Modelling and Closed‐loop Control of Complex Tube Forming Based on an Optimized Application of Martensite Formation in Austenitic Stainless Steels

Metastable austenitic steels undergo deformation‐induced martensitic transformation which can lead to a distinct increase of fatigue strength at an optimal volume fraction of martensite. This effect was used in the present study to define the local strength behaviour of a structural component part for the very high cycle fatigue (VHCF) regime. The investigation was on a discontinuous two‐stage forming process that consists of U‐O‐forming and rotary draw bending and results in a cross tube of a trailer coupling as exemplary dynamically loaded component. The volume fraction of martensite can be adjusted by means of plastomechanical simulation of the forming process and its parameters as part of the online process control. The formation of martensite shows a strong dependence on forming parameters (e.g. temperature and strain‐rate) and batch variations. These disturbance variables can only be taken into account by a closed‐loop control. Non‐isothermal material models were analysed according to their simulation accuracy of the martensite evolution. For the online control various hierarchical mathematical models were studied with regard to time effort and model error.     

Effect of Cold Deformation on Phase Evolution and Mechanical Properties in an Austenitic Stainless Steel for Structural and Safety Applications

 Abstract： The effects of cold deformation on the formation of strain induced α′ martensite and mechanical properties of an austenitic stainless steel have been examined. X-ray diffraction analysis has revealed that 30% and 40% cold rolling have resulted in the formation of 24% and 315% martensite respectively. Microstructural investigation has demonstrated that the formation of martensite is enhanced with increase in the percent deformation at 0 ℃. Investigation of mechanical properties reveals that hardness, yield strength and tensile strength values increase where as percent elongation drops with increasing deformation. The fractographic observation corroborates the tensile results. Examination of sub-surface at the fractured end of the tensile sample manifests that void/microcrack nucleation occurs in the interfacial regions of the martensite phase as well as at the austenite-martensite interface.     

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).     

C、Cr、Ni和Mn含量对304不锈钢变形诱发马氏体相变的影响

用X射线衍射技术研究了304奥氏体不锈钢变形诱发马氏体相变倾向对成分的敏感性.在液氮内的拉伸结果表明: C、Mn、Cr和Ni的含量从标准范围的上限降到下限,显著增大形变诱发马氏体相变倾向和形变强化能力,尤其是C、Mn、Cr和Ni的含量接近下限的钢在室温下拉伸形成ε马氏体和α'马氏体,其中α'马氏体快速形成使流变应力迅速上升;此外发现,在液氮温度下,变形早期ε马氏体与α'马氏体同时存在,α'马氏体的体积分数累积到约70%后,ε马氏体消失.     

Structure Evolution and Solidification Behavior of Austenitic Stainless Steel in Pulsed Magnetic Field

 To understand the solidification behavior of austenitic stainless steel in pulsed magnetic field, the solidification process is investigated by means of the self made high voltage pulse power source and the solidification tester. The results show that the solidification structure of austenitic stainless steel can be remarkably refined in pulsed magnetic field, yet the grains become coarse again when the magnetic intensity is exceedingly large, indicating that an optimal intensity range existed for structure refinement. The solidification temperature can be enhanced with an increase in the magnetic intensity. The solidification time is shortened obviously, but the shortening degree is reduced with the increase of the magnetic intensity.     

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.     

Static Recrystallization Behavior of 316LN Austenitic Stainless Steel

The static recrystallization of 316LN austenitic stainless steel was studied by double-pass hot compression tests on a Gleeble-3500 thermomechanical simulator. The specimens were compressed at the deformation temperatures of 950, 1050, 1150 °C, strain rates of 0.01, 0.1, 1s^?1, strains of 0.1, 0.15, 0.2, and intervals of 1 — 100 s. The results show that the volume fraction of static recrystallization of 316LN increases with the increase of deformation temperature, strain rate, strain and interval, which indicates that static recrystallization occurs easily under the conditions of higher deformation temperature, higher strain rate and larger strain. Deformation temperature has significant influence on static recrystallization of 316LN. The volume fraction of static recrystallization could easily reach 100% at higher deformation temperatures. By microstructure analysis, it can be concluded that the larger the volume fraction of static recrystallization, the more obvious the grain refinement. The static recrystallization activation energy of 317 882 J/mol and the exponent n of 0.46 were obtained. The static recrystallization kinetics was established. The predicted volume fraction of static recrystallization is in good agreement with the experimental results.     

Some Strengthening Methods for Austenitic Stainless Steels

Austenitic stainless steels possessing good corrosion resistance have recently found growing applications as a constructional material. In this instance, increasing strength properties, which are typically quite low, is of great interest. Due to the low stacking fault energy, strain hardening of alloyed austenite is efficient for increasing tensile strength without impairing ductility seriously. In addition, certain grades are unstable, so that cold working creates strain‐induced martensite that enhances strengthening. Grain size refinement to micrometer scale or even finer can also increase the yield strength, still providing good ductility. In the present paper dislocation and phase transformation strengthening and thereby properties achievable in temper rolled austenitic stainless steels are discussed. Strengthening by the reversion annealing is also described and excellent results achievable are shown. Finally, the effect of bake hardening through the static strain ageing is presented. Long‐term research work in various projects indicates that the current knowledge of strengthening of austenitic stainless steels is close to the industrial utilisation.     

Influence of Pre-Transformed Martensite on Work-Hardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work-hardening behavior under the uniaxial tensile condition. The X-ray diffractometer (XRD) was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre-transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work-hardening curve of prestrained specimens observably changes with true strain. The work-hardening exponent n of stainless steel decreases with the increase of pre-transformed martensite. The achievement is a significant contribution to the process design during pressing.     

