DESCRIPTION Title of Invention: Method of Production of Brazed Joint and Such a Brazed Joint Technical Field [0001] The present invention relates to a method of production of a brazed joint and to such a brazed joint, in particular is suitable for use for brazing together the sheet surfaces of a plurality of ~t~~l sheets. Background Art [0002] In the past, the practice had been to produce a hot stamped part by a hot stamping process. At that time, a blank comprised of a plurality of steel sheets joined . ,,. '">··· :· .. o:· ~~·'-''··:· ~(' . at the surfaces- is supplied to· t"he hot stamping pro'c::ess. Due to this, the strength and rigidity of the hot stamped part are raised. In this case, one or more steel sheets are joined at a region of the main steel sheet requiring larger strength and rigidity than other regions. As 20 applications for such hot stamped parts using such blanks, for example, there are component parts of monocoque bodies (unit construction bodies) such as front side members or center pillars etc. [0003] To join such a main steel sheet and a 25 reinforcing steel sheet, there is the technique of using brazing. [0004] PLT 1 discloses the art of placing a brazing filler metal (hard solder, same below) between the surfaces of the main steel sheet and reinforcing steel 30 sheet and welding them to provisionally fasten the steel sheets and brazing filler metal, then heating the steel sheets to a temperature higher than the shaping temperature of the steel sheets and hot press-forming them and then cooling the steel sheets in the die after 35 shaping so as to cause the brazing filler metal to solidify. [0005] PLT 2 discloses the art of clamping a brazing - 2 - filler metal with a solidus temperature of 1050°C or less and a liquidus temperature of 700°C or more between the surfaces of a main steel sheet and reinforcing steel sheet, placing the blank in a heating furnace to heat it 5 to 900°C to 1050°C, then quenching and press-forming (hot stamping) it. [0006] PLT 3 discloses the art of coating a sheet surface with a brazing filler metal, heating the coated main steel sheet for the quenching and making the brazing 10 filler metal melt, joining the surface of the main.steel sheet and the surface of the reinforcing steel sheet through this melted brazing filler metal, then quenching and press-forming (hot stamping) the same. Citation List 15 Patent Literature [0007] PLT 1: Japanese Patent Publication No. 2002- 178069A PLT 2: Japanese Patent Publication No. 2004-141913A PLT 3: Japanese Patent Publication No. 2011-88484A 20 PLT 4 : Japanese Patent Publication No. 2014-200840A PLT 5: Japanese Patent Publication No. 2015-166099A Nonpatent Literature [0008] NPLT 1: Takedachi, A. ''Prediction of Ac1, Ac3, and Ms Transformation Points of Steel by Empirical 25 Formula", Heat Treatment, 41 (3), p. 164 to p. 169 NPLT 2: Choquet, P. et al., Mathematical Model for Predictions of Austenite and Ferrite Microstructures in Hot Rolling Processes, IRSID Report, St.Germain-en-Laye, 1985.7p. 30 Summary of Invention Technical Problem [0009] However, in the arts described in PLTs 1 to 3, locations arise where the brazing filler metal is not filled between the steel sheets. Clearances are liable to 35 remain between the steel sheets. Therefore, in the arts described in PLTs 1 to 3, it is not easy to sufficiently - 3 - improve the strength and rigidity of hot stamped parts. [0010) Therefore, the inventors proposed in PLT 4 the technique of applying pressure in the thickness direction to a plurality of metal sheets between which a brazing 5 filler metal is clamped at the time of the heating step for hot stamping or in the period from the heating step until transfer to the hot stamping step. According to this technique, it is possible to prevent clearance from remaining between the steel sheets as in the arts 10 described in PLTs 1 to 3. [0011] Further, the inventors proposed in PLT 5 using a brazing filler metal having a solidus temperature above the heating temperature and below the solidus temperature of the metal sheets in the hot stamping step so as to 15 produce a hot, stamped part having a strength and rigidity substantially equal to a hot stamped part described in the above-mentioned PLT 4 without making great changes to the hot stamping process. [0012) On the other hand, for example, in auto parts 20 etc. where both lighter weight and higher strength are being sought, further higher strength is being demanded for application of hot stamped parts (brazed joints} obtained by brazing together a plurality of steel sheets and hot stamping them. 25 [0013) However, the specific tensile shear strength (TSS} and