Soft magnetic steel for electromagnetic valve and method for manufacturing the same

Abstract

A soft magnetic steel for solenoid valves comprises, by weight: 0.06%-0.08% C, not exceeding 0.05% Si, 1.05%-1.20% Mn, 0.065%-0.075% P, and 0.29%-0.32% S; and 0.003%-0.01% Nb, 0.0018%-0.01% Ti, and 0.003%-0.03% Mo. This soft magnetic steel possesses excellent mechanical properties and stable magnetic properties, effectively improving the yield rate and reducing manufacturing costs in gearbox solenoid valve manufacturing. This invention also provides a method for manufacturing the soft magnetic steel for solenoid valves.

Description

Technical Field

[0001] This invention belongs to the field of steel, and specifically relates to a soft magnetic steel for electromagnetic valves and its manufacturing method. Background Technology

[0002] The transmission solenoid valve is a core component of an automatic transmission. The transmission controller controls the actuation of the solenoid valve to move components such as the clutch, brake, and torque converter, thereby achieving automatic gear shifting. The transmission solenoid valve controls the on / off state and flow direction of pressurized fluid in the pipeline, regulates pressure, and eliminates hydraulic shock. It requires a rapid and precise response throughout its service life. Therefore, the soft magnetic steel used to manufacture transmission solenoid valves has very high requirements, needing to simultaneously meet good mechanical strength, machinability, and stable magnetic properties. Currently, the armature and magnetic sleeve of the solenoid valve are largely manufactured using imported 11SMn30 steel. However, the cost of imported materials is currently high, and the magnetic properties, especially key indicators such as coercivity and magnetic flux density, fluctuate greatly, making precise control impossible. This results in a high scrap rate and low production efficiency in actual production. Currently, the industry's understanding of the impact of soft magnetic steel processing parameters on magnetic properties is insufficient. Mass-produced soft magnetic steel often fails to achieve ideal magnetic properties, with significant fluctuations in magnetic properties between different batches. Determining the right manufacturing parameters for soft magnetic steel materials with stable magnetic properties that meet design requirements remains a challenge for the industry. Therefore, providing a soft magnetic steel for solenoid valves with stable magnetic properties and its manufacturing method is of great significance for reducing production costs, improving product quality, and increasing the localization rate of key components in the automotive industry. Summary of the Invention

[0003] The purpose of this invention is to provide a soft magnetic steel with stable magnetic properties, thereby reducing the scrap rate of solenoid valve products caused by fluctuations in magnetic properties. This invention further provides a method for manufacturing the solenoid valve steel with stable magnetic properties.

[0004] According to one aspect of the present invention, a soft magnetic steel with stable magnetic properties is provided, the composition of which, by weight, comprises: 0.06%-0.08% C, not exceeding 0.05% Si, 1.05%-1.20% Mn, 0.065%-0.075% P and 0.29%-0.32% S, and 0.003%-0.01% Nb, 0.0018%-0.01% Ti and 0.003%-0.03% Mo.

[0005] Carbon (C) affects the hardness of steel and has a significant impact on its mechanical properties. However, as an austenite-forming element, C negatively affects magnetic properties. Therefore, the C content in conventional 11SMn30 steel is further limited to 0.06%-0.08%. Sulfur (S) is beneficial for improving the machinability of steel, but excessive S will impair its mechanical properties. Nitrogen (Nb), Ti, and Mo are ferrite-forming elements that are beneficial for improving the magnetic properties of steel.

