DESCRIPTION GRAIN-ORIENTED ELECTRICAL STEEL SHEET EXTREMELY EXCELLENT IN MAGNETIC PROPERTIES AND METHOD OF PRODUCTION OF SAME TECHNICAL FIELD The present invention relates to a method for producing grain-oriented electrical steel sheet used mainly as a core of a transformer etc. BACKGROUND ART Various technologies have been proposed for stably producing a grain-oriented electrical steel sheet excellent in magnetic properties having a magnetic flux density B8 (magnetic flux density in magnetic field of 800 A/m) exceeding 1.9T. The methods of production in the case containing Al as an inhibitor can be classified into the first to third, that is, three types of, technologies shown in Table 1 according to the slab heating temperature. Table 1 (Table Removed) The first technology is the complete solid solution non-nitridation type, that is, a method of heating a slab from 1350°C to an ultra-high temperature of 1450°C at the highest, then holding the slab at that temperature for a time long enough to uniformly heat (soak) the entire slab. This causes the MnS, A1N, and other substances having inhibitor capabilities to completely dissolve and causes them to function as the inhibitors required for secondary recrystallization. This complete solid-solubilization simultaneously becomes a means for eliminating the difference in inhibitor strength due to the slab position as well and, in this point, is advantageous for realizing stable secondary recrystallization. In the case of this technology, however, irrespective of the fact that the complete solid-solubilization temperature for securing the amount of inhibitor required for secondary recrystallization is not that high thermodynamically, in actual industrial production, the temperature cannot help becoming an ultra-high temperature in order to secure productivity and a uniform solid solution state of the slab as a whole. Improvement has been attempted, but actual production involves a variety of problems. For example, 1) securing the hot rolling temperature is difficult according to the position and when it cannot be secured, in-slab deviation of the inhibitor strength occurs, therefore poor secondary recrystallization occurs, 2) coarse grains are easily formed at the time of slab heating, the coarse grain parts cannot be secondary recrystallized, and streak-like poor secondary recrystallization occurs, 3) the slab surface layer melts and becomes molten slag and enormous labor becomes necessary for maintenance of the heating furnace, 4) giant edge cracks are easily formed in the steel strip after hot rolling, and so on. Further, in this technology, as disclosed in ISIJ International, Vol. 43 (2003), No. 3, pp. 400 to 409, Acta Metall., 42 (1994), 2593, KAWASAKI STEEL TECHNICAL REPORT, Vol. 29 (1997)3, 129-135, it is widely known that the Goss orientation sharpness deteriorates when performing nitridation after decarburization annealing up to the start of the secondary recrystallization in order to supplement the inhibitors. Further, it is well known that poor secondary recrystallization occurs when an amount of nitrogen is small at the time of melting. The second technology is a (sufficient) precipitation nitridation type. As disclosed in Japanese Patent Publication (A) No. 59-56522, Japanese Patent Publication (A) No. 5-112827, Japanese Patent Publication (A) No. 9-118964 etc., this performs the slab heating at a temperature less than 1280°C and performs the nitridation from after the decarburization annealing to the start of the secondary recrystallization. In this method, as shown in for example Japanese Patent Publication (A) No. 2-182866, control of the mean grain size of primary recrystallized grains after the decarburization annealing to within a content range, usually a range from 18 to 35 µm, is very important for performing the secondary recrystallization well. Further, the amount of substances having an inhibitor capability in solid solution in the steel exerts a large influence upon the growth potential of primary recrystallized grains. Therefore, in this technology, in order to make sizes of the primary recrystallized grains in the steel sheet uniform, for example, Japanese Patent Publication (A) No. 5-295443 discloses a method of making the solute nitrogen at the time of the slab heating low to suppress non-uniform precipitation occurring in a later process. From the viewpoint of reduction of the amount of solid solution, the actual slab heating temperature is desirably 1150°C or less. In this technology, however, no matter how strictly_ the chemical compositions are adjusted, the inhibitor substances cannot be left completely coarsely precipitated as they are, so the primary recrystallized grain size tends not to be constant. Therefore, in actual production activities, in order to obtain a suitable primary recrystallized grain size, the conditions of the primary recrystallization annealing (particularly the temperature) are adjusted for each coil. For this reason, the production process becomes troublesome. Further, the formation of the oxide layer in the decarburization annealing is not constant. Therefore, sometimes poor formation of the glass film occurs. The third technology is the mixed type. As shown in Japanese Patent Publication (A) No. 2000-199015, the slab heating temperature is set to 