Mold Powder Research Articles

Heating Microscopy and its Applications

Heating microscopes were commercially introduced 60 years ago, and they immediately gained an important role in material science and engineering thanks to their wide field of application. Nowadays, these instruments are used in both industry and research in many sectors: traditional and advanced ceramic industries, the glass industry, power plants, the metallurgical sector, and, generally, in all fields where the melting behavior of materials has to be determined. Heating microscopy techniques can be successfully applied to the direct characterization of glazes, glasses, frits, ashes, mold powders for continuous casting, coals, slags, etc.—during a heat treatment.

Effects of Mold Powder Chemistry on Shell Growth, Momentum and Heat Transfer in a Billet Mold

Effects of mold powder chemistry on shell growth and thinning have been studied using Computer Fluid Dynamic techniques under conditions of constant casting speed and steel superheat for a peritectic steel in a billet caster. Two mold powders were considered; a basic powder suitable for peritectic steels and an acid powder not recommendable for this steel in order to emphasize the importance of chemistry on shell stability. Numerical results indicate a strong interaction between powder composition and steel flow-heat transfer phenomena. The acid powder creates recirculating flows at both sides of the entry jet that transport sensible heat to the shell inducing its remelting and thinning leading, eventually, to a strand breakout. Meanwhile, the basic powder induces a single recirculating flow in the internal radius side of the mold without severe shell thinning. A colder meniscus is predicted using the acid powder which is in agreement with the casting practice experience. Powder infiltration of the basic powder in between the mold hot wall and the strand provides a powder shell with a macroscopically smooth surface while the acid powder yields irregular infiltration. Buoyancy forces along the mold working height and mold curvature play a fundamental role on the generation of the recirculating flows. Interaction between powder chemistry and fluid flow-heat transfer are two-way coupled phenomena that must be considered for powder design purposes.

The Influence of Carbonaceous Material on the Melting Behaviour of Mould Powder

AbstractThe melting behaviour of mould powder during continuous casting is an important consideration with respect to caster performance, production rate and steel quality. In this experimental study the effect of different carbonaceous materials on the melting characteristics of mould powders was evaluated. Using X‐ray diffraction, different types of carbon were quantitatively characterized in terms of their internal structure and reactivity experiments were conducted to investigate potential relationships between the structural morphology of carbons and their reactivity. High temperature microscopy and drip test experiments were then used to investigate the melting behaviour of mould powders containing different carbonaceous materials. From the results obtained, correlations were established between the structural factors, chemical reactivity and melting behaviour.

Research on One-Time Forming Process of Matrix PDC Bits

Due to the difficulty in the ceramic mould forming process of matrix PDC bits and low production efficiency, a manufacturing mould technology of graphite powder forming process was developed. The technical program is to formulate a circuit of silicone rubber forming process, determine the forming process of first mould, rubber mould and graphite powder mould, and sinter bit. The results show that manufacturing bit mould to sinter PDC bits with this method can simplify the circuit, increase production efficiency, and improve bit quality. The resulting bit is also of high precision, exquisite appearance and good consistency

The Influence of Mould Powder Properties on Shell Formation in Continuous Casting of Steels

AbstractFor peritectic steel grades the shell formation is uneven resulting in quality problems such as surface cracks. Production of these steels calls for good control and knowledge of the critical factors for shell formation. By optimisation of process parameters, it is possible to enable production of crack sensitive slabs for heavy plate. This also gives a unique possibility to study the interaction between process parameters and shell formation. The paper presents the results from two large investigations with this ambition covering the total of 27 + 30 heats of 220 tons of steel at SSAB EMEA‐Oxelösund and concludes how surface cracks can be significantly decreased.

