Normalizing-free low-iron-loss high-grade non-oriented silicon steel and preparation method and application thereof

Abstract

The invention relates to normalizing-free low-iron-loss high-grade non-oriented silicon steel as well as a preparation method and application thereof, and belongs to the technical field of cold rolling and steel for household appliances. The preparation method of the normalizing-free low-iron-loss high-grade non-oriented silicon steel comprises the treatment procedures of steelmaking, continuous casting, hot rolling, acid pickling, cold rolling and continuous annealing, and the cold rolling procedure comprises the steps of primary cold rolling, cover annealing and secondary cold rolling. On the basis of a secondary cold rolling method, the normalizing-free high-grade non-oriented silicon steel with low iron loss and excellent electromagnetic performance is prepared on the premise that the normalizing procedure is omitted by optimizing component design and technological parameters such as rolling and annealing, and the normalizing-free high-grade non-oriented silicon steel is stable in component and pure in steel quality.

Description

Technical Field

[0001] This invention relates to a non-normalized, low-iron-loss, high-grade non-oriented silicon steel, its preparation method, and its application, belonging to the technical field of cold-rolled and household appliance steel, specifically to the technical field of non-oriented electrical steel products. Background Technology

[0002] Several domestic enterprises with high-grade steel production capabilities have developed and matured 50W470 steel, characterized by low iron loss and excellent electromagnetic properties, meeting the technical specifications for steel used in large and medium-sized motors, and representing one of the future development trends in electrical steel. There are generally two conventional manufacturing processes for 50W470: one is a two-stage cold rolling and intermediate annealing method; the other is a normalizing and single-stage cold rolling method. Currently, the normalizing and single-stage cold rolling method is commonly used in China. The normalizing process, through high-temperature and time-holding treatment, optimizes the microstructure and deformation texture of the hot-rolled sheet, prevents corrugation defects, and ultimately improves the magnetic properties of the finished product. However, the normalizing process generally uses relatively high temperatures, which has a certain impact on production costs and strip surface quality; most importantly, implementing the normalizing process requires enterprises to add specialized normalizing treatment equipment, which undoubtedly significantly increases equipment investment costs and subsequent maintenance costs for small and medium-sized enterprises. Therefore, there is an urgent need in this field for a new technical solution that can still ensure that silicon steel products possess excellent electromagnetic properties such as low iron loss and high permeability without the normalizing process.

[0003] This invention, based on a two-stage cold rolling method, optimizes the composition design and process parameters such as rolling and annealing. By eliminating the normalizing process, it still ensures low iron loss and high magnetic permeability in silicon steel products. Particularly for production lines without normalizing equipment, the method described in this invention can produce high-grade non-oriented silicon steel products of comparable quality, providing a feasible approach for small and medium-sized enterprises to produce high-grade non-oriented silicon steel. Summary of the Invention

[0004] This invention provides a non-normalizing, low-iron-loss, high-grade non-oriented silicon steel, its preparation method, and its application. Specifically, by adopting a low-silicon, high-manganese, low-iron-loss chemical composition and controlling the purity of the steel, while adjusting and optimizing process parameters such as hot rolling (heating temperature, final rolling temperature, coiling temperature), cold rolling (reduction ratio distribution, bell annealing), and continuous annealing (temperature and strip speed), a non-normalizing, high-grade non-oriented silicon steel with both low iron loss and excellent electromagnetic properties is prepared. Its composition is stable and the steel is pure.

[0005] This invention employs a two-stage cold rolling and bell-type annealing process. Through a low-Si, high-Mn composition design, a reasonable distribution of the reduction rates during the two cold rolling stages (the first stage uses a critical reduction rate design), and an optimized bell-type annealing regime, high-grade non-oriented silicon steel with low iron loss for high-efficiency motors is successfully prepared under normalization-free conditions. This product exhibits an iron loss below 3.30 and a magnetic induction above 1.71. Compared to similar products, its overall magnetic performance is optimal, meeting the requirements for high-efficiency motors.

