22MnB5钢溶质分配系数及TiN析出对微观偏析的影响

为了提高凝固过程中溶质微观偏析模型计算的准确性,基于溶质分配系数及夹杂物析出对溶质偏析的重要影响,定量分析了溶质分配系数以及TiN析出对凝固过程溶质元素含量的影响,为微合金高强钢凝固过程研究提供理论参考.针对22MnB5钢建立了耦合TiN析出热力学模型的溶质微观偏析模型,并探究了温度及钢液凝固路径对溶质分配系数的影响规...     

GCr15轴承钢大方坯凝固过程微观偏析模型及应用研究

建立了考虑δ/γ相变的GCr15轴承钢大方坯连铸凝固两相区溶质微观偏析模型,并应用于220 mm ×260 mm铸坯的凝固传热。结果表明:通过模型可以获得高碳钢精确的固液相线温度,以及温度与固相率的关系;GCr15轴承钢大方坯凝固过程仅析出γ相,凝固末期S、P和C元素的偏析严重;固相率越大,冷却速率对偏析度的影响更明显;S和P元素含量以及冷却速率对零塑性温度（ZDT）影响较大;采用基于凝固传热模型优化的连铸工艺后,铸坯中心碳偏析指数控制在0.961.05,且铸坯未产生内裂纹。     

Investigation of Centreline Segregation and Centreline Porosity in CC ‐ Slabs

Improvements of solidification processing in conventional or near net shape casting depend on sophisticated methods of macroscopic examination of central unsoundness and inhomogeneity of the cast material. As long as remarkable deviations referring to the quality features exist between the slab centre and the bulk material such methods should be looked after. Two semi‐macroscopic methods of segregation analysis are discussed in this paper: firstly, the potential of the emission spectral analysis combined with sectioning of a sample and secondly, the computer aided micro‐probe analysis. Both methods are restricted to small local areas. The proneness of elements and steel grades with respect to micro‐ and macrosegregation as well as distribution‐ and segregation coefficients are determined. The effect of a diffusion anneal on homogenisation of segregation is studied. Last not least the precipitation of sulphides, phosphides or carbonitrides is estimated. Ultrasonic detection of the central unsoundness of conventional slabs or thin slabs is a new approach towards a quantitative macroscopic examination. The sample size is 400 mm x 300 mm x 30 mm. To achieve a low ultrasonic noise using a 5 MHz‐transducer the samples are heat treated for grain refinement. “Dog bone porosity”, macroscopic topographical misfit between the upper and lower solidifying shells, periodical corner cracks or systematic transverse centre cracks are detected. Obviously, segregations do not reflect the ultrasonic beam. However, it can be presumed that an interrelation exists between porosity and segregation, although these might be locally apart from each other. This new ultrasonic test of the central unsoundness of continuous cast (c.c.) slabs gives valuable arguments to machine builders and maintenance people to decide on the right concepts for strand guidance, support rollers and secondary cooling. Soft reduction can be optimized and variations in casting speed counteracted by dynamic means. R&D service of this kind can help steelmakers when decisions are impending to build new casting machines or revamp old ones.     

Computer simulation of the brittle‐temperature‐range (BTR) for hot cracking in steels

The hot‐cracking susceptibility of a material is usually evaluated by means of a characteristic temperature range, the so‐called brittle‐temperature‐range (BTR). The upper bound of the BTR is determined by the zero‐strength‐temperature (ZST), the lower limit is defined by the zero‐ductility‐temperature (ZDT). In the present work both ZDT and ZST are related to the current amount of residual liquid during solidification, which in turn is evaluated from a modified Scheil‐Gulliver micro‐segregation model, which takes into account back diffusion of interstitial alloying components. This model is particularly suitable for application to high‐order alloy systems. The calculated values of the ZDT are compared to numerous experimental results from literature and good agreement is achieved. It is demonstrated that the application of the classical Scheil‐Gulliver model does not give feasible results.     

