Title of Invention | DRAWING QUALITY Al-KILLED REPHOSPHORISED STEEL SHEETS HAVING TENSILE STRENGTH OF 340-440 MPA |
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Abstract | The present invention is provided with a development of drawing quality Al- killed rephosphorised steel sheets having tensile strength of 340-440 MPa comprising a steel having the composition %C = 0.04-0.06, %Mn = 0.3-.15, %S = 0.01 max, %P = 0.05-0.09, %Si = 0.01 max, %AI = 0.04-0.05, N (ppm) = 35-50; characterized in that Al-killed steel by addition of Phosphorus and Manganese and subsequent heat treatment and Thermo- Mechanical Treatment develop secondary work embrittlement properties like a strength of 340-440 MPa, drawability and weldability. |
Full Text | FIELD OF THE INVENTION The present invention relates to the development of an Al-killed rephosphorised high strength sheet steel. More particularly the invention relates to a development of drawing quality Al-killed rephosphorised steel sheets having Tensile Strength of 340-440 MPa UTS formable and weldable steel sheet, adaptable to mainly for manufacturing automotive body panels. BACKGROUND OF THE PRESENT INVENTION The modern automobile industry is driven by some important factors, such as (a) Energy saving, or less fuel consumption (b) Less emission (c) Improved passenger safety (d) Low cost manufacturing, etc. In order to satisfy all these basic conditions, the modern car manufacturing involves more high strength steel grades, which should also be combined with certain degree of formability. The formable high strength steels are usually expensive (high strength interstitial free steel), and the manufacturing steps requires special facility like continuous annealing. The present invention gives a solution to this 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. Therefore, continuous annealing facility is not required. PRIOR ART Al-killed drawing quality or extra deep drawing quality steels are known to be excellent materials for automotive stamping applications. Though the drawability is inferior to that of interstitial free steels, they find substantial application due to their formability, combined with the cost advantage. The rephosphorised versions of this family of steels offer a vital alternative to high strength interstitial free steels of strength levels, between 340 to 440 MPa UTS. More importantly, the drawability values of rephosphorised Al-killed grades are comparable with those of IFHS grades, while IFHS is a non-aging grade. This grade has never been manufactured anywhere else in India before. Drawbacks connected with hither to known processes and advantage of the present invention The worldwide trend is to process high strength formable steel grades through continuous annealing line, which is the best route for producing interstitial free high strength steels. In India, there is no continuous annealing line for processing uncoated steel sheet. Therefore, the steel manufacturers are compelled to produce high strength formable grades through batch annealing only. Manufacturing of interstitial free based high strength steels through batch annealing is extremely difficult due to the complex metallurgy of those grades. Therefore, Al-killed rephosphorised grades, though slightly inferior in forming property, offer great advantage in terms of easy processing and expenditure. The main advantages are: a) Can be produced through batch annealing route b) Combination of strength and substantial formability c) An economic option for automotive industry OBJECTS OF TH INVENTION It is therefore an object of the present invention to propose an improved grade of steel sheet having 340 to 440 MPa UTS, formable and weldable quality adaptable to manufacture of automotive body panels which eliminates the disadvantages of prior art. Another object of the present invention is to propose an improved grade of steel sheet having 340 to 440 MPa UTS, formable and weldable quality adaptable to manufacture of automotive body panels which is applicable and usable for a wide range of automotive application. A further object of the present invention is to propose an improved grade of steel sheet having 340 to 440 MPa UTS, formable and weldable quality adaptable to manufacture of automotive body panels which is suitable for producing through batch annealing route. A still further object of the present invention is to propose an improved grade of steel sheet 340 to 440 MPa UTS, formable and weldable quality adaptable to manufacture of automotive body panels which is cost savings. A yet further object of the present invention is to propose an improved grade of steel sheet having 340 to 440 MPa UTS, formable and weldable quality adaptable to manufacture of automotive body panels which saves energy consumption. SUMMARY OF THE INVENTION The invented product is a steel grade for automotive stamping applications. It is a low carbon based steel composition, strengthened by suitable additions of phosphorus and