Cold rolled steel low residual magnetism annealing process

Abstract

The application discloses a cold-rolled steel low-residual-magnetic annealing process, which comprises the following steps: placing steel into an annealing furnace and heating to 670-700 DEG C, keeping for 55-70 min to eliminate deformation strengthening and residual stress; then reducing the temperature of the steel to 480-520 DEG C at an average cooling speed less than 1.5 DEG C / min; then heating the steel to 780-810 DEG C, keeping for 170-190 min to increase the grain size and eliminate the residual magnetic field in the material through the movement of the thermal activated magnetic domain wall; injecting a protective atmosphere, reducing the temperature of the steel to 640-670 DEG C at an average cooling speed less than 1 DEG C / min, and performing magnetic gas annealing to reduce the oxides and impurities in the steel, improve the magnetic crystal anisotropy of the magnetic material, improve the magnetic permeability and the magnetic saturation induction intensity of the magnetic material, and finally make the steel reach an extremely low residual magnetism with a magnetic field intensity less than 1 a / cm and good magnetic properties.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of steel heat treatment, in particular to a low residual magnetic annealing process of cold-rolled steel. BACKGROUND

[0002] Cold-rolled steel sheet is an indispensable special functional material with excellent soft magnetic properties, which is widely used in relays, magnetic shields, electromagnetic valves, DC motors and various instruments and meters. When cold-rolled steel is rolled into shape, as the thickness of the steel sheet decreases, the surface free magnetic pole energy (static magnetic energy) and the domain wall energy per unit domain wall area increase, the magnetic domain wall moving resistance increases, residual magnetism is formed, and the magnetic properties of the cold-rolled steel are reduced.

[0003] The current heat treatment process, such as the invention patent with the patent number "CN202010761229.2" and the name "Preparation method of high-performance oriented silicon steel ultra-thin strip", and the US patent with the publication number US1965559A, all disclose a secondary cold-rolling method for producing magnetically stable cold-rolled sheets by intermediate annealing. The annealing process is set between the two cold-rolling processes, and the final secondary annealing is performed. Since the annealing process is between the two cold-rolling processes, after the first annealing, the product needs to be taken out of the furnace for cold rolling, and then put into the furnace, which is low in work efficiency, difficult in production, and high in production environmental requirements. SUMMARY

[0004] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, the present application proposes a low residual magnetic annealing process of cold-rolled steel, which can improve the production difficulty of low residual magnetic cold-rolled steel sheet and improve the production efficiency.

[0005] The low residual magnetic annealing process of cold-rolled steel according to the first aspect of the present application comprises the following steps:

[0006] Step 1, placing the rolled steel into an annealing furnace and heating to 670-700℃, and holding for 55-70min;

[0007] Step 2, reducing the temperature of the steel to 480-520℃ at an average cooling rate of less than 1.5℃ / min;

[0008] Step 3, heating the steel to 780-810℃ and holding for 170-190min;

[0009] Step 4, injecting a protective atmosphere into the annealing furnace, reducing the temperature of the steel to 640-670℃ at an average cooling rate of less than 1℃ / min;

[0010] Step 5, reducing the temperature of the steel to 180-210℃;

[0011] Step 6: Remove the steel from the heating furnace and cool it to room temperature, or allow the steel to cool to room temperature with the furnace.

[0012] The low residual magnetic annealing process for cold-rolled steel according to embodiments of the present invention has at least the following beneficial effects: In the present invention, primary crystallization is first performed in step 1: the cold-deformed metal is heated to 670℃~700℃ and held for 55min~70min, so that the deformed grains are transformed back into uniform equiaxed grains to eliminate deformation strengthening and residual stress. Then, in step 2, cooling annealing is performed, controlling the cooling rate, with an average cooling rate of less than 1.5℃ / min, to avoid the generation of residual stress due to excessively rapid temperature drop, and to reduce the generation of transition grains and the impact on secondary crystallization. In step 3, the steel is heated to 780℃~810℃ for secondary crystallization and held for 170min~190min. n. Increase the grain size and eliminate the residual magnetic field in the material by thermally activating the movement of magnetic domain walls. In step 4, inject a protective atmosphere for cooling and annealing to form magnetic annealing, which reduces the oxides and impurities in the material, improves the magnetocrystalline anisotropy of the magnetic material, and increases its magnetic permeability and magnetic saturation induction intensity. In step 4, by controlling the average cooling rate to less than 1℃ / min, the oxides and impurities can be fully reduced, and the steel is kept at a high temperature to activate the movement of magnetic domain walls to eliminate the residual magnetic field in the material, thereby further improving the magnetocrystalline anisotropy of the magnetic material, so that the steel achieves extremely low residual magnetism, magnetic field strength <1a / cm, and has good magnetic properties.

