Research on the Construction and Performance of Novel Steel Composite Materials
Supervisor:yang ke dan yi yin
With the development of science and technology,more and more products with excellent quanlity and abundant functionalities have been exploited and provided.Therefore,more and higher requirements are put forward on the most widely utilized structural metal materials in industrial fields,especially steels.On one hand,higher strength and sufficient toughness are needed to meet the loading capabilities.On the other hand,some certain functional performances,such as magenetic,optical,electrical,acoustic properties,are preferred.In terms of realizing the functional properties of material,composite materials have certain congenital design and structural advantages.In this study,a maraging steel(C400)with ultra-high strength,relative low ductility and toughness,and excellent soft magnetic properties,and a low strength,great ductility and toughness and non-magnetic commercial austenite stainless steel(316L)were selected as the original materials for fabrication of the steel composite.Inspired by the concept of "brick wall" structure or layer structure with alternated distribution of hard and soft phases discovered in nature creatures such as mother pearl shellfish,a entirely novel steel composite which not only can minimize the shortcomings of the original materials at the maximum extent,but also possess excellent mechanical perfprmance as well as new physical properties,has been developed.At first,hot deformation behavior of C400 was studied,which laid the theoretical foundation for the subsequent study of microstructure evolution of original materials in the preparation process of the composite materials.Secondly,the effect of processing parameters on the interface bonding behavior during vacuum hot pressing was studied on a Gleeble 3800.The fabrication feasibility of hierarchical structure composites comprised of ultrahigh strength C400 and relatively soft and ductile 316L was explored,and small size C400/316L multi-layer composites were successfully prepared.Moreover,the block C400/316L multi-layer composites with large size were prepared by vacuum hot pressing and rolling.Their mechanical properties,interface characteristics,microstructures,strengthening and toughening mechanism and magnetic performance were analyzed and studied,respectively.Finally,ultra-thin C400/316L multi-layer composites with different thicknesses were fabricated by asynchronous rolling,and their mechanical properties,interface characteristics and magnetic properties of the strips were investigated.The main results of this study are as follows:(1)Study on the hot deformation behavior of an ultra-high-strength maraging steelHot-processing behavior of an ultra-high strength Fe-Ni-Co-based maraging steel was studied in temperature range of 900-1200℃ and strain rate range of 0.001-10 s-1.The results showed that the deformation temperature and strain rate had significant effects on the flowing stress of C400.With increase of the deformation temperature and decrease of the strain rate,the flowing stress was significantly reduced.Due to the high content of Mo,a higher activation energy Q(439.311 kJ/mol)was required in the process of hot deformation,indicating that dynamic recrystallization was difficult to occur in the hot deformation process,and that the dynamic recovery sub-microstructure and the deformed microstructure were in the dominance.The thermal processing maps presented large "instability zone,and limited hot-working parameter windows.The optimum windows were 1100-1200℃/0.001-1 s-1 and 1000-1100℃/0.001-0.1 s-1.The hyperbolic sine law equation embracing the Z parameter was depicted as follows:Z=ε exp(493111/RT)=0.9916×1018[sinh(0.0069×σ)]-4 808(2)Feasibility study of multi-layer steel compositesThe fabrication feasibility of C400/316L heterogeneous multi-layer steel composites was explored.The results showed that in the process of vacuum hot pressing,high vacuum atmosphere was the key factor to obtain excellent interface bonding,and the appropriate amount of deformation reduction can promote the integration of interface,while the subsequent heat treatment process had no deteriorating effect on the interface bonding.After vacuum hot pressing,the interfaces under different deformation conditions were very clear,straight,and no obvious defects and oxides were observed.At the same time,slight mutual diffusion phenomenon occurred on both sides of the interface and "bite" structure in micro-scale was formed.Via vacuum hot pressing and subsequent heat treatment,C400/316L multi-layer composites with great hardness difference and good interface combination were successfully obtained.(3)Study on preparation and performance of bulk multi-layer steel compositesC400/316L multi-layer composite in large scale with 5 layers,9 layers and 11 layers were prepared by vacuum hot pressing and rolling.The micro-structural characteristics,mechanical properties and magnetic performance were studied,respectively.The results showed that due to the coordinated deformation of strong interface,an additional stress field was formed near the interface,which promoted the formation of a unique double-gradient nanostructure near the C400/316L interface:gradient nano-mechanical twin in 316L layer/gradient lath martensite in C400 layer.The C400/316L multi-layer composites under cold-rolling and solution states performed great bending deformation abilities.While after solution and aging,the average bending strength could reach up to 2344 MPa,indicating less poor bending deformation abilities.If direct aging after cold rolling was adopted,the bending strength can be increased by more than 30%because of the retained cold deformation reinforcement.In the process of crack propagation,due to the role of soft phase and interface,the crack deflection,bridge,and even delamination occurred,which greatly consumed the energy and delayed the crack propagation.Therefore,the bending curves presented typical step-like characteristics.In addition,the average uniaxial tensile strength of C400/316L composites under cold-rolling state were 1205 MPa,while the tensile strength could be increased to 1374 MPa and 1815-1893 MPa respectively,after solution and aging and direct aging treatment.Compared with the reduction of plasticity caused by premature neck shrinkage of C400 layer,the promotion effect of interface was not obvious and the final plasticity was not significantly improved,although the interface had a positive contribution to the plasticity of multi-layer composites after crack initiation at the outermost C400 layer.In addition,due to the attenuation,delay and dissipation effect of heterogeneous interface,gradient nano/micro structure interface and nano-precipitations,the C400/316L multi-layer composites exhibited great anti-high-speed impact performance.At the same time,the multi-layer composites also showed excellent soft magnetic performance.(4)Study on preparation and performance of ultra-thin multi-layer steel compositesUltra-thin C400/316L multilayer composite strips with thickness of 0.04-0.4 mn were successfully prepared via asynchronous rolling method.With increase of the deformation,the interface gradually changed from straight to curved,and even the phenomenon of neck shrinkage and disorder occurred.The average tensile strength of 11-layer ultra-thin strip under cold-rolling state can reach up to 1610MPa,and the strip also showed excellent soft magnetic characteristics with high saturation magnetization(122 to158 emu/g).In addition,the mechanical and magnetic properties of the ultra-thin strips can be adjusted effectively by heat treatment.