不同热处理工艺对Fe-30Mn-3Si-4Al TWIP钢力学组织性能的影响

采用拉伸性能测试、金相观察、SEM和EDS等方法研究了不同热处理工艺对Fe-30Mn-3Si-4AlTWIP钢微观组织、拉伸力学性能及断口形貌的影响,并采用X射线衍射仪测定材料的物相组成。结果表明,冷却速度越快,TWIP钢的延伸率和强度越高;热处理后其室温组织为含有退火孪晶的单一奥氏体,冷却速度越小,奥氏体晶粒和退火孪晶的尺寸越大。拉伸时发生典型的延性断裂,在拉伸过程中退火孪晶转变成形变孪晶,使材料的塑性提高。     

超细晶奥氏体在两相区大变形后的瞬态组织

将一种低碳结构钢循环加热淬火得到超细晶粒奥氏体,再以20℃/s的速率将其冷却至两相区进行真应变量为2的大变形,分析了形变后的瞬态组织.结果表明:用该工艺制备的超细晶奥氏体在两相区的高速大变形的后期,始终呈现应变硬化特征,并伴随有一定程度的形变诱导相变或铁素体动态再结晶等软化行为;同时,在较低温度快速大变形容易在试样的个别碳过饱和区导致应变诱导孪晶马氏体组织的生成,且随着形变温度降低孪晶马氏体量增加-循环加热淬火前的原始组织影响奥氏体内碳浓度分布,在一定程度上影响冷却变形过程的应力应变行为和形变后的瞬态组织.     

爆炸和滚压强化对不锈钢表面层组织结构的影响

对奥氏体不锈钢表面进行了爆炸强化、滚压强化和喷丸强化。爆炸强化时表面层内出现形变孪晶栅栏,位错线具有方向性、呈密布排列,局部山现ε马氏体、α马氏体和奥氏体超细晶粒的区域;滚压强化后表面层内也有就、孪晶栅栏出现,但位错以胞状结构的形式分布在奥氏体基体上;喷丸强化的表面层内孪晶栅栏十分致密,位错线仍以方向性排布,并无位错胞出现。表面层内组织结构出现差异,主要与变形速度和变形量有关,高速和较大的变形量可诱发α,ε马氏体转变和形成超细奥氏体晶粒,慢速变形时能促使位错胞的形成。本文还讨论了孪晶栅栏的形成方式及其内部构造。     

304不锈钢箔材在不同应变速率下的拉伸性能研究

304不锈钢是一种常用的奥氏体不锈钢.在拉伸应变过程中,应变速率的变化会诱发马氏体转变量和转变速率,以及内部组织滑移线、位错、层错、形变孪晶密度的转变量和转变速率的不同,从而表现出不同的应变硬化行为.本文针对0.1 mm厚度304奥氏体不锈钢箔材,从断后伸长率,断面收缩率,屈服强度,抗拉强度及硬化指数5个方面,研究了室温条件下不同应变速率对其拉伸性能的影响.实验结果表明：马氏体转变理论同样适用于304奥氏体不锈钢箔材, 且0.1 mm厚度304不锈钢存在“越薄越脆,越小越强”的尺寸效应现象;同时,0.1 mm厚度304奥氏体不锈钢箔材拉伸力学性能随应变速率的变化主要表现在以下几方面：断后延伸率和断面收缩率均随着应变速率的增加而降低;低应变速率时,随着应变速率的增加屈服强度增大,而抗拉强度随应变速率的提高呈现减弱的相反规律;高应变速率下,304奥氏体不锈钢的强度主要由材料本身性能决定,应变速率的改变对强度的影响较小;准静态低应变速率下,硬化指数随应变速率增大而升高,较高应变速率下,硬化指数与应变速率变化无关.     

