Tank Car Traction Bolster Device [0001] The application claims the priority of a Chinese patent application entitled "tank car traction bolster device", filed with the Chinese Patent Office on December 01, 2010, with an application No. 201010568841.4, all the contents of which are 5 hereby incorporated by reference in its entirety. FIELD OF THE INVENTION [00021 The present invention relates to technical field of a railway tank car, in particular to a tank car traction bolster device. BACKGROUND TECHNOLOGY 10 [0003] At present, tens of thousands of existing railway tank car traction bolster structures in China adopt the structures of bolster-beam lower middle cover plate. Referring to FIGs.1 and 2, the structure of the bolster-beam lower middle cover plate is located between a traction beam and a top center plate, and the traction beam is formed by welding two pieces of Z-shaped steel; a bending area in the structure is lapped and welded 15 on the bottom surface of each bolster-beam underside cover plate. [0004] Owing to the excitation from track irregularity, the tank car bears complex dynamic load, and the structure of the tank car traction bolster device bears complex dynamic stress. As the structure of the bolster-beam lower middle cover plate is located between the traction beam and the top center plate, the traction beam is formed by 20 welding the two pieces of Z-shaped steel, wherein the floating-sinking working condition of a tank car body is taken for example, the vertical vibration of a bogie is transferred to the top center plate via a lower center plate and then to the traction beam, an integrated rear draft lug and center plate seat and the bolster-beam underside cover plates via the bolster-beam lower middle cover plate, so the bolster-beam lower middle cover plate is 25 located on a direct transfer path of the dynamic load and can bear alternating dynamic stress in harsh stress conditions. [0005] As the bolster-beam lower middle cover plate has a bending area on the structure, negative impacts are caused on both the traction bolster assembly and the stress of the traction bolster in the actual manufacturing process. The impacts are not recognized for a long time, and the original design is always used in the structure. [0006] The first impact is that the presence of the bending area has an effect on the symmetry degree after the assembly of the bolster-beam lower middle cover plate and the traction beam are paired, and the reason is mainly that bending is not a precise molding 5 process, and a big error in symmetry at two sides of the same bolster-beam lower middle cover plate exists; meanwhile, the traction beam used for assembly with the bolster-beam lower middle cover plate is formed by welding two pieces of Z-shaped steel, and the error in width of the lower wing surfaces of the Z-shaped steel can also affect the symmetry degree after assembly. With the two main factors, the bolster-beam lower middle cover 10 plate assembled onto the lower wing surfaces of the Z-shaped steel is asymmetric in relation to the traction beam (the bigger symmetric tolerance is about 20mm), so a local gap is caused between the lower wing surface of the Z-shaped steel at one side of the traction beam and the bolster-beam lower middle cover plate, as shown in FIG.3. [00071 According to the inventors of the present invention, the results of the detailed 15 computational study on structure stress finite elements and the strength test study show that the local gap has an obvious effect on the stress of the structure of the bolster-beam lower middle cover plate: the structure of the bolster-beam lower middle cover plate is affected in transferring the dynamic load from the top center plate to the lower wing surfaces of the Z-shaped steel with larger stiffness as well as the cover plate at the lower 20 side of the bolster beam; and the structure of the bolster-beam lower middle cover plate bears the dynamic load that can not be transferred due to the gap and bears the alternating dynamic stress in bending area. The research results show that when the bolster-beam lower middle cover plate and the bolster-beam underside cover plates are in good contact state, the stress level of the bolster-beam lower middle cover plate structure 25 is about 60Mpa, which occurs near the bending area; while the local gap is bigger, the stress level of the bolster-beam lower middle cover plate structure is about 167Mpa, which occurs near the bending area, but obviously bigger than the former one. [0008] The second adverse impact is that the bending area of the bolster-beam lower middle cover plate is formed by a bending