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。     

汽车尾气排放系统低温端用铁素体不锈钢开发

随着汽车工业的快速发展,以降低汽车自身重量和提高材料性能为目的的材料开发,得到了汽车部件制造厂商的重视.铁素体不锈钢因不含有贵重金属镍,明显降低材料成本,同时,铁素体不锈钢又具有比奥氏体不锈钢更好的抗点蚀、晶间腐蚀等性能,广泛用于汽车尾气排放系统中.介绍了宝钢超低碳、氮Cr17Mo汽车尾气系统低温端用铁素体不锈钢的开发过程.该钢种具备良好的力学性能、成型性能及耐腐蚀性能,可以满足汽车排气系统冷端部位的中管、消声器和尾管的使用性能要求.     

304与301B奥氏体不锈钢在线固溶热处理工艺研究

通过双道次热压缩和热轧试验研究了在线固溶热处理工艺对304和301B奥氏体不锈钢组织和性能的影响,提高终了压缩温度、冷却速度,降低卷取温度,可以改善奥氏体不锈钢微观组织,减少碳化物析出,提高抗晶间腐蚀能力和力学性能.将该工艺应用于工业生产的304奥氏体不锈钢热轧卷板,测试结果表明304各项性能得到改善,冷轧前退火工序可被在线固溶取代.     

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。     

应变速率对高氮奥氏体不锈钢塑性流变行为的影响

研究了室温拉伸时应变速率对高氮奥氏体不锈钢18%Cr-18%Mn-0.65%N力学性能和塑性流变行为的影响。结果表明,随应变速率的升高,试验钢的屈服强度Rp0.2升高,而抗拉强度Rm及塑性略有降低;在各应变速率下,试验钢的塑性流变行为均可以用Ludwigson模型进行描述;应变速率的升高对试验钢流变方程参数的影响如下：1）强度系数K1、应变硬化指数n1和n2减小,试验钢的加工硬化能力降低;2）真实屈服强度TYS降低;3）瞬变应变εL减小,表明升高应变速率能够促进位错多系滑移和交滑移。     

Energy Absorption Behaviour of Austenitic and Duplex Stainless Steels in a Crash Box Geometry

The improvement of the passive safety plays an important role in the development of new steels for automotive parts. At the same time aspects of weight reduction as well as the industrial feasibility have to be considered. Powered by these objectives, the development and application of new steel concepts for various purposes is promoted. For the present investigation especially weight reduction combined with an improvement of the passive safety are emphasised. As example one representative part of the body structure, the crash box, is considered. At the moment different steel grades (dual phase‐, TRIP‐and HSLA‐steels) as well as fibre reinforced materials are applied. New materials for this special purpose have to exhibit outstanding formability, a high capacity to absorb energy during a possible crash and should be cost effective compared to already existing material concepts. During this project different grades of austenitic stainless steels with varying stability were compared to duplex stainless steels and a TRIP grade with regard to their possible application as crash‐box material. The austenitic grades show excellent gradual formability according to their strength level. All of them exhibit an extraordinary strain hardening behaviour. The duplex grades show a lower formability but on a much higher yield level. Besides the determination of classical material data such as uni‐ and multi‐axial flow curves, dynamic tensile tests and forming tests for the determination of forming limit curves were performed. The material data were used in the simulation of a drop tower test which is commonly used to evaluate the performance of different materials in car components. The results were then evaluated with regard to the absorbed energy, the folding behaviour and the resulting forces.     

新型汽车排气系统用铁素体不锈钢的冷凝液腐蚀

为了开发既经济又具有良好耐冷凝液腐蚀性能的汽车排气系统用不锈钢,采用冷凝液腐蚀试验方法,对一系列的铁素体不锈钢进行了10周期的腐蚀评价研究。研究结果表明:Cr当量高于17%的铁素体不锈钢与Cr的质量分数为17%的铁素体不锈钢耐冷凝液腐蚀性能相当,且平均腐蚀失重量均小于6 g/m2,平均最大腐蚀深度均小于0.03 mm。在此试验结果的基础上,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、10、20周期的冷凝液腐蚀试验,并使用极值分析方法对三种周期冷凝液腐蚀试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。     

316L奥氏体不锈钢的氮合金化

采用金相显微镜、XRD、拉伸试验机及高低温冲击试验机等,并结合Thermo-calc软件计算研究了氮对316L奥氏体不锈钢微观组织、析出相、力学性能和耐点蚀性能的影响.结果表明:氮合金化能够抑制316L不锈钢中σ相和Chi相的析出,增加Cr2N的析出倾向,对奥氏体晶粒细化不明显;氮的添加能够提高316L不锈钢的室温强度和-100℃以上温度的夏比冲击功,降低-100℃以下的夏比冲击功,但对室温拉伸塑性影响不明显.此外,氮能够改善316L不锈钢的耐点蚀能力.     

