Strip Casting Research Articles

New Technology to Produce 1 GPa Low Carbon Microalloyed Steels from Cast Strip

Global economy requires steel with further increasing mechanical properties and simultaneously decreasing price. In mass manufacturing three major methods can be used to increase strength: (i) increase microalloying element additions (increases cost), (ii) decrease deformation temperature and (iii) increase cooling rate after high temperature processing (both can be challenging for equipment). Thin strip casting is an effective way to reduce cost as it brings a reduction in number of deformation passes and shortens the production line. However, the mechanical properties can be missed due to insufficient microstructure development. In this article, we investigate a recently proposed technology based on Austenite Conditioning followed by Accelerated Cooling and Warm Deformation (AC2WD). Two low carbon steels microalloyed with either 0.012Ti or 0.1Mo-0.064Nb-0.021Ti (wt.%) were subjected to three processing modifications of the AC2WD-technology with two, one or no deformation of cast microstructure in the austenite temperature field. The Ti- and MoNbTi-steels exhibited 685–765 MPa and 880–950 MPa of the yield stress, 780–840 MPa and 1035–1120 MPa of tensile strength, and 20–30% and 22–24% of elongation to failure, respectively. The nature of strengthening mechanisms associated with the AC2WD-technology is discussed on the basis of detailed microstructure characterisation.

Characterization of cermet coatings and its effect on the responding heat transfer performance in strip casting process

In this study, 4 types materials, i.e., WC-12Co, WC-10Co4Cr, NiCr-Cr3C2 and Ni60 were coated on the bare copper to simulate the wear resistant materials on the surface of strip casting rolls. Then, these coatings were characterized by the field emission scanning electron microscopy (FE-SEM), energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), and laser scanning confocal microscope (LSCM). Next, the heat transfer performance of those coatings was evaluated through the droplet solidification tests. The results indicated that the maximum interfacial heat flux and the total heat-removed is varying with different coatings, in which the highest values of 6.25 MW/m2 and 7.21 MJ/m2 in the case of bare copper system could be reduced to 2.62 MW/m2 and 4.22 MJ/m2 with the application of WC-10Co4Cr coating. The heat transfer performance of different coatings is determined by the comprehensive effects of the characteristics of coatings, such as microstructure, porosity, phase composition, adhesion property, and thickness, etc. Furthermore, the results demonstrate that the Ni60 coating with a low porosity (0.78%), good adhesion property and thin thickness (14.1 μm), etc., shows a better heat transfer capability, through which the maximum heat flux is 5.24 MW/m2 and total heat removed is 5.78 MJ/m2, and shows great potential to be used in the strip casting process.

The Application of Centrifugal Atomization Method for Preparation of Rapidly Solidified Nd-Fe-B Flakes Used for Production of Permanent Magnets

Nd-Fe-B-type permanent magnets are used in applications that require a high-energy product/volume ratio in order to reduce weight. Automotive industry, hard drives, or wind turbines are just examples of an application where their use can be found. Conventional casting techniques reveal the formation of a high quantity of α-Fe and large Nd-rich regions. On the other hand, new techniques, like strip casting, melt-spinning and centrifugal atomization, produce homogeneous and fine-scaled microstructures. This paper discusses the application of rapid solidification by means of the centrifugal atomization method for preparation of Nd-Fe-B flakes. The effect of alloy composition and various process parameters of centrifugal atomization on the microstructure of rapidly solidified Nd-Fe-B alloy were investigated. The microstructures and the phase composition were examined by metallographic techniques, namely optical and scanning electron microscopy. Additionally, the influence of the processing parameters on the microstructures of as-cast flakes and subsequent magnetic properties of the prepared magnets will be discussed.

Study of the Heat Transfer Behavior and Naturally Deposited Films in Strip Casting by Using Droplet Solidification Technique

Oxide films that naturally deposit on the surface of the twin-roll mold during strip casting greatly influence the heat transfer from molten steel pool to the mold wall, which further affect the quality of casting product. In this study, a droplet solidification technique has been developed to simulate the initial process of solidification and film deposition during strip casting process. The results suggest that the maximum heat flux increases firstly (to 5201.5 kW/m2 for industrial pure Fe and to 4242.1 kW/m2 for stainless steel) and then decreases (to 4700.3 kW/m2 for industrial pure Fe and to 2037.8 kW/m2 for stainless steel) with the repeat of dropping tests. Furthermore, the roughness and thickness of the films formed on the surface increase with the successive addition of the solidifying material on the prior film. The compositions of the films are detected mainly as oxides containing O, Fe, Si, Mn, S, and Cu for the industrial pure Fe sample and they are O, Si, S, Mn, Cr, and Cu for the stainless steel sample. The deposited film with a thickness (54 μm for industrial pure Fe and 82 μm for stainless steel) and a roughness (24.5 nm for industrial pure Fe and 36.6 nm for stainless steel) allows a better wetting behavior between the molten steel and mold surface, resulting in an increase of actual contact area, and an enhancement of nucleation rate, which then in turn promote the interfacial heat transfer during the initial solidification of molten steel.

