Steel plate manufacturing method

Abstract

Provided is a method for manufacturing a steel sheet having a sheet width of more than 1600 mm, capable of achieving both suppression of galling caused by pickup generated through contact and friction between the steel sheet and a hearth roll, and suppression of meandering of the steel sheet, during continuous annealing of a cold-rolled steel sheet.　In an in-furnace region where the steel sheet temperature in a heating step of continuous annealing is in a range of 100-400°C, the ratio [C / Wr] of the crown amount C (mm) of the hearth roll installed to the roll width Wr (mm) is set at 0.00025-0.00082, and the unit tension applied to the steel sheet is set at 0.80 kgf / mm2 to 1.35 kgf / mm2, inclusive. Even in the case of a steel plate having a quarter-elongation profile prone to galling and meandering, both suppression of the occurrence of galling and suppression of meandering can be achieved, and, contrary to conventional common technical knowledge, the effect of a significant decrease in meandering in particular can be obtained.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing steel sheets by cold rolling followed by continuous annealing, and more particularly to a method for manufacturing steel sheets with a width exceeding 1600 mm. [Background technology]

[0002] Generally, steel sheets for automobile body panels are manufactured by cold rolling followed by continuous annealing, and these sheets require excellent surface appearance quality. However, in continuous annealing furnaces, pickup can occur in response to fluctuations in the contact pressure between the steel sheet and the hearth rolls (fluctuations within or between steel sheet coils). Pickup is a phenomenon in which easily oxidizable elements such as Mn and Si contained in the steel sheet concentrate on the surface of the steel sheet during the annealing process, forming oxides, and these oxides adhere to the surface of the hearth rolls. This pickup can cause scratches on the steel sheet. These scratches are more likely to occur in wide steel sheets with a width exceeding 1600 mm, and are particularly noticeable in steel sheets exhibiting a so-called quarter-stretch shape. Generally, a quarter-stretch shape refers to a steel sheet that has a large elongation during cold rolling around the 1 / 4 position on both the left and right sides in the width direction, resulting in a wavy shape at that position (a quarter-stretch shape in the narrow sense). In contrast, in this application, the term "quarter-stretched steel sheet" refers not only to steel sheets with such a narrow sense of quarter-stretched shape, but also to steel sheets that have a large elongation and wavy shape around the 1 / 4 position on either the left or right side in the width direction. In other words, a steel sheet that has a large elongation and wavy shape around the 1 / 4 position on either the left or right side in the width direction is called a quarter-stretched steel sheet.

[0003] In continuous annealing of steel plates, a major challenge is preventing the plate from meandering. Typically, hearth rolls are fitted with a roll crown (convex crown) to apply a centering force to the steel plate and prevent meandering (for example, Patent Document 1). With wide steel plates exceeding 1600 mm in width, the centering force due to contact and friction between the steel plate and the hearth roll becomes large, creating areas with high contact pressure between the steel plate and the hearth roll, making it easier for pick-up to occur. In particular, when the steel plate exhibits a quarter-stretch shape, areas with high contact pressure between the steel plate and the hearth roll are especially likely to occur, making pick-up even more likely. For this reason, the above-mentioned scratches are more likely to occur in wide steel plates exceeding 1600 mm in width, and are especially likely to occur in steel plates with a quarter-stretch shape. Furthermore, when the steel plate feeding speed exceeds 90 mpm, the contact pressure between the steel plate and the hearth roll increases, making it particularly easy for scratches to occur during pickup formation.

[0004] Possible measures to reduce pickup by lowering the contact pressure between the steel plate and the hearth roll include (i) reducing the steel plate tension, (ii) reducing the hearth roll crown amount, and (iii) reducing the roll roughness.

[0005] However, reducing the steel plate tension (unit tension) as described in (i) above increases the risk of steel plate meandering and leads to a decrease in productivity. Furthermore, our own investigations have shown that for wide steel plates with a width exceeding 1600 mm, reducing the unit tension does not effectively eliminate scratches caused by picking. For this reason, it is difficult to achieve both scratch prevention and meandering prevention with the above-mentioned measure (i).

[0006] Furthermore, the addition of a roll crown to the hearth roll is intended to prevent meandering. Therefore, according to conventional technical common sense, reducing the amount of hearth roll crown as described in (ii) above, similar to reducing steel plate tension, is thought to reduce the centering force, thereby decreasing the ability to correct meandering and increasing the risk of meandering.

