Critical ausforming temperature to promote isothermal bainitic transformation in prior-deformed austenite

The effects of deformation temperature on phase transformation and microstructure in nanostructured bainite steel were studied. The results indicate that the deformed austenite with a strain of 0.3 at 300°C presents accelerated kinetics of bainitic transformation. However, the amount of bainite in ausformed austenite then reduces with the increase in deformation temperature. A critical deformation temperature, determining whether the bainitic transformation can be promoted, was found in deformed austenite. In addition, the thickness of bainite plate in deformed austenite reduces with the decrease in ausforming temperature. The adjacent bainite ferrite plates grow up interactively, and the intersection angle is about 60–73°. A lower ausforming temperature contributes to a more serious cross-growth phenomenon of bainite plates.     

Microstructures and Mechanical Properties of Si-AI-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP （transformation induced plasticity） steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10^-2 to 10^-4 s^-1, the volume fraction of transformed martensite from retained austenite, as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

Banded microstructure in a low-alloyed eutectoid steel

The possible influence of banded microstructure on the fracture performance of a high-carbon steel is investigated using electron microscopy and X-ray diffraction techniques. The banded microstructure was found to be alternating layers of bainite and tempered martensite/retained austenite. Transformation of bainite was promoted by a combined effect of segregation of alloying elements, non-uniform thermal gradient across the steel, and the insufficient austenitisation. The transformation of bainite along the prior austenite grain boundaries (PAGBs) introduces a non-uniform strain distribution at bainite/martensite interface and a higher stress concentration, which may eventually lead to the failure of the heavy section in a brittle intergranular manner. Results provide insights to the importance of understanding the bainitic transformation in such eutectoid steel systems.     

Microstructures and Mechanical Properties of Si-Al-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10-2 to 10-4 s-1, the volume fraction of transformed martensite from retained austenite,as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

奥氏体变形对铌微合金钢贝氏体相变的影响

用Gleeb le-1500热模拟机研究了铌微合金钢奥氏体变形后连续冷却条件下贝氏体相变规律.研究表明,随变形温度的升高,先共析铁素体量较少,贝氏体的板条束变宽.在连续冷却条件下,贝氏体的转变量随变形量的增加而减少,随应变速率的增加而减少,但应变速率对贝氏体转变量的影响随冷却速度的增加而减弱.随着变形后保温时间的延长,奥氏体稳定性增加,在较快冷却条件下转变产物中存在残余奥氏体.     

含磷和钒热轧TRIP钢组织控制及力学性能研究

为探讨含磷和钒热轧TRIP钢的组织控制和力学性能,采用不同变形温度（900和800℃）,研究其相变行为,并在此基础上进行热轧试验.研究表明：随着冷速增加,变形温度对铁素体相变开始温度（Ar3）的影响逐渐增大;相同冷速条件下,变形使贝氏体相变开始温度（Bs）升高;变形对贝氏体相变的促进作用,随着变形温度的降低而减弱.终轧...     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Effect of hot deformation on phase transformation kinetics of 86CrMoV7 steel

The time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams of 86CrMoV7 steel with and without hot deformation were constructed by means of dilatometry, metallography and transmission electron microscopy (TEM). The results show that the pearlite and bainite transformations of 86CrMoV7 steel can be promoted and the microstructure can be refined by hot deformation. The undissolved carbides associated with hot deformation increase the inhomogeneity of carbon distribution in deformed austenite. The inhomogeneities of the austenite increase the number of nucleation sites for pearlite and bainite, and promote pearlite and bainite formation, which result in refinement of both the pearlite and bainite microstructures. In contrast, the undissolved carbides do not play a direct role on the pearlite and bainite transformation of 86CrMoV7 steel in the absence of hot deformation.     

