Solidification control in continuous casting of steel

An integrated understanding of heat transfer during solidification, friction/lubrication at solid-liquid interface, high temperature properties of the solidifying shell etc. is necessary to control the continuous casting process. The present paper elaborates upon the knowledge developed in the areas of initial shell formation, mode of mould oscillation, and lubrication mechanism. The effect of these issues on the caster productivity and the quality of the product has been discussed. The influence of steel chemistry on solidification dynamics, particularly with respect to mode of solidification and its consequence on strength and ductility of the solidifying shell, has been dealt with in detail. The application of these basic principles for casting of stainless steel slabs and processing to obtain good quality products have been covered.     

Thermal stress evaluation in the steel continuous casting process

Two numerical models for the thermal stress and plastic strain analysis in the solid shell at the initial stage of a steel continuous casting process, are presented and their performances evaluated. First, a numerical procedure based on a natural extension of the semi‐analytical study proposed by Weiner and Boley (J. Mech. Phys. Solids 1963; 11 :145–154) is introduced and validated by comparison with their semi‐analytical 1‐D solution. The basic hypotheses of this model, and particularly the extended plane strain condition with uniform axial strain, are afterwards tested in more complex and realistic situations by comparing results with those obtained by an alternative numerical scheme presented by Fachinotti (Doctoral Thesis, Universidad Nac. Litoral, Argentina, 2001) and Fachinotti and Cardona (Internal Report, CIMEC, sent to IJNME, 2002). Finally, results of a typical and representative 3D simulation, corresponding to a slab continuous casting process, are shown. Copyright © 2005 John Wiley & Sons, Ltd.     

Properties of reacto-thermitically sintered stainless steel

Abstract

Reacto-thermitic sintering (RTS) is a novel process capable of producing near net shape powder metallurgical stainless steel components with negligible interconnected porosity at sintering temperatures of 1150°C. It utilises chemical reactions between oxides present on the surface of conventional stainless steel powders and small quantities of reactive additives to produce a transient liquid phase. This liquid can be frozen during cooling to consolidate the component without slumping. Previous work by the authors has studied the fundamental mechanisms underpinning the evolution of this liquid phase and its subsequent behaviour in eliminating porosity. The present paper investigates the effect of varying the quantity of reactive additive on the dimensional stability, mechanical properties, and corrosion resistance of stainless steel specimens produced by RTS in the light of this work.     

Influence of chemistry on transverse cracking during continuous casting of medium C high N steel billets

Abstract

Corner cracking in medium C electric arc steel was experienced after continuous casting of square billets. This led to the investigation of the influence of high N levels on hot ductility. Steels microalloyed with V, Ti and B were subjected to in situ melting followed by cooling according to typical off-corner temperature profiles. Steels containing moderate amounts of V resulted in good ductility at the estimated off-corner temperature during unbending of 900°C due to the suppression of precipitation. Ti additions to V, Al, Si and B containing steels, although beneficial at low N levels, are detrimental to hot ductility when the N content is high due to a higher precipitate volume fraction. High Ti–B steels produced the worst hot ductility in medium C steels containing high N.     

Hot corrosion mechanism of tundish plaster with steel slags in continuous casting

This study concerns the chemical reactions involved and the phases formed during penetration of slags of variable composition into porous plaster structure of tundish. Tundish plaster is mainly composed of MgO with minor amounts of SiO₂ and impurities, with a grain size of less than 1 mm, inorganic or organic fibers in order to decrease density and provide porosity for insulation, plasticizers and stiffening agents and some other additions. Tundish slag analysis for different grades of steel (7176D and 1191D, according to DIN standard) at different sequences, indicated a very variable composition of CaO (11–42%), SiO₂ (28–46%), Al₂O₃ (6–12%), MgO (11–20%), MnO (0–13%) and some minor variations of Fe, FeO and TiO₂. Experimental work indicated that slag when penetrated into pores of plaster, develop the phases of Monticellite (CaO · MgO · SiO₂) and Merwinite (CaO_1.5 · MgO_0.5 · SiO₂) around MgO particles and decrease the liquidus temperature from 2,800 °C to about 1,500 °C and provide dissolution of MgO grains in steel making process. Calculation based on two kinetic equations developed for diffusion controlled dissolution, indicated that the dissolution of MgO in tundish plaster is not a diffusion controlled process and is affected by turbulent flow parameters. Phase diagram of CaO–SiO₂–MgO indicates that decreasing SiO₂ to below 20% and increasing CaO content to as high as possible, increases the liquidus temperature to above 2,000 °C. Sources of SiO₂ in the process are the rice husk addition, which is used as an insulating material on top of the melt, and the slag flux addition. These sources should be reduced to as low a level as possible. This fact does not affect the fluidity of slag which is required for inclusion removal. Fluidity of slag comes from low melting point eutectics in CaO–Fe₂O₃ and CaO–FeO (about 1,200 °C) due to iron oxide on top of the steel melt.     