Fabrication of a Novel TRIP‐Aided Ni‐Free Lean Duplex Stainless Steel with Good Corrosion Resistance by Twin‐Roll Strip Casting and Its Deformation Mechanisms

The thin strip of novel TRIP‐aided Ni‐free lean duplex stainless steel with good corrosion resistance is fabricated by twin‐roll strip casting. The microstructure, corrosion resistance, and mechanical properties of the present Ni‐free steel (DSS‐2) are investigated in detail in this study. Meanwhile, comparison with duplex stainless steel containing about 2% Ni (DSS‐1) is also conducted to discuss the effect of alloying Ni on corrosion resistance and mechanical properties. The DSS‐2 has the more excellent mechanical properties than the comparison steel DSS‐1 due to the strain induced martensitic transformation during plastic deformation, which the ultimate tensile strength and total elongation are measured to be about 960 MPa and 55%, respectively. The strain induced martensitic transformation in DSS‐2 will be suppressed with the increase of solid solution treatment temperature and strain rate. The austenitic stability is more dependent on element partitioning in austenite phase than on grain size of austenite for the present TRIP‐aided Ni‐free lean duplex stainless steel. The novel Ni‐free steel has the better corrosion resistance and mechanical properties than the austenitic stainless steel SUS304, which may potentially be applied to substitute SUS304 in some fields as anti‐corrosive structural material.

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.

Effect of Cold Rolling Process on Microstructure, Texture and Properties of Strip Cast Fe-2.6%Si Steel.

The use of twin-roll strip casting for the preparation of non-oriented silicon steel has attracted widespread attention in recent years, but related reports are limited. In this study, both one- and two-stage cold rolling with three intermediate annealing temperatures were employed to produce strip cast non-oriented silicon steel. The evolution of the microstructure and texture through the processing routes and its effect on magnetic properties were studied. Compared with one-stage rolling, two-stage rolling increased the in-grain shear bands and the retention of Cube texture in the cold rolled sheets, thereby promoting the nucleation of favorable Goss and Cube grains and restraining the nucleation of harmful {111}<112> grains. With the increase in intermediate annealing temperature, the η-fiber texture in annealed sheets was gradually enhanced, and the average grain size was increased, leading to significant improvement of magnetic properties.

Effect of annealing after strip casting on microstructure, precipitates and texture in non-oriented silicon steel produced by twin-roll strip casting

A 3.1 mm thick non-oriented Fe-1.0% Si steel as-cast strip was produced by twin-roll strip casting. Samples with/without strip annealing were cold rolled to 0.5 mm and then annealed. The development of microstructure, precipitate and texture was characterized along the entire processing route. It was found that annealing after strip casting had a minor influence on the average grain size and texture of the as-cast strip. A large number of particles precipitated and coarsened during the strip annealing process. Meanwhile, precipitates hindered dislocation slip and crystal rotation such that more significant in-grain shear bands were formed in cold rolled sheets. During recrystallization annealing process, the newly formed fine precipitates in directly cold-rolled sheets consumed stored strain energy and also hindered grain growth. But the initial precipitates in the annealed strip grew further and the effect of pinning of dislocations and grain boundary was lost. Therefore, annealed sheets with strip annealing process showed homogeneous equiaxed grains and the recrystallization texture was characterized by strong λ-fiber texture, which led to high permeability.

