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Low Carbon Bake Hardening Steel Sheets Produced Through Batch Annealing Route And A Process For Its Production.
Abstract: ABSTRACT TITLE: LOW CARBON BAKE-HARDENING STEEL SHEETS PRODUCED THROUGH BATCH ANNEALING ROUTE AND A PROCESS FOR ITS PRODUCTION. The present invention relates to high strength rephosphorized Low Carbon Bake hardening steel sheet having excellent formability and surface finish, and a method for manufacturing the same through batch annealing route. The cold rolled batch annealed bake hardening steel sheet obtained by the process is intended mainly for outer panel material for automobile application where the strength of the drawn parts can be further increased by paint baking process. Importantly, the developed steel grade is a low cost low carbon steel with carbon wt% ranging from 0.01-0.02 wt% and micro alloying with B, having yield strength YS of not less than 220 MPa, before bake hardening, average r-value of higher than 1.6 and a Bake hardening (BH) value of not less than 35 MPa, favouring application in low tare weight dent resistant car body fabrication leading to improved fuel efficiency.
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Notices, Deadlines & Correspondence
Notices, Deadlines & Correspondence
Patent Information
Application #
Filing Date
17 February 2015
Publication Number
34/2016
Publication Type
INA
Invention Field
MECHANICAL ENGINEERING
Status
Email
anjanonline@vsnl.net
Parent Application
Patent Number
Legal Status
Grant Date
2021-04-20
Renewal Date
Applicants
JSW STEEL LIMITED
JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051
MAHARASHTRA,INDIA.
Inventors
1. SINGH, Rajan Kumar
Product Design and Quality Control(PDQC) Department,HSM2 Glass House(1st Floor),
JSW Steel Limited,
Vijaynagar works,
Toranagallu, Bellary-583275,
Karnataka, India.
2. MISHRA, Devasish
Product Design and Quality Control(PDQC) Department,HSM2 Glass House(1st Floor),
JSW Steel Limited,
Vijaynagar works,
Toranagallu, Bellary-583275,
Karnataka, India.
3. SHARMA, Sanjay
Cold Rolling Mill(CRM)-2 Office,
JSW Steel Limited,
Vijaynagar works,
Toranagallu, Bellary-583275,
Karnataka, India.
4. RATHORE, Gajraj Singh
Glass House(1st Floor),
JSW Steel Limited,
Vijaynagar works,
Toranagallu, Bellary-583275,
Karnataka, India.
5. CHANDRAWANSHI, Madhawan
Product Design and Quality Control(PDQC) Department,HSM2 Glass House(1st Floor),
JSW Steel Limited,
Vijaynagar works,
Toranagallu, Bellary-583275,
Karnataka, India.
Specification
CLIAMS:We Claim:
1. Low carbon bake-hardening steel comprising 0.01-0.02 weight% C, Si:<0.02 weight% Mn 0.4-0.5 weight%, 0.005-0.015 weight% S , 0.03-0.06 weight% sol Al , 0.05-0.07 weight% P, N less than 0.004 % , 0.0008-0.002 weight % B and the remainder being Fe and unavoidable impurities having yield strength YS of not less than 220MPa before bake hardening and enabling an average “r” value of 1.6 or greater and a bake hardening value of not less than 35MPa.
2. Low carbon bake-hardening steel as claimed in claim 1 which is having 0.2< (B/N) wt %< 1 and and 81.7 and BH Index of 30-50 MPa. In this steel, costly Nb and Ti micro alloying were used to fix C and N to make steel soft and to impart desired bake hardenability.
There has been thus a need in the related field to developing bake hardening steel through batch annealing route which would be cost effective and yet provide still higher strength coupled with desired dent resistance and favourable drawability(r), Stetchability(n) and bake hardenability(BH) to suit fabrication automobile body components by judicious selection of chemical composition of steel produced and continuously cast to slab as well as controlled parameters for processing through the route of hot rolling, cold rolling, batch annealing, and skin pass rolling to produce sheets of required gauge thickness.
