Twin Roll Casting

Technological progress and industrialization of twin-roll thin strip casting and rolling

The twin-roll thin strip casting and rolling technology can form a millimeter-scale thin strip at one time. Due to high-speed condensation, grain refinement, less component segregation, and obvious advantages in energy saving and emission reduction, many materials that are difficult to form, such as high-silicon electrical steel, high-strength The use of this process in steel has also made breakthroughs, but there are still some problems in realizing industrialization. This paper introduces the latest progress of twin-roll thin strip casting and rolling technology and discusses the industrialization issues.

1 The latest development of twin-roll thin strip casting and rolling technology

1.1 Castrip production line of Nucor Steel Corporation

1) Main process technical parameters

In 1994, BHP/IHI built a 1345mm thin strip casting and rolling production line in BLUESCOPE/IHI, Australia. In 2000, it was relocated to the Crawfordsville plant of Nucor Corporation of the United States. IHI, BHP, and Nucor Steel Corporation of the United States cooperated to develop The thin strip casting and the rolling process was developed, and the production line was named Castrip. In May 2002, the reconstructed Castrip production line was hot-tested (the width of the casting machine was 1345mm). The production line mainly produces low-carbon steel with an annual design capacity of 500,000. Ton. In 2009, another Cast rip production line (casting machine width 1680mm) was built in Arkansas, mainly producing low carbon steel.

2) The main process flow

The first Cast rip production line: electric furnace+VOD+LF+Castrip.

The second Castrip production line: Electric furnace+VTD+LF+Castrip.

3) Main product quality and performance

(1) Surface quality and edge quality

The surface quality of the cast strip is compared with the traditional hot continuous rolling as shown in Table 2. Since the edge of the cast strip has a cast-like structure with a depth of about 2mm, the Cast rip steel strip needs to be trimmed before it can be sold or cold rolled.

(2) Internal quality and microstructure

Since Cast rip uses two-roll casting strips and does not use mold slag, as long as the ladle slag is well controlled, there are very few non-metallic inclusions in the cast strip products, and there are almost no large-particle non-metallic inclusions.

The solidification time of Castr rip casting strip is much faster than that of thin slab casting and traditional thick slab casting, so the structure of Cast rip casting strip is similar to that of thin slab casting and rolling and traditional thick slab casting + hot rolling. There is a big difference compared to.

Table 3 presents the microstructure comparison between Cast rip products and traditional hot tandem products. It can be seen that due to the extremely fast condensation rate of the cast strip, the microstructure of Cast rip products presents polygonal ferrite, Widmanrite and acicular ferrite, and the grain size is large.

(3) Product process performance

Nucor’s inspection results show that the hole expansion performance, anisotropy performance and cold bending performance of the cast strip after rolling and controlled cooling are better than those of similar traditional hot continuous rolling products.

◆Hole expansion performance: Although the strength of the cast strip after rolling (UCS380) is higher than that of the traditional hot continuous rolling product HR360/HR300, its hole expansion depth is much larger than that of HR360/HR300, which shows that the acicular ferrite inside the cast strip after rolling That is to increase the strength, but also improve the toughness.

◆Anisotropy: The difference between r45 and r0 and r90 of cast strip products after Casttrip rolling is small, so the rm value is high and Δr is small. Cupping test results show that the lugs are small and are particularly suitable for stamping or deep drawing steel.

◆ Flattening performance of straight seam welded pipe: Casttrip products (UCS, after cold rolling and annealing, elongation 23%) with straight seam welded pipe have been subjected to extreme flattening test and partial cold bending test results after flattening. The role of ferrite, the material has excellent toughness.

1.2 European Eurostrip thin strip casting and rolling production line

The Eurostrip thin strip casting technology was jointly developed by Arcelor and ThyssenKrupp. The first continuous casting unit was established at the Krefeld plant in ThyssenKrupp Nirosta (KTN), Germany. el d), started in December 1999, and in June 2001 added an in-line 4-high hot rolling stand and an in-line induction heater. In 2003, a double coiler system was installed to realize “endless casting and rolling”, thus fully reaching the industrial scale and starting the commercial production of austenitic stainless steel.

1) The main process flow

UHP electric arc furnace + AOD refining + Eurostrip, the main product is austenitic stainless steel.

2) Main product quality and performance

Surface quality is especially important for stainless steel, where appearance requirements are very strict. The main problems that need to be solved in the production of surface defect-free products are: avoiding microcracks and avoiding oxide “scum”. Oxides (dross) on the surface of the strip are rare and small but have an impact on the acceptance of products, especially stainless steel products.

For carbon steel, the surface quality of the product meets the requirements of the end use. By taking appropriate measures along the entire process line to further improve the oxidation situation in the sensitive stage, the degree of scale formation on the strip surface is reduced, which greatly eliminates the adverse effect on the strip quality, especially in the hot rolling stage. The technological characteristics of strip casting and rolling cause strip roughness (5μm) to be larger than that of traditional hot strips (1.8μm). In-line hot rolling reduces it to about 2 μm, which can even meet some direct uses, such as raw materials for galvanizing lines.

TKS-NR tested the mechanical properties of the material, and its ultimate deep drawing rate and arch height were comparable to those of traditional process products. For carbon steel products, the mechanical properties are close to the standard EN 10025 requirements.

1.3 Baosteel Baostrip Industrial Production Line

In May 2001, the thin-strip casting and rolling industrialization project was officially listed in Baosteel’s scientific and technological development plan, and was identified as a major scientific research project of the group company, named “Research and Development of Key Technologies and Complete Sets of Thin-strip Casting and Rolling Industrialization”, Baoling Company participated in the joint research project team. After six years of research, a pilot-scale thin strip casting and rolling test line was built, and nearly 280 heat tests were carried out on this unit line to achieve Whole furnace casting and coiling of SUS304 stainless steel and low and medium-grade carbon steel.

In 2012, Baoling Heavy Industry invited experts to jointly develop, and achieved many achievements in production process research and development and equipment development. To this end, an industrialized unit with a diameter of 800×1430mm was built. Its main technical parameters are shown in Table 5. The industrialized unit was built in Ningbo Steel Plant. The first coil of steel was produced in early 2014. After more than one year of trial production, the preparation thinnest hot-rolled coil is 0.90mm×1430mm. Since the products in trial production are only used for container plates and the cost is high, the project won the 2016 Metallurgical Science and Technology Special Award.

1.4 WISCO High Silicon Steel Thin Strip Casting and Rolling Testing Machine

1) Development history of thin strip casting and rolling of WISCO

WISCO started to carry out research on thin strip casting and rolling since 2004. From 2007 to 2009, as a key scientific research project of WISCO, it completed the “Preliminary Research on Thin Strip Casting and Rolling Electrical Steel”. “Experimental research on silicon electrical steel” was included in the key scientific research project plan of WISCO.

2) Pilot line of thin strip casting and rolling high silicon electrical steel

In February 2012, the Ministry of Science and Technology of the People’s Republic of China and Wuhan Iron and Steel signed the “High-Silicon Electrical Steel for Energy-saving Motors” according to the “Administrative Measures for the National High-tech Research and Development Program (863 Program)” (Guoke Faji [2011] No. 363). “Development” project task book, project number: 2012AA03A506.

The subject is to produce 0.10-0.30mm thick 6.5%Si non-oriented electrical steel by thin strip casting + warm rolling technology. By the end of 2016, the experimental research content and objectives stipulated in the task statement were completed, and the acceptance was passed in April 2017.

1.5 Research and Development Progress of Thin Strip Casting and Rolling at Northeastern University

Northeastern University has successfully produced oriented silicon steel, non-oriented silicon steel, and 6.5% Si electrical steel using its thin strip casting and rolling test equipment. High magnetic induction, high-grade non-oriented silicon steel has been successfully prepared, and the magnetic induction index B50 is better than that of existing products at home and abroad by 0.03-0.04T. It provides a short process, low difficulty, and low cost to manufacture high-efficiency non-oriented silicon steel without heating, normalization treatment, and two-step cold rolling and intermediate annealing. technical prototype.

2) Provide a new process flow of the short process, low difficulty and low cost to manufacture oriented silicon steel without high-temperature heating and nitriding treatment, providing a technical prototype for industrial production, and successfully prepared 0.27mm thick ordinary oriented silicon steel, The magnetic induction index B8 reaches 1.85T, which is comparable to the existing CGO products at home and abroad; the 0.23mm thick high magnetic induction oriented silicon steel is successfully prepared, and the B8 reaches 1.94T, which is better than the existing Hi-B products at home and abroad.

3) A full-process high magnetic induction oriented silicon steel process technology based on ultra-low carbon composition design has been formed, and a 0.27mm thick high magnetic induction oriented silicon steel has been successfully prepared. The B8 reaches 1.94T, which is superior to the existing Hi-B products at home and abroad. It provides a short, low-difficulty, and low-cost process for manufacturing grain-oriented silicon steel without high-temperature heating, nitriding, or decarburization, and provides a technical prototype for the industrial production of grain-oriented silicon steel strip casting and rolling.

4) A full-process high-silicon oriented silicon steel process technology based on ultra-low carbon composition design has been formed, and 4.5%Si and 6.5%Si oriented silicon steels with a thickness of 0.18-0.23mm have been successfully prepared. B8 reaches 1.78T and 1.74T respectively, which is significantly superior for foreign products. It provides a short, low-difficulty, and low-cost manufacturing process of 4.5%Si and 6.5%Si oriented silicon steel using warm rolling and cold rolling technology without high-temperature heating, decarburization, and nitriding treatment. It is oriented silicon steel. The industrial production of thin strip casting and rolling provides a technological prototype.

In April 2016, Hebei Jingye Iron and Steel Co., Ltd. cooperated with Northeastern University and invested nearly 400 million yuan to build a thin strip casting and rolling line with an annual output of 400,000 tons of silicon steel in Hebei Jingye.

2 Discussion on the industrialization of twin-roll thin strip casting and rolling technology

Since the 1980s, the industrialization of thin-strip cast-rolled steel materials has reached a second climax, but the results are far from meeting expectations. Therefore, the industrialization of twin-roll strip casting and rolling technology needs to be discussed in depth.

2.1 Manufacturing cost issues

1) Electricity fee: Considering the use of scrap steel as raw material, the electricity consumption is about 1000kWh/t or more through the electric furnace + VOD + LF smelting and refining process. In the case of extremely low electricity prices in the United States, this cost can be ignored.

2) Argon gas (Ar) cost: When the twin-roll thin strip casting is cast, the molten pool formed by the twin-roller and the side sealing plate is an open type, and the mold flux cannot be used, only the argon gas protection with continuous argon blowing can be used. The price is high and the consumption is large. The consumption of argon in the Castrip production line is 11Nm3/t.

3) Refractory material (including side sealing plate): average 4 heats per pour, that is, 440 tons of molten steel, the refractory material is $3860 per pouring time, that is, the consumption per ton of steel is $8.77. If each pouring time is 1 heat, the consumption per ton of steel is $8.77. for $35. Once the casting process fluctuates, the side sealing plate is easily damaged, and the cost increases several times.

4) Consumption of twin-roll casting machine: The two crystallization roll sleeves of the twin roll casting machine are made of beryllium copper material, which is expensive. Once the casting process fluctuates, the roll sleeve will be damaged, and the cost will increase several times.

5) Metal yield: Castrip production line is the most industrialized production line in the world, its metal yield is only 91%, and both raw material and manufacturing costs will increase by 9%.
Therefore, the problem of the high cost of thin strip casting and rolling needs to be solved from the above five aspects.

2.2 Process technical issues

The process technology problem is the selection of steel smelting method and ladle capacity. It mainly includes the following aspects.

1) Castrip adopts electric furnace smelting, and the aluminum content in the steel is easy to control. The low-silicon and non-silicon electrical steels produced by Casttrip all contain extremely low aluminum, and the problem of pouring and plugging seldom occurs; while the production line using converter smelting uses aluminum Deoxidation, the aluminum content in molten steel is high, and water plugging accidents often occur. Therefore, no matter which smelting method, the aluminum content in molten steel must be controlled to a minimum (<0.05%). Even high-efficiency motor steel needs to adhere to this in principle.

2) In the thin strip casting and rolling production line, the ladle capacity should not be too large, the molten steel capacity is large, the pouring time is long, the heat dissipation is fast, and the temperature control is difficult, and many problems such as low superheat casting are prone to occur, which is not conducive to the structure control of the strip. The practice has proved that it is appropriate to choose 0.02% of the production capacity for the ladle capacity. For a production line with a production capacity of 500,000 tons, the ladle capacity is about 100 tons.

3) Selection of casting roll material, rotational speed, and cooling water flow rate. It can be seen from the curve of the surface temperature of the two crystallization roll sleeves of Be-Cu and steel material that the maximum temperature of the steel material roll is 828.8 ℃, which is much higher than that of the Be-Cu material roll, which is not suitable for making double rolls Thin strip roll sleeves. From the curve of the equivalent stress on the surface of the crystallization roll with time at different speeds, it can be seen that for a roll diameter of 500-600mm in diameter, the speed should not exceed 60m/min. For the roll diameter of 500-600mm in diameter, the distance between the cooling water pipe and the roll surface should not exceed 25mm; for the roll diameter of 500-600mm in diameter, the larger the cooling water, the smaller the temperature and equivalent stress on the surface of the crystallization roll, but the effect is not large.