Numerical Analysis of Electromagnetic Field and Flow Field in High Casting Speed Slab Continuous Casting Mold With Traveling Magnetic Field

In a continuous casting process, it is essential to prevent the surface defects which are caused by the mold powder entrapments. It is well known that the decrease in the molten steel flow velocity just under the free surface is one of the most effective methods for the prevention of mold powder entrapments. For this purpose, the electro-magnetic level stabilizer (EMLS) has been developed, which is applied to a low frequency alternating magnetic field moving from the narrow face of the mold to the mold center below the nozzle exits. In this study, the effect of the EMLS on the molten steel flow is investigated. Numerical simulation of the electromagnetic field and the molten steel flow in a mold were carried out. Simulation results indicate that, due to the electromagnetic force, the molten steel is forced to flow toward the magnetic field traveling direction in the region where the magnetic field is imposed. The molten steel flow is decelerated in proportion to the imposed electromagnetic force. Consequently, the molten steel flows toward the mold center near the free surface with a smaller imposed electromagnetic force, and it flows toward the nozzle at the nozzle side and toward the narrow face at the narrow face side with a larger imposed electromagnetic force. However, the magnitude of the electromagnetic force is decided by the current intensity and frequency, a suitable imposed electric current can be chosen to minimize the flow velocity and also the amount of mold powder entrapments.

Study on the Production of Ti Micro-Alloyed High Strength Hot Rolled Steel by CSP Process

Ti micro-alloyed high strength hot rolled steel was developed in Valin Lianyuan Steel in the CSP line. The cleanliness of liquid steel was good enough for thin slab casting after LF refining. The mould powder was adjusted for stabilizing the heat flux of thin slab continuous casting mould. Homogeneous microstructure consisting of ferrite and pearlite was obtained in the hot rolled steel plates by the improving of rolling process. The nano-scale precipitates of Ti(C, N) and Nb(C, N) is the main strengthening mechanism. The yield strength of developed hot rolled plate is higher than 660 MPa and the tension strength is 760 MPa. The ductile-brittle transition temperature is below -60 degree Celsius. The developed Ti alloyed steel with designed composition fulfils the requirements of 600 MPa grade steel for engineering machinery.

Longitudinal surface cracks of thin slabs

Based on the production practice of medium carbon thin slabs in the CSP plant, the reasons and influencing factors for the formation of longitudinal cracks were investigated, and some industrial measures were taken to eliminate the cracks. The results show that the efficient solutions to reduce longitudinal cracks are improving the performance of the mold powder, stabilizing the mold heat flux, and maintaining a proper taper of the mold during casting. Proper pouring temperature and secondary cooling also play important roles in preventing longitudinal surface cracks.

Inclusions Generated during Ingot Casting of Tool Steel

AbstractAn industrial trial study was carried out at Uddeholm Tooling AB, Sweden, to investigate the mechanisms of the formation of inclusions, especially macro inclusions during ingot casting. Strong evidence of reoxidation was observed. Two types of inclusions generated during casting were detected in the ingot. Inclusions of Type C1 originated from mould powder with reoxidation product Al2O3 as a part of it. Inclusions of this type were usually small. The number of Type C1 inclusions increased from the centre of the ingot toward the side wall of the mould. Type C2 inclusions were macro size. They could be the results of reoxidation and erosion of runner.

Prevention of corner cracks in slab continuous casting

Surface corner cracks are sometimes observed on slabs of high strength low alloyed steel grades, requiring expensive scarfing. To avoid surface crack formation, we can act on the steel intrinsic ductility loss through the limitation of S and N contents: S < 30 ppm and N < 40 ppm are well adapted thresholds. A Ti/N ratio higher than 3.6 and a limitation of Al*N product lead to a significant improvement of the quality results. In addition, the continuous casting process must be optimized. It aims at limiting the thermomechanical strains faced by the slab surface during solidification and cooling, particularly when the slab corner temperature is in the range of the ductility trough. The main process actuators are the mould powder, the secondary cooling curves and the limitation of the misalignments in the top part of the caster, especially the transitions between the mould foot-rolls, the vertical and the bending segments.

Surface inclusions and their evolution on strip rolled on CSP line and in following cold rolling process

Hot and cold rolled strip samples with surface defects such as black pockmark, sliver and central cracking were collected from a compact strip production line followed by cold rolling. Microstructures and chemical compositions of the strip with defects were studied by scanning electronic microscopy and energy dispersive X-ray spectroscopy. Four typical types of defects on strip including surface spherical mould powder entrapment, centre spherical mould powder entrapment, surface linear mould powder entrapment and centre linear mould powder entrapment were studied by hot and cold rolling experiment in laboratory. It was observed that spherical mould powder entrapment was transformed into the surface black pockmark after the hot rolling and cold rolling process. Centre spherical mould powder entrapment led to central fracture in the hot and cold rolling process. Surface linear mould powder entrapment resulted in sliver and centre linear mould powder entrapment led to severe sliver.

