DESC:FIELD OF THE INVENTION

The present invention relates to High Strength Low Alloy steel sheet having yield strength of 300MPa minimum suitable for automotive structural parts like rear cross member and shock absorber and a method of producing the same through continuous annealing process. The steel having composition comprising 0.02Wt% to 0.07Wt% of Carbon; 0.3Wt% to 0.7 wt% of Manganese; 0.01 or less wt% of Sulphur; 0.020Wt% to 0.06 wt% of Aluminum; 0.006 or less wt% of Nitrogen; 0.008Wt% to 0.025 wt% of Niobium; and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 18 for excellent hole expansion. The slab having above composition is hot rolled , cold rolled , continuous annealed followed by overaging and skin pass elongation with selected process parameters to achieve minimum Yield strength of 300MPa and Total Elongation of 28 % or more with good Phosphatbility, hole expansion ratio and spot weldability. Cold rolled High Strength Low Alloy steel used in structural application with desired formability requirements, excellent hole expansion ratio.

BACKGROUND OF THE INVENTION

In press forming of automobile body components from sheet metal, Hole expansion ratio is an important factor to be considered. If hole expansion ratio is very less, formability of the sheet metal will be poor and crack tend to be generated at the cross section. In the present invention material is produced with high yield ratio to reduce the weight of the automobile component with high tensile strength and reducing the thickness of the material with good hole expansion ratio to prevent crack during the forming. The material is produced with good surface where excellent phosphatibility can be obtained with good spot weldability.

Granted Patent CN103131843 relates to automotive structural parts with high strength low alloy steel cold rolled sheet HC340LA stabilized continuous annealing process, belonging to the field of automotive steel production technology with yield strength minimum of 340Mpa by adding Niobium and titanium for strengthening by reducing the variation in mechanical properties within 30Mpa. According to the said prior art there arises the problem of accelerated ageing as steel strip is coiled at temperature of 565°C to 595°C and after annealing, when aged at 340°C to 380°C. The material produced using the chemistry disclosed patent CN103131843 which results in poor weldability, phosphatibility, Poor Ageing Resistance and very less hole expansion ratio.

There has been thus a need in the related filed to developing High Strength Low Alloy Steel with improved ageing guarantee and better weldability for automotive structural applications. Accelerated ageing prevention is important in this grade, because after production if yield strength is around 380 Mpa at the higher level and if end use of material is after 3 to 4 months then due to accelerated ageing yield strength will increase to 410 Mpa to 420Mpa, which results as poor spring back resistance formability during the component manufacturing.

The present invention aims to solve the problem of prior art by providing good ageing resistance for 6 months, prevents accelerated ageing with better weldability, hole expansion ratio and phosphatibility processed through continuous annealing process suitable for automotive structural applications. Hole expansion test is the most commonly used method to evaluate the edge cracking resistances. Hole Expansion is an observable fact that occurs during forming whenever the Component is withdrawn from tool set. Hole Expansion Ratio decreases with increase in Tensile strength of material.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide High Strength Low alloy Steel sheet having improved Hole Expansion Ratio, with better Spot weldability, excellent phosphatibility and a method for manufacturing the same through continuous annealing route.

A further object of the present invention is directed to provide High Strength Low alloy Steel sheet having selective composition along with processing through hot rolling, cold rolling, continuous annealing followed by temper rolling with selective parameters at each stage to ensure to obtain desired property with excellent surface to achieve phosphatibility , Hole expansion ratio and excellent spot weldability.

A still further object of the present invention is directed to provide High Strength Low alloy Steel sheet with Spot weldability wherein to achieve Carbon equivalent is 0.27 or less, cold rolled steel is continuously annealed with selected soaking temperature to achieve the desired properties.

Yet another object of the present invention is directed to provide High Strength Low alloy Steel sheet with Excellent phosphatibility and of producing the same wherein amount manganese addition is less than 0.7 and silicon addition is 0.035% or less where formation of oxides on the surface is restricted.

Yet another object of the present invention is directed to provide High Strength Low alloy Steel sheet with Excellent phosphatibility and of producing the same wherein amount manganese addition is less than 0.7 and silicon addition is 0.035% or less can be used for hot dip galvanizing and galvannealing.

