High magnetic induction oriented silicon steel with excellent plate shape and production method

Abstract

This invention discloses a method for producing high-magnetic-induction oriented silicon steel with excellent strip shape. Before high-temperature annealing, a layer of magnesium oxide release agent is coated on the surface of the strip. The particle size of the magnesium oxide meets the following requirements: 1μm≤D50≤7μm and 9μm≤D90≤18μm. The partial pressure ratio during continuous decarburization annealing is 0.1~0.4. The high-magnetic-induction oriented silicon steel product prepared by the method of this invention has a strip length with warping deformation reduced from about 3000m to less than 200m, effectively reducing the length of the warping deformation.

Description

Technical Field

[0001] This invention relates to the field of grain-oriented silicon steel manufacturing technology, specifically to a high magnetic induction grain-oriented silicon steel with excellent plate shape and its production method. Background Technology

[0002] Grain-oriented silicon steel is a soft magnetic material used in the core manufacturing of transformers and related electrical products. High-magnetic-induction grain-oriented silicon steel refers to grain-oriented silicon steel with a magnetic induction intensity of B800 (magnetic induction intensity of grain-oriented silicon steel sheet under an applied magnetic field strength of 800 A·m) ≥ 1.88 Tsla. The process involves heating continuously cast slabs with a silicon content of 2–4.5% and containing certain inhibitory components (such as Al, N, etc.) to 1100–1250℃, hot-rolling to 1.8–2.8 mm, then normalizing and annealing at around 1100℃, followed by a single cold rolling to a finished thickness of 0.15 mm–0.35 mm. This is followed by decarburization and primary recrystallization annealing, application of a high-temperature annealing release agent, and then high-temperature long-term annealing at a maximum temperature of around 1200℃. Finally, an insulating coating is applied to the surface, the material is dried, and then subjected to stretching and leveling annealing to obtain the finished high-magnetic-induction grain-oriented silicon steel.

[0003] During the aforementioned high-temperature, long-duration annealing process, grain boundary slip caused by high-temperature creep can easily lead to strip deformation, resulting in a central bulge (ship-shaped, see appendix). Figure 1 Generally, these deformations can be eliminated through subsequent stretching and leveling annealing. If the bulge height h caused by deformation is less than 30 mm, applying tension at a plate temperature of around 800°C to achieve an elongation of less than 0.3% during stretching and leveling will flatten the strip without affecting the magnetic properties of the finished product. If the bulge height h is greater than or equal to 30 mm, stretching and annealing at a plate temperature of 800°C with an elongation of 0.3% to 0.75% will generally flatten the strip. However, due to excessive elongation, secondary grain deformation occurs, leading to a deterioration in orientation and magnetic properties. If the bulge height h is greater than or equal to 30 mm, using a smaller elongation (<0.3%) will not effectively eliminate the central bulge deformation, resulting in warping defects (see Appendix). Figure 2 When grain-oriented silicon steel sheets with poor sheet shape are used to manufacture core laminations, the lamination factor decreases and the no-load performance of the core deteriorates. Therefore, it is necessary to reduce the deformation of high-magnetic-induction grain-oriented silicon steel during high-temperature annealing to achieve excellent magnetic properties and sheet shape.

[0004] To improve strip deformation during high-temperature annealing, Nippon Steel's patent JPH03177518A suggests arranging C-type electromagnetic induction heating at the strip edge during continuous decarburization annealing to coarsen the edge grains and prevent edge deformation during high-temperature annealing. Kawasaki's patent JP2001262233A suggests that for Bi-added high magnetic induction oriented silicon steel, the winding tension of the outer ring (at least 20mm thick) should be ≥60MPa, and the dwell time in the high-temperature annealing temperature range of 900 to 1100℃ should not exceed 40 hours, which can reduce the amount of deformation at the lower end face edge. JFE patent JP2006274343A suggests placing a metal ring on the inner ring of the upper end face of the vertical coil before high-temperature annealing. The ring is inserted into the inner core and suspended at the top to prevent inner ring distortion. Nippon Steel patent JPH551643A suggests coating the lower end face of the outer ring of the vertical coil in the annular furnace with insulating material to prevent deformation caused by thermal stress from the contact between the lower end of the steel coil and the base plate. Kawasaki patent JPS61124529A proposes that, to prevent strip deformation, the edges are not rolled to the finished thickness during cold rolling. This way, during continuous annealing and coiling, only the thicker edge is rolled up, and placing this thicker edge on the lower end face of the annular furnace can reduce the deformation of the lower end face edge of the vertical coil. The deformation of the lower end face of the coil is a concern. Kawasaki Patent JPH11246913A2 suggests that during continuous annealing, the tension (T1, T2, T3) of the inner, middle, and outer rings should satisfy T3≥T1≥T2, and the coating amount (W1, W2, W3) of the corresponding parts should satisfy W3≤W1≤W2, which can reduce edge waviness and edge deformation. Kawasaki Patent JP2003171719A (2001) proposes that during cold rolling, the thickness of the lower end face and the middle of the ring furnace should be consistent, while the thickness of the upper end face should be reduced to prevent edge deformation of the lower end face. Nippon Steel Patent JPH0297622A points out that by coarsening the grains at the edge of the lower end face of the ring furnace during continuous annealing, the purpose of preventing edge deformation during high-temperature annealing can be achieved. The above patented technologies focus on solving the deformation of the lower end face edge of vertical steel coils during high-temperature annealing, and do not address the problem of central bulging.

