Description:
FIELD OF THE INVENTION:

The present invention relates to a cold-rolled micro-alloyed high-strength steel grade for automobiles by batch-annealing technology in Hydrogen atmosphere (95%). More particularly, this invention is directed to the development of high-strength cold-rolled steel grade sheet / Coils suitable for use in an automobile body and having a tensile strength (TS) of not less than 410 MPa, an average r-value of not less than 1.00, strain hardening exponent not less than 0.20 and excellent elongation.
The invention also relates to a method for manufacturing the same.

BACKGROUND AND PRIOR ART OF THE INVENTION:

In the field following relevant prior art patents with their brief disclosures are mentioned:

CN101265550A discloses cold rolling automobile plate for micro-alloy high-strength steel stamping and producing method thereof. The steel contains chemical compositions by the following weight percentages: C 0.04 - 0.10%, Si 0.03 - 0.05%, Mn 0.7 - 0.9%, P less than or equal to 0.030%, S less than or equal to 0.030%, Al 0.02 -0.07%, Nb 0.03 -0.07%, Fe and other unavoidable impurities, the rest. The production method includes the following steps of molten iron desulphurization, converter combined blowing, vacuum treatment, continuous casting, hot continuous rolling, stream cooling, coiling ( acid cleaning and steel rolling), batch annealing, leveling up, and finishing, The steel has excellent mechanical properties, simple chemical compositions, and maneuverable production method, which facilitates popularization and application.

CN103160754A discloses a micro alloy steel cast steel wheel which comprises, the following chemical components by wt. : C 0.55-0.70%, Si greater than or equal to 0.15%, Mn 0.50 - 1.00%, P less than or equal to 0.030%, S less than or equal to 0.040%, Cr 0.08 - 0.30%, Mo 0.05 - 0.20%, Ni less than or equal to 0.25%, Cu less than or equal to 0.35%, V less than or equal to 0.04%, Al less than or equal to 0.060%, Ti less than or equal to 0.03%, Nb less than or equal to 0.05% and the balance of Fe. According to the invention, the micro scale alloy is added for casting to form the micro-alloy steel cast wheel which has a certain superiority on a railway high speed heavy load aspect.

EP1291447A1 discloses cold-rolled steel sheet having excellent strain aging hardening properties and method for producing the same. The slab composition including by mass, C not more than 0.15%, Al not more than 0.02%, N 0.0050 - 0.0250% and in which Si + Mn/5 + 10P is adjusted to less than 0.44 and the ratio N/Al is adjusted to less than 0.3.

Indian Patent No. 227599 (763/KOL/2006) discloses development of cold rolled batch annealed bake hardening steel article and a process of preparing the same for automobile application. The steel composition comprising an ultra low carbon steel in weight % C < 0.0035, Mn 0.2 -0.6, Al < 0.03, S <0.02, P 0.03 - 0.06, N < 0.0050, and Nb and/or Ti < 0.02.

Though the above referred prior art steels also has the applications in the automobile industry, the present invention endeavors to improvise the technology further in the steel making.

In the quest for fuel economy, automobile manufacturers needed to reduce weight and this translated to reduced thickness of steel sheets. This led to the explosive growth in the use of higher and higher strength steels for the manufacture of automobiles. Their applications can be broadly classified into outer body panels and body-in-white parts. In the case of outer body panels, the major driver is weight reduction. This translates to more formable steels for manufacture of complex design parts as well as part consolidation. In addition, reduction of gauge for weight savings has led to the use of high strength steels. As the tensile strength increases above 350MPa drawability, weldability and bending performance deteriorates and it's hard to form the steel to desired automotive component with suitable surface property.

The present invention gives a solution to the problem. Chemical composition and processing route are such that the manufacturing of this steel grade is less expensive. Moreover, this grade is exclusively suitable for batch annealing process.

OBJECTS OF THE INVENTION:

The object of the present invention is to provide for an improved cold-rolled formable quality steel having tensile strength more than 410 MPa to manufacture of automobile body which eliminates the disadvantages of prior art and is applicable and usable for a wide range of automobile applications.

A further object of the present invention is to provide for an improved cold-rolled formable quality steel having tensile strength more than 410 MPa suitable for producing through batch annealing route.

A further object of the present invention is to provide for an improved cold-rolled formable quality steel having tensile strength more than 410 MPa with excellent elongation, r-bar, strain hardening exponent (n-bar).

