Title of Invention | A METHOD TO DETERMINE THE SHELF LIFE OF BAKE HARDENING SHEETS IN AN INDUSTRIAL PRACTICE |
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Abstract | The shelf life is defined as the maximum period of storage of finally processed cold rolled annealed sheets at room temperature prior to use, without showing any stretcher strain in the component after forming. Aging kinetics of cold rolled-annealed ultra low carbon bake hardening steel has been studied in the temperature range of 50- 100°C and based on these results, the activation energy for the aging process was determined as 21-24 kCal/mole, which is higher than that of diffusion of carbon or nitrogen in iron. Using this activation energy, Hundy equation was modified. The shelf life predicated using Hundy equation is significantly lower than that of actual test data, whereas the predictions made using modified Hundy equation (from higher activation energy) is close to the actual room temperature aging data. In the present invention a simple method has been developed to determine the shelf life of ultra low carbon steel in an industrial practice. |
Full Text | 2 FIELD OF THE INVENTION The Invention relates to the development of a method to determine the shelf life of bake hardening steel in industrial practice. BACKGROUND OF THE INVENTION The steel for automobile body panel applications requires higher strength for better dent resistance and good formabillty for defect free formation of the component of automobile. In view of the above, bake hardening steel was developed having a good combination of yield strength and formabillty and it gives rise to an appreciable increase in yield strength of 30-50 MPa during commercial baking operation (around 17 °C / 20min) of the formed component This extra increase in strength leads to an improvement in dent resistance. A thinner gauge of this material can also be used replacing the conventional thicker material. In advanced automobile, use of bake hardening steels is well established due to the additional strength achieved during the commercial baking operation. This bake hardening is primarily achieved by the presence of controlled amount of interstitial carbon in solid solution after cold roiling and annealing. Ultra low carbon (ULC) steel is the prime material for bake hardening (BH) grade, since it possesses excellent formability and it is easier to control the solute carbon to a desired level at the 3 steel making stage. In addition, these steels can easily be processed through various routes such as batch annealing, hot dip galvanizing gaivannealing route and continuous annealing route. However, automakers are concerned about the room temperature aging problem in bake hardening steels due to inappropriate amount of solute elements, which may cause stretcher strain during forming at automobile manufacturer's place. This leads to a rejection of material adding to an extra cost for manufacturing. The requirement is that the material should have sufficient shelf life so that it should not degrade during the transportation and storage before the forming operation. The knowledge of the shelf life is necessary for both steel makers and automakers. For steel makers, it is important to know the shelf life in order to control the inventory and decide a period for its storage, transportation and its delivery, whereas for automobile component manufacturers it is useful in deciding its storage period and controlling inventory to produce the automobile components without any rejection. There is no standard available to evaluate the shelf life of bake hardening steel and hence different automakers use their own technique for its evaluation. Some of the them are using the conventional method i.e. strain aging, which is mainly based on the rise in the flow stress after accelerated aging test at 100 °C after 7.5-10% tensile deformation. The method does not suit for predicting the room temperature aging behaviour of ultra low carbon steel in skin-passed condition, specially to be used for manufacturing auto-components, where stretcher strain caused by yield point 4 elongation of the input material is the main concern. Hundy has presented an expression for simulating the aging behaviour at different temperature based on Cottrell-Bilby model an the prediction made by it deviates from the actual room temperature test data in case of skin-passed or temper rolled material. Carrying out actual room temperature test for shelf life evaluation is not possible in view of long time and large number of samples required for this. Thus there is a need to formulate a simple method for the prediction of shelf life in an industrial practice. In view of the limited work in this area, the present invention focuses on evaluating the strain aging behaviour of ultra low carbon bake hardening steel and then formulating a simple method to determine the shelf life applicable In an industrial practice. OBJECTS OF THE INVENTION - the object of the invention Is to determine the shelf life of bake hardening steel in an industrial practice. - another object of the invention is to reduce the rejection loss of bake hardening steel for steel manufacturers and ancillary users. - further object of the Invention is to optimise production / inventory control of steel manufacturers and ancillary users. 