Microstructural Control And Evolution Of Ultrafine Grained Q&P Steel

With the increasing need in fields of the automotive lightweight and the optimization of steel product structure, the development of the third generation advanced high strength steel (AHSS) becomes one of the hotspots in the research of many companies and scientists. Because of its novel heat treatment concept, quenching and partitioning (Q&P) steels is one of the most promising AHSS steels and have received significant attention. In this research, through the design of thermomechanical treatment and the control of microstructural evolution, a new kind of ultrafine grained Q&P steel is developed with the combination of grain refinement method and Q&P process. The process design, carbon partition behavior and micro structure evolution characterization of this ultrafine grained Q&P steel have been systematically studied. The main work and results are as follows.On the basis of traditional Q&P steel, three compositions are designed with different C or Mn contents to investigate the effect of these elements to the phase transformation and the formation of ultrafine microstructure. The transformation behavior is investigated by dynamic continuous cooling curve, Thermo-calc and modified super-element model. The results indicate that increasing Mn dramatically decrease the intercritical temperature and enlarge the three-phase region of ferrite, austenite and cementite, and slow the diffusive phase transformation. At the meantime, C increase the Ae3 temperature and cementite solution temperature. The three-phase region is also enlarged and the critical cooling rate for martensitic transformation is decreased.Ultrafine grained Q&P steels with ferrite grain size around 1μm have been fabricated through annealing of tempered and deformed martensite. The tempering process decreases the hardness of as-quenched martensite and the deformation load, which is beneficial to the following cold rolling process, while the tempering temperature has very few effects on the grain sizes of annealed steels. The annealing temperature, time and heating rate have great influences on the formation of ultrafine microstructure, which are associated with the formation of austenite, the dissolution of cementite and the recrystallization of matrix. The effect of partial quenching temperature is investigated and the optimal partial quenching temperature is calculated by modified CCE model. The largest amount of retained austenite and the best mechanical property are obtained when adopting the optimal partial quenching temperature, but when using other temperatures which are not far away from the calculated one. the mechanical properties are not changed greatly. The partitioning processes are compared between ultrafine grained and coarse grained Q&P steels. Ultrafine grained Q&P steel has a considerable amount of retained austenite only after partitioning for 10s which indicate that ultrafine grained Q&P steel has a quicker austenite stabilization progress.Assuming the immobility of phase boundaries in the partitioning process, a C partition model is established on the basis of probable volume change resulted by the diffusion of C between BCC and FCC phase, which is compared with the experiment results. When the partitioning process develops to a certain degree, the experimental volume dilation is bigger than the calculated one, which indicates that in the real partitioning process not only C diffusion but also other competition phase phenomena exist. Kinetic simulations have been conducted by Dictra to the partitioning process, in which the grain size effect is taken into account in both Fe-C binary and Fe-C-Mn ternary systems. The result indicates that both the C depletion in martensite and enrichment in austenite are more pronounced in the ultrafine grained Q&P system, and the phase boundary shows a bidirectional characterization. Mn slows the C diffusion to some extent. When considering the effect of carbide precipitation, the simulation shows that the precipitation of cementite dramatically decrease the enrichment of C to the austenite, which act as the "trap" of C.The ultrafine microstructure evolution has been investigated by means of various experimental methods. During cold rolling many cell-like structures is formed which shows ring-like select area diffraction patterns, indicating that many large misorientation boundaries exist in the microstructure and make provision for the formation of ultrafine grains. During heating. the pinning effect of carbides inhibits the growth of grains and when reaching the annealing temperature, austenite islands take over this effect of carbides while undissolved carbides could still give a strong pinning effect at the same time. Therefore the growth of ferrite is restrained. A recrystallization-phase transformation interaction model during heating has been established. The simulation result shows that high heating rate decreases the recrystallization fraction during heating. which facilitates the interaction between recrystallization and austenite transformation and thus refines the microstructure. The effect of ultra-grain refinement to the microstructure and mechanical property of Q&P steels has been investigated. The refinement of microstructure gives a significant increase of yield and ultimate strength without the decrease of ductility. Although ultrafine grained Q&P steel exhibits a weaker instantaneous work hardening index, pronounced TRIP effect guarantees the sustained work hardening and comparable ductility to coarse grained Q&P steel. The effect of retained austenite to the ultrafine grained steel is also highlighted by comparing with other second phases. Two-dimensional representative volume element model is used to investigate effects of grain sizes and phase distribution to the stress and strain evolution as well as the martensitic transformation during deformation. The results shows that transformed martensite inhibits the further transformation of austenite nearby, while in the condition that the first martensite exists, the microstructure distribution of ultrafine grained Q&P steels is beneficial to the ultimate transformed fraction of retained austenite.