Hot-rolled steel sheet temper rolling method and hot-rolled steel strip production method

Abstract

Provided are: a hot-rolled steel sheet temper rolling method capable of performing temper rolling to form a hot-rolled steel strip having a target shape by correcting a shape defect even in the case of a hot-rolled steel sheet having high strength; and a hot-rolled steel strip production method.　The present invention provides a hot-rolled steel sheet temper rolling method for temper rolling a hot-rolled steel sheet by a temper rolling mill, the method including: an acquisition step for acquiring specifications and temper rolling conditions of a hot-rolled steel sheet to be subjected to temper rolling; a calculation step for determining, on the basis of the specifications and the temper rolling conditions of the hot-rolled steel sheet, the elongation rate, the rolling load, and the bender amount at the start of temper rolling and after the start of temper rolling; and a temper rolling step for temper rolling the hot-rolled steel sheet on the basis of the elongation rate, the rolling load, and the bender amount determined in the calculation step. In the calculation at the start of the temper rolling in the calculation step, the bender amount is calculated so as to be within a predetermined proportion with respect to the upper and lower limit values of the device specification of the temper rolling mill, and the elongation rate is calculated so as to be within the range of a lower limit value determined from the shape of the hot-rolled steel sheet before the temper rolling and a predetermined upper limit value, and, in the calculation after the start of the temper rolling in the calculation step, the bender amount is calculated so as to be within the lower and upper limit values of the device specification of the temper rolling mill, and the elongation rate is calculated so as to be within the range of the lower limit value determined from the shape of the hot-rolled steel sheet before the temper rolling and the predetermined upper limit value.

Description

Temper rolling method for hot-rolled steel sheet and manufacturing method for hot-rolled steel strip

[0001] The present invention relates to a method for temper rolling a hot-rolled steel sheet and a method for producing a hot-rolled steel strip.

[0002] In recent years, customers have been demanding flat steel sheets. For example, tolerances for flattened or wavy steel sheets, such as those shown in Figures 1(a) and 1(b), have become stricter, making quality assurance for flat steel sheet shapes an important issue.

[0003] Poor shape of steel plate can cause threading problems in processes after temper rolling, resulting in reduced yield. For this reason, flat steel plate is required for products other than the final product. In the hot rolling process for producing hot-rolled steel plate, shape control of the hot-rolled steel plate is performed using mechanisms such as work roll benders, work roll shifts, and pair crosses, which are shape control actuators in the rear stands of the finishing mill.

[0004] In this process, steel sheets are generally rolled to have a selvedge wave tendency with a steepness of 1 to 2%, taking into consideration runnability and water-ride characteristics on the run-out table at the exit of the finish rolling process, and winding characteristics on a coiler. Since the shape of the steel sheet changes during winding on a coiler and the cooling process in the coil yard, it is not easy to measure the shape of the steel sheet during the hot rolling process. For this reason, many steel sheets are sent to a temper rolling line to have their shape corrected. In the temper rolling line, steel sheets are temper rolled for the purposes of not only shape correction, but also cutting defects, surface inspection, and removing stretcher strain.

[0005] In general rolling, the difference in elongation that a steel sheet has as a shape defect is relatively smaller than the difference in elongation imparted by rolling. Therefore, under ideal conditions where the work rolls do not deform, the defective shape can be corrected simply by imparting an elongation to the steel sheet through rolling. However, in temper rolling, the elongation imparted to the steel sheet is small, at approximately 0.3 to 2.0%, so only a strain equivalent to the difference in elongation that the steel sheet has as a shape defect can be imparted. Therefore, in order to flatten the shape of a steel sheet after temper rolling, it is necessary to ensure a minimum elongation imparted to the steel sheet over its entire length. Here, elongation (ε) is a quantity that indicates the magnitude of deformation of the steel sheet during the temper rolling process, which is defined by the thickness of the steel sheet before temper rolling and the thickness of the steel strip after temper rolling, as shown in the following equation (1).

