Hydrogen Reduction Galvanizing And Coating Structure Formation Mechanism Of Hot-Rolled Strip Steel

Hot dip galvanized strip is one of the fastest growing type of steel in recent years and widely used in construction,automotive and appliance industry.Traditional production of hot galvanized steel always includes acid pickling.However,the pickling process will reduce the strip metal yield and bring serious industrial pollution.Hydrogen reduction of hot rolled steel sheet directly use the heat during galvanizing and eliminates the separate scale removal process,which can simplify the production process and reduce production costs.With the purpose of development of hydrogen reduction galvanizing technology,this paper studied the hydrogen reduction of hot mill scale and the coating structure formation mechanism during galvanizing.The main contents are as following.The mass transformation process during thermogravimetric test was calculated by mathematical modeling of the oxide scale reduction.The results show that under the given experimental conditions,the gaseous diffusion has little influence on the reaction.In Ar-10%H₂ atmosphere,the reduction underwent a long incubation period at 550oC.Increasing in reaction temperature can effectively reduce the incubation period.However,reaction rate significantly reduced during the later stage of reduction above 600oC.Further incresing the temperature to above 750oC,the reaction rate resumed to increase with temperature.The kinetics of scale reduced in Ar-20%H₂atmosphere in the temperature range of 370-550oC were systemicly studied.The results showed that the controlling mechanism of the reaction is the two-or three-dimensional growth of nuclei.The reaction apparent activation energy has a significant change in the vicinity of the 410oC.Below 410oC,the activation energy is 78.8 kJ/mol and decreased to 31.8 kJ/mol above 410oC.The continuous annealing reduction of oxide scale was studied using thermal simulator.The weight loss results agree well with the Thermogravimetric results.A dense reduced iron formed on the surface of unreduced oxides results in the decreas of reaction rate above 600oC.The study also found that increasing in hydrogen concentration is more effective on the reaction rate at 550oC.According to the feature of oxide scale reduction at low and high temperature,an innovative two-stage soaking process was proposed.The steel was first reduced at low temperature（550oC）for a period of time,and then heated to high temperature（800oC）for a fast and completely reduction.Under the two-stage process,the oxide scale can be totally reduced in 120-150 s in50%H₂ atmosphere.The coating structure formation process of hot-dip galvanizing on the reduced scale was studied.The surface of the reduced scale is loose and porous.Large quantity of Fe-Zn alloys formed in the coating galvanized in low Al content zinc baths.Increasing in bath Al content can inhibit the growth of Zn Fe alloy.Bending tests showed that the adhesion of the coating increases with the decrease of Zn-Fe alloy content and decreases with the increase of the residual oxides.Salt spray test showed that the residual oxides has little effect on the corrosion rate of plate sample.The coating structure formation process of hot-dipped Zn-1.43%Al-1.59%Mg coating on hydrogen reduced scale was studied.The coating composed of Zn,Al and MgZn₂ phases.As the increasing cooling rate,coating structure turn finer and ratio of eutectic structure increase.Under sufficient reduction,the coating adherence of hydrogen reduced specismen is equivalent to the pickled specimen.Neutral salt spray test results show that the Zn-1.43%Al-1.59%Mg coating has excellent corrosion resistance.The coating also has excellent cuts and cracks protection ability.