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TWIP钢选择性氧化及Fe-Si-Mn-Sb部分相关系的研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/9/20 8:42:35  

摘要:先进的高强度钢(AHSS)具有优异的强度和延展性,获得越来越多的关注,其中孪晶诱发塑性(TWIP)钢还具有优良的成形性和高的能量碰撞吸收能力,被广泛应用于汽车工业。Mn、Si和Al是获得机械性能所需的基本合金元素,但是这些元素会在连续退火过程中发生选择性氧化,导致热浸镀锌涂层的表面恶化。因此,要改善TWIP钢的可镀性,必须深入了解其表面的选择性氧化行为。

本文实验测定了Fe-Sb-Si三元体系部分相平衡关系,获得该体系的热力学参数。此外,通过实验并结合热力学计算,研究了TWIP 980钢在不同的氧压、退火温度和时间等条件下表面氧化物形成规律。主要工作如下:

1、利用平衡合金法并结合扫描电子显微镜、能谱仪和X射线衍射仪等测试技术,本实验测定了Fe-Sb-Si三元系的部分等温截面的相关系。Fe-Sb-Si三元体系700°C等温截面存在5个三相区:(1) (αFe) + FeSb + FeSb2,(2) (αFe) + FeSi + FeSb2,(3) (Liq.) + FeSi +FeSb2,(4) (Liq.) + FeSi + FeSi2,(5) (Liq.) + (Si) + FeSi2;Fe-Sb-Si三元体系900°C等温截面也存在5个三相区,分别是(1)(αFe) + FeSb + (Liq.),(2)(αFe) + (Liq.) +Fe5Si3,(3)Fe5Si3 + (Liq.) + FeSi,(4)(Liq.) + FeSi + FeSi2和(5)(Liq.) + FeSi2 +(Si)。截面中未发现三元化合物的存在,且Sb不能溶解到Fe-Si二元相。利用热力学外推法和Pandat软件计算出的700°C和900°C的Fe-Sb-Si三元系等温截面的相关系与实验相图完全一致。

2、综合运用场发射扫描电镜(FE-SEM)、电子背散射衍射(EBSD)、X射线光电子能谱仪(XPS)等技术分析了不同工艺参数条件下TWIP 980钢表面的选择性氧化现象。随着氧压的增加,TWIP钢表面MnO、SiO2数量开始增多,形貌由颗粒状向薄膜状转变,Al2O3开始趋向于内氧化;随着退火时间的延长,TWIP钢表面MnO、SiO2数量开始增加,Mn2SiO4、MnAl2O4等复合氧化物开始生长;在低氧压下,部分简单氧化物如SiO2、MnO更加趋于内氧化。退火温度主要影响TWIP钢表面氧化物的数量和分布状态,不能根本改变氧化物的类型。尽量降低气氛的氧压,可以减少氧化物的形成和长大。提供更高的氧压,可以促进表面元素趋向内氧化,或者,缩短退火的保温时间,都可以抑制TWIP钢表面的选择性氧化。

3、通过FactSage软件Fe-Mn-Si-C,Fe-Mn-Al-C和Fe-Mn-Si-Al-C体系在低氧压和820°C条件下的热力学计算与模拟,得出初生氧化相图。根据Fe-Mn-Si-C体系初生氧化相图,表面最初形成的氧化物为SiO2,随后形成MnSiO3和Mn2SiO4,当Si元素消耗殆尽,剩下的Mn则继续氧化形成MnO简单氧化物;根据Fe-Mn-Al-C体系初生氧化相图,优先生成Al2O3,Al元素消耗的同时,析出MnAl2O4,最终当Al元素全部消耗殆尽时,剩余Mn元素形成MnO。TWIP 980钢先参照Fe-C-Mn-Al合金体系的氧化规律进行氧化,其表面先后形成Al2O3、MnAl2O4、SiO2、MnSiO3或Mn2SiO4和MnO等氧化物,氧化物形成的种类与实验完全一致。

Advanced high strength steels (AHSS) haveattracted more and more attention due to their excellent strength andductility. Among them, twining induced plasticity (TWIP) steel also hasexcellent formability and high energy absorption capacity, widely used inautomobile industry. Mn, Si and Al are the basic alloying elements repuired toobtain mechanical properties, but these elements can be selectively oxidized duringcontinuous annealing, which results in deterioration of the surface afterhot-dip galvanization. Therefore, it is necessary to understand the selectiveoxidation behavior of its surface in order to improve the platability of TWIPsteel.

