| In order to eliminate the red scale,the heating procedures simulated that applied in the industrial reheating furnace were used in the present study according to the formation mechanism of red scale.The high-temperature oxidation behavior of low carbon silicon-containing steels was systematically investigated by the thermogravimetric analysis,microstructural characterization,energy spectrum analysis,XRD phase analysis and in-situ observation experiments.In addition,the relationship between the oxygen volume fraction and the passivation behavior of low carbon silicon-containing steel was analyzed.Furthermore,the morphology of the binary eutectic Fe2SiO4/FeO and its formation mechanism were clarified.The effects of beginning oxidation temperature,heating rate and cooling rate on the oxidation behavior of the test low carbon silicon-containing steel were studied,aiming at decreasing the net-like Fe2SiO4/FeO.Moreover,the relationship between the melting temperature of Fe2SiO4/FeO and the content of P and S was determined by in-situ observation.Results indicate that:?1?The phase of each layer in High-1.21Si and Low-0.25Si steels were almost the same,which were determined to be Fe2O3?the uppermost layer?,FeO and a small amount of Fe3O4?the middle layer?,and Fe2SiO4/FeO?the innermost layer?.The bright area is FeO,whereas Fe2SiO4 appears mainly as the dark area.The total thickness of the oxide scale increased with the silicon content.?2?Fe2SiO4/FeO presented graininess morphology when the heating temperature was lower than the melting temperature of Fe2SiO4/FeO,conversely,the Fe2SiO4/FeO showed obvious net-like morphology when the heating temperature was higher than the melting temperature of Fe2SiO4/FeO.?3?Fe2SiO4/FeO apperaed obvious net-like morphology in the High-1.21Si steel when the heating temperature was higher than the melting temperature of Fe2SiO4/FeO;while no obvious net-like morphology was observed in the Low-0.25Si steel.The content of Fe2SiO4/FeO as well as its penetration depth both increased with the silicon.?4?The passivation period in the High-1.21Si steel occurred at the oxygen concentrations ranging from 2.0 vol.%to 3.0 vol.%,whereas no visible passivation period was observed at the oxygen concentrations?1.5 vol.%.Moreover,the duration of passivation period increased with the increasing oxygen concentration.?5?There was no significant effect of oxygen concentration on the normial oxidation temperature?NOT,average value:481?C?during the heating-up period of the High-1.21Si steel.The intense oxidation temperature?IOT?under different oxygen concentration were almost the same and lower than the melting temperature of Fe2SiO4/FeO,illustrating that the the IOT had no apparent relationship with the melting of Fe2SiO4/FeO.The finshining oxidation temperature?FOT?increased with the oxygen concentration during the cooling stage.?6?The oxidation rate of High-1.21Si steel remained unchanged when the temperature reached one certain value.Furthermore,the oxidation rate of the isothermal stage increased linearly with the oxygen concentration,which was attributed to the dynamic equilibrium between the promotion and inhibition effects on ion diffusion.?7?The effect of silicon on mass gain depended on the oxygen concentration and the oxidation time.The mass gain of low-silicon steel was greater than that of high-silicon steel at 1260?C in the oxygen concentrations of 1.0 vol.%and 2.0 vol.%,even when the isothermal holding time was 90 minutes.However,the mass gain curve of low-silicon steel intersected with that of high-silicon steel at 1260?C in the oxygen concentrations of 3.0 vol.%and 4.0 vol.%at an isothermal holding time of 90 minutes.The critical time?crosspoint?was postponed with decreasing oxygen concentration.In addition,the oxidation mass curve of High-1.21Si steel oxidized in a heating process same as industrial furnace conditions followed linear rule.?8?The beginning oxidation temperature had no significant effect on the oxidation mass gain and the penetration depth of Fe2SiO4/FeO when temperature was lower than the intense oxidizing temperature of silicon-containing steel.However,the oxidation mass gain and the penetration depth of Fe2SiO4/FeO both decreased with the increasing of beginning oxidation temperature when temperature was higher than the intense oxidizing temperature.The oxygen atmosphere must be adjusted at the temperature higher than the IOTs because the adjustment of oxygen atmosphere before the IOTs had no significant influence on the oxidation process of silicon-containing steel.Sometimes,weak reduction atmosphere might be used in forepart of industrial furnace to reduce the oxidation.It also demonstrated that the adoption of weak reduction atmosphere before IOTs has no effect on the oxidation mass gain and the distribution of Fe2SiO4/FeO.The ending temperature of weak reduction atmosphere must be after the IOTs.?9?To decrease the ending temperature of the firstly-heating stage and increase the heating rate of the second-heating stage can shorten the standing time during the high-temperature stage of the low carbon silicon-containing steels.Consequently,the net-like morphology Fe2SiO4/FeO was apparently reduced.?10?The thickness of oxide scale formed on low carbon silicon-containing steel was thinnered at a fast cooling rate.Therefore,to increase the cooling rate can reduce the thickness of oxide scale and amount of Fe2SiO4/FeO under the same heating and isothermal holding conditions.Additionally,the solidifying point of Fe2SiO4/FeO decreased apparently from 1169.3?C to 1130.1?C at a higher cooling rate of 300?C/min.The decreasing solidifying point of Fe2SiO4/FeO was beneficial to the descaling because the liquid Fe2SiO4/FeO was easier to be removed.?11?The actual melting temperature of Fe2SiO4/FeO in High-1.21Si steel was about 1170?C,which was close to the theoretical melting temperature of Fe2SiO4/FeO.The melting point decreased with the increasing P and S contents,and the melting point of Fe2SiO4/FeO reduced up to 954.2?C and 1101.3?C when the content of P and S reached 0.115 wt.%and 0.012 wt.%,respectively.The addition of P caused the formation of ternary eutectic compound Fe3?PO4?2/Fe2SiO4/FeO in the silicon-containg area,while the binary compound FeO/FeS with melting point of 940?C fomed in the matrix near the interface between the Fe2SiO4 and matrix.Therefore,it is easy to wipe off the red scale when the descaling temperature is higher than the melting point of Fe2SiO4/FeO by addition of moderate P and S in the steels.?12?type-1 liquid Fe2SiO4/FeO was more likely to form a net-like morphology than type-2 liquid Fe2SiO4/FeO.The penetration depth of type-1 liquid Fe2SiO4/FeO was also larger at the same oxidation degree.Therefore,type-1 liquid Fe2SiO4/FeO should be avoided in order to eliminate red scale defect.Net-like Fe2SiO4/FeO may be alleviated by two methods:decreasing the oxygen concentration in the heating furnace before the melting point of Fe2SiO4/FeO is reached and increasing the reheating rate before the melting point.In addition,FeO is distributed with a punctiform or lamellar morphology on Fe2SiO4. |
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