| As an important basic industry of the national economy,the green and lowcarbon transformation and high-quality development of my country’s steel industry play an important role in the country’s carbon dioxide emissions peaking before 2030 and the realization of carbon neutrality before 2060.Although my country’s hot-rolled steel plates have basically met the urgent needs of various fields of national economic construction,some products are difficult to meet the needs of the high-end market due to problems such as easy surface corrosion.The whole process of hot-rolled steel plate production is carried out at high temperature,and the formation of oxide scale from the heating to the coiling stage is carried out throughout.In-depth research on the influence of alloy composition,heating,rolling and cooling process parameters on the microstructure of oxide scale and related mechanisms is crucial to breaking through the key common technologies for accurate surface quality control of hot-rolled products.In this paper,three typical hot-rolled steel sheets of ordinary carbon steel(Q355),high-strength steel(Q690)and weathering steel(S450AW)are used as the research objects,the high temperature oxidation behavior during heating was systematically studied by means of thermogravimetric analyzer,scanning electron microscope,electron probe,Raman spectrometer,X-ray diffraction analyzer and other experimental methods.The high temperature oxidation mechanism of the experimental steel was revealed by the measurement of the bonding force between the iron oxide scale and the steel substrate.The effects of different deformations during rolling on the microstructure and texture of the oxide scale on the surface of the steel sheet were studied,and the structure type and crystallographic data of the oxide scale were obtained.From the point of view of the relationship between the coincident site lattice and grain boundary type and the integrity of the oxide scale,the orientation relationship and the formation mechanism of the oxide phases of different structural types in the oxide scale are revealed.The effects of coiling temperature and cooling rate on the eutectoid transformation of FeO in oxide scale were studied,the types of Fe3O4 grain boundaries were characterized,and their effects on corrosion resistance were revealed.The results of the study are as follows.(1)When heated in air and CO2+O2 atmosphere respectively,the thickness of oxide scale on the surface of Q355,Q690 and S450AW steels increases with the increase of heating temperature.At the same heating temperature,Q355 steel has the thickest oxide scale,followed by Q690 steel,and S450AW steel is the thinnest.Since the content of alloying elements in the three experimental steels increases in turn,the enrichment of alloying elements at the interface hinders the transport process of iron ions and oxygen ions,and inhibits the high-temperature oxidation behavior of steel plates.The thickness of the oxide scale on the surface of the steel plate under the air atmosphere is higher than that under the CO2+O2 gas condition,the addition of CO2 gas reduces the oxidation driving force of the steel plate,and the oxidation rate constant is lower than that under the air condition,which improves the high temperature oxidation resistance of the steel sheet.(2)The bonding force between the oxide scale and the substrate of the three experimental steels under different heating processes was measured by the drawing method.The results show that with the increase of the heating temperature,although the thickness of the oxide scale increases gradually,its bonding force with the substrate first decreases and then increases.When the heating temperature is 900～1150 ℃,with the increase of the heating temperature,the difference between the elastic modulus of the oxide scale and the substrate micro-region increases with the increase of the temperature.The larger the difference,the poorer the ability of coordinating deformation of the oxide scale interface,resulting in a gradual decrease in the bonding force between the oxide scale and the substrate.When the heating temperature is 900℃,the difference between the elastic modulus of the steel matrix interface and the oxide layer is 36 MPa,the difference is 49 MPa at 1100℃,and the difference is as high as 90 MPa at 1150℃.When the heating temperature rises to 1200℃,the bonding force between the iron oxide scale and the matrix increases slightly.This is because the mixed phase of Fe2SiO4/FeO and FeCr2O4/FeO rises to the melting point and then melts,and the liquid phase immerses into the matrix under the action of external force.The metal lattice voids disperse the external stress and inhibit the peeling of the oxide scale.