Research And Application Of Hot Dip Galvanizing Of Hot-rolled Steel Strip Without Pickling

Due to the high surface quality and good corrosion resistance,hot dip galvanized steel strip and plate is widely used in building,household appliance and automobile.However,the conventional hot dip galvanizing process has a series of deficiency.Acid pickling,as an important step of the original plate preparation,create environment pollution and increase the cost.At the same time,for the high-strength steel,because of the selective oxidation of manganese and silicon during reduction annealing,the oxidation products exacerbate the wettability of the matrix and create bare spots on the galvanized steel plate.To solve this problem,the steel strip is usually pre-oxidized before reduce annealing,the pre-oxidation increase the cost and reduced the production efficiency.Thus the hot dip galvanizing process need to reduce energy consumption and reduce pollution emissions to achieve short process production.In the present paper,a novel processing method for producing zinc-coated steel was designed and tested,in which the pickling step and pre-oxidation are skipped.This short process can increase efficiency and reduced cost.At the same time,bare spots caused by selective oxidation could be eliminated effectively.Start with the controlling of hot rolled oxide scale,cold-rolling with oxide scale,phase transformation of oxide scale during heating,reduction of oxide scale in low concentration hydrogen and galvanizing of hot-rolled steel strip without pickling were investigated.The main original works and innovative achievements of this dissertation are as follows:1.High temperature oxidation of low carbon steel was studied by using Setsys Evolution 1750 thermal gravimetric analyzer(TGA).It was found that the oxidation kinetics initially followed the linear law at the early stage of the reaction.When the oxide scale grows to a certain thickness,the oxidation kinetics starts to deviate from the linear rate law,and the oxidation will follow the parabolic law.The oxidation activation energy of the tested low carbon is about 204.097 kJ-mol-1.Using the isothermal transformation data,the phase transformation of oxide scale during cooling was simulated calculated.2.Evolution of oxide scale on hot-rolled low carbon steel strip in cold rolling was investigated by cold rolling without pickling on an experimental 4-high mill.It was found that micro-cracks formed in the oxide scale near the entry to the roll bite,running perpendicular to the rolling direction.Density of cracks increases near the bite entry.If the reduction of single pass is lower than 16%,only cracking happened in the oxide scale.When the reduction is higher than 16%,interface shear stress is likely to beyond the adhesive strength of oxide scale,spalling and powering will occur,and the oxide scale will be broken seriously.The critical reduction of single pass increase to 23%when the rolling lubrication oil was used.Under the same total reduction condition,multi-pass rolling is favor to protect the oxide scale.3.The phase transformation of oxide scale during heating was systematically investigated by using differential scanning calorimeter(DSC)and thermal simulation experiment.During heating,eutectoid structure transformed to wustite in reverse when the temperature was higher than 570?,followed by the reverse transformation of magnetite.The transformation of eutectoid structure is happened mainly in the temperature range of 590 ?640 ?.While the transformation of magnetite continue to 750 ?.Due to the difference between the density of wustite and magnetite,a lot of pores distributed in the formed wustite.4.Reduction of oxide scale on hot rolled steel strip in low concentration was systematically investigated by TGA,and the structure of reduction products was analyzed by using electron probe x-ray micro-analyzer(EPMA).There are many pore through the total oxide scale,which provided the passageway for the hydrogen,furthermore these pores increase the surface areas for reduction.Thus cold rolling can promote the reduction effectively.Reduction of oxide scale in hydrogen has the general characteristics of gas solid reaction.The whole reaction process is distributed into three stages:induction period,acceleration period and deceleration period.The partial pressure of hydrogen effect the reduction rate directly.The reduction in high partial pressure of hydrogen has a shorter induction period and a higher reaction rate.The kinetic plots of the isothermal reduction of oxide scale were fitted using mathematical models.Phase-boundary-controlled reaction and three-dimensional growth of nuclei was found to be the controlling mechanism for the reduction at 800? and 500?,respectively.Reduction product is varied dependent on reduction temperature.When the oxide scale is reduced at low temperature(lower than 600?).the product is porous iron;while the product is dense iron when the reduction temperature is higher than 600?.At the early stage of the reduction,the reduction rate increase as temperature increase.However,after reduction for 6 minutes,the reduction at 600-700 ? slow down,and a "rate-minimum" occurred in this temperature range.While the outlayer dense iron acting as a barrier against the gas diffusion was responsible for the observed "rate-minimum" at 700 ?.5.The galvanizing of hot rolled steel strip without pickling was investigated in laboratory.The results show that the structure of zinc coating is effected by reduction process,the zinc liquid composition,the residual oxide scale and the surface state of the substrate.When the reduction is carried out at temperature lower than 600 ?,the reduction products is porous iron,which increase the roughness of the substrate.If the content of aluminum is lower,? phase and? phase will formed in the coating,these iron-zinc intermetallic phase deteriorate the formability of the coating.When the reduction is carried out at high temperature,the reduction products is dense iron,and the substrate have a relatively good surface quality,however,because of the hot rolled steel strip have a rough surface,the content of aluminum need to be improved to 0.6%.The residual oxide scale on the surface of the reduced steel strip acts as a physical barrier that inhibits Fe and Zn reactions.The inhibition effect of the residual oxide scale was found to be similar to that of the Fe2Al5 layer.According the research finding in laboratory,the manufacture of hot-dip galvanized steel strip without hydrogen descaling was carried out in industrial condition.The products have a good surface quality,and the coating meet the "2T" bending requirements.