Method for controlling spot defects of an alloyed hot-dip galvanized steel sheet

Abstract

The application discloses a kind of alloying hot galvanizing steel plate spot defect control methods, the method includes: control the heating furnace in hot rolling process between 150min to 200min with strip steel in furnace length, the heating furnace discharge temperature of strip steel is between 1150 ℃ to 1180 ℃, and the rough rolling temperature of hot rolling is between 1000 ℃ to 1030 ℃;Control the rolling section S1 rack to S n Rack in cold rolling acid rolling process in turn to strip steel is rolled, wherein, S1 to S n‑1 The roughness of the roll of the rack is the first roughness, S n The roughness of the roll of the rack is the second roughness, the first roughness is less than the second roughness, and n is the total number of rolling section racks;Control the oxygen content ≤10ppm in annealing section in cold rolling galvanizing process, and control the hydrogen content between 2% to 5% in furnace nose.This method can effectively improve the alloying spot defect of outer plate, improve the quality of steel plate.

Description

Technical Field

[0001] This invention relates to the field of galvanized alloy technology, and in particular to a method for controlling spot defects in alloyed hot-dip galvanized steel sheets. Background Technology

[0002] Alloyed hot-dip galvanized steel (GA) products are widely used in automotive body white due to their excellent weldability, paintability and high corrosion resistance. Compared with ordinary hot-dip galvanized (GI) products, the surface quality control of GA products is more difficult. Even the smallest defects on the substrate can affect the zinc-iron alloying diffusion process and thus have a 'magnification' effect.

[0003] Alloying spots are a unique defect on the surface of GA (Gasoline Automotive) outer panels. They can be formed by hot-rolled iron oxide scale, pickling residue, substrate oxidation, and the introduction of zinc oxide film. Alloying spots are generally considered to be caused by inconsistent alloying levels. Macroscopically, they form areas of varying brightness on the surface, appearing as visible dots, streaks, patches, and peak-shaped white defects. Therefore, effectively controlling the formation of alloying spots on GA outer panels is a major challenge in GA automotive outer panel production. Summary of the Invention

[0004] This application provides a method for controlling stain defects in alloyed hot-dip galvanized steel sheets, which can effectively improve the alloying stain defects on the outer sheet and enhance the quality of the steel sheet.

[0005] In a first aspect, the present invention provides the following technical solution through an embodiment of the present invention:

[0006] A method for controlling blemish defects in alloyed hot-dip galvanized steel sheets includes: controlling the time the strip spends in the furnace during the hot rolling process to be between 150 min and 200 min, the furnace exit temperature of the strip to be between 1150℃ and 1180℃, and the hot rolling roughing temperature to be between 1000℃ and 1030℃; and controlling the temperature from stand S1 to stand S2 in the cold rolling pickling process. n The stand rolls the strip steel sequentially, with S1 to S... n-1 The surface roughness of the rolls on the mill stand is the first roughness, S. n The roughness of the rolls of the stand is the second roughness, the first roughness is less than the second roughness, and n is the total number of stands in the rolling section; the oxygen content in the annealing section of the cold rolling galvanizing process is controlled to be ≤10ppm, and the hydrogen content in the furnace nose is controlled to be between 2% and 5%.

[0007] Preferably, S1 to S n-1 The roughness of the rolls on the stand is between 0.4 μm and 1.0 μm.

[0008] Preferably, the Sn The roughness of the rolls on the frame is between 2.0 μm and 2.6 μm.

[0009] Preferably, the method further includes: controlling the cleaning section of the cold-rolled galvanizing process to clean the strip steel, such that the residual oil on the surface of the strip steel after cleaning is ≤450mg / m². 2 Surface residual iron ≤450mg / m 2 .

[0010] Preferably, the method further includes: controlling the Al content of the zinc pot in the galvanizing section to be between 0.120% and 0.150% in the cold rolling galvanizing process, and the alloying temperature of the galvanizing section to be between 460°C and 500°C.

[0011] Preferably, the method further includes: controlling the pickling speed of the pickling section in the cold rolling pickling process to be between 150 m / min and 200 m / min, and the cold rolling speed to be ≤1000 m / min.

[0012] Preferably, the roughing mill in the hot rolling process includes an R1 mill and an R2 mill, and the method further includes: controlling the R1 mill to start one pass of descaling water; controlling the R2 mill to start H passes of descaling water, where H is a natural number and H≥4.

