Properties Research Of Low Carbon Bainite Fishplate Materials Used For The High-speed Railway

The rail joint bar(abbreviated as fishplate)is important section that is connecting the rail joint and its quality will influence thesecurity of railway transportation directly The fishplate was made with B6 and B7 steel originally, but with the development of modern railway to a high speed, heavy load and high density transport, these materials can't satisfy the current situation. This component was made of B6, B7 steel and 56Nb microalloy steel, which are both far from meeting the higher demand of fishplate materials in that, with the tendency of high speed, heavy load and high density in terms of modern rail transportation, since 1977, the rail have speeded up for 6 times at home and the rail transportation became heavier and busier with the roaring development of economy in our country. Over years, a lot were studied with quite great success by researchers overseas on rail and fishplate materials.The thesis, based on the history of low carbon bainitic steel and the transformation mechanism, briefly introduces the strengthening mechanism of bainitic steel, studies on and use of fishplate materials both at home and abroad as well as their smelting, chemical components, organizations, properties etc.. Then, structural design, work condition, stress state and invalid reason of the fish plate, etc. are analysed.The studied fishplate material is unquenched air cooled bainitic steel which is optimized in its properties via controlled rolling and controlled cooling, and its microstructure at room temperature is observed. In accordance with state standards, comparative study is made between the tensile properties (tensile strength s_b, yield strengths s_0.2, d andΨ) and impact properties (A_KV) of new types of bainite fishplate materials at room temperature and low temperature; images of tensile fracture and impact fracture are observed via SEM at different temperatures, and the method of cracking is covered; the microstructure and the method of strengthening are observed through TEM.The tensile and impact properties of the two mainly studied new types of bainite fishplate materials are both superior to B7 steel which is in use at home. At room temperature and -55℃, the yield strengths of material A are respectively 722MPa and 738MPa, the elongation percentages respectively 60.7% and 41.7%; at the same temperatures, the yield strengths of material B are respectively 568.5 MPa and 700 MPa, the elongation percentages respectively 63.5% and 67.1%. At room temperature, the impact properties A_KV of A and B are respectively 40J and 90J, and their strength and toughness perfectly meet the specifications said in TB/T 2345-93 43-75kg/m Technical Requirement to Goods Supply of Rail-used Fishplate, but the low temperature toughness is a little bit inferior to that of fishplate made in German, which can be improved via low temperature tempering.Observation via TEM and metallographic analysis Shows that the microstructures of the two types of bainitic steels are respectively fine-grain granular bainite and meta-bainite, which is related to the rather high silicon content of this type of steel. Silicon impedes carbide from precipitating, which, consequently, makes the unchanged austenite be rich in carbon and carbide-free bainite results, and counteracts adverse effect to cementite.According to the study on the tested steel in lab, the thesis suggests that the two types of tested bainitic steels are up to the demands of properties of fishplate material set by Railway Ministry, therefore they can replace the B7 steel and 56Nb which are in use at home and become key materials supporting the speeding up of domestic rail transportation.