| Air-cooled Mn-series bainitic steels have been applied in many areas because ofits advantages as follows: good combination of strength and toughness, simpleprocess and low cost, energy saving and reducing the pollution of the environment. Vand Ti are usually added in steels to improve strength and toughness of the steelthrough refining grains and second phase dispersion as well as the technology of con-trolled rolling and cooling process.Based on the alloying design and phase transfor-mation theory of Mn-series air cooling bainitic steels, V micro-alloying technology isapplied. Effect of V on the mechanical performance, microstructure and phase trans-formation of steels are studied. Morphology, chemistry composition, size and distri-bution of vanadium precipitates were studied by use of TEM and EDS. A new kind ofvanadium microalloyed super-high strength rebar was obtained and industrializedproduction was carried out sucessfully.The main results of this dissertation are listedbellow:(1). For the steels with different carbon content and srrength level, the tensilestrength and yield strength are increased obviously by adding0.04%V, elongationand area reduction remainning basically unchanged. As V content is increased from0.03%to0.08%, the toughness of steels are improved obviously wherea the strengthincreases slightly.(2). Precipitates in the steel containing0.04%V distribute on the ferrite and dis-location band, containing spheric, spheroidicity, cubic, rectangular and other irregularshapes.The result of EDS indicates that the precipitates are mostly complex precipi-tates containing both V and Ti, those only containing V are not observed.(3). The tensile strength and yield strength are increased obviously by adding0.5%Cr, meanwhile, thoughness, elongation and area reduction remainning basicallyunchanged.(4). The steel containing0.04%V has stronger resistance to tempering than thesteel wothout V, and presents secondary hardening phenomenon. (5). Air-cooled super strength finish rolled rebar of industrialization trial produc-tion reaches the mechanical performance as follows: Φ20mm Rel＞1400MPa, Rm＞1600MPa; Φ25mm Rel＞1200MPa, Rm＞1500MPa; Φ32mm Rel1140~1200MPa,Rm1450~1500MPa. Elongation is9.0%, and the total elongation with maximumstress is5.0%. All the mechanical properties meet the demand of PSB1080level, andreaches PSB1200level. |
