The oxidation kinetics and composition of oxide scales on low carbon steel(SPHC)were studied during isothermal oxidation.Thermogravimetric analyzer(TGA)was used to simulate isothermal oxidation process of SPHC for 240min under air condition,and the temperature range was from 500to 900 ℃.Scanning electron microscope(SEM)was used to observe cross-sectional scale morphology and analyze composition distribution of oxide scales.The morphology of oxide scale was classical three-layer structure.Fe2O3developed as whiskers at the outermost layer,and interlayer was perforated-plate Fe3O4 while innermost layer was pyramidal FeO.From the oxidation curves,the oxidation mass gain per unit area with time was of parabolic relation and oxidation rate slowed down.On the basis of experimental data,the isothermal oxidation kinetics model was derived and oxidation activation energy of SPHC steel was 127.416kJ/mol calculated from kinetics data.
The influences of coiling temperature and cooling condition on structural transformation of the hot-rolled tertiary oxide scale formed under continuous cooling conditions were studied by thermal gravimetric analyzer.The fourth oxide scale formed under different conditions were classified and plotted.Because the oxide scale structure transformation is diffusion-controlled and the transformation law is similar to "C" curve,the eutectoid transformation nose temperature is 450 °C.Under condition of low temperature and high cooling rate,ion diffusion behavior is restricted so that the eutectoid reaction is suppressed,resulting in that the fourth oxide scale is mainly made up of pre-eutectoid Fe3O4 and Fe O without eutectoid products.From scale structure transition diagram,the eutectoid reaction process was affected by coiling temperature and cooling rate,leading to various scale structures.
Guang-ming CAOTeng-zhi WURong XUZhi-feng LIFu-xiang WANGZhen-yu LIU
A new hot-dip galvanizing method was employed on hot-rolled low carbon steel.The effects of Al contents on microstructure,micro-hardness and corrosion resistance of Zn-Al alloy coatings were systematically investigated.Phase composition,microstructure and element distribution in Zn-Al alloy coatings were analyzed using X-ray diffraction(XRD)and electron probe micro analysis(EPMA),respectively.It is found that Al content(0.6-6.0 wt.%)in galvanizing zinc affects surface quality and adhesion between coatings and matrix in the newly developed method.In addition,with increasing Al content,micro-hardness significantly increased due to the increase in Zn-Al eutectoid phases.Potentiodynamic polarization and electrochemical impedance spectroscopy(EIS)also revealed that increase in Al plays a noticeable role in improving the corrosion resistance of Zn-Al alloy coatings.
Zhi-feng LiYong-quan HeGuang-ming CaoJun-jian TangXiang-jun ZhangZhen-yu Liu
采用热重法研究了中铬铁素体不锈钢的高温氧化行为,测定出不同温度条件下的氧化增重曲线,其氧化增重符合抛物线规律,由于溶质铬元素的选择性氧化形成致密的氧化层阻碍离子扩散造成实验钢的氧化激活能较高为560 k J/mol。通过氧化铁皮表面形貌观察,在氧化铁皮表面生成部分异常长大的突起铁-铬尖晶石相,并随着氧化温度升高,有增多和长大趋势。当氧化温度为1100℃时,氧化铁皮出现了分层,氧化铁皮最外层为富铁的氧化物(Fe O和Fe3O4为主),将突起铁-铬尖晶石相包裹住。结合热轧粘辊实验结果表明,突起的铁-铬尖晶石容易划伤轧辊表面形成裂纹源,在后续循环轧制过程中急剧的冷热循环和轧制变形,促进了裂纹形成和扩展,为热轧粘辊提供了形核地点。高温轧制促进表面富铁的氧化物的生成,消除突起铁-铬尖晶石相可有效的降低热轧粘辊。
Because of the effect of silicon on the formation of oxide scale,red scale is the main surface defect of hotrolled Fe-Si plate,making the scale difficult for descaling compared with carbon steel.Thermogravimetric analyzer(TGA)is used to simulate isothermal oxidation process of Fe-1.5Si alloy for 60min under air condition,and the temperature range is from 700to 1 200℃.Electron probe microanalysis(EPMA)is used to observe cross-sectional scale morphology and analyze elemental distribution of the scale.Relational graph of temperature,scale thickness and scale structure is obtained.It is found that scale structure(outer Fe oxide layer+inner FeO/Fe2SiO4layer+internal Si oxide precipitates)is almost unchanged with temperature except at 1 000and 1 200℃.At 1 000℃internal Si oxide precipitates cannot be found at the subsurface of the alloy,and at 1 200℃FeO/Fe2SiO4not only forms a layer as usual but also penetrates into the outer Fe oxide layer deeply.
LIU Xiao-jiangCAO Guang-mingHE Yong-quanJIA TaoLIU Zhen-yu
Hot-rolled Fe-(0.75-2.20)Si(mass%)alloys were oxidized in dry air at 600-1200 ℃.The oxidation process was carried out by thermal gravimetric analysis(TGA).At 600-1150 ℃,oxidation gain curves were approximately parabolic.Electron probe microanalysis(EPMA)was applied to investigate cross-section morphology of oxide layer and element distribution across the layer.At lower temperature of 700℃,the oxide layer consisted of internal oxidation zone(IOZ),inner Si-rich layer(conglomerate of fayalite and magnetite)and outer hematite layer,while at higher temperature of 1200℃,fayalite and wustite were observed in external oxide scale.Liquidus temperature of fayalite was detected by differential scanning calorimetry(DSC).Through comparing the oxidation mass gain and parabolic rate constant of the alloys,it was found that oxidation resistance of Fe-Si alloy was enhanced by increasing Si content below 1150 ℃ while increasing Si content of the alloy resulted in higher oxidation rate above 1150 ℃ owing to the liquid fayalite formation.
Xiao-jiang LIUYong-quan HEGuang-ming CAOTao JIATeng-zhi WUZhen-yu LIU
During hot rolling process metals will inevitably oxidize because of high temperature and air condition.In order to guarantee the surface quality,acid pickling is applied to remove the oxide scale while waste acid will do harm to the environment.Faced with the problem,by means of reduction process of hot-rolled plates,the oxide scale will be reduced to iron,so that acid pickling is unnecessary.One pass cold rolling procedure was applied.The compression ratios of hot-rolled plates with oxide scale were 10%,18%,26%and 31%,respectively.After that,samples mentioned above including a sample without deformation were separately reduced under hydrogen atmosphere condition(5% H2 +95% Ar in volume percent)at 600-1 000℃.The thermal gravimetric apparatus(TGA)was used to establish accurate experimental condition and obtain complete mass loss data.Field emission electron probe microanalysis(EPMA)was applied to analyze scale morphology change and composition distribution through the oxide scale.It was found that the sample with 26%compression ratio could be reduced completely at 900℃ which was favorable to galvanization.
Xiao-jiang LIUGuang-ming CAOYong-quan HEMing YANGZhen-yu LIU
