METHOD FOR PREVENTING MEANING OF STEEL PLATE AND METHOD FOR MANUFACTURING STEEL PLATE

Abstract

To provide a method of suppressing meandering of a steel sheet after water quenching in a continuous processing line for the steel sheet such as continuous annealing and surface treatment such as plating having a water quenching device.SOLUTION: In a continuous processing line for a steel sheet such as continuous annealing and surface treatment such as plating having a water quenching device, connection conditions of the terminal ends and the starting ends of the steel sheets are connected so as to make the same sheet width with each other or the sheet width on the starting end side narrow, and the sheets are made to pass. This makes it possible to suppress meandering in a furnace after passing the water quenching device, thereby making it possible to stably pass the sheet through the continuous processing line, thereby improving productivity by increasing the passing speed of the sheets, and preventing damage to the steel sheets due to meandering.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for suppressing meandering of a steel sheet in a furnace in a continuous treatment line such as a continuous annealing line having a water quenching device or a surface treatment line such as a plating line, and in particular to a method for suppressing meandering of a steel sheet due to shape defects that occur in the steel sheet after water quenching, and a method for manufacturing a steel sheet. [Background technology]

[0002] In recent years, there has been a strong demand for improved fuel efficiency in automobiles from the perspective of protecting the global environment. To this end, there has been an active movement to reduce the thickness of automobile components and the weight of the vehicle body itself. However, simply thinning the thickness of automobile components reduces the strength of the vehicle body, so it is necessary to maintain strength even when the thickness is thinned. Furthermore, from the perspective of ensuring passenger safety in the event of a collision, there is a strong demand for higher strength automobile bodies. To meet these demands, efforts are being made to achieve both lighter weight and higher strength in automobile components. Cold-rolled steel sheets and hot-dip galvanized steel sheets, which are the raw materials for automobile components, are generally manufactured in continuous processing lines such as continuous annealing furnaces.

[0003] In manufacturing using a continuous annealing furnace, methods for increasing the strength of steel sheets (hereinafter also referred to as strips) include adjusting the steel sheet composition, such as by adding Si to achieve solid solution strengthening, but also by transforming the internal structure of the steel sheet into a high-strength structure by rapid cooling using a water quenching device. This allows the amount of alloying elements added to be reduced, making it possible to reduce production costs.

[0004] In a continuous annealing furnace, steel sheets are passed over hearth rolls located at the top and bottom of the furnace. Continuous annealing furnaces have sections such as a heating zone, a soaking zone, a cooling zone, and an overaging zone, and continuous annealing is achieved by passing the steel sheets through these zones. In a continuous steel sheet processing line such as a continuous annealing furnace, the starting and ending ends of each steel sheet are welded together while the sheet passing speed is adjusted with a looper, allowing the steel sheets to be passed as a continuous steel sheet.

[0005] One of the important issues in carrying out continuous annealing is to prevent the steel sheet from meandering and pass it straight through the furnace. If the meandering is not sufficiently prevented, the steel sheet will come into contact with the furnace wall, causing problems such as damage to the equipment and scratches on the steel sheet, which will slow down the sheet passing speed on the line and reduce productivity. Furthermore, if the damage to the steel sheet is severe, the steel sheet will break, causing the line to be stopped for a long time, further reducing productivity.

[0006] A typical method for suppressing meandering is to install a center position control (CPC) device, which measures and continuously monitors the widthwise position of the steel sheet, and suppress meandering using a steering device attached to the hearth roll. Patent Document 1 also discloses a technology for preventing meandering and buckling by installing hearth rolls with different roughness levels in the upper and lower furnaces of a continuous annealing furnace. Patent Document 2 discloses a technology for detecting the non-contact width between the tapered portion of the hearth roll and the strip in the meandering direction, and cooling the detected portion of the strip that is not in contact with the roll taper with gas, thereby increasing the contact area with the roll due to thermal contraction of the strip and enhancing the spooling effect to suppress meandering. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 8-283872 [Patent Document 2] Japanese Patent Application Publication No. 4-333528 Summary of the Invention [Problem to be solved by the invention]

[0008] However, Patent Document 1 is limited to the detection of meandering within a heating furnace. Furthermore, as the steel sheet threading rate increases, wear occurs due to contact friction with the steel sheet, which may result in a decrease in roll roughness and a decrease in meandering suppression capability. Patent Document 2 also has the drawback of requiring strip cooling gas for meander suppression and, when targeting water-quenched steel sheets, being virtually ineffective in suppressing meandering after quenching because the steel sheet temperature has already dropped to approximately room temperature. Furthermore, conventional meander suppression technology using CPC equipment can cause excessive meander correction and delayed response in cases of L-shaped welding or when the offset in the width direction is very large, which can actually promote meandering. This can lead to the need to stop the continuous steel strip processing line and reduce production volume.

