The present invention relates to producing of cold rolled batch annealed bake hardening steels. Such steels possess higher yield and tensile strength than conventional IF steels, excellent formability, bake hardening and shelf life and can be used such as automobile applications in those components, which are put for commercial paint baking operation.
BACKGROUND OF THE INVENTION
Commercial bake hardened steels possessing superior dent behaviour as compared to conventional extra deep drawing steel resulting in fuel savings of the automobile by gauge reduction or improvement in dent resistance in case of same thickness is reckoned since Jong for its development.
The present invention has successfully developed bake hardening steel through trials forming automobile components such as inner, outer and side panels of automobiles manufacturers'premises.
In the prior art low carbon aluminium killed steel, conventionally known as extra deep drawing (EDD) grade was primarily used for auto body applications. In view of the better formability requirements, the commercial application of Interstitial Free (IF) steel came into practice in early eighties. These steels showed good formability properties but dent resistance was poor. The steel for automobile body panel applications requires higher strength for better dent resistance and good formability for the defect free formation of the component designed for the automobile. In view of the above, a new steel called bake hardening steel was developed in which the initial yield strength may vary to > 180 MPa, which gives rise to an appreciable increase in yield strength of 30-50 MPa during commercial baking operation of the formed component. This extra increase in strength will
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lead to improvement in dent resistance or a thinner gauge material can be used for the same purpose.
The mechanism of bake hardening is similar to classical strain-aging phenomenon. The rise in strength is caused by the interaction between solute atoms and dislocations. During baking of the press-formed components, the increase in the flow stress mainly occurs by the segregation of carbon to dislocations (Cottrell atmosphere formation and locking) and precipitation of carbides.
The bake hardenability is primarily achieved by the presence of controlled amount of solute carbon in solution after cold rolling, annealing and skin passing.
DESCRIPTION OF THE INVENTION
The bake hardening steel is easier to produce through continuous annealing route due to advantage of higher temperature of annealing and higher cooling rate. Production of these steels through batch annealing route has been difficult task for the steelmakers because of the low temperature of annealing and slow cooling process. In the present invention, bake-hardening steel with 30-50 MPa BH and shelf life > 3 months has been developed through Batch Annealing route. A judicious selection of chemistry, a precise control of hot rolling, cold rolling, annealing and skin pass parameters are the enabiers to obtain required amount of carbon in solid solution for a good combination of bake hardening strength and shelf life. The shelf life is defined as the period up to which can be formed into component without any stretcher strain (Luder Lines).
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Main objective of the invention is to prepare bake hardening steels with the following salient features
(i) Bake hardening steei sheet with Mn-P alloyed ultra low carbon chemistry has been produced through Batch Annealing route with
YS>180 MPa, r > 1.7 and n-value>0.18.
(ii) The steel possess bake hardening strength in the range of 30-50 MPa with shelf life of > 3 months.
Another main objective of the invention is to achieve its primary objective to develop a compatible - bake hardening strength and shelf life of the bake hardening steel by providing controlled amount of solute carbon in solution while sequencing through cold rolling, annealing and skin passing.
A further objective of the invention is to select steel composition based on ultra low carbon along with some solid solution strengthening elements Mn and P and with micro alloying additives Nb and/or Ti if necessary.
A still further objective of the invention is to provide good combination of bake hardening strength and shelf life of the bake hardening steel article by judicious selection of composition of the steel and a precise control of hot rolling, cold rolling, annealing and skin pass parameters.
The present invention relates to a cold rolled batch annealed bake hardening steel article formed by producing an ultra low carbon steel in a LD converter of composition in weight % C - 65% deformation, the resulted cold rolled coils being annealed in batch annealing furnace with optimum annealing cycle and the annealed coils being given skin pass rolling with deformation > 0.8%, the resulted steel article being characterization evaluated for Bake Hardening through the course of giving a test specimen of Ihe resulted cold rolled steel an elongation of 2%, heating at 170°C for 20 minutes in a furnace and then air cooled and finally calculating for difference between yield stress after baking and flow stress after 2% elongation.
