DESC:FIELD OF THE INVENTION:

The present invention relates to High Strength Low Alloy steel sheet suitable for automotive cross member structural parts produced using continuous annealing process. According to an aspect the present invention is directed to provide high strength low alloy steel having yield strength of minimum 340 Mpa. The steel having by weight percent carbon 0.05-0.08, Manganese 0.4-0.8, Phosphorous 0.02 or less, Titanium-0.02-0.06, Niobium-0.01-0.03, Nitrogen -0.006max, Aluminum- 0.02-0.06, balance iron and essentially the usual balance impurities, wherein Ti/N ratio 6 to 12, is hot rolled with Finishing temperature of 860°C or more with Run out table cooling rate of 8°C/Sec to 15°C/Sec, then cold rolled, annealed using continuous annealing at temperature of 800°C or less with residence time of 94 Sec or less, with skin pass elongation of 1.5% or more to achieve Yield strength of 340Mpa min with yield ratio greater than 0.75 and no YPE after accelerated ageing. Cold rolled High Strength Low Alloy steel sheet is suitable for use in structural application with desired formability requirements, spring back resistance better weldability and excellent phosphatibility.

BACKGROUND ART

In the past decades major developments are taken for the use of high strength steel in the transport vehicles to reduce the dead weight. Cold rolled high strength low alloy steel is easy to fabricate the different automotive components. The steel does not only have higher tensile strength but also has good weldability but with desired work hardening and formability. All these desirable properties are achieved by aiming the desirable chemistry and processing using the desired process parameters.

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 and very less ageing resistance

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 410Mpa 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 430Mpa to 440Mpa, which results as spring back and bending related problems 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 and phosphatibility processed through continuous annealing process suitable for automotive structural applications. Spring back is an observable fact that occurs during forming whenever the Component is withdrawn from tool set. Spring back happens due to relaxation in elastic behavior is not uniform and the shape of the pressing will not be exactly match as the shape of the punch that has been used in its manufacture. The most important fact that affects spring back is yield stress, so to avoid the problem yield strength to tensile strength ratio is to be maintained. In the present invention the yield ratio is more than 0.75 which help in preventing spring back while forming and Ti is added to prevent the increase in yield strength due to ageing which helps in avoiding spring back problem in materials.

OBJECTS OF THE INVENTION

It is thus the basic object of the present invention to solve the problem of prior art by providing High Strength Low Alloy steel sheet with good ageing resistance for 6 months, prevent accelerated ageing with better weldability and phosphatibility processed through continuous annealing process suitable for automotive structural applications.
Another object of the present advancement is directed to developments related to steel chemistry and processing of High Strength Low Alloy steel sheet which would be able to address the problems concerning spring back especially directed to the yield stress and enabling maintaining desired yield strength to tensile strength ratio.

Yet another object of the present invention is directed to developments related to steel chemistry and processing which would help in preventing spring back while forming and also to prevent the increase in yield strength due to ageing which in turn would help in avoiding spring back problem in materials.

A still further object of the present invention is directed to provide high strength low alloy steel sheet having high yield strength of 340Mpa or more with YS/TS ratio more than 0.75 and no YPE after accelerated ageing.

A still further object of the present invention is directed to provide high strength low alloy steel sheet wherein ratio of Mn/(Al+Si) is selectively maintained to control the deterioration of surface property and ratio of Ti/N controlled to achieve the yield strength in desired range.

SUMMARY OF THE INVENTION

Thus according to the basic aspect of the present invention there is provided a steel composition suitable for producing high strength low alloy steel cold rolled steel through continuous annealed route with ageing resistance and weldability comprising:
0.05Wt % to 0.08Wt% percent of Carbon;
0.4wt% to 0.8Wt% of Manganese;
0.02wt% to 0.06Wt% of Aluminum;
0.01Wt% to 0.03Wt% of niobium;
0.02Wt% to 0.06Wt% of Titanium;
Up to 0.006Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, Wherein Ti to N is in ratio of 6 to 12.

A still further aspect of the present invention is directed to said high strength low alloy cold rolled continuous annealed steel sheet including Si, composition of Si is based on ratio of Mn/(Al+Si) greater than 5.

