FIELD OF THE INVENTION
The present invention relates to a variety of high strength cold rolled formable grade (YS: 280 Mpa min. and rm :1.25 min.) Niobium micro alloyed steel and also to a process for its manufacture through continuous casting-batch annealing route. The high strength cold rolled steels involves selective alloying alongwith grain refinement and precipitation hardening. The invention is further directed to achieving the preferred microstructure in hot band to ensure strength levels. Additionally the invention is directed to obtaining good combination of strength and ductility such as the end properties of High Yield Strength (280 Mpa min), High UTS (370 Mpa min) with good % elongation such as (26min.) at 50mm GL and adequate formability with rm:1.25 min and FLD0=42%(1.28 mm). The resultant grade of steel of the invention have good potential for use in structural and safety related parts of automobiles, as also these grades are found to meet the needs of other applications such as white goods, roll formed sections and structural profiles in construction and engineering sectors, industrial storage racks, panels, decks etc.
BACKGROUND AND PRIOR ART
The fuel consumption of automobiles being proportional to weight of car and the ever-increasing trend in the oil prices globally, the demand for weight reduction in automobile application has been focusing attention on replacement of mild steel with thinner gauge high strength low alloy steels all over the world. These grades are basically low Carbon Aluminium killed micro-alloyed steels, where strength is essentially increased by grain refinement and precipitation hardening. High strength cold rolled formable grades are mainly being produced worldwide through continuous annealing route with lean alloy chemistry. It is well known in the art of steel making and rolling that achieving high strength in cold rolled and batch annealed is more difficult than in hot rolled steels. It has been observed that grain coarsening and coalescence of carbides /carbonitrides that takes place during slow heating and long soaking times at batch annealing temperature causes loss in yield strength by more than 100 Mpa in comparison to that of hot band.
There was therefore the persistent need to deal with essential parameters and critical issues for development of the process of a grade of steel involving selection of suitable alloy design, modification of hot rolling practice and optimized annealing cycle for achieving high strength with good formability, such that the steel thus produced is capable of use for high strength structural and safety related applications with reduced over all sections /weights.
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OBJECTS OF THE INVENTION
The basic object of the present invention is therefore, directed to a High strength (YS:280 Min.) cold rolled formable (The average plastic strain ratio value rm =1.25 min) grade of Niobium alloyed steel and developing a process for its manufacture by the continuous casting-batch annealing route.
A further object of the present invention is directed to a high strength cold rolled formable steel with selective Carbon content to facilitate defect free concast slab and also achieve solid solution strengthening in cobmbination with grain refinement and precipitation hardening, by suitable alloy addition.
A further object of the present invention is directed to a high strength cold rolled formable steel directed to obtain the desired properties in the resulting grade of steel such as the yield strength, ultimate tensile strength and total % elongation in 50mm gauge length of the processed cold rolled sheets varied between 280-320 Mpa, 370-410 Mpa and 26-33% respectively and further target the average plastic strain ratio value (rm) between 1.25 -1.3.
A further object of the present invention is directed to a high strength cold rolled formable steel which could achieve a weight reduction, in selective automobile application, upto about 20% over conventional cold rolled mild steel.
A further object of the present invention is directed to a high strength cold rolled formable niobium alloyed steel, having the advantageous good potential for use in structural and safety related parts of automobiles, these grades can meet the needs of other applications such as white goods, roll formed sections and structural profiles in construction and engineering sectors, industrial storage racks, panels, decks etc.
SUMMARY OF THE INVENTION:
Thus according to the basic aspect of the present invention there is provided Cold rolled Niobium alloyed steel obtained through batch annealing route comprising:
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a combination of high strength YS: 280Mpa min. and formability rm :1.25 min.
A further aspect of the present invention of said cold rolled Niobium alloyed steel comprising yield strength, Ultimate tensile Strength and total % elongation in 50mm gauge length of the processed cold rolled sheets varying in the range of 280-320Mpa, 370-410Mpa and 26-33 respectively.
A still further aspect of the present invention of said cold rolled Niobium alloyed steel wherein the average plastic strain ratio value (rm) is in the range of 1.25 -1.3 and FLDo of about 42% (for 1.28mm thick strip).
