形变强化和逆相变细晶强化对Fe-18Cr-8Ni钢组织和性能的影响北大核心CSCD

奥氏体不锈钢较低的屈服强度限制了它在结构件中的使用。采用形变和相逆转变方法分别制备出了高屈服强度的奥氏体不锈钢。利用X射线衍射仪、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和万能试验机分别对奥氏体钢进行组织表征和力学性能测试,结果表明粗大的奥氏体晶粒在形变过程中形成位错、剪切带、应变诱导马氏体等组织,相逆转变方法获得了超细的无缺陷等轴奥氏体晶粒。形变强化和细晶强化均能明显提高奥氏体不锈钢屈服强度(280 MPa提升至550 MPa)的同时保持较好的塑性(伸长率46%和55%)。     

C和Mn元素配分行为对冷轧中锰TRIP钢组织性能的影响

采用冷轧+两相区温轧退火(CR+WR+IA)热处理工艺,研究了两相区退火时间对超细晶铁素体与奥氏体中组织形貌演变、C和Mn元素配分行为以及力学性能的影响。结果表明,冷轧试验钢经两相区形变退火处理后,获得了由铁素体、残余奥氏体或新生马氏体组成的超细晶复相组织。在645℃随退火时间的延长,形变马氏体向逆相变奥氏体配分的C、Mn元素增多,C、Mn元素富集位置增加,同时富Mn区形变马氏体回复再结晶现象明显;伴随少量碳化物溶解,试验钢的屈服强度由741 MPa持续降低到325 MPa。两相区退火10 min时,试验钢力学性能最佳,此时抗拉强度达到最大值1 141 MPa,断后伸长率及均匀伸长率分别为23.6%和18.1%,强塑积达到26.928 GPa·%。     

7.9Mn-1.4Si-0.07C钢高强韧机理研究

通过热轧、温轧、奥氏体化、两相区退火处理得到7.9Mn-1.4Si-0.07C钢板,该材料的拉伸强度及塑性随奥氏体化温度不同而具有显著差异.奥氏体化温度降低,室温下奥氏体含量升高,综合力学性能提高.当奥氏体化温度由900℃降低为800℃时,所得到钢板的奥氏体体积分数由15%增加到28%,拉伸强度由1 150 MPa提高到1 340 MPa,塑性由21%提高至27%.实验钢优异的力学性能源于其中大量的超细铁素体及奥氏体,细晶强化使其具有超高强度,铁素体基体及变形过程中奥氏体向马氏体相变提供了良好的塑性.基体组织中的位错强化,形变诱导马氏体转变的TRIP效应等是增强该钢板加工硬化能力的主要因素.     

C和Mn元素配分行为对冷轧中锰TRIP钢组织性能的影响

采用冷轧+两相区温轧退火（CR+WR+IA）热处理工艺,研究了两相区退火时间对超细晶铁素体与奥氏体中组织形貌演变、C和Mn元素配分行为以及力学性能的影响。结果表明,冷轧试验钢经两相区形变退火处理后,获得了由铁素体、残余奥氏体或新生马氏体组成的超细晶复相组织。在645℃随退火时间的延长,形变马氏体向逆相变奥氏体配分的C、Mn元素增多,C、Mn元素富集位置增加,同时富Mn区形变马氏体回复再结晶现象明显;伴随少量碳化物溶解,试验钢的屈服强度由741持续降低到325MPa。两相区退火10min时,试验钢力学性能最佳,此时抗拉强度达到最大值1141MPa,断后伸长率及均匀伸长率分别为236%和181%,强塑积达到26928MPa·%。     

反复轧制过程中TA1纯钛板的组织演化及强化机制

采用反复轧制工艺制备了超细晶TA1纯钛板。通过金相、透射电镜、X射线衍射、扫描电镜等手段,分析了纯钛板在反复轧制过程中,不同的应变量所对应的组织形貌特点,并测试了强度、塑性,观察了宏观断口与微观形貌。结果表明:纯钛在常规轧机上经过反复轧制可显著细化晶粒,晶粒尺寸由轧制前的80μm降至120 nm;强度则随着轧制应变量的增加而提高,当Von Mises等效应变为2.4时,平均屈服强度提高到678 MPa,是轧制前粗晶的3倍多;位错及其交互作用是细化晶粒的主要机制,在高密度位错区域由于位错的交互作用而形成了位错胞和亚晶粒,最终演变成超细晶粒;细晶强化和加工硬化是导致纯钛轧制后强度显著提高的主要原因。     

反复轧制过程中TA1纯钛板的组织演化及强化机制

采用反复轧制工艺制备了超细晶TA1纯钛板。通过金相、透射电镜、X射线衍射、扫描电镜等手段,分析了纯钛板在反复轧制过程中,不同的应变量所对应的组织形貌特点,并测试了强度、塑性,观察了宏观断口与微观形貌。结果表明:纯钛在常规轧机上经过反复轧制可显著细化晶粒,晶粒尺寸由轧制前的80μm降至120 nm;强度则随着轧制应变量的增加而提高,当Von Mises等效应变为2.4时,平均屈服强度提高到678 MPa,是轧制前粗晶的3倍多;位错及其交互作用是细化晶粒的主要机制,在高密度位错区域由于位错的交互作用而形成了位错胞和亚晶粒,最终演变成超细晶粒;细晶强化和加工硬化是导致纯钛轧制后强度显著提高的主要原因。     

TWIP钢室温静态拉伸下的组织形态和变形机制

在室温下对退火Fe-24Mn-1Si-1.5Al-0.045CTWIP钢进行了不同程度的拉伸变形,采用JEM-2100透射电子显微镜对变形后的组织形貌进行表征和分析。研究结果表明:在变形初期,晶粒内存在着大量位错,它们相互缠结,呈胞状结构。在此阶段,位错滑移为主要变形机制。随着变形量的增加,形变孪晶在晶界等处形成,孪生机制被激活,孪生和滑移机制相互竞争。双孪生系统在大多数晶粒内先后被激活,孪生和滑移机制相互交割,起到动态细化晶粒的作用,使强度显著提高。在变形后期,试验钢的变形机制主要是TRIP效应,以及孪生与滑移的相互作用而诱发了去孪生机制,层状组织出现,孪晶特征减弱,从而导致样品的局部变形和失效。     

固溶温度对节约型双相不锈钢TRIP/TWIP行为的影响

 通过微拉伸、电子背散射（EBSD）、透射电子显微镜（TEM）等手段,研究了具有亚稳奥氏体相的节约型双相不锈钢在1 000～1 200 ℃范围内不同固溶温度下的组织与性能的演变规律;探讨了固溶温度对形变诱导塑性（TRIP/TWIP）的作用机制。结果表明,随着固溶温度的升高,抗拉强度与伸长率均先升高后降低,而亚稳奥氏体相比例由74%（1 000 ℃）降低到37%（1 200 ℃）;1 050 ℃固溶时,试验钢表现出最佳综合性能,抗拉强度达到960 MPa,伸长率达到62%,强塑积达到60 GPa·%。在经拉伸变形的微观结构中形变诱导马氏体与形变孪晶共存,表明试验钢中亚稳奥氏体相的变形机制主要受TRIP及TWIP共同控制,从而导致其塑性变形过程呈现多阶段应变硬化特征,而钢中铁素体相的变形机制主要变形为位错的滑移。     

室温ECAP变形纯钛孪生行为研究

室温下对纯钛进行多道次等径弯曲通道变形（ECAP）,分别采用光学显微镜（OM）、透射电镜（TEM）、电子背散射衍射仪（EBSD）、室温拉伸和显微硬度观察,测试纯钛变形过程组织演变和力学性能变化规律,探讨纯钛室温变形机制和孪生行为。结果表明,纯钛ECAP变形过程中出现■拉伸孪晶和■压缩孪晶,随着挤压道次的增大,孪晶数量先增大后减小。孪晶的出现有效改变晶格取向,激发进一步位错滑移,辅助塑性变形过程,使纯钛显微组织有效细化,经过4道次ECAP变形,平均晶粒尺寸由约63.79μm细化至约2.81μm。1道次变形后晶粒细化效果最显著,平均晶粒尺寸比变形前减小约94%;随着变形道次的增加,晶粒细化效果减弱,4道次变形后平均晶粒尺寸累积减小约95.6%。同时,大量位错、孪晶和亚晶的形成,使得位错、孪晶以及亚晶之间的相互作用加强,显著提高了纯钛的屈服强度和显微硬度,4道次变形后,屈服强度从215 MPa增加到600 MPa,增幅为179%;显微硬度从HV 129增加到HV 200。由于1道次变形后晶粒细化效果最显著,并且出现大量孪晶和位错,屈服强度与硬度的增幅也最大。     

底材和退火对纳米/微米晶复合304不锈钢组织和力学性能的影响

通过铝热反应熔化法在铜底材和玻璃底材上制备块体纳米晶/微米晶复合的304不锈钢,研究了不同底材和退火工艺对钢的组织和力学性能的影响。研究发现2种底材上制备的不锈钢均包含纳米晶和微米晶,铜底材和玻璃底材上制备的材料的晶粒尺寸分别为28.8nm和30.0 nm,退火后变为21.4 nm和22.7nm。铜底材上制备的304不锈钢抗拉强度,屈服强度和伸长率分别为521 MPa,279 MPa和8.1%,退火后分别为1 001 MPa,475 MPa和9.3%。玻璃底材上制备的304不锈钢抗拉强度、屈服强度和伸长率分别为358 MPa,221 MPa和7.5%,退火后分别为1 023 MPa,461 MPa和8.6%。表明等温退火能够提高纳米晶/微米晶复合304不锈钢的性能。     

铸态304L奥氏体不锈钢等径角挤压变形研究

 研究了铸态304L奥氏体不锈钢在等径角挤压(ECAP)变形过程中显微组织的演变过程。结果表明,经4道次剪切变形后树枝晶破碎、原始粗大晶粒碎化。显微组织的变化过程可归纳为：原始粗晶粒→晶粒被滑移带分割→位错发展形成高密度位错墙,与滑移带共同作用形成胞块结构→应变增加形成层片状界面→形成大角度晶界的细小晶粒。表明铸态304L奥氏体不锈钢经ECAP变形后塑性变形机制主要由滑移完成。     

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。     

Microstructure Refinement and Property Improvement of Metastable Austenitic Manganese Steel Induced by Electropulsing

Grain refinement efficiency of electropulsing treatment(EPT)for metastable austenitic manganese steel was investigated.The mean grain size of original austenite is 300μm.However,after EPT,the microstructure exhibits a bimodal grain size distribution,and nearly 70vol.%grains are less than 60μm.The refined austenite results in ultrafine martensitic microstructure.The tensile strengths of refined austenitic and martensitic microstructures were improved from 495to 670,and 794to 900MPa respectively.The fine grained materials possess better fracture toughness.The work-hardening capacity and wear resistance of the refined austenitic microstructure are improved.The reasonable mechanism of grain refinement is the combination of accelerating new phase nucleation and restraining the growth of neonatal austenitic grain during reverse transformation and rapid recrystallization induced by electropulsing.     

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     

Effect of Grain Size on Mechanical Properties of Nickel-Free High Nitrogen Austenitic Stainless Steel

The fine grained structures of nickel-free high nitrogen austenitic stainless steels had been obtained by means of cold rolling and subsequent annealing.The relationship between microstructure and mechanical properties and gain size of nickel-free high nitrogen austenitic stainless steels was examined.High strength and good ductility of the steel were found.In the grain size range,the Hall-Petch dependency for yield stress,tensile strength,and hardness was valid for grain size ranges for the nickel-free high nitrogen austenitic stainless steel.In the present study,the ductility of cold rolled nickel-free high nitrogen austenitic stainless steel decreased with annealing time when the grain size was refined.The fracture surfaces of the tensile specimens in the grain size range were covered with dimples as usually seen in a ductile fracture mode.     

Recrystallization kinetics of an austenitic high-manganese steel subjected to severe plastic deformation

The evolution of the microstructure and the properties of an austenitic high-manganese steel subjected to severe deformation by cold rolling and subsequent recrystallization annealing is investigated. Cold rolling is accompanied by mechanical structural twinning and shear banding. The microhardness and microstructural analysis of annealed samples are used to study the recrystallization kinetics of the high-manganese steel. It is shown that large plastic deformation and subsequent annealing result in rapid development of recrystallization processes and the formation of an ultrafine-grained structure. A completely recrystallized structure with an average grain size of 0.64 μm forms after 30-min annealing at a temperature of 550°C. No significant structural changes are observed when the annealing time increases to 18 h, which indicates stability of the recrystallized microstructure. The steel cold rolled to 90% and annealed at 550°C for 30 min demonstrates very high strength properties: the yield strength and the tensile strength achieve 650 and 850MPa, respectively. The dependence of the strength properties of the steel on the grain size formed after rolling and recrystallization annealing is described by the Hall–Petch relation.     

High performance nano-structured stainless steel sheet

High-strength steels have been attracting more and more attention of people,Unfortunately.deterioration of ductility limited their applications.To solve this problem,a nano-structured stainless steel sheet is developed to combine high strength and high ductility.Processing of the surface mechanical attrition treatment(SMAT) was introduced to obtain a nano-grain layer on the double surface of the stainless steel sheet.The microstructure of the nanostructured steel sheet is characterized by an alternate distribution of coarse grained layer and nanocrystalline layer.Then the dual surface nano-crystallized stainless steel sheets were co-warm rolled at 500℃.The experimental results reveal that the mechanical properties of the nanostructured steel exhibit high yield strength in the range of 700 -950 MPa and tensi le strength higher than 930 MPa.Moreover,elongation to fracture reaches to 15%-48%, together with a uniform elongation stabilized to 13%-45%.