Study On Microstructure And Mechanical Properties Of The New C Grade Link-Chain Steel For Mining

The link-chain steels with chemical compositions (C:0.20～0.28%,Si:0.17～0.8%, Mn:1.2-2.0%, Mo:0.15-0.50%,Cr≤0.8%) were prepared by vacuum melting. The ingots are forged at 850℃～1200℃and processed into test materials. Microstructures analysis in different heat treatment states by optics microscope and SEM. The CCT curve of the experimental steels was investigated by means of full automatic phase transformation tester of Formaster-IV. Mechanical property measurements at different conditions, weld microstructure of preproduction of the link-chain in the new materials industralized conditions and properties of product demand were conducted.Continuous cooling transformation of undercooling austenite about the new c grade link-chain steel for mining revealed that: the critical cooling rate to gain all martensite is 20℃/s. At the cooling rate ranging from 20℃/s to 5℃/s, it can obtain martensite and bainite structures. The mixed microstructure of ferrite and granular bainite could be obtained from 2 to 0.05℃/s. The microstructure of the steel is granular bainite and the granular bainitic structure formed by ferrite matrix and uniformly distributed m/a components can be obtained under normalized or hot-rolling state. The hardness was 20HRC～23HRC and it can meet the requirements of the shearing blanking to the steel. Hardenability test showed that: The maximum full hardening diameters is 35 mm meanwhile the martensite microstructure in the core part of the tested steel. Fine lath martensite can be obtained in the core part of the steel with diameters of 26 mm under 880℃(20℃water quenching)and 940℃(37℃water quenching). Good comprehensive mechanical properties, such as yield strengthσs1020MPa～1050Mpa, tensile strength after 400℃tempering. The impact energy of the test steel under different process of hardening and tempering revealed that, the crack extension zone of the impact fracture presented tearing ridge and fracture characteristics. The fracture surfaces of the steel composed of dimples and exhibit a tough fracture character after 500℃tempering. The test round chain had a good welding technicality of flash butt welding and the ductile microstructure of low carbon bainitic structure were found at welding lines. Compared with 25MnV, low carbon bainitic structure was fine after induction heat treatment, temper troostite can be obtained in the welding seam as same as the matrix. The tensile properties were superior to the requirements of national standards. The effect of inhibiting grain growth at high temperature by means of Nb micro-alloying was obvious and fine austenite grain can be kept. A remarkable effects showed that inhibiting grain growth under high temperature which the steel contained Nb was better than V or no Nb in it.