Strip Casting Research Articles

Effects of HDDR treatment on structure, morphology and particle size of Nd-Fe-B powders prepared by ball milling of Nd-Fe-B strip cast alloys

In this work we perform HDDR treatment on Nd-Fe-B strip cast alloys and investigate the structure, morphology and particle size of ball-milled original and HDDR powders. After HDDR treatment, the columnar Nd2Fe14B grains are refined into equiaxed polyhedral grains with average size of 2.04 µm and the Nd-rich phase redistributes at triple junctions or along the grain boundaries. During ball milling process, the columnar grains are broken into monocrystalline powders by transgranular fracture while the HDDR equiaxed grains are cracked into single crystalline powders along the grain boundaries, thus the HDDR powders show equiaxed polyhedral morphology with higher sphericity and present more Nd-rich phase crystal planes than original powders. The HDDR powders exhibit smaller particle size and narrower size distribution (d50 = 1.92 µm, relative span = 1.463) than original powders (d50 = 2.51 µm, relative span = 1.705). Moreover, the HDDR powders with smaller particle size present higher magnetization (131.4 emu/g) than original powders (128.2 emu/g), which illustrates the reduction of surface damage due to intergranular fracture. Therefore, we believe that this HDDR powders are conducive to the preparation of high coercivity sintered Nd-Fe-B magnets.

Effect of cooling route on microstructure and mechanical properties of twin-roll casting low carbon steels with an application of oxide metallurgy technology

The oxide metallurgy was employed in the fabrication of twin-roll strip casting low carbon steels. The quantitative dependences of the microstructure and mechanical properties on cooling parameters were revealed and emphatically investigated. After twin-roll strip casting, a two-step cooling route closed to the actual process was designed and the cooling rates exhibited great influences on the proportion of polygonal ferrite (PF) and acicular ferrite (AF). With increasing cooling rates in both steps, the AF volume fraction raised from 0.04 to 0.75 because the rapid cooling restrained the formation of PF. The AF laths were induced by fine Al–Si–Ti–Mn composite oxides (0.2–2 μm), leading to a heterogeneous microstructure consisting of PF and AF clusters with high proportion of large-angle (≥15°) grain boundaries. It was found that the AF laths kept K–S relationship with austenite and AF laths with collinear growth direction belong to the same codirectional variants group in one AF cluster. With the increase of AF volume fraction, the yield strength (YS) and the ultimate tensile strength (UTS) were improved, while the strip retained excellent elongation. The improved YS was mainly ascribed to the grain boundary strengthening and dislocation strengthening.

Topical Issues of the Technological Development of Advanced Electrical Steel Grade Production in Russia

The growing competition in the global steel market in the field of rolled products potentially implies an efficient, economical and environmentally friendly approach to the production of electrical steel. The following key elements of advanced strip steel casting are discussed: innovations in the use of cast strips for the production of the initial hot-rolled billet and cold-rolled steel from electrical anisotropic and isotropic steel, as well as amorphous steel strips for magnetic cores of electrical machines, which have been commercially applied in world practice.

Texture evolution in twin-roll strip cast non-oriented electrical steel with strong Cube and Goss texture

Increasing magnetically favorable <100>//ND texture components is a key challenge in the preparation of high-efficiency non-oriented electrical steels. In this study, an Fe-1.3 wt% Si steel with strong Cube ({100}<001>) and Goss ({110}<001>) texture was successfully produced by novel twin-roll strip casting, cold rolling and annealing process. The microstructure and texture of the material was characterized by optical microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The origin and formation mechanism of texture are described from the perspective of deformation and recrystallization behavior of specifically oriented grains. It was observed that initial Cube rotated toward {013}<031>-{110}<110> besides rotation toward {001}<130>-{001}<120> during cold rolling. In addition, new Cube deformation bands were developed in the deformed {115}<051>-{115}<161> grains. Cube components were partly retained as large block, small band structure and crystallite after heavy cold rolling. The Cube deformation structures served as nucleation sites of new Cube grains. The shear band within {114}<110>, {112}<110> and {111}<112> matrix also provided some Cube nuclei. Morphology change from near bar-shaped to equiaxed occurred during the growth of Cube and Goss grains. The formation of recrystallization texture is attributed to the oriented nucleation mechanism, and the orientation pinning and size effects that impacted the intensity of texture component. The low thickness of strip and coarse solidification microstructure with strong {100} texture are the decisive factors to obtain strong Cube and Goss texture in strip-cast non-oriented electrical steel.

