| The Interstitial-Free Steel has deep drawing property and no time-effect.It can be processed into complex parts and reduce the weight of automobile.It meets the requirements of automobile safety,weight reduction,energy saving and environmental protection.It is widely used in automobile steel plate manufacturing industry.Since IF steel is used from the product to the user,it often has to go through a long period of inventory and turnover transportation.The IF steel surface is prone to dew condensation,and presents rust.Therefore,studying the mechanism of rust defects on IF steel has important practical and theoretical significance for improving the corrosion resistance of IF steel.In this paper,the effect of Mn content and heat treatment on corrosion resistance of oxide film on the IF steel surface was studied by electrochemical method.The metallographic structure,grain orientation,composition and structure of oxide film on IF steel under different conditions were analyzed by means of metallographic microscope,XRD,EBSD and XPS.The formation mechanism of oxide film was explored on the basis of grasping the law of the synergistic effect of alloy elements on the corrosion resistance.Mn content has obvious influence on corrosion resistance of oxidation film on IF steel surface.With the increase of Mn content,the corrosion resistance of the oxide film on the surface of IF steel decreases gradually.The main reason is that with the increase of Mn content,the grain orientation of IF steel becomes more obvious and stable,but the {111}<110>and {111}<112>orientations gradually increase,the large angle grain boundaries gradually extend from the outside to the inside,and the percentage of small angle orientation difference slightly increases,which leads to the texture strengthening of<111>/ND and the precipitation of Mn oxide from the oxide film on the surface of IF steel.When different amount of manganese oxide is contained in the surface oxide of IF steel,the density of acceptor in the oxide film and the density of the inner layer of the electric double layer oxide film are changed,and then the corrosion resistance of the oxide film is affected.Electrochemical analysis shows that Mn content has little effect on the corrosion resistance of IF steel substrate.The wet-heat test shows that the corrosion of IF steel surface is mainly uniform pitting corrosion,and the corrosion resistance of the oxide film of IF steel sample decreases with the increase of Mn content.In the 3.5%NaCl electrolyte,the heat treatment on the corrosion resistance of IF steel substrate is minimal.In the 0.1×10-4 mol/L Na2SO4 electrolyte.After heat treatment,the oxide content of Mn in the oxide film on the surface of the IF steel increases,and the oxide content of Fe decreases sharply.XRD and EBSD analyses show that the grain orientation preference of the template after annealing is not obvious,and the main orientations of the corrosion resistance of IF steel {111}<110>and {111}<112>are reduced.The corrosion resistance of the model is reduced after the heat treatment.The corrosion resistance of the oxide film on IF steel surface of is affected by the composition of different alloys.The electrochemical study shows that the order of corrosion resistance of oxide film on IF steel surface with different alloys composition is DQ-IF>340DQ-IF>SEDQ-IF>340BH-IF.Corrosion resistance of oxide film on IF steel surface is improved with decreasing the content of manganese for IF steel substrate.With the increase of wMn content in the range of 0-0.5%,the corrosion resistance of oxidation film on IF steel surface decreases,but that can be controlled by the synergistic effect of alloying Nb,Al and Ti elements.However,when wMn is above 0.5%,the corrosion resistance of the oxide film on IF steel surface decreases obviously,and adjusting the content of alloying elements Nb,Al and Ti has little effect on the corrosion resistance.Based on the analysis of corrosion resistance of oxide film on IF steel surface,the types and contents of alloying elements affect the soaking temperature of IF steel during heat treatment.When the alloy element Mn content of IF steel is low without Nb element,the soaking temperature of IF steel is higher,for example,the soaking zone temperature of DQ-IF steel with wMn content 0.12%is 830?.However,when the alloy element Mn content of IF steel contained element Nb is high,the soaking temperature of IF steel is lower,which is the soaking zone temperature of 340BH-IF steel with wMn content 0.60%is 780?.Taking the corrosion resistance of IF steel surface oxide as evaluation index,the heat treatment system of 340BH-IF steel was optimized.That is the temperature of soaking zone 780?,Outlet temperature 160?,cooling water temperature normal temperature.And the quality of cooling water is distilled water.As the surface roughness Ra of cold-rolled steel sheet increases,the corresponding self-corrosion potential of the cold-rolled steel sheet first increases and then decreases.When the roughness Ra is controlled from 1.20 to 1.25?m,the surface of the cold-rolled steel sheet has a good corrosion resistance.In the electrochemical pickling process,the oxide film on the surface of the cold rolled steel sheet is more likely to be corroded and dissolved.In the cold-rolled steel sheet with no oxide film protected or poor oxide film compactness,the corrosion resistance reduces as the surface roughness Ra and the peak number Rpc decreases.The value difference between the surface potential of the cold-rolled steel sheet and the potential at the cracked oxide film are related to the occurrence of a "flat yellow spot".The smoothing flat yellow spot defects are mainly the oxidative corrosion,and the state of the oxide film on the surface affects the corrosion resistance of the steel plate.The corrosion resistance of the non-degreased and degreased annealed sheet is better than that of the flat surface of the steel sheet where the oxide film is cracked.Under the action of the leveling liquid,the passivation occurred on the surface of the cold-rolled steel sheet and then a new oxide film formed.Different additives have different effects on the film formation rate on the surface of the cold rolled steel sheet.An addition of the phosphate and the molybdate can effectively improve the film formation rate of passivation film on the surface of cold rolled sheet.With the increase of the silicate concentration in the leveling solution,the film formation rate of the passive film on the surface of the cold rolled steel sheet first increases and then decreases.When the silicate concentration in SP3F/A leveler was 6.3%,the film formation rate is faster and good corrosion resistance.Based on heat treatment,alloy elements and the modification of the surface physical state of IF steel,the corrosion resistance of oxide film on IF steel surface can be improved,which improve the market competitiveness,but also to provide a valuable reference for designing the new IF steel. |
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