Energetics of Plastic Deformation of Metastable Austenitic Stainless Steel

The change in the internal energy during uniaxial tensile deformation of austenitic stainless steels EN 1.4301 (AISI 304) and EN 1.4318 (AISI 301LN) was determined by measuring the extent of γ→α'‐martensite transformation and the temperature increase of the samples. From the results the fraction of the stored energy of cold work and the free energy change related to the strain‐induced γ→α'‐martensite transformation were determined. The fraction of stored energy varied around 0.4. With the metastable steel grades the free energy change related to the γ→α'‐martensite transformation was found to vary between ‐98 MJ/m³ and ‐206 MJ/m³ depending on the austenite stability of the steel. Furthermore, the magnitude of the mechanical driving force was estimated by comparing the results with the free energy change of thermally induced transformation.     

302HQA钢冷变形过程组织演化分析

通过室温压缩实验,研究了奥氏体不锈钢302HQA的冷变形行为,组织分析和X射线衍射分析表明,冷变形过程产生了形变马氏体,进一步磁性检测表明:随着变形量的增加,形变马氏体的量也增加;在小变形条件下,变形速率对形变马氏体的生成量影响较小;当变形量增加到50%时,随着变形速率的增加,由于变形热效应的影响,形变马氏体含量反而有所下降。     

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.     

Dilatometric Study of Continuous Annealing of a Cold-Rolled Austenitic Stainless Steel

The sequence of structural changes produced in two deformed microstructures of austenitic stainless steel elaborated by a multipass unidirectional cold rolling (CR) and a two-step one, to a 75% thickness reduction, is followed by dilatometric experiments. The two materials show different dilatometric behaviors. X-ray diffraction and microhardness measurements are performed to underlie the observed dilatometric behaviors. The material subjected to multipass unidirectional CR shows an unusual dilatometric behavior. The first heating stage leads to the occurrence of the recovery reaction in competition with the ε-martensite reversion. When the temperature increases between 550 and 780 °C, the reversion of deformation-induced α-martensite takes place and leads to a complicated dilatometric anomaly. Further increase in temperature leads to the occurrence of the recrystallization transformation. However, the material subjected to two-step CR shows a quite usual dilatometric behavior which is explained by the occurrence of several reactions in the following order: 1) T < 300 °C, the recovery reaction, 2) 300 < T < 680 °C, the ferro- to paramagnetic transformation of α-martensite, the reversion of ε-martensite, and the athermal reversion of α-martensite, 3) 680 < T < 760 °C, thermal reversion of DIM, and then 4) at T > 760 °C, the recrystallization.     

奥 氏 体 不 锈 钢 的 再 结 晶 动 力 学

 采用双道次压缩和应力松弛2种实验方法,研究了含铌奥氏体不锈钢347热变形后的再结晶动力学特征,确定了AVRAMI方程表达式,同时通过定量金相技术分析了对应2种实验方法的淬火试样中再结晶组织所占份数。比较不同实验方法所得结果可知：定量金相分析结果与应力松弛法结果较吻合,而与双道次压缩实验结果差异较大。仅进行1次应力松弛实验就可以得到一条再结晶率与时间的关系曲线,可大大减少实验量。     

氢致奥氏体不锈钢晶格畸变

对304L和316L奥氏体不锈钢试样在充氢后和充氢同时进行X射线衍射分析,观察到在充氢过程中存在奥氏体晶格膨胀-收缩-膨胀的现象;在充氢后时效一段时间的情况下,存在奥氏体晶格收缩-膨胀-收缩的现象,并初步讨论了可能的原因。     

含氮奥氏体不锈钢的敏化行为

通过65％硝酸浸泡法和电化学动电位再活化法定量评价18—8奥氏体不锈钢的晶间腐蚀倾向，并利用分析电子显微镜确定晶界的贫铬区，利用透射电子显微镜及电子衍射技术确定晶界析出物的结构。试验结果表明：适量增加氮含量可提高18—8奥氏体不锈钢的耐晶间腐蚀性能。敏化状态下再活化率Rn在氮的质量分数为0．16％时最低；当氮的质量分数大于0．16％时，贫铬区最低铬含量随氮含量的增加而降低；贫铬区宽度在氮的质量分数为0．16％时最窄，氮含量继续提高则贫铬区宽度扩大。     

铜对奥氏体抗菌不锈钢性能的影响

 研究了不同铜含量的奥氏体抗菌不锈钢的抗菌性能、腐蚀性能和力学性能。样品经过热轧、抗菌热处理、冷轧、退火处理后,进行了抗菌试验、腐蚀试验等。试验结果表明,随着铜含量的增加,抗菌性逐渐提高;其耐点蚀性随着铜含量的增加呈提高的趋势;强度随着铜含量的增加先减后增,塑性先增后减。     

Effect of Cold Rolling on Microstructure and Mechanical Properties of AISI 301LN Metastable Austenitic Stainless Steels

 The microstructure and mechanical properties evolution of AISI 301LN metastable austenitic stainless steels during cold rolling were investigated. A wide range of cold thickness reduction (10%-80%) was carried out in a four-high rolling mill at ambient temperature. The X-ray and Feritscope MP30 were used to identify the strain-induced α′-martensite phase and its volume fraction, respectively. The microstructure was observed by optical micrograph and the mechanical properties were determined by tensile tests and microhardness. The results show that the strain-induced α′-martensite nucleated at the shear bands intersections and the growth of α′-martensite occurred by the repeated nucleation of new embryos. The volume fraction of strain-induced α′-martensite increased with increasing the cold rolling reduction. In addition, the percentage increased in the tensile strength is the same as that of hardness. The ratio between the average tensile strength and the average microhardness was found to range between 2.82 and 3.17.