Method for manufacturing a steel sheet

Abstract

The present invention provides a method for manufacturing a steel sheet, which, in the manufacturing of a steel sheet having a sheet width of more than 1600 mm, can simultaneously suppress the occurrence of scratches caused by sticking due to contact and friction of the steel sheet with a furnace bottom roll and the occurrence of snake marks in the steel sheet when continuously annealing the steel sheet after cold rolling. In a heating process of the continuous annealing, in a region of the furnace where the temperature of the steel sheet reaches a range of 100 to 400°C, the ratio [C / Wr] of the amount of crown C (mm) of the furnace bottom roll provided to the roll width Wr (mm) is set to be 0.00025 or more and 0.00082 or less, and the unit tension applied to the steel sheet is set to be 0.80 kgf / mm 2 or more and 1.35 kgf / mm 2 or less. Even for a quarter wave-shaped steel sheet, which is particularly prone to scratches and snake marks, the present invention can simultaneously suppress the occurrence of scratches and snake marks, and can achieve a particularly significant improvement in snake marks, unlike conventional technical common sense.

Description

Technical Field

[0001] This invention relates to a method for manufacturing steel sheets that undergo continuous annealing after cold rolling, and particularly to a method for manufacturing steel sheets with a width greater than 1600 mm. Background Technology

[0002] Typically, automotive exterior steel sheets are manufactured through continuous annealing after cold rolling, requiring excellent surface finish. However, in continuous annealing furnaces, adhesion can sometimes occur due to variations in the contact pressure between the steel sheet and the furnace bottom rolls (within or between steel coils). Adhesion refers to the phenomenon where easily oxidizable elements such as Mn and Si in the steel sheet accumulate on the surface during annealing, forming oxides that adhere to the surface of the furnace bottom rolls. This adhesion can sometimes cause scratches on the steel sheet. These scratches are more likely to occur on wide steel sheets (those wider than 1600 mm), and are particularly prone to appearing on sheets exhibiting a so-called quarter-wave shape. Generally, a quarter-wave steel sheet refers to a sheet that has significant elongation during cold rolling around the left and right quarters of its width, exhibiting a wavy shape at that position (in a narrow sense). In contrast, this application refers not only to the narrow definition of quarter-wave steel plate, but also to steel plates that extend significantly and have a wavy shape near the 1 / 4 mark on either side of the plate width direction, and are thus called quarter-wave steel plates. That is, steel plates that extend significantly and have a wavy shape near the 1 / 4 mark on either side of the plate width direction are called quarter-wave steel plates.

[0003] In continuous annealing of steel plates, preventing the steel plate from snakes is a major challenge. Typically, to apply a centering force to the steel plate and thus prevent snakes, the hearth rollers are given a roll crown (convex crown) (e.g., Patent Document 1). For wide steel plates with a width greater than 1600 mm, the centering force generated by the contact and friction between the steel plate and the hearth rollers increases, creating areas of high pressure on the contact surface, making adhesion more likely. This is especially true when the steel plate has a quarter-wave shape, where areas of high pressure on the contact surface are more likely to form, thus increasing the likelihood of adhesion. Therefore, scratches, as described above, are more likely to occur on wide steel plates with a width greater than 1600 mm, particularly on quarter-wave steel plates. Furthermore, when the steel plate's throughput speed reaches 90 mpm or higher, the contact surface pressure between the steel plate and the hearth rollers further increases, leading to adhesion and making scratches particularly likely.

[0004] As a countermeasure to reduce adhesion and thus reduce the contact surface pressure between the steel plate and the furnace bottom roll, one could consider (i) reducing the steel plate tension, (ii) reducing the furnace bottom roll convexity, and (iii) reducing the roll roughness.

[0005] However, reducing the steel sheet tension (unit tension) as described in (i) increases the risk of steel sheet serpentine movement, leading to reduced productivity. Furthermore, according to the inventors' research, for wide steel sheets with a width greater than 1600 mm, even with reduced unit tension, scratches caused by adhesion are difficult to eliminate. Therefore, the countermeasures described in (i) are insufficient to simultaneously suppress scratch generation and serpentine movement.

[0006] Furthermore, the crowning of the furnace bottom rolls is applied to prevent snake-like behavior. Therefore, following conventional technical knowledge, it can be assumed that reducing the crowning of the furnace bottom rolls as described above (ii) will, like reducing the steel plate tension, lead to a decrease in snake-like correction ability due to the reduction in centering force, thus increasing the risk of snake-like behavior.