The present invention is directed to provide a cold rolled batch annealed bake hardening steel sheet mainly intended for outer panel material for automobile application where the strength of the drawn parts can be further increased by paint baking process. The invention also is directed to a method of producing High-strength rephosphorized cold rolled steel sheet manufactured through batch annealing route appropriate for use in an outer panel and the like of an automobile body and having a yield strength YS of not less than 220 MPa, before bake hardening, average r-value of higher than 1.6 and a Bake hardening (BH) value of not less than 35 MPa.
OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide high strength rephosphorized Low Carbon Bake hardening steel sheet having excellent formability and surface finish, and a method for manufacturing the same through batch annealing route.
A further object of the present invention is directed to provide high strength rephosphorized Low Carbon Bake hardening steel sheet with improved formability in combination with required dent resistance suitable for application in automobile outer panel fabrication.
A still further object of the present invention is directed to provide high strength rephosphorized Low Carbon Bake hardening steel sheet wherein bake hardening steel sheet with Mn-P-B alloyed low carbon chemistry has been produced through Batch Annealing route.
Another object of the present invention is directed to provide high strength rephosphorized Low Carbon Bake hardening steel sheet wherein high Strength and bake hardening with excellent drawability is provided by controlling the amount of solute carbon in solid solution by controlled micro alloying (Boron) with solid solution strengthening elements(Phosphorus and Manganese ), selecting suitable hot rolling, cold rolling ,batch annealing and skin pass parameters.
Yet another object of the present invention is directed to provide high strength rephosphorized Low Carbon Bake hardening steel sheet having the properties of YS>220 MPa, r > 1.6 and n-value>0.19 and BH-Index >35 MPa making it an appropriate material for automobile outer panel material with excellent economic potential.
A further object of the present invention is directed to provide high strength rephosphorized Low Carbon Bake hardening steel sheet suitable for using a higher strength material with lower gauge in place of higher gauge material with less strength to reduce the body weight of automobile vehicle which helps in improving fuel efficiency and reducing CO2 emission.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide low carbon bake-hardening steel comprising 0.01-0.02 weight% C, Si:<0.02 weight% Mn 0.4-0.5 weight%, 0.005-0.015 weight% S , 0.03-0.06 weight% sol Al , 0.05-0.07 weight% P, N less than 0.004 %, 0.0008-0.002 weight % B and the remainder being Fe and unavoidable impurities having yield strength YS of not less than 220MPa before bake hardening and enabling an average “r” value of 1.6 or greater and a bake hardening value of not less than 35MPa.
A further aspect of the present invention is directed to said low carbon bake-hardening steel which is having 0.2< (B/N) wt %< 1 and and 81.6) makes it a appropriate material for automobile outer panel material with excellent economic potential. The invented cold rolled steel grade is provided with High Strength and Bake hardening with excellent drawability by controlling the amount of solute carbon in solid solution by controlled micro alloying (Boron) with solid solution strengthening elements(Phosphorus and Manganese ) , selecting suitable Hot rolling cold rolling , annealing and skin pass parameters.
In accordance with an embodiment of the present invention, molten steel of above stated composition range is processed through LD converter route followed by RH-Degassing treatment of molten steel for getting the desired level of C and other alloying elements. Molten steel is then casted into slabs in continuous casting machine followed by hot rolling the slabs. The hot roll finishing temperature is kept at least 8700C (870-890 0C preferably) with coiling temperature of 6200C (6100C -6300C preferably). After hot rolling strips are pickled in acid medium (HCl) having concentrations between 4-20% and cold rolled with a minimum cold reduction of 75 percent (preferably 75-80 %) to achieve higher plastic strain ratio post annealing. Following cold rolling, the steel strip is batch annealed. In batch annealing, a coil of cold rolled steel strip is heated at a slow heating rate of about 40-45 0C /Hour in a batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core as H2 has very high heat conductivity. In addition, 100% H2 atmosphere is beneficial in avoiding graphitization and achieving excellent surface reflectivity 98% or more. Annealing cycle of 690-710 °C was implied. Maintaining the cold spot temperature of 6900C or high and hot spot temperature 710 °C or less to achieve a higher BH-Index of 35MPa or more along with superior r –bar value of 1.6 and above.