5 - yet another object of the invention is to formulate a simple method for ascertainment of shelf life of bake hardening steel. DETAILED DESCRIPTIONS Of THE PREFERRED EMBODIMENT Cold rolled ultra low carbon (ULC) steels alloyed with Mn and P were taken for present investigations. The bake hardening strength in these steels were found to vary from 31 to 43 MPa. For assessing the kinetics of aging, the tensile specimens were aged for various periods at different aging temperatures (Ta) i.e. 50, 70, 85 and 100 °C. The yield point elongation (YP-EL) was then measured by conducting tensile test. To study the room temperature aging behaviour, the samples were aged for various periods in oil bath furnace maintained at 30 °C and then yield point elongation were similarly measured. For determination of the bake hardening strength, tensile specimens were pre-strained to 2%, and then heated at 170 °C for 20 minutes in an oil bath furnace. The aged sample was again tested on a tensile machine to measure lower yield stress. BH strength was evaluated as the difference between lower yield stress after baking and flow stress after 2% elongation. 6 Kinetics of aging process was assessed using following equation, which is derived form of Johnson - Menl - Avrami (3 - M - A) equation: In (ty) = C + Q / RT — (1) Where ty = time for a given fraction of transformation, and Q = Activation energy for the governing process. In the present analysis, the time for yield point appearance was used as ty. The In (ty) was plotted for different steels as a function of 1 / T and the activation energy calculated from the slope for the different steels was found to be in the range of 21 - 24 kCal / mole. Hundy derived the following correlation using Cottrell-Bilby equation log [t/t] = K [l/Tr-l/T] - log [T/Tr] — (2) Where, K = constant = Q / R (log10e), Q = Activation energy, R = gas constant, (for carbon, K = 4400; for nitrogen, K = 4000), Tr = room temperature (K), T = artificial aging temperature (K), t = aging time at room temperature, Tr t= aging time at artificial aging temperature T. It enables the aging time at any temperature to be converted to an equivalent aging time at room temperatures. The constant K was derived from the activation energy for the diffusion of C or N in iron. The above equation is reported to deviate from the actual room temperature test data in temper rolling condition. This 7 could be due to the higher activation energy in the skin passed or temper rolled material. The average activation energy determined in the present results was used to re-calculate the exact K-value of the Hundy equation and thus modified Hundy equation can be represented as follows: Log [tr/tr] = 5030 [1/Tr-1/T] - Log [T/Tr ]- (3) The shelf life was predicted by both Hundy equation and modified Hundy equation. The pedictions made by Hundy equations are much lower than that of actual room temperature test data, whereas the prediction made using modified equation matches well with them. In the present work, the shelf life was defined as the time of room temperature aging when the yield point elongation becomes 0.2%. This level of point elongation may be acceptable by the automobile manufacturers. To determine the exact shelf life (time for YPEL- 0.2%), it is necessary to conduct a large number of tests at a high temperature for simulation. In the present invention a simple method has been designed for evaluating the shelf life in an industrial practice, YP-EL value at the aging temperature of 100 °C for lh was selected as the aging parameter for accelerated ageing test. A relationship between the shelf life as a function of YP-EL for 100 °C/lh has been established as shown in fig. 2. The laid down criteria is shown in table-III. Based on this relationship, the 8 expected shelf life can be assessed by a single simple test and can be utilized in industrial condition as a routine basis. The selecting "100 °C" as the test temperature is important since this temperature can be achieved accurately in a boiling water bath and it can be used in place of using furnace. A typical graph for the experimental evaluation of shelf life for such sample steel is shown in Fig 1. The stretcher strain in the formed component Is related to the yield point elongation of the material before forming. It has been observed that there is no occurrence of stretcher strain up to a certain YPEL value. A limit of 0.2% YPEL for no stretcher strain has been considered. One can decide any test temperature or aging time for simulation. In the present method, aging parameter as 100 °C for lh has been selected for the convenience of the test in industrial condition. Test temperature as 100 °C has been selected since this is the boiling point of water. The samples can be heat-treated in boiling water easily without any variation in temperature. An appropriate time of lh has been decided since too long time period may not be possible for conducting this test on routine basis in an industrial condition. Shorter time period may result in a very low yield point elongation causing an error in measurement. 9 Similar aging parameter (100 °C/lh) has also been suggested in a paper reported by Hundy for simulation for aging process. The relationship has been established and shown In Fig 2. And shelf life can be ascertained from the chart as shown in Table - III from above experiments. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 1. Fig - 1 shows an experimental evaluation of shelf life and its validation with the actual room temperature test (assumed as 30 °C). 2. Fig - 2 shows relationship between the shelf life and yield point elongation. 3. Fig - 3, shows relationship between Aging time and Yleldpolnt Elongation 4. Fig - 4, shows slopes of various steel. 