[0007] Furthermore, as shown in Patent Document 2, in order to prevent snake-like movement, the bottom roller needs to be roughened. Therefore, reducing the roughness of the roller in (iii) above will reduce the friction on the roller surface, resulting in a decrease in centering force, and thus increasing the risk of snake-like movement. In addition, the inventors' research results have confirmed that even if the roller roughness is reduced, the occurrence of scratches will not be sufficiently reduced.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 7-138656

[0011] Patent Document 2: Japanese Patent Application Publication No. 63-65028 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] As mentioned above, when manufacturing wide steel plates with a width greater than 1600mm through continuous annealing, conventional techniques have struggled to simultaneously suppress scratches and prevent snake-like patterns.

[0014] Furthermore, especially with wide-width materials (steel plates) in a quarter-wave shape, the contact surface pressure between the steel plate and the furnace bottom rolls is relatively high in a portion of the plate width direction, making it prone to adhesion to the rolls and thus resulting in particularly significant scratches. Moreover, the greater difference in centering force in the width direction of wide-width materials in a quarter-wave shape makes the risk of snake-like movement particularly high. Therefore, for quarter-wave steel plates, it has always been considered more difficult to simultaneously suppress scratches and snake-like movement.

[0015] Therefore, the purpose of this invention is to provide a method for manufacturing steel plates that, when continuously annealing cold-rolled steel plates for manufacturing steel plates with a width greater than 1600 mm, can simultaneously suppress scratches caused by adhesion resulting from contact and friction between the steel plate and the furnace bottom rollers, as well as suppress the serpentine movement of the steel plate.

[0016] In addition, steel plates with a width greater than 1600mm, especially quarter-wave steel plates, are prone to both scratches and serpentine patterns. Furthermore, scratches are more likely to occur when the steel plate passing speed in the production line reaches 90mpm or more.

[0017] Therefore, the present invention provides a method for manufacturing steel plates that can simultaneously suppress scratches and prevent serpentine movement, even when the object is a quarter-wave steel plate and the plate passing speed in the production line is 90 mpm or higher.

[0018] Methods for solving problems

[0019] In order to solve the above-mentioned problems, the inventors have conducted intensive research on a method that can simultaneously suppress the generation of scratches and suppress snake-like behavior during continuous annealing of steel plates, and the following insights have been obtained.

[0020] First, the inventors conducted a serpentine simulation on a steel plate with a width of 1800 mm to investigate the relationship between the convexity of the furnace bottom roller and the serpentine movement. In this simulation, for this wide steel plate as the object, different results were obtained: even if the convexity of the furnace bottom roller was relatively small to a certain extent, its impact on the serpentine movement was also small.

[0021] Furthermore, a serpentine simulation was conducted on a 1800mm wide steel plate with a quarter-wave shape (a shape in which the plate floats within 400mm of the edge, without generating a centering force), and the same investigation was carried out. In this simulation, contrary to conventional technical common sense, by reducing the crown of the furnace bottom roll by about 30% compared to the usual crown, a reduction in serpentine movement was obtained compared to the case where the usual crown was used. This is attributed to the fact that no centering force is generated within 400mm of the edge; therefore, the lower the crown, the less the difference in centering force in the plate width direction, and thus the suppression of serpentine movement.

[0022] As stated above, based on the serpentine simulation of a steel plate with a width of 1800 mm, the following results were obtained, which differ from previous technical common sense.

[0023] (i) For wide steel plates, even if the convexity of the furnace bottom roll is relatively small, the effect on the serpentine motion is also small.

[0024] (ii) In particular, for wide steel plates with a quarter-wave shape, the serpentine behavior is improved by reducing the convexity.

[0025] On the other hand, regarding the occurrence of scratches in the continuous annealing furnace, a special investigation was conducted into which part of the furnace the scratches occurred. The results showed that the furnace bottom rollers that caused the scratches were those installed in the steel plate temperature range of 100–400°C.

