Low-carbon Bainitic Steel, And The Strength And Toughness Mechanism

High strength low carbon bainitic steel is a kind of steels which was developed in 20 years in the world. It has three characteristics:high strength, high ductility and excellent weld property. It is the outcome combing modern metallurgy with physical metallurgy research works are performed on low carbon bainitic steel all over the world.In this paper, steel ingot for low carbon bainitic was used as the experimental sample, which is untempered due to restriction on production technology, so impact toughness is unstable. The most important works are getting the structures converting curves under dynamic and static states by the thermodynamic simulating tester Gleeble 1500D and analyzing and observing the microstructures under different cooling situations, including final cooling temperature and cooling rate. By scanning electric camera and transmission electric camera, we researched structures shapes and the influence between the toughness and inclusions, quantity, size. Also, researched the relations obdurability to crystal orientation, large angel boundary or small angle boundary, and the major conclusions are as follows:(1) It was shown that lathy bainite increased, the volume fraction of MA phase reduced and the shape from strip became into globosity with final cooling temperature reducing in the same rate of cooling conditions. But in same final cooling temperature, granular bainite became slightly shiner, the dimension of MA phase diminished and the number decreased with cooling rate advancing.(2) The higher cooling rate had, the higher the tensile and yield strength and the bigger impact toughness but lower elongation percentage got. In the same cooling rate, strength increased and yield ratio makeup from 0.74 to 0.81, but elongation percentage reduced, the impact toughness became unconspicuous with final cooling temperature.(3) The scanning electron microscope shows that crack in the roots exist large size inclusions for the vast majority of Al,Ca oxides. The area around large pieces of inclusion is frail segment, which is prone to take place cleavage fracture. So scavenging material and reducing inclusion can improve the toughness of materials. (4) It was found that block divided by small angle boundary, mainly controlled strength, and packet divided by high angle boundary, mainly controlled toughness.