| The automobile inventory and sales are growing along with the economic and technological development in China, and hence the energy consumption and traffic accidents increase year by year. A large number of traffic accidents are caused by insufficient tensile strength of automobile beam. Car lightweight is the most efficient measure to cope with energy depletion and traffic safety without sacrificing the strength of auto body structure. It is one of ways for the traditional auto industry to adapt the strategy of sustainable development by designing high strength steel used for automobile beam. In addition, with the exploitation of circumpolar and extremely cold area, the usage frequency of automobile gradually raises in low-temperature environment which bring great challenge to the body structure steel's toughness. At present, the optimization design of the beam steel is mainly focused on the strength and cold bending behavior. However, little attention is paid to the toughness on the condition of high strength, especially the low-temperature toughness. In this work a series of 700 MPa grade Nb-B micro-alloying steel with a mixture microstructure of acicular ferrite and granular bainite are designed, and the effect of elements Ti, V and Ni on the refinement and mechanical properties have been studied. The main results are shown as follows:The influences of microalloying elements on precipitation characteristics of second phases are calculated by use of Thermo-calc software, such as precipitation temperature, type, composition and quantity. The results show that the critical precipitation temperature of NbC is about 1120? with the addition of 0.04% Nb in the tested steel. However, NbC is displaced by ?Ti, Nb?C with addition of Ti. The critical precipitation temperature of ?Ti, Nb?C increases from 1180? to 1240? with increasing Ti from 0.04% to 0.07%, and its thermal stability is higher. At the same time, the precipitation quantity increases along with increment of Ti content. There is only ?Ti, Nb, V?C precipitate with the addition of 0.04% V. V-rich MC could precipitate when V is 0.08%.The effect of second phase on austenite grain growth is studied based on the thermodynamic calculation results. The results show that the grain coarsening temperature is between 950? and 1000? for Ti-free steel, whereas it rises to the range from 1000? to 1050? for Ti-bearing steels. The grain coarsening temperature does not increase with higher Ti content, but the mixed crystal can be controlled, which is due to the pinning effect of ?Ti, Nb?C and Ti?C, N? which have relatively higher thermal stability. Besides, the refinement is not obvious by adding Ni and V, and no single VC or VN has been detected in the V-bearing samples.The dynamic continuous cooling transformation behaviors of the tested steels are measured by Gleeble 3500 thermosimulator. The mixed microstructure of acicular ferrite and granular bainite could be obtained when the cooling rate is between 2?·s-1 and 10 ?·s-1. The volume fraction of island in granular bainite decreases with addition of Ti, however, it increases with addition of Ni and V. The misorientation between acicular ferrite and granular bainite is high-angle boundary. The matrix of granular bainite is block ferrite and the islands are mainly irregular bulk retained autenite when the cooling rate is below 5?·s-1. In the range of 5?·s-1?10?·s-1, the matrix of granular bainite is composed of lath like ferrite and discontinuous thin retain auteinte film could be observed between them.The volume fraction of static austenitic recrystallization decreases below 950? for the tested steel. It is obvious that the static recrystallization is significantly retarded by the addition of Ti. The apparent activation energy increases by 60 kJ·mol-1 for the sample with 0.04% Ti. It is observed that the main reason for the decline of recrystallization rate is attributed to the pinning effect of titanium carbonitride. But the influence of Ni and V on the recrystallization is not obvious at high temperatures.It is favorable for acicular ferrite nucleation and growth when the relaxation time is shorter than 50 s and the polygonal ferrite appears when the relaxation time is extended to 100 s. By lowering the coiling temperature or adding of Ti, Ni and V, the islands in granular bainite could be effectively refined. The islands can transform into upper bainite, lower bainite, lath bainite, retained autenite or twin martensite when the coiling temperature is 550?. The nanohardness of the islands are from 2.6 GPa to 5.78 GPa.The ingots are hot rolled by small rolling mill in laboratory. The mixture of acicular ferrite and granular bainite can be obtained when the finish rolling temperature is 850? and with a relaxation about 50 s, cooling rate is 10?·s-1 and coiling temperature is 550?. The tensile strength of the 7 mm hot rolled plates is greater than 710 MPa, and the plates have perfect cold bending performance that can reach the initial design criterion.The Charp impact toughness decreases with increasing of Ti content, at the same time ductile-brittle transition temperature ?DBTT? rises. The upper-shelf energy ?USE? is 85 J for the Ti-free steel with a DBTT of -38?. However, the USE and DBTT is 81 J and -30? for the steel containing 0.04% Ti. The USE further decreases to 72 J for the steel containing 0.07% Ti with a DBTT of -19?. The precipitation and aggregation of ?Ti, Nb??C, N? particles on prior austenite grain boundary lead to the dislocation piling up, which leads to a decrease of Charp impact toughness. The{101} texture, which is conducive to the expanding of ductile crack, decreases with addition of Ti. Whereas the {001} texture, which promotes the cleavage fracture, increases with addition of Ti. The increasing of {001} texture deteriorates toughness and increases the DBTT. The large grain boundaries ?>15°? which could hinder crack propagation, increases with addition of Ti, and it would increase the risk of brittle fracture. |
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