A further object of the present invention is to provide for an improved cold-rolled formable quality steel having tensile strength more than 410 MPa with good cost effectiveness.

A further object of the invention is to provide for an improved cold-rolled formable quality steel having tensile strength more than 410 MPa in the form of sheets/ coils.

A further object of the invention is to provide for a process to manufacture the improved cold-rolled formable quality steel and also in sheet / coil form having tensile strength more than 410 MPa usable for a wide range of automobile applications.
SUMMARY OF THE INVENTION:

The present invention relates to a cold-rolled micro-alloyed high-strength steel grade for automobile structural application with a UTS >410 MPa having excellent elongation (>25%), YS/UTS >85%, r-bar>1.0 and strain hardening exponent (n-bar)>0.20; manufactured through an economic processing route. It is a low carbon based steel composition, strengthened by suitable addition of manganese and micro-alloyed by Nb & Ti.

The developed steel grade composition comprising chemical elements in terms of mass percent:
C: 0.03-0.08 %; ?n: 1.20% max; Si: 0.35% max; Al: 0.02-0.05%; P: 0.020 % or less; S: 0.008% or less; ?: 0.006% or less; Ti: 0.030% or less; Nb: 0.04 or less; and the balance being Fe and other unavoidable impurities.

The processing schedule including steel making, hot rolling, cold rolling and annealing have been determined appropriately so as to achieve desired drawability and strength. The microstructure of the steel after cold-rolling and batch-annealing comprises predominantly ferrite grains with uniformly fine size of the order of ASTM number 9-10, the grains possessing equiaxed or polygonal shape, and wherein the hot-rolling and cold-rolling process parameters adapted being 860-900°C, 600-640°C and 635-700°C respectively for finish rolling, coiling, and batch annealing, and >60% and 1-2% respectively being cold rolling deformation, and skin pass elongation.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:

FIGURE 1 shows the microstructure (optical micrograph) of the invented ISC410LA grade steel.

--------------------------------------------------------------------------------------------------------------------

---------------------------------------------------------------------------------------------------------------------
DETAILED DESCRIPTION OF THE INVENTION:

The present invention relates to a cold-rolled micro-alloyed high-strength steel grade (ISC410LA) for automobiles by batch-annealing technology in Hydrogen atmosphere (95%).
The present invention particularly relates to a high-strength cold-rolled steel grade (ISC410LA) Sheets/Coils suitable for use in an automobile body and having a tensile strength (TS) of not less than 410 MPa, an average r-value of not less than 1.00, strain hardening exponent not less than 0.20 and excellent elongation.

The invention includes a method for manufacturing the same.

The developed steels grade (ISC410LA) possesses high strength primarily by micro-alloying elements contributing to fine carbide precipitation, substitution and interstitial strengthening, and grain-size refinement.

The processing schedule includes steel making, hot rolling, cold rolling and annealing have been determined appropriately so as to achieve the desired drawability and strength.

Following Table 1 gives the Heat Chemistry of specific embodiments of invented ISC410LA grade steels apart from Fe and other unavoidable impurities in mass percents.

Table 1: Heat Chemistry of invented ISC410LA grade steels

C Mn P S Si Al Nb Ti [N]
0.07 1.14 0.012 0.005 0.319 0.04 0.028 0.025 0.057
0.07 1.12 0.019 0.006 0.250 0.04 0.035 0.020 0.055

Process route for the development of ISC410LA grade steel is :
? Primary steel making by Basic Oxygen Furnace
? Secondary steel refining in Ladle furnace for fine tuning of alloying element and desulphurization
? Continuous slab casting
? Hot charging of slabs to reheat furnace
? Slab drop out @ around 1200°C-1250oC
? Rough Rolling finish @ 1080oC or less
? Finish rolling @ around 880 ± 20oC
? Coiling @ 600°C-640°C
? Acid pickling and Cold rolling with >60% reduction
? Batch annealing @ Hot spot temperature 670°C-700°C& Cold spot temperature 635oC-670oC
? Skin pass rolling @ 1.0-2.0% temper elongation.

The following Table 2 gives the mechanical properties of different invented ISC410LA grade steel as Coils.

