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DP1180双相钢选择性氧化实验研究及热力学分析

作者:优质期刊论文发表网  来源:www.yzqkw.com  发布时间:2019/11/1 9:33:19  

摘要:在汽车行业快速发展的过程中,先进高强度钢板由于其良好的力学性能和成形性能,在汽车行业的应用十分广泛。近年来,先进高强钢发展十分迅速,已有多种钢种得到广泛应用,现发展比较完善的主要有双相钢(DP)、相变诱发塑性钢(TRIP)等钢种。由于汽车对钢板耐蚀性的要求高,先进高强钢必须通过热浸镀锌来提高其抗腐蚀性能。热浸镀锌是提高钢板耐蚀性最为经济、有效的方式之一。先进高强钢主要指的是通过添加一定含量的合金元素,如Mn、Cr、Si等元素,通过固溶强化使得钢板具有较高的强度,从而实现其优异的力学性能。但是在连续镀锌线上的退火阶段,这些与氧亲和力较大的合金元素会与保护气氛中极少量的水汽发生反应,在钢板表面形成氧化物,这些氧化物会阻碍钢板与锌液接触,恶化钢板表面润湿性能,最终可能导致钢板表面发生大面积的漏镀现象,严重影响钢板的表面质量。因此系统研究合金元素的氧化行为,对高强钢的进一步发展十分重要。但是目前结合热力学分析,研究工艺参数对高强钢选择性氧化影响的工作比较少。所以,本文通过实验研究双相钢的选择性氧化行为,并使用热力学计算软件FactSage对Fe-Mn-Cr-Si-O体系的相平衡进行计算讨论,为研究双相钢的选择性氧化问题提供热力学分析。

为了对先进高强钢的选择性氧化问题进行深入研究,本文选择了其中一种典型的钢种,DP1180双相钢进行选择性氧化实验研究。将DP1180双相钢在820 oC,不同氧压(10-23、10-25和10-29)下分别退火60 s、120 s、240 s,并利用场发射扫描电镜(FE-SEM)、电子背散射衍射(EBSD)和X射线光电子能谱(XPS)等多种分析手段,研究了退火时间和退火气氛(氧压)对DP1180双相钢选择性氧化的影响。结合FE-SEM分析、EBSD分析、XPS分析,得出结论退火60 s时,表面还没有形成明显凸出的氧化物颗粒。退火120 s时,80%左右基体还没有被氧化,主要存在的氧化物为简单氧化物SiO2和复杂氧化物MnCr2O4,还含有少量的MnO。退火240s时,氧化物数量明显增加,除了上述三种氧化物外,还存在复杂氧化物MnSiO3和Mn2SiO4,但是数量很少。且发现随着对金属及金属氧化物加热温度的增加,即氧压的增加,表面氧化物数量也增加。

本文采用FactSage软件对Fe-Mn-Cr三元系初生氧化相图进行热力学计算,得出1000 oC下Fe-Mn-Cr-O四元等温截面。截面中存在三种氧化物,分别为MnCr2O4、MnO和Cr2O3,MnCr2O4主要与Fe-Cr一侧的合金平衡,MnO主要与富Mn区域的合金平衡,而Cr2O3只能与几乎纯Cr区域的合金平衡。并借助SEM-EDS和XRD,通过实验测定了Fe-Mn-Cr-O四元系1000 oC截面中MnO和MnCr2O4两种氧化物的分界线,一共确定了 11个相区,分别为(1) MnCr2O4 + Bcc、(2) MnCr2O4 + Bcc + High-sigma、(3) MnCr2O4+High-sigma、(4) MnCr2O4 + Fcc + Bcc、(5) MnCr2O4 + Fcc、(6) MnO + Bcc、(7) MnO + Bcc + High-sigma、(8) MnO +High-sigma、(9) MnO + Fcc、(10)Cr2O3 + MnCr2O4 + Bcc和 (11) MnCr2O4 + MnO + Fcc。实验结果表明实验测定的氧化物分界线与计算所得的氧化物分界线相比,向Fe-Cr一侧偏移。Fe在氧化物MnO和MnCr2O4中的溶解度随着平衡合金中Fe含量的增加而增加,而Cr在MnO中的溶解度随着平衡合金中的Cr含量的降低而降低。

根据FactSage软件计算的Fe-Mn-Cr-O、Fe-Mn-Si-O和Fe-Mn-Cr-Si-O体系的氧化相图,可以用来解释DP1180双相钢选择性氧化过程。成分为2.5%Mn-0.54%Cr-0.28%Si-96.5%Fe-0.18%C的DP1180双相钢在820 oC退火时,首先形成复合氧化物MnCr2O4和简单氧化物SiO2。Mn、Cr含量降低,生成MnO。因为Mn含量比较高,进一步氧化MnO会和SiO2反应形成复合氧化物MnSiO3和Mn2SiO4。