manganese. The processing schedules, including steel making, hot rolling, cold rolling and annealing, have been determined appropriately so as to achieve desired drawability and strength. The material is also adequately weldable. Chemical composition C% Mn% S% P% Si% Al% N (ppm) 0.04-0.06 0.3-1.5 0.01 0.05-0.09 0.01 0.04-0.05 35-50 max max Physical processing a) hot charging of slabs to reheat furnace b) slab drop out @ around-1200°C c) finish rolling @ higher than Ar3 temperature - 900 - 910°C d) coiling @ 550 - 560°C e) cold rolling @ 66 - 74% f) batch annealing @ hot spot temperature - 700°C - 720°C and cold spot temperature 620 - 640° (table suitably adjusted) g) skin pass rolling @ 1.0% temper elongation Test results (two results are given below for each grade of Al-killed rephos family): Secondary work embrittlement property: OK upto - 30°C (tested for rephos - 340 and rephos - 390, rephos - 440 was not tested, but expected to show same property due to same phosphorus content as in rephos - 390) Variables (a) Chemical parameters: C, P and Mn - Increase strength of the steel (b) Physical parameters: Slab drop out temperature, finish rolling temperature, coiling temperature, cold reduction and batch annealing temperature, all have significant effects on final mechanical properties of all the steel grades. WE CLAIM: 1. Drawing quality Al-killed rephosphorised steel sheets having tensile strength of 340-440 MPa comprising a steel having following composition: %C = 0.40-0.06 %Mn= 0.3-1.5 %S = 0.01 max %P = 0.05-0.09 %Si = 0.01 max %AI = 0.04-0.05 N (ppm) = 35-50 characterized in that Al-killed steel by addition of Phosphorus and Manganese and subsequent heat treatment and Thermo-Mechanical Treatment develop secondary work embrittlement properties like a strength of 340-440 MPa, drawability and weldability. 2. The process to convert liquid steel to steel sheet comprising of following steps: - manufacturing of slab from the liquid sheet; - hot charging the slab to reheat furnace; - finish rolling @ higher than Ar3 temperature 900-910°C; - coiling @ 550-560; - cold rolling @ 66-74%; - batch annealing @ hot spot temperature 700°C and cold spot temperature 620°C; and - skin pass rolling @ 1.0% tempered elongation; characterized in that Al-killed steel having a ferrite microstructure after hot rolling is being converted to a pancaked ferritic microstructure after subsequent cold rolling and annealing treatment producing a good secondary work embrittlement property and weldability. 3. The process to convert liquid steel to steel sheet as substantially described and illustrated herein. ABSTRACT TITLE: DRAWING QUALITY AI-KILLEP REPHOSPHORISED STEEL SHEETS HAVING TENSILE STRENGTH OF 340-440 PMa The present invention is provided with a development of drawing quality Al- killed rephosphorised steel sheets having tensile strength of 340-440 MPa comprising a steel having the composition %C = 0.04-0.06, %Mn = 0.3-.15, %S = 0.01 max, %P = 0.05-0.09, %Si = 0.01 max, %AI = 0.04-0.05, N (ppm) = 35-50; characterized in that Al-killed steel by addition of Phosphorus and Manganese and subsequent heat treatment and Thermo- Mechanical Treatment develop secondary work embrittlement properties like a strength of 340-440 MPa, drawability and weldability. |
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00943-kol-2007-correspondence others 1.1.pdf
00943-kol-2007-correspondence others.pdf
00943-kol-2007-description complete.pdf
943-KOL-2007-AMANDED CLAIMS-1.1.pdf
943-KOL-2007-AMANDED CLAIMS.pdf
943-KOL-2007-CORRESPONDENCE 1.4.pdf
943-KOL-2007-CORRESPONDENCE-1.1.pdf
943-KOL-2007-CORRESPONDENCE-1.2.pdf
943-KOL-2007-CORRESPONDENCE-1.3.pdf
943-KOL-2007-DESCRIPTION (COMPLETE) 1.1.pdf
943-KOL-2007-DESCRIPTION (COMPLETE)-1.2.pdf
943-KOL-2007-EXAMINATION REPORT REPLY RECIEVED 1.1.pdf
943-KOL-2007-EXAMINATION REPORT.pdf
943-KOL-2007-GRANTED-ABSTRACT.pdf
943-KOL-2007-GRANTED-CLAIMS.pdf
943-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf
943-KOL-2007-GRANTED-DRAWINGS.pdf
943-KOL-2007-GRANTED-FORM 1.pdf
943-KOL-2007-GRANTED-FORM 2.pdf
943-KOL-2007-GRANTED-SPECIFICATION.pdf
943-KOL-2007-PETITION UNDER RULE 137.pdf
943-KOL-2007-REPLY TO EXAMINATION REPORT 1.1.pdf
943-KOL-2007-REPLY TO EXAMINATION REPORT.pdf
Patent Number | 255484 | |||||||||||||||
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Indian Patent Application Number | 943/KOL/2007 | |||||||||||||||
PG Journal Number | 09/2013 | |||||||||||||||
Publication Date | 01-Mar-2013 | |||||||||||||||
Grant Date | 26-Feb-2013 | |||||||||||||||
Date of Filing | 29-Jun-2007 | |||||||||||||||
Name of Patentee | TATA STEEL LIMITED | |||||||||||||||
Applicant Address | RESEARCH AND DEVELOPMENT AND SCIENTIFIC SERVICES DIVISION JAMSHEDPUR-831 001, INDIA | |||||||||||||||
Inventors:
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PCT International Classification Number | B01D39/12 | |||||||||||||||
PCT International Application Number | N/A | |||||||||||||||
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