Effect of Deep Cryogenic Treatment on Formation of Reversed Austenite in Super Martensitic Stainless Steel

The effect of deep cryogenic treatment on the formation of reversed austenite(RA)in super martensitic stainless steel was investigated.RA was found to form in steels without(A)and with(B)deep cryogenic treatment.The volume fraction of RA initially increased and then decreased with increasing tempering temperature over 550-750 ℃for the two steels,which were quenched at 1 050 ℃.In addition,for both with and without deep cryogenic treatment,the RA content reached a maximum value at 650 ℃ although the RA content in steel B was greater than that in steel A over the entire range of tempering temperatures.Furthermore,the hardness(HRC)of steel B was greater than that of steel A at tempering temperatures of 550-750 ℃.From these results,the basic mechanism for the formation of RA in steels A and B was determined to be Ni diffusion.However,there were more Ni-enriched points,a lower degree of enrichment,and a shorter diffusion path in steel B.It needed to be noted that the shapes of the RA consisted of blocks and stripes in both steels.These shapes resulted because the RA redissolved and transformed to martensite along the martensitic lath boundaries when the tempering temperature was 650-750 ℃,and a portion of RA in the martensitic lath divided the originally wider martensitic laths into a number of thinner ones.Interestingly,the RA redissolved more rapidly in steel B and consequently resulted in a stronger refining effect.     

Research Progress and Development Tendency of Nitrogen-alloyed Austenitic Stainless Steels

Research progress on nitrogen-alloyed austenitic stainless steels was expounded through the development of steel grades.In addition,hot topics in the research of nitrogen-alloyed austenitic stainless steels were discussed,including the solubility of nitrogen,brittle-ductile transition,and welding.On this basis,it was proposed that the future development tendency of nitrogen-alloyed austenitic stainless steels lied in the three fields of high-performance steels,resource-saving steels,and biologically friendly steels.The problems encountered during the research of nitrogen-alloyed austenitic stainless steels were discussed.     

Numerical Modeling of δ‐Ferrite Dissolution Kinetics in Austenitic Stainless Steels

In the present paper a numerical model that describes the diffusion processes of Ni and Cr between ferrite and austenite in Fe‐Cr‐Ni ternary systems has been developed by using the finite difference method (FDM). The model employed the tie‐line relationships given by Kajihara and Kikuchi. The moving ferrite/austenite interface was determined by solving the mass balance equations using the Newton‐Raphson method. The model has been applied to predicting the dissolution kinetics of ferrite in austenitic stainless steels, which were processed by welding, continuous casting and strip casting, respectively. The accuracy of the model has been confirmed to be quite satisfactory as compared with measured values, with advantages over another numerical model developed by Vitek et al. This model was reproduced in the present work.     

High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying and Adding Nitrided Ferroalloys

 A simple and feasible method for the production of high nitrogen austenitic stainless steels involves nitrogen gas alloying and adding nitrided ferroalloys under normal atmospheric conditions. Alloying by nitrogen gas bubbling in Fe Cr Mn Mo series alloys was carried out in MoSi2 resistance furnace and air induction furnace under normal atmospheric conditions. The results showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. Nitrogen content of 069% in 18Cr18Mn was obtained using air induction furnace by bubbling of nitrogen gas from porous plug. In addition, the nickel free, high nitrogen austenitic stainless steels with sound and compact macrostructure had been produced in the laboratory using vacuum induction furnace and electroslag remelting furnace under nitrogen atmosphere by the addition of nitrided alloy with the maximum nitrogen content of 081%. Pores were observed in the ingots obtained by melting and casting in vacuum induction furnace with the addition of nitrided ferroalloys and under nitrogen atmosphere. After electroslag remelting of the cast ingots, they were all sound and were free of pores. The yield of nitrogen increased with the decrease of melting rate in the ESR process. Due to electroslag remelting under nitrogen atmosphere and the consequential addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. There existed mainly irregular Al2O3 inclusions and MnS inclusions in ESR ingots, and the size of most of the inclusions was less than 5 μm. After homogenization of the hot rolled plate at 1 150 ℃×1 h followed by water quenching, the microstructure consisted of homogeneous austenite.     

