A method for improving the spalling of iron oxide scale from hot-formed steel

Abstract

This invention belongs to the field of steel rolling, specifically disclosing a method for improving the removal of iron oxide scale from hot-formed steel. The method includes the following steps: slab heating, hot rolling, laminar flow cooling, coiling, coil storage, pickling, and cold rolling. The laminar flow cooling step employs a single-stage water cooling process at the head, followed by air cooling from the single-stage water cooling stage until coiling. The coiling temperature is controlled within the range of 650–700℃. The hot rolling head uses a dry-head process with a dry-head length ≥30m. This method can induce micro-cracks on the strip surface, thereby improving the problem of iron oxide scale removal from the surface of hot-formed steel after hot stamping.

Description

Technical Field

[0001] This invention relates to the field of steel rolling, and more specifically to a method for improving the removal of iron oxide scale from hot-formed steel. Background Technology

[0002] Hot stamping technology is one of the important ways to reduce vehicle body weight and improve the impact resistance and crashworthiness of automobiles. With the advancement of energy conservation and emission reduction efforts in the automotive industry, the application ratio of hot-formed steel in vehicle bodies is gradually increasing. Currently, the main product types used include four categories: hot-rolled pickled, cold-rolled, annealed, and coated. During the hot forming process, the high-temperature sheet metal has a 5-10 second transfer time in the air. During this period, cold-rolled and cold-rolled annealed products will form iron oxide scale. This scale is prone to peeling off during subsequent stamping, which has an adverse effect on the surface quality of parts, stamping efficiency, and mold lifespan.

[0003] Patent application CN111411298A discloses a method for preparing a low-strength, scale-resistant hot-formed steel coil. This method controls the surface roughness of the strip by adjusting the roughness of the cold-rolled rolls to reduce scale shedding. However, the roll roughness decreases with wear during production, requiring chrome plating to maintain the roughness, which increases equipment costs. Patent application CN106702278A discloses a method for producing hot-stamped steel sheets with uncoated scale-controlling properties. This method controls the atmosphere in the furnace during the aging process to directly generate a pre-oxidized film on the strip surface, achieving scale-free stamping. However, this method is only applicable to annealed steel sheets. Summary of the Invention

[0004] The purpose of this invention is to provide a method for improving the removal of iron oxide scale from hot-formed steel. By matching the hot and cold rolling processes, the microstructure of the strip surface is changed, thereby improving the problem of scale removal without increasing equipment costs. It is also applicable to both cold-rolled and cold-rolled annealed products.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for improving the removal of iron oxide scale from hot-formed steel includes the following steps: slab heating, hot rolling, laminar flow cooling, coiling, steel coil storage, pickling, and cold rolling.

[0006] Furthermore, in the slab heating process of the present invention, the slab exit temperature is 1280~1310℃, and the heating time above 1250℃ is ≥90min.

[0007] Furthermore, the hot rolling process described in this invention includes rough rolling and finish rolling, with the finish rolling temperature ranging from 880 to 900°C.

[0008] Furthermore, in the laminar flow cooling process described in this invention, a single-stage water cooling process is adopted at the head, and air cooling is adopted from the single-stage water cooling to the winding. The winding temperature is controlled within the range of 650 to 700°C. The hot rolling head adopts a dry head process with a dry head length ≥30m.

[0009] Furthermore, in the steel coil storage process described in this invention, after the steel coil is unloaded from the coiler, it is placed in an air-cooled environment for 4 to 6 hours, and then placed in an insulation pit for slow cooling. After being kept in the insulation pit for 48 hours, it is removed from the pit.

[0010] Furthermore, in the pickling and cold rolling processes described in this invention, the total cold rolling reduction rate is ≥70%.

[0011] Furthermore, the method described in this invention is applicable to hot-formed steel with the following chemical composition and mass percentage: C: 0.20-0.26%, Si: 0.15-0.30%, Mn: 1.00-1.50%, Als: 0.010-0.060%, Ti: 0.020-0.050%, Cr: 0.15-0.35%, B: 0.0020-0.0050%, P≤0.030%, S≤0.010%, N≤0.0060%, with the remainder being iron and unavoidable impurities.

[0012] Furthermore, the thickness of the hot-formed steel described in this invention is 0.8–1.8 mm.

[0013] The method described in this invention can be applied to two types of products: cold-rolled hardened products and cold-rolled annealed products.

[0014] The inventive principle and beneficial effects of the technical solution of this invention are as follows: By using the cooling process of hot rolling, heat preservation of steel coils, and control of large reduction rate in cold rolling, micro-cracks appear on the surface of the strip after cold rolling, thereby enhancing the bonding force between iron oxide scale and the matrix during subsequent hot stamping and improving the problem of iron oxide scale detachment.

[0015] Because hot-formed steel has a high alloy content and strong hardenability, this invention utilizes a process of slab heating, hot rolling, laminar flow cooling, and slow cooling of the hot coil to obtain a low-strength matrix and a hot-rolled surface, providing a basis for the appearance of surface cracks after subsequent cold rolling. Experiments have shown that, when matched with the aforementioned hot-rolled base material, a stable microcracked surface can be obtained when the cold rolling reduction rate is controlled above 70%.

[0016] The present invention adopts a high winding temperature and a front-end single-stage water cooling process for laminar flow cooling. Compared with conventional processes, the hot-rolled microstructure tends to coarsen. Therefore, a large reduction rate control is adopted for hot rolling and cold rolling to avoid the impact of grain coarsening on the performance after hot forming.