[0013] Preferably, the height of the descaling manifold in the finishing mill during the hot rolling process is between 110 mm and 130 mm. The method further includes controlling the descaling pressure of the finishing mill to be between 22 MPa and 24 MPa, and the final rolling temperature to be between 900°C and 930°C.

[0014] Preferably, the cleaning section in the cold-rolled galvanizing process cleans the strip steel by controlling the cleaning section to use abrasive wire brush rollers made of tungsten carbide or corundum to clean the strip steel.

[0015] Preferably, the method further includes controlling the dew point temperature of the annealing section to be below -40°C.

[0016] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0017] The method for controlling spot defects provided in this invention includes: in the hot rolling process, controlling the time the strip spends in the furnace to be between 150 and 200 minutes, the furnace exit temperature of the strip to be between 1150°C and 1180°C, and the hot rolling roughing temperature to be between 1000°C and 1030°C. By reducing the heating time and using low-temperature rolling to lower the rolling temperature, the oxidation of the steel plate is reduced, effectively avoiding the formation of hot-rolled iron oxide scale, and thus, to some extent, avoiding the formation of alloying spots. In the cold rolling and pickling process, using a stand with a roll roughness ranging from low to high helps improve the surface condition of the strip. By controlling the S with a lower reduction rate... n The mill uses a larger roll roughness to press the steel plate, which is more conducive to the formation of a local oxide film on the steel plate surface, helping to improve the surface condition of the strip and reduce the generation of defects. In the cold rolling galvanizing process, the oxygen content in the annealing section is controlled to ≤10ppm, and the hydrogen content in the furnace nose is controlled between 2% and 5%. Precise control of the furnace atmosphere effectively prevents the oxidation of the substrate and zinc liquid, thus avoiding the formation of alloying spots to a certain extent. This application starts from the defect generation mechanism of hot-rolled iron oxide scale, substrate oxidation, and zinc liquid oxide film introduction. In the hot rolling process, low temperature tapping is used to control the heating time. In the pickling process, a roll roughness control strategy of low at the beginning and high at the end is used to improve the surface condition of the strip. In the galvanizing process, the dew point temperature is strictly controlled. These measures provide comprehensive control and regulation of defects throughout the entire process, greatly eliminating alloying hot-dip galvanized steel plate spot defects and significantly improving the surface quality of the steel plate. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart illustrating a method for controlling blemishes on alloyed hot-dip galvanized steel sheets, as provided in an embodiment of the present invention. Detailed Implementation

[0020] This application takes into account that due to the long heating temperature and time of hot rolling, local oxidation may still exist on the substrate and zinc liquid, and also takes into account the influence of the roughness of the pickling rolls on the substrate surface, the problem of alloy spots in the conventional technology still exists.

[0021] In view of this, the embodiments of this application provide a method for controlling the stain defects of alloyed hot-dip galvanized steel sheets, which can effectively improve the alloying stain defects and improve the quality of the steel sheets.

[0022] The overall technical solution of this application embodiment is as follows:

[0023] A method for controlling blemish defects in alloyed hot-dip galvanized steel sheets includes: controlling the time the strip spends in the furnace during the hot rolling process to be between 150 min and 200 min, the furnace exit temperature of the strip to be between 1150℃ and 1180℃, and the hot rolling roughing temperature to be between 1000℃ and 1030℃; and controlling the temperature from stand S1 to stand S2 in the cold rolling pickling process. n The stand rolls the strip steel sequentially, with S1 to S... n-1 The surface roughness of the rolls on the mill stand is the first roughness, S. n The roughness of the rolls of the stand is the second roughness, the first roughness is less than the second roughness, and n is the total number of stands in the rolling section; the oxygen content in the annealing section of the cold rolling galvanizing process is controlled to be ≤10ppm, and the hydrogen content in the furnace nose is controlled to be between 2% and 5%.

[0024] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0025] Firstly, the embodiments of the present invention provide a method for controlling stain defects in alloyed hot-dip galvanized steel sheets, specifically, as follows: Figure 1 As shown, the method includes the following steps S101 to S103.

[0026] Step S101: Control the time the strip is in the furnace during the hot rolling process to be between 150 min and 200 min, the furnace exit temperature of the strip to be between 1150℃ and 1180℃, and the hot rolling roughing temperature to be between 1000℃ and 1030℃.