[0013] In some embodiments of the present invention, a protective atmosphere is injected into the annealing furnace, the protective atmosphere being hydrogen, nitrogen, or a hydrogen-nitrogen mixture, and the dew point of the protective atmosphere being -40°C to -30°C.

[0014] In some embodiments of the present invention, the protective atmosphere is a hydrogen-nitrogen mixture with a nitrogen content of less than 5%.

[0015] In some embodiments of the present invention, the chemical composition of the steel is: 0.06% < Mn < 0.15%, 0.03% < Si < 0.05%, 0.005% < Al < 0.02%, C < 0.01%, 0.01% < P < 0.015%, S < 0.01%, N < 0.005%, O < 0.005%, 0.03% < Sn < 0.15%, with the balance being Fe and other unavoidable impurities.

[0016] In some embodiments of the present invention, the chemical composition of the steel is 0.1% < Mn < 0.15% and 0.005% < S < 0.01%.

[0017] In some embodiments of the present invention, the chemical composition of the steel is 0.005% < Al < 0.01%, a protective atmosphere is injected into the annealing furnace, the protective atmosphere is a hydrogen-nitrogen mixture, the nitrogen content is less than 5%, and the dew point of the protective atmosphere is -40°C to -30°C.

[0018] In some embodiments of the present invention, in step 3, the average size of the ferrite grains is greater than 100 μm.

[0019] In some embodiments of the present invention, the thickness of the steel is less than 8 mm.

[0020] In some embodiments of the present invention, in step 1, the average heating rate is less than 12°C / min.

[0021] In some embodiments of the present invention, in step 3, the average heating rate is less than 8°C / min. Detailed Implementation

[0022] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0023] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0024] In the description of this invention, the use of "first" and "second" is for the purpose of distinguishing technical features only, and should not be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated or the order of the technical features indicated.

[0025] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0026] In related technologies, the magnetic properties of magnetic materials such as cold-rolled steel are affected during the cold rolling process. Magnetic materials consist of magnetic and non-magnetic grains, with magnetic grains composed of magnetic atoms. These magnetic atoms can generate manufacturing defects during processing, such as oxides, inclusions, and dislocations. These defects affect the permeability and saturation magnetic induction of the magnetic material, and also increase domain wall energy and resistance to domain wall movement, leading to residual magnetism.

[0027] The low residual magnetic annealing process for cold-rolled steel according to an embodiment of the present invention includes the following steps:

[0028] Step 1: Place the rolled steel into an annealing furnace and heat it to 670℃~700℃, and hold it for 55min~70min.

[0029] Step 2: Reduce the temperature of the steel to 480℃~520℃, with an average cooling rate of less than 1.5℃ / min;

[0030] Step 3: Heat the steel to 780℃~810℃ and hold for 170min~190min;

[0031] Step 4: Inject a protective atmosphere into the annealing furnace to reduce the temperature of the steel to 640℃~670℃, with an average cooling rate of less than 1℃ / min.

[0032] Step 5: Reduce the temperature of the steel to 180℃~210℃;

[0033] Step 6: Remove the steel from the heating furnace and cool it to room temperature, or allow the steel to cool to room temperature with the furnace.

[0034] In this invention, a primary crystallization is first performed in step 1: the cold-deformed metal is heated to 670℃~700℃ and held for 55min~70min, causing the deformed grains to revert to uniform equiaxed grains, thereby eliminating deformation strengthening and residual stress. Then, in step 2, cooling annealing is performed, controlling the cooling rate to an average rate of less than 1.5℃ / min to avoid excessively rapid temperature drops that could generate residual stress and reduce the formation of transition grains, thus minimizing the impact on secondary crystallization. In step 3, the steel is heated to 780℃~810℃ for secondary crystallization, held for 170min~190min, increasing the grain size and activating the crystals through thermal processing. The movement of magnetic domain walls eliminates the residual magnetic field in the material. In step 4, a protective atmosphere is injected for cooling annealing, forming magnetic annealing, which reduces the oxides and impurities in the material, improves the magnetocrystalline anisotropy of the magnetic material, and increases its magnetic permeability and magnetic saturation induction intensity. In step 4, by controlling the average cooling rate to less than 1℃ / min, the oxides and impurities can be fully reduced, and the steel is kept at a high temperature. This activates the movement of magnetic domain walls to eliminate the residual magnetic field in the material, thereby further improving the magnetocrystalline anisotropy of the magnetic material, making the steel achieve extremely low residual magnetism, magnetic field strength <1a / cm, and good magnetic properties.

[0035] In step 5, the rate of temperature reduction can be increased compared to step 4, with the average cooling rate increasing to 10℃ / min, or the temperature can be reduced along with the furnace. At this point, the steel is already in a relatively stable state, which can increase the annealing speed and improve production efficiency.

[0036] It is conceivable that, in some embodiments of the present invention, the average heating rate in step 1 is less than 12°C / min.

[0037] It is conceivable that, in some embodiments of the present invention, the average heating rate in step 3 is less than 8°C / min.

[0038] It is conceivable that, in some embodiments of the present invention, the protective atmosphere is hydrogen, nitrogen, or a hydrogen-nitrogen mixture, and the dew point of the protective atmosphere is -40°C to -30°C. By controlling the dew point, oxidation is controlled, resulting in a more uniform oxide layer that can achieve a better pickling effect in subsequent pickling processes.

[0039] It is conceivable that, in some embodiments of the present invention, the protective atmosphere is a hydrogen-nitrogen mixture with a nitrogen content of less than 5%. Hydrogen can penetrate into the interior of the material, reducing oxides and impurities therein, thereby restoring magnetic properties. The hydrogen concentration and temperature are controlled to a high level to ensure the treatment effect. Adding a small amount of nitrogen can control the oxidation of the steel surface and help reduce hydrogen embrittlement of the steel.

[0040] Furthermore, hydrogen has high thermal conductivity, which can improve the efficiency of annealing.

[0041] It is conceivable that, in some embodiments of the present invention, the chemical composition of the steel is: 0.06% < Mn < 0.15%, 0.03% < Si < 0.05%, 0.005% < Al < 0.02%, C < 0.01%, 0.01% < P < 0.015%, S < 0.01%, N < 0.005%, O < 0.005%, 0.03% < Sn < 0.15%, with the balance being Fe and other unavoidable impurities.

[0042] During the steel forming process, Mn and S react to form MnS. The ratio of Mn to S affects the grain size. In this invention, to obtain larger grains, when 0.06% < Mn < 0.15% and S < 0.01%, larger grains can be obtained, resulting in better magnetic effects.

[0043] In this invention, Al is a deoxidizing element in steel, but it affects ferromagnetic properties. N and Al easily form AlN during steel forming, which inhibits grain growth, resulting in finer grains. Therefore, in this invention, it is necessary to control the content of Al to be 0.005% < Al < 0.02% and N < 0.005% to maintain the physical properties of the steel and reduce its impact on grain growth, thereby obtaining better magnetic properties.

[0044] It is conceivable that, in some embodiments of the present invention, the chemical composition of the steel is controlled with the ranges of Mn and S being 0.1% < Mn < 0.15% and 0.005% < S < 0.01%, which can result in larger grains and allow the steel to maintain good magnetic properties.

[0045] It is conceivable that, in some embodiments of the present invention, the chemical composition of the steel is 0.005% < Al < 0.01%, a protective atmosphere is injected into the annealing furnace, the protective atmosphere is a hydrogen-nitrogen mixture, the nitrogen content is less than 5%, the dew point of the protective atmosphere is -40°C to -30°C, by controlling the content of Al and nitrogen, the production of AlN is reduced, the impact on grain growth is reduced, and the oxides in the steel can be better reduced, the resistance to magnetic domain wall movement is small, the elimination effect of residual magnetic field is better, and the residual magnetism of the steel is reduced.

[0046] It is conceivable that, in some embodiments of the present invention, in step 3, the average size of the ferrite grains is greater than 100 μm, so that the cold-rolled steel has better magnetic properties.

[0047] It is conceivable that, in some embodiments of the present invention, the thickness of the steel is less than 8 mm, so as to achieve a good magnetic annealing effect.

[0048] It is conceivable that, in some embodiments of the present invention, when the thickness of the steel is set to 1mm to 3mm, it has a better magnetic annealing effect, low residual magnetism, and superior magnetic properties.

[0049] Example 1

[0050] Step 1: Place the rolled steel into an annealing furnace and heat it to 680℃, then hold it for 60 minutes.

[0051] Step 2: Cool down to 500℃ for 180 minutes;

[0052] Step 3: Increase the temperature to 800℃ and hold for 180 minutes;

[0053] Step 4: Inject a protective atmosphere into the annealing furnace, then cool down to 650℃ for 180 minutes.