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, W, Cr and Mo each by content in the range of 0.001 to 0.03 %.
Another 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:
0.05Wt % to 0.08Wt% percent of Carbon;
0.4wt% to 0.8Wt% of Manganese;
0.02wt% to 0.06Wt% of Aluminum;
0.01Wt% to 0.03Wt% of niobium;
0.02Wt% to 0.06Wt% of Titanium;
Up to 0.006Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, Wherein Ti to N is in ratio of 6 to 12; and
ii) carrying out steel sheet manufacturing including hot rolled, pickled, cold reduced and continuous annealing such as to reach to excellent ageing resistance of atleast 6 months at room temperature with desired weldability with Carbon equivalent in range 0.22 to 0.29.

Yet another aspect of the present invention is directed to said process comprising:
a) Hot rolling of said steel sheet with Finishing Temperature more than 860 °c with ROT cooling rate 8-15°C/Sec; and
b) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled to thickness less than 45% of initial Thickness.

A further aspect of the present invention is directed to said process further comprising:
a) Soaking said steel at temperature range between 770°C to 800 °C with residence time 50-94 Sec;
b)Slow cooling further said steel at temperature range 630 °C to 680 with cooling rate of 1°C /Sec to 5 °C/sec;
c) Rapid cooling of said steel at temperature 480°C or less with cooling rate 10°C /Sec to 18°C /Sec;
d) Over ageing of said steel at temperature range between 340 °C to 420with residence time of 180-350Sec; and
e) Skin passing of said steel between 1.5% to 2.3%.

Yet another aspect of the present invention is directed to said high strength low alloy cold rolled continuous annealed steel sheet which is obtained involving the selective steel composition and controlled continuous annealing for desired strength following Furnace Speed >= 0.56*SS TEMP + 1.25* SCS TEMP-(469.56 + 3323.5* C + 146* MN + 11191* TI + 9473* NB)

A still further aspect of the present invention is directed to said steel composition capable of generating Tensile Strength (MPa) based on the elemental constitution and continuous annealing conditions including soaking, slow cooling and furnace speed conditions predetermined based on = 518 + 765 C + 34 MN + 2574 TI + 2179 NB - 0.13 SS TEMP - 0.29 SCS TEMP + 0.23 Furnace Speed.

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 800 °C;
b) Slow cooling further said steel at temperature range 630 °C to 680°C;
c) Rapid cooling of said steel at temperature 480°C or less;
d) Over ageing of said steel at temperature range between 340 °C to 420°C;
e) Skin passing of said steel between 1.5% to 2.3%; and
f) Continuous annealing Furnace speed of said steel between 120mpm to 210mpm.

A further aspect of the present invention is directed to said high strength low alloy cold rolled continuous annealed steel sheet having yield strength in the range of 340 Mpa to 420 Mpa with YS/TS ratio more than 0.75.

A still further aspect of the present invention is directed to said High strength low alloy cold rolled continuous annealed steel sheet wherein the ratio of of Mn/(Al+Si) to be greater than 5.

A still further aspect of the present invention is directed to said high strength low alloy cold rolled continuous annealed steel sheet having
a) Phosphate grain size less than 3.5µm; and
b) Coating Weight 2-3g/m2.

A still further aspect of the present invention is directed to said process for said steel sheet having excellent weldability and spring back resistance, wherein the process is controlled such as to achieve anyone or more of:
a) Yield strength from 340-420 Mpa with YS/TS ratio more than 0.75; and
b) No Yield point elongation after accelerated ageing;
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 non limiting illustrative figure and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

Figure 1: shows graphically the effect of Ti/N ratio on Yield Strength.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWING AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT

The present invention relates to cold rolled high strength low alloy steel with high tensile strength and yield strength, good formability, weldability and phosphatability and method of manufacturing the same as described hereunder.

Following abbreviations have been used to describe the present invention:
SS- Soaking Section
SCS – Slow Cooling Section
RCS-Rapid Cooling Section
OAS Over-ageing section
SPM-Skin Pass Elongation
YPE-Yield Point Elongation
HSM-Hot Strip Mill
UTS-Ultimate Tensile Strength
YS-Yield Strength
El – Elongation
SPM-Skin Pass Mill
FT-Finishing Temperature
ROT- Run Out Table

The present invention relates to a method for producing the cold rolled high strength low alloy steel containing by weight percent Carbon of 0.05 to 0.08%, 0.4 to 0.8% of Manganese with alloying elements of titanium 0.02-0.06% and niobium 0.01-0.03% respectively, and Solid solution strengthening by (manganese and silicon). In hot rolled product by keeping the Finishing temperature above 860°C grain refinement is achieved. The steel is cooled in Run out table with cooling rate of 8-15°C/Sec micro alloying additions are retained in the solid solutions and during the subsequent annealing they precipitate even more finely dispersed particles by not dissolving at higher annealing temperature of 750°C or more.

