1. Production status
At present, according to the different formation methods of inhibitors, the industrial production technology of oriented electrical steel can be divided into two types. One is the "inherent inhibitor method", in which the formation elements of the inhibitor are added during smelting, and the coarse precipitates (such as MnS, AlN, MnSe, etc.) precipitated during solidification are completely dissolved by high-temperature heating of the slab (> 1300 ℃) , And then re-precipitate in the form of fine dispersion during the subsequent hot rolling or normalization, acting as an inhibitor. The other is the "obtain inhibitor method", which adjusts the aluminum and nitrogen content in the steel during smelting, reheats the slab at a low temperature (1100-1200 ℃), does not require the inhibitor to be completely dissolved, and before high temperature annealing Nitriding treatment is used to obtain new fine and dispersed nitride inhibitors to enhance the suppression ability.
Because the production of oriented electrical steel in thin slab continuous casting and rolling process has many advantages compared with the traditional process, many countries are competing to carry out research in this area. The most representative foreign countries are the Italian AST company and the German Thyssen company, both of which have carried out the production of oriented electrical steel, and have applied for related patents in this research field; the continuous casting center of the Central Iron and Steel Research Institute has conducted a lot of research work, and at the same time The technology was successfully applied to Wuhan Iron and Steel CSP process production line, and trial production of oriented silicon steel has been successfully carried out.
When the Italian AST company uses the CSP process to produce CGO and Hi-B steel, it uses the "inherent inhibitor method" combined with the "obtain inhibitor method", and the heating temperature of the thin slab is 1150-1300 ° C. The hot-rolled sheet is subjected to two-stage normalization, one-time cold rolling, and decarburization annealing, followed by high-temperature gas nitriding at about 900 ° C. The production of ordinary oriented silicon steel and highly magnetically oriented silicon steel reaches the typical values ​​of Nippon Steel Z8 and Z6H, respectively. . Thyssen in Germany also adopts the same inhibitor design ideas and process flow. The heating temperature of the thin slab is 1150 ℃, and the rolling reduction and final rolling temperature are controlled during hot rolling. The magnetic induction value B8 of the finished product can reach 1.89 without nitriding treatment. T, increased to 1.93T after nitriding treatment. The process of producing oriented silicon steel by AST Steel Company adopts electric furnace process, and the production efficiency is low. The production line stopped production of silicon steel in October 2005. The TSCR process has changed from the original electric furnace process to the converter process. The TSCR process of the converter process has obvious advantages in production efficiency and cost. AST Steel has realized the production of oriented silicon steel on the TSCR production line.
The National Continuous Casting Center of the General Iron and Steel Research Institute simulated the thin slab continuous casting and rolling process in the laboratory. The heating temperature of the thin slab was 1150-1180 ℃. The CGO and Hi-B steels were successfully trial-produced by the "inherent inhibitor method". The magnetic properties of the finished products were respectively Reached the level of 27Q140, 30Q130 and 30QG130. In addition, Hi-B steel was successfully trial-produced using the "inherent inhibitor method" in combination with the solid-phase nitriding method, and the magnetic properties of the finished product reached the level of 30QG120. Through the "Eleventh Five-Year" National Science and Technology Support Project "New Technology Research on Oriented Electrical Steel Production for Thin Slab Continuous Casting and Rolling", Wuhan Iron and Steel Co., Ltd. and the continuous casting center cooperated in research and development, and successfully conducted trial production of ordinary oriented silicon steel using the CSP process. The magnetic properties reach the level of 30Q130 and 27Q140.
2. Technical analysis of the production of oriented electrical steel 2.1. Organization and characteristics of inhibitors The cold-rolled oriented electrical steel (CGO) obtained by trial production of the thin slab continuous casting and rolling process on the laboratory simulation demonstration line of the National Continuous Casting Center of the Central Iron and Steel Research Institute ) As an example, introduce the structure and inhibitor characteristics of oriented electrical steel produced by thin slab continuous casting and rolling process. The trial production process is shown in the original figure 1.
Original Figure 1 is a schematic diagram of the process flow of laboratory oriented electrical steel in trial production 2.1.1. Organization evolution rule Observation of the structure of the hot-rolled sample of oriented silicon steel trial produced in the laboratory simulation TSCR process, from the surface to the surface of 1/5, 1/4 thickness The decarburized layer mainly contains recrystallized ferrite grains. The grains are irregular polygonal with a size of about 5-40 μm and an average size of about 15.6 μm. The transition layer and the center layer located in the center area contain deformed grains , Sub-grains and partially recrystallized grains. There is fine pearlite in the transition layer and the center layer, and the elongated coarse ferrite area is between the network carbides. The structure of the primary cold-rolled sample is shown in the original figure 2 (a). The grain shape changes. As the amount of deformation increases, the equiaxed grains in the hot-rolled sheet extend in the rolling direction. The structure after intermediate annealing is as shown in the original figure 2 (b). The rolled deformed structure undergoes recovery and recrystallization, and the formed grains are more regular, gradually approaching the equiaxed grains. The grain size is about 10-105 μm, and the size shows obvious unevenness. The structure of the secondary cold-rolled sample is shown in the original Figure 2 (c). Due to the large amount of deformation, some grain boundaries become blurred, and the grains are difficult to distinguish and appear as fibrous stripes. The microstructure of the cold-rolled sample after decarburization annealing is shown in the original figure 2 (d). Recovery and recrystallization occurred after decarburization annealing, the primary grains were more fine and uniform, and the average grain size was about 15.5 μm. The secondary crystals in the finished product The average size of the particles is 4-5cm, and the average is about 4.6cm, indicating that the secondary recrystallization of oriented silicon steel produced by the laboratory simulation TSCR process is well developed.
Original Figure 2 shows the structure of thin slab continuous casting and rolling process to produce CGO oriented electrical steel 2.1.2. Characteristics of inhibitors Inhibitors play a beneficial role in the decarburization annealing and high temperature annealing process, so the strength of the inhibitor After decarburization annealing, its distribution in the steel is determined. The samples after the secondary cold rolling recovery annealing of oriented silicon steel were subjected to the initial recrystallization test, and the distribution of the inhibitor Cu2S in the initial recrystallization samples after recovery annealing was observed, as shown in FIG. 3. Among them, point A is typical Cu2S particles. Through multi-field statistics, the size of Cu2S particles in the first recrystallized sample is about 20-40nm, the average diameter is about 25nm, the distribution density is about 2.13 × 1014 particles / cm3, volume fraction It is 0.167%. The AlN formed in the steel is shown as points B and C in FIG. The auxiliary inhibition effect of AlN makes the growth of primary crystal grains better suppressed, which is beneficial to the development of secondary recrystallization.
Original Figure 3 shows the distribution of inhibitors in the first recrystallized sample after recovery annealing 2.2. The technical advantages of the thin slab continuous casting and rolling process and the traditional process for the production of oriented electrical steel and the difference in thermal history are shown in the following table. Compared with the traditional slab process, the technical advantages are as follows.
The original chart shows the comparison of the production process of oriented electrical steel in the thin slab continuous casting and rolling process and the traditional slab process.
1) The slab heating temperature is low and the time is short. Compared with the traditional slab process, the thickness of the thin slab process (<100mm) is much smaller than the thickness of the traditional continuous casting slab (200-300mm). The surface temperature of the thin slab> 950 ℃ enters the soaking furnace, after heating at a temperature of 1100-1200 ℃ for a short time (<1h), it can ensure the uniformity of the temperature from the core of the slab to the surface layer. Thin slab heating at low temperature and short time can avoid or reduce the problems caused by high temperature heating (1300-1400 ℃) in the traditional slab process. For example, high-temperature heating is easy to cause coarse grains in the slab, and hot cracks appear during heating or rolling. At the same time, the edge cracking of the hot-rolled plate obviously causes a large amount of trimming, which reduces the quality, output and yield of the product.
2) The thickness of the hot-rolled coil can be less than 1.2mm, and the ultra-thin oriented silicon steel product with a thickness of ≤0.23mm can be produced by one-time cold rolling method. At the same time, the thin slab hot rolling process does not require rough rolling, and the cast slab can be directly rolled into a hot rolled coil with a thickness of 2.0-2.5mm. In addition, in terms of controlling the hot-rolled sheet shape, the strip thickness and hot crown control accuracy of the TSCR process is more than double that of the traditional process. This will also be very beneficial for the production of high magnetic orientation oriented electrical steel by a large cold reduction method. Compared with the traditional slab process, the process of producing oriented silicon steel in the TSCR process is greatly shortened, the production efficiency will be greatly improved, and the production cost will also be greatly reduced.
3) The as-cast structure is fine and uniform, which is beneficial to the control of the structure. Because the solidification speed of the thin slab (thickness about 50mm) in the crystallizer and the cooling rate distribution at high temperature (above 1400 ° C) are about 1 order of magnitude higher than that of the traditional thick slab (thickness about 210mm), its original cast structure Compared with thick slabs, it is more fine and uniform. The size of the equiaxed crystal is generally less than 1.5mm, so the electromagnetic stirring process can be omitted. The cast billet is heated at a low temperature for a short time, and the structure is not easy to roughen. At the same time, the grain size of the cast billet is small, so it is easier to obtain a favorable initial grain size, and it can increase the yield rate and reduce the production cost.
4) The second phase precipitates are fine, which is conducive to the control of inhibitors. Due to the fast solidification speed of the thin slab, the texture is more uniform, the macrosegregation and the center looseness are small, and the anisotropy is not obvious, so that the element segregation of the thin slab is slightly smaller than that of the thick slab (the degree of segregation of the thin slab in the middle of the slab) Only 20% of the thick billet) helps to reduce the size of AlN, MnS and other precipitates and distribute them evenly. The research results show that the second phase precipitates in the cast slab produced by the CSP process are finely dispersed, and the average size is not more than 60nm. At the same time, because the thin slab has a low soaking temperature and short time, it is conducive to controlling the precipitation. Roughening and growing, keeping a large amount of precipitates can still be distributed in thin slabs with fine dispersion.
5) The type of inhibitor obtained is more favorable, and the inhibitory ability of the inhibitor may be higher. When the low temperature slab heating process (1150-1300 ℃) is used to produce Hi-B steel, because the inhibitor cannot be fully dissolved, in order to enhance the inhibitory ability of the inhibitor, generally adopt a low temperature nitriding treatment at about 750 ℃ ​​before high temperature annealing, and Thin slab process adopts high temperature nitriding treatment at about 900 ℃. The high-temperature nitriding treatment can not only increase the rate of nitrogen infiltration and diffusion into the steel sheet, so as to directly form part of the required AlN inhibitor, rather than the final inhibitor mainly based on (Al, Si) N after low-temperature nitriding treatment. Therefore, the inhibitory effect of the inhibitor may be stronger than that of the low-temperature slab process, and the magnetic properties of the finished product are better.
2.3. Technical difficulties In order to realize the production of oriented electrical steel in the process of thin slab continuous casting and rolling, the main technical difficulty that must be solved is to ensure the precipitation amount, distribution and size of the inhibitor to meet the requirements of the inhibitor, that is, to ensure that the high temperature annealing process There are a sufficient number of finely dispersed inhibitors to inhibit the growth of the first grain.
Because the heating furnace temperature in the thin slab continuous casting and rolling process is only 1150-1200 ℃, the use of traditional inhibitors and their control is no longer applicable to the thin slab continuous casting and rolling process. Therefore, the core of the production of oriented electrical steel using the "TSCR + inherent inhibitor method" is to adapt to the formulation of the inhibitor plan for the thin slab process. It is necessary to combine the TSCR process with the structure and texture characteristics of the oriented silicon steel to carry out the thermodynamic and kinetic analysis of the inhibitor. Ensure the inhibitory effect of the inhibitor under low temperature heating conditions. The core of the research on the production of oriented electrical steel using the “TSCR + obtaining inhibitor method†not only includes the formulation of the inherent inhibitor before nitriding treatment, but more importantly, the formulation of the gaseous nitriding process. Including the combined sequence of decarburization annealing and gaseous nitriding process, the level of nitriding temperature, etc., so that nitrogen can quickly infiltrate into the steel strip in a short time to obtain the required appropriate amount of nitriding and the type, size and size of nitride precipitation phase Distribution, and in the high temperature annealing and heating stage conversion to obtain the appropriate type, quantity, size and distribution of effective nitride inhibitor (AlN or (Si, Al) N) to meet the requirements of the inhibitor. At the same time, in the decarburization annealing, gaseous nitriding and high temperature annealing processes, the suitable primary recrystallization size and grain uniformity, the appropriate nitriding amount and the effective nitride inhibitor volume fraction, and the appropriate secondary recrystallization size are met . The unification of these three can ensure the excellent and stable magnetic properties of the finished product.
3. The development trend adopts nitriding treatment to obtain the required inhibitor in the steel. Since the inherent inhibitor in the steel is not required to be completely dissolved, it can not only greatly reduce the soaking temperature of the slab, but also overcome the aluminum in the steel during smelting. Fluctuation of nitrogen content to stably produce Hi-B steel with excellent magnetic properties. Therefore, the main development trend of using thin slab continuous casting and rolling process to produce oriented electrical steel is to adopt the "obtain inhibitor method", that is, nitriding treatment to stably produce Hi-B steel. At the same time, the use of high temperature nitriding has also become the development trend of nitriding treatment. It can not only increase the rate of nitrogen infiltration and diffusion into the steel sheet, so that nitrogen can be better diffused into the steel strip, affecting the type of nitride precipitation during the nitriding With distribution. And make the inhibitor in the final steel exist in different forms, and in the process of nitriding, it may directly form part of the required AlN inhibitor. However, the use of high temperature nitriding needs to ensure that the size of the first recrystallized grains is in the appropriate range, that is, the grains are not easy to grow during the high temperature nitriding process.
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