含钼铁素体不锈钢的高温氧化行为

 采用增重法研究了排气系统用Type444铁素体不锈钢在1000℃空气中的高温抗氧化性能,并结合X射线衍射(XRD)、扫描电镜(SEM)、能谱分析(EDS)及辉光光谱(GDS)分析技术,对氧化膜的形貌和组成进行了分析。结果表明：Type444铁素体不锈钢在1000℃下连续氧化100h,氧化动力学曲线按照抛物线线型氧化规律变化,氧化激活能为(277. 5±10) kJ·mol-1,表面氧化膜由连续致密的Cr2O3组成,具有良好的高温抗氧化性能。铌和钼元素扩散至Fe2(Nb,Mo) Laves相,其氧化物颗粒会充当沉淀物从氧化膜表面逆扩散至基体和氧化膜界面处的孔洞里,从而降低了其在高温下的氧化速率,提高了抗高温氧化性能。     

日新制钢汽车排气系统用耐热铁素体不锈钢的开发

为了减轻空气污染和缓解全球变暖,各个国家均出台了尾气排放标准,且限制标准越来越严格。在此背景下,多种技术已被应用以满足相关要求,如提高燃油燃烧率、催化效率,降低排气管热容量等。而这一系列措施均会导致汽车尾气的最高温度上升。近些年,汽车排气系统零件主要用铁素体不锈钢制作,因为铁素体不锈钢的热膨胀系数小,耐循环氧化和耐高温疲劳性能均优于奥氏体不锈钢。介绍了日新制钢新研发的汽车排气系统用耐热不锈钢,以及该钢种的研发现状和发展趋势。Nb、Mo、Cu在700℃时的固溶强化和析出强化对提高铁素体不锈钢的高温强度非常有效,而添加Nb、Mo、W主要是在900℃时起到固溶强化作用。Cr,Si和Mn可降低铁素体不锈钢(含14%Cr)在大气中950℃保温200h的氧化程度。特别是添加0.8%及以上的Mn可有效提高氧化皮的附着力。事实证明,日新制钢开发的NSSHR-1(14Cr-1Mn-0.9Si-Nb)和NSSHR-2(10Cr-0.9Si-Nb-Ti)铁素体不锈钢与441和439不锈钢相比,具有更高的耐热性和成形性。     

Nb对0Cr11铁素体不锈钢组织和性能的影响

 铁素体不锈钢与奥氏体不锈钢相比具有成本低、热膨胀系数低和耐应力腐蚀等优点,所以被广泛应用到汽车排气系统、家用电器和建筑等领域。研究了不同的Nb含量对铁素体不锈钢显微组织和力学性能的影响。研究结果表明：Nb有细化晶粒的作用,随着Nb含量的增加,晶粒的平均尺寸减小;由于合金元素Nb和Ti的加入,形成了TiN、NbC 和 Fe2Nb析出相,其透射电镜观察结果与Thermal calc计算结果一致;材料的抗拉强度和显微硬度随着Nb含量的增加而增加,这是由于Nb的固溶强化和析出强化共同作用的结果。     

铌含量对17%Cr超纯铁素体不锈钢组织与高温强度的影响

随着汽车行业的发展,汽车排气系统热端用铁素体不锈钢的使用温度越来越高,高温性能成为限制其发展的关键性能。研究铌含量对铁素体不锈钢组织与高温性能的影响可以指导钢种设计和工业化生产。选取了2种17%Cr超纯铁素体不锈钢进行高温时效试验,结果表明:铌的质量分数为0.2%时,铌没有明显的强化作用,950℃高温下晶粒迅速长大,高温强度迅速降低;铌的质量分数为0.4%时,在950℃高温下时效,材料中析出弥散第二相粒子Fe2Nb,阻止晶粒长大,提高强度,防止高温引起的材料软化。材料的高温强度是由固溶铌以及铌析出物的共同作用引起的,当固溶铌析出形成析出物时,固溶铌含量减少引起的强度降低以及析出引起强度升高的竞争导致了材料的强度变化。     

