DANIELI concepts and experiences in thin slab casting and rolling technology for HRC production—the evolution of a winning process route to face increasing market requirements

The direct rolling process for hot strip production,where the thin slab caster is connected directly to the mill,has gained market share rapidly because of its remarkable advantages in terms of energy savings and investment cost over the conventional hot strip mills. However,the unquestionable advantages of the first-generation applications of this plant concept also entail significant limitations both in productivity and steel grades that can be produced. Since his first pioneering applications,Danieli considered strategic the development of new technical solutions specifically conceived to overcome these limitations with the goal of increasing plant production volumes and enlarging steel grade product mix,in order to cover the gap between "Conventional mill" and "Thin slab casting and rolling" process routes. In order to reach this goal,Danieli has developed a complete portfolio of plant lay outs adopting Thin Slab Casting and Rolling technologies,each of them conceived to guarantee the optimal CAPEX and OPEX parameters in fitting with market requirements our Customer intend to target.in terms of productivity,steel grades and coil dimension product mix. Danieli TSR(Thin Slab Rolling) fTSR(flexible Thins Slab Rolling) QSP(Quality Strip Production) and ETR(Extra Thin Rolling) plant configurations are analyzed in this paper. With this diversified approach,Danieli solutions are most appropriate answers to thin slab casting and rolling to produce hot rolled coils with superior quality and an extremely diversified range of steel grades. Already,this approach has allowed Danieli plants to:①exceed the threshold production of 3.0 Mt/a with 2 casting strands in operation as done in Tangshan Iron and Steel plant in P.R.China since 2005;②expand the product mix to include virtually all the steel grades used for flat product applications,including the most demanding ones,such as peritectic(in Essar Algoma Canada and Benxi Iron and Steel,China),micro-alloyed,and silicon steels,for the most sophisticated applications,such as automotive and pipe manufacturing,including Arctic applications,(as done in OMK plant in Russia);③extend the range of final strip thicknesses to include ultra thin gauges,down to 0.8 mm(as in Ezz Flat Steel,in Egypt).

Optimization design of wide face water slots for medium-thick slab casting mold

A three-dimensional finite-element model has been established to investigate the thermal behavior of the medium-thick slab copper casting mold with different cooling water slot designs. The mold wall temperatures measured using thermocouples buried in different positions of the mold with the original designed cooling system were analyzed to determine the corresponding heat flux profile. This profile was then used for simulation to predict the temperature distribution and the thermal stress distribution of the molds. The predicted temperatures during operation matched the plant measurements. The results showed that the maximum temperature, about 635 K in the wide hot surface, was found about 60 mm below the meniscus and 226 mm from the center of the mold. For the mold with the type I modified design, there was an insignificant decrease in temperature of about 5 K, and for the mold with the type II modified design, the maximum temperature was decreased by about 15 K and the temperature of the hot surface was distributed more uniformly along the length of the mold. The corresponding maximum thermal stress at the hot surface of the mold was reduced from 408 MPa to 386 MPa with the type II modified design. The results indicated that the modified design II is beneficial to the increase of mold life and the quality of casting slabs.

Modeling and control of dynamic soft reduction in continuous slab casting

The centerline segregation in continuous casting steel is known to be formed by the interdendritic enriched liquid flow at the solidification end in strand.At present,several methods,such as thermal soft reduction, electromagnetic stirring and dynamic soft reduction,are available to reduce the center segregation and to improve the internal quality.Although some methods could alleviate center segregation to some extent,they can also give rise to new problems.For instance,thermal soft reduction can lead to surface cracking during casting of some steels and electromagnetic stirring can result in white band.Dynamic soft reduction is an effective technology for the improvement of strand quality in continuous slab casting.In this paper,the key parameters of dynamic soft reduction technology,such as soft reduction region,the total reduction amount,for low carbon steel have been studied based on experimental and theoretical analysis.The soft reduction region was related to the segregation of chemical composition,slab bulging of narrow side and avoiding internal crack of slab.The total reduction amount was composed of natural thermal shrinkage and actual reduction amount depending on critical strain.For low carbon steel with slab dimension of(950 - 1 950) mm×170 mm,the solid fraction range in soft reduction region was recommended from 0.4 to 0.8,the total reduction amount was 2.0 -2.8 mm,and the optimized reduction rate was 0.4 - 1.2 mm/min,respectively.Furthermore,a dynamic soft reduction system in continuous slab casting, Visual Cast-Dynamic,has been developed by multithreading method.It demonstrated that the dynamic soft reduction model in this study was correct with the ability of online control.

