Batch annealing method for steel sheet coils and cold-rolled steel sheet

Abstract

This invention provides a technology to suppress shape defects that may occur in batch-annealed cold-rolled steel sheets during subsequent processes after batch annealing. [Solution] The batch annealing method for steel plate coils according to the present disclosure includes a step A of raising the ambient temperature inside the inner cover of a batch-type annealing furnace from room temperature to a target temperature of -300°C, a step B of raising it thereafter to a target temperature of -100°C, a step C of raising it thereafter to a target temperature, and a step D of maintaining it at the target temperature, wherein the target temperature is 450°C or more and 750°C or less, and in steps B and C, the difference ΔQ of the decrease in ambient temperature rise Q(t) is -50°C / h or more and less than 0°C / h.

Description

Technical Field

[0001] The present disclosure relates to a batch annealing method for steel sheet coils and cold-rolled steel sheets.

Background Art

[0002] Generally, in batch annealing of steel sheet coils formed by winding steel sheets, in order to shorten the annealing time, the heating temperature or the ambient temperature of the batch annealing furnace is rapidly increased. On the other hand, if the heating temperature or the ambient temperature is rapidly increased in this way, a temperature difference occurs in the steel sheet coil, and it is also known that there are problems caused by performing annealing for a long time in a state where such a temperature difference has occurred.

[0003] For example, in Patent Document 1, a batch annealing furnace having an inner cover, an outer cover located outside the inner cover, and a burner installed on the outer cover is prepared, and a steel sheet coil formed by winding a steel sheet is placed at the upper end in the inner cover, and the inner cover is heated by the burner to anneal the steel sheet coil. A batch annealing method for a steel sheet coil, comprising a temperature rising step of raising the temperature of the entire steel sheet coil from room temperature to a target annealing temperature range, and a soaking step of holding the entire steel sheet coil in the target annealing temperature range. The temperature rising step includes a step of holding the ambient temperature in the inner cover at a first temperature within the target annealing temperature range, a step of lowering the temperature from the first temperature, a step of holding at a second temperature lower than the lower limit temperature of the target annealing temperature range and higher than the temperature of the lowest temperature point of the steel sheet coil, and a step of raising the temperature to a third temperature within the target annealing temperature range. A batch annealing method for a steel sheet coil is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] According to the technology disclosed in Patent Document 1, the temperature difference between the highest temperature point and the lowest temperature point of the steel sheet coil and the time difference during which the highest temperature point and the lowest temperature point are maintained within the target annealing temperature range can be reduced. However, the total time from the start of temperature rise to the end of cooling of the steel sheet coil in the annealing process becomes significantly longer than the conventional conditions, resulting in significant reduction in productivity and significant increase in energy consumption as problems.

[0006] In the process of considering both reducing the temperature difference between the highest temperature point and the lowest temperature point of a steel sheet coil formed by winding a cold-rolled steel sheet and shortening the annealing time, the inventors found that simply suppressing the temperature difference between the highest temperature point and the lowest temperature point of the steel sheet coil in the annealing process tends to make the mechanical properties of the steel sheet coil non-uniform. Furthermore, the inventors newly discovered the problem that shape defects such as waves (ear waves) are likely to occur in the cold-rolled steel sheet during the continuous feeding in the post-process after annealing.

[0007] In view of such circumstances, an object of the present disclosure is to provide a technique for suppressing shape defects that may occur in the post-process after batch annealing in a cold-rolled steel sheet.

Means for Solving the Problems

[0008] The inventors focused on the thermal history of the hot point (HP) and cold point (CP) of the steel sheet coil in the annealing process as the cause of shape defects occurring in the post-process after batch annealing. As a result, it was found that the thermal history around the time when the ambient temperature in the batch annealing furnace reaches near the target temperature is related to this shape defect. Then, through further investigation, the inventors found that by controlling the descending difference ΔQ of the temperature rise Q(t) described below with respect to the ambient temperature in the batch annealing furnace, the non-uniformity of the heat treatment applied to the steel sheet coil can be suppressed, and the shape defects that may occur in the post-process after batch annealing can be suppressed.

[0009] Based on the above findings, the summary structure of this disclosure is as follows: (1) A batch annealing method for steel sheet coils, comprising an inner cover, an outer cover covering the inner cover, and a burner for heating the inner cover, wherein a steel sheet coil, which is made by winding cold-rolled steel sheet into a coil shape and placed inside the inner cover, is annealed using a batch-type annealing furnace, The ambient temperature inside the inner cover is Step A involves raising the temperature from room temperature to the target temperature of -300°C, Subsequently, step B is performed to raise the temperature to the target temperature of -100°C, Subsequently, step C is performed to raise the temperature to the target temperature, Subsequently, step D is performed to maintain the temperature at the target temperature, Includes, The target temperature to be reached is between 450°C and 750°C. A batch annealing method for steel plate coils, characterized in that in steps B and C, the temperature rise Q(t) of the ambient temperature defined by the following equation (1) is used to set the temperature drop difference ΔQ defined by the following equation (2) to be -50°C / h or more and less than 0°C / h. Note Q(t) = T(t) - T(t-1) ... (1) ΔQ={Q(t2)-Q(t1)} / (t2-t1) ···(2) formula However, T(t) is the ambient temperature t time after the start of process A, t1 is the elapsed time from the start of process A until Q(t) begins to decrease, and t2 is the elapsed time from the start of process A until Q(t) decreases to at least 10°C.

[0010] (2) The batch annealing method for steel plate coils according to (1) above, wherein the outer surface temperature of the inner cover heated by the burner is 50°C or less above the target temperature.

[0011] (3) The batch annealing method for steel sheet coils according to (1) or (2) above, wherein the thickness of the cold-rolled steel sheet is 100 μm or less.

