Implementation of an YSZ coating material to prevent clogging of the submerged entry nozzle (SEN) during continuous casting of Ce-treated steels

The possibility to reduce clogging in the submerged entry nozzle (SEN) during continuous casting of Ce alloyed stainless steels has been studied. This was done by implementing a new plasma coating material, consisting of yttria-stabilised zirconia (YSZ). The coating was first tested in pilot–plant trials; where the amount of steel teemed through the plasma-coated nozzles was monitored. Thereafter, samples of the coatings from the nozzles were studied using a FEG-SEM equipped with EDS. In addition, the coating material was tested in industrial trials where the performance was judged with respect to the clogging tendency detected as the movement of the stopper rod. The results from both the pilot–plant trials and industrial trials showed that the use of an YSZ coating led to a decreased clogging tendency during the casting in comparison to when using an uncoated SEN. Specifically, the pilot–plant trials showed that the clogging factor was lowered when implementing the YSZ coatings. Furthermore, the industrial trials showed that the clogging tendency, measured by the stopper rod movements, were lower when implementing the YSZ coating.     

Reaction performances of mould slags with different SiO2 contents for 321 stainless steel

In the present study, laboratory-scale experiments and thermodynamic calculation between the mould slags and the steel were performed to investigate reaction performances of mould slags during continuous casting (CC) of 321 stainless steel. The results show that in the final Slag1 (traditional mould slag), SiO₂ decreased by 3.86?wt-% and TiO₂ increased by 4.85?wt-%, whereas the melting temperature, viscosity and crystallization rate increased noticeably. Perovskite (CaTiO₃) and TiN were precipitated during the steel-slag reaction. Compared to Slag1, both properties and constituents of Slag2 (newly developed) with low SiO₂ changed moderately, despite TiO₂ in the final Slag2 increased by 3.96?wt-%. The precipitation of TiN and the reactivity between steel and slag were evaluated by the thermodynamic method. A comprehensive enthalpy change of steel-slag reaction was estimated to elucidate the formation of frozen steel or clogging based on the experimental results and the thermodynamic calculation.     

Effect of Nozzle Base Material on the Rate of Clogging during the Continuous Casting of Aluminum‐Killed Steels

One approach to solving the problem of nozzle clogging during the continuous casting of aluminum‐killed steels is through careful selection of nozzle materials. In this study, the rate of clogging was measured while casting steel through simulated nozzles produced from alumina, zirconia, magnesia, zirconia‐graphite, and alumina‐graphite ‐ three common base materials and two common carbon‐containing nozzle materials. Spent nozzles were characterized using optical and cathodoluminescence microscopy. Interactions between the nozzles and steel were not observed in the alumina, zirconia, and zirconia‐graphite nozzles. Slight interactions were observed in the magnesia nozzles as alumina inclusions within the steel interacted with the nozzle to produce alumina‐magnesia spinel. Greater amounts of interaction were observed with alumina‐graphite nozzles. No statistical differences in the mean rate of clogging were observed between all of the pure oxide nozzles and zirconia‐graphite nozzles. However, the alumina‐graphite nozzles clogged at a much higher rate than the other nozzles. The higher rate of clogging is thought to be due to refractory‐steel interactions.     

连铸中间包上水口及浸入式水口结瘤物分析

水口结瘤一直是困扰连铸生产的问题,对连铸生产中发生结瘤的中间包上水口和浸入式水口从上到下做了全面的解剖分析,通过SEM、EDS、X-RD等手段对水口的结瘤物进行分析研究。结果表明,中间包上水口结瘤严重,钢水完全凝结;在浸入式水口渣线以上,发生了严重的结瘤,厚度约为16 mm,渣线下的结瘤较轻,结瘤物主要是Al2O3夹杂着钢水凝结而成,由于钢水中的酸溶铝含量较高,水口处Al2O3的富集使钢水凝结,堵塞水口。根据研究得出,中间包上水口和浸入式水口结瘤堵塞的原因为钢水酸溶铝含量较高,钢水在水口处温降过大、拉速过低等。     

Studies of the decarburisation phenomena during preheating of submerged entry nozzles (SEN) in continuous casting processes

Decarburisation of the submerged entry nozzles (SEN) during the preheating process was investigated based on plant trials and thermodynamic modelling at three different steel plants. During the trials the preheating processes were mapped, the temperature profiles were registered and post-mortem studies of the SENs with scanning electron microscopy were performed. Typically, the glass/silicon powder will form a dense and protective layer inside the SEN when heated over 1100°C. However, this study found that the temperature distribution inside the SEN did not always reach this critical temperature. Thus, decarburisation of the SEN was found at all steel plants. The overall results illustrate that the control of the preheating process needs to be improved at all steel plants. It is suggested that future research should be focused on the development of new coating materials to prevent decarburisation of the refractory base material, which would decrease the chances of clogging during casting.     

