FIELD OF INVENTION The present invention relates to a cold-rolled steel sheet with improved strength and ductility. The invention further relates to a method of manufacturing a cold-rolled sheet. BACKGROUND OF INVENTION In the automotive industry there is an increasing demand for lighter vehicles with reduced fuel consumption. This necessitates the replacement of conventional low strength steels with higher strength steels so that thinner sections can be used and reduction in the weight of the auto components is possible without compromising on safety and functional requirements. Moreover, auto components have quite complex shapes and so the high-strength steel sheets to be used must also have excellent formability. Safety relevant parts which require high energy absorption capacity or resistance to crash need to have both high strength and high formability. Low alloy Transformation Induced Plasticity (TRIP) aided steels are multiphase steels which combine excellent strength and ductility properties and can be used for the applications mentioned above. Conventional TRIP aided steels or TPF (Matsumura, 0., Sakuma, Y. and Takechi, H. Transactions ISD, 21 (1987), pp. 570-579) have a- polygonal ferrite matrix and bainite and retained austenite as other phases. The retained austenite gradually transforms to martensite during deformation resulting in a continuously increasing strain hardening exponent and an enhanced strength-ductility balance. Other varieties of TRIP aided steels include TBF (NISSHIN STEEL TECHNICAL REPORT, No. 43, December 1980, pp. 1-10) and TAM (Sugimoto, K., Kanda, A., Kikuchi, R., Hashimoto, S., Kashima, T. and Ikeda, S. ISD International, 42 (2002), pp. 910- 915) steels. In TBF the matrix phase is bainitic ferrite and not polygonal ferrite. The second phase is retained austenite whereas TAM steel consists of annealed martensite as the matrix phase and bainite and retained austenite as other phases. Amongst the known steel grades, it is easy to attain high strength in TBF steels, because of its hard bainitic structure. Another advantage of the TBF steel is its ease of manufacturing for example, TBF steel is easily producible through one thermal processing (continuous annealing or plating). However, the ductility of TBF steel is not very high as the bainite does not easily accommodate the volume expansion associated with the transformation of retained austenite to martensite (Matsumura, O., Sakuma, Y. and Takechi, H. Transactions ISD, 21 (1987), pp. 570-579). TPF steels on the other hand, have very good ductility owing to the presence of ferrite in the matrix which being a softer phase allows easy accommodation of the volume expansion associated with the austenite to martensite transformation (Matsumura). However, the strength of ferrite being much lower than that of the bainite, the overall strength of the steel is less. US20080251160 describes a method of producing a TRIP aided steel, having 30-65 % bainitic ferrite, 30-50% polygonal ferrite and 5-20% retained austenite, which possesses a tensile strength - total elongation balance to the tune of 23000MPa% or more, and a balance of tensile strength vis-a-vis uniform elongation to the tune 14700 MPa% or more. OBJECTS OF THE INVENTION It is therefore an object of the invention to propose a cold-rolled steel sheet having an improved strength and ductility, which eliminates the disadvantages of the prior art. Another object of the invention is to propose a cold-rolled steel sheet having an improved strength and ductility, which is adaptable to automotive industry in particular for manufacturing of high crash resistant auto-components. A still another object of the invention is to propose a cold-rolled steel sheet having an improved strength and ductility, which possesses a right balance amongst the several mechanical properties. A further object of the invention is to propose a process for manufacturing a cold-rolled steel having improved strength and ductility. A still further object of the invention is to propose a process for manufacturing a cold- rolled steel having improved strength and ductility, which implements the steps of alloying a TRIP -aided steel, optimization of the volume fractions of the bainite and polygonal ferrite in the microstructure. Detail description of the invention The cold rolled steel sheet with improved strength and ductility according to the present invention contains in weight percent 0.15-0.35% of C, 1.0-3.0% of Mn, 0.80-1.50% of Al; 0.35-0.65% of Si, 0.10% or less (not including 0%) of Nb, 1.0% or less (not including 0%) of Ti, maximum 0.015% of S, maximum 0.030% of P, maximum 0.010% of N, the remaining being substantially iron and incidental impurities. The cold rolled steel sheet with improved strength and ductility according to the present invention has a microstructure comprising 60-80% of bainitic ferrite, 10-30% of polygonal ferrite, 10-15% of retained austenite and 0-5% martensite. The cold-rolled steel sheet of the invention exhibits an excellent balance between its yield strength and its uniform elongation; a right balance between its yield strength and its total elongation; a desired balance between its tensile strength and its uniform elongation; and a true balance between its tensile strength and its total elongation. The inventive method of manufacturing the cold-rolled steel sheet enables the product to possess an yield strength and uniform elongation in the range of 12000 MPa% to 15000 MPa%, yield strength and total elongation in the range of 15000 MPa% to 19500 MPa%, tensile strength and uniform elongation in the range of 14800MPa% to 19000MPa% and tensile strength and total elongation in the range of 18500MPa% to 24400MPa%. The steel sheet according to the present invention is adaptable in a wide spectrum of industrial fields including the automobile industry, the electric industry and the machinery industry. Particularly, this superior strength and ductility balance of the produced steel sheet provides a high potential for high energy absorption typically essential in the automotive industry for parts requiring high crash resistance. For manufacturing the cold-rolled steel sheet with excellent strength and ductility according to the present invention, the steel sheet is hot rolled, pickled, cold rolled and then soaked at a temperature Ts expressed by the formula (1) below and then quenched from this temperature down to the bainitic transformation range around 340°C to 440°C, held in this temperature range for a time less than 1000 s. A3-50(° C.) 745 MPa. 6. The process as claimed in claims 2 to 5, wherein the produced steel sheet comprises a tensile strength > 925 MPa. 7. The process as claimed in claims 2 to 6, wherein the produced steel sheet comprises a uniform elongation in the range of 15% to 20%. 8. The process as claimed in claims 2 to 7, wherein the produced steel sheet comprises a total elongation in the range of 20% to 25%. The invention relates to a cold rolled steel sheet with improved strength and ductility, including in percent by mass: 0.15-0.35% of C; 1.00-3.00% of Mn; 0.80-1.50% of Al; and 0.35-0.65% of Si; the microstructure of the steel comprising: 60-80% of bainitic ferrite; 10-30% of polygonal ferrite; 10-15% of retained austenite; and 0-5% of martensite. The invention further relates to a process of manufacturing a cold rolled steel sheet having improved strength and ductility.