[0006] ε = (H ｉｎ -H ｏｕｔ ) x 100 / H ｏｕｔ ... (1) In the above formula (1), ε is the elongation rate (%), and H ｉｎ is the thickness (m) of the steel plate before temper rolling, and H ｏｕｔ is the thickness (m) of the steel strip after temper rolling.

[0007] For this reason, in the temper rolling process, upper and lower limits are set for the elongation to be imparted to the steel sheet, and operation is carried out so that the elongation falls within the upper and lower limit range over the entire length of the steel sheet. However, since the deformation resistance of the steel sheet is strain rate dependent, there is a tendency for operation to be carried out at an elongation close to the upper limit at the tip of the steel sheet where the rolling speed is slow, and at an elongation close to the lower limit at the center of the steel sheet where the rolling speed is fast.

[0008] Under these operating conditions, when the temper rolling speed exceeds 500 mpm or when the upper and lower limits of the elongation percentage are narrow due to customer requests, it becomes difficult to keep the elongation percentage within the target range over the entire length of the steel plate. In this case, the elongation percentage at the head and tail ends of the steel plate is intentionally deviated from the target range, or the elongation percentage is adjusted by manually manipulating the roll gap. However, since the latter method can also cause problems such as meandering and shrinkage when rolling at high speeds, the former method is the most commonly used solution. However, the former method of deviating from the target elongation percentage at the head and tail ends not only reduces the yield, but also causes problems such as a shortened length of the steel strip after temper rolling and reduced productivity due to the long time required to trim off the defective portions.

[0009] In order to solve such problems, Patent Document 1 discloses a temper rolling method in which the elongation, which changes depending on the temper rolling speed, can be controlled within a range of upper and lower limits, while the change in shape due to the change in rolling load can be corrected by operating a work roll bender, which is a shape control actuator.

[0010] Japanese Patent Application Laid-Open No. 2020-32462

[0011] However, in recent years, steel sheets have become increasingly stronger. Therefore, there is a wide range of needs for shape correction by temper rolling, from mild steel, which is a common low-carbon steel, to hard steel, such as high-strength steel sheets with high carbon content and other additives. In particular, high-strength steel sheets are deformed due to the influence of cooling conditions and transformation during coiling after cooling, and the steepness of the steel sheet after hot rolling can be high, exceeding 1 to 2%. In such cases, the technology disclosed in Patent Document 1, which requires that the elongation be within a predetermined upper and lower limit, has a problem in that shape defects may not be corrected even when the shape control actuator is operated under preset or recalculated conditions.

[0012] The present invention aims to solve the above problems and to provide a method for temper rolling a hot-rolled steel sheet and a method for manufacturing a hot-rolled steel strip, which can correct shape defects even in the case of a high-strength hot-rolled steel sheet and temper roll it into a hot-rolled steel strip of a target shape.

[0013] The means for solving the above problems are as follows: [1] A method for temper rolling a hot-rolled steel sheet by temper rolling the hot-rolled steel sheet using a temper rolling mill, the method comprising: an acquisition step of acquiring specifications and temper rolling conditions of the hot-rolled steel sheet to be temper rolled; a calculation step of calculating an elongation, a rolling load, and a bender amount at the start of temper rolling and after the start of temper rolling based on the specifications and temper rolling conditions of the hot-rolled steel sheet; and a temper rolling step of temper rolling the hot-rolled steel sheet using the elongation, the rolling load, and the bender amount calculated in the calculation step, wherein in the calculation at the start of temper rolling in the calculation step, the bender amount is used to calculate the elongation, the rolling load, and the bender amount. a temper rolling method for a hot-rolled steel sheet according to [1], wherein the calculation step calculates the bender amount so that it is within a predetermined ratio of upper and lower limit values ​​of the equipment specifications of the rolling mill, and the elongation is calculated so that it is within a range between a lower limit value determined from the shape of the hot-rolled steel sheet before temper rolling and a predetermined upper limit value, and in the calculation after the start of temper rolling in the calculating step, the bender amount is calculated so that it is within upper and lower limit values ​​of the equipment specifications of the temper rolling mill, and the elongation is calculated so that it is within a range between a lower limit value determined from the shape of the hot-rolled steel sheet before temper rolling and a predetermined upper limit value. [2] The temper rolling method for a hot-rolled steel sheet according to [1], wherein the calculation step calculates an elongation difference using a steepness of the hot-rolled steel sheet before temper rolling, and calculates a lower limit value of the elongation based on the elongation difference. [3] The temper rolling method for a hot-rolled steel sheet according to [2], wherein the calculation step determines the lower limit value of the elongation based on the strength of the hot-rolled steel sheet before temper rolling. [4] A method for producing a hot-rolled steel strip, comprising temper rolling a hot-rolled steel sheet by the temper rolling method for a hot-rolled steel sheet according to any one of [1] to [3] to produce a hot-rolled steel strip.

