Study On Microstructure And Properties Of Q355NH Weathering Steel Prepared By Supersonic Fine Particle Bombarding

Weathering steel is the high-strength,low-alloy steel which provides a significantly higher corrosion resistance than the regular carbon steel.With the modernization of the national economy,in many fields such as heavy-duty equipment,lightweight design,energy-saving vehicles and so on,requirements that high strength,superior corrosion resistance and low cost for steel structural materials are put forward.It is of theoretical significance and engineering value to develop a type of weathering steels with better mechanical properties and corrosion resistance.For the purpose of investigating the mechanisms of the plastic deformation and the corrosion resistance of the surface nanocrystallization,in the work,the surface morphology,mechanical and electrochemical corrosion properties of Q355 NH weathering steel were studied by changing SFPB process times(15 min,30 min and 45 min).The finite element model of shot peening strengthening for single shot was established using ABAQUS software.The effects of shot peening speed(60 m/s,80 m/s,100 m/s and 120 m/s)and shot peening angle(30,45,60,75 and 90 degrees)on surface residual stress,equivalent plastic strain and surface roughness of sheet metal were analyzed.The research results showed that:(1)The surface average grain size of the samples after the SFPB treatments with different process duration reached nanometer level.With the increase of the processing time,the tensile strength improved linearly.At the same time,the change of the elongation was contrary to the tensile strength.There had the gradient distribution of the microhardness along the thickness direction.The fracture modes of untreated and treated samples for 15 minutes had the ductile fracture characteristics.The specimens treated for 30 min and 45 min showed mainly cleavage fracture modes accompanied by slight ductile fracture.The grain boundaries not only blocked the slip of dislocations,but also coordinated the strain to improve the plasticity and toughness of the material due to the existence of a large number of grain boundaries on the metal surface.(2)The electrochemical corrosion test results in 3.5% NaCl showed that the corrosion resistance of the sample steels increased at first and then decreased,which in turn were samples for 15 min,untreated,30 min and 45 min.It is beneficial to the rapid formation of passive film and the corrosion resistance of the material when the weathering steel surface had a short(15 min)the plastic deformation time.However,with the extension of treatment time(30 min and 45 min),the surface roughness,micro-strain and defect density of materials increased obviously.It was difficult to form a complete passive film on the surface due to the autocatalytic process dominated by anodic dissolution,which caused the weakness of the corrosion resistance.(3)The velocity and angle of shot peening have obvious effects on the residual stress,equivalent plastic strain and surface roughness of metal sheets.Rising the shot peening velocity will caused the increase of surface roughness,maximum residual compressive stress and overall residual compressive stress layer depth.When the incident angle of the shots raised from 30° to 75°,the maximum residual compressive stress and the surface roughness increased obviously.But,the influence of incident angle on the residual stress field and surface roughness changed little as the incident angle was greater than 75°.Considering the influence of shot peening angle on residual stress and surface roughness,it is better to select in the range of 75° to 90° in the actual shot peening process.The change of surface microstructures of metal materials will have a significant impact on their mechanical and corrosion properties.The application range of traditional materials can be effectively expanded by appropriate surface treatment methods.The study provides theoretical and experimental support for the development and application of non-spraying weathering steel,and some ideas for the microstructure design in the process of surface treatment of metal materials.