Following batch annealing, resultant steel was given an optimum skin pass elongation of (1.6 ±0.2) % mainly to avoid occurrence of any stretcher strain along with high yield strength of more than 220 MPa.
The resultant steel was evaluated for Bake Hardening by straining the steel sample to 2%, heating in a furnace at 170°C for 20 minutes followed by air cooling and lastly calculating for difference between yield strength post baking and yield strength after 2% elongation .
Complete description of Inventive steel and comparative steel grades are illustrated in following table I and II:
Table I- Compositions of the invented steel sheets along with some comparative examples.
Composition wt%
Steel Sample Number C Mn S P Si Al N B Al/N B/N Example
1 0.01 0.45 0.008 0.06 0.004 0.055 0.0032 0.001 17.19 0.31 Inventive Steel
2 0.015 0.45 0.008 0.06 0.005 0.057 0.0032 0.0009 17.81 0.28 Inventive Steel
3 0.017 0.43 0.008 0.06 0.007 0.06 0.003 0.001 20.00 0.33 Inventive Steel
4 0.02 0.47 0.008 0.065 0.004 0.06 0.003 0.001 20.00 0.33 Inventive Steel
5 0.015 0.42 0.01 0.07 0.004 0.055 0.003 0.0015 18.33 0.50 Inventive Steel
6 0.012 0.45 0.008 0.05 0.005 0.06 0.0034 0.002 17.65 0.59 Inventive Steel
7 0.017 0.43 0.008 0.06 0.004 0.06 0.004 0.0015 15.00 0.38 Inventive Steel
8 0.02 0.5 0.008 0.05 0.006 0.052 0.003 0.0008 17.33 0.27 Inventive Steel
9 0.01 0.45 0.01 0.065 0.004 0.055 0.0032 0.002 17.19 0.63 Inventive Steel
10 0.015 0.5 0.01 0.06 0.005 0.07 0.0032 0.0018 21.88 0.56 Inventive Steel
11 0.025 0.45 0.008 0.06 0.004 0.055 0.0034 0.0009 16.18 0.26 Comparative Example
12 0.03 0.47 0.008 0.06 0.005 0.057 0.003 0.001 19.00 0.33 Comparative Example
13 0.016 0.45 0.008 0.065 0.004 0.06 0.003 0.0031 20.00 1.03 Comparative Example
14 0.017 0.6 0.01 0.07 0.006 0.055 0.003 0.0015 18.33 0.50 Comparative Example
15 0.015 0.45 0.008 0.08 0.004 0.06 0.004 0.0016 15.00 0.40 Comparative Example
16 0.01 0.5 0.008 0.03 0.005 0.052 0.0034 0.0014 15.29 0.41 Comparative Example
17 0.003 0.3 0.005 0.035 0.01 0.045 0.004 11.25 Comparative Example
18 0.004 0.4 0.005 0.04 0.01 0.055 0.003 18.33 Comparative Example
19 0.015 0.5 0.01 0.05 0.005 0.07 0.0032 0.0002 21.88 0.06 Comparative Example
20 0.012 0.5 0.008 0.062 0.006 0.06 0.003 20.00 0.00 Comparative Example
21 0.01 0.45 0.008 0.06 0.004 0.055 0.0032 0.001 17.19 0.31 Comparative Example
Table II- Hot rolling, cold rolling, annealing parameters along with the mechanical properties of respective steel sheets.