[0009] An object of the present invention is to provide a method for suppressing meandering of a steel sheet after water quenching in a continuous steel sheet processing line, such as continuous annealing and surface treatment such as plating, which has a water quenching device. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the present invention aims to suppress meandering of steel plates that have been quenched by a water quenching device in a cooling zone of a continuous processing line equipped with a water quenching device, by connecting the end and start ends of the steel plates so that the plate width is the same or narrower, and is a method of suppressing meandering that is particularly effective for steel plates where either or both of the widths of the trailing steel plate and the leading steel plate are 900 mm or more. The means for solving the above problems are as follows. [1] A method for preventing the meandering of steel plates in a continuous steel plate processing line, in which the width of the succeeding steel plate is connected so that it is equal to or smaller than the width of the preceding steel plate. [2] The method for suppressing meandering of a steel plate according to [1], wherein either or both of the width of the trailing steel plate and the width of the leading steel plate are 900 mm or more. [3] The method for suppressing meandering of a steel plate according to [1] or [2], wherein the continuous processing line for the steel plate is equipped with a heating device and a water quenching device. [4] The method for suppressing meandering of a steel plate according to [3] or [4], wherein either or both of the trailing steel plate and the preceding steel plate have a martensite structure after passing through the water quenching device. [5] The method for suppressing meandering of a steel plate according to any one of [1] to [4], wherein the ratio of the plate width of the preceding steel plate to the plate width of the following steel plate is 1.0 to 1.3. [6] A method for manufacturing a steel sheet, using the method for suppressing meandering of a steel sheet according to any one of [1] to [5]. [Effects of the Invention]

[0011] According to the present invention, in a continuous processing line for performing surface treatment, such as a continuous annealing furnace equipped with a water quenching device, meandering in the furnace after the water quenching device can be suppressed by connecting the starting and ending ends of a steel sheet so that they have the same width or the width at the starting end is narrower. As a result, stable sheet running on the continuous processing line is possible, improving productivity by increasing the sheet running speed and suppressing damage to the steel sheet due to meandering. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing an example of a continuous annealing line in which a method for suppressing meandering of a steel sheet according to an embodiment of the present invention can be implemented. [Figure 2] FIG. 4 is a diagram showing the connection conditions of steel plates in the present embodiment. [Figure 3] 10A and 10B are diagrams illustrating a meandering mechanism in the present embodiment. [Figure 4] FIG. 10 is a diagram showing the calculated position of warpage and the assumed position of cooling unevenness in the simulation of this embodiment. [Figure 5] FIG. 10 is a diagram showing the results of a simulation of the amount of change in warpage due to uneven cooling in the steel sheet width direction in this embodiment. [Figure 6] FIG. 10 is a diagram showing the results of a simulation of the amount of change in warpage due to temperature difference caused by uneven cooling in the steel sheet width direction in this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figure 1 is a schematic diagram showing an example of a continuous annealing line in which a method for suppressing meandering of a steel sheet according to this embodiment can be implemented. The continuous annealing furnace in FIG. 1 refers to the zones from the heating zone 4 to the soaking zone 5, the cooling zone 7, the overaging zone 9, and the air-cooling zone 11, followed by the post-treatment zone 12. The heating zone 4 has a heating device, and the cooling zone 7 has a water quenching device 8. The steel sheet is passed through hearth rolls 6 provided in each zone of the continuous annealing furnace and annealed. The method for suppressing meandering of a steel sheet according to this embodiment is preferably applied to a steel sheet that is quenched by the water quenching device 8 provided in the cooling zone 7 and passed through the overaging zone 9. The continuous annealing furnace shown in FIG. 1 is an example of a continuous treatment line. However, the continuous treatment line is not limited to a continuous annealing furnace, and may be any line that has a cooling device such as a water quenching device.

[0014] In a continuous annealing line, the end and beginning of each steel sheet are welded together and run as a continuous steel sheet. This connection can be classified into three connection conditions shown in Figure 2. Figure 2(a) shows a connection condition in which the leading sheet width is narrow and the trailing sheet width is wide. Figure 2(b) shows a connection condition in which the leading sheet width is wide and the trailing sheet width is narrow. Figure 2(c) shows a connection condition in which the leading and trailing sheets have the same width. The connection condition shown in Figure 2(a) is referred to as a wide-up connection, the connection condition shown in Figure 2(b) is referred to as a narrow-down connection, and the connection condition shown in Figure 2(c) is referred to as an identical sheet width connection. In this embodiment, the connection is made so that the width of the trailing steel sheet is equal to or smaller than the width of the leading steel sheet, as shown in Figure 2(b) or Figure 2(c). This prevents the steel sheets from meandering. Note that identical sheet width includes a case in which the difference in width between the connected sheets is within 10 mm and can be considered to be substantially the same sheet width.

