Thin Strip Casting Process Research Articles

Effect of Annealing Time on Texture Evolution of Fe–3.4 wt% Si Nonoriented Electrical Steel

Herein, the effect of annealing time on the texture evolution in Fe–3.4 wt% Si non‐oriented electrical steel is investigated. Strip samples are cast using a vacuum sampling method, which simulate the solidification conditions of an industrial twin roll thin strip casting (TRSC) process. As‐cast samples with different carbon and sulfur (C&S) levels are hot rolled (HR) with varying levels of hot deformation, cold rolled (CR) to 0.35 mm thickness, and then annealed at 1050 °C for different holding times (1, 6, 24 h). To Fe–3.4 wt% Si nonoriented electrical steel, the observed texture evolution can be divided into different stages as annealing time is increased from 1 to 24 h. With increasing annealing time, the fraction of Goss texture decreases initially and then increases again through the consumption of grains with other textures. With additional time, a decrease of pinning force due to precipitate coarsening results in normal grain growth, resulting in an increase of grain size. In this step, Cube grains can form from rotated Goss grains. A model for core loss is presented and used to explain the core loss results.

Applying crystallographic orientation analysis for exploring deformation mechanism of 6.5 wt.% Si electrical strip during twin-roll thin strip casting process

The study described here is aimed at illuminating deformation mechanism involved in Twin-roll Thin Strip Casting Process (TRSC) with respect to high-permeability 6.5 wt.% Si electrical strip (6.5 wt.% Si strip) in terms of crystal plasticity (CP) theory, and a Visco-plastic Self-consistent model (VPSC) was utilized to explore the deformation mechanism by applying crystallographic orientation (OR) analysis. Results underscore that the activation of overall potential single slip system with regard to studied 6.5 wt.% Si strip including { 110 } < 111 > , { 112 } < 111 > as well as { 123 } < 111 > can contribute to the OR conversion from Cube texture to Rotated-Cube texture, α-fiber texture as well as γ-fiber texture, while with different evolution rate during deformation. The potential multi-slip system of { 112 } < 111 > + { 123 } < 111 > with average number of activated slip system per grain ~9.1–9.4, and multi-slip system of { 110 } < 111 > + { 112 } < 111 > + { 123 } < 111 > with average number of activated slip system per grain ~10.4–10.8 are identified to be the predominant slip systems during deformation by comparing to the actual OR features obtained from TRSC-processed 6.5 wt.% Si strip. The CP-based method proposed in present study is deemed as a reference to investigate deformation behavior involved in the other semi-solid forming process.

Comparison of cracking sensitivity between high-permeability 1.5 wt% Si steel and 6.5 wt% Si steel in mushy zone

Twin-roll thin strip casting process (SC) has been utilized in laboratory to fabricate typical high-permeability electrical steel (HPES) such as 1.5 wt% Si and 6.5 wt% Si steel with ideal crystallographic orientation and excellent permeability after a series of heat treatments. Whereas, there exists a problem associated to the cracking sensitivity in high temperature mushy zone, which further restricts the industrial application of SC to produce HPES. Mechanical behavior of 1.5 wt% Si and 6.5 wt% Si steels in high temperature mushy zone were comparatively studied by Gleeble-3800 physical simulation system under strain rate 3 s−1, systematically. The high temperature brittle region composed of zero strength temperature (ZST) and zero ductility temperature (ZDT) in the mushy zone were quantitatively determined, and the critical strain threshold associated to hot crack initiation in the brittle region were further evaluated following empirical model proposed by Y Won. Results show that the tensile strength and fracture ductility in the mushy zone are both closely associated with the varying volume fraction of network-shaped structure during solidification, and are decreased with decreasing solid fraction concerning studied 1.5 wt% Si and 6.5 wt% Si steels. The high temperature brittle region corresponding to the studied 1.5 wt% Si and 6.5 wt% Si steels in mushy zone are separately confirmed to be 1459 ~ 1465℃ and 1390 ~ 1427℃, and the thresholds of critical strain for hot cracks initiation in high temperature brittle region are calculated to be ~ 1.18% and ~ 0.24%, respectively. Considering from perspective of width of high temperature brittle region and critical strain threshold for hot crack initiation, the cracking sensitivity of 6.5 wt% Si steel in the mushy zone is more serious compared to 1.5 wt% Si steel.