Assuring Microstructural Homegeniety in Dual Phase and Trip Steels

The presence of ferrite/pearlite bands in dual phase and TRIP assisted steels is a consequence of microchemical segregation which causes mechanical properties anisotropy. Such inhomogeneous phase distribution produces a lowering of the mechanical properties such as fracture behaviour. This anisotropy is commonly not accounted in micromechanics computations which often assume a random distribution of phases in the solid. The present paper deals with an integral model for this undesirable band formation accounting for the solute segregation caused by solidification, microcomponent diffusion present in the austenitisation process, and the nucleation of the transformed phase in segregated regions. In the present work, the model was applied to two industrial grade dual phase steels and two TRIP assisted steels. The influence of such parameters on band formation is summarised in a number of “band prevention plots”, which are aimed at providing the optimum processing conditions for ferrite/pearlite band prevention.     

Structural Morphologies and Deformaiton Characteristics of Semi‐solid Type 304 Stainless Steel during Solidificaiton and Remelting

Microstructural evolutions of type 304 stainless steel and the related mechanical property of flow stress in semi‐solid state are investigated. The evolutions of microstructure during solidification, partial remelting of a hot‐rolled billet and partial remelting of a cast billet are compared with respect to structural morphologies in the semi‐solid state. Various structural morphologies, such as the linear and multilayered liquid/austenite/δ‐ferrite structure, globular liquid/δ‐ferrite structure and dendrite structure, are characterized using optical micrographs and an EPMA (electron probe microanalyzer). The various structural morphologies in the semi‐solid state are influenced not only by the phase transformation but also by the previous treatment of type 304 steel, such as hot rolling and casting. Furthermore, a series of hot compression tests are conducted for various combinations of deformation rate and deformation temperature in the semi‐solid state, to measure the flow stress and the change in microstructure resulting from plastic deformation. Flow stress, phase segregation, microfracture and distortion of solid particles during and after the hot compression test are strongly affected by the structural morphology in the semi‐solid state, such as the dendrite structure, nonglobular structure and globular structure. Semi‐solid type 304 stainless steel with dendrite structure exhibits the highest flow stress, which is about three times that of steel with globular structure, although the testing temperature and deformation rate are controlled to be the same. This is a result of the higher bonding force between solid particles and lower fluidity of the liquid phase of the dendrite structure than those of the globular structure, which exhibits excellent fluidity of the liquid phase and rotation of solid particles.     

Assessment of Segregation in Hot Tear Cracks

In solidified slabs different types of segregations such as crystal segregation (micro segregation), centre segregation (macro segregation) and hot tear segregation (HTS) may occur. The present paper examines the segregation behaviour of different elements in hot tear cracks depending on the carbon content. The aim of this work is to determine the segregation factors in the hot tear cracks filled with residual melt and compare with micro and macro segregation. Within the scope of this examination, a microanalytical assessment was made of eight slab samples with different steel grades each showing different types of hot tear cracks that had been healed up by an inflow of residual melt. The hot tear cracks are located outside the primary dendrites in the dendritic interstices, parallel or transversal to the direction of casting. Segregation in the cracks healed up by residual melt depends on the carbon content and will become more pronounced as the carbon content increases. The intensity of segregation for the various elements in the hot tear cracks (hot tear crack segregation) is between that of crystal segregation and centre segregation. The thickness of the segregated zone in the hot tear crack area is 30 ‐ 50 μm. Apart from an enrichment of the alloying elements manganese, silicon and chromium, the healed up hot tear cracks also contain secondary precipitates of sulphides and niobium‐titanium‐carbonitrides. Towards the slab centre, the latter can cause, among other things, the development of niobium‐titanium‐carbonitrides (Nb_1‐x, Ti_x)(C_1‐y, N_y) networks in the primary dendritic interstices.     

Flow Modelling in Continuous Casting of Round Bloom Strands with Electromagnetic Stirring

For the majority of continuously cast semis for stringent applications such as wire rods, rails, seamless tubes, etc. an optimal cleanness, surface and sub‐surface quality as well as a minimum of centre line segregation and central shrinkage porosity is almost mandatory. In‐mould electromagnetic stirring (M‐EMS) has a number of functions but its primary purpose is to help breaking down coarse columnar dendritic solidification structure to produce a finer dendritic structure and a larger proportion of equiaxed grains. The rotary effect of M‐EMS has also an important impact on the cleanness of the steels, which can be explained by its action on exogenous and endogenous inclusions. The modelling of turbulent flow, steel temperature, solidification, exogenous inclusion transport and electromagnetic stirring with mathematical and ‐ as far as possible ‐ physical models are presented, including PIV‐measurements on a 1:1 scale water‐model of the strand.     