[0006] Furthermore, the soft magnetic steel meets the following performance requirements: tensile strength ≥ 300 MPa, yield strength ≥ 210 MPa, hardness HBW of 110-150, coercivity of 150 A / m-200 A / m; and magnetic flux density ≤ 1.20T ≤ B. 1000 ≤1.42T, 1.46T≤B 2000 ≤1.58T, 1.65T≤B 5000 ≤1.70T,

[0007] 1.77T≤B 10000 ≤1.82T, 1.92T≤B 20000 ≤1.96T, 2.01T≤B 30000 ≤2.05T. Simultaneously meeting both mechanical and magnetic performance requirements is essential for successful application in solenoid valve manufacturing and reducing product scrap rates. Typically, solenoid valves using 11SMn30 steel require a coercivity Hc ≤ 250A / m. The inventors discovered in practice that further limiting the coercivity range to 150A / m-200A / m enables solenoid valves made from this steel to achieve more stable response speeds and accuracy, reducing the likelihood of deviations and improving product yield.

[0008] Furthermore, the grain size grade in the microstructure of this soft magnetic steel is 3-5. A fine grain structure is beneficial for improving the mechanical properties of the steel, while larger grains are necessary to maintain stable magnetic properties.

[0009] According to another aspect of the present invention, a method for manufacturing soft magnetic steel is provided, the method comprising the following steps: hot rolling a steel billet at an initial rolling temperature of 870℃-920℃ and a final rolling temperature of 910℃-940℃, and processing it into bars; cold drawing the hot-rolled bars with a surface area reduction rate of 7%-18%; and holding the cold-drawn bars at 780℃-860℃ for 8-20 hours to complete the magnetic heat treatment.

[0010] The above heat treatment process enables the steel used in the solenoid valve to form a grain structure of suitable and uniform size, and the precipitates are evenly and dispersed. This allows the steel to have good mechanical and machinability properties, as well as stable magnetic properties. The coercivity, remanence, magnetic saturation strength and magnetic induction can all be controlled within the design requirements without exceeding the tolerance.

[0011] Furthermore, the composition of the bar used in this manufacturing method includes: 0.06%-0.08% C, no more than 0.05% Si, 1.05%-1.20% Mn, 0.065%-0.075% P, and 0.29%-0.32% S, as well as 0.003%-0.01% Nb, 0.0018%-0.01% Ti, and 0.01%-0.03% Mo. Steel manufactured within the composition range of commercially available 11SMn30 steel cannot achieve stable magnetic properties, and tends to exceed tolerances in indicators such as coercivity and magnetic flux density. Therefore, it is necessary to further limit the content of C, which is detrimental to magnetic properties, and increase elements such as Nb, Ti, and Mo, which help improve magnetic properties, based on the general composition of 11SMn30 steel.

[0012] Preferably, the cold drawing process reduces the surface area by 10.25%-13.22%, the heating temperature for magnetic heat treatment is 820±20℃, and the magnetic heat treatment time is 12h.

[0013] Furthermore, the manufacturing method also includes a step of melting the bar stock, wherein the residual oxygen content is controlled to be between 40 ppm and 100 ppm during tapping, and the sulfides in the bar stock meet SEP1572 standard 2.2 or 2.3, with an inclusion grade of less than 2. To achieve good mechanical, machinability, and magnetic properties, strict control of the bar stock's microstructure is required. Controlling the residual oxygen content reduces oxide inclusions; sulfide distribution affects the steel's machinability.

[0014] Furthermore, before hot rolling the bar, it is first heated to 1170℃-1190℃ and held at that temperature for 2-4 hours. This preheating and holding process ensures a uniform temperature distribution inside and outside the bar before hot rolling, reducing structural defects within the bar.

[0015] Furthermore, in the magnetic heat treatment process, the bar is first preheated to 650±20℃ and held for 2 hours, then heated to 820±20℃ for 3.5 hours; after holding at this temperature, it is first cooled to 650±20℃ for 7 hours, and then cooled in the furnace to 200±20℃ before being removed from the furnace. The heating and cooling processes are controlled during the magnetic heat treatment to ensure a uniform microstructure in the semi-finished steel product.

[0016] Furthermore, the manufacturing method also includes a grinding step, in which the magnetically heat-treated steel semi-finished product is ground to the required dimensions. After recrystallization, the magnetically heat-treated steel semi-finished product has a uniform structure and cannot be subjected to further mechanical deformation such as drawing or upsetting, which would introduce deformation and residual stress, otherwise it would lead to unstable magnetic properties.

[0017] Furthermore, the grinding step involves a milling depth of 0.02mm-0.03mm on each side. Excessive milling depth increases labor time, while insufficient milling depth results in substandard product surface quality. Attached Figure Description

[0018] Figure 1a This is a schematic diagram of a partial structure of a solenoid valve in one embodiment;

[0019] Figure 1b This is a schematic diagram of the magnetic sleeve and armature structure in one embodiment;

[0020] Figure 1c This is a schematic diagram of the cross-sectional structure of the magnetic sleeve in one embodiment;

[0021] Figure 1d This is a schematic diagram of the cross-sectional structure of the armature in one embodiment;

[0022] Figure 2 This is a curve showing the range of magnetic induction intensity B in steel under different magnetic field strengths in one embodiment.

[0023] The purpose of the above-described drawings is to provide a detailed description of the invention so that those skilled in the art can understand the technical concept of the invention, and not to limit the invention. For the sake of brevity, the above-described drawings only schematically depict the structures related to the technical features of the invention, and do not depict the complete structure and all details strictly according to actual scale. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0025] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment herein. The phrase appearing in various places in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive, independent, or alternative embodiments. Those skilled in the art will understand that the embodiments herein can be combined with other embodiments without conflict of principle.

[0026] Automotive automatic transmissions incorporate various solenoid valves to control the transmission's hydraulic system, enabling complex actions such as gear shifting and torque conversion. As the structure and function of automatic transmissions become increasingly complex, the requirements for the response speed and motion precision of these solenoid valves are also rising. In one embodiment, such as... Figure 1a In the solenoid valve shown, the materials of the solenoid armature 2 and the magnetic sleeve 1 are crucial to the overall performance of the solenoid valve. (Combined with...) Figures 1b-1dThe magnetic sleeve 1 is fixedly connected to the solenoid valve housing 3. The armature 2 is nested in the cavity 11 inside the magnetic sleeve 1. When the solenoid valve is energized, the magnetic sleeve 1 and the armature 2 generate a magnetic field in the same direction. The lower armature 2 is attracted by the magnetic sleeve 1 and moves to the left along the axial direction. The armature 2 is provided with a mounting hole 21 for the valve stem 4. One end of the valve stem 4 passes through the through hole 12 at the end of the magnetic sleeve 1 and is fixedly connected to the mounting hole 21. When the armature 2 moves, it drives the solenoid valve valve stem 4 to move. The valve stem 4 is provided with a radially penetrating through hole (not shown). When the valve stem 4 moves to different positions, the through hole can be connected to or offset from different oil passages 6a or 6b to achieve oil passage adjustment. The other end of the valve stem 4 is connected to the reset device 5. Under the action of the reset device 5, once the magnetic force between the magnetic sleeve 1 and the armature 2 weakens, the valve stem 4 will be pushed to the right. Thus, the magnitude of the magnetic force between the magnetic sleeve 1 and the armature 2 is controlled by the strength of the current of the solenoid valve, thereby achieving precise control of the hydraulic circuit. Because the armature 2 and magnetic sleeve 1 require high structural precision, the magnetic material of the solenoid valve needs to possess excellent machinability to ensure dimensional accuracy during machining; it also needs to meet strength requirements to achieve the design life under long-term, high-frequency, and high-intensity service conditions; and simultaneously, it needs to have stable magnetic properties to ensure the response speed and accuracy of the solenoid valve's actuation. Currently, solenoid valves are typically manufactured using free-machining 11SMn30 steel. Domestically produced 11SMn30 steel is usually only used as a raw material for machining, lacking control over its magnetic properties and thus failing to meet the manufacturing requirements of solenoid valves. Even imported special steel often exhibits deviations in key properties such as coercivity, leading to increased product scrap rates. Currently, there is limited research on 11SMn30 steel in this field, and there is a lack of understanding and technical accumulation regarding how to precisely control the steel's composition and processing technology to obtain products with stable magnetic properties. Because the process from raw material smelting to finished product processing of 11SMn30 steel is relatively long, each step will affect the final magnetic properties. Steel producers, machining companies, or solenoid valve manufacturers find it difficult to effectively link the factors affecting the quality of the final product in their research and optimization of process parameters within their respective processes. Therefore, it is always difficult to provide steel products that meet the quality requirements.

[0027] To solve the above problems, the inventors conducted extensive research and experiments, starting from the raw material stage to track and analyze the complete process of solenoid valve production and processing. They comprehensively analyzed and verified the smelting, heat treatment, and machining processes, and proposed a solenoid valve steel with stable magnetic properties and its manufacturing method.

[0028] According to one aspect of the present invention, a steel for solenoid valves with stable magnetic properties is provided. Conventional commercial 11SMn30 steel, used as a machining raw material, has a composition by weight of 0.04%-0.1% C, not exceeding 0.09% Si, 0.09%-1.26% Mn, not exceeding 0.15% P, and 0.25%-0.35% S. The solenoid valve steel with stable magnetic properties provided in this embodiment of the invention further requires a C content of 0.06%-0.08%, a Si content not exceeding 0.05%, a Mn content of 1.05%-1.20%, a P content of 0.065%-0.075%, and a S content of 0.29%-0.32%, and further requires the addition of 0.003%-0.01% Nb, 0.0018%-0.01% Ti, and 0.01%-0.03% Mo. C and Mn are austenite-forming elements, which are beneficial for improving strength and toughness, but detrimental to the magnetic properties of the finished steel. Si, Nb, Ti, and Mo are ferrite-forming elements, which are beneficial to the magnetic properties of the finished steel. Nb and Ti are used to adjust and improve the mechanical properties of the finished steel. The finished steel for this solenoid valve has a grain size of 3-5, exhibiting good mechanical properties, machinability, and stable magnetic properties. The tensile strength of the finished steel for this solenoid valve is ≥300MPa, and the yield strength is...

[0029] ≥210MPa, hardness HBW is 110-150. Regarding magnetic properties, the coercivity Hc is 150A / m-200A / m; the magnetic induction intensity B of the steel used in this solenoid valve meets the following numerical ranges under different external magnetic field strengths H: Figure 2 The BH curve shown represents the magnetic induction intensity B at 1000 A / m. 1000 Magnetic induction intensity B at 2000 A / m 2000 Magnetic induction intensity B at 5000 A / m 5000 ,

[0030] Magnetic induction intensity B at 10000 A / m 10000 Magnetic induction intensity B at 20000 A / m 20000 And the magnetic induction intensity B at 30000 A / m 30000 Each satisfies 1.20T≤B 1000 ≤1.42T,

[0031] 1.46T≤B 2000 ≤1.58T, 1.65T≤B 5000 ≤1.70T, 1.77T≤B 10000 ≤1.82T,

[0032] 1.92T≤B 20000 ≤1.96T, 2.01T≤B 30000≤2.05T. The above performance indicators ensure that the steel used in this solenoid valve, while possessing good mechanical properties, can meet the requirements of rapid response and precise operation of the solenoid valve.

[0033] According to another aspect of the present invention, a method for manufacturing steel for a solenoid valve is provided, the method comprising the following steps:

[0034] First, the steel billet is hot-rolled, with the initial rolling temperature controlled at 870℃-920℃ and the final rolling temperature controlled at 910℃-940℃. The hot rolling process breaks down the columnar crystals formed during the solidification of the billet, transforming the microstructure into relatively uniform and fine equiaxed crystals. This promotes the redistribution of precipitates in the billet microstructure, forming dispersed, uniform, and fine strengthening phases. The billet is then processed into bars.

[0035] Next, the bars are cold-drawn with appropriate parameters, and finally subjected to magnetic heat treatment to obtain stable magnetic properties and qualified mechanical properties. Typically, bars obtained after hot rolling are cold-drawn within the range of 7%-18%, and then subjected to magnetic heat treatment at 780℃-860℃ for 8-20 hours to obtain soft magnetic steel with stable magnetic properties.

[0036] Specifically, for the steel used in this embodiment, hot-rolled bars are first prepared, and then the surface area reduction rate of cold drawing is selected to be 10.25%-13.22%, with the preferred optimal surface area reduction rate q = 12.8%. The cold drawing process introduces plastic deformation into the bar, which serves as the driving force for solid-state phase transformation during subsequent heat treatment.

[0037] Finally, the semi-finished steel after cold drawing is heated to above the Curie temperature for demagnetization. Through iterative testing, the optimal process is determined to be a magnetic heat treatment at 780℃-860℃ for 12 hours, preferably 820±20℃. This allows for sufficient recrystallization of the deformed grains, resulting in a uniform and fine grain structure, ensuring the grain size meets the requirements for mechanical and magnetic properties. It should be understood that the magnetic heat treatment temperature can be adaptively adjusted within a specific temperature range depending on the degree of deformation during cold drawing.

[0038] The chemical composition of the bar is controlled as follows: 0.06%-0.08% C, no more than 0.05% Si, 1.05%-1.20% Mn, 0.065%-0.075% P and 0.29%-0.32% S, as well as 0.003%-0.01% Nb, 0.0018%-0.01% Ti and 0.003%-0.03% Mo. Among these elements, carbon (C) enhances the mechanical properties of steel but negatively impacts its magnetic properties; manganese (Mn) enhances the toughness of steel but promotes austenite formation, thus impairing the magnetic properties of the finished product; excessive phosphorus (P) and sulfur (S) significantly weaken mechanical properties, but when controlled within a certain range, they can significantly improve the machinability of the steel; silicon (Si) significantly affects the magnetic properties of steel, requiring precise control of its content. After long-term research, the inventors discovered that to achieve a balance between the mechanical and magnetic properties of the steel used in this solenoid valve, additional Nb, Ti, and Mo are needed. These elements improve both the magnetic and mechanical properties of the steel. During the bar smelting process, the residual oxygen content is controlled between 40ppm and 100ppm at tapping, the sulfides in the bars meet SEP1572 standard 2.2 or 2.3, and the inclusion grade is less than 2, ensuring the stability of the finished steel's microstructure and enabling subsequent processing to obtain finished steel with qualified mechanical properties and stable magnetic properties.

[0039] Before hot rolling the bars, they are first heated to 1170℃-1190℃ and held at that temperature for 2-4 hours. Preferably, at least one preheating treatment is performed at a temperature not exceeding 950℃ to ensure uniform temperature throughout the bar before further heating. The temperature, stages, and holding time of this preheating treatment should be adapted to the actual heating capacity of the heat treatment furnace. During the heating process, the cooling rate is controlled by adjusting the flow rate of cooling water and cooling air in the hot rolling equipment, maintaining the initial rolling temperature at 870℃-920℃ and the final rolling temperature at 910℃-940℃. Too low a temperature will result in insufficient hot rolling, while too high a temperature will lead to excessively large grain sizes, impairing the mechanical properties of the finished steel.

[0040] In the magnetic heat treatment stage, the bar is first preheated to 650±20℃ and held for 2 hours, then heated to 820±20℃ and held for 12 hours for magnetic heat treatment. After 7 hours, it is cooled to 650±20℃ and then cooled in the furnace to 200±20℃ before being removed from the furnace, completing the heat treatment process. Those skilled in the art should understand that for general heat treatment furnaces, given the initial temperature, target temperature, and heating / cooling time, the furnace can achieve temperature feedback regulation based on its own PLC control system, thereby completing the temperature control of the heating / cooling process. The heating / cooling process is achieved at an approximately uniform rate. Due to limitations in the heating and heat dissipation capabilities of different heat treatment furnaces and the accuracy of the controller, the temperature may fluctuate reasonably within the normal heat treatment parameter range.

[0041] During cold drawing, as the deformation increases, the magnetic properties of the finished material undergo higher-order changes. The stability of the finished product's magnetic properties is highly sensitive to the reduction rate during cold drawing. Furthermore, applying plastic deformation to the material after magnetic heat treatment affects the magnetic properties, and machining introduces a residual stress layer on the surface, which also significantly interferes with the steel's magnetic properties. Therefore, the dimensions of the steel used in this solenoid valve need to be controlled during the hot rolling stage. After cold drawing, the bar dimensions should be close to the finished product dimensions, retaining only machining allowance. After magnetic heat treatment, the bar is ground into the finished steel product. In a preferred embodiment, the single-sided grinding amount is controlled at 0.02mm-0.03mm. Too little grinding will result in substandard steel surface roughness, while too much grinding will lead to excessive processing loss and reduced production efficiency.

[0042] The above method can obtain soft magnetic steel rods with stable magnetic properties in mass production, which can be used for the manufacture of solenoid valves. The overall pass rate is significantly better than that of imported 11SMn30 steel, which effectively improves production efficiency and reduces the manufacturing cost of parts.

[0043] In long-term practice, the inventors tried the research and development route of first locking the heat treatment parameters and then adjusting the cold drawing deformation rate. However, after a lot of attempts, the inventors realized that once the reduction rate reaches a certain value during the cold drawing deformation process, the material plastic deformation will cause grain boundary slip, and the magnetic properties of the sample will change nonlinearly, making it difficult to grasp the law of magnetic property change. Finally, the inventors determined the above-mentioned process development method.

[0044] In one embodiment of the present invention, soft magnetic steel for a gearbox solenoid valve is manufactured using the following method.

[0045] First, steel is smelted, and its composition is controlled by weight as follows: C 0.06%-0.08%, Si ≤0.05%, Mn 1.05%-1.20%, P 0.065%-0.075%, S 0.29%-0.32%, Cr ≤0.15%, Ni ≤0.10%, Mn 0.01%-0.03%, Nb 0.003%-0.01%, Ti 0.018%-0.01%, and Mo 0.01%-0.03%. The principle of composition control is to adjust the elemental ratios to favor magnetic properties while ensuring qualified mechanical properties. Molten iron is obtained by smelting in a blast furnace, then transferred to a converter for steelmaking. The molten steel is then transferred to a refining furnace for further refining, and finally continuously cast to obtain steel billets. The sulfide content in steel should be tested according to SEP1572 standard, and its typical field of view should conform to chart 2.2 or 2.3 to ensure the machinability of the finished steel. During tapping from the smelting furnace, the residual oxygen content should be controlled at 40-100 ppm. The slag removal process should be standardized to prevent foreign inclusions from contaminating the steel. Intrinsic inclusions should be fine, uniform, and dispersed, with an inclusion grade of less than 2.

[0046] Next, the steel billet undergoes hot rolling. Before hot rolling, the billet is first heated using a segmented heating method, gradually raising it to 1170℃-1190℃ and holding it at that temperature for 2-4 hours. Preheating can begin at below 950℃ to ensure uniform temperature throughout the billet. In other embodiments, two or more preheating stages can be used depending on the heating and holding capacity of the heat treatment furnace. The initial rolling temperature of the billet is controlled at 870℃-920℃ by adjusting the flow rates of water and air cooling in the hot rolling equipment, and the final rolling temperature is controlled at 910℃-940℃, thus completing the billet rolling process. During rolling, the columnar crystals formed during the solidification of the billet are broken up, solidification defects in the microstructure are eliminated, precipitated phases are redistributed, and the microstructure is improved. Because subsequent cold drawing severely limits the plastic deformation introduced into the billet, the dimensions of the billet must be controlled during hot rolling to ensure that the dimensions of the billet after cold drawing are close to the finished product dimensions, retaining only a machining allowance. In this embodiment, based on the finished product dimensions, and with a machining allowance of 0.02mm on each side, the hot-rolled billet is processed into a round bar with a diameter of 19±0.1mm according to the reduction rate.

[0047] The hot-rolled bars were cold-drawn using a combined drawing machine, with the surface reduction rate q controlled at 11.24%. The bars were drawn to a diameter of 17.9 mm, followed by straightening, cutting, polishing, and flaw detection. Compared to the final product, the cold-drawn bars retained a machining allowance of 0.02 mm to 0.03 mm on each side.

[0048] Magnetic heat treatment was performed on the cold-drawn bars using a gas-fired bell-type furnace. First, the bars were preheated to 648℃ and held for 2 hours, then heated to 825℃ and held for 12 hours after 3.5 hours. After holding at this temperature, they were cooled to 650℃ for 7 hours, and then cooled in the furnace to 200℃ before being removed from the furnace. This process was automated by inputting the target temperature and time into the heat treatment furnace. After magnetic heat treatment, the plastic deformation introduced during cold drawing drives grain recrystallization. The holding temperature, time, and cooling rate during the subsequent cooling process have a crucial impact on the final microstructure. The grain size after magnetic heat treatment is grade 3-5 to simultaneously meet the requirements of the material's mechanical and magnetic properties.

[0049] Finally, the steel that has undergone magnetic heat treatment is ground to the required dimensions, with a grinding amount of 0.02mm-0.03mm per side. After magnetic heat treatment, the steel should not be subjected to plastic deformation or cutting processes that alter the grain structure or introduce high residual stress, in order to ensure the stability of the magnetic properties of the finished product.

[0050] The mass-produced solenoid valve steel obtained after the above processing meets the following mechanical properties: tensile strength of 357MPa, yield strength of 265MPa, and hardness HBW of 128; magnetic properties: coercivity of 188A / m; and magnetic flux density under different applied magnetic fields: B 1000 =1.433T, B 2000 =1.568T, B 5000 =1.693T, B 10000 =1.807T, B 20000 =1.953T,

[0051] B 30000 =2.048T. Both the mechanical and magnetic properties meet the design requirements. Furthermore, this steel has excellent machinability. Using this steel to manufacture gearbox solenoid valves results in fast response speeds, precise operation, and good durability, enabling long-term service within automatic gearboxes. This effectively improves the yield rate of solenoid valve production lines, reduces the risk of scrap, and optimizes manufacturing costs.

[0052] In the first comparative example, the steel composition was controlled at C content of 0.04%-0.06% and Si content of...

[0053] With a content of ≤0.09%, Mn content of 0.99%-1.26%, P content of ≤0.15%, and S content of 0.25%-0.35%, after hot rolling, cold drawing, and magnetic heat treatment as in the example, the test result showed a tensile strength of 290 MPa, which is unqualified in terms of mechanical properties.

[0054] In the second comparative example, the steel composition was controlled at C content of 0.06%-0.08% and Si content of...

[0055] The sample, with a content of ≤0.09% Mn (0.99%-1.26%), ≤0.15% P, and 0.25%-0.35% S, underwent the same hot rolling, cold drawing, and magnetic heat treatment as the example. While its mechanical properties met the design standards, its magnetic properties deviated significantly, with a coercivity of 270 A / m, exceeding the upper limit. Excessive coercivity leads to a prolonged response time between the magnetic sleeve and armature when the solenoid valve current changes, resulting in sluggish valve operation and affecting control accuracy.

[0056] In the third comparative example, the steel composition and hot rolling processing parameters were controlled in the same way as in the first embodiment. During the cold drawing process, the bar was cold-drawn from a diameter of 19 mm to 18.5 mm. After undergoing the same magnetic heat treatment as in the first embodiment, it was further lightly drawn to a diameter of 17.5 mm. The resulting sample was tested and found to have a coercivity of Hc = 350 A / m, exceeding the limit, due to the plastic deformation introduced by the light drawing.

