Research On Forming Behavior And Surface Composite Passivation Of Galvanized Non Quenched And Tempered Fastener Steel

In recent years, with the rapid development of automobile, rail transportation and construction machinery in our country, the top fasteners especially the high quality galvanized fasteners which require of high performance and light weight of automobile are urgently demanded. Due to the domestic fastener steel have many issues after quenching and tempering process, such as strong toughness mismatching, easy generation of crackles and crevices, low work hardening rate, surface chromate passivation and so on. So, a large amount of high strength fastener steel of automobile still need to import. However, the non quenched and tempered steel fasteners prepared by galvanized composite passivation treatment can get rid of steel annealing before cold drawn, leave out quenching and tempering after forming the bolts, shorten the production cycle, reduce the energy consumption, effectively avoid the problem of surface oxidation, decarburization, the workpiece deformation and six chromium pollution caused by heat treatment and coat-plating, so it got popular increasingly by fastener manufacturing industry. This paper chose the high strength bainite non quenched and tempered fasteners steel as the research object. The microstructure evolution and cold forming behavior of experiment steel during the hot forming process was studied; silane passivation composited with inorganic salt and properly adding Si O₂ and Ce O₂ nanoparticles was processed on the surface of galvanized fasteners, the corrosion resistance and self-healing property of the passivation film were enhanced by the synergistic effect of the inorganic-organic passivation. The main work of this paper is as follows:The influence of deformation temperature, deformation rate and cooling rate on the microstructure transformation of the non quenched and tempered steel was studied by thermal simulation experiments. The microstructure evolution rule of the experimental steel during the reducing-sizing process was analyzed, rolling experiments was conducted and rolling process parameters were determined. The experimental results showed that with lower deformation temperature, larger deformation and higher cooling rate（1¹0/s）, the microstructure was finer and the mechanical performance is better. Because of the fast cooling rate, the hardness and strength of the material are higher than those of the double cooling rate, in addition, the process of reducing deformation temperature, fast cooling first and slow cooling later were beneficial to the precipitation of granular bainite. The analysis showed that the temperature of deformation was from 780℃ to 550℃ at 5℃/s, which then be cooled to room temperature at 1 ℃ /s can make the distribution of the bainite precipitation particles uniform and fine. The experimental steel only occurred a static recrystallization in the reducing-sizing stage and the degree of recrystallization was the highest in center. Besides, grain size from the surface to the center gradually increased and the greater the amount of recrystallization, the finer the grain. The industrial rolling experiments showed that the inlet temperature of the reducing-sizing machine set at 780℃⁸50℃, cooling from 550℃ to 600℃ at 4⁵℃/s after rolling process, then cooling at 1² ℃/s, this rolling process can meet the requirements of the galvanized non quenched and tempered fastener steel’s microstructure and properties.The cold forming behavior of experimental steels was studied by room temperature upsetting experiment, the critical damage value of the ductile fracture criterion and material constants were also calculated, and the results were applied to the analysis of heading six flange bolts and six angle cylindrical bolt forming process of ductile fracture. Using the coupled thermo-mechanical simulation method, established a finite element model of bolt cold forging, analyzed the influence of mold structure, friction coefficient, cold heading speed on the stress and strain, forming load, temperature distribution, metal flow and cold upsetting cracking effect of the six flange bolt cold forging process. The results showed that the surface of experimental steel cracked when the high compression ratio of the cold heading is 1/3, and concluding that the steel forming limit diagram, the critical damage value of Oyane ductile fracture criterion was 0.67731, the material constant B was 0.8263. The finite element simulation results of cold heading bolt six angle flange surface showed that the third station die entry angle 120° ¹25° was more appropriate, the friction factor of 0.3 metal with good property, and cold speed had little effect on the cold heading process. Comparing damage evolution of the two typical bolts in cold heading process, it showed that bainitic non quenched steel can be used in the manufacture of the six flange bolts, but fracture in manufacturing inner six angle cylindrical bolt tended to be greater, the results coincided with the actual situation, which can be used to guide the production of fastener products.This work obtained the inorganic-silane composite passivation process on galvanized non quenched and tempered fasteners through the single variable test and orthogonal test. In this work, silane KH560 and KH570 as the main component of film formed a physical shielding layer obstructing the corrosive substances of the outsides, in addition, the corrosion resistance and self healing property of the passivation film were enhanced by the synergistic effect of the inorganic-organic passivation, the passivation film was prepared by the addition of vanadium, titanate, the zirconium salts, Si O₂ and Ce O₂ nanoparticles, combined with the silane. The performance of silane passivation film was detected and characterized by salt water immersion test, copper sulfate drop accelerated corrosion test, electrochemical test, scratch corrosion test and neutral salt spray test, and its adhesion, passivation film film weight, conductivity, water resistance properties etc were tested. The average thickness of the silane composite passivation film is less than 5μm, the average weight of the film is 194.5mg/m2, the adhesion is level 1, the water resistance and conductivity are good and the coating is compact and uniform.The surface morphology of silane composite passivation film was characterized and analyzed by LSCM and SEM, the molecular structure was characterized by FT-IR, XRD and EDS, and the corrosion resistance of the passive film was detected by Mott-Schottky, EIS, TAFEL and SECM, the mechanism of film formation and corrosion resistance were analyzed, and the mechanism of corrosion resistance was established. The experimental results showed that the main composition of the silane composite passivation film were O, Si, C elements, the structure of the surface layer is the silane crosslinking Si-O-Si structure and some silane hydrolysis substances. The silane film covered on the surface of the zinc layer, barring water and other corrosive medium to the substrate by physical function, and the silane formed Si-O-Zn bonds through the chemical function, and there were a large number of silicon hydroxyl produced, and a complex Si-O-Si structure was formed. The inorganic salts and nanoparticles were added in the network structure of the silane film, the defects and cracks of the silane film were filled, which increased the density of the silane film, gave the film self-healing ability and effectively prevented the zinc matrix from being corroded.