On a nonlinear stefan problem arising in the continuous casting of steel

Summary The continuous casting process employed in the steel industry is a many faceted big industrial problem which has given rise to many sub-problems. Here, we examine the problem involving the determination of the solid-liquid steel interface and we develop and extend a previously proposed model, which incorporates heat transfer through two layers of solid and liquid mould powder and the interface between the solid powder and the mould wall. The problem simplifies to the classical Stefan problem except that the condition on the boundary is nonlinear. Integral formulation procedures are used to establish the normalized pseudo steady state temperature as an upper bound to the normalized actual temperature. The pseudo steady state approximation yields an upper bound on the interface position, which an independent numerical enthalpy scheme confirms to be an extremely accurate approximation for the parameter values occurring in practice. The present work is important since it provides a simple method for the prediction of the solid-liquid steel interface and a bounding procedure which can be used to validate other estimates.List of symbols D flux thickness atz ^*=0 - H enthalpy - L latent heat of steel - M the half thickness of the cast steel - Q heat flux - R interface thermal contact resistance - S _m ^* melting temperature of steel - T ^* temperature - T normalized temperature - T _m ^* melting temperature of mould powder - T ^* temperature of cooling water - T _w ^* temperature on mould wall - T _u ^* temperature of solid flux on its interface with mould wall - T ₀ ^* temperature on casting surfaceT ^*(0,z ^*) - U casting speed - X ^*(z ^*) physical coordinate of the steel phase change boundary - X(z) non-dimensional coordinate of the steel phase change boundary - c specific heat of steel - h(z ^*) thickness of liquid flux layer - k thermal conductivity of steel - ks thermal conductivity of solid flux layer - k _l thermal conductivity of liquid flux layer - m surface heat transfer coefficient - s(z ^*) thickness of solid flux layer - t time - , , positive constants given by (3.2) - constant given by (3.5) - coefficient of linear thermal expansion of steel - angle shown in Figure 2 - positive constant defined by (M-D)/2 - (z) positive parameter - (z ^*) amount of contraction of steel - density - (z) positive parameter used in (5.7) and (5.8)     

钢铁连铸用铜合金结晶器

结晶器是钢铁连铸设备的重要部件，其用途是把浇注的钢水成形并生成足够厚度的凝壳，防止铸锭带移向２次冷却带时跑钢。以往，结晶器是选择导热性良好的脱氧铜制作的。实践表明，脱氧铜结晶器在使用中易变形和磨损，寿命很短。因此，研制、开发新型铜合金结晶器材料，便成为钢铁连铸工程中的重要课题。本文综合性地介绍了这方面的发展。     

Finite element numerical simulation on thermo-mechanical behavior of steel billet in continuous casting mold

The thermo-mechanical behavior of a thin, growing shell during the early stages of solidification in a continuous casting mold is very important to the ultimate quality of the final billet. A two-dimensional, transient finite element model has been developed to treat the heat flow and deformation of the solidifying shell in the continuous casting billet mold as a coupled phenomena. The major application of the model is to predict the extent of the gap between the mold and the shell and focus on the influence of mold taper on the thermo-mechanical behavior of the steel billet to help to understand the formation of off-corner cracks and break-outs in the solidifying shell. The calculations indicate that the gap is initially formed at the corner of the billet, where heat transfer is greatly reduced. Insufficient mold taper contributes to a hot spot in the off-corner region, which corresponds to the lowest shell thickness. At the same time, the solidifying front on the diagonal of the billet is subjected to an excessive mechanical strain, which causes the off-corner cracks and even the break-outs.     

Metallurgical investigation on fatigue failure of stainless steel chain in a continuous casting machine

Stainless steels strips (chains) are used for the connection of dam blocks in belt casting machines. Thermal cycling and repetitive stressing under complex loading conditions due to tension and bending are the most frequent function modes during production. Samples from fractured stainless steel strips used for the connection of dam blocks in a copper rod continuous casting line, were sent for failure investigation. Optical and scanning electron microscopy for structural and fractographic evaluation along with mechanical testing are used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the failure was caused by bending fatigue which assisted also by thermal cycling, initiated from the strip surface and followed by ductile final overload fracture. Final fracture occurred via ductile failure, when the remaining strip cross sectional area reaches a critical size, becoming unable to sustain the operating load. Review of the service history (operating conditions, e.g. process design, applied loads, thermal cycles), in combination to the examination of a potential substitution of the material to a more heat (and fatigue) resistant one are suggested as further fatigue damage preventive actions.     