The present invention relates to high strength low alloy steel processed through continuous annealing for use in automotive structural parts having the tensile strength of 510 Mpa or more and yield strength of 420Mpa or less with elongation of minimum 25%, and with ensured accelerated ageing resistance for 6 months to prevent spring back, better weldability and phosphatibility and YS/TS of 0.75 or more.

Cold rolled high strength steel having chemistry 0.05%-0.08% of Carbon, 0.4% -0.8% of manganese, 0.02% -0.06% of Titanium, 0.01% -0.03% of Niobium, 0.03 %or less Silicon, 0.02%or less Phosphorous, 0.006 % or less Nitrogen, 0.02% -0.06% Aluminum and remaining Fe. Slab is produced with the above said chemistry and hot rolled with finishing mill temperature at 860°C or more with run out table cooling rate in range of 8-15°C/Sec for a better surface i.e. with minimum amount of scale formation , Hot rolled coil is pickled with optimum acid concentration to remove the scales on the surface and cold rolled with 55% or more reduction. After cold reduction coil is passed through continuous annealing section where first the emulsion present on the surface is cleaned using electrolytic cleaning and then passed through heating section with temperature of 750°C or more with residence time of 150 to 350Sec and soaked at the soaking section with temperature of 770°C or more with residence time of 50-94Sec where recrystallization happens and slow cooled in slow cooling section with temperature of 630°C or more where residence time is 40-80Sec with cooling rate of 4°C/Sec or less and rapidly cooled at rapid cooling section with cooling rate of 10°C/Sec to 18°C/Sec and over aged at overaging section with 340°C or more with residence time of 180-350Secs . Process of annealing cycle from heating section to overaging section takes place in 7 min to 16 min. After the strips being annealed, it passes through Skin pass mill where temper rolling is done with skin pass elongation of 1.5% or more high skin pass elongation is given to avoid yield point elongation. This ensures to achieve yield strength of minimum 340Mpa.

The present invention relates to cold rolled high strength steel used for structural application where requirement of forming is very less whereas requires high strength. The present steel produced with high yield strength of 340Mpa or more with YS/TS ratio more than 0.75 and no YPE after accelerated ageing.

Moreover, the High strength Low Alloy steel is having selective composition to ensure better weldability with Carbon equivalent in the range of 0.22 to 0.29.

The strength is obtained by combination of chemical composition and the desired process parameter to achieve the desired properties. The strength is particularly obtained by carbon and manganese by solid solution strengthening, precipitation hardening by carbides and nitrides formation by addition of titanium and niobium.

The micro alloying elements titanium and niobium are added for precipitation strengthening of the material. Titanium added in the steel for carbides and nitrides and which is also effective in the improvement of notch toughness of the Heat Affected Zone of the weld joints by the formation of TiN which provides good weldability of the material.

Niobium is also for strengthening, but delays the recrystallization and recovery of worked structure of austenite. When niobium content is between 0.01 to 0.03 the annealing temperature to be set below 800°C for full recrystallization if processed above 800 then strengthening by precipitation hardening will be reduced and decrease in tensile strength will occur thus adversely affecting to achieve the targeted properties.

The present invention relates to manufacturing of high strength cold rolled steel using continuous annealing. The hot metal is process through the LD convertor for refining the hot metal and removing the impurities and then through ladle reheating of addition of alloying elements and then casted through continuous casting.The produced slab is hot rolled with desired finishing mill temperature and cooled in run out table at desired rate of cooling. The produced hot rolled temperature is pickled and cold rolled with 55% or more, reduced to desired thickness. The cold rolled steel is cleaned using electrolytic cleaning and then annealed with optimum temperature and then skin passed with 1.5% or more to obtain the high yield strength and to ensure the ageing guarantee.

Steel grade according to present invention comprising by weight percent carbon 0.05-0.08, Manganese 0.4-0.8, Phosphorous 0.02 or less, Silicon<0.03, Titanium-0.02-0.06, Niobium-0.01-0.03, Nitrogen-0.006max, Aluminum- 0.02-0.06 balance iron and essentially the usual balance impurities. The Titanium and Niobium is added as carbide and nitride a precipitate which increases the strength of the material. The justification for maintaining the above composition involving selective weight percent range of each of the constituents are as follows:

Carbon (0.05-0.08%wt %) – Carbon ranging from 0.05 to 0.08 is used for increasing the tensile strength of the material. To achieve the minimum tensile strength of 410Mpa or more, minimum 0.05% 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 0.05 to 0.08% to achieve the desired properties. Carbon equivalent less than 0.29% is thus ensured resulting in better weldability.

Manganese (0.4-0.8% Wt%) – Manganese acts as a solid solution strengthener, 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 0.4% is required which will act as a grain refinement, but upper limit should be maintained to 0.8% to achieve the desired range. If manganese contents go beyond 0.8% 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. To control the detoriation of surface property the ratio of Mn/(Al+Si) to be greater than 5, if the ratio is less than that, then chance of oxide formation is more and in turn poor phosphatability. So it is desired to maintain manganese level at 0.4% to 0.8%.

Niobium (0.01-0.03%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 0.01 to 0.03 the annealing temperature to be set below 800°C for full recrystallization if processed at 800°C then strengthening by precipitation hardening will be reduced and decrease in tensile strength will occur affecting to achieve the desired properties.
Minimum niobium required for strengthening is 0.01% to achieve minimum YS of 340Mpa by precipitation strengthening and maximum niobium to be 0.03 to achieve the property in the desired range.

Aluminum (0.02-0.06wt%) – Aluminum acts as a deoxidizing agent but when present more than 0.06% it generates inclusion which is one of the possible causes for Edge sliver, therefore the aluminum present should be 0.06% or less. Very less Al 0.02 or less, will lead to leave free Nitrogen.

Nitrogen (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.

Titanium (0.02-0.06 wt%)-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 0.02% is required to achieve Yield strength of 340Mpa minimum. Titanium is a grain refiner because it retards austenite grain growth by formation of titanium nitride. Small percentage of titanium first forms titanium nitride, by increasing titanium further more forms titanium carbo-sulphides which in turn provide sulphide shape control advantageous in providing good surface.
The formation of titanium carbide occurs after the formation of titanium carbosulphides which helps in precipitation strengthening the minimum requirement of titanium is 0.02% derived from Minimum Ti Required to prevent accelerated ageing and then after precipitation strengthening, As per Ti(Min)=47/14*N (Wt %) is 0.02% to fix 0.006% of Nitrogen. But when it exceeds the 0.06% the strength of material increases drastically. To achieve the desired yield strength from 340 to 420 Mpa and also to prevent accelerated ageing which causes spring back in the materials titanium level should be maintained between 0.02-0.06.

As TiN determines the variation in the yield strength ,the ratio of Ti/N has to be controlled between 6 to 12, to achieve the yield strength between 340MPa to 420Mpa.

Silicon (0.03 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 0.03 wt% or less.

Zr, Bi, V, W, Cr and Mo (less than 0.03 weight %) – In present invention these elements are carbides and nitride formers which increases yield strength by formation of precipitates. It also depends on the size and amount of carbide present. These elements is also effective in grain refiner; addition of these elements give combined effect of precipitation strengthening and grain refinement therefore for present invention the level at least one element from Zr, Bi, V, W, Cr and Mo each by content in the range of 0.001 to 0.03 %.

Bases on the experimental data generated, a emprical equation has been developed here to calculate the Tensile strength, for present inventive grades having the composition range as described above
Tensile Strength (Mpa) = 518 + 765 C + 34 MN + 2574 TI + 2179 NB - 0.13 SS TEMP - 0.29 SCS TEMP + 0.23 Furnace Speed.

Further based on experimental data following process condition is developed:
Furnace Speed >= 0.56*SS TEMP + 1.25* SCS TEMP-(469.56 + 3323.5* C + 146* MN + 11191* TI + 9473* NB)

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.

Phosphatibility test procedure – Firstly alkali degreasing was performed 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 grains. Grain size < 3.5 µm is considered as excellent phosphatibility. The phosphate coating weight with 2 -3g/m2 was measured using the XRF method.

Complete description of Inventive steel and comparative steel grades are illustrated as per the following tables I to IV
Table I- Compositions of the invented steel sheets along with some comparative examples.
Table II- Hot rolling, cold rolling, annealing parameters of inventive and comparative steels.
Table III- Mechanical Properties of samples.
Table IV – Ageing Properties of samples.

Table I

Table II-

S. no Thick Red% FT ROT Cooling Rate°C/S SS SCS RCS OAS Skin pass Furnace Speed
1 1.15 59 830 9 762 628 482 402 1.8 166
2 1.03 66 860 8 804 651 480 380 1.8 190
3 1.03 66 897 13 803 661 484 392 1.82 168
4 1.5 61 855 11 778 670 454 375 1.82 100
5 1 64 895 12 786 643 432 369 1.50 120
6 1.5 61 899 13 779 640 478 394 1.80 130
7 1.03 66 910 16 806 665 480 398 1.81 220
8 1.75 56 898 13 827 687 471 390 1.81 120
9 1.5 67 895 12 775 652 437 398 1.80 175
10 1.15 59 895 13 780 633 479 403 1.79 180
11 1.55 66 930 17 802 650 480 402 2.03 125
12 1.55 66 875 12 788 658 483 405 2.03 126
13 1.5 67 893 13 775 652 437 398 1.80 175
14 1.5 61 910 13 772 653 473 369 2.30 125
15 1.45 62 875 10 811 696 478 390 1.78 160
16 1.15 59 850 9 762 620 482 402 1.79 166
17 1.45 64 902 12 782 631 440 385 1.79 140
18 1.45 62 883 13 774 639 481 414 1.81 140
19 1.6 60 878 12 774 663 427 345 1.81 102
20 1.45 62 887 11 766 634 482 381 1.80 109
21 1.45 62 905 13 774 639 481 414 1.81 140
22 1.15 59 887 11 785 645 483 404 1.37 176
23 0.7 71 905 13 781 639 430 382 1.40 140
24 2 57 899 12 779 662 481 391 1.80 120
25 1.45 64 868 11 776 630 440 379 1.80 140
26 1.55 66 875 12 790 643 481 404 2.03 123
27 1 64 920 15 786 643 432 369 1.68 100
28 1.75 56 840 7 793 632 470 386 2.23 96
29 1.5 61 860 11 781 652 474 387 2.00 150
30 0.9 65 862 10 782 640 450 384 1.79 210
31 0.9 65 868 11 780 640 450 384 1.78 208
32 0.9 65 868 11 780 640 450 384 2.40 208

Table III
S.no YS YPE TS YS/TS ELONGATION Phosphate Crystal Size, Coating Weight I/C Remarks
1 421 0 532 27.2 C High C, High TS,
2 369 0 456 0.81 31.8 I
3 345 0 427 0.81 35.7 3, 2.5 I
4 315 0 450 0.7 33.9 C Low Ti/N Ratio, Low YS
5 346 0 457 0.76 32.4 I
6 414 0 506 0.82 27.6 2.8 , 2 I
7 333 0 427 35.1 7 , 3.8 C Low Mn Low YS, Poor Phosphatability ratio Mn/(Al+Si) <5
8 345 0 425 0.81 38.5 I
9 399 0 498 0.80 31.4 I
10 422 0 526 26.5 C High Nb High YS ,
11 335 0 431 37.4 C Low Nb Low YS
12 339 0 436 0.78 36.6 I
13 399 0 495 0.81 31.4 I
14 391 0 481 0.81 29.3 I skin pass high level
15 344 0 441 0.78 36.8 I
16 425 0 532 27.2 C Low SCS High YS
17 370 0 464 0.80 33.3 I
18 368 0 461 0.80 32.3 I
19 390 0 490 0.80 31.5 I
20 342 0 439 0.78 35.8 I
21 368 0 461 0.80 32.3 I
22 402 1.28 498 31.2 C High C YPE
23 404 1.13 484 30 C LOW SPM YPE
24 365 0 466 0.78 32.1 I
25 365 0 461 0.79 35.5 I
26 360 0 443 0.81 33.2 I
27 345 0 457 0.75 32.4 I
28 335 0 420 36.2 C Low Furnace Speed Low YS
29 435 0 512 20.1 C High Ti/N ratio high ys
30 409 0 505 0.81 30.8 I
31 415 0 514 0.81 25.5 I
32 440 0 520 0.85 21.3 C High Skin Pass Elongation high Ys

I –Inventive, C- Comparative

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)
6 346 Nil Nil 352 0 Nil Nil 352 0 Inventive steel- Aging pass for 6 hrs
20 368 Nil Nil 373 0 Nil Nil 373 0 Inventive steel- Aging pass for 6 hrs
21 390 Nil Nil 394 0 Nil Nil 395 0 Inventive steel- Aging pass for 6 hrs
33 400 Nil Nil 403 0 Nil Nil 403 0 Inventive steel- Aging pass for 6 hrs

Example 1-from table I and II it is shown that sample number 2 and 3 with carbon range 0.057%, 0.05 range with cold reduction of 55% or more with soaking temperature 770°C or more shows tensile strength in the range of 456Mpa and 427Mpa. In sample number 1 where carbon level is 0.084% the tensile strength observed is more than desired level (i.e.) 532Mpa, range of carbon to be maintained from 0.05% to 0.08%.As carbon increases the tensile strength of the material increases and yield strength also increases whereas elongation decreases.

Example 2 – Sample number 3 where manganese with 0.4% with tensile strength of 427 and yield strength of 345 Mpa with elongation of 35.7%. In sample number 31 where manganese is 0.8%, tensile strength observed is 514Mpa and yield strength 415MPa .Whereas in sample number 1 where manganese is 0.83% and tensile strength observed is 532 Mpa, out of the desirable range, level of manganese to be maintained between 0.4% to 0.8%, it also improves the phosphatibility with the grain size of less than 3µm. As the tensile strength increases with increase in percent of manganese, and also yield strength increases, whereas elongation decreases drastically.

Example 3-Sample number 7 where ratio of Manganese to addition of aluminum and silicon is 4.5 surface qualities will be very poor and fails to achieve phosphatibility whereas in inventive example where silicon is restricted less than 0.03, ratio is greater than 5 good phosphatibility is achieved.

Example 4 – Sample number 5 and 6 where titanium present is 0.02% and 0.06% with yield strength of 346 Mpa and 414 Mpa whereas in sample number 4 titanium attained is 0.003% and yield strength tested is 315Mpa and the ratio of Ti/N is less than 5 to achieve yield strength minimum 340 Mpa ratio of Ti/N should be minimum 5 as shown in Figure 1. Whereas in sample 32 where titanium is 0.07% and ratio Ti/N is 19 and yield strength is 440Mpa, so the ratio should be 12 or less to achieve yield strength 420Mpa or less. As titanium increases yield strength of the material increase titanium in range of 0.005 to 0.07 was taken and yield strength observed is 340 to 420 with titanium range of 0.02% to 0.06%. Yield Strength increases with increase in Ti %, 0.01 % of titanium increase 28Mpa.

Example 5 – Sample number 3 and 13 where range of niobium is 0.016 and 0.03, yield strength obtained is 345Mpa and 399Mpa with skin pass elongation of 1.82 % and 1.80% respectively. In sample number 7 and 10 niobium 0.007% ,0.032 yield strength observed is 333Mpa and 422Mpa .The range of niobium to obtain the desired property range should be between 0.01 to 0.03% to obtain the yield strength range of 340Mpa to 410Mpa.

Example 6– Sample number 14 and 15 where processed with soaking temperature 772°C and 811°C, tensile strength observed is 481Mpa and 441Mpa but when soaking temperature is 770°C or less recrystallization will not occur and material behavior will be very poor.

Example 7 - As slow cooling temperature increase the yield strength decreases, slow cooling temperature to be maintained 630°C or more to achieve the yield strength in the range of 340Mp to 420Mpa.In sample number 15 and 17 where slow cooling temperature is 696°C and 630°C where yield strength is observed 344Mpa and 370Mpa, whereas in sample number 16 where SCS is 620°C yield strength observed is 425Mpa which is outside the desired range, slow cooling temperature to be maintained between 630°C to 700°C.

Example 8- In sample number 19, 20 and 21 where over ageing temperature is 344°C, 380°C and 414°C where tensile strength of 490Mpa, 439Mpa and 461Mpa. As overaging temperature increases tensile strength decreases, temperature to be maintained between 340°C to 420°C to obtain desired tensile strength of 510Mpa or less.

