1 Technical background
Steel production is a typical process industry. The traditional process production process is affected by the connection of processes. There are repeated cooling and heating processes of steel billets, which leads to large energy consumption and CO2 emissions. Fully tapping the connection technology of casting-rolling processes, realizing near-net-shape manufacturing in the steel preparation process, reducing or eliminating the heating or heating links in the connection process of casting and rolling processes, and greatly reducing the processing process are of great significance for promoting energy conservation and emission reduction of steel materials and accelerating the green and low-carbon transformation of the steel industry. The twin-roll thin strip casting and rolling technology realizes the effective connection of casting-rolling processes. After forming, the cast strip does not need to be heated again. Usually, one hot rolling is used to complete the forming process, which greatly reduces the number of rolling passes and reduces the energy consumption and CO2 emissions of the hot rolling process. Practice shows that this technology has the following advantages: 1) The casting-rolling process is integrated. The molten steel is formed into a cast strip by two counter-rotating crystallizers. There is no need to heat the cast strip. It directly enters the rolling process to obtain a thin strip of a certain thickness. The rolling process eliminates the heating furnace or induction heating supplementary heat link, which saves energy and reduces consumption, and is green and environmentally friendly. 2) The grain morphology, precipitation distribution and texture characteristics of the sub-rapid solidification process meet the performance control requirements of electrical steel. The unique sub-rapid (100-1000℃/s) solidification characteristics of thin strip casting and rolling enable the microstructure of the cast strip to flexibly control the equiaxed crystal and columnar crystal morphology, eliminate segregation, and suppress the precipitation of the second phase, thereby greatly improving the favorable organization and texture of the final product of electrical steel, significantly improving the magnetic induction and reducing iron loss. 3) The hot rolling process of thin strip casting and rolling electrical steel is short, which can also reduce the subsequent processing volume, and the subsequent cold rolling process eliminates normalization, decarburization/nitriding and other links, further reducing energy consumption and carbon emissions. The average energy consumption per ton of steel is reduced by 40kg of standard coal by eliminating annealing heat treatment processes such as normalization, decarburization and nitriding. In recent years, Northeastern University has conducted exploratory research on the production of three types of steel, non-oriented silicon steel, oriented silicon steel and special-purpose silicon steel by thin strip casting and rolling process, and compiled the research results into a book “Mechanism of Microstructure and Performance Control of Twin-roll Thin Strip Continuous Casting High-performance Electrical Steel”. The research results mainly include special solidification phenomena in casting and rolling, exploration of new inhibitor design and control methods, exploration of abnormal growth mechanism, and evolution law of silicon steel microstructure and magnetic properties under short process conditions, etc., providing theoretical guidance for the research and practice of industrialization technology of thin strip casting and rolling silicon steel.
2 Research progress
2.1 Theoretical research progress of non-oriented silicon steel strip casting
The crystal rotation and shear band deformation behaviors of specially oriented grains in the cast strip during cold rolling were systematically studied, and the process conditions for controlling the casting and rolling parameters to obtain ideal annealing texture and magnetic properties were proposed. The solidification structure of Fe-1.3%Si steel has coarse and uniform grains, forming some {100} and {110} oriented components. The specially oriented grains in the middle of the casting strip tend to complete the plastic deformation process through geometric softening via shear band deformation. The high-density dislocation unit cell of the shear band obtains Cube and Goss η orientations under the influence of minimum strain energy. The shear band orientation in the {110}<110> oriented grains is mainly Cube, while the {111}<112> oriented grains form a very strong Goss oriented shear band structure. The shear band deformation of oriented grains is not sufficient. Recrystallization is carried out through directional nucleation on a large number of Cube-oriented shear bands to form a favorable Cube texture, which significantly improves the magnetic properties of the annealed sheet (Figure 1).
Figure 1 Cube grain nucleation and growth during cold-rolled shear band deformation of thin strip casting non-oriented silicon steel
2.2 Progress in theoretical research on thin strip casting oriented silicon steel
1) The evolution law of Goss texture of oriented silicon steel under thin strip continuous casting conditions was systematically studied. Different from the conventional process, the Goss texture of thin strip continuous casting oriented silicon steel mainly originates from the shear band inside the γ-cold deformed grains and is evenly distributed along the thickness direction of the plate, solving the key Gauss texture “seed” problem of thin strip continuous casting oriented silicon steel (Figure 2).
Figure 2 reveals the origin of secondary recrystallized Goss grains in oriented silicon steel
2) Based on the characteristics of sub-rapid solidification of the thin strip continuous casting process, an innovative ultra-low carbon composition design of oriented silicon steel was proposed, which eliminated the subsequent decarburization annealing and further simplified the production process; it overcame the AlN inhibitor of conventional carbon-containing oriented silicon steel. The inherent problem of uneven precipitation distribution was solved, which significantly improved the utilization efficiency of the inhibitor and successfully prepared high magnetic induction oriented silicon steel with a magnetic property B8 of more than 1.9T.
2.3 Research progress of thin strip casting and rolling special silicon steel
2.3.1 Preparation technology and mechanism of strong {100} texture non-oriented silicon steel
The ultimate goal of the development of non-oriented silicon steel is to develop a new generation of strong {100} textured non-oriented silicon steel and minimize magnetic anisotropy and intra-periodic magnetic induction fluctuations. In the traditional process, since the ingot undergoes a large rolling compression, the finished plate has a strong γ texture and a weak Cube texture, resulting in a lower magnetic induction of the product. The two major characteristics of the thin strip continuous casting process, namely sub-rapid solidification and near-net forming, give silicon steel a unique evolution law of microstructure, providing a feasible technical route for the preparation of strong {100} texture non-oriented silicon steel. Through the unique controllable abnormal growth phenomenon of {100} oriented grains in thin strip casting and rolling, combined with tension reciprocating rolling and simple heat treatment process, the proportion of Cube oriented grains can be controlled, thereby obtaining excellent magnetic properties. The rolling direction magnetic induction B50 of the 0.5mm thick annealed plate reaches 1.87T, while the transverse magnetic induction reaches 1.86T. Its magnetic induction value is significantly higher than that of similar products currently published (Figure 3). The corresponding iron loss values
Figure 3: Microstructure and texture control of high magnetic induction {100} textured non-oriented silicon steel
2.3.2 Microstructure and property control of high magnetic induction oriented Fe-6.5%Si steel
Oriented Fe-6.5%Si steel (also known as super silicon steel) with uniform Goss texture has significant advantages such as high magnetic permeability, low coercivity and low iron loss. The magnetostriction value of this super silicon steel in the <001> direction is 0, making it the most ideal soft magnetic alloy material for high-frequency iron cores. Due to the increase in Si content, the matrix of Fe-6.5%Si steel is single-phase ferrite at the full solidus temperature, which makes the grains very easy to coarsen, and the matrix has very poor room temperature plasticity, which cannot meet the rolling requirements at all. Therefore, in order to smoothly realize the rolling process and obtain perfect secondary recrystallization, it is required to have a fine grain size before rolling and to dissolve enough grain growth inhibitors. Cast-rolled silicon steel has played a key process advantage in solving the key problem of insufficient toughness of super silicon steel, and has achieved the following technological innovations and progress: 1) It was found that the single-phase BCC structure of the Fe-6.5%Si alloy casting strip is uniformly distributed in the solidified shell There are a large number of Σ3 and Σ9 grain boundaries, accounting for 21.9% and 9% respectively. Among them, the Σ3 grain boundary has typical twin characteristics and forms unique solidification twins. 2) A large number of Σ3 grain boundaries enable the Fe-6.5%Si alloy strip to be formed by warm rolling + cold rolling, and some Σ3 grain boundaries are distributed in the primary recrystallization matrix and are formed during the secondary recrystallization annealing process. It plays a role in controlling the normal growth of grains. For the first time, low-energy and high-angle grain boundaries were introduced into the secondary recrystallization process of oriented silicon steel. 3) The Goss abnormal growth structure with accurate orientation and perfect development was obtained, with B8 magnetic induction value of 1.77T and P10/50 value of 0.52W/kg. The magnetic induction exceeds similar studies by more than 0.1T (Figure 4).
Figure 4 Development of high magnetic induction oriented Fe-6.5Si steel
3 Development and prospects
The unique sub-rapid solidification and near-net forming characteristics of thin strip continuous casting have unique advantages in the preparation of high-performance electrical steel. It is very likely to provide solutions for the development of a new generation of high-performance silicon steel products, and is expected to become a breakthrough in the technical bottleneck of electrical steel preparation, bringing new opportunities for the leapfrog development of my country’s electrical steel industry.