A further object of the present invention is directed to provide High Strength Low alloy Steel sheet with high yield ratio and excellent hole expansion ratio to avoid cracking during the forming process.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a steel composition suitable for producing high strength low alloy cold rolled steel sheet through continuous annealed route comprising:
0.02Wt% to 0.07 wt% of Carbon;
0.3Wt% to 0.7 wt% of Manganese;
0.01 or less wt% of Sulphur;
0.020Wt% to 0.06 wt% of Aluminum;
0.006 or less wt% of Nitrogen;
0.008Wt% to 0.025 wt% of Niobium;
and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 18;
favouring producing steel sheets having anyone or more
a) Yield strength from 300-380 Mpa and Tensile Strength 380 Mpa or more;
b) Hole Expansion ratio of 80% to 95%;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

A further aspect of the present invention is directed to said steel composition comprising Ti; wherein composition of Ti is based on ratio of (Ti+Al)/N to be 12 to 35.

A still further aspect of the present invention is directed to said steel composition as claimed in anyone of claims 1 or 2, further comprising at least one element from Zr, Bi, V, Mo and Cr each by content in the range of 0.002 to 0.03 %.

A still further aspect of the present invention is directed to a high strength low alloy cold rolled continuous annealed steel sheet comprising :
0.02Wt% to 0.07 wt% of Carbon;
0.3Wt% to 0.7 wt% of Manganese;
0.01 or less wt% of Sulphur;
0.020Wt% to 0.06 wt% of Aluminum;
0.006 or less wt% of Nitrogen;
0.008Wt% to 0.025 wt% of Niobium;
and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 18;
and having anyone or more of
a) Yield strength from 300-380 Mpa and Tensile Strength 380 Mpa or more;
b) Hole Expansion ratio of 80% to 95%;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

Another aspect of the present invention is directed to said high strength low alloy cold rolled continuous annealed steel sheet comprising Ti; wherein composition of Ti is based on ratio of (Ti+Al)/N to be 12 to 35.

Yet another aspect of the present invention is directed to said high strength low alloy cold rolled continuous annealed steel sheet further comprising at least one element from Zr, Bi, V, Mo and Cr each by content in the range of 0.002 to 0.03 %.

A further aspect of the present invention is directed to a process for the manufacture of steel sheet comprising:
i) providing a selective steel composition for slab generation comprising:
0.02Wt% to 0.07 wt% of Carbon;
0.3Wt% to 0.7 wt% of Manganese;
0.01 or less wt% of Sulphur;
0.020Wt% to 0.06 wt% of Aluminum;
0.006 or less wt% of Nitrogen;
0.008Wt% to 0.025 wt% of Niobium;
and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 12; and
ii) carrying out steel sheet manufacturing including hot rolled, pickled, cold reduced and continuous annealing such as to reach with excellent hole expansion ratio, spot weldability and excellent phosphatibility.

A still further aspect of the present invention is directed to said process comprising:

a) Hot rolling said slabs with finishing temperature of 870°C to 910°C and run out table cooling rate between 10 °C /Sec to 13°C/Sec to obtain hot rolled strips of desired thickness;
b) Processing said hot rolled strips through pickling coupled with tandem cold rolling mill, to remove the oxide layer present on the surface and cold reduction in range of 55% to 71%.

A still further aspect of the present invention is directed to said process, further comprising:

a) Soaking said steel at temperature range between 770°C to 830°C with residence time of 50Sec to 100Sec;
b) Slow cooling further said steel at temperature range of 630°C to 680°C with cooling rate of 1-4°C/Sec;
c) Rapid cooling of said steel at temperature in range of 450°C to 510°C with cooling rate of 15 °C/Sec;
d) Over ageing of said steel at temperature range between 360°C to 410°C more with residence time of 180-350secs; and
e) Skin passing of said steel between 1.7% to 2.0%.

A still further aspect of the present invention is directed to a high strength low alloy cold rolled continuous annealed steel sheet which is obtained involving the selective steel composition and controlled continuous annealing for better spot weldability following
[[C]%+[Mn]%/6+(Cr+Mo+Zr)%/10+[Ti]%/2+[Nb]%/3+[V]%/7+{UTS}/900+{H}/20]< 0.27 , UTS in Ksi, H – Thickness in Inches,[M]- % of Mass.