[0006] A method for preparing low-iron-loss, high-grade non-oriented silicon steel without normalization, the method comprising steelmaking, continuous casting, hot rolling, pickling, cold rolling, and continuous annealing processes, wherein the cold rolling process comprises a primary cold rolling, a bell-type annealing, and a secondary cold rolling step, wherein... The primary cold rolling step employs a critical reduction rate method, with a reduction rate of 5% to 15%. The bell-type annealing step uses a hydrogen atmosphere to first heat the temperature from room temperature to 230-260°C at a heating rate of 100-150°C / h; then heat it to 650-750°C at a heating rate of 35-45°C / h, hold it at that temperature for 6-10 h, and then cool it with the furnace to 280-330°C; finally, rapidly cool it to 80-120°C at a cooling rate of 25-30°C / h. The secondary cold rolling step is performed in 4 to 5 passes, with a reduction rate of 25% to 45% in the first 3 to 4 passes and a reduction rate of ≤12% in the last pass.

[0007] Preferably, the cold rolling step is performed in a single-stand 6-roll mill.

[0008] Preferably, the bell-type annealing step is carried out in a bell-type annealing furnace.

[0009] Furthermore, the present invention performs intermediate annealing between two cold rolling processes, which can store energy for recrystallization and effectively improve recrystallization power.

[0010] In the above technical solution, the specific operations of steelmaking and billet casting can be achieved using existing technologies.

[0011] In the above technical solution, the hot rolling process adopts a hot charging and hot delivery process to heat the slab to the furnace exit temperature. After the slab is heated, it is first rough rolled to obtain an intermediate slab with a thickness of 30~40 mm, and then finished rolled to obtain a hot rolled strip with a thickness of 2.3~2.5 mm. After cooling and coiling, a hot rolled steel coil is obtained. The furnace charging temperature is ≥500℃, the furnace exit temperature is 1120~1150℃, the finishing rolling temperature is 1000~1050℃, the final rolling temperature is 880~920℃, and the coiling temperature is 680~720℃.

[0012] Furthermore, the raw material composition of the slab is as follows: C: ≤0.003%, Si: 1.55%~1.75%, Mn: 0.70%~0.90%, P: 0.030%~0.045%, S: ≤0.0025%, Al: 0.50%~0.70%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, compared with products of the same grade, this invention adopts a low-Si, high-Mn composition design, which can effectively reduce iron loss, increase beneficial texture components, improve magnetism, and control corrugation defects; in addition, it controls the purity of molten steel and fully utilizes the role of Mn element.

[0014] Furthermore, the steel charging temperature of the present invention is ≥500℃ and the furnace exit temperature is 1120~1150℃. Under the above conditions, the solid solution and precipitation of second phases such as MnS and AlN can be effectively controlled.

[0015] Furthermore, the finishing rolling temperature of 1000~1050℃, the final rolling temperature of 880~920℃, and the coiling temperature of 680~720℃ described in this invention can effectively increase the grain size and optimize the microstructure under the above conditions. In addition, the thickness of the obtained hot-rolled steel coil is controlled at 2.3mm~2.5mm, which is beneficial to the optimization and realization of the reduction rate of the cold rolling process.

[0016] Furthermore, the thickness of the slab is 225~235 mm.

[0017] In the above technical solution, the pickling process uses hydrochloric acid solution for pickling.

[0018] In the above technical solution, the continuous annealing process has an annealing temperature of 900~960℃ and a strip speed of 80~120 m / min.

[0019] In the above technical solution, the method includes a post-processing step: rolling an insulating varnish onto the surface of the strip steel obtained after a continuous annealing process to obtain low-iron-loss, high-grade non-oriented silicon steel that is free from normalization.

[0020] Another object of the present invention is to provide a low-iron-loss, high-grade non-oriented silicon steel obtained by the above method.

[0021] Furthermore, the magnetic induction intensity of the low-iron-loss high-grade non-oriented silicon steel is ≥1.70 T, and the iron loss value P1.5 / 50 ≤3.5 W / kg.

[0022] Another object of the present invention is to provide the application of the above-mentioned low-iron-loss, high-grade non-oriented silicon steel in high-efficiency motors.

[0023] The beneficial effects of this invention are: (1) This invention uses a single-stand 6-roll mill for secondary cold rolling to produce high-grade non-oriented silicon steel. It adopts a “low at the beginning and high at the end” reduction rate distribution mode and uses the critical reduction rate method in the first rolling to optimize the texture and energy storage distribution. In addition, a low-temperature hood annealing process is used between the two rolling processes for intermediate annealing. This process can store energy for recrystallization, promote uniform growth of finished grain size, and reduce costs. It is more economical than the traditional high-temperature normalization and intermediate continuous annealing process.

[0024] (2) The finished steel obtained by the present invention is superior to or equivalent to high-grade non-oriented silicon steel produced by normalization and one cold rolling in terms of key performance indicators such as magnetic induction intensity and iron loss, thus achieving a good balance between performance and cost. Attached Figure Description

[0025] Figure 1 This is a diagram of the low-temperature hood-type annealing process in Example 1.

[0026] Figure 2 This is a diagram of the low-temperature hood-type annealing process in Example 2.

[0027] Figure 3 This is a diagram of the low-temperature hood-type annealing process in Example 3.

[0028] Figure 4 The image shows the metallographic structure of the non-oriented silicon steel products obtained in Example 1 and Comparative Example 1. Detailed Implementation

[0029] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0030] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0031] One of the specific implementation methods: A method for preparing low-iron-loss, high-grade non-oriented silicon steel without normalization includes the following steps: (1) Steelmaking and continuous casting processes Raw materials were weighed according to the chemical composition design, and steelmaking was carried out by hot metal pretreatment desulfurization, converter smelting, and RH refining. The chemical composition of the final molten steel, in mass percentage, is as follows: C: ≤0.003%, Si: 1.55%~1.75%, Mn: 0.70%~0.90%, P: 0.030%~0.045%, S: ≤0.0025%, Al: 0.50%~0.70%, with the balance being Fe and unavoidable impurities. Subsequently, the molten steel was continuously cast into slabs with a thickness of 225~235 mm using existing technology.

[0032] (2) Hot rolling process The hot-charged slab obtained in step (1) is heated to the furnace exit temperature using a hot-charging process. It is first rough-rolled to obtain an intermediate slab with a thickness of 30~40 mm, and then finished-rolled to obtain a hot-rolled strip with a thickness of 2.3~2.5 mm. After cooling and coiling, a hot-rolled steel coil is obtained. The furnace charging temperature is ≥500℃, the furnace exit temperature is 1120~1150℃, the finishing rolling temperature is 1000~1050℃, the final rolling temperature is 880~920℃, and the coiling temperature is 680~720℃.

[0033] (3) Pickling and cold rolling processes The hot-rolled steel coil obtained in step (2) is pickled with hydrochloric acid and then cold-rolled using a single-stand 6-roll mill, including a first cold rolling, a bell-type annealing, and a second cold rolling step. The primary cold rolling step employs a critical reduction rate method, with a reduction rate of 5% to 15%. The bell-type annealing step uses a hydrogen atmosphere, and the temperature is first heated from room temperature to 230-260°C at a heating rate of 100-150°C / h; then heated to 650-750°C at a heating rate of 35-45°C / h, held at that temperature for 6-10 h, and then cooled in the furnace to 280-330°C; finally, it is rapidly cooled to 80-120°C at a cooling rate of 25-30°C / h. The secondary cold rolling step is performed in 4 to 5 passes, with a reduction rate of 25% to 45% in the first 3 to 4 passes and a reduction rate of ≤12% in the last pass.

[0034] (4) Continuous annealing process The strip obtained in step (3) is subjected to continuous annealing, wherein the annealing temperature is 900~960℃ and the strip speed is 80~120 m / min.

[0035] In the above technical solution, the method includes a post-processing step: rolling an insulating varnish onto the surface of the strip steel obtained after a continuous annealing process to obtain low-iron-loss, high-grade non-oriented silicon steel that is free from normalization.

[0036] Example 1 A method for preparing low-iron-loss, high-grade non-oriented silicon steel without normalization includes the following steps: (1) Steelmaking and continuous casting processes Raw materials were weighed according to the chemical composition design, and steelmaking was carried out by hot metal pretreatment desulfurization, converter smelting, and RH refining. The chemical composition of the final molten steel, in mass percentage, is as follows: C: 0.0022%, Si: 1.60%, Mn: 0.85%, P: 0.033%, S: 0.0020%, Al: 0.60%, with the balance being Fe and unavoidable impurities. Subsequently, the molten steel was processed into slabs with a thickness of 230 mm using continuous casting equipment according to existing technology.