[0007] Furthermore, as shown in Patent Document 2, hearth rolls need to be roughened to prevent meandering. Therefore, reducing the roll roughness as described in (iii) above reduces friction on the roll surface and decreases the centering force, thus increasing the risk of meandering. In addition, the inventors' studies have confirmed that reducing the roll roughness does not sufficiently reduce the occurrence of scratches. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 7-138656 [Patent Document 2] Japanese Patent Application Publication No. 63-65028 [Overview of the project] [Problems that the invention aims to solve]

[0009] As described above, when manufacturing wide steel plates with a width exceeding 1600 mm through continuous annealing, it has been difficult to achieve both the suppression of dents and the suppression of meandering using conventional technology.

[0010] Furthermore, in particular, wide steel sheets with a quarter-stretch shape are prone to scratches because there are areas in the width direction where the contact pressure between the steel sheet and the hearth roll is high, making the roll more susceptible to pick-up. Moreover, wide steel sheets with a quarter-stretch shape have a large difference in centering force in the width direction, which increases the risk of meandering. For these reasons, it has been considered more difficult to suppress both scratches and meandering when working with steel sheets with a quarter-stretch shape.

[0011] Therefore, the object of the present invention is to provide a method for manufacturing steel sheets that can suppress both scratching caused by pickup resulting from contact and friction between the steel sheet and the hearth roll, and suppress meandering of the steel sheet, when cold-rolled steel sheets are continuously annealed in order to manufacture steel sheets with a width exceeding 1600 mm.

[0012] Furthermore, among steel plates with a width exceeding 1600 mm, quarter-stretched steel plates in particular are prone to both scratches and meandering, and scratches are more likely to occur when the steel plate passing speed in the line exceeds 90 mpm.

[0013] Therefore, the present invention aims to provide a method for manufacturing steel sheets that can suppress both the occurrence of scratches and meandering, even when the target is a steel sheet with a quarter-stretch shape and when the sheet passing speed in the line is 90 mpm or more. [Means for solving the problem]

[0014] In order to solve the above problems, the inventors diligently investigated a method that can suppress both the occurrence of oak defects and meandering during the continuous annealing of steel plates, and as a result obtained the following findings.

[0015] First, the inventors conducted a simulation of meandering in a steel plate with a width of 1800 mm and investigated the relationship between the crown amount of the hearth roll and the meandering. In this simulation, contrary to expectations, the results were obtained that, for wide steel plates like the one in question, even if the crown amount of the hearth roll is relatively small, the effect on meandering is small.

[0016] Furthermore, a similar investigation was conducted by performing a meandering simulation on a 1800mm wide steel plate that simulated a quarter-stretch shape (a shape in which the plate lifts in the 400mm range from the edge and no centering force is generated). In this simulation, contrary to conventional technical wisdom, it was found that reducing the crown amount of the hearth roll by about 30% compared to the conventional crown amount reduced the meandering compared to when the conventional crown amount was used. This is thought to be because, since no centering force is generated in the 400mm range from the edge, the difference in centering force in the plate width direction is reduced as the crown amount decreases, and as a result, the occurrence of meandering is suppressed.

[0017] As described above, according to the simulation of the meandering of a steel plate with a plate width of 1800 mm, the following results different from the conventional technical common sense were obtained. (i) In a wide steel plate, even if the crown amount of the hearth roll is somewhat small, the influence on meandering is small. (ii) Particularly in a wide steel plate having a quarter elongation shape, meandering is rather improved by reducing the crown amount.

[0018] On the other hand, regarding the occurrence of scratching in the continuous annealing furnace, particularly, an investigation was made on which part in the continuous annealing furnace the scratching occurs. As a result, it was found that the hearth roll causing the scratching is the hearth roll provided in the range of 100 to 400 °C as the steel plate temperature.