A still further aspect of the present invention is directed to said process carried out under controlled operative conditions such as to achieve said steel sheet having anyone or more of:
a) Yield strength from 300-380 MPa and Tensile Strength 380 Mpa or more;
b) Hole Expansion ratio of 80% to 95%;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying illustrative drawings and examples.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: Shows graphically the effect of (Ti+Al)/N ratio on Yield strength.
Figure 2: Shows graphically the effect of Mn/(C+S) ratio on Hole Expansion Ratio.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING FIGURES AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT
The present invention is directed to provide High strength low alloy steel sheets with excellent hole expansion ratio, spot weldability and excellent phosphatibility suitable for automobile part applications.

Steel of present invention consist of 0.02Wt% to 0.07 wt% of Carbon; 0.3Wt% to 0.7 wt% of Manganese; 0.01 or less wt% of Sulfur; 0.020Wt% to 0.06 wt% of Aluminum; 0.006 or less wt% of Nitrogen; 0.008Wt% to 0.025 wt% of Niobium; and the balance being Fe and inevitable or associated impurities also being present, Wherein Mn/(C+S) ratio to be 6 to 18. Titanium and Niobium is added as carbide and nitride a precipitator which increases the strength of the material.

With the aim of obtaining High strength low alloy steel sheets with yield strength of 300MPa minimum alongwith excellent hole expansion ratio, spot weldability and excellent phosphatibility, through continuous annealing route, effect of Metallurgical factors affecting the mechanical properties and microstructure are described hereunder in details.

Carbon (0.02-0.07wt %) – Carbon ranging from 0.02 to 0.07 is used for increasing the tensile strength of the material. To achieve the minimum tensile strength of 380Mpa or more minimum 0.02% of carbon is required. Excessive amount of carbon increases the tensile strength significantly reduces the ductility and deteriotes the cold rolling properties hence the upper limit of carbon to maintained in the range of 0.02 to 0.07% to achieve the desired properties. Carbon equivalent less than 0.27 resulting in better weldability. The hole expansion ratio is a key indicator to evaluate stretch flanging performance of steel sheets. To achieve good hole expansion ratio Mn/(C+S) ratio to be reduce hard phase formation thus providing good formability and lesser inclusions, hole expansion ratio achieved in present invention is in range of 80 to 95 % as shown in Fig. 2.

Manganese (0.3-0.7%Wt%) – Manganese acts as a solid solution strengthening, increase in manganese content increases the tensile strength, the drastic increase in tensile strength by addition of manganese happens not only because of solid solution strengthening but also by ferrite grain refinement. To achieve the minimum strength level 0.3% is required which will act as a grain refinement, but upper limit should be maintained to 0.7% to achieve the desired range. If manganese contents go beyond 0.7% yield strength level of steel will increase more than 420 Mpa which will be out of the range, also affects the surface by formation of manganese oxide on the surface which affects the phosphatibility of the material. So it is desired to maintain manganese level at 0.3% to 0.7%.

Niobium (0.008-0.025 wt %) –Niobium increases yield strength by formation of precipitates. It also depends on the size and amount of niobium carbide present. Niobium is also effective in grain refiner; addition of niobium give combined effect of precipitation strengthening and grain refinement thereby increase the strength by 20 to 30mpa per 0.01% of niobium.
But Niobium delays the recrystallization and recovery of worked structure of austenite. When niobium content is ranges from 0.008 to 0.025 .Minimum niobium required for strengthening is 0.008% to achieve minimum 300Mpa by precipitation strengthening and maximum niobium to be 0.025 to achieve the property in the desired range.

Aluminum (0.025-0.05 wt %) – Aluminum acts as a deoxidizing agent but when present more than 0.05% it generates inclusion which is one of the possible causes for Edge sliver, therefore the aluminum present should be 0.05% or less. Very less Al 0.02 or less, will lead to leave free Nitrogen.

(Ti+Al)/N (12-35)-Titanium in the low carbon steels forms carbides and nitrides to provide grain refinement and precipitation strengthening and it also acts as a sulphide control, For effective strengthening using titanium minimum 0.025% is required to achieve Yield strength of 300Mpa minimum. Small percentage of titanium first forms titanium nitride, but it does not provide precipitate strengthening as it is coarser, by increasing titanium further more forms titanium carbo-sulphides which in turn provide sulphide shape control advantageous in providing good surface, to achieve the minimum strength using titanium as a grain refinement (Ti+Al)/N to be 12 to 35 as shown in Fig. 1. When it exceeds the 35 the strength of material increases drastically. To achieves the desired yield strength from 300 to 380 Mpa and also to prevent accelerated ageing which causes spring back in the materials titanium level should be maintained such that (Ti+Al)/N to be 12 to 35.

