Method for manufacturing hot-rolled steel sheet and hot-rolled steel sheet

Abstract

This method for manufacturing a hot-rolled steel sheet includes: a step for obtaining a rough-rolled steel sheet by hot rough-rolling a slab; a step for obtaining a rolled steel sheet by hot finish-rolling the rough-rolled steel sheet; a step for winding the rolled steel sheet in a coil shape; and a step for cooling the wound coil at an average cooling rate of 0.5°C / min to 0.7°C / min in a temperature range from the winding temperature of the coil to 500°C. The hot-rolling parameter defined by formula (1) below is more than 3.95. T1: rough-rolling outlet side temperature (K), T 2: finish-rolling inlet side temperature (K), T3: finish-rolling outlet side temperature (K), T4: cooling intermediate temperature (K), T5: coil winding temperature (K), t: sheet passing time (seconds) from the start of rough-rolling to winding in a coil shape, Q: 125140 [J / mol], and R: 8.314 [J / (K mol)].

Description

Hot-rolled steel sheet manufacturing method and hot-rolled steel sheet

[0001] The present disclosure relates to a method for manufacturing a hot-rolled steel sheet and a hot-rolled steel sheet.

[0002] There has been a demand for improved occupant safety in vehicles, and for this purpose, the strength of vehicle bodies has been improved. On the other hand, against the backdrop of worsening issues such as global warming, efforts to improve the fuel efficiency of automobiles are accelerating. It is known that reducing the weight of vehicle bodies is an effective way to improve fuel efficiency.

[0003] In order to achieve collision safety while reducing the weight of automobiles, steel sheets have been further strengthened, and for example, high-strength cold-rolled steel sheets with tensile strengths of 980 MPa or higher are being put into practical use. In order to achieve a tensile strength of 980 MPa or higher, it is necessary to add solid-solution strengthening elements to the steel, and examples of the solid-solution strengthening elements include Si and Mn.

[0004] In the production of high-strength steel sheets containing solute Si, a slab obtained by casting is subjected to hot rolling, pickling, cold rolling, and annealing. A scale layer made of iron-based oxides is formed on the surface of the hot-rolled steel sheet obtained by hot rolling, and when the hot-rolled steel sheet is then wound into a coil at a high temperature, a hot-rolled steel sheet 1 having a layer structure as shown schematically in Fig. 1 is obtained.

[0005] The hot-rolled steel sheet 1 shown in Fig. 1 includes a steel sheet body 20 and a scale layer 10 covering a surface 20a of the steel sheet body 20. The steel sheet body 20 mainly includes a steel sheet base 21 containing Si element in a solid solution state, and a steel sheet base 21 containing Si element in a solid solution state at least in part as SiO ２ The scale layer 10 is composed of an internal oxide layer 22 containing Si in a state (oxide state). It is preferable that at least half of all the Si elements contained in the internal oxide layer 22 are in an oxide state, and it is particularly preferable that all the Si elements are in an oxide state. However, Si in a solid solution state may also be present in the internal oxide layer 22. The internal oxide layer 22 is formed on the surface 20a side of the steel plate body 20, and the steel plate substrate 21 is covered with the internal oxide layer 22. The scale layer 10 is composed of an iron oxide layer 11 containing iron-based oxides and a reduced iron layer 12 made of reduced iron.

[0006] Such a layer structure is formed because Si, an easily oxidizable element, is dissolved in the steel sheet. During and after hot rolling, an iron oxide layer 11 is formed on the surface of the hot-rolled steel sheet. When the hot-rolled steel sheet is wound around a coil while still at a high temperature and slowly cooled after hot rolling, oxygen (O) contained in the iron oxide layer 11 diffuses toward the inside of the steel sheet. Since Si has a higher oxygen affinity than iron, the diffused oxygen combines with Si dissolved in the steel sheet to form SiO ２ This results in SiO ２ An internal oxide layer 22 containing Si is formed. Meanwhile, oxygen in the iron oxide layer 11 bonds with the dissolved Si, thereby reducing a portion of the iron oxide layer 11. As a result, a reduced iron layer 12 is formed on the surface of the iron oxide layer 11.

[0007] A hot-rolled steel sheet 1 is pickled before cold rolling to remove a scale layer 10. Insufficient removal of the scale layer 10 by the pickling treatment can cause poor appearance of the final product (cold-rolled steel sheet) and adversely affect optional surface treatments performed after cold rolling. In particular, a problem has arisen in that the reduced iron layer 12 in the scale layer 10 remains without being completely removed by the pickling treatment. Therefore, a method for reliably removing the reduced iron layer 12 by the pickling treatment has been sought. For example, techniques are known for preventing the reduced iron layer 12 from remaining after the pickling treatment by suppressing the amount of reduced iron produced and thinning the reduced iron layer 12 (e.g., Patent Documents 1 and 2).

[0008] On the other hand, when zinc plating is performed as a surface treatment for a cold-rolled steel sheet, if the amount of dissolved Si on the surface of the cold-rolled steel sheet is large, the galvanizability is reduced. In order to improve the galvanizability, it is effective for the surface of the cold-rolled steel sheet to be covered with an internal oxide layer 22 (for example, Patent Document 2). The dissolved Si in the internal oxide layer 22 is at least partially oxidized to SiO ２ As a result, the concentration of solute Si in the internal oxide layer 22 is reduced, improving the galvanizability of the cold-rolled steel sheet.

[0009] JP 2017-222887 A JP 2022-136964 A

[0010] As described above, the internal oxide layer 22 is formed by winding a hot-rolled steel sheet into a coil while it is still at a high temperature and slowly cooling it. At this time, the cooling rate is faster in the portions of the coil that are more exposed to the outside air (the edge portions in the coil width direction) than in other portions (the central portion in the coil width direction), and the internal oxide layer 22 is less likely to form. That is, compared to the central portion in the coil width direction of the hot-rolled steel sheet, the edge portions in the coil width direction may have less of the internal oxide layer 22. As a result, a large amount of solute Si may remain on the surface 20a of the steel sheet body 20. Therefore, when a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet is galvanized, the galvanizability of the edge portions may be reduced, which may result in poor plating (uneven plating).

