Application Status and Development Trend of Continuous Casting Reduction Technology: A Review

Abstract

Continuous casting is a dominant steelmaking process due to its steady-state nature, enhanced yield of steel production, and low consumption of energy and manpower. However, the production of defect-free cast products is still a big challenge, as the internal defects, including macro-segregation of alloying elements, cracks, and pores, can be easily formed during the solidification process, which seriously deteriorates the microstructure and mechanical properties of the cast products. Therefore, this paper puts forward the common solutions to overcome these problematic issues. The reduction technology can effectively improve the center segregation of the billet, bloom, and slab. The history of the development of the reduction technology is summarized and classified according to the variations in research trend. Furthermore, the basic principles and parameters of reduction technology are described to implement them in the actual production process. This paper compares the similarities and differences between soft reduction (SR) and heavy reduction (HR) technologies with a particular focus on the theoretical research of HR technology and further elaborates the key parameter and equipment problems during implementation of HR. Moreover, this paper also considers the HR technology adopted by Baosteel as a case study, which helped to put forward some viewpoints for the future development of reduction technology.

1. Overview of Reduction Technology

1.1. The Continuous Casting Technology

Continuous casting process of steelmaking is a dominant cast method in which the molten steel is solidified into partly finished billet, bloom, or slab. The different steel grades are cast into a wide variety of dimensions and a large amount, approximately 95% of the world’s steel, is manufactured by the continuous casting process. Continuous casting is a more significant and distinguishable process than the other casting methods due to its steady-state nature, high quality, the enhanced yield of steel production, and low consumption of energy and manpower. However, the production of defect-free cast products is still a big challenge, and it needs further research and development to control the internal defects during casting of difficult steel grades The different continuous casting methods are shown in

Table 1. The different continuous casting methods.

Type	Advantage	Shortcoming	Application	Challenge
Vertical type [1]	Inclusions can easily float and dissipate heat.	Large height, High investment cost, difficult maintenance.	Zhong Yuan Special Steel Company, et al.	How to reduce equipment height and investment cost.
Vertical type with bending [2,3]	The height of the whole machine is less than others.	Transverse corner cracks can form.	Dillinger Steel Plant in Germany, et al.	How to eliminate the cracks in straightening process.
Circular arc type with straight mould [4]	Float of inclusions, low investment cost.	Crack defects can easily produced.	NKK, Nippon Steel, et al.	How to eliminate the cracks in straightening process.
Circular-arc type [5]	Low investment cost, beneficial to improve the quality.	Inclusions can easily segregate at the inner arc.	NSC Muroran Works, et al.	How to control the inclusion formation.
Horizontal type [6]	Low investment cost, high surface quality.	Low production capacity.	Fukuyama works of nippon kokan, et al.	How to increase production capacity.

. Solidification is the last step of the continuous casting process, which is started in the copper mold and is of great importance to obtain the desire microstructure before the further operation and delivery of cast products. Several physiochemical processes occur in the mushy zone during casting and ingot solidification process, which include interdendritic flow of molten metal, solute redistribution, and crystallization. During solidification, the outer surface of the strand solidified earlier and the molten metal remained in the center region, which causes a temperature gradient from outer shell towards the center region. The solid/liquid interface evolution is the primary reason for the macro-structural changes. Moreover, the alloy segregation, porosities, surface, internal cracks, and non-metallic inclusions are common casting defects, which can easily form during the solidification process and need to compensate during continuous casting process. Reduction technologies are of great importance to control these internal defects formation. The main objective of reduction technology is to apply a small reduction amount on the surface of cast products at the solidification end to suppress the solidification shrinkage volume. The implementation of a small reduction amount on the surface of ingot is an effective method to reduce the center segregation of alloying elements and internal defects in cast products during the solidification process, which will be further summarized in the following sections.

1.2. The Formation Mechanism of Billet Segregation

The continuous casting billet is mainly prone to defects, including center segregation, shrinkage cavity, and other microstructural irregularities. The formation mechanism of different types of defects is not similar under the different conditions, and thereby different research methods and solutions were proposed to overcome this dilemma.

During the solidification process of the billet, the non-uniformity of local heat dissipation leads to the center segregation [7]. There are different theories about the causes of segregation, such as thermo-solutal convection, thermal shrinkage, grain bridging, solidification shrinkage, and solid shell bulging. Aboutalebi et al. [8] used the continuum model to simulate the redistribution of solute flow process and found that the center segregation was gradually formed during the solidification process of the slab. Janssen et al. [9] demonstrated the thermal shrinkage theory and showed that the decrease of temperature caused the volume shrinkage of billet, and the molten steel was flown towards the center region, which led to the positive segregation in billet. Suzuki [10] found that the rejected solute was concentrated below the grains bridging in the etched strand sample. Murao [11] studied the crystal bridge theory and calculated the solute transport through grain bridging. Flemings [12] and Ludwig [13] proposed that the shell bulging was the main reason for the centerline segregation, which was further proved by Kajitani et al. [14].