[0026] Therefore, for steel plates of various widths (1000–1850 mm), an investigation was conducted on the balance between roll crown measurement and unit tension, which simultaneously suppresses scratches and snake-like behavior, for the bottom rolls installed in furnaces where the steel plate temperature reaches 100–400 °C. The results showed that for wide steel plates with a width greater than 1600 mm, by setting a small roll crown measurement, previously considered inappropriate due to concerns about snake-like behavior, and controlling the unit tension within a specified range, both scratches and snake-like behavior were suppressed. Furthermore, the effect was particularly significant for steel plates with a quarter-wave shape, which are inherently prone to scratches and snake-like behavior; especially regarding snake-like behavior, it was found to be reduced compared to the commonly used roll crown measurement. It was further found that an optimal roll crown measurement exists that can suppress scratches and snake-like behavior to a higher level, and by optimizing this roll crown measurement, the steel plate throughput speed can be increased.

[0027] This invention is based on the following insights, the main points of which are as follows.

[0028] [1] A method for manufacturing a steel plate, wherein the steel plate has a width greater than 1600 mm, When continuously annealing cold-rolled steel sheets, in the furnace region where the steel sheet temperature reaches the range of 100–400°C during the heating process, the ratio of the convexity C (mm) of the furnace bottom roll to the roll width Wr (mm) [C / Wr] is set to be above 0.00025 and below 0.00082, and the unit tension applied to the steel sheet is set to 0.80 kgf / mm. 2 Above and 1.35 kgf / mm 2 the following.

[0029] [2] According to the manufacturing method of [1] above, the shape pattern coefficient Λ1 of the steel plate to be continuously annealed is -0.8 or more and 0.8 or less (but does not include greater than -0.05 and less than 0.05), or the shape pattern coefficient Λ2 is -1.2 or more and 1.2 or less (but does not include greater than -0.05 and less than 0.05).

[0030] [3] According to the manufacturing method of [1] or [2] above, the ratio of the convexity C (mm) of the bottom roller to the width Wr (mm) of the roller [C / Wr] is 0.00045 or more and 0.00075 or less.

[0031] Invention Effects

[0032] According to the present invention, when continuously annealing cold-rolled steel sheets for manufacturing steel sheets with a width greater than 1600 mm, it is possible to simultaneously suppress scratches caused by adhesion resulting from contact and friction between the steel sheet and the furnace bottom rolls, as well as suppress the snake-like behavior of the steel sheet. Furthermore, even for quarter-wave steel sheets, which are particularly prone to scratches and snake-like behavior, it is possible to simultaneously suppress scratch formation and snake-like behavior, and in particular, to achieve a significant improvement in snake-like behavior. Therefore, according to the present invention, high-quality steel sheets can be manufactured efficiently without the occurrence of through-sheet defects caused by snake-like behavior.

[0033] Furthermore, according to the present invention, by optimizing the convexity of the furnace bottom roller, even when the plate passage speed in the production line is 90 mpm or higher, it is possible to both suppress the generation of scratches and suppress serpentine movement. Therefore, high-quality steel plates can be manufactured at high speeds without plate passage failures caused by serpentine movement. Attached Figure Description

[0034] Figure 1 This is an illustrative diagram that schematically shows the shape of the bottom roller (roller body). Detailed Implementation

[0035] The present invention is a manufacturing method for continuously annealing cold-rolled steel plates, characterized in that, in order to suppress the occurrence of snake-like patterns and scratches on the steel plates during continuous annealing, the heating process is carried out under specific conditions.

[0036] Snakeing and scratches in steel sheets produced during continuous annealing are particularly noticeable on steel sheets with a width greater than 1600 mm. The suppression effect on snakeing and scratches provided by this invention is significantly achieved on such wide steel sheets. Therefore, this invention is intended for manufacturing steel sheets with a width greater than 1600 mm. Furthermore, a width of 1700 mm or more is preferred, and 1800 mm or more is more preferable. On the other hand, there is no particular upper limit to the width, but it is preferably set to 2000 mm or less.

[0037] Adhesion, which causes scratches, is particularly pronounced during the heating process of continuous annealing. As mentioned above, it is determined that this is caused by contact with the bottom rollers located in the furnace zone (referred to as the preheating zone) where the steel plate temperature reaches 100–400°C. Furthermore, it is determined that the ease with which scratches occur strongly depends on the contact surface pressure of the bottom rollers located in the furnace zone within the aforementioned steel plate temperature range. This contact surface pressure is significantly affected by the crown shape and unit tension of the bottom rollers. In addition, the ease with which scratches occur is also significantly affected by the crown shape and unit tension of the bottom rollers.

