steel by nitride formation and precipitation, favouring its use in direct drawing applications, free of stretcher strain marks/coil break defects, without the need of additional process such as tempered rolling or stretch leveling, thus favouring wide application and use of such steel grade in automobile components, compressor shells etc.
(Figure 1)