Nitrogen (0- 0.006max wt %) –Nitrogen present in the high strength steel containing the titanium and niobium increase strength by formation of nitride precipitates but when present more than 0.006%, it results in the presence of free nitrogen and results in yield point elongation and deteriotes ageing property so nitrogen should maintained at 0.006% maximum.

Silicon (0.035 or less wt %) - Silicon deteriorates plating /surface properties as well by forming SiO2 type of oxide (Scale). It is advantageous to add as low an amount of silicon in the steel as is possible, the added amount of silicon is preferably 0.035 wt% or less.

Zr, Bi, V, Mo and Cr in the range of 0.002 to 0.03 wt % - each of from Zr, Bi, V, Mo and Cr act as carbide former and/or nitride former and/or solid solution strengthening elements, however adding each of these elements in an amount more than 0.03 wt% unnecessarily adds up to the cost of the steel and also increases the yield strength of materials.

Method of Manufacturing:

Slabs are casted with the chemistry said above and hot rolled with slab reheating temperature of below 1220°C to attain roughing mill delivery temperature below 1060°C, to prevent the surface defects like rolled in scale, finishing temperature of 860°C to 900°C and then winded with run out table cooling rate between 10 °C /Sec to 13°C/Sec and processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduction of 55-71% with reduction distribution between 20-30% from 1st stand to 4th stand and max 5 % reduction at stand 5th to create proper roughness at Pickling cum Tandem Cold Mill.

After pickled and cold rolled steel strip is processed through continuous annealing line, where electrolytic cleaning removes emulsion present on the surface. Cleaned surface passes through the preheating section where the strip is heated and then passes through soaking section where it is heated above 770°C or more with residence time of 50Sec to 100Sec where complete recrystallization takes and makes the steel softer to get desired properties of UTS 480Mpa or less. Sheet passes through slow cooling section with 680°C or less with cooling rate of 1°C/Sec to 4°C/Sec and rapidly cooled to 450°C with cooling rate of 6°C/Sec to 15°C/Sec and overaged at 360°C or more with residence time 180-350 sec to reduce the solute carbon with residence time of 180-350Secs, then processed through skin-pass mill with 1.7% or more elongation to eliminate the yield point elongation. Additionally Cold rolled sheet processed can be coated using galvanizing process and used as Galvannealed and Galvanized steel for similar applications.

Method of evaluating phosphatability

Phosphating process can be defined as the treatment of a metal surface so as to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal which provides excellent corrosion resistance and paint ability to steel surface .The coating is formed as a result of a topochemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film.
To evaluate phosphatability firstly alkali degreasing was performed on steel sheet at 40°C for 120 sec using FC-E2032 chemical manufactured by NIHON PARKERIZING India Pvt Ltd to the obtained cold rolled steel sheet without any oil/grease on surface. Degreasing was followed by water rinsing and then surface conditioning at room temperature for 30 seconds using PL-Z chemical manufactured by NIHON PARKERIZING India Pvt Ltd. Phosphate treatment using PB-L3020 chemical, manufactured by NIHON PARKERIZING India Pvt was done at 40°C for 90 seconds. Subsequently, the surface after phosphate treatment was observed under a Scanning electron microscope using Secondary Electron image mode. Average grain size was measured assuming circular phosphate crystals. Crystal size < 3µm is considered as excellent for phosphatability. The phosphate coating weight was measured using the XRF method and steel sheet with average coating weight after zinc phosphate chemical conversion coating of 1.5-2.5 g/m2 is considered having excellent phosphatability.

Hole Expansion Test: The hole expansion ratio is a key indicator to evaluate stretch flanging performance of steel sheets, which is usually obtained by hole expanding test using cylindrical or conical punch. According to ISO 16630-2009 (Metallic materials -- Sheet and strip -- Hole expanding test).

Ageing Test: Process steel strip is made it to dumbbell shape and heated using an oil bath at 100°C for 6 hours and tested again, while testing if yield point elongation is observed 0.3 or less then the shelf life of the product will be 6 months at room temperature as per Hundys equation. Ageing test is done for 6 Hr which is equivalent to 6 month as per Hundys equation.

The High Strength Low Alloy steel sheet with excellent hole expansion ratio, spot weldability and excellent phosphatibility according to the present invention are illustrated with help of following examples and the experimental data as presented in tables I-IV.

