低碳-硅-锰系TRIP钢的组织演变规律研究

采用彩色金相、SEM、TEM和X射线衍射技术研究了低碳-硅-锰TRJP钢在单向拉伸状态下的组织演变规律.结果表明,TRIP钢变形前的组织为F、B和残余奥氏体,经拉伸变形后部分残余奥氏体在应变作用下转变为孪晶结构的马氏体,提高了钢的强度;TRIP钢的断裂为韧性断裂,位于F晶界处的残余奥氏体发生相变从而松弛了应力,延缓了断裂的产生,使TRIP钢板获得高塑性.     

铆螺钢不变形和变形条件下连续冷却的相变行为

在Gleeble-1500热模拟试验机上进行铆螺钢的热模拟实验,通过研究其不变形条件下和与实验室轧机轧制变形量相一致的变形条件下的连续冷却相变行为,建立了相应的静态和动态CCT图,通过扫描电镜(SEM)对其组织进行观测。结果表明,由于形变增加了形核位置和能量,加速了相变,在热变形的CCT图中,变形使铁素体、珠光体和贝氏体转变线向高温区的左移。最快的冷却速度获得了全部的马氏体组织;随冷却速度降低,粒状贝氏体、多边形铁素体和珠光体组织形成;快冷抑制了铁素体和珠光体形成,使硬度增高;硅、锰和铬合金元素使CCT图中的珠光体和贝氏体转变线右移;在变形条件下以3.3℃/s~16.7℃/s的冷速冷却时,能够得到多边形铁素体、粒状贝氏体和残余奥氏体组织。由于组织中残余奥氏体的存在,有助于产生相变诱发塑性(TRIP)效应,铆螺钢实际轧制时可能能够获得满意的冷镦性能。     

On the role of martensitic transformation on damage and cracking resistance in TRIP-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J_R curves and CTOD measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite.     

Influence of Isothermal Bainite Transformation Time on Microstructure and Mechanical Properties of Hot-Dip Galvanized TRIP Steel

The influence of isothermal bainitic transformation (IBT) time on microstructure and mechanical properties of hot-dip galvanized TRIP steel with 0.20C-1.50Mn-1.2Al-0.26Si was investigated using optical microscopy, x-ray diffraction (XRD), Transmission Electron Microscope (TEM), dilatometry, and mechanical testing. This steel has high tensile strength of over 780 MPa with elongation more than 22%. The microstructure of the steel mainly consisted of ferrite, bainite, retained austenite, and martensite. The metastable austenite remaining after bainitic transformation will be transformed into martensite at the final cooling stage. The IBT time affects retained austenite content. When the IBT increased from 10 to 60 s, the amount of retained austenite increased from 9.40 to 15.42%, correspondingly. The IBT time also affects the strain hardening behavior. The n value characteristics of samples for IBT time from 10 to 30 s are similar to those of DP steel; however, the n value of specimen with IBT of 60 s shows features typical of TRIP steel.     

0.2C-1.5Mn-1.5Si相变诱发塑性(TRIP)钢板在烤漆过程中的组织和力学性能变化

本文系统地研究了0.2C-1.5Mn-1.5Si TRIP钢板在烤漆硬化处理过程中组织和力学性能的变化规律.结果表明,在烤漆过程中,TRIP钢板中处于亚稳态的残留奥氏体发生了贝氏体相变,经烤漆处理的TRIP钢板屈服强度提高,表现出一定程度的烤漆硬化性.     

A study of microstructure,transformation mechanisms and correlation between microstructure and mechanical properties of a low alloyed TRIP steel

Differently heat treated samples of a low alloyed TRIP steel have been investigated using electron diffraction techniques in SEM and TEM. Aim was, first, to discriminate the microstructure constituents, austenite, ferrite, bainite, and martensite, second to gain information on the γ–α phase transformation mechanisms and third to correlate the mechanical properties and the microstructure of the samples. Bainite always occurs in conjunction with an orientation gradient in the surrounding ferrite matrix. It consists of fine lamellae of ferrite and austenite which show a sharp Kurdjumov–Sachs orientation relationship with each other. This was interpreted in terms of a displacive bainite formation mechanism. The microstructure is formed by growth of γ-grains during intercritical annealing and shrinking of these grains during the subsequent cooling without nucleation of new α-grains. The transformation first occurs reconstructively into ferrite and then, at lower temperature, displacively into bainite. The mechanical properties of differently heat treated samples are most strongly influenced by the amount and distribution of carbon in the retained austenite and by the degree of recovery in bainite and austenite.     

等温淬火温度对含铌TRIP钢组织和力学性能的影响

利用金相显微镜、X射线衍射等方法研究了0.15C-1.46Si-1.56Mn-0.06Nb冷轧TRIP钢板等温淬火温度对组织和力学性能的影响。结果表明,试验钢最佳的临界热处理工艺:在840℃两相区保温180 s,再快速冷却到420℃并在该温度保温240 s,进行贝氏体等温转变处理。采用这种热处理工艺,试验钢的微观组织为铁素体+贝氏体+残留奥氏体,其中铁素体占72%,贝氏体占20%,残留奥氏体占8%,可获得较佳的相变诱发塑性和较好的强韧性配合,其强塑积可达到2.5×104MPa.%,提高或降低贝氏体等温淬火温度都会降低强塑积。结果还表明,在840℃,适当的延长热处理时间可以提高残留奥氏体体积分数及残留奥氏体的碳含量,有助于提高材料的强塑积。     

EFFECT OF AUSTEMPERING ON TRANSFORMATION INDUCED PLASTICITY OF HOT ROLLED MULTIPHASE STEELS

Effect of austempering on the transformation induced plasticity (TRIP) of hot rolled multiphase steel was investigated. Polygonal ferrite, granular bainite, and a large amount of stabilized retained austenite could be obtained in the hot rolled multiphase steel. Strain induced martensite transformation (SIMT) of retained austenite and TRIP effectively occur under straining owing to austempering after hot rolling, and mechanical properties of the present steel remain at a relatively high constant value for austempering at 400℃. The mechanical properties of the steel exhibited a good combination of tensile strength (791MPa) and total elongation (36%) because the stability of retained austenite is optimal when the steel is held for 20min.     

空冷弛豫对铌微合金化热轧TRIP钢组织性能的影响

通过热轧试验研究了轧后空冷弛豫时间对铌微合金化热轧TRIP中厚板组织和力学性能的影响,结合光学显微镜、扫描电镜、透射电镜及XRD分析了弛豫过程各组成相的尺寸、含量变化以及铁素体基体上的位错组态和微合金碳氮化物的析出行为.结果表明,采用820 ℃终轧并弛豫80～120 s后超快冷的工艺可以得到含有铁素体、贝氏体和残留奥氏体的多相组织,弛豫时间显著影响铁素体晶粒尺寸、铁素体量以及铁素体基体上的位错密度和沉淀析出量.随着弛豫时间的延长,试验钢的屈服强度和抗拉强度呈降低趋势.空冷弛豫80 s时,试验钢的抗拉强度、伸长率和强塑积分别达到820 MPa,37.5%和30750 MPa·%的最大值.     

低碳Si-Mn系TRIP钢的热处理工艺对组织的影响

低碳Si-Mn系TRIP钢有着复杂的显微组织，主要由多边形铁素体（F）＋无碳贝氏体（B）＋残留奥氏体（AR）组成。本试验采用了彩色金相法，并结合X－ray衍射、SEM和TEM等手段研究了低碳Si-Mn系TRIP钢显微组织与工艺的关系，发现随着两相区退火温度的升高，最终显微组织中铁素体基体体积分数变小，并且贝氏体量增多，残留奥氏体的稳定性呈起伏式变化；在贝氏体转变区的等温温度过高或过低，均使最终显微组织中残留奥氏体体积分数减少；在贝氏体转变区等温时，所形成贝氏体表现出粒状的特征。     

Effects of Austempering after Hot Deformation on the Mechanical Properties of Hot Rolled Si-Mn TRIP Steel Sheets

Excellent mechanical properties are obtained by austempering after hot deformation without subsequent heat treatment in the present Si-Mn TRIP steel sheets. Isothermal holding time after finishing rolling has affected the mechanical properties of this steel. The results show that the sample exhibits a good combination of ultimate tensile strength and total elongation when it is held at the bainite transformation temperature after hot deformation. The stability of retained austenite increases with an increase of isothermal holding time, and a further increase in the holding duration results in a decrease of it. The tensile strength, total elongation and strength ductility reach the maximum values (774MPa, 33% and 25542MPa% respectively) for this sort of hot rolled Si-Mn TRIP steel using the optimal technology.     

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.     

Effect of Microstructure on Tensile Behavior and Mechanical Stability of Retained Austenite in a Cold-Rolled Al-Containing TRIP Steel

In the present study, a quenching treatment prior to two-stage heat treatment was conducted on a Fe–0.28 C–1.55 Mn–2.06 Al transformation-induced plasticity steel to tailor the final microstructure. Compared with the microstructure of the ferrite, bainite and blocky retained austenite obtained by conventional two-stage heat treatment, the microstructure subjected to quenching plus two-stage heat treatment was composed of the ferrite, lath bainite and film-like retained austenite. The corresponding tensile behavior and mechanical stability of retained austenite were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the mechanical stability of blocky retained austenite grains is lower and most of them transform to martensite during the tensile deformation, which leads to higher ultimate tensile strength and instantaneous work hardening exponent. Film-like retained austenite has relatively higher stability, which could cause sustained work hardening and high ductility as well as product of strength and elongation.     

HSLA钢连续冷却过程的相变与组织研究

在Gleeble—1500热模拟试验机上研究了20SiMn3NiA钢在不同连续冷却条件下相和组织变化,用热膨胀法测定了该钢的连续冷却转变曲线（动态CCT曲线）。研究结果表明,20SiMn3NiA钢中的Mn、Ni、Si等合金元素能有效地阻止铁素体和珠光体的形成,故20SiMn3NiA钢的过冷奥氏体连续冷却转变曲线只有马氏体和贝氏体相变区。当临界冷却速度大于1℃／s时,20SiMn3NiA钢就可以获得板条状马氏体组织,且随着冷却速度的增大,马氏体组织变得越来越细。与静态CCT曲线相比,形变使动态CCT曲线的Ms点升高,奥氏体稳定性降低,形变细化了马氏体和贝氏体组织,使20SiMn3NiA钢在1℃／s的冷却速率下产生较高的强度。     

Effect of Prior Cold Deformation on the Stability of Retained Austenite in GCr15 Bearing Steel

In this work, the effect of prior cold deformation on the stability of retained austenite in GCr15 bearing steel was investigated after quenching and tempering treatment. The thermal stability was evaluated by calculating thermal activation energy for decomposition of retained austenite using differential scanning calorimeter. The mechanical stability was investigated according to the strain-induced martensitic transformation behavior of retained austenite under the standard compression testing. It is found that the prior cold deformation not only accelerates the carbide dissolution during the austenitization process but also contributes to the carbon partitioning in the tempering stage due to the higher density of phase boundaries, which results in the improvement of the thermal stability of retained austenite. Due to the enhanced carbide dissolution, the higher carbon content in the prior austenite will intensify the isotropic strain of martensitic transformation. As a consequence, the film-like retained austenite is likely to form under a higher hydrostatic pressure and thus shows a higher mechanical stability. Additionally, it is noteworthy that the benefits of the prior cold deformation to the stability of retained austenite would be saturated when the cold deformation degree is larger than 40%.     

退火工艺对TRIP980冷轧钢板显微组织和力学性能的影响

以冷轧TRIP980钢为研究对象,探讨了退火温度、过时效温度和过时效时间对钢板组织性能的影响。结果表明:退火温度从800 ℃降低至760 ℃,随着奥氏体化程度的降低和原奥稳定性的增强,冷却后组织中硬相含量更低,残奥含量更高,宏观表现为拉伸强度降低、伸长率提高;过时效温度从360 ℃提高至400 ℃,随着贝氏体体积分数的提高,拉伸强度提高;过时效时间从600 s延长至1500 s,随着硬相贝氏体的软化和残奥稳定性增大,拉伸强度降低、伸长率提高。     

冷却介质对ZG30CrMn2Si2NiMo组织和力学性能的影响

研究了不同冷却介质对ZG30CrMn2Si2NiMo铸钢力学性能及组织的影响.试验结果表明,奥氏体化后空冷ZG30CrMn2Si2NiMo组织为板条贝氏体型铁素体和残余奥氏体组织,奥氏体化后淬火ZG30CrMn2Si2NiMo组织为板条马氏体、贝氏体型铁素体和残余奥氏体组织.ZG30CrMn2Si2NiMo获得板条马氏体、贝氏体型铁素体和残余奥氏体组织时,具有良好的力学性能.     

Stress-assisted martensitic transformations in steels: A 3-D phase-field study

A 3-D elastoplastic phase-field model is developed for modeling, using the finite-element method, the stress-assisted martensitic transformation by considering plastic deformation as well as the anisotropic elastic properties of steels. Phase-field simulations in 3-D are performed by considering different loading conditions on a single crystal of austenite in order to observe the microstructure evolution. The thermodynamic parameters corresponding to an Fe–0.3% C steel as well as the physical parameters corresponding to commercial steels, acquired from experimental results, are used as input data for the simulations. The simulation results clearly show the well-known Magee effect and the Greenwood–Johnson effect. The results also show that even though the applied stresses are below the yield limit of the material, plastic deformation initiates due to the martensitic transformation, i.e. the well-known transformation-induced plasticity (TRIP) phenomenon. It is concluded that the loading conditions, TRIP as well as autocatalysis play a major role in the stress-assisted martensitic microstructure evolution.     

Effect of Retained Austenite on Properties of Austempered Ductile Iron

Austempered ductile iron (ADI) exhibits a favourable combination of strength and toughness, and has been used as a substitute for quench-tempered or carburise-quenched steel. A characteristic feature of bainite transformation of cast iron, as opposed to carbon steel, is that precipitation of carbide is suppressed by the high concentration of silicon. Thus, a favourable structure, consisting of bainitic ferrite and retained austenite without carbide, can be provided by the optimum austempering treatment. Such microstructure and the mechanical properties of the iron are significantly affected by the conditions of the austempering treatment and the chemical composition. In this study, several grades of ductile iron were austempered under various conditions. The relationship between the impact strength, the quantity of retained austenite and the isothermal transformation curve was investigated. The stability of the retained austenite is considered important, because ADI contains a large amount of retained austenite which contributes to the improvement of ductility and toughness and which may transform to martensite when held at low temperature or subjected to stress. In this study, the stability of the retained austenite at low temperatures was examined by holding or stressing to establish the relations between transformation and temperature, stress and strain.

When the austempering time is short, the untransformed austenite partially transforms to martensite during air cooling, due to the lower carbon content, resulting in lower impact strength. As the austempering time increases, the untransformed austenite is stabilised by carbon-enrichment and there is little transformation to martensite, resulting in a large amount of retained austenite and higher impact strength. When the austempering time becomes much longer, the carbon-enriched austenite decomposes, presumably to bainitic ferrite and carbide, decreasing impact strength. In increasing the silicon content, precipitation of carbide in bainite is suppressed and both the maximum impact value and the content of retained austenite increase. The decreasing rates after the maxima through an additional isothermal holding becomes smaller.

By holding at temperatures down to –40°C, the decrease in retained austenite and the increase in hardness are both small. The retained austenite is stable under stress lower than that required to cause plastic deformation. Compressive stress hinders the martensitic transformation, because the transformation is accompanied by volume expansion.     

不同碳含量贝氏体合金钢等温淬火后的组织与性能

研究了3种碳含量(0.22C、0.34C、0.45C)的贝氏体钢在960℃奥氏体化+Ms点以上10～50℃等温淬火工艺下碳含量对贝氏体组织转变和力学性能的影响。结果表明,3种试验钢经过等温淬火处理后均获得由贝氏体铁素体和残留奥氏体相间分布组成的无碳化物贝氏体组织;随着碳含量的降低,贝氏体相变时间显著缩短,贝氏体铁素体板条变厚,硬度和抗拉强度呈下降趋势,但冲击性能显著提高,这主要是与低碳钢贝氏体转变温度更高,贝氏体铁素体板条粗大但高碳含量的大块状残留奥氏体减少有关。