In order to solve the problem of center segregation, different technologies have emerged to improve the internal quality of the billet [15,16], such as low superheat casting [17], electromagnetic stirring at the solidification end [18], and SR technology [19], which are shown in

Table 2. The different technologies to overcome the center segregation in continuous casting products.

Technical Methods	Advantage	Shortcoming
Electromagnetic stirring at solidification end [23]	Increases the number of equiaxed crystals in liquid cavities ensures the composition uniformity of liquid steel and decreases the macro-segregation.	The mixing position is difficult to control, and the maintenance cost is high; easy to cause negative segregation of white bright band; does not work when the solid fraction will large.
Low superheat casting [24]	Increases the number of equiaxed crystals in liquid cavities, controls the formation of columnar crystals and center macro-segregation.	The nozzle is easy to be blocked at very low temperature; not conducive to inclusion floating; easy to cause semi-macro-segregation.
Soft reduction [15,16]	Reduces the center segregation and controls the internal cracks formation.	High requirements for equipment and control parameters; high requirements for the end position of liquid core; poor dynamic adjustment applicability.

. Currently, the SR technology has been extensively studied and applied in the production process [20,21,22], which played a significant role in continuous casting process to improve the product quality. However, there are still some technical problems which are required to be overcome, e.g., the large variations in the sections of continuous casting billet and control of casting speed, which make it difficult to determine an accurate reduction position.

1.3. Basic Principle of Reduction Technology

In the later solidification stage of the billet, the fluidity of the molten steel reduces, and the solidification shrinkage cannot be compensated, which leads to the formation of gas pores and shrinkage porosity [12]. In addition, the serious impact of the defect on the product quality cannot be completely eliminated due to the reduced fluidity of molten steel. Moreover, macro-segregation is caused by the relative flow of molten metal enriched with alloying elements during solidification, which is also a serious internal defect and can be increased from a few mm to cm or even larger. The solubility of carbon, phosphorus, sulfur, and other constituents is quite variable in the solid and liquid phases, which have more solubility in the liquid phase.

The formation of centerline segregation is due to the factors which are responsible for the motion of molten steel during the continuous casting process. The relative flow of molten metal is mainly due to the three aspects: (1) turbulent flow by molten metal feeding from tundish, (2) natural convection due to change of density which in turn resulted from the concentration variation and temperature gradient, and (3) fluid flow due to the deformation of solid state, shell bulging, solidification, and thermal shrinkage. Centerline segregation is directly related to the transport phenomenon, which in turn is associated with fluid flow, heat transfer, and steel composition. The previous research studies [25,26,27,28,29] have shown that the reduction technology can effectively improve the internal quality of the billet, reducing the center porosity and segregation, which is one of the key technologies to improve the slab internal quality during the continuous casting production process. Li et al. [30] developed a two-dimensional thermal–mechanical coupled model to investigate the deformation behavior of round bloom during soft reduction (SR) in the reduction force mode. They found that the shrinkage porosity in the center of the round bloom almost vanished with the implementation of reduction process. The changes in the solidification structure of billet were investigated with the reduction amounts of 1, 2, 3, and 4 mm, respectively, at a cooling time of 120 s and 150 s, which showed that the shrinkage porosity was significantly decreased at a reduction amount of 2 mm and a cooling time of 120 s [31]. For the billet or bloom soft reduction pretreatment, the continuous caster uses pulling and withdrawal units with fixed cylinders to adjust the roll gap between the one set of rolls. For slab reduction pretreatment, the clamping cylinders are fixed at the entrance and exit to control the reduction rate at the solidification crater end. The main purpose of soft reduction technology is to control the formation of macro-segregation by suppressing of volume shrinkage at final solidification stage and interrupt the suction flow of residual and segregated molten metal, as shown in Figure 1. The basic principle of reduction technology [19,32,33] is to apply an external force in an appropriate area of cast product during the solidification process, which compensates for the volume shrinkage at the solidification end. The implementation of small reduction amount on the surface of billet or slab is an effective method to reduce the center segregation of alloying elements and internal defects in cast products during solidification process, which is known as soft reduction. The applied reduction amount breaks the dendrites, promotes the dendrites nucleation, refines the grains, increases the number of equiaxed dendrites, reduces the center segregation, and improves the internal quality of the cast products.

The numerous studies have shown that the reduction technology can effectively reduce the center segregation and shrinkage porosity [22,34,35], which improved the internal quality of the billet, achieved the production of high-quality billet, and provided a strong background for the development of industrial intelligent and precise parameters. The reduction technology is the state-of-art in the continuous casting process due to its easiest and most effective implementations to control the center segregation in the cast products. The effect of SR on center segregation is more complicated, and it is difficult to fully reveal its mechanism by experimental research. The main functions of reduction technology are as follows [32,33,36]:

(1)

It compensates the solidification shrinkage at the center of billet, reducing or eliminating the internal shrinkage cavity and porosity formed by the volume shrinkage at the solidification end.

(2)

Under the extrusion effect of applied reduction amount, the molten steel enriched with solute elements is forced to flow in the opposite of the casting direction, which controls the transverse flow of molten steel, and reduces the center segregation.

(3)

The applied reduction amount can significantly break the “crystal bridge” and homogenizes the residual melt between the dendrites and controls the carbon macro-segregation, which further promotes the reverse flow of melt, and homogenizes the composition of constituents elements at the solidification end. The homogenization of melt further promotes the dendrites nucleation, refines the grains, increases the number of equiaxed dendrites, and improves the internal quality of billet and slab.

1.4. Development History of Reduction Technology

NKK Company of Japan invented a SR sector composed of segmented rolls in the late 1970s. They adjusted the roll gap in a certain area, but it was not applicable at a large scale due to insufficient understanding of equipment factors and soft reduction mechanism. Nippon Steel increased the roll gap at the solidification end of the billet in the 1980s [37]. This was the base of an early static soft reduction technology. However, the roll gap was not adjusted during the casting process. In 1986, Fukuyama Steel Works of Japan introduced the soft reduction set up, which consisted of the small pitch combined rolls (S.P.S). Moreover, the center segregation of slab was significantly reduced by a pouring speed of 0.75 m/min. However, the static reduction technology was not practical due to complex conditions such as the change in pulling speed.

Nippon Steel and NKK also proposed an alternative soft reduction technology. For example, Nippon Steel changed the middle part of the roll, and put forward the disc roll SR method; NKK put forward the concept of artificial bulging soft reduction and applied it to the slab caster. The above SR technologies are developed on the basis of static SR technology, which fails to break the limitations of static SR technology but provides a new idea for improving the center segregation of the cast products.

Dynamic SR technology was developed on the basis of static SR technology. In 1997, the roll gap was controlled by the proportional valve developed by VAI [38], as they used remote function, which can dynamically adjust the roll gap size according to the solidification end point of the billet to achieve the effect of dynamic reduction, and that was called the dynamic SR technology. This technology was applied to the Rautaruukki slab caster in Finland [39], which was the first slab caster in the world with a full dynamic casting flow guidance composed of 15 segments smart sector. It was equipped with the Dyncool dynamic secondary cooling control model to calculate and control the volume of cooling water, and then the automatic slab taper control system (ASTC) was used to adjust the taper of the roll gap to achieve the dynamic SR. Since then, German SMS Demag Company has developed and implemented the dynamic SR technology to the slab caster. In 1999, this technology was used by Salzgitter Iron and Steel Plant in Germany, which improved the internal quality of billets. For the billet or bloom SR pretreatment, the continuous caster uses pulling and withdrawal units with fixed cylinders to adjust the roll gap between the one-set rollers. For slab reduction pretreatment, the clamping cylinders were fixed at the entrance and exit to control the reduction rate at the solidification crater end. The main purpose of SR technology is to control the formation of macro-segregation by suppressing volume shrinkage at the final solidification stage and interrupt the suction flow of residual and segregated molten metal.

With the development of heavy industry, our requirements for the high quality and better performance of steel products are also increased [40], which promotes the production of continuous casting billets such as large section square billets and wide thick plates. The continuous casting billet is further characterized by low casting speed, slow internal heat dissipation, low solidification rate, and developed columnar dendrites. However, the SR technology has little effect on the large sectioned continuous casting billet. In recent years, HR technology has gained much attention.

The HR technology is based on soft reduction technology, which is suitable for the production of large billets. The billets with large sections have more serious internal porosity than the small section billets, which is difficult to eliminate [41]. Nippon Steel proposed NS Bloom Large Reduction Technology [42]. They used convex rolls to implement a large reduction amount on the solidified bloom, which significantly improved the central segregation and reduced the internal defects in rolled products. However, the fluctuation of casting speed and superheat leads to a significant deviation between the solidification end point and the adjusted reduction position, which also affects the reduction effect. Posco, South Korea, proposed the technology of using two sector segments to perform heavy reduction on the casting products (PosHARP, POSCO Heavy Strand Reduction Process [43]). SR was applied in the first segment before the solidification end, while the HR was applied in the second segment at the solidification end point, which significantly reduced the internal defects of cast product. Sumitomo Metal has put forward PCCS (Porosity Control of Casting Slab) technology in which HR is applied on a thick and wide slab at the solidification end, which significantly improves the internal quality of the slab [44]. Cheng et al. [40] and Wu et al. [41] developed the integrated HR technology at the solidification end and applied it in the production process of bearing steel. Moreover, the center porosity of rolled bars was reduced from 2.0–2.5 to 1.0 by using integrated HR technology. Previous research reports [7,30,45,46] have shown that the reduction technology can change the internal structure of the billet and improve its internal quality. The development history of reduction technology is shown in

Table 3. Development history of reduction technology.

Name	Time of Investigation	Technical Name	Application	Ref.
Static SR	Late 1970s	Segmented roll sector of small roll diameter	NKK Japan	[47]
	1980s	Enlarge roll gap contraction	Nippon Steel	[37]
	1986	Small pitch combined roll	Fukuyama Steel Works, Japan	[48]
	Late 1980s	Artificial bulging soft reduction (ISBR)	NKK Japan	[49]
	Early 1990s	Disc roll soft reduction (DRSR)	Nippon Steel	[50]
Dynamic SR	Late 1990s	Hydraulic clamping sector	VAI, SMS Demag, etc.	[38]
HR	recent years	Crown roll large reduction amount	Nippon Steel, POSCO, etc.	[43]

.