[0015] The main cause of steel sheet meandering is poor shape during water quenching. In the production of high-strength steel sheets, the steel sheet is often cooled in one go from a high temperature range at approximately 1000°C / s (immersion cooling), causing a phase transformation of the steel sheet structure. This phase transformation, also known as martensitic transformation, causes the steel sheet's volume to expand during the structural transformation, resulting in deformation in the width and length directions. Due to poor shape in the width direction of the steel sheet, such as one-sided elongation caused by this phase transformation, the centering force in the hearth rolls inside the furnace of a continuous annealing line becomes uneven in the width direction of the sheet, as shown in Figure 3(b), causing meandering in the direction of travel.

[0016] Defective steel sheet shape can be attributed to the steel sheet shape before immersion cooling, but is primarily caused by uneven phase transformation during immersion cooling due to uneven cooling in the width direction. The greater the uneven cooling in the width direction, the more uneven the temperature of the steel sheet becomes across the width, leading to uneven phase transformation timing across the width. Uneven phase transformation timing across the width, as described above, leads to defective steel sheet shape across the width. To investigate the effects of the joining conditions and uneven cooling shown in Figures 2(a) and 2(b), we used computer simulations to calculate the amount of warpage during water quenching when cooling on one side of the steel sheet in the width direction is delayed, resulting in uneven cooling, and when there is no uneven cooling, as shown in Figure 4(b). We then evaluated the change in warpage as a form defect. As shown in Figure 4(a), the cross section 1 m from the weld point of the preceding sheet is designated as Section A, and the cross section 1 m from the weld point of the succeeding sheet is designated as Section B. The difference in warpage in the graphs in Figures 5(a) and 5(b) refers to the difference in warpage measured at cross sections A and B, respectively. Figures 5(a) and (b) show the simulated strip temperature distribution and the difference in warpage after water quenching at each strip width position on cross section A of the leading steel sheet and cross section B of the trailing steel sheet. In Figure 5, the strip width positions are defined as DR (the narrow side of the steel sheet), OP (the opposite side), and Ce (the center). The midpoint between DR and Ce is defined as q-Dr, and the midpoint between Ce and OP is defined as q-OP. When there is no cooling unevenness in the width direction, as shown in Figure 5(a), the effect on shape defects is small regardless of the splicing conditions. However, when there is cooling unevenness in the width direction, as shown in Figure 5(b), shape defects are significant with a wide-up splice like Figure 2(a) but are small with a narrow-down splice like Figure 2(b). Figure 5 shows the amount of warping of the steel plate near the weld due to temperature unevenness in each connection method, and it shows that the greater the cooling unevenness, the greater the amount of warping. These methods for suppressing steel plate meandering can be applied to various steel plate manufacturing methods.

[0017] In the method for suppressing meandering of steel sheets according to this embodiment, the ratio of the width of the leading steel sheet to the width of the trailing steel sheet is preferably 1.0 to 1.3. Although the detailed reason is unknown, when the ratio of the width of the leading steel sheet to the width of the trailing steel sheet is 1.0 to 1.3 (including the same width), the portion that will not become the weld between the leading steel sheet and the trailing steel sheet, i.e., the range of both end portions of the leading steel sheet, is small, so it is presumed that the uneven cooling in the width direction of both unconstrained end portions is also reduced and the difference in the amount of warpage is also reduced. Therefore, it is preferable that the ratio of the width of the leading steel sheet to the width of the trailing steel sheet is 1.0 to 1.3.

[0018] In the method for suppressing meandering of a steel sheet according to this embodiment, it is preferable that the width of either or both of the trailing steel sheet and the leading steel sheet is 900 mm or more. Although the detailed reason is unknown, it is presumed that the wider the steel sheet width, the larger the range in which non-uniformity in structural changes occurs due to uneven cooling during water quenching, and therefore, when either or both of the leading steel sheets have a width of 900 mm or more and are widened, the amount of warpage becomes large. Therefore, it is preferable to apply the present invention when either or both of the leading steel sheets have a width of 900 mm or more.

[0019] Furthermore, when either or both of the trailing steel sheet and the preceding steel sheet have a martensite structure after passing through a quenching device, if uneven cooling occurs during quenching, deformation due to martensite transformation will occur unevenly, resulting in a large amount of warpage. Therefore, it is preferable to apply the method for suppressing meandering of steel sheets according to this embodiment when either or both of the trailing steel sheet and the preceding steel sheet have a martensite structure after passing through a quenching device.

[0020] Uneven cooling in the width direction of the plate is difficult to completely eliminate because it is influenced by various conditions, such as uneven plate temperature before quenching and plate running speed. For this reason, technology to reduce warpage even when uneven cooling occurs is important, and narrowing down or connecting plates with the same width is effective in suppressing meandering in the overaging zone downstream of the water quenching device. [Example]

[0021] Examples of the present invention will be described below.