[0037] (2) Hot rolling process The slab obtained in step (1) is fed into the slab heating furnace by hot charging and hot loading. The furnace loading temperature is 550℃. After being heated to 1130℃, it is rolled into an intermediate slab with a thickness of 33 mm through multiple rough rolling passes. Then, it is rolled into a hot-rolled strip with a thickness of 2.5 mm through multiple finishing rolling passes. The average final rolling temperature is 894℃. Finally, it is cooled and coiled into a hot-rolled steel coil with an average coiling temperature of 705℃.

[0038] (3) Pickling and cold rolling processes After pickling the hot-rolled steel coil obtained in step (2) with hydrochloric acid, it was cold-rolled using a single-stand 6-roll mill. Specifically, the first cold rolling process was carried out using the critical reduction rate method, with a reduction rate of 7%, to obtain a strip with a thickness of 2.3 mm. Subsequently, an intermediate low-temperature bell-type annealing process was carried out, using a hydrogen atmosphere to heat the strip from room temperature to 240°C at a heating rate of 120°C / h. Then, it was heated from 240°C to the target holding temperature of 680°C and held for 9 hours at a heating rate of 41.5°C / h. After being cooled from 680°C to 300°C in the furnace, it was rapidly cooled to the furnace exit temperature of 100°C at a cooling rate of 25.5°C / h. The specific process is as follows: Figure 1 As shown; finally, a second cold rolling process was carried out using a single-stand 6-roll mill, with 4 passes. The reduction rate of the first 3 passes was 38%, and the reduction rate of the last pass was 9.5%, resulting in a strip with a finished thickness of 0.5 mm.

[0039] (4) Continuous annealing process The strip steel obtained in step (3) is annealed in a continuous annealing furnace at a temperature of 920°C and a strip steel speed (unit operating speed) of 100 m / min. Finally, insulating varnish is rolled onto the surface of the strip steel to obtain the above-mentioned non-oriented silicon steel with low iron loss and no normalization.

[0040] Example 2 (1) Steelmaking and continuous casting processes Raw materials were weighed according to the chemical composition design, and steelmaking was carried out by hot metal pretreatment desulfurization, converter smelting, and RH refining. The chemical composition of the final molten steel, in mass percentage, is as follows: C: 0.0020%, Si: 1.65%, Mn: 0.80%, P: 0.035%, S: 0.0022%, Al: 0.63%, with the balance being Fe and unavoidable impurities. Subsequently, the molten steel was used in continuous casting equipment to produce slabs with a thickness of 230mm according to existing technology.

[0041] (2) Hot rolling process The slab obtained in step (1) is fed into the slab heating furnace by hot charging and hot loading. The furnace loading temperature is 500℃. After being heated to 1150℃, it is rolled into an intermediate slab with a thickness of 35mm through multiple rough rolling passes. Then, it is rolled into a hot-rolled strip with a thickness of 2.5mm through multiple finishing rolling passes. The average final rolling temperature is 902℃. Finally, it is cooled and coiled into a hot-rolled steel coil with an average coiling temperature of 716℃.

[0042] (3) Pickling and cold rolling processes After pickling the hot-rolled steel coil obtained in step (2) with hydrochloric acid, it was cold-rolled using a single-stand 6-roll mill. Specifically, the first cold rolling process was carried out using the critical reduction rate method, with a reduction rate of 10%, to obtain a strip with a thickness of 2.25 mm. Then, an intermediate low-temperature bell-type annealing process was carried out, using a hydrogen atmosphere to heat the strip from room temperature to 245°C at a heating rate of 122°C / h. Next, it was heated from 245°C to the target holding temperature of 700°C and held for 8 hours at a heating rate of 43.5°C / h. After being cooled from 700°C to 310°C in the furnace, it was rapidly cooled to the furnace exit temperature of 90°C at a cooling rate of 26.5°C / h. The specific process is as follows: Figure 2 As shown; finally, a second cold rolling process was carried out using a single-stand 6-roll mill, with 4 passes. The reduction rate of the first 3 passes was 42%, and the reduction rate of the last pass was 12%, resulting in a strip with a finished thickness of 0.5 mm.

[0043] (4) Continuous annealing process The strip steel obtained in step (3) is annealed in a continuous annealing furnace at a temperature of 900°C and a strip steel speed (unit operating speed) of 110 m / min. Finally, insulating varnish is rolled onto the surface of the strip steel to obtain the above-mentioned non-oriented silicon steel with low iron loss and no normalization.

[0044] Example 3 (1) Steelmaking and continuous casting processes Raw materials were weighed according to the chemical composition design, and steelmaking was carried out by hot metal pretreatment desulfurization, converter smelting, and RH refining. The chemical composition of the final molten steel, in mass percentage, is as follows: C: 0.0019%, Si: 1.70%, Mn: 0.90%, P: 0.035%, S: 0.0025%, Al: 0.65%, with the balance being Fe and unavoidable impurities. Subsequently, the molten steel was processed into slabs with a thickness of 230 mm using continuous casting equipment according to existing technology.

[0045] (2) Hot rolling process The slab obtained in step (1) is fed into the slab heating furnace by hot charging and hot loading. The furnace loading temperature is 520℃. After being heated to 1120℃, it is rolled into an intermediate slab with a thickness of 34 mm through multiple rough rolling passes. Then, it is rolled into a hot-rolled strip with a thickness of 2.5 mm through multiple finishing rolling passes. The average final rolling temperature is 911℃. Finally, it is cooled and coiled into a hot-rolled steel coil with an average coiling temperature of 702℃.

[0046] (3) Pickling and cold rolling processes After pickling the hot-rolled steel coil obtained in step (2) with hydrochloric acid, it was cold-rolled using a single-stand 6-roll mill. Specifically, the first cold rolling process was carried out using the critical reduction rate method, with a reduction rate of 12%, resulting in a strip with a thickness of 2.2 mm. Subsequently, an intermediate low-temperature bell-type annealing process was performed, using a hydrogen atmosphere to heat the strip from room temperature to 235°C at a heating rate of 128°C / h. Then, it was heated from 235°C to the target holding temperature of 730°C and held for 7 hours at a heating rate of 44.6°C / h. After being cooled from 730°C to 300°C in the furnace, it was rapidly cooled to the furnace exit temperature of 100°C at a cooling rate of 26.2°C / h. The specific process is as follows: Figure 3 As shown; finally, a second cold rolling process is carried out using a single-stand 6-roll mill, with 4 passes. The reduction rate of the first 3 passes is 40%, and the reduction rate of the last pass is 10%, resulting in a strip with a finished thickness of 0.5 mm.

[0047] (4) Continuous annealing process The strip steel obtained in step (3) is annealed in a continuous annealing furnace at a temperature of 940°C and a strip steel speed (unit operating speed) of 120 m / min. Finally, insulating varnish is rolled onto the surface of the strip steel to obtain the above-mentioned non-oriented silicon steel with low iron loss and no normalization.

[0048] Comparative Example 1 The chemical composition of this comparative example is the same as that of Example 1. The difference is that the pickling and cold rolling processes in step (3) are different. All other operations are the same as those in Example 1. Specifically, the hot-rolled steel coil obtained in step (2) is pickled with hydrochloric acid and then normalized using a high-temperature normalizing system. The normalizing temperature is 900℃ and the strip speed in the process section is 40 m / min. Then, a single-stand 6-roll mill is used for rolling. The rolling is divided into 5 passes. The reduction rate of the first 4 passes is 40%, and the reduction rate of the last pass is 10%. The thickness of the finished product is 0.5 mm.

[0049] Comparative Example 2 The difference between this comparative example and Example 3 is that the chemical composition of the molten steel in step (1) is different. By mass percentage, C: 0.0025%, Si: 1.90%, Mn: 0.45%, P: 0.040%, S: 0.0020%, Al: 0.40%, with the balance being Fe and unavoidable impurities. All other operations are the same as in Example 3.