[0027] Step S102: Control the rolling section S1 stand in the cold rolling and pickling process to S... n The stand rolls the strip steel sequentially, with S1 to S... n-1 The surface roughness of the rolls on the mill stand is the first roughness, S. n The surface roughness of the rolls in the stand is the second surface roughness, the first surface roughness is less than the second surface roughness, and n is the total number of stands in the rolling section;

[0028] Step S103: Control the oxygen content in the annealing section of the cold-rolled galvanizing process to ≤10ppm, and control the hydrogen content in the furnace nose to between 2% and 5%. It should be noted that the steel plate targeted in this application can be IF steel plate, i.e., interstitial steel, or of course, other types of steel plate.

[0029] In the specific implementation process, the hot rolling adopts a low-temperature furnace exit process, controlling the furnace exit temperature of the strip steel between 1150℃ and 1180℃, and the furnace heating time between 150min and 200min. By reducing the heating temperature and shortening the heating time, the oxidation of the steel plate is reduced, effectively avoiding the formation of hot-rolled iron oxide scale, and to a certain extent avoiding the formation of alloying spots.

[0030] The roughing process employs low-temperature rolling, controlling the hot-rolled roughing temperature of the strip steel between 1000℃ and 1030℃. Lowering the rolling temperature helps reduce steel plate oxidation. This application addresses the issue of reducing the degree of oxidation in steel plates after hot rolling by controlling the furnace heating temperature, heating time, and rolling mill temperature. This achieves normal rolling while improving surface defects on the steel plate, thus controlling production costs and improving steel plate quality.

[0031] For example, the furnace exit temperature of the strip steel is 1155℃ or 1160℃, the furnace heating time is 160min, 170min or 180min, and the hot rolling roughing temperature is 1010℃ or 1015℃.

[0032] In a specific embodiment, the roughing mill in the hot rolling process includes an R1 mill and an R2 mill. The control method further includes: controlling the R1 mill to start one pass of descaling water; and controlling the R2 mill to start H passes of descaling water, where H is a natural number and H≥4.

[0033] For example, the roughing mill adopts a "1+5" mode, that is, the R1 mill is controlled to start one descaling water pass; and the R2 mill is controlled to start five descaling water passes. Compared with the traditional descaling method, this application improves the removal effect of primary iron oxide scale by increasing the number of descaling passes.

[0034] Of course, as other alternative embodiments, rough rolling can also adopt the "1+6" mode, "1+7" mode, etc.

[0035] Furthermore, to effectively remove iron oxide scale from the steel plate surface, the height of the descaling manifold in the finishing mill during the hot rolling process can be set between 110mm and 130mm. Control methods can also include controlling the descaling pressure of the finishing mill between 22MPa and 24MPa, and the final rolling temperature between 900℃ and 930℃. The final rolling temperature refers to the temperature of the hot-rolled strip when it leaves the final finishing mill.

[0036] This application improves the removal of secondary iron oxide scale by setting the height of the descaling manifold between 110mm and 130mm and increasing the descaling pressure. For example, the descaling pressure is 23MPa or 24MPa, and the final rolling temperature is 910℃ or 920℃.

[0037] In the cold rolling and pickling process, the pickling speed in the pickling section is controlled between 150 m / min and 200 m / min, and the cold rolling speed is ≤1000 m / min. For example, the pickling speed is 160 m / min, 170 m / min, 180 m / min or 190 m / min, and the rolling speed is 900 m / min or 800 m / min.

[0038] Furthermore, check whether the reflectivity of the cold-rolled strip is above 70%. If not, the emulsion can be filtered again to ensure that the reflectivity meets the requirements.

[0039] In the cold rolling and pickling process, control the rolling section S1 stand to S... n The stand rolls the strip steel sequentially, from S1 to S... n-1 The surface roughness of the mill rolls is set between 0.4 μm and 1.0 μm, S n The surface roughness of the rolls on the mill stand is set between 2.0 μm and 2.6 μm. Rolling the strip using a mill stand with progressively higher roll roughness helps improve the strip's surface condition by controlling the S-axis with a lower reduction rate. n The stand uses a larger roll roughness to press the steel plate, which is more conducive to the formation of a local oxide film on the steel plate surface, which helps to improve the surface condition of the strip and reduce the occurrence of steel plate blemishes.

[0040] For example, the rolling section contains 5 stands. The roll roughness of stands S1, S2, S3 and S4 is between 0.4 μm and 1.0 μm, and the roll roughness of stand S5 is between 2.0 μm and 2.6 μm.