Recrystallization behavior in a low-density high-Mn high-Al austenitic steel undergone thin strip casting process

The recrystallization behavior of a high-Mn high-Al lightweight steel (Fe-28.4Mn-8.3Al-1.27 C (wt%)) with 50% cold rolling deformation is investigated at the annealing temperature of 800 °C, 900 °C and 1000 °C. The detailed microstructure evolution is characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The partial recrystallized bimodal austenite grains and κ-phase (Fe, Mn)3AlC form at 800 °C, while fully recrystallized austenite grains without κ-phase appear at 900 °C and 1000 °C. With the increase in annealing temperature, the increased frequency of annealing twin boundaries reduces the average austenitic grain size and the dispersion of the austenite grain size effectively. The effect of austenite grain size on the tensile properties is discussed. The strain hardening behavior is also investigated by Hollomon analysis and C-J analysis, and the later one is better to explain the strain hardening behavior in different four stages. The recrystallization behavior significantly improves the tensile toughness from 59 MJ/m3 to 436 MJ/m3 at the expense of tensile strength decrease from 1460 MPa to 890 MPa, due to the homogenous fine austenite grains and high frequency of high misorientation angle. The steel with fully recrystallized austenite grains of 10.0 µm annealed at 900 °C exhibits the optimum mechanical properties with excellent tensile strength of 965 MPa, ductility of 48.3%, and good toughness of 400 MJ/m3.

Application of computational fluid dynamics to steelmaking processes

Today, the numerical simulation of fluid dynamic phenomena (CFD) in steelmaking applications is state of the art. The latest simulation results for the BOF and AOD as well as the thin strip casting process are described. The basic influence of the process parameters, e.g. blowing and stirring rate, vessel dimension, location of top lance, number and arrangement of porous plugs, on the converter melt flow and mixing time is simulated. In the case of thin strip casting, the free melt flow on the moving belt is influenced by an induced electromagnetic field. The respective MHD (magneto-hydrodynamic) is presented.

Microstructure analysis of AISI 304 stainless steel produced by twin-roll thin strip casting process

The microstructure of AISI 304 austenite stainless steel fabricated by the thin strip casting process were investigated using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). The microstructures of the casting strips show a duplex structure consisting of delta ferrite and austenite. The volume fraction of the delta ferrite is about 9.74vol% at the center and 6.77vol% at the surface of the casting thin strip, in vermicular and band shapes. On account of rapid cooling and solidification in the continuous casting process, many kinds of inclusions and precipitates have been found. Most of the inclusions and precipitates are spherical complex compounds consisting of oxides, such as, SiO 2, MnO, Al 2O 3, Cr 2O 3, and FeO or their multiplicity oxides of MnO·Al 2O 3, 2FeO·SiO 2, and 2MnO·SiO 2. Many defects including dislocations and stacking faults have also formed during the rapid cooling and solidification process, which is helpful to improve the mechanical properties of the casting strips.

Simulation of Inelastic Deformation and Thermal Mechanical Stresses in Twin-Roll Casting Process of Mg Alloy

Twin-roll thin strip casting process combines casting and hot rolling process into a single step; in which melt and solid states exist in the casting region, so its deformation is more complex than rolling process. In this paper, Anand’s model, a unified visco-plasticity constitutive model, was employed to simulate the highly nonlinear behavior in the twin-roll casting process. Anand model’s parameters were regressed based on compression tests at various temperature and strain rate for magnesium alloy AZ31. To calculate the thermal mechanical stresses, the thermal flow of twin-roll casting process was simulated firstly; then stresses were calculate, in which the temperature field result of thermal flow was imposed as body load, and a small displacement load along roller tangential direction was imposed simultaneously in order to simulate rolling action. The deformation results can well describe the forward slip zone, backward slip zone and melt eddy zone in the casting region. Based on the results, the applicability of Anand’s model on twin-roll casting process was discussed.

Application of Anand's constitutive model on twin roll casting process of AZ31 magnesium alloy

Twin-roll thin strip casting process combines casting and hot rolling into a single process, in which thermal stress and thermal mechanical stress were involved. Considering the high temperature gradient, the existing of liquid and solid regions and rolling deformation, suitable constitutive model is the key to describe the process. Anand's model is a temperature-dependent, rate-dependent and unified of creep and plasticity model and the Jaumann derivative was employed in Anand's model which makes the constitutive model frame-indifferent or objective, therefore the highly nonlinearities behavior in the twin-roll casting process can be simulated. The parameters of the Anand's model were regressed based on the compression tests of AZ31 magnesium alloy. The simulation results reveal that the Anand's model can well describe the deformation characteristics of twin-roll casting process. Based on the simulation results, the form of evolution equations in Anand's model was discussed.

