Effect Of Pretreatment On Iron Scale Reduction

Entering the 21st century,the development of China's steel industry is dominated by the coordinated allocation of"resources,environment and energy".There are many kinds of iron and steel products,among which galvanized sheet surface quality has good wear resistance and corrosion resistance performance,wide application fields,the market demand is great.The pickling in the traditional hot galvanized sheet production process will produce a lot of waste acid,which will pollute the environment and increase the production cost.In the face of the above problems,the new process of reducing hot dip galvanizing without pickling was studied at home and abroad.The technological process is:hot rolling-pretreatment-reduction annealing-hot dip galvanizing.The new process eliminates pickling,which is more environmentally friendly and efficient.Therefore,according to the process link of hot-rolled strip reduction without pickling,the pretreatment and reduction process in the process link are targeted.In this paper,SPHC low carbon hot rolled steel plate was used as the experimental material,and the experimental material was pretreated firstly.(1)Leveling simulation experiment to study the deformation behavior of the iron scale under the cold rolling with a pressure reduction rate of 8%;(2)The wet spray pretreatment experiment was designed to study the morphology characteristics of the iron scale after treatment;At the same time,prepare for the follow-up experimental research.The smooth-treated and un-smooth-treated samples were subjected to thermogravimetric analysis(TG)for reduction kinetics experiment under the atmosphere of 30%hydrogen and nitrogen mixture,and the influencing factors in the reaction and reduction kinetics study were compared and analyzed.In order to study the reduction law and the change law of the oxidized iron sheet at different reduction temperature and different reduction time after wet spray treatment,the reduction experiment under the atmosphere of 30%hydrogen and nitrogen mixture was carried out by vacuum tube furnace.The reduced samples were examined by scanning electron microscope(SEM)on the surface and section.The conclusions are as follows:When the SPHC hot rolled steel plate was smoothed under 8%pressure,the surface crack direction was perpendicular to the rolling direction,the crack size was uniform,the surface of the iron scale did not flake off,and the surface quality reached the state of only cracking but not peeling.After the wet spray pretreatment,some micro-cracks will appear on the surface of the oxide scale,with irregular distribution,small number of cracks,different sizes of cracks,not obvious extending in all directions,and the existence of some pits will cause the surface to be slightly uneven.Reduction kinetics experiment results show that the reaction temperature of hot rolled iron sheet in 30%H₂concentration of hydrogen and nitrogen mixture at500-900?has the characteristics of gas-solid reaction.The reaction will go through four stages:induction period,acceleration period,deceleration period and reduction end period.Among them,the induction period will become shorter with the increase of temperature.The reduction reaction of SPHC steel treated by wet spraying process on the surface of vacuum tube furnace was concentrated on the oxide surface at 500?.The reduction reaction occurred in the iron oxide cortex at 600?,and the reduction product was porous iron.The reduction products at 700?were porous iron and dense iron,and the extension of reduction time was conducive to the transformation of porous iron into dense iron.The reduction product at 800-900?is mainly dense iron.