Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si Mn TRIP steel was investigated by means of microstructural ob servation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

微合金化控轧控冷钢筋纵向金相组织研究

 对微合金化控轧控冷钢筋的纵向金相组织进行了研究,并分析了不同成分试验钢纵向“条带”组织的差异及形成原因。研究结果表明：偏析元素（P、Si、Mn等）在轧制过程中沿轧制方向呈条状分布,是20MnSi、20MnSiV钢产生带状组织的原因。铌及其碳氮化物的溶质拖曳和“钉扎”作用,使20MnSiNb钢的奥氏体未再结晶轧制温度提高到1050℃,在冷却过程中,先共析铁素体在形变奥氏体晶界和内部变形带均匀析出,随后沿形变奥氏体晶界（在先共析铁素体与奥氏体的界面上）生成珠光体带,最后在形变奥氏体晶粒内部形成贝氏体条。研究条件下优势形核点的排序为：形变奥氏体晶界和形变奥氏体晶内变形带、偏析元素和夹杂、再结晶奥氏体晶界。     

Effect of Niobium on Transformations From Austenite to Ferrite in Low Carbon Steels

Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite.However,the effects of niobium on transformation behaviour have not been fully characterised and understood to date.This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents,together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature.It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content.When austenite grain sizes are consistent,the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.     

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

 The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bainitic steel during continuous cooling. Continuous cooling transformation (CCT) curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln［1/(1-f)］ and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth patterns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agreement with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.     

Austenite Grain Growth and the Surface Quality of Continuously Cast Steel

Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, delta-ferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains.     

奥氏体形变对50CrV4钢相变行为的影响

利用Thermecmastor-Z型热模拟试验机,结合金相显微镜(OM)、扫描电镜(SEM)、维氏硬度计等,系统研究了奥氏体区变形对50CrV4钢连续冷却相变和等温相变规律的影响。建立了试验钢动态CCT曲线。研究结果表明,奥氏体变形能促进连续冷却转变过程中铁素体-珠光体、贝氏体转变,但亦可提高奥氏体的机械稳定性,进而抑制马氏体转变,Ms点由331.6℃(奥氏体未变形)降低至291℃(950℃下变形50%+890℃下变形50%,变形速率均为5s-1,变形后冷速为20℃/s)。当轧后冷速小于0.5℃/s时,试验钢中可获得铁素体+珠光体组织。此外,在研究不同变形量对试验钢等温相变规律影响时发现,650℃等温时,试验钢中发生铁素体-珠光体相变。随着变形量的增加(由30%增加至50%),其等温相变动力学加快(相变完成时间由197.6s减小至136.5s),铁素体体晶粒尺寸、珠光体片层间距减小,硬度增加。     

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

超低碳贝氏体钢快速加热奥氏体长大过程中移动晶界上硼的反常偏聚

利用径迹显微照相技术研究了超低碳贝氏体钢焊接热影响区在焊接热循环快速加热过程中硼在奥氏体晶界上的偏聚行为。发现以高密度位贝氏体为原始组织的材料进行快速加热时，新形成的奥氏体晶粒边界上在很高温度下仍会出现反常的晶界硼偏聚。用晶界位错驰原制对这种新的非平衡现象进行了讨论。     

Austenite Formation in Plain Low-Carbon Steels

In this study, austenite formation from hot-rolled (HR) and cold-rolled (CR) ferrite-pearlite structures in a plain low-carbon steel was investigated using dilation data and microstructural analysis. Different stages of microstructural evolution during heating of the HR and CR samples were investigated. These stages include austenite formation from pearlite colonies, ferrite-to-austenite transformation, and final carbide dissolution. In the CR samples, recrystallization of deformed ferrite and spheroidization of pearlite lamellae before transformation were evident at low heating rates. An increase in heating rate resulted in a delay in spheroidization of cementite lamellae and in recrystallization of ferrite grains in the CR steel. Furthermore, a morphological transition is observed during austenitization in both HR and CR samples with increasing heating rate. In HR samples, a change from blocky austenite grains to a fine network of these grains along ferrite grain boundaries occurs. In the CR samples, austenite formation changes from a random spatial distribution to a banded morphology.     

Simulation of austenite decomposition in continuous cooling conditions: a cellular automata-finite element modelling

Transformation of austenite to ferrite under continuous cooling condition was investigated. The heat conduction problem was managed by finite element method while two-dimensional cellular automata modeling was simultaneously performed to predict the progress of austenite decomposition using a two-step algorithm to reduce surface-to-volume ratio. Continuous cooling experiments on low carbon steel were made and the ferrite structure was determined and compared with the simulation data. The predicted and the experimental results demonstrated an acceptable consistency and the activation energy for ferrite growth was determined as 171 kJ/mole. The rate of ferrite transformation increased under examined continuous cooling conditions owing to higher nucleation rate. Moreover, the initial austenite grain size has shown a significant impact on the rate of transformation e.g. in air-cooled samples as the austenite grain size decreased from 24 to 34 µm, the mean ferrite grain size decreased about 8 µm.     

Influence of Vanadium on Grain Growth of Deformation Induced Ferrite in Low Carbon Steel during Continuous Cooling Process

The effects of vanadium on the deformation induced ferrite transformation (DIFT) in low carbon steel during heavy deformation at the temperature slightly higher than Ar₃ and on the coarsening of DIFT microstructure during the continuous cooling processes after deformation were investigated using a thermo‐simulator. The results show that vanadium has little effect on the volume fraction of DIFT microstructure under heavy deformation, whereas the deformation induced ferrite (DIF) grains are refined with increasing vanadium content. The steel containing a small amount of vanadium exhibits similar velocity of grain growth compared to vanadium free steel, while the vanadium remarkably inhibits grain growth in a steel containing a high amount of vanadium during the continuous cooling process. The diffusion activation energy of grain boundaries for all the tested steels is calculated and the influencing mechanism of vanadium on the grain growth during the continuous cooling process is discussed.     