高锰TWIP/TRIP 钢研究进展与应用

综述了近来年高锰孪晶诱发塑性/相变诱发塑性(TWIP/TRIP)钢的研究进展和实际应用情况。介绍了晶粒尺寸对TWIP钢变形机制的影响,观察了形变孪晶随晶粒尺寸变化的演变过程。总结了形变诱导马氏体和形变诱导孪晶的演变机理。简述了碳化钒(VC)沉淀粒子对高锰TWIP/TRIP钢延迟断裂及加工硬化行为的影响,并对VC沉淀粒子与形变孪晶的交互作用机制进行了探讨。阐述了双辊铸轧工艺制备高锰TWIP/TRIP钢薄带的近终成型工艺过程及显微组织的演变机理,并探讨了工程应用的前景。     

18-8型奥氏体不锈钢在低周疲劳变形下的微观组织特征

利用透射电镜系统地研究了18—8型奥氏体不锈钢在低周疲劳变形下的微观组织,包括奥氏体位错组态,形变孪晶和形变马氏体。发现奥氏体位错组态与应变幅和温度有关,形变马氏体主要在 SB 交截处形核,在室温和低温下由于母相变形方式的不同而产生不同的组织特征。     

应变速率对热冲压淬火-配分钢显微组织与力学性能的影响EI北大核心CSCD

采用分离式Hopkinson压杆对热冲压淬火-配分(HS-Q&P)钢在0~12000 s^(-1)应变速率范围内进行动态压缩实验,利用SEM,EBSD,XRD等分析表征手段探究动态压缩过程中试样的变形行为。结果表明:实验钢在不同速率下的变形行为基本相似且分为3个阶段,在平台处应力有小幅度增加,增幅更多体现在应变上。在压缩过程中出现的绝热升温会带来软化效应。残余奥氏体的存在会提高实验钢的强度和塑性变形能力。钢中残余奥氏体发生相变诱导塑性(transformation induced plasticity,TRIP)效应减少的体积分数与马氏体增加的体积分数基本一致,证明TRIP效应为钢中主要的强化机制。同时,通过SEM可观测到残余奥氏体发生TRIP效应转变成细小针状马氏体,随着应变速率增加,晶格畸变越来越严重,EBSD图像中可以观测到部分形变孪晶,在不同应变速率下,〈001〉取向的晶粒都会更容易产生形变孪晶。     

层错能对Fe-Mn-C系TRIP/TWIP钢变形机制影响

对三种不同层错能(SFE)Fe-Mn-C系TWIP钢的变形机制进行了研究.结果表明:在淬火态下,TWIP钢组织为全奥氏体,奥氏体晶粒内存在少量退火孪晶.TWIP钢的层错能随着C、Mn含量的增加而增加.层错能为7 mJ/m2时,变形后出现大量ε马氏体,且随着应变量的增大,ε马氏体峰增强,表现为单一的TRIP效应;层错能为12 mJ/m2时,应变诱导γ→ε→α或γ→α的转变及形成少量形变孪晶,表现为TRIP/TWIP效应;层错能为18 mJ/m2时,变形后形成大量形变孪晶,表现为单一的TWIP效应,抗拉强度和延伸率分别达到851 MPa及49%.随着层错能增加,TWIP钢的断裂机制由沿晶断裂转变为以韧窝为主的塑性断裂.     

316L不锈钢在表面机械滚压处理时的形变诱导马氏体相变和组织细化过程

采用表面机械滚压处理(SMRT)在316L不锈钢表面制备出梯度纳米结构(GNS)表层,研究了SMRT对GNS表层中的相组成和微观组织演变的影响机制。结果表明:经SMRT后316L不锈钢表层的奥氏体相发生形变诱导马氏体相变,且马氏体含量随着SMRT压下量的增大而增多;微观组织的细化过程先后经历了高密度位错生成和交互作用、形变孪生、形变诱导马氏体相变和马氏体晶粒细化过程,最终在最表层形成以马氏体相为主、晶粒尺寸~55 nm的纳米晶组织。     