cross tension strength (CTS} of the brazed joint described in PLTs 1 to 3 are not described, so the strength of the brazed joint was not sufficiently raised. Control (management} of the temperature of the steel 30 sheets in the hot stamping step has not been studied from the viewpoint of raising the joint strength of a brazed joint. [0014) Therefore, the present invention has as its object the provision of a brazed joint having excellent 35 tensile strengths (TSS and CTS} and a method of production of the same. Solution to Problem - 4 - [0015] To solve the above problem, the inventors took note of and studied the control (management) of the temperature of steel sheets in the hot stamping step. As a result, they discovered that the Ar3 point in a region 5 near the brazing filler metal at the steel sheets becomes higher than the inherent Ar3 point of the steel sheets. Further, they discovered that by controlling the quenching start temperature in the hot stamping step to the inherent Ar3 point of the steel sheets or less, it is 10 possible to make the degree of quenching at. th~·region near the brazing filler metal of the steel she~ts and the degree of quenching at other regions different to form a softened region in the region near the brazing filler metal of the steel sheets and thereby raise the tensile ·15 strength of the· brazed joint and thereby completed·· the present invention. [0016] The aspects of the present invention are as follows: (1) A method of production of a brazed joint comprising a 20 heating step of heating a sheet combination having a plurality of steel sheets superposed so that the sheet surfaces face each other and a first brazing filler metal placed in a predetermined region between the surfaces of two mutually adjoining steel sheets among the plurality 25 of steel sheets by a heating temperature of an Ac3 point of the steel sheets or more and a hot stamping step of quenching and shaping the sheet combination heated by the heating step, wherein the first brazing filler metal has a liquidus temperature of less than the heating 30 temperature, before the start of quenching in the hot stamping step, the Ar3 point in a region near the brazing filler metal at the steel sheets is higher than the Ar3 point of the steel sheets, and the quenching start temperature of the sheet combination at the hot stamping 35 step is made a temperature of the Ar3 point of the steel sheet or less. (2) The method of production of a brazed joint according - 5 - to (1), further comprising a step of obtaining a sheet combination where instead of the first brazing filler metal, a second brazing filler metal having a solidus temperature over the heating temperature in the heating 5 step and less than the solidus temperature of the steel sheets is placed in a predetermined region between two steel sheets and a brazing step of heating the sheet combination to a temperature over a liquidus temperature of the second brazing filler metal and less than the 10 solidus temperature of the steel sheets before ·the· heating step, then cooling down to less than the solidus temperature of the second brazing filler metal to braze the steel sheets. (3) The method of production of a brazed joint according 15 to (1) or· (2), wherein the heating step heats· the ·sheet combination while applying pressure to the sheet combination in the thickness direction of the steel sheets. (4) The method of production of a brazed joint according 20 to (2), wherein the brazing step heats the sheet combination while applying pressure to the sheet combination in the thickness direction of the steel sheets. (5) The method of production of a brazed joint according 25 to any one of (1) to (4), wherein the plurality of steel sheets are comprised of one or both of nonplated steel sheets not plated on their surfaces and galvanized steel sheets or aluminum plated steel sheets. [0017] (6) A brazed joint comprising a plurality of 30 steel sheets overlaid so that their surfaces face each other and a brazing filler metal placed in a predetermined region between the surfaces of two mutually adjoining steel sheets in the plurality of steel sheets, the two steel sheets being joined by the brazing filler 35 metal, in which brazed joint, a relationship between an average hardness of positions near the brazing filler metal in the steel sheets contacting the brazing filler - 6 - metal and an average hardness of representative positions of a base metal region in the steel sheets satisfies the following (A) : Average hardness of positions near brazing filler metal 5 (HV)+50HV region filler (~m) (HV) metal (HV) metal .. (HV) {HV) 1-1 A 454 450 440 335 40 2-2 