A still further aspect of the present invention of said cold rolled Niobium alloyed steel comprising selectively Carbon 0.08% by wt max., Mn in amounts of 0.4 to 0.8 % by wt., Si 0.2 max, S 0.02 max, P in amounts of 0.02 max and Al 0.02 min and Nb 0.015 max by wt.
Yet further aspect of the present invention of said cold rolled Niobium alloyed steel comprising a steel composition (wt %) of C=0.07, Mn = 0.8, Si = 0.09, S = 0.013, P=0.02, AI = 0.026 and Nb = 0.014.
According to a further important aspect of the present invention there is provided a process for the manufacture of cold rolled Niobium alloyed steel following continuous casting-batch annealing route, comprising:
providing a selective alloy composition with controlled Niobium addition comprising carbon upto 0.08% by wt., Mn in amounts of 0.4 to 0.8 % by wt., Si 0.2 max, S 0.02 max, P in amounts of 0.02 max and Al 0.02 min and Nb 0.015 max by wt;
subject the above selective composition to stages of selective hot rolling followed by batch annealing to thereby achieve combined effect of grain refinement and precipitation hardening.
A still further aspect of the present invention directed to a process for the manufacture of cold rolled Niobium alloyed steel following continuous casting-batch annealing route, comprising:
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providing a selective alloy composition comprising carbon upto 0.08% by wt., Mn in amounts of 0.4 to 0.8 % by wt., Si 0.2 max, S 0.02 max, P in amounts of 0.02 max and Al 0.02 min and Nb 0.015 max by wt;
subject the above selective composition to stage of rolling comprising hot rolling comprising finish rolling in the selective temperature range of 850 to 870°C and coiling in the selective temperature range of 640 to 660°C; and
following a selective annealing cycle comprising 0-550 C for 7 to 9Hrs., soaking at 550 C for upto 10 hrs. max and 550-670 C for about 3 Hrs, soaking at 670 C for 16 Hrs max.
Another aspect of the present invention provides a process for the manufacture of cold rolled Niobium alloyed steel following continuous casting-batch annealing route comprising:
wherein said hot rolled coils were further pickled and cold reduced to thickness range froml.2 to 1.8 mm for desired favored application in automobile and other structural formed sections;
said cold rolled steel product were subjected to said selective annealing cycle to
achieve said desired properties, comprising:
heating of the steel from ambient temperature to 550°C in 7 to 9 hrs time;
soaking at 550°C for 10 hours max.;
further heating of the steel from 550°C to 670°C in 3 hrs time, and
soaking at 670°C for 16 hrs max;
A still further aspect of the present invention directed to a process for the manufacture of cold rolled Niobium alloyed steel following continuous casting-batch annealing route wherein desired grain size of 10-15 urn in hot band is achieved by maintaining FRT and CT at 850°C and 650°C, respectively.
The present invention and its objects and advantages are described in further details with particular reference to the accompanying figures.
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BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES :
Figure 1: is the schematic representation of the simulation test program for batch annealing.
Figure 2: is the illustration of grain size obtained at various finishing and coiling temperatures.
Figure 3: is the illustration of Iso-yield stress curves for combination of Nb content and annealing temperature.
Figure 4: is the forming limit diagram for 1.28 mm HSCR 26 sheet.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURES:
The present invention as already described, is directed to produce a grade of steel having desired combination of strength e.i. YS: 280 Mpa min. and formability rm:1.25 min. in cold rolled Niobium alloyed steel through batch annealing route and a process for its manufacture.
The technological measures taken to establish the process of manufacture said high strength cold rolled formable steel involved the following parameters:
(a) Alloy design: The Carbon content was restricted below 0.08% to avoid peritectic
reaction to facilitate defect free concast slab. Manganese addition was done for
solid solution strengthening and Niobium addition adopted for grain refinement
and precipitation hardening.
