FIELD OF THE INVENTION
The present invention relates to a composition of low Carbon (0.04-0.06 wt % max) Boron added (5-20ppm) cold rolled formable (rm: 1.7 min) batch annealed steel and process for developing the same. More particularly, the invention relates to a method for production of low Carbon boron containing aluminium killed steel wherein said process is directed to attaining specified low B/N atomic ratio up to 0.3, instead of focusing on absolute Boron content, whereby availability of aluminium and nitrogen ensured in the solid solution for its precipitation during batch annealing as the governing factor, even if boron is present in the steel, resulting in improved mean value of the plastic anisotropy ratio (rm: 1.7 min) value, high grain aspect ratio and a coarse pancake structure for desired improved forming properties in the resulting steel product. The process of the invention is directed to developing cold rolled batch annealed formable steel (rm: 1.7) even with higher nitrogen up to 80 ppm, relatively higher carbon content (0.04-0.06 wt %) and Si (0.035 wt% max) for the Extra Deep Drawing (EDD) variety of steel. All such steels are continuously cast to 210mm thick slabs which are hot rolled to 3.8 mm thickness, finish rolled and coiled and HR coils are further reduced to 1.2 mm thickness which are finally batch annealed to provide desired formable properties in the end products. The process of the invention is thus having prospects of wide industrial application in steel plants for production of low carbon steel with selective boron, aluminium and nitrogen additions for extra deep drawing quality formable steel products with favourable rm value and coarse grained microstructure of preferred aspect ratio adapted to suit a variety of end use/applications.
BACKGROUND OF THE INVENTION
It is well known in the related art of steel making that to achieve high formability in batch annealed CR sheet steel for extra deep drawing characteristic, a comprehensive study of the steel composition and correlation of the mean plastic strain ratio vis-a-vis desired microstructure in finished steel is of extreme importance. It is known in the art from recent experiments/researches carried out, that small Boron addition in continuous annealed low carbon steel has shown beneficial effects. The optimum combination of low yield strength and high rm value has been reported at stoichiometric boron addition. The improvement on

the value of rm is attributable to the ferrite grain growth after recrystallisation of the annealed steel. It is also experienced in the art that boron addition in selective wt percent in the batch annealed Interstitial free (IF) steel used extensively for reducing cold work embrittlernent. However, no significant experimentation has been reported to study the influence of boron in batch annealed low carbon aluminium killed steel, particularly in respect of improving the forming properties. The experiments with boron addition revealed that it results in substantial deterioration in the [111] texture development and rm value. The prior art experiments and researches further concluded based on experimental observations that absolute boron rather than the B/N ratio is the main factor responsible for such deterioration in rm value. It is also apparent from the related prior knowledge that no systematic investigation has been carried out on the microstructure development during the batch annealing of boron containing aluminium killed steel and or to explore establishing a correlation therebetween the grain growth with B/N ratio as a determining factor to attain favourable rm value.
There has thus been a need in the related art to ascertaining a process for production of low carbon born added aluminum killed cold rolled batch annealed steel to achieve desired formability in terms of rm value, preferably 1.7 min wherein the B/N atomic ratio has the significant bearing on microstructure, crystalline grain growth with improved aspect ratio of grains and finding correlation therebetween, so as to affirm that high rm value required for desired formability of batch annealed CR sheets is attributable to the B/N ratio and not the absolute boron content in steel. The present invention seeks to solve the limitations of prior art as to the reliance on absolute boron content as a means for determining the rm value to achieve formability properties in CR sheet, by way of systematically investigating and ascertaining the effect of availability of the aluminum and nitrogen in solid solution and precipitation during batch annealing as the determining factor, even if boron is present in steel, for desired improved rm value vis-a-vis the selective B/N atomic ratio.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing a composition of tow carbon (0.04-0.06%) boron added aluminum killed cold rolled and batch annealed steel having improved formability (rm: 1.7 min) and a process for its production