Globular Crystals in the Center of Roll Cast Aluminum Alloy Strips

This study investigated the crystallization position and formation mechanism of globular crystals at the center area in the thickness direction of aluminum alloy strips cast by a high-speed twin roll caster. Twin roll casters for single strips and clad strips were used, as well as twin roll casters equipped with a cooling slope. The globular crystals were formed from dividing arms of dendrites of the solidified layer facing the center area at the roll gap. The arms of isolated dendrite also divided. No globular crystals were formed at the interfaces of clad strips with different solidification temperatures because of the temperature gradient at the interface which inhibited division of the dendrite arms. It was demonstrated that globular crystals at the center area of the thickness direction were formed by dendrite-arm-dividing at the roll gap by the strip casting clad strip. Experiments by semisolid-strip casting with the cooling slope showed that globular crystals in the molten metal existed in the solidification layers.

Changing from a Vertical Burr to a Horizontal Burr in the Vertical-Type High-Speed Twin-Roll Caster

Strip casting using a side dam plate produces a vertical burr at the strip edge. In the present study, changing this vertical burr into a horizontal burr using a burr changer is proposed. The burr changer was placed inside the side dam plate. The burr changer was made from mild steel and an insulator sheet and cut along the shape of the roll. The burr changer was placed so as to prevent exhaustion of semisolid metal between the side dam plate and the roll-side surface. When the position of the burr changer was appropriate, the vertical burr changed into a horizontal burr. The horizontal burr was flat. The width of the horizontal burr was affected by the lowest position of the burr changer and became narrower as the lowest position of the burr changer approached the roll gap position.

Novel PrNd-Lean Ce-Based R₂Fe₁₄B Permanent Magnets With High Performance

In this article, the phase compositions, microstructures, and magnetic properties of PrNd-free Ce-Fe-B-based sintered magnets with the nominal composition of (Ce0.78Y0.17La0.05)16.28Febal.B7.44M1.94 (at.%: M = Co, Al, Cu, and Nb) have been investigated. It shows that a lamellar crystalline structure is formed in the Ce-Fe-B-based strip casting (SC) alloy. The Y-substituted Ce-Fe-B-based SC alloy is mainly composed of R2Fe14B and RFe2 phases. It suggests that Y substitution is helpful for depressing the $\alpha $ -Fe phase generated in the SC alloy. The island-like intermediate phases exist in the RFe2 region, which are probably caused by the incomplete peritectic reaction during alloy solidification. The Curie temperature $T_{\mathrm {c}}$ of the Y-substituted Ce-Fe-B-based alloy is about 463 K, which is higher than that of the Ce2Fe14B compound ( $T_{\mathrm {c}} =424$ K). For the experimental Ce-Fe-B-based sintered magnet without PrNd addition, the remanence $B_{\mathrm {r}}$ of 8.31 kG, the coercivity $H_{\mathrm {cj}}$ of 0.92 kOe, and the maximum energy product (BH)max of 5.30 MGOe have been obtained. A good combination of coercivity $H_{\mathrm {cj}}$ (= 5.57 kOe) and maximum energy product (BH)max (= 20.17 MGOe) has been achieved in the Ce-Fe-B-based blended magnet with PrNd/ $\sum $ RE = 25% prepared using a binary main phase (BMP) method. The heterogeneous elemental distributions and microstructure characteristics of the BMP magnet are also analyzed.

Microstructure and mechanical properties of Al-7.9Zn-2.7Mg-2.0Cu (wt%) alloy strip fabricated by twin roll casting and hot rolling

High Zn content Al-7.9Zn-2.7Mg-2.0Cu (wt. %) aluminum alloy strip with 5.5 mm thickness was successfully fabricated by twin roll casting (TRC). Fine grain size, second phase and second dendrite arm spacing (SDAS) were obtained in the cast strip due to the high cooling rate during TRC process. Short-time annealing treatment of 480 ºC-1 h could effectively reduce the second phases, and the elongation of the strip was improved. The TRC alloy sheet with 2.0 mm thickness was fabricated by the annealing treatment (480 ºC-1 h) and hot rolling. The yield strength, tensile strength and elongation along the rolling direction of the TRC sheet after solution treatment, pre-aging and natural aging were 398 MPa, 590 MPa and 15.74%, respectively. After bake-hardening, the yield strength, tensile strength and elongation along the rolling direction were 550 MPa, 605 MPa and 10.07%, respectively.