[0011] To eliminate uneven coating, it is sufficient to form an internal oxide layer 22 of sufficient thickness even at the edge portions of the hot-rolled steel sheet in the coil width direction. However, forming a thick internal oxide layer 22 means that more oxygen is supplied from the iron oxide layer 11, which results in more reduced iron being generated and an increase in the thickness of the reduced iron layer 12. The increase in the thickness of the reduced iron layer 12 causes a problem in that the reduced iron layer 12 is more likely to remain after pickling of the hot-rolled steel sheet (i.e., the pickling ability is reduced). In this specification, the degree to which the scale layer 10 (particularly the reduced iron layer 12) can be easily removed by pickling is referred to as "pickling ability."

[0012] Thus, if the internal oxide layer 22 of the hot-rolled steel sheet is thickened in order to improve the galvanizability of the cold-rolled steel sheet, the reduced iron layer 12 becomes thicker, and the pickling ability of the hot-rolled steel sheet deteriorates. Therefore, a method is needed that can easily remove even a thick reduced iron layer 12 by pickling. However, according to conventional technical common sense, the means for improving the pickling ability of a hot-rolled steel sheet is to make the reduced iron layer 12 thinner (Patent Documents 1 and 2), and no study has been conducted on a means for improving the pickling ability of a hot-rolled steel sheet having a thick reduced iron layer 12.

[0013] Therefore, in order to provide a hot-rolled steel sheet having both excellent galvanizability and excellent pickling properties, an object of the present invention is to provide a hot-rolled steel sheet that can achieve excellent pickling properties even when the reduced iron layer 12 is thick, and a method for manufacturing the same.

[0014] A first aspect of the present invention is a method for producing a hot-rolled steel sheet, the method comprising the steps of: rough hot rolling a slab to obtain a rough-rolled steel sheet; finish hot rolling the rough-rolled steel sheet to obtain a rolled steel sheet; winding the rolled steel sheet into a coil; and cooling the wound coil at an average cooling rate of 0.5°C / min to 0.7°C / min in a temperature range from the coiling temperature of the coil to 500°C, wherein the hot-rolling parameter defined by the following formula (1) is greater than 3.95. Here, T １ : Rough rolling exit temperature (K), T ２ : Finish rolling entry temperature (K), T ３ : Finish rolling exit temperature (K), T ４ : Cooling intermediate temperature (K), T ５ : coil winding temperature (K), t: sheet threading time (seconds) from the start of the rough rolling to being wound into a coil, Q: 125140 [J / mol], and R: 8.314 [J / (K mol)].

[0015] Aspect 2 of the present invention is the T ４ is in the range of 1073.15K to 1173.15K.

[0016] Aspect 3 of the present invention is the T １ is in the range of 1373.15K to 1473.15K.

[0017] A fourth aspect of the present invention is the method for producing a hot-rolled steel sheet according to any one of the first to third aspects, wherein the chemical composition of the slab satisfies the following: C: 0.08% by mass or more and 0.30% by mass or less; Si: more than 0.5% by mass and 3.0% by mass or less; Mn: 1.5% by mass or more and 3.0% by mass or less; Cr: more than 0% by mass and 1.0% by mass or less; P: more than 0% by mass and 0.10% by mass or less; S: more than 0% by mass and 0.05% by mass or less; Al: more than 0% by mass and 1.0% by mass or less; and N: more than 0% by mass and 0.010% by mass or less, with the balance being Fe and inevitable impurities.

[0018] A fifth aspect of the present invention is a hot-rolled steel plate comprising a steel plate body and a scale layer covering a surface of the steel plate body, wherein the steel plate body is located on the surface side, and at least a part of Si element is SiO ２ and a steel plate base material covered with the internal oxide layer and containing Si element in a state of solid solution Si, wherein the scale layer comprises an iron oxide layer in contact with the surface of the steel plate body and a reduced iron layer covering the iron oxide layer, wherein, in a cross-sectional view in the thickness direction, the average thickness of the internal oxide layer is 10.0 μm or more, and a ratio of the area of ​​the reduced iron layer to the area of ​​the scale layer is less than 29%.

[0019] A sixth aspect of the present invention is the hot-rolled steel sheet according to the fifth aspect, wherein the chemical composition satisfies the following: C: 0.08% by mass or more and 0.30% by mass or less; Si: more than 0.5% by mass and 3.0% by mass or less; Mn: 1.5% by mass or more and 3.0% by mass or less; Cr: more than 0% by mass and 1.0% by mass or less; P: more than 0% by mass and 0.10% by mass or less; S: more than 0% by mass and 0.05% by mass or less; Al: more than 0% by mass and 1.0% by mass or less; and N: more than 0% by mass and 0.010% by mass or less, with the balance being Fe and inevitable impurities.

[0020] According to an embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having excellent plating properties and pickling properties, and a method for manufacturing the same.

[0021] Fig. 1 is a schematic cross-sectional view of a hot-rolled steel sheet according to an embodiment of the present invention. Fig. 2 is a schematic cross-sectional SEM image of a hot-rolled steel sheet produced in Example Test No. 5. Fig. 3 is a graph showing the relationship between the reduced iron rate and the lowest pickling temperature (minimum pickling temperature) at which the reduced iron layer can be completely removed.

[0022] The present inventors have conducted extensive research into a hot-rolled steel sheet 1 ( FIG. 1 ) including a steel sheet body 20 and a scale layer 10, to find a means for improving galvanizability by thickening the internal oxide layer 22 provided on the surface side of the steel sheet body 20, while ensuring sufficient pickling ability even when the reduced iron layer 12 included in the scale layer 10 is thick. As a result, the present inventors have found for the first time that a factor determining pickling ability is the content of the reduced iron layer 12 in the scale layer 10 (reduced iron rate), and that pickling ability can be improved by keeping the reduced iron rate low, thereby completing the present invention. Hereinafter, a hot-rolled steel sheet 1 according to an embodiment of the present invention will be described with reference to the drawings.

[0023] <Hot-rolled steel sheet 1> Fig. 1 is a schematic diagram showing a cross section in the thickness direction of a hot-rolled steel sheet 1. The hot-rolled steel sheet 1 comprises a steel sheet body 20 and a scale layer 10 covering a surface 20a of the steel sheet body 20. The steel sheet body 20 includes an internal oxide layer 22 provided on the surface 20a side, and a steel sheet base material 21 covered by the internal oxide layer 22. The scale layer 10 includes an iron oxide layer 11 in contact with the surface 20a of the steel sheet body 20, and a reduced iron layer 12 covering the iron oxide layer 11.