The Microstructure and Processing in Twin Roll Casting of Magnesium Alloy Strip

Magnesium has a hexagonal close-packed crystal structure which makes it more difficult to deform than aluminum. Conventional strip production usually requires several process steps to reach the final strip thickness. Strip casting can reduce some of the process steps and make strip processing simpler and easier, especially for alloys with poor deformability. The twin-roll casting process can directly obtain strips with thickness less than one or two millimeters. In this paper, the metallurgical characteristics of the twin-roll strip casting process were analyzed and discussed. A laboratory scale vertical twin roll caster in prepared AZ31 magnesium alloy strips, with 1.0 to 2.0 mm thick and 150mm wide. Process stability in the thin strip casting process of the alloy has been studied, the casting temperature (superheat) was proven to be a key factor influencing process stability and casting strip quality. The as-cast microstructure of the alloy was analyzed and evaluated by optical microscopy, which showed that the as-cast microstructure was composed of developed dendrites when the superheat was high, and of a rosebush-like structure when the superheat was low. When the casting temperature was close to the liquid phase temperature，the as-cast microstructure became global or grainy, and the grain size of the cast strip was very small. This remarkably improved the deformability of the as-cast strip.

Modeling of Thermo-Mechanical Stresses in Twin-Roll Casting of Aluminum Alloys

A two-dimensional steady state thermo-mechanical stress model was developed using the commercial structural analysis package ANSYS, to compute the stresses arising from thermal gradients and mechanical loads applied to strip and roll during a horizontal twin-roll thin strip casting process. The finite element mesh and the nodal temperature values obtained from the fluid flow, heat transfer, and solidification model developed on FIDAP, were transported to ANSYS. Mechanical loads like roll separating force and strip exit tension were also applied on the strip. A visco-plastic constitutive relation has been used to describe the behavior of solidifying aluminum alloy. The effects of inlet velocity of the melt and contact strip/roll heat transfer coefficient on the resultant stress profile in the strip and the rolls were investigated.

Strip formation and process stability in twin roll strip casting

Process stability in the thin strip casting process of steel has been investigated with a laboratory pilot caster. Depending on the steel grade and the process parameters irregular process states could be observed. It has been proved experimentally that the observed behaviour is not caused by the regulation system. Investigations of microsections on the cast strips have shown evidence of periodical changes in heat flux during solidification and strip formation as the reason for observed oscillating process variables. Similar to conventional flat rolling forward and backward slip between strip and casting rolls during the joining of the shells can occur. Under certain combinations the process parameters and the cast material a backward‐slip can lead to local lift‐offs of the solidified shells. This circumstance significantly disturbs the heat flux and consequently the solidification, even periodically.

Wear of refractory ceramics against nickel

In the thin strip steel casting process, molten steel is retained between two rotating rolls and two fixed refractory side dams. The shape and size of the plates obtained are determined by the roll diameter and the depth of molten steel in the tundish. The fixed side dams sliding against the rolls are subjected to high stresses due to localised temperature differences. The plate temperature varies from steel melting temperatures of 1500–1600°C to roll contact and back side dam temperatures of 200–400°C. The purpose of this paper is to show the possibility, in this case of dry friction, of identifying the governing parameters of refractory wearing from the third body point of view. The refractory wearing on nickel is governed by both contact pressure and temperature around the contact. At temperature lower than a threshold value, the wear can be controlled through applied load. At higher temperature, the control of wear decreases significantly. The influence of high temperature can be reduced by optimising the refractory (composition and microstructure). This study was very helpful to develop refractory materials meeting the requirements of thin strip casting process.

The Myosotis project : progress report

The thin strip casting process developped by the Myosotis Project has now reached a level raising the question of an industrialization.The last improvements deal with the optimization of mould hydrodynamics, the lateral confinement and the casting rolls thermomechanics.Several sequences with a duration of more than 4 h have been cast at the Isbergues plant. The maximum duration is 5 h, i.e. 225 tons and nearly 11 km of steel strip with 865 mm wide rolls.The surface quality obtained after cold processing is suitable for saleable applications (decoration tubes, etc.).