[0014] In the method for temper rolling a hot-rolled steel sheet according to the present invention, the hot-rolled steel sheet is temper-rolled by calculating the elongation in temper rolling to be equal to or greater than a lower limit determined based on the shape of the hot-rolled steel sheet before temper rolling. This makes it possible to correct shape defects even in high-strength hot-rolled steel sheets, and to produce hot-rolled steel strips with a target shape.

[0015] Fig. 1 is a schematic diagram showing an example of a shape defect of a steel sheet. Fig. 2 is a flow diagram showing a process flow of a temper rolling method for a hot-rolled steel sheet according to an embodiment of the present invention. Fig. 3 is a schematic diagram explaining the crown amount of a hot-rolled steel sheet.

[0016] Hereinafter, the present invention will be described in detail with reference to the embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments. In the method for temper rolling a hot-rolled steel sheet according to this embodiment, a work roll bender, which is generally provided in a temper rolling mill as a shape control actuator, is used.

[0017] The method for temper rolling a hot-rolled steel sheet according to this embodiment includes an acquisition step, a calculation step, and a temper rolling step. The acquisition step is a step of acquiring the specifications and temper rolling conditions of the hot-rolled steel sheet to be temper rolled. The calculation step is a step of determining the elongation at the start of and after the start of temper rolling, the rolling load, and the bender operation amount (hereinafter simply referred to as "bender amount") of the work roll bender based on the specifications of the hot-rolled steel sheet and the temper rolling conditions acquired in the acquisition step. In this calculation step, the elongation at the start of and after the start of temper rolling is calculated so as to be within a range between a lower limit determined from the shape of the hot-rolled steel sheet before temper rolling and a predetermined upper limit.

[0018] The temper rolling process is a process for producing a hot-rolled steel strip by temper rolling a hot-rolled steel sheet with the elongation, rolling load, and bender amount at and after the start of temper rolling that are calculated in the calculation process. In this embodiment, the shape of the hot-rolled steel sheet before temper rolling may be referred to as an entry shape, and the shape of the hot-rolled steel strip after temper rolling may be referred to as an exit shape.

[0019] 2 is a flow chart showing the flow of the acquisition process and the calculation process in the temper rolling method for hot-rolled steel sheet according to the present embodiment. The acquisition process and the calculation process in the temper rolling method for hot-rolled steel sheet according to the present embodiment will be described in detail with reference to FIG.

[0020] First, the specifications of the hot-rolled steel sheet to be temper rolled and the temper rolling conditions are acquired (Step S1). This Step S1 is the acquisition process. The specifications of the hot-rolled steel sheet include at least the thickness, width, crown, tensile strength, and upper and lower limits of the elongation of the hot-rolled steel sheet. The temper rolling conditions include at least the work roll diameter of the temper rolling mill, the entry tension, the exit tension, the rolling speed at the start of rolling, and the rolling speed after the start of rolling. However, instead of the tensile strength of the hot-rolled steel sheet, the yield stress of the hot-rolled steel sheet may be acquired.

[0021] Next, the elongation rate of temper rolling acquired in step S1 is set (step S2), and the rolling load of temper rolling is calculated (step S3). Various prediction formulas are used to calculate the rolling load. In the temper rolling method for a hot-rolled steel sheet according to this embodiment, the rolling load is calculated using the following formula (2) from the viewpoint of calculation time and ease of maintenance.

[0022] In the above formula (2), P Ｒｏｌｌ is the rolling load (tonf), and H ｉｎ is the entry tension (kgf / mm ２ ) ε is the elongation rate (%) of temper rolling, T ｉｎ is the entry tension (kgf / mm ２ ) and T ｏｕｔ is the exit tension (kgf / mm ２ ) R ０ is the reference roll diameter (m), R is the roll diameter (m), and T Ｓ０ is the reference tensile strength (MPa), and T Ｓ is the tensile strength (MPa), and Vel ０ is the reference rolling speed (mpm), and Vel is the rolling speed (mpm). Here, the reference roll diameter, reference tensile strength, and reference rolling speed are the average or representative values ​​of the roll diameter, tensile strength, and rolling speed of steel plates grouped by a predetermined strength level. W is the width (m) of the steel plate. ０ ~a ７ , C １ , C ２ is an adjustment correction coefficient. The reference yield stress and yield stress of the steel plate may be used instead of the reference tensile strength and tensile strength. Among the above symbols, symbols common to formulas (3) to (11) described later have the same meaning.

[0023] The adjustment correction coefficients can be determined from actual values ​​when a hot-rolled steel sheet is reduced by a temper rolling mill. Specifically, the setting values ​​(load, bender) of the actuators of the temper rolling mill and the actual values ​​(load, bender) are acquired, and the adjustment correction coefficients can be determined by fitting so that the least squares value of the error between these values ​​approaches zero. The actual values ​​to be acquired may be those acquired immediately after the start of rolling, and in order to improve accuracy, it is preferable to acquire actual values ​​for multiple points or multiple types of hot-rolled steel sheets. The adjustment correction coefficients in equations (3) to (11) described later can also be determined using a similar procedure. The adjustment correction coefficients are used to improve the accuracy of the setting values ​​of the rolling actuators immediately after the start of reduction of the hot-rolled steel sheet.

[0024] Next, the bender operation amount that will result in a flat delivery profile when temper rolling is performed under the conditions of steps S2 and S3 is calculated (step S4). In the temper rolling method for hot-rolled steel sheet according to this embodiment, the bender operation amount is calculated using the following equations (3) and (4) in consideration of calculation time and ease of maintenance. The bender operation amount that will result in a flat delivery profile is calculated as the minimum amount of the sum of the square of the following equation (3) and the square of the following equation (4), as shown in the following equation (5).

[0025] In the above formulas (3) to (5), P Ｂｅｎ is the vendor volume (tonf / chock), and b ０ ~b ３ , c １ ~c ３ is the adjustment correction coefficient, and Λ ２ is the flatness of the steel plate at the width edge (-), and Λ ４ is the flatness (-) of the steel plate at a position approximately 0.7 times the width edge. (-) means that it is dimensionless.

[0026] Fig. 3 is a schematic diagram illustrating the crown amount of a hot-rolled steel sheet. Cr in the above formulas (3) to (5) is the crown amount (µm) of the inlet-side hot-rolled steel sheet, which is defined as Cr = (Hc - He) / 2 using the dimensions He and Hc shown in Fig. 3.

[0027] The Cr in the above formulas (3) to (5) is preferably measured over the entire longitudinal area in a previous process such as a hot rolling process. When the crown is not measured over the entire longitudinal area, the crown measured at one point in the longitudinal direction may be considered to be formed over the entire area, and the calculation may be performed by reflecting the crown over the entire longitudinal area.

[0028] Next, in step S4, it is confirmed whether the bender amount calculated falls within a predetermined constraint range. This constraint differs between the start of temper rolling and after the start of temper rolling. Therefore, first, it is confirmed whether it is the start of rolling (rolling TOP) (step S5). If it is the rolling TOP (step S5: Yes), it is confirmed whether the bender amount calculated in step S4 falls within ±20% of the upper and lower limit values ​​of the equipment specifications (step S6). The ±20% upper and lower limit values ​​in step S6 are an example of a predetermined ratio with respect to the upper and lower limit values ​​(100%). The ratio is not limited to ±20%, but may be ±10% or ±30%. Here, ± indicates the direction of the load applied when adjusting the bender amount, with the upper limit corresponding to "+" and the lower limit corresponding to "-".

[0029] In this way, at the start of temper rolling, the bender amount is set so as to be within a predetermined ratio of the upper and lower limit values. This makes it possible to avoid the risk that the bender amount will be set to a value close to the upper or lower limit value of the equipment specifications at the start of temper rolling, which will limit the bender operation amount during temper rolling and make it impossible to adjust the bender operation amount for shape correction.

[0030] For example, if the upper and lower limits of the bender amount in the equipment specifications are ±60 tonf / chock, the bender constraint is set to ±10 tonf / chock at the start of temper rolling. In this way, by starting the bender operation amount at the start of temper rolling from about ±10 tonf / chock, the bender amount can be increased to the upper limit of 60 tonf / chock during temper rolling, making it possible to correct the shape of the hot-rolled steel sheet.

[0031] On the other hand, if the rolling is not at the TOP in step S5 (step S5: No), it is confirmed whether the bender amount calculated in step S4 is within the range of the upper and lower limit values ​​of the equipment specifications (step S7). Since the bender amount after temper rolling can be used up to the upper and lower limit values ​​of the equipment specifications, it is sufficient if it is within that range.

[0032] In steps S6 and S7, if the bender operation amount is outside the range of the upper and lower limit values ​​of the bender constraint (steps S6 and S7: No), the rolling force and elongation rate are corrected so that the bender amount falls within the constraint (step S8). In step S8, if the bender operation amount exceeds the upper limit of the bender constraint, the difference ΔP between the bender operation amount calculated in step S4 and the bender constraint is calculated using the following equation (6): Ｂｅｎ On the other hand, if the bender operation amount is smaller than the lower limit of the bender constraint, the difference ΔP between the bender operation amount calculated in step S4 and the bender constraint is calculated using the following equation (7). Ｂｅｎ Calculate.

[0033] In the above formula (6), P Ｂｅｎｕｌ is the upper limit of the vendor constraint, and in the above equation (7), P Ｂｅｎｌｌ is the lower limit of the bender constraint. Next, the rolling load is corrected using the following equation (8), and the corrected elongation is calculated using the following equations (9) to (11).

[0034] In the above formulas (8) to (11), P Ｒｏｌｌ new is the predicted rolling load (tonf) after the correction, and εnew is the elongation rate (%) after the correction. This elongation rate is the elongation rate in which the bender operation amount falls within the range of the upper and lower limit values. C ３ is the adjustment correction coefficient.

[0035] On the other hand, if the bender operation amount is within the range of the upper and lower limit values ​​of the bender constraints (steps S6 and S7: Yes), or after the rolling load and elongation rate are corrected in step S8, the upper and lower limit values ​​of the elongation rate are calculated (step S9).

[0036] The upper and lower limit values ​​of the elongation calculated in step S9 are determined based on the customer's request, the entry thickness of the steel plate, and the target thickness of the steel strip, and the lower limit value is determined based on the entry shape of the hot-rolled steel plate. The feature of the temper rolling method for hot-rolled steel plate according to this embodiment is the calculation of this elongation. For this reason, the calculation method of the upper and lower limit values ​​of the elongation will be described in detail below.

[0037] In calculating the upper and lower limit values ​​of the elongation percentage, it was found that in the case of a high-strength hot-rolled steel sheet with a poor entry shape, the calculation of the elongation percentage by the method disclosed in Patent Document 1 does not correct the poor shape, resulting in a deterioration in yield in the next process or at the customer's site. When the cause of this was investigated, it was found that the higher the strength of the hot-rolled steel sheet and the worse the entry shape, the higher the elongation percentage required for correction by temper rolling, and that the elongation percentage calculated in the conventional method is insufficient.

[0038] That is, in the conventional technology, a target value of the elongation percentage in temper rolling was set in consideration of the required plate thickness tolerance and the load capacity of the mill, and then the upper and lower limits of the elongation percentage were calculated within the range of the plate thickness tolerance while leaving room for shape correction. As a result, in some cases the lower limit of the elongation percentage was 0% or a negative value (if it is negative, it is rounded up to 0%). Therefore, the conventional method of setting the target value of the elongation percentage and the upper and lower limits of the elongation percentage in consideration of control was insufficient for correcting large shape defects in high-strength hot-rolled steel plates.

[0039] Therefore, in the temper rolling method for hot-rolled steel sheet according to this embodiment, the lower limit of the elongation rate is calculated from the entry shape. Specifically, based on the defective shape of the hot-rolled steel sheet, the lower limit is calculated so that the elongation rate can correct the defective shape. As a result, even if the hot-rolled steel sheet is high in strength and has a large defective shape, the defective shape of the steel sheet can be corrected by temper rolling at the elongation rate or higher.

[0040] The lower limit of the elongation is preferably calculated based on the elongation difference of the hot-rolled steel sheet before temper rolling so as to be equal to or greater than the elongation difference. Here, the elongation difference is an index showing the shape of the steel sheet calculated using the steepness of the hot-rolled steel sheet before temper rolling and the following formula (12):

[0041] δ=(π / 2) ２ ×(λ / 100)２ (12) In the above formula (12), δ is the difference in elongation percentage (%), and λ is the steepness (%).

[0042] For example, if the steepness of the hot-rolled steel sheet before temper rolling is 3%, the difference in elongation is calculated to be approximately 0.2%, so in this case, the lower limit of the elongation is 0.2% or more. Furthermore, it is preferable to adjust the lower limit of the elongation in consideration of the strength of the hot-rolled steel sheet. Specifically, even if the hot-rolled steel sheet has a similar degree of defective shape, it is preferable to increase the elongation of the temper rolling if the strength of the hot-rolled steel sheet is high.

[0043] For example, the relationship between the elongation difference and the elongation required to obtain the shape straightening effect for each steel grade of each strength is confirmed in advance through laboratory experiments, and a table summarizing the relationship between the elongation difference and the elongation required to obtain the shape straightening effect for each steel grade is constructed. Using this table, the elongation required to improve the steepness of a hot-rolled steel sheet of a specified steel grade is determined, and this elongation is set as the lower limit. In this way, by determining the elongation in temper rolling to be higher than the elongation difference depending on the strength of the hot-rolled steel sheet, the shape can be changed significantly even if the work roll bender amount is the same, thereby further improving the shape straightening effect of the hot-rolled steel sheet.

[0044] On the other hand, the upper limit of the elongation may be an upper limit determined in advance based on the plate thickness required for the next process or the product, and may be calculated in the same manner as in the method disclosed in Patent Document 1, with the upper limit being approximately 3%. An elongation greater than 3% is not preferable because it may exceed the mill load capacity of the temper rolling mill or may actually worsen the shape of the hot-rolled steel strip after temper rolling. Thus, in step S9, the lower limit of the elongation in temper rolling is calculated from the entry shape, and the upper limit of the elongation in temper rolling is calculated so as to fall within a predetermined range.

[0045] Next, it is confirmed whether the elongation calculated in step S2 or the elongation corrected in step S8 is within the range of the upper and lower limit values ​​of the elongation calculated in step S9 (step S10). If the elongation is outside the range of the upper and lower limit values ​​(step S10: No), the elongation is corrected so that it is within the range of the upper and lower limit values ​​determined in S9, and the rolling load and bending amount are recalculated based on the corrected elongation (step S11).

[0046] On the other hand, if the elongation is within the range of the upper and lower limits (step S9: Yes), or after correcting the elongation in step S11, it is confirmed whether the calculation has been completed up to the tail end of the hot-rolled steel sheet (step S12). If the calculation has been completed up to the tail end of the hot-rolled steel sheet (step S12: Yes), the calculation is terminated. On the other hand, if the calculation has not been completed up to the tail end of the hot-rolled steel sheet (step S12: No), the process returns to step S2, and the same calculation is repeated again from step S2.

[0047] Once the elongation, rolling load, and bender amount at and after the start of temper rolling are determined in this way, a temper rolling process is carried out in which the hot-rolled steel sheet is temper-rolled using a temper rolling mill that reflects these set values. This makes it possible to correct shape defects even in high-strength hot-rolled steel sheets, and to produce hot-rolled steel strips with the desired shape.

[0048] As described above, in the temper rolling method according to this embodiment, in the calculation step, the elongation rate in temper rolling is determined so as to be equal to or greater than the lower limit determined based on the entry shape. This makes it possible to determine the elongation rate, rolling load, and bender amount at the start of temper rolling and after the start of temper rolling that can correct the shape to the target shape. Then, by temper rolling a hot-rolled steel sheet using a temper rolling mill that reflects these, it is possible to correct shape defects even in high-strength hot-rolled steel sheets, and to produce a hot-rolled steel strip with the target shape.

[0049] If the elongation rate is corrected in step S11, there is no solution that can correct the shape of the hot-rolled steel sheet within the constraints of the bender amount. In this case, since a poor delivery shape reduces the yield, the bender amount is set to a value close to the upper limit of the elongation rate determined in step S9, and the bender amount constraint is allowed to be exceeded. In step S12, the end of rolling is determined at the tail end. However, the hot-rolled steel sheet after the start of rolling may be divided into multiple regions in the longitudinal direction, and the above calculation may be performed for each of the multiple divided regions. For example, the number of divisions may be nine. In this way, by dividing the hot-rolled steel sheet into multiple regions in the longitudinal direction, the above calculation can be performed not only for the two regions before and after the start of rolling, but also for each divided region.

[0050] Next, an example will be described in which a hot-rolled steel strip was produced by temper rolling a hot-rolled steel sheet using the following temper rolling mill. The specifications of the temper rolling mill and the dimensions of the hot-rolled steel sheet are as follows.

[0051] <Temper rolling mill> Mill type: 4Hi Backup roll diameter: 1100mm Work roll diameter: 560mm Maximum rolling speed: 800mpm <Hot-rolled steel sheet> Entry thickness: 1.6-3.0mm Sheet width: 800-1200mm Entry tension: 2t Exit tension: 8t Tensile strength: 3 types: 350MPa, 450MPa, 600MPa

[0052] In the comparative example, the elongation percentages at the start of and after the start of temper pass rolling were calculated using the method disclosed in Patent Document 1, i.e., with an upper and lower limit range of ±0.6% from a target value according to customer demand. On the other hand, in the inventive example, in addition to the above ±0.6%, a lower limit value of the elongation percentage was calculated based on the difference in elongation percentage of the hot-rolled steel sheet before temper pass rolling, and the upper and lower limit values ​​of the elongation percentages at the start of and after the start of temper pass rolling were calculated by further restricting them by the lower limit value.

[0053] In the case of soft hot-rolled steel sheets, an upper limit value of the elongation rate was used that prioritized the constraints of the plate thickness tolerance, and in the case of hard steel sheets, an upper limit value of the elongation rate was used that prioritized the mill load capacity. These conditions were applied, and the hot-rolled steel sheets were temper-rolled while controlling the bender amount and rolling load using the method described in Patent Document 1. The shape evaluation index Λ2 (flatness of the steel strip at the width end) was measured for the hot-rolled steel strip after temper rolling. When I-unit was within 10 for a distance of 90% or more of the total length of the steel strip, it was evaluated as very good (◎), and when it was within 20, it was evaluated as good (○). On the other hand, when I-unit was greater than 20 for a distance of 90% or more of the total length of the steel strip, it was evaluated as poor (×) since the shape defect could not be corrected. These evaluation results are shown in Table 1 below.

[0054]

[0055] Nos. 1 to 3 are comparative examples. In the comparative examples, a target elongation value of ±0.6% was calculated from the mill load capacity and the thickness tolerance required by the customer, and this value was used as the upper and lower limits of the elongation. In the steel plate inlet shape, the steepness was small and the steel plate was soft (Nos. 1 and 2), and the shape after temper rolling was very good or good (◎ or ◯). However, in the steel plate with a large steepness and a hardness (No. 3), the shape after temper rolling was poor (×), and shape correction by temper rolling was insufficient.

[0056] Nos. 4 to 10 are examples of the invention. In the examples of the invention, the upper and lower limits of the elongation percentage, which are target values, were calculated taking into consideration the mill load capacity and the thickness tolerance required by the customer, and the lower limit of the elongation percentage was set to be equal to or greater than the elongation percentage difference determined from the steepness of the entry steel plate shape. As a result, regardless of the strength or entry steel plate shape, the shape of the steel strip after temper rolling was very good or good (◎ or ◯), confirming that defective shapes of steel plates can be sufficiently corrected by temper rolling.

[0057] Under condition No. 7, the shape after temper rolling was very good (◎), whereas under condition No. 8, in which the tensile strength of the hot-rolled steel sheet subjected to temper rolling was high, the shape after temper rolling was good (◯). In contrast, under condition No. 10, in which the elongation rate in temper rolling was increased by 0.1% as the tensile strength of the steel sheet increased, the shape after temper rolling was very good (◎). From these results, it was confirmed that, although the delivery shape deteriorates as the strength of the hot-rolled steel sheet subjected to temper rolling increases, the deterioration of the delivery shape can be suppressed by increasing the lower limit of the elongation rate in temper rolling in response to the increase in strength.

[0058] From these results, it was confirmed that by temper rolling a hot-rolled steel sheet with an elongation rate in temper rolling that is equal to or greater than the lower limit determined based on the shape of the hot-rolled steel sheet before temper rolling, it is possible to correct shape defects even in high-strength hot-rolled steel sheets, and to produce hot-rolled steel strips with the desired shape.

Claims

1. A method for temper rolling a hot-rolled steel plate by temper rolling the hot-rolled steel plate using a temper rolling mill, comprising: an acquisition step of acquiring the specifications and temper rolling conditions of the hot-rolled steel plate to be temper rolled; a calculation step of calculating an elongation rate, rolling load and bender amount at the start of temper rolling and after the start of temper rolling based on the specifications and temper rolling conditions of the hot-rolled steel plate; and a temper rolling step of temper rolling the hot-rolled steel plate using the elongation rate, rolling load and bender amount calculated in the calculation step, wherein in the calculation at the start of temper rolling in the calculation step, the bender amount is calculated to be within a predetermined ratio of upper and lower limit values ​​of the equipment specifications of the temper rolling mill, and the elongation rate is calculated to be within a range between a lower limit value determined from the shape of the hot-rolled steel plate before temper rolling and a predetermined upper limit value, In the calculation after the start of temper rolling in the calculation step, the bender amount is calculated so that it is within upper and lower limit values ​​of the equipment specifications of the temper rolling mill, and the elongation is calculated so that it is within a range between a lower limit value determined from the shape of the hot-rolled steel sheet before temper rolling and a predetermined upper limit value.

2. The method for temper rolling hot-rolled steel sheet according to claim 1, wherein in the calculation step, the elongation difference is calculated using the steepness of the hot-rolled steel sheet before temper rolling, and the lower limit value of the elongation is calculated based on the elongation difference.

3. The method for temper rolling a hot-rolled steel sheet according to claim 2, wherein in the calculation step, the lower limit of the elongation is determined based on the strength of the hot-rolled steel sheet before temper rolling.

4. A method for producing a hot-rolled steel strip, comprising temper rolling a hot-rolled steel sheet using the method for temper rolling a hot-rolled steel sheet according to any one of claims 1 to 3 to produce a hot-rolled steel strip.

Images