Solidification and crystallization properties of CaO-SiO 2-Na 2O based mold fluxes

Crystallization properties play an important role in keeping a smooth running of continuous casting process and high surface quality of cast strands. To reduce fluorine pollution in slag, a new type of CaO-SiO 2-Na 2O (CSN) based mold flux was studied. The solidification and crystallization properties, including crystallization temperature, crystallization ratio and solidification mineragraphy, were measured, which were compared with the CaO-SiO 2-CaF 2 (GF) mold flux. The results show that the crystallization performance is equal to the high fluoride mold powder and CSN can be used for peritectic steel grades sensitive to longitudinal cracking in continuous casting.

Mould powder investigations for high speed casting

Mould powders significantly determine the stability of the continuous casting process of steel. The processes leading to melting of mould powder and solidification of mould slag were studied in situ using high temperature X-ray diffraction with additional powder X-ray diffraction and microscopic techniques. It was shown that during heating, a powder shows a specific sequence of phase relations before melting takes place. During cooling and solidification, one or more crystalline phases can be formed. The findings on high temperature properties were confirmed by the analyses of slag rims and slag films, obtained from the Corus thin slab caster. Finally, the in depth characterisation, together with some calculations on mould slag was compared with plant data of the thin slab caster. Results from this work give a better understanding of the mould powder functions at the thin slab caster and are being used to guide mould powder design for the current and even higher casting speeds.

Effect of Properties of Mold Powder Entrapped into Molten Steel in a Continuous Casting Process

Mold powder plays an important role in the continuous casting process for high quality steel production. Qualitatively, it is well known that mold powder entrapment causes surface defects in steel sheets, and high viscosity of the mold powder prevents the entrapment of the mold powder into the molten steel in ultra-low carbon steel.This paper deals quantitatively with the entrapment of mold powder caused by suction by Karman's vortex in a laboratory experiment. First, in a water model experiment using oil as a substitute for mold powder, the effects of the viscosity of the oil and the interfacial tension between the water and oil were investigated. Following this, hot model experiments were carried out with mold powder and molten steel. The amount of mold powder entrapment was affected by the viscosity of the molten mold powder and the interfacial tension between the molten steel and mold powder. As the result, the following equation for mold powder entrapment was obtained.

Water Model Experiments on the Effect of an Argon Bubble on the Meniscus Near the Immersion Nozzle

Water model experiments have been carried out to understand the behavior of the meniscus of molten steel near the immersion nozzle. The meniscus is disturbed by a large argon bubble rising along the immersion nozzle. Water, silicone oil, and air are used as models for molten steel, mold powder, and argon, respectively. A cylindrical rod is used as a model for the immersion nozzle. The contact angle of a water droplet on the rod is adjusted to become nearly the same as that of a molten steel droplet on the immersion nozzle by coating repellent on the rod surface. A single air bubble of a predetermined volume is released from a cap-shaped container to attach to the bottom of the rod. The behavior of the bubble passing across the interface between water and silicone oil is observed with a high-speed video camera to make clear the entrapment of silicone oil droplets into the water bath.

Optimisation of Electromagnetic Stirring in Steel Billet Caster by Using Image Processing Technique for Improvement in Billet Quality

Determination of best combination of current and frequency of an electromagnetic stirrer (EMS) of a billet caster is of prime importance for ensuring good internal, surface as well as subsurface quality of billets. In the present study, this optimization of current and frequency values is achieved very efificiently using image processing techniques. The EMS currents and frequency are varied between 240-300 A and 3-5 Hz respectively during casting and corresponding billet samples are collected. Samples are also collected from beginning, middle and end of casting heats in order to asses the influence of tundish superheat. The samples thus collected are scanned using ultrasonic C-scanner to get the full color image of samples. The grabbed images are processed and analysed using Matlab Image Processing Toolbox in RGB color space. Finally, data of various defects as obtained from image processing, in each grades of steel are compared for determining the best combination of EMS parameters. It has been found that length of equiaxed zone increases significantly with increasing EMS current up to 280 A, almost for all close casting grades. Increasing EMS current beyond 280 A increased the length of equiaxed zone marginally but chances mould powder entrapment at the surface of the billet and erosion of submerged entry nozzle is more. Central shrinkage number of subsurface and central crack also reduced significantly up to 280 A EMS current. Marginal improvement in billet quality is also observed when 3 Hz EMS frequency is used but the improvement in the quality is not significant.

Tools and techniques for use in development of mould powders

The tools and techniques used within Corus UK to improve mould powder performance and product quality are reviewed. Mould thermal monitoring, which allows real time assessment of heat transfer, is a critical tool in the development of mould powders. Thermal analysis, viscosity measurements, flux–mould plate interactions, steel velocity profiles in the mould and slag film properties also play a part in continuing studies. Case studies of improvements in process performance achieved using these methods are presented.

Heat Flux Through Slag Film and Its Crystallization Behavior

An experimental apparatus for simulating copper mold is used to quantify the heat flux through the slag film and to obtain a solid slag for further determining its crystallization behavior. The result indicates that both the chemical composition of the mold powder and the cooling rate have an important influence on the heat flux through the slag film. With increasing the binary basicity, the heat flux of slag film decreases at first, reaches the minimum at the basicity of 1.4, and then increases, indicating that the maximum binary basicity is about 1.4 for selecting “mild cooling” mold powder. The heat transfer through the slag film can be specified in terms of the crystalline ratio and the thickness of the slag film. Recrystallization of the solid slag occurs and must be considered as an important factor that may influence the heat transfer through the solid slag layer.

Mould Powder Requirements for High-speed Casting

Mould powders play an important role in the stability of the continuous casting process of steel at all casting speeds. The main functions of mould slag (i.e. molten powder) are to provide sufficient lubrication and to control the heat transfer in horizontal direction between the developing steel shell and the copper mould. Several authors have reviewed the composition, properties and operational experience of mould powders for conventional slab casting [1-2]. However, only a few publications have been issued on the design and use of mould powders for thin slab casting [3-4]. Compared with conventional slab casting, process control in thin slab casting is more critical and it can be assumed that mould powder demands for thin slab casting become more critical as well [5-6]. Thus a more extensive understanding of mould powder properties and functions is needed to meet the increasing demands of high-speed thin slab casting. Nevertheless, a lot of improvements (mould powder developments) are based on trial and error and are focussing on casting speeds up to 2 m/min (conventional slab casting). A general approach is to relate the chemical composition of a mould powder to the operational behaviour (in-mould behaviour) during casting. In doing this, some physical properties of the mould slag like the viscosity and the melting point are addressed as well. However, this approach will not automatically result in a suitable mould powder or an in-depth knowledge on powder design and required slag properties. At Corus IJmuiden, a project started in the framework of the Netherlands Institute for Metals Research with the aim to develop mould powders suitable for high-speed thin slab casting with a maximum casting speed up to 8 m/min. For this work, a fundamental understanding and quantification of the melting and solidification behaviour of mould slag as well as the functions of mould powder is needed.

A novel solid freeforming method using simultaneous part and mould construction

PurposeThe purpose of this paper is to present a new powder‐based solid freeforming method based on conventional furnace sintering after co‐deposition of mould and part powder materials.Design/methodology/approachBased on acoustic powder deposition, both mould and part powder materials are delivered simultaneously into the forming area according to the cross section of the 3D computer file. The part is formed in the form of loose powder surrounded by the mould powder again in a loose state. The whole assembly can then be sintered by a conventional method and the mould powder, which has a higher sintering temperature than that of part powder, remains in the loose state after sintering and can be removed.FindingsComplex‐shaped components containing re‐entrant cavities and the capability of being made with 3D functional gradients can be rendered directly as a powder preform suitable for subsequent compaction or direct sintering in a conventional furnace. The flowability and compatibility of the powders need to be selected carefully and the track distance between part/mould powders is important for forming a vertical wall.Research limitations/implicationsThe main factors affecting building from powder tracks are identified, including the effect of track distance at an interface on integrity, discontinuous feeding on bends and the effects of fill strategies. The flow rates of the part and mould powder as well as their geometrical maps are controlled computationally. Materials and instrumental aspects are discussed.Originality/valueThis paper describes a method to produce complex‐shaped object without residual stress and expensive lasers and the process could be modified to include 3D functional gradients.