FIELD OF THE INVENTION The present invention relates to a novel composition of continuously cooled hot rolled high strength steel with minimum 400 VHN (which is approximately >1200 MPa UTS) and minimum 20% retained austenite in the final microstructure (to provide an elongation of minimum 20%) adaptable to automotive structural applications. BACKGROUND OF THE INVENTION Application of ultra high strength (UHS) steel sheet increased multifold during the recent past in an effort to reduce the weight of the overall vehicle. However, because of its poor formability, the UHS steel sheet is not easily applicable to a wide variety of automotive components. Hence, the workability required for the UHS steel sheet demands stringent and additional processing. Therefore, it is necessary to develop a hot rolled thin steel sheet with high tensile strength combined with uniform elongation for application to automotive component such as long and cross member including bumpers. Technology for producing UHS-Steel is disclosed in the known documents1'2,3, where major part of strengthening are contributed by the nano-sized bainitic ferrite sheaves. Admittedly, the prior art technology is enabled to produce highest strength in the Steel grade, the biggest concern in these steels, known as "Superbainite" is the long isothermal holding time at a lower temperature (as high as 5 days at 200°C) to allow the progress of transformation to form extremely fine bainite sheaves. This long holding time vis-a-vis the "Superbainite" imposes a restriction in manufacturing of such steel through a continuous production line. The second disadvantage is the lower elongation which is typically 7 % at a strength range of 2260 MPa. This limited elongation does not allow the steel to be used in wider areas of applications where strength and ductility both are equally important. Another disadvantage is the alloy composition of the steel, wherein the amount of C in steel typically lies in the range of 0.8 - 1.0 wt% along with Ni and Co. Such a C concentration does not allow the steel to be welded and the use of Ni and Co makes the steel very costly. Researchers4,5 are continuously trying for reduction of the amount of C and increasing the elongation. Nevertheless, the current research is also not directed to evolve a solution towards production of such steels through a continuous production line. The composition of said prior art steel also contains elements like Ni and Mo making the steel costlier. The proposed invention has been developed to solve the difficulties of combination of excellent ductility and high formability in hot rolled ultra high strength steel sheet for automotive long and cross member applications by developing a carbide-free bainitic steel strengthened by very fine bainite plates with hardness minimum 400 VHN (~ tensile strength of minimum 1200 MPa) minimum and the presence of minimum 20% retained austenite in the final microstructure to provide an elongation of minimum 20%. 4 OBJECTS OF THE INVENTION It is therefore, an object of the present invention to propose a steel composition for producing hot rolled high strength carbide-free bainitic steel sheet, which eliminates the disadvantages of prior art. Another object of the present invention is to propose a method of producing hot rolled high strength carbide-free bainitic steel sheet having 70-80% bainite with minimum 20% retained austenite with a minimum carbon content of 0.90% C. A further object of the present invention is to propose a method of producing hot rolled carbide-free bainitic steel sheet, which increases strength of the steel product due to the transformation of austenite into carbide free bainite at a coiling temperature of 350°C -550°C. A still further object of the present invention is to propose a method of producing hot rolled carbide free bainitie steel sheet which yields a hardnes value of minimum 400 VHN (>1200 MPa tensile strength) and 20% minimum retained austenite in the microstructure to provide at least 20% elongation. SUMMARY OF THE INVENTION Accordingly, there is provided a new grade of ultra high strength steel, consisting of: carbon from 0.2 to 0.5 wt%; silicon content from 1.1 to 2.1 wt%; manganese content from 1.0 to 2.0 wt%, chromium content between 0.5 to 3.0 wt%, aluminium from 0.01 to 0.07 wt%; sulphur and nitrogen content 0.008 wt% and 0.005 maximum, respectively, the produced steel grade exhibiting a hardness of more than 400 VHN. BREIF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 Calculated TTT diagram for the inventive steel Figure 2 Calculated To curve for the inventive steel composition Figure 3a Calculated amount of retained austenite as a function of isothermal transformation temperature in accordance with the invention Figure 3b Calculated ratio of film type to blocky type austenite as a function of isothermal transformation temperature in accordance with the invention Figure 4 Calculated strength of the steel in accordance with the invention Figure 5 Comparison between the cooling profile of a hot rolled coil with the cooling rate applied in Gleeble in accordance with the invention Figure 6 Optical microstructure of the newly developed steel Figure 