A mold simulator for the continuous casting of steel: Part I. The development of a simulator

Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations. The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous casting process by a laboratory scale simulator.     

An asymptotic model of the mold region in a continuous steel caster

A model is developed to simulate the solidification of the steel shell in the mold region of the continuous casting process. Conduction-dominated temperature fields in the mold, mold flux, steel shell, and molten steel regions are determined through the development of an evolution equation for the solidifying front. This equation is derived in the limit of small aspect ratio, mold width to height, using asymptotic methods. These results are coupled with a lubrication-theory model for the mold flux region. This model assumes a temperature-dependent viscosity for the mold flux and allows for solidification of the flux at temperatures below a critical value. System response to changing casting speeds, superheat, mold wall temperatures, and mold flux properties is investigated.     

低碳钢和中碳钢连铸方坯初始凝固的对比研究

以低碳钢和中碳钢为研究对象,围绕不同连铸工艺参数对方坯初始凝固行为的影响,利用CA-FE耦合模型模拟实际连铸过程结晶器内方坯的初始凝固行为,考察拉速和过热度对方坯出结晶器坯壳厚度的影响,对比二者出结晶器横截面枝晶微观形貌.研究表明:过热度和拉速增加均能使出结晶器坯壳厚度下降,而拉速的影响更为显著.不同钢种在相同条件下出结晶器坯壳厚度下降梯度不同.过热度越低柱状晶越致密细小,利于提高连铸坯质量,拉速对柱状晶的影响相对较小.由于出结晶器坯壳安全厚度限制,过热度取15℃,低碳钢拉速不能超过2.2 m·min-1,中碳钢拉速不能超过2.5 m·min-1,据此针对不同钢种设计不同拉速可提高连铸效率.同时,模型结果显示低碳钢出结晶器时刻柱状晶更为发达.      

高速连铸结晶器内凝固传热行为及其均匀性控制

从分析高拉速包晶钢板坯连铸结晶器内凝固传热行为特征入手,首先阐明拉速对结晶器内的界面热阻、凝固坯壳的温度与应力分布的影响规律,研究发现拉速超过1.6 m·min?1时,界面热阻明显增加,拉速由1.4 m·min?1提升至1.6 m·min?1和1.8m·min?1时,出结晶器坯壳厚度相应减少约10%,其发生漏钢的危险不断增加;在此基础上,阐述了结晶器的内腔结构、保护渣、振动与液面控制等控制结晶器内坯壳凝固均匀性的相关技术。要实现高速连铸,首要应考虑结晶器内腔结构的优化设计,使其能更好地迎合凝固坯壳的生长,研制适合包晶钢等凝固特点的专用连铸保护渣至关重要,铸坯鼓肚控制也是保障高拉速液面稳定的关键。      

连铸保护渣性能与钢种、工艺参数关系的初探

从低、中、高碳钢等钢种本身性能及凝固收缩特点出发研究了不同钢种用保护渣性能特点,并分析了拉速、铸坯断面形状、尺寸及结晶器振动特性等连铸工艺参数对保护渣性能的要求,找出了不同钢种及工艺条件对保护渣性能设计的特点。     

Simulation of thermal behavior during peritectic steel solidification in slab continuous casting mold

Thermal behavior of the solidifying shell in continuous casting mold is very important to final steel products.In the present work,one two-dimension transient thermal-mechanical finite element model was developed to simulate the thermal behavior of peritectic steel solidifying in slab continuous casting mold by using the sequential coupling method.In this model,the steel physical properties at high temperature was gotten from the micro-segregation model withδ/γtransformation in mushy zone,and the heat flux was obtained according to the displacement between the surface of solidifying shell and the hot face of mold as solidification contraction,the liquid-solid structure and distribution of mold flux,and the temperature distribution of slab surface and mold hot face,in addition,the rate-dependent elastic-viscoplastic constitutive equation was applied to account for the evolution of shell stress in the mold.With this model,the variation characteristics of surface temperature,heat flux, and growth of the solidifying shell corner,as well as the thickness distribution of the liquid flux,solidified flux,air gap and the corresponding thermal resistance were described.     

Effect of Grade on Thermal–Mechanical Behavior of Steel During Initial Solidification

Thermal–mechanical analysis of solidification is important to understand crack formation, shape problems, and other aspects of casting processes. This work investigates the effect of grade on thermal–mechanical behavior during initial solidification of steels during continuous casting of a wide strand. The employed finite element model includes non-linear temperature-, phase-, and carbon content-dependent elastic–viscoplastic constitutive equations. The model is verified using an analytical solution, and a mesh convergence study is performed. Four steel grades are simulated for 30 seconds of casting without friction: ultra-low-carbon, low-carbon, peritectic, and high-carbon steel. All grades show the same general behavior. Initially, rapid cooling causes tensile stress and inelastic strain near the surface of the shell, with slight complementary compression beneath the surface, especially with lower carbon content. As the cooling rate decreases with time, the surface quickly reverses into compression, with a tensile region developing toward the solidification front. Higher stress and inelastic strain are generated in the high-carbon steel, because it contains more high-strength austenite. Stress in the δ-ferrite phase near the solidification front is always very small, owing to the low strength of this phase. This modeling methodology is a step toward designing better mold taper profiles for continuous casting of different steels.     

特厚板连铸保护渣系列规划

 针对特厚板连铸工艺的特点,分析了传统的中厚板连铸保护渣与特厚板连铸保护渣的作用特征差异。根据不同钢种在结晶器内的凝固特性,对新钢特厚板连铸保护渣进行了系列规划,分为高碳钢连铸保护渣、包晶钢连铸保护渣、中碳低合金钢连铸保护渣3大类。在此基础上,提出了保护渣熔化温度、黏度、转折温度、结晶比例的控制范围。生产实践表明,设计的保护渣浇铸过程结晶器内状况良好,渣面无结团、结块现象,液渣层厚度合适,保护渣消耗量正常,铸坯表面质量优良,连铸生产工艺顺行。     

Development of mould fluxes based on lime–alumina slag system for casting high aluminium TRIP steel

Abstract

With excellent strength and ductility properties, transformation induced plasticity (TRIP) steels are superior to ordinary commercial steels and have great application potential in the automobile industry. However, the continuous casting of TRIP steels is associated with challenges owing to its high aluminium content, approximately 0?5–1?8%. The reaction of aluminium near the steel/slag interface continuously changes the basicity of conventional lime–silica based mould fluxes causing variation in the flux physical properties between the solidifying shell and mould wall. Therefore, the heat transfer process is unstable and the surface quality of the as-cast slab is not guaranteed.

To reduce the reactivity of aluminium in TRIP steel during casting, it is suggested to replace the lime–silica based fluxes with a lime–alumina based mould flux, based on laboratory measurements and industrial trials.

The trials showed great improvement in the slab surface quality and no depression defect was noticed so far.     

不锈钢(304HC)浇铸过程保护渣成分变化及其对渣性能影响

通过不锈钢(304HC)连铸过程中对结晶器内液渣现场取样、分析,探索了结晶器内渣-钢界面化学反应及钢中杂夹的吸收造成渣成分的变化,及其对熔渣性能的影响.研究表明,渣中Al2O3、Cr2O3含量有所增长,F减少,碱度下降,并造成保护渣使用性能发生变化,粘度有波动、凝固温度升高及结晶率下降.研究结果为更合理地设计不锈钢连铸保护渣配方,进一步降低不锈钢铸坯表面缺陷提供了依据.     

The effect of carbon content on solidification of steel in the continuous casting mold

This investigation examines the effect of steel carbon content on microsegregation and strand deformation during the first stage of solidification in the continuous casting mold. Calculation of microsegregation for phosphorus indicates a minimum at 0.10 wt pct C, and a maximum around 0.25 wt pct C of the decrease in solidus temperature. This leads to very different effective shell thicknesses and determines whether or not the strand shell can contract. As a result, mainly steels around 0.10 wt pct C can produce a finite gap in the early stages of strand formation, explaining the pronounced waviness of the surface of such steels. On the other hand, steels with more than 0.20 wt pct C are forced by ferrostatie pressure to remain in contact with the mold wall leading to uniform shell growth and smooth strand surfaces but also undergoing enhanced mold friction.     

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

 The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product. The multiphase flow phenomena of molten steel, steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation, and the effect of electromagnetic brake (EMBR) and argon gas blowing on the process were investigated. The relationship between wavy fluctuation height near meniscus and the level fluctuation index F, which reflects the situation of mold flux entrapment, was clarified. Moreover, based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification, the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux, temperature and stress of shell in slab continuous casting mold were described.     

钢的软接触电磁连铸技术的研究进展

钢的软接触电磁连铸技术(SoftContactElectromagneticContinuousCasting)是利用高频交变电磁场在结晶器内铸坯初始凝固区施加电磁压力来减少钢液与结晶器壁的接触压力,从而减小结晶器振动对铸坯表面质量的影响,降低拉坯阻力和减弱初始凝固点的传热来提高铸坯表面质量。分析了实现钢的软接触电磁连铸在结晶器结构、材质以及电磁场参数等方面需要解决的问题,并介绍了该技术的最新研究成果:高频调幅磁场及无结晶器振动的电磁连铸技术。     

Crystallization Characteristics of CaO-Al2O3-Based Mold Flux and Their Effects on In-Mold Performance during High-Aluminum TRIP Steels Continuous Casting

Crystallization behaviors of the newly developed lime-alumina-based mold fluxes for high-aluminum transformation induced plasticity (TRIP) steels casting were experimentally studied, and compared with those of lime-silica-based mold fluxes. The effects of mold flux crystallization characteristics on heat transfer and lubrication performance in casting high-Al TRIP steels were also evaluated. The results show that the crystallization temperatures of lime-alumina-based mold fluxes are much lower than those of lime-silica-based mold fluxes. Increasing B₂O₃ addition suppresses the crystallization of lime-alumina-based mold fluxes, while Na₂O exhibits an opposite effect. In continuous cooling of lime-alumina-based mold fluxes with high B₂O₃ contents and a CaO/Al₂O₃ ratio of 3.3, faceted cuspidine precipitates first, followed by needle-like CaO·B₂O₃ or 9CaO·3B₂O₃·CaF₂. In lime-alumina-based mold flux with low B₂O₃ content (5.4 mass pct) and a CaO/Al₂O₃ ratio of 1.2, the formation of fine CaF₂ takes place first, followed by blocky interconnected CaO·2Al₂O₃ as the dominant crystalline phase, and rod-like 2CaO·B₂O₃ precipitates at lower temperature during continuous cooling of the mold flux. In B₂O₃-free mold flux, blocky interconnected 3CaO·Al₂O₃ precipitates after CaF₂ and 3CaO·2SiO₂ formation, and takes up almost the whole crystalline fraction. The casting trials show that the mold heat transfer rate significantly decreases near the meniscus during the continuous casting using lime-alumina-mold fluxes with higher crystallinity, which brings a great reduction of surface depressions on cast slabs. However, excessive crystallinity of mold flux causes poor lubrication between mold and solidifying steel shell, which induces various defects such as drag marks on cast slab. Among the studied mold fluxes, lime-alumina-based mold fluxes with higher B₂O₃ contents and a CaO/Al₂O₃ ratio of 3.3 show comparatively improved performance.     

高拉速薄板坯连铸保护渣剪切变稀性质

 针对高拉速薄板坯连铸保护渣现场使用过程中卷渣风险加剧、黏结报警频发等问题,通过使保护渣产生非牛顿流体行为,从而有效解决上述问题。该种新型保护渣具有剪切变稀的特性,即在较低剪切速率下具有较高黏度、在较高剪切速率下具有较低黏度。基于现场数据,计算出高拉速薄板坯结晶器钢液面表面区域的剪切速率为10~90 s-1,结晶器弯月面及以下区域的剪切速率可达120~1 600 s-1。采用旋转圆筒法研究了Al₂O₃对保护渣剪切变稀性质的影响。采用Oswald-De Waele幂律模型对剪切变稀行为进行了定量分析。结果表明,随着Al₂O₃含量的增加,保护渣剪切变稀性质先增强后减弱,Al₂O₃质量分数为8.61%的试样剪切变稀性质最强,其流动性指数最低达到0.764 4。研究发现,非牛顿流体保护渣具有的剪切变稀性质能够满足在结晶器钢液面表面区域和弯月面及以下区域内对保护渣黏度的要求。基于高拉速薄板坯连铸的具体工艺参数,建立结晶器内多相耦合模型,通过模型计算发现,保护渣的剪切变稀性质增强不仅会明显降低剪切卷渣的风险,提高结晶器弯月面区域液渣流入的均匀性,而且在结晶器弯月面及以下区域具有更厚的液态渣膜,更容易实现全程液态润滑,同时提高了渣耗量,进一步剖析了保护渣剪切变稀性质的作用效果。本研究为开发非牛顿流体高拉速薄板坯连铸保护渣提供了理论依据。     

A Study for Initial Solidification of Sn-Pb Alloy During Continuous Casting: Part II. Effects of Casting Parameters on Initial Solidification and Shell Surface

The initial shell solidification of liquid steel in the mold has significant influence on both surface and internal quality of the final slab, and it is mainly determined by the high transient high temperature thermodynamics occurring in the mold. This study investigated the effects of casting parameters like casting temperature, mold oscillation frequency, and stroke on the initial solidification of a Sn-Pb alloy through the use of a mold simulator to allow the clear understanding of the inter-relationship between irregular shell solidification, heat transfer, negative strip time (NST), and casting conditions. Results suggested that the shell surface oscillation marks (OMs) are strongly depending upon the fluctuations of meniscus responding temperatures and heat flux. An abrupt sudden fluctuation of high frequency temperature and heat flux at the meniscus during the NST would deteriorate the shell surface and leads to deep OMs. The fluctuations of responding temperature and heat flux are determined by the NST, meniscus solidification, and oil infiltration, which in turn are influenced by casting conditions, like casting temperature, oscillation frequency, stroke, etc.     

高锰高铝钢纯净度控制关键技术研究进展

摘要：汽车轻量化对低密度高锰高铝钢的需求日趋迫切,然而高锰高铝钢的大规模化生产仍受制于纯净度控制技术与连续浇注工艺。首先基于高锰高铝钢的成分特点,阐述了高锰高铝钢的微观组织特征,然后通过热力学模拟了高锰高铝钢中夹杂物的形成与演变规律,分析了不同Al和Mn含量对夹杂物的类型和含量的影响规律,最后总结了近年来国内外学者关于高锰高铝钢中夹杂物形成理论和实验研究,钢中高铝含量显著影响Al2O3、AlN及AlON的竞相析出以及影响MnS等夹杂的析出行为,进而从精炼渣、耐火材料和保护渣角度分析了高锰高铝钢冶炼特点并指出了其纯净度控制的关键方向。     

TiC颗粒强化型马氏体耐磨钢的性能研究

采用模铸、连铸两种工艺工业化试制一种TiC颗粒强化型马氏体耐磨钢,分析了TiC颗粒的析出规律,对比研究了试验钢与传统马氏体耐磨钢的组织、力学性能及耐磨性能。试验结果表明:凝固速度越大,TiC析出相越细;轧制压缩比越大,颗粒分布越均匀;TiC颗粒强化马氏体钢强度与传统马氏体钢相当,韧性有所降低;微米级的TiC可以有效提高材料的磨粒磨损性能,试验钢磨损失重仅为同等硬度传统马氏体钢的70%;耐磨性能的提高主要是因为在磨粒磨损条件下,微米级TiC硬质点可以破碎磨砺、钝化尖角、阻断磨痕。     

基于反算热流的结晶器内流动--传热--凝固耦合模拟

基于Navier-Stokes动量方程和湍流低雷诺数k-ε方程,综合考虑能量守恒和钢液凝固与糊状区对流动过程的影响,建立了描述结晶器内钢液流动、传热及凝固过程的三维耦合数学模型.以实测温度和结晶器反问题模型计算出的热流为边界条件,模拟计算了结晶器内钢水的流动、传热和凝固行为.钢液流动决定结晶器内的温度和热流分布,铸坯凝固受钢液流动和结晶器热流双重因素的影响.建立的模型以及由此得到的铸坯凝固非均匀特征可为进一步考察浇铸过程中纵裂和其他表面缺陷提供借鉴和参考.      

高铝钢用低反应型保护渣成分对其黏度的影响

摘要：汽车轻量化有助于保护环境、节约能源,高铝钢有利于减轻汽车质量同时维持强塑性。但由于连铸过程中传统结晶器保护渣界面反应的制约,高合金钢铸坯质量和操作流畅性受到很大影响,引起裂纹、漏钢等问题。不仅会造成安全事故,还会增加成本。低反应型CaO Al2O3系保护渣相对于传统保护渣,SiO2质量分数在6%~10%之间,［Al］和(SiO2)渣钢界面反应程度显著减弱,具有提高铸坯质量和确保操作顺行的潜力。设计此类保护渣时应该考虑渣钢界面反应、渣中元素向钢液中富集对铸坯质量的影响以及可能的结晶相种类。探讨了低反应型保护渣中成分对黏度变化机制的影响,即熔渣结构的变化、渣系过热度的变化和结晶相的变化。分别讨论了CaO/Al2O3、B2O3、Na2O、Li2O和CaF2在CaO Al2O3渣系中的作用,旨在为满足高铝钢连铸生产的新一代低反应型保护渣系的设计与优化提供思路与便利。