| In the 21st century,resource,energy and environment have become a priority theme for the development of steel industry in China,which is the key to support its sustainable development.In the current steel production process,acid pickling is widely used in steel enterprises as a process to remove the oxide scale on steel strip,but the waste acid produced seriously damages the ecological environment.At the same time,in order to ensure the effect of pickling and improve the production efficiency,a large amount of strong acids has been widely used,such as "hydrochloric acid" and "sulfuric acid",which causes the surface defects of "over pickling" and "lack pickling",and then seriously affects the quality of the products.Therefore,in the face of the adjustment of the national macro policies and in order to adapt to the basic policy of energy conservation and emission reduction in China,the downstream production enterprises urgently need the steel products with "easy pickling" and even "free pickling" which can alleviate the destruction of waste acid emissions to the ecological environment.With the development of freight logistics and the increase of transportation cost,the demand for heavy truck load is increasing gradually in the downstream automobile factory.The strength level and thickness specification of steel materials are constantly improved.Thepowder oxides is serious after the stamping forming steel plate which is prepared by the original "free pickling" steel control technology.It can not meet the requirement of "free pickling" steel.The optimization of thermo mechanical control process is the key to solve the problems which coordinate the guarantee of the mechanical properties of steel and the achievement of "free pickling" production of high-strength steel.In addition,according to hot dip galvanizing of hot-rolled steel strip without pickling,it is the premise for popularizing and applying this technology to improve the reduction efficiency,the coating structure and the corrosion resistance of the product.In view of the core issues in the industrialization process of hot-rolled "free pickling"steel and the technology of hot dip galvanizing of hot-rolled steel strip without pickling,this paper focuses on the high temperature characteristics of oxide scale and the transformation rule of coating structure.The key problems of the effect of Cr element on growth mechanism of oxide scale,the deformation coordination of oxide scale and substrate at high temperature,the structure transformation rule of oxide scale,the efficient reduction mechanism under the H2 condition,the composition of zinc liquid under the technology of hot dip galvanizing of hot-rolled steel strip without pickling,are investigated.The main results of this paper are as follows.(1)The oxidation behavior of experimental steel at 1050?1300? in dry air was studied,and the oxidation kinetics model was established,which indicated that the oxidation kinetics curves of the experimental steel was in a straight line at the early stage of oxidation,but the oxidation kinetic curves transformed parabolic law with the prolongation of oxidation time.The oxidation rate obviously decreased with the increase of Cr content,the complete layer of FeCr204 formed at the surface of substrate hindered the ion diffusion,which greatly improved the high temperature oxidation resistance of the experimental steel.(2)The high temperature oxidation behaviors of the experimental steel at 1050? under different wet atmosphere conditions were studied.The results indicated that the reaction between water vapor and FeCr2O4 generated a volatile hydroxide CrO2(OH)2 in the wet atmosphere.A large number of pore defects formed at the scale/substrate interface,which destroyed the protection of the FeCr2O4 layer and accelerated the high temperature oxidation rate of the experimental steel.Additionally,the oxidation rate increased with the increasing of water vapor volume fraction in the wet atmosphere.(3)The final surface quality of the product was determined by the condition of the oxide scale on the surface of hot rolled steel.In view of the problem of surface quality control,the deformation coordination of the oxide scale on the experimental steel during the thermal deformation process was systematically studied.The results indicated that the high temperature plasticity of the oxide scale was directly determined by the deformation temperature.The oxide scale can be deformed approximately equal to the substrate when the deformation was 10%at 950?1050?,which ensured the completeness of the oxide scale after hot deformation.Additionally,the transformation behavior of FeO in oxide scale during isothermal process was systematically studied by isothermal transformation experiment,and then the TTT(Time,Temperature,Transformation)diagram of oxide scale was established.The main temperature range of eutectoid transformation was 300?450?,and the temperature of the eutectoid transformation at the tip of the nose was 400?.In addition,the final structure was determined by the original structure of oxide scale after isothermal transformation.The pore defects formed in the oxide scale grown in wet atmosphere,which inhibited the growth of eutectoid structure,and then caused the C-type curve in TTT diagram to shift to the right.(4)Aiming at the low reduction rate of oxide scale in the technology of hot dip galvanizing of hot-rolled steel strip without pickling,the reduction reaction of oxide scale on experimental steel in the mixed atmosphere of 20%H2-Ar was studied.The reduction kinetic curves of heating and isothermal reduction were established,and the influence rule of oxide scale condition on the reduction efficiency was obtained.The results indicated that a large number of cracks formed inside the oxide scale after the pretreatment process,which provided channels for gas diffusion and promoted the reduction process.At the same time,the mechanisms of reduction at different temperatures were obtained.At low temperature,the"gas-solid" phase interfacial reaction was controlled by the diffusion of ions and electrons in the solid phase,and the reduction product was porous iron.The "gas-solid" phase interfacial reaction changed to be controlled by the nucleation and growth of new phase with the increase of reduction temperature.The reduction product formed a dense pure iron layer on the surface,which hindered the direct contact between gas phase and solid phase.(5)Aiming at the characteristics of the hot dip galvanizing of hot-rolled steel strip without pickling,the effect of Al mass percent in zinc liquid on the microstructure,properties and corrosion resistance of alloy coating was investigated.The results indicated that a complete Fe2Al5 suppression layer formed at the interface with the increase of Al mass percent,which reduced the formation of the brittle phase of Fe-Zn and improved the plasticity of the coating.At the same time,the microstructure of the coating changed from pure ?-Zn phase to lamellar Zn-Al eutectic solid solution with ?-Al phase and ?-Zn phase alternately distributed with the increase of Al mass percent.The formation of lamellar structure increased the number of grain boundaries in the coating,thereby improving the surface hardness of coating,which presented obvious grain boundary strengthening when subjected to external force.The ?-Al phase with negative potential was first corroded and a compact passivation film was formed on the surface when the coating was subjected to electrochemical corrosion,which significantly improved the corrosion resistance of the alloy coating.(6)In combination with the above research,a trial production of hot rolled "free pickling"process was carried out.Aiming at the thick specification(? 8 mm)and high strength(?700 MPa)QStE650TM automobile beam steel,the composition design scheme of "increasing Cr,reducing Si,reducing Nb" was proposed.Through the optimization of hot rolling process,the thickness of oxide scale was reduced effectively,and the eutectoid transformation of FeO was suppressed.During the stamping process,the amount of oxide powder on the surface of steel strip significantly decreased,and the application requirement of "free pickling" steel was achieved.The related technology has been successfully popularized in many hot rolling production lines of domestic iron&steel enterprises,more than 6000 tons production have been produced,and great application results have been achieved.According to the technology of hot dip galvanizing of hot-rolled steel strip without pickling,the microstructure uniformity and adhesion of the coating were greatly improved by the integrated control of the pretreatment process,the reduction process of oxide scale and the composition of zinc liquid.The surface quality of the hot dip galvanized sheet through "free pickling" technology was great,which reached the requirement of the forming performance of 180° bending of coating without cracking and shedding. |
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