Method for improving 1500mpa grade cold-rolled hot-formed steel flat coil and application thereof

Abstract

The present application relates to the technical field of cold-rolled hot-formed steel production, and particularly relates to a method for improving 1500MPa-grade cold-rolled hot-formed steel flat coil and application thereof. The method comprises the following steps: reducing the length of the casting blank; controlling the finishing temperature to be 890±15 DEG C; increasing the hot-rolled thickness specification; adopting U-shaped coiling, the temperature of the head and tail of the steel coil within a range of 60m is 20-30 DEG C higher than the target coiling temperature, the target coiling temperature is controlled to be 700±10 DEG C; and the coiling tension is controlled in six sections. The present application increases the overall rigidity of the steel coil, enhances the ability of the steel coil to resist gravity and radial pressure of the flat coil, reduces the degree of flatness of the flat coil, improves the finishing and coiling temperature, specifies the cooling mode of the steel coil, increases the transformation of austenite to ferrite and pearlite, digests austenite, inhibits the transformation of austenite to bainite with large phase transition expansion, and thus reduces the occurrence of flatness.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled hot-formed steel production technology, specifically to a method for improving 1500MPa grade cold-rolled hot-formed steel flat coils and its application. Background Technology

[0002] Cold-rolled hot-formed steel is one of the main raw materials used in the production of automotive parts. Among them, 1500MPa grade cold-rolled hot-formed steel is the most widely used. Users heat special cold-rolled strip steel to the austenitic temperature and then quickly stamp it. The steel is then held under pressure in a die and quenched to obtain a martensitic structure, resulting in parts with ultra-high strength while maintaining high dimensional accuracy. It is commonly used for automotive B-pillars, C-pillars, front and rear anti-collision beams, bumpers, and reinforcement components. Currently, steel mills face many technical challenges in the production of cold-rolled hot-formed steel. One of the most significant challenges affecting production is the defect of hot-rolled flat coils in 1500MPa grade cold-rolled hot-formed steel. In actual production, the thicker the hot-rolled coil, the smaller or absent the flattening. For 1500MPa grade cold-rolled hot-formed steel coils with a thickness (H≥4.0mm), the high rigidity of the coil itself can resist the radial pressure caused by the coil's own weight and phase transformation expansion, thus preventing flattening or causing only slight flattening, which does not affect subsequent production processes. Flattening mainly occurs in hot-rolled thin specifications (H<4.0mm). The steel coil itself has poor rigidity, and the difference between the maximum and minimum inner diameter of the steel coil is greater than 40mm. The flattening is serious and affects the production of the next process. It often requires manual cutting of the coil core and hot rolling re-coiling to make up for it, resulting in defects such as surface scratches or abrasions. This seriously affects the production rhythm, increases production costs, reduces the yield, and reduces the steel mill's profits.

[0003] Patent application CN 111408625 A discloses a method for controlling the flattening of hot-rolled advanced high-strength steel coils. This method modifies the surface roughness of the rolls to increase the friction between the steel coil layers, thereby improving resistance to flattening. During the laminar flow stage, two different cooling rates are used to rapidly transform eutectoid ferrite, consuming austenite. During the coiling stage, a large coiling tension of 50-60 MPa is used, with tension coefficient distribution to improve the support of the inner coiled strip and resist residual radial pressure between the coil layers. Furthermore, the coil is rotated for 30-40 seconds after coiling to allow more time for austenite phase transformation. The shortcomings of this method are: ① Increasing the roughness of the finishing rolls increases grinding time and production costs; producing easily flattened steel grades requires specific rolls, reducing production efficiency; ② Laminar flow segmented control places high demands on production line equipment, automation levels, and model control, which many steel mill hot-rolling production lines cannot achieve; ③ The 30-40 seconds coiling time reduces production efficiency.

[0004] Patent application CN 113458152 A provides a control method for eliminating flattening of hot-rolled high-strength steel strip. This method involves configuring information acquisition and command execution devices on the hot rolling production line. The model first controls and receives the strip thickness, finishing and coiling temperatures, and laminar flow mode. Then, based on the detected thickness and temperature information, it automatically adjusts the cooling water output and controls the actual coiling temperature to promote the phase transformation of ferrite and pearlite. The model adjusts the coiling tension to 1.3-1.6 times to improve the stability of the steel coil. After coiling, the steel coil undergoes a slight flattening in the horizontal direction on the coiler within a fixed tail time. At this time, the steel coil is placed on a transport trolley by rewinding 90°, and the coil shape is restored by being flattened again during the transportation process. However, this method is only applicable to hot-rolled high-strength steel with a yield strength of 550-630MPa and a tensile strength of 640-730MPa, and has limitations. It is not suitable for cold-rolled hot-formed steel of the 1500MPa level. Moreover, the temperature and tension control model system of this method requires production line upgrades and equipment additions, which increases costs. In addition, in this method, the steel coiler rotates 90° counterclockwise to place the transport trolley, and the flattening of the coil during transportation offsets the original flattening. However, some high-strength steels are flattened during the slow cooling process, such as hot-formed steel, so this method is ineffective.

[0005] Patent application CN 108754104 A provides a method for eliminating defects in hot-rolled flat coils of 590MPa grade duplex steel. This method promotes the transformation of austenite into ferrite and pearlite by using specific finishing rolling temperatures, laminar intermediate temperatures, cooling rates, and coiling temperatures exceeding the bainite transformation temperature, thereby reducing bainite transformation. Furthermore, it controls the tension to reduce slippage between coil layers and improves the overall stiffness of the coil against flattening forces. However, this method is limited to 590MPa and specific steel grades and is not applicable to cold-rolled hot forming of 1500MPa grade steel. Summary of the Invention

[0006] To address the technical problems of thin-gauge cold-rolled hot-formed steel flat coils with a strength of 1500MPa, this invention provides a method for improving 1500MPa-level cold-rolled hot-formed steel flat coils and its application. This invention is easy to implement, requires no additional cost, and provides good improvement in flat coil performance.

[0007] In a first aspect, the present invention provides a method for improving 1500MPa grade cold-rolled hot-formed steel flat coils, comprising:

[0008] The length of the cast billet should be controlled between 5.5 and 8.5 m.

[0009] The finishing rolling temperature is controlled at 890±15℃;

[0010] When the cold-rolled thickness is 0.6 mm ≤ h ≤ 0.9 mm, control the hot-rolled thickness to be 3.0 mm; when the cold-rolled thickness is 0.9 mm < h ≤ 1.2 mm, control the hot-rolled thickness to be 3.4 mm; when the cold-rolled thickness is 1.2 mm < h ≤ 1.5 mm, control the hot-rolled thickness to be 3.8 mm; when the cold-rolled thickness is 1.5 mm < h ≤ 1.8 mm, control the hot-rolled thickness to be 4.2 mm; when the cold-rolled thickness is 1.8 mm < h ≤ 2.1 mm, control the hot-rolled thickness to be 4.5 mm; when 2.1 mm < h ≤ 2.5 mm, control the hot-rolled thickness to be 5.4 mm;

[0011] Adopt U-shaped coiling. The temperature within the range of 60 m at the head and tail of the steel coil is 20 - 30 °C higher than the target coiling temperature, and control the target coiling temperature to be 700 ± 10 °C;

[0012] The coiling tension is controlled in six segments, which are T1, T2, T3, T4, T5, and T6 respectively from the head to the tail of the strip steel, and T2 > T1 > T3 > T4 > T5 > T6.

[0013] Furthermore, before coiling, laminar cooling is carried out on the cold-rolled steel coil in a full-section sparse cooling mode.

[0014] Furthermore, the head tension is controlled by two segments of T1 and T2, the tension of T1 is 27 - 31 KN, and the tension of T2 is 32 - 40 KN; the middle tension is controlled by two segments of T3 and T4, and the tension of T3 and T4 is 23 - 28 KN; the tail tension is controlled by two segments of T5 and T6, and the tension of T5 and T6 is 17 - 25 KN.

[0015] Furthermore, place the steel coil in the middle of other hot-rolled coils, avoid stacking, and cool slowly, so that more retained austenite can transform into ferrite and pearlite.

[0016] Furthermore, an increase in the coiling temperature will cause a decrease in the strength of the annealed finished product. Compensation is carried out in the annealing process. On the basis of the original annealing and temper rolling elongation rate of 0.7% - 0.8%, increase the annealing and temper rolling elongation rate by 0.2% to increase the yield strength; reduce the original annealing rapid cooling temperature by 15 °C based on 470 °C, and reduce the original aging temperature by 15 °C based on 450 °C to increase the tensile strength of the finished product, making the performance of the finished product fluctuate less than before.