[0019] Therefore, for steel plates with various plate widths (1000 to 1850 mm), the balance between the roll crown amount and the unit tension that can suppress both scratching and meandering was investigated for the hearth roll provided in the furnace where the steel plate temperature is 100 to 400 °C. As a result, for wide steel plates with a plate width exceeding 1600 mm, it was found that by setting a small roll crown amount, which was conventionally considered inappropriate due to concerns about meandering, and controlling the unit tension within a predetermined range, both the occurrence of scratching and meandering can be suppressed. Among them, in the case of a steel plate having a quarter elongation shape, since it is a steel plate that is originally likely to cause scratching and meandering, the effect is remarkable, and particularly for meandering, it was found that it is rather reduced compared to the case of using the conventionally generally used crown amount. Furthermore, it was also found that there is an optimal roll crown amount that can suppress the occurrence of scratching and meandering at a higher level, and by optimizing this roll crown amount, it is possible to increase the passing speed of the steel plate.

[0020] The present invention has been made based on such findings and has the following gist. [1] A method for manufacturing a steel plate having a plate width exceeding 1600 mm, When cold-rolled steel sheets are continuously annealed, in the furnace region where the steel sheet temperature is in the range of 100 to 400°C during the heating process, the ratio of the crown amount C (mm) to the roll width Wr (mm) of the hearth rolls [C / Wr] should be 0.00025 or more and 0.00082 or less, and the unit tension applied to the steel sheet should be 0.80 kgf / mm². 2 More than 1.35kgf / mm 2 The following is a method for manufacturing steel plates. [2] A method for manufacturing steel sheets according to [1], wherein the steel sheet to be continuously annealed has a shape mode coefficient Λ1 of -0.8 or more and 0.8 or less (excluding those greater than -0.05 and less than 0.05), or a shape mode coefficient Λ2 of -1.2 or more and 1.2 or less (excluding those greater than -0.05 and less than 0.05). [3] A method for manufacturing steel plates according to the manufacturing method of [1] or [2], wherein the ratio [C / Wr] of the crown amount C (mm) of the hearth roll to the roll width Wr (mm) is 0.00045 or more and 0.00075 or less. [Effects of the Invention]

[0021] According to the present invention, when manufacturing steel plates with a width exceeding 1600 mm, it is possible to suppress both scratches caused by pickup resulting from contact and friction between the steel plate and the hearth roll during continuous annealing of cold-rolled steel plates, and to suppress meandering of the steel plate. Furthermore, even with quarter-stretch steel plates, which are particularly prone to scratches and meandering, it is possible to suppress both scratches and meandering, and the effect of significantly improving meandering is obtained. Therefore, according to the present invention, high-quality steel plates can be efficiently manufactured without causing problems with plate feeding due to meandering.

[0022] Furthermore, according to the present invention, by optimizing the crown amount of the hearth roll, it is possible to suppress both the occurrence of scratches and meandering even when the sheet metal speed in the line is 90 mpm or higher. As a result, high-quality steel sheets can be manufactured at high speed without causing sheet metal feeding problems due to meandering. [Brief explanation of the drawing]

[0023] [Figure 1] A schematic diagram illustrating the shape of the roll (roll body) of a hearth roll. [Modes for carrying out the invention]

[0024] The present invention relates to a manufacturing method for cold-rolled steel sheets, characterized in that the heating process is carried out under specific conditions in order to suppress meandering and the occurrence of defects in the steel sheets during continuous annealing.

[0025] The meandering and scratching of steel plates during continuous annealing are particularly noticeable in steel plates with a width exceeding 1600 mm, and the suppression effect of the present invention on meandering and scratching is significantly obtained in such wide steel plates. Therefore, the present invention is intended for steel plates with a width exceeding 1600 mm. Furthermore, it is preferable to target plates with a width of 1700 mm or more, and more preferably 1800 mm or more. On the other hand, there is no particular upper limit to the plate width, but it is preferable to set it to 2000 mm or less.

[0026] Pickup, which causes scratches, occurs particularly significantly during the heating process of continuous annealing. As mentioned above, it was found to be caused by contact with the hearth rolls installed in the furnace region (called the preheating zone) where the steel sheet temperature is 100-400°C. Furthermore, it was found that the likelihood of scratches occurring strongly depends on the contact pressure with the hearth rolls installed in the furnace region within the above steel sheet temperature range. This contact pressure is significantly influenced by the crown shape and unit tension of the hearth rolls. Similarly, the likelihood of meandering is also significantly influenced by the crown shape and unit tension of the hearth rolls.

[0027] Therefore, in the present invention, when cold-rolled steel sheets are continuously annealed, in the furnace region where the steel sheet temperature is in the range of 100 to 400°C during the heating process, the roll crown of the hearth roll installed there and the unit tension applied to the steel sheet are set as follows. (i) Ratio of hearth roll crown amount C (mm) to roll width Wr (mm) [C / Wr]: 0.00025 or more and 0.00082 or less (ii) Unit tension to be applied to the steel plate: 0.80 kgf / mm 2 More than 1.35kgf / mm 2 below Figure 1 schematically shows the planar shape of a hearth roll (roll body), which has a convex crown whose outer diameter is larger at the center of the roll and decreases towards the end of the roll. This convex crown acts as a centering force on the steel plate in contact with the hearth roll, allowing the steel plate to be automatically centered. Figure 1 shows the crown amount C and the roll width Wr. Both are dimensions of the hearth roll before use (i.e., crown amount C is the initial crown amount). The crown amount C can be calculated as C = (D1 - D2) / 2, where D1 is the diameter of the center of the roll body and D2 is the diameter of the end of the roll body.

[0028] The diameter of the roll body is not particularly limited, but for example, D1 is preferably 800 mm or more, more preferably 900 mm or more, and even more preferably 950 mm or more. Also, D1 is preferably 1200 mm or less, more preferably 1100 mm or less, and even more preferably 1050 mm or less. Also, D2 is preferably 801 mm or more, more preferably 901 mm or more, and even more preferably 951 mm or more. Also, D2 is preferably 1205 mm or less, more preferably 1105 mm or less, and even more preferably 1055 mm or less.

[0029] Conventionally, to prevent meandering, it was considered desirable to set the crown amount C of the hearth roll to around 2.2 mm (approximately 2.0 to 2.4 mm), and it was thought that if meandering was a particular problem, it was necessary to increase the crown amount and increase the centering force. However, the inventors of this invention investigated hearth rolls installed in the furnace region where the steel plate temperature is in the range of 100 to 400°C during the heating process of continuous annealing, and discovered a different fact. Specifically, they found that for steel plates with a width of more than 1600 mm, the effect on meandering is small even if the crown amount is less than 2.0 mm, and conversely, for steel plates with a quarter-stretch shape, reducing the crown amount improves meandering. Furthermore, they found that for steel plates with a width of more than 1600 mm, reducing the crown amount significantly suppresses the occurrence of scratches.

[0030] Further investigation revealed that the optimal crown amount C for a hearth roll is determined by its relationship with the roll width Wr. Specifically, it was found that setting the ratio [C / Wr] of crown amount C (mm) to roll width Wr (mm) between 0.00025 and 0.00082 suppresses both scratching and meandering. The ratio [C / Wr] for hearth rolls in conventional technology is above 0.00082 (generally 0.0013 or less), and therefore, this range of 0.00025 to 0.00082 is smaller than the level of conventional technology.

[0031] For the reasons stated above, in the present invention, the ratio [C / Wr] of the crown amount C (mm) to the roll width Wr (mm) of the hearth roll installed in the furnace region where the steel sheet temperature is in the range of 100 to 400°C during the heating process of continuous annealing is set to 0.00025 or more and 0.00082 or less.

[0032] If the ratio [C / Wr] is less than 0.00025, the crown is too small, causing meandering. Therefore, the ratio [C / Wr] should be 0.00025 or higher. On the other hand, if the ratio [C / Wr] is greater than 0.00082, the crown is too large, and the effect of suppressing scratches is not sufficiently obtained. Therefore, the ratio [C / Wr] should be 0.00082 or lower. Here, the amount of crown of the hearth roll used in this invention is smaller than that of conventional hearth rolls, and therefore, a ratio [C / Wr] of 0.00082 or lower is a lower level than that of conventional hearth rolls, but the effect on meandering is small. Furthermore, as will be described later, in the case of quarter-stretched steel plates, a ratio [C / Wr] of 0.00082 or lower is actually more effective in improving meandering.

[0033] From the viewpoint of further suppressing the occurrence of scratches, it is preferable that the ratio [C / Wr] be 0.00045 or higher. Also, from the viewpoint of further suppressing the occurrence of scratches, it is preferable that the ratio [C / Wr] be 0.00075 or lower. Furthermore, by setting the ratio [C / Wr] within this range, the occurrence of scratches can be further suppressed, so the steel plate feeding speed can be increased, and it becomes possible to achieve a steel plate feeding speed of 90 mpm or higher.