Microstructure, tensile and fatigue properties of high strength Al 7075 alloy manufactured via twin-roll strip casting

High strength Al 7075 alloy was manufactured via twin-roll strip casting (TRC) process, and its microstructure, tensile and fatigue properties were investigated. Commercial Al 7075 alloy fabricated via direct-chill (DC) casting process was also used for comparison. T6 and T651 heat treatments were implemented for both the materials to precipitate MgZn2(η) phases. The TRC alloy showed globular grain shape with η phases evenly distributed at the grain boundary and matrix interior, while the DC alloy showed elongated grains due to hot forging process and η phases clumped together at grain boundaries. The TRC alloy’s yield strength was 518.5 MPa, tensile strength was 578.2 MPa, and elongation was 6.9%. Comparing both the alloys’ mechanical properties, the TRC alloy’s strength was at least about 40 MPa higher and elongation was about 4% lower than the DC alloy. The TRC alloy showed about 20 MPa higher fatigue (fatigue limit) than the DC alloy. The DC alloy was observed to have coarse cracks on fatigue fractured surface. By contrast, the TRC alloy showed uniform fractured surface without coarse cracks, and the evenly-distributed η phases improved fatigue resistance efficiently. The present study sought to examine the correlation among TRC process-led microstructure, tensile and high-cycle fatigue properties while discussing the strengthening mechanism.

Zr55Cu30Al10Ni5 amorphous alloy sheets with large plasticity fabricated by twin-roll strip casting

Zr55Cu30Al10Ni5 (at.%) amorphous sheets with thickness of ~0.5 mm and width of ~75 mm were successfully fabricated by twin-roll strip casting. The amorphous alloy sheets fabricated using a high rolling force exhibited a high-energy state, a high free-volume content, and structural inhomogeneity, which led to an engineering strain greater than 62% in compression at room temperature. This study provides an effective method for fabricating large amorphous alloy sheets with good plasticity.

Intrinsic properties, phase constitution and microstructure of novel RE-Fe-B strip-casting alloys based on misch metal

In order to reduce the RE-Fe-B material cost and balance the utilization of rare earth sources, misch metal substitution for Nd in the sintered magnet becomes of many interests. Under the process of fabricating high magnetic properties magnet, the strip-casting (SC) technique plays a key role, a series of rapidly solidified [(PrNd)1-xMMx]13.8FebalCo0.5Cu0.1Al0.6Ga0.07B5.9 (x = 0 ~ 1.00) (MM, Misch Metal) strips were manufactured. Subsequently, the phase constitution, microstructure and the intrinsic magnetic properties were measured. There are always stable RE2Fe14B phase in the strips with MM, which is similar to Nd-Fe-B strip. Whereas, when x ≥ 0.50, there exists REFe2 phase inevitably, and the amount of REFe2 phase gradually increases with increasing MM content. Magnetic measurements indicated that intrinsic magnetic properties, such as the saturation magnetization Ms and magneto-anisotropy field HA, deteriorate gradually by substituting PrNd with MM except for x = 0.35. For x = 0.35, the abnormal phenomenon is ascribed to Ce valence shifting toward +3 that is caused by lattice expanding. In addition, when x ≥ 0.50, La and Ce is prior to aggregate in the RE-rich phase clusters, and the content of Fe rises with the increase of MM, moreover, the interfaces between of RE2Fe14B phase and RE-rich phase become indistinct.

Effects of boron content on the microstructure and mechanical properties of twin-roll strip casting borated steel sheets

High borated steel sheets containing 0.25 wt% to 4.0 wt% boron including hypoeutectic (0.28 wt% B and 2.11 wt% B), eutectic (2.43 wt% B) and hypereutectic (4.01 wt% B) compositions were tried to be fabricated by a novel strip casting technology, and the effects of boron content on sub-rapid solidification behavior and subsequent microstructural evolution together with mechanical properties was studied. It was found that the morphology of borides depended greatly on the boron content. When increasing boron, the morphological change of borides was network-like → grainy → cluster-like → plate-like. Various orientation relationships between different morphological borides and γ-Fe in as-cast steels were also identified. After subsequent hot-rolling and solution-treating, ultra-fine (<10 μm) borides were obtained in hypoeutectic and eutectic steels. Benefiting from that, excellent mechanical properties was achieved, which was much better than that of steels prepared by traditional ingot casting. Particularly, when the boron content approached eutectic composition, of which the effect on the microstructure and mechanical properties was more sensitive. The steels containing 2.11 wt% and 2.43 wt% boron exhibited different morphological borides with a total elongation of 14.1% and 8.0%, respectively. Moreover, a thin hypereutectic borated steel sheet was also prepared, but the borides were very large and the total elongation was very low. The correlation between microstructure and strength was clarified based on the Hall-Petch behavior and intermetallic strengthening effect. Smaller γ-grains and higher volume fraction of borides were responsible for higher strength. The plastic behavior of steels was influenced by strain-hardening rate that was determined by the volume fraction of γ-Fe matrix. A better sustainable strain-hardening capability was beneficial to extend the stage of uniform deformation and enhance the ductility. In addition, the mechanical properties of steels were also decided by the fracture mechanism that was dominated by the size of borides.

Size effects of Y-enriched intermetallic precipitates in Fe-20Cr-5Al-0.25Y alloy on the spallation resistance of thermally grown α-Al2O3 scales

Standard ingot casting and twin-roll strip casting (TRSC) were used to process Fe-20Cr-5Al-0.25Y (wt.%) alloy, in which Y-enriched intermetallic precipitates with different sizes had been formed. Cyclic oxidation tests were carried out at 1200℃ in air, and the size effects of precipitates in alloy on the spallation resistance of thermally grown α-Al2O3 scales were investigated. The results clearly demonstrated that the spallation resistance of α-Al2O3 scales could be improved by refining the Y-enriched precipitates, which was attributed to the fact that the small precipitates can generate high density of interfacial oxides to effectively key the alumina scale to the substrate.

Secondary Recrystallization Behavior in Fe-3%Si Grain-oriented Silicon Steel Produced by Twin-roll Casting and Simplified Secondary Annealing

Grain-oriented silicon steels were produced by the shortest processing route involving twin-roll strip casting, two-stage cold rolling with intermediate annealing, and simulated continuous annealing. The secondary recrystallization behavior of grain-oriented silicon steels under different inhibition conditions was in-situ observed by combining the confocal laser scanning microscopy (CLSM) and electron backscattered diffraction (EBSD) techniques. The results revealed that the optimal temperature of secondary recrystallization showed a proportional relationship with the Zenner pinning force. In the case of weak pinning force, the abnormal grain growth occurred quickly at ~1050 °C. The corresponding growth rates were in the range of 60–1400 μm/min and decreased gradually as the secondary recrystallization proceeded. In the case of strong pinning force, the incubation time and onset temperature of the secondary recrystallization was significantly increased, but the total time of the secondary recrystallization was obviously shortened from 685 s to 479 s, and the final magnetic induction of B8 was increased from 1.7 T to 1.85 T. After the secondary annealing, some island grains and coarse primary grains were retained. The formation of island grain was related to the low migration of grain boundaries. The findings of coarse γ- grains indicated that the primary grain size also played a crucial role during secondary recrystallization, apart from the primary recrystallized texture, which attracted more attention previously.

Casting of High Aluminum Content AM Series Magnesium Alloys by Using a Horizontal Twin Roll Caster

In this paper, twin roll casting of magnesium alloys with high aluminum content such as, Mg-11 mass%Al-0.2 mass%Mn, Mg-12 mass%Al-0.2 mass%Mn, Mg-13 mass%Al-0.2 mass% have been performed for the purpose of use as an original material for hot forging. Also the mechanical properties of the cast materials were examined. A 10 miri-meters thick strip was cast by the use of a horizontal twin roll caster. The microscopic observation was conducted to investigate into the precipitation of the metal compounds such as Mg17Al12, and the Vickers hardness of the cast strips test were performed. From the result of the roll casting experiments, a 10 mm thick strip was continuously cast at a roll speed of 3.1 m/min. The average grain size of the casting strips was about 46 micron meters. When the aluminum content was 13%, the hardness of the twin roll cast (TRC) strips became 1.7 times higher than that of extruded AZ 31. Also, a uniaxial compression test at elevated temperature was conducted to obtain a true strain-true stress curve for examining possibility of direct hot forging (DHF) of TRC magnesium alloys with high aluminum content.

Numerical Modeling of Transport Phenomena in the Horizontal Single Belt Casting (HSBC) Process for the Production of AA6111 Aluminum Alloy Strip

In this research study, numerical modelling and experimental casting of AA6111 strips, 250 mm wide, 6 mm thick, was conducted. The velocity of the molten AA6111 alloy at the nozzle slot outlet was raised to 2 m/s, whilst the belt speed was kept at 0.3 m/s. The numerical model demonstrates considerable turbulence/fluctuations in the flow of the molten AA6111 alloy in the HSBC process, rendering its free surface highly non-uniform and uneven. These discontinuites in the flow resulted from the sudden impact of molten metal onto the inclined refractory plane, and then onto the slowly moving belt. However, it has been determined that these surface variations are rapidly damped, and as such are not detrimental to final strip surface quality. Any surface perturbations remaining can be eliminated via hot plastic deformation. The experimental findings are in accordance with the model predictions. Furthermore, at high metal heads inside the delivery launder, the molten metal was observed to be flowing inwards towards the center of the strip, thereby filling the centre depression region, formed otherwise. The model predictions were validated against experimental findings. A surface roughness and microstructural analysis was also conducted to determine the surface and bulk quality of the as-cast strip.

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.

Research on cast-rolling force in twin-roll continuous strip casting based on uncertainty analysis

The uncertainties of various parameters, including casting temperature, rolling velocity and roll gap are investigated in twin-roll continuous strip casting process for S304 stainless steel. A sample-based stochastic model based on Latin hypercube sampling is established to evaluate the influence of various uncertain parameters on the height of Kiss Point and the cast-rolling force. A polynomial mathematical model combined with numerical simulation is built with full factorial design approach. The convergence of variance is used to represent the variability of the input parameters, and Inter Quartile Range (IQR) is adopted to qualify the uncertainty of output parameters. The IQR analysis shows that casting temperature is the most dominant factor among all output parameters including roll gap and rolling velocity.

Revealing the recrystallization behavior of an excellent strip casting 4.5 wt% Si non-oriented electrical steel

An excellent 4.5 wt%Si non-oriented electrical steel was fabricated by warm rolling and isothermal annealing of twin-roll cast strip. The detailed evolution of microstructure and texture was studied using the combination of electron back-scattering diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscope (TEM) for revealing the recrystallization behavior. The favorable {100}〈013〉 nuclei originate in the shear bands within deformed {111}⟨11-2⟩ and {110}⟨1-10⟩ grains. Based on the EBSD analysis, oriented nucleation is the predominant mechanism for the formation of strong {100}⟨013⟩ texture. The rearrangement of dislocations and cell transformation to sub-grains were characterized. The mechanisms of (sub-)grain coalescence, as well as strain induced grain boundary migration, take effect simultaneously, during the nucleation.

Mg-Based Materials with Quasiamorphous Phase Produced by Vertical Twin-Roll Casting Process

Metallic materials with micron grains, submicron grains, or amorphous structures have attracted great interest in recent decades owing to their excellent mechanical properties and corrosion resistance. Compared with traditional forming processes, rapid solidification technology has shown great superiority and potential in the preparation of materials in such structures. In this study, fine-grained quasiamorphous Mg-based alloy strips fabricated by a twin-roll strip casting process were explored using simulation and experimental methods. The concept of critical casting speed was proposed to reflect the optimum casting conditions. The product of critical casting speed and strip thickness was used to evaluate the cooling capacity of the casting system. Based on simulation results, a twin-roll strip-casting experiment was performed on a Mg-rare earth alloy. A novel puddle-like microstructure of the as-cast alloy strip was obtained. Tensile testing results showed that the novel strip exhibited improved ductility.