Finite element numerical simulation on thermo-mechanical behavior of steel billet in continuous casting mold

The thermo-mechanical behavior of a thin, growing shell during the early stages of solidification in a continuous casting mold is very important to the ultimate quality of the final billet. A two-dimensional, transient finite element model has been developed to treat the heat flow and deformation of the solidifying shell in the continuous casting billet mold as a coupled phenomena. The major application of the model is to predict the extent of the gap between the mold and the shell and focus on the influence of mold taper on the thermo-mechanical behavior of the steel billet to help to understand the formation of off-corner cracks and break-outs in the solidifying shell. The calculations indicate that the gap is initially formed at the corner of the billet, where heat transfer is greatly reduced. Insufficient mold taper contributes to a hot spot in the off-corner region, which corresponds to the lowest shell thickness. At the same time, the solidifying front on the diagonal of the billet is subjected to an excessive mechanical strain, which causes the off-corner cracks and even the break-outs.     

A power transducer system for the ultrasonic lubrication of the continuous steel casting

A critical point in the continuous steel casting process exists in the meniscus zone of the cooled mould, i.e., the region in which the steel stream flowing out of the tundish nozzle starts to solidify. This is a critical point because of the sticking that occurs between the solid shell of steel and the mould. In this work, a new system for the ultrasonic lubrication of the continuous steel casting is proposed and experimentally tested. The basic idea is to excite one of the mould's natural vibration modes by means of a distributed ultrasonic source. This source is composed of an array of power emitters, with each of them placed upon an antinode of the mould. An experimental characterization of the vibrational behavior of a square mould was first carried out. The most active resonance modes of the mould were detected with an experimental technique based on a simple impedance measurement. The modal shape of the selected mode, and hence the position of antinodes, was obtained by means of interferometer measurements. Additional experimental investigations were performed by exciting mould vibrations with up to four piezoceramic disks placed on different sets of antinodes. Some positioning criteria to maximize the superposition effect were derived. Measurements were obtained through excitation of the mould with up to four Langevin-type power emitters, designed and manufactured to work at the mould's selected resonance frequency. These measurements have shown that, by increasing the number of emitters, the ultrasonic power transmitted to the mould and, consequently, the maximum available displacement, increases. Other practical advantages of the proposed system are highlighted and discussed.     

Experimental study on hot melt splashes during electromagnetic continuous casting silicon

Hot melt splashes often take place during electromagnetic casting silicon, which break the stable pulling process and endanger the safety of equipments in the vacuum chamber. Investigations are carried out to analyze the causes of melt splashes and put forward measures to prevent the happening of them. Pores with different sizes are observed in perpendicular sections of the left ingots, their surface compositions are analyzed by EDS. The results indicate that they are rich in O, Al, Ca and Mg elements, which means that the driving pressures of hot melt splashes are mainly the vapor tension of SiO, Al, Ca and Mg. Calculated results show that the volatilization of those elements is inevitable, so the vapor excluding and pressure releasing are key issues for preventing hot melt splashes, both of which are affected by the height and top position of the liquid dome. Therefore, controlling of those parameters properly can efficiently prevent the hot melt splashes. The optimal dome top position is from 6 mm to 16 mm lower than the coil top position.     

Interdendritic microsegregation development of high-strength low-alloy steels during continuous casting process

A 1-D model based on DICTRA software was used to simulate Mn microsegregation in high-strength low-alloy steels during continuous casting. The experimentally determined (using a cumulative profiling method) segregation results were in good agreement with the modelling results. Steels undergoing solidification via a peritectic reaction had a larger segregation range than non-peritectic steels ascribed to trapping of the alloying atoms in liquid by austenite acting as a diffusion barrier. Subsequent, post-solidification cooling through the single phase austenite field decreased the microsegregation level, although the last γ→α phase transformation did affect the segregation profile in the solute-depleted dendrite centre. Simulation indicates that segregation levels could be reduced by decreasing either secondary dendrite arm spacing or adopting faster cooling rates through solidification.     