[0024] The average value (average thickness) of the thickness 22t of the internal oxidation layer 22 measured in a cross section in the thickness direction (a cross section perpendicular to the surface of the hot-rolled steel sheet 1) is set to 10.0 μm or more. Since the amount of solute Si on the surface 20a of the steel sheet body 20 can be sufficiently reduced, the galvanization properties of the cold-rolled steel sheet obtained from the hot-rolled steel sheet 1 can be improved. The average thickness of the internal oxidation layer 22 is preferably 10.5 μm or more, and more preferably 11.0 μm or more. The upper limit of the thickness of the internal oxidation layer 22 is not particularly limited from the viewpoint of galvanization properties. However, if the internal oxidation layer 22 becomes thick, the reduced iron layer 12 becomes thick, which may affect pickling properties. Therefore, the thickness of the internal oxidation layer 22 is preferably 13.0 μm or less, and more preferably 12.5 μm or less.

[0025] The thickness of the internal oxide layer 22 is measured in a cross section perpendicular to the rolling direction, near the center in the width direction of the hot-rolled steel sheet 1. The thickness of the internal oxide layer 22 will be described in detail later.

[0026] The ratio of the area of ​​the reduced iron layer 12 to the area of ​​the scale layer 10 in the cross section in the thickness direction (reduced iron rate) is set to less than 29%. By keeping the reduced iron rate below 29%, pickling properties can be improved even if the reduced iron layer 12 is thick. The reduced iron rate is measured near the center in the width direction of the hot-rolled steel sheet 1 in the cross section perpendicular to the rolling direction. The reduced iron rate will be described in detail later.

[0027] The "reduced iron rate" in this specification refers to the ratio of the area of ​​the reduced iron layer 12 to the area of ​​the scale layer 10, which is taken as 100%, in a cross-sectional view of the hot-rolled steel sheet 1 (for example, a cross-sectional SEM image of the hot-rolled steel sheet 1), and can be expressed by the following formula (2): reduced iron rate (%) = S １ / S ０ × 100... (2) where S ０ (μm ２ ) is the area of ​​the scale layer 10 in a cross-sectional view, and S １ (μm ２ ) is the area of ​​the reduced iron layer 12 in a cross-sectional view.

[0028] The thickness of the reduced iron layer 12 is correlated with the thickness of the internal oxide layer 22. Therefore, in order to make the internal oxide layer 22 thicker than a certain thickness, the reduced iron layer 12 cannot be made thinner. The thickness of the reduced iron layer 12 is determined by the area S of the reduced iron layer 12 in a cross-sectional view. １ Therefore, in order to ensure a sufficient internal oxidation layer 22, the area S of the reduced iron layer 12 is approximately proportional to １ The area S of the reduced iron layer 12 must be set to a certain value. １ In order to suppress the reduced iron rate to less than 29% while increasing the area S of the scale layer 10 to a certain extent, ０ The area S of the scale layer 10 can be increased. ０ corresponds to the sum of the area of ​​the reduced iron layer 12 and the area of ​​the iron oxide layer 11 in a cross-sectional view. Therefore, in order to suppress the reduced iron rate, it is necessary to increase the area of ​​the iron oxide layer 11 in a cross-sectional view. In other words, if the iron oxide layer 11 is formed thick to increase the overall thickness of the scale layer 10, the reduced iron rate can be reduced even if the reduced iron layer 12 is thick.

[0029] The relationship between the reduced iron ratio and the pickling property will now be described in detail. FIG. 3 is a graph showing the relationship between the reduced iron ratio and the pickling property, created based on data from the examples described later. The lowest pickling temperature at which the reduced iron layer 12 can be completely removed (referred to as the "minimum pickling temperature") is used as an indicator of the pickling property. A 10% hydrochloric acid solution was used, and the pickling treatment was performed by immersion in the solution for 60 seconds. Hot-rolled steel sheets 1 with different reduced iron ratios were prepared, and pickling treatment was performed at various pickling temperatures to determine the minimum pickling temperature. The graph in FIG. 3 plots the "minimum pickling temperature" on the vertical axis and the "reduced iron ratio" on the horizontal axis. The pass / fail criteria for the judgment are whether the reduced iron layer 12 can be completely removed at a typical pickling temperature (70°C). In other words, a minimum pickling temperature of 70°C or less is evaluated as "good pickling property." The graph in FIG. 3 confirms that the pickling property of the hot-rolled steel sheet 1 is good when the reduced iron ratio of the scale layer 10 is less than 29%.

[0030] Although the reason why the reduced iron ratio affects the pickling properties has not been elucidated, it is presumed as follows. The scale layer 10 is composed of a reduced iron layer 12 and an iron oxide layer 11. When the area of ​​the scale layer 10 in a cross-sectional view of the hot-rolled steel sheet 1 is taken as 100%, if the ratio of the area of ​​the reduced iron layer 12 (reduced iron ratio) is less than 29%, the ratio of the area of ​​the remaining iron oxide layer 11 (referred to as the "iron oxide ratio") will be 71% or more. The iron oxide ratio is approximately proportional to the ratio of the thickness of the iron oxide layer 11 to the total thickness of the scale layer 10. Therefore, a high iron oxide ratio means that the iron oxide layer 11 is thick.

[0031] The solubility of the iron oxide layer 11 in the pickling solution used in the pickling treatment is higher than that of the reduced iron layer 12. Therefore, when the hot-rolled steel sheet 1 is pickled, the iron oxide layer 11 dissolves preferentially in the pickling solution. Because the reduced iron layer 12 is less soluble in the pickling solution, the reduced iron layer 12 takes on a film-like form and exists in a state slightly floating above the surface 20a of the steel sheet body 20. If the film-like reduced iron layer 12 peels off from the surface 20a of the steel sheet body 20, the reduced iron layer 12 can be removed. However, the reduced iron layer 12 may reattach to the surface 20a of the steel sheet body 20, which may cause a decrease in pickling properties. In order to facilitate peeling of the reduced iron layer 12 from the surface 20a of the steel sheet body 20 and to prevent reattachment, it is considered advantageous to have as wide a gap as possible between the film-like reduced iron layer 12 and the surface 20a of the steel sheet body 20 when the film-like reduced iron layer 12 is floating above the surface 20a of the steel sheet body 20.