[0034] Here, the hearth roll used in the present invention is not particularly limited in terms of conditions other than the ratio [C / Wr], but typically the crown amount C is 0.5 mm or more and less than 2.0 mm, and the roll width Wr is 2000 mm or more and 2400 mm or less. The crown amount C is preferably 0.5 mm or more, more preferably 1.0 mm or more, and even more preferably 1.5 mm or more. Furthermore, the crown amount C is preferably less than 2.0 mm, more preferably 1.8 mm or less, and even more preferably 1.6 mm or less. Furthermore, the roll width Wr is preferably 2000 mm or more, more preferably 2100 mm or more, and even more preferably 2200 mm or more. Furthermore, the roll width Wr is preferably 2400 mm or less, more preferably 2300 mm or less, and even more preferably 2250 mm or less.

[0035] Generally, the unit tension applied to the steel sheet in the heating process of continuous annealing should be controlled appropriately within a range of 1.0 kgf / mm before and after. If the unit tension is significantly lower than this range, the steel sheet will snake, while if it is significantly higher, scratching defects are likely to occur. Therefore, in the present invention, in the furnace area where the steel sheet temperature ranges from 100 to 400 °C in the heating process of continuous annealing, the unit tension applied to the steel sheet is set to be 0.80 kgf / mm or more and 1.35 kgf / mm or less. It is preferable that the unit tension is 0.90 kgf / mm or more, and more preferably 1.00 kgf / mm or more. Also, it is preferable that the unit tension is 1.25 kgf / mm or less, more preferably 1.15 kgf / mm or less, and even more preferably 1.10 kgf / mm or less. 2 By using a hearth roll having a specific ratio [C / Wr] with the crown amount suppressed as described above in the furnace area where the steel sheet temperature ranges from 100 to 400 °C and is likely to cause scratching defects, and controlling the unit tension within the above range, it is possible to suppress both the occurrence of scratching defects and snaking. 2 In the heating process of continuous annealing, especially in the furnace area where the steel sheet temperature is 200 to 300 °C, it is preferable that the unit tension applied to the steel sheet is 1.00 kgf / mm or more and 1.20 kgf / mm or less. It is preferable that the unit tension is 1.05 kgf / mm or more, and more preferably 1.00 kgf / mm or more. It is preferable that the unit tension is 1.15 kgf / mm or less, and more preferably 1.20 kgf / mm or less. 2 2 2 2 2 2 2 2

[0036] 2 2 2 2 2 2 ​​​​​​​​​​​​​​​The following is more preferable. The unit tension is calculated by dividing the value measured by the load cell by the cross-sectional area of ​​the steel plate that has passed through. This is because, under these conditions, when the line speed is 100 mpm or higher, the effect of reducing the contact pressure between the steel plate and the roll due to the reduction of the centering force is obtained, and the effect of suppressing scratches is significantly obtained. For this reason, in this case, it is preferable that the line speed be 100 mpm or higher. It is also preferable that it be 130 mpm or lower.

[0037] As mentioned above, the meandering and scratching of steel plates are particularly pronounced in steel plates with a width exceeding 1600 mm, especially in steel plates with a quarter-stretch shape. However, the present invention is also effective for steel plates with a quarter-stretch shape. In particular, the improvement effect on suppressing meandering is greater for steel plates with a quarter-stretch shape.

[0038] In this invention, a quarter-stretched steel sheet is defined as a steel sheet having a shape mode coefficient Λ1 of -0.8 or more and 0.8 or less (excluding those greater than -0.05 and less than 0.05), or a shape mode coefficient Λ2 of -1.2 or more and 1.2 or less (excluding those greater than -0.05 and less than 0.05).

[0039] Furthermore, as mentioned above, the reason why both shape mode coefficients Λ1 and Λ2 exclude the range of "greater than -0.05 and less than 0.05" is that this range corresponds to a high degree of flatness in the steel plate, and is therefore excluded from the range of quarter-elongation shapes.