[0041] For example, the roll roughness of stands S1, S2, S3, and S4 is 0.5µm, 0.6µm, 0.7µm, 0.8µm, or 0.9µm, respectively. Alternatively, the roll roughness of stands S1, S2, S3, and S4 gradually increases, i.e., the roll roughness of stand S1 is 0.5µm, stand S2 is 0.6µm, stand S3 is 0.7µm, stand S4 is 0.8µm, and so on. The roll roughness of stand S5 is 2.0µm, 2.2µm, 2.4µm, or 2.6µm.

[0042] The cold-rolled galvanizing process mainly includes a cleaning section, an annealing section, and a galvanizing section. In the cold-rolled galvanizing process, the cleaning section is crucial for cleaning the strip steel, ensuring that the residual oil on the strip steel surface is ≤450mg / m² after cleaning. 2 Surface residual iron ≤450mg / m 2 .

[0043] Optionally, in order to effectively ensure the cleaning effect, the cleaning section of the cold-rolled galvanizing process is controlled to clean the strip steel. Specifically, this may include: controlling the cleaning section of the cold-rolled galvanizing process to use abrasive wire brush rollers made of tungsten carbide or corundum to clean the strip steel.

[0044] Furthermore, the oxygen content in the annealing zone is controlled to be ≤10ppm, and the hydrogen content in the furnace nose is controlled between 2% and 5%. This application employs precise control of the atmosphere in the furnace nose area to effectively prevent oxidation of the substrate and molten zinc, thus avoiding the formation of alloying spots to a certain extent. In addition, the dew point temperature in the annealing zone can be controlled below -40℃.

[0045] For example, the hydrogen content in the furnace nose is 5%, and the dew point temperature of the annealing section is controlled at -35°C or -30°C.

[0046] Furthermore, to effectively prevent the introduction of zinc oxide film, the Al content in the zinc pot of the galvanizing section is controlled between 0.120% and 0.150% during the cold rolling galvanizing process, and the alloying temperature of the galvanizing section is controlled between 460℃ and 500℃. By controlling the Al content in the zinc pot, the formation of an inhibitory layer in localized areas of the zinc pot can be avoided, thus preventing the inhibitory layer from hindering the alloying reaction and resulting in more uniform final alloying. This application achieves precise control of the alloying temperature, resulting in more stable performance of the alloyed hot-dip galvanized steel sheet.

[0047] For example, the zinc pot has an Al content of 0.120%, 0.130%, 0.14%, or 0.150%, and an alloying temperature of 460°C or 470°C.

[0048] The method for controlling defects in alloyed hot-dip galvanized steel sheets provided in this application starts from the generation mechanism of defects such as hot-rolled iron oxide scale, substrate oxidation, and zinc liquid oxide film, and carries out comprehensive control and regulation of the whole process, which greatly eliminates alloying defects and significantly improves the surface quality of the outer sheet.

[0049] Based on the control method provided in this application, three embodiments and three comparative examples are disclosed below to intuitively demonstrate the improved effect of the control method provided in this application. The process parameters for each step in the embodiments and comparative examples, as well as the staining conditions of the resulting alloyed galvanized sheets, are shown in Table 1. Table 1 below lists the process parameters for the hot rolling-pickling process:

[0050] Table 1

[0051]

[0052] In Table 1 above, landscape paintings are divided into three levels: 0-2. Level 0 represents no landscape painting, which has no effect on alloying marks; Level 1 represents slight landscape painting, which has a minor effect on alloying marks; Level 2 represents moderate to severe landscape painting, which has a significant effect on alloying marks.

[0053] As can be seen from Table 1, the higher the furnace exit temperature, the longer the heating time, the higher the roughing rolling temperature, the fewer the R2 descaling passes, the lower the finishing rolling descaling manifold height, and the lower the descaling pressure, the higher the grade of the landscape painting. Conversely, the lower the grade of the landscape painting is when the furnace exit temperature, furnace heating time, roughing rolling temperature, R2 descaling passes, finishing rolling descaling manifold height, finishing rolling descaling pressure, final rolling temperature, pickling speed, and rolling temperature are all close to or within the range given in this application.

[0054] Table 2 below shows the process parameters for the cold rolling-galvanizing process:

[0055] Table 2

[0056]

[0057] In Table 2 above, alloying defects are classified into three levels: 0-3. Level 0 represents no defects and can be released; Level 1 represents minor defects, which may be released to a very small number of users; Level 2 represents slightly severe defects; and Level 3 represents severe defects. Defects of Level 2 or above are strictly prohibited from being released.