In other words, it is essential to apply an external force on the billet to generate the reduction amount, regardless of the type of reduction technology. The applied reduction amount at the solid–liquid interface can significantly break the dendrites, which increases and reduces the fraction of equiaxed and columnar dendrites, reduces the shrinkage porosity, and improves the internal quality of cast products. However, the types of reduction technology and process conditions such as reduction interval, reduction amount, reduction rate, cooling rate, and casting speed at different reduction zones lead to different solidification structures and the mechanical properties of the final product. Therefore, experimental and numerical simulation was performed by metallurgical scientists with regard to the above-mentioned parameters and they detected the micro-transformation of solidified microstructure, evolution of shrinkage porosity, macro-segregation behavior and strength-ductility synergy of cast products. Moreover, the obtained optimized parameters were successfully used in the actual production process.

1.5. Classification of Reduction Technology

There are two types of SR technology, including static and dynamic SR. Static SR refers to implementing the reduction amount at a specific location, while in the dynamic SR the position of reduction can be adjusted by changing of process parameters and the size of roll gap can be controlled. The two different types of reduction forces acted on the solid–liquid two phase zone during the implementation of reduction technology [42,51]: the first is thermal stress, which is responsible for the volume shrinkage at the solidification end. However, the disadvantage of this method is that it is not favorable for the growth of equiaxed crystals and the energy conservation; the other is mechanical stress, which compensates for the solidification shrinkage of the billet. Moreover, thermal and mechanical stresses cause the deformation of the cast products, and cracks will occur when the deformation exceeds the allowable limit during the operation of the continuous caster. According to different reduction equipment, SR can be divided into roller type SR and forging type SR [42]. The roller type SR is easy to operate, while the forging type soft reduction requires high equipment accuracy. HR technology has been developed in recent years. Nippon Steel [52], Sumitomo Metal [44,53], and POSCO [54,55] have used HR technology for continuous casting production. MCC continuous casting [56] has achieved good industrial application results by using the HR technology to improve the central segregation and porosity of the billet. During HR technology, a small reduction amount is implemented before the solidification end, which reduces the macro-segregation of alloying elements and porosity without inducing the internal cracks formation due to the large deformation, while the large reduction amount is implemented at and after the solidification end to reduce or eliminate the porosity. The classification of reduction technology is shown in

Table 4. Classification of reduction technology.

Reduction Technology	Force	Reduction Mode
SR technology	Thermal stress, Mechanical stress	Roller type Forged type Forced cooling at solidification end
HR technology	-	Single point reduction Multipoint reduction

.

There are two meanings of SR from the perspective of reduction amount: one is to apply a minimum reduction amount of less than 6 mm at the solidification end, while the other one is to apply a large reduction amount of 15–20 mm at the liquid metal core [57]. The reduction amount will be applied at the core of liquid metal before the solidification end, which is also called liquid core SR. The position and amount of SR can be different from the liquid core soft reduction, but the principle of reduction will be the same. Therefore, SR can be understood as a special form of liquid core reduction. The difference between the two types of SR is shown in Figure 2.

Liquid metal rolling, also known as casting rolling, is different from the first two types of reduction. It can be referred to as the process of rolling when there will be molten metal at the core of cast product to prepare the semi-finished products. This type of rolling is also used to crystallize molten metal.

There are two types of reduction technology, including SR and HR. The difference between HR and SR technology is as follows.

The HR technology at the solidification end of continuous casting billet is developed on the basis of the SR technology. The basic principle and technical research methods are basically same for both technologies. The main differences between HR and SR are as follows:

(1)

The reduction interval is different, as the SR is implemented at solid fraction range of 0.3–0.8, while the HR is implemented at the solidification end.

(2)

SR can only be achieved by adjusting the roll gap, while HR has to be concerned with pressure control besides the roll gap.

(3)

SR has a major role to overcome internal defects formation in ordinary steel grades, medium and low carbon steels, while the HR has outstanding results for high-quality steels, medium and high carbon steels. The similarities and differences between HR and SR technology are shown in

Table 5. Differences between HR and SR technology.

-	Reduction Parameter	Section	Casting Speed	Steel Grade	Effect
SR	Small reduction amount, forward reduction interval, implemented at small solid fraction.	Small section billet	High casting speed	Medium and low carbon steel, alloyed steel	Improve the center segregation and center porosity.
HR	Large reduction amount, backward reduction interval, implemented at high solid fraction.	Large section billet	Low casting speed	Medium and high carbon steel, alloyed steel	Improve center segregation and eliminates porosity.

.

2. Research on Reduction Technology

Prior research reports [22,58,59,60] have investigated the dynamic reduction technology from the perspective of numerical simulation and analyzed it by using the solidification heat transfer theory and finite element difference method to establish mathematical model. Moreover, the nail shooting experiment was conducted to explore the solid fraction with regard to the different reduction amount, reduction interval, reduction rate, and other parameters [19,32,33,34]. The detailed research progress and development status of reduction technology have been elaborated in the following sections.