液氮电脉冲处理超细晶316L不锈钢的低温拉伸性能

在液氮环境用电脉冲(EPT)工艺优化冷轧316L奥氏体不锈钢的微观组织和力学性能，研究了电脉冲处理后样品的室温和低温拉伸性能及其变形机制。结果表明：液氮电脉冲处理后的冷轧316L不锈钢可得到再结晶组织。输入电脉冲的能量不同，其再结晶比例也不同, EPT-7.5LN样品可产生完全再结晶组织。在不同温度下电脉冲处理样品的拉伸实验结果表明，在77 K的拉伸强度-塑性匹配远比在293 K时的高。透射电镜的表征结果表明，样品在293 K的拉伸变形机制以位错和变形孪晶为主，在77 K的拉伸变形时则发生了大量形变诱导马氏体相变。正是大量的马氏体相变及其随后的位错滑移变形使材料的加工硬化能力显著提高，从而使其塑性增强。进一步分析表明，产生变形机制差异的主要原因是，在低温下这种材料的层错能显著降低。     

Formation of ultrafine grained microstructure in the austenitic stainless steel and its impact on tensile properties

Commercial grade AISI 316L austenitic stainless steel was heavily cold rolled to 90% of thickness reduction. The cold rolled material was subjected to repetitive annealing treatment for short duration of 45-60 s at various temperatures. The microstructure of the cold rolled and after annealing was studied by optical as well as transmission electron microscope. The microstructural examination of the specimens after repetitive annealing process revealed the formation of ultrafine grain size microstructure. It was also noted that depending on the processing condition the grain size distribution varied widely. The tensile testing of the annealed specimen showed that the yield strength increased by 4-5 times that of the coarse grained material. However, a loss in the strain hardening ability was observed in these specimens. A good combination of yield strength and ductility for ultrafine grained stainless steel as compared to the coarse grained material could be obtained by the optimization of the microstructure.     

Grain size dependence of mechanical, corrosion and tribological properties of high nitrogen stainless steels

Austenitic stainless steels have been indispensable for the progress of technology during the last 80 years. Due to the cost of nickel and to the prospective of allergic reactions caused by this element, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low nickel content. In order to maintain the austenitic microstructure, nickel reduction is balanced with nitrogen addition. Nitrogen addition to austenitic stainless steels is also very effective for improving yield strength and corrosion resistance without reducing ductility and toughness. In order to further increase the strength, it is possible to combine the effect of nitrogen addition and grain refining. The purpose of this study is to examine the relationship between microstructures and mechanical, corrosion and tribological properties of a high nitrogen stainless steel with an ultrafine grained structure.     

Influence of grain size on hot ductility of plain C–Mn steels

Abstract

The influence of grain size on the hot ductility of 0·19 and 0·65wt-%C steels of the C–Mn type has been determined. For the low-carbon steel, a gram Size increase from 70 to 180 μm had only a small influence on hot ductility, as measured by tensile reduction in area values. However, increasing the grain size to 290 μm raised the temperature at which ductility started to fall by 50°C. In the finer grained steels it is believed that the ductility trough starts at the Ar₃ temperature when films of ferrite form round the stronger austenite grains. Ductility soon recovers as the temperature is lowered because of a thickening of the ferrite and a consequent reduction of strain concentration at the boundaries, so that only a narrow trough is observed. In coarser grained steels it is considered that deformation induced ferrite can have a pronounced influence on hot ductility over a wide range of temperatures leading to a wide ductility trough. Refining the grain size had an even greater influence on the hot ductility of the 0·65wt-%C steel. Intergranular tensile fracture at coarse grain size was by grain boundary sliding in the austenite resulting in a very wide ductility trough. Refining the grain size prevented intergranular failure occurring in the γ down to the lowest temperature examined: 700°C. Although the main influence of grain size is in controlling the width of the trough, the depth also increased with an increase in grain size.