Another object of the present invention is directed to examine the beneficial effect of boron addition in small wt % in steel and to investigate the factors responsible and control on the formable property of the end product.
A further object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the experiment confirmed that it is the B/N atomic ratio and not the absolute boron that has direct bearing on the favored improvement in formability.
A still further object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the experiments confirm that the high rm value with low B/N ratio(up to 0.3) is attributable to sufficient availability of aluminium and nitrogen in solid solution for precipitation during batch annealing of steel improving the formability property id EDD quality steel.
A still further object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the investigation suggest that the resulting microstructure in CR sheet steel after recrystallisation and batch annealing the grain shape anisotropy has been characterized through grain aspect ratio, as the ratio of major and minor grain length in a pancake structure favor higher forming properties.
A still further object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the amount of boron, nitrogen and aluminium in steel is optimized to ensure low B/N atomic ratio up to 0.3 and coarse pancake grains/microstructure in resulting steel product desirable for forming properties.
A still further object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the process would favor

desired formability property in resulting steel even with higher nitrogen (up to 80ppm), relatively low carbon content (0.04-0.06wt %) and Silicon (0.035 wt% max).
According to yet another object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the variation of mean aspect ratio is studied for varying B/N atomic ratio to establish the trend and correlation coefficient between the variables.
A still further object of the present invention is directed to developing a process for production of low carbon cold rolled boron added aluminium killed and batch annealed steel adapted to achieve improved formability (rm: 1.7 min), wherein the process condition i.e. hot rolling parameters, finishing and coiling temperatures, % cold reduction, batch annealing cycle remain identical as that practiced during existing processing of Extra Deep Drawing Steel.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to Low carbon batch annealed steel composition for manufacture of improved batch annealed steel with improved rm value comprising:
C: 0.04-0.06 Wt%; Mn.: 0.15 -0.20 Wt%; S:upto 0.02 Wt%; P: upto 0.02 Wt%; Si: 0.025-0.035Wt%; Al: 0.02-0.04Wt%; B 10-50ppm;

N.-40-80 ppm;
B/N Atomic Ratio : 0.2-0.8
A further aspect of the present invention is directed to Low carbon batch annealed steel composition adapted for producing batch annealed steel with mean plastic anisotropy ratio (rm) value in the range of 1.74 to 1.12.
A still further aspect of the present invention is directed to Low carbon batch annealed steel composition wherein with increased B/N ratio the aspect ratio of steel decreases.
A still further aspect of the present invention is directed to Low carbon batch annealed steel composition adapted for favouring improved mean plastic anisotropy ratio (rm) value, high grain aspect ratio and coarse pancake structure for desired forming properties.
According to yet another aspect of the present invention is directed to a process for the manufacture of cold rolled batch annealed formable steel comprising:
(i) providing steel composition having selectively
C: 0.04-0.06 Wt%;
Mn.: 0.15-0.20 Wt%;
S:upto 0.02 Wt%;
P: upto 0.02 Wt%;
Si: 0.025-0.035Wt%;
Al: 0.02-0.04Wt%;
B 10-50ppm;
N:40-80 ppm;
B/N Atomic Ratio : 0.2-0.8; and
(ii) processing the same involving hot rolling, cold reduction and batch annealing.

A still further aspect of the present invention is directed to sad process wherein the said hot rolling parameters including finishing at 880°C +/- 10°C and coiling 570°C +/- 10°C temperature, % cold reduction 60 to 70, batch annealing cycle comprising of 50-60 hours cycle, Intermediate heating upto 550 C and finally to 670 C.
A still further aspect of the present invention is directed to said process comprising providing optimum amount of boron, nitrogen and aluminium adapted to favour achieving coarse pancake structure suited for improved formability.
The present invention and its objects and advantages are described in greater details with reference to the accompanying non limiting illustrative figures and example.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the graphical plot of the grain aspect ratio against the varying B/N atomic ratio in the low carbon that illustrate the logarithmic trend in the nature of variation and computing the correlation coefficient therefrom.
Figure 2(a) and 2(b): is the illustration of the micrographic image of the resulting microstructure of the boron added low carbon aluminum killed cold rolled batch annealed steel, showing the change in microstructure from coarse pancake structure to equiaxed structure with increase in B/N ratio.
DETAILED DESCRIPTION OF THE INVENTION WITH REFRENCE TO THE ACCOMPANYING DRAWINGS
The present invention relates to developing a process for production of low carbon (0.04-0.06 wt %) boron added aluminium killed cold rolled batch annealed steel adapted to providing desired improved formability properties (rm:1.7 min) corresponding to low B/N

atomic ratio in the range of 0.2 to 0.8 and preferably upto 0.3, rather than absolute boron content. The invention is thus directed to systematic investigation of microstructural evolution during batch annealing of boron containing aluminium killed steel particularly qualifying B/N ratio for improving the mean plastic strain ratio (rm) value. The high rm value with low B/N atomic ratio is attributable to sufficient availability of Al and N to precipitate during batch annealing, even when boron is present, which is established for the first time by way of the present invention. The variation of grain aspect ratio for varying B/N ratio has been experimentally studied and graphical plot of trend and computation of correlation coefficient therebetween is done. It has been established that the B/N ratio is important factor responsible for improving rm value to achieve improved formability in cold rolled batch annealed steel rather than the absolute Boron content in steel. Optimum amount of boron, nitrogen and aluminium can lead to coarse pancake structure ideally suited for improved formability.
The objects and advantages of the present invention is described in greater details with reference to the following accompanying drawings and example:
Reference is first invited to the accompanying Figure 1 that graphically illustrate the trend of variability of the grain aspect ratio characterizing the grain shape anisotropy, plotted with variation in the B/N atomic ratio of different composition of boron containing low carbon aluminium killed cold rolled batch annealed steel that has direct impact on the forming property of steel. As a result of change in B/N ratio from 0.2 to 0.8, the rm value decreases from 1.74 to 1.12 due to change in grain aspect ratio. It is interesting to note that aspect ratio of steel B (B/N:0.185) is found to be higher as shown by dotted line in Figure 1, as compared to that of steel A (B/N: 0). As the B/N ratio increases, the aspect ratio generally decreases. A definite logarithmic trend emerged while correlating B/N ratio vis-a-vis aspect ratio wherefrom a good value of correlation coefficient is computed e.g.R2= 0.94.
Reference is now invited to the accompanying Figure 2a & 2b that illustrate the micrograph of the microstructure of the resulting cold rolled batch annealed steel obtained following the process according to the invention. The change in microstructure from pancaking to equiaxed with increasing B/N ratio has been shown in the figures e.g. 2(a) for B/N ratio 0.2 and 2(b) for B/N ratio:0.8.

The above and other aspects of the process according to the present invention is illustrated with the help of the following non-limiting Example I.
Example I:
The experiment comprised systematic study on the microstructure evolution during batch annealing of boron containing aluminium killed steel, particularly quantifying B/N ratio for improving the rm value as a measure of improving the formability of conventional Extra Deep Drawing steel.
The study is carried out on the industrially produced low carbon steel with varying B/N atomic ratio ranging from 0.2 to 0.8. The chemical composition on weight percent basis, used for the present experimental investigation is given as the following :
Chemical composition of steel (Wt. %)used for the study:
C: 0.04-0.06; Mn: 0.15-0.20; S: 0.02 max; P: 0.02 max; Si: 0.025-0.035; Al: 0.02-0.04; B: 10-50 ppm; N: 40-80 ppm and the B/N Atomic Ratio: 0.2-0.8;
All the steels are continuously cast to 210 mm thick slabs and then hot rolled to 3.8mm thickness. The hot rolled (HR) bands are finish rolled at 880 ±10 °C and coiled at 570 ± 10°C. HR coils are cold reduced to 1.2 mm thickness and are then batch annealed. The coiling temperature was kept less than 600°C for all the steels as maintained typically in coiling temperature for processing of conventional EDD steel.
Observed Results:
The present experimental investigation showed significant difference in rm value in the industrial steels wherein the observation comprised:

(i)Testing of steels carried out to investigate the influence on forming property, having similar absolute boron content e.g. 20 ppm and different B/N atomic ratio.
(ii) For increase in B/N ratio from 0.2 to 0.8, the rm value decreased from 1.74 to 1.12.
(iii) The high rm value with low B/N ratio can be attributed to sufficient availability of Al and N to precipitate during batch annealing which is identified for the first time in this experiment.
(iv) A systematic microstructure evolution is carried out to ascertain the importance of B/N ratio, rather than the absolute boron content in steel, influencing average plastic strain (rm) value.
(v) The grain shape anisotropy has been characterized through grain aspect ratio which is measured as the ratio of major and minor length of grains.
(vi) A graphical plot of variation of aspect ratio with varying B/N atomic ratio.
(vii) As the B/N ratio increases, the aspect ratio decreases in general.
(Viii) It is exceptional to note that aspect ratio of steel 'B' having B/N:0.185 is found to be higher as compared to that of steel 'A' having B/N:0.8
(ix) A definite logarithmic trend obtained while correlating B/N ratio vis-a-vis aspect ratio showing a good correlation co-efficient (R2=0.940).
(x) The change in microstructure of resulting boron added cold rolled batch annealed steel CR sheets observed with increasing B/N ratio shown e.g. from pancaking at B/N ratio=0.185 to equiaxed for B/N ratio=0.8.
The above experimental study concludes that it is not the absolute boron content but the B/N ratio that controls the properties of batch annealed aluminium killed steel. Availability of Al and N in solid solution is the main governing factor for its precipitation during batch annealing even if boron is present in steel. Optimum amount of boron, nitrogen and aluminium can lead to coarse pancake structure ideally suited for improved formability.

