晶粒细化对奥氏体不锈钢高温力学性能的影响

摘要：对粗晶201LN奥氏体不锈钢采用60%冷变形结合700℃退火120s工艺制备超细晶奥氏体不锈钢,研究晶粒细化对奥氏体不锈钢高温力学性能的影响。利用光学显微镜、扫描电子显微镜、透射电子显微镜、电子背散射衍射技术对粗晶和超细晶奥氏体钢进行了组织表征,并使用万能试验机测试20和650℃环境下力学性能。结果显示粗晶奥氏体不锈钢经过冷变形结合退火工艺处理,平均晶粒尺寸由18μm细化为0.9μm,屈服强度由383MPa提高到704MPa,而伸长率由63.8%下降到46.3%,表明晶粒细化能有效提高奥氏体不锈钢屈服强度的同时较小损害塑性,TEM证实其形变机制均为形变诱导马氏体和孪生协同作用。当温度由20℃提高到650℃时,粗晶奥氏体不锈钢屈服强度和伸长率分别下降到180MPa和28.1%,超细晶奥氏体不锈钢屈服强度和伸长率分别为384MPa和24.2%。这表明在650℃高温环境下细晶强化作用仍然有效,粗晶和超细晶奥氏体不锈钢也有较好的塑性,其形变机制分别变为位错滑移和位错滑移+层错+孪生。     

氧化物弥散强化钢的强化机理

采用粉末冶金法制备出成分为Fe-12.5Cr-2.5W-0.4Ti-0.02V-0.4Y₂O₃(12Cr-ODS,质量分数,%)的铁素体钢.通过电镜观察及力学性能测试等手段研究了12Cr-ODS铁素体钢的组织与性能,并定量计算了不同强化机制对合金屈服强度的贡献.电镜观察发现12Cr-ODS钢为等轴的铁素体组织,平均晶粒尺寸为1.5μm,不同尺寸氧化物在基体中均匀分布.力学性能测试结果表明12Cr-ODS钢具有优异的室温拉伸性能,屈服强度达到738 MPa.合金主要强化机制为氧化物弥散强化、氧化物弥散强化钢加工强化、热错配位错强化和晶界强化机制,各种强化机制计算得到的理论屈服强度为750 MPa,与实测值吻合较好.      

形变热处理工艺对403Nb马氏体耐热钢组织和性能的影响

403Nb是一种马氏体耐热不锈钢,主要用于制作火电机组汽轮机叶片,该钢是在AISI 403不锈钢的基础上,通过添加一定质量分数的Nb和V 发展起来的。通过与传统热处理工艺对比,研究了形变热处理工艺(即高温固溶后在奥氏体亚稳态区变形后空冷)对马氏体耐热钢403Nb的组织和性能的影响。结果表明：形变热处理可以使403Nb钢组织明显细化,马氏体板条宽度由传统热处理的0.3 μm减小到0.1 μm,而且生成大量弥散的纳米级MX碳化物。形变热处理对403Nb钢组织改变的直接结果是可以大幅度提高403Nb钢的屈服强度,在常温拉伸时由传统热处理的800 MPa提高到1 180 MPa,在高温(700 ℃)拉伸时由246.5 MPa提高到341.4 MPa,其屈服强度分别提高了47.55%和38.5%。     

1000MPa级TRIP钢的组织性能及强化机理

鉴于热镀锌TRIP钢在成分和工艺上的特殊性,试验采用低Si含P以及V微合金化的成分设计思路,并进行镀锌连续退火工艺模拟,分析其微观组织和性能。探讨了残余奥氏体稳定性的影响因素及强化机理。结果表明,添加P和采取较高的冷轧变形量均有利于提高残余奥氏体量及其稳定性。在残余奥氏体的形变诱导马氏体相变、贝氏体和V析出的强化作用以及铁素体晶粒细化的共同作用下,抗拉强度和强塑积分别达到1 035 MPa和25 875 MPa·%。     

细晶强化和位错强化对中锰马氏体钢的强化作用

 研究了碳和锰含量对淬火中锰马氏体钢的位错密度、残余奥氏体含量、晶粒尺寸等组织结构以及室温力学性能的影响。借助于SEM、EBSD、TEM和XRD表征了材料的微观组织,探讨了马氏体钢的强化机制。结果表明:随着碳含量增加,淬火中锰钢的位错密度和残余奥氏体体积分数逐渐增加,板条束和板条块尺寸逐渐细化,大角晶界百分数逐渐增加,强度逐渐升高;增加锰含量能够提高马氏体钢的位错密度和抗拉强度。分析认为,位错强化和细晶强化是淬火中锰马氏体钢的主要强化机制。马氏体板条尺寸是马氏体抗拉强度的结构控制单元,而原奥氏体晶粒尺寸则是马氏体屈服强度的结构控制单元。     

多向锻造对超高强度不锈钢组织及力学性能的影响

通过光学显微镜(OM)、扫描电子显微镜(SEM)、背散射电子衍射(EBSD)、透射电子显微镜(TEM)对多向锻造淬火后的马氏体超高强度不锈钢的显微组织进行定量表征,分析多向锻造对试验钢显微组织的影响;同时进行力学性能测试,分析多向锻造对试验钢力学性能的影响,讨论不同强化机制对试验钢强度的贡献。结果表明,随着锻造道次的增加,试验钢的原奥氏体晶粒、马氏体板条束(Packet)、板条块(Block)尺寸均逐渐细化,5个道次后原奥氏体晶粒从226.1细化到3.2 μm,Packet尺寸从106.1细化到2.9 μm,Block尺寸从2.3细化到1.5 μm;试验钢的力学性能显著提升,屈服强度由1 030增加至1 175 MPa,冲击功吸收功由140增加至194 J,伸长率也从9.3%增加到了11.6%;试验钢强度的提升主要归功于位错强化与细晶强化并以位错强化为主,且位错强化与细晶强化并不遵从线性叠加强化机制,而更接近均方根叠加机制。     

23.8%Mn TRIP/TWIP钢的组织性能及强化机制

研究了锰含量(质量分数)为23.8%的低碳高锰钢的力学行为和组织演变,并对其强化机制进行了探讨.结果表明:23.8%Mn TRIP/TWIP钢的屈服强度约为300 MPa,抗拉强度可达610 MPa,断裂延伸率可达到63%.实验钢拉伸变形呈连续屈服,其应变硬化指数n值约为0.48.该钢在变形初期的强化机制以应变诱发孪生为主,变形后期出现应变诱发马氏体相变.位错与形变孪晶、马氏体之间的相互作用也对强度的增加做出贡献.     

500 MPa级Nb Ti微合金化方矩形管用钢的强化机制

采用光学显微镜和透射电子显微镜等对500 MPa级Nb Ti微合金化方矩形管用钢的组织与性能进行了分析，研究了其强化机制。结果表明，终轧温度和卷取温度对试验钢的组织和力学性能有显著影响，在研究的温度范围内，终轧温度和卷取温度的降低均有利于获得更加细小的铁素体晶粒与细小弥散的第二相析出物；当卷取温度不变时，随着终轧温度的下降，屈服强度、抗拉强度和断后伸长率均升高；当终轧温度不变时，随着卷取温度的逐渐下降，屈服强度和抗拉强度呈现出先上升后下降的规律，而断后伸长率呈现出单调上升的规律；试验钢在终轧温度为840 ℃和卷取温度为570 ℃时可获得最优的综合力学性能，其屈服强度和抗拉强度分别为537和578 MPa，断后伸长率为33.5%；细晶强化是试验钢最主要的强化机制，由晶粒细化引起的强度增量占总强度的49%～51%，由固溶强化引起的强度增量次之，占总强度的23%～27%，由析出强化引起的强度增量较小，仅占总强度的3.8%～8.2%。     

内氧化法制备MgO弥散强化铁基材料

采用机械合金化—低温表面氧化—高温内氧化—还原处理制备MgO弥散强化铁粉后再经放电等离子(SPS)烧结制备MgO弥散强化铁基材料,并通过SEM和EDS对材料的组织和断口进行分析。结果表明:添加MgO能够细化晶粒,并均匀地分布于基体中,MgO颗粒尺寸200nm~1μm。添加MgO强化后,材料的拉伸断口由粗大的韧窝变成细小的等轴韧窝;MgO弥散强化铁基材料烧结体的室温力学性能得到有效提高,Fe+1.0%MgO的抗拉强度为342.6MPa,屈服强度为276.3MPa,硬度为61HRB,相对于纯铁分别提高了20.5%、54.2%和84.8%。     

塑性变形方法对奥氏体不锈钢力学性能的影响

在试验的基础上研究了经2种塑性变形(锻造、等径角挤压变形)的奥氏体不锈钢完全再结晶后的组织与性能.研究结果表明,完全再结晶退火后,经锻造、等径角挤压变形得到的晶粒尺寸与屈服强度的关系有明显的差异,塑性变形方法对Hall-Petch关系有显著的影响;等径角挤压变形试验料的晶粒尺寸满足Hall-Petch关系;超大塑性变形(ECAP)会降低奥氏体不锈钢完全再结晶后的屈服强度.     

添加微米级ZrC颗粒的低碳微合金钢强韧性研究

真空条件下,在低碳微合金钢中添加体积分数为1．1％的微米级ZrC陶瓷颗粒,在φ450mm热轧试验机上进行轧制,将ZrC颗粒作为形变核心和奥氏体及形变诱导铁素体的再结晶核心。热轧后晶粒被细化到5．5um,钢板抗拉强度和屈服强度分别达到635MPaN517．5MPa,钢板强韧化的机制是综合引入了形变强化、相变强化、第二相粒子强化及细晶强化的结果。热轧钢板的显微组织为铁素体加少量珠光体,钢中存在大量第二相粒子,并呈弥散分布。     

Recrystallization Behavior of a Heavily Deformed Austenitic Stainless Steel During Iterative Type Annealing

The study describes evolution of the recrystallization microstructure in an austenitic stainless steel during iterative or repetitive type annealing process. The starting heavily cold deformed microstructure consisted of a dual phase structure i.e., strain-induced martensite (SIM) (43 pct in volume) and heavily deformed large grained retained austenite. Recrystallization behavior was compared with Johnson Mehl Avrami and Kolmogorov model. Early annealing iterations led to reversion of SIM to reversed austenite. The microstructure changes observed in the retained austenite and in the reverted austenite were mapped by electron backscatter diffraction technique and transmission electron microscope. The reversed austenite was characterized by a fine polygonal substructure consisting of low-angle grain boundaries. With an increasing number of annealing repetitions, these boundaries were gradually replaced by high-angle grain boundaries and recrystallized into ultrafine-grained microstructure. On the other hand, recrystallization of retained austenite grains was sluggish in nature. Progress of recrystallization in these grains was found to take place by a gradual evolution of subgrains and their subsequent transformation into fine grains. The observed recrystallization characteristics suggest continuous recrystallization type process. The analysis provided basic insight into the recrystallization mechanisms that enable the processing of ultrafine-grained fcc steels by iterative type annealing. Tensile properties of the processed material showed a good combination of strength and ductility.     

Ti-V-Mo微合金化22MnB5钢中析出相及其强化作用

使用温成形替代热成形可以避免热成形过程中表面氧化等问题,但热成形常用22MnB5钢在高温回火后出现明显的软化现象.而通过向钢中添加Ti、V、Mo等微合金元素可以在钢中形成细小的析出相以及细化晶粒,起到提高强度的作用,从而可以解决该问题.因此,通过在22MnB5钢中添加Ti、V、Mo微合金元素,利用OM(光学显微镜)、F...     

冷轧变形量对高氮奥氏体不锈钢Mnl7Crl9N0.6组织和性能的影响

对感应炉-电渣重熔冶炼的节镍型高氮奥氏体不锈钢Mn17Cr19N0.6的3mm热轧板进行变形量10%～60%的冷轧及拉伸实验。结合金相组织观察及XRD物相分析,研究高氮奥氏体不锈钢冷变形过程中微观组织变化规律,得出结论:在冷轧过程中,随着变形量的增加,实验钢中晶粒的形状由块状到压扁拉长状,滑移从单滑移为主到交滑移,孪晶最终被分割破碎。实验钢在不同冷轧变形量后的组织均为单一的奥氏体相,并没有出现其他相,实验钢在冷变形过程中没有发生马氏体转变,因此,实验钢在冷轧过程中没有通过相变强化,以形变强化为主,抗拉强度从冷轧变形量为10%时的1045 MPa升高至变形量为60%时的1880MPa,因此通过冷变形可以制备出不同强度级别且组织为单一奥氏体的特种材料。     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

Nanograined/Ultrafine-Grained Structure and Tensile Deformation Behavior of Shear Phase Reversion-Induced 301 Austenitic Stainless Steel

We describe here an electron microscopy study of shear reversion-induced nanograined/ultrafine-grained (NG/UFG) structure and evolution of tensile strained microstructure in metastable type 301 austenitic stainless steel. The NG/UFG structure with grain size in the range of 200 to 500 nm was obtained by severe cold deformation and controlled annealing in the narrow temperature range of 973 to 1073 K (700 to 800 °C). The different stages of annealing involve the following: (a) transformation of strain-induced martensite to highly dislocated lath-type austenite, (b) formation of dislocation-cell structure and transformation to recovered austenite structure with defect-free subgrains, and (c) coalescence of subgrains to form a NG/UFG structure concomitant with a completely recrystallized structure, and consistent with martensitic shear-type phase reversion mechanism. The optimized cold working and annealing treatment resulted in NG/UFG material with a high yield strength (~1000 MPa) and high ductility (~30 pct) combination. Multiple deformation mechanisms were identified from postmortem electron microscopy examination of tensile strained NG/UFG 301 austenitic stainless steel and include dislocation glide and twinning. The evidence of heterogeneous nucleation of overlapping stacking faults and partial dislocations points toward deformation     

钛微合金化HRB400E钢筋组织性能及强化机理

为系统研究含钛钢连续冷却相转变和强化机理,利用热模拟试验机、高分辨透射电镜及金相显微镜等设备进行试验。结果表明,低冷速下(0.5~1 ℃/s),组织主要为铁素体和珠光体;冷速逐步增加(1~5 ℃/s),贝氏体组织出现,且贝氏体比例逐渐增加;高冷速后(5~10 ℃/s),组织以贝氏体为主。含钛试验钢强化机制为析出强化和细晶强化。晶粒内部弥散析出10~20 nm的TiN。优化冷速为(1.5±0.5) ℃/s开展20 mm HRB400E钢筋工业试制,屈服强度不小于430 MPa,断后伸长率不小于20%,最大力总伸长率不小于15%,强屈比不小于1.4。     

Load sharing between austenite and ferrite in a duplex stainless steel during cyclic loading

The load sharing between phases and the evolution of micro- and macrostresses during cyclic loading has been investigated in a 1.5-mm cold-rolled sheet of the duplex stainless steel SAF 2304. X-ray diffraction (XRD) stress analysis and transmission electron microscopy (TEM) show that even if the hardness and yield strength are higher in the austenitic phase, more plastic deformation will occur in this phase due to the residual microstresses present in the material. The origin of the microstresses is the difference in coefficients of thermal expansion between the two phases, which leads to tensile microstresses in the austenite and compressive microstresses in the ferrite. The microstresses were also found to increase from 50 to 140 MPa in the austenite during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. The cyclic loading response of the material was, thus, mainly controlled by the plastic properties of the austenitic phase. It was also found that initial compressive macrostresses on the surface increased from −40 to 50 MPa during the first 10³ cycles. After the initial increase of microstresses and macrostresses, no fading of residual stresses was found to occur for the following cycles. A good correlation was found between the internal stress state and the microstructure evolution. The change in texture during cyclic fatigue showed a sharpening of the deformation texture in the ferritic phase, while no significant changes were found in the austenitic phase.