Table 2: Mechanical properties of different invented ISC410LA grade steel as coils

Coil Id Thickness, mm YS, MPa UTS, MPa %El YS/UTS MPa n-bar r-bar
A79811 1.200 351.0 461.0 30.0 0.76 --- 1.09
A79815 1.200 370.0 485.0 30.0 0.76 --- 1.15
A79601 1.600 386.0 505.0 32.0 0.76 0.201 1.20
A79614 1.600 365.0 451.0 32.0 0.81 0.201 1.20

The microstructure (optical micrograph) of the invented ISC410LA grade steel after cold-roiling and batch-annealing is shown in the accompanying Figure 1.

The invented steel is found to be predominantly comprising fine ferrite grains with uniform sizes of the order ASTM number 9-10. The grains were found to possess equiaxed and polygonal shape. The process parameters during the entire process chain of the material couple with steel chemistry contributes towards an optimum microstructure, resulting into an excellent combination of mechanical properties.

ADVANTAGES OF THE INVENTION
- Optimum chemistry and process parameters of the steel resulted in consistent and favorable mechanical properties suitable for automotive steel products.
- The material achieved high tensile strength without exceeding the maximum limit of yield strength and the minimum limit of elongation.
- Minimum usage of micro-alloying and other elements in the steel resulted into excellent balance of strength and ductility required by automotive manufacturers.
- Uniformly fine scale of the microstructure has been achieved with the appropriate process parameters and chemistry of the steel, without sacrificing the mill productivity.

Although the foregoing description of the invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of general illustration and is not intended to limit the invention to only the particular embodiments and applications disclosed. It would be apparent to those having ordinary skill in the art that a number of changes, modifications, variations or alterations to the invention as disclosed herein may be made, none of which depart from the spirit or scope of the present invention. All such changes, modifications, variations and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.

Claims:WE CLAIM:

1. Cold-rolled high strength steel having tensile strength of at least 410 MPa with composition in terms of weight % comprising of:
C : 0.03-0.08 % , ?n : 1.20% max,
Si : 0.35% max, Al : 0.02-0.05% ,
P : 0.020 % or less, S : 0.008 % or less,
? :0.006 % or less, Ti : 0.030% or less,
Nb : 0.04 or less
and the balance being Fe and other unavoidable impurities.

2. Cold-rolled high strength steel as claimed in claim 1 is of ISC410LA grade and has the following heat chemistry of elements in weight %:
C 0.07, Mn 1.14, P 0.012, S 0.005, Si 0.319, Al 0.04, Nb 0.028, Ti 0.025, N 0.057 and the balance being Fe and other unavoidable impurities.

3. Cold-rolled high strength steel as claimed in claim 1 is of ISC410LA grade and has the following heat chemistry of elements in weight % :
C 0.07, Mn 1.12, P 0.019, S 0.006, Si 0.250, Al 0.04, Nb 0.035, Ti 0.020, N 0.055 and the balance being Fe and other unavoidable impurities.

4.. Cold- rolled high strength steel as claimed in claims 1 to 3 is having the following properties :
Yield strength of 340 MPa or more,
Tensile strength of 410 MPa or more,
YS/ UTS ratio of 0.85 or less
Total elongation of 25% or more,
Strain hardening coefficient of 0.20 or more and
r-bar of 1.0 or more.

5. Cold-rolled high strength steel as claimed in claims 1 to 5 is having properties as required in the automobile industry.
7. Cold-rolled high strength steel as claimed in claims 1 to 6 is in the form of sheets /coils.

8. A process for the manufacture of cold-rolled high strength steel sheet/coil as claimed in claims 1 to 7 comprising the steps of:
i) Producing the steel having the mentioned composition by processing heat from basic oxygen furnace, Ladle refining without vacuum degassing and continuously cast in slabs;
ii) Subjecting said slabs to reheating at a temperature in the range from 1200°C to 1250°C;
iii) Subjecting said reheated slab to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
iv) Subjecting said rough rolled steel to finish rolling with finish mill exit temperature 880 ± 20oC;
v) Coiling the finish rolled steel with average run out table cooling rate of 8°C/second or more; coiling temperature less than 600-640oC;
vi) Acid Pickling and Cold rolling the said hot rolled steel Coils with cold reduction of 60% or more ;
vii) Batch Annealing in Hydrogen atmosphere (95%) of cold rolled steel coil at hot spot temperature 670-700oC and Cold spot temperature 635-670oC; and
viii) Skin pass rolling @ 1% temper elongation.