[0038] Therefore, in this invention, when continuously annealing the cold-rolled steel sheet, in the furnace area where the steel sheet temperature reaches the range of 100 to 400°C during the heating process, the roll crown of the furnace bottom roller and the unit tension applied to the steel sheet are set as follows.

[0039] (i) The ratio of the crown of the furnace bottom roller C (mm) to the roller width Wr (mm) [C / Wr]: ≥0.00025 and ≤0.00082; (ii) Unit tension applied to the steel plate: 0.80 kgf / mm 2 Above and 1.35 kgf / mm 2 the following.

[0040] Figure 1 This diagram schematically illustrates the planar shape of the furnace bottom roll (roller body), giving it a convex shape with a larger outer diameter in the center and gradually decreasing towards the ends. This convex shape allows a centering force to be applied to the steel plate in contact with the furnace bottom roll, enabling the steel plate to automatically center itself. Figure 1 The figure shows the crown measurement C and the roll width Wr. Both are the dimensions of the furnace bottom roll before use (i.e., the crown measurement C is the initial crown measurement). The crown measurement C is calculated by setting the diameter of the central part of the roll body as D1 and the diameter of the end of the roll body as D2, using C = (D1 - D2) / 2.

[0041] It should be noted that there is no particular limitation on the diameter of the roller body. For example, D1 is preferably 800 mm or more, more preferably 900 mm or more, and even more preferably 950 mm or more. Furthermore, D1 is preferably 1200 mm or less, more preferably 1100 mm or less, and even more preferably 1050 mm or less. Additionally, D2 is preferably 801 mm or more, more preferably 901 mm or more, and even more preferably 951 mm or more. Furthermore, D2 is preferably 1205 mm or less, more preferably 1105 mm or less, and even more preferably 1055 mm or less.

[0042] Previously, to prevent snake-like behavior, it was considered preferable to set the crown angle C of the furnace bottom rollers to around 2.2 mm (approximately 2.0 mm to approximately 2.4 mm). Especially when snake-like behavior became a problem, it was necessary to further increase the crown angle to enhance centering force. However, the inventors conducted research on furnace bottom rollers positioned in the furnace region where the steel plate temperature reaches 100–400°C during continuous annealing heating processes, and discovered a different fact. Specifically, it was found that for steel plates with a width greater than 1600 mm, even a crown angle less than 2.0 mm had a relatively small impact on snake-like behavior; conversely, for steel plates with a quarter-wave shape, reducing the crown angle could improve snake-like behavior. Furthermore, it was found that for steel plates with a width greater than 1600 mm, reducing the crown angle significantly suppressed the formation of scratches.

[0043] Furthermore, further research revealed that the optimal crown C of the furnace bottom roller depends on its relationship with the roller width Wr. Specifically, it was determined that by making the ratio of crown C (mm) to roller width Wr (mm) [C / Wr] greater than 0.00025 and less than 0.00082, both scratching and serpentine motion can be suppressed. In the prior art, the ratio [C / Wr] of the furnace bottom roller is greater than 0.00082 (typically less than 0.0013), therefore, the range of 0.00025 to 0.00082 is smaller than that of the prior art.

[0044] For the reasons mentioned above, in this invention, the ratio [C / Wr] of the convexity C (mm) of the furnace bottom roller, which is set in the furnace area where the steel plate temperature reaches 100 to 400°C during the continuous annealing heating process, to the roller width Wr (mm) is set to 0.00025 or more and 0.00082 or less.

[0045] If the ratio [C / Wr] is less than 0.00025, the crown is too small, and thus snake-like behavior will occur. Therefore, the ratio [C / Wr] is set to 0.00025 or higher. On the other hand, if the ratio [C / Wr] is greater than 0.00082, the crown is too large, and thus the scratch suppression effect cannot be sufficiently obtained. Therefore, the ratio [C / Wr] is set to 0.00082 or lower. Here, the crown of the furnace bottom roller used in this invention is smaller than that of conventional furnace bottom rollers. Therefore, although the ratio [C / Wr] of 0.00082 or lower is lower than that of conventional furnace bottom rollers, the effect on snake-like behavior is small. In addition, as described below, in the case of quarter-wave steel plates, a ratio [C / Wr] of 0.00082 or lower is actually effective in improving snake-like behavior.

