Computer modelling for thin strip twin-roll casting

In twin-roll casting, the influence of the heat-transfer rate at the metal–mold boundary on the thickness and the proportion of solidifying metal at the exit from the water-cooled roller mold is analyzed. The corresponding analytical curves are plotted for the production of moderate-carbon and stainless steel strip (thickness 1–6 mm). The proposed method may be used in the development of twin-roll technology and equipment.     

Evolution of microstructure and texture during casting of AISI 304 stainless steel strip

The solidification behavior of AISI 304 stainless steel strip was studied using a melt/substrate contact apparatus, whereby a copper substrate embedded in a moving paddle is rapidly immersed into a steel melt to produce thin (∼1-mm gage) as-cast coupons. For cases where other casting conditions were kept constant, the effect of substrate topography and melt superheat on the development of microstructure and texture during solidification was studied using electron backscatter diffraction (EBSD) and optical microscopy. It was found that nucleation and growth of grains during solidification were influenced both by substrate topography and melt superheat. A ridged substrate produced a high density of randomly oriented grains at the chill surface with the preferred growth of 〈001〉-oriented grains perpendicular to the substrate wall producing a coarse columnar grain structure exhibiting a strong 〈001〉 fiber texture at the strip center. In contrast, a smooth substrate resulted in a lower nucleation density to produce a very coarse-grained columnar microstructure with moderate and essentially constant 〈001〉 fiber texture throughout the strip thickness. By the manipulation of casting parameters, it is possible to produce strip-cast austenitic stainless steel with a particular microstructure and texture.     

Mathematical modelling of heat transfer in horizontal continuous thin strip casting of steel on a multi-layered casting bed

Presentation of the stages of mathematical modelling technique of heat transfer process in a horizontal continuous thin strip casting of steel on a multi-layered bed. Analytical solution of the mechanistic model of the process. Also presentation of a balance between sensible and latent heat of the metal, and radiation losses from the upper surface and conduction through the “substrate”. Calculation of the time to achieve complete solidification. Results from the model and comparison with experimental data.     

Twin roll strip casting of low carbon steels

Abstract

Strip casting of low carbon steels has been investigated using a laboratory twin roll machine with copper rolls. The following grades have been studied: a deep drawing low carbon (LC) steel, a LC steel with titanium addition, and a high strength low alloy (HSLA) grade. The casting behaviour of these steels has been examined, and the observed surface defects classified. Defects are most pronounced for the LC grade and significantly less for the HSLA steel and the LC steel with titanium addition. The as cast structure has been analysed. It can be modified by post-cast treatment, e.g. by normalisation or in line rolling. The mechanical properties of cold rolled and annealed strip materials and their textures are presented. Satisfactory sheet properties can be achieved both as hot band and as cold rolled sheet when adequate treatment steps are applied. Consequences for strip casting applications and future research are discussed.     

Solidification and solidification cracking in nitrogen-strengthened austenitic stainless steels

The solidification behavior of three heats of nitrogen-strengthened austenitic stainless steel was examined and was correlated with solidification mode predictions and with hot cracking resistance. The heat of NITRONIC* 50 solidified by the austenitic-ferrite mode, and the NITRONIC 50W and NITRONIC 50W - Nb heats solidified by the ferritic-austenitic mode. This behavior was in good agreement with predictions based on Espy’s formulas for Cr and Ni equivalents. Both the NITRONIC 50W and NITRONIC 50W + Nb welds contained primary delta-ferrite, with the latter weld and the NITRONIC 50 weld also containing some eutectic ferrite. Solute profiles in austenite near the eutectic ferrite showed decreasing Fe and increasing Cr, Ni, Mn, and Mo relative to austenite in the dendrite cores. Numerous Nb-rich precipitates were found on the eutectic ferrite/austenite interfaces and within the eutectic ferrite. The precipitates were mainly Nb(C, N), with some Z-phase, a Nb-rich nitride, also detected. One instance of the transformation of eutectic ferrite to sigma-phase was observed to have occurred during cooling of the NITRONIC 50 weld. Hot cracking was seen in the NITRONIC 50 and NITRONIC 50W + Nb welds and resulted from the formation of a niobium carbonitride eutectic in the interdendritic regions. In the absence of Nb, the NITRONIC 50W heat formed no observable eutectic constituents and did not hot crack. The presence of hot cracks in the NITRONIC 50W + Nb weld indicates that solidification by the ferritic-austenitic mode did not counteract the effects of small Nb additions.     

