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Fe-(Mn,Si)-Sn合金的选择性氧化及相关相平衡研究

作者:润色论文网  来源:www.runselw.com  发布时间:2019/11/1 9:35:56  

摘要:随着汽车车身轻量化概念的提出,一代又一代先进高强钢(AHSS)相继出现。其中TWIP钢因较高的强度、较好的延伸率和碰撞吸能能力等特性,成为一种理想的汽车用钢。但是,在连续镀锌退火过程中,TWIP钢中大量存在的合金元素(尤其是Mn高达25 wt.%)会发生选择性氧化,产生氧化物恶化钢板的镀锌性能。因此,要改善TWIP钢的可镀性必须深入了解其表面选择性氧化行为。

本文通过实验研究了简单的模型合金在不同的退火气氛、退火温度和时间等条件下表面氧化物形成特性,并结合热力学计算模拟和优化,进而推测出含较多合金元素的TWIP钢表面复杂的氧化情况。此外,实验测定了Fe-Mn/Si-Sn三元体系部分界面相平衡关系,补充了热力学数据库并验证其准确性。主要工作如下:

(1) 利用据平衡合金法并结合扫面电镜、能谱仪和X射线衍射仪等测试技术,实验测定了Fe-Mn-Sn和Fe-Si-Sn三元系的部分等温截面的相关系。在此两个三元系均中均未发现三元化合物的存在。Fe-Mn-Sn 三元系450oC等温截面中存在一个晶格常数随着合金相中Fe含量的增加逐渐减小的连续固溶体相(Fe, Mn)Sn2;Mn在固溶体相FeSn中的最大溶解度可以达到26.8 at.%;Fe在Mn-Sn二元体系中的金属间化合物Mn3Sn2、Mn3Sn 和βMn的最大溶解度分别达到46.2 at.%、37.3 at.%和34.8 at.%。Fe-Si-Sn三元系700 oC和890 oC等温截面中所有Fe-Si二元化合物中Sn的溶解度几乎为零;此外,利用热力学数据外推出的700 oC和890 oC等温截面与实验测定结果完全一致。

(2) 在关于Fe-Mn-Si三元合金的选择性氧化研究中,利用热力学计算软件Pandat,模拟出Fe-Mn-Si体系的初生氧化相图,分析了Fe-Mn-Si三元合金表面选择性氧化行为。当Mn含量小于 3 wt.%时,随着退火温度的增加,合金表面形成的复合氧化物会转变为SiO2;当退火温度超过900 oC时,形成MnSiO3 比Mn2SiO4更易稳定存在;此外,采用FactSage模拟不同氧压条件下Fe-Mn-Si三元合金在820 oC退火时产生的选择性氧化现象,发现随着氧压的增大,高Mn含量(15~30 wt.%) 的合金表面优先产生Mn2SiO4 随后逐渐变成MnO与Mn2SiO4共存的状态。结合热力学计算结果对Fe-25Mn-3Si三元合金进行选择性氧化实验研究。结果表明,低氧压下合金表面稳定存在氧化物为MnO和MnSiO3/Mn2SiO4,高氧压下其表面出现少量的Mn3O4;随着氧压的增大,合金表面产生氧化物的数量增加;随着退火时间的延长,合金表面产生的氧化物在逐渐长大,且其形貌由颗粒状转变为蠕虫状。

(3) 综合运用电子背散射衍射(EBSD)、场发射扫描电镜(FE-SEM)、X射线光电子能谱仪(XPS)等技术分析了不同实验条件下Fe-Mn和Fe-Mn-Sn合金表面的选择性氧化现象,结果显示其氧化物主要是由MnO和极少量的Mn3O4组成;合金中Sn元素优先扩散到基体表面,降低氧的渗透能力,阻碍氧与Mn的接触,减少氧化物的形成;此外, Sn的添加影响氧化物分布形式使颗粒状弥散态趋于聚集成片状。

With the concept of lightweight car body,generations of advanced high-strength steels (AHSS) have emerged one by one.Among these steels, the twinning-induced plasticity (TWIP) steel has become anideal automotive steel due to the most excellent strength, formability andcollision energy absorption. However, there is a large number of alloyingelements, especially Mn up to 25 wt.%, among in the TWIP steel. They can causethe poor surface quality of the hot-dip Zn coating as they form selectiveoxides on the surface during the continuous annealing. Therefore, in order toimprove the platability of TWIP steel, it is necessary to further understandthe surface selective oxidation behavior.