[0040] When the shape mode coefficient Λ1 of the steel plate is between -0.8 and 0.8 (excluding values ​​greater than -0.05 and less than 0.05), it represents a state where the elongation is greater on either the left or right side in the width direction of the steel plate. In steel plates of this shape, meandering usually occurs significantly, but by applying the present invention, meandering can be greatly reduced. Furthermore, when the shape mode coefficient Λ2 of the steel plate is between -1.2 and 1.2 (excluding values ​​greater than -0.05 and less than 0.05), it represents a state where the elongation is greater on both the left and right sides in the width direction of the steel plate (corresponding to a quarter-elongation shape on both sides). In steel plates of this shape, meandering usually occurs significantly, but by applying the present invention, meandering can be greatly reduced.

[0041] Whether a steel plate has a quarter-stretch shape can be determined, for example, by the following methods (1) or (2). (1) The shape of the steel plate is measured with a shape detector, and the shape mode coefficient Λ1 is calculated based on the measurement results. Calculate Λ². (2) Based on past performance (relationship between manufacturing conditions and shape mode coefficients Λ1 and Λ2), the shape mode coefficients Λ1 and Λ2 are determined from the manufacturing conditions of the steel sheet.

[0042] The shape mode coefficient Λ1 represents the magnitude of elongation at one side, and Λ2 represents the magnitude of elongation at the middle and at the ends. These coefficients can be evaluated, for example, based on the methods described in References 1 and 2. [Reference document 1] Japanese Patent Application Laid-open No. 61-255710 [Reference 2] The Iron and Steel Institute of Japan, 5th Edition Steel Handbook Committee (ed.), "5th Edition Steel Handbook, Volume 2: Rolling and Secondary Processing," p. 218, The Iron and Steel Institute of Japan, 2014. In the case of (1) above, the shape of the steel sheet should be measured using a shape detector between the time of cold rolling and when it is introduced into the continuous annealing furnace (for example, when the coil is unwound during continuous annealing). In this case, the entire length of the coil may be measured, or only a part of the coil in the longitudinal direction (for example, the leading edge) may be measured. Known techniques may be used for the shape detector and the method of measuring the shape of the steel sheet using this shape detector.

[0043] Next, other manufacturing conditions for the present invention will be described.

[0044] The present invention involves continuous annealing of cold-rolled steel sheets, but plating may be performed after annealing. In other words, the steel sheets produced by the present invention also include plated steel sheets that are plated after annealing. Examples of plating treatments include, but are not limited to, hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, electro-galvanized nickel-zinc plating, hot-dip aluminum plating, tin plating, hot-dip aluminum-silicon plating, and hot-dip zinc-aluminum plating.

[0045] The thickness of the steel plate is not particularly limited, but is usually between 0.1 and 4.0 mm. A thickness of 0.1 mm or more is preferred, 0.3 mm or more is more preferred, and 0.5 mm or more is even more preferred. Furthermore, a thickness of 4.0 mm or less is preferred, 1.0 mm or less is more preferred, and 0.8 mm or less is even more preferred.

[0046] Steel sheets before cold rolling are manufactured, for example, by continuous casting followed by hot rolling and pickling.

[0047] The chemical composition of the steel sheet is not particularly limited, but for example, it may contain, by mass%, C: greater than 0% and 0.3% or less, Si: greater than 0% and 2% or less, Mn: greater than 0% and 5% or less, P: greater than 0% and 0.2% or less, S: greater than 0% and 0.03% or less, N: greater than 0% and 0.03% or less, and sol.Al: greater than 0% and 0.5% or less. Preferably, it is C: 0.0001% or more, Si: 0.001% or more, Mn: 0.01% or more, P: 0.001% or more, S: 0.0001% or more, N: 0.0005% or more, and sol.Al: 0.001% or more. Furthermore, if necessary, one or more of the following elements may be included: Nb: greater than 0% and 0.3% or less, Ti: greater than 0% and 0.3% or less, Cr: greater than 0% and 1% or less, Mo: greater than 0% and 1% or less, V: greater than 0% and 0.5% or less, Cu: greater than 0% and 1% or less, Ni: greater than 0% and 1% or less, B: greater than 0% and 0.01% or less, Sn: greater than 0% and 0.3% or less, Sb: greater than 0% and 0.3% or less, Co: greater than 0% and 1% or less, Zr: greater than 0% and 0.5% or less, W: greater than 0% and 0.5% or less, Ca: greater than 0% and 0.01% or less, Mg: greater than 0% and 0.01% or less, and REM: greater than 0% and 0.01% or less. The remainder is iron and unavoidable impurities.

