Title of Invention: Full Cone Spray Nozzle Technical Field [OOOl] The present invention relates to a full cone spray nozzle which for example is used for cooling, washing, etc. in the process of production of steel sheet and sprays a liquid in a full cone shape. Background Art [0002] A "full cone spray nozzle" is a nozzle in which the liquid which is discharged from the nozzle is sprayed in a conical shape. "Full cone" means the droplets of the discharged liquid are filled to the center of the cone. [0003] A full cone spray nozzle generally has a tubular nozzle body inside of which there is a vane structure which has swirl flow generating means. There are various shapes of vane structures, but the liquid which is supplied from the upstream end of the nozzle body passes the vane structure and flows to the downstream end of the nozzle body during which time the swirl flow generating means of the vane structure makes it swirl and form an eddy current. [00041 The liquid which flows to the downstream side of the nozzle body in this way is sprayed from the downstream end of the nozzle body in a full cone shape. [0005] PLT 1 discloses a full cone spray nozzle which has a bore at the center part of the vane structure and is provided with a swirl flow generating means comprised of a plurality of swirl paths which are formed in an inclined direction at the outer circumferential surface of the vane structure. This full cone spray nozzle aims at generating a spray pattern of a uniform flow rate distribution by a wide angle (65 to 75") with a uniform flow rate distribution. [0006] PLT 2 discloses a full cone spray nozzle which lacks the center bore of the vane structure and makes the vane structure as a whole an X-shape. According to this full cone spray nozzle, it is possible to generate a spray pattern which has a bell-shaped flow rate distribution which has a maximum flow rate at the center 5 of the spray region of a narrow spray angle (about 30° or less). [00071 PLT 3 discloses a nozzle which has channel grooves in an inclined direction at the outer circumference of the vane structure, has a downstream 10 side of the vane structure formed into a cone shape, and ejects a hollow cone shaped spray. A "hollow cone shaped spray" is a spray which is cone shaped at its outside, but does not have droplets of the discharged liquid filled to the center of the cone. Therefore, according to 15 this nozzle, it possible to give a swirl force to a low pressure liquid and generate a fine, stable hollow cone spray, but a full cone spray is not produced. Citations List Patent Literature 20 [OOOS] PLT 1. Japanese Patent Publication No. 2005- 508741A PLT 2. Japanese Patent Publication No. 2005-058899A PLT 3. Japanese Patent Publication No. 2005-052754A Summary of Invention 2 5 Technical Problem [OOOS] In the process of production of steel sheet, for example, when cooling steel sheet after hot rolling, spray nozzles are used to spray cooling water on the steel sheet. 30 [OOlO] To use spray nozzles for cooling steel sheet, it is demanded that it be possible to obtain a strong, uniform spray impact and a uniform water flow rate distribution across the entire region being sprayed. If the spray impact is weak, the cooling ability is 3 5 inferior. If the spray impact and the flow rate distribution are not uniform, over-cooling etc. occur in part of the region of the steel sheet and, as a result, the characteristics of the steel sheet are adversely affected. [OOll] Here, the "water flow rate distribution" means the distribution of the flow rate density of fluid per unit area in a spray region on a flat surface when projecting the spray on to a flat surface. Further, the "spray impact" means the pressure of the fluid which strikes a flat surface when the spray is projected onto a flat surface. [00121 Even if using a conventional spray nozzle, if raising the inflow pressure of the liquid from the inlet of the spray, a strong uniform spray impact and uniform flow rate distribution can be obtained. However, raising the inflow pressure requires an increase in the pumps. This is not desirable from the cost viewpoint. [0013] The full cone spray nozzle of PLT 1 requires an axial flow by the center bore of the vane structure in order to obtain a uniform water flow rate distribution in a wide angle spray region. However, it is in practice difficult to obtain a uniform water flow rate distribution due to the effects of dimensional tolerances and pressure fluctuations in the liquid. The flow rate of the center part of the spray area easily becomes greater. However, if just using a vane structure which does not have a center bore so as to decrease the flow rate at the center part of a wide angle use spray nozzle, conversely the flow rate near the center part will fall and a uniform spray pattern will no longer be able to be obtained (see FIG. 5C) . [0014] The full cone spray nozzle of PLT 2 is one for obtaining a bell-curve type spray pattern. The further from the center, the weaker the spray impact. Therefore, when used for cooling steel sheet, good cooling is not possible. [00151 The nozzle of PLT 3 is one which imparts a swirl force to a low pressure liquid and generates a hollow 'cone type spray pattern which has a weak spray impact and fine liquid droplets. This cannot be applied for generating a full cone spray by a high pressure liquid with a strong spray impact. [0016] An object of the present invention is to 5 provide a full cone spray nozzle which is suitable for example for cooling steel sheet in the process of production of steel sheet and which has a strong, uniform spray impact across the entire sprayed region even without increasing the inflow pressure. 10 [0017] That is, the object is to realize a nozzle which has the characteristic of the amount of liquid reaching an object (in the case of the present invention, the flat surface to be cooled) per unit area per unit time being substantially constant at the circle at the 15 bottom of the cone. Furthermore, in the nozzle of the present invention, the object is to increase the velocity by which the fluid impacts the object over that of the conventional nozzle, strengthen the spray impact, and improve the cooling ability by the same inflow pressure. 20 Solution to Problem [00181 The inventors in particular engaged in in-depth studies on a structure of a full cone spray nozzle which gives the necessary spray impact in the spray region required for cooling steel sheet in particular without 25 raising the inflow pressure and furthermore which achieves a uniform water flow rate distribution. [00191 When made a structure with a bore at the center part of the vane structure inside the nozzle, as explained above, the uniformity of the flow rate 30 distribution is not good, so the inventors studied in detail a structure with no bore at the center part of the vane structure. The "vane structure" referred to here is the part 2 which gives swirl at the inside of the nozzle which forms the swirl path 7 which is shown in FIG. 1 or 35 FIG. 3. [0020] When made a structure with no bore at the center part of the vane structure inside the nozzle, as explained above, the flow rate distribution easily becomes an inverted bell curve. However, as a result of studies of the inventors, it was learned that even in a structure with no bore at the center part of the vane 5 structure, by providing channels of a suitable width and depth at the circumference of the vane structure, particularly the downstream side, a full cone spray nozzle which has a spray angle suitable for cooling steel sheet etc. can be obtained. 10 [00211 However, even if simply making the nozzle a structure with no bore at the center part of the vane structure and making the channels around the vane structure suitable sizes, the pressure loss inside of the nozzle is large and a strong spray impact cannot be 15 obtained. [00221 The inventors engaged in further studies. As a result, they learned that by providing a projecting part at the downstream side of the vane structure and, furthermore, setting the swirl flow chamber at the 20 downstream side of the vane structure to a suitable size, it is possible to obtain a full cone spray nozzle which can reduce the pressure loss inside of the nozzle and which can form a spray pattern which has a strong spray impact across a broad range of the spray area without 25 raising the fluid pressure. COO231 Furthermore, they discovered that by making the downstream side projection a combination of a columnar shape and conical shape, it is possible to make the size of the swirl flow chamber more suitable and as a result 30 it is possible to obtain a full cone spray nozzle which can reduce the pressure loss inside the nozzle more and furthermore which can form a spray pattern which has a strong spray impact across a broad range of the spray area. 35 LO0241 Note that, sometimes an upstream side projection is provided at the upstream side of the vane structure and sometimes it is not, but from the viewpoint of stabilization of the flow rate, it is understood that it is also possible to provide the upstream side projection at the upstream side of the vane structure. [0025] The present invention was made based on the 5 above findings and has as its gist the following: I00261 (1) A full cone spray nozzle comprising: a nozzle body having a fluid inlet at an upstream end and a spray orifice at a downstream end; a vane structure of an axial direction length W and 10 diameter D arranged at an intermediate position inside of the nozzle body so that an outer circumferential surface contacts the inside of the nozzle body; a plurality of channel grooves of a width T and a depth H in an outer circumferential surface of the vane 15 structure; a downstream side projecting part at a downstream side of the vane structure; and a swirl flow chamber of axial direction length L which is a space formed by an inside wall surface of the nozzle 20 body, the vane structure, and the spray orifice, wherein 0.25