Whole process metallurgical behavior of the high-abundance rare-earth elements LRE (La, Ce and Y) and the magnetic performance of Nd0.75LRE0.25-Fe-B sintered magnets

Balanced usages of the high-abundance rare-earth elements, such as La, Ce and Y, have attracted tremendous interests of the industrial and scientific societies because of the economic and environmental merits. One of the key issues is of understanding the whole process metallurgical behavior of the high-abundance rare-earth elements in order to optimize the microstructures for a high magnetic performance of the RE-Fe-B sintered magnets, which contain a high concentration of the high-abundance rare-earth elements. In this work, we systematically characterized the metallurgical behaviors during the whole process of strip casting, sintering and annealing for the magnets (Nd0.75LRE0.25)30.5FebalAl0.1Cu0.1B1 (LRE = La, Ce and Y, wt. %). During the strip casting (SC), La and Ce mainly distribute in the grain boundary phase while Y distributes uniformly in the 2:14:1 matrix phase grains. Under the sintering and annealing, La is squeezed out from the 2:14:1 matrix phase to segregate in the grain boundaries, while Ce and Y migrate from the grain boundary to the 2:14:1 matrix phase. The crystalline structures of matrix phase and grain boundary phase of Nd-La/Ce/Y-Fe-B are tetragonal phase (space group P42/mnm) and hcp-Re2O3. The investigations of the grain boundary of Nd-La/Ce/Y-Fe-B magnets demonstrate that the grain boundary of the magnet with Ce is thicker than that of magnets with La and Y. The different metallurgical behaviors of elements La, Ce and Y are attributed to the different solidification temperature and substitution energy of Re2Fe14B phases. Core-shell grain structure was formed in the case of Y-containing magnets. Elemental analysis demonstrates that Y prefers to locate in the core of the grain boundary and Nd distributes in the shell of the grain boundary.

Influence of hot deformation on texture and magnetic properties of strip cast non-oriented electrical steel

The present study focuses on improving the magnetic properties and decreasing the anisotropy in non-oriented electrical steel by optimizing {100} recrystallization texture. As-cast Fe-1.3%Si strip with {100} texture produced by strip casting was subjected to hot rolling in the ferrite region, cold rolling, and recrystallization annealing. Magnetic properties and texture evolution after different stages of processing were studied. Annealed sample without hot rolling exhibited pronounced Cube and Goss texture, which led to high permeability but induced a large difference (∼0.15 T) in magnetic induction B50 between the maximum at 0° and minimum at 45° to the rolling direction. The introduction of hot rolling with 17–40% reduction weakened the intensity of recrystallization texture and had small influence on the nature of texture and magnetic induction. However, relatively complete {100} recrystallization texture was developed in the sample with hot rolling of 55% reduction. On the other hand, the average grain size of annealed sheets gradually increased with the increased hot rolling reduction. As a result, the magnetic induction and the core loss was optimized together with the improvement of anisotropy. The development of recrystallization texture is discussed on the basis of the deformed microstructure and nucleation mechanism, while the magnetic properties are correlated to the magnetic quality of the texture.

Significant effect of as-cast microstructure on texture evolution and magnetic properties of strip cast non-oriented silicon steel

In this study, two types of as-cast microstructure produced by strip casting were cold rolled and annealed to investigate the effect of initial microstructure on the textural evolution and magnetic properties of non-oriented silicon steel. The results indicated that the cold-rolled sheets of coarse-grained strip with pronounced {100} components exhibited stronger λ fiber (〈100〉//ND) and weaker γ fiber (〈111〉//ND) texture as composed to the fine-grained strip with strong Goss ({110}〈001〉) texture. After annealing, the former was dominated by η fiber (〈001〉//RD) texture with a peak at {110}〈001〉 orientation, while the latter consisted of strong {111}〈112〉 and relatively weak {110}〈001〉 texture. In addition, a number of precipitates of size ∼30–150 nm restricted the grain growth during annealing, resulting in recrystallization of grain size of ∼46 μm in the coarse-grained specimen and ∼41 μm in the fine-grained specimen. Ultimately, higher magnetic induction (∼1.72 T) and lower core loss (∼4.04 W/kg) were obtained in the final annealed sheets of coarse-grained strip with strong {100} texture.

Numerical Analysis of Twin-Roll Casting of Strips With Profiled Cross-Section

The relatively high production costs of innovative materials with tailored properties such as Tailor Welded Blanks, Patchwork Blanks, Tailor Heat Treated Blanks and Tailor Rolled Blanks are responsible for a growing interest in new cost-effective production methods. One of the promising energy-saving and environmental friendly technologies for the production of tailored blanks is twin-roll casting. In the study a new alternative method for twin-roll casting of strips with profiled cross-section is proposed, which uses one or more preloaded endless steel strips with an antiadhesive coating for profiling of the formed strip on a pair of the common cylindrical shells. As a primary stage for the practical process design, numerical simulation of the process using the finite element software package ANSYS is realized. In this way, dependencies of the strip elements outlet temperature, deformation zone length and elements outlet speed on the varied strips thickness and total solidification-deformation zone length are established. Based on the simulation results, a procedure for the twin-roll casting process design is suggested.