(b) Hot rolling practice: A thermo-mechanical simulation study, using Gleeble 3500C,
was carried out before the actual plant trial, to optimize the finishing and coiling
temperatures for achieving the requisite micro-structure in hot rolled steel to
ensure strength levels required in hot band. The principle of alloy addition was
maintained by selective use of the chemical composition of steel(Wt%) for said
simulation study in Gleeble 3500C as follows :
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C-0.07%, Mn-0.8%, Si-0.09%, S-0.013%, P-0.02%, AI-0.026%, Nb-0.014% ;
Reference is now invited to figure 1, that schematically represent the simulation test program, wherein the rolling of the selected sample of steel material, having said selective composition, starting at a temperature of 1000°C and carrying out rolling in six passes, with varying the finishing temperatures at 900°C, 875°C and 850°C and the corresponding coiling temperatures were maintained at 650°C, 625°C and 600°C. The results of such simulation study was investigated in terms of grain structure obtained for different selective combination of the finish roll temperature (FRT) and the coiling temperature (CT), directed to obtain a deterministic combination of parameters in an optimal way to ensure the achievement of desired high strength and cold rolled formable steel at the end of the process and which could be followed as the standardized measure in the actual plat operation.
Reference is now invited to figure 2, that illustrate the various grain sizes obtained and measured during the said simulation study with a preferred selection of chemistry of the starting steel material, for different combinations of the finish rolling and the coiling temperatures, the findings of the simulation study revealed that - (i) at high FRT, ferrite grain size remains unaffected by CT; (ii) finest grains were obtained with the combination of FRT at 850°C and the CT at 600°C; (iii) desired grain size of 10-15 ?m in hot band can be achieved by maintaining FRT and CT at 850°C and 650°C, respectively.
(c) Annealing cycle: As complete recrystallisation with minimal grain growth and least precipitate coarsening favours in achieving good combination of strength and ductility, suitable annealing cycles for each grade was specially designed. The accompanying figure 3 illustrates the iso-yield stress curves for combination of Nb content and annealing temperature, and the respective no recrystallisation zone for each combination were identified from literature. The annealing cycle was designed to achieve said desired properties, comprising:
heating of the steel from ambient temperature to 550°C in 7 to 9 hrs time; soaking at 550°C for 10 hours max.;
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further heating of the steel from 550°C to 670°C in 3 hrs time, and soaking at 670°C for 16 hrs max;
In actual plant operation for producing said high strength niobium alloyed, cold rolled formable steel, after standardizing the process adopting said simulation study according to the present invention, was carried out in following sequence/route: each heat were processed through the LD/LF to continuous caster, then the slabs subjected to hot rolling at selective FRT and CT combination, then these are subjected to pickling, cold rolling, batch annealing and lastly the skin pass. Slabs were hot rolled with the selected parameters on the basis of simulation study such as finish rolling temperature of 860 ±10°C and coiling temperature of 650 ±10°C. The hot rolled coils were further pickled and were cold rolled to 1.2 to 1.8mm thickness, then annealed as per designed cycle and the skin passed for surface treatment.
The properties obtained in the resulting steel at the end of such process sequence with selective parameters as already determined, was found to be Yield strength (YS) in the range of 280-320 Mpa, the Ultimate tensile strength (UTS) in the range of 370-410 Mpa and a total percent elongation with 50mm gauge length in the range of 26-33%. The average plastic strain ratio value, rm, was found to be between 1.25-1.3, ensuring the desired formability of the steel sheet.
To understand the material behaviour further, under drawing as well as stretching condition, Forming Limit Diagram was evaluated for 1.28mm sheet thickness and corresponding FLD0 was found to be 42%, which is considered to be adequate for moderate cold forming applications. The resulting steel product when applied in chassis of autorickshaws, an advantageous weight reduction of about 9.4% could be effected due to improved strength and cold formability achieved with reduced section/thickness, as compared to conventional : structural steels.
Thus the selective grade of steel obtained as the product according to the invention following the developed and established process, having high strength and reasonably good hot/cold formability, is having good potential for use in various structural and safety related parts of automobiles, and also capable to meet the needs of other segments like roll formed sections and structural profiles used in construction and engineering sectors, industrial storage racks, panels, decks etc.
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We claim:
1. Cold rolled Niobium alloyed steel obtained through batch annealing route comprising:
a combination of high strength YS: 280Mpa min. and formability rm :1.25 min.
2. Cold rolled Niobium alloyed steel as claimed in claim 1 comprising yield strength,
Ultimate tensile Strength and total % elongation in 50mm gauge length of the
processed cold rolled sheets varying in the range of 280-320Mpa, 370-410Mpa and
26-33 respectively.
3. Cold rolled Niobium alloyed steel as claimed in anyone of claims 1 or 2 wherein the
average plastic strain ratio value (rm) is in the range of 1.25 -1.3 and FLDo of about
42%(for 1.28mm thick strip).
4. Cold rolled Niobium alloyed steel as claimed in anyone of claims 1 to 3 comprising
selectively Carbon 0.08% by wt max., Mn in amounts of 0.4 to 0.8 % by wt., Si 0.2
max, S 0.02 max, P in amounts of 0.02 max and Al 0.02 min and Nb 0.015 max by
wt.
5. Cold rolled Niobium alloyed steel as claimed in anyone of claims 1 to 4 comprising a
steel composition (wt %) of C=0.07, Mn=0.8 , Si=q.09,S=0.013,P=0.02,AI=0.026
and Nb=0.014.
6. A process for the manufacture of cold rolled Niobium alloyed steel following
continuous casting-batch annealing route, comprising:
providing a selective alloy composition with controlled Niobium addition comprising Carbon upto 0.08% by wt., Mn in amounts of 0.4 to 0.8 % by wt., Si 0.2 max, S 0.02 max, P in amounts of 0.02 max and Al 0.02 min and Nb 0.015 max by wt;
subject the above selective composition to stages of selective hot rolling followed by cooling and batch annealing to thereby achieve combined effect of grain refinement and precipitation hardening.
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7. A process for the manufacture of cold rolled Niobjum alloyed steel following
continuous casting-batch annealing route as claimed in claim 6 comprising:
providing a selective alloy composition comprising Carbon upto 0.08% by wt, Mn in
amounts of 0.4 to 0.8 % by wt., Si 0.2 max, S 0.02 max, P in amounts of 0.02 max
and Al 0.03 min and Nb 0.015 max by wt;
subject the above selective composition to stage of rolling comprising hot rolling comprising finish rolling in the selective temperature range of 850 to 870°C and cooling in the selective temperature range of 640 to 660°C; and
following a selective annealing cycle comprising 0-550 C for 7 to 9Hrs., soaking at 550 C for upto 10 hrs. max and 550-670 C for about 3 Hrs, soaking at 670 C for 16 Hrs max.
8. A process for the manufacture of cold rolled Niobium alloyed steel following
continuous casting-batch annealing route as claimed in anyone of claims 6 or 7
comprising:
wherein said hot rolled coils were further pickled and cold reduced to thickness range froml.2 to 1.8 mm for desired favored application in automobile and other structural formed sections;
said cold rolled steel product were subjected to said selective annealing cycle to
achieve said desired properties, comprising:
heating of the steel from ambient temperature to 550°C in 7 to 9 hrs time;
soaking at 550°C for 10 hours max.;
further heating of the steel from 550°C to 670°C in 3 hrs time, and
soaking at 670°C for 16 hrs max;
9. A process for the manufacture of cold rolled Niobium alloyed steel following
continuous casting-batch annealing route as claimed in anyone of claims 6 to 8
wherein desired grain size of 10-15 urn in hot band is achieved by maintaining FRT
and CT at 850°C and 650°C, respectively.
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10. A high strength (YS: 280 Mpa min.) cold rolled formable (rm:1.25 min.) steel and a process for its manufacture; substantially as hereindescribed and illustrated with reference to accompanying examples/figures.


Dated this 11th day of January, 2007.

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Niobium micro alloyed high strength cold rolled formable grade (YS: 280 Mpa min. and rm :1.25 min.) of steel and a process for its manufacture through continuous casting-batch annealing route. A thermo-mechanical study directed to selectively provide for the finishing and coiling temperatures for achieving the preferred microstructure having grain size of 10-15 xm in hot-band favoured to ensure end properties of the steel with defined optimum parameters in terms of FRT and CT for suitable batch annealing cycles for the grade such as good combination of strength and ductility in the range of High Yield Strength of 280 Mpa min, High UTS of 370 Mpa min. with good % elongation of 26min. at 50mm GL and formability with rm:1.25 min and FLD0=42%(for 1.28 mm thick sheet), adequate for moderate forming applications. The resultant grade of steel having good potential for use in structural and safety related parts of automobiles and structural profiles in construction and engineering sectors.