It is thus possible by way of the present invention to developing a process for production of low carbon boron added aluminium killed cold rolled batch annealed steels to achieve improved average plastic strain ratio (rm) value. It is established through the experimentation that it is not the absolute boron but the B/N atomic ratio that controls the properties of batch annealed aluminium killed steel. The process also showed that the availability of Al and N in solid solution is the main governing factor for its precipitation during batch annealing even if boron is present in steel. It is further established experimentally that low B/N ratio up to 0.3 resulted in improved mean plastic strain ratio (rm) value, high grain aspect ratio and coarse pancake structure are the desirable factors favoring improved forming properties. The present invention is thus applicable for development of cold rolled batch annealed formable steel (rm: 1.7 min) even with higher nitrogen upto 80 ppm, low carbon content (0.04-0.06wt %) and Si (0.035 wt% max), while the hot rolling parameters including the finishing and coiling temperature, % cold reduction, batch annealing cycle maintained identical to that followed for existing Extra Deep Drawing steel variety. The present invention is thus capable of producing steels with high formability in a simple, reliable, cost effective manner in steel plants, to meet the requirements of a number of end applications in automobile components or like different other industries.

WE CLAIM:
1. Low carbon batch annealed steel composition for manufacture of improved batch
annealed steel with improved rm value comprising:
C: 0.04-0.06 Wt%;
Mn.: 0.15 -0.20 Wt%;
S:upto 0.02 Wt%;
P: upto 0.02 Wt%;
Si: 0.025-0.035Wt%;
Al: 0.02-0.04Wt%;
B 10-50ppm;
N:40-80 ppm;
B/N Atomic Ratio : 0.2-0.8
2. Low carbon batch annealed steel composition as claimed in claim 1 adapted for producing batch annealed steel with mean plastic anisotropy ratio (rm) value in the range of 1.74 to 1.12.
3. Low carbon batch annealed steel composition as claimed in anyone of claims 1 or 2 wherein with increased B/N ratio the aspect ratio of steel decreases.
4. Low carbon batch annealed steel composition as claimed in anyone of claims 1 or 3 adapted for favouring improved mean plastic anisotropy ratio (rm) value, high grain aspect ratio and coarse pancake structure for desired forming properties.
5. A process for the manufacture of cold rolled batch annealed formable steel comprising:
(i) providing steel composition having selectively C: 0.04-0.06 Wt%;

Mn.: 0.15 -0.20 Wt%;
S:upto 0.02 Wt%;
P: upto 0.02 Wt%;
Si: 0.025-0.035Wt%;
Al: 0.02-0.04Wt%;
B 10-50ppm;
N:40-80 ppm;
B/N Atomic Ratio : 0.2-0.8 and
(ii) processing the same involving hot rolling, cold reduction and batch annealing.
6. A process as claimed in claim 5 wherein the said hot rolling parameters including finishing at 880°C +/- 10°C and coiling 570°C +/- 10°C temperature, % cold reduction 60 to 70, batch annealing cycle comprising of 50-60 hours cycle, Intermediate heating upto 550 °C and finally to 670 °C.
7. A process as claimed in anyone of claims 5 or 6 comprising providing optimum amount of boron, nitrogen and aluminium adapted to favour achieving coarse pancake structure suited for improved formability.
8. Low carbon batch annealed steel composition for manufacture of improved batch annealed steel with improved rm value and its process for manufacture substantially as hereindescribed and illustrated with reference to the accompanying figures.

The present invention relates to composition for low Carbon (0.04-0.06 wt % max) Boron added (5-20ppm) cold rolled formable (rm: 1.7 min) batch annealed steel and a process for its manufacture. More particularly, the low Carbon boron containing aluminium killed steel wherein having specified low B/N atomic ratio up to 0.3, independent of absolute boron, wherein availability of aluminium and nitrogen ensured in the solid solution for its precipitation during batch annealing to achieve improved mean value of the plastic anisotropy ratio (rm: 1.7 min) value, high grain aspect ratio and a coarse pancake structure providing enhanced forming properties in resulting steel. The process is directed to developing low carbon cold rolled batch annealed formable steel (rm: 1.7) even with higher nitrogen up to 80 ppm, carbon content (0.04-0.06 wt %) and Si (0.035 wt% max). The process of the invention is thus having prospects of wide industrial application in steel plants for production of low carbon batch annealed formable EDD steel products with improved rm value to suit a variety of end use/applications.