A 454 450 435 330 35 3 3 502 None A 495 490 370 40 ----4-4 A ·----·-5-0-2- ------ -~9~ ---- 494 350 ---42- - [0122] Table 3 850°C quenching 700°C quenching (comparative examples) {invention examples) r- Average Average Average hardness Average hardness hardness of of Thickness hardness of of Thickness Cb~~inaticin B_razing J:t!p_:de~;ent,\ ti Vi:' positions of L·epresenta ti ve positions o:::: filler positions of near softened positions of near softened of sheets metal base metal brazing region base metal brazing region region filler (~) region filler I~) (HV) metal {HV) metal (HV) (HV) 5 5 A 550 545 530 380 40 6 6 550 None A 543 535 400 36 [0123] In Table 2 and Table 3, the "Combination of sheets" corresponds to the numbers shown in Table 1 ("No.") "Combinations of sheets" of "1-1", "2-2", "3-3", ''4-4", ''5-5", and "6-6'' respectively show use of two of the No. 1, No. 2, No. 3, No. 4, No. 5, and No. 6 steel 10 sheets shown in Table 1. The same is true in the later explained Table 4 and Table 5 as well. [0124] Further, in Table 2 and Table 3, ''A'' in the column of "brazing filler metal" shows use of Cu-Sn20% braze (solidus temperature 770°C, liquidus temperature 15 930°C) as the brazing filler metal. The same is true in the later explained Table 4 and Table 5 as well. [0125] Further, in Table 2, "850°C quenching" shows that the quenching start temperature X in the hot stamping step was made 850°C. "700°C quenching" shows that 20 the quenching start temperature X in the hot stamping step was made 700°C. As explained above, lhe Ar3 points of - 35 - the No. 1 steel sheet and No. 2 steel sheei are 727"C, while the Ar3 points of the No. 3 steel sheet and No. 4 steel sheet are 7ll"C. Therefore, ''850"C quenching'' are examples not satisfying the condition of the above- 5 mentioned formula (2) (are comparative examples). On the other hand, "700"C quenching" are examples satisfyin::; the condition of the above-mentioned formula (2) (are invention examples). [0126] Similarly, in Table 3, ''850"C quenching" shows . - -· ·- -- ·-- 10 that the quenching start temperature X in the hot stamping step was made 850"C. "650"C quenching" shows that the quenching start temperature X in the hot stamping step was made 650"C. As explained above, the Ar3 point of the No. 5 -steel '3ll'E!cc-t is 680"C, wnile-the 'Ar3 p1oint''Of the 15 No. 6 steel sheet is 693"C. Therefore, "850"C quenching" are examples not satisfying the condition of the abovementioned formula (2) (are comparative examples). On the other hand, "650"C quenching" are examples satisfying the condition of the above-mentioned formula (2) (are 20 invention examples). [0127] In addition, in Table 2 and Table 3, "Average hardness of representative positions of base metal region", "Average hardness of positions near brazing filler metal", and "Thickness of softened region'' were 25 all explained in the present embodiment. The same is true in the later explained Table 6 and Table 7 as well. [0128] However, here, the region satisfying the abovementioned formula (9) is designated as the softened region. That is, a region with a value of the Vicker's 30 hardness smaller than the value of the average hardness (HV) of representative positions of the base metal region minus 50HV was specified as the ''softened region''. [0129] As shown in Table 2 and Table 3, it will be understood that if the condition of formula (2) is not 35 satisfied, the average hardness of positions near the brazing filler metal will not fall much at all from the - 36 - average hardness of the representative positions of the base metal region. Further, the softened region is not formed. On the other hand, if the condition of formula (2) is satisfied, the average hardness of positions near 5 the brazing filler metal falls 50HV or more from the average hardness of representative positions of the base metal region. Further, it will be understood that a. softened region having a thickness of 30 ~m or more is formed. 10 [0130] Evaluation of Hot Stamped Part FIG. 5 is a view showing the shape of a test piece of a tensile shear test. Between the surfaces of two steel sheets of the same number ("No.") in Table 1 (for example, No. 1 steel sheets), thickness 30 ~brazing 15 filler metal was placed to produce a sheet combin.ad_on (test piece of tensile shear test). [0131] As shown in the top figure of FIG. 5, two steel sheets of widths of 25 mm and lengths of 90 mm were set so that the entireties of the regions in the width 20 direction were overlaid and regions of 25 mm were overlaid from the front end along the long direction. A brazing filler metal was placed at the entire overlapping region (25 mmx25 mm region). [0132] FIG. 6 is a view showing the shape of a test 25 piece of a cross tensile test. Between the surfaces of two steel sheets of the same number ("No.") in Table 1 (for example, No. 1 steel sheets), thickness 30 ~m brazing filler metal was placed to produce a sheet combination (test piece of cross tensile test). 