process on a numerical control bending 2 machine; and the bending process is a plastic molding process which leads to residual stress in the bending area. Due to upward bending, as for the bolster-beam lower middle cover plate, the lower surface bears the residual tensile stress. The residual stress test results show that the value of the residual tensile stress is close to 10OMpa. As for the 5 bolster-beam lower middle cover plate bearing alternating stress, bigger residual stress in bending area is equivalent to the increase of average stress value born by the bending area, thus reducing the carrying capacity of the structure and being bad for the fatigue strength of the structure. [0009] The third adverse impact is that due to various factors such as manufacturing 10 error in the bending process and the rebounding of the plate after bending, the accuracy of the bending radius of the bolster-beam lower middle cover plate cannot be guaranteed, leading to the occurrence of the bigger gap when the bolster-beam lower middle cover plate and the bolster-beam underside cover plate are welded. In order to reduce the gap during assembly welding, the bolster-beam lower middle cover plate has to be bent 15 upwards by using toolkits. Referring to FIG.4, the stress of the bolster-beam lower middle cover plate near the bending area can be further increased. [0010] Meanwhile, the bolster-beam lower middle cover plate of the existing railway tank car is 10mm in thickness and is made of Q345A materials, so the bolster-beam lower middle cover plate lacks of sufficient strength reservation. 20 [0011] Due to all the adverse impacts mentioned above, the risk of structural failure of the tank car traction bolster device is increased, leading to longitudinal fatigue cracks along the car body near the bending area of the bolster-beam lower middle cover plate when the railway tank car is in service. SUMMARY 25 [0012] The present invention provides a tank car traction bolster device in order to solve the problem of fatigue cracks in the structure of the bolster-beam lower middle cover plate in the prior art. [0013] The technical scheme of the present invention for the technical problems is that the tank car traction bolster device, comprising a traction beam as well as bolster-beam 3 underside cover plates and a bolster-beam lower middle cover plate that are arranged under the traction beam; the bolster-beam underside cover plates are installed at two sides of the bolster-beam lower middle cover plate; the bolster-beam lower middle cover plate is in a flat plate structure. 5 [0014] Each of the bolster-beam underside cover plates is provided with a longitudinal stiffener plate near a bending area of traction beam. [0015] The bolster-beam lower middle cover plate is 10-16mm in thickness. [0016] The bolster-beam lower middle cover plate is made of Q45ONQR1 materials. [0017] Bolster beam webs are matched with the structure of the bolster-beam lower 10 middle cover plate. [0018] The tank car traction bolster device of the present invention has the following advantages: [00191 1. The local gap between the lower wing surface of each Z-shaped steel at one side of the traction beam and the bolster-beam lower middle cover plate is eliminated, thereby 15 improving the fatigue strength reliability of the structure. [0020] 2. The bending residual stress in the original structure is eliminated, thereby improving the fatigue strength reliability of the structure. [0021] 3. The high-strength assembly problem in the original structure during the assembly of the bolster-beam lower middle cover plate and the bolster-beam underside 20 cover plate is solved. [0022] 4. The strength reservation of the bolster-beam lower middle cover plate is improved. [0023] 5. The structure is simple, so that the tank car traction bolster device can adapt to the existing production equipment and technologies of railway tank cars. 25 BRIEF DESCRIPTION OF THE DRAWINGS [0024] In order to more clearly describe the technical scheme in the embodiments of the present invention or the prior art, the drawings needed in description of the embodiments or the prior art are briefly described below; obviously, the drawings in the following description represent some embodiments of the present invention, and other drawings 4 can also be obtained by the person skilled in this field without creative work. [0025] FIG.1 is a structural view of the traction bolster of the prior art in an ideal operating status. [0026] FIG.2 is a partially enlarged structural view of FIG.1. 