1200 to 1350°C and the nitridation is made essential in the same way as the second technology. In order to avoid the ultra-high slab heating temperature exceeding 1350°C in the first technology, the slab heating temperature is lowered. The insufficient inhibitor strength along with this is made up for by the nitridation. This technology is further classified into two types. One is the partial solid solution nitridation type (partial precipitation nitridation type), and the other is the complete solid solution nitridation type as represented by Japanese Patent Publication (A) No. 2001-152250. In the former, it is not easy to make the solid solution state industrially uniform in the steel sheet (coil) as a whole. On the other hand, in the latter, the contents of the inhibitor elements are reduced to enable the elements to enter solid solution, therefore a non-uniform state of inhibitors seldom occurs. This is a very logical and effective technology. This third technology classifies inhibitors into a primary inhibitor for determining the primary recrystallized grain size and a secondary inhibitor for making the secondary recrystallization possible. The primary inhibitor naturally contributes to the secondary recrystallization as well. Due to the presence of the primary inhibitor, the fluctuation in grain size after the primary recrystallization becomes small. Particularly, in the latter complete solid solution type, the primary recrystallized grain size does not change in the usual temperature range, therefore, it is not necessary to change the primary recrystallization annealing conditions for adjustment of the grain size, and the glass film is formed extremely stably. As the primary inhibitor, the inhibitor substances used in the first technology (for example, A1N, MnS, MnSe, Cu-S, Sn, Sb, etc.) are mainly used. However, to reduce the slab heating temperature, their contents are required to be small. The secondary inhibitor is the A1N which is formed nitrided and these primary inhibitors after the decarburization annealing and up to the start of the secondary recrystallization. Further, the above Japanese Patent Publication (A) No. 2001-152250 also discloses BN as a primary inhibitor. However, N bonds with Al as well, therefore actually sometimes the secondary recrystallization becomes unstable when Al and B are simultaneously contained. As a problem common to the above three technologies, the fact that the suitable ranges of the contents of the required inhibitor substances (particularly Al and N) are narrower in comparison with the process capability at the time of melting in the steelmaking may be mentioned. Therefore, conventionally, the method of adjusting the production conditions using the acid-soluble Al (hereinafter referred to as "solAl") minus the N equivalent, that is, A1R, as a parameter is disclosed in the first and second technologies. In the first technology, for example Japanese Patent Publication (A) No. 60-177131 prescribes adjustment of a soaking time or cooling rate of the annealing before the last cold rolling and/or any of the series of process conditions by the A1R value. Further, in the second technology, Japanese Patent Publication (A) No. 7-305116 prescribes a ratio of N2 in the atmosphere at the time of the final annealing according to an equation of the A1R. Japanese Patent Publication (A) No. 8-253815 adds Bi and prescribes the temperature of the annealing before the last cold rolling according to the equation of A1R. Japanese Patent Publication (A) No. 8-279408 includes Ti and defines the nitridation amount according to the equation of A1R considering TIN. DISCLOSURE OF THE INVENTION In the case of the third technology, the primary recrystallization annealing temperature dependency of the primary recrystallized grain size is negligibly small. However, if the contents of the inhibitor ingredients, particularly Al and N and further the Ti exerting an influence upon the formation of A1N, fluctuate, sometimes the secondary recrystallization behavior becomes unstable. When the A1R is large, in order to secure the magnetic properties, it is necessary to increase the nitridation amount in the later process. The reason for this is currently considered to be as-follows. If the A1R is large, A1N precipitates large after the annealing before the last cold rolling and the primary grain size becomes large, but the effect of the primary inhibitor as the secondary inhibitor becomes strong, therefore the secondary recrystallization start temperature becomes higher. With this as is, the inhibitor strength is not sufficient in terms of quality with respect to the higher temperature, the balance of the grain size and inhibitor is lost, and poor secondary recrystallization results. Therefore, it is necessary to strengthen the secondary inhibitor by the nitridation so as to correspond to the higher secondary recrystallization temperature, and the need arises for increasing the nitridation amount. Namely, if the secondary recrystallization temperature rises, it is necessary to strengthen the inhibitor strength. Further, the degree of change of the inhibitor strength becomes large (the change of strength of the inhibitor is sudden at a high temperature), so coarse inhibitors may become necessary. However, if the nitridation amount is made large, the glass film suffers from defects of metal exposure, and the defect ratio (rejection rate) remarkably increases. On the other hand, if the A1R is small, A1N precipitates small after the annealing before the last cold rolling and the primary grain size becomes small, therefore the secondary recrystallization start temperature does not become higher and the nitridation amount may be kept small. However, if A1R is too small, as disclosed in Non-Patent Document 1, the secondary recrystallization nuclei forming positions spread out over the entire sheet thickness. Therefore, not only the grains of the sharp Goss orientation in the vicinity of the surface layer, but also at the center layer the grains of dispersed-Goss orientation are secondary recrystallized, and the magnetic properties deteriorate. In this way, if the A1R changes, the secondary recrystallization behavior and in turn the sharpness of the Goss orientation changes. However, at the melting stage, it is difficult to control the ranges of the ingredients of Al, N, and Ti to narrow ranges, therefore countermeasures for easing the influence of fluctuations of these ingredients have been desired. The fact that a grain-oriented electrical steel sheet is produced through many processes after hot rolling is well known. In the present invention, the slab heating temperature is not made extremely high or low, production is possible by a conventional hot rolling mill, no special slab heating apparatus is needed, the inhibitor strength is kept content in the processes after the hot rolling even when the ingredients unavoidably fluctuate, and a grain-oriented electrical steel sheet extremely good in magnetic properties can be produced. The present invention provides a method of production of a grain-oriented electrical steel sheet applying high temperature slab heating using A1N as a main inhibitor of secondary recrystallization which makes effective use of the later process of nitridation prohibited in the past due to deterioration of the magnetic properties and thereby obtains a grain-oriented electrical steel sheet extremely excellent in magnetic characteristics. The present invention comprises the following: (1) A method of production of a grain-oriented electrical steel sheet extremely excellent in magnetic properties comprising reheating a slab comprising, by mass%, C: 0.025 to 0.10%, Si: 2.5 to 4.0%, Mn: 0.04 to 0.15%, solAl: 0.020 to 0.035%, N: 0.002 to 0.007%, S and Se, as Seq (S equivalents) = S + 0.406xSe, 0.010 to 0.035%, Ti < 0.007%, and a balance of Fe and unavoidable impurities to 1280°C or more and a solid solution temperature of the inhibitor substances or more, hot rolling it to form a hot rolled steel strip, annealing the hot rolled strip and cold rolling it one time or two or more times while intermediate annealing it in between, or omitting the annealing of the hot rolled strip and cold rolling it two or more times while intermediate annealing it in between, decarburization annealing it, nitriding it after the decarburization annealing in a mixed gas of hydrogen, nitrogen, and ammonia in the strip running state, coating an annealing separator mainly consisting of MgO, and applying final annealing, said method of production of a grain-oriented electrical steel sheet characterized by making a ratio of precipitation of the N contained in the steel strip after the hot rolling as A1N 20% or less, making a circle equivalent mean grain size (diameter) of the primary recrystallized grains after completion of the decarburization annealing 7 jam to less than 20 jam, making the nitrogen increase AN (mass%) in the nitridation within a range of Equation (1), and making the nitrogen contents Nl and N2 (each surface, mass%) of a 20% thickness portion of one surface of the steel sheet (strip) within a range of Equation (2). 0.007-([N]-14/48x[Ti])) in the primary recrystallization texture is broad, and further the intensity of S9 to Goss orientation becomes weak, therefore a high magnetic flux density is not obtained. Further, when it exceeds 92%, the Goss orientation intensity ({110}<001>) in the primary recrystallization texture becomes extremely weak, and the secondary recrystallization becomes unstable. The last cold rolling may be performed at ordinary temperature, but it is known that the primary recrystallization texture is improved and the magnetic properties become extremely good when at least 1 pass is performed holding the steel within a temperature range from 100 to 300°C for 1 minute or more. Regarding the mean grain size (diameter of circle equivalent area) of the primary recrystallized grains after the decarburization annealing, in for example Japanese Patent Publication (A) No. 07-252532, the mean grain size of the primary recrystallized grains is made 18 to 35 um. In the present invention, however, it is necessary to make the mean grain size of primary recrystallized grains 7 um to less than 20 um. This is an extremely important point in the present invention for making the magnetic properties (particularly the watt loss) good. Namely, if the primary recrystallized grain size is small, from the viewpoint of the texture as well, the volume percentage of Goss orientation grains becoming nuclei of the secondary recrystallization becomes large in the stage of the primary recrystallization. Further, since