[0046] From the viewpoint of further suppressing scratch formation, the ratio [C / Wr] is preferably set to 0.00045 or higher. Furthermore, from the viewpoint of further suppressing scratch formation, the ratio [C / Wr] is preferably set to 0.00075 or lower. Moreover, by setting the ratio [C / Wr] to this range, scratch formation can be further suppressed, thus increasing the plate throughput speed, and allowing the plate throughput speed to be set to 90 mpm or higher.

[0047] Here, for the hearth roller used in this invention, conditions other than the [C / Wr] ratio are not particularly limited, but it is generally configured such that the crown C is 0.5 mm or more and less than 2.0 mm, and the roller width Wr is 2000 mm or more and less than 2400 mm. The crown 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 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 roller width Wr is preferably 2000 mm or more, more preferably 2100 mm or more, and even more preferably 2200 mm or more. Furthermore, the roller width Wr is preferably 2400 mm or less, more preferably 2300 mm or less, and even more preferably 2250 mm or less.

[0048] Typically, the unit tension applied to the steel sheet during the continuous annealing heating process is controlled at 1.0 kgf / mm². 2 The appropriate tension is approximately 0.80 kgf / mm². If the unit tension is significantly lower than this value, serpentine motion will occur; conversely, if it is significantly higher, scratches are likely to occur. Therefore, in this invention, the unit tension applied to the steel plate in the furnace region where the steel plate temperature reaches the range of 100–400°C during the continuous annealing heating process is set to 0.80 kgf / mm². 2 Above and 1.35 kgf / mm 2 The unit tension is set to 0.80 kgf / mm. 2 The preferred value is 0.90 kgf / mm. 2 The above is more preferably set to 1.00 kgf / mm 2 That's all. Additionally, the unit tension is set at 1.35 kgf / mm. 2 Hereinafter, the preferred value is set to 1.25 kgf / mm. 2 Hereinafter, a more preferred value is 1.15 kgf / mm 2 Hereinafter, a further preferred value is set at 1.10 kgf / mm. 2 the following.

[0049] In furnace zones where steel plates prone to scratches reach temperatures of 100–400°C, by using furnace bottom rolls with a specific ratio [C / Wr] that suppresses convexity as described above, and by controlling the unit tension within the aforementioned range, both scratch formation and serpentine movement can be suppressed.

[0050] Furthermore, in the continuous annealing heating process, especially in the furnace region where the steel plate temperature reaches 200–300°C, the unit tension applied to the steel plate is preferably set to 1.00 kgf / mm. 2 Above and 1.20 kgf / mm 2 The unit tension is preferably set to 1.00 kgf / mm.2 The above is more preferably set at 1.05 kgf / mm 2 The above. The preferred unit tension is set at 1.20 kgf / mm. 2 Hereinafter, a more preferred value is 1.15 kgf / mm 2 The following is a further explanation. Furthermore, the unit tension is calculated by dividing the value measured by the force sensor by the cross-sectional area of ​​the steel plate passing through. Additionally, this is because, under this condition, when the production line speed is 100 mpm or higher, a reduction in the contact pressure between the steel plate and the roller due to the reduced centering force can be achieved, significantly suppressing scratches. Therefore, in this case, the production line speed is preferably set to 100 mpm or higher. Furthermore, it is preferably set to 130 mpm or lower.

[0051] As mentioned above, the occurrence of snake-like patterns and scratches in steel plates is significant in steel plates with a width greater than 1600 mm, especially in steel plates with a quarter-wave shape. However, the present invention is also effective for steel plates with a quarter-wave shape. In particular, the improvement effect is greater for steel plates with a quarter-wave shape in terms of snake-like pattern suppression.

[0052] In this invention, a quarter-wave steel plate is defined as a steel plate with a shape mode coefficient Λ1 of -0.8 or more and 0.8 or less (but 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 (but excluding those greater than -0.05 and less than 0.05).

[0053] It should be noted that the shape pattern coefficients Λ1 and Λ2 mentioned above do not include "greater than -0.05 and less than 0.05", because the flatness of the steel plate in this range is high, and therefore it is excluded from the range of quarter-wave shape.

[0054] When the steel plate shape pattern coefficient Λ1 is -0.8 or higher and 0.8 or lower (but not greater than -0.05 and less than 0.05), it indicates that the elongation on either side of the steel plate in the width direction is large. For steel plates of this shape, significant serpentine behavior usually occurs, but by applying this invention, serpentine behavior can be significantly reduced. Furthermore, when the steel plate shape pattern coefficient Λ2 is -1.2 or higher and 1.2 or lower (but not greater than -0.05 and less than 0.05), it indicates that the elongation on both sides of the steel plate in the width direction is large (both sides conform to a quarter-wave shape). For steel plates of this shape, significant serpentine behavior usually occurs, but by applying this invention, serpentine behavior can be significantly reduced.

[0055] Whether a steel plate is a quarter-wave shape can be determined, for example, by the following methods (1) or (2).

[0056] (1) The shape of the steel plate is determined by a shape detector, and the shape mode coefficients Λ1 and Λ2 are calculated based on the measurement results.

[0057] (2) Based on past performance (the relationship between manufacturing conditions and shape pattern coefficients Λ1 and Λ2), the shape pattern coefficients Λ1 and Λ2 are obtained from the manufacturing conditions of the steel plate.

[0058] Furthermore, the shape pattern coefficient Λ1 represents the magnitude of unilateral elongation, and Λ2 represents the magnitude of mid-section elongation and end elongation. These coefficients can be evaluated, for example, based on the methods described in References 1 and 2.

[0059] [Reference 1] Japanese Patent Application Publication No. 61-255710

[0060] [Reference 2] Japan Iron and Steel Association, 5th Edition Steel Handbook Committee, ed., *5th Edition Steel Handbook, Volume 2: Rolling and Secondary Processing*, p. 218, Japan Iron and Steel Association, 2014.

[0061] In the case described in (1) above, the shape of the steel sheet can be determined using a shape detector during the period from cold rolling to before it is introduced into the continuous annealing furnace (e.g., when the coil is uncoiled during continuous annealing). Alternatively, the entire length of the coil can be measured, or only a portion of the coil's length direction (e.g., the front end portion) can be measured. The shape detector used and the method for determining the shape of the steel sheet using this shape detector can employ known techniques.

[0062] Next, other manufacturing conditions of the present invention will be described.

[0063] This invention involves continuously annealing cold-rolled steel sheets (cold-rolled steel sheets), but a plating process can also be performed after annealing. That is, steel sheets manufactured using this invention also include plating sheets that undergo plating after annealing. Examples of plating processes include hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, electro-zinc-nickel plating, hot-dip aluminizing, tin plating, hot-dip aluminized silicon plating, and hot-dip zinc-aluminum plating, but are not limited to these.

[0064] The thickness of the steel plate is not particularly limited, and is usually 0.1 to 4.0 mm. The plate thickness is preferably 0.1 mm or more, more preferably 0.3 mm or more, and even more preferably 0.5 mm or more. Furthermore, the plate thickness is preferably 4.0 mm or less, more preferably 1.0 mm or less, and even more preferably 0.8 mm or less.

[0065] Steel plates before cold rolling are manufactured, for example, by continuous casting followed by hot rolling and pickling.

[0066] The chemical composition of the steel plate is not particularly limited. For example, by mass%, it may contain: C: greater than 0% and less than 0.3%, Si: greater than 0% and less than 2%, Mn: greater than 0% and less than 5%, P: greater than 0% and less than 0.2%, S: greater than 0% and less than 0.03%, N: greater than 0% and less than 0.03%, and sol.Al: greater than 0% and less than 0.5%. Preferably, the composition 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.

[0067] Furthermore, depending on the requirements, it may also contain one or more of the following: Nb: greater than 0% and less than 0.3%; Ti: greater than 0% and less than 0.3%; Cr: greater than 0% and less than 1%; Mo: greater than 0% and less than 1%; V: greater than 0% and less than 0.5%; Cu: greater than 0% and less than 1%; Ni: greater than 0% and less than 1%; B: greater than 0% and less than 0.01%; Sn: greater than 0% and less than 0.3%; Sb: greater than 0% and less than 0.3%; Co: greater than 0% and less than 1%; Zr: greater than 0% and less than 0.5%; W: greater than 0% and less than 0.5%; Ca: greater than 0% and less than 0.01%; Mg: greater than 0% and less than 0.01%; REM: greater than 0% and less than 0.01%. The balance is iron and unavoidable impurities.

[0068] The mechanical properties of the steel sheet are effective when the tensile strength (TS) after annealing is 200 MPa or more and 340 MPa or less. That is, TS is preferably 200 MPa or more. In addition, TS is preferably 340 MPa or less.

[0069] Example

[0070] Cold-rolled steel sheets with a width of 1830 mm and a thickness of 0.75 mm are passed through a hot-dip galvanizing production line (CGL) to manufacture alloyed hot-dip galvanized steel sheets. Specifically, the cold-rolled steel sheet is continuously annealed, then hot-dip galvanized, and subsequently alloyed to produce alloyed hot-dip galvanized steel sheets. The throughput speed in the continuous annealing furnace is set at 120 mpm. Regarding the furnace bottom rollers installed in the furnace zone (preheating zone) where the steel sheet temperature reaches 100–400°C during the continuous annealing heating process, the ratio of the crown C (mm) to the roller width Wr (mm) is adjusted by using furnace bottom rollers with adjusted crown dimensions [C / Wr].

[0071] In each embodiment (inventive example, comparative example), the steel plates of group A and group B were subjected to continuous annealing and plating treatment, respectively. The shape mode coefficients of the steel plates were obtained by measuring the shape of the steel plates using a shape detector installed on the downstream side of the rolling mill in the cold rolling production line, and calculating the shape mode coefficients Λ1 and Λ2 based on the measurement results.

[0072] (1) Group A steel plates

[0073] • Steel plate shape: quarter-wave

[0074] Shape mode coefficient Λ1: 0.2–0.8

[0075] Shape mode coefficient Λ2: 0.3~1.1

[0076] • Roll weight: 20 tons

[0077] • Quantity of rolls: 1000 rolls

[0078] (2) Group B steel plates

[0079] • Steel plate shape: flat shape

[0080] Shape mode coefficient Λ1: -0.004 to 0.04

[0081] Shape mode coefficient Λ2: -0.04 to 0.04

[0082] • Roll weight: 20 tons

[0083] • Quantity of rolls: 1000 rolls

[0084] The snake-like amount, average snake-like amount, and snake-like occurrence rate of the steel plate were measured and calculated as follows. The center position of the steel plate was calculated based on the value of the online edge detector, and the snake-like amount was determined by the difference between this center position and the roll center position. This calculation was performed every 10 meters to calculate the average snake-like amount. Additionally, the maximum snake-like amount for each coil was determined, and the snake-like occurrence rate was calculated as "(number of coils with a maximum snake-like amount of 30mm or more) / (total number of coils) × 100%".

[0085] In addition, by using an online surface defect meter and visual inspection by inspectors to check the front and back of each roll of material along its entire length, the occurrence of scratches is investigated, and the scratch incidence rate is calculated as "(number of rolls with scratches) / (total number of rolls) × 100"%.

[0086]

[0087] In Table 1, No. 2 and No. 3 are comparative examples with excessively high [C / Wr] values, representing examples of existing technology. No. 11 is a comparative example with excessively low [C / Wr] values. Furthermore, No. 8 is a comparative example with excessively high unit tension, and No. 9 is a comparative example with excessively low unit tension. These comparative examples all show that the "[Scratch Occurrence Rate] + [Serve Occurrence Rate] (%) of all roll materials" is a relatively high value. In contrast, the "[Scratch Occurrence Rate] + [Serve Occurrence Rate] (%) of all roll materials in the present invention example has a maximum of only 15%, which is suppressed to a low value.

Claims

1. A method for manufacturing a steel plate, wherein the steel plate has a width greater than 1600 mm, When continuously annealing cold-rolled steel sheets, in the furnace region where the steel sheet temperature reaches the range of 100–400°C during the heating process, the ratio of the convexity C (mm) of the furnace bottom roll to the roll width Wr (mm) [C / Wr] is set to be above 0.00025 and below 0.00082, and the unit tension applied to the steel sheet is set to 0.80 kgf / mm. 2 Above and 1.35 kgf / mm 2 the following.

2. The method for manufacturing a steel plate according to claim 1, wherein, The shape mode coefficient Λ1 of the steel plate to be continuously annealed is greater than -0.8 and less than 0.8 (but not greater than -0.05 and less than 0.05), or the shape mode coefficient Λ2 is greater than -1.2 and less than 1.2 (but not greater than -0.05 and less than 0.05).

3. The method for manufacturing the steel plate according to claim 1 or 2, wherein, The ratio of the convexity C (mm) of the furnace bottom roller to the roller width Wr (mm) [C / Wr] is greater than 0.00045 and less than 0.00075.

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