[0058] As can be seen from Table 2, the alloying stain grade is higher when the roughness of the S5 stand rolls is smaller, the number of abrasive filament brush roll pairs is fewer, and the hydrogen content in the furnace nose is lower. Conversely, the alloying stain grade is lower when the roughness of the S1-S4 rolls, the roughness of the S5 roll, the reflectivity, the number of abrasive filament brush roll pairs, the residual oil and iron, the oxygen content in the annealing section, the dew point in the annealing section, the hydrogen content in the furnace nose, the aluminum content in the zinc pot, and the alloying temperature are all close to or within the range given in this application.

[0059] Comparative experiments show that the method for controlling defects in alloyed hot-dip galvanized steel sheets for automotive outer panels, as described in this application, greatly eliminates alloying defects and significantly improves the surface quality of the outer panels.

[0060] In summary, the method for controlling defects in alloyed hot-dip galvanized steel sheets provided by this invention addresses the generation mechanisms of defects such as hot-rolled iron oxide scale, substrate oxidation, and zinc oxide film introduction. In the hot rolling process, low-temperature tapping is used to control heating time, while descaling water is used to control iron oxide scale and surface defects. In the pickling process, a roll roughness control strategy with lower roughness at the beginning and higher roughness at the end is employed to improve the surface condition of the strip. In the galvanizing process, strict control is maintained over cleaning quality and the atmosphere in the furnace nose area. This comprehensive control and regulation of defects throughout the entire process essentially eliminates alloyed defects, significantly improving the surface quality of the outer sheet.

[0061] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0062] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for controlling stain defects in alloyed hot-dip galvanized steel sheets, characterized in that, include: The time the strip spends in the heating furnace during the hot rolling process is controlled to be between 160 min and 200 min, the furnace exit temperature of the strip is between 1150℃ and 1155℃, and the hot rolling roughing temperature is between 1000℃ and 1015℃. Controlling the rolling section from stand S1 to S in the cold rolling and pickling process n The strip steel is rolled sequentially from stand S1 to stand S2. n-1 rack, then from S n-1 rack to S n The surface roughness of the rolls on the mill stand ranges from low to high, with S1 to S... n-1 The surface roughness of the rolls on the mill stand is the first roughness, S. n The surface roughness of the rolls in the stand is the second surface roughness, the first surface roughness is less than the second surface roughness, and n is the total number of stands in the rolling section; The oxygen content in the annealing section of the cold-rolled galvanizing process should be controlled to be ≤10ppm, and the hydrogen content in the furnace nose should be controlled between 2% and 5%. S1 to S n-1 The roughness of the rolls in the frame is between 0.4 μm and 1.0 μm; The S n The roughness of the rolls on the frame is between 2.0 μm and 2.6 μm.

2. The method as described in claim 1, characterized in that, Also includes: The cleaning section of the cold-rolled galvanizing process is controlled to clean the strip steel, and the residual oil on the strip steel surface after cleaning is ≤450mg / m². 2 Surface residual iron ≤450mg / m 2 .

3. The method as described in claim 1, characterized in that, Also includes: The Al content of the zinc pot in the galvanizing section of the cold rolling galvanizing process is controlled between 0.120% and 0.150%, and the alloying temperature of the galvanizing section is controlled between 460°C and 500°C.

4. The method as described in claim 1, characterized in that, Also includes: The pickling speed in the pickling section of the cold rolling and pickling process is controlled between 150 m / min and 200 m / min, and the cold rolling speed is ≤1000 m / min.

5. The method as described in claim 1, characterized in that, The roughing mill in the hot rolling process includes an R1 mill and an R2 mill, and the method further includes: Control the R1 mill to start one pass of descaling water; Control the R2 mill to start the descaling water for H passes, where H is a natural number and H≥4.

6. The method as described in claim 5, characterized in that, In the hot rolling process, the height of the descaling manifold in the finishing mill is between 110mm and 130mm, and the method further includes: The descaling pressure of the finishing mill is controlled between 22 MPa and 24 MPa, and the final rolling temperature is controlled between 900°C and 930°C.

7. The method as described in claim 1, characterized in that, The cleaning section in the controlled cold-rolled galvanizing process cleans the strip steel, including: The strip steel is cleaned using abrasive filament brush rollers made of tungsten carbide or corundum in the cleaning section of the cold rolling galvanizing process.

8. The method as described in claim 1, characterized in that, Also includes: The dew point temperature of the annealing section is controlled to be below -40°C.

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