5. Table - III shows a simulated relationship data of YPEL at 100 °C / lh (%) and shelf life. 10 SUMMARY OF THE INVENTION The shelf life is defined as the maximum period of storage of finally processed cold rolled annealed sheets at room temperature prior to use, without showing any stretcher strain in the component after forming. Aging kinetics of cold rolled-annealed ultra low carbon bake hardening steel has been studied in the temperature range of 50- 100°C and based on these results, the activation energy for the aging process was determined as 21-24 kCal/mole, which is higher than that of diffusion of carbon or nitrogen in iron. Using this activation energy, Hundy equation was modified. The shelf life predicated using Hundy equation is significantly lower than that of actual test data, whereas the predictions made using modified Hundy equation (from higher activation energy) is close to the actual room temperature aging data. In the present invention a simple method has been developed to determine the shelf life of ultra low carbon steel in an industrial practice. 11 WE CLAIM 1) A method to determine the shelf life of bake hardening steel In industrial practice. 2) The method as claimed under claim 1 characterised in that preparation of test data of yield point elongation at different temperatures and time for samples of cold rolled ultra low carbon steels (ULC) alloyed with Mn and P. 3) The claim as claimed under claim 1 and 2 characterised by pre straining of the said samples to 2% and heating at 170 °C for 20 minutes, and evaluation of bake hardening strength as difference between lower yield stress after baking and flow stress after 2% elongation. 4) The claim as claimed under claim 1, 2 and 3 characterised in that determination of activation energy from the test data and slope of various steels (In (ty) VSV1/T) as shown in fig - 4. 12 5) The claims as claimed under claims 1, 2, 3 and 4 characterised in that simulation of strain aging and determination of shelf life through plotting the graphs as shown in Fig - 1 and Fig - 2 from the derived test data. The shelf life is defined as the maximum period of storage of finally processed cold rolled annealed sheets at room temperature prior to use, without showing any stretcher strain in the component after forming. Aging kinetics of cold rolled-annealed ultra low carbon bake hardening steel has been studied in the temperature range of 50- 100°C and based on these results, the activation energy for the aging process was determined as 21-24 kCal/mole, which is higher than that of diffusion of carbon or nitrogen in iron. Using this activation energy, Hundy equation was modified. The shelf life predicated using Hundy equation is significantly lower than that of actual test data, whereas the predictions made using modified Hundy equation (from higher activation energy) is close to the actual room temperature aging data. In the present invention a simple method has been developed to determine the shelf life of ultra low carbon steel in an industrial practice.The shelf life is defined as the maximum period of storage of finally processed cold rolled annealed sheets at room temperature prior to use, without showing any stretcher strain in the component after forming. Aging kinetics of cold rolled-annealed ultra low carbon bake hardening steel has been studied in the temperature range of 50- 100°C and based on these results, the activation energy for the aging process was determined as 21-24 kCal/mole, which is higher than that of diffusion of carbon or nitrogen in iron. Using this activation energy, Hundy equation was modified. The shelf life predicated using Hundy equation is significantly lower than that of actual test data, whereas the predictions made using modified Hundy equation (from higher activation energy) is close to the actual room temperature aging data. In the present invention a simple method has been developed to determine the shelf life of ultra low carbon steel in an industrial practice. |
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01055-kol-2007-correspondence others 1.1.pdf
01055-kol-2007-correspondence others.pdf
01055-kol-2007-description complete.pdf
1055-KOL-2007-(20-12-2011)-ABSTRACT.pdf
1055-KOL-2007-(20-12-2011)-CLAIMS.pdf
1055-KOL-2007-(20-12-2011)-CORRESPONDENCE.pdf
1055-KOL-2007-(20-12-2011)-DESCRIPTION (COMPLETE).pdf
1055-KOL-2007-(20-12-2011)-DRAWINGS.pdf
1055-KOL-2007-(20-12-2011)-FORM-1.pdf
1055-KOL-2007-(20-12-2011)-FORM-2.pdf
1055-KOL-2007-(20-12-2011)-OTHERS.pdf
1055-KOL-2007-(29-08-2011)-AMANDED CLAIMS.pdf
1055-KOL-2007-(29-08-2011)-AMANDED PAGES OF SPECIFICATION.pdf
1055-KOL-2007-(29-08-2011)-DESCRIPTION (COMPLETE).pdf
1055-KOL-2007-(29-08-2011)-DRAWINGS.pdf
1055-KOL-2007-(29-08-2011)-EXAMINATION REPORT REPLY RECIEVED.pdf
1055-KOL-2007-(29-08-2011)-FORM 1.pdf
1055-KOL-2007-(29-08-2011)-FORM 2.pdf
1055-KOL-2007-(29-08-2011)-OTHERS.pdf
1055-KOL-2007-(29-08-2011)-PETITION UNDER RULE 137-1.1.pdf
1055-KOL-2007-(29-08-2011)-PETITION UNDER RULE 137.pdf
1055-KOL-2007-CORRESPONDENCE 1.1.pdf
1055-KOL-2007-CORRESPONDENCE.pdf
1055-KOL-2007-EXAMINATION REPORT.pdf
1055-KOL-2007-GRANTED-ABSTRACT.pdf
1055-KOL-2007-GRANTED-CLAIMS.pdf
1055-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf
1055-KOL-2007-GRANTED-DRAWINGS.pdf
1055-KOL-2007-GRANTED-FORM 1.pdf
1055-KOL-2007-GRANTED-FORM 2.pdf
1055-KOL-2007-GRANTED-LETTER PATENT.pdf
1055-KOL-2007-GRANTED-SPECIFICATION.pdf
1055-KOL-2007-REPLY TO EXAMINATION REPORT.pdf
Patent Number | 252778 | ||||||||
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Indian Patent Application Number | 1055/KOL/2007 | ||||||||
PG Journal Number | 22/2012 | ||||||||
Publication Date | 01-Jun-2012 | ||||||||
Grant Date | 30-May-2012 | ||||||||
Date of Filing | 30-Jul-2007 | ||||||||
Name of Patentee | TATA STEEL LIMITED | ||||||||
Applicant Address | JAMSHEDPUR | ||||||||
Inventors:
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PCT International Classification Number | B21D22/22; B21D22/02; B21D22/20 | ||||||||
PCT International Application Number | N/A | ||||||||
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PCT Conventions:
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