30 [0133] As shown in the top figure of FIG. 6, two steel sheets of widths 50 mm and lengths 150 mm were placed so that 50 mmx50 mm square regions at their centers overlapped. A brazing filler metal was placed at the entire overlapping region (50 mmx50 mm region) . 35 [0134] The thus produced sheet assemblies (test pieces for tensile shear tests and test pieces for cross tensile 5 - 37 - tests) were subjected to heating steps and hot stamping steps under the same conditions as the above-mentioned brazed joint (see section on ''Evaluation of Brazed Joint") [0135] A brazed joint obtained by the hot stamping step (test piece for tensile strength test) was subjected to a tensile shear test by a technique based on JIS Z3136 to measure the tensile shear strength (TSS). Further, the direction of tension at the time of the tensile shear 10 ~test: was the di.cection of the white arrows shown in the bottom figure of FIG. 5. [0136] Further, a brazed joint obtained by the same hot stamping step ·(test piece for· cross tensile test) was subje~ted to a cross tensile test by a technique based on 15 JIS Z3i37 ·to m~asure the cross tensile strength (CTS). Further, the direction of tension at the time of the cross tensile test was the direction of the white arrows shown in the bottom figure of FIG. 6. [0137] In a hot stamped part such as the hat shaped 20 member 110 shown in FIG. 1, the form of the imparted stress differs depending on the location. The form of the stress can be approximated by the combination of stresses when force acts in the directions of the white arrows shown in the bottom figures of FIG. 5 and FIG. 6. 25 Therefore, when satisfying the above-mentioned formula (2), compared with when not, if both the tensile shear strength (TSS) and cross tensile strength (CTS) are improved, it can be said that the joint strength is improved no matter how the brazed joint is shaped. 30 Therefore, here, a hot stamped part is evaluated by evaluating the tensile shear strength (TSS) and the cross tensile strength (CTS) . The results are shown in Table 4 to Table 7. - 38 - [0138] Table 4 TSS (kN) I CTS (kN) Combination Brazing Quenching start temp_erature (oC) of sheets filler metal 850 700 850 700 (comparative (invention (comparative (invention examples) examples) examples) examples) 1-1 A 31.8 40.5 3.3 4.7 2-2 A 34.55 46.5 3.7 5.22 3-3 A 33.6 42.3 3.5 4 4-4 A 35.7 48.2 3 3.5 1-1 8 36.1 40.5 4. 4 6.7 2-2 B 40.3 45.1 4. 2 6.2 3-3 8 38.5 42.6 4.6 7 4-'l B 43.7 4'1. 5 3 .·6 '' ,. ·.;· 5.5 [0139] Table 5 TSS (kN) .CTS (kN) Combination Brazing Quenching start temperature (OC) of sheets filler metal 850 650 850 650 (comparative .( inyent ion ~-comp.tt:r;:at.igs; .(invent ion examples) examples) examples) examples) 5-5 A 32.6 38.3 3.2 4.5 6-6 A 30.5 36.6 2.9 4.4 5-5 8 33.2 49.6 3.1 4 6-6 B 34.5 47.5 2.7 3.3 5 [0140] Table 6 Average Average hardness Quenching hardness of of Thickness representative positions of start TSS positions of near softened temperature Remarks (kN) base metal brazing region (oC) region filler (Jlm) (HV) metal (HV) 850 34.55 4 60 460 0 Comparative example 770 31.9 455 454 0 Comparative example 735 33.3 450 440 0 Comparative example 718 39.05 440 388 23 Invention example 694 46.5 435 330 35 Invention example 670 54.1 430 300 40 Invention example 635 49.9 420 2 98 60 Invention example 620 35 370 270 100 Reference example 580 32 350 250 130 Reference example - 39 - [0141] Table 7 Average Average hardness Quenching hardness of of Thickness representative positions of start CTS positions softened temperature of near Remarks (kN) base metal brazing region ('C) region filler (f!m) (HV) metal (HV) ~50 3.7 460 460 0 Comparative example 765 3.8 455 454 0 Comparative example 740 3.7 445 440 0 Comparative example :-- 720 5 440 388 20 Invention example ·- 696 5.4 435 330 35 Invention example 671 6.5 . 430 300 40 Invention example 633 5.5 420 2 98 65 Invention example 623 3.9 370 270 100 Reference example 581 3.7 350 250 130 Reference example [01.42]· ·. In:·.--.Tab1c~ -1 and Table ~; "R" in the co_l-umn .. ;f ''brazing filler metal'' shows use of Cu-Zn30% braze 5 (solidus temperature 900°C, liquidus temperature 930°C) as the brazing filler metal. [0143] The "quenching (start) temperature" in Table 4 to Table 7 shows the quenching start temperature X at the hot stamping step. Table 4 shows the measurement results 10 for the tensile shear strength (TSS) and the cross tensile strength (CTS) for the cases where the "quenching (start) temperature" is 850°C and 700°C. As explained while referring to Table 2, the measurement results when the "quenching (start) temperature" is 850°C are examples 15 not satisfying the condition of the above-mentioned formula (2) (are comparative examples). On the other hand, the measurement results when the "quenching (start) temperature" is 700°C are examples satisfying the condition of the above-mentioned formula (2) (are 20 invention examples). Further, Table 5 shows the measurement results of the tensile shear strength (TSS) and cross tensile strength (CTS) for the cases where the "quenching (start) temperature" is 850°C and is 650°C. As explained while referring to Table 3, the measurement - 40 - results when the "quenching (start) temperature" is 850°C are examples not satisfying the condition of the abovementioned formula (2) (are comparative examples). On the other hand, the measurement results when the "quenching 5 (start) temperature" is 650°C are examples satisfying the condition of the above-mentioned formula (2) (are invention examples). [0144] As shown in Table 4 and Table 5, it will be understood that regardless of the combination of the 10 sheets and the type of the brazin-g fiiler ·metai, -compared with when the condition of the formula (2) is not satisfied, when it is satisfied, both the tensile shear strength (TSS) and cross tensile strength (CTS) can be 15 improved. ~;.,_),,:,; ;~-· !'.:~r,·:~ [0145] Further, Table 6 and Table 7 show the measurement results of the tensile shear strength (TSS) and cross tensile strength (CTS) for the cases where the "quenching temperature" (quenching start temperature X in hot stamping step) differs for the case of the 20 "combination of sheets" of "2-2". The measurement results of Tables 6 and 7 are graphed and shown in FIGS. 7 and 8. [0146] As shown in Table 6 and Table 7, it will be understood that if the "quenching temperature" becomes lower, both the tensile shear strength (TSS) and cross 25 tensile strength (CTS) tend to become larger. [0147] As explained above, the Ar3 point of the No. 2 steel sheet is 727°C. In Table 6 and Table 7, the measurement results when the "quenching temperature" exceeds this temperature become examples not satisfying 30 formula (2) (are comparative examples). On the other hand, the measurement results when the "quenching temperature" is this temperature or less become examples satisfying formula (2) (are invention examples) . [0148] As shown in Table 6 and Table 7, when changing 35 the range of the "quenching temperature", as explained with reference to Table 2 and Table 3, it will be - 41 - understood that if the condition of formula (2) is not satisfied, the average hardness of positions near the brazing filler metal will not fall much at all from the average hardness of representative positions of the base 5 metal region. This will be visually apparent from FIGS. 7 and B. Further, the softened region was not formed. On the other hand, it will be understood that if the condition of formula (2) is satisfied, the average hardness of positions near the brazing filler metal falls 10 50 HV or more from·the average hardness of representative positions of the base metal region. This will be visually apparent from FIGS. 7 and B. Further, it will be understood that a softened region having a thickness of 10 ~ or more is formed. 15 [0149] In particular, wl1en the quenching start temperature X is (Ar3 point -100) (°C) to the Ar3 point (°C), the base metal region is sufficiently quenched and the hardness rises. Further, the softened region formed can ease the stress concentration at the brazing filler 20 metal, so both the tensile shear strength (TSS) and cross tensile strength (CTS) become larger. This will be visually understood from FIGS. 7 and B. [0150] Further, in the present embodiment, the results of combinations of the same types of sheets are shown. 25 Other explanations of the combinations of sheets will be omitted. However, the inventors confirmed that a similar trend is obtained as explained in the present embodiment for the combination of different types of sheets. [0151] Summary 30 In the above way, in the present embodiment, the sheet combination 200 comprised of the steel sheets 210, 220 between which a brazing filler metal 231 rs sandwiched is heated by a temperature of the Ac3 point of the steel sheets (matrix) or more, then is hot stamped using as the 35 quenching start temperature X the temperature of the Ar3 point of the steel sheet (matrix) to thereby produce the - 42 - brazed joint. By doing this, it is possible to make the Vicker's hardness at positions of the steel sheets 210, 220 near the brazing filler metal 230 (average hardness of positions near brazing filler metal) smaller than the 5 value of the inherent Vicker's hardness of the steel sheets 210, 200 (average hardness of representative positions of base metal region) minus 50 HV. That is, it is possible to lower the hardness of regions near the brazing filler metal 231 of the steel sheets 210, 220 10 (softened regions.) l Therefore, it .is possible to· .control the temperature of the steel sheets at the hot stamping step so as to raise the joint strength of the brazed joint. Accordingly, it is possible to produce a high strength brazed joint without greatly changing the 15 · conventional process of production'· of· hot stamped parl:s. [0152] Modification In the present embodiment, in the heating step, the sheet combination 200 being heated was pressed in the thickness direction of the steel sheets 210, 220. However, as 20 described in the specification of PLT 4, this pressing operation may be performed after the heating step and before the hot stamping step. Further, if performing the above such pressing operation, it is possible to reliably keep a clearance from forming between the steel sheets 25 210, 220 and the brazing filler metal 231, so this is preferable, but this pressing operation does not necessarily have to be performed. [0153] Further, in the present embodiment, the explanation was given with reference to the example of 30 the case of brazing in the heating step like in the art described in the specification of PLT 4. However, as shown in the art described in the specification of PLT 5, it is also possible to perform the brazing before the heating step. 35 [0154] That is, instead of the first brazing filler metal 231 having a liquidus temperature lower than the heating temperature at the heating step, a second brazing - 43 - filler metal 232 having a solidus temperature higher than the heating temperature in the hot stamping heating step ("heating step" in the present embodiment) and lower than the solidus temperature of the steel sheets 210, 220 is 5 used to braze the steel sheets 210, 220 (brazing step). The second brazing filler metal 232 is an example of the brazing filler metal 230. It differs from the first brazing filler metal 231 in the liquidus temperature and solidus temperature, but may be the same or different in 10 oth,~r propert'ies, shape, etc. Further, regarding, i:he placement of the brazing filler metal 232 in the brazing step or the provision of the softened region, in FIGS. 2 to 4, read the description ''brazing filler metal 230" as "brazing filler metal 232''. 15 {\Ji!:>Sj ·'·in t:he bl"azing step,· the· sheet· combination 200 set in the fixture is heated so that the temperature of the brazing filler metal 232 becomes a temperature exceeding the liquidus temperature of the brazing filler metal 232 and lower than the solidus temperature of the 20 steel sheets 210, 220. Further, at this time, in the same way as explained while referring to FIG. 3, the sheet combination 200 is preferably heated while applying pressure. The sheet combination 200 heated in this way is cooled until the temperature of the brazing filler metal 25 232 falls below the solidus temperature of the brazing filler metal 232. [0156] After that, the heating step, hot stamping step, and shot blasting step explained in the present embodiment are performed in that order. Even if heating 30 the brazed sheet combination 200 in the heating step, since the solidus temperature of the brazing filler metal 232 is higher than the heating temperature, the brazing filler metal 232 does not become a liquid phase and the solid phase state can be maintained. Further, in the same 35 way as when using the first brazing filler metal 231, before the start of quenching at the hot stamping step, the Ar3 point of the regions near the brazing filler - 44 - metal in the steel sheets becomes higher than the Ar3 point of the steel sheets. [0157] Therefore, it is possible to form the abovementioned softened region at the steel sheets and not 5 only raise the tensile strength of the brazed joint, but also reliably prevent liquid metal embrittlement cracking at the time of hot stamping. Further, the brazing is performed as a separate step from the heating step before the hot stamping, so management of the brazing becomes J.O ·easy and the. brazing conditions cari be ·kept·. from becoming restricted. Further, even if the atmosphere of the heating step before the hot stamping is a nonreducing atmosphere, it is possible to prevent oxidation of the brazing filler metal from making brazing impossible. Due 15 to the above, it· is possible to r'aT's'"e" 'the joint· 'Strength of the brazed joint without making major changes in the conventional hot stamping process. The above effect obtained by using such a second brazing filler metal was confirmed by experiments by the present inventors. 20 Further, when performing this, the brazing is completed when starting the heating step, so in the heating step, it is also possible not to apply the above-mentioned pressure. [0158] Further, in the present embodiment, the 25 explanation was given with reference to the example of a case of two steel sheets joined planarly. However, the number of steel sheets joined planarly may be three or more as well. In this case, brazing filler metal is placed between each two facing steel sheets. 30 [0159] Further, the measurement positions of temperature explained in the present embodiment, including the examples and modifications as well, were all positions of the center of the brazing filler metal in the thickness direction of the steel sheets. 35 [0160] Further, the present embodiments, including the examples and modifications, all only show examples of specific means for working the present invention. The - 45 - technical scope of the present invention must not be !imitatively interpreted based on the same. That is, the present invention can be worked in various ways without departing from its technical idea or main features. CLAIMS Claim 1. A method of production of a brazed joint comprising a heating step of heating a sheet combination 5 having a plurality of steel sheets superposed so that the sheet surfaces face each other and a first brazing filler metal placed in a predetermined region between the surfaces of two mutually adjoining steel sheets among the plurality of steel sheets by a heating temperature of an 10 Ac3 point of the steel sheets or more and ·15 a hot stamping step of quenching and shaping the sheet combination heated by the heating step, wherein the first brazing filler metal has a liquidus temperature of less than the heating temperature, before the start of quenching in the hot stamping step, the Ar3 point in the region near the brazing filler metal at the steel sheets is higher than the Ar3 point of the steel sheets, and the quenching start temperature of the sheet 20 combination at the hot stamping step is made a temperature of the Ar3 point of the steel sheet or less. Claim 2. The method of production of a brazed joint according to claim 1, further comprising: a step of obtaining a sheet combination where 25 instead of the first brazing filler metal, a second brazing filler metal having a solidus temperature over the heating temperature in the heating step and less than the solidus temperature of the steel sheets is placed in the predetermined region between two steel sheets and 30 a brazing step of heating the sheet combination to a temperature over a liquidus temperature of the second brazing filler metal and less than the solidus temperature of the steel sheets before the heating step, then cooling down to less than the solidus temperature of 35 the second brazing filler metal to braze the steel sheets. Claim 3. The method of production of a brazed joint - 47 - according to claim 1 or 2, wherein the heating step heats the sheet combination while applying pressure to the sheet combination in the thickness direction of the steel sheets. 5 Claim 4. The method of production of a brazed joint according to claim 2, wherein the brazing step heats the sheet combination while applying pressure to the sheet combination in the thickness direction of the steel sheets. 10 Claim 5. The method of production of a brcfzced· joint 15 according to any one of claims 1 to 4, wherein the plurality of steel sheets are comprised of one or both of nonplated steel .sheets not plated on their surfaceB and galvanized steel sheets or aluminum plated steel sheets. Claim 6. i\ brazed joint comprising a plurality of steel sheets overlaid so that their surfaces face each other and a brazing filler metal placed in a predetermined region between the surfaces of two mutually 20 adjoining steel sheets in the plurality of steel sheets, the two steel sheets being joined by the brazing filler metal, in which brazed joint, a relationship between an average hardness of positions near the brazing filler 25 metal in the steel sheets contacting the brazing filler metal and an average hardness of representative positions of a base metal region in the steel sheets satisfies the following (A) : Average hardness of positions near brazing 30 filler metal (HV)+50HV