A method for preventing production of 65Mn flat coil of fine blanking steel

Abstract

The application discloses a production method for preventing flat coiling of fine-blanking steel 65Mn, and comprises the following hot continuous rolling steps: the coiling temperature is controlled at pearlite phase transition nose point temperature + (5-35) DEG C; the coiling tension is controlled by using gradient tension; after the coiling is completed, the steel coil is rotated with the mandrel to keep the shape; and the spraying cooling water is closed during the whole process of coiling to uncoiling. The method can reduce the content of residual austenite, improve the rigidity of the steel coil matrix on the basis of retaining the structure of inhibiting net ferrite and reducing cold rolling strip breakage, solve the quality defect of the steel coil flat coiling, retain a small amount of island ferrite structure obtained by controlling the cooling of the front hot rolling, retain the double-scale pearlite + island ferrite structure, be compatible with the five-rack cold continuous rolling anti-breakage technology, not damage the anti-breakage performance, realize the double technology effects of anti-breakage and anti-flat coiling, adapt to the hot rolling production of 1.8mm-4.5mm thin specification 65Mn fine-blanking steel, and is simple in process, does not need additional alloy addition, and is strong in industrialization adaptability.

Description

Technical Field

[0001] This invention relates to a rolling method, and more particularly to a method for producing 65Mn fine-stamped steel coils that prevents flattening. Background Technology

[0002] 65Mn is a high-carbon pearlitic precision stamping steel, widely used in the manufacture of precision stamped parts and elastic metal components due to its high strength and excellent wear resistance. Under the production conditions of a five-stand cold rolling mill, this mill is characterized by high inter-stand tension, high single-stand reduction rate, and large continuous rolling deformation. 65Mn raw materials produced by conventional hot rolling processes contain a network ferrite structure. During high-tension, high-reduction cold rolling, the ferrite preferentially undergoes plastic deformation, inducing microcracks that propagate along the network structure, easily leading to cold-rolled strip breakage.

[0003] To address the aforementioned strip breakage issue, existing mature processes employ a segmented controlled cooling and ferrite-reducing hot-rolling preparation scheme. This involves regulating the matrix microstructure through low-temperature phase transformation, eliminating network ferrite, and significantly reducing the strip breakage rate in five-stand cold continuous rolling. However, this type of low-temperature phase transformation process results in a lower coiling temperature, incomplete phase transformation reaction within the coil, and a large amount of untransformed austenite remaining in the microstructure after uncoiling. The austenite matrix has low strength and poor rigidity, causing plastic collapse under its own weight after the coil detaches from the coiling mandrel support, resulting in flattened coil defects.

[0004] The existing technology has the following drawbacks: (1) Phase transformation residual defects: The existing anti-breakage hot rolling process adopts a low temperature cooling system. After coiling, the steel coil contains residual untransformed austenite. The soft austenite cannot support the weight of the steel coil, which is the fundamental metallurgical mechanism of flat coil formation. (2) Unreasonable winding temperature: Conventional low temperature winding is far from the nose point of pearlite phase transformation, the phase transformation kinetics are slow, the phase transformation is delayed, and the deformation continues after unwinding; the further away from the reasonable winding temperature, the greater the flat roll deformation, and the closer the difference between the major and minor axes is to the critical value of 50mm. (3) Unreasonable tension system: Traditional constant tension winding results in insufficient interlayer friction during strip speed increase, loose interlayer bonding of steel coil, and easy slippage and collapse; (4) Coarse uncoiling process: The coil is unloaded directly after winding without a shaping and pressure holding stage. There is no rigid support during the phase transformation process, which exacerbates the deformation of thin-gauge steel coils. (5) External cooling interference: During the coiling process, the pinch roller sprays water randomly onto the steel coil, causing local rapid cooling and uneven temperature field, which further enriches the residual austenite and worsens the flat coil defect. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a production method that can improve the strength of the matrix and prevent the flat coil of 65Mn fine stamping steel.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following hot continuous rolling steps: the coiling temperature is controlled at the pearlite phase transformation nose point temperature + (5~35)℃; the coiling tension is controlled by gradient tension; after coiling, the steel coil rotates with the mandrel to maintain its shape; the spray cooling water is turned off throughout the entire process from coiling to uncoiling.

[0007] Furthermore, the winding tension is controlled using the following gradient tension control: the tension is dynamically increased according to the strip travel distance, with the winding tension controlled at 78kN to 82kN in the 0-100m range, 83kN to 87kN in the 100-200m range, 88kN to 97kN in the 200-300m range, and the tension kept constant at 88kN to 97kN after 300m.

[0008] Furthermore, the steel coil rotates with the mandrel at a speed of 42-47 s / revolution, and the shape-maintaining rotation time is controlled to be 2-3 min.

[0009] Furthermore, it is applicable to fine-stamped steel 65Mn with thicknesses ranging from 1.8mm to 4.5mm.

[0010] The beneficial effects of adopting the above technical solution are as follows: (1) Solving the flat roll problem from the metallurgical mechanism: Based on the TTT curve, the winding temperature is controlled at about 20°C above the pearlite nose point, which greatly reduces the residual austenite, improves the matrix strength, and fundamentally eliminates the flat roll caused by the self-weight collapse of the soft phase. (2) Retain the excellent microstructure of the strip breakage prevention: This invention is used in conjunction with the previous strip breakage patent. It does not change the microstructure control effect of the segmented cooling in the front section. The matrix is ​​still double-scale pearlite + 1% to 3% island ferrite, which takes into account the strip breakage prevention capability of five-stand cold continuous rolling. (3) Strong process adaptability: The tension gradient, mandrel rotation and water cooling processes are all compatible with existing hot rolling production lines, requiring no hardware modification and no increase in production costs; (4) Wide product coverage: Stable compatibility with the full range of thin specifications from 1.8mm to 4.5mm, with the difference between the long and short axes ≤46mm under all process boundary conditions, and all meet the ≤50mm qualified judgment standard; (5) High production stability: It avoids industrial pain points such as local rapid cooling, insufficient tension, and premature unwinding. The steel coil has a regular shape and no defects such as collapsed coil, tower shape, or misaligned layer, which greatly improves the success rate of pickling and cold rolling. (6) This invention is based on the TTT phase transformation curve of 65Mn steel, precisely controlling the coiling temperature above the phase transformation nose point. Combined with gradient coiling tension, mandrel delayed shaping, and water-avoidance cooling control, it reduces the residual austenite content. While retaining the microstructure that suppresses network ferrite and reduces cold-rolled strip breakage, it improves the rigidity of the steel coil matrix and solves the quality defects of flat coils caused by low-temperature material preparation processes. This invention retains a small amount of island ferrite microstructure obtained from the controlled cooling of the previous hot rolling stage, and retains the dual-scale pearlite + island ferrite microstructure. It is compatible with the five-stand cold continuous rolling anti-strip breakage process, does not damage the anti-cold-rolled strip breakage performance, and achieves the dual process effects of anti-strip breakage and anti-flat coil. It is suitable for the hot rolling production of 1.8mm to 4.5mm thin-gauge 65Mn precision stamping steel. The process is simple, requires no additional alloying, and has strong industrial adaptability. Detailed Implementation

[0011] The present invention will now be described in further detail with reference to specific embodiments.

[0012] This method for producing 65Mn precision-stamped steel coils to prevent flattening uses 65Mn precision-stamped steel with the following chemical composition (wt): C: 0.62%–0.72%, Si: 0.20%–0.30%, Mn: 0.50%–1.50%, P≤0.015%, S≤0.005%, Als: 0.02%–0.05%, N≤0.006%, with the balance being Fe and unavoidable impurities. The above composition has high purity and extremely low levels of impurity elements, and fluctuations within the composition range do not affect the phase transformation law and coil shape control.

[0013] This method for producing 65Mn steel coils to prevent flattening is suitable for all thin-gauge products from 1.8mm to 4.5mm, including both hot and cold rolling processes. The process is as follows: (1) Hot rolling process: including hot rolling and coiling processes.

[0014] The coiling process involves controlling the coiling temperature at the pearlite transformation nose temperature + (5~35)℃. Since the pearlite transformation nose temperature of this 65Mn steel is 580℃, the coiling temperature is controlled at 600℃±15℃. If the coiling temperature is too low, the austenite transformation driving force is insufficient, resulting in a large amount of residual austenite, and the soft matrix cannot withstand its own weight, causing collapse. This method controls the coiling temperature approximately 20℃ above the transformation nose, i.e., 600℃±15℃, placing the steel coil in a slow and stable pearlite transformation range. During the coiling process, austenite is fully transformed into high-strength pearlite, significantly reducing the residual austenite content, improving the high-temperature rigidity of the steel coil, and eliminating the cause of flattening from the metallurgical source.

[0015] The coiling tension is controlled using the following gradient tension control: the tension is dynamically increased according to the strip travel distance. In the 0-100m range, the coiling tension is controlled at 78kN-82kN, preferably 80kN; in the 100-200m range, the coiling tension is controlled at 83kN-87kN, preferably 85kN; in the 200-300m range, the coiling tension is controlled at 88kN-97kN, preferably 90kN; and after 300m, the tension is kept constant at 88kN-97kN, preferably 95kN. In this way, the interlayer friction is increased by increasing the tension, forming uniform compressive stress and suppressing interlayer slippage and collapse.

[0016] After winding, the coil is not unwound immediately. The coil rotates at a constant speed with the mandrel, with a rotation speed of 42-47 s / revolution, preferably 45 s / revolution. The shape-preserving rotation time is controlled at 2-3 minutes. In this way, most of the phase transformation is completed under rigid support, the coil shape is fixed, and collapse is prevented during unwinding.

[0017] The pinch rolls and cooling water spray are turned off throughout the entire process from coiling to uncoiling to prevent any cooling water from coming into contact with the strip and avoid uneven phase transformation and enrichment of residual austenite caused by local rapid cooling.

[0018] (2) Cold rolling process: Cold rolling is carried out using a five-stand cold rolling mill.

[0019] (3) After adopting the above method, the microstructure of the cold-rolled product is a dual-scale pearlite + 1% to 3% island ferrite microstructure, the difference between the major and minor axes of the inner ring of the steel coil is ≤46mm, and the flat coil defect rate of the cold-rolled product is reduced to 3% or less, which is far lower than the average flat coil defect rate of ≥40% of conventional processes. Example 1

[0020] (1) The chemical composition of the 65Mn steel is: C=0.65%, Si=0.25%, Mn=1.00%, P=0.012%, S=0.003%, Als=0.035%, N=0.004%, with the balance being Fe and unavoidable impurities; the pearlite phase transformation nose point temperature is 580℃.

[0021] (2) Hot continuous rolling: Coiling temperature 585℃; Coiling tension adopts gradient tension system: 80kN for 0~100m, 85kN for 100~200m, 90kN for 200~300m, and constant 95kN after 300m; After coiling is completed, the coil is not unloaded immediately, the mandrel is kept rotating for 45s / revolution, and the rotation time is 2.5min; The cooling water of the pinch roll is turned off throughout the process.

[0022] (3) Cold rolling: Cold rolling is carried out using a five-stand cold rolling mill.

[0023] (4) Test results: The thickness of the cold-rolled steel coil was 1.8 mm, the difference between the major and minor axes of the inner ring was 46 mm, and there was no flattened coil; the microstructure was a dual-scale pearlite + 1% to 3% island ferrite microstructure. Example 2

[0024] (1) The chemical composition of the 65Mn steel is: C=0.70%, Si=0.28%, Mn=1.20%, P=0.010%, S=0.002%, Als=0.040%, N=0.003%, with the balance being Fe and unavoidable impurities; the pearlite phase transformation nose point temperature is 580℃.

[0025] (2) Hot continuous rolling: Coiling temperature 615℃; Coiling tension adopts gradient tension system: 80kN for 0~100m, 85kN for 100~200m, 90kN for 200~300m, and constant 95kN after 300m; After coiling is completed, the coil is not unloaded immediately, the mandrel is kept rotating for 45s / revolution, and the rotation time is 3min; The cooling water of the pinch roll is turned off throughout the process.

[0026] (3) Cold rolling: Cold rolling is carried out using a five-stand cold rolling mill.

[0027] (4) Test results: The thickness of the cold-rolled steel coil was 4.5 mm, the difference between the major and minor axes of the inner ring was 18 mm, and there was no flattened coil; the microstructure was a dual-scale pearlite + 1% to 3% island ferrite microstructure. Example 3

[0028] (1) The chemical composition of the 65Mn steel is: C=0.68%, Si=0.22%, Mn=0.80%, P=0.011%, S=0.004%, Als=0.030%, N=0.005%, with the balance being Fe and unavoidable impurities; the pearlite phase transformation nose point temperature is 580℃.

[0029] (2) Hot continuous rolling: Coiling temperature 600℃; Coiling tension adopts gradient tension system: 78kN for 0~100m, 83kN for 100~200m, 88kN for 200~300m, and constant 97kN after 300m; After coiling is completed, the coil is not unloaded immediately, the mandrel is kept rotating for 44s / revolution, and the rotation time is 2min; The cooling water of the pinch roll is turned off throughout the process.

[0030] (3) Cold rolling: Cold rolling is carried out using a five-stand cold rolling mill.

[0031] (4) Test results: The obtained cold-rolled steel coil has a thickness of 3.0 mm, a major-minor axis difference of 27 mm, and no flat coils; the microstructure is a dual-scale pearlite + 1% to 3% island ferrite microstructure. Example 4

[0032] (1) The chemical composition of the 65Mn steel is: C=0.62%, Si=0.20%, Mn=1.50%, P=0.014%, S=0.005%, Als=0.020%, N=0.006%, with the balance being Fe and unavoidable impurities; the pearlite phase transformation nose point temperature is 580℃.

[0033] (2) Hot continuous rolling: Coiling temperature 595℃; Coiling tension adopts gradient tension system: 79kN for 0~100m, 86kN for 100~200m, 92kN for 200~300m, and constant 95kN after 300m; After coiling is completed, the coil is not unloaded immediately, the mandrel is kept rotating for 42s / revolution, and the rotation time is 3min; The cooling water of the pinch roll is turned off throughout the process.

[0034] (3) Cold rolling: Cold rolling is carried out using a five-stand cold rolling mill.

[0035] (4) Test results: The obtained cold-rolled steel coil has a thickness of 2.0 mm, a major-minor axis difference of 33 mm, and no flat coils; the microstructure is a dual-scale pearlite + 1% to 3% island ferrite microstructure. Example 5

[0036] (1) The chemical composition of the 65Mn steel is: C=0.72%, Si=0.30%, Mn=0.50%, P=0.015%, S=0.0045%, Als=0.050%, N=0.0055%, with the balance being Fe and unavoidable impurities; the pearlite phase transformation nose point temperature is 580℃.

[0037] (2) Hot continuous rolling: Coiling temperature 610℃; Coiling tension adopts gradient tension system: 82kN for 0~100m, 87kN for 100~200m, 97kN for 200~300m, and constant 97kN after 300m; After coiling is completed, the coil is not unloaded immediately, the mandrel is kept rotating for 47s / revolution, and the rotation time is 2.5min; The cooling water of the pinch roll is turned off throughout the process.

[0038] (3) Cold rolling: Cold rolling is carried out using a five-stand cold rolling mill.

[0039] (4) Test results: The obtained cold-rolled steel coil has a thickness of 4.0 mm, a major-minor axis difference of 38 mm, and no flat coils; the microstructure is a dual-scale pearlite + 1% to 3% island ferrite microstructure.

Claims

1. A production method for preventing flattening of 65Mn precision-stamped steel coils, characterized in that, The hot continuous rolling process includes the following steps: the coiling temperature is controlled at the pearlite phase transformation nose point temperature + (5~35)℃; the coiling tension is controlled by gradient tension; after coiling, the steel coil rotates with the mandrel to maintain its shape; the spray cooling water is turned off throughout the entire process from coiling to uncoiling.

2. The production method for preventing flattening of 65Mn fine-stamped steel coils according to claim 1, characterized in that, The winding tension is controlled by the following gradient tension: the tension is dynamically increased according to the strip travel distance. The winding tension is controlled at 78kN to 82kN in the 0-100m range, 83kN to 87kN in the 100-200m range, 88kN to 97kN in the 200-300m range, and the tension is kept constant at 88kN to 97kN after 300m.

3. The production method for preventing flattening of 65Mn fine-stamped steel coils according to claim 1, characterized in that: The steel coil rotates with the mandrel at a speed of 42-47 seconds per revolution, and the shape-preserving rotation time is controlled to be 2-3 minutes.

4. A production method for preventing flattening of 65Mn fine-stamped steel coils according to claim 1, 2, or 3, characterized in that: It is suitable for fine stamping steel 65Mn with a thickness of 1.8mm to 4.5mm.