[0032] The gap between the reduced iron layer 12 and the surface 20a of the steel sheet body 20 depends on the thickness of the iron oxide layer 11 that was present therebetween. If the iron oxide layer 11 is thin, the gap between the reduced iron layer 12 and the surface 20a of the steel sheet body 20 becomes narrower, and as a result, it is presumed that the film-like reduced iron layer 12 is less likely to peel off from the surface 20a of the steel sheet body 20 and is more likely to re-adhere. In other words, if the iron oxide layer 11 is thin, the pickling property becomes poor. If the iron oxide layer 11 is thick, the gap between the reduced iron layer 12 and the surface 20a of the steel sheet body 20 becomes wider, and as a result, it is presumed that the film-like reduced iron layer 12 is more likely to peel off from the surface 20a of the steel sheet body 20 and is less likely to re-adhere. In other words, if the iron oxide layer 11 is thicker, the effect of improving the pickling property is expected.

[0033] The present inventors have conducted extensive research into the thickness of the iron oxide layer 11 required to obtain good pickling properties, and have found that a sufficient pickling property can be obtained when the iron oxide ratio is 71% or more (i.e., the reduced iron ratio is less than 29%).

[0034] The reduced iron percentage is preferably 28% or less, more preferably 27% or less, and particularly preferably 25% or less. From the viewpoint of improving pickling properties, there is no particular lower limit for the reduced iron percentage, but the reduced iron layer 12 needs to be present to some extent in order to ensure the thickness of the internal oxide layer 22. From the viewpoint of ensuring the thickness of the internal oxide layer 22, the reduced iron percentage is preferably more than 17%, more preferably 18% or more, and particularly preferably 19% or more.

[0035] Identification of each layer constituting the hot-rolled steel sheet 1, measurement of the thickness of the internal oxide layer 22, and measurement of the areas of the scale layer 10 and the reduced iron layer 12 are performed by SEM observation of a cross section of the hot-rolled steel sheet 1. The SEM observation of the cross section is performed on a cross section perpendicular to the rolling direction of the hot-rolled steel sheet 1.

[0036] Fig. 2 is a schematic diagram of a cross-sectional SEM image of a hot-rolled steel sheet 1 produced as Sample No. 5 in the example. The hot-rolled steel sheet 1 includes a steel sheet body 20, which appears light gray in the cross-sectional SEM image, and a scale layer 10 covering the surface 20a of the steel sheet body 20. The scale layer 10 includes an iron oxide layer 11 (dark gray in the cross-sectional SEM image) in contact with the surface 20a of the steel sheet body 20, and a reduced iron layer 12 (light gray in the cross-sectional SEM image) covering the iron oxide layer 11. In the example of Fig. 2, most of the iron oxide layer 11 is covered by the reduced iron layer 12, but a portion of the iron oxide layer 11 is not covered by the reduced iron layer 12 and is exposed to the outside. The iron oxide layer 11 may be partially covered by the reduced iron layer 12 as shown in Fig. 2, or may be completely covered by the reduced iron layer 12 as shown in Fig. 1.

[0037] When the internal structure of the steel plate body 20 is observed in detail in the cross-sectional SEM image, the vicinity of the surface 20a is a slightly darker gray than the interior of the steel plate body 20 (the steel plate substrate 21), and many fine crack-like lines are observed. This portion near the surface 20a is the internal oxidation layer 22. The internal oxidation layer 22 has fine color unevenness, so it can be easily distinguished from other layers (the steel plate substrate 21 and the iron oxide layer 11) by examining the SEM image. On the other hand, the steel plate substrate 21 is a slightly lighter gray than the internal oxidation layer 22, and there are almost no crack-like lines (although several lines are observed near the boundary with the internal oxidation layer 22). Furthermore, the steel plate substrate 21 has almost no color unevenness.

[0038] As described above, the steel sheet substrate 21 contains Si element in a solid solution state (solid solution Si), whereas in the internal oxidation layer 22, at least a part of the Si element is in an oxide state (SiO ２ ) by XPS or Auger spectroscopy. ２ By specifying the Si distribution, the solute Si distribution, and the content ratios thereof, it is also possible to specify the range of the steel sheet substrate 21 and the range of the internal oxide layer 22.

[0039] Next, the thickness measurement of the internal oxide layer 22 will be described. In the cross-sectional SEM observation, when the total length of the hot-rolled steel sheet 1 in the rolling direction is taken as 100%, a cross section perpendicular to the rolling direction is exposed in a range excluding 25% of the head side and 25% of the tail side. In the cross section, the vicinity of the surface near the center in the width direction of the hot-rolled steel sheet 1 (for example, a range from the surface to a depth of about 30 μm) is observed. The SEM observation is performed, for example, at a magnification of 2000 times and a field of view of 60 μm in the width direction and 40 μm in the vertical direction. The SEM image is acquired with the vertical direction of the field of view roughly aligned with the thickness direction of the hot-rolled steel sheet 1.

[0040] In the obtained cross-sectional SEM image ( FIG. 2 ), the surface 20a of the steel sheet body 20 and the boundary surface 22b between the internal oxidation layer 22 and the steel sheet substrate 21 are identified. As described above, the internal oxidation layer 22 and the steel sheet substrate 21 can be easily distinguished visually, so the boundary surface 22b is identified manually. Note that the boundary surface 22b may also be identified using image processing software, for example, by binarization. Next, the image processing software is used to determine the area of ​​the region surrounded by the surface 20a, the boundary surface 22b, and the vertical frame line of the field of view (i.e., the internal oxidation layer 22). The average thickness of the internal oxidation layer 22 is determined by dividing this area by the width dimension (60 μm) of the field of view. This average value is treated as the "thickness 22t of the internal oxidation layer 22" (see FIG. 1 ).