the primary recrystallized grain size is small, the number of Goss nuclei is relatively large as well. The absolute number thereof increases about quintuple in the case of the present invention compared with the case where the mean radius of the primary recrystallized grains is 18 to 35 um, therefore the secondary recrystallized grain size becomes relatively small as well. As a result of this, the watt loss is remarkably improved. Further, the start of the secondary recrystallization occurs near the surface layer of the sheet thickness, but when the primary recrystallized grain size is small, the selectivity in the sheet thickness direction of the Goss secondary recrystallization nucleus growth increases, and the Goss secondary recrystallization texture becomes sharp. When the grain size is less than 7 um, the secondary recrystallization temperature is extremely lowered, and the Goss orientation sharpness becomes poor. When the grain size becomes 20 jam or more, the secondary recrystallization temperature rises, and the secondary recrystallization becomes unstable. Usually, as the primary recrystallized grain size, when the slab heating temperature is made 1280°C or more and the inhibitor substances are made completely solid-solute, even if the annealing temperature before the last cold rolling and the decarburization annealing temperature are changed, the grain size substantially becomes within a range of 9 (j,m to less than 20 fam. In the present invention, in comparison with the technology of the sufficient precipitation nitridation type (second technology), the mean grain size of the primary recrystallized grains is made small, and the nitridation amount is made small. Due to these, the driving force for grain boundary movement (grain growth: secondary recrystallization) becomes larger and the secondary recrystallization starts in an earlier stage in the temperature heating up stage of the last final annealing (at a lower temperature). Due to this, in actual circumstances where the secondary recrystallization annealing is carried out in a coil state by box type annealing, with the method of causing secondary recrystallization in a constant heating up rate, the temperature histories of the different positions of the coil are similar, so the non-uniformity of magnetic properties according to coil positions of the secondary recrystallization is remarkably reduced, and the magnetic properties are stabilized to an extremely high level. The decarburization annealing is carried out under known conditions, that is, at 650 to 950°C for 60 to 500 seconds in accordance with the strip (sheet) thickness as well, preferably for 80 to 300 seconds in a nitrogen and hydrogen mixed wet atmosphere. At this time, if the heating rate from the start to the temperature up to 650°C is made 100°C/sec or more, the primary recrystallization texture is improved and the magnetic properties become good. In order to secure the heating rate, various methods may be considered. Namely, there are electrical resistance heating, induction heating, directly energy input heating, and so on. If the heating speed is made fast, the amount of Goss orientation is enriched in the primary recrystallization texture and the secondary recrystallized grain size becomes smaller as known by Japanese Patent Publication (A) No. 1-290716 etc. Applying nitridation to the steel sheet after the decarburization annealing and before the start of the secondary recrystallization is essential in the present invention. As that method, a method of mixing a nitride (CrN, MnN, etc.) with the annealing separator at the time of the high temperature annealing and a method of nitridation in a mixed gas of hydrogen, nitrogen, and ammonia in a state where the strip is run after the decarburization annealing are known. Either method can be employed, but the latter method is practical in industrial production, so the present invention is limited to the latter. The nitridation is to secure the N to be bonded with the acid-soluble Al and secure the inhibitor strength. If the amount thereof is small, the secondary recrystallization becomes unstable. Further, if the amount is large, the Goss orientation sharpness extremely deteriorates and defects of exposure of the ground iron (matrix) in the primary film frequently occur. The upper limit of the nitrogen amount after the nitridation must be the amount exceeding the N of the Al equivalent as A1N. The reason for this is not yet clear, but the inventors think as follows. When the temperature becomes high during the secondary recrystallization annealing, the A1N functioning as the inhibitor dissolves and go into solid solution to be weakened. In this case, however, since diffusion of N is easy, if the content (nitridation amount) is small, this weakening is fast, and the secondary recrystallization becomes unstable. In this way, for thermal stabilization of the inhibitor, N larger than the A1N equivalent is necessary. In this case, Al is sufficiently fixed, therefore the weakening of the inhibitor is slow, and the selective growth of the Goss secondary recrystallization nuclei is secured extremely largely. By combining the above influences, the nitridation amount AN (mass%) is adjusted within the range defined in the following Equation (1). 0.007-([N]-14/48x[Ti])