[0012] (4) The batch annealing method for steel sheet coils according to any one of (1) to (4) above, wherein steps A to D are performed in a hydrogen atmosphere.

[0013] (5) The batch annealing method for steel plate coils according to (4) above, wherein hydrogen gas is intermittently or continuously injected into the inner cover from the start of step A to the end of step D.

[0014] (6) Step B is to set the ambient temperature Step B1, which raises the temperature to the target temperature of -200°C, Subsequently, step B2 is performed to raise the temperature to the target temperature of -100°C, It further includes, A batch annealing method for steel sheet coils according to any one of (1) to (5) above, wherein the temperature rise Q(t) in step C is smaller than the temperature rise Q(t) in step B1.

[0015] (7) The batch annealing method for steel sheet coils according to (6) above, wherein the temperature rise Q(t) in step C is 25°C or less.

[0016] (8) The temperature rise Q(t) has its maximum value in step A of steps A to D, A batch annealing method for steel sheet coils according to any one of (1) to (7) above, wherein the temperature rise Q(t) in step A is 30°C or more and 100°C or less.

[0017] (9) The batch annealing method for steel sheet coils according to (8) above, wherein the temperature rise Q(t) in the temperature range in step B where the ambient temperature is 300°C or higher has a minimum value of 60°C or less and a maximum value of 65°C or less.

[0018] (10) In a cold-rolled steel sheet with a thickness of 100 μm or less, A cold-rolled steel sheet characterized in that the difference between the yield strength at the center in the width direction of the cold-rolled steel sheet and the yield strength at the edges in the width direction is 30 MPa or less.

[0019] (11) Cold-rolled steel sheet as described in (10) above, having a yield strength of 450 MPa or less. [Effects of the Invention]

[0020] According to one embodiment of the present disclosure, it is possible to suppress shape defects that may occur in batch-annealed cold-rolled steel sheets during subsequent processes after batch annealing. [Brief explanation of the drawing]

[0021] [Figure 1] This is a schematic diagram showing the configuration of a batch-type annealing furnace according to one embodiment of the present disclosure, along with the state of the steel sheet coil. [Figure 2] This graph shows the time evolution of ambient temperature, HP temperature, and CP temperature in Invention Example 1. [Figure 3] This is a graph showing the time change of the temperature rise Q in Invention Example 1. [Figure 4] This graph shows the time-dependent changes in ambient temperature, HP temperature, and CP temperature in Comparative Example 1. [Figure 5] This graph shows the time change of the temperature rise Q in Comparative Example 1. [Figure 6] This is a graph showing the time change of the temperature rise Q in Invention Example 2. [Figure 7] This graph shows the time change of the temperature rise Q in Comparative Example 2. [Modes for carrying out the invention]

[0022] Prior to describing preferred embodiments of this disclosure, we will now describe in detail the experiments conducted by the inventors to achieve both a reduction in the temperature difference between the HP and CP of the steel coil and a reduction in the annealing time.

[0023] <Experiment> In the prior art disclosed in Patent Document 1, the ambient temperature inside the annealing furnace was rapidly increased to a high temperature range in order to shorten the annealing time. However, the temperature of the outer periphery of the steel coil also rises rapidly as a result, and the temperature of the HP of the steel coil rises to a high temperature range close to the target temperature range, although it does not reach the target temperature range. As a result, recrystallization occurs partially before the CP of the steel coil in the HP of the steel coil. Therefore, the inventors investigated a method for controlling the ambient temperature in the annealing process, and in particular investigated a method for controlling it gradually in the high temperature range.

[0024] First, the inventors of this invention have determined that the ambient temperature in the annealing process is Step A involves raising the temperature from room temperature to the target temperature of -300°C, Subsequently, process B involves raising the temperature to the target temperature of -100℃, Subsequently, process C is performed to raise the temperature to the target temperature, Subsequently, process D is performed to maintain the target temperature, The process was divided into four parts. The inventors then attempted to slow down the temperature rise in the high-temperature range by making the temperature rise in process C slower than in the conventional process. As a result, when the HP of the steel coil reached the target temperature, the temperature difference between the HP and CP of the steel coil was sufficiently small at 30°C, and consequently, the difference in the maintenance time around the target temperature between the HP and CP of the steel coil was also reduced. However, it was newly discovered that this alone was insufficient to prevent shape defects such as waves from occurring in the cold-rolled steel sheets during subsequent processes after batch annealing.

[0025] The inventors believed that in order to suppress this shape defect, not only control of the temperature difference between HP and CP of the steel coil was necessary, but also other control measures. After further investigation, they found that there are conditions under which this shape defect can be significantly suppressed even when the maximum temperature difference within the steel coil during the annealing process is around 80°C. Specifically, the inventors focused on the difference in the change in temperature rise between HP and CP of the steel coil. As a result, it was found that this shape defect can be suppressed by controlling the decrease difference corresponding to the slope of the temperature rise when transitioning from process B to process C, compared to the ambient temperature one hour prior. This is thought to be because this control makes it possible to approximate the temperature rise changes of HP and CP of the steel coil, and consequently, the non-uniformity of the heat treatment applied to the steel coil can be suppressed.

[0026] Preferred embodiments of this disclosure will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0027] <Batch-type annealing furnace> Referring to Figure 1, the configuration of the batch-type annealing furnace 100 in this embodiment will be described. The batch-type annealing furnace 100 comprises an inner cover 2, an outer cover 4 that covers the inner cover 2, and a burner 6 that heats the inner cover 2.