[0041] Measurement of the areas of the scale layer 10 and the reduced iron layer 12 will now be described. A cross-sectional SEM image is acquired under the same observation conditions (observation position, magnification, and field of view) as those used to measure the thickness of the internal oxide layer 22, and the area of ​​the scale layer 10 is identified in the cross-sectional SEM image. Next, the cross-sectional SEM image of the area of ​​the scale layer 10 is binarized using image processing software, and the area S of the high-brightness region (corresponding to the reduced iron layer 12) is determined. １ The area S of the entire scale layer 10 is measured. ０ From these areas, the reduced iron rate is calculated using the above-mentioned formula (2). When a part of the reduced iron layer 12 is separated as shown in FIG. 2, the area of ​​the separated part is also included in the area S of the reduced iron layer 12. １ Add up to.

[0042] (Chemical composition) The chemical composition of the hot-rolled steel sheet 1 is not particularly limited, but the effects of the hot-rolled steel sheet according to the embodiment of the present invention are particularly exhibited when the steel sheet base 21 of the hot-rolled steel sheet 1 contains solute Si. In other words, the hot-rolled steel sheet 1 preferably contains Si. Below, Si and other elements that may be contained, and their preferred contents will be described. Note that the "chemical composition of the hot-rolled steel sheet 1" refers, more precisely, to the chemical composition of the steel sheet base 21 of the hot-rolled steel sheet 1.

[0043] In one preferred embodiment, the chemical composition of the hot-rolled steel sheet 1 satisfies the following: C: 0.08% by mass or more and 0.30% by mass or less, Si: more than 0.5% by mass and 3.0% by mass or less, Mn: 1.5% by mass or more and 3.0% by mass or less, Cr: more than 0% by mass and 1.0% by mass or less, P: more than 0% by mass and 0.10% by mass or less, S: more than 0% by mass and 0.05% by mass or less, Al: more than 0% by mass and 1.0% by mass or less, and N: more than 0% by mass and 0.010% by mass or less, with the balance being Fe and inevitable impurities. Each element will be described below.

[0044] [C: Preferably 0.08% by mass or more, 0.30% by mass or less] C is an element effective in improving the strength of a cold-rolled steel sheet (hereinafter simply referred to as "cold-rolled steel sheet") manufactured from the hot-rolled steel sheet according to an embodiment of the present invention. When contained in the steel together with Si, and optionally together with Mn, C is a particularly effective strengthening element for ensuring the tensile strength of the cold-rolled steel sheet. Furthermore, C is also an element necessary for ensuring retained austenite and improving workability. To effectively exert this effect, the C content is preferably 0.08% by mass or more, more preferably 0.11% by mass or more, and even more preferably 0.13% by mass or more. While a high C content is preferable from the viewpoint of ensuring the strength of the cold-rolled steel sheet, excessive C content may deteriorate corrosion resistance, spot weldability, and workability. Therefore, the C content is preferably 0.30% by mass or less, more preferably 0.25% by mass or less, and even more preferably 0.23% by mass or less.

[0045] [Si: preferably more than 0.5 mass% and not more than 3.0 mass%] Si is an inexpensive steel strengthening element and does not significantly affect the workability of cold-rolled steel sheets. Furthermore, Si is an element that can suppress the decomposition of retained austenite, which is useful for improving the workability of cold-rolled steel sheets, to form carbides. To effectively exert this effect, the Si content is preferably more than 0.5 mass%, more preferably 1.0 mass% or more, even more preferably 1.1 mass% or more, and particularly preferably 1.2 mass% or more. While there is no particular upper limit for the Si content, if the Si content is too high, the solid-solution strengthening effect of Si may become significant, resulting in an increased rolling load. Therefore, from the viewpoint of, for example, production stability, the Si content is preferably not more than 3.0 mass%, more preferably not more than 2.7 mass%, and even more preferably not more than 2.5 mass%.

[0046] [Mn: Preferably 1.5% by mass or more and 3.0% by mass or less] Like Si, Mn is an inexpensive strengthening element for steel and is effective in improving the strength of cold-rolled steel sheets. Mn, when contained in steel together with Si and, if necessary, C, is a particularly effective strengthening element for ensuring the tensile strength of cold-rolled steel sheets. Furthermore, Mn stabilizes austenite and contributes to improving the workability of cold-rolled steel sheets by generating retained austenite. To effectively exert this effect, the Mn content is preferably 1.5% by mass or more, more preferably 1.8% by mass or more, and even more preferably 2.0% by mass or more. However, if the Mn content is too high, the ductility of the cold-rolled steel sheet may decrease, adversely affecting the workability of the cold-rolled steel sheet and further reducing the weldability of the cold-rolled steel sheet. From these viewpoints, the Mn content is preferably 3.0% by mass or less, more preferably 2.8% by mass or less, and even more preferably 2.7% by mass or less.

[0047] [Cr: preferably more than 0 mass% and 1.0 mass% or less] Cr is an element effective in improving the strength of a cold-rolled steel sheet. Furthermore, Cr is an element that improves the corrosion resistance of a cold-rolled steel sheet and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheet. Similarly to B and Ti, Cr is also an element that is effective in improving the delayed fracture resistance of a cold-rolled steel sheet. Therefore, Cr can be contained in an amount that does not affect the strength and workability, such as elongation, of the cold-rolled steel sheet. Although the Cr content may be 0 mass%, to effectively exert these effects, the Cr content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.01 mass% or more. On the other hand, excessive Cr content may deteriorate the workability, such as elongation, of the cold-rolled steel sheet. Therefore, the Cr content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.6 mass% or less.

[0048] [P: preferably more than 0 mass% and 0.10 mass% or less] P is an element that is inevitably present as an impurity element. If the P content is excessive, it may deteriorate weldability. Therefore, the P content is preferably suppressed to 0.10 mass% or less, more preferably 0.08 mass% or less, and even more preferably 0.05 mass% or less.

[0049] [S: preferably more than 0 mass% and 0.05 mass% or less] S is an element that is inevitably present as an impurity element. Usually, steel inevitably contains about 0.0005 mass% of S. Excessive S content may form sulfide-based inclusions, promote hydrogen absorption in a corrosive environment, deteriorate the delayed fracture resistance of the cold-rolled steel sheet, and deteriorate the weldability and workability of the cold-rolled steel sheet. Therefore, the S content is preferably suppressed to 0.05 mass% or less, more preferably 0.010 mass% or less, and even more preferably 0.005 mass% or less.