[0005] Kawasaki Patent JPH10265854A provides a good understanding of the mechanism of high-temperature creep in steel strips: Steel strips that have undergone primary recrystallization have grain sizes ranging from 15 to 30 μm, resulting in low high-temperature strength and numerous grain boundaries. Under temperatures above 950°C, grain boundary slip easily occurs, leading to strip deformation. In contrast, steel strips that have undergone secondary recrystallization have grain sizes exceeding 30 mm, resulting in higher high-temperature strength and less creep. Therefore, if secondary recrystallization occurs in the strip before 950°C, the grains grow larger and enter the high-temperature stage, reducing creep and minimizing strip deformation. Furthermore, during high-temperature annealing, the heat generated diffuses from the outer ring inwards, resulting in a higher temperature in the outer ring and causing deformation to extend from the outer ring to the inner ring. Therefore, this patent proposes that for the outer ring (accounting for 0.2-3% of the total wall thickness), by reducing the primary grain size to 0.90 times that of the inner ring grain size, the secondary recrystallization temperature of the outer ring reaches 0.95 times that of the inner ring. This results in preferential secondary recrystallization in the outer ring, improved high-temperature strength, less strip deformation, and no deformation in the inner ring. However, this patent only addresses the deformation of the lower edge of the high-temperature annealed vertical coil. Furthermore, with the advent of high-magnetic-induction grain-oriented silicon steel, its secondary recrystallization temperature has increased to over 1000℃. Reducing the grain size of the outer ring of the grain-oriented silicon steel using the above method is no longer effective in lowering its secondary recrystallization temperature below 950℃. If the grain size is below 18μm, performance defects may occur. Therefore, this method is no longer applicable.

[0006] Kawasaki Patent JP2001303137A proposes that by changing three physicochemical indicators, including the angle of repose of magnesium oxide, and ensuring that the proportion of particles larger than 40μm is <5%, the coiling tension of the decarburizing annealing coating unit is in the range of 49MPa to 147MPa. After high-temperature annealing, there are no plate shape defects and the coil shape is good. However, the relevant mechanism is not explained, and the change of magnesium oxide has a significant impact on the quality of the bottom layer, making it impossible to achieve both plate shape and surface finish. Kawasaki Patent JP2003166018A proposes that by controlling the temperature difference in the inner circumferential direction of the steel coil to be less than 20° during cooling, the warping can be reduced to 100m. However, due to the characteristics of high-temperature annealing heating, this method is difficult to industrialize. Summary of the Invention

[0007] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a high magnetic induction oriented silicon steel with excellent strip shape and a production method thereof, thereby reducing the length of strip bending deformation.

[0008] To achieve the above objectives, the present invention provides a method for producing high magnetic induction oriented silicon steel with excellent sheet shape. Before high-temperature annealing, a layer of magnesium oxide release agent is coated on the surface of the strip steel. The particle size of the magnesium oxide satisfies the following conditions: 1μm≤D50≤7μm and 9μm≤D90≤18μm. The partial pressure ratio during continuous decarburization annealing is 0.1~0.4.

[0009] Furthermore, the amount of magnesium oxide applied is 3.5–8.5 g / mm². 2 .

[0010] Furthermore, the difference between the coating thickness of the thickest region and the coating thickness of the thinnest region of the magnesium oxide is no greater than 0.5 g / mm². 2 .

[0011] Furthermore, when the strip is coiled after continuous decarburization annealing, the outer coil winding tension is 45-120 MPa. The outer coil refers to the area from the outermost ring to 1 / 10 to 1 / 6 of the winding thickness along the radial winding thickness direction after continuous decarburization annealing and magnesium oxide coating.

[0012] A high magnetic induction oriented silicon steel is also provided, which is prepared by the production method described above.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: the high magnetic induction oriented silicon steel finished product prepared by the production method of the present invention has a strip length of flexural deformation that is shortened from about 3000m to less than 200m, effectively reducing the length of strip flexural deformation. Attached Figure Description

[0014] Figure 1 This is a schematic diagram showing the bulge in the middle of the strip after high-temperature annealing.

[0015] Figure 2 The finished steel strip is curved.

[0016] Figure 3 Schematic diagram of heating high-temperature annealed steel coils;

[0017] Figure 4 A schematic diagram showing the deformation of the outer ring developing towards the inner ring;

[0018] Figure 5 This is a schematic diagram showing the proportion of silicon dioxide under different partial pressure ratios. Detailed Implementation

[0019] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can understand it.

[0020] Investigations revealed that the mechanism of the central bulge in the strip steel is as follows: High-temperature annealing widely employs gas heating technology, and existing annular furnaces, tunnel furnaces, and single-unit bell-type annealing furnaces all use open flame burners for heating. A schematic diagram of this heating process is attached. Figure 3 Under the heating conditions of the burner, the outer ring of the steel coil receives the most heat, and high-temperature creep deformation occurs first. After deformation occurs, it further develops inward along the direction of the thickness of the vertically coiled section (see appendix). Figure 4 This mechanism is consistent with the phenomenon observed in actual industrial production, namely, that the outer ring deforms more, and the deformation decreases as you move towards the inner ring, until the deformation disappears.

[0021] Therefore, the present invention reduces strip deformation in the following two ways:

[0022] 1) Effectively buffers the deformation of the outer ring as it propels into the inner ring;

[0023] 2) Reduce the driving force that causes deformation.

[0024] Based on the aforementioned mechanism of strip deformation occurrence and development, two factors promote the deformation of the outer ring towards the inner ring: First, heat is conducted from the area with higher heat intensity to the inner ring, increasing the temperature of the strip in the direction of the inner ring and making it more prone to deformation. Second, the deformation of the outer ring strip is replicated to the adjacent inner ring due to the coiling tension (i.e., when the bending direction of the outer ring strip deformation is towards the inner ring of the coil, it generates compressive stress on the inner ring strip; conversely, when it bends outward, it creates a non-contact space with the inner ring strip, making it easier for the inner ring to deform outward when it expands due to heat). Correspondingly, two factors alleviate the deformation of the outer ring towards the inner ring: one is the interlayer release agent, whose loose structure can buffer the aforementioned compressive stress. As is well known, before high-temperature annealing, a layer of anti-adhesion release agent is applied to the surface of the strip. The magnesium oxide-based separator between the strip layers affects the development of deformation because the particle size distribution and porosity of the separator components have different effects on heat conduction efficiency and deformation buffering effect. Secondly, the amorphous inner oxide layer mainly composed of silicon dioxide on the strip surface can prevent heat from spreading to the inner ring due to its low thermal conductivity (SiO2 has a thermal conductivity of 2.8, while Fe has a thermal conductivity of 50), thereby reducing the driving force for inner ring deformation.

[0025] Therefore, the present invention employs the following production method to produce high magnetic induction oriented silicon steel with excellent sheet shape:

[0026] 1) Before high-temperature annealing, a layer of magnesium oxide release agent is applied to the surface of the strip steel. Under the premise that the particle size distribution of magnesium oxide satisfies the following relationship, the buffering effect of magnesium oxide between layers is optimal: 1μm≤D50≤7μm and 9μm≤D90≤18μm.

[0027] If D50 < 1 μm or D90 < 9 μm, the magnesium oxide density is too high, which cannot play a buffering role, and the heat transfer efficiency is too high, creating conditions for the deformation of the inner ring; if D50 > 7 μm or D90 > 18 μm, although the buffering effect is better, it is easy to print pit defects on the strip surface.

[0028] 2) The application rate of magnesium oxide is 3.5–8.5 g / mm. 2 .

[0029] Magnesium oxide coating amount less than 3.5 g / mm 2At this point, due to the winding tension, it is no longer able to buffer the compressive stress generated by the deformation of the outer ring, and warping develops from the outer ring to the inner ring, causing deformation of the entire roll. Therefore, the coating amount of magnesium oxide should not be less than 3.5 g / mm²; when the coating amount is greater than 8.5 g / mm², it should be less than 3.5 g / mm². 2 If the volume ratio of the coil after winding ([coil weight ÷ (strip density × coil volume)] × 100%) is less than 88%, the coil will collapse and loosen after high-temperature annealing, and will be scrapped because it cannot be uncoiled in the subsequent stretching and leveling annealing unit.

[0030] 3) The outer coil tension also significantly affects the buffering capacity of magnesium oxide. Excessive coil tension causes the magnesium oxide coating to lose its elasticity, directly transferring the compressive stress generated by the outer coil deformation inwards. Verification has shown that when the outer coil tension exceeds 120 MPa, warping deformation increases dramatically. However, when the coil tension is less than 45 MPa, coil collapse occurs, resulting in severe loosening after high-temperature annealing, making unwinding impossible in the stretching and leveling annealing unit. Therefore, the suitable outer coil tension range is 45–120 MPa. The outer coil referred to above refers to the area from the outermost coil to 1 / 10–1 / 6 of the coil thickness along the radial winding thickness direction after continuous decarburization annealing and magnesium oxide coating winding.

[0031] 4) The uniformity of the release agent application also affects the uniformity of strip deformation. Generally, the grooves on the coating roller surface use a consistent groove shape and size. However, due to the force characteristics of a two-roll coating machine, the two ends of the coating roller are subjected to greater force, while the middle is subjected to less force, resulting in a coating thickness that is thicker in the middle and thinner at the edges. The coating amount in the middle is 2 g / mm thicker than at the edges. 2 The above describes the process. Under winding tension, the intermediate magnesium oxide coating is compressed, reducing its ability to buffer the compressive stress generated by the deformation of the outer ring. This transfers the deformation to the adjacent inner strip, causing the warping deformation to develop towards the core of the coil. Experimental studies have shown that the coating uniformity (coating amount in the thickest area - coating amount in the thinnest area) needs to reach 0.5 g / mm². 2 Only by keeping it inside can the warping deformation of the outer ring be prevented from expanding into the inner ring.

[0032] 4) The following describes in detail the effect of the silica oxide layer on the strip surface on the warping deformation. Different partial pressure ratios can be calculated by using a continuous decarburization annealing furnace with different furnace dew points and hydrogen contents.

[0033] formula for partial pressure ratio:

[0034] The higher the partial pressure ratio, the thinner the amorphous SiO2 inner oxide layer on the surface of the strip (see Appendix). Figure 5Amorphous SiO2 has a low thermal conductivity, which can prevent the diffusion of localized high temperatures caused by the burner heating characteristics into the inner ring. Experiments have shown that when the partial pressure ratio exceeds 0.4 and the heating time is insufficient, the oxide layer thickness inside SiO2 is thinner, the thermal resistance decreases, and the length of the warping deformation increases. When the partial pressure ratio is less than 0.1, due to insufficient moisture in the atmosphere, the continuous decarburization ability decreases, and the carbon content of the finished product cannot reach the target range (<30ppm), resulting in magnetic aging. Moreover, a partial pressure ratio less than 0.1 causes the oxide layer thickness inside SiO2 to reach more than 4μm, resulting in deterioration of magnetism. Therefore, the partial pressure ratio is 0.1 to 0.4.

[0035] Example

[0036] A continuously cast billet containing 3.32% silicon, 0.027% acid-soluble aluminum, 0.0072% nitrogen, 0.088% chromium, and no more than 0.1% total manganese, sulfur, and phosphorus, with the remainder being iron and unavoidable impurities, is heated to 1200℃, hot-rolled to a thickness of 2.2mm, then normalized annealed at 1100℃ for 3 minutes, pickled, cold-rolled to a finished thickness of 0.30mm, then decarburized and coated with a high-temperature annealing release agent mainly composed of magnesium oxide, followed by high-temperature annealing at 1200℃ for 20 hours. Finally, an insulating coating is applied and a stretching and leveling anneal is performed. The above decarburization annealing and release agent coating processes, as shown in Table 1, yielded 5 different finished rolls. The length of the outer ring deformation indicates that the warping defects in the high magnetic induction oriented silicon steel produced using this invention are significantly reduced.

[0037] Table 1

[0038]

[0039] As shown in Table 1, after the implementation of the present invention in the continuous decarburization annealing coating unit for oriented silicon steel, the length of the strip steel that undergoes warping deformation in the finished high magnetic induction oriented silicon steel product is shortened from about 3000m to less than 200m.

Claims

1. A method for producing high magnetic induction oriented silicon steel with excellent plate shape, characterized in that: Before high-temperature annealing, a layer of magnesium oxide release agent is applied to the surface of the strip steel. The particle size of the magnesium oxide meets the following requirements: 1μm≤D50≤7μm and 9μm≤D90≤18μm; during continuous decarburization annealing, the partial pressure ratio P H2O / P H2 The value is 0.1 to 0.

4. The amount of magnesium oxide applied is 3.5–8.5 g / mm². 2 The difference between the coating thickness of the thickest region and the coating thickness of the thinnest region of magnesium oxide shall not exceed 0.5 g / mm². 2 The outer coil tension of the strip after continuous decarburization annealing is 45-120 MPa. The outer coil refers to the area from the outermost ring to 1 / 10 to 1 / 6 of the coil thickness along the radial winding thickness direction after continuous decarburization annealing and magnesium oxide coating.

2. A high-magnetic-permeability grain-oriented silicon steel, characterized in that: The high magnetic induction oriented silicon steel is prepared by the production method described in claim 1.

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