Micro‐Segregation Behavior of Solute Elements in the Mushy Zone of Continuous Casting Wide‐Thick Slab

Based on the assumption that the transverse cross‐section of dendrites is a regular hexagon, a coupled macro‐heat transfer and solute diffusion model was developed to describe micro‐segregation behavior of solute elements in the mushy zone during wide‐thick slab continuous casting. The heat transfer model and the micro‐segregation model were validated by the industrial tests and the experiment measurements in literatures, respectively. The results show that the cooling rate decreases from 51.1°C s^?1 at the surface to 0.13°C s^?1 at the center of the wide‐thick slab of peritectic steel continuous casting. The effective cooling rate equaling to 1.78°C s^?1 mainly depends on mold cooling and varies little with casting speed. The micro‐segregation behavior of solute elements in the mushy zone depends on back diffusion ability and local equilibrium at interfaces. Compared with other elements in the steel, phosphorus, and sulfur elements exhibit a much higher segregation ratio at the end of the solidification and are more significantly affected by the initial carbon content.     

A numerical scheme for solidification of an alloy

In modeling the solidification of an alloy, two central numerical problems are:1. The calculation of thermal and species transport fields which satisfy the conservation equations and are also consistent with the underlying thermodynamics.2. The treatment of local scale solute diffusion.In this paper, in the context of modeling inverse segregation in a uni-directionally solidified casting, a recently proposed implicit⧹explicit time integration scheme for coupling thermal and solutal fields is presented and possible ways of capturing the local scale solute diffusion in a macroscopic model are explored. A key element in this work is the validation of the proposed approach by comparison with a sophisticated similarity solution. © 1998 Canadian Institute of Mining and Metallurgy.     

Modelling of Manganese Sulphide Formation during Solidification,Part I: Description of MnS Formation Parameters

Precipitations of second phases in steel grades influence the mechanical properties of the material significantly. Manganese sulphides especially affect the ductility properties at high as well as at low testing temperatures. For a sufficient appreciation of the influence of MnS precipitates on the properties, a proper knowledge about the parameters in regard to the size and distribution of MnS precipitates is necessary. This paper proposes a model to describe the solidification of a medium carbon steel grade, the micro‐segregation of manganese und sulphur and the subsequent precipitation of MnS. Especially for the calculation of the mean radius of globular MnS precipitates after solidification, a new approach is presented regarding the MnS size as a function of the manganese and sulphur content, the cooling rate during solidification and the secondary dendrite arm spacing. The model is validated by means of quench tests in combination with light‐optical and scanning electron microscopic evaluations of the size distribution of MnS precipitates as well as EDX analysis for the composition of the formed sulphide phase.     

Effects of Re and Ru on the Solidification Characteristics of Nickel-Base Single-Crystal Superalloys

The solidification characteristics, such as the phase transformation temperature and partition coefficient, were investigated in four Ni-base single-crystal superalloys with varied contents of Re and Ru. Differential scanning calorimetry (DSC) analysis reveals that the liquidus and solidus temperatures decrease with increasing levels of Re additions. However, the Ru only has negligible influences on the liquidus and solidus temperatures. The partition coefficient of constituent elements is investigated by using the directional solidification followed quenching (DSQ) technique. Re is found to reduce the segregation of Re and W, whereas Ru only has negligible effects on the segregation of Re and W. For comparison, the matrix scanning is performed on the representative dendritic structures. With increasing withdrawal rate, Re and Ru additions show the stronger influences on segregation behavior of alloying elements, particularly for Al and Ta.     

Effects of relative motion between consumable electrodes and mould on solidification structure of electroslag ingots during electroslag remelting process

Abstract

Steel solidification process control, especially in the solidification process of high alloy steel, and improvement of the solidification structure have been increasingly gaining interest among metallurgists, particularly the electroslag workers. To further develop the electroslag remelting (ESR) process and to improve the advantage of the ingot solidification structure, the effects of relative motion between the consumable electrodes and the mould (namely, mould rotation) on chemical element distribution were observed in this study, as well as the compact density changes in electroslag ingots. Experiment results show that applying relative motion between the mould and the consumable electrodes in ESR results in a more uniform chemical element distribution in the electroslag ingots. Compared with the electroslag ingot of conventional ESR, maximum segregation of carbon could decrease from 3·19 to 1·146, and statistical segregation decreased from 0·2636 to 0·0608. Maximum segregation of chromium could decrease from 1·316 to 1·253, and statistical segregation decreased from 0·2753 to 0·1201. The compact density for the stationary mould increased from 0·7693 to a compact density of 0·9501 for the rotating mould. The improvement in the solidification structure of the electroslag ingot can be attributed to mould motion, which led to the generation of a shallow pool and the improvement of the solidification structure. But the excessive rotation rate is harmful to solidification structure instead due to the molten metal pool motion caused by violent slag pool motion.     

Micro Semi‐solid Manufacturing ‐ A New Technological Approach towards Miniaturisation

One of the most important prerequisites to meet the increasing demand for efficient technologies for micro‐part production is constituted by the ability to overcome existing process limitations by new innovative technological approaches. By the introduction of a new process variant based on a hybrid material condition between solid and liquid state, such an approach is presented. This micro semi‐solid manufacturing technology, so‐called Micro‐Thixoforming, was initiated, on the one hand, by being aware of the technological limits of existing micro‐forming and micro‐casting processes and, on the other hand, by a comprehensive understanding of the special rheological mechanisms of metallic materials in semi‐solid state finally establishing the desired potential to follow the trend towards miniaturisation with drastically reduced process restrictions. However, this promising potential can only be successfully exploited when the initial idea, which is based on phenomenological considerations, can be transferred to a process technology with sufficient practical relevance. Therefore, the presented new integrated process concept for Micro‐Thixoforming is particularly characterised by the application of unconventional solutions for the main process steps: raw material conditioning, thermal pre‐processing, semi‐solid forming and thermal post‐processing. To give an indication of the innovative character of the chosen practical solutions liquid metal jet technology, LASER‐induced plasma shockwaves and high pressure water jet should be mentioned. However, what is even more important in this context is the ability not only to realise a process concept but, beyond that, to recognise the further potential regarding new strategies for material design arising from the availability of this process. Such a strategy e.g. consists of utilising the well‐known segregation effect, which notably often is negatively associated with semi‐solid forming. However, for the envisaged technological approach, controlled segregation aims at a defined adjustment of functionally graded properties for the produced micro part.     

镍基高温合金GH4151的偏析及相析出行为

摘要：利用电子探针(EPMA)、场发射扫描电镜（SEM）及差热分析（DTA）研究了GH4151合金的元素偏析行为、铸态组织特征以及析出相种类,并对合金凝固过程进行讨论。结果表明：GH4151合金凝固过程中,W元素偏聚于枝晶干,Mo、Nb、Ti元素偏聚于枝晶间,Co、Cr、Al元素几乎不发生偏析,Nb、Ti元素偏析较重。GH4151铸锭心部为粗大的等轴晶,主要析出相包括强化相γ′相、一次碳化物、η相、（γ+γ′）共晶相以及Laves相,其中枝晶间分布的η相、（γ+γ′）共晶相和Laves相为低温脆性相在凝固末期形成,扩大了合金的凝固区间,从而导致合金热裂敏感性增加。     

钢锭中硫化物偏析的研究

对钢锭凝固过程进行实测和计算机凝固模拟计算，研究了钢锭凝固时的偏析问题，结果表明凝固模拟计算是预测分析大型钢锭凝固进行的有效手段。由此，通过凝固冷却条件能预测钢锭中产生偏析的可能性，并进一步寻求减少和消除偏析的方法。     