7 Microstructure of the newly developed steel as seen under Scanning Electron Microscope Figure 8 TEM photograph showing the alternate austenite and bainite structure and the bainite thickness of the newly developed steel Figure 9 XRD profile sowing the goodness of matching between the simulated and experimentally obtained data. The black dots represent the experimental data points, the red line represents the simulated profile and the blue line denotes the differences at each points between the experimentally obtained data with the simulated profile. BRIEF DESCRIPTION OF THE ACCOMPANYING TABLES Table 1 Chemical composition of the steel according to the invention. Table 2 Results obtained after full profile Rietveld refinement of the XRD data in accordance with the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION According to the invention a steel composition is proposed which can produce a steel with hardness of 400 VHN minimum (equivalent to an UTS of minimum 1200 MPa) and 20% minimum austenite retained in the final microstructure to produce an elongation of 20% minimum after hot rolling and coiling in a typical hot strip mill. In designing carbide-free bainitic steel which can deliver a very high strength (>400 VHN), one must avoid the formation of diffusional products such as polygonal ferrite and cementite. The formation of cementite from austenite during cooling can be taken care of by the addition of suitable amount of Si which prevents cementite formation due to its very low solubility in cementite. To avoid the possible formation of other high temperature diffusional product named polygonal ferrite, the diffusional bay of the time-temperature- transformation (TTT) diagram must be shifted to the right side so that even in a moderate cooling rate ferrite does not form. MUCG83 program6 was used to get the idea about the possible time-temperature combination for diffusional and displasive transformation. During the progress of bainitic transformation, the whole of the austenite grain does not transform instantaneously to bainite. When the first plate of bainite forms, it rejects its excess carbon which it can not accommodate, into the adjacent austenite. Further advancement of transformation therefore is associated with a lower free energy change due to the higher carbon content of austenite from which bainite forms. Finally a time is reached when the free energies of both residual austenite and bainitic ferrite of the same composition becomes identical and therefore any further transformation becomes thermodynamically impossible. To represents the locus of all the points, on a temperature versus carbon concentration plot, where the stress-free austenite and ferrite of identical composition (with respect to both interstitial and substitutional alloying elements) are having same free energy7,8. The bainitic transformation can progress by successive nucleation of subunits of bainitic ferrite till the carbon concentration in the remaining austenite reaches to its theoretical limit which is defined by the To curve. It leads to the conclusion that the maximum amount of bainite which can be produced at any given transformation temperature is restricted by the retained austenite carbon concentration which can not exceed the limit given by the To curve. Bainitic transformation normally happens at such a temperature wherethe diffusion of any elements except carbon is extremely negligible. Hence it can be considered that during bainitic transformation no other diffusional reaction interacts with it and the temperature is too high for other diffusionless transformation to occur. The carbon enrichment in austenite from adjacent bainitic-ferrite plates makes it thermally stable at room temperature and it will only transform to martensite during deformation exhibiting a TRansformation Induced Plasticity (TRIP) effect. Consequently, the total amount of carbon in the steel becomes a function of the volume fraction and carbon content of retained austenite and bainite and can be expressed as where Vr is the volume fraction of retained austenite after the transformation is over; x, Cr and Qare the carbon concentrations in bulk, retained austenite and in bainite, respectively. Vr can be obtained from X-ray diffraction analysis and can be obtained both from the XRD analysis as well as from theoretical considerations using To. During TRIP effect, austenite transforms to martensite and the material gets work hardened. As a consequence, it is very much essential to have a certain amount of austenite remain untransformed at the ambient temperature so that TRIP effect can occur. For this reason, the Ms temperature of this retained austenite has to be lower than the room temperature, but the corresponding Md temperature should be close to the maximum temperature reached during collisions9. After the isothermal bainitic transformation, there could be regions of untransformed and residual austenite which may transform to martensite on further cooling / quenching. The morphology of this kind of austenite is blocky and exhibits triangular shape in two dimensional sections. Austenite could also be present there in the microstructure as thin films