Solidification and microstructure of a high alloy stainless steel

The steel considered in the present work can be classified as a 6Mo austenitic stainless steel, which has a much higher corrosion resistance than many other commercially available stainless steels. However, because of macrosegregation during solidification of ingot casting, the high alloy content in the steel, especially Mo, N, C, enhances precipitation of intermetallic phases such asa phase. Such precipitation mainly occurs in the center of the material and has normally no significant effect on mechanical properties or surface corrosion resistance. The introduction of modern production methods such as continuous casting of slabs for stainless sheet production has reduced the macrosegregation tendency and related precipitation. However, the microsegregation in a small scale may be enhanced as the higher cooling rate prevents the diffusion in the solidified region. In the present work, the continuous casting solidification process was simulated in a laboratory gradient     

Development of laboratory scale thin strip caster and investigation of direct casting of AISI 304 stainless steel

Abstract

An unequal diameter (1 : 3), two roll thin strip casting machine has been designed and fabricated for investigation of the direct casting of thin strip on a laboratory scale. The system consists of a preheatable shallow tundish with online heating facility, water cooled rotating rolls (chill and auxiliary) for solidification of the liquid metal, and a stripper assembly. The machine has a variable speed. Roll gap setting and roll pressure adjustment are two important features of the machine. It is also possible to vary the placement angle of the auxiliary roll with respect to the chill roll. A heat transfer model was developed, based on experimental casting results. Experiments were conducted using AISI 304 grade stainless steel. Up to 100 kg of steel was cast without interruption into strips of widths 100 and 200 mm and thickness varying between 1 and 2 mm. Some of the process parameters affecting the quality of the strip were identified.     

双辊薄带连铸生产不锈钢技术探讨

不锈钢是双辊薄带连铸工艺重点开发的钢种之一.综述了不锈钢的分类和性能特点,简要介绍了不锈钢的几种生产型式,详细讨论了双辊薄带连铸生产不锈钢的技术要点和难点.     

Texture enhancement by inoculation during casting of ferritic stainless steel strip

Melt/substrate contacting experiments, designed to approximate conditions encountered during strip casting, were carried out to produce as-cast ferritic stainless steel strip. The results show that inoculation of the melt to produce TiN particles, together with casting onto a smooth substrate, results in the optimum conditions for nucleation and subsequent growth of an exceedingly high volume fraction of ferrite grains with 〈001〉 oriented within a few degrees of the normal direction (ND) of the strip surface. It is argued that, during casting, TiN particles either nucleate or deposit onto the substrate with 〈001〉 parallel to the ND, and since these particles exhibit crystallographic features similar to δ-ferrite, subsequent epitaxial growth inherits the initial particle orientation. Such oriented nucleation of ferrite from a smooth substrate results in the optimum heat-transfer conditions for further growth of dendrites with 〈001〉 perpendicular to the substrate, thus producing the intense through-thickness 〈001〉//ND fiber texture in the as-cast strip. The potential for producing grain-oriented silicon iron by direct strip casting is outlined.     