MST/420     

Effect of retained austenite characteristics on fatigue behavior and tensile properties of transformation induced plasticity steel

Transformation induced plasticity steels are commonly used for automotive industry due to their high strength and high ductility. These steels achieve good balance of strength and ductility due to transformation of retained austenite to martensite during deformation. In this study, effect of retained austenite characteristics on fatigue and tensile property of conventional CMnSi steel is evaluated. Tensile and fatigue test were carried out at room temperature. After mechanical tests, fractography observations were carried out by scanning electron microscopy. All samples show reasonably high values of tensile strength and fatigue limit. Results of fatigue test show that fatigue performance of this steel improved by increasing volume fracture of retained austenite.     

Effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel

The effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel have been investigated. Yield strength is reduced by sensitisation, but ultimate tensile strength is nearly unaffected. Strain-hardening behaviour is changed by sensitisation, too. Although sensitisation may induce martensite formation near grain boundary, twin boundary, and austenite/martensite interface, the sensitisation-induced martensite does not exert a great influence on tensile behaviour in the 304 steel. In the unsensitised condition, martensitic transformation in the steel bulk induced by prior deformation and liquid-nitrogen immersion also does not change strain-hardening behaviour in the present steel.     

The role of grain size in static and cyclic deformation behaviour of a laser reversion annealed metastable austenitic steel

Different grain sizes were created in a metastable 17Cr‐7Mn‐7Ni steel by martensite‐to‐austenite reversion at different temperatures using a laser beam. Two fully reverted material states obtained at 990°C and 780°C exhibited average grain sizes of 7.7 and 2.7 μm, respectively. The third microstructure (610°C) consisted of grains at different stages of recrystallization and deformed austenite. A hot‐pressed, coarse‐grained counterpart was studied for reference. The yield and tensile strengths increased with refined grain size, maintaining reasonable elongation except for the heterogeneous microstructure. Total strain‐controlled fatigue tests revealed increasing initial stress amplitudes but decreasing cyclic hardening and fatigue‐induced α′‐martensite formation with decreasing grain size. Fatigue life was slightly improved for the 2.7‐μm grain size. Contrary, the heterogeneous microstructure yielded an inferior lifetime, especially at high strain amplitudes. Examinations of the cyclically deformed microstructure showed that the characteristic deformation band structure was less pronounced in refined grains.     

Tensile deformation of a duplex Fe-20Mn-9Al-0.6C steel having the reduced specific weight

The room temperature deformation characteristics of a duplex Fe-20Mn-9Al-0.6C steel with the reduced specific weight of 6.84 g/cm³ in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800-1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m². Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.     

Effect of grain refinement on strain hardening and fracture in austenitic stainless steel

The present study aims to investigate the effect of grain refinement on strain hardening behaviour and fracture surface characteristics in 316LN austenitic stainless steel (ASS). The ASSs with varying grain sizes were obtained through 90% cold rolled reduction and subsequently phase reversion annealing treatment. The results showed that the grain refinement from coarse-grained (CG) structure to ultrafine-grained (UFG) structure increased the yield strength whilst maintaining a reasonable ductility. The strain hardening curves in all the samples were divided into three stages. The fractures in all the samples were ductile fracture with dimples. The subtle differences in the strain hardening behaviour and fracture surface characteristics among the samples with various grain sizes from CG structure to UFG structure were influenced by the deformation mechanisms of austenite.     

Enhanced tensile properties of 316L steel via grain refinement and low-strain rolling

Excellent strength–ductility synergy of metallic materials is significant for their industrial applications. This study presents a fine-grained 316L stainless sheet (average grain size of ～5?µm) with a good combination of strength and ductility achieved via low-strain cold rolling (rolling strain of 30%). The fabricated steel sheet exhibits maximum yield strength and ultimate tensile strength values of 1045 and 1080?MPa, respectively, with a uniform elongation of 7%. Experimental results confirm that the high density of dislocations, strain-induced martensitic phase, and deformation twins together contribute to the high strength of the rolled stainless steel. Moreover, its good ductility is attributable to the strain-induced martensitic transformation and deformation twins.