Microstructure, Tensile and Fatigue Properties of Al–5 wt.%Mg Alloy Manufactured by Twin Roll Strip Casting

This study investigated the microstructure, tensile and fatigue properties of Al–5 wt.%Mg alloy manufactured by twin roll strip casting. Strips cast as a fabricated (F) specimen and a specimen heat treated (O) at 400 °C/5 h were produced and compared. In the F specimen, microstructural observation discovered clustered precipitates in the center area, while in the O specimen precipitates were relatively more evenly distributed. Al, Al6(Mn, Fe), Mg2Al3 and Mg2Si phases were observed. However, most of the Mg2Al3 phase in the heat-treated O specimen was dissolved. A room temperature tensile test measured yield strength of 177.7 MPa, ultimate tensile strength of 286.1 MPa and elongation of 11.1% in the F specimen and 167.7 MPa (YS), 301.5 MPa (UTS) and 24.6% (EL) in the O specimen. A high cycle fatigue test measured a fatigue limit of 145 MPa in the F specimen and 165 MPa in the O specimen, and the O specimen achieved greater fatigue properties in all fatigue stress conditions. The tensile and fatigue fracture surfaces of the above-mentioned specimens were observed, and this study attempted to investigate the tensile and fatigue deformation behavior of strip cast Al–5 wt.%Mg based on the findings.

Effect of hot band annealing on microstructure, texture and magnetic properties of non-oriented electrical steel processed by twin-roll strip casting

In the present study, an as-cast strip of Fe-3.2Si wt% non-oriented electrical steel was processed successfully by a new compact and low cost twin-roll strip casting process. After hot rolling, both the hot rolled bands with and without annealing were subjected to cold rolling and final annealing. The aim of the study is to elucidate the effect of hot band annealing on the microstructure, texture and magnetic properties of non-oriented electrical steel processed by twin-roll strip casting using a combination of optical microscopy (OM), X-ray diffraction (XRD), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The study indicated that the AlN and MnS particles precipitated during hot band annealing process affected crystal rotation of the cold rolled sheet. {1 0 0}〈0 1 1〉 texture with a higher intensity and {1 1 0}〈1 1 0〉 texture with a lower intensity was observed after cold rolling compared to the cold rolled sheet without hot band annealing. The recrystallization and growth behavior of grains was impeded because of the precipitation during final annealing, and a strong λ-fiber texture with a smaller average grain size was obtained in the annealed sheet without hot band annealing process. However, the recrystallization texture in the final annealed specimen subjected to the hot band annealing process was characterized by pronounced {4 1 1}〈1 4 8〉 texture and the average grain size was significantly larger than without hot band annealing. The results suggested that hot band annealing is a suitable route to remarkably improve the magnetic properties of non-oriented electrical steel.

Effects of plastic straining on the corrosion resistance of TRIP-aided lean duplex stainless steels

In the present paper, the effects of plastic deformation on the corrosion resistance of strip cast TRIP (transformation-induced plasticity)-aided stainless steel (19Cr-DSS) and conventional lean duplex stainless steel (LDX2101) were studied by observations of deformed microstructure and electrochemical experiments. The electrochemical results indicate that the corrosion susceptibility of experimental steels in 3.5% NaCl solution is increased with increasing deformation amounts. The Mott–Schottky analysis revealed that the passive films formed on both 19Cr-DSS and LDX2101 exhibit the n-type and p-type semiconducting behaviors regardless of strains. With the increase in true strain, the acceptor and donor densities are both continuously increased. The total density of acceptor and donor in 19Cr-DSS is increased more rapidly with increasing strain than that of LDX2101, indicating that the corrosion resistance of 19Cr-DSS is more sensitive to plastic deformation compared to that of LDX2101. For the TRIP-aided 19Cr-DSS, the deterioration of corrosion resistance after plastic deformation may have been caused by the decrease in low-Σ CSL boundaries especially the twin (Σ3) boundaries, the increase in dislocations, and the formation of strain-induced martensite during plastic deformation.