[0050] [Al: preferably more than 0 mass% and 1.0 mass% or less] Al is an element that has a deoxidizing effect. To effectively exert this effect, the Al content is preferably more than 0 mass%, more preferably 0.005 mass% or more, and even more preferably 0.02 mass% or more. If the Al content is excessive, inclusions such as alumina may increase, which may deteriorate the workability of the cold-rolled steel sheet. Therefore, the Al content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0051] [N: preferably more than 0 mass% and 0.010 mass% or less] N is an element that is inevitably present as an impurity element. If the N content is excessive, nitrides may be formed, which may deteriorate the workability of the cold-rolled steel sheet. In particular, when the cold-rolled steel sheet contains B to improve hardenability, N combines with B to form BN precipitates, which inhibits the hardenability-improving effect of B. Therefore, the N content is preferably suppressed to 0.010 mass% or less, more preferably 0.008 mass% or less, and even more preferably 0.005 mass% or less.

[0052] [Balance] The balance is Fe and inevitable impurities. The inevitable impurities include trace elements (e.g., As, Sb, Sn, etc.) that are introduced depending on the conditions of raw materials, materials, manufacturing equipment, etc. The aforementioned P, S, and N are generally preferable as their contents are lower, and therefore can also be considered inevitable impurities. However, these elements are specified as above because the present invention can achieve its effects by limiting their contents to specific ranges. For this reason, in this specification, the "unavoidable impurities" that make up the balance are a concept that excludes elements whose composition ranges are specified.

[0053] In addition to the above components, other well-known optional components may also be contained to the extent that they do not impair strength or sufficient bendability. Optional components include Cu, Ni, Ti, Nb, V, and B. These optional components are described below.

[0054] [Cu: preferably more than 0 mass% and 1.0 mass% or less] Like Cr, Cu is an element that is effective in improving the strength of the cold-rolled steel sheet and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheet, thereby improving the corrosion resistance of the cold-rolled steel sheet. To effectively exert these effects, the Cu content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.05 mass% or more. Furthermore, from the viewpoint of the workability of the cold-rolled steel sheet, the Cu content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0055] [Ni: preferably more than 0 mass% and 1.0 mass% or less] Like Cr and Cu, Ni is an element that is effective in improving the strength of a cold-rolled steel sheet and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheet, thereby improving the corrosion resistance of the cold-rolled steel sheet. To effectively exert these effects, the Ni content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.05 mass% or more. Furthermore, from the viewpoint of ensuring the workability of the cold-rolled steel sheet, the Ni content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0056] [Ti: preferably more than 0 mass% and 0.15 mass% or less] Like Cr, Cu, and Ni, Ti is an element that is effective in improving the strength of a cold-rolled steel sheet and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheet, thereby improving the corrosion resistance of the cold-rolled steel sheet. Similarly to B and Cr, Ti is also an element that is effective in improving the delayed fracture resistance of the cold-rolled steel sheet, and therefore can be contained in an amount that does not affect the strength and workability, such as elongation, of the cold-rolled steel sheet. To effectively exert these effects, the Ti content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.05 mass% or more. Furthermore, from the viewpoint of ensuring the workability of the cold-rolled steel sheet, the Ti content is preferably 0.15 mass% or less, more preferably 0.12 mass% or less, and even more preferably 0.10 mass% or less.

[0057] [Nb: preferably more than 0 mass% and 0.15 mass% or less] Nb is an element that is effective in improving the strength of cold-rolled steel sheets and also acts to refine austenite grains after quenching, thereby improving the toughness of the cold-rolled steel sheets. To effectively exert this effect, the Nb content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.005 mass% or more. On the other hand, an excessive Nb content may generate a large amount of carbides, nitrides, or carbonitrides, which may deteriorate the workability or delayed fracture resistance of the cold-rolled steel sheets. Therefore, the Nb content is preferably 0.15 mass% or less, more preferably 0.12 mass% or less, and even more preferably 0.10 mass% or less.

[0058] [V: preferably more than 0 mass% and 0.15 mass% or less] Like Nb, V is an element that is effective in improving the strength of cold-rolled steel sheets and also acts to refine austenite grains after quenching, thereby improving the toughness of the cold-rolled steel sheets. To effectively exert this effect, the V content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.005 mass% or more. On the other hand, if the V content is excessive, like Nb, a large amount of carbides, nitrides, or carbonitrides may be formed, which may deteriorate the workability or delayed fracture resistance of the cold-rolled steel sheets. Therefore, the V content is preferably 0.15 mass% or less, more preferably 0.12 mass% or less, and even more preferably 0.10 mass% or less.

[0059] [B: preferably more than 0 mass% and 0.005 mass% or less] B is an element useful for improving the hardenability and weldability of cold-rolled steel sheets. Like Ti and Cr, B is also effective in improving the delayed fracture resistance of cold-rolled steel sheets. Therefore, B can be added in an amount that does not affect the strength and workability, such as elongation, of the cold-rolled steel sheets. To effectively exert these effects, the B content is preferably more than 0 mass%, more preferably 0.0002 mass% or more, even more preferably 0.0003 mass% or more, and particularly preferably 0.0004 mass% or more. On the other hand, if the B content is excessive, these effects may saturate, and ductility may decrease, resulting in poor workability. Therefore, the B content is preferably 0.005 mass% or less, more preferably 0.004 mass% or less, and even more preferably 0.003 mass% or less.

[0060] <Method for manufacturing hot-rolled steel sheet 1> The method for manufacturing hot-rolled steel sheet 1 includes the steps of: rough rolling a slab in the hot state to obtain a rough-rolled steel sheet (rough rolling step); finish-rolling the rough-rolled steel sheet in the hot state to obtain a rolled steel sheet (finish rolling step); winding the rolled steel sheet into a coil (coil winding step); and cooling the wound coil at an average cooling rate of 0.5°C / min to 0.7°C / min in a temperature range from the coil winding temperature to 500°C (coil cooling step). The hot-rolling parameter defined by the following formula (1) is set to be greater than 3.95 at each temperature T １ ~T ５is controlled.

[0061] Here, T １ : Rough rolling exit temperature (K), T ２ : Finish rolling entry temperature (K), T ３ : Finish rolling exit temperature (K), T ４ : Cooling intermediate temperature (K), T ５ : coil winding temperature (K), t: sheet threading time (seconds) from the start of the rough rolling to being wound into a coil, Q: 125140 [J / mol], and R: 8.314 [J / (K mol)].

