FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)


TITLE OF THE INVENTION High strength microalloyed electric resistance welded steel tubes
APPLICANTS
TATA MOTORS LIMITED, an Indian company
Having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India

INVENTORS
Mr. Hariharan Venkatraman and Mr. Kanad Karandikar
Indian National of TATA MOTORS LIMITED,
an Indian company having its registered office
at Bombay House, 24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner
in which it is to be performed


FIELD OF INVENTION
The present invention deals with the base metal chemistry of electric resistance welded cross member tube. More particularly this invention deals with the level of microalloy elements and level of carbon in the base metal of tube that will make the tube weldable and will ensure consistency of mechanical properties along the length of the tube. This newly developed electric resistance welded grade tube will have properties better than cold drawn electric resistance welded grade specified in IS 3074.
BACKGROUND OF INVENTION
Pipes are widely used for structural parts in automobiles, structural parts includes parts like chassis cross members. These pipes needs to exhibit very good welding consistency as these parts are subjected to twisting and turning during normal service life of the vehicle. To ensure this normally cold drawn electric resistance welded tubes; grades mentioned in standard IS 3074 are used. Cold drawn electric resistance welded as drawn tube grade 1 mentioned in IS 3074 contain 0.12%C (max), 0.60% Mn (max), 0.04% S (max) and 0.04 P (max). Tensile strength and Yield strength of this tube is 430 MPa and 370 MPa respectively. Limitations of this tube are, firstly strength achieved by cold drawing is at the cost of reduction in percentage elongation of the tube and due to cold drawing consistency in mechanical properties along and across the length of the tube is difficult to maintain. On the other hand conventional electric resistance welded tube grade 3 which have; 0.3 5%C (max), 1.30% Mn (max), 0.04% S (max) and 0.04% P (max) tube cannot be used for the said application because yield strength of electric resistance welded tube grade 3 is 270 MPa which is lesser than cold drawn electric resistance welded as drawn tube grade 1 by 100 MPa. Also welding consistency along the length of the tube is difficult to maintain in electric resistance welded tube grade 3. Further to this percentage elongation, which is
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an important parameter as far as pipe bending and its performance under the test conditions is considered, is less and is 10% min.
OBJECT OF THIS INVENTION
1. The main object of this invention is to develop an ERW tube which will have properties comparable to or better than CEW1 AD tube and which will have a good welding consistency along the length of the tube.
2. Yet another object of this invention is to develop a low cost tube which will replace existing CEW1AD tube.
3. Yet another object of this invention is to develop an ERW tube which will have good formability and weldability.
SUMMARY OF INVENTION
The present invention describes a cost effective, weldable and high strength electric resistance welded tube. Chemical composition of this tube is 0.04 - 0.06% C, 0.5 - 0.7% Mn, 0.01 -0.11% Si, 0.02 % P (max), 0.002 - 0.02% S, 0.0 - 0.02 %Nb, 0.0 - 0.02% Ti, 0.0 - 0.02% V and N of 50 parts per million. This novel ERW tube has lower carbon content which makes this tube weldable. Also this tube contains microalloying elements such as Ti, Nb, and V which increases the yield strength of the tube. This tube though has a higher cost than electric resistance welded tube grade 3 is cheaper than cold drawn electric resistance welded as drawn tube grade 1 due to elimination of annealing, phosphating and cold drawing operations.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a graph of tensile plot of representative tensile sample of newly developed ERW grade
DETAILED DESCRIPTION OF INVENTION
The present invention provides an alloy for pipes which has Chemical composition 0.04 -0.06% C, 0.5 - 0.7% Mn, 0.01 - 0.11% Si, 0.02 % P (max), 0.002 - 0.02% S, 0.0 - 0.02 %Nb, 0.0 - 0.02% Ti, 0.0 - 0.02% V, Ti + Nb + V > 0.02 min and N of 50 ppm
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The following are the reasons for choice of the above specified chemical composition: Carbon (C): Carbon plays an important role in weldability of the tube and the basic strength of the tube. Carbon less than 0.4% results in steel of insufficient strength and carbon higher than 0.2% may pose a problem in welding of the tube. This is because weldability which is characterized by carbon equivalent (Ceq) as per American society of welding and given by following formula Ceq = %C + (%Mn)/6 + (%Ni+%Cu)/15 + (%Cr+%Mo+%V)/5 goes high. To avoid this carbon content was restricted to 0.04 - 0.06% and to take care of drop in strength due to lower carbon content micro alloying elements like Niobium (Nb), Titanium (Ti) and Vanadium (V) were added. These elements without increasing the carbon equivalent increase the strength by refining the grain size.
Manganese (Mn):An alloy containing Mn less than 0.5% poses problems with strength. On the other hand Mn above 1.3% poses problems in continuous casting of steel. This is due to segregation of Mn along the width of the concast. To avoid this Mn content was kept as 0.5 - 0.7%.
Silicon (Si) and Aluminium (Al): These two elements are added to steel as a
deoxidizer.
Si above 0.8 % makes steel difficult to roll. However both the elements are added to
deoxidize the steel. Si can be added in the range 0.01% - 0.1% and Al can be added in
the range 0.02% - 0.06%.
Phosphorus (P) and sulphur (S):To avoid any problems with formability of this tube phosphorus and sulphur level are kept as low as possible. However S in the range of 0.002% to 0.02% and Phosphorus up to 0.02% are acceptable.
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Titanium (Ti), Vanadium (V) and Niobium (Nb): Titanium forms TiN precipitate when Ti is present up to 0.02% and for this N has to be 50 ppm. 50ppm nitrogen level is dependent on stochiometric ratio of Ti: N. This TiN precipitates avoid grain coarsening in the slab heating stage of the tube and makes austenite grain size smaller. Further to this when V and Nb are present up to 0.02% refine the ferrite grain size during transformation of steel microstructure from austenite to ferrite under different processing routes. However total microalloying content should not be less than 0.02 % i.e. Ti + Nb + V > 0.02 min
Grain size of this newly developed grade is 9 and above. This is required as grain size plays important role in achieving required yield strength of 370 MPa. Grain size of 9 can be achieved in two ways either by adding microalloying elements or by thermo mechanical rolling i.e. by mechanically reducing the grain size when material is in hot condition and further reducing the grain size by phase transformation. This newly developed grade utilizes both the mechanisms to reduce the grain size and gain strength. When this steel is rolled and formed in to tube gives excellent weld strength, variation in properties along and across the tube length is minimal and tube is better in formability judged by drift, flattening and flaring tests. To illustrate this invention an electric resistance welded tube having minimum alloy composition in the range specified was made and tested. Properties of this tube were compared to cold drawn electric resistance welded as drawn tube grade 1. Yield strength which is a function of grain size which in turn affected by microalloying content and processing parameters. Processing parameters can alone give yield strength of 370 MPa but to have better factor of safety microalloying elements are added. These elements can increase the yield strength by 70MPa which is evident from tensile strength test graph attached.
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TABLES:

%C
0.052
%Mn 0.700
%Si 0.110
%P 0.010
%S 0.002
%A1 0.029
%V 0.020
% Ti 0.000
%Nb 0.000

Table I
Parameters CEW1 AD Newly developed ERW tube
Mechanical Properties of base metal:
TS 453.36-476.9 476 - 483
YS 365.1-462.3 440 - 442
%EL 9.4-10.2 16-18

Mechanical Properties of welded region:
TS 457.8 605-612
YS 442.7 574 - 579
%EL 7.4 12 - 12.4
Formability tests:
Drift 59.1%, 48.4%, 53.7% 56.6%, 55.0%, 53.0%

Cracks observed No cracks observed (Test
stopped due to test rig
limitations)
Flattening Passed Passed

Full flattening(Ot) Full flattening ( Ot)
Crush test Passed Passed

Full crushing Full crushing
Hydro test (1015 Psi for 5 sec min) Passed Passed
Eddy current test Passed Passed
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The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.
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WE CLAIM
1. A high-strength low-carbon electric resistance welded (ERW) grade tube being
formed of a base metal composition comprising essentially of:
about 0.04 - 0.06% C, about 0.5 - 0.7% Mn, about 0.01- 0.11% Si, upto 0.02% P, about 0.002 - 0.02% S; and
microalloying elements not less than 0.02% to increase the yield strength of said tube.
2. The high-strength low-carbon electric resistance welded (ERW) grade tube as claimed in claim 1, wherein said microalloying elements comprises about 0.0 - 0.02 % Nb, about 0.0 - 0.02% Ti, and about 0.0 - 0.02% V.
3. The high-strength low-carbon electric resistance welded (ERW) grade tube as claimed in claim 2, wherein said composition further comprises about 50ppm of N to form TiN precipitate to avoid grain coarsening in a slab heating stage of said tube and to make austenite grain size smaller.
4. The high-strength low-carbon electric resistance welded (ERW) grade tube as claimed in claim 1, having a grain size of 9 and above and an yield strength of more than 440 Mpa in the base metal and more than 575 Mpa in the welded region.
5. The high-strength low-carbon electric resistance welded (ERW) grade tube as claimed in claim 4, wherein said grain size is achieved by adding said microalloying elements and by thermo mechanical rolling of said tube.
6. Chassis cross members made of high-strength low-carbon electric resistance welded (ERW) grade tube as claimed in claims 1 to 5.
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7. A high-strength low-carbon electric resistance welded (ERW) grade tube as hereinabove described with reference to the accompanying drawings.

Dated this 24th day of March 2009