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The continuous cooling transformation process and the organization performance of transformation product was investigated by means of dilatation test during single pass deformation on Gleeble-3500. The result shows that bainite transition can be happened when the cooling speed about 3??/s on 460B steel with boron, and the grain size is coarse and hardness is higher when the cooling speed (0. 1, 0. 3, 0. 5??/s)is slowly, it is more tiny than without boron. Boron can make the CCT curves shift to right, the transition from austenite to ferrite and pearlite is restrained, the hardenability of the steel are obviously improved, the cooling speed from austenite to martensite is lower and Ac3, Ac1, Ar3, Ar1, Ms is dropped.     

Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel

 The transformation behavior and microstructural characteristics of a low carbon high Nb-bearing microalloyed pipeline steel have been investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves (CCT) of the tested steel was constructed. High Nb content and deformation enhancing the formation of acicular ferrite; the microstructures are range from PF, QF to AF with increasing cooling rates from 0.5 to 50℃/s and dominated by acicular ferrite in a broadened cooling rate higher than 5℃/s. The chaotic microstructure consists of non-equiaxed ferrite and interwoven ferrite laths distributed high density dislocations and sununits. The results of isothermal holding show that acicular ferrite microstructure is formed in region of 550-600℃. With the holding time or temperature increased, some low misorientations boundaries change to high misotrentationn as dislocations moving and grain boundaries coarsening.     

Effect of Boron Precipitation Behavior on the Hot Ductility of Boron Containing Steel

The effect of boron (B) precipitation behavior on the hot ductility of B containing steel was investigated. Hot ductility of B containing steel was sensitive to the cooling rate (CR) in the range of 1 to 20 K/s (1 to 20 °C/s), whereas that of B-free steel showed little change with CR. Increased CR causes deepening and widening of the ductility trough in B containing steel. Particle tracking autoradiography (PTA) analysis and transmission electron microscope (TEM) image of the samples show that boron nitride (BN) particles form along prior austenite grain boundaries, and that as CR increases, these particles become smaller and more numerous. This increase in the number of small BN precipitates may promote intergranular fracture, leading to a decrease in hot ductility in the lower austenite temperature region (1173 to 1273 K (900 to 1000 °C)). Furthermore, the formation of filmlike ferrite at ~1123 K (850 °C) causes a decrease in the hot ductility of this steel regardless of B addition and CR.     

Embrittlement Mechanism due to Slow Cooling During Quenching for M152 Martensitic Heat Resistant Steel

 The brittleness of M152 martensitic heat resistant steel due to slow cooling during quenching was experimentally investigated by means of mechanical property test, TEM and XRD analysis. The results showed that the nonreversal brittleness of M152 steel due to slow cooling during quenching was caused by the continuous precipitation of M23C6 along prior austenite grain boundaries and of M2C along prior residual austenite film. The residual austenite in the steel was unstable and may decompose due to the precipitation of second phase during the process of slow cooling after quenching. The low cooling rate in the temperature range from 820 to 660℃ has a strong effect on the impact toughness of the steel, the precipitation of second phase in the same temperature range results in nonreversal brittle.     

Dilatometric Studies on Copper Added Titanium‐Boron Steels

An attempt has been made to examine the effect of Cu addition in low carbon Ti ‐ B microalloyed steel on their continuous cooling transformation behaviour by dilatometric study. It has been demonstrated that addition of Cu by an amount of 1.5 wt% in Ti – B microalloyed steel effectively lowers the transformation start temperature. Addition of Ni (0.79 wt%) in 1.5 wt% Cu‐added Ti–B microalloyed samples further lowers the transformation temperature of austenite even at cooling rates comparable with air ‐ cooling condition. The microstructural investigation of the samples subjected to the dilatometric study under different cooling rates suggests the possibility of obtaining pearlite free multiphase microstructures in 1.5 wt% Cu – and 0.79 wt% Ni – added Ti – B microalloyed steel by air – cooling from the austenite region.     

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The transformation behavior of Nb-V micro-alloyed low carbon austenite steel during continuous cooling was investigated through Gleeble-3800 themomechanical simulator and metallographic analysis. The experimental results show that the dynamic CCT diagrams shift to the left and upper compared with the static ones, the begin temperature of A????-Fe transformation is gradually lower with the increase of cooling rate. The high temperature deformation improves Ferrite and Pearlite transformation and decreases Ferrite transformation zone. When the cooling rate is lower, the deformation improves Bainite transformation; when the cooling rate is higher, the deformation restrains Bainite transformation.     

Phosphorus segregation in austenite in Fe–P–C,Fe–P–B and Fe–P–C–B alloys

The equilibrium grain boundary segregation of phosphorus was investigated in Fe–P–C, Fe–P–B and Fe–P–C–B alloys after austenitising at temperatures ranging from 825–1100 °C. The grain boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. Phosphorus, carbon and boron segregate to the austenite grain boundaries. The segregation of P in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. Boron and, in a lesser degree, carbon were found to decrease the grain boundary concentration of phosphorus. The results can be explained by assuming equilibrium segregation and mutual displacement of these elements in austenite.     

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

During the continuous casting of low‐carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second‐phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation.