Thick steel plate for high fatigue flat spring and method for manufacturing the same

Abstract

The present application relates to a kind of thick steel plate for high fatigue disc spring and its manufacturing method, belong to metallurgy field.The steel plate optimizes component design, using continuous casting-rolling-continuous furnace heat treatment process to produce thick steel plate for high fatigue life disc spring.The spring steel plate produced by this method can reach 2800mm in width, flat, flatness≤3mm / m, thickness difference 0.3mm, hardness≤252HBW, reduce surface milling processing amount, easy to process and shape, improve disc spring material utilization rate.The steel plate is high in purity, inclusion face scanning index≤3.5, and the disc spring produced has a service life of more than 2 million times.The thick steel plate for disc spring produced by continuous casting-rolling is higher in purity, higher in production efficiency and higher in size accuracy compared to die casting-forging disc spring steel plate, and the material yield is increased by more than 10%, which has important popularization value.

Description

Technical Field

[0001] This invention relates to the field of metallurgy, and more particularly to a spring steel plate and its manufacturing method. Background Technology

[0002] Disc springs, with their conical disc shape, have a shorter stroke, require less space, are easier to assemble and maintain, and are highly economical compared to traditional springs. They are widely used in various industrial engineering fields such as brakes, shock absorbers, bearing preload safety overload devices, and clamping devices, and also in some specialized fields such as the nuclear, aerospace, and military industries. Spring manufacturers primarily purchase disc spring steel plates and then, according to their own processes and requirements, stamp or machine the steel plates into hollow ring spring components. These components are then produced through heat treatment, precision milling, and other processes.

[0003] Currently, standard disc spring parts (thickness <15mm) on the market are made by stamping steel strips or flat steel into rings, followed by further processing. Another type is medium-to-large disc spring parts with a thickness of 15mm or more, which use die-cast blanks. These blanks are repeatedly heated and forged into round steel of different sizes, and after annealing, they are stamped into rings using a large stamping press. Because disc spring parts have diverse inner and outer diameters, the steel used as raw material for disc springs comes in various sizes and quantities, resulting in a low raw material yield. Furthermore, the blank production process is unstable, leading to large fluctuations in the dimensional accuracy of the forged round steel, and low utilization rates in stamping and subsequent finishing. Currently, the maximum deviation in the inner and outer diameters of medium-to-large disc spring ring parts produced from forged round steel is 10mm, with a thickness allowance of 5mm. The precision milling of disc springs is time-consuming, labor-intensive, and costly. In addition, disc springs require high fatigue life, high dimensional accuracy, and high material purity and decarburization layer. Blanks produced using medium-frequency furnaces or small ingots experience large fluctuations in material purity control, resulting in unstable quality of the disc spring ring parts.

[0004] Chinese patent document CN111349846A discloses a method for producing high-strength, high-toughness spring flat steel, and patent document CN109161803A discloses a method for producing 1550MPa grade spring flat steel. The spring flat steel produced by these methods has a width ≤150mm, while large-specification disc springs typically have an outer diameter of 300-1000mm, failing to meet usage requirements. While continuously rolled thick steel plates offer high dimensional utilization, spring steel plates are medium-to-high carbon alloy steel plates, exhibiting severe segregation and porosity at the center of the slab. This results in significant differences between the surface and core microstructure of the heat-treated spring steel, leading to a short service life. Furthermore, when the steel plate width exceeds 2000mm, the internal stress after rolling is high, causing severe warping. Additionally, the high strength of rolled spring steel plates makes disc spring forming difficult.

[0005] Currently, few domestic steel companies produce low-segregation spring steel plates with a width greater than 1600mm and a thickness greater than 15mm. In view of this situation, there is a need to obtain a stable, efficient, and high-quality thick steel plate for disc springs. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of current thick steel plates, such as severe segregation and poor plate shape, optimize element composition, realize the production of low segregation continuous casting slabs, and use medium and heavy plate rolling mills to produce 15-40mm thick dished spring steel plates with low segregation, flat plate shape, and stable quality, replacing the high energy consumption and low cost-effectiveness of the die casting-forging process for producing spring steel plates.

[0007] The technical solution adopted by this invention to solve the above problems is as follows: a thick steel plate for high fatigue disc springs and its manufacturing method. The steel plate has a thickness of 15-40 mm, a segregation bandwidth ≤ 80 micrometers, a flatness ≤ 3 mm / m, a thickness difference within the same plate of less than 0.3 mm, and a hardness ≤ 252 HBW. The chemical composition of the steel plate, by mass percentage, is: C: 0.48%–0.52%, Si: 0.15%–0.27%, Mn: 0.70%–0.90%, Cr: 0.90%–1.10%, V: 0.10–0.15%, P: ≤ 0.015%, S: ≤ 0.003%, H ≤ 0.0001%, N+H+O ≤ 0.0065%, with the balance being Fe and unavoidable impurity elements.

[0008] The following is a detailed description of the component design in this invention:

[0009] Both carbon (C) and manganese (Mn) are elements that can improve the strength and hardness of steel plates, but excessively high C and Mn contents can easily lead to segregation and uneven microstructure. This invention employs narrow compositional control, with C content ranging from 0.48% to 0.52% and Mn content from 0.70% to 0.90%.

[0010] Si (Si) is a deoxidizing element in steel, used for solid solution strengthening to improve its strength and hardness. When the Si content is below 0.10%, the deoxidation effect is poor; when the Si content is high, silicate inclusions in the molten steel increase, and the purity decreases. In this invention, the Si content is controlled at 0.15-0.27%.

[0011] Cr is a key element for improving the strength and hardness of spring steel. However, excessive Cr will reduce the toughness of the material and introduce more Ti, which can easily produce sharp and hard oxides during smelting, affecting the fatigue life of disc springs. This invention controls its content to 0.90%–1.10%.

[0012] V: V(C, N) can be dispersed and precipitated, significantly improving the strength and hardness of steel, but the cost is high. This invention controls its content to be between 0.10-0.15%.

[0013] H: A harmful gaseous element. High H content easily leads to white spots and widens microcracks in steel plates, causing processing cracks. This invention strictly controls the H content to no more than 0.00010%.

[0014] O and N: Harmful gaseous elements. High content of these elements can easily produce hard, sharp-angled inclusions, severely reducing the fatigue life of springs. This invention employs LF refining and RH vacuum processes to strictly control the O content to no more than 0.0010% and the N content to no more than 0.0040%.

[0015] The method for manufacturing thick steel plates for high-fatigue disc springs provided by this invention includes the following specific process steps:

[0016] (1) High-purity steelmaking: The molten iron undergoes pretreatment, converter smelting, LF refining, and RH vacuum degassing in sequence. After pretreatment, the sulfur content of the molten iron is no more than 0.002%. During converter steelmaking, a deep dephosphorization smelting method is adopted, and slag is retained during tapping to reduce phosphorus reversion. The LF process uses high-basicity white slag refining and steel-slag mixing technology during tapping to effectively control the phosphorus and sulfur content. In addition, the fluctuation range of Si and Mn alloying elements and the content of harmful elements such as phosphorus and sulfur are strictly controlled. The residual elements are required to be: Ti: ≤0.003%; Sb: ≤0.003%; Sn: ≤0.005%; As: ≤0.007%. The argon flow rate is high at the beginning and low at the end to ensure uniform stirring of the molten steel and avoid secondary oxidation. During RH vacuum degassing, the high vacuum time is increased to no less than 20 minutes, and the total time is no less than 45 minutes. The molten steel must contain the following components: P ≤ 0.015%, S ≤ 0.003%, H ≤ 0.0001%, O ≤ 0.0010%, N ≤ 0.0040%.

[0017] (2) Low-segregation continuous casting: The entire process is oxidation-free continuous casting, employing automatic slag detection and a low superheat-constant casting speed process. A light reduction of 4-7mm is used in continuous casting, with a small-large-small distribution method to reduce liquid surface fluctuations and enhance the removal of non-metallic inclusions. The cooling rate is controlled in the second cooling section of continuous casting, ensuring that the light reduction section acts on the slab's solids content within the range of 0.5-0.9, improving center segregation in the continuously cast slab. This produces continuously cast slabs with a thickness of 150mm and a width of 1800-2800mm. The slab exhibits low-magnification center segregation grade C0.5, center porosity grade 0.5, and no other defects.

[0018] (3) Hydrogen diffusion and slow cooling: After the continuous casting slab comes off the line, it is placed in a slow cooling pit at high temperature to diffuse hydrogen and slowly cool down to below 200°C to promote the diffusion of hydrogen in the slab.

[0019] (4) Rolling: The billet is heated in a furnace at a high temperature to allow the segregated components to fully dissolve and diffuse. The heating and holding temperature is ≥1210℃. After exiting the furnace, it is descaled by high-pressure water, and then rolled to optimize the slab shape. The initial rolling temperature is set to >1050℃. The first 2-3 passes use high deformation reduction rate rolling, with the maximum reduction rate in the maximum pass being >25%, and the reduction rate in the last pass not exceeding 10%.

[0020] (5) High-temperature normalizing: In order to further reduce the segregation of the steel plate and improve the uniformity of the steel plate structure, high-temperature diffusion normalizing is adopted. The temperature is 920±10℃ and the furnace time is 2.5~3.0min / mm.

[0021] (6) Tempering: Tempering heat treatment is carried out in a roller hearth continuous furnace at a tempering temperature of 740±10℃ and a holding time of 3.5~4.5min / mm. Before entering the furnace for tempering, the steel plate needs to be shot blasted to remove the surface iron oxide scale; after exiting the furnace for tempering, it is straightened while still warm to ensure that the flatness of the steel plate is ≤3mm / m.

[0022] The present invention has the following features or advantages:

[0023] This invention employs a full-process high-purity, low-segregation control technology. Through narrow compositional control of C, Si, Mn, and Cr, and the addition of an appropriate amount of V, the strength of the steel plate is improved while reducing the tendency for elemental segregation. The refining-vacuum degassing process reduces the content of harmful elements such as P, S, O, N, and H. The constant drawing speed-low superheat-precision light reduction process further reduces center segregation in the slab, ensuring a low-magnification center segregation of C grade 0.5, a center porosity of 0.5, and no other defects.

[0024] This invention employs a combination of high-temperature billet heating and high-temperature normalizing to further improve center segregation in steel plates, achieving a segregation bandwidth of no more than 80μm. Furthermore, a continuous furnace high-temperature tempering process is used to relieve stress and promote uniform microstructure in the thick spring steel plate, facilitating subsequent cutting and processing of disc spring components.

[0025] This invention uses high-quality continuously cast slabs to roll thick steel plates and employs optimized plate shape control rolling technology. The aim is to produce spring steel plates with high thickness accuracy and flat plate shape, thereby reducing the milling allowance during the machining of disc spring rings, significantly improving the yield, and reducing production costs.

[0026] Compared with the prior art, the advantages of this invention are as follows:

[0027] The present invention provides a method for manufacturing thick steel plates for high-fatigue disc springs, which optimizes composition design and enables the production of low-segregation continuously cast slabs. This method changes the traditional production process of medium and large disc spring rings, which involves casting, forging, and stamping. It overcomes the technical difficulties of low purity and unstable quality in traditional processes, significantly improving the dimensional accuracy and production efficiency of thick steel plates for disc springs. The produced thick steel plates for disc springs have a flatness ≤3mm / m, a thickness difference of 0.3mm between plates, and a hardness ≤252HBW. Attached Figure Description

[0028] Figure 1 The near-surface microstructure of the disc spring steel is 100×.

[0029] Figure 2 The steel core of the disc spring is constructed with a 100× mesh, and... Figure 1 The structure shown is consistent, with uniform surface and core structures. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0031] Example 1

[0032] Production steps of thick steel plates for disc springs:

[0033] 1. After smelting molten iron and clean scrap steel in a converter to remove harmful element phosphorus (P), the molten steel is then smelted in an LF refining furnace for 45 minutes. After desulfurization and alloy composition adjustment, the molten steel undergoes a 25-minute vacuum degassing treatment in an RH vacuum refining furnace to further reduce the content of harmful gaseous elements in the molten steel. The final composition of the molten steel before continuous casting is: C: 0.50%, Si: 0.27%, Mn: 0.83%, P: 0.01%, S: 0.0005%, Cr: 1.0%, V: 0.12%, H: 0.6ppm, O: 12ppm, N: 24ppm, with the balance being Fe and impurity elements.

[0034] 2. Molten steel is continuously cast into slabs with a thickness of 150mm*1900mm using a straight-arc continuous casting machine. During continuous casting, an oxidation-free protective casting method is used, with superheat controlled at 10-20℃, a constant casting speed of 1.2m / min, and a light reduction technique. The slab exhibits low-magnification center segregation of C 0.5, center porosity of 0.5 grade, and no center cracks, corner cracks, or triangular cracks. After the continuously cast slab is removed from the line at high temperature, it undergoes slow cooling and hydrogen diffusion in a pit.

[0035] 3. The continuously cast billet is transferred to the 3500mm rolling mill and heated in a walking beam furnace. The soaking temperature of the billet is 1220℃, and the soaking time is 2 hours. The initial rolling temperature of the continuously cast billet is 1050℃. The single-pass reduction rate of the first three rolling passes is ≥15%, the maximum reduction rate is 25%, and the reduction rate of the last pass is 9.8%. Finally, it is rolled into a steel plate with a thickness of 9.75mm, a width of 1900mm, and a length of 19000mm.

[0036] 4. After the hot-rolled steel sheet cools to room temperature, it enters a roller hearth continuous furnace for high-temperature normalizing at 920℃. After air cooling, the normalized steel sheet undergoes tempering heat treatment at 740℃ for 4.0 min / mm. After tempering, the steel sheet is removed from the furnace and air-cooled. After cooling, the steel sheet undergoes shot blasting to remove rust. The steel sheet also needs to be straightened using a straightening machine, with a final flatness of 3 mm / m.

[0037] Example 2

[0038] Production steps of thick steel plates for disc springs:

[0039] 1. After smelting molten iron and clean scrap steel in a converter to remove harmful element phosphorus (P), the molten steel is then smelted in an LF refining furnace for 45 minutes. After desulfurization and alloy composition adjustment, the molten steel undergoes a 25-minute vacuum degassing treatment in an RH vacuum refining furnace to further reduce the content of harmful gaseous elements in the molten steel. The final composition of the molten steel before continuous casting is: C: 0.50%, Si: 0.27%, Mn: 0.84%, P: 0.01%, S: 0.0005%, Cr: 1.0%, V: 0.12%, H: 0.5ppm, O: 12ppm, N: 25ppm, with the balance being Fe and impurity elements.

[0040] 2. Molten steel is continuously cast into slabs with a thickness of 150mm*2800mm using a straight-arc continuous casting machine. During continuous casting, an oxidation-free protective casting process is employed, with superheat controlled at 10-20℃ and a constant casting speed of 0.5m / min. A light reduction technique is also used, resulting in low-magnification center segregation of C 0.5, center porosity of 0.5 grade, and no center cracks, corner cracks, or triangular cracks. After the continuously cast slab is removed from the line at high temperature, it undergoes slow cooling and hydrogen diffusion in a pit.

[0041] 3. The continuously cast billet is transferred to the 4300mm rolling mill and heated in a walking beam furnace. The soaking temperature of the billet is 1250℃, and the soaking time is 2 hours. The initial rolling temperature of the continuously cast billet is 1050℃, and the final rolling temperature is 860℃. The single-pass reduction rate of the first three rolling passes is ≥15%, the maximum pass reduction rate is 30%, and the final pass reduction rate is 8%. Finally, it is rolled into a steel plate with a thickness of 36mm, a width of 2800mm, and a length of 15000mm.

[0042] 4. After the hot-rolled steel sheet cools to room temperature, it enters a roller hearth continuous furnace for normalizing at 920℃. After normalizing, the steel sheet is air-cooled and then tempered at 740℃ for 4.0 min / mm. After tempering, the steel sheet is removed from the furnace and air-cooled. After cooling, the steel sheet undergoes shot blasting to remove rust. The steel sheet also needs to be straightened using a straightening machine, with a final flatness of 2 mm / m.

[0043] Table 1. Detection results of inclusions (DIN 50602 standard M method) and decarburized layer in the examples.

[0044] hardness flatness Thickness difference of the same plate Inclusion Index Example 1 231HBW 3mm / m 0.2mm 3.2 Example 2 220HBW 2mm / m 0.3mm 3.2

[0045] After the disc spring ring parts produced in the above embodiments are sent to customers for processing into disc springs, fatigue life tests are conducted. The single disc spring produced in Embodiments 1 and 2 has a fatigue test of >2 million cycles, and the combined 10-piece spring has a fatigue test of ≥700,000 cycles, far exceeding the purchase requirement of 130,000 cycles. It can be seen that the disc spring ring parts produced by this invention are of excellent quality.

[0046] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A method for manufacturing a thick steel plate for a high-fatigue disc spring, characterized in that: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.48%–0.52%, Si: 0.15%–0.27%, Mn: 0.70%–0.90%, Cr: 0.90%–1.10%, V: 0.10–0.15%, P: ≤ 0.015%, S: ≤ 0.003%, H: ≤ 0.0001%, N+H+O: ≤ 0.0065%, with the balance being Fe and unavoidable impurity elements; The manufacturing process steps include, I. High-purity steelmaking: The molten iron undergoes pretreatment, converter smelting, LF furnace refining, and vacuum degassing in sequence. After pretreatment, the S content of the molten iron is ≤0.002%. Deep dephosphorization is carried out during converter smelting, and slag is left at tapping to reduce phosphorus reversion. High-basicity white slag refining is used in LF furnace refining, and two-thirds of the refining slag is carried at tapping to control the increase of P and S content. In addition, the fluctuation of Si and Mn alloying elements is controlled. The residual elements are Ti: ≤0.003%; Sb: ≤0.003%; Sn: ≤0.005%; As: ≤0.007%. Argon gas is used to stir the molten steel. The argon flow rate in the early stage of smelting is 300-600L, and the argon flow rate in the later stage of smelting is 100-150L. The high vacuum treatment time is increased during vacuum degassing. The high vacuum holding time is not less than 20min, and the total time is not less than 45min. II. Low Segregation Continuous Casting: Continuous casting without oxidation protection throughout the entire process, employing automatic slag detection, low superheat (below 20℃), constant casting speed process, and light reduction of 4-7mm within the solid fraction range of 0.5-0.9 in the continuous casting billet; producing 150mm thick continuous casting slabs with low magnification mass center segregation of C 0.5 grade, center porosity of 0.5 grade, and no other defects; 3. Hydrogen diffusion and slow cooling: After the continuous casting slab comes off the line, it is placed in a slow cooling pit at high temperature to diffuse hydrogen. The pit is cooled to below 200°C to promote hydrogen diffusion. IV. Rolling: The billet is heated at high temperature in the furnace to allow the segregated components to fully dissolve and diffuse. After descaling in the furnace, it is rolled. The initial rolling temperature is >1050℃. The first 2-3 passes are rolled with a high deformation reduction rate, with the maximum reduction rate of the first pass being >25%. The reduction rate of the last pass is not greater than 10%. After rolling, 2-4 passes of hot straightening are performed. V. High-temperature normalizing: High-temperature diffusion normalizing is adopted, with a heating temperature of 920±10℃, a furnace time of 2.5~3.0min / mm, and air cooling; VI. Tempering: The tempering temperature is 740±10℃, the tempering time in the furnace is 3.5~4.5min / mm, after which the furnace is removed and air-cooled, shot blasting is performed.

2. The method according to claim 1, characterized in that: The steel plate is produced with a thickness of 15-40mm and a width of 1800-2800mm.

3. The method according to claim 1, characterized in that: The steel plate has a segregation bandwidth of ≤80 micrometers, a flatness of ≤3mm / m, a thickness difference of ≤0.3mm, and a hardness of ≤252HBW.

4. The method according to claim 1, characterized in that: Step 1: Control the following parameters throughout the entire steel smelting process: P: ≤0.015%, S: ≤0.003%, H: ≤0.0001%, O: ≤0.0010%, N: ≤0.0040%.

5. The method according to claim 1, characterized in that: Step 2: Width of continuous casting slab: 1800-2800mm. The reduction amount in the continuous casting light reduction range is distributed according to 2-4-2mm. The cooling rate is controlled by atomized water spray in the second cooling section of continuous casting, so that the light reduction section acts on the slab solid fraction range of 0.5-0.9, which improves the center segregation of the continuous casting slab.

6. The method according to claim 1, characterized in that: Step 4: The continuous casting slab is heated and held at a temperature ≥1210℃ to accelerate the diffusion of segregated elements in the steel and to ensure that V (C, N) in the slab is fully dissolved.

7. The method according to claim 1, characterized in that: Step 6: The steel plate is subjected to tempering heat treatment in a roller hearth continuous furnace. Before entering the furnace, the steel plate is shot blasted to remove the surface iron oxide scale. After tempering and exiting the furnace, the steel plate is straightened at a warm temperature to ensure that the flatness of the steel plate is ≤3mm / m.

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