Cyclic Deformation Behaviour of Three Austenitic Cast CrMnNi TRIP/TWIP Steels with Various Ni Content

This study presents the cyclic deformation behaviour of three high‐alloyed austenitic cast steels which are characterized by different chemical compositions leading to different austenite stabilities and stacking fault energies. Thus, depending on the chemical composition different deformation mechanisms arise which have a significant influence on the cyclic deformation behaviour and life time relations. The materials were characterized under total‐strain control. The fatigue life relations of Basquin and Manson‐Coffin are applied successfully for all steel variants. The cyclic stress‐strain response is described using the Ramberg‐Osgood relationship. It is shown that the parameters n' and K' depend strongly on the accumulated plastic strain λ_p. The mechanical properties are discussed together with microstructural investigations of deformation structures and martensitic transformations as well as twinning, respectively.     

Effect of Nitrogen on the Deformation Behaviour of High‐Nitrogen Austenitic Stainless Steels

The deformation behaviour of high‐nitrogen austenitic steels with the base composition of Fe‐18Cr‐10Mn containing various contents of nitrogen was investigated. Two deformation modes including deformation‐induced martensitic transformation (DIMT) and deformation twinning (DT) were observed depending on the nitrogen content. In the alloys with lower nitrogen contents, γ→?→α' martensitic transformation sequentially occurred, whereas DT acted as a main deformation mode and DIMT was suppressed in the alloys with increasing nitrogen content. Both DIMT and DT showed strong crystallographic orientation dependence. The competing mechanism between them was discussed in terms of the variation of stacking fault energy with nitrogen content.     

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.     

Spontaneous and Deformation‐Induced Martensite in Austenitic Stainless Steels with Different Stability

The fraction and microstructure of spontaneous and deformation‐induced martensite in three austenitic stainless steels with different austenite stability have been investigated. Samples were quenched in brine followed by cooling in liquid nitrogen or plastically deformed by uniaxial tensile testing at different initial temperatures. In‐situ ferritescope measurements of the martensite fraction was conducted during tensile testing and complemented with ex‐situ X‐ray diffractometry. The microstructures of quenched and deformed samples were examined using light optical microscopy and electron backscattered diffraction. It was found that annealing twins in austenite are effective nucleation sites for spontaneous α'‐martensite, while deformation‐induced α'‐martensite mainly formed within parallel shear‐bands. The α'‐martensite formed has an orientation relationship near the Kurdjumov‐Sachs (K‐S) relation with the parent austenite phase even at high plastic strains, and adjacent α'‐martensite variants were mainly twin related (<111> 60° or Σ3).     

等径角挤压变形奥氏体不锈钢的组织演变

 用实验方法研究了奥氏体不锈钢在等径角挤压冷变形(路径RC)过程中组织变化。实验结果表明：当剪切方向与孪晶带方向成一定角度时，在剪切力的作用下，孪晶逐渐由大块孪晶→由剪切带分割的孪晶(楼梯状)→小块状→奥氏体亚晶或马氏体晶粒；部分孪晶在剪切力作用下，剪切带可直接碎化成具有大角度位向差的细小晶粒(奥氏体亚晶+马氏体晶粒)，可发生马氏体相变；当剪切方向与孪晶带方向相同时，孪晶带区域也可发生马氏体转变；3道次变形后，具有明显特征的孪晶已很少，此后继续进行剪切变形，孪晶碎化组织(含马氏体)和奥氏体剪切滑移带(含碎化晶粒)的变形以剪切滑移方式进行，当奥氏体的滑移遇到阻力时，可局部形成局部形变孪晶来协调变形；随变形道次的增加，马氏体转变也越多，在多次剪切以及道次中的交叉滑移作用下，马氏体板条逐渐被高密度位错墙分割而碎化成细小的晶粒；8道次变形后，可获得60~230 nm的等轴晶粒。     

Sensitization Behaviour of Manganese‐Alloyed Austenitic Stainless Steels

Manganese is an essential alloying element in advanced austenitic stainless steels with specific properties such as high resistance to harsh corrosive environments, high strength or low material costs. These materials are often used for welded constructions which have to be highly corrosion resistant. Hence it has to be ensured that the heat input during welding does not initiate the precipitation of chromium carbide resulting in a susceptibility to intergranular corrosion. This leads to the question whether the sensitization behaviour of manganese‐alloyed austenitic stainless steels is comparable to that of the well‐known conventional chromium nickel austenites. In the present work the effect of heat‐input on the susceptibility of the CrNi‐steel 1.4301 and the CrNiMn‐steel 1.4376 to intergranular corrosion (IGC) was considered. Investigations were carried out by corrosion testing in the so‐called Strauss‐Test to elucidate the effect of the annealing temperatures on the microstructure. Furthermore, the influence of heat treatment on the mechanical properties was evaluated by tensile testing. As a result, it could be demonstrated that manganese‐alloyed austenitic stainless steels like grade 1.4376 exhibit a sensitization behaviour very similar to the conventional austenitic steel grades. The same kinds of tests on intergranular corrosion resistance can be applied for both types of materials.     

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。     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

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。     

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.     

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.