According to the invention there is also provided a process of producing a cold rolled batch annealed bake hardening steel and article produced from the steel comprising the steps of producing an ultra low carbon steel in a LD converter of composition 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, by controlling through RH degasser to maintain required level of carbon and other impurities, continuously casting the steel into slabs, hot rolling the slabs and finishing the resulted strip at required temperature ranges and coiling according to the specification, coid rolling the hot rolled coils with > 65% deformation, the resulted cold rolled coils being annealed in batch annealing furnace with optimum annealing cycle and the annealed coils being given skin pass rolling with deformation > 0.8%, the resulted steel article being characterization evaluated for Bake Hardening through the course of giving a test specimen of the resulted cold roiled steel an elongation of 2%, heating at 170°C for 20 minutes in a furnace and then air cooled and finally calculating for difference between yield stress after baking and flow stress after 2% elongation.
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DETAILED DESCRIPTION OF THE INVENTION
The invention will be better understood from the following description with reference to the accompanying drawing and illustrations in which
Figure 1 represents a flow chart of the process for producing a bake hardening steel sheet.
The steel sheet is produced through judicious selection of steel composition produced through LD process, its processing, slab casting and sequencing through hot railing, cold roiling, batch annealing and skin pass roiiing.
A proper selection of chemistry is very important for achieving a combination of strength, formability and bake hardening properties. The steel selected in the present investigation is based on ultra low carbon along with some solid solution strengthening elements such as Mn and P. Increasing Mn and P in steel would increase both yield strength and ultimate tensile strength. Aluminium has been primarily used as a de-oxidizer. Sulphur and Nitrogen have been kept low.
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The chemical composition selected for bake hardening steel is shown in the following Table 1.


The commercial heat with the above composition was made in 130 tonne LD converter.
The liquid steel was processed through RH degasser to achieve the required level of carbon and control the other impurities. The liquid steel was continuously cast into slabs at Slab Caster. Slabs were hot rolled at Hot Strip Mill (HSM) with appropriate finish rolling temperature (FRT) and coiling temperature (CT).
The hot rolled coils were cold rolled at cold rolling mill (CRM) with >65% deformation.
The cold rolled coils were annealed in batch annealing furnace (BAF) with suitable annealing cycle. These coils were given skin pass deformation of >0.8% at Skin Pass Mill (SPM) at Cold Rolling Mill.
The yield strength (YS), ultimate tensile strength (UTS) and elongation of the cold rolled (CR)-annealed-skin passed samples were evaluated using Instron Machine. Drawability (r), was evaluated using Modul r- tester. Stretchability (n-value) and bake hardening (BH) properties were also evaluated by tensile test method. For studying the room temperature aging behaviour, the samples were preserved at room temperature for a period over three months and then tested for tensile properties (yield strength, tensile strength, elongation, n value) and bake hardening (BH) strength. The method for evaluating the bake hardening strength is described below.
Experimental determination of Bake Hardening (BH) strength
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For evaluating the BH strength, tensile specimen was given an elongation of 2%, heated at 170 °C for 20 min in a furnace and then air-cooled. The aged sample was again tested for tensile properties. The yield point of the final test sample was noted and BH strength was evaluated as the difference between lower yield stress after baking and flow stress after 2% elongation.
Typical properties:
Typical properties of cold rolled strips in skin passed condition are shown in Table 2.
Table 2: Typical properties of cold rolled (CR) strip after annealing and skin pass operation

It may be seen that even after 3 months, the properties are almost similar. Yield point elongation (YPEL) was not observed initially in skin passed condition as well as after three months of holding the samples at room temperature.
The finally processed cold rolled strips were tried for manufacturing of automobile components of three wheeler, four wheeler and trucks at automobile manufacturer's premises. The components were successfully formed on the trial basis and found to be suitable for this purpose.