High-strength steel have been generallyapplied in the auto-motive industry due to high intensity and greatformability. In recent years, high-strength steel have been developed andcommercialized maturely, for instance, dual-phase steel and transformation-inducedplasticity steel. However, the sophisticated environment of automobilesrequires the high-strength steel for outstanding corrosion resistance. In orderto further improve the corrosion resistance, the high-strength steel have to behot-dip galvanized. Hot-dip zinc coatings have been one of most high-efficiencyapproach to improve the resistance of steel to corrosion. Nonetheless, for theadjunction of the alloy elements (Mn, Cr, Si), the selective oxidation problemof high-strength steel during recrystallization annealing period in preferenceto galvanizing section has generated extensive problems. An understanding ofthe selective oxidation problem of alloy elements (Mn, Cr, and Si) isfantastically momentous for practical application of high-strength steel.However, rare fundamental researches on the effect of process parameters on thesurface selective oxidation of high-strength steel. In this work, experimentalinvestigations have been conducted on the selective oxidation process of dualphase steel and oxidation diagram of the Fe-Mn-Cr-Si-O system, to research theselective oxidation process of dual phase steel.

In this work, dual phase steel 1180 waschosen to study the selective oxidation behavior. Annealing experiments werecarried out at 820℃ for 60 s, 120 s and 240 s, under the oxygen partialpressure of 10-23, 10-25 and 10-29 respectively. FE-SEM、EBSD and XPS were used to analyse the experimental results. When theannealed time was 60 s, no apparent particles formed on the surface. When theannealed time was 120 s, 80 percent of substrate was not oxidated. The mainoxides were SiO2, MnCr2O4 and very little MnO formed. When the time reached 240s, the amount of oxides increased visibly and MnSiO3 and Mn2SiO4 formed on thesurface apart from oxides SiO2, MnCr2O4 and MnO. Moreover, the amount of oxidesaugmented along with the increasing oxygen partial pressure.

Thermodynamic computations of theFe-Mn-Cr-O oxidation diagram at 1000 oC was performed with FactSage software.MnCr2O4, MnO and Cr2O3 coexisted with Fe-Mn-Cr alloys in the section. MnCr2O4can equilibrate with alloys close to the Fe-Cr side. MnO is in equilibrium withMn-rich region and Cr2O3 can equilibrate with almost pure Cr. The 1000 oCisothermal section of the Fe-Mn-Cr-O system was determined by Scanning ElectronMicroscopy equipped with Energy Dispersive X-ray Spectrometer and X-ray powerdiffraction. The 1000 oC isothermal section of the Fe-Mn-Cr-O system wasstudied experimentally. The oxide phase boundary between MnCr2O4 and MnO wasgained. Eleven three-phase equilibrium regions, (1) MnCr2O4 + Bcc, (2) MnCr2O4+ Bcc + High-sigma, (3) MnCr2O4+ High-sigma, (4) MnCr2O4 + Fcc + Bcc, (5)MnCr2O4 + Fcc, (6) MnO + Bcc, (7) MnO + Bcc + High-sigma, (8) MnO + High-sigma,(9) MnO + Fcc, (10) Cr2O3 + MnCr2O4 + Bcc and (11) MnCr2O4 + MnO + Fcc, havebeen detected. In comparison with the calculated phase diagram, theexperimental oxide phase boundary of MnCr2O4 and MnO turns towards the Fe-Crside partially. The solubilities of Fe in MnCr2O4 and MnO phases augmented withthe increasing Fe content in alloys. Meanwhile, the solubilities of Cr in MnOdecreased with the diminishing Cr content in alloys.

The oxidation diagram of Fe-Mn-Cr-O,Fe-Mn-Si-O and Fe-Mn-Cr-Si-O were calculated by FactSage software and could beused to explain the selective oxidation process of dual phase steel 1180. Forthe composition of dual phase steel 1180(2.5%Mn-0.54%Cr-0.28%Si-96.5%Fe-0.18%C), MnCr2O4 and SiO2 will be formed first.Then MnO can be formed together with the decreasing content of Mnand Cr. Subsequently, with the reaction between MnO and SiO2, MnSiO3 andMn2SiO4 will be formed.

关键词:双相钢;选择性氧化;相图;Fe-Mn-Cr-O;热浸镀锌

Dual phase steel;Selectiveoxidation;Phase diagram;Fe-Mn-Cr-O;Hot-dip galvanization

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