In this paper, the partial phaseequilibrium relationship of Fe-Sb-Si ternary system has been measuredexperimentally, and the thermodynamic parameters of the system have beenobtained. In addition, the formation of oxide on the surface of TWIP 980 steelunder different oxygen pressure, annealing temperature and time was studied byexperiments and thermodynamic calculation. The main work is as follows:

1. The correlation of partial isothermalcross sections of the Fe-Sb-Si ternary system has been measured by means ofequilibrium alloying method combined with scanning electron microscopy, energydispersive spectrometer and X-ray diffractometer. There are five three-phasezones in the 700°C isothermal cross section of the Fe-Sb-Si ternary system: (1)(αFe) + FeSb + FeSb2,(2) (αFe) + FeSi + FeSb2,(3) (Liq.) + FeSi + FeSb2,(4) (Liq.) + FeSi+ FeSi2 and (5) (Liq.) + (Si) + FeSi2; There are also five three-phase zones inthe 900°C isothermal cross section of the Fe-Sb-Si ternary system: (1)(αFe) +FeSb + (Liq.),(2)(αFe) + (Liq.) + Fe5Si3,(3)Fe5Si3 + (Liq.) + FeSi,(4)(Liq.) + FeSi +FeSi2 and (5)(Liq.) + FeSi2 + (Si).No ternary compounds are found in thesection, and Sb can not dissolve into the Fe-Si binary phase.The correlationsof the isothermal cross sections of the Fe-Sb-Si ternary system are in goodagreement with the experimental phase diagrams by thermodynamic extrapolationmethod and Pandat software.

2. Selective oxidation of TWIP 980 steelunder different process parameters was analyzed by field emission scanningelectron microscopy(FE-SEM), electron backscatter diffraction(EBSD) and X-rayphotoelectron spectroscopy(XPS). With the increase of oxygen pressure, thenumber of MnO and SiO2 on the surface of TWIP steel increases, the morphologychanges from granular to thin film, and Al2O3 tends to internal oxidation.Withthe prolongation of annealing time, the number of MnO and SiO2 on the surfaceof TWIP steel increases, and Mn2SiO4 and MnAl2O4 begin to grow. At low oxygenpressure, some simple oxides such as SiO2 and MnO tend to internal oxidation.Annealing temperature mainly affects the number and distribution of oxides onthe surface of TWIP steel, and it can not fundamentally change the type ofoxides. The formation and growth of oxides can be reduced by minimizing the oxygenpressure in the atmosphere. Providing higher oxygen pressure can promote theinternal oxidation of surface elements, or the selective oxidation of TWIPsteel can be inhibited by shortening the holding time of annealing.

3. Thermodynamic calculation and simulationof the Fe-Mn-Si-C, Fe-Mn-Al-C and Fe-Mn-Si-Al-C systems under low oxygenpressure and 820°C were carried out by FactSage software, and the primaryoxidation phase diagram was obtained. According to the primary oxidation phasediagram of the Fe-Mn-Si-C system, the initial oxide on the surface is SiO2,then MnSiO3 and Mn2SiO4 are formed. When the Si element is exhausted, theremaining Mn continues to be oxidized to form simple MnO oxides. TWIP 980 steelis first oxidized according to the oxidation rules of Fe-C-Mn-Al alloy system.Al2O3, MnAl2O4, SiO2, MnSiO3 or Mn2SiO4 and MnO oxides are successively formedon the surface of TWIP 980 steel, and the types of oxide formation are in goodagreement with the experiments.

关键词:TWIP钢;相平衡;选择性氧化;热力学计算

TWIP steel; phase equilibrium; selectiveoxidation; thermodynamic calculation

上一篇:Mn基和Fe置换Ni-Mn-Sn基 Heusler合金 晶体结构、磁与磁热性能研究     下一篇:退火处理对Co-Cr-Fe-Ni-Ti多主元合金显微组织及硬度的影响
 
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