[0017] The setting of the dry end is to achieve the phase transformation gradient after hot rolling to improve the flatness problem of hot coil; on the other hand, it is to compensate for the temperature drop when the inner ring of the hot coil contacts the winding mandrel and the air. Attached Figure Description

[0018] Figure 1 The microstructure of the surface of hot-formed steel after cold rolling in Example 2; Figure 2 The microstructure of the surface of hot-formed steel after cold rolling in Comparative Example 2; Figure 3 The morphology of the iron oxide scale on the surface after thermoforming in Example 2; Figure 4 The morphology of the iron oxide scale on the surface of Comparative Example 2 after thermoforming. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments.

[0020] The method of this invention is for the most widely used 1500MPa level product. The chemical composition and mass percentage of the product are as follows: C: 0.20-0.26%, Si: 0.15-0.30%, Mn: 1.00-1.50%, Als: 0.010-0.060%, Ti: 0.020-0.050%, Cr: 0.15-0.35%, B: 0.0020-0.0050%, P≤0.030%, S≤0.010%, N≤0.0060%, with the remainder being iron and unavoidable impurities. The steel strip composition of each embodiment and comparative example is shown in Table 1.

[0021] Table 1. Chemical composition and content (wt%) of steel strips in each embodiment and comparative example.

[0022] This invention provides a method for improving the removal of iron oxide scale from hot-formed steel, comprising the following steps: slab heating → hot rolling → laminar flow cooling → coiling → warehousing cooling → pickling and cold rolling → (annealing); specific process control is as follows: (1) Slab heating: The slab thickness is 230mm, the slab exit temperature is 1280~1310℃, and the heating time above 1250℃ is ≥90min.

[0023] (2) Hot rolling: After the slab is heated, it is rough rolled and fine rolled to achieve the target thickness of the hot-rolled substrate. The final rolling temperature range is 880~900℃.

[0024] (3) Laminar flow cooling: The head single-section water cooling process is adopted. After the single-section water cooling, the winding is air-cooled. The winding temperature control range is 650~700℃, and a dry head with a length ≥30m is set.

[0025] (4) Cooling in the warehouse: After the hot-rolled steel strip is unloaded from the coiler, it is left to cool for 4 to 6 hours, and then transferred to the heat preservation pit for slow cooling for 48 hours. After leaving the pit, it is transferred to the cold rolling process for leveling and cold rolling or directly to cold rolling without leveling, depending on the requirements of the coil shape and plate shape of the cold rolling.

[0026] (5) Pickling and cold rolling: After hot pickling, cold rolling is performed to the finished thickness, and the total reduction rate of the cold rolling process is ≥70%. After cold rolling, annealing can be carried out according to the downstream user's requirements for product performance. The annealing method can be continuous annealing or shroud annealing.

[0027] The steel strip thickness and specific process control of each embodiment and comparative example are shown in Table 2; among them, Embodiment 1 and Comparative Example 1 are produced by casting billets in the same furnace, and Embodiment 2 and Comparative Example 2 are produced by casting billets in the same furnace.

[0028] Table 2. Thickness and process control of finished steel plates in each embodiment and comparative example.

[0029] The surface microstructures of the hot-formed steel after cold rolling in Example 2 and Comparative Example 2 are as follows: Figure 1 and Figure 2 As shown; by Figure 1 and Figure 2 It can be seen that after applying the technical solution of the present invention, more obvious microcracks appear on the cold-rolled surface, which provides a basis for improving the bonding force of iron oxide scale.

[0030] The sheet metal of each embodiment and comparative example was thermoformed. The thermoforming process was as follows: the total heating time of the sheet metal was 260 seconds, the heating process was carried out by gradually increasing the temperature from 850 to 950°C, the heating process was under nitrogen protection throughout, and the time from the sheet metal exiting the furnace to the transfer to the stamping die was 6.8 seconds.

[0031] The surface oxide scale morphologies of Example 2 and Comparative Example 2 after thermoforming are as follows: Figure 3 and Figure 4 As shown; by Figure 3 and Figure 4 It can be seen that the iron oxide scale on the surface of the product using the technical solution of the present invention has strong adhesion after thermoforming and no peeling occurs. Figure 4 The comparative product showed a large area of ​​sheet metal peeling off after stamping.

[0032] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for improving the removal of iron oxide scale from hot-formed steel, characterized in that, The method includes the following steps: slab heating, hot rolling, laminar flow cooling, coiling, steel coil storage, pickling, and cold rolling.

2. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, In the slab heating process, the slab exit temperature is 1280~1310℃, and the heating time above 1250℃ is ≥90min.

3. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, The hot rolling process includes roughing and finishing rolling, with the finishing rolling temperature ranging from 880 to 900°C.

4. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, The laminar flow cooling process adopts a single-stage water cooling process at the head, followed by air cooling from the single-stage water cooling to the coiling. The coiling temperature is controlled within the range of 650 to 700°C. The hot rolling head adopts a dry-head process with a dry-head length ≥30m.

5. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, In the steel coil storage process, after the steel coil is unloaded from the coiler, it is placed in an air-cooled environment for 4 to 6 hours before being placed in an insulated pit for slow cooling.

6. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, The pickling and cold rolling processes have a reduction rate of ≥70%.

7. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, The method is applicable to hot-formed steel with the following chemical composition and mass percentage: C: 0.20-0.26%, Si: 0.15-0.30%, Mn: 1.00-1.50%, Als: 0.010-0.060%, Ti: 0.020-0.050%, Cr: 0.15-0.35%, B: 0.0020-0.0050%, P≤0.030%, S≤0.010%, N≤0.0060%, with the remainder being iron and unavoidable impurities.

8. The method for improving the removal of iron oxide scale from hot-formed steel according to claim 1, characterized in that, The thickness of the hot-formed steel is 0.8 to 1.8 mm.

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