Method for manufacturing steel sheet

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

Steel plate manufacturing method

[0001] The present invention relates to a method for producing a steel sheet by performing continuous annealing after cold rolling, and particularly to a method for producing a steel sheet having a width of more than 1600 mm.

[0002] Generally, steel sheets for automotive exterior panels are produced by cold rolling followed by continuous annealing, and excellent surface appearance quality is required for these steel sheets. However, in continuous annealing furnaces, pickup can occur due to fluctuations in the contact pressure between the steel sheet and the hearth roll (variations within or between steel sheet coils). Pickup is a phenomenon in which easily oxidizable elements contained in the steel sheet, such as Mn and Si, concentrate on the steel sheet surface during the annealing process to form oxides, which then adhere to the hearth roll surface. This pickup can cause scratches on the steel sheet. These scratches are likely to occur in wide steel sheets with a width exceeding 1600 mm for the following reasons, and are particularly likely to become apparent in steel sheets exhibiting a so-called quarter-stretched shape. A quarter-stretched steel sheet generally refers to a steel sheet that is significantly elongated near both left and right quarter positions in the sheet width direction during cold rolling, resulting in a corrugated shape at these positions (a steel sheet with a quarter-stretched shape in the narrow sense). In contrast to this, in the present application, the term "quarter-stretched steel sheet" refers not only to a steel sheet having such a narrow definition of a quarter-stretched shape, but also to a steel sheet having a large elongation and a wave shape near either the left or right 1 / 4 position in the sheet width direction. That is, a steel sheet having a quarter-stretched shape is a steel sheet having a large elongation and a wave shape near either the left or right 1 / 4 position in the sheet width direction.

[0003] Preventing meandering of the steel sheet is a major issue in continuous annealing of steel sheets. Hearth rolls are typically provided with a roll crown (convex crown) to apply a centering force to the steel sheet to prevent meandering (see, for example, Patent Document 1). In wide steel sheets exceeding 1600 mm in width, the centering force due to contact and friction between the steel sheet and the hearth roll increases, resulting in partial regions of high contact pressure between the steel sheet and the hearth roll, making pickup more likely to occur. In particular, when the steel sheet has a quarter-stretched shape, partial regions of high contact pressure between the steel sheet and the hearth roll are particularly likely to occur, making pickup more likely to occur. For this reason, the above-mentioned scratches are likely to occur in wide steel sheets exceeding 1600 mm in width, and particularly in steel sheets with a quarter-stretched shape. Furthermore, when the steel sheet threading speed is 90 mpm or more, the contact surface pressure between the steel sheet and the hearth roll increases, and therefore, scratches are particularly likely to occur along with the generation of pick-up.

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

[0005] However, reducing the steel sheet tension (unit tension) as described in (i) above increases the risk of steel sheet meandering, leading to reduced productivity. Furthermore, the inventors' investigations have revealed that in wide steel sheets with a width exceeding 1600 mm, it is difficult to eliminate pick-up scratches even if the unit tension is reduced. Therefore, it is difficult to simultaneously suppress the occurrence of scratches and meandering using the method described in (i) above.

[0006] Furthermore, the purpose of providing a roll crown to the hearth roll is to prevent meandering. Therefore, according to conventional common technical knowledge, the reduction in the amount of hearth roll crown described in (ii) above is considered to reduce the ability to correct meandering due to a reduction in centering force, similar to a reduction in the tension of the steel sheet, and to increase the risk of meandering.

[0007] Furthermore, as shown in Patent Document 2, it is necessary to impart roughness to the hearth roll to prevent meandering, and therefore, reducing the roll roughness as described in (iii) above reduces the friction on the roll surface and decreases the centering force, thereby increasing the risk of meandering. Furthermore, as a result of studies by the present inventors, it was confirmed that reducing the roll roughness does not sufficiently reduce the occurrence of scratches.

[0008] JP-A-7-138656 JP-A-63-65028

[0009] As described above, when a wide steel sheet having a width exceeding 1600 mm is produced through continuous annealing, it has been difficult to simultaneously suppress the occurrence of scratches and the meandering using conventional techniques.