(3)The microstructure of oxide scale on the surface of high-strength steel Q690 and weathering steel S450AW under different rolling conditions was studied by EBSD analysis,and the grains of Fe3O4 and Fe2O3 phases in the scale were reconstructed.The results show that the Fe3O4 phase in the oxide layer has a twophase heterogeneous morphology,Fe3O4 in the oxide layer comprises columnar grains,and Fe3O4 near the substrate comprises spherical grains.When the rolling reduction rate is 10%,the oxide scale remains basically intact,and the Schmitt factor of Fe3O4 in the {114}<110>direction is 0.4945.When the rolling reduction ratio increased to 18%,the oxide scale had been partially cracked,and the{441}<110>Schmitt factor of Fe3O4 decreased to 0.4621.At this time,the surface Fe2O3 grains are embedded in the Fe3O4 layer.When the reduction rate is 27%,the oxide scale still shows cracking phenomenon,and the Fe3O4 grains in the{112}<112>direction have the lowest Schmitt factor value of 0.4389.When the grains are plastically deformed,the high Schmitt factor grains undergo plastic deformation before the low Schmitt factor grains.By introducing the Schmitt factor,the evolution behavior of texture during high temperature plastic deformation is elucidated.(4)The texture evolution behavior of Fe3O4 and Fe2O3 phases of oxide scale on the surface of Q690 steel under different rolling conditions was studied by EBSD analysis technology,and the relationship between surface cracks of oxide scale and special grain boundaries was established.The results show that as the rolling reduction ratio increases from 10%to 27%,the proportion of small-angle grain boundaries in the oxide scale increases from 28.6%to 38.9%.Fe3O4 formed a γtexture parallel to the oxide growth direction,while Fe2O3 formed a {0001}<1010>fibrous texture.The increase in the proportion of low-angle grain boundaries in the oxide scale and the existence of lattice grain boundaries at low-dimensional coincidence positions,such as the 60°/<111>(Σ3a)grain boundary of Fe3O4,and the 59.5°/<1210>(Σ13b)grain boundary of Fe2O3 and 90.5°/<0110>(Σ19b)grain boundaries,which hinder the formation and propagation of cracks.(5)The phase transformation mechanism of iron oxide scale on the surface of Q690 steel and S450AW steel at different coiling temperatures and cooling rates was studied by thermal simulation experiments.The results show that the eutectoid transformation process of Q690 steel and S450AW steel follows the "C" curve relationship.The nose tip temperature of the eutectoid transformation of the oxide scale on the Q690 steel surface is 420～510 ℃,and the nose tip temperature of the oxide scale on the S450AW steel surface is 420～480℃.For S450AW steel,when the coiling temperature decreases from 610℃ to 410℃,the proportion of Σ3 andΣ13b grain boundaries in cubic Fe3O4 first increases and then decreases.When the coiling temperature is 510℃,the Fe3O4 low-dimensional coincident site lattice Σ3 and Σ13b grain boundaries account for the highest proportion.These lowdimensional coincident site lattice grain boundaries can be used to enhance crack resistance and improve the oxide scale on the surface of the steel plate.(6)The corrosion resistance of oxide scale on the surface of S450AW steel with different structural types was evaluated by electrochemical experiments and salt spray experiments.The results show that,compared with the steel substrate samples,the corrosion resistance of the samples with oxide scale is better than that of the samples without oxide scale.The self-corrosion current of the sample with oxide scale moves to the left,and the self-corrosion potential moves to the positive.The self-corrosion current of Type Ⅰ(Fe3O4 phase at the interface)sample is higher than that of Type Ⅱ(FeO phase at the interface)sample.The porosity of Type Ⅱspecimens(15.9%)is lower than that of Type Ⅰ specimens(25.8%)through the measurement of porosity,indicating that Type Ⅰ specimens are more prone to corrosion.The corrosion weight gain law of Type Ⅰ and Type Ⅱ samples was studied by salt spray experiments.The results showed that the corrosion weight gain trend of the samples was parabolic,and the corrosion weight gain trend within 48 h was larger,which is the chemical reaction controlled by the interface;after 48 h,the reaction is controlled by diffusion.The corrosion weight gain trend of Type Ⅱspecimens is lower than that of Type Ⅰ specimens,indicating that Type Ⅱ specimens have better corrosion resistance.In summary,by optimizing the heating process parameters,the bonding force between the oxide scale and the substrate is reduced.During the rolling process,the proportion of low-angle grain boundaries and special grain boundaries of the oxide scale is increased by adjusting the reduction ratio.During the coiling process,the proportion of the lattice at the special overlapping position is increased by adjusting the coiling temperature and cooling rate.It can provide theoretical basis for the control of the structure type of oxide scale at different stages,and provide technical support for the optimization of the whole process. |
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