Example 9- Skin pass elongation increases the yield strength decreases at certain point it becomes constant and again starts increasing, at lower skin pass elongation the yield point elongation is not suppressed completely, when yield point elongation is suppressed yield strength becomes constant and then increases. When skin pass elongation is greater than 1.5% the yield strength observed is yield point elongation completely suppressed and strength more than 340Mpa. In sample number 5 where SPM elongation is 1.5% and yield strength observed is 346Mpa with no yield point elongation, in sample number 24, 25 and 26, skin pass elongation is 1.8 and 2% where yield strength observed is 365Mpa, 365Mpa and 360Mpa with no yield point elongation, whereas in sample number 22 and 23 where skin pass elongation is 1.37 and 1.4 yield point elongation is observed so optimum skin pass elongation to 1.5% or more but in sample 31 and 32 keeping all other parameter same and varying only skin pass elongation 1.78 and 2.4 so that Yield strength obtained is 415MPa and 420MPa . Thus to obtain desired yield strength skin pass elongation to be 2.3 % or less.

Thus the present advancement as disclosed above is directed to High Strength Low Alloy steel sheet suitable for automotive cross member structural parts produced using continuous annealing process. More particularly, the advancement is targeted at high strength low alloy steel having selectively Yield strength of 340Mpa min and Elongation of 21 min with ageing resistance upto 6 months and yield ratio more than 0.75. Cold rolled High Strength Low Alloy steel with compromising moderate n value and r value for high strength used in structural application with desired formability requirements, Spring back resistance better weldability and excellent phosphatibility.

,CLAIMS:We Claim:
1. Steel composition suitable for producing high strength low alloy steel cold rolled steel sheet through continuous annealed route with ageing resistance and weldability comprising:
0.05Wt % to 0.08Wt% percent of Carbon;
0.4wt% to 0.8Wt% of Manganese;
0.02wt% to 0.06Wt% of Aluminum;
0.01Wt% to 0.03Wt% of niobium;
0.02Wt% to 0.06Wt% of Titanium;
Up to 0.006Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, Wherein Ti to N is in ratio of 6 to 12.

2. A high strength low alloy cold rolled continuous annealed steel sheet of claim 1 including Si, composition of Si is based on ratio of Mn/(Al+Si) 5 to 16 preferably greater than 5.

3. A high strength low alloy cold rolled continuous annealed steel sheet of anyone of claims 1 or 2 , further comprising at least one element from Zr, Bi, V, W, Cr and Mo each by content in the range of 0.001 to 0.03 %.

4. A process for the manufacture of steel sheet comprising:
i) providing a selective steel composition for slab generation comprising:
0.05Wt % to 0.08Wt percent of Carbon;
0.4wt% to 0.8Wt% of Manganese;
0.02wt% to 0.06Wt% of Aluminum;
0.01Wt% to 0.03Wt% of niobium;
0.02Wt% to 0.06Wt% of Titanium;
Up to 0.006Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, Wherein Ti to N is in ratio of 6 to 12; and
ii) carrying out steel sheet manufacturing including hot rolled, pickled, cold reduced and continuous annealing such as to reach to excellent ageing resistance of atleast 6 months at room temperature with desired weldability.

5. A process as claimed in claim 4 comprising:
a) Hot rolling of said steel sheet with Finishing Temperature 860°C to 910°C preferably more than 860 °C with ROT cooling rate 8-15°C/Sec; and
b) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled to thickness less than 45% of initial Thickness.

6. A process as claimed in claim 4 or 5, further comprising:
a. Soaking said steel at temperature range between 770°C to 800 °C with residence time 50-94 Sec;
b. Slow cooling further said steel at temperature range 630 °C to 680 with cooling rate of 1°C /Sec to 5 °C/sec;
c. Rapid cooling of said steel at temperature 480°C or less with cooling rate 10°C /Sec to 18°C /Sec;
d. Over ageing of said steel at temperature range between 340 °C to 420with residence time of 180-350Sec; and
e. Skin passing of said steel between 1.5% to 2.3%.

7. A high strength low alloy cold rolled continuous annealed steel sheet of anyone of claims 4 to 6 which is obtained involving the selective steel composition and controlled continuous annealing for desired strength following Furnace Speed >= 0.56*SS TEMP + 1.25* SCS TEMP-(469.56 + 3323.5* C + 146* MN + 11191* TI + 9473* NB)

8. A process as claimed in anyone of claims 4 to 7 for said steel sheet having excellent weldability and spring back resistance, wherein the process is controlled such as to achieve anyone or more of:
a) Yield strength from 340-420 Mpa with YS/TS ratio more than 0.75;
b) No Yield point elongation after accelerated ageing to prevent spring back;
c) Phosphate grain size less than 3.5µm; and
d) Coating Weight 2-3g/m2.

Dated this the 10th day of August, 2016
Anjan Sen
Anjan Sen & Associates
(Applicants Agent)