[0048] The mechanical properties of the steel sheet are most effective when the tensile strength (TS) after annealing is between 200 MPa and 340 MPa. Specifically, a TS of 200 MPa or higher is preferable, and a TS of 340 MPa or lower is also preferable. [Examples]

[0049] A cold-rolled steel sheet with a width of 1830 mm and a thickness of 0.75 mm was passed through a continuous galvanizing line (CGL) to produce an alloyed hot-dip galvanized steel sheet. Specifically, the cold-rolled steel sheet was continuously annealed, then hot-dip galvanized, and subsequently alloyed to produce an alloyed hot-dip galvanized steel sheet. The sheet passing speed in the continuous annealing furnace was set to 120 mpm. For the hearth rolls installed in the furnace region (preheating zone) where the steel sheet temperature is in the range of 100 to 400°C during the heating process of continuous annealing, the ratio of crown amount C (mm) to roll width Wr (mm) [C / Wr] was adjusted by using hearth rolls with an adjusted crown amount.

[0050] In each embodiment (inventive example, comparative example), steel sheets from Group A and Group B were subjected to continuous annealing and plating treatments, respectively. The shape mode coefficients of the steel sheets were determined by measuring the shape of the steel sheets with a shape detector installed downstream of the rolling mill in the cold rolling line, and calculating the shape mode coefficients Λ1 and Λ2 based on the measurement results. (1) Steel plates of Group A • Steel plate shape: Quarter elongation shape Shape mode coefficient Λ1: 0.2~0.8 Shape mode coefficient Λ2: 0.3~1.1 • Coil weight: 20 tons • Number of coils: 1000 (2) Steel plates of Group B ·Steel plate shape: flat shape Shape mode coefficient Λ1: -0.004~0.04 Shape mode coefficient Λ²: -0.04~0.04 • Coil weight: 20 tons • Number of coils: 1000 The amount of meandering, the average amount of meandering, and the meandering occurrence rate of the steel plates were measured and calculated as follows: The center position of the steel plate was calculated from the values ​​of the online edge detector, and the amount of meandering of the steel plate was measured from the difference from the center position of the roll. The average amount of meandering was calculated by doing this every 10m. In addition, the maximum amount of meandering was determined for each coil, and the meandering occurrence rate was calculated by "(number of coils with a maximum meandering of 30mm or more) / (total number of coils) × 100"%.

[0051] Furthermore, the presence or absence of scratches was investigated by inspecting the entire length of each coil on both sides using an online surface defect meter and visual inspection by inspectors. The scratch occurrence rate was calculated by determining "(number of coils with scratches) / (total number of coils) × 100"%.

[0052] [Table 1]

[0053] In Table 1, No. 2 and No. 3 are comparative examples with a ratio [C / Wr] that is too high and comparable to the level of the prior art, No. 11 is a comparative example with a ratio [C / Wr] that is too low, No. 8 is a comparative example with a unit tension that is too high, and No. 9 is a comparative example with a unit tension that is too low. All of these comparative examples show a high value for "total coil [scratch occurrence rate] + [serpentine occurrence rate] (%)". In contrast, the present invention example shows that the "total coil [scratch occurrence rate] + [serpentine occurrence rate] (%)" is at most 15%, and all values ​​are kept low.

Claims

1. A method for manufacturing steel plates with a width exceeding 1600 mm, When cold-rolled steel sheets are continuously annealed, in the furnace region where the steel sheet temperature is in the range of 100 to 400°C during the heating process, the ratio of the initial crown amount C (mm) of the hearth roll to the roll width Wr (mm) [C / Wr] should be 0.00025 or more and 0.00082 or less, and the unit tension applied to the steel sheet should be 0.80 kgf / mm ２ 1.35kgf / mm or more ２ The following is a method for manufacturing steel plates.

2. A method for manufacturing a steel sheet according to claim 1, wherein the steel sheet to be continuously annealed has a shape mode coefficient Λ1 of -0.8 or more and 0.8 or less (excluding values ​​greater than -0.05 and less than 0.05), or a shape mode coefficient Λ2 of -1.2 or more and 1.2 or less (excluding values ​​greater than -0.05 and less than 0.05).

3. The method for manufacturing a steel sheet according to claim 1 or 2, wherein the ratio [C / Wr] of the crown amount C (mm) to the roll width Wr (mm) of the hearth roll is 0.00045 or more and 0.00075 or less.

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