entrapped between two neighbouring sub-units of bainite. Experimental observations had shown that this later variation of austenite morphology contains a higher amount of C than the islands of residual austenite resulting from the geometrical partitioning of the prior austenite grain by different crystallographic variants of bainite sheaves10. This effect was reflected in the martensitic decomposition of the islands of residual austenite when quenching to room temperature from the isothermal transformation temperature whereas the entrapped films are fully retained11. If the volume fraction of bainite could be increased, the volume fraction of such blocky type austenite would be decreased; its C content would increase, leading to a decreased final martensite volume fraction. However, as the degree of transformation to bainite at a particular isothermal transformation temperature is restricted by thermodynamics of the incomplete reaction phenomena, the only way to increase the volume fraction of bainite would then be by lowering the temperature. It was suggested that the ratio of volume fraction of film / blocky morphology should be 0.9 for obtaining a good combination of strength - ductility and optimum property. In a microstructure where the constituent phases are only austenite and bainite, the sources of strengthening are very much limited. The steel can develop its strength from the solid solution strengthening of austenite and bainite, very fine plates of bainites and from the dislocations generated during the isothermal bainitic transformation. The theoretical strength of this kind of microstructures can be expressed after Bhadeshia8 as where KL,KD and Kp are constants, aFe\s the strength of pure annealed iron, 1300 MPa range which is outstanding considering the simple chemistry and processing of this selected alloy. References 1. H. K. D. H. Bhadeshia, Materials Science and Engineering -/I, Volume 481 - 482, pp. 36 - 39, 2008; 2. F. G. Caballero, H. K. D. H. Bhadeshia, K. J. A. Mawella, D. G. Jones and P. Brown, Materials Science and Technology, Volume 18, pp. 279 - 284, 2002; 3. C. Garcia-Mateo, F. G. Caballero and H. K. D. Bhadeshia, ISU International, Volume 43, pp. 1238 - 1243, 2003 4. F. G. Caballero, M. J. Santofima, C. Capdevila, C. G. Mateo and C. G. De Andres, ISU International, Volume 46, pp. 1479 - 1488, 2006; 5. F. G. Caballero, M. J. Santofima, C. Garcia Mateo, J. Chao and C. Garcia de Andres, Materials and Design, Volume 30, pp. 2077 - 2083, 2009 6. http://www.msm.cam.ac.uk/phase-trans/ 7. H. K. D. H. Bhadeshia and D. V. Edmonds, Acta Metallurgica, Vol. 28, 1980, 1265 8. H. K. D. H. Bhadeshia, Bainite in Steels, 2nd edition, The Institute of Materials, London 9. B. C. DeCooman, Current Opinion in Solid State and Materials Science, Vol. 8, pp. 285 - 303, 2004 10. H. K. D. H. Bhadeshia and D. V. Edmonds, Acta Metallurgica, Vol. 28, pp. 1265 - 1273, 1980 11. H. K. D. H. Bhadeshia and D. V. Edmonds, Metallurgical Transactions A, Vol. 3, pp . 895, 1979. 12. S. B. Singh and H. K. D. H. Bhadeshia, Materials Science and Engineering A, Vol. 245, pp. 72 - 79, 1998 13.J. R. Cahoon, W. H. Broughton and A. R. Kutzak, Metallurgical Transactions, Volume 2, pp. 1979 - 1983, 1971 14. E. J. Pavlina and C. J. Van Tyne, Journal of Materials Engineering and Performance, Vol. 17, No. 6, pp. 888 - 893, 2008 15. D. J. Dyson and B. Holmes, Journal of the Iron and Steel Institute, pp. 469 - 474, 1970 WE CLAIM 1. A new grade of ultra high strength steel, consisting of: carbon from 0.2 to 0.5 wt%; silicon content from 1.1 to 2.1 wt%; manganese content from 1.0 to 2.0 wt%, chromium content between 0.5 to 3.0 wt%, aluminium from 0.01 to 0.07 wt%; sulphur and nitrogen content 0.008 wt% and 0.005 maximum, respectively, the produced steel grade exhibiting a hardness of more than 400 VHN. 2. The Steel grade as claimed in claim 1, wherein the final microstructure of the steel contains 77-80 bainite with at least 20% retained austenite. 3. The steel grade as claimed in claim 1 or 2, wherein the retained austenite is thermally stable at room temperature and is transformable to martensite during deformation only. 4. The steel grade as claimed in claim 1 and 2, wherein the bainite is nano- sized bainite of 80-200 nm. 5. The steel grade as claimed in claim 1, 2 wherein the nano-sized bainite as claimed in claim 4 can be produced during a coil cooling situation. The invention relates to a continuously cooled bainitic steel composition is proposed to a steel with hardness of 400 VHN (which is approximately an UTS of 1200 MPa minimum) and 20% retained austenite (to provide total elongation of minimum 20%) for application in automotive industries. The steel will first be made in a steel converter having the composition in wt% C: 0.20-0.50, Mn: 1.0- 2.0, Si: 1.1-2.1, Cr: 0.5-3.0, S: 0.008 max, P: 0.025 max, Al: 0.01 -0.07, N: 0.005 max, cast into slab, hot rolling the slab into strip at finish rolling temperature (FRT) 850-950 C and coiling the strip at coiling temperature 350- 550 C. During the natural cooling of the coil, the bainitic transformation will continue and the desired properties will be developed when the coil cools down to room temperature.