[0022] The steel sheets were annealed using a continuous annealing line equipped with a water quenching device as shown in Figure 1. The steel sheets used had a chemical composition, in mass%, of C: 0.10% to 0.35%, Si: 0.01% to 2.0%, Mn: 0.8% to 2.35%, P: 0.05% or less, S: 0.005% or less, Al: 0.005% to 0.10%, N: 0.0060% or less, V: 0.001% to 1.0%, and Ti: 0.001% to 0.3%. In addition to the above, the high-strength steel plate further contains one or more of the following elements: Nb: 0.001% or more and 0.3% or less; Cr: 0.001% or more and 1.0% or less; Mo: 0.001% or more and 1.0% or less; Ni: 0.01% or more and 1.0% or less; Cu: 0.01% or more and 1.0% or less; B: 0.0002% or more and 0.005% or less; Sb: 0.001% or more and 0.05% or less; REM: 0.0002% or more and 0.05% or less; Mg: 0.0002% or more and 0.05% or less; and Ca: 0.0002% or more and 0.05% or less; and has a plate thickness of 1.0 to 2.3 mm and a plate width of 750 to 1250 mm. The high-strength steel sheets described above were water-quenched under conditions of a sheet temperature of 870°C or higher in the soaking zone, a sheet temperature of 730°C or higher in the cooling zone before water-quenching, and a water temperature between 10 and 40°C during water-quenching. The quenching transformed the steel sheet structure into a martensite structure. The quenched structure was observed under an SEM to confirm the presence or absence of an acicular structure. The quenched high-strength steel sheets were confirmed to have an acicular structure, confirming the precipitation of a martensite structure. The sheets were then tempered in the overaging zone at a sheet temperature of 150 to 200°C. Table 1 shows the joining conditions for the leading and trailing steel sheets and the amount of meandering of the steel sheets under each condition.

[0023] [Table 1]

[0024] The aforementioned meandering amount value is the maximum widthwise deviation of the center of the steel sheet within the overaging zone through which the steel sheet passes after water quenching, with the center of the hearth roll widthwise being 0 mm. In continuous annealing furnaces, the line must be slowed down when meandering exceeds 175 mm. Therefore, the threshold for determining meandering amount was set at 175 mm, and cases where meandering of 175 mm or more occurred were marked as "X." Furthermore, a value of less than 175 mm was marked as "pass," while a value of 100 mm to 175 mm was marked as "good," and a value of less than 100 mm was marked as "excellent." Because cooling unevenness in the steel sheet width inevitably occurs, meandering exceeding the threshold occurred with wide-up connections. However, narrow-down connections prevented meandering of 175 mm or more. A tendency for meandering to increase was observed, particularly when either the leading or trailing steel sheet width was 900 mm or greater. However, narrow-down connections showed improved meandering compared to wide-up connections. [Explanation of symbols]

[0025] 1: Rewinder 2: Welding machine 3: Inlet looper 4: Heating zone 5: uniform temperature 6: Hearth Roll 7: Cooling zone 8: Water quenching device 9: Overaging zone 10: Location of meandering 11: Air-cooled zone 12: Post-processing zone 13: Exit looper 14: Temper mill 15: Side trimmer 16: Winding machine 21: Quenching tank 22: Water surface 23: Steel plate 24: Roll

Claims

1. A method for suppressing meandering of a steel plate, in which, in a continuous processing line for steel plates, before water quenching, when connecting a trailing steel plate so that the plate width of the trailing steel plate is smaller than the plate width of the leading steel plate, the ratio of the plate width of the leading steel plate to the plate width of the trailing steel plate is 1.1 to 1.

3.

2. The method for suppressing meandering of a steel plate according to claim 1 , wherein one or both of the plate width of the trailing steel plate and the plate width of the leading steel plate is 900 mm or more.

3. The method for suppressing meandering of a steel plate according to claim 1 , wherein the continuous processing line for the steel plate comprises a heating device and a water quenching device.

4. The method for suppressing meandering of a steel plate according to claim 2 , wherein the continuous processing line for the steel plate comprises a heating device and a water quenching device.

5. The method for suppressing meandering of steel sheets according to claim 3 , wherein one or both of the trailing steel sheet and the preceding steel sheet have a martensitic structure after passing through the water quenching device.

6. The method for suppressing meandering of steel plates according to claim 4 , wherein one or both of the trailing steel plate and the preceding steel plate have a martensitic structure after passing through the water quenching device.

7. A method for manufacturing a steel sheet, using the method for suppressing meandering of a steel sheet according to any one of claims 1 to 6.

Images