Steel Sample Number Hot rolling parameters cold rolling parameters Properties (COLD ROLLED)
Finishing Temperature 0C Coiling Temperature 0C cold rolling % reduction BAF Annealing Cycle,0C SPM Elongation Yield Strength (MPa) Tensile Strength (MPa) El%-CR r-bar BH Index(MPa) n-value
1 870 620 75.0 700/670 1.4 228 351 41.33 1.93 39.7 0.21
2 870 620 75.0 700/670 1.6 237 358 41.1 1.9 38 0.206
3 870 620 75.0 700/670 1.6 241 361 39.6 1.82 37 0.2
4 870 620 75.0 700/670 1.6 252 365 39.3 1.71 36 0.194
5 870 620 75.0 700/670 1.6 242 368 39.4 1.74 40 0.2
6 870 620 75.0 700/670 1.6 241 347 40.1 1.89 41 0.204
7 870 620 75.0 700/670 1.6 249 363 39.5 1.8 39 0.2
8 890 620 75.0 700/670 1.6 244 361 39.9 1.72 35 0.196
9 890 620 75.0 700/670 1.6 232 358 40.8 1.83 40 0.21
10 890 620 75.0 700/670 1.8 245 365 40.1 1.71 41 0.2
11 870 620 75.0 700/670 1.6 262 376 37.1 1.5 31 0.187
12 870 620 75.0 700/670 1.6 269 387 36.5 1.41 27 0.184
13 870 620 75.0 700/670 1.6 260 372 37.4 1.55 41 0.186
14 870 620 75.0 700/670 1.6 263 380 37.1 1.56 33 0.19
15 870 620 75.0 700/670 1.6 267 385 37.2 1.6 44 0.19
16 870 620 75.0 700/670 1.6 200 310 41.3 1.85 34 0.2
17 870 620 75.0 700/670 1.6 182 290 42.1 1.92 41 0.22
18 870 620 75.0 700/670 1.6 210 315 41.5 1.9 40 0.21
19 870 620 75.0 700/670 1.6 237 356 39.8 1.8 30 0.2
20 870 620 75.0 700/670 1.6 242 347 40 1.82 28 0.198
21 870 570 70.0 700/670 1.6 235 365 39.9 1.5 32 0.2
As can be seen from Table 1 and 2, Inventive steels of Nos. 1 to 10 were produced by strictly controlling the content of C, Mn ,P, B, sol Al, N to satisfy the condition of 0.01-0.02 weight% C, Si:<0.02 weight% Mn 0.4-0.5 weight%, 0.005-0.015 weight% S , 0.03-0.06 weight% sol Al , 0.05-0.07 weight% P, N less than 0.004 % , 0.0008-0.002 weight % B and the remainder being Fe and inevitable impurities, additionally provided that composition satisfies the following relationships of 0.2<(B/N)wt%<1. Inventive steels of Nos. 11 to 20 showing composition, process parameter and mechanical properties some comparative steel.
The selective composition of the steel grade as claimed vis-à-vis the intended properties in the cold rolled sheet achieved are illustrated by way of the following examples. Also the effect of changing annealing temperature on BH index, Yield strength, UTS and r-value for steel sample 1 have been shown:
Example 1:
Comparative example number 11 and 12 from table 1 and table 2 satisfies all the terms of the present invention except for C wt% which is 0.025wt% and 0.03 wt% respectively in present examples and higher side than the invention range of 0.01-0.02 wt%. As can be seen from table-II , the consequence of keeping the C wt% more than the inventive range can be judged by the lower r-value and lower BH –index than the claimed for steel sample number 11 and 12.
On the other hand keeping the carbon level below the claimed weight % of 0.01 will result in lower Yield strength than 220MPa and lower UTS than 320 MPa as claimed in the present invention. Effect of low carbon wt% can be seen by taking example steel sample number 17 and 18 from table- I and table- II.
Example 2:
The B wt% range in the present invention is 0.0008-0.002 wt%. The reason behind that can be explained by taking the example of steel sample number 19 and 20 from table I and Table II. Keeping the B level of 0.0002 wt% results in lower BH-Index of 30 MPa as compared with steel sample number 2 with B wt% of 0.0009 and respective BH Index of 38 MPa which is 26.6 % higher than sample number 19 . Similarly Steel sample number 20 having no Boron added has a respective BH-Index of 28 MPa, which is 37% lower than BH-index of Steel sample number 6 having B wt% of 0.002.
