2020 Volume 60 Issue 10 Pages 2165-2175
Complex section products have significant lightweight and functional characteristics, and the twin-roll casting (TRC) is characterized by high efficiency and short flow. Hence, the TRC process for fabricating complex section products combines both advantages and will have a broader market demand and application prospect. In this paper, the research progress in recent years is reviewed, and novel TRC processes are divided into three categories according to the product section characteristics, namely transverse variable profiled strip, longitudinal variable profiled strip, and circular section products. The essence of the TRC process of complex section products is to change the steady-state characteristics in time series and the uniform characteristics in spatial distribution into transient or nonuniform. The technical principle, deformation characteristics, influence mechanism, are systematically analyzed. The current challenges and future directions of the TRC process are discussed.
The twin-roll casting (TRC) process is a near-ending forming technology combining rapid solidification and hot rolling deformation. Compared with other processes, it has advantages such as short flow, low energy consumption, low production cost, etc., which is regarded as the most revolutionary technology in the metallurgical industry in the 21st century.1,2,3) At present, the most representative TRC processes for steel making include the Castrip of United States, the Eurostrip of Europe, the Postrip of Korea, and the Hikar of Japan, among which the Castrip has the highest level of industrialization.4) In China, The Baosteel Group used 15 years to develop critical technologies, and the Baostrip was successfully commercialized in 2014. The Shagang Group realized industrial production through the introduction of the Castrip and independent innovation in 2019. The total energy consumption is expected to be reduced to 1/5 of that of the hot rolling process, and the carbon dioxide emission is expected to be reduced to 1/4. Compared with steel making, non-ferrous metal making is more industrialized. Taking Al alloys as an example, the industrial production of part of 1xxx, 3xxx, 5xxx, 6xxx, and 8xxx series has been realized.5,6) Hence, with the development of green and sustainable industrial processes, the TRC process has become an internationally recognized industry focus.
Laminated metal clad material (LMCM) is a new type of structural and functional material formed by metal components with different physical and mechanical properties and bonded in the interface.7,8,9) They have the excellent features of each metal component and can be widely used in power electronics, bridge construction, marine engineering, aerospace engineering, etc. The development of the TRC process in producing single metal strips provides a practical basis for developing novel TRC processes of LMCMs. It is expected to solve the problems, such as severe overcapacity of single metal materials, unreasonable product structure, and insufficient supply in high-quality application.10)
In recent years, a large amount of research work has been conducted on fabricating LMCMs by TRC processes, mainly focusing on the process layout scheme, numerical simulation method, metal component collocation, and product quality control. Haga et al.11,12,13,14,15,16) carried out experimental studies to develop novel TRC processes using both liquid substrate and liquid cladding, such as single-roll caster, twin-roll caster, three-roll caster, and multistage tandem caster, etc. The process stability and the critical technical problems of high-speed TRC processes were analyzed. Bimetallic, trimetallic, and polymetallic clad strips were successfully prepared, and the speed reached 16–30 m/min. Huang et al.17,18,19,20) studied numerical simulation methods of TRC processes and established a transient thermal-mechanical coupling model and a steady-state thermal-flow coupling model. The distribution of stress field, temperature field, flow field was revealed, and Cu/Al, Ti/Al, Invar/Cu clad strips were successfully prepared. Vidoni et al.21) fabricated austenitic stainless steel/carbon steel clad strip and characterized the microstructure, shear strength, and diffusion layer thickness of the bonding interface. Subsequently, Munster et al.22,23) fabricated high manganese steel/austenitic stainless steel clad strip with 25 meters long and studied the effect of rolling and heat treatment on the interface bonding strength. In the future, the secondary formability and deep processing performance of LMCMs will become a research focus.24,25,26)
At present, most current researches on the TRC process focus on the single metal strip or clad strip. Hence, the section of the products is rectangular. Complex section products, non-rectangular section products, have significant lightweight and functional characteristics, which can be widely used in aerospace engineering, rail transit, etc. The TRC processes for producing complex section products have the dual advantages of complex section products and the TRC process, which will have a broader market demand and application prospects. Based on the present research work, the TRC processes for producing complex section products are divided into three categories. Process characteristics, technical difficulties, and product performance are systematically analyzed, and future directions are discussed.