445J2超纯铁素体不锈钢和316L超低碳奥氏体不锈钢在溴化锂溶液的点蚀分析

445J2铁素体不锈钢由于高的导热率、低的热膨胀系数以及良好的耐蚀性能使得其作为溴冷机中一些部件的良好候选材料,本文采用电化学测试方法对比研究了445J2超纯铁素体不锈钢(/%:0.01C,22.5Cr, 1.9Mo, 0.27Nb, 0.20Ti, 0.09Al, 0.36Cu, 0.015P,0.001S,0.015N)和316L奥氏体不锈钢(/%:0.002C,16.8Cr, 10.19Ni, 2.02Mo, 0.025P,0.0008S)在20～60℃0.1～1M的溴化锂溶液中的点蚀行为,并采用扫描电镜(SEM)和能谱分析仪(EDS)对电化学结果进行表征。结果表明,随着LiBr温度和浓度的升高,两种钢腐蚀电流密度增大,点蚀电位降低,耐点蚀性变差;氧化物和硫化物夹杂会引起两种钢的点蚀;高含量的Cr以及Mo、Ti、Nb、Al等合金元素使445J2钢具有优异的耐点蚀性能。     

Stainless Steels for Exhaust Lines

Exhaust lines of cars have been strongly evolving during the last decade to meet the various requirements of the automotive industry and the new environment regulations. This development tends to favour stainless steel grades to replace cast iron for the exhaust manifold and aluminized low carbon steel for pipes and mufflers. Among the various stainless steel grades, the proportion of ferritic grade increases for the hot part of the exhaust system as its price is lower and more stable than austenitic or refractory grades, and as it exhibits a better resistance to cyclic oxidation. A large and rapid development of stainless ferritic grades has been based on Ti and Nb stabilization. Adding niobium presents many advantages for corrosion resistance, deep‐drawability, and mainly creep resistance at high temperature. The paper is focused on stainless steel with 14 wt% chromium, stabilized with Nb. Intermetallic precipitation is studied after a 100 h holding time at different temperatures (sag tests). It is shown that, depending on the test temperature and on the chemical composition of the steel, Fe₂Nb or Fe₂Nb₃ intermetallic particles can precipitate, which affects the creep resistance: Fe₂Nb₃ being more stable at higher temperature. Some impacts on the alloy design dedicated to exhaust line application are presented as a conclusion of the study.     

Overview on Basic Types of Hot Rolling Textures of Steels

The present study gives a review on basic types of crystallographic textures developing during hot rolling of polycrystalline steels. The results are grouped into three fundamental classes of textures. The first group comprises pure Fe, some weakly bonded B2 and DO₃ structured intermetallics, as well as closely related alloys such as ferritic low carbon and microalloyed interstitial free steels. The second group includes highly alloyed corrosion‐resistant ferritic stainless and Fe‐Si transformer steels. Typical examples are steels with about 10 wt.%‐17 wt.% Cr, with about 3 wt.% Si, as well as body centered cubic transition metals such as Ta, Mo, and Nb which do not undergo any phase transformation during hot rolling. The third group comprises stable and instable austenitic stainless steels for instance on the basis of larger amounts of Cr and Ni or on Mn as well as duplex steels. Most L1₂ structured intermetallic alloys can also be assigned to this group. The suggested classification scheme is discussed in terms of different processing parameters, thermodynamics, microstructure, and crystallographic aspects.     

Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high‐temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe? Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms – precipitation–hardening, grain refinement, and solid‐solution strengthening – were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high‐temperature strength about 40%, also after long‐term annealing. The contribution of the Laves phase precipitates on the high‐temperature strength is rather small due to their rapid coarsening.     