[0028] A steel plate coil C is placed inside the inner cover 2. Specifically, a base 8 is provided at the bottom of the inner cover 2, and the steel plate coil C is placed on the base 8 in an up-end configuration. More specifically, two to four steel plate coils C are stacked in the height direction via spacers 10. "Up-end" refers to the state in which the central axis of the steel plate coil C is perpendicular or substantially perpendicular to the ground.

[0029] The burner 6 is positioned so that the flame is directed toward the outer surface of the inner cover 2. Therefore, when the inner cover 2 is heated by the burner 6, more specifically when the outer surface of the inner cover 2 is heated by the burner 6, the ambient temperature inside the inner cover 2 rises due to thermal radiation from the inner cover 2, and the steel plate coil C placed inside the inner cover 2 is indirectly heated. In other words, the ambient temperature in the annealing process, including processes A to D, is controlled by heating the inner cover 2 with the burner 6. The flame power of the burner 6 can be appropriately adjusted according to the target ambient temperature inside the inner cover 2. However, from the viewpoint of making the subsequent temperature drop difference ΔQ more stable at -50°C / h or higher, it is preferable to adjust the flame power of the burner 6 so that the outer surface temperature of the inner cover 2 heated by the burner 6 is 50°C or less above the target temperature. From this viewpoint, conventional control methods such as repeatedly extinguishing and reigniting the burner 6 after raising the outer surface temperature of the inner cover 2 to a considerably high temperature, such as the target temperature +200°C, are undesirable. Note that "external surface temperature" refers to the temperature at the point on the outer surface of the inner cover 2 closest to the burner 6. Furthermore, as will be explained in more detail later, the target temperature is between 450°C and 750°C.

[0030] The arrangement and number of burners 6 are not particularly limited, but from the viewpoint of making the ambient temperature inside the inner cover 2 uniform or substantially uniform, it is preferable to have 2 to 4 burners at the same height and at equal intervals in the circumferential direction of the outer cover 4. Furthermore, from the viewpoint of utilizing the convection of the ambient gas inside the inner cover 2, it is preferable to position the burners 6 below the outer cover 4 so that the flame is emitted toward the height of the inner cover 2 corresponding to the lowest stage of the steel plate coil C.

[0031] The ambient temperature inside the inner cover 2 is measured over time, at least during the annealing process, by a temperature sensor (not shown) placed inside the inner cover 2. The temperature sensor is preferably placed on a base 8 inside the inner cover 2. A known or arbitrary thermocouple can be used as the temperature sensor.

[0032] <Steel plate coil> Referring to Figure 1, the steel sheet coil C in this embodiment will be described. The steel sheet coil C is formed by winding a cold-rolled steel sheet into a coil shape, and has, for example, a cylindrical shape. As described above, a temperature difference occurs in the steel sheet coil C during batch annealing. Specifically, in each of the steel sheet coils C placed on the upper end within the inner cover 2, the position corresponding to HP in Figure 1 is the most prone to temperature increase, and the position corresponding to CP in Figure 1 is the least prone to temperature increase. Hereinafter, this embodiment will be described focusing on HP and CP of the uppermost steel sheet coil C, but this disclosure is not limited thereto.

[0033] The composition of the cold-rolled steel sheet constituting the steel sheet coil C is not particularly limited, but for example, it can have a composition containing, by mass%, C: 0.0001% to 0.15%, Si: 0.001% to 0.5%, Mn: 0.01% to 1.0%, P: 0.001% to 0.05%, S: 0.0001% to 0.02%, Al: 0.0005% to 0.20%, N: 0.0001% to 0.0040%, with the remainder being Fe and unavoidable impurities. The cold-rolled steel sheet according to this embodiment may also contain one or more additional components selected from Ti, Nb, B, Cu, Ni, Sn, and Cr.

[0034] Furthermore, the cold-rolled steel sheet constituting the steel sheet coil C may have a surface treatment layer, such as an iron-nickel alloy layer, formed on at least one surface of the cold-rolled steel sheet. The iron-nickel alloy layer is an alloy layer containing Fe and Ni, and the alloy may be in the form of a solid solution, eutectoid / eutectic, or compound (intermetallic compound), or these may coexist.

[0035] Furthermore, while the thickness of the cold-rolled steel sheet constituting the steel sheet coil C is not particularly limited, the effect of suppressing shape defects that may occur in subsequent processes after batch annealing becomes more pronounced when the thickness is 100 μm or less (i.e., when it is a steel foil). This is because, for steel foil, if the amount of heat transferred by radiation from the surface layer of the sheet is the same as that of a steel sheet, the proportion of the heat added to the total thickness is larger than that of a steel sheet, making it more prone to softening. Also, for the same reason, the properties of steel foil are more easily altered by even slight temperature changes. The thickness of the cold-rolled steel sheet can be, for example, 25 μm or more.

[0036] <Annealing process> Referring to Figure 2, the annealing process in the batch annealing method according to this embodiment will be described in detail. In this embodiment, it is important to control the ambient temperature inside the inner cover 2 as follows. That is, in the batch annealing method according to this embodiment, the ambient temperature inside the inner cover 2 is controlled as follows. Step A involves raising the temperature from room temperature to the target temperature of -300°C, Subsequently, process B involves raising the temperature to the target temperature of -100℃, Subsequently, process C is performed to raise the temperature to the target temperature, Subsequently, process D is performed to maintain the target temperature, This includes the following. The target temperature is the plate temperature required to obtain the desired properties of the steel plate or surface-treated steel plate, and is set appropriately from the range of 450°C to 750°C. Below 450°C, local differences in grain growth are less likely to occur, and the effect of suppressing shape defects that may occur in subsequent processes after batch annealing is less likely to appear. Above 750°C, ΔQ tends to be less than -50°C / h, and local differences in grain growth are more likely to occur. The target temperature is a common value for the ambient temperature and for the HP and CP of the steel plate coil C.

[0037] Process A corresponds to initial heating, process B corresponds to intermediate heating, process C corresponds to final heating, and process D corresponds to maintenance heating. Specifically, process D is the process of maintaining the ambient temperature after the ambient temperature has risen to the target temperature until the CP reaches the target temperature, and the maintenance time can be, for example, 10 minutes or more and 15 hours or less. During process D, the inner cover 2 is continuously heated by the burner 6 to raise the temperature of the CP, but from the viewpoint of avoiding overheating of the HP, it is preferable to control the burner 6 by intermittently extinguishing and reigniting it within the range of target temperature + 20°C or less for both the ambient temperature and the HP, and it is more preferable to keep the temperature below target temperature + 10°C with such control. For example, if the target temperature is 590°C, it is preferable to have a temperature of 590°C or more and 610°C or less, and more preferable to have a temperature of 590°C or more and 600°C or less. Refer to Figure 2, T A This represents the target ambient temperature to be reached in process A, which is 290°C. Also, T B2 This refers to the target ambient temperature to be reached in process B (more specifically, process B2, which will be described later), and in this case it is 490°C. Also, T C T in Figure 2 represents the target ambient temperature to be reached in process C (i.e., the target ambient temperature to be reached in this annealing method), which is 590°C. Therefore, in the example shown in Figure 2, the ambient temperature inside the inner cover 2 is raised from room temperature to 290°C in process A, from 290°C to 490°C in process B, from 490°C to 590°C in process C, and maintained in process D until the uppermost CP of the steel sheet coil C reaches 590°C. Note that T in Figure 2 B1 This refers to the target ambient temperature to be reached in process B1, which will be described later, and in this case is 390°C. However, this disclosure is not limited to these terms.

[0038] Furthermore, the batch annealing method according to this embodiment is characterized in that, in steps B and C, the difference in the decrease of the ambient temperature rise Q ΔQ is -50°C / h or more and less than 0°C / h. The temperature rise Q is defined by the following equation (1), and the decrease difference ΔQ is defined by the following equation (2).

[0039]

number

[0040]

number

[0041] In equation (1) above, T(t) represents the ambient temperature t time after the start of process A. In equation (2) above, t1 represents the elapsed time from the start of process A until Q(t) begins to decrease. Also, t2 represents the elapsed time from the start of process A until Q(t) decreases to at least 10°C.

[0042] Here, the ambient temperature T(t) shows an upward trend globally, even if it fluctuates only slightly locally until it reaches the target temperature. "Locally" refers to the measurement time interval of the ambient temperature T(t), which is, for example, 30 seconds to 5 minutes. "Globally" refers to a time scale that is, for example, 5 to 10 times the measurement time interval. Q(t) also fluctuates only slightly locally, influenced by the ambient temperature T(t), but Q(t) begins to show a downward trend globally after t1 has elapsed since the start of process A. Therefore, t1 represents the elapsed time from the start of process A until Q(t) begins to decline on that time scale, and t2 represents the elapsed time from the start of process A until Q(t) falls to at least 10°C on that time scale.

[0043] <Descent difference ΔQ> The technical significance of setting the aforementioned temperature difference ΔQ to -50°C / h or more and less than 0°C / h will be explained below, with appropriate reference to Invention Example 1 (Figures 2 and 3) and Comparative Example 1 (Figures 4 and 5) described later.

[0044] Referring to Figure 3, from the viewpoint of suppressing shape defects that may occur in cold-rolled steel sheets in the post-annealing process, the Q at HP of the uppermost steel sheet coil C HP The curve shape and the Q at CP of the steel plate coil C located at the top. CPIt is important to approximate the curve shape of. That is, Q HP 's curve shape and Q CP By approximating the curve shapes of and, the thermal history of HP and the thermal history of CP in the recrystallization process and grain growth process in the steel sheet coil C can be approximated, and the non-uniformity of the heat treatment applied to the steel sheet coil C can be suppressed. As a result, the variation in mechanical properties (specifically, yield strength) within the steel sheet coil C is suppressed, and shape defects that may occur in subsequent processes after batch annealing are suppressed.

[0045] Q HP 's curve shape and Q CP To approximate the curve shapes of and, it is important to set the descending difference ΔQ to at least -50 °C / h or more. Here, as shown in FIG. 5, Q HP 's curve shape and Q CP If the curve shapes of and deviate significantly, the recrystallization process and grain growth process in the HP and CP of the steel sheet coil C will differ significantly, and shape defects are likely to occur in subsequent processes after batch annealing. Specifically, in the annealing process of the steel sheet coil C, after recrystallization occurs, grain growth begins. However, as a result of the heating method differing depending on the position in the steel sheet coil C in the case of batch annealing, when the thermal history is different, the position that reaches the recrystallization start temperature earlier and the greater the amount of heat applied after reaching the recrystallization start temperature, the easier the softening of that position progresses. Also, it is considered that the aspect of recrystallization and the aspect of grain growth also differ depending on the way heat is applied in the vicinity of the timing when recrystallization and grain growth start. The cause of this deviation is, as shown in FIG. 5, Q when transitioning from process B to process C HPThe large difference in temperature rise is particularly significant because the HP is located on the outermost surface of the coil and is therefore highly susceptible to the effects of temperature changes in the ambient gas, while the CP is the last to rise in temperature after the surrounding steel plates have warmed up. As a result, the temperature rise is smoothed out, and the temperature rise Q is less affected by rapid changes in ambient temperature. In other words, if the ambient temperature is kept constant until the target temperature is reached, as in the conventional method, the plate temperature at the HP will rise along with the ambient temperature at a relatively large rate, while the plate temperature at the CP will rise at a relatively small rate. In contrast, if the temperature drop difference ΔQ is set to at least -50°C / h, the time change of the temperature rise Q becomes gradual, and the Q when transitioning from process B to process C becomes HP The difference in value can be suppressed. As a result, Q HP The curve shape and Q CP The curve shape can be approximated. The reason why the downward difference ΔQ is set to less than 0°C is that the ambient temperature inside the inner cover 2 tends to rise globally until it reaches the target temperature, but the temperature rise Q of the ambient temperature tends to fall globally when transitioning from process B to process C.

[0046] The following describes an example of a control method for setting the temperature drop difference ΔQ to be between -50°C / h and 0°C / h. However, this disclosure is not limited to this example.

[0047] To keep the temperature drop difference ΔQ between -50°C / h and 0°C / h, it is effective to control the heating rates of processes B (especially process B2, which will be described later) and C. That is, process B is Step B1 involves raising the ambient temperature to the target temperature of -200°C, Subsequently, step B2 is performed to raise the ambient temperature to the target temperature of -100°C, When this is included, in order to suppress the length of the annealing process and reduce the difference in heat between HP and CP so that the temperature drop difference ΔQ is -50°C / h or more, the heating rate D in process B2 is defined by equation (3) below. B2 It is preferable to control the heating rate D within the following range: B2It is preferable to control the temperature between 16°C / h and 50°C / h, more preferably between 16°C / h and 47°C / h, even more preferably between 21°C / h and 45°C / h, and even more preferably between 30°C / h and 45°C / h. Also, the heating rate D in step C, as defined by equation (4) below, is also preferable. C It is effective to control the temperature between 3°C / h and 15°C / h. In Figure 2, the target temperature is 590°C, and the ambient temperature inside the inner cover 2 is raised from 290°C to 390°C in process B1 and from 390°C to 490°C in process B2, but this disclosure is not limited to this.

[0048]

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[0049]

number

[0050] This makes it possible to slow down the temperature rise Q of the ambient temperature, Q HP The curve shape and Q CP Since the curve shape can be approximated, shape defects that may occur in subsequent processes after batch annealing can be suppressed. The heating rate in other processes can be appropriately set according to the composition of the cold-rolled steel sheet, but from the viewpoint of keeping the annealing process time down and keeping the difference in heat between HP and CP down, the heating rate D of process A is set. A For example, it is preferable to set the temperature to 40°C / h or higher and 60°C / h or lower, and more preferably to 42°C / h or higher and 58°C / h or lower. From a similar viewpoint, the heating rate D of step B1 B1 For example, it is preferable to set the temperature between 35°C / h and 60°C / h, and more preferably between 40°C / h and 58°C / h. Here, the heating rate D AThe heating rate D is defined as the temperature difference (°C) between the first time point when process A starts and the ambient temperature is within the range of 40°C to 50°C, and the second time point when process A ends, divided by the elapsed time (h) from the first time point to the second time point. B1 is the heating rate D B2 and D C Similarly, it is defined as the temperature difference (°C) between the start and end of process B1 divided by the time required for process B1 (h). Furthermore, from the viewpoint of avoiding overheating of the steel plate coil C, it is even more preferable to control the outer surface temperature of the inner cover 2 heated by the burner 6 to the target temperature + 50°C or less.

[0051] Furthermore, it is preferable that the ambient temperature continues to rise in steps B1 and B2, and it is even more preferable that there are no steps to maintain or lower the ambient temperature for more than one hour throughout the entirety of step B, and it is even more preferable that step B consists of steps B1 and B2 in which the ambient temperature continues to rise.

[0052] Here, from the viewpoint of obtaining the purification effect of impurities on the surface of the steel sheet coil C by the reducing effect of the atmospheric gas, it is preferable that steps A to D be carried out in a hydrogen atmosphere. Here, when the thickness of the cold-rolled steel sheet constituting the steel sheet coil C is 100 μm or less (i.e., when it is a steel foil), the surface area of ​​the steel sheet coil C is larger than that of a normal steel sheet. Therefore, the carbon on the surface of the steel sheet coil C reacts with hydrogen in the atmospheric gas more than usual, and the hydrogen concentration in the atmospheric gas decreases significantly, especially during the annealing process. This causes the problem of prolonged annealing time if the purging of hydrogen gas into the inner cover 2 is completed in one step before the start of annealing, as in the conventional method. Therefore, when the thickness of the cold-rolled steel sheet constituting the steel sheet coil C is 100 μm or less, it is preferable to intermittently or continuously inject hydrogen gas into the inner cover 2 from the start of step A to the end of step D. This makes it possible to shorten the annealing time while obtaining the purification effect of impurities. From this viewpoint, it is preferable to maintain a hydrogen concentration of 90 volume% or more in the atmosphere during steps A to D, and the flow rate of hydrogen gas is 3 m 3 It is preferable to set it to 5m or more.3 It is more preferable to set the flow rate to 30 m³ or higher. The upper limit of the hydrogen gas flow rate also depends on the equipment volume, but if the flow rate is too high, the gas temperature may drop, so 30 m³ is preferable. 3 It is preferable to keep the flow rate below / h. For example, the hydrogen gas flow rate may be kept within the above range by intermittently injecting a predetermined amount of hydrogen gas at predetermined time intervals (e.g., every hour) from the start of process A to the end of process D. The hydrogen concentration can be measured using a known or any hydrogen concentration meter. The dew point of the atmosphere inside the inner cover 2 at this time can be -20°C or lower.

[0053] However, if hydrogen gas is injected intermittently or continuously, the rate of heating of the ambient temperature will also increase due to the good thermal conductivity of hydrogen gas. Consequently, the temperature rise Q of the ambient temperature will also become steeper. HP The curve shape and Q CP The curve shape deviates significantly. Therefore, in order to slow down the temperature rise Q of the ambient temperature, the heating rate D in process B2 is B2 It is preferable to control the temperature to 50°C / h or less, and more preferably to 45°C / h or less. On the other hand, the heating rate D B2 The lower limit is not particularly limited, but it is preferable to set it to 15°C / h or higher from the viewpoint of shortening the annealing time. The heating rate in the other steps can be set appropriately according to the composition of the cold-rolled steel sheet, etc., as in the method described above.

[0054] This makes it possible to slow down the temperature rise Q of the atmosphere temperature, even when hydrogen gas is injected intermittently or continuously. HP The curve shape and Q CP The curve shape can be approximated. Furthermore, from the viewpoint of avoiding overheating of the steel plate coil C due to the good thermal conductivity of hydrogen gas, it is preferable to control the outer surface temperature of the inner cover 2 heated by the burner 6 to the target temperature + 50°C or less. However, this disclosure is not limited thereto, and steps A to D may be carried out in an atmosphere mainly composed of nitrogen (for example, about 95% by volume of nitrogen). In this case, it is preferable to control the outer surface temperature of the inner cover 2 to the target temperature - 5°C or less.

[0055] <Temperature rise Q> The ambient temperature rise Q is maximized in process A among processes A, B, C, and D, and it is preferable that the temperature rise Q in process A be between 30°C and 100°C. This is because a temperature of 30°C or higher allows for a shorter annealing time, and a temperature of 100°C or lower helps to minimize energy loss.

[0056] The ambient temperature rise Q is preferably such that in the temperature range of process B where the ambient temperature is 300°C or higher, the minimum value is 60°C or less and the maximum value is 65°C or less. HP The curve shape and Q CP This is because it is possible to approximate the curve shape. More specifically, in the temperature range in which recrystallization occurs in the steel plate coil C and the temperature range just before the temperature in which recrystallization occurs, by setting the minimum value of the temperature rise Q to 60°C or less and the maximum value to 65°C or less, it is possible to suppress the temperature rise of HP from becoming too large relative to the temperature rise of CP. As a result, Q HP The curve shape and Q CP This suppresses large differences in the curve shape and reduces the possibility of differences in mechanical properties between HP and CP. From this perspective, it is preferable that the maximum value of the temperature rise Q is 60°C or less, that is, in the temperature range where the ambient temperature of process B is 300°C or higher, the temperature rise Q is 60°C or less. In particular, it is preferable that the minimum value of the temperature rise Q in process B2 is 60°C or less and the maximum value is 65°C or less, and it is more preferable that the maximum value of the temperature rise Q in process B2 is 60°C or less. If the temperature rise Q is greater than 0°C, the ambient temperature will not decrease compared to one hour earlier.

[0057] It is preferable that the ambient temperature rise Q in process C be smaller than the ambient temperature rise Q in process B1. This is to suppress overheating of the steel sheet coil C. From the viewpoint of suppressing overheating, it is preferable that the ambient temperature rise Q in process C be 45°C or less. From the viewpoint of more stably suppressing overheating, it is more preferable to keep the temperature rise Q at 25°C or less during process C, and even more preferable to keep it at 15°C or less. From a similar viewpoint, it is preferable that the ambient temperature rise Q in process C gradually decreases as it approaches process D. For example, it is preferable to gradually decrease the temperature rise Q so that the temperature rise Q immediately after the start of process C is 45°C or less and 25°C or more, and the temperature rise Q immediately before the end of process C is 15°C or less. The temperature rise Q in process B1 can be appropriately set according to the component composition of the cold-rolled steel sheet, but it is preferable to keep it at 30°C or more from the viewpoint of suppressing the length of the annealing process, and it is preferable to keep it at 65°C or less from the viewpoint of suppressing the difference in heat quantity between HP and CP and making the decrease difference ΔQ -50°C / h or more.

[0058] <Effects> The batch annealing method for steel sheet coil C according to the embodiment described above is characterized in that, in steps B and C, the difference ΔQ between the temperature rise Q and the ambient temperature T inside the inner cover 2 is at least -50°C / h. With this configuration, Q HP The curve shape and Q CP Because the curve shape can be approximated, the non-uniformity of the heat treatment applied to the steel plate coil C can be suppressed. As a result, shape defects that may occur in subsequent processes after batch annealing can be suppressed.

[0059] Examples of post-batch annealing processes include, for example, sheet feeding processes that apply forces such as tension, pressure, or stress to cold-rolled steel sheets, or other sheet feeding processes. Examples of the former include shape correction processes, secondary cold rolling, temper rolling, or surface treatment processes such as plating or coating. Examples of the latter include sheet feeding processes such as rewinding or slitting (width adjustment). Sheet feeding processes that apply forces such as tension, pressure, or stress to cold-rolled steel sheets are particularly prone to shape defects. In this disclosure, "secondary cold rolling" refers to cold rolling performed after batch annealing, as opposed to cold rolling performed before batch annealing, and secondary cold rolling may be repeated, or annealing and secondary cold rolling may be repeated.

[0060] <Cold rolled steel plate> The following describes, for example, a cold-rolled steel sheet obtained by a manufacturing method including a batch annealing method for the steel sheet coil C according to the embodiment described above. This cold-rolled steel sheet may be a cold-rolled steel sheet after batch annealing, or a cold-rolled steel sheet that has undergone the post-processing described above after batch annealing.

[0061] The cold-rolled steel sheet according to this embodiment is characterized by having a thickness of 100 μm or less, and in particular by having a difference of 30 MPa or less between the yield strength at the center of the width direction of the cold-rolled steel sheet and the yield strength at the edges in the width direction. "Center of the width direction" means, for example, a region that includes the center of the width of the cold-rolled steel sheet and extends from the center of the width direction toward both sides within 25%, preferably within 20%, of the width (hereinafter, each region will also be referred to as the "center"). "Edge of the width direction" means, for example, a region that extends from one end of the width direction of the cold-rolled steel sheet toward the center of the width direction within 25%, preferably within 20%, of the width (hereinafter, each region will also be referred to as the "one end"), and a region that extends from the other end of the width direction of the cold-rolled steel sheet toward the center of the width direction within 25%, preferably within 20%, of the width (hereinafter, each region will also be referred to as the "other end"). If the plate width is narrow and it is difficult to obtain a test specimen of sufficient area within 20%, the area within 25% may be used for evaluation.

[0062] Such cold-rolled steel sheets can be obtained, for example, by the batch annealing method of the steel sheet coil C described above, Q HP The curve shape and Q CP This is because the curve shape can be approximated to suppress variations in yield strength, which is a mechanical property within the steel sheet coil. With such cold-rolled steel sheets, variations in yield strength are suppressed, which can prevent shape defects that may occur in subsequent processes after batch annealing.

[0063] In addition, the cold-rolled steel sheet according to this embodiment preferably has a thickness of 25 μm or more. Furthermore, the cold-rolled steel sheet according to this embodiment preferably has a yield strength of 450 MPa or less.

[0064] Although one embodiment of the present disclosure has been described in detail above with reference to the attached drawings, the present disclosure is not limited to this embodiment. It is clear to any person with ordinary skill in the art to which the present disclosure belongs that various modifications or alterations can be conceived within the scope of the technical idea set forth in the claims, and these will naturally be understood to fall within the technical scope of the present disclosure. [Examples]

[0065] (Examples of inventions and comparative examples) For each example of the invention and comparative example, a steel sheet coil was prepared by winding a cold-rolled steel sheet into a coil shape, with a composition of C:0.039%, Si:0.02%, Mn:0.19%, P:0.0014%, S:0.0006%, Al:0.046%, and N:0.0023% by mass, with the remainder being Fe and unavoidable impurities. Then, batch annealing was performed using the batch-type annealing furnace shown in Figure 1, under the annealing conditions shown in Table 1. For each example of the invention and comparative example, the coil mass was 15 tons, the coil length was 35,000 m, the coil outer diameter was 1,700 mm, and the coil inner diameter was 508 mm. In addition, for each example of the invention and comparative example, the width of the cold-rolled steel sheet constituting the steel sheet coil was 900 mm, and the thickness was 60 μm. In addition, for each example of the invention and comparative example, the measurement time interval for the ambient temperature, HP temperature, and CP temperature was 1 minute. Also, heating rate D of process AA The ambient temperature at the start of the measurement was 50°C.

[0066] [Table 1]

[0067] In Table 1, T A This refers to the target ambient temperature to be reached in process A, and D A This refers to the heating rate in process A, and Q in process A. min This represents the minimum value of the temperature rise Q in process A, and Q in process A max This represents the maximum value of the temperature rise Q in process A. Also, T B1 This refers to the target ambient temperature to be reached in process B1, and D B1 This refers to the heating rate in process B1, and Q in process B1. min This represents the minimum value of the temperature rise Q in process B1, and Q in process B1 max This represents the maximum value of the temperature rise Q in process B1. Also, T B2 This refers to the target ambient temperature to be reached in process B2, and D B2 This refers to the heating rate in process B2, and Q in process B2. min This represents the minimum value of the temperature rise Q in process B2, and Q in process B2 max This represents the maximum value of the temperature rise Q in process B2. Also, T C This refers to the target ambient temperature to be reached in process C (i.e., the target ambient temperature to be reached in this annealing method), and D C This refers to the heating rate in process C, and Q in process C. min This represents the minimum value of the temperature rise Q in process C, and Q in process C max This represents the maximum value of the temperature rise Q in process C. Also, Q in process D min This represents the minimum value of the temperature rise Q in process D, and Q in process D max This represents the maximum temperature rise Q in process D. The annealing time refers to the elapsed time from the start of process A to the end of process D.

[0068] (Evaluation method: strength of surrender) Cold-rolled steel sheets of Invention Examples 1-1, 1-2, and 1-3, which were batch annealed under the same annealing conditions as Invention Example 1, and cold-rolled steel sheets of Comparative Examples 1-1, 1-2, and 1-3, which were batch annealed under the same annealing conditions as Comparative Example 1, were subjected to secondary cold rolling followed by shape correction. Subsequently, test specimens were taken from the center, one end, and the other end of the final cold-rolled steel sheet in accordance with JIS Z 2241:2022, and the yield strength was evaluated in accordance with the tensile test of the said standard. The test specimens were taken so that the tensile direction was parallel to the rolling direction. A tensile testing machine (ORIENTEC Tensilon RTC-1350A universal material testing machine) was used as the apparatus for the tensile test. The measurement conditions were room temperature and a tensile speed of 10 mm / min. The evaluation results are shown in Table 2.

[0069] In Table 2, "center" refers to the area including the center of the width of the final cold-rolled steel sheet and extending from that center outwards to both sides within 20% of the sheet width; "one end" refers to the area extending from one end in the width direction of the final cold-rolled steel sheet outwards within 20% of the sheet width towards the center of the sheet width; and "other end" refers to the area extending from one end in the width direction of the final cold-rolled steel sheet outwards within 20% of the sheet width towards the center of the sheet width.

[0070] (Evaluation method: Shape evaluation) Cold-rolled steel sheets of Invention Examples 1-1, 1-2, and 1-3, which were batch annealed under the same annealing conditions as Invention Example 1, and cold-rolled steel sheets of Comparative Examples 1-1, 1-2, and 1-3, which were batch annealed under the same annealing conditions as Comparative Example 1, were subjected to secondary cold rolling followed by shape correction. Subsequently, test pieces of a predetermined length were taken from the final cold-rolled steel sheets, and the height of the convex portion, which was shaped as a wave or elongation, was measured with a ruler to evaluate shape defects at the widthwise ends of the cold-rolled steel sheets. A "good" rating was given if the height of the wave or elongation was less than 1.5 mm at both ends of the final cold-rolled steel sheet, and a "poor" rating was given if the height of the wave or elongation was 1.5 mm or more at either end of the final cold-rolled steel sheet. The evaluation results are shown in Table 2.

[0071] [Table 2]

[0072] (Explanation of evaluation results) As is clear from Table 1, in Comparative Example 1, the temperature drop difference ΔQ in processes B and C was -52.0°C / h, which was below -50°C / h. In Comparative Example 2, the temperature drop difference ΔQ in processes B and C was -55.0°C / h, which was below -50°C / h. Therefore, as shown in Figures 5 and 7 respectively, Q HP The curve shape and Q CP The curve shape deviated significantly from the expected shape. As a result, as is clear from Table 2, in Comparative Examples 1-1, 1-2, and 1-3, the difference in yield strength between the central part and one end of the final cold-rolled steel sheet exceeded 30 MPa, and waves or elongation with a height of 1.5 mm or more occurred.

[0073] In contrast, as is clear from Table 1, in Invention Example 1, the temperature drop difference ΔQ in steps B and C was -11.0°C / h, which was -50°C / h or higher. Also, in Invention Example 2, the temperature drop difference ΔQ in steps B and C was -15.0°C / h, which was -50°C / h or higher. Therefore, as shown in Figures 3 and 6 respectively, Q HP The curve shape and Q CP The curve shape was similar to that of the original. As is clear from Table 2, in Invention Examples 1-1, 1-2, and 1-3, the difference in yield strength between the central part and the final cold-rolled steel sheet was 30 MPa or less at both ends, and the shape was also good. Furthermore, when comparing the annealing time at the same target temperature, the extension of the annealing time in Invention Example 1 compared to the annealing time in Comparative Example 1 was within 12% and 30%, respectively, and was not prolonged. [Industrial applicability]

[0074] According to this disclosure, it is possible to suppress shape defects that may occur in batch-annealed cold-rolled steel sheets during subsequent processes after batch annealing. [Explanation of Symbols]

[0075] 100: Batch-type annealing furnace 2: Inner cover 4: Outer cover 6: Burner 8: Pedestal 10: Spacer C: Steel plate coil

Claims

1. In a batch annealing method for steel sheet coils, a batch-type annealing furnace comprising an inner cover, an outer cover covering the inner cover, and a burner for heating the inner cover is used to anneal a steel sheet coil, which is made by winding cold-rolled steel sheet into a coil shape and placed inside the inner cover, The ambient temperature inside the inner cover is Step A involves raising the temperature from room temperature to the target temperature of -300°C, Subsequently, step B is performed to raise the temperature to the target temperature -100°C, Subsequently, step C is performed to raise the temperature to the target temperature, Subsequently, step D is performed to maintain the temperature at the target temperature, Includes, The target temperature to be reached is between 450°C and 750°C. A batch annealing method for steel plate coils, characterized in that in steps B and C, the temperature rise Q(t) of the ambient temperature defined by the following equation (1) is used to set the temperature drop difference ΔQ defined by the following equation (2) to be -50°C / h or more and less than 0°C / h. Note Q(t)=T(t)-T(t-1)...Equation (1) ΔQ={Q(t2)-Q(t1)} / (t2-t1)...Equation (2) However, T(t) is the ambient temperature t time after the start of process A, t1 is the elapsed time from the start of process A until Q(t) begins to decrease, and t2 is the elapsed time from the start of process A until Q(t) decreases to at least 10°C.

2. The batch annealing method for steel plate coils according to claim 1, wherein the outer surface temperature of the inner cover heated by the burner is 50°C or less above the target temperature.

3. The batch annealing method for steel sheet coils according to claim 1, wherein the thickness of the cold-rolled steel sheet is 100 μm or less.

4. The batch annealing method for steel sheet coils according to claim 3, wherein steps A to D are performed in a hydrogen atmosphere.

5. The batch annealing method for steel plate coils according to claim 4, wherein hydrogen gas is intermittently or continuously injected into the inner cover from the start of step A to the end of step D.

6. Step B is to change the ambient temperature Step B1 involves raising the temperature to the target temperature -200°C, Subsequently, step B2 is performed to raise the temperature to the target temperature -100°C, It further includes, The batch annealing method for steel sheet coils according to claim 1, wherein the temperature rise Q(t) in step C is smaller than the temperature rise Q(t) in step B1.

7. The batch annealing method for steel sheet coils according to claim 6, wherein the temperature rise Q(t) in step C is 25°C or less.

8. The temperature rise Q(t) has its maximum value in step A of steps A to D, The batch annealing method for steel plate coils according to claim 1, wherein the temperature rise Q(t) in step A is 30°C or more and 100°C or less.

9. The batch annealing method for steel sheet coils according to claim 8, wherein in step B, the temperature rise Q(t) in the temperature range where the ambient temperature is 300°C or higher has a minimum value of 60°C or less and a maximum value of 65°C or less.

10. In cold-rolled steel sheets with a thickness of 100 μm or less, A cold-rolled steel sheet characterized in that the difference between the yield strength at the center in the width direction of the cold-rolled steel sheet and the yield strength at the edges in the width direction is 30 MPa or less.

11. The cold-rolled steel sheet according to claim 10, wherein the yield strength is 450 MPa or less.

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