[0017] Furthermore, the chemical elements and their contents of the cold-rolled hot-formed steel are C 0.21% - 0.26%, Si 0.15% - 0.45%, Mn 1.20% - 1.50%, Al 0.020% - 0.050%, B 0.0020% - 0.0040%, Ti 0.025% - 0.035%, P ≤ 0.020%, S ≤ 0.020%, and the balance is Fe and unavoidable impurities.

[0018] Secondly, the present invention provides an application of the above method in the preparation of 1500MPa grade cold-rolled or galvanized hot-formed steel hot-rolled coils.

[0019] The beneficial effects of this invention are as follows:

[0020] This invention specifies shorter billet lengths to reduce coil weight and flattening. It increases hot-rolled thickness by specification without compromising cold-rolling production difficulty, thereby increasing coil stiffness and enhancing resistance to gravity and radial pressure, thus reducing flattening. By increasing finishing and coiling temperatures and modifying the laminar cooling mode, specific regulations are made to increase the transformation of austenite to ferrite and pearlite, digesting austenite and inhibiting the transformation of austenite to bainite with high phase transformation expansion, thereby reducing flattening. Furthermore, this invention provides solutions to the impact of process adjustments on finished product performance, resulting in minimal fluctuations in product performance. This invention also implements segmented control of coil winding tension to increase coil stiffness and stability. Additionally, this invention limits the coil cooling environment to allow for slow cooling, similarly increasing the transformation of austenite to ferrite and pearlite, reducing bainite formation, and improving flattening. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is the morphology of the 1500MPa grade cold-rolled hot-formed steel coil of Embodiment 1 of the present invention.

[0023] Figure 2 The hot-rolled steel coil of 1500MPa grade cold-rolled hot-formed steel is shown in Comparative Example 1 of this invention. Detailed Implementation

[0024] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0025] A method for improving the cold-rolled hot-formed steel flat coil of 1500MPa grade includes the following steps: smelting to form a billet, hot rolling (roughing, finishing, laminar flow cooling, coiling, cooling), cold rolling, and annealing.

[0026] In the step of smelting to form a casting blank, the length of the casting blank is reduced, and the length of the casting blank is controlled to be 5.5 - 8.5 m. After hot rolling the shorter casting blank, the weight of the steel coil is light, and the ability of the steel coil's own stiffness to resist the coiling gravity and the radial pressure of volume expansion is enhanced.

[0027] In the hot rolling finishing rolling step, the finishing rolling temperature is controlled to be 890 ± 15 °C. This temperature is the starting temperature of the transformation from austenite to ferrite, which is conducive to the transformation from austenite to ferrite.

[0028] In the hot rolling step, according to the principle that the thicker the hot rolling thickness, the smaller the degree of flat coiling, the hot rolling thickness specification corresponding to the cold rolling thickness is increased. Specifically, when the cold rolling thickness is 0.6 mm ≤ h ≤ 0.9 mm, the hot rolling thickness is increased by 0.4 mm to 3.0 mm; when the cold rolling thickness is 0.9 mm < h ≤ 1.2 mm, the hot rolling thickness is increased by 0.6 mm to 3.4 mm; when the cold rolling thickness is 1.2 mm < h ≤ 1.5 mm, the hot rolling thickness is increased by 0.6 mm to 3.8 mm; when the cold rolling thickness is 1.5 mm < h ≤ 1.8 mm, the hot rolling thickness is increased by 0.7 mm to 4.2 mm; when the cold rolling thickness is 1.8 mm < h ≤ 2.1 mm, the hot rolling thickness is increased by 0.7 mm to 4.5 mm; when the cold rolling thickness is 2.1 mm < h ≤ 2.5 mm, the hot rolling thickness is increased by 0.7 mm to 5.4 mm. After the hot rolling thickness is increased, the cold rolling reduction rate is increased by 4% - 10%, and cold rolling can be carried out normally. For hot-formed steel, increasing the hot rolling thickness has little impact on the finished product performance.

[0029] For the laminar flow mode, the full-section sparse cooling with a slower cooling rate is adopted. Changing from the front-end concentrated cooling with a fast cooling rate to the full-section sparse cooling with a slow cooling rate can make the austenite transform into ferrite and pearlite.

[0030] In the coiling step, U-shaped coiling is adopted. The temperature within the range of 60 m at the head and tail of the steel coil is 20 - 30 °C higher than the target coiling temperature. The target coiling temperature is controlled to be 700 ± 10 °C. Increasing the coiling temperature is beneficial to the transformation from austenite to ferrite and pearlite, reduces the transformation of the remaining austenite at the head and tail of the strip during the slow cooling process of the steel coil into bainite, and reduces the phase transformation expansion amount at the head and tail of the steel coil.

[0031] The coiling tension is controlled in six segments of T1, T2, T3, T4, T5, and T6. Segment control can well adjust the tension of different coil layers of the entire coil strip. The head tension > the middle tension > the tail tension, T2 > T1 > T3 > T4 > T5 > T6. Through appropriate tension distribution and setting, the stability between coil layers is increased, the stiffness of the steel coil is improved, and the ability to resist the radial pressure caused by gravity and phase transformation expansion is enhanced. Specifically, the head tension is controlled by two segments of T1 and T2, the T1 tension is 27 - 31 KN, and the T2 tension is 32 - 40 KN; the middle tension is controlled by two segments of T3 and T4, and the tension is 23 - 28 KN; the tail tension is controlled by two segments of T5 and T6, and the tension is 17 - 25 KN.

[0032] After coiling, the steel coil is placed among other hot-rolled coils, stacking is prohibited, and it is cooled slowly so that the remaining austenite can transform into more ferrite and pearlite.

[0033] Increasing the coiling temperature can alter the decrease in strength of annealed finished products. However, this can be mitigated by increasing the annealing elongation by 0.2% to increase the yield strength, or by reducing the annealing rapid cooling and aging by 15℃ to increase the tensile strength of the finished product, resulting in minimal fluctuations in the product's performance.

[0034] CR1500HF steel is used to produce 1500MPa grade cold-rolled hot-formed steel. The elemental composition and content are as follows:

[0035] Table 1 Chemical composition of CR1500HF (wt%, balance Fe and unavoidable impurities)

[0036]

[0037] Examples 1-4 and Comparative Examples 1-4 were set up. Examples 1-4 were carried out according to the method of the present invention, with a billet length of 5.5-8.5m and a finishing rolling temperature of 890±15℃. Laminar flow cooling mode was adopted, using full-section sparse cooling. The target coiling temperature was 700±10℃, with the head and tail sections each 60m long exceeding the target coiling temperature by 20-30℃. The coiling tension was controlled in six segments: T1 and T2 at the head, T3 and T4 in the middle, and T5 and T6 at the tail, with the order T2>T1>T3>T4>T5>T6. The range of T1 was 27-31KN, T2 was 32-40KN, T3 and T4 were 23-28KN, and T5 and T6 were 17-25KN. After coiling, the steel coil was placed among other hot coils for slow cooling; stacking for slow cooling was prohibited. Specific parameters are recorded in Tables 2 and 3.

[0038] Table 2 Relationship between cold-rolled thickness and hot-rolled thickness in Examples 1-4 (mm)

[0039]

[0040] Table 3 Process parameters for Examples 1-4

[0041]

[0042] The relationship between cold-rolled and hot-rolled thicknesses for Comparative Examples 1-4 is shown in Table 4. The billet length requirement is 8-12m, the finishing rolling temperature is 860±10℃, laminar flow cooling uses front-end centralized cooling, the coiling temperature is 640±10℃, and the coiling tension is controlled in two stages, with a tension range of 15-25KN. After coiling, the steel coil is not subjected to slow cooling. The hot rolling process for Comparative Examples 1-4 is shown in Table 5.

[0043] Table 4 Relationship between cold-rolled thickness and hot-rolled thickness in Comparative Examples 1-4 (mm)

[0044]

[0045] Table 5 Process parameters of Comparative Examples 1-4

[0046]

[0047] The maximum and minimum diameters of the cores obtained in Examples 1-4 and Comparative Examples 1-4 were measured respectively, and the difference between the maximum and minimum inner diameters was calculated. An inner diameter difference ≤ 30mm does not affect subsequent production processes and meets the standard; an inner diameter difference > 30mm affects subsequent production processes, resulting in severe flattening and not meeting the standard. The results are shown in Table 6 below.

[0048] Table 6. Measurement results (mm) of core dimensions in Examples 1-4 and Comparative Examples 1-4

[0049]

[0050] After the hot-rolled coils of Examples 1-4 and Comparative Examples 1-4 were slowly cooled, they were cold-rolled and then annealed to obtain the finished products. In Examples 1-4, the rapid cooling temperature was controlled at 455℃, the aging temperature at 435℃, and the leveling elongation was 0.9%. In Comparative Examples 1-4, the annealing rapid cooling temperature was 470℃, the aging temperature at 450℃, and the leveling elongation was 0.7%. Performance tests were conducted on the cold-rolled hot-formed steel products obtained in Examples 1-4 and Comparative Examples 1-4. The results are shown in Table 7. The performance of the finished products obtained in the examples is not significantly different from that of the comparative examples. This indicates that Examples 1-4 effectively eliminated the impact of increased coiling temperature on the finished product performance by increasing the annealing leveling elongation, rapid cooling temperature, and aging temperature.

[0051] Table 7 Properties of Cold-Rolled Hot-Formed Steel Products

[0052]

[0053] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method of improving a 1500 MPa grade cold rolled hot formed steel flat coil, characterized in that, The process comprises: The process comprises: The smelting step reduces the length of the casting blank and controls the length of the casting blank to be 5.5-8.5 m; The finishing temperature of the hot rolling step is controlled to be 890±15℃; The hot rolling step increases the corresponding hot rolling thickness specification according to the principle that the thicker the hot rolling thickness, the smaller the flatness of the coil, and controls the hot rolling thickness to be 3.0 mm when the cold rolling thickness is 0.6mm≤h≤0.9mm, 3.4 mm when the cold rolling thickness is 0.9mm<h≤1.2mm, 3.8 mm when the cold rolling thickness is 1.2mm<h≤1.5mm, 4.2 mm when the cold rolling thickness is 1.5mm<h≤1.8mm, 4.5 mm when the cold rolling thickness is 1.8mm<h≤2.1mm, and 5.4 mm when the cold rolling thickness is 2.1mm<h≤2.5mm; The coiling step adopts U-shaped coiling, and the temperature of the head and tail of the coil within a range of 60 m is 20-30℃ higher than the target coiling temperature, and the target coiling temperature is controlled to be 700±10℃; The coiling tension is controlled in six sections, and the tension from the head to the tail of the strip is T1, T2, T3, T4, T5 and T6 respectively, and T2>T1>T3>T4>T5>T6.

2. The method of claim 1, wherein, Before coiling, the cold-rolled coil is subjected to laminar flow cooling in a full-section sparse cooling mode.

3. The method of claim 1, wherein, The head tension is controlled by T1 and T2, the T1 tension is 27-31KN, and the T2 tension is 32-40KN; the middle tension is controlled by T3 and T4, the T3 and T4 tension is 23-28KN; the tail tension is controlled by T5 and T6, the T5 and T6 tension is 17-25KN.

4. The method of claim 1, wherein, The coil is placed in the middle of other hot-rolled coils to avoid stacking and slow cooling.

5. The method of claim 1, wherein, The annealed coil is annealed to increase the annealing flattening elongation to 0.9%-1.0%, reduce the annealing fast cooling temperature to 455℃, and reduce the aging temperature to 435℃.

6. The method of claim 1, wherein, The chemical elements and contents of the cold-rolled hot-formed steel are C 0.21%-0.26%, Si 0.15%-0.45%, Mn 1.20%-1.50%, Al 0.020%-0.050%, B 0.0020%-0.0040%, Ti 0.025%-0.035%, P≤0.020%, S≤0.020%, and the balance is Fe and unavoidable impurities.

7. The method of claim 1, wherein, The chemical elements and contents of the cold-rolled hot-formed steel are C 0.23%, Si 0.45%, Mn 1.50%, Al 0.045%, B 0.0040%, Ti 0.030%, P 0.012%, S 0.016%, and the balance is Fe and unavoidable impurities.

8. The use of the method of any one of claims 1-7 in the preparation of a 1500MPa grade cold-rolled or galvanized hot-formed steel hot-rolled coil.

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