55SiMnMo钢正火组织的精细结构和晶体学特征

55SiMnMo 钢的正火组织由无碳化物上、下贝氏体,马氏体和残余奥氏体组成。贝氏体含有中脊线,组织单元和位错;马氏体含有孪晶;残余奥氏体含有高密度位错。奥氏体的位向关系和惯习面表明它以切变机制生成。从界面富碳和界面应力弛豫两方面阐述了贝氏体组织单元以及贝氏体片条的生成机制。     

The approach to equilibrium during tempering of a bulk nanocrystalline steel: an atom probe investigation

A local electrode atom probe has been used to analyze the solute partitioning during bainite transformation in a novel, nanocrystalline bainitic steel. Atom probe results show the absence of any partitioning of substitutional elements between the phases involved. The results are fully consistent with the diffusionless transformation of austenite to bainite. However, substitutional elements are expected to redistribute approaching an equilibrium phase boundary as the mixture of bainitic ferrite and retained austenite is tempered. The compositional analysis of the austenite/ferrite interface by atom probe tomography indicates that retained austenite decomposes during tempering before equilibrium is reached at the interface.     

Fracture toughness of nanostructured railway wheels

This paper describes nanostructured railway wheels made of Si–Mn–Mo–V low-carbon steel through an advanced metallurgy process and fabrication technology. The microstructure of the wheels, particularly in the rim portion, is composed of carbide-free bainite that consists of bainitic ferrite laths and retained austenite films along the lath boundaries. The thickness of the laths and films is in nanometer scale. For comparison, traditional pearlite–ferrite wheel steels are also investigated. Test results show that carbide-free bainite steel is superior to pearlite–ferrite steel not only in yield strength but also in fracture toughness. Theoretical explanation of these phenomena is also elucidated.     

冷却速度对10CrNi5MoV钢焊缝金属相变的影响

采用Gleeble-3500热模拟试验机测量了10CrNi5MoV钢焊缝金属在不同冷却速率下奥氏体连续冷却过程中的温度-膨胀曲线,利用杠杆定律,得到了不同冷却速率下相变动力学曲线,分析了冷却速率对焊缝金属相变的影响。结果表明,不同冷却速率下焊缝金属的奥氏体转变动力学曲线均呈S型,冷却速率为60 ℃/s、30 ℃/s、15 ℃/s时,奥氏体转变速率与温度的曲线呈单峰状,冷却速率为6 ℃/s,奥氏体转变速率与温度的曲线表现为贝氏体、粒状贝氏体相变的双峰转变。      

Analysis on bainite transformation in reheated low-carbon bainite weld metals

A combination of laser scanning confocal microscopy (LSCM) and dilatometry was utilised to simultaneously qualitatively analyse the morphological evolution of bainite by examination of in situ observed micrographs and to quantitatively investigate the amount of bainite transformation by studying dilatometry data. Shorter bainite structures form for smaller prior austenite grain sizes and lower cooling rates, which causes greater bainite transformation to occur in the latter stages of the transformation process. On the other hand, the amount of lath-shape sub-structure increases due to a higher cooling rate. In addition, the surface relief presents greater height and the peak and valley values stay farther away from the horizontal line for specimens at a higher cooling rate. In addition, a lower strain energy per unit volume gives rise to greater bainite transformation compared to that from abundant driving forces.     

Alloy optimisation of bainitic steel for large plastic mould

Based on the phase transformation theories, especially the T₀ concept of bainite transformation, alloy optimisation of bainitic steel with carbides has been carried out aiming at the produce of plastic mould with large cross-section. The effect of manganese and silicon on proeutectoid ferrite and bainite transformation is explored by dilatometric analysis, XRD and different microscopy techniques. The results show that after the alloy optimisation, the transformation of proeutectoid ferrite is suppressed and when the cooling rate is lower than 0·1°C?s^??1, the new lower bainite transformation appears by decreasing carbon capacity of austenite and promoting carbide precipitation. Industrial production proves that the optimised alloy SDP1 can meet the demand for the plastic mould with the thickness of 1050?mm.     

Phase transformations during the decomposition of austenite below Ms in a low-carbon steel

The purpose of this study is to investigate and understand the phase transformations during the decomposition of austenite, which occurs during isothermal treatments below the martensite start temperature (M_s) in a low-carbon steel. Isothermal holding treatments after rapid cooling to various temperatures (forming a controlled volume fraction of initial martensite) were carried out in a dilatometer. Results obtained by dilatometry, microstructural characterization and hardness were analyzed. This combination of results shows that the microstructures formed below the M_s temperature are mainly bainitic, mixed with tempered martensite. The kinetics of isothermal bainite formation was described by a nucleation-based transformation model. The complex competition and interactions between their transformation mechanisms during the isothermal holding at different temperature regimes are discussed.     

Fatigue crack propagation behavior of multiphase microstructure in a quenched and partitioned medium-carbon bainitic steel

A medium-carbon steel was treated by the bainitic isothermal transformation plus quenching and partitioning (B-QP) process to obtain bainite/martensite/retained austenite multiphase microstructure, and its fatigue crack propagation (FCP) behavior was evaluated in contrast with BAT (bainite austempering) sample with fully bainite microstructure. Results show that B-QP sample exhibits a lower FCP rate and higher fatigue threshold ΔK_th (12.6 MPa·m^1/2). Moreover, the FCP path of B-QP sample displays a strongly tortuosity and more crack branching due to more filmy retained austenite (7.2%) and higher percentage of high angle misoriented boundaries (68%). The larger crack tortuosity and the secondary cracks as result of crack branching are primarily responsible for the lower FCP rate of B-QP sample. In addition, the FCP rate curve of B-QP sample shows a pronounced small plateauing at the near-threshold zone, which can be ascribed to the mechanical twinning that occurred in the filmy retained austenite.     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Investigation on decomposition behavior of austenite under continuous cooling in vanadium microalloyed steel (30MSV6)

In the present study, investigations are focused on microstructural evolution and the resulting hardness during continuous cooling transformation (CCT) in a commercial vanadium microalloyed steel (30MSV6). Furthermore, the effects of cooling rate and austenite grain size (AGS) on CCT behavior of the steel have been studied by employing high-resolution dilatometry. Quantitative metallography accompanied with scanning electron microscopy (SEM) has efficiently confirmed the dilatometric measurements of transformation kinetics and austenite decomposition products. A semi-empirical model has been proposed for prediction of microstructural development during austenite decomposition of the steel and the resultant hardness. The model consists of 8 sub-models including ferrite transformation start temperature, ferrite growth, pearlite start temperature, pearlite growth, bainite start temperature, bainite growth, martensite start temperature and hardness. The transformed fractions of ferrite, pearlite and bainite have been described using semi-empirical Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach in combination with Scheil's equation of additivity. The JMAK rate parameter for bainite has been formulated using a diffusion-controlled model. Predictions of the proposed model were found to be in close agreement with the experimental measurements.     

3Cr2Mo塑料模具钢连续冷却相变行为

为了调节塑料模具钢3Cr2Mo的组织,以实现在线预硬化,使用Gleeble1500热模拟试验机、光学显微镜以及透射电子显微镜等研究3Cr2Mo钢变形及未变形奥氏体的连续冷却相变行为及相变组织.实验结果表明,3Cr2Mo钢奥氏体稳定性较高,在所研究的实验条件下,连续冷却过程中没有出现先共析铁素体和珠光体,而是发生贝氏体和马氏体相变.热变形使奥氏体发生了机械稳定化,贝氏体相变推迟到较低温度下才完成.随着冷却速度的降低,贝氏体的形态由常规板条状变成粒状,最终可获得粒状贝氏体组织.     

Transformation of austenite remaining after intercritical rolling to ferrite

Abstract

The impact of austenite deformation in the intercritical range on the rate of transformation in continuous cooling to ferrite, pearlite, bainite or martensite has been studied. The austenite associated with the rolled ferrite is much higher in carbon content, which does not influence the pearlite transformation but retards bainite and martensite. Furthermore, in comparison with rolling of stable austenite the increased strain hardening of the intercritically cooled austenite accelerates the formation of ferrite and pearlite (+ 10–30°C) and refines them but retards the bainite and martensite transformations (?20–40°C). At the intermediate cooling rate near 16 K s^?1, these several influences combined with near doubling of the ferrite production give rise to the suppression of bainite formation and to maximum increased delay of martensite start.