Determination of the Hot Cracking Tendency of a Twin-Roll Cast AZ31 Magnesium Alloy by Means of Gleeble Tests

The knowledge about the formation of hot cracking in magnesium alloys, such as in twin-roll cast magnesium sheets and strips, is fundamental for a good quality of the strips during the further processing by rolling or welding and minimize the reject. Hot cracking often occurs in the so-called mushy zone, when solid phases and melt coexist, at temperatures where the material no longer exhibits ductility. For the evaluation of the hot cracking tendency of an alloy, the width of the HTBR (High-temperature brittleness range) can be used. On the basis of a test on a Gleeble HDS-V40, the HTBR was determined for a twin-roll cast AZ31 magnesium alloy. The transition between ductile forming behaviour and complete brittle reaction of the AZ31 alloy is confirmed by the observation of the fracture surfaces (determination of the fracture type) in the scanning electron microscope (SEM) and is located at 555 °C. The HTBR shows a range 35 K.

The Effect of Nb Micro-alloying on the Bainitic Phase Transformation Under Strip Casting Conditions

The effect of Nb concentration on the transformation from austenite to bainitic ferrite has been examined under simulated strip casting conditions. Nb concentration was found to delay the nucleation of bainite, but accelerated its growth. It is suggested that the delay in nucleation increases the driving force for transformation, which results in an increase in the growth rate of the bainite. The bainite/austenite interfaces are proposed to move too quickly to suffer appreciable solute drag.

On Goss Orientation in Strip Cast Grain‐Oriented Silicon Steel

In this study, the origin of Goss texture during intermediate annealing and Goss texture development during secondary annealing of strip cast grain‐oriented silicon steel are studied by electron backscattered diffraction (EBSD). The study indicates that Goss grains originate inside the shear bands of deformed {111}&lt;112&gt; and {111}&lt;110&gt; grains. Compared to {111}&lt;110&gt; grains, {111}&lt;112&gt; grains provide more number of nucleation sites for Goss grains. During subsequent recrystallization process, Goss grains exhibit a smaller growth rate than the average value of all the recrystallized grains. The development mechanism of Goss texture is concluded as oriented nucleation. Prior to secondary annealing, high fraction of high‐energy boundaries (20°–45° misorientation angle) are observed in the vicinity of Goss grains, while significantly low fraction of Σ5 + Σ7 + Σ9 boundaries are observed. During secondary annealing, the domination of high energy boundaries around Goss grains is maintained, but the fraction of Σ5 + Σ7 + Σ9 boundaries decrease to be similar to the matrix grains. After the onset of the abnormal grain growth, the growing Goss grains continue to consist of high fraction of high‐energy boundaries. These results are consistent with the high energy (HE) boundary model, which is used to explain the abnormal grain growth in the current strip casting route.

Microstructure and texture development during deformation and recrystallisation in strip cast AA8011 aluminum alloy

In the present work, microstructure and texture development during deformation and recrystallisation of a twin roll cast (TRC) AA8011 aluminum alloy was investigated. TRC AA8011 alloy with initial thickness 7.2 mm and Fe:Si ratio ∼0.93 was cold rolled in a laboratory rolling mill to 2 mm final thickness. The cold rolled sheet was recrystallized at 375 °C and 450 °C for different times. The hardness decreased with increase in recrystallisation time for both temperatures, but there was no effect on centre line segregation (consisting predominantly of Al-Fe-Si intermetallics). XRD confirmed the presence of β-AlFeSi particles at all the stages of recrystallisation. The initial TRC sheet had a very weak texture close to brass ({110}<112>). After 72% cold rolling (final thickness, 2 mm), strong brass, copper ({112} <111>) and S ({123}<634>) texture i.e. metal type texture developed. Low temperature annealing (4 h at 375 °C) was found to give optimum combination of deformation texture and cube recrystallisation texture ({100} <100>) expected to improve the forming behavior of cold rolled and annealed sheet.

Abnormal growth of {100} grains and strong Cube texture in strip cast Fe-Si electrical steel

High-grade non-oriented electrical steel was successfully processed by controlling texture using strip casting technology. Remarkable abnormal grain growth of {100} grains occurred in the as-cast strip after isothermal annealing. This phenomenon is considered to be the synergistic effect involving the size advantage of coarse columnar grains and surface energy, where the normal grain growth was inhibited by thermal grooving in the surface layer. Special rotation path was identified during the cold rolling process and metastable {118} 〈110〉–{113} 〈110〉 texture dominated the deformation texture. The final annealed sheet exhibited strong Cube texture together with significantly superior magnetic properties.