[0057] In the fourth comparative example, the steel composition was controlled in the same way as in the first embodiment. The initial rolling temperature was controlled at 870℃-910℃ during the hot rolling process. Due to improper control of the cooling rate, the final rolling temperature was 800℃. After the same cold drawing and magnetic heat treatment as in the first embodiment, the mechanical and magnetic properties of the sample were found to be out of tolerance. The tensile strength was only 280MPa, and the coercivity exceeded the upper limit by 290A / m, which did not meet the design requirements.

[0058] The purpose of the above embodiments is to provide a further detailed description of the present invention in conjunction with the accompanying drawings, so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the claims of the present invention, optimization or equivalent substitution of the involved components, methods and steps, as well as combination of implementation methods in different embodiments without causing a conflict of principles, all fall within the protection scope of the present invention.

Claims

1. A method for manufacturing soft magnets for solenoid valves, characterized in that, Includes the following steps: The steel billet is hot rolled at an initial rolling temperature of 870℃-920℃ and a final rolling temperature of 910℃-940℃, and then processed into bars. The bar is cold-drawn, and the surface area reduction rate of the bar after cold drawing is 7%-18%. The cold-drawn rods are held at 780℃-860℃ for 8-20 hours to complete the magnetic heat treatment. The composition of the rod includes: 0.06%-0.08% C, no more than 0.05% Si, 1.05%-1.20% Mn, 0.065%-0.075% P, and 0.29%-0.32% S. And, 0.003%-0.01% Nb, 0.0018%-0.01% Ti and 0.01%-0.03% Mo.

2. The method for manufacturing soft magnetic steel according to claim 1, characterized in that, The cold drawing process reduces the surface area by 10.25%-13.22%, and the heating temperature for magnetic heat treatment is 820±20℃, with a magnetic heat treatment time of 12h.

3. The method for manufacturing soft magnetic steel according to claim 1, characterized in that, It also includes the step of smelting bar stock, wherein the residual O content is controlled at 40ppm-100ppm during tapping, the sulfides in the bar stock meet SEP1572 standard 2.2 or 2.3, and the inclusion grade is less than 2.

4. The method for manufacturing soft magnetic steel according to claim 1, characterized in that, Before hot rolling the bar, the bar is first heated to 1170℃-1190℃ and held for 2-4 hours.

5. The method for manufacturing soft magnetic steel according to claim 1, characterized in that, In the magnetic heat treatment process, the cold-drawn bar is first preheated to 650±20℃ and held for 2 hours, and then heated to the magnetic heat treatment temperature for 3.5 hours. After holding for 7 hours, it is first cooled to 650±20℃, and then cooled in the furnace to 200±20℃ before being taken out of the furnace.

6. The method for manufacturing soft magnetic steel according to claim 1, characterized in that, It also includes a grinding step, in which the magnetically heat-treated bar is ground to the required size to obtain a finished steel product.

7. The method for manufacturing soft magnetic steel according to claim 6, characterized in that, The processing amount in the grinding step is 0.02mm-0.03mm on one side.

8. A soft magnet for a solenoid valve, characterized in that, Manufactured using the soft magnetic steel manufacturing method as described in any one of claims 1 to 7.

9. The soft magnet for the solenoid valve according to claim 8, characterized in that: The soft magnetic steel has a tensile strength ≥300MPa, a yield strength ≥210MPa, and a hardness HBW of 110-150. Coercivity is 150A / m-200A / m; The magnetic field strength satisfies: 1.20T≤B 1000 ≤1.42T, 1.46T≤B 2000 ≤1.58T, 1.65T≤B 5000 ≤1.70T, 1.77T≤B 10000 ≤1.82T, 1.92T≤B 20000 ≤1.96T, 2.01T≤B 30000 ≤2.05T; The grain size grade in the steel is 3-5.

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