New Argon Stopper Technology for Slab Casting

Argon addition into the casting channel reduces the amount of clogging on the refractory wall to allow long sequence casting of aluminium-killed steel grades.The key to minimum surface defects on the final product is to establish the optimum quantity by optimising the gas distribution below the stopper nose.This is achieved by new argon stopper technology which supports low gas quantities with homogeneous gas and bubble distributions to float non-metallic inclusions(NMI)in the continuous casting mould.

Microstructure,texture and precipitates of grain-oriented silicon steel produced by thin slab casting and rolling process

A grain-oriented silicon steel strip with AlN as main inhibitor was produced by thin slab casting and rolling（TSCR）process.The microstructure,texture and precipitates of the hot-rolled strip were investigated by use of optical microscope（OM）,X-ray diffractometer,transmission electron microscope（TEM）and energy dispersive spectroscope（EDS）.The result shows that the microstructure and texture exhibit a through-thickness gradient similar to that of the hot-rolled strip produced by conventional high-temperature slab-reheating process;the preferred orientation varies from {110}〈001〉in the surface layer to{001}〈110〉in the center layer,and the Goss texture with a maximum intensity mainly concentrates on the surface layer.In addition,some other texture components,for example rotated Goss texture,form in the 1/4thickness layer,which are not observed in the hotrolled strip produced by conventional high-temperature slab-reheating process.The precipitates in the hot-rolled strip are mainly（Mn,Cu）S and AlN compound particles with dimension of 100-200 nm,and the fine precipitates are significantly less than that in the hot-rolled strip produced by conventional high-temperature slab-reheating process.Moreover,the areal density of the fine precipitates in the center layer is more than that in the surface layer.

Effect of Asynchronous Adjustments of Clamping Cylinders on Triangular Crack of Slab Castings Under Application of Soft Reduction

Soft reduction is known to be one of the best ways to improve the internal quality of slab castings such as center segregation, center porosity, centerline or triangular zone cracks, which is based on a proper adoption of the amount of reduction upon the given final solidification zone through roll gap adjustments. The synchronization of the clamping cylinders for roll gap adjustments should be very important to the application of soft reduction, including the synchronization of the clamping cylinders adjustments in the same and different segments. The synchronization of clamping cylinders adjustments is mainly affected by the adjustable accuracy of the four position-controlled clamping cylinders mounted in the upper frames of the segments according to a predetermined transformation relationship between the signals of displacement sensors and aimed roll gap, which, however, is also influenced by the installation accuracy, the precision of displacement sensors, the deformation of the segment frames and/or its bearing pedestals. Due to the actual asynchronous adjustments of the four clamping cylinders, the dynamic soft reduction operation is normally applied at non-ideal mechanical conditions. Here 7 possible situations of asynchronous adjustments of the local segments which may induce gap deviation have been presented. The roll gap deviation in the soft reduction region of a slab casting has been studied by a 3-D visco-elastic plastic FEM model, through which the additional inter-roll bulging, the related triangular cracks induced by one kind of the possible asynchronous adjustment situation and the effectiveness of soft reduction have been analyzed. A critical tolerance for the gap adjustments has been proposed for better contribution of soft reduction to the internal quality of slabs.

LATTICE BGK MODEL SIMULATION OF ASYMMETRIC FLOW INSIDE A CONTINUOUS SLAB CASTING MOLD

The incompressible lattice Bhatnager-Gross-Krook （BGK） model of computational fluid dynamics, from which the unsteady incompressible Navier-Stokes equations can be exactly derived with the limit of small Mach number, was established in continuous casting mold. An asymmetric flow pattern in the two-dimensional central plane of continuous slab casting mold was simulated, and the flow pattern is not stationary but changes over frequently if the Reynolds number is larger than 3000 or so. The results are found to be in excellent agreement with previous experimental results.

Model control technologies of dynamic secondary cooling and soft reduction for slab continuous casting

Dynamic model control technologies of secondary cooling and soft reduction of Baosteel are introduced. Model principle and control system architecture are summarized, as well as functions and features. Finally, applications of model technologies are discussed. The self-developed dynamic secondary cooling model and the dynamic soft reduction model have been applied on several casting machines inside and outside Baosteel, desired control effects were achieved with good stability and reliability. Temperature measurement results verified the correctness of model.

Morphology and Precipitation Kinetics of MnS in Low-Carbon Steel During Thin Slab Continuous Casting Process

The morphology of manganese sulfide formed during thin slab continuous casting process in low-carbon steel produced by compact strip production （CSP） technique was investigated. Using transmission electron microscopy analysis, it was seen that a majority of manganese sulfides precipitated at austenite grain boundaries, the morphologies of which were spherical or close to the spherical shape and the size of MnS precipitates ranged from 30 nm to 100 nm. A mathematical model of the manganese sulfide precipitation in this process was developed based on classical nucleation theory. Under the given conditions, the starting and finishing precipitation temperatures of MnS in the continuous casting thin slab of the studied low-carbon steel are 1 189 ℃ and 1 171 ℃, respectively, and the average diameter of MnS precipitates is about 48 nm within this precipitation temperature range. The influences of chemical components and thermo-mechanical processing conditions on the precipitation behavior of MnS in the same process were also discussed.

Strain in solidifying shell of continuous casting slabs

Two-dimension unsteady heat transfer model was applied to obtain the surfacetemperature and the shell thickness of continuous casting slabs during the process ofsolidification. On the basis of which, the mathematical model of strain at the interface of solidand liquid steel was set up. Through which, the strain in the solidifying shell under normal andabnormal operation conditions was gained. The results indicate that the strain is small under thenormal operation conditions and the internal crack never happens. However, when the variation of theroll gap is above 2 mm, the strain caused by which is greater than that caused by bulging.Furthermore, the total strain exceeds the critical one and the internal crack is the result. So itis of great importance to maintain the fine state of continuous casting machine to avoid theappearance of internal crack.

INVESTIGATION ON HOT DUCTILITY AND STRENGTH OF CONTINUOUS CASTING SLAB FOR AH32 STEEL

By means of Gleeble-1500 testing machine, the simulation of continuous casting process forAH32 steel was carried out and hot ductility and strength were determined. The cracking sensitivity was studied under the different temperatures and strain rates. The Precipitations of AIN at different temperatures and the fractures of high-temperature tensile samples were observed by using TEM （transmission electron microscope） and SEM （scanning electron microscope）. The factors affecting the brittle temperature zone were discussed.

Flow and Temperature Fields in Slab Continuous Casting Molds

In order to develop super-board and super-thick slabs, the flow and temperatur fields were studied in slab continuous casting molds under different practical conditions, such as slab dimensions, with-drawing slab speed, design of nozzles, and superheat tempera-ture. The results showed that it is preferred to incline nozzle bores downwards and the submerged depth of the nozzles is best kept be-tween 250-300 mm. In addition, the solidified shell is thicker at the wide face than that at the narrow face, while the thin points alongthe wide face ekist both in the center and in the some area toward each respective end.

Modeling of unsteadiness of fluid flows and level fluctuations in thin slab continuous casting moulds

Unsteady fluid flows and level fluctuations in a thin slab continuous casting mould have significant influence on product quality. In this study, the phenomena concerning transient flow features and free surface motions were analyzed by means of the large eddy simulation （LES） software with the smagorinsky SGS model--VisualCast （VCast） II, where the Simpler algorithm on a body-fitted mesh was used to resolve governing equations. A series of water analog experiments on the fluid flow and the surface wave in the moulds of thin slab continuous casting were also performed. The results of fluid regions, middle of vortex and level fluctuation from digital simulations were identical with the results of the water analog experiments.

Study Segregation of Alloying Elements in Continuous Casting Slab with Valence Electron Structure

By calculating the electron structures of the phases that phosphor, sulfur and alloying elements dissolving inγ-Fe, the reason why alloying elements can bring centerline segregation in continuous casting slab (CCS) with nA, the number of electrons on the strongest covalent bonds, and the structure formation factor S were investigated, and an electron structural criterion to control and to eliminate the centerline segregation was advanced. Basing on this, the electron structures of a part of rare earth phosphides and sulfides are calculated, the physical mechanism that rare earth elements can control the segregation of phosphor and sulfur is analyzed, and the criterion is well verified.

Numerical simulation of fluid flows in a slab continuous casting mold under electromagnetic stirring

This study established a three-dimensional mathematical model to determine the fluid flow in a slab continuous casting mold under an electromagnetic stirring force. The flow structure and distribution were studied with respect to different continuous casting parameters and stirring current. Based on the calculation results, the mold flux entrapment index in free surface and velocity uniformity index were used to evaluate the flow field in the mold. The theoretical basis for the optimization of the flow field structure was provided. The study also suggested an optimization method for electromagnetic stirring parameters.

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.

Research on the trajectory of non-metal inclusions with the flow in the mold during the solidification of continuous casting slabs

By means of the numerical simulation method,the mathematical model of inclusions movement in the mold is established under the condition of austenitic and fen-itic stainless steel slab production. According to the simulation results, the main zones for inclusion particles accumulation were found and many factors that affected floating-up probability of inclusion particles were identified. These factors include the inclusion particle size, the casting speed and the slab width, etc. It is believed that the inclusion particle size is the key one among these factors.

Physical modeling and numerical simulation of electromagnetic stirring in a slab continuous casting mold

Through physical modeling and numerical simulation,the flow field in a slab continuous casting mold with electromagnetic stirring is measured under different casting parameters and stirring currents. To qualitatively evaluate the flow field in the mold, two indexes,i, e., mold flux entrapment and velocity uniformity, are proposed. Based on these two indexes, some optimized stirring parameters under different casting conditions can be determined.

Technological Investigation on the Continuous Casting of Mid-Width Thin Slabs

Continuous casting of thin slabs is a key state project for 7th and sth five -year plans. On thebasis of foundamental works , CISRI condueted the tests of 102 heats in Lanzhou Steel Works in the periodof January 1991- June 1992. Three slab assortments of 50× 900 mm . 70 × 900 mm , 70 × 500 mm were ex-amined. In June of 1992, 412 t steel of 46 heats were cast with the efficiency of 91. 3 %. This. result meetsthe requiremeni of the state in this period. The mould, hedt transfer . casting technology and factors influ-encing slab’s qualities are investigated as well.

The optimization of mold fluxes used for casting the peritectic steel of heavy slabs at Baosteel

In order to solve the problem of the high surface longitudinal crack ratio of heavy peritectic steel slabs produced by the No. 3 continuous caster at Baosteel,the physical properties of the original mold flux and the optimized mold flux were compared in a comprehensive way by using analytical measures, such as a slag film heat-flow simulator, a thermowire molten flux crystallization tester and an X-ray diffractometer in the laboratory. The results reveal that one of the major reasons for the cracks is the poor heat transfer ability of the original mold flux. However, the optimized mold flux with a high basicity features a high crystallizing rate,low crystallization temperature and low heat-flow density. Therefore, the optimized mold flux is more suitable for casting peritectic steel by the heavy slab continuous caster. The test results show that the slabs produced by using the optimized mold flux had no surface longitudinal crack in four test casts, while the surface longitudinal crack ratio of the slabs produced by using the original mold flux was 5%. The optimized mold flux can effectively prevent slab surface longitudinal cracks from occurring.