Studies On Aging Behavior Of Cu-enriched Nano Cluster Strengthened HSLA Steels

Nanoscale Cu-enriched-cluster-strengthened steels can be applied to shipbuilding,marine engineering,automobile industry,infrastructure,heavy duty constructions,high-pressure pipes,bridge building,large container etc.due to the high strength,good toughness,corrosion resistance and excellent weldability.The mechanical properties of these kinds of steels are dependent on the composition and processing routes,specifically solid solution treatment,themomechanical treatment and aging processes.The precipitation of nanoscale clusters are mainly controlled by the aging processes.It is significant to study the effects of aging on the precipitation of nanoscale clusters since composition,crystal structure,size and distribution of nanoscale clusters can significantly affect the strengthening mechanisms and finally the mechanical properties.In this study,both vacuum induction furnace and vaccum arc meltor were used to prepare the nanoscale cluster-strengthened steels with various compositions.The preparation processes were investigated in detail.Considering the large specimen size of steels prepared by vacuum induction furnace,the effects of thermomechanical control processing(TMCP)on the matrix microstructure and mechanical properties of the steels prepared by vacuum induction furnace were investigated.The effects of composition and aging processes on the microstructure and mechanical properties of the steels prepared by vacuum arc meltor were investigated because it is convenient to prepare specimens with various compositions.And then the effects of casting methods,TMCP,aging processes on the microstructure and mechanical properties were compared and discussed.Optical microscope,scanning electron microscope,X-ray diffraction technique were applied to characterize the matrix microstructure while small angle neutron scattering(SANS)and 3-D atom probe tomography(APT)were applied to characterize the nanoscale clusters.The research results show that the microstructures are mainly polygonal ferrites after casting and TMCP.TMCP can improve both strength and ductility of the steels.It is evident that a combination of hot-rolling and cold-rolling can optimize the microstructure and improve the mechanical properties,as compared to solo hot rolling.Aging time and aging temperature can affect the precipitation of nanoscale clusters while there is no evident effect on matrix microstructure.Aging stages consist of under-aged state,aging peak and over-aged state.With same aging time,aging temperature speeds up the aging effect leading to a short time to aging peak state.Lowering the aging temperature can increase the aging time to peak aging while the peak strengthen are increased.Under the condition of solid solution,fracture surface after tensile test demonstrates a ductile microvoid accumulation fracture mode,consistence with the ductile fracture.After aging,the fracture surface is still dominated by ductile fracture mod.With the increase in strengthening,specifically at aging peak state,some cleavage fracture can be observed,indicating that matrix microstructures in this study have not reached the optimum state and can be improved further.The results from both SANS and APT indicate that nanoscale precipitates emerge upon aging.With increase in aging time,the cluster size increases.The nanoscale clusters contains mainly elements Cu、Ni、Al.The precipitation of nanoscale clusters induces the aging effects,including under-aged state,aging peak state and over-aged state.