Study On The Microstructure And Properties Of Low Carbon Bainitic Steel Q460E

The heat treatment of normalizing or normalizing + tempering of different temperatures was proceeded to a low carbon bainitic Q460E steel provided by the company. Through the method of optical microscope (OM), scanning electron microscope (SEM), electronic tensile testing machine, microhardness tester and impact energy tester, the effects of temperatures at which normalizing or normalizing + tempering proceeded on the microstructure and mechanical properties of the testing steel were studied, the evolution of microstructure and variation of mechanical properties with different heating treatment were explored and the reasons for these were discussed. It aims to get better mechanical properties and practical technologies, to promote and widen the application of this kind of steel, to provide the theoretical basis for the exploitation of other new bainitic steels. It has a great significance to lowering self-weight and extenting service life of engineering structure, energy conservation, environmental protection, and etc.The results are as follows. The main microstructure in the full normalized (FN) steel is granular bainite. The fine M-A islands dispersedly distribute in ferrite. With increasing of normalizing temperature, the amount of M-A islands increased, while the island chord length and the space between islands reduced, and these lead to an increment of the strength. The microstructure in the sub-temperature normalized (SN) steel is mainly ferrite and bainite colonies. With increasing of temperature, fine bainite colonies are arranged from scattered to dense; the growing austenite grains merge into a bigger one. After cooling M-A islands similar to that in the FN specimen and uneven ferrite are able to be observed. The strength increased with increasing of temperature of the SN, until up to the temperature of 780℃, at which the strength is the highest. If the temperature keeps on increasing, the strength will be reduced. The mechanical properties of the SN are nearly equal to those of the FN, all of which have well ductility and strength. Mechanical properties of normalized Q460E could reach the requirement of some medium carbon quenched and tempered steel.When tempering at 200℃to 400℃, the microstructure of full normalized steel is more stabilized and its mechanical properties are better. The higher temperature of the SN is, the higher temper resistance of microstructure and mechanical properties is. The mechanical properties of the SN are proximate the same with the FN in the same temperature.When tempering at 500℃to 600℃, the impact toughness of the SN at a low temperature, -40℃, and plasticity are widely increased. While tempering at 600℃, the low-temperature impact toughness of the FN is greatly reduced, the reason for this can be attributed to the carbide precipitated at grain boundaries of austenite and brims of M-A islands.