Phase precipitation and segregation behavior of nickel based superalloy GH4151

The element segregation behavior, as cast microstructure characteristics and precipitation phase types of GH4151 alloy were studied by electron probe microanalysis (EPMA), field emission scanning electron microscopy (SEM) and differential thermal analysis (DTA), and the solidification process of the alloy was discussed. During the solidification of GH4151 alloy, element W is segregated to the dendrite cores, elements Mo, Nb and Ti are segregated to the interdendritic regions, and elements Co, Cr, Al are not segregated, elements Nb and Ti are biased more heavily. The core of GH4151 ingot is coarse equiaxed grains, whose main precipitated phases include γ′, MC carbide, η phase, (γ+γ′) eutectic phase and Laves phase. At the end of solidification, low temperature brittle phases such as η phase, (γ+γ′) eutectic phase and Laves phase are formed, which enlarge the solidification range and increase the hot cracking sensitivity of the alloy.     

Experimental Investigations and Computer Simulation of the Liquid Fraction Evolution during the Solidification of Alloys Suitable for Semi‐Solid Processing

One important parameter for the processing of materials by semi‐solid forming is the actual distribution of the solid and liquid phases in the semi‐solid range. This parameter defines the process stability for the forming step. Therefore it is necessary to obtain information about the materials behaviour in the semi‐solid state for different materials grades. This kind of information can be obtained by experimental studies in the interesting temperature range or by calculations with simulation programs using thermodynamic data validated by experiments. This work shows the results of experimental studies and thermodynamic calculations of the solidification and heat treatment behaviour of the aluminium alloy A319 and the steel X210CrW12. The experimental studies of solidification and heat treatment of these alloys were carried out using a differential thermal analysis system (DTA). The theoretical fraction of liquid content was calculated from the DTA signal by using a software module called Corrdsc. The experimental data obtained were used to validate the thermodynamic simulations of the solidification of semi‐solid alloys. The simulations of the solidification process were carried out for equilibrium conditions, with the Scheil‐Gulliver model as well as with diffusion calculations. The equilibrium and Scheil‐Gulliver calculations were performed by the program Thermo‐Calc, and the diffusion by the program DICTRA. The required thermodynamic and mobility data for multicomponent systems were taken from the data bases COST 507 light alloys, TCFE2000 Steel/Alloys and MOB2 mobility and from newly added data. The comparison of calculated phase transformations and fractions of liquid content with experimental data revealed a good agreement.     

Study on Mitigating Center Macro‐Segregation During Steel Continuous Casting Process

In the current paper, Methods of enlarging the area for the distribution of segregation solutes were introduced to mitigate center macro‐segregation in steel billets and steel slabs during continuous casting process, which cost less and have significant effect. The location of center macro‐segregation is relative to the shape of liquid‐core at the solidification end during steel continuous casting. A method of dissymmetrical cooling on different surfaces, by which the area for the precipitation of segregation solutes was enlarged, was introduced to mitigate the center macro‐segregation in billets during continuous casting process. Method of optimizing the uniformity of solidified shell in the transverse direction was introduced to mitigate the center macro‐segregation in steel slabs. The uniform cooling intensity along the transverse direction guaranteed a regular solidification end in the continuous casting slab, which aided in the effective application of dynamic soft reduction technology. A relevant 2‐D heat transfer model was developed for the optimization of uniform solidification. The current method was applied to the industrial slab continuous casting using the heat transfer model. The results indicated a better industrial slab quality with much less center macro‐segregation after the use of the method.     

Evolution of Secondary Phases Formed upon Solidification of a Ni-Based Alloy

The solidification of UNS N08028 alloy subjected to different cooling rates was studied, where primary austenite dendrites occur predominantly and different amounts of sigma phase form in the interdendritic regions. The solidification path and elemental segregation upon solidification were simulated using the CALPHAD method, where THERMO-CALC software packages and two classical segregation models were employed to predict the real process. It is thus revealed that the interdendritic sigma phase is formed via eutectic reaction at the last stage of solidification. On this basis, an analytical model was developed to predict the evolution of nonequilibrium eutectic phase, while the isolated morphology of sigma phase can be described using divorced eutectic theory. Size, fraction, and morphology of the sigma phase were quantitatively studied by a series of experiments; the results are in good agreement with the model prediction.