Experimental study on hot melt splashes during electromagnetic continuous casting silicon

Hot melt splashes often take place during electromagnetic casting silicon, which break the stable pulling process and endanger the safety of equipments in the vacuum chamber. Investigations are carried out to analyze the causes of melt splashes and put forward measures to prevent the happening of them. Pores with different sizes are observed in perpendicular sections of the left ingots, their surface compositions are analyzed by EDS. The results indicate that they are rich in O, Al, Ca and Mg elements, which means that the driving pressures of hot melt splashes are mainly the vapor tension of SiO, Al, Ca and Mg. Calculated results show that the volatilization of those elements is inevitable, so the vapor excluding and pressure releasing are key issues for preventing hot melt splashes, both of which are affected by the height and top position of the liquid dome. Therefore, controlling of those parameters properly can efficiently prevent the hot melt splashes. The optimal dome top position is from 6 mm to 16 mm lower than the coil top position.     

Phase transformation behavior in a multipurpose dental casting gold alloy during continuous heating

Phase transformation in a multipurpose dental casting gold alloy during continuous heating was studied by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. The behavior can be explained by the following reaction sequences in the nodule: ₁(FCC) + ₂ (L1₂) ₁(fcc) + ₂(L1₂) + (L1₀), where fcc is face centred cubic. A discontinuous precipitation with very fine nodules contributed to the hardening and the growth produced the softening. This multipurpose gold alloy is characterized by the introduction of a PtZn ordered phase with L1₀ structure instead of a CuAu I phase.     

A 2.5D finite element model for bending and straightening in continuous casting of steel slabs

This paper presents a bi‐dimensional slice model of the continuous casting process developed to focus on the risk of transverse cracking during bending and straightening of steel slabs. The model is based on the finite element method and it integrates both thermal and mechanical aspects: temperature evolution, solidification, stress and strain developments. Generalized plain strain conditions are applied in the casting direction, allowing taking account of the extraction force applied to the slab as well as strains in this direction. The model also includes an original solution to counteract the generally wrong modelling of slab bulging with such slice models. The model has been applied to an industrial case of slab casting. Some numerical results illustrate the accuracy of the model compared to results of other models, measurements and observations on the caster. Transverse cracks are predicted to be the most likely to occur at the edge on the upper face, at the end of straightening of the slab. This is due to the combination of low ductility of the material with tensile stress and elongation in the casting direction in the straightening zone. This conclusion has been confirmed by the examination of slabs that present transverse cracks. Copyright © 2006 John Wiley & Sons, Ltd.     

Determination of boundary condition of a multi-crystalline silicon billet during continuous casting

Boundary conditions are always complicated and not readily available during continuous casting, especially for the multi-crystalline silicon materials. In order to improve the situation, the temperature variation curves for certain points in the multi-crystalline silicon have been obtained using the experimental apparatus under different cooling conditions. Based on the temperature measurements, the heat transfer coefficients in the second cooling zone and the interface between the multi-crystalline silicon and mould or bottom block have been calculated applying the inverse method and numerical simulation. The calculated results have been validated by comparing the predicted temperatures with the measured ones.     

A note on casting speed of Ohno continuous casting process

Abstract

The Ohno continuous casting process, known as the OCC process, is a heated mould crystallisation process that permits the generation of single crystal or cast products with a unidirectional structure. In this process, solidification takes place at the mould exit. Thus, the understanding of the process parameters is crucial to the successful application of this technology. The present note is aimed at clarifying the often misunderstood factors influencing the casting speed of the process.     

The intermediate transformation of Mn-Mo-Nb steel during continuous cooling

The continuous cooling transformation diagram for low-carbon low-alloy steel containing 0.05% C, 1.99% Mn, 0.31% Mo and 0.06% Nb was constructed by dilatometry and metallography. The intermediate transformation between martensite and polygonal ferrite involves two typical stages: the formation of ferrite matrix and the formation of microphases. Four intermediate transformation products obtained from various cooling rates and designated B₁, B₂, A₁ and A₂, were studied. The B₁ and B₂ structures are composed of pockets of parallel ferrite laths and interlath microphases, which are films of retained austenite in B, and are fragments of retained austenite or martensite or martensite-retained austenite (M-A) constituents in B₂. The B₁ structure is further characterized by the appearance of martensite particles inside the ferrite laths. The A₁ structure is comprised of the randomly arranged ferrite groups. Each group contains several short ferrite laths in the same crystallographic orientation and granular M-A constituents or martensite located at the rim of ferrite laths or groups. The A₂ structure is morphologically analogous to Widmanstätten ferrite. The formation mechanisms of these products are also discussed.