Reanalysis of solidification behaviour from the microstructure in near-net-shape casting of steels

Numerical calculations revealing the relationship between the as-cast structure and cooling conditions in near-net-shape casting of steels are presented. The solidification behaviour of steels of different composition was investigated for different process conditions with a one-dimensional heat-flow model. The dependence of the secondary dendrite arm spacing (SDAS) on the distance from the chill surface was determined on the basis of the empirical relationship between local solidification time and SDAS. The numerical results were compared with experimental values of SDAS for a stainless steel, a carbon tool steel and a high-speed steel, resp. Reasonable agreement of model calculations with the experimentally determined SDAS was obtained utilizing time-dependent effective heat-transfer coefficients of the order of 2.5 to 4.5 · 10³ W/m²K for typical thin-slab dimensions and 6.0 · 10³ W/m²K for thin-strip casting.     

Plasma-melted nitrogen-bearing cast stainless steels—microstructure and tensile properties

Alloys of Fe-Cr-Ni and Fe-Cr-Mn were plasma arc-melted and chill cast in the form of ingots. Exposure of liquid melts to a nitrogen plasma for the purpose of adding nitrogen to the above alloys was used to dissolve varying amounts (up to 0.32 wt pct) of nitrogen. Carbon and sulfur were varied up to 0.5 and 0.056 wt pct, respectively. The alloys were evaluated for their monotonic behavior. It was observed that while strength and ductility parameters increased considerably with increasing nitrogen and carbon contents, both these parameters deteriorated with sulfur content. The analysis of the present results, along with the data from the literature, suggests that the strength parameters are predominantly chemistry dependent, particularly controlled by the nitrogen and carbon contents. The results also show that the alloys produced by this relatively new technique, plasma arc-melting, are comparable to those produced by other standard techniques. A.K. SINGH, formerly Postdoctoral Fellow, Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University.     

双辊薄带钢连铸侧封技术研究

侧封问题是双辊薄带连铸技术的一个关键性问题。分固体板侧封和电磁侧封两个方面综述了侧封技术的发展状况,分析了它们的优缺点。     

Mathematical modelling and computer simulation of the cooling process of reused substrates in horizontal continuous thin strip casting cycles of steel on a two-layered moving casting bed

Investigation of the substrate materials with a non-sacrificial manner during the cycles of continuous thin strip casting of steel on a two-layered moving casting bed and their thermal behaviour between the casts, as these cool, after the cast product removal. Formulation of the mathematical model of the cooling process of a re-used substrate for a period between casts in a continuous strip casting plant as well as the obtaining of the analytical solution of the model. Also a presentation of computer simulation of this process on flowcharts for the main program and subroutines. Prediction of the temperature distribution in fused silica substrates during the time of cooling process between separate casts as well the calculation of the time needed for the critical service region and temperature. Demonstration of the simulation results and comparison with the reality for a substrate combination of 3 cm fused silica on mild steel.     

Theoretical modelling of an isokinetical steel feeding system for near-net-shape strip casting

During near-net-shape casting as considered here, liquid steel in thicknesses between 5 and 15 mm has to be put onto a cooled belt recirculating with speeds of 30 to 60 m/min. On the belt the liquid steel solidifies, before it can be subjected to one or two in-line hot deformation steps. The requirement that the surface and the shape of the strips should already have the best possible structure upon being cast means that high demands are placed above all on the liquid steel feeding system. Especially the liquid metal should ideally be cast with the lowest possible turbulence and at a casting speed which is equal to the speed of the conveyor belt or cooled conveyor belt. In other words, the relative speed between the cast liquid and the cooled conveyor should become “zero” in order that the cast liquid can be more or less “laid” onto the cooled conveyor (known as isokinetic feeding). Different kinds of feeding liquid metals onto a circulating belt are reported with main emphasis on the theoretical modelling of the so-called underpressure feeding systems. It is shown that, on the one hand, the casting speed can be regulated independently of the input mass flow rate into the tundish and, on the other hand, the liquid can be accumulated in the tundish or discharged from it at constant casting speed.     

Analysis and prevention of cracking during strip casting of AISI 304 stainless steel

In this study, a microstructural investigation was conducted on the cracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the cracked regions showed that most of the cracks were deep, sharp, and parallel to the casting direction. They initiated at the tip of dendrites and propagated along the segregated liquid films between primary dendrites, indicating that they were typical solidification cracks. This cracking phenomenon was closely related to the inhomogeneous solidification of cast strips, represented by depressions, i.e., uneven and somewhat concave areas on the strip surface. The depressions, which were unavoidable in flat rolls due to the presence of a gas gap between the roll and the cast strip, were finely and evenly distributed over the cast strip surface by intentionally providing homogeneous roughness on the roll surface; then, the number and size of cracks were considerably reduced. In addition, the nitrogen gas atmosphere, which retained high solubility in the melt during cooling and good wettability with the roll surface, was successfully used to prevent cracking, because the thickness of the gas gap was minimized.     

Water modelling of level fluctuation in thin slab continuous casting mould

Abstract

A water modelling experiment was conducted to study the meniscus instability in a continuous thin slab casting mould using particle image visualisation. The results show that the level fluctuation, circulation centre position and jet impinging depth are unsteady and periodic with a similar period. The probability distributions of the fluctuating meniscus and wave height have been obtained with the highest frequency near the average position. The flow pattern and meniscus profile may be momentarily asymmetrical, and the phase difference of level fluctuation in the two sides of mould centreline is a half period. The average meniscus profile, the highest and lowest meniscus positions are generally symmetrical about the mould centreline. The wave height mainly depends on the jet impinging depth and circulation centre position. The wave height increases as the jet impinging position rises and the circulation centre approaches to the submerged entry nozzle.     

Strip properties and processing in twin roll casting of stainless and low carbon steels

Metallurgical features of the twin‐roll strip casting process are discussed. The as‐cast structure of two austenitic stainless steels is evaluated using EPMA and metallography. Cleanness levels as reached in twin‐roll strip casting are compared with conventional and thin slab casting. In‐line annealing is described as a method for grain refinement. The scale layer on twin‐roll cast strip is characterised for low carbon and stainless steels.     

Mechanical properties of type 304 stainless steel sheet produced by twin-roll strip casting process

Tensile properties and formability of type 304 stainless steel sheets which were cast by a twin-roll type strip caster and thereafter cold-rolled are examined and compared with those of conventionally processed sheets. The results exhibit that the strip casting processed sheets have mechanical properties almost equal to the conventional ones. Furthermore, it is found that the degree of plastic anisotropy caused by crystallographic anisotropy in the present sheets is weaker than that in the conventional ones.     

Solidification microstructure and temperature field during direct chill casting of Al-16Si alloy

By properly controlling casting parameters such as pouring temperature, casting velocity and water flux, direct chill (DC) casting can be employed to produce refined microstructure in the hypereutectic Al-Si alloys without chemical modification. This refined microstructure is characterized of fine primary Si particles, fully developed dendritic Al halos and fine coupled eutectics. In this work, in situ measurements of temperature field in the mould during DC casting of Al-16Si alloy at casting velocity of 2.17 mm/s, 3.5 mm/s and 4.34 mm/s at a pouring temperature of 800°C were performed. The results show that the primary Si phase nucleated at considerable undercooling (about 27°C to 38°C) and the growth temperature of dendritic Al halos was 7°C to 8°C below the equilibrium eutectic temperature. In the center regions of the DC cast billet, halos are fully developed because the G_l/R value is low.     

Solidification in continuous casting of aluminum

During casting of aluminum ingot by a new process utilizing level pouring, variations in cooling rate within the surface band occur as a result of the discontinuous nature of metal flow. Associated segregation contributes to a marked cyclic pattern of the macrostructure in that region. The kinetics of metal flow into the surface zone are explained graphically using a sequence of macrographs. The proposed mechanism is also supported by results of thermal measurements and Electron Microprobe analyses.