Table I- Compositions of the invented steel sheets
Table II- Hot rolling, cold rolling, annealing parameters.
Table III – Mechanical properties of respective steels.
Table IV – Ageing Properties of samples.

Table I:

I-Inventive, C – Comparative
CE- [C]%+[Mn]%/6+(Cr+Mo+Zr)%/10+[Ti]%/2+[Nb]%/3+[V]%/7+{UTS}/900+{H}/20< 0.27 , UTS in Ksi, H – Thickness in Inches,[M]- % by Mass.
Table II:
Sample FT ROT Cooling Rate °C/Sec Thk Cold Reduction SS SCS RCS OAS SPM
1 874 10 1.9 60 798 665 481 396 1.83
2 901 13 1.2 63 819 664 488 389 1.81
3 901 13 1.2 63 815 665 477 389 1.80
4 897 13 1 67 816 650 480 394 1.80
5 889 12 1.2 63 795 667 464 395 1.80
6 884 11 1.55 61 798 636 478 374 1.82
7 894 13 1 67 814 650 480 385 1.80
8 901 13 1.2 63 815 665 477 389 1.80
9 899 13 1.03 66 797 642 481 390 1.90
10 889 12 1.2 63 808 645 477 384 1.83
11a) 897 13 1.03 66 772 635 480 383 1.80
11b) 897 13 1.03 66 760 635 480 383 1.80
12a) 895 12 1.5 63 833 685 480 367 1.71
12b) 895 12 1.5 63 845 685 480 367 1.71
12C) 895 12 1.5 63 845 685 480 367 1.5
13 877 11 1.18 63 769 622 459 362 2.00
14 887 12 1.2 63 821 711 506 409 1.80
15a) 897 13 1.03 66 784 641 480 390 2
15b) 897 13 1.03 66 784 641 480 390 2.2
16 896 12 1.25 63 828 700 483 401 1.80
17 909 12 2 66 844 697 486 394 1.62
18 900 15 1.23 62 803 652 480 397 1.60
19 893 12 0.9 65.38 801 681 468 408 1.8
20 895 13 0.9 65.38 780 649 450 382 1.79
21 875 12 0.9 65.38 782 640 450 384 1.79

Table III:
Sample YS YPE TS ELONGATION Hole Expansion Ratio Phosphate Grain Size µm Coating Weight g/m2
1 314 0 403 34.3 92.9
2 369 0 469 24.9 87.6
3 369 0 469 24.9 90.9 3 2.2
4 336 0 424 29.2 82.0 2.5 1.8
5 360 0 470 27 94.3 3 2.5
6 330 0 424 28.5 86.3
7 337 0 428 27.8 80.4 3 1.9
8 369 0 469 24.9 83.2
9 350 0 432 27.3 81.2 2.8 2.3
10 325 0 416 33.6 85.7
11a) 371 0 451 26.3 92.4 2.5 3
11b) 385 0 465 26.3
12a) 315 0 420 29.1 80.6
12b) 295 0 400 34
12C) 283 0 402 34.5
13 329 0 424 30.4 80.8
14 343 0 443 31.7 80.8
15a) 372 0 449 25 80.8
15b) 385 0 449 25 80.8
16 246 0 371 38.2 75.4
17 277 0 320 43.3
18 396 0 501 23.7 4 3.5
19 404 0 475 28.7 4.5 3.3
20 394 0 460 29.3 72.0
21 409 0 485 30.8 75.0

Table IV:
Steel Initial YS After 3 Hours, at 100 deg C YPE, After 6 Hours, at 100deg C YPE,<0.3% Remarks
LYP UYP YS LYP UYP YS
(MPa) (MPa) (MPa) (MPa)
6 330 Nil Nil 332 0 Nil Nil 332 0 Inventive steel- Aging pass for 6 hrs
8 369 Nil Nil 370 0 Nil Nil 370 0 Inventive steel- Aging pass for 6 hrs
10 325 Nil Nil 330 0 Nil Nil 332 0 Inventive steel- Aging pass for 6 hrs
17 277 320 300 300 5.5 322 300 300 6 Comparative steel- Ageing Test Failed

Example 1- In sample 1 and 5 where carbon percentage is 0.02 and 0.07 and its tensile strength are 405 and 470MPa. Whereas in sample no 18 where C is 0.077% tensile strength obtained is 501Mpa which is out of scope of present invention, carbon is restricted to 0.07%, in sample 17 where C is 0.015% tensile strength is 320Mpa which is below the desired level, carbon is maintained in range of 0.02Wt% to 0.07Wt%.

