| In recent years,steel strips have developed towards zero surface defects and high dimensional accuracy directions,which improves the requirement for lubrication technology during hot rolling process.The surface defects and material loss associated with surface oxidation of steel strips also restrict the development of high-quality products.Nowadays,composite nanoparticles obtained through chemical synthesis or other means with excellent properties,which are then dispersed as composite nanofluids,have provided a new route for the development of high-performance lubricants.Based on their good film-forming ability and application in anti-corrosion coatings,the spread of nanoparticles on hot rolled strips can inhibit the oxidation of metal,and even diffuse into the matrix to improve surface properties of rolled steel strips.Therefore,the synergistic lubrication mechanism of composite nanofluid was studied in this paper.And the surface-effect of nanofluid for steel hot rolling including surface quality improvement,metal oxidation inhibition and diffusion of nanoparticles into steel matrix was revealed.Water-based MoS2-Al2O3 composite nanofluid with superior dispersion stability,wettability and tribological properties was prepared by solvothermal method as lubricant for steel hot rolling.At the optimum concentration of 2 wt.%,the PB value and average friction coefficient were 697 N and 0.073.This was attributed to the interfacial physical and tribochemical reactions between nanofluid and metal surface during friction processes.A double layer tribofilm in the thickness of about 23 nm composed of adsorption film and chemical reaction layer was formed.The adsorption film on the top contained ultrafine Al2O3,MoS2 particles and amorphous substance,and the reaction layer at the bottom mainly composed of Fe3O4,Fe2O3 and Fe2(SO4)3.Molecular dynamics(MD)simulations showed that the Al2O3 in MoS2-Al2O3 nanoparticles moved with 91%rolling and 9%sliding motion,and the interlayer sliding of MoS2 transformed 72.3%of the friction on metal surface into intra-lamellar friction.The diffusion of S atoms to Al2O3 particle promoted its rolling motion and prevent it from embedding into softer Fe matrix.The rolling of Al2O3 in turn enhanced the interlayer sliding of MoS2.The combined effect of above factors contributed to the excellent lubrication performance.The application of MoS2-Al2O3 nanofluid could reduce the rolling force,roll rebound and rolled surface roughness by 26.9%,35.7%and 25.9%,respectively.MoS2-Al2O3 particles decomposed and oxidized to MoS2,Al2O3 and MoO3 on high temperature surface.These particles reduced the friction and wear in hot rolling process through ball-bearing,interlayer sliding,polishing and mending effect.At the same time,the material and energy transfer between strips surface and nanofluid induced a series of microstructure evolutions.The grain size of substrate and oxide scale decreased obviously.The deformed grain proportion and local misorientation reduced,indicating alleviation of residual stress and deformation.Al2O3 mainly deposited and adsorbed on hot rolled surface,and MoS2 diffused along the thickness direction to form FeS and FeMo4S6 phases.Through transition state search in quantum chemical calculation,Mo and S atoms in MoS2 migrated to Fe lattice through substitutional and interstitial diffusion to form FeS and FeMo4S6 solid solutions.The diffusion barrier was 0.84 eV and 0.54 eV,respectively.Then,using a combination of MD simulations and response surface method,a multivariate nonlinear mathematical model,dc=f(T,p,t),was established for the diffusion depth dc of Mo and S atoms along rolled strips surface with the variation of temperature T,interfacial pressure P and diffusion time t.The deposition layer with a thickness of 193 nm formed by Al2O3 particles.Due to the physical adsorption and penetration barrier of Al2O3 to oxidizing molecules,the MSD in final diffusion state of O2 and H2O molecules at 1300℃decreases by 20.7%and 43.7%,respectively.And the activation energy of oxidation reaction increased from 123.6 kJ·mol-1 to 141.8 kJ·mol-1.As a result,the high temperature oxidation of strip steel during hot rolling process was significantly suppressed that the average oxide scale thickness decreases from 60.4 μm to 39.5μm,and the proportion of high-valence oxide(Fe2O3)also decreased.The denser oxide and diffusion layer on rolled strip surface induced by MoS2Al2O3 nanofluid reduced the electrochemical corrosion rate in NaCl solution by about 91.2%.The mechanism was Cl--dominated oxygen absorbing corrosion with Fe3O4,Fe2O3 and FeOOH as corrosion products.The adsorption and diffusion behaviors of H2O,Cl-,H3O+and OH-on crystal surfaces were analyzed by MD simulations.Al2O3 crystal had the strongest adsorption effect to corrosion media,and Al2O3 and FeMo4S6 exhibited stronger barrier effect on their diffusion.Besides,the reduction of surface defects reduced the migration rate of corrosion media to Fe matrix and avoided local concentration distribution of charges.The alleviation of residual stress and deformation of rolled surface weakened the pitting sensitivity of steel strips,contributing to the enhancement of corrosion resistance. |
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