Hot Rolling Process And Structure Property Under The Welding Thermal Cycle Of Ti Bearing Dual-phase Steel

Hot rolled dual-phase steel is used mainly for welding structures, such as car wheels, beams, longitudinal beam. This article makes Ti bearing dual-phase steel under different cooling processes and then get the hot rolled steel under different welding thermal cycle, it's organization evolution law and welding performance is studied. The main work is as follows:(1) Thermal expansion method was employed to determine the CCT curves and transformation behavior with continuous cooling for tested steel. At the same time, different hot rolling processes were carried out for Ti bearing Dual-phase Steel at laboratory. The article has researched the steel's structure and mechanical properties under different cooling condition and composition. Under the condition of continuous cooling, the increase of Ti content can strongly inhibit the formation of ferrite and pearlite, promote bainite transformation and refine bainite lath. In addition, the yield strength and tensile strength of hot rolled dual-phase steel increase significantly with the rise of Ti content, but the yield strength increment is greater than that of tensile strength, which results in the increase of yield ratio. Along with the increase of the content of Ti,the impact energy has reduced,and the impact performance has also declined.(2) By measuring the SH-CCT diagram of tested steel with welding thermal simulation, the influence of cooling rate on microstructure and hardness of CGHAZ was obtained to reveal the phase transition of HAZ at different cooling rates. The results indicate that, With the increasing of cooling rate, it will promote bainite transformation. The higher peak temperatures can make the CCT curve move to the right; higher peak temperature can also inhibit the formation of ferrite and pearlite, bainite formation and refine bainite lath.(3) The welding thermal simulation was adopted to study the effect of the welding processing parameters(t8/s), the content of Ti, the original organization on the microstructure and properties of heat affected zone. The results show that,under the welding thermal cycle, the microstructure of HAZ is tiny and uniform at the higher cooling rate(when t8/5 is 5s), the granular bainite is increasing with the reducing cooling speed. However,the experimental steel which has more content of Ti has much bulky granular bainite at low cooling rate. In addition, the result shows that, experimental steel with low content of Ti has good welding performance, the amount of its heat input range is larger, but experimental steel with higher content of Ti has relatively poorer performance.(4) By using the impact experiment and fracture morphology scan, the experimental steel's impact fracture mechanism was studied under the different welding process, the results show that, the experimental steel with low Ti content fracture morphology is primary with ductile fracture, the cracks near impact fracture surface had been obviously changed in the path when extending to the ferrite grain boundary, so it has good impact property. Under high cooling rate, as the original austenite grain size in HAZ was very small and uniform, the secondary cracks crack arrest at the original austenite grain boundary, and its cracking unit path are very small, so the experimental steel with high content of Ti showed good ability to hinder crack propagation. However, at low cooling rate, the coarse granular bainite has weaker ability to hinder the crack extension. So it has lower impact absorbing energy.