Technical field [0001] The present invention relates to a continuous annealing apparatus of a manufacturing method and a steel sheet of the steel sheet, in particular, C of 0.050% by mass or more, Si and 0.10 wt% or more, a tensile strength of 780MPa or more which contains Mn than 1.20 wt% the method for producing a high-strength steel sheet, and to a continuous annealing apparatus of a steel sheet suitable for the method of manufacturing the steel sheet. BACKGROUND [0002] Recently, low fuel consumption and CO 2 for the purpose of vehicle weight reduction and crashworthiness improvement of aimed at reducing emissions, in the automotive field, there is a growing need to use a high strength steel sheet such as a vehicle body or parts. Recently, a tensile strength of 780MPa or more, or have been used high-strength steel sheet, such as more than 980 MPa. 　However, when increasing the strength of the steel sheet, material properties such as generally moldability (processability) is deteriorated. On the other hand, these high strength steel plate is molded in large quantities and at low cost as with mild steel plate by press working, it is required to be provided as various members. Therefore, the high strength steel sheet of the above, along with high strength, high ductility, and good processability is also required. [0003] 　Here, the tensile strength high-strength steel sheet of more than 780 MPa, high ductility, and, in order to impart good workability, generally, the alloy elements such as Si and Mn have been made be added to the steel. 　However, with respect to the steel sheet containing Si and Mn, when performing the annealing of maximum temperature 800 ~ 900 ° C. is oxidized and deposited and concentrated in the surface layer of the Si and Mn of the steel sheet, Si oxide on the surface and Si-Mn oxide there is a fear that exposed. Thus when the Si oxide or Si-Mn oxide is exposed on the surface of the steel sheet, and plating property deterioration during the plating process, problems such as decrease in chemical conversion treatability before painting occurs. [0004] 　For such problems, for example, Patent Document 1, the high strength steel sheet Si content is 0.4 to 2.0 mass%, when subjected to heat treatment, direct flame reducing burner flame-reducing furnace of the air ratio was reduced steel sheet in the reducing atmosphere of less than 0.6 to 0.9, in terms of controlled thin Si oxide film, an indirect heating furnace for performing hydrogen reduction water pressure P H2 O and hydrogen partial pressure P H2 logarithm log (P H2 O / P H2 to) by -1.6 or -0.5, and suppress the Si oxide is exposed on the surface of the steel sheet, a technique for improving the plating of the steel sheet Proposed. [0005] 　Patent Document 2, in a continuous annealing line and a direct flame heating furnace indirect reduction furnace, prior to the reduction in the reducing furnace, after oxidizing the steel sheet by heating a direct flame, the highest temperature of the steel sheet the T (923K ≦ T ≦ 1173K) when a logarithmic logP of oxygen partial pressure in the atmosphere in the furnace O2 and, -0.000.074 × T 2 + 0.105 × T-0.2 × [Si %] 2 + 2.1 × [Si%] - 98.8 ≦ logP O2 ≦ -0.000.078 × T 2 by reducing the range of + 0.107 × T-90.4, excellent in formability method of producing a high strength galvannealed steel sheet has been proposed. [0006] 　Further, Patent Document 3, when continuous annealing the cold rolled steel sheet, the surface of the steel sheet by heating the steel sheet by using a direct heat burner steel temperature is above air ratio 0.95 at 550 ° C. or higher at Atsushi Nobori is oxidized, then, the steel sheet temperature by heating the steel sheet after heating to a higher 750 ° C., the dew point is soaking annealing in an oven at -25 ° C. or less with an air ratio than 0.89 a flame burner by a technique with improved chemical conversion treatability it has been proposed. [0007] 　Further, Patent Document 4, when subjected to continuous annealing of the steel sheet, the heating process, -40 ° C. The dew point of the atmosphere in the case of the heating furnace temperature is 600 ° C. or higher A ° C. or less (650 ≦ A ≦ 780) follows and, by the heating furnace temperature is the dew point of the atmosphere and -10 ° C. or higher in the case of a ° C. greater than B ° C. or less (800 ≦ B ≦ 900), technology with improved chemical conversion treatability is proposed ing. [0008] 　Patent Documents 5 and 6, when performing the continuous annealing comprising the preheating step and the temperature rise step and the recrystallization step the steel plate, in the preheating step, the water vapor partial pressure ratio of hydrogen partial pressure in the continuous annealing atmosphere (P H2 O / P H2 that) is controlled to satisfy the following conditions (1) from the relationship between the preheating temperature Tp, the Atsushi Nobori step, a recrystallization temperature Tr and 650 ° C. ~ 900 ° C., annealing atmosphere water vapor partial pressure ratio of relative hydrogen partial pressure (P H2 O / P H2 ) satisfies the following conditions (2) from the relationship between the recrystallization temperature Tr, and so that the heating rate is 1 ~ 20 ° C. / sec control should be given, and in the recrystallization step, hydrogen partial vapor partial pressure ratio (P for pressure of the annealing atmosphere H2 O / P H2 satisfied) from the relationship between the recrystallization temperature Tr following equation (3) conditions, and the retention time of 40 to 600 By controlling so as to, technologies with improved chemical conversion treatability has been proposed. (1): log (P H2 O / P H2 ) ≦ -2.8 × 10 -6 Tp 2 + 6.8 × 10-3Tp-4.8 (2) below: 5.3 × 10-8 × Tr 2 + 1.4 × 10 -5 × 0.01 ≦ Tr-P H2O / P H2 of ≦ Tr × 10-7 6.4 × 2 + 1.7 × 10 -4 Tr-0.1 (. 