Manufacturing method of hot rolled steel sheet

Abstract

To provide a method capable of increasing productivity by enabling a pickling load to be reduced because the occurrence of internal oxidation of high-tension steel sheet can be inhibited by a totally new method.SOLUTION: A method of manufacturing a hot-rolled steel sheet is characterized as follows: a steel material including 0.5-2.0 mass% of Si, 1.0-3.0 mass% of Mn and 0.1-2.0 mass% of Al is hot-rolled to be turned into the hot-rolled steel sheet; and an Sn inclusion adheres to a surface of the hot-rolled steel sheet prior to a roll-up step in and after a final descaling step, when the hot-rolled steel sheet is rolled up in a coil form.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a hot-rolled steel sheet, and more particularly to a method for suppressing internal oxidation of a high-tensile steel sheet containing Si, Mn, and Al. [Background technology]

[0002] In recent years, the use of high-strength steel sheets in automobiles has been increasing in order to achieve both weight reduction and crashworthiness. High-strength steel sheets often contain relatively large amounts of easily oxidizable elements such as Si, Mn, and Al to ensure strength.

[0003] In steel sheets that contain a lot of Si, Mn, and Al, an abnormal layer called an internal oxide layer forms in the base steel just below the scale during the hot rolling process. The internal oxide layer is formed by selective oxidation of any one or a combination of elements that are particularly susceptible to oxidation: Si, Mn, and Al. The higher the steel sheet temperature is and the longer it is kept at high temperature, the thicker the internal oxide layer grows.

[0004] The internal oxide layer is difficult to remove by pickling and requires a large number of steps in the pickling process, which is a factor that significantly reduces the productivity of hot-rolled steel sheets. Furthermore, if the internal oxide layer remains, it may cause a decrease in the surface quality and weldability of the hot-rolled steel sheet after pickling, and may also cause defects during the production of cold-rolled steel sheets using the hot-rolled steel sheet as a raw material, which may also cause deterioration in the chemical conversion treatability and platability of the cold-rolled steel sheet.

[0005] Patent Document 1 discloses that by adding appropriate amounts of elements such as Sb, Sn, and Te to the slab composition of a hot-rolled steel sheet, these elements are concentrated at the interface between the scale and the base steel during hot rolling to form a concentrated layer (barrier layer), thereby making it possible to suppress the formation of an internal oxide layer in the hot-rolled steel sheet. However, although there is no problem if the content of these elements such as Sb, Sn, and Te is appropriate, if the content is too high, the ductility of the steel may decrease, and thus the formability may decrease.

[0006] Patent Document 2 is based on the idea that internal oxidation of the steel surface can be suppressed by suppressing decarburization, and discloses that when a hot-rolled steel sheet is wound into a coil, a carburizing agent is placed on the steel sheet surface, which carburizes the steel surface and suppresses decarburization of the steel sheet surface, thereby suppressing internal oxidation.

[0007] Patent Document 3 is based on the idea that the progress of internal oxidation on the surface of steel material progresses via CO and CO2 gases, and that the progress of internal oxidation can be suppressed by forcibly excluding these CO and CO2. It discloses that by inserting spacers between the plates of a coil, the CO and CO2 gases remaining between the plates wound in a coil can be discharged, and as a result, internal oxidation can be suppressed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2017-31454 A [Patent Document 2] JP 2017-124424 A [Patent Document 3] JP 2019-84546 A Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, high-tensile steel coiled at high temperature has a problem that its pickling property is very poor, which inhibits productivity. In order to solve this problem, various studies have been made. Patent Document 1 attempts to suppress the formation of an internal oxidation layer in a hot-rolled steel sheet by adding an appropriate amount of elements such as Sb, Sn, and Te to the slab composition of the hot-rolled steel sheet. Patent Document 2 attempts to suppress internal oxidation by disposing a carburizing agent on the surface of the steel sheet when the hot-rolled steel sheet is coiled. And Patent Document 3 attempts to suppress internal oxidation by discharging CO and CO2 gases remaining between the sheets wound in a coil by inserting a spacer between the sheets of the coil. Thus, various studies have been made to suppress internal oxidation, but further improvements and development of different methods are still required.

[0010] The present invention aims to provide a method that can suppress the occurrence of internal oxidation of high tensile steel sheet, reduce the load of pickling, and improve productivity by using a completely new technique that has not been known previously. [Means for solving the problem]

[0011] The present invention provides the following means. [1] A method for producing a hot-rolled steel sheet, comprising hot-rolling a steel material containing, by mass, 0.5-2.0% Si, 1.0-3.0% Mn, and 0.1-2.0% Al to obtain a hot-rolled steel sheet, and when winding the hot-rolled steel sheet into a coil, adhering an Sn-containing substance to the surface of the hot-rolled steel sheet after the final descaling step and before the coiling step. [2] The manufacturing method of the hot-rolled steel sheet according to [1], characterized in that a foil of the Sn-containing material is supplied to the surface of the hot-rolled steel sheet after the steel material is rolled by a finishing rolling mill and cooled by a run-out table (ROT) and before being wound by a down coiler, and the foil is wound into a coil together with the hot-rolled steel sheet. [3] The method for producing hot-rolled steel sheet according to [1], characterized in that the powder of the Sn-containing material is scattered on the surface of the hot-rolled steel sheet after the steel material is rolled by a finishing rolling mill and cooled by a run-out table (ROT) and before being wound by a down coiler, and the powder is wound into a coil together with the hot-rolled steel sheet. [4] The method for producing a hot-rolled steel sheet according to [1], characterized in that the liquid containing the Sn-containing substance is sprayed onto the surface of the hot-rolled steel sheet at at least one location of the entry side of a finishing rolling mill, between the stands of the finishing rolling mill, the exit side of the finishing rolling mill, the run-out table (ROT), and before winding by a down coiler, and then wound into a coil together with the hot-rolled steel sheet. Effect of the Invention

[0012] According to the present invention, it is possible to suppress the generation of internal oxidation in high tensile steel sheets. With this reduction in internal oxidation, the load of pickling can be reduced, and productivity can also be improved. In addition, since internal oxidation can be suppressed, there is no need to lower the coiling temperature of the steel sheet (in other words, the coiling temperature of the steel sheet can be increased), and it is possible to reduce the rolling load in the subsequent cold rolling process. Furthermore, since the embodiment of the present invention does not add elements to the steel material (slab) for suppressing internal oxidation, it is also possible to eliminate the influence of such elements on the mechanical properties of the steel material, such as the workability. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram conceptually showing the progress of internal oxidation of Si, Mn, and Al. [Diagram 2] FIG. 2 is a cross-sectional view of the test material used in the examples. [Diagram 3] FIG. 3 is a diagram showing how various substances are attached to the surface of a steel sheet using a jig in the examples. [Figure 4] Figure 4 is a chart summarizing the maximum internal oxidation depth for each deposited element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present invention provides a method for producing a hot-rolled steel sheet, which is characterized in that when a steel material containing, by mass, 0.5-2.0% Si, 1.0-3.0% Mn, and 0.1-2.0% Al is hot-rolled and wound into a coil, a Sn-containing substance is adhered to the surface of the steel sheet after the final descaling step and before the coiling step.

[0015] The steel sheet targeted by the present invention is a steel sheet containing 0.5-2.0 mass% Si, 1.0-3.0 mass% Mn, and 0.1-2.0 mass% Al, which is a so-called high-tensile steel. From the viewpoint of the quality of high-tensile steel, the contents of each element of Si, Mn, and Al are specified, but on the other hand, these elements can cause internal oxidation. That is, Si is an element added to ensure strength, but a Si content of 0.5% or more by mass may cause internal oxidation. In addition, a high Si content may deteriorate workability, so the Si content is set to 2.0 mass% or less. Mn is also an element added to improve strength, but a Mn content of 1.0% or more by mass may cause internal oxidation. In addition, a high Mn content may deteriorate workability, so the Mn content is set to 3.0 mass% or less. Al is also an element added to improve strength and corrosion resistance, but a Al content of 0.1% or more by mass may cause internal oxidation. In addition, since a high Al content can also deteriorate workability, the Al content is set to 2.0 mass% or less. Therefore, the present invention targets a steel sheet containing 0.5 to 2.0 mass% Si, 1.0 to 3.0 mass% Mn, and 0.1 to 2.0 mass% Al.

[0016] Steel containing Si, Mn and Al in the above content is so-called high tensile steel, which is hard at low temperatures and difficult to cold roll, and is quickly coiled after hot rolling. Therefore, the coiling temperature of high tensile steel is typically 600°C to 900°C, and the above coiling temperature can be appropriately adopted in one embodiment of the present invention. Generally, if the temperature is less than 600°C, the steel sheet may become hard and coiling may become difficult. Therefore, the lower limit of the coiling temperature may be 600°C. On the other hand, the higher the coiling temperature, the softer the steel sheet becomes, and coiling becomes easier. Therefore, in order to facilitate coiling, coiling may be performed at 650°C or higher. However, if the coiling temperature is too high, the ease of coiling will reach a plateau and heating energy will be wasted, so the upper limit of the coiling temperature may be 900°C.

[0017] In the coiling temperature range described above, in a normal high tensile steel (that is, a high tensile steel not treated with the special measures of the present invention etc.), internal oxidation of Si, Mn and Al occurs in the coiled state.

[0018] Here, the mechanism by which internal oxidation occurs will be explained. This has been conventionally known as follows. That is, in the above-mentioned coiling temperature range, Si, Mn, and Al are easily oxidized when oxygen is supplied. The steel sheet is in contact with the air before coiling, and oxide scale is already present on the surface of the coiled steel sheet. The oxide scale on the surface of the coiled steel sheet serves as the oxygen source, and the oxygen diffuses inward into the steel and reacts with Mn, Si, and Al in the steel to form oxides. These oxides that form inside the steel are called internal oxides. To help understand, the process is shown diagrammatically in Figure 1.

[0019] Thus, internal oxidation is a state in which oxides of Si, Mn, and Al are dispersed inside the steel in its metallic state from the surface of the steel to the surface. The oxides of Si, Mn, and Al may be individual oxides, or may be oxides of two or more of them combined. These oxides may be distributed at the grain boundaries of the steel, or may be distributed within the grains. When oxides are distributed at the grain boundaries, they are sometimes called grain boundary oxidation, but here we will refer to this condition as internal oxidation as well.

[0020] The internal oxide layer formed according to this mechanism is a state in which oxides are dispersed inside the steel deeper than the surface oxide scale, and a long period of pickling is required to remove it.

[0021] As a result of intensive research, the present inventors have found that internal oxidation can be suppressed by adding Sn from the outside to the surface of the hot-rolled steel sheet before coiling the hot-rolled steel sheet. Although not wishing to be bound by a particular theory, the following is considered. Since the melting point of Sn is about 232°C and the coiling temperature of a normal hot-rolled steel sheet is 600°C or higher, Sn added to the surface of the hot-rolled steel sheet easily melts. On the other hand, an oxide scale layer is formed on the surface of the hot-rolled steel sheet, and this oxide scale layer is a layer formed by external oxidation of Fe in the base material of the hot-rolled steel sheet, and its structure is relatively coarse and dense compared to the base material of the hot-rolled steel sheet. Therefore, the Sn added from the outside melts and penetrates the coarse and dense oxide scale, and reaches the interface between the oxide scale and the base material. Since the base material is relatively dense, Sn does not penetrate into the inside of the base material. In this way, Sn is concentrated and present at the interface between the base material of the hot-rolled steel sheet and the oxide scale. In this case, even if oxygen is supplied from the oxide scale, the Sn concentrated at the interface acts as an oxygen diffusion barrier and inhibits the oxygen from diffusing into the base material, thereby inhibiting internal oxidation.

[0022] In fact, the inventors conducted laboratory experiments to experimentally reproduce the heat treatment that generates internal oxidation, assuming coiling in hot rolling. Specifically, the heat treatment was performed with Sn foil sandwiched between overlapping steel sheets. As a result, it was confirmed that Sn derived from the sandwiched Sn foil, i.e., Sn added from outside, was concentrated at the interface between the scale and base material of the steel sheet, and that internal oxidation was suppressed in the base material of the steel sheet.

[0023] Based on the above findings, in one embodiment of the present invention, when a hot-rolled steel sheet is wound into a coil, an Sn-containing substance is adhered to the surface of the hot-rolled steel sheet after the final descaling step and before the coiling step.

[0024] Typically, a method for producing a hot-rolled steel sheet includes the following steps in the following order, and these steps can be appropriately adopted in one embodiment of the present invention. Even if some of these steps are omitted, the effects of the present invention can be obtained as long as the conditions of the present invention are satisfied. (Heating Step) In the heating step, the steel material is heated as necessary so as to obtain a desired steel sheet structure. (Descaling process) In this process, scale formed on the surface of the steel material is removed as necessary. (Rough rolling process) In this process, the steel material is roughly rolled into a rough bar using a rough rolling mill. The rough rolling mill may be equipped with a plurality of roll pairs (stands) arranged in a row, or may be of the reverse type. (Final descaling step) In this step, scale formed on the surface of the rough bar is removed. If scale remains, it may cause indentation marks in the subsequent finish rolling step. (Finish rolling process) In this process, the descaled rough bar is roughly rolled using a finishing rolling mill to produce a hot-rolled sheet (also called a hot-rolled steel sheet). The finishing rolling mill is exemplified by one equipped with a plurality of roll pairs (stands) arranged in a row. (Cooling step) In this step, the finish-rolled hot-rolled sheet is cooled on a run-out table (ROT) so as to obtain a desired steel sheet structure. (Coiling process) In this process, the cooled hot-rolled sheet (hot-rolled steel sheet) is wound into a coil using a down coiler.

[0025] The timing for attaching the Sn-containing material to the surface of the hot-rolled steel sheet is limited to after the final descaling process and before the coiling process. This is because if the Sn-containing material is attached before the final descaling process, the Sn-containing material will be removed by the subsequent final descaling process, making it difficult to effectively attach the Sn-containing material to the surface of the hot-rolled steel sheet. On the other hand, by attaching the Sn-containing material to the surface of the rolled steel sheet before the coiling process, the Sn-containing material can be wound into a coil together with the hot-rolled steel sheet.

[0026] The Sn inclusions attached to the surface of the hot-rolled steel sheet are present between the hot-rolled steel sheets wound into a coil. The melting point of Sn is about 232°C, and since the coiling temperature of normal hot-rolled steel sheets is 600°C or higher, the Sn in the Sn inclusions attached to the surface of the hot-rolled steel sheet can easily melt. On the other hand, a layer of oxide scale is formed on the surface of the hot-rolled steel sheet, and this oxide scale layer is a layer formed by the external oxidation of Fe in the base material of the hot-rolled steel sheet, and its structure is relatively coarse and dense compared to the base material of the hot-rolled steel sheet. Therefore, the Sn in the Sn inclusions attached to the surface of the hot-rolled steel sheet melts and penetrates the coarse and dense oxide scale, and reaches the interface between the oxide scale and the base material. Since the base material is relatively dense, Sn does not penetrate into the inside of the base material. In this way, Sn is concentrated and present at the interface between the base material of the hot-rolled steel sheet and the oxide scale. Therefore, even if oxygen is supplied from the oxide scale, the Sn concentrated at the interface acts as a diffusion barrier for the oxygen and inhibits the oxygen from diffusing into the base material, thereby inhibiting internal oxidation.

[0027] The Sn-containing material is not particularly limited as long as the Sn in the Sn-containing material melts at the winding temperature and the effect of the present invention can be obtained, and may be metallic Sn or a Sn compound. Metallic tin may be tin alone or a tin alloy. As the tin alloy, solder, bronze, pewter, Babbitt metal, Wood's alloy, Galinstan, etc. may be used. As the Sn compound, tin chloride, tin oxide, tin sulfide, tin sulfate, etc. may be used.

[0028] The form of the Sn-containing material is not particularly limited as long as the effects of the present invention can be obtained, and it may be in the form of foil, powder, liquid, or the like.

[0029] When the Sn-containing material is in the form of a foil, the foil of the Sn-containing material may be supplied to the surface of the hot-rolled steel sheet after cooling by the ROT and before winding by the down coiler, and may be wound into a coil together with the hot-rolled steel sheet. At least a part of supplying the foil of the Sn-containing material and winding the foil of the Sn-containing material together with the hot-rolled steel sheet may be performed using a laminate winding device. The Sn of the wound foil-like Sn-containing material melts and concentrates at the interface between the base material of the hot-rolled steel sheet and the oxide scale, acting as an oxygen diffusion barrier and suppressing internal oxidation. The thickness of the foil may be appropriately adjusted so that it can be wound together with the hot-rolled steel sheet. In addition, the planar dimensions of the foil may be appropriately adjusted depending on the range in which internal oxidation is to be suppressed, and may be approximately the same as the planar dimensions of the hot-rolled steel sheet. In this case, it is possible to suppress internal oxidation on almost the entire plane of the hot-rolled steel sheet. The ends of the hot-rolled steel sheet in the sheet width direction may be trimmed (cut), and the planar dimensions of the foil may be adjusted according to the location to be trimmed to reduce costs.

[0030] When the Sn-containing material is in powder form, the powder of the Sn-containing material may be scattered on the surface of the hot-rolled steel sheet after cooling by the ROT and before winding by the down coiler, and the hot-rolled steel sheet may be wound into a coil shape together with the hot-rolled steel sheet. The powder of the Sn-containing material may be scattered using a powder spraying device. The Sn in the wound powder of the Sn-containing material melts and is concentrated at the interface between the base material of the hot-rolled steel sheet and the oxide scale, acting as an oxygen diffusion barrier and suppressing internal oxidation. The shape and dimensions of the powder may be appropriately adjusted so that the desired scattering can be achieved and so that the hot-rolled steel sheet can be wound together with the hot-rolled steel sheet. In addition, the range of scattering of the powder may be appropriately adjusted according to the range in which internal oxidation is desired to be suppressed, and may be approximately the same as the planar dimensions of the hot-rolled steel sheet. In this case, it is possible to suppress internal oxidation in almost the entire plane of the hot-rolled steel sheet. The end of the hot-rolled steel sheet in the sheet width direction may be trimmed (cut), and the range of scattering of the powder may be adjusted according to the trimmed portion to reduce costs.

[0031] When the Sn-containing material is in liquid form, the liquid containing the Sn-containing material may be sprayed onto the surface of the hot-rolled steel sheet at at least one of the following locations: the entry side of the finishing rolling mill, between the stands of the finishing rolling mill, the exit side of the finishing rolling mill, the run-out table (ROT), and before winding by the down coiler, and may be wound into a coil together with the hot-rolled steel sheet. The locations listed above are generally equipped with equipment capable of supplying cooling water, lubricants, etc. to the steel sheet and equipment in the manufacture of the hot-rolled steel sheet, and the liquid Sn-containing material may be sprayed alone or together with cooling water, lubricants, etc. using such equipment. The Sn in the wound liquid Sn-containing material melts and concentrates at the interface between the base material and the oxide scale of the hot-rolled steel sheet, acting as a diffusion barrier for oxygen and suppressing internal oxidation. The viscosity and concentration of the liquid may be appropriately adjusted so that the desired spraying can be performed and so that the liquid can be wound together with the hot-rolled steel sheet. The liquid spray range may be adjusted as appropriate depending on the range in which internal oxidation is desired to be suppressed, and may be approximately the same as the planar dimensions of the hot-rolled steel sheet. In this case, it is possible to suppress internal oxidation over almost the entire planar surface of the hot-rolled steel sheet. The ends of the hot-rolled steel sheet in the sheet width direction may be trimmed (cut), and the liquid spray range may be adjusted depending on the trimmed area to reduce costs.

[0032] The presence or absence of internal oxidation can be determined as follows: After the coil has cooled to room temperature, it is retrieved, a sample piece is cut out from the coil, and the cross section of the sample piece is observed with a SEM (scanning electron microscope), and mapping and quantitative analysis of Si, O (oxygen), Mn, Al, and Fe are performed with an EPMA (electron probe micro analyzer) to observe the condition of the internal oxidation layer. In this specification, if the deepest point of the internal oxidation is 0.5 μm or less from the scale / steel interface, the internal oxidation layer is evaluated as having no problems.

[0033] In an embodiment of the present invention, a Sn-containing material is attached to the surface of a hot-rolled steel sheet and the hot-rolled steel sheet is wound into a coil. The Sn in the Sn-containing material melts and concentrates at the interface between the base material of the hot-rolled steel sheet and the oxide scale, acting as an oxygen diffusion barrier and suppressing the formation of internal oxidation in the hot-rolled steel sheet. This in turn reduces the load of pickling and improves productivity. EXAMPLES

[0034] The present invention will be described below with reference to examples. However, the present invention should not be construed as being limited by the examples.

[0035] The test material used was a hot-rolled steel sheet of 440 MPa-class TRIP steel (1.2% Si by mass, 1.3% Mn by mass, 0.3% Al by mass, 2.4 mm thick). This steel type was manufactured at a CT of 400°C, so no internal oxidation occurred, but an oxide scale was present on the surface of the steel sheet (Fig. 2). This hot-rolled steel sheet was cut into 20 mm squares, and two sheets were stacked together. Elements that are expected to have an inhibitory effect based on past studies were attached between the two sheets under the conditions shown in Table 1, and the sheets were held in place with a jig (Fig. 3) in an infrared imaging furnace with N2 gas flow at 700°C for 1 hour. The stack was then cooled and removed, and its cross section was observed using a SEM to confirm the state of internal oxidation. The internal oxidation was dispersed in dots in the base material of the hot-rolled steel sheet, more specifically, near the interface between the base material of the hot-rolled steel sheet and the oxide scale, and could be identified on the SEM screen.

[0036] [Table 1]

[0037] Based on the results of SEM observations of internal oxidation, the maximum internal oxidation depth within the observation field is organized by element in Figure 4. The state of internal oxidation changes depending on the type of element, and compared to the case where only black scale material is overlaid and no element is used as the standard, the maximum internal oxidation depth when C is attached is slightly shallower, and when Sn is attached it is less than half of the case where no element is used, demonstrating a high internal oxidation suppression effect. On the other hand, when Al is attached, the maximum internal oxidation depth is more than twice as deep.

[0038] More detailed observation of Sn adhesion, which has a large effect in suppressing internal oxidation, revealed that it had an effect of suppressing internal oxidation not only in the depth direction of the observed cross section, but also in the width direction. Specifically, in the cases of all materials other than Sn, the internal oxidation spread continuously in the horizontal direction of the observed cross section along the interface between the base material of the hot-rolled steel sheet and the oxide scale, but in the case of Sn, the internal oxidation did not spread in the horizontal direction, and some areas were found where the internal oxidation was completely suppressed.

[0039] As described above, it was confirmed that internal oxidation can be suppressed extremely effectively by attaching elements, especially Sn, to the steel sheet surface. To further consider the mechanism of internal oxidation suppression by attaching elements to the steel sheet surface, element mapping and quantitative analysis of Fe, Sn, Si, Mn, Al, O (oxygen), etc. were performed by EPMA analysis. As a result, it was confirmed that Sn was concentrated at the scale / base metal interface. From this, it is presumed that the Sn element attached to the steel sheet surface migrated to the scale / base metal interface and acted as an oxygen diffusion barrier, which is the same effect as that of the previously known addition of Sn to the base metal (slab composition).

Claims

1. A method for producing a high tensile steel sheet, comprising hot rolling a steel material containing, by mass%, 0.5 to 2.0% Si, 1.0 to 3.0% Mn, and 0.1 to 2.0% Al to obtain a hot rolled steel sheet, and when winding the hot rolled steel sheet into a coil, adhering a Sn-containing substance to a surface of the hot rolled steel sheet after a final descaling step and before a coiling step.

2. 2. The method for producing a high tensile steel sheet according to claim 1, characterized in that a foil of the Sn-containing material is supplied to a surface of the hot-rolled steel sheet after the steel material is rolled by a finishing rolling mill and cooled by a run-out table (ROT) and before being wound by a down coiler, and the foil is wound into a coil together with the hot-rolled steel sheet.

3. 2. The method for producing a high tensile steel sheet according to claim 1, characterized in that the powder of the Sn-containing material is scattered on a surface of the hot-rolled steel sheet after the steel material is rolled by a finishing rolling mill and cooled by a run-out table (ROT) and before being wound by a down coiler, and the powder is wound into a coil together with the hot-rolled steel sheet.

4. 2. The method for producing a high tensile steel sheet according to claim 1, characterized in that the liquid containing the Sn-containing substance is sprayed onto a surface of the hot-rolled steel sheet at at least one location selected from the entry side of a finishing rolling mill, between the stands of the finishing rolling mill, the exit side of the finishing rolling mill, a run-out table (ROT), and before winding by a down coiler, and then wound into a coil together with the hot-rolled steel sheet.

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