NUMERICAL INVESTIGATION OF COUPLED TURBULENT FLOW, HEAT TRANSFER, AND MACROSCOPIC SOLIDIFICATION IN A VERTICAL TWIN-ROLL THIN-STRIP CASTER

A vertical twin-roll continuous thin-strip casting process for stainless steel has been mathematically modeled. The model takes into account the coupled turbulent flow, heat transfer, and macroscopic solidification aspects of the process. A low-Reynolds-number K-ε turbulence model was used to account for the turbulent effects. The transport equations for the wedge-shaped caster's cavity were solved using a boundary-fitted nanorthogonal coordinate system. A control-volume-based, iterative finite difference scheme on a staggered grid was used to solve the discretized transformed equations. The SIMPLE algorithm was employed to resolve the velocity-pressure coupling in the momentum equations. The parameters examined in this study include the Darcy coefficient and turbulent damping factor for the mushy region, the roll gap, and the inlet nozzle width. The effects of these process parameters on the turbulent flow and temperature fields, the turbulent eddy viscosity distributions, and the extents of the mushy and solidified regions were ascertained. A change of the Darcy coefficient above 800 did not show any significant effect on the results. The three types of liquid-fraction-dependent turbulent damping factor used for modeling the mushy region did not show a measurable effect on the solidified shell thickness but showed a noticeable influence on the velocity and temperature distributions in both liquid and mushy regions. For a fixed width of the nozzle, an increase in the roll gap increased the penetration depth of the plunging inlet jet but decreased the extent of the mushy region. The solidified shell profile was insensitive to the change of the roll gap. For a fixed roll gap, an increase in the width of the nozzle decreased the penetration depth of the nozzle but increased the extent of the mushy region, while the solidified shell thickness remained practically unaffected.

Transient thermal model of the continuous single-wheel thin-strip casting process

A transient heat-transfer model (STRIP1D) has been developed to simulate the single-roll continuous strip-casting process. The model predicts temperature in the solidifying strip coupled with heat transfer in the rotating wheel, using an explicit finite difference procedure. The model has been calibrated using strip thickness data from a test caster at ARMCO Inc. (Middletown, OH) and verified with a range of other available measurements. The strip/wheel interface contact resistance and heat transfer were investigated in particular, and an empirical formula to calculate this heat-transfer coefficient as a function of contact time was obtained. Wheel temperature and final strip thickness are investigated as a function of casting speed, liquid steel pool depth, superheat, coatings on the wheel hot surface, strip detachment point, wheel wall thickness, and wheel material.

A simple analytical model for a single roll thin strip casting process

A simple analytical model is described for a single roll thin strip casting process. This model was developed using fluid mechanics only. Molten metal flow in the puddle in the strip moving direction was treated as a boundary layer flow, whereas the crosswise expansion of the puddle was taken as a potential flow. By assuming a flow speed distribution so that the predictions of the model conform to experimental measurements, this model allows the effects of operating parameters including cast speed and nozzle location angle on the strip formation to be predicted and an optimum operating condition window to be proposed. Occurrence of strip jamming can also be interpreted based on the predicted puddle profile.

A single-roll thin-strip casting process

A single-roll thin-strip casting process has been described and the effect of operating parameters on the strip quality experimentally studied concerning mainly casting speed and nozzle location angle. As a result, an optimum operating condition has been established which shows that the nozzle location angle is an important factor. A large nozzle location angle tends to lead to strip jamming, whereas a small angle gives rise to production of narrow strips with poor surface quality.

Heat and fluid flow simulation of the melt-drag single-roll strip casting process

The melt-drag single-roll thin strip casting process is investigated via a numerical model, which takes into account fluid flow and heat transfer with phase change. Boundary-layer coupled equations for momentum and energy along with a generalized enthalpy formulation extend the computation to the liquid, mushy and solid zones of the cast. The process variables investigated are heat transfer coefficient at roll/cast interface, casting speed and pool melt temperature, and they are analysed in terms of strip thickness and growth rate. The parametric study is conducted for AISI SAE1020 steel. Preliminary findings indicate that the heat transfer coefficient at the roll-caster interface has to some extent similar importance in determining the final strip thickness to the residence time which is a function of pool height and drum speed.

Thin‐strip casting of steel with a twin‐roll caster – discussion of product defects of ≍ 1 mm‐Fe6%Si‐strips

After a former detailed study of the thin strip casting process now defects of the strip and their avoidance are discussed. Hence it is deduced that the casting process should be carried out with small rolling forces. Thus the process of strip cooling has to be separated from the process of strip forming, and a cooling device has to be established close to the casting rolls.