5 [0027] FIG.3 is a structural view of the traction bolster of the prior art in an actual operating status (1). [0028] FIG.4 is a structural view of the traction bolster of the prior art in an actual operating status (2). [0029] FIG.5 is a structural view of the tank car traction bolster device of an embodiment 10 of the present invention. [0030] FIG.6 is an A-direction structural view of FIG.5. [0031] FIG.7 is a structural view of the bolster-beam lower middle cover plate of an embodiment of the present invention. [0032] FIG.8 is a structural view of the stiffener of an embodiment of the present 15 invention. DETAILED DESCRIPTION [00331 In order to make more clear the purposes, the technical schemes and the advantages of embodiments of the present invention, the technical schemes of embodiments of the present invention are clearly and completely described in 20 combination with the accompanying drawings as follows. Obviously, the described embodiments are only part of embodiments of the present invention, and based on the embodiments of the present invention described herein, all other embodiments of the present invention obtained by a person skilled in the field without creative work are within the scope of the present invention. 25 [00341 Referring to FIG.5, FIG.6 and FIG.7, the present invention relates to a tank car traction bolster device; a preferred implementation mode is that the traction bolster device comprises an end beam 1, a traction beam 8 located in the middle of the end beam 1 and arranged perpendicularly to the end beam 1, and side beams 2 located at the two ends of the end beam 1 and perpendicular to the end beam 1; one ends of the side beams 2 are 5 connected with the end beam 1; the other ends of the side beams 2 are provided with bolster-beam webs 5 in parallel to the end beam 1; a bolster-beam support plate 6 is arranged at the middle section of each bolster-beam web 5; an integrated rear draft lug and center plate seat 9 is installed in the cavity of the traction beam 8 near the middle part 5 thereof; a front draft lug 11 is arranged in the inner cavity of the traction beam 8 near the end beam 1; a side pipe support 4 and a junction plate 3 for fixation and connection are arranged at one end of each bolster-beam web 5 connected with each side beam 2; a bolster-beam lower middle cover plate 10 is located under the traction beam 8, and bolster-beam underside cover plates 7 connected with the bolster-beam webs 5 are 10 respectively arranged at the two sides of the bolster-beam lower middle cover plate 10. The traction beam 8 is made of Z-shaped steel, the lower wing surface of the Z-shaped steel is regarded as the locating benchmark to guarantee the flatness of the lower wing surface; the bolster-beam lower middle cover plate 10 is placed on the lower wing surface of the Z-shaped steel; and a top center plate 12 is placed on the bolster-beam lower middle 15 cover plate. The bolster-beam lower middle cover plate 10 is in a flat plate structure, and the structures of the bolster-beam underside cover plates 7 and the bolster-beam webs 5 are matched with the structure of the bolster-beam lower middle cover plate 10. [0035] The thickness of the bolster-beam lower middle cover plate 10 of the present invention can be 10-16mm; in addition, the materials of the bolster-beam lower middle 20 cover plate 10 can be Q45ONQR1 with better strength reservation and low temperature toughness. [0036] Referring to FIG.5 and FIG.8, the bolster beam of an embodiment of the present invention comprises the junction plates 3, the side pipe supports 4, the bolster-beam webs 5, the bolster-beam support plates 6 and the bolster-beam underside cover plates 7. The 25 side pipe support 4 can be made in a form of a side pipe or a side cover plate, etc. The bending position of the connection of each bolster-beam underside cover plate 7 and the traction beam 8 can be provided with a longitudinal stiffener 13 to enhance the strength and support ability of the bolster beam; the longitudinal stiffener 13 can be vertically installed on each bolster-beam underside cover plate 7. 6 [0037] Finally, the above embodiments are only used for illustrating the technical schemes of the present invention, rather than limiting; despite of detailed description with the reference of the embodiments of the present invention, the person skilled in the field can understand that the technical schemes recorded in the embodiments can be modified, 5 or the technical features can be replaced by equivalents; these modifications or the replacements do not make the essence of the corresponding technical schemes depart from the spirit and the scope of the technical schemes of all the embodiments of the present invention. 7