On the Relationship between Mechanical Properties and Mechanisms of Plastic Deformation in Metastable Austenitic Steels

The requirements of the automotive industry for materials exhibiting increased structural performances are continuously increasing. These materials must remain ductile during the forming operations while also exhibiting improved strength and energy absorption capacities. New highly alloyed steel grades have been studied for a few years now, due to their exceptional mechanical properties resulting from interactions between dislocation plasticity, transformation plasticity and mechanical twinning. This study deals with the mechanical properties of steel grades presenting high manganese compositions. At room temperature, several phase transformations (γ→α', γ→? and ?→α′) were found to occur when the samples are deformed. The effect of different annealing conditions on the mechanical properties and the transformation sequence is analysed. The evolution of the work‐hardening of the samples is interpreted in connection with the kinetics of the phase transformations.     

A Semi‐Empirical Model for the evolution of Retained Austenite via Bainitic Transformation in Multiphase TRIP Steels

Multiphase TRIP steels exhibit unique combinations of strength and cold formability, characteristics especially desirable in automotive applications. This behaviour is largely determined by the amount and stability of the retained austenite dispersion in the microstructure, produced by a two‐stage heat‐treatment, consisting of intercritical annealing followed by an isothermal bainitic treatment. The amount and stability of retained austenite is mainly determined by the proper selection of the temperature and temporal duration of the bainitic treatment. In the present work an approach is presented which allows for the calculation of the amount of retained austenite in the microstructure, as a function of bainitic treatment conditions. The approach is based on the physical characteristics of the bainitic transformation and on the stabilizing effects of the formation of bainitic ferrite in austenite. Each bainitic ferrite platelet is considered to chemically stabilize a part of the surrounding austenite due to carbon rejection. The spatial and temporal extent of this stabilization is determined by solving the corresponding carbon‐diffusion problem, and thus the amount of retained austenite contributed by any individual platelet is determined. Subsequently, the evolution of the population of the platelets in the entire microstructure is determined and, thus, the volume fraction of retained austenite as a function of transformation time is calculated. Application and comparison of calculations with experimental results, obtained from four different multiphase TRIP steel compositions, exhibited very good qualitative and quantitative agreement.     

低镍不锈钢生产中的若干冶金学问题

高氮低镍不锈钢具有良好的强度、延展性及耐腐蚀性能，且可以降低晶界的腐蚀敏感性。同时，以低成本的氮代替昂贵的镍可以降低生产成本。简要回顾了我国及世界不锈钢产业的发展历程，分析了低镍不锈钢中氮的作用及不锈钢中加氮工艺等一系列问题，并对不锈钢生产中存在的若干冶金学问题进行了总结。     

临界区退火处理对0.15C-1.5Mn-1.5Al冷轧TRIP钢相变和力学性能的影响

利用热膨胀仪、热处理实验、拉伸实验和X衍射实验研究了不同临界退火温度对0.15C-1.5Mn-1.5Al TRIP钢相变、残余奥氏体特征和力学性能的影响.结果表明:不同临界退火温度下的热膨胀曲线具有相同的变化过程,两相区奥氏体含量随临界退火温度的升高而上升,而其碳含量则下降;临界退火温度影响试样的残余奥氏体特征和力学性能,820℃退火处理时残余奥氏体的体积分数为14%,碳的质量分数为1.36%;力学性能也是820℃退火时最佳,强塑积可达25400 MPa·%,且变形过程中具有高的瞬时加工硬化指数(n值).     

氮对304奥氏体不锈钢组织和力学性能的影响

在０Ｃｒ１８Ｎｉ９奥氏体不锈钢成分基础上，加入一定的氮，并使钢中的镍含量控制在标准下限含量的条件下，研究了氮对组织和力学性能的影响。结果表明：加氮后钢的强度提高，奥氏体稳定不变，固溶态组织不变，而敏化后晶界析出物类型有所不同。     

Work Hardening Behavior and Stability of Retained Austenite for Quenched and Partitioned Steels

Both microstructure and mechanical properties of low alloy steels treated by quenching and partitioning (Q&P)process were examined.The mixed microstructure of martensite and large-fractioned retained austenite (about 27?3%)was characterized and analyzed,excellent combinations of total elongation of 1 9% and tensile strength of 1 835 MPa were obtained,and three-stage work hardening behavior was demonstrated during tensile test.The en-hanced mechanical properties and work hardening behavior were explained based on the transformation-induced plas-ticity effect of large-fractioned austenite.     

汽车排气系统用409型铁素体不锈钢的开发研究

描述汽车排气系统的结构及其对材料的特殊要求,介绍不锈钢分公司409型铁素体不锈钢的开发过程.该钢具有良好的高温性能、成形性及耐腐蚀性,可以较好地满足汽车排气系统用钢的需求.