我国汽车用超纯铁素体不锈钢的应用和发展

以排气系统为中心,介绍了国内外超纯铁素体不锈钢的开发和应用现状,并对我国超纯铁素体不锈钢的生产和发展进行了展望。     

Study on the corrosion properties of 0Cr11 ferritic stainless steel for automotive exhaust systems

This study investigates the corrosion properties of 0Cr11Ti and 0Cr11NbTi ferritic stainless steels(FSS) for automotive exhaust systems.The results indicate that the base metal and weld seam of 0Cr11NbTi steel exhibit better intergranular and condensate corrosion resistant properties because carbon and nitrogen are stabilized by Nb and Ti,and the precipitation of Cr carbide is retarded in grain boundaries.     

Carbide precipitation and high-temperature strength of hot-rolled high-strength,low-alloy steels containing Nb and Mo

The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)_MC//(100)_ferrite, [011]_MC//[010]_ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles.     

汽车排气系统用铁素体不锈钢的应用及腐蚀失效评价

最近几年,随着对全球环境问题的关注,汽车燃油的经济性和汽车尾气排放的等级化要求不断提高,铁素体不锈钢更多用于汽车排气系统零部件。铁素体不锈钢性能得到了明显改善,使铁素体不锈钢具有低的价格和良好性能,欧美及日本汽车几乎全部使用超纯铁素体不锈钢用于排气系统零部件制造。介绍了汽车排气系统的构成及对各部件常用不锈钢的性能要求、排气系统零部件的主要失效形式和所用不锈钢材料的腐蚀评价方法。     

建筑维护系统用高耐蚀性铁素体不锈钢的研发与应用

研究了Mo、Nb、Ti对22.5%Cr铁素体不锈钢力学性能、耐腐蚀性能的影响,研究结果表明,Mo主要以固溶状态存在于铁素体不锈钢中,通过固溶强化提高了铁素体不锈钢的强度;在Ti/Nb双稳定铁素体不锈钢中,由于粗大TiN颗粒的形成和金属间化合物在晶界的析出,钢的韧性明显降低;在Nb单稳定铁素体不锈钢中,Nb以固溶状态和Nb(C,N)析出相存在于铁素体基体中,Nb(C,N)弥散分布于铁素体晶内,并作为σ相的形核质点,避免了σ相在晶界的析出,从而改善了钢的低温韧性。在此基础上研制出建筑维护系统用的高耐蚀性铁素体不锈钢B445R。测试结果表明,B445R在氯离子溶液中具有比316L更优越的耐点腐蚀性能,而且具有良好的成形性能和焊接性能,这些优点使其成为沿海地区建筑维护系统理想的装饰材料。     

Ultra-Pure Ferritic Stainless Steels Grade, Refining Operation, and Application

The grades of ultra-pure ferritic stainless steels, especially the grades used in automobile exhaust system, were reviewed. The dependence of properties on alloying elements, the refining facilities, and the mechanism of the reactions in steel melts were described in detail. Vacuum, strong stirring, and powder injection proved to be effective technologies in the melting of ultra-pure ferritic stainless steels. The application of the ferritic grades was also briefly introduced.     

新型汽车排气系统用铁素体不锈钢的冷凝液腐蚀研究

 铁素体不锈钢中Cr元素对其耐腐蚀性能起着非常重要的作用。本文采用实验室冷凝液腐蚀实验方法,对一系列汽车排气系统用铁素体不锈钢在实验室模拟冷凝液中的腐蚀性能进行了研究。所有钢种经过10周期的实验室冷凝液腐蚀实验后,研究结果表明：Cr当量高于17%的铁素体不锈钢与含17%Cr的铁素体不锈钢耐实验室冷凝液腐蚀性能相当,且平均腐蚀失重量均小于6g/m2,平均最大腐蚀深度均小于0.03mm。在此实验结果的基础上,对新开发的439M型铁素体不锈钢和409L型铁素体不锈钢进一步开展5周期、10周期、20周期的实验室冷凝液腐蚀实验,并使用极值分析方法对三种周期冷凝液腐蚀实验后样品的最大点蚀深度进行统计分析,研究结果表明,新开发的439M型铁素体不锈钢的预测寿命是409L的1.6倍。     

Ultra-Pure Ferritic Stainless Steels-Grade,Refining Operation,and Application

The grades of ultra-pure ferritic stainless steels,especially the grades used in automobile exhaust system,were reviewed.The dependence of properties on alloying elements,the refining facilities,and the mechanism of the reactions in steel melts were described in detail.Vacuum,strong stirring,and powder injection proved to be effective technologies in the melting of ultra-pure ferritic stainless steels.The application of the ferritic grades was also briefly introduced.     