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The steel sheet developed in the present work can be used for the automobile applications in those components, which are put for commercial paint baking operation. Some examples of these components are side, outer, inner body panels. The developed steel in the present work possesses higher yield and tensile strength than conventional IF steel, excellent formability (r & n-value), bake hardening strength of 30-50 MPa and, good shelf life. With the use of this material, a thinner gauge sheet will replace the thicker material of lower strength thus decreasing the weight of the automobile, which will reduce the weight of the vehicle and further improve fuel efficiency of automobiles. Due to higher strength of this material as compared to the conventional material, it would be better in terms of dent resistance and could lead to a better safety of the automobiles and the passengers as well. The resulted article as proposed in the description and illustration of the invention no doubt will have large commercial demand in automobile industries.
The invention as narrated herein should not be read in a restrictive manner as many modifications, adaptations and changes can be made to form many embodiments beyond the illustrated one within the scope and limit of the appended claims.
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WE CLAIM
1. A eold rolled batch annealed bake hardening steel article formed by
producing an ultra low carbon steel in a LD converter of composition in weight %
C - <0.0035, Mn 0.2-0.6, Ai -<0.03, S-<0.02, P - 0.03 - 0.06, N - <0.0050 and
Nb and/or Ti <0.02f by controlling through RH degasser to maintain required
level of carbon and other impurities, continuously casting the steel into slabs, hot
rolling the slabs and finishing the resulted strip at required temperature ranges
and coiling according to the specification, cold rolling the hot rolled coils with >
65% deformation, the resulted cold rolled coils being annealed in batch
annealing furnace with optimum annealing cycle and the annealed coils being
given skin pass rolling with deformation > 0.8%, the resulted steel article being
characterization evaluated for Bake Hardening through the course of giving a
test specimen of the resulted cold rolled steel an elongation of 2%, heating at
170°C for 20 minutes in a furnace and then air cooled and finally calculating for
difference between yield stress after baking and flow stress after 2% elongation.
2. An article as claimed in claim 1 wherein the steel produced is based on
ultra law carbon along with some solid solution strengthening elements such as
Mn, P and micro alloying elements Nb and/or Ti.
3. An article as claimed in claim 1 wherein the yield strength (YS), ultimate
tensile strength (UTS), elongation of the cold rolled annealed - skin passed
samples and drawability (r), stretchability (n-value) and bake hardening (BH)
properties were evaluated by tensile test method.
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4. An article as claimed in the preceeding claims wherein the developed steel
possesses higher yield and tensile strength than conventional Interstitial Free
(IF) steel and have compatible formability, bake hardening strength of 30-50
MPa and good shelf life.
5. A steel composition as claimed in claim 1 comprising in wt % C-<0.0035,
Mn-0.2-0.6, Al <0.03, S-<0.02, P-0.03-Q.06, N-<0.0050 and Nb and/or Ti <0.02.
6. An article as claimed in the preceeding claims wherein the article formed
is a steel sheet.
7. A steel sheet as claimed in the preceeding claims wherein thinner gauge
sheet of improved properties than the conventional IF steel sheet can be used In
automobile components such as side, outer, inner body panels etc thus reducing
the weight of a vehicle and improving fuel efficiency of automobiles.
8. A steel sheet as claimed in claims 5,6&7, has shelf life more than 3
months.
9. A cold rolled batch annealed bake hardening steel article as herein
described and illustrated.
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10. A process of producing cold rolled batch annealed bake hardening steel and article produced from the steel comprising the steps of producing an ultra low carbon steel in a LD converter of composition in weight % C - <0.0035, Mn 0.2-0.6, Al - 65% deformation, the resulted cold rolled coils being annealed in batch annealing furnace with optimum annealing cycle and the annealed cotfs being given skin pass rolling with deformation > 0.8%, the resulted steel article being characterization evaluated for Bake Hardening through the course of giving a test specimen of the resulted cold rolled steel an elongation of 2% heating at 170°C for 20 minutes in a furnace and then air cooled, and finally calculating for difference between yield stress after baking and flow stress after 2% elongation.

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