Keeping the B level above 0.002 wt % shows higher BH-Index, but it has a major drawback of lower r-value, high YS AND High UTS than the claimed values which is not desirable for drawability. Keeping the B wt% of 0.0025 or more also results in surface defects like pitting, coil brakeage problem due to excess free boron.
The nature of variability of BH Index with Bwt% and Cwt% has been shown graphically in accompanying Figure 1 & Figure 2 respectively.
Example 3 :
The effect of P content on BH-index is shown graphically in accompanying Figure 3. Here, the claimed P wt% range can be justified from Table I and Table II by taking example of steel sample number 15 and 16. Steel sample number 16 with P wt% of 0.03 which is lower than the minimum target wt% of 0.05 shows reduced BH-index and YS 34 MPa and 200 MPa respectively. On contrary steel sample number 15 with P wt% of 0.08 having higher than the maximum target wt% of 0.07 shows higher BH Index of 44 MPa, but as a consequence of higher P wt% drawability i.e. r-value drops significantly with respect to the target value . In addition, UTS and Yield strength of steel sample number 15 is higher than the target value which ultimately bring down the drawability of steel.
Example 4:
The effect of Batch annealing cycle (actual strip temperature during annealing) on mechanical properties and BH-Index of cold rolled batch annealed bake hardenable steel is shown in following table III. Chemistry has been kept the same for all trials in table III which is of sample number 1. BH-index along with r–value drops significantly as the annealing temperature (Actual strip temperature during annealing) decreases.
At annealing temperature of 690 0C and below the resultant r-value is less than target r-value 1.6. Hence it is advisable to keep the annealing temperature (actual strip temperature) more than 690 0C.keeping the annealing temperature higher than results in lower yield strength than the target value of 220MPa.
Example 5:
Keeping the coiling temperature to 570 ºC resulted in low BH index and Low r-bar value as apparent in steel sample number 21. Whereas, steel sample number 1 with same chemistry and other process parameters results in better BH Index of more than 35 with good r-value of more than 1.6. Keeping lower coiling temperature also resulted in wrinkling problem on the edge of the strip. To overcome these harms coiling temperature it set to 610-630 ºC
Table III- Effect of changing annealing temperature on BH index, Yield strength, UTS and r-value for steel sample number 1.
S.No steel chemistry cold reduction % BAF Annealing Cycle,0C strip temperature,0C Skin pass eleongation Yield Strength (MPa) Tensile Strength (MPa) El%-CR r-bar BH Index
(MPa)
1 sample 1 75 720/700 720 1.41 215 337 42 1.95 41
2 sample 1 75 710/690 710 1.41 225 351 41.33 1.93 39.7
3 sample 1 75 700/680 698 1.41 227 355 40.3 1.6 39
4 sample 1 75 700/670 690 1.2 222 351 39.1 1.63 36
5 sample 1 75 700/665 670 1.8 246 368 38.8 1.5 33
6 sample 1 75 690/660 660 1.6 249 371 38.1 1.44 30
The effect of changing annealing temperature on BH index has been shown graphically in accompanying Figure 4.
It is thus possible by way of the present invention to provide a cold rolled batch annealed bake hardening steel sheet mainly intended for outer panel material for automobile application where the strength of the drawn parts can be further increased by paint baking process. The invention is also directed to a process for producing high-strength rephosphorized cold rolled steel sheet manufactured through batch annealing route appropriate for use in an outer panel and the like of an automobile body and having a yield strength YS of not less than 220 MPa, before bake hardening, average r-value of higher than 1.6 and a Bake hardening (BH) value of not less than 35 MPa. The bake hardening steels produced according to the present invention is characterized by its low yield strength before forming making it more drawable. After drawing, coated with paint and given a paint baking heat treatment at 1700C for 20 minutes, a significant enhance in yield strength can be observed by paint baking which makes the material good dent resistant favouring advantageous application in automobile body outer panel or the like.
