The present disclosure relates to threaded joints used for connecting steel pipes. Background technology [0002] In oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), oil well pipes such as casings and tubing are used to mine underground resources. Oil country tubular goods are made up of steel pipes that are connected in sequence, and threaded joints are used to connect them. [0003] The types of threaded joints for steel pipes of this type are roughly divided into coupling type and integral type. In the case of the coupling type, of the pair of pipe materials to be connected, one pipe material is a steel pipe and the other pipe material is a coupling. In this case, male threaded portions are provided on the outer circumferences of both end portions of the steel pipe, and female threaded portions are provided on the inner circumferences of both end portions of the coupling. Then, the male threaded portion of the steel pipe is screwed into the female threaded portion of the coupling, whereby both are fastened and connected. In the case of the integral type, the pair of pipe materials to be connected are both steel pipes, and no separate coupling is used. In this case, a male screw portion is provided on the outer circumference of one end of the steel pipe, and a female screw portion is provided on the inner circumference of the other end. Then, the male threaded portion of one steel pipe is screwed into the female threaded portion of the other steel pipe, whereby both are fastened and connected. [0004] Generally, the joint part of the pipe end where the male threaded part is formed is called a pin because it contains an element inserted into the female threaded part. On the other hand, the joint portion of the pipe end portion on which the female threaded portion is formed is referred to as a box because it contains an element for receiving the male threaded portion. Both these pins and boxes are tubular because they are the ends of the tubing. [0005] In recent years, the development of deep wells, which are in harsh environments of high temperature and high pressure, has been increasing, and the threaded joints for oil country tubular goods used in these wells are required to further improve the sealing performance. Especially in deep wells, the tensile / compressive load and the external pressure are high, so it is necessary to improve the sealing performance against the external pressure under the tensile / compressive load. [0006] Threaded joints for oil country tubular goods exhibit high sealing performance at the sealing part. Generally, the diameter of the pin seal is larger than the diameter of the box seal. Therefore, in the fastened state, both the sealing portions are fitted to each other and are in close contact with each other to be in a tight-fitting state, and a sealing portion is formed by metal contact. The difference between the diameter of the pin seal and the diameter of the box seal is called the "seal interference". The larger the amount of seal interference, the higher the seal contact force, and good sealing performance can be obtained. [0007] In order to improve the sealing performance against external pressure, it is effective to increase the wall thickness of the seal part of the pin on which the external pressure acts. As a result, the diameter reduction resistance of the seal portion of the pin when an external pressure is applied to the threaded joint is increased, so that the substantial decrease in the amount of seal interference is small and the decrease in the seal contact force is small. In addition, in order to exhibit stable sealing performance even under high tensile / compressive load, a certain length of the threaded portion and the area of ​​the shoulder surface are also required. [0008] International Publication No. 2015/194160 (Patent Document 1) discloses a threaded joint for steel pipes that improves sealing performance against internal pressure and external pressure (see paragraph 0020). In this threaded joint, the shoulder surface, the first seal surface, the first male / female thread portion, the auxiliary shoulder surface, the annular portion, the second seal surface, and the second seal surface are arranged in order from the tip of the pin (the end of the oil country tubular goods). It has a male / female thread (see paragraph 0063 and FIG. 6). [0009] Hereinafter, the first male threaded portion is referred to as an "inner male thread", the first female threaded portion is referred to as an "inner female thread", the second male threaded portion is referred to as an "outer male thread", and the second female threaded portion is referred to as an "outer female thread". Collectively, "internal thread", external male thread and external female thread are collectively referred to as "external thread", and a threaded joint provided with internal thread and external thread is referred to as "two-step threaded joint". [0010] Japanese Unexamined Patent Publication No. 10-89554 (Patent Document 2) discloses a slim type threaded joint for oil country tubular goods in which only the torque shoulder portion has high strength and is excellent in compression resistance and corrosion resistance (see paragraph 0016). This threaded joint manages the threaded part for screwing and fastening the pin part and the box part, the sealing part for ensuring sealing performance against the internal pressure and / or external pressure of the oil well pipe, and the tightening torque. It is an integral type threaded joint for an oil well pipe provided with a torque shoulder portion of the above, and the strength of the torque shoulder portion is higher than that of other parts of the joint and the main body of the oil well pipe (see paragraph 0018 and FIG. 1). This is also a "two-step threaded joint". [0011] International Publication No. 2017/104282 (Patent Document 3) discloses a threaded joint for steel pipes that can ensure excellent sealing performance and suppress the occurrence of crevice corrosion (see paragraph 0017). This threaded joint defines the amount of interference δ shld between the intermediate shoulders so that the inner and intermediate shoulders come into contact with each other at the same time (see paragraphs 0050 to 0057 and FIG. 3). This is also a "two-step threaded joint". Prior art literature Patent documents [0012] Patent Document 1: International Publication No. 2015/194160 Patent Document 2: Japanese Unexamined Patent Publication No. 10-89554 Patent Document 3: International Publication No. 2017/104282 Patent Document 4: PCT / JP2018 / 039964 Outline of the invention Problems to be solved by the invention [0013] The present inventors have newly found that seizure is likely to occur on one of the internal thread and the external thread when starting to fasten a two-step threaded joint. [0014] An object of the present disclosure is to provide a threaded joint for steel pipes in which seizure is less likely to occur on both internal and external threads. Means to solve problems [0015] The present inventors have newly obtained the following findings as a result of analyzing the cause of seizure on one of the internal thread and the external thread when starting to fasten a two-step threaded joint. When fastening a two-stage threaded joint, there are cases where the external thread comes into contact after the internal thread comes into contact, and there are cases where the internal thread comes into contact after the external thread comes into contact. At the start of fastening, since the screws are not completely engaged, the fastening proceeds in a so-called misalignment state in which the axes of the pins and the box do not completely match. Therefore, when the internal screw comes into contact first, an excessive load is applied to the internal screw, and when the external screw comes into contact first, an excessive load is applied to the external screw. With internal or external threads that are subject to excessive load, the surface pressure is locally high, so seizure is likely to occur. [0016] The threaded joint for steel pipes according to the present disclosure includes a tubular pin and a tubular box in which the pin is screwed and fastened to the pin. The pin includes a tapered inner male thread, a tapered outer male thread, and a pin intermediate shoulder surface. The internal male screw is located on the tip end side of the pin and is formed on the outer peripheral surface of the pin. The external male screw is located on the side opposite to the tip of the pin and is formed on the outer peripheral surface of the pin. The pin intermediate shoulder surface is formed between the inner and outer male threads. The box includes a tapered inner female thread, a tapered outer female thread, and a box intermediate shoulder surface. The internal female thread corresponds to the internal male thread and is formed on the inner peripheral surface of the box. The outer female thread corresponds to the outer male thread and is formed on the inner peripheral surface of the box. The box intermediate shoulder surface faces the pin intermediate shoulder surface and comes into contact with the pin intermediate shoulder surface when the pin and the box are fastened. [0017] Each of the inner male thread, the outer male thread, the inner female thread, and the outer female thread includes a complete threaded portion. Each of the fully threaded portions includes an inner end located on the tip side of the pin and an outer end located on the opposite side of the pin tip. The pins and boxes fastened to each other have a longitudinal section that includes the shaft of the steel pipe. The Cartesian coordinate system introduced on the vertical section is the x-axis set for the axis of the steel pipe, the y-axis set for the radius of the steel pipe, the plane including the pin intermediate shoulder plane and the box intermediate shoulder plane, and the axis of the steel pipe. It has an origin (0,0) set at the intersection with. [0018] The above-mentioned threaded joint for steel pipe satisfies the following formulas (1) and (2). X 1 + L 1 Repeated fastening / disassembling tests specified in ISO 13679 standard were conducted for multiple specimens with different dimensions, and the presence or absence of seizure was confirmed. Each specimen is a coupling type threaded joint having the basic structure shown in FIG. The common test conditions are shown below. [0090] (1) Dimensions of steel pipe 7-5 / 8 [inch] x 1.06 [inch] (outer diameter 193.7 [mm], wall thickness 27.0 [mm]) 10-1 / 8 [inch] x 0.8 [inch] (outer diameter 257.2 [mm], wall thickness 20.3 [mm]) (2) Steel pipe grade API standard P110 (carbon steel with a nominal yield stress of 110 [ksi]) [0091] Table 2 summarizes the test conditions and evaluations of each specimen. [Table 2] [0092] In the specimen 1, only the external screw is in contact and the internal screw is not in contact at the start of fastening, so that the contact becomes unstable. Therefore, as the fastening progresses, an excessive load is applied to the external screw, and seizure occurs in the external screw in the first fastening / disassembling test. [0093] In the specimens 2 to 4, since the internal screw and the external screw start contacting at almost the same time, the contact is stable from the start of fastening to the completion of fastening. Therefore, a uniform load was applied to both the internal screw and the external screw, and seizure did not occur, and the fastening / disassembling test was successful three times. Code description [0094] 10: Pin 12: Nose 13: Seal surface inside the pin 14: Inner male screw 16: Pin intermediate seal surface 17: External male screw 18: Pin middle shoulder surface 20: Box 22: Recess 23: Seal surface inside the box 24: Internal female screw 26: Box intermediate seal surface 27: External female screw 28: Box middle shoulder surface 141: Completely threaded part in the pin 171: Completely threaded part outside the pin 241: Completely threaded part inside the box 271: Completely threaded part outside the box 141i, 171i, 241i, 271i: Inner end 141o, 171o, 241o, 271o: outer end The scope of the claims [Claim 1] It is a threaded joint for steel pipes With a tubular pin, Equipped with a tubular box into which the pin is screwed in and fastened to the pin, The pin is The tapered inner male screw located on the tip side of the pin and formed on the outer peripheral surface of the pin, A tapered external male screw located on the opposite side of the tip of the pin and formed on the outer peripheral surface of the pin, Includes a pin intermediate shoulder surface formed between the inner male screw and the outer male screw. The box is Corresponding to the inner male screw, the tapered inner female screw formed on the inner peripheral surface of the box, Corresponding to the external male screw, the tapered external female screw formed on the inner peripheral surface of the box, Includes a box intermediate shoulder surface that faces the pin intermediate shoulder surface and is in contact with the pin intermediate shoulder surface when the pin and the box are fastened. Each of the internal male screw, the external male screw, the internal female screw, and the external female screw includes a complete thread portion, and each of the complete thread portions has an inner end located on the tip end side of the pin and the tip of the pin. Including the outer edge located on the opposite side of The pin and the box fastened to each other have a vertical cross section including the axis of the steel pipe, and the Cartesian coordinate system introduced on the vertical cross section includes the x-axis set for the axis of the steel pipe and the steel pipe. The y-axis set to the radius of It has an origin (0,0) set at the intersection of the plane including the intermediate shoulder surface and the box intermediate shoulder surface and the axis of the steel pipe. A threaded joint for steel pipes that satisfies the following formulas (1) and (2). X 1 + L 1