Transverse variable profiled (TVP) strip refers to the strip section changes along the width direction (WD), which is perpendicular to the rolling direction (RD). The product section is determined by the grooves formed by rolls. Hence, the essence is that the grooves changes along the WD. The most common method is to machine grooves along the WD on the roll surface,27) as shown in Fig. 1(a). Besides, the profiled strip mold can be preloaded on the traditional roll surface,28) as shown in Fig. 1(b). Although the groove is not evenly distributed along the WD, each segment along the RD can be simplified to the traditional rolling process at the stable state.
Vidoni et al.29) studied the influence of symmetry grooves on the TRC process. TVP steel strips with a thickness difference of 1 mm between the edge and the center and a transition angle of 45° were successfully produced. No defects such as hot melt and crack exist on the strip surface. However, the microstructure of thick-zone is different from that of thin-zone, resulting in the uneven performance along the WD. Based on this basis, Daamen et al.30) optimized the geometric parameters of the grooves. The results show that the dimensional accuracy and microstructure uniformity can be improved effectively by reducing the thickness difference between thick-zone and thin-zone and by adopting a smoother transition angle. Furthermore, Daamen et al.31) compared the advantages and disadvantages of welding, rolling, and TRC. It indicates that the maximum constraint of the TRC process depends on the maximum thickness difference between the thick-zone and the thin-zone, which can be achieved when the product performance meets the requirements. Through numerical simulations, Vidoni et al.27,32) found that the temperature distribution and microstructure uniformity can be improved by regulating the interfacial heat transfer coefficient between the roll and the molten melt. The roll surface was selectively sprayed, that is, NiCr coating was sprayed on the roll surface in the thin-zone to reduce the thermal conductivity. The results of the infrared thermal imager show that the temperature uniformity of thick-zone and thin-zone was significantly improved. Besides, with the subsequent hot rolling, the shrinkage porosity can be substantially reduced, and the dimensional accuracy and mechanical properties of the TVP strips can be improved.
In the traditional TRC process, the cast-rolling zone is approximate V-shaped and evenly distributed along the WD. The kissing point (KP) is the interface of solid (semi-solid) and liquid. It is the core of the process control, but an indirect factor that affects product performance. In the thermoplastic deformation process, the temperature, strain, and strain rate are the direct factors. Therefore, the KP in the cast-rolling zone is uniform when the flow, heat transfer and solidification are controlled uniformly along the WD. The essence is that the temperature, strain and strain rate are uniform, thus ensuring the product performance uniformity, such as dimensional accuracy, surface quality, microstructures, mechanical properties, etc. For the TRC process of TVP strips, the cast-rolling zone is not evenly distributed along the WD due to the grooves. Hence, the uniformity of the KP is not equivalent to the uniformity of the temperature, strain, and strain rate in the deformation process. Due to the nonuniform geometry along the WD, the most significant technical difficulty is to control the metal flow, heat transfer, and solidification to ensure the uniformity of temperature, strain, and strain rate. That is also the key to ensure product performance uniformity. At present, the decrease of thickness difference and the optimization of interfacial heat transfer coefficient are helpful to improve the KP uniformity. However, the KP control strategy under intricate grooves still needs to be further explored.
Dong33) attempted to fabricate TVP clad strips by the TRC process. Only one roll was machined with grooves to construct the asymmetric cast-rolling zone, as shown in Fig. 2(a). The solid Cu strip and liquid Al were fed into the cast-rolling zone simultaneously. Cu/Al clad strips were successfully prepared, which verified the feasibility, as shown in Fig. 2(b). The technical essentials of fabricating clad strips are different from those of fabricating single metal strips. In addition to ensuring the mechanical property uniformity of each metal component, the interfacial bonding strength uniformity also needs to be ensured. However, the interfacial bonding strength is closely related to interfacial element reaction-diffusion, temperature, rolling pressure, etc. Hence, product performance uniformity is more difficult to control during the fabrication of TVP clad strips.
Based on the TRC process, Song34) used asymmetric grooves to fabricate SiCp/Al particle-reinforced composite strips. Compared with fabricating single metal strips, surface cracks are more likely to occur in fabricating composite strips. Combined with 3-D thermal-flow coupling simulation results, local roll surface coating technology was adopted to improve the temperature distribution uniformity of the cast-rolling zone. SiCp/Al particle-reinforced composite strips with excellent surface quality were successfully prepared, as shown in Fig. 2(c). In general, particle-reinforced or fiber-reinforced composite material usually has a higher viscosity than single metal at the same temperature. That leads to a deterioration in the liquidity. Moreover, the distribution uniformity and distribution orientation of the reinforcement have a significant influence on product performance. Hence the metal flow, heat transfer, and solidification in the cast-rolling zone are more challenging to control when casting composite materials.
From entering the cast-rolling zone to altogether leaving the cast-rolling zone, the molten melt undergoes two processes: solidification and plastic deformation. The vertical TRC can fabricate Al alloy at the high-speed, but the contact status between the roll and the molten melt would deteriorate. Therefore, crack, surface pit or other defects are prone to occur on the product surface. Yamashiki et al.35) studied the effect of V-shaped micro-groove on improving the contact status, as shown in Fig. 3. Stable production was achieved with V-shaped micro-groove width of 0.45 mm, depth of 0.2 mm, and spacing of 0.1 mm. There are three types of initial contact status between the micro-groove and the molten melt, which are full-fill, half-fill, and un-fill. Full-fill causes the solidified metal to be unable to separate from the roll when leaving the cast-rolling zone, resulting in sticky issues. The heat transfer between the roll and molten melt depends on the contact area. Hence, un-fill causes leak issues because the contact area is minimal. Therefore, half-fill under the action of surface tension is the ideal state. Besides, micro-plastic deformation will occur below the KP, which is conducive to further improving the surface quality.
Schematic diagram of the TRC process of TVP strips with micro-grooves.35) (Online version in color.)
Longitudinal variable profiled (LVP) strip refers to the strip section changes along the RD. The traditional roll section is circular, and the position of the roll can be approximately considered to be constant. Therefore, the essence of fabricating TVP strips is to change the contact status between the roll and the workpiece, which means that the traditional steady rolling process needs to be altered to a transient rolling process. At present, there are mainly four methods to fabricate LVP strips.
The first method is to machine corrugated curves along the circumference of one roll or two rolls. Haga et al.36) studied the influence of micro-groove direction on the surface quality, such as RD, WD or cross direction, etc. The results indicate that the strip with surface texture can be produced directly by the TRC process, and the surface texture can be flattened without cracking by the subsequent cold rolling process, as shown in Fig. 4. Combining the research of Yamashiki et al.,35) it can be inferred that the reasonable micro-groove on the roll surface can improve the contact status between the roll and molten melt. That is advantageous to inhibit surface crack, and it could help to solve the production problem of alloys, which are challenging to be produced by the TRC process in the past.
Schematic diagram of the TRC process of LVP strips with micro-grooves.36) (Online version in color.)
The second method is that the two rolls are cylinders, but a rigid mold with waves along the RD is used during the rolling. In the TRC process of LMCM, the rigid mold with waves along the RD can be regarded as the substrate strip with microscopic surface roughness. Stolbchenko et al.37) developed an online polishing device for fabricating steel/Al clad strips. The initial surface roughness Ra of the steel strip is 0.5 μm, and other surface roughness of 4.2, 10.9, 19.8, and 22.1 μm can be obtained by changing the sandpaper specification. The electron probe microanalysis (EPMA) results show that the polishing improved the purity of the steel/Al interface, as shown in Fig. 5(a). The interfacial bonding strength with the surface roughness of 4.2 μm is higher than that of the unpolished surface. However, the interfacial bonding strength tends to decrease with the continuous increase of surface roughness. The main reason is that the deep grooves with sharp edges are dominant on the surfaces with high surface roughness. Similar to the effect of the micro-groove on the roll surface, the contact status at the solid-liquid interface and the reaction-diffusion under high temperature and rolling pressure at the solid-solid interface will become worse.
Consider the effect of deformation, Ji38) analyzed the influence of the substrate surface roughness on the interfacial bonding process, as shown in Fig. 5(b). In the non-deformation region, when the surface groove is small, good contact can be achieved. When the surface groove is large, there is a dead zone of metal flow near the bottom of the groove. Moreover, porosity defects may occur in the solidification process due to the uneven filling of liquid metal. During the severe plastic deformation process, the metal embedded in the groove will undergo shear fracture under the action of interfacial slip. When the groove is half-filled, there is a tendency to push the cladding into the groove under interfacial pressure in the subsequent deformation process. However, the substrate at the edge of the groove will produce the reaction force, which will hinder that tendency. Besides, due to the interfacial slip, the peak of the groove is ground, and the interface diffusion layer is broken, resulting in inclusion particles. The particles will move with the interfacial slip, and they will accumulate and form defects when these particles reach the unfilled grooves.
To sum up, the metal in the cast-rolling zone undergoes physical state transition of liquid, semi-solid and solid, and undergoes significant plastic deformation under high temperature and rolling pressure. Hence, the TRC process belongs to the typical thermal-fluid-mechanical-microstructure multi-field coupling problem, which needs to analyze the problem and explain the mechanism from multiple angles. As mentioned before, the flow, heat transfer, and solidification in the TRC process determine the temperature, strain, and strain rate of the deformation process, and then determine the product performance. The micro-groove on the roll surface or the surface roughness of the substrate can be regarded as the rough surface at different sizes. Therefore, the micro contact status of the solid-liquid interface under the action of the surface tension and the micro plastic deformation mechanism of the solid-solid interface under the high temperature and rolling pressure are two essential technical challenges. At present, the interaction between micro contact behavior of the solid-liquid interface and flow, heat transfer, and solidification has not been revealed. The relationship between the micro plastic deformation mechanism of the solid-solid interface and the interfacial bonding criterion and the crack stopping mechanism has not been clarified. Therefore, based on the theories of interfacial wetting, heat and mass transfer, and micro-forming, etc., the theory system of interface microtopography customization oriented by demand will be a valuable research direction to fine control the product performance. For example, the micro-groove on the roll surface is used to form the surface texture on strips while ensuring the normal separation of the roll and the strip. However, in the TRC process of LMCMs, the substrate and the cladding need to form a complete interfacial bonding without separation or microcosmic defects.
In the preparation of LMCMs, both TRC and rolling have serious problems of warping due to the deformation discordance between the substrate and the cladding. Similar to the first method, Wang et al.39) proposed a corrugated + flat rolling (CFR) process for bimetallic clad strips based on macro-waves, including two stages, as shown in Fig. 6(a). Firstly, a corrugated roll and a flat roll are used for corrugated rolling to prepare the workpiece with macro-waves. Then, two flat rolls are used for flat rolling to flatten the workpiece with macro-waves again. Wang et al.40,41) successfully prepared flat and straight AZ31B/5052 clad strip at 400°C by at the pass reduction rate of 35% and 30%, respectively. The first pass is to construct macro-waves, as shown in Fig. 6(b), and the second pass is to eliminate marco-waves, as shown in Fig. 6(c). Hence, the initial interfacial bonding can be realized through the first pass, and the complete interfacial bonding can be realized through the second pass. The cache and release of the marco-wave play a crucial role in coordinating the plastic deformation of substrate and cladding, and it is better to deploy the metal component that is difficult to deform at the corrugated roll side. The strong shear deformation between peaks and troughs can refine grains, improve texture, improve deformation coordination, and reduce residual stress. Besides, the formed spatial bonding interface would contribute to improving the contact area, as shown in Fig. 6(c). All the factors mentioned above work together to improve product performance.
Similar to the second method, Wang et al.42) proposed a rolling process assisted with corrugated mold, as shown in Fig. 7(a). Experimental results indicate that the AT63 Mg alloy strip prepared by this method has no obvious crack on edges. The basal texture can be effectively weakened to obtain higher fracture elongation and strong strain hardening. The mechanism weakening the basal texture was revealed. During traditional rolling, the stress exerted on the workpiece by the upper and lower rolls is symmetrically distributed, resulting in symmetrical deformation. Hence, the workpiece possesses the intense basal texture, and the basal poles parallel to the ND, as shown in Fig. 7(b). In Fig. 7(c), a corrugated mold is placed between the bottom surface of the workpiece and the lower roll in the first pass. The stress applied to the workpiece by the upper roll and the corrugated mold is asymmetric, resulting in asymmetric deformation. As a result, the asymmetric stresses promote a much larger texture spreading along RD and significantly reduced texture intensities. The second pass is similar to the normal rolling, but it is also an asymmetric deformation process because the workpiece is corrugated. Therefore, after two-pass rolling, the texture of the workpiece is much weaker than that of the traditional rolling process. Sun et al.43) and Chen et al.44) used similar methods to fabricate AZ31 alloy strips and Al/Ti/Al clad strips and obtained similar results.
Rolling process assisted with corrugated mold: (a) schematic diagram, (b) first pass, (c) second pass.42) (Online version in color.)
The third method is that the two rolls are cylinders, and one of the rolls is raised and pressed repeatedly to change the thickness of the roll gap. There are two kinds of roll gap control modes in the TRC process. One is the constant roll gap control mode, that is, the position of the roll is not changed during the TRC process to ensure the strip thickness is not changed. The other is the constant rolling force control mode, that is, the rolling force is constant during the TRC process. Rolling force mainly depends on KP, roll gap, and deformation resistance, etc. When the change of process parameters causes the fluctuation of KP, it is necessary to change the roll gap to maintain constant rolling force. Hence, the position of one roll is fine-tuned. The constant rolling force control mode is similar to the third method, but in the TRC process, that mode is usually used to ensure the uniformity of strip thickness rather than used to produce LVP strips. However, the rolling process, similar to the third method, has made significant progress in fabricating LVP strips in recent years.45,46,47)
Du et al.48,49,50) proposed a vibration TRC process, as shown in Fig. 8(a). The two rolls rotate in reverse, and the roll on the right reciprocates with micro-amplitude vibration in the vertical direction. Figure 8(b) shows the macrostructure of the Al alloy in the cast-rolling zone. Under the condition of non-vibration, the columnar crystal was very developed. The strip presented anisotropy, and the centerline macrosegregation was observed. With the vibration amplitude of 0.2 mm and frequency 30 Hz, the growth trend of grains along the vertical direction of the roll surface was terminated. Bulky columnar crystal size decreased substantially, and fine equiaxed grains appeared at the center of the molten pool. Figure 8(c) shows the steel strip prepared at different vibration frequencies. The steel strip has obvious vibration marks, which can be completely eliminated in subsequent finishing rolling. The essence of the vibration TRC process is asymmetric deformation. In the liquid zone of the molten pool, the high-frequency vibration of the roll has the effect of the disturbance, which stops the grain growth trend along the normal direction of the roll surface. Except for the mechanical vibration, a similar effect can also be obtained by electromagnetic stirring,51) ultrasonic vibration,52) etc. In the solid zone of the molten pool, reciprocating rolling can strengthen the shear deformation and promote the dynamic recrystallization and grain refinement. Therefore, the sizeable columnar crystal structure decreases, and the small equiaxed crystal structure increases, to inhibit centerline macrosegregation and improve product performance. Grydin et al.53) constructed the asymmetric TRC process by driving only one roll and obtained the similar conclusion.
The fourth method is to change the product specifications by adjusting the delivery device. Smith et al.54) proposed a Fata Hunter Optiflow system to achieve online replacement of product specifications. By separating the nozzle from the side dam, the side dam can slide into the nozzle and move horizontally along the WD. At the maximum speed of 1.5 mm/s, a width increase of 200 mm incrementally lasted for 2 hours without stopping, verifying the feasibility of the process. However, the problems that may be encountered after increasing the speed have not been reported. For example, the effect of movement on seal reliability, side dam lifetime, and KP position is not clear yet. Martin et al.55) proposed a variable-width TRC technology based on the electromagnetic side dam. Online adjustment of the strip width was realized by switching the unilateral static electromagnetic side dam. Experimental results revealed that there is an inevitable delay when the electromagnetic side dam is opened, but the instantaneous response when the electromagnetic side dam is closed. Besides, the electromagnetic side dam could also produce the electromagnetic stirring effect on the liquid metal in the cast-rolling zone. Compared with moving mechanical side dams, the switching of the electromagnetic side dam is more responsive and has more application prospects. However, there are still many important problems that need to be solved in the industrial production, such as the precise control of the KP, layout scheme of the electromagnetic side dam, optimization of the response speed, distribution of the temperature field, etc.
Based on the traditional TRC process, Ji et al.56,57) proposed a method for fabricating LMCMs with circular section, as shown in Fig. 9. Rolls possess round grooves, and the cast-rolling zone is formed by rolls and conformal mold. The solid pipe or bar is used as the substrate, which is fed into grooves by the guiding device. A special annular delivery device is used to cast the molten melt continuously and evenly into the cast-rolling zone. Under the combined action of high temperature and rolling pressure, a reliable interfacial bonding can be formed between the substrate and cladding.
Schematic diagram of the TRC process of LMCMs with circular section.57) (Online version in color.)
The section evolution of the cast-rolling zone was obtained through an emergency stop and quick cooling method,58) as shown in Fig. 10. The thickness of cladding at ND and TD is asymmetric. Above the KP, the fluidity of the molten melt ensures the continuity of the process. Below the KP, the cladding metal is forced to undergo significant plastic deformation as the roll rotates. However, the plastic deformation is mainly concentrated at the ND side, while there is no direct plastic deformation at the TD side. Hence, the metal would flow from the ND side to the TD side. The extrusion pressure caused by the metal flow at the TD side is the main reason to promote the interfacial bonding, which is weaker than that at the ND side.
Schematic diagram of the forming mechanism of the TRC process of circular section products.58) (Online version in color.)
Therefore, the forming mechanism that the circular section products can be produced by the TRC process can be summarized as the fluidity of molten melt and plastic deformation flow of solid metal. Besides, the section of the substrate and the cladding is annulus or circular. Even if only mechanical interlocking forms between the substrate and cladding, they would not separate under general service conditions due to the contact stress between the inner layer and outer layer. Hence, products can be used as finished products or as raw materials for subsequent deep processing.
However, two problems need further analysis and optimization. The first problem is that when fabricating bimetallic clad pipes, the substrate is a hollow pipe. The flattening of the pipe may happen when the rolling force is too large.59) Therefore, the critical flattening condition of the substrate needs to be studied in the future. The second question is that the nonuniform distribution of the geometry, heat transfer, plastic deformation along the circumference will cause the nonuniform of the mechanical properties, microstructures, interfacial bonding strength.60) The product can meet the general purpose, such as mechanical structure, heat exchangers, etc. However, it is challenging to meet the technical requirements of performance uniformity under special service conditions, such as railway power transmission line, transmission pipeline of the nuclear power plant, etc. Therefore, the control strategy of product performance uniformity based on the forming mechanism needs to be developed.
The shape and size of the product depend on the grooves formed by rolls, such as circle, rectangle, square, etc. However, for large size castings, it is difficult to establish a continuous process because molten melt may not solidify in the cast-rolling zone. Sidelnikov et al.61) suggested a casting-rolling-extrusion integration process, through arranging a cooling crystallizer above the rolls and an extrusion mold below the rolls. The molten melt experiences controlled-solidification in the cooling crystallizer, high-speed deformation between the rolls, and it is finally extruded by the extrusion mold. The workpiece can be processed either in the pure solid state, as shown in Fig. 11(a), or in the semi-solid state, as shown in Fig. 11(b).
Schematic diagram of the casting-rolling-extrusion integration process: (a) in solid state, (b) in semisolid state.61)
The TRC process is a part of the whole production process. There are pretreatment processes, such as rotating mold or electromagnetic mold, or deep processing technologies, such as rolling, stamping, etc. Hence, the process of pouring molten melt directly into the cast-rolling zone is called the TRC-extrusion process. This process can be used to produce complex section bars, as shown in Fig. 12(a), or hollow section pipes, as shown in Fig. 12(b). The experimental results show that the 6082 alloy bar prepared by the TRC-extrusion method can meet the requirements of EN 755-2 standard after heat treatment. The wires prepared on this basis can meet the requirements of EN 754-2. Therefore, the production cost of the TRC-extrusion process is lower than that of the traditional hydraulic extrusion process, which is suitable for industrial production and has a good development prospect in the future.
Schematic diagram of the TRC-extrusion: (a) solid product, (b) hollow product.61)
Through a large number of theoretical and experimental studies, the feasibility and superiority of the TRC process for fabricating complex section products have been proved. It has attracted considerable attention from the industry and has become one of the hot research directions. The TRC process is mainly determined by two parts, namely the forming mold and the casting system. The forming mold primarily consists of the rolls, which coordinate with each other. The casting system mainly consists of the delivery device and the side dam. The essence of the TRC process for fabricating complex section products is to break the steady-state characteristics in time series and the uniform characteristics in spatial distribution and transform them into the transient characteristics in time series or the nonuniform in spatial distribution. Therefore, the current implementation methods can be summarized as changing the grooves or profiles of the rolls, layout scheme of the rolls, movement of the rolls, coordinating control strategy between the delivery device and the side dam, etc.
From the perspective of deformation, the TRC process of complex section products is a nonuniform deformation in time series or spatial distribution, which has become an important development direction in the field of metal processing. With the improvement of product performance requirements and the promotion of green and sustainable development industry, the TRC process of complex section products will have more substantial market competitiveness. However, as a link in the whole industrial chain, there are still several key issues that need to be solved before it can be fully put into industrial production and integrated into the industrial chain. The main development trends in the future are as follows:
(1) Expand the range of materials that can be fabricated by the TRC process and improve product performance.
At present, the TRC process of Al alloy in industrial production has been relatively mature. However, due to the limited length of the crystallization zone and cooling capacity, it is impossible to produce alloys with a significant temperature difference between solidus and liquidus through process optimization. Hence, there are only a few varieties that can be produced by the TRC process. Besides, only a handful of companies in the world have mastered the TRC technology of steel, and their products are mainly traditional carbon steels. Therefore, it is a crucial problem to expand the product category. The TRC process has significant advantages in production cost and cycle, but deep processing performance of TRC products is usually worse than that of hot-rolled products. Most of TRC products are generally used as billets, which require further processing. Therefore, it is equally important to improve the comprehensive properties of TRC products.
Many metals are still facing the problem of difficult processing. For example, single metals with body-centered-cubic structures, such as Mg and Ti, are prone to cracking and presents significant anisotropy during plastic processing. LMCMs have significant deformation incongruity and residual stress, due to the substantial difference in mechanical properties between the substrate and the cladding. The particle-reinforced or fiber-reinforced metals have high strength but low plasticity and poor secondary formability. The nonuniform deformation under high temperature and high pressure is expected to solve the above problems.
(2) Establish the complete theoretical system of nonuniform plastic forming.
Most of the traditional metal plastic processing techniques can be simplified into plane stress or plane strain problems to simplify the analysis process. However, the core of nonuniform plastic forming is a complex multi-direction stress deformation condition formed by the nonuniform physical state, temperature field, deformation boundary, contact conditions, etc. that change with time series. Traditional rolling theory can not describe all its characteristics. Hence, it is urgent to establish the basic theoretical system of nonuniform plastic forming. The KP is the core of the process stability control, but it cannot be directly obtained due to the shielding of the side dam. The approximate position of the KP can be inferred according to the rolling force. However, to control the KP accurately, it is necessary to construct the semisolid-solid rheological constitutive equation and analyze the flow field and temperature field. Besides, the analysis of the asymmetric deformation boundary is helpful to reveal the generation mechanism of warping defects and residual stress, which is the foundation of the coordinate deformation control theory. Then, the toughening mechanism of single metals and interfacial bonding mechanism of LMCMs need to be revealed to lay the foundation of product performance control.
(3) Control the microstructure-property uniformity under nonuniform boundary conditions.
The nonuniform geometric boundary inevitably leads to the nonuniform heat transfer boundary, which determines the solidification process and finally affects the quality of the strip. Therefore, the corresponding characterization method of product performance uniformity of complex section products needs to be formulated. Besides, the microstructure-property uniformity control strategy under nonuniform boundary conditions needs to be established based on the cooling channel optimization, groove optimization, thermal resistance customization of roll surface, etc. The TRC process is characterized by high efficiency, and its capacity mainly depends on continuity. At present, there are two factors. One is the wear of the rolls and the heat loss of the side dam. The other is the uniformity of anti-sticky coating on the roll surface. The service life depends on the weakest position. For the nonuniform boundary conditions, the effective service life assessment method, the surface wear repair method, the remanufacturing technology of the roll, and the anti-sticky coating technology need further research, which directly determines the production cost.
(4) Promote the coordinated development of the whole industrial chain of the TRC process of complex section products.
The market determines demand, and service requirements determine product performance. Therefore, expanding the potential applications of complex section products will help accelerate the industrialization process. In turn, the development of the industry will promote the progress of technology. The TRC process of complex section products is located at the upstream of the whole industrial chain. The products need to go through controlled cooling and heat treatment processes. And most products mainly used as raw materials for hot rolling, cold rolling, stamping, etc. However, due to the particular geometric structure of the complex section products, prior technical standards are not fully applicable. The processing technology at the downstream of the whole industrial chain needs to be innovated. Therefore, aiming at the service performance requirements, whole industry chain collaborative development mode will become a necessary stage for the TRC process of complex section products to enter the industrial application truly.
This project is supported by the National Natural Science Foundation of China (51974278), the Natural Science Foundation of Hebei Province Distinguished Young Fund Project (E2018203446), the Graduate Student Innovation Project of Hebei Province (CXZS201803, CXZZBS2019047), and the China Scholarship Council.
