Study On Microstructure And Mechanical Properties Of 590MPa Hot-rolled Hteel With High Hole Expansion Ratio

In the field of automobile, high hole expansion steel(FBsteel), made up of a large amount of ferrites and a small amount of bainites, receives extensive attention as its good performance. This steel is mainly used to manufacture auto parts, which always have complex sharp and higher requirements of stretch flangeable. Therefore, it is so important to investigate microstructure and mechanical properties of the steel.In this paper, a low C hot rolling FBsteel containing Nb and Ti is studied. Law of continuous cooling phase transformation of the FBsteel has been investigated by determining CCT curves. Simultaneously, the effects of different rolling process parameters on microstructure and hardness of the FBsteels have been investigated by thermal simulation experiment. The pre-production trial has been carried on in AnGang 1780 hot rolling line, and FBsteel sheets with good performance have been obtained.The results are as following:1, Based on thermal dilation experiment, statistic and dynamic CCT curves of the test steel have been obtained, and law of continuous cooling phase transformation is investigated. The results show that compared with statistic CCT, ferrite and bainite phase transformation area is enlarged in dynamic CCT. And with cooling rate improved, hardness of the test steels increase. When the cooling rate are equal, hardness of the test steels in statistic conditions are higher.2. With the decrease of finishing temperature, the ferrite content of the microstructure is improved, and average grain size is decreased. Microhardness of ferrite is decreased and microhardness of bainite is improved.3. With the decrease of air cooling temperature, the amount of the ferrite, significantly fine grains, in the microstructure is increased firstly and then decreased. Microhardness of the ferrite is increased and the bainite is increased firstly and then decreased.4. With the increase of air cooling time, the amount of the ferrite is increased and the average grain size is increased. Microhardness of the ferrite is decreased and the bainite is increased.5. With the decrease of coiling temperature, the ferrite content and average grain size almost remain unchanged. The microstructure of the bainite change where the transformation of slab martensite to lower bainite occurs and microhardness of the bainite is increased.6. The results of the pre-production trial show that the final microstructure of three different thickness sheets is made up of ferrite and bainite. The ultimate tensile strength is higher than 600 MPa, and the longitudinal ductility is higher than 19%. The hole expansion ratio is higher than 100%, which is greater than national standard requirement.