4) Development of side sealing plate. According to the requirements of thin strip casting and rolling on the side sealing plate, in 2013, the Wuhan Iron and Steel Research Institute undertook the Beijing Science and Technology Commission’s “development and development of side sealing plate for thin strip casting and rolling” project, and passed the acceptance in 2015. The developed BN+SIALON The E-side sealing plate has been proved to be superior to Saint-Gobain’s performance.

5) Selection of crystallizing roller diameter. The experimental simulation results show that the temperature difference on the surface of the crystallization roll is related to the diameter of the roll as follows. Although the larger the diameter of the crystallization roll, the larger the size of the side sealing plate, the more molten steel in the molten pool, and the more stable the casting and rolling process is. The more molten steel, the larger the contact surface with the roll surface, the higher the temperature of the contact zone, and the higher the temperature of the surface of the crystallization roll, which leads to the greater temperature difference on the surface of the crystallization roll, which leads to the increase of surface defects such as cracks in the strip. . On the contrary, the smaller the diameter of the crystallizing roll, the smaller the alternating temperature difference on the surface after stabilizing the cast strip, and the smaller the possibility of thermal fatigue damage; the smaller the size of the side sealing plate, the lower the production test cost. Beyond φ800mm, this is the reason why Castrip has always insisted on φ500mm crystallizing rollers.

6) Selection of crystallizing roller length. The lengths of crystallizing rollers used by various companies are varied, but Castrip has always insisted on small roller diameters and large widths, that is, crystallizing rollers with a diameter of φ500mm and a width of 1345-2000mm. The main reason is that there is a triple zone in the molten pool, the temperature of the end face of the crystallization roll is the lowest, and the solidification of the molten steel on the end face of the roll is faster, which leads to the thickening of the edge of the cast strip, and the affected area is about 50mm. Therefore, the surface of the crystallization roll should be wide and not narrow, but if the roll surface is too wide, it will bring the difficulty of uniform distribution of molten steel in the lateral direction of the roll surface. The problem with the uniform flow, the length of the crystallization roller is ≥1250mm.

7) Control of casting force and cracks. The casting force actually has a close relationship with the casting speed (ie the rotational speed of the casting roll). The higher the casting speed, the more unsolidified molten steel, the smaller the casting force, the smaller the casting force, and the fewer transverse cracks on the cast strip. The casting force is too large, which means that the cast strip is completely solidified above the double-roll matching point. The process of reaching the matching point is the double-roll hot rolling process, which not only affects the shape of the roll gap and is prone to cracks, but also causes cracks due to widening. side cover. Therefore, it is necessary to precisely control the position of the anastomosis point to ensure the stability of the small casting and rolling force.
The heat transfer coefficient h of the liquid phase region of the molten pool is about 104w/(m2·k). The temperature at the interface between the molten steel and the casting rolls will vary by several hundred degrees. When reaching the solid phase region in the molten pool, surface tension will be generated, and the h value will also increase. In a very short time, the temperature difference at the interface will be significantly smaller, which is equivalent to a thermal load shock and is more obvious in the area close to the surface layer. This impact will generate large internal stress, which is prone to crack formation. In the case of low casting force, the change in his small, the thermal load shock is also slowed down, and the resulting stress may also be small enough not to initiate cracks. Therefore, in order to reduce cracks, the temperature difference at the interface between the molten steel and the casting roll must be reduced, which is also the reason for choosing a small diameter crystallization roll.

3 Conclusion

Thin strip casting and rolling can effectively inhibit the segregation of Cu, S, P, and other inclusion elements in the steel matrix, so as to realize the effective comprehensive utilization of inferior mineral resources (such as high phosphorus, high sulfur, high copper ores or scrap steel, etc.), saving valuable resources; Utilizing the advantages of rapid solidification of thin strip casting and rolling, it can produce materials that are difficult to form such as high silicon steel and high strength steel and has also made breakthroughs, but there is still a difficult process before industrialization. Under the current development environment of controlling the total amount of iron and steel, eliminating outdated production capacity, optimizing product structure, and realizing energy conservation and emission reduction, it is recommended that qualified enterprises increase their efforts to promote industrialization in order to develop standard industrialized production lines.

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