2.1. Experimental Research

The cast billet is prone to serious defects such as segregation of alloying elements, central porosity, and internal crack defects. The influence of reduction parameters such as reduction amount, cooling time, and reduction interval, etc., on the internal quality of the casting billet was investigated by different researchers through laboratory experiments. The data obtained from the experimental research are more accurate compared to the numerical simulation data, and it is also beneficial to find the possible problems which could be faced during the actual production process.

Some researchers carried out experimental research from the perspective of theoretical analysis to study the reduction parameters. Ali et al. [61] investigated the optimized reduction amounts by analyzing the final microstructure of medium carbon steel via EBSD and X-ray computer tomography. They found that the reduction amount of 2 mm and 4 mm is enough to compensate the solidification shrinkage porosity, while the size of the shrinkage cavity was significantly increased ~2 times with the increase of the reduction amount to 6 mm. Yao et al. [62] studied the solid–liquid temperatures and mechanical properties of GCr15 bearing steel. According to the experimental results and heat transfer model, the reduction interval and optimized reduction amount was proposed for dynamic SR technology. The optimized process parameters, reduced total carbon content in the middle and later stage of solidification, setting of reasonable reduction interval, and reduction amount can improve the central segregation of cast products and resulted in the homogeneous microstructure. Bleck et al. [63] performed the soft reduction experiments by using a laboratory casting machine, while the solid fraction in the core area of billets was varied. They found that the center segregation, porosity, chemical homogeneity of constituent elements, and equiaxed crystal zone were significantly improved with the implementation of soft reduction, especially when the solid fraction was about 0.9. Wang et al. [64] explored the factors which affected the reduction efficiency of bloom during SR. The results showed that the center solid fraction affected the reduction efficiency of bloom, while the reduction amount, the dimensions of casting strand and the diameter of roller have little effect on reduction efficiency. Zong, et al. [65] investigated the internal cracking surrounding primary carbides in high carbon steel as-cast blooms induced by SR. It was found that the carbides precipitation in the vicinity of existing internal cracks was located at the middle of the surface and centerline of the bloom, which further increased the existence of the segregated bands in the hot-rolled wire rods. Ali et al. [66] investigated the effect of SR amount increasing from 1–4 mm with a cooling time of 120 s and 180 s to reduce the internal crack defects. The results showed that the minimum reduction amount was enough to compensate the solidification shrinkage defects and center segregation of medium carbon steel ingots. Jin et al. [67] improved the internal quality of 42 CrMo steel bloom and found that the SR has little effect on the solidification structure, but the central compactness and alloying segregation was improved significantly. The size of the largest porosity point at the bloom center was reduced from 1106 μm × 608 μm to 600 μm × 334 μm, while the carbon segregation index was reduced from 0.889~1.095 to 0.962~1.064. It can be inferred that the soft reduction technology can significantly reduce the internal defects and improve the internal quality of cast products. The difference just existed in the experimental schemes and equipment, but these all are explored on the basis of the actual production process. The experimental research parameters include but are not limited to experimental steel grades, reduction parameters, and detection methods, etc.

2.2. Numerical Simulation Research

The steel plants are constantly trying to manufacture higher quality products with low cost. However, the segregation of alloying elements, central porosity, internal crack defects, and non-metallic inclusions are common casting defects, which can easily generate [68,69] during the solidification process and have an adverse effect on the final product. The association of software engineers and steel industries is an effective campaign to optimize the parameters of continuous casting, mainly the reduction section. Therefore, numerical simulation can easily detect the physical phenomenon behind the formation of internal defects during the implementation of reduction technology.

Liu et al. [70] developed a thermal–mechanical coupled FEM model to predict proper soft reduction amount for continuous casting slab. Kajatani et al. [14] used a finite-volume scheme to simulate the deformation-induced macro-segregation in continuous casting steel. It is shown that the positive centerline segregation of carbon in the slab is well reproduced with this model. Guan et al. [71] developed a multiphase solidification model, combining turbulent fluid flow, heat transfer, solute transport with back diffusion, and shell deformation. The simulation results showed that a large reduction applied just before the solidification end could significantly reverse the flow of solute-enriched melt.

The temperature field, stress field, liquid core flow field, and other complex fields during solidification process were analyzed by means of numerical simulation, and different mathematical models were established to explore the relevant parameters of reduction technology. Numerical simulation has lower cost, higher efficiency, and visual detection compared to experimental research, which gives an insight into reduction technology. Jiang et al. [15] used numerical simulation to study the flow of molten steel and solute distribution in the continuous casting slab by using reduction technology. The simulation results showed that the velocity of molten steel enriched with solute elements was restrained after the implementation of reduction technology, which significantly reduced the center segregation.

