Technical Field The present invention relates to a cold-rolled steel sheet. Background Art Recently, the reduction of weight of automobile bodies has increasingly been demanded with the aim of improving the fuel efficiency of automobiles. One/of the measures to reduce an automobile body weight is to use a steel material having a high strength. However, as the strength of a steel material increases, the press forming of the steel material becomes increasingly difficult. This is because, generally, as the strength of a steel material increases, the yield stress of the steel material increases and, further, the elongation thereof decreases. To cope with the above problem, a steel sheet that makes use of strain induced transformation of retained austenite (hereunder referred to as "TRIP steel"), and the like, have been invented to improve elongation and these technologies are disclosed in Japanese Unexamined Patent publications No. 861-157625 and No. H10-130776, for example. However, an ordinary TRIP steel sheet inevitably requires a large amount of 8i to be contained, as a result the performance of chemical conversion treatment and hot-dip galvanization on the surface of the steel sheet deteriorates and, therefore, the members to which the steel sheet is applicable are limited. In addition, in a retained austenite steel, a large amount of C must be added in order to secure a high strength and, as a result, problems of welding, such as nugget cracks, arise. With regard to the performance of chemical conversion treatment and hot-dip galvanization on the surface of a steel sheet, inventions that aim to reduce the Si amount in a retained austenite TRIP steel are disclosed in Japanese Unexamined Patent Publications No. H5-247586 and No. 2000-345288. However, through the inventions, though an improvement of the performance of chemical conversion treatment and hot-dip galvanization, as well as ductility, can be expected, an improvement in the aforementioned weldability cannot be expected. Moreover, in the case of a TRIP steel of 980 MPa or more in tensile strength, the yield stress is very high and, therefore, the problem has been that the shape freezing property of the steel deteriorates at the time of pressing or the like. Further, in the case of a high strength steel sheet of 980 MPa or more in tensile strength, the occurrence of delayed fracture is a concern. Another problem is that, as a TRIP steel sheet contains a large amount of retained austenite, voids and dislocations are formed, in quantity, at the interface between a martensite phase formed by strain induced transformation and other phases in the vicinity of the martensite phase, hydrogen accumulates the interface and, then, delayed fracture occurs. Further, as a technology of reducing a yield stress, a dual phase steel (hereunder referred to as "DP steel") containing ferrite has so far been known as disclosed in Japanese Unexamined Patent Publication No. S57-155329. However, the technology requires that a cooling rate after recrystallization annealing is 30°C/sec. or more and the cooling rate is insufficiently achieved in an ordinary hot-dip galvanizing line. Furthermore, the target tensile strength of the steel sheet is 100 kg/mm2 at the highest and therefore a high strength steel sheet having sufficient formability has not always been realized. Disclosure of the Invention The object of the present invention is, by solving the aforementioned problems of the prior art, to realize a high strength steel sheet excellent in formability and the performance of chemical conversion treatment and galvanization, and a method for producing the steel sheet in an industrial scale. The present inventors, as a result of earnestly studying a high strength steel sheet excellent in formability, have found that, in the case of a DP steel having a low yield stress, a high strength steel sheet capable of securing an elongation higher than before can be produced industrially by optimizing the steel components and, namely, by regulating the balance between the amounts of Si and Al and the value of TS (a target strength) to specific ranges and, particularly, by adjusting the addition amount of Al. By the present invention, realized is a high strength steel sheet wherein ductility is improved to an extent comparable with, or similar to, a conventional retained austenite steel, chemical converted coating treatment and hot-dip galvanization is improved by reducing Si and, moreover, the properties are less deteriorated even when alloying plating is applied. Further, the present invention provides a DP steel that allows retained austenite to be unavoidably included at 5% or less and substantially does not contain retained austenite so as not to incur the problems of delayed fracture and secondary working embrittlement. The tensile strength of a high strength steel sheet according to the present invention ranges from 590 to 1,500 MPa and the effects of the present invention are particularly conspicuous with a high strength steel sheet of 980 MPa or more. The present invention is based on the above technological concept and the gist of the present invention is as follows: (1) A high strength steel sheet excellent in formability, chemical converted coating treatment and hot-dip galvanizing, characterized in that: said steel sheet contains, in mass, 0.03 to 0.20% C, 0.005 to 0.3% Si, 1.0 to 3.1% Mn, 0.001 to 0.06% P, 0.001 to 0.01% S, 0.0005 to 0.01% N, 0.2 to 1.2% Al, and not more than 0.5% Mo, with the balance consisting of Fe and unavoidable impurities; the amounts of Si and Al in mass % and the target strength (TS) of said steel sheet satisfy the following expression (1); and the metallographic structure of said steel sheet contains ferrite and martensite; (0.0012 x [target strength TS] - 0.29 - [Si])/2.45 (Ac3 - 500)/10a .... (3) a = 0.6[C] + 1.4[Mn] + 3.7[Mo] - 0.87, where/ X is a cooling rate in terms of °C/sec., Ac3 is expressed in terms of °C, [C] is the amount of C, [Mn] that of Mn, and [Mo] that of Mof each in terras of mass %. Brief Description of the Drawings Figure 1 is a graph showing the ranges of Al and Si for each target strength TS. Figure 2 (a) is a graph showing the relationship between the performance of chemical conversion treatment and hot-dip galvanization and the amounts of Mn and B in the case of 0.4% Al, and Figure 2 (b) is a graph showing the relationship between the performance of chemical conversion treatment and hot-dip galvanization and the amounts of Mn and B in the case of 1.2% Al. Figure 3 is a graph showing the relationship between the cooling rate for securing ductility and the chemical components. Best Mode for Carrying out the Invention The embodiments of the present invention will be hereunder explained in detail. Firstly, the reasons for regulating the chemical components and the metallographic structure of a high strength steel sheet according to the present invention will be explained. C is an essential component from the viewpoint of securing strength and as the basic element to stabilize martensite. When a C amount is less than 0.03%, the strength is insufficient and a martensite phase is not formed. On the other hand, when a C amount exceeds 0.2%, strength increases excessively, ductility is insufficient, weldability deteriorates, and therefore the steel cannot be used as an industrial material. For those reasons, a C amount is regulated in the range from 0.03 to 0.2%, preferably from 0.06 to 0.15%, in the present invention, Mn must be added from the viewpoint of securing strength and, in addition, is an element that delays the formation of carbides and is effective for the formation of ferrite. When an Mn amount is less than 1.0%, strength is insufficient, the formation of ferrite is also insufficient, and ductility deteriorates. On the other hand, when an Mn amount exceeds 3.1%, hardenability increases more than necessary, as a result martensite is formed abundantly and, thus, strength increases, as a result the variation of product quality increases, ductility is insufficient, and therefore the steel cannot be used as an industrial material. For those reasons, an Mn amount is regulated in the range from 1.0 to 3.1% in the present invention. Si is an element that is added from the viewpoint of securing strength and generally to secure ductility. However, when Si is added in excess of 0.3%, the chemical converted coating treatment and hot-dip galvanization deteriorates. Therefore, an Si amount is set at 0.3% or less in the present invention, and further, when importance is placed on hot-dip galvanization, a preferable Si amount is 0.1% or less. Furthermore, Si is added as a deoxidizer and for the improvement of hardenability. However, when an Si amount is less than 0.005%, the deoxidizing effect is insufficient. Therefore, the lower limit of an Si amount is set at 0.005%. P is added as an element to strengthen a steel sheet in accordance with a required strength level. However, when the addition amount of P is large, P segregates at grain boundaries and, as a result, local ductility deteriorates. Further, P also deteriorates weldability. Therefore, the upper limit of a P amount is set at 0.06%. The lower limit of a P amount is set at 0.001%, because the decrease of a P amount beyond the figure causes the refining cost to increase at the stage of steelmaking. S is an element that forms MnS and, by so doing, deteriorates local ductility and weldability, and therefore it is better that S does not exist in a steel. For that reason, the upper limit of an S amount is set at 0.01%. The lower limit of an S amount is set at 0.001%, because, like P, decreasing an S amount beyond this figure causes a refining cost to increase at the stage of steelmaking. Al is the most important element in the present invention. The addition of Al accelerates the formation of ferrite and improves ductility. In addition, Al is an element that does not deteriorate the performance of chemical conversion treatment and hot-dip galvanization even when Al is added in quantity. Furthermore, Al functions also as a deoxidizing element. An Al addition of 0.2% or more is necessary for the improvement of ductility. On the other hand, when Al is added excessively, the above effects are saturated and rather a steel becomes brittle. For that reason, the upper limit of an Al amount is set at 1.2% N is an element that is unavoidably included. When N is contained excessively, not only an aging property deteriorates but also the amount of precipitated AlN increases and the effect of Al addition is reduced. For that reason, a preferable N amount is 0.01% or less. On the other hand, excessive reduction of an N amount causes the cost to increase in a steelmaking process and, therefore, it is generally preferable to control an N amount to about 0.0005% or more. In general, large amounts of alloying elements must be added in order to produce a steel sheet having a high strength and in which the formation of ferrite is suppressed. For that reason, the fraction of ferrite in a structure decreases, the fraction of the second phase increases, and therefore elongation decreases considerably particularly in a DP steel of 980 MPa or more. To cope with this, the measures of the addition of Si and the reduction of Mn are mostly taken. However, the former measure causes the performance of chemical conversion treatment and hot-dip galvanization to deteriorate, the latter measure causes a strength to be hard to secure and, therefore, these measures are not usable for a steel sheet as intended in the present invention. In this light, the present inventors, as a result of intensive studies, found that when the amounts of Al, Si and the value of TS were controlled so as to satisfy the following expression (1), a sufficient ferrite fraction was secured and an excellent elongation was secured; (0.0012 x [target strength TS] - 0.29 - [Si])/2.45 (Ac3 — 500)/iDa... (3) a = 0.6[C] + 1.4[Mn] - 3.7[Mo] — 0.87 where, X is a cooling rate in terms of °C/sec, Ac3 is expressed in terms of °C, [C) is the amount of C, [Mn) that of Mn and [Mo) that of Mo, each in terms of mass %, and successively applying skin-pass rolling.