Cyclic-oxidation behaviours of the powder-metallurgy TiAl–4Nb–3Mn and TiAl–2Nb–2Mo beta-gamma alloys

For this study, cyclic-oxidation tests for the powder-metallurgy TiAl–4Nb–3Mn and TiAl–2Nb–2Mo alloys were carried out in air between room temperature and 900°C, and the oxidation behaviours under cyclic oxidation were compared with those under isothermal oxidation. The morphologies and structures of the oxides are identical to those from isothermal oxidation, except for the Mn-oxide formation on the top surface in the case of the TiAl–4Nb–3Mn alloy. The growth rate of the oxides in the TiAl–2Nb–2Mo alloy is slower and more stable than that of the oxides in the TiAl–4Nb–3Mn alloy, and severe internal oxidation is also evident in the latter alloy; however, the weight gain showed a sudden drop in the TiAl–2Nb–2Mo alloy that is mainly due to the enhanced formation of titanium nitrides at the interface.     

Effect of different mo contents on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of CD4MCU cast duplex stainless steels with different Mo contents of 0, 2, and 4 pct, respectively, were examined in the present study. The polarization and the in-situ slow-strain-rate (SSR) tests were conducted in a 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking (SCC) with different Mo contents. The addition of Mo, which is a strong ferrite stabilizer, affected the microstructure of the present alloy and, eventually, the tensile and corrosion behaviors in a complex manner. The tensile properties of CD4MCU cast duplex stainless steel, for example, were found to be determined by the volume fraction of hard ferritic phase, the presence of the second precipitates of soft austenitic phase in the ferrite matrix, and the shape of the austenitic phase. The addition of 2 pct Mo was detrimental to the corrosion properties of CD4MCU cast duplex stainless steel due to the significant increase in the volume fraction of ferritic phase. With the addition of 4 pct Mo, however, the resistances to pitting corrosion and SCC recovered to those of the specimen without Mo. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mo contents was discussed based on the micrographic and fractographic observations.     

汽车排气系统用不锈钢及焊管制造技术

铁素体不锈钢具有良好的耐氧化性、低的热膨胀系数、高的抗腐蚀性能以及更为经济的成本,使得铁素体不锈钢更多地用于汽车排气系统零部件。欧美及日本汽车全部使用不锈钢材料特别是铁素体不锈钢用于排气系统零部件制造以满足汽车轻量化及严格的尾气排放要求。其中,汽车排气系统中60%以上零件为管件。介绍了汽车排气系统的构成、常用不锈钢品种及各部件的性能要求,并阐述了汽车用不锈钢焊管的主要焊接加工方式。     

Comparison of Work Hardening Behaviour of Ferritic‐Bainitic and Ferritic‐Martensitic Dual Phase Steels

In this research, the stress‐strain curves of two types of dual phase steels, namely ferritic‐bainitic and ferritic‐martensitic steels with 0.16%C and 1.2% Mn have been obtained using tensile tests. Both steels were intercritically annealed under different conditions and the ferritic bainitic steels subsequently quenched in a salt bath, while the ferritic martensitic steels were water quenched. The stress‐strain data of the specimens were checked using Hollomon's equation. The results showed that both types of dual phase steels had two stages of work hardening and each stage had a different work hardening exponent. The effects of volume fraction of hard phases (bainite and martensite) on ultimate tensile strength, total elongation and work hardening exponent were also investigated. The results indicated that with increasing volume fraction of hard phase the UTS was increased whereas the work hardening exponent and total elongation were decreased.