SR has a major impact on controlling the internal quality of cast products, but it can also lead to crack formation. The shape of rollers can significantly reduce the crack formation during the continuous casting process, as the convex-shaped roller improves the reduction efficiency and reduces the cracks formation and micro-segregation, which does not require upgrading the equipment, and can also reduce equipment wear. Zhou et al. [58] performed the FEM simulation and investigated that the convex roll can increase the deformation at the slab center, which further increases the reduction efficiency up to 15.10 pct. Ma et al. [59] investigated the effect of SR on the center macro-segregation during the continuous casting process, and developed a mathematical model that included fluid flow and solute transfer. Moreover, this model was further incorporated with a soft reduction model. The simulation results revealed that the carbon enrichment occurred at the solidification front, while the center segregation ratio at the center region was 0. 1.11 and 1.09, respectively, with soft reduction.

2.3. Industrial Field Research

Due to the complex production conditions of steel industry, reduction technology might have some limitations. For example, the combined effect of different factors such as casting speed, steel grade, and cooling water intensity might be problematic during the production process of cast steel. The optimization of reduction parameters according to the actual production process is very helpful to improve the quality of the casting billet.

Han et al. [72] established a dynamic control model for the SR of slab, according to the reduction parameters, which were inversely optimized with consideration of roll gap, reduction interval, feedback information of the model, and metallurgical process standards. This significantly maintained the production of high-quality slabs during the slab continuous casting process. The solidification of high-alloyed steel (0.4C–1.5Mn–2Cr–0.35Mo–1.5Ni) resulted in a high temperature gradient of solidified shell with the formation of columnar crystal, which contributed to the center segregation and cracking due to the high carbon content and was significantly improved by optimized SR parameters [33]. Chen et al. [16] conducted the industrial experiments on machine soft reduction (MSR) and improved the internal quality of the bearing billet. They found that the suitable casting condition for bearing steel (GCr15) should have a casting speed of 0.88 m/min and MSR rate of 2.4 mm/m or 2.1 mm/min for the 9# caster in NISCO.

The theoretical research on reduction technology can be combined as shown in Figure 3. The comparison of experimental and numerical simulations can be used to optimize the reduction technology. Moreover, numerical simulation parameters can also be optimized.

2.4. Technical Parameters of Reduction Technology

In the application process of SR technology, it is necessary to consider the process parameters and equipment conditions, which will affect the internal quality of cast products. Therefore, in-depth research on the process parameters and related equipment is required.

2.4.1. Investigation of Process Parameters

(1)

Reduction interval

The reduction interval should be at the position where the solid fraction (fs) in the center of the billet will be 30%~70% [73]. The reduction position has no fixed value due to the differences in equipment conditions, steel grade section, composition, etc. However, the reduction position needs to be corrected according to the specific situation during the actual production process. It is more difficult but important to accurately predict the reduction position in the case of different steel grades, casting speeds, and other parameters. The determination of reduction position is the key factor for the success of SR technology [74,75,76]. Flemings et al. [73] showed that the center segregation mainly occurred in the solid–liquid two-phase area at the end of solidification. Through the further research on the location of center segregation, the solid–liquid two-phase area is specifically divided into three different areas according to the fluidity of liquid steel between dendrites, which are named as the solid phase area, the solid–liquid two-phase area, and the liquid phase area. If the SR is carried out at an early stage, there will be less dendrites in the billet, and a high fraction of molten steel with free flow. The solute distribution in this reduction area will be uniform, and the SR will have no obvious effect. If the SR is implemented at later stage, then the number of dendrites in the billet will be large and there will be a narrow flow of residual steel with high viscosity, which will lead to porosity, shrinkage cavity, and center segregation. The nail shooting method (tracer method) [77], heat-tracing calculation [78], pressure feedback detection of slab shell at the solidified end [79], and electromagnetic ultrasonic (EMAT) detection [80] are commonly used methods for the detection of solidification end point. Currently, heat-tracing methods are commonly used, but these have some limitations due to inaccuracy in boundary conditions and physical parameters, which leads to large deviations between predicted and actual values.

A nail shooting experiment was used to determine the thickness of solidified shells to further calculate the solid fraction of the cast products. The shooting pin has sulfide, which diffuses rapidly due to its low melting point when the shooting pin enters into the liquid phase of the cast product. Kawawa et al. [77] divided the cross-section of Billet into three regions: 1, 2, and 3, as shown in Figure 4 [77]. In Zone 1, the nail keeps its original shape, and the sulfide does not diffuse. In Zone 2, some amount of sulfide around the nail is diffused. In Zone 3, the nail is completely melted, and the sulfide is also fully diffused.

(2)

Reduction ratio

The reduction ratio [26,32,75,81] refers to the reduction amount per unit length in the casting direction, expressed in mm/m, and is one of the most important parameters of SR technology. The much smaller reduction ratio is not enough to compensate for the volume shrinkage during the solidification process of continuous casting billet. Therefore, the effect of soft reduction technology will not be significant, which cannot reduce the shrinkage cavity, porosity, and center segregation. The large reduction ratio exceeds the maximum deformation rate that the steel grade can bear, which leads to crack formation in the billet. The reduction ratio is related to the deformation rate of the steel. Therefore, the deformation generated by the maximum reduction ratio should not exceed a certain critical value for different steel grades, or it will cause internal cracks and damage the equipment. The reduction ratio should be within a suitable range [82], as the much larger and smaller reduction ratios will cause inverted “V” shaped and “V” shaped segregation, respectively.