We Claim:
1. Low carbon bake-hardening steel comprising 0.01-0.02 weight% C, Si:<0.02 weight% Mn 0.4-0.5 weight%, 0.005-0.015 weight% S , 0.03-0.06 weight% sol Al , 0.05-0.07 weight% P, N less than 0.004 % , 0.0008-0.002 weight % B and the remainder being Fe and unavoidable impurities having yield strength YS of not less than 220MPa before bake hardening and enabling an average “r” value of 1.6 or greater and a bake hardening value of not less than 35MPa.
2. Low carbon bake-hardening steel as claimed in claim 1 which is having 0.2< (B/N) wt %< 1 and and 8
Documents
Application Documents
| # | Name | Date |
|---|---|---|
| 1 | 501-MUM-2015-IntimationOfGrant20-04-2021.pdf | 2021-04-20 |
| 1 | Complete Specification for submission-17-02-2015.pdf ONLINE | 2015-02-17 |
| 2 | Complete Specification for submission-17-02-2015.pdf | 2015-02-17 |
| 3 | FORM 3.pdf ONLINE | 2018-08-11 |
| 4 | FORM 3.pdf | 2018-08-11 |
| 5 | Figures 1-4.pdf ONLINE | 2018-08-11 |
| 6 | Figures 1-4.pdf | 2018-08-11 |
| 6 | 501-MUM-2015-DRAWING [27-01-2020(online)].pdf | 2020-01-27 |
| 7 | 501-MUM-2015-Form 1-160715.pdf | 2018-08-11 |
| 7 | 501-MUM-2015-FER_SER_REPLY [27-01-2020(online)].pdf | 2020-01-27 |
| 8 | 501-MUM-2015-FORM-26 [27-01-2020(online)].pdf | 2020-01-27 |
| 8 | 501-MUM-2015-Correspondence-160715.pdf | 2018-08-11 |
| 9 | 501-MUM-2015-FER.pdf | 2019-08-09 |
| 10 | 501-MUM-2015-FER.pdf | 2019-08-09 |
| 10 | 501-MUM-2015-OTHERS [27-01-2020(online)].pdf | 2020-01-27 |
| 11 | 501-MUM-2015-Correspondence-160715.pdf | 2018-08-11 |
| 11 | 501-MUM-2015-FORM-26 [27-01-2020(online)].pdf | 2020-01-27 |
| 12 | 501-MUM-2015-FER_SER_REPLY [27-01-2020(online)].pdf | 2020-01-27 |
| 12 | 501-MUM-2015-Form 1-160715.pdf | 2018-08-11 |
| 13 | 501-MUM-2015-DRAWING [27-01-2020(online)].pdf | 2020-01-27 |
| 13 | Figures 1-4.pdf | 2018-08-11 |
| 14 | 501-MUM-2015-COMPLETE SPECIFICATION [27-01-2020(online)].pdf | 2020-01-27 |
| 14 | Figures 1-4.pdf ONLINE | 2018-08-11 |
| 15 | 501-MUM-2015-CLAIMS [27-01-2020(online)].pdf | 2020-01-27 |
| 15 | FORM 3.pdf | 2018-08-11 |
| 16 | FORM 3.pdf ONLINE | 2018-08-11 |
| 16 | 501-MUM-2015-ABSTRACT [27-01-2020(online)].pdf | 2020-01-27 |
| 17 | Complete Specification for submission-17-02-2015.pdf | 2015-02-17 |
| 17 | 501-MUM-2015-PatentCertificate20-04-2021.pdf | 2021-04-20 |
| 18 | Complete Specification for submission-17-02-2015.pdf ONLINE | 2015-02-17 |
| 18 | 501-MUM-2015-IntimationOfGrant20-04-2021.pdf | 2021-04-20 |
Search Strategy
| 1 | SearchStrategy-501-MUM-2015_29-07-2019.pdf |