In this paper, the experimental researchand thermodynamic calculation about the surface oxide characteristics of simplemodel alloys were used to predict the selective surface oxidation behavior ofthe complex TWIP steel. In addition, the Fe-Mn/Si-Sn ternary phase diagram wasmeasured and tested by thermodynamic calculation. The main contents are asfollows:

(1) In this study, some isothermal sectionsof the Fe-Mn-Sn and the Fe-Si-Sn ternary system were established withequilibrium alloying method by using scanning electron microscopy with energydispersive spectrometer and X-ray diffractometer. There is no existence of ternarycompounds in this investigation. In the Fe-Mn-Sn ternary system at 450oC, acontinuous solid solution phase (Fe, Mn)Sn2 was identified. The maximumsolubility of Mn in the FeSn phase can reach 26.8 at.%, and the Fe solubilitiesin the binary Mn-Sn compounds, Mn3Sn2, Mn3Sn and βMn, are 46.2 at.%, 37.3 at.%and 34.8 at.%, respectively. In the Fe-Si-Sn ternary system at 700 oC and 890oC, Sn cannot dissolve into the Fe-Si binary phases. Furthermore, thermodynamicextrapolation of the Fe-Si-Sn system was carried out. Calculated phaserelationship agreed well with experimental results. The primary oxidationdiagram of Fe-Si-Sn ternary system was also calculated.

(2) In the investigation about theselective surface oxidation behavior of Fe-Mn-Si ternary alloys, the primaryoxidation diagram of Fe-Mn-Si ternary system was calculated by Pandat. When theMn content is less than 3 wt.%, as the annealing temperature increases, thecomposite oxides formed on the surface are converted into SiO2. When theannealing temperature above 900 oC, MnSiO3 is more stable than Mn2SiO4.Moreover, according to the thermodynamic simulation about the selectiveoxidation of Fe-Mn-Si ternary alloys annealed at 820 oC under different oxygenpressure conditions by Factsage, as the oxygen pressure increases, the alloywith high Mn content (15~30 wt.%) preferentially generates Mn2SiO4 and thengradually becomes a state where MnO and Mn2SiO4 coexist on the surface. Basedon the thermodynamic calculation results, the selective oxidation experiment ofFe-25Mn-3Si ternary alloy was carried out. The results show that MnO andMnSiO3/Mn2SiO4 are both formed at low oxygen pressure and additional Mn3O4 athigh oxygen pressure. Increasing the oxygen pressure generally leads to morepronounced surface oxidation. As the annealing duration increases, the oxideproduced on the surface of the alloy gradually grows, and its morphologychanges from granular to vermicular.

(3) The surface selective oxidation ofFe-Mn and Fe-Mn-Sn alloys under different experimental conditions was analyzedby using the electron backscattered diffraction (EBSD), field emission scanningelectron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS)techniques. The results indicates that the oxides are mainly composed of MnOand a very small amount of Mn3O4 and a considerable amount of Sn is found tosegregate to the surface of model alloys. The addition of Sn can prevent andchanged the distribution and morphology of surface oxides from granular toflake.

关键词:Fe-Mn-Sn;Fe-Si-Sn;相平衡;选择性氧化;TWIP钢

Fe-Mn-Sn; Fe-Si-Sn; phase equilibrium;selective oxidation; TWIP steel

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