[0054] Step 5: Cool the furnace to 200℃.

[0055] Step 6: Remove from the oven.

[0056] The chemical composition of the steel is as follows: Mn: 0.15%, Si: 0.05%, Al: 0.01%, C: 0.01%, P: 0.015%, S: 0.01%, N: 0.005%, O: 0.005%, Sn: 0.15%, with the balance being Fe and other unavoidable impurities.

[0057] The protective atmosphere contains 97% hydrogen and 3% nitrogen.

[0058] Tests showed that the steel strength met the design requirements for SPCC steel (the Japanese standard (JIS) for "general-purpose cold-rolled carbon steel sheets and strips"). The average ferrite grain size was approximately 105 μm, the magnetic field strength was 0.9 a / cm, and the residual magnetism was low.

[0059] Example 2

[0060] In the chemical composition of the steel, S was adjusted to 0.005%, Al to 0.005%, and Mn to 0.1%, while the others were the same as in Example 1.

[0061] Testing revealed that the steel strength was lower than that of Example 1, but it still met the design requirements for SPCC steel. The average ferrite grain size was approximately 110 μm, the magnetic field strength was 0.8 a / cm, and the residual magnetism was low.

[0062] Comparative Example 1

[0063] In step 4, the cooling time is set to 100 min, that is, the cooling rate is 1.5℃ / min, and everything else is the same as in Example 1.

[0064] Testing revealed that the steel strength was slightly lower than that of Example 1, but still met the design requirements for SPCC steel. The average ferrite grain size was approximately 85 μm, the magnetic field strength was 1.2 a / cm, and the residual magnetism was greater than that of Example 1.

[0065] Comparative Example 2

[0066] The chemical composition of the steel was adjusted as follows: S was adjusted to 0.02%, Al was adjusted to 0.02%, and the rest were the same as in Example 1.

[0067] Testing revealed that the steel strength was higher than that of Example 1, but still met the design requirements for SPCC steel. The average ferrite grain size was approximately 80 μm, the magnetic field strength was 1.5 a / cm, and the residual magnetism was greater than that of Example 1.

[0068] Of course, the present invention is not limited to the above-described embodiments. Those skilled in the art can make equivalent modifications or substitutions without departing from the spirit of the present invention. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

1. A low residual magnetic annealing process for cold-rolled steel, characterized in that, Includes the following steps: Step 1: Place the rolled steel into an annealing furnace and heat it to 670℃~700℃, and hold it for 55min~70min. Step 2: Reduce the temperature of the steel to 480℃~520℃, with an average cooling rate of less than 1.5℃ / min; Step 3: Heat the steel to 780℃~810℃ and hold for 170min~190min; Step 4: Inject a protective atmosphere into the annealing furnace. The protective atmosphere is a hydrogen-nitrogen mixture or hydrogen with a nitrogen content of less than 5%. The dew point of the protective atmosphere is -40℃ to -30℃, which reduces the temperature of the steel to 640℃ to 670℃, with an average cooling rate of less than 1℃ / min. Step 5: Reduce the temperature of the steel to 180℃~210℃; Step 6: Remove the steel from the heating furnace and cool it to room temperature, or allow the steel to cool to room temperature in the furnace. The chemical composition of the steel is as follows: 0.06% < Mn ≤ 0.15%, 0.03% < Si ≤ 0.05%, 0.005% < Al ≤ 0.01%, C ≤ 0.01%, 0.01% < P ≤ 0.015%, S ≤ 0.01%, N ≤ 0.005%, O ≤ 0.005%, 0.03% < Sn ≤ 0.15%, with the balance being Fe and other unavoidable impurities.

2. The low residual magnetic annealing process for cold-rolled steel according to claim 1, characterized in that, In the chemical composition of the steel, 0.1% < Mn < 0.15% and 0.005% < S < 0.01%.

3. The low residual magnetic annealing process for cold-rolled steel according to claim 1, characterized in that, In step 3, the average size of the ferrite grains is greater than 100 μm.

4. The low residual magnetic annealing process for cold-rolled steel according to claim 1, characterized in that, The thickness of the steel is less than 8 mm.

5. The low residual magnetic annealing process for cold-rolled steel according to claim 1, characterized in that, In step 1, the average heating rate is less than 12°C / min.

6. The low residual magnetic annealing process for cold-rolled steel according to claim 1, characterized in that, In step 3, the average heating rate is less than 8°C / min.