[0050] According to GB / T 3655 standard, the magnetic properties of the non-oriented silicon steel products obtained in Examples 1-3 and Comparative Examples 1 and 2 were tested, and the results are shown in Table 1. It can be seen that compared with silicon steel obtained by normalizing treatment and a single cold rolling process, the silicon steel produced by the normalizing-free intermediate annealing and two-rolling method provided by this invention has superior electromagnetic properties, with iron losses generally equal to or lower than the former, and also possesses superior magnetic induction intensity. A comparison shows that silicon steel produced by the method of this invention and silicon steel made with ordinary silicon-manganese composition has essentially the same electromagnetic properties. The non-normalizing, low-iron-loss, high-grade non-oriented silicon steel produced by the method of this invention achieves an iron loss below 3.30 and a magnetic induction intensity above 1.71, reaching the advanced level in China. Compared with normalizing and single rolling methods, this invention is more suitable for the development of production lines without electromagnetic stirring and normalizing equipment, and for the production of high-grade silicon steel products. The method is simple, easy to operate, and can quickly yield results without investment, making it of significant value for widespread application.

[0051] Table 1. Performance test results of the non-oriented silicon steel products obtained in Examples 1-3 and Comparative Examples 1-2

[0052] Figure 4 The images show the metallographic structures of the finished non-oriented silicon steel products obtained in Example 1 and Comparative Example 1. It can be seen that the average grain size of the high-grade non-oriented silicon steel product obtained in Example 1 using the method described in this invention is larger than the grain size of the comparative example. The non-oriented silicon steel obtained by the method described in this invention has a larger grain size, fewer grain boundaries, lower domain wall movement resistance, reduced iron loss, and superior magnetic properties.

Claims

1. A method for preparing low-iron-loss, high-grade non-oriented silicon steel without normalization, characterized in that: The preparation method includes steelmaking, continuous casting, hot rolling, pickling, cold rolling, and continuous annealing processes. The cold rolling process includes a first cold rolling, a bell-type annealing, and a second cold rolling step. The primary cold rolling step employs a critical reduction rate method, with a reduction rate of 5% to 15%. The bell-type annealing step uses a hydrogen atmosphere to first heat the temperature from room temperature to 230-260°C at a heating rate of 100-150°C / h; then heat it to 650-750°C at a heating rate of 35-45°C / h, hold it at that temperature for 6-10 h, and then cool it with the furnace to 280-330°C; finally, rapidly cool it to 80-120°C at a cooling rate of 25-30°C / h. The secondary cold rolling step is performed in 4 to 5 passes, with a reduction rate of 25% to 45% in the first 3 to 4 passes and a reduction rate of ≤12% in the last pass.

2. The preparation method according to claim 1, characterized in that: The hot rolling process employs a hot charging and hot delivery process to heat the slab to the furnace exit temperature. After rough rolling, an intermediate slab with a thickness of 30-40 mm is obtained, followed by finish rolling to obtain a hot-rolled strip with a thickness of 2.3-2.5 mm. Subsequently, after cooling and coiling, hot-rolled steel coils are obtained. The furnace charging temperature is ≥500℃, the furnace exit temperature is 1120-1150℃, the finish rolling temperature is 1000-1050℃, the final rolling temperature is 880-920℃, and the coiling temperature is 680-720℃.

3. The preparation method according to claim 2, characterized in that: The raw material composition of the slab, based on mass percentage, is as follows: C: ≤0.003%, Si: 1.55%~1.75%, Mn: 0.70%~0.90%, P: 0.030%~0.045%, S: ≤0.0025%, Al: 0.50%~0.70%, with the balance being Fe and unavoidable impurities.

4. The preparation method according to claim 2, characterized in that: The thickness of the slab is 225~235 mm.

5. The preparation method according to claim 1, characterized in that: The continuous annealing process has an annealing temperature of 900~960℃ and a strip speed of 80~120 m / min.

6. The preparation method according to claim 1, characterized in that: The method includes a post-processing step: rolling an insulating varnish onto the surface of the strip steel obtained after a continuous annealing process to obtain low-iron-loss, high-grade non-oriented silicon steel that is free from normalization.

7. Low-iron-loss, high-grade non-oriented silicon steel prepared by the method according to any one of claims 1 to 6.

8. The low-iron-loss, high-grade non-oriented silicon steel according to claim 7, characterized in that: The low-iron-loss, high-grade non-oriented silicon steel has a magnetic induction intensity ≥1.70 T and an iron loss value P1.5 / 50 ≤3.5 W / kg.

9. The application of the low-iron-loss, high-grade non-oriented silicon steel as described in claim 7 in high-efficiency motors.

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