Development of indices for quantification of nozzle clogging during continuous slab casting

Abstract

Deposition of alumina inclusions inside the submerge entry nozzle is a long standing problem in continuous casting of aluminium killed steel. Clogging of the casting nozzle severely affects productivity and also causes degradation of product quality. Unfortunately, very few numerical tools are available for precise quantification of the magnitude of clogging. Tools for quantifying the extent of clogging were developed during the present work and have been integrated with the control system of a running continuous slab casting shop. Reasonable match has been observed between the estimation from the developed program and the actual observations in the casting shop and subsequent products. In addition to process control, the tools were also found useful for comparing the castability of different steel grades. This feature has been utilised to study nozzle clogging during casting of steel grades that are commonly believed to have poor castability.     

40Cr钢浸入式水口结瘤分层结构的形成机理

  为明确转炉→LF精炼→连铸工艺条件下生产40Cr钢浸入式水口结瘤分层结构的形成机理,采用扫描电镜、阴极发光仪以及FactSage 7.1热力学软件,对40Cr钢水口结瘤形貌及分层结构进行了研究。结果表明,浸入式水口结瘤物分为水口脱碳层、凝钢层、夹杂物堆积层、凝钢层和表面疏松层。分析了40Cr钢在连铸过程中钢中Al2O3、Al2O3 CaO(SiO2)和Al2O3 MgO尖晶石在水口内壁上的分层沉积现象。结合现场工艺,研究了双凝钢层的形成机理,并通过热力学计算解释了结瘤物的形成机理。     

Minimisation of calcium additions to low carbon steel grades

Abstract

This investigation aimed to determine the in plant feasibility of decreasing, to an amount close to the minimum literature value, the calcium addition to liquid steel for prevention of alumina buildup during continuous casting. Six plant trials were carried out at calcium additions of 0·14 kg/t of steel (reduced from the original 0·19 kg/t), added to the second ladle of a two or three ladle sequence. Total oxygen samples were taken at the ladle furnace and tundish to determine total oxygen and nitrogen contents of the steel. The total oxygen content at the ladle furnace varied between 19 and 26 ppm, with a slight degree of reoxidation between the ladle furnace and the caster. Alumina clogging was successfully prevented by the addition of 0·14 kg calcium/t of steel during the first five trials. During the sixth trial the submerged entry nozzle (SEN) failed and, although the stopper behaved as if clogging occurred, this behaviour was caused by the poor perfomance of the SEN rather than actual clogging. Microanalysis of inclusions in steel samples revealed a distribution in degrees of modification between different inclusions, and the formation of a substantial amount of CaS (which is taken to indicate overmodification, based on equilibrium calculations). However, the CaS is mostly associated with at least partly liquefied oxide inclusions, which is likely to reduce the potential clogging effect of solid CaS.     

Influence Factors of SEN Clogging of Rare-Earth Steel Continuous Casting and Solve Ideas of Japanese Steel Enterprises

The clogging behavior of submerged entry nozzle (SEN) is the main reason restricting the continuous casting of steel, which seriously affects the application and popularization of rare-earth (RE) steel. Through literature analysis, herein, the main influencing factors and mechanism of RE steel continuous casting immersion outlet clogging, and sorts out the main ideas and practices of relevant Japanese enterprises in solving SEN clogging in recent years, are summarized, mainly including the following: 1) optimizing SEN materials to reduce nozzle clogging reactions; 2) using Ar-blowing SEN; 3) applying electromagnetic field to eliminate charge and inhibiting clogging reaction; and 4) and regulating the composition of molten steel to reduce the possibility of clogging, etc. The combination of these advantages and measures is a feasible way to solve the nozzle clogging of RE steel continuous casting nozzle.     

Collision-controlled growth of composites in casting nozzles

Like the open-stream nozzles and submerged entry nozzles on c.c. tundishes, ladle nozzles can also become blocked. Accumulations of Al₂O₃, CaO · 2Al₂O₃, CaO · Al₂O₃, MgO · Al₂O₃ and Ti₃O₅ particles were observed on five sleeves removed from rotary ladle nozzles. The type of particles suspended in the steel depends on metallurgy. Growth takes place in bushy ramification in a coral-shaped structure and is controlled by flow separation, interfacial turbulence and gradient collision. Heat transfer on the nozzle wall is impaired. Steel which has infiltrated freezes if a heat sink is present. A stable composite of steel and oxide is formed. CaS depositions in c.c. tundish submerged entry nozzles are also examined. These CaS deposits become coated with viscous aluminates of the (C₁₂A₇ + CA) type, prior, here too, to impregnation with steel. The overall result is a reduction in interfacial energy.     

Prevention of overgrowth on submerged nozzles during casting of steel deoxidized by aluminum

An initial reaction layer forms on the submerged entry nozzles; this layer is followed by bulk deposits that primarily contain endogenous alumina and steel inclusions. It would therefore be of some interest to prevent the formation of this initial layer opposite the refractory wall of the submerged nozzle. The presence of carbon and SiO₂ on the surface of the nozzle facilitates formation of a loose layer of alumina deposits. Refractories with various coatings were tested in contact with nonmetallic inclusions, liquid steel, and synthetic slag. It was found that a coating of pure alumina or ZrO₂ reduces overgrowth on the submerged nozzles and shows high chemical resistance to liquid steel.     

F-free mould powders for low carbon steel slab casting – technological parameters and industrial trials

The choice of the mould powder for slab casting is a difficult task because mould powders have many important functions during the continuous casting of steel. CaF₂ is a key ingredient in conventional mould slags since it reduces the viscosity, the liquidus temperature and the break temperature. Fluorine in mould powders is undesirable from the environmental and health points of view due to the following reasons: (i) evolves easily from slags, producing health-injurious gaseous substances, such as hydrofluoric acid; (ii) creates problems for storage and utilisation of solid waste and (iii) causes machinery corrosion. Aim of the present work is to describe the development of a new F-free mould powder for low carbon steel slab casting replacing CaF₂ with B₂O₃. Laboratory tests and industrial trials were performed considering the technological parameters viscosity, break temperature and crystallisation tendency. From laboratory tests it was concluded that important technological parameters are similar when comparing the F-bearing mould powder (reference) and the new F-free B-bearing mould powder for low carbon steel slab casting: viscosity at 1300°C, break temperature, and crystallisation tendency. It was observed during industrial trials that a significant decrease of the submerged entry nozzle erosion was observed. The results were similar when comparing the F-bearing and the F-free performance: slag pool thickness measurements, melting behaviour, Al₂O₃ absorption, mould powder consumption and slabs superficial quality.     

Effects of clogging, argon injection, and continuous casting conditions on flow and air aspiration in submerged entry nozzles

The inter-related effects of nozzle clogging, argon injection, tundish bath depth, slide-gate opening position, and nozzle-bore diameter on the steel flow rate and pressure in continuous-casting slide-gate nozzles are quantified using computational models of three-dimensional (3-D) multiphase turbulent flow. The results are validated with measurements on operating steel continuous slab-casting machines and are presented for practical conditions with the aid of an inverse model. Predictions show that initial clogging may enhance the steel flow rate due to a potential streamlining effect before it becomes great enough to restrict the flow channel. The clogging condition can be detected by comparing the measured steel flow rate to the expected flow rate for those conditions, based on the predictions of the inverse model presented here. Increasing argon injection may help to reduce air aspiration by increasing the minimum pressure, which is found just below the slide gate. More argon is needed to avoid a partial-vacuum effect at intermediate casting speeds and in deeper tundishes. Argon flow should be reduced during shallow tundish and low casting speed conditions (such as those encountered during a ladle transition) in order to avoid detrimental effects on flow pattern. Argon should also be reduced at high casting speed, when the slide gate is open wider and the potential for air aspiration is less. The optimal argon flow rate depends on the casting speed, tundish level, and nozzle-bore diameter and is quantified in this work for a typical nozzle and range of bore diameters and operating conditions.     

Interaction of alumina inclusions in steel with calcium-containing materials

Clogging of tundish and submerged entry nozzles (SENs) adversely impacts productivity and quality in the continuous casting of aluminum-killed steels. Clogging results from an accretion layer that develops on the inside surface of the nozzle and restricts steel flow. Current nozzle refractories often react with molten steel to form solid by products that promote clogging. Nozzle materials that are inert with the liquid steel or react with accretions to form liquid reaction products could inhibit or eliminate clogging. Static experiments were conducted to investigate the stability between calcium-based materials and aluminum-killed steel. The results indicate that both calcium titanate and calcium zirconate react with alumina to form calcium aluminates. However, only the reaction between alumina and calcium titanate yielded calcium aluminate chemistries that were molten at steel casting temperatures. Liquid reaction products are preferred since they would be removed from the nozzle by the steel flow, thereby preventing accretion formation and clogging.     

Fluid flow and heat transfer with nail dipping method in mould during continuous casting process

ABSTRACT

The character and cause of sliver defect on IF steel sheet surface are studied by means of SEM, unstable flow in mould could induce surface velocity and level fluctuations, leading to surface defects during continuous casting of steel. The nozzle clogging is a serious problem during the continuous casting of steel, due to its influence on the casting operations and products quality. In this study, the nail dipping method for measuring surface velocity and flow direction in molten steel were employed. The fluid flow in mould of whole casting sequence was investigated, especially during the nozzle clogging conditions. The results showed that when nozzle clogging occurred in the 7th heat, the flow velocities on R and L side of nozzle were 0.280 and 0.402?m/s, respectively. The surface defect ratio of hot-rolled and cold-rolled plates increases with the increase of heat flux deviation on both sides of the mould copper. The different clogging per cent on both sides of the nozzle will lead to asymmetry flow, the surface velocity is higher with the small clogging per cent side compared to that of relatively large per cent clogging side.     

Controlling Nozzle Clogging by Secondary Steelmaking Without Reheating

Ever since continuous casting was introduced, nozzle clogging has been a recurring subject of metallurgical research and publications. Nozzle clogging is caused by the deposition of non‐metallic inclusions – on the casting equipment – which are solid at the temperature of casting. Calcium treatment already represents a possible solution to eliminate this phenomenon, but there are special steel grades which do not allow its application either because they are price sensitive or for certain other reasons.³ Cleaning the steel melt by agitation is an alternative solution, but it has time constraints due to the cooling of the melt. Over extensive cooling can be prevented by heating the melt (in ladle), but its introduction and operational costs per heat are often too high for an individual steelmaking plant to afford. The goal of our research is to find metallurgical solution to reduce the rate of nozzle clogging in plants which do not possess a ladle furnace and the time devoted to treatment or the price of the product do not allow Ca‐treatment to be applied.     

CSP浇注无取向硅钢水口堵塞研究

国内某厂采用BOF—LF—RH—CSP工艺生产无取向硅钢(Al含量0.3%～0.4%),通过对某一浇次每炉的系统取样(RH喂钙前、RH喂钙后、中间包稳态浇注及铸坯等),并采用光学显微镜观察,SEM分析,以及对水口堵塞物的矿相分析研究等,明确了水口堵塞物的结构和成分,简单阐述了水口堵塞的机理。重点研究了钙处理生成大量含CaS的夹杂物对水口堵塞的影响。通过改进生产工艺,在满足产品质量要求的前提下,使CSP生产无取向硅钢的连浇炉数从5炉提高到8炉及以上,提高了生产率。     

含钛焊丝钢小方坯连铸水口的结瘤机制

 利用扫描电镜对含钛焊丝钢中夹杂物性质及连铸水口结瘤物的物相组成进行了分析,并结合热力学计算研究了水口结瘤的形成机制。结果表明,LF 精炼出站钢液中存在大量Al2O3、TiO2夹杂物,并不断附着沉积在水口内壁形成氧化铝型、氧化钛型或两者结合的结瘤物,连浇炉数仅为4次。通过优化钢中［Al］-［Ti］-［O］关系,控制铝质量分数在钛-铝竞争氧化平衡线之上,即w(［Ti］)/w(［Al］)4/3>84.49 且w(［Ti］)/w(［Al］)>7.46,当钢液中w(［Al］)<0.0068%时,能够降低Al2O3夹杂比例,有效减轻水口结瘤,连浇炉数提升至6炉次。     

提高方圆坯铸机连浇炉数的工艺及装备技术

 攀钢钒提钒炼钢厂2009年10月中旬投产的方圆坯连铸机单中间包连浇炉数较低,影响了炼钢生产组织。分析了浇注铝镇静钢水口堵塞原因,通过研究脱氧工艺、夹杂物变性处理工艺及优化二冷喷嘴结构与布置方式、改进拉矫设备及工艺,改善了钢水可浇性,提高了铸机拉速,铝镇静钢因水口堵塞造成的流次断浇率由投产初期的12.9%降至2.0%以下,单中间包连浇炉数由投产初期的4～5炉提高到7～10炉,硅镇静钢单中间包连浇炉数由6～9炉提高到8～14炉,从而保证了方圆坯铸机生产顺行,降低了生产成本。     

Up‐scaling of a Low Temperature PACVD Process for the Deposition of α/γ‐Nanocrystalline Alumina Coated Tools for Thixocasting of 1.2208 steel

An industrial‐scale pulsed plasma‐assisted chemical vapor deposition (PACVD) process for crystalline alumina growth was developed. To obtain a homogeneous coating thickness distribution over complex geometries and large dimensions, a suitable gas injection system was designed. A phase formation diagram for alumina coatings as a function of pulse length and cathode voltage has been compiled, allowing for the deposition of dense α/γ‐Al₂O₃ coatings at a substrate temperature of 590 °C. Moulds coated with an α/γ‐Al₂O₃ coating were utilized in steel thixocasting at temperatures of ～1400 °C. The coatings were intact after thixocasting and showed significantly improved chemical wear resistance compared to plain steel moulds.