3) the formula: P H2O / P H2 of <. 5. × 10. 3 -8 × Tr 2 + 1.4 × 10 -5 × Tr-0.01 [0009] 　Further, Patent Document 7, Si, the steel sheet containing Mn and Al in a specific ratio, the logarithmic ratio of hydrogen partial pressure and the steam partial pressure of the atmospheric gas in the reducing furnace is, -1.39 ≦ log (P H2 O / P H2 production method of galvanized steel sheet is controlled so as to satisfy) ≦ -0.695 is disclosed. [0010] 　However, the technique described in Patent Documents 1-7 described above has the following problems. 　In other words, Patent Documents 1 to 3 have a direct flame heating section is characterized by controlling the air ratio of the direct fire burner. However, recently, as a continuous annealing apparatus for performing annealing with respect to steel sheets, those constituted by all indirect heating furnace without a direct flame heating unit is the mainstream. In a continuous annealing furnace which does not have such direct flame heating unit, as described in Patent Documents 1-3, a method of controlling the air ratio of the direct fire burners can not be applied. Further, as in Patent Documents 1 to 3, the method of oxidizing the steel sheet at direct flame burner before reduction, due to such a high temperature deterioration of the combustion gas calorific value variation of the burner equipment will not be able to secure a predetermined air ratio , the thicker the resulting oxide film as a result, the build-up (raised) to the hearth rolls in the furnace can not be prevented from is formed. Oxide film formed by the direct flame furnace, by adhering the peeling roll surface of a steel plate while the steel sheet is wound around the furnace rolls, so to generate a flaw press the steel sheet, which is not preferable. 　Further, when controlling the atmosphere in the furnace under the conditions described in Patent Document 4, although the oxide of Si is exposed on the surface can be suppressed, since the dew point is required to be -10 ° C. or higher, decarburization proceeds of steel sheet, tensile strength and fatigue strength of the steel sheet there is a problem such as lowered. Particularly, in the high strength steel sheet containing a large amount of C, the strength reduction due to decarburization is a serious problem. 　Further, the conditions described in Patent Documents 5 and 6, the degradation of the external variations and production facilities moisture content in the gas, wherein (1), wherein P in (3) H2 O / P H2 value and the dew point or the like there is also a problem that it is difficult to ensure the proper conditions. 　In Patent Document 7, when producing a hot-dip galvanized steel sheet using a horizontal reduction furnace, the hydrogen concentration in the atmospheric gas in the furnace to be 10% or more has been suggested. However, the hydrogen concentration in the atmospheric gas is 10% or more, because it requires special equipment, and requires a large capital investment on the application. CITATION Patent Document [0011] Patent Document 1: Japanese Patent 2007-191745 JP Patent Document 2: Japanese Patent 2006-233333 JP Patent Document 3: Japanese Patent 2013-253322 JP Patent Document 4: Japanese Patent 2012-072452 JP Patent Document 5: Japanese Patent 2008-069445 JP Patent Document 6: Japanese Patent 2008-121045 JP Patent Document 7: Japanese Patent 2012-12683 JP Summary of the Invention Problems that the Invention is to Solve [0012] 　The present invention was made in view of the circumstances described above, the by controlling the atmosphere during annealing in a continuous annealing furnace having no direct flame heating unit, which was oxidized Si inside of the steel plate steel to suppress the Si oxide is exposed on the surface, and a method of manufacturing a decarburization high-strength steel sheet which can suppress the progress of a steel plate, and the continuous annealing apparatus of a steel sheet suitable for the method of manufacturing the high-strength steel sheet an object of the present invention is to provide a. In the present invention, the high strength, indicating that tensile strength is not less than 780 MPa. Means for Solving the Problems [0013] 　In order to solve the above problems, the present inventors have made intensive studies to obtain the following findings. 　During the heating of (a) annealing in the range of steel sheet temperature of 750 ° C. from 700 ° C., internal oxidation of Si, and decarburization is started. 　(B) 700 ℃ ~ 800 ℃ , particularly, in the range of 750 ° C. from 700 ° C., water pressure P in the atmosphere in the furnace H2O and hydrogen partial pressure P H2 equation log consisting Prefecture (P H2O / P H2 of) by adjusting the appropriate range, it is possible to suppress the decarburization causes internally oxidized Si. 　(C) In addition, 700 ° C. or less, or even in the range of heating the steel sheet at 800 ° C. greater, log (P H2 O / P H2 by adjusting the) within a proper range, to prevent the acid oxidation of the surface of the steel sheet, or promote internal oxidation, and it is possible to suppress the decarburization. 　(D) the time of heating the steel sheet at 800 ° C. greater than the dew point of the atmosphere in the furnace is to be less than -10 ° C., to suppress the decarburization, it is possible to prevent a decrease in strength. [0014] 　The present invention was made based on the above findings. It shows the summary below. 　(1) steel sheet production method according to an embodiment of the present invention has a tensile strength of a manufacturing method of the above high strength steel sheet 780 MPa, as a chemical composition, in mass%, C: 0.050 ~ 0.40% , Si: 0.10 ~ 2.50%, Mn: 1.20 ~ 3.50%, Cr: 0 ~ 0.80%, Ni: 0 ~ 5.00%, Cu: 0 ~ 3.00%, nb: 0 ~ 0.10%, Mg : 0 ~ 0.010%, Ti: 0 ~ 0.10%, B: 0 ~ 0.010%, Mo: 0 contained to 0.5%, and the balance Fe and impurities, as the impurity, P: 0.100% or less, S: 0.010% or less, Al: 1.200% or less, N: 0.0100% or less, the limit steel sheet to, 750 ° C. by heating to a temperature range of ~ 900 ° C., continuous annealing factory performing continuous annealing by holding 0-300 seconds at this temperature range Has the continuous annealing process, when performing the heating to said temperature range, and when performing the hold at the temperature range, the hydrogen concentration in the atmosphere in the furnace, and less than 10 vol%, the steel sheet of when the temperature is 700 ° C. or less, the water pressure P in the atmosphere of the furnace H2O and hydrogen partial pressure P H2 consisting Metropolitan a relation log (P H2O / P H2 of furnace body average value of) the range of the following formula (i), when the temperature of the steel sheet is below 700 ° C. ultra 800 ° C., the log (P in the atmosphere of the furnace H2 O / P H2The furnace body average of) the range of the following formula (ii), when the temperature of the steel sheet is 800 ° C. greater than the log (P in the atmosphere of the furnace H2 O / P H2 furnace average of) was in the range of the following formula (iii), and the dew point less than -10 ° C.. 　　　-3.01 　In the continuous annealing step in the method for manufacturing a steel sheet according to the present embodiment, the temperature of the steel sheet 1 is when the internal oxidation and decarburization of Si is 700 ° C. ultra 800 ° C. or less initiated, water pressure P in the atmosphere in the furnace of the continuous annealing apparatus 10 H2 O and hydrogen partial pressure P H2 is a relational expression consisting Metropolitan log (P H2 O / P H2 of furnace body average value of) -1.36 greater, and less than -0.07. log (P H2 O / P H2 ) to within the above range, it is possible to oxidize the Si inside of the steel sheet 1, it is possible to suppress the Si oxide is exposed on the surface of the steel sheet 1, the decarburization it is possible to suppress the occurrence. As a result, tensile strength, fatigue strength can be secured, further plating resistance, we are possible to produce a superior high-strength steel sheet chemical conversion treatability. Here, log is the common logarithm. [0039] 　Above relations log (P H2 O / P H2 when the furnace body average value of) is -1.36 or less, the internal oxidation of Si is not sufficiently generated, Si oxide is exposed on the surface of the steel sheet 1 , plating properties, chemical conversion treatment deteriorates. On the other hand, the above-mentioned relation log (P H2 O / P H2 when the furnace body average value of) is -0.07 or more, decarburization proceeds and the strength of the steel sheet 1 may be reduced. 　To further reduce the decarburization of a steel plate as well as further suppress the exposure of the surface of the Si oxide in the temperature of 700 ° C. Ultra 800 ° C. below the temperature range of the steel sheet, water pressure P in the atmosphere in the furnace H2O and hydrogen partial pressure P H2 consisting Metropolitan relation log (P H2O / P H2 of furnace body average value of) -1.00 ≦ log (P H2O / P H2 be in the range of) ≦ -0.67 It is preferred. [0040] 　Here, 700 ° C. log (P in steel temperature range of super 800 ° C. or less H2 O / P H2 of defining a), only in the range of 750 ° C. from 700 ° C. is an internal oxide formation temperature range log (P H2 O / P H2 of furnace body average value of) -1.36 　In the method for manufacturing a steel sheet according to the present embodiment, when the temperature of the steel sheet of 800 ° C. greater (when heated and a time holding), log (P H2 O / P H2 of furnace body average value of), -3.01 　When the temperature of the steel sheet of 700 ° C. or less, log (P H2 O / P H2 furnace average of) the -3.01 greater, less than -0.07, i.e., -3.01