Study On Reduction Annealing And Hot Dip Galvanizing Of Low Carbon Steel Scale

The steel used in this experiment is SPHC steel.Thermogravimetric analyzer was used to study the reduction kinetics of the smoothed pre-treated experimental steel.In the mixture of 5%and 15%hydrogen and nitrogen,isothermal reduction was conducted for60min at a temperature ranging from 500℃to 900℃.The laws of the oxidation scale reduction reaction of low carbon steel with different hydrogen concentrations changing with time and temperature were found out.The surface and section morphology of the reduced iron scale were observed by scanning electron microscope.The reduction products and mechanism of iron scales at different hydrogen concentrations and reduction temperatures were studied.SPHC steel was treated by spray pretreatment and leveling pretreatment respectively,and no pickling reduction hot dip galvanized trial production is in the industrial environment.Then the surface and section morphology of the coating were observed by scanning electron microscope,and the adhesion of the coating was tested by bending experiment.The experimental results are as follows:According to the reduction kinetic curves of the iron scale on the flat sample surface at 5%H₂and 15%H₂and 500℃to 900℃,respectively,the influencing factors of reduction reaction are composed of phase boundary controlled reaction,nuclear nucleation and growth and diffusion,among which,the phase boundary controlled reaction,nuclear nucleation and growth are the main controlling factors.The reduction product was porous iron,and the reduction temperature was 500℃and 600℃,and the reduction mechanism is that H₂diffuses from the outer layer of the iron scale to the inner layer.When the reduction temperature is 700℃,it is the transition stage of porous iron to dense iron,and porous iron coexists with dense iron.At 800℃and900℃,the reduced product is dense iron,with the increase of temperature or the decrease of H₂concentration,the grain size of reduced products will increase.The reduction mechanism is that a dense reduced iron film is first formed in the outermost layer of the iron scale.The iron film prevents the continuous diffusion of hydrogen and the reduction reaction is carried out outwardly at the interface.There is a thick oxide sheet at the interface between the flat galvanized sheet and the substrate,which leads to poor adhesion between the zinc layer and the substrate.After the first and second injection pretreatment,the residual oxide scale at the interface between the zinc layer and the substrate is relatively thin,so the adhesion between the zinc layer and the substrate is good.At the same time,the oxide sheet at the interface can prevent the alloying of zinc layer and steel matrix and avoid the generation of Fe-Zn composite phase.