[0062] Each temperature (T １ ~T ５ ) is the temperature measured at the center of the width direction of the steel sheet at each measurement position by a radiation thermometer. In general, the cooling intermediate temperature is the temperature at the center of the width direction of the coil measured by a radiation thermometer installed at a position 1 / 4 to 1 / 3 from the final stand of the finishing rolling mill when the total length of the cooling zone (runout table) from the finishing rolling mill to the coil winding position is set to 1. ４ The intermediate cooling temperature measured at a position one-third of the way from the final stand of the finishing mill is used.

[0063] [Regarding Formula (1)] The thickness of the iron oxide layer 11 is determined by the temperature and time of the hot rolling process. It is expected that the higher the temperature of the steel sheet during hot rolling and the longer the time required for hot rolling, the greater the thickness of the iron oxide layer 11 and the lower the reduced iron ratio in the scale layer 10. Therefore, a "hot rolling parameter" such as Formula (1) was newly defined as an index for determining the thickness of the iron oxide layer 11 formed on the hot-rolled steel sheet 1, and the relationship between the hot-rolling parameter determined from the manufacturing conditions of the hot-rolled steel sheet 1 and the reduced iron ratio of the resulting hot-rolled steel sheet 1 was confirmed. As a result, it was found that the reduced iron ratio was less than 29% when the hot-rolling parameter was more than 3.95. The hot-rolling parameter is preferably 4.10 or more, more preferably 4.20 or more, and particularly preferably 4.30 or more.

[0064] The above formula (1) is based on the general formula (3) (Arrhenius formula) for calculating the thickness of the iron oxide layer 11. As shown in formula (3), the thickness x of the iron oxide layer 11 can be estimated from the temperature T and time t during oxidation (A is a constant).

[0065] To easily compare the relationship between the thickness of the iron oxide layer 11 and the reduced iron rate, the inventors created formula (1) based on formula (3). In formula (1), Q is the activation energy of Fe diffusion in iron oxide (FeO) (125,140 [J / mol]), and R is the gas constant (8.314 [J / (K mol)]).

[0066] On the left side of formula (1), the terms in the braces have the following meanings: １ The first term including (rough rolling delivery temperature): This term relates to the thickness of the iron oxide layer 11 formed during rough rolling. The formed iron oxide layer 11 is reduced in thickness by the subsequent finish rolling, so it is multiplied by a coefficient of 0.2. ２ The second term including (finish rolling entry temperature): This term relates to the thickness of the iron oxide layer 11 formed between rough rolling and finish rolling. The formed iron oxide layer 11 is reduced in thickness by the subsequent finish rolling, so it is multiplied by a coefficient of 0.2. ３ The third term including (finish rolling delivery temperature): This term relates to the thickness of the iron oxide layer 11 formed during finish rolling.

[0067] ・T ４ The fourth term including (intermediate cooling temperature): This term relates to the thickness of the iron oxide layer 11 formed between the end of finish rolling and the measurement position of the intermediate cooling temperature. ５ Item 5 including (coil winding temperature): This item relates to the thickness of the iron oxide layer 11 that is formed from the measurement position of the intermediate cooling temperature until the hot-rolled steel sheet 1 is wound into a coil. After the hot-rolled steel sheet 1 is wound into a coil, the surface of the hot-rolled steel sheet 1 does not come into contact with the air except for the steel sheet portion wound on the outermost side of the coil, so it is assumed that no further iron oxide layer 11 is formed.

[0068] [Regarding each manufacturing process] (Rough rolling process) Molten steel adjusted to a predetermined chemical composition is melted and cast into a slab by continuous casting or the like. The chemical composition of the slab is the same as that of the hot-rolled steel sheet to be manufactured. Rough rolling can be performed using a known rough rolling mill. The outlet temperature T １ (K) is preferably 1373.15K to 1473.15K (1100°C to 1200°C), more preferably 1383.15K to 1453.15K (1110°C to 1180°C), and particularly preferably 1403.15K to 1428.15K (1130°C to 1155°C).

[0069] (Finish Rolling Step) The rough-rolled steel sheet obtained in the rough rolling step is finish-rolled in a hot state. The finish rolling can be performed using a known finishing mill. The inlet temperature T ２ (K) is preferably 1223.15K to 1373.15K (950°C to 1100°C), which can promote the formation of the iron oxide layer 11. ２ is more preferably 1273.15K to 1363.15K (1000°C to 1090°C), and particularly preferably 1283.15K to 1353.15K (1010°C to 1080°C).

[0070] Finish rolling exit temperature T ３ (K) is preferably 1073.15K to 1273.15K (800°C to 1000°C), more preferably 1173.15K to 1253.15K (900°C to 980°C), and most preferably 1193.15K to 1233.15K (920°C to 960°C).

[0071] (Coiling process) The rolled steel sheet obtained in the finish rolling process is coiled. It is preferable to cool the rolled steel sheet in a cooling zone (runout table) between the finish rolling mill and the coil winding position. This allows the cooling rate of the rolled steel sheet to be appropriately controlled, and therefore the temperature of the rolled steel sheet during coil winding (coil winding temperature T ５ In order to check and control the cooling rate of the rolled steel plate at this time, the cooling intermediate temperature T４ The cooling intermediate temperature T ４ The temperature T is preferably 1073.15 K to 1173.15 K (800° C. to 900° C.), which can promote the formation of the iron oxide layer 11. ４ is more preferably 1083.15K to 1153.15K (810°C to 880°C), and particularly preferably 1098.15K to 1143.15K (825°C to 870°C).

[0072] Coil winding temperature T ５ Since it affects the thickness 22t of the internal oxide layer 22, it is important to appropriately control the coil winding temperature T ５ (K) is preferably 823.15K to 1023.15K (550°C to 750°C), more preferably 873.15K to 973.15K (600°C to 700°C), and particularly preferably 913.15K to 963.15K (640°C to 690°C).

[0073] (Coil Cooling Process) The wound coil is cooled to a coil winding temperature T ５ The coil is cooled at an average cooling rate of 0.5°C / min to 0.7°C / min in the temperature range from 100°C (500°C) to 500°C (773.15K). During cooling of the coil, oxygen (O) diffuses from the iron oxide layer 11 into the steel sheet body 20, forming an internal oxide layer 22 and a reduced iron layer 12. The thicknesses of the reduced iron layer 12 and the internal oxide layer 22 are determined by the cooling time of the wound coil (particularly the cooling time to 500°C). If the cooling rate of the coil exceeds 0.7°C / min, the cooling time of the coil becomes short, oxygen (O) diffusion into the steel sheet body 20 becomes insufficient, and the average thickness of the internal oxide layer 22 may become less than 10.0 μm. If the cooling rate of the coil is less than 0.5°C / min, the cooling time may become excessively long. The average cooling rate is calculated using the following formula: ５ (min) is the coil winding temperature T ５ Average cooling rate (°C / min) = (T ５ -773.15) / t ５

[0074] (Hot-rolled steel sheet (test material)) Molten steel having a predetermined chemical composition was melted and then continuously cast into a slab. The slab was heated to 1200°C, then roughly rolled and finish rolled, and then wound into a coil and cooled to produce a hot-rolled steel sheet (test material). The temperature conditions T １ ~T ５ The rolling time t was as shown in Table 1.

[0075] The chemical composition of the molten steel used for each test number was as follows. Note that "unavoidable impurities" include P, S, and N in amounts within the ranges described above. - Test Nos. 1 to 9: C content 0.22 mass%, Si content 1.7 mass%, Mn content 2.0 mass%, Cr content 0.5 mass%, Al content 0.04 mass%, the balance being Fe and unavoidable impurities. - Test Nos. 10 to 11: C content 0.13 mass%, Si content 1.0 mass%, Mn content 2.35 mass%, Cr content 0.25 mass%, Al content 0.02 mass%, the balance being Fe and unavoidable impurities.

[0076] The hot-rolled steel sheets thus obtained were subjected to calculation of the reduced iron ratio and a pickling test. The results are shown in Table 1.

[0077] (Calculation of reduced iron ratio) A cross section perpendicular to the rolling direction was prepared at approximately the center of the length of the hot-rolled steel sheet, and the surface vicinity (range from the surface to a depth of approximately 30 μm) near the center of the width direction of the hot-rolled steel sheet was observed using an SEM. The SEM observation was performed at a magnification of 2000 times and a field of view of 60 μm in width direction × 40 μm in length direction. The scale layer 10 in the cross-sectional SEM image was binarized using image processing software, and the area S of the high-brightness region (corresponding to the reduced iron layer 12) was calculated. １ and the area S of the entire scale layer 10 ０ The reduced iron ratio was calculated from these areas using the above-mentioned formula (2).

[0078] (Pickling test) A plurality of 50 mm square test pieces were cut from the hot-rolled steel sheet at approximately the center of the length and the center of the width. 10% hydrochloric acid was used as the pickling solution. After immersion in the pickling solution for 60 seconds, both sides of the test pieces were visually observed to determine the minimum pickling temperature at which the reduced iron layer 12 was completely removed.

[0079]

[0080] The hot-rolled steel sheets of Test Nos. 3, 5 to 8, 10, and 11, which were produced under manufacturing conditions that satisfied the requirements of the embodiment, had internal oxide layer thicknesses of 10.0 μm or more, and it was confirmed that the cold-rolled steel sheets had high galvanizability after cold rolling. Furthermore, the hot-rolled steel sheets of Test Nos. 3, 5 to 8, 10, and 11 were produced under conditions where the hot-rolling parameter exceeded 3.95, and the reduced iron ratio was less than 29%. The results of the pickling test of these hot-rolled steel sheets confirmed that the minimum pickling temperature was 70°C or less, and that the pickling ability was good.

[0081] The hot-rolled steel sheets of Test Nos. 1, 2, and 9 were produced under conditions where the hot-rolling parameter was 3.95 or less, and therefore had a reduced iron content of 29% or more. The results of the pickling tests on these hot-rolled steel sheets confirmed that the minimum pickling temperature exceeded 70°C, and that the pickling properties were poor. The hot-rolled steel sheet of Test No. 4 had a high cooling rate of the coil, which resulted in insufficient diffusion of oxygen (O) from the iron oxide layer 11 to the steel sheet body 20, resulting in an internal oxide layer thickness of less than 10.0 μm. This is thought to be why the galvanizability of the cold-rolled steel sheet after cold rolling was poor.

[0082] This application claims priority from Japanese Patent Application No. 2024-046923, filed March 22, 2024. Japanese Patent Application No. 2024-046923 is incorporated herein by reference.

[0083] REFERENCE SIGNS LIST 1 hot-rolled steel sheet 10 scale layer 11 iron oxide layer 12 reduced iron layer 20 steel sheet body 20a surface of steel sheet body 21 steel sheet substrate 22 internal oxide layer 22t thickness of internal oxide layer 22b boundary surface

Claims

1. A method for producing a hot-rolled steel sheet, comprising: a step of hot-rough rolling a slab to obtain a rough-rolled steel sheet; a step of hot-finish rolling the rough-rolled steel sheet to obtain a rolled steel sheet; a step of winding the rolled steel sheet into a coil; and a step of cooling the wound coil at an average cooling rate of 0.5°C / min to 0.7°C / min in a temperature range from the coiling temperature of the coil to 500°C, wherein the hot-rolling parameter defined by the following formula (1) is greater than 3.

95. Here, T １ : Rough rolling exit temperature (K), T ２ : Finish rolling entry temperature (K), T ３ : Finish rolling exit temperature (K), T ４ : Cooling intermediate temperature (K), T ５ : coil winding temperature (K), t: sheet threading time (seconds) from the start of the rough rolling to being wound into a coil, Q: 125140 [J / mol], and R: 8.314 [J / (K mol)].

2. Said T ４ The method for producing a hot-rolled steel sheet according to claim 1, wherein the temperature is in the range of 1073.15K to 1173.15K.

3. Said T １ The method for producing a hot-rolled steel sheet according to claim 1, wherein the temperature is in the range of 1373.15K to 1473.15K.

4. A method for producing a hot-rolled steel sheet according to claim 1, wherein the chemical composition of the slab satisfies the following: C: 0.08 mass% or more and 0.30 mass% or less; Si: more than 0.5 mass% and 3.0 mass% or less; Mn: 1.5 mass% or more and 3.0 mass% or less; Cr: more than 0 mass% and 1.0 mass% or less; P: more than 0 mass% and 0.10 mass% