低碳钢形变强化相变的组织细化

利用热模拟压缩变形试验研究了应变速率,形变温度和应变量对Q235级别低碳20钢过冷奥氏体形谱强化相谱的组织演变规律,探讨了了奥氏体昌粒控制对形变强化相变的影响,分析了组织细化的原因,结果表明,形变强烈促进过冷奥氏体相变,过冷奥氏体在800－740℃温度范围名义变形量为70％,应变速率为Is^-1,可获得了平均截径为2－3um及小于2um的铁索体细晶与珠光体混合组织,还观察到在局部细小铁素体晶粒的晶界上渗碳体以离异珠光体形式析出的现象,适当控制奥氏体晶粒尺寸有利于形变强化强晶组织的获得。     

奥氏体不锈钢晶粒细化对形变机制和力学性能的影响

利用相逆转变原理采用冷变形使得亚稳奥氏体转变为形变马氏体,采用不同温度和时间退火分别获得纳米晶/超细晶和粗晶奥氏体不锈钢。通过拉伸实验得到不同晶粒尺寸的奥氏体不锈钢力学性能,采用透射电镜观察形变组织结构并利用扫描电镜观察断口特征。结果表明:高屈服强度纳米晶/超细晶奥氏体不锈钢通过形变孪晶获得优良塑性;而低屈服强度的粗晶奥氏体不锈钢发生形变诱导马氏体效应,得到良好的塑性;两组具有不同形变机制的奥氏体不锈钢拉伸断口均为韧性断裂。形变机制由形变孪晶转变为形变诱导马氏体归因于晶粒细化导致奥氏体稳定性大幅度提高。     

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。     

ZrC/奥氏体相界面形变诱导铁素体相变超细化机理

利用Gleeble-1500热模拟试验机进行单轴热压缩实验,研究了含ZrC粒子的低碳钢在形变诱导相变过程中ZrC粒子对铁素体晶粒细化的影响及铁素体形核的基本特性.结果表明:一定粒径和体积分数的ZrC粒子弥散分布于基体相中时,能够阻碍位错的运动,形成集中形变区,加速形变诱导相变的进程,因而提高铁素体形核率,导致铁素体晶粒细化;ZrC/奥氏体相界面上形变诱导铁素体相变具有形核位置不饱和性、新生α相超细晶的特点;在应变条件下,铁素体晶粒在〈111〉方向择优取向,晶粒内部存在一定量的小角度晶界,由于铁素体动态再结晶的发生,组织进一步细化.ZrC/奥氏体相界面铁素体晶粒的超细化机理是形变诱导相变、铁素体动态再结晶及ZrC粒子弥散强化三者同时作用的结果.     

超细晶粒钢制备工艺及机制与传统控轧控冷(TMCP)钢的异同

总结对比了传统控轧控冷(或热机械控制处理,TMCP)钢与超细钢开发制备所用的新型TMCP工艺的特征及其冶金机制.轧前急冷、低温加工与大应变变形(强加工)是超细钢制备工艺的3个必要条件.TMCP的晶粒细化主要靠加工硬化奥氏体的静态铁素体转变.新工艺晶粒细化主要靠形变诱导动态铁素体相变.     

GCr15轴承钢激光表面熔凝强化的研究

本文研究了 GCr15轴承钢经激光表面熔凝处理后显微组织的变化,讨论了激光表面熔凝的强化机理。激光表面熔凝强化的本质是马氏体的孪晶强化,残余奥氏体的形变强化、碳的固溶强化及枝晶细化强化等。     

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.     

基体组织和加工过程对不锈钢餐、厨具铬离子析出量的影响

研究了不锈钢基体组织和加工过程中形变程度、应力分布及加工温度对不锈钢餐、厨具铬离子析出量的影响。结果表明:马氏体不锈钢的耐腐蚀性比奥氏体不锈钢的差,铬离子析出量更大;加工过程导致奥氏体不锈钢发生马氏体相变,同时产生大量的位错和缺陷,使其耐腐蚀性降低,铬离子析出量增多;加工过程中局部的应力集中导致该部分区域马氏体相变量大,铬离子析出量增多;加工温度控制在形变诱发马氏体转变温度Md以上时,马氏体相变量显著减少,铬离子析出量得到明显控制。     

过共析钢在过冷奥氏体形变过程中的组织超细化

用Gleeble1500热模拟试验机进行单轴热压缩实验,研究了过共析钢在过冷奥氏体形变过程中的组织演变规律.结果表明:过冷奥氏体变形可以抑制网状渗碳体的形成,过冷奥氏体动态相变只得到珠光体组织,在继续变形过程中珠光体发生动态球化,得到超细化(α θ)复相组织.提高形变温度使过冷度降低,阻碍过冷奥氏体动态相变的进行;而降低应变速率使变形时间延长,有利于过冷奥氏体动态相变和珠光体动态球化的进行,但得到的超细化复相组织较为粗人.     

热变形奥氏体状态对珠光体转变动力学的影响

本文在40Cr 钢上研究了热变形奥氏体状态对珠光体转变运力学的影响,所得结论如下:1)在本文的试验条件下,高温形变有促使珠光体转变的作用;2)奥氏体状态必然对以后的相变动力学及相变产物的组织与性能有显著影响,研究形变对钢的相变以及相变后的组织与性能的影响,必须明确相变前奥氏体所处的状态,即加工硬化状态或再结晶状态等等;3)于再结晶状态下,形变对珠光体转变的促进作用,是再结晶细化晶粒作用的反映。晶粒愈细小,单位体积内的晶界面积愈大,从而使相变的核心数目增多,导致转变速度的加快。     

The microstructure evolution and tensile properties of medium-Mn steel heat-treated by a two-step annealing process

The martensitic hot-rolled 0.3C-6Mn-1.5Si(wt％)steel was annealed at 630℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃for 10 min.The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines.A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally,proved the above process is feasible."Austenite memory"was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite,leading to lath-shaped morphology of austenite and fer-rite grains."Austenite memory"disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization,resulting in globular microstructure and refinement of both austenite and ferrite grains.The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation,but the dislocation strengthening played a decisive role on the yielding behavior.The tensile curves change from continuous to discontin-uous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement.A method by controlling the cold-rolled reduction is proposed to avoid the Liiders strain.     

管线用超低碳钢中针状铁素体的形成及强韧化行为

通过对一种管线用超低碳钢的变形奥氏体相变工艺的分析，提出了能够获得针状铁素体为主的组织的控制热加工工艺（TMCP）制度，研究了针状铁素体的结构特征和力学特性，结果表明，与管线用中，低碳钢相比较，实验用钢尽管具有很低的碳含量（0．025％），但在当前优化的TMCP工艺下能够获得优良的力学性能，即具有相当的强度和高的冲击韧性，针状铁素体的结构特征提高了材料的力学性能。值得注意的是，在当前肝硬化的TMCP工艺下，针状铁素体晶界上存在一层薄膜，这层薄膜对管线用超低碳钢的强韧性具有重要的作用。     

Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel

In this study,the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic(RAFM) steel friction stir welded joints were investigated.When a W-Re tool is used,the recommended welding parameters are 300 rpm rotational speed,60 mm/min welding speed and10 kn axial force.In stir zone(SZ),austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths.The microstructure in SZ contains lath martensite with high dislocation density,a lot of nano-sized MX and M_3C phase particles,but almost no M_(23)C_6 precipitates.In thermal mechanically affect zone(TMAZ)and heat affect zone(HAZ),refinement of prior austenite and martensitic laths and partial dissolution of M_(23)C_6 precipitates are obtained at relatively low rotational speed.However,with the increase of heat input,coarsening of martensitic laths,prior austenite grains,and complete dissolution of M_(23)C_6 precipitates are achieved.Impact toughness of SZ at-20?C is slightly lower than that of base material(BM),and exhibits a decreasing trend with the increase of rotational speed.     

Microstructure Evolution of a 10Cr Heat-Resistant Steel during High Temperature Creep

The microstructure evolution of a 10Cr ferritic/martensitic heat-resistant steel during creep at 600℃ was investigated in this work.Creep tests demonstrated that the 10Cr steel had higher creep strength than conventional ASME-P92 steel at 600℃.The microstructure after creep was studied by transmission electron microscopy,scanning electron microscopy and electron probe microanalysis.It was revealed that the martensitic laths were coarsened with time and eventually developed into subgrains after 8354 h.Laves phase was observed to grow and cluster along the prior austenite grain boundaries during creep and caused the fluctuation of solution and precipitation strengthening effects,which was responsible for the two slope changes on the creep rupture strength vs rupture time curve.It was also revealed that the microstructure evolution could be accelerated by stress,which resulted in the lower hardness in the deformed part of the creep specimen,compared with the aging part.     

Quantitative microstructural characterisation of Fe–30Ni alloy after martensitic transformations by means of stereological and magnetic methods

In the Fe–30Ni alloy investigated a martensitic transformation can occur both during quenching or plastic deformation. Martensite formed during plastic deformation, depending on the thermo-mechanical treatment applied, exhibits a different morphology from that achieved during quenching and forms the so-called composite-like structure. The morphology and volume fraction of martensite depends both on strain and temperature. In the present studies Fe–30Ni alloy was deformed by monotonic rolling in one path and perpendicular rolling in the temperature range M_D–M_S. The aim of the investigations was a determination of martensite volume fraction depending on the strain and temperature. To examine the influence of strain, the alloy was deformed by rolling in one path or perpendicular rolling at a temperature of − 30 °C, in the strain range of 10–30%. The dependence of temperature was investigated by rolling with 30% strain in a temperature range from − 30 °C to − 80 °C. The variants of thermo-mechanical treatment performed enabled us to achieve different martensite morphologies and volume fractions. Microstructural analysis was performed by means of light microscopy and transmission electron microscopy. The results of quantitative microstructural analysis of martensite and retained austenite volume fractions formed in different thermo-mechanical treatments were compared with those obtained by magnetic measurements. The fraction of deformation-induced martensite determined varied from 2% to 86%. The partial volume fractions V_V^MF of martensite formed in different deformation directions were also determined. It was found that the influence of the temperature on the martensite volume fraction is more pronounced than the influence of strain.     

Reverse phase transformation of martensite to austenite in stainless steels: a 3D phase-field study

The martensitic transformation of austenite as well as the reversion of martensite to austenite has been reported to significantly improve mechanical properties of steels. In the present work, three dimensional (3D) elastoplastic phase-field simulations are performed to study the kinetics of martensite reversion in stainless steels at different annealing temperatures. The input simulation data are acquired from different sources, such as CALPHAD, ab initio calculations, and experiments. The results show that the reversion occurs both at the lath boundaries as well as within the martensitic laths, which is in good agreement with the experimental observations. The reversion that occurs within the laths leads to splitting of a single martensite lath into two laths, separated by austenite. The results indicate that the reversed austenite retains a large extent of plasticity inherited from martensite.     

Nb、Ti微合金钢中碳氮化物固溶与再析出的研究

利用光学显微镜(OM)、透射电子显微镜(TEM)研究了再加热温度、奥氏体区变形温度和组织转变温度的变化对Nb、Ti微合金钢组织性能及其碳氮化物固溶与再析出行为的影响.结果表明:钢中加入铌,主要利用铌的碳氮化物在奥氏体形变过程中的再析出,抑制形变奥氏体的再结晶,在随后的组织演变过程中细化了组织;而钢中加入较高含量的钛,主要利用钛的碳化物在铁素体中的析出,产生明显的沉淀强化作用.这主要是铌、钛的碳氮化物固溶后,在奥氏体和铁素体中再析出的不同所造成的.钢中复合加入Nb-Ti后既起到细化晶粒的作用,又起到析出强化的作用.细晶强化既提高钢的强度又提高钢的韧性,但沉淀强化在大幅提高钢的强度的同时恶化了钢的韧性.     

Further investigations on martensites in Fe-0.5 wt% C and Fe-0.5wt% C-24 wt% Ni melt spun ribbons

The types of martensites occurring in Fe–C and Fe–Ni–C melt spun ribbons in the as-solidified and after heat treatment conditions has been investigated using transmission electron microscopy. It has been found that melt spinning of Fe–0.5 wt% C induces a dislocated martensitic structure at room temperature even for grains as fine as 3m. The martensite laths are separated by thin layers of retained austenite. The martensite/austenite orientations are controlled by both the K-S and N-W relationships. Some of the martensite laths exhibit a twin relation. The dislocations within the laths are mainly screw. The counterpart alloy, Fe–0.5 wt% C–24 wt% Ni exhibits 100% austenite under similar conditions. Increasing the austenite grain size upon annealing enhances martensitic transformation. The present work is concentrated on the details of butterfly martensite obtained in these ribbons.     

Modeling the heterogeneous effects of retained austenite on the behavior of martensitic high strength steels

The effects of pockets of retained austenite on the behavior of martensitic steels have been investigated. A dislocation-density based crystalline plasticity and specialized finite-element formulation were used to investigate how f.c.c. austenite pockets interact with b.c.c. martensitic laths. Quasi-static and dynamic analyses were undertaken to investigate how the effects of the orientations of parent austenite grains and different crystallographic interfaces affect shear strain localization, strength, and toughness. It is shown that the orientations of the parent austenite grain have a significant effect on the dominance of specific interfacial slip systems, and this subsequently affects whether the retained austenite has incompatible slip with martensitic laths, for low austenite Euler angles, or compatible slip with martensitic laths, for high values of austenite Euler angles.