[0010] Furthermore, in particular, with quarter-stretched wide-width steel sheets (steel sheets), there are areas in the width direction where the contact pressure between the steel sheet and the hearth roll is high, making pickup on the rolls more likely, and therefore the occurrence of scratch marks is particularly pronounced. Furthermore, with quarter-stretched wide-width steel sheets, the difference in centering force in the width direction is large, making the risk of meandering particularly high. For this reason, it has been thought that it is more difficult to simultaneously suppress the occurrence of scratch marks and meandering for quarter-stretched steel sheets.

[0011] Therefore, an object of the present invention is to provide a method for manufacturing a steel sheet that can simultaneously suppress scratches caused by pickup that occurs due to contact and friction between the steel sheet and a hearth roll and suppress meandering of the steel sheet when continuous annealing a cold-rolled steel sheet in order to manufacture a steel sheet having a width exceeding 1600 mm.

[0012] Furthermore, among steel sheets with a width exceeding 1600 mm, quarter-stretched steel sheets are particularly susceptible to both scratches and meandering, and scratches are more likely to occur when the threading speed of the steel sheet in the line is 90 mpm or higher.

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

[0014] In order to solve the above problems, the present inventors have conducted extensive research into a method that can simultaneously suppress the occurrence of scratches and the occurrence of meandering during continuous annealing of steel sheets, and as a result, have obtained the following findings.

[0015] First, the inventors performed a simulation of meandering for a steel plate having a width of 1800 mm to examine the relationship between the crown amount of the hearth roll and meandering. In this simulation, a result different from the expectation was obtained, that is, for a wide steel plate as the target, even if the crown amount of the hearth roll is small to some extent, the influence on meandering is small.

[0016] Furthermore, a simulation of meandering was carried out for a steel plate having a width of 1,800 mm, which simulates a quarter-stretched shape (a shape in which the plate is lifted within 400 mm from the edge, and no centering force is generated), and a similar investigation was carried out. Contrary to conventional common technical knowledge, this simulation showed that by reducing the crown amount of the hearth roll by about 30% from the conventional general crown amount, meandering was reduced compared to when the conventional general crown amount was used. This is thought to be because, since no centering force is generated within 400 mm from the edge, the lower the crown amount, the less the difference in centering force in the plate width direction, and as a result, the occurrence of meandering is suppressed.

[0017] As described above, the simulation of meandering for a steel plate with a width of 1,800 mm yielded the following results that are different from conventional common technical knowledge: (i) For wide steel plates, even if the crown amount of the hearth roll is somewhat small, the effect on meandering is small; and (ii) In particular, for wide steel plates with a quarter elongation shape, reducing the crown amount actually improves meandering.

[0018] On the other hand, the occurrence of scratches in a continuous annealing furnace was investigated, particularly in terms of the location in the continuous annealing furnace where scratches occur. As a result, it was found that the hearth rolls causing scratches were those installed in a steel sheet temperature range of 100 to 400°C.

[0019] Therefore, the balance between roll crown amount and unit tension capable of suppressing both scratch marks and meandering was investigated for steel sheets of various widths (1000 to 1850 mm) using hearth rolls installed in a furnace where the steel sheet temperature is 100 to 400°C. As a result, it was found that for wide steel sheets with a width of more than 1600 mm, both scratch mark occurrence and meandering can be suppressed by using a small roll crown amount, which was previously considered inappropriate due to concerns about meandering, and controlling the unit tension within a predetermined range. Furthermore, the effect is particularly pronounced for steel sheets with a quarter elongation shape, which is inherently prone to scratch marks and meandering. It was found that meandering in particular is reduced compared to the crown amount commonly used in the past. Furthermore, it was found that there is an optimal roll crown amount that can more effectively suppress scratch mark occurrence and meandering, and that optimizing this roll crown amount can increase the steel sheet threading speed.

[0020] The present invention was made based on these findings and is summarized as follows: [1] A method for manufacturing a steel sheet having a width exceeding 1600 mm, wherein, when a cold-rolled steel sheet is continuously annealed, in a furnace region where the steel sheet temperature is in the range of 100 to 400°C in the heating process, the ratio [C / Wr] of the crown amount C (mm) to the roll width Wr (mm) of the hearth roll to be installed is set to 0.00025 or more and 0.00082 or less, and the unit tension applied to the steel sheet is set to 0.80 kgf / mm ２ 1.35kgf / mm or more ２ [2] A method for manufacturing a steel sheet in the manufacturing method of [1] above, wherein the steel sheet to be continuously annealed has a shape mode coefficient Λ1 of -0.8 or more and 0.8 or less (however, excluding more than -0.05 and less than 0.05), or a shape mode coefficient Λ2 of -1.2 or more and 1.2 or less (however, excluding more than -0.05 and less than 0.05). [3] A method for manufacturing a steel sheet in the manufacturing method of [1] above or [2] above, 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.

[0021] According to the present invention, when a cold-rolled steel sheet having a width exceeding 1600 mm is continuously annealed, it is possible to simultaneously suppress the occurrence of scratches due to pickup caused by contact and friction between the steel sheet and a hearth roll, and to suppress meandering of the steel sheet. Furthermore, even in a quarter-elongated steel sheet, which is particularly susceptible to scratches and meandering, it is possible to simultaneously suppress the occurrence of scratches and suppress meandering, and in particular, it is possible to achieve the effect of significantly improving meandering. Therefore, according to the present invention, it is possible to efficiently produce high-quality steel sheet without the occurrence of sheet threading trouble 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 threading speed in the line is 90 mpm or more. As a result, high-quality steel sheet can be produced at high speed without the occurrence of sheet threading trouble due to meandering.

[0023] An explanatory diagram showing the roll (roll barrel) shape of a hearth roll.

[0024] The present invention is a manufacturing method in which cold-rolled steel sheet is subjected to continuous annealing, and is characterized in that the heating step is carried out under specific conditions in order to suppress meandering of the steel sheet and the occurrence of scratches during continuous annealing.

[0025] The occurrence of meandering and scratches in steel sheets during continuous annealing is particularly likely to become apparent in steel sheets with a sheet width exceeding 1600 mm, and the effect of suppressing the occurrence of meandering and scratches according to the present invention is significantly achieved in such wide steel sheets. Therefore, the present invention targets steel sheets with a sheet width exceeding 1600 mm. Furthermore, the target sheet width is preferably 1700 mm or more, and more preferably 1800 mm or more. On the other hand, the upper limit of the sheet width is not particularly limited, but it is preferably 2000 mm or less.

[0026] Pickup, which causes scratches, occurs particularly remarkably in the heating process of continuous annealing, and as mentioned above, it has been found that it occurs due to contact with the hearth roll installed in the furnace region where the steel sheet temperature is 100 to 400°C (region called the preheat zone). It has also been found that the likelihood of scratches occurring is strongly dependent on the contact pressure with the hearth roll installed in the furnace region where the steel sheet temperature ranges. The contact pressure is significantly affected by the crown shape and unit tension of the hearth roll. Similarly, the likelihood of meandering is significantly affected by the crown shape and unit tension of the hearth roll.

[0027] For this reason, in the present invention, when a cold-rolled steel sheet is 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 applied to the steel sheet: 0.80 kgf / mm ２ 1.35kgf / mm or more ２ FIG. 1 below shows a schematic diagram of the planar shape of a hearth roll (roll barrel), which has a convex crown where the outer diameter is larger at the center of the roll and becomes smaller toward the ends of the roll. This convex crown applies a centering force to the steel sheet that comes into contact with the hearth roll, allowing the steel sheet to be automatically centered. FIG. 1 also shows the crown amount C and 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 by C = (D1 - D2) / 2, where D1 is the diameter of the center of the roll barrel and D2 is the diameter of the end of the roll barrel.

[0028] The diameter of the roll barrel 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. D1 is preferably 1200 mm or less, more preferably 1100 mm or less, and even more preferably 1050 mm or less. D2 is preferably 801 mm or more, more preferably 901 mm or more, and even more preferably 951 mm or more. 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 has been 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 that when meandering is particularly problematic, it is necessary to increase the crown amount and centering force. However, the present inventors have studied hearth rolls 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, and have found a different fact. That is, they have found that for steel sheets with a width of more than 1600 mm, even if the crown amount is less than 2.0 mm, the effect on meandering is small, and conversely, for steel sheets with a quarter elongation shape, reducing the crown amount improves meandering. Furthermore, they have found that for steel sheets with a width of more than 1600 mm, reducing the crown amount significantly suppresses the occurrence of scratches.

[0030] Further investigation revealed that the optimum crown amount C of the hearth roll is determined by its relationship with the roll width Wr. Specifically, it was found that both the occurrence of scratches and meandering can be suppressed by setting the ratio [C / Wr] of the crown amount C (mm) to the roll width Wr (mm) to be 0.00025 or more and 0.00082 or less. The ratio [C / Wr] of hearth rolls in conventional techniques exceeds 0.00082 (generally 0.0013 or less). Therefore, this range of 0.00025 or more and 0.00082 or less is smaller than the level of the conventional techniques.

[0031] For the above reasons, in the present invention, the ratio [C / Wr] of the crown amount C (mm) to the roll width Wr (mm) of a hearth roll installed in an area inside a furnace where the steel sheet temperature is in the range of 100 to 400°C in the heating step 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, resulting in meandering. Therefore, the ratio [C / Wr] is set to 0.00025 or more. On the other hand, if the ratio [C / Wr] is more than 0.00082, the crown is too large, resulting in insufficient effect in suppressing scratches. Therefore, the ratio [C / Wr] is set to 0.00082 or less. Here, the crown amount of the hearth roll used in the present invention is smaller than that of a conventional hearth roll. Therefore, although a ratio [C / Wr] of 0.00082 or less is lower than that of a conventional hearth roll, the effect on meandering is small. Furthermore, as will be described later, in the case of a quarter-stretched steel sheet, a ratio [C / Wr] of 0.00082 or less is actually more effective in improving meandering.

[0033] From the viewpoint of further suppressing the occurrence of scratches, the ratio [C / Wr] is preferably set to 0.00045 or more. Also, from the viewpoint of further suppressing the occurrence of scratches, the ratio [C / Wr] is preferably set to 0.00075 or less. Furthermore, by setting the ratio [C / Wr] in this range, the occurrence of scratches can be further suppressed, and therefore the threading speed of the steel sheet can be increased, and it becomes possible to set the threading speed of the steel sheet to 90 mpm or more.

[0034] Here, the hearth roll used in the present invention is not particularly limited in terms of conditions other than the ratio [C / Wr]. However, 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. 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. The roll width Wr is preferably 2000 mm or more, more preferably 2100 mm or more, and even more preferably 2200 mm or more. 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] Normally, the unit tension applied to the steel sheet during the heating process of continuous annealing is 1.0 kgf / mm ２ If the unit tension is significantly lower than this, meandering will occur, while if it is significantly higher than this, scratches will be more likely to occur. For this reason, in the present invention, the unit tension applied to the steel sheet in the furnace region where the steel sheet temperature is in the range of 100 to 400°C in the heating step of continuous annealing is set to 0.80 kgf / mm ２ 1.35kgf / mm or more ２ The unit tension shall be 0.80 kgf / mm or less. ２ or more, 0.90 kgf / mm ２ It is preferable that the pressure is 1.00 kgf / mm or more. ２ It is more preferable that the unit tension is 1.35 kgf / mm or more. ２ or less, 1.25 kgf / mm ２ It is preferable that the resistance be 1.15 kgf / mm or less. ２ More preferably, it is 1.10 kgf / mm or less. ２In the furnace region where the steel sheet temperature, at which scratches are likely to occur, is 100 to 400°C, by using a hearth roll having a specific ratio [C / Wr] that suppresses the crown amount as described above and controlling the unit tension within the above range, it becomes possible to suppress both the occurrence of scratches and meandering.

[0036] In the heating process of continuous annealing, the unit tension applied to the steel sheet is 1.00 kgf / mm, particularly in the furnace region where the temperature of the steel sheet is 200 to 300°C. ２ Over 1.20 kgf / mm ２ The unit tension is preferably 1.00 kgf / mm or less. ２ It is preferable that the pressure is 1.05 kgf / mm or more. ２ It is more preferable that the unit tension is 1.20 kgf / mm or more. ２ It is preferable that the resistance be 1.15 kgf / mm or less. ２ It is more preferable to set it to 1 / 3000 or less. The unit tension is calculated by dividing the value measured by the load cell by the cross-sectional area of ​​the steel sheet that has passed through. This is because, under these conditions, when the line speed is 100 mpm or more, the effect of reducing the contact pressure between the steel sheet and the roll due to the reduction in centering force is obtained, and the effect of suppressing scratches is significantly obtained. Therefore, in this case, the line speed is preferably 100 mpm or more. It is also preferable to set it to 130 mpm or less.

[0037] As described above, the occurrence of meandering and scratches in steel sheets is particularly pronounced in steel sheets having a quarter elongation shape among steel sheets having a width of more than 1600 mm, but the present invention is also effective for steel sheets having a quarter elongation shape. In particular, the effect of improving meandering suppression is greater in steel sheets having a quarter elongation shape.

[0038] Here, in the present 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 more 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 more than −0.05 and less than 0.05).

[0039] As described above, the reason why the range of "greater than -0.05 and less than 0.05" is excluded for both the shape mode coefficients Λ1 and Λ2 is because the flatness of the steel sheet is high in this range, and it is excluded from the range of the quarter elongation shape.

[0040] When the shape mode coefficient Λ1 of the steel sheet is -0.8 or more and 0.8 or less (excluding more than -0.05 and less than 0.05), this indicates a state in which the elongation of the steel sheet is large on either the left or right side in the width direction. In steel sheets with such a shape, meandering usually occurs significantly, but by applying the present invention, this meandering can be significantly reduced. In addition, when the shape mode coefficient Λ2 of the steel sheet is -1.2 or more and 1.2 or less (excluding more than -0.05 and less than 0.05), this indicates a state in which the elongation of the steel sheet is large on both the left and right sides in the width direction (corresponding to a quarter elongation shape on both sides). In steel sheets with such a shape, meandering usually occurs significantly, but by applying the present invention, this meandering can be significantly reduced.

[0041] Whether a steel sheet has a quarter elongation shape can be determined, for example, by the following method (1) or (2): (1) Measure the shape of the steel sheet with a shape detector, and calculate the shape mode coefficients Λ1 and Λ2 based on the measurement results. (2) Determine the shape mode coefficients Λ1 and Λ2 from the manufacturing conditions of the steel sheet based on past performance (the relationship between the manufacturing conditions and the shape mode coefficients Λ1 and Λ2).

[0042] The shape mode coefficient Λ1 represents the magnitude of the one-sided elongation, and Λ2 represents the magnitude of the center elongation and end elongation. These coefficients can be evaluated based on the methods described in References 1 and 2, for example. [Reference 1] JP 61-255710 A [Reference 2] "5th Edition Steel Handbook, Volume 2, Rolling and Secondary Processing," edited by the 5th Edition Steel Handbook Committee of the Iron and Steel Institute of Japan, p. 218, Iron and Steel Institute of Japan, 2014. In the case of (1) above, the steel sheet shape can be measured using a shape detector after cold rolling and before being introduced into a continuous annealing furnace (e.g., during coil rewinding for continuous annealing). In this case, the entire length of the coil can be measured, or only a portion of the coil in the longitudinal direction (e.g., the tip end portion) can be measured. The shape detector used and the method for measuring the steel sheet shape using this shape detector can be any known technology.

[0043] Next, other production conditions of the present invention will be explained.

[0044] In the present invention, a cold-rolled steel sheet (cold-rolled steel sheet) is continuously annealed, but a plating treatment may be performed after annealing. That is, the steel sheet produced by the present invention also includes a plated steel sheet that is plated after annealing. Examples of plating treatments include, but are not limited to, hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, electrozinc-nickel 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 0.1 to 4.0 mm. The thickness is preferably 0.1 mm or more, more preferably 0.3 mm or more, and even more preferably 0.5 mm or more. The thickness is preferably 4.0 mm or less, more preferably 1.0 mm or less, and even more preferably 0.8 mm or less.

[0046] The steel sheet before cold rolling is produced, for example, through continuous casting, hot rolling and pickling.

[0047] The chemical composition of the steel sheet is not particularly limited, but may contain, for example, in mass %, C: more than 0% and 0.3% or less, Si: more than 0% and 2% or less, Mn: more than 0% and 5% or less, P: more than 0% and 0.2% or less, S: more than 0% and 0.03% or less, N: more than 0% and 0.03% or less, and sol. Al: more than 0% and 0.5% or less. Preferably, the contents are 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 may be contained: Nb: more than 0% and 0.3% or less, Ti: more than 0% and 0.3% or less, Cr: more than 0% and 1% or less, Mo: more than 0% and 1% or less, V: more than 0.5% and 0.5% or less, Cu: more than 0% and 1% or less, Ni: more than 0% and 1% or less, B: more than 0.01% and 0.01% or less, Sn: more than 0% and 0.3% or less, Sb: more than 0.3% and 0.3% or less, Co: more than 0% and 1% or less, Zr: more than 0% and 0.5% or less, W: more than 0% and 0.5% or less, Ca: more than 0.01% and 0.01% or less, Mg: more than 0% and 0.01% or less, and REM: more than 0.01% and 0.01% or less. The balance is iron and unavoidable impurities.

[0048] 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, the TS is preferably 200 MPa or more. Also, the TS is preferably 340 MPa or less.

[0049] A cold-rolled steel sheet having a width of 1830 mm and a thickness of 0.75 mm was passed through a hot-dip galvanizing line (CGL) to produce a galvannealed steel sheet. That is, the cold-rolled steel sheet was continuously annealed, hot-dip galvanized, and subsequently alloyed to produce a galvannealed steel sheet. The sheet passing speed in the continuous annealing furnace was 120 mpm. For hearth rolls installed in the furnace region (preheat zone) where the steel sheet temperature is in the range of 100 to 400°C during the heating step of continuous annealing, hearth rolls with adjusted crown amounts were used, thereby adjusting the ratio [C / Wr] of the crown amount C (mm) to the roll width Wr (mm).

[0050] In each example (invention example, comparative example), steel sheets of the following Group A and Group B were each subjected to continuous annealing and plating. 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 sheets of Group A Steel sheet shape: quarter-stretched shape Shape mode coefficient Λ1: 0.2 to 0.8 Shape mode coefficient Λ2: 0.3 to 1.1 Coil weight: 20 tons Number of coils: 1,000 (2) Steel sheets of Group B Steel sheet shape: flat shape Shape mode coefficient Λ1: -0.004 to 0.04 Shape mode coefficient Λ2: -0.04 to 0.04 Coil weight: 20 tons Number of coils: 1,000 The meandering amount, average meandering amount, and meandering occurrence rate of the steel sheets were measured and calculated as follows. The center position of the steel sheet was calculated from the value of the online edge detector, and the meandering amount of the steel sheet was measured from the difference from the center position of the roll. The meandering amount was calculated every 10 m to calculate the average meandering amount. In addition, the maximum meandering amount was determined for each coil, and the meandering occurrence rate was calculated by calculating "(number of coils with a maximum meandering amount of 30 mm or more) / (total number of coils) × 100"%.

[0051] In addition, the presence or absence of scratching was checked by inspecting the front and back surfaces of each coil over its entire length using an online surface defect meter and visual inspection by an inspector, and the scratching occurrence rate was determined by calculating "(number of coils with scratching) / (total number of coils) × 100" percentage.

[0052]

[0053] In Table 1, Nos. 2 and 3 are comparative examples in which the ratio [C / Wr] is too high, and are comparative examples of the level of the prior art. No. 11 is a comparative example in which the ratio [C / Wr] is too low. No. 8 is a comparative example in which the unit tension is too high, and No. 9 is a comparative example in which the unit tension is too low. All of these comparative examples show high values ​​for "[Rate of Scratch Occurrence] of All Coils + [Rate of Meandering Occurrence] (%)." In contrast, in the examples of the present invention, "[Rate of Scratch Occurrence] of All Coils + [Rate of Meandering Occurrence] (%)" is kept low, with a maximum of 15%.

Claims

1. A method for manufacturing steel sheets with a width exceeding 1600 mm, in which, 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) of the hearth roll to the roll width Wr (mm) [C / Wr] is set to 0.00025 or more and 0.00082 or less, and the unit tension applied to the steel sheet is set to 0.80 kgf / mm ２ 1.35kgf / mm or more ２ A method for manufacturing a steel plate as follows:

2. The method for producing 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 more 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 more than -0.05 and less than 0.05).

3. A method for manufacturing a steel sheet according to claim 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.

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