Example 2-In sample 4 and 5 where manganese percentage are 0.3Wt% and 0.7Wt% tensile strength observed is 424Mpa and 470MPa.whereas in sample 16 and 18 where manganese is 0.25 and 0.77, tensile strength obtained is 370MPa and 501MPa is out of scope of present invention, Manganese in present invention is restricted in range of 0.3Wt% to 0.7Wt%.

Example 3 –In sample 5 and 6 where niobium is 0.025wt% and 0.008Wt%, yield strength obtained is 360MPa and 330MPa, whereas in sample no 21 niobium is 0.042wt% yield strength obtained is 409Mpa out of scope of present invention, but in sample 17 where niobium is very negligible yield strength is less, so the range of niobium to be maintained between 0.008 wt% to 0.025Wt%.

Example 4 – In sample 11a and 12a where SS temperature is 772°C and 883°C where tensile strength obtained is 451MPa and 421MPa.whereas in sample 11b and 12b SS temperature is 760°C and 845°C where tensile strength is 465 and 400, yield strength is 385Mpa and 295Mpa is out of scope of invention, SS temperature to be in range of 770°C to 830°C.

Example 5 – In sample 12 a) and 15a) where spm elongation are 1.71% and 2% where yield strength obtained is 315MPa and 372MPa, whereas in sample 20 where spm elongation is 2.2% ys obtained is 385MPa is out of scope of present invention, in sample No. 12 where spm elongation is 1.5% yield strength is 283Mpa is out desired range, spm elongation to 1.7% to 2% to achieve desired range of yield strength.

Example 6 – In sample 10 and 11 where ratio of (Ti+Al)/N is 13 and 35 respectively where yield strength obtained is 325Mpa and 371MPa, whereas in sample 16 and 17, ratio is 10 and 11 where yield strength obtained is 246Mpa and 277 Mpa. In sample no. 17 ageing test failed for 3 hours and 6 hours as shown in table IV. There is no yield point elongation after 3 or 6 hours of ageing test in the sample no 6, 8 and 10 as shown in table IV. So, the ratio of (Ti+Al) and N to be maintained between 12 to 35 to achieve yield strength in the range of 300 to 380 Mpa.

Example 7 – In sample no. 1 and 4 where Mn/(C+S) is 18 and 6, hole expansion ratio obtained is 92.9% and 82% whereas in sample 20 and where the ratio is 21 and 4, hole expansion ratio obtained is 72% and 75% which is out of scope from present invention.

It is thus possible by way of the present invention to provide 300MPa yield strength level High Strength Low Alloy steel sheet suitable for automotive structural parts produced through continuous annealing process. The steel having 0.02Wt% to 0.07Wt% of Carbon; 0.3Wt% to 0.7 wt% of Manganese; 0.01 or less wt% of Sulphur; 0.020Wt% to 0.06 wt% of Aluminum; 0.006 or less wt% of Nitrogen; 0.008Wt% to 0.025 wt% of Niobium; and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 18 for excellent hole expansion. The slab is hot rolled with Finishing temperature of 870°C to 910 °C with Run out table cooling rate between 10 °C /Sec to 13°C/Sec, then cold rolled with reduction 55% to 71%, Continuous annealing by heating the steel strip at the rate of 3-100C/sec to annealing temperature of 770 °C -830°C, Holding at Soaking section temperature for 100seconds or less , slow cooling at 1-4 °C/sec to temperature of 630°C-680°C with residence time of 80 sec or less, Rapid cooling rate at 15 °C/sec or less to temperature 450°C or more and overaging temperature of 360°C or more with residence time of 350 sec or less, with skin pass elongation of 1.7 or more% to achieve minimum Yield strength of 300MPa and Total Elongation of 28 % or more with good Phosphatability, hole expansion ratio and spot weldability. Cold rolled High Strength Low Alloy steel used in structural application with desired formability requirements, excellent hole expansion ratio.
,CLAIMS:We Claim:
1. Steel composition suitable for producing high strength low alloy cold rolled steel sheet through continuous annealed route comprising:
0.02Wt% to 0.07 wt% of Carbon;
0.3Wt% to 0.7 wt% of Manganese;
0.01 or less wt% of Sulphur;
0.020Wt% to 0.06 wt% of Aluminum;
0.006 or less wt% of Nitrogen;
0.008Wt% to 0.025 wt% of Niobium;
and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 18; favoring producing steel sheets having anyone or more
a) Yield strength from 300-380 Mpa and Tensile Strength 380 Mpa or more;
b) Hole Expansion ratio of 80% to 95%;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

2. Steel composition as claimed in claim 1 comprising Ti; wherein composition of Ti is based on ratio of (Ti+Al)/N to be 12 to 35.

3. Steel composition as claimed in anyone of claims 1 or 2, further comprising at least one element from Zr, Bi, V, Mo and Cr each by content in the range of 0.002 to 0.03 %.

4. A high strength low alloy cold rolled continuous annealed steel sheet comprising:
0.02Wt% to 0.07 wt% of Carbon;
0.3Wt% to 0.7 wt% of Manganese;
0.01 or less wt% of Sulphur;
0.020Wt% to 0.06 wt% of Aluminum;
0.006 or less wt% of Nitrogen;
0.008Wt% to 0.025 wt% of Niobium;
and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 18;
and having anyone or more of
a) Yield strength from 300-380 Mpa and Tensile Strength 380 Mpa or more;
b) Hole Expansion ratio of 80% to 95%;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

5. A high strength low alloy cold rolled continuous annealed steel sheet of claim 4 comprising Ti; wherein composition of Ti is based on ratio of (Ti+Al)/N to be 12 to 35.

6. A high strength low alloy cold rolled continuous annealed steel sheet of anyone of claims 4 or 5, further comprising at least one element from Zr, Bi, V, Mo and Cr each by content in the range of 0.002 to 0.03 %.

7. A process for the manufacture of steel sheet comprising:
i) providing a selective steel composition for slab generation comprising:
0.02Wt% to 0.07 wt% of Carbon;
0.3Wt% to 0.7 wt% of Manganese;
0.01 or less wt% of Sulphur;
0.020Wt% to 0.06 wt% of Aluminum;
0.006 or less wt% of Nitrogen;
0.008Wt% to 0.025 wt% of Niobium;
and the balance being Fe and inevitable or associated impurities also being present, wherein Mn/(C+S) ratio to be 6 to 12; and
ii) carrying out steel sheet manufacturing including hot rolled, pickled, cold reduced and continuous annealing such as to reach with excellent hole expansion ratio, spot weldability and excellent phosphatibility.

8. A process as claimed in claim 7 comprising:

i) Hot rolling said slabs with finishing temperature of 870°C to 910°C and run out table cooling rate between 10 °C /Sec to 13°C/Sec to obtain hot rolled strips of desired thickness; and
ii) Processing said hot rolled strips through pickling coupled with tandem cold rolling mill, to remove the oxide layer present on the surface and cold reduction in range of 55% to 71%.

9. A process as claimed in claim 7 or 8, further comprising:

a. Soaking said steel at temperature range between 770°C to 830°C with residence time of 50Sec to 100Sec;
b. Slow cooling further said steel at temperature range of 630°C to 680°C with cooling rate of 1-4°C/Sec;
c. Rapid cooling of said steel at temperature in range of 450°C to 510°C with cooling rate of 15 °C/Sec;
d. Over ageing of said steel at temperature range between 360°C to 410°C more with residence time of 180-350secs; and
e. Skin passing of said steel between 1.7% to 2.0%.

10. A high strength low alloy cold rolled continuous annealed steel sheet of anyone of claims 7 to 9, which is obtained involving the selective steel composition and controlled continuous annealing for better spot weldability following
[[C]%+[Mn]%/6+(Cr+Mo+Zr)%/10+[Ti]%/2+[Nb]%/3+[V]%/7+{UTS}/900+{H}/20]< 0.27, UTS in Ksi, H – Thickness in Inches,[M]- % of Mass.

11. A process as claimed in anyone of claims 7 to 10 carried out under controlled operative conditions such as to achieve said steel sheet having anyone or more of:
a) Yield strength from 300-380 MPa and Tensile Strength 380 Mpa or more;
b) Hole Expansion ratio of 80% to 95%;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

Dated this the 24th day of August, 2016
Anjan Sen
Of Anjan sen & Associates
(Applicants Agent)