The previous research reports [40,41,83,84] investigated the values of reduction ratio for slab casting and showed that the reduction ratio decreases linearly along the casting direction. The maximum and minimum values of the reduction ratio at lower casting speed were greater than the maximum and minimum values at higher casting speed. However, the average reduction ratio decreases linearly with the casting speed.

The reduction ratio theoretical model can be evaluated by the following equation [26,81].

$$\frac{dH}{dz}=-2\frac{{dY}_{suf}}{dz}=2\frac{{\displaystyle {\int }_0^{Y_{suf}}{\displaystyle {\int }_0^{X_{suf}}\left(\frac{\partial \rho }{\partial z}\right)dxdy}}}{{\displaystyle {\int }_0^{X_{suf}}\overline{\rho }{|}_y-Y_{suf}dx}}$$

(1)

where X_suf indicates the position of the right surface of the narrow plane in the x direction and Y_suf indicates the position of the upper surface of the wide plane in the y direction. H is the slab thickness, dH/dz is the reduction rate, and ρ represents the density.

(3)

Total reduction amount

The total reduction amount is related to the reduction ratio [85]. The optimized reduction amount is that which can compensate the volume shrinkage of the molten steel at the solidification end, decrease the flow of the molten steel enriched solute elements, and reduce the center segregation [86,87,88,89]. If the reduction amount is too large, it will lead to internal cracks formation, which will also adversely affect the service life of the SR equipment. The increasing reduction amount was always accompanied with a high stress concentration at the two-phase zone, which is mainly responsible for the cracks initiation which further propagate from the stress concentrated toward the stress-free area [90]. If the reduction amount is too small, the effect of SR will not be obvious, as the effective reduction will not shift to the core of cast product. Moreover, the effect of reduction amount will also be negligible with the increase of shell thickness. The shell thickness implies a high solid fraction at the center of slab, which restricts the compression during implementation of reduction amount and decreases the effectiveness of reduction technology to reduce porosities.

Three factors should be considered for the implementation of the reduction amount: (1) The reduction amount should compensate the solidification shrinkage of the billet and along with reduced center segregation; (2) The values of reduction amount should not much larger as to avoid the internal cracks formation; (3) The applied force during the soft reduction process should not damage the stiffness and service life of the section frame and pinch roller of the continuous caster.

2.4.2. Research on Reduction Equipment

In addition to the exploration of process parameters, the researchers also improved and optimized the equipment factors related to the use of the SR technology, and they achieved good results [84,91,92]. Liu et al. [91] found that the clamping cylinder significantly affected the formation of volume shrinkage by using the roll gap model. They explored and optimized the SR parameters by studying the clamping cylinder from the perspective of equipment stability and accuracy. Moreover, the optimization of the clamping cylinder has remained a hot research topic to improve the product quality. Cheng et al. [92] compared the influence of different number of reduction rolls on the internal porosity of the slab by using single and multiple rolls to enhance the efficiency of SR technology. Currently, several researchers focused on the exploration of reduction process parameters and the adjustment of equipment related to reduction technology to implement the optimized parameters in the actual production process [51,56,90]. For example [93], the control of cooling water intensity and casting speed in the secondary cooling zone can accurately determine the reduction interval. Different reduction-related equipment, such as the shape of reduction rollers, i.e., flat or convex shape rollers, will also affect the SR technology. The optimization of combined reduction process parameters and reduction equipment resulted in the outstanding quality of cast product, while the improper selection of the reduction parameters has adverse effects on the equipment life and the internal quality of the product. Moreover, the improper adjustment of equipment and insufficient accuracy will lead to misjudgment of reduction parameters and even serious accidents during the actual production process. The impact of these parameters and equipment on billet quality is shown in Figure 5.

2.4.3. Factors Affecting the Reduction Technology

There are different factors which have important roles in controlling the internal quality of cast products and they are briefly discussed in the above sections. Herein, the optimized and influential factors can be concluded to solve the internal segregation and porosity defects of wide and thick continuous casting billet and slab. Wu et al. [41] explored the effect of roller differential speed on the porosity formation during the implementation of HR technology. They altered the roller speed, and further confirmed it by numerical simulation and industrial tests. The results showed that the control of speed difference could effectively improve the effect of HR and reduce the internal porosity of billet. Dong et al. [94] predicted the type of porosity formation in billet through numerical simulation with implementation of HR. They changed the number of reduction rollers (equipment parameters), actual reduction amount, and reduction position (process parameters), and carried out a comparative experiment between SR and HR. The results showed that the effect of multi-roll HR was better than that of single-roll HR and SR. Therefore, it can be inferred that HR can effectively eliminate the porosity formation. HR technology has also been widely used in different industries, including POSCO, South Korea, Sumitomo Metal, Japan, etc. The group of Zhu et al. [40,41,74,81,84] explained the key influencing factors and equipment optimization to improve the efficiency of reduction technology as follows.

(1)

They developed an online detecting model for the solidification end of the continuous casting billet to investigate the feasible reduction position, which included heat-tracing model, thermal imaging of the billet surface, and nails shooting to calibrate the solid fraction of billet. Through verification with the actual production, the error of shell thickness was detected as 1.5%.

(2)

To achieve the optimized reduction amount, the formula of the minimum theoretical reduction at different positions was evaluated by using the concept that the reduction amount at the solidification end should be higher than the solidification shrinkage of billet.

(3)

The reduction interval was evaluated according to the principle that the solidifying metal should have a minimum degree of solute segregation after the implementation of reduction amount, which is considered as the best reduction interval. Furthermore, the segregation of different solute elements after the implementation of reduction technology was studied, and the formula for improving the degree of element segregation was obtained.

3. Industrial Implementation of Reduction Technology

The industrial application of reduction technology mainly focuses on dynamic SR and HR, while static reduction is not practical due to its limitations. Dynamic SR technology was first successfully applied in Nippon Steel [56,92] and now it has been widely used in industrial production [13,18,19,22,41,45,95]. Some industrial applications of reduction technology are shown in

Table 6. Application of reduction technology in industry [13,18,19,22,41,45,95].

Enterprise	Type	Steel Grade	Reduction Amount/mm	Solid Fraction fs	Casting Speed m/min	Improvement Effect
Muroran Factory	SR	High carbon steel	6~9	0.35~0.5	-	Center segregation
Sumitomo Metal	SR	Bearing steel	6~9	0.4~0.8	-	Center segregation
Pohang, South Korea	SR	High carbon steel	6~7	0.2~0.8	-	Carbon segregation index decreases to 1.1
Xingtai Steel	SR	Bearing steel	8.3~12.5	0.4~0.96	0.65~0.75	Center and V-shape segregation
Thyssen, Germany	SR	High carbon steel	-	0.2~0.7	6.0~8.5	Slab homogeneity
MCC Continuous casting	HR	72A	5~20	-	-	Eliminate porosity
Pangang Group	HR	Bearing steel	-	0.7~0.92	-	Improve billet quality

.

The industrial experiment of HR technology was conducted in Baosteel industry [96]. The relevant test parameters during HR are mentioned. As the continuous caster has a vertical arc with a radius of 10 m, it can produce slabs with a thickness of 220 mm, 250 mm, and 300 mm, and has an annual output of 2.3 million tons. Moreover, a single point reduction was adopted.

The production was carried out according to the relevant parameters selected from the experiment. The theoretical design of the reduction parameter is different from the actual reduction. Although the HR amount was insufficient, it still improved the internal quality of the slab. When the slab thickness was 250 mm and the reduction amount was 9~10 mm, the slab porosity was significantly improved. The key factors affecting the deficiency of target reduction are the adoption of single point reduction and the improper selection of reduction rollers.

4. Prospects of Reduction Technology

This paper elucidates the development history of reduction technology and describes the basic principles, along with the classification of different types of reduction technology. Moreover, this paper focuses on the theoretical research of HR technology and elaborates the key parameter and equipment problems during the implementation of HR. Taking the No.3 slab continuous caster of Baosteel as an example, the actual industrial test and application effect of the HR technology are explained and put forward some viewpoints on the future development of reduction technology.

The basic principle of reduction technology is to apply an external force in an appropriate area of billet during the solidification process, which compensates the volume shrinkage at the solidification end and improves the internal quality. The research on the reduction technology includes the reduction amount, reduction interval, reduction ratio parameters, and numerical simulation. However, the effects of secondary cooling water intensity, alloy composition, mechanical equipment, etc. on the center segregation and cracks of the billet have rarely been discussed.

5. Future Perspective

Currently, the main problems during implementation of reduction technology are as follows.

(1)

How to measure the temperature distribution of the billet during solidification and how to calculate the volume shrinkage at the solidification end to provide a basis for the theoretical calculation of the reduction parameters. New research methods and ideas should be adopted for the reduction technology, such as the effect of alloy composition on the mechanical properties of cast products, the relationship between cooling water intensity and billet solidification, etc., and a breakthrough in the quantification of reduction parameters.

(2)

The research on the secondary cooling process is not enough to implement it in the practical application of the reduction technology, especially the relationship between the water intensity in the secondary cooling zone and solidification.

(3)

It is difficult to accurately judge the position of the solidification end of the cast products, especially when the casting speed and cooling water intensity changed. The selection of an appropriate reduction interval and amount also needs to be further investigated. The exact control of solidification end point is needed to improve the internal quality of cast products. Further research should be conducted on the transition states of different steel grades and the fluidity of molten steel in the two-phase zone, along with the optimization in the design of reduction rollers.

(4)

How to use the reduction technology for industrial production, as the investigations on reduction technology are mainly discontinuous, and a complete research system of reduction technology has not been designed. According to the actual working conditions and the current investigations, a systematic research system of reduction technology should be designed to automatically adjust the reduction parameters such as reduction interval, reduction amount, and reduction equipment.

(5)

With the progress of HR technology, the mechanical equipment also needs to be upgraded to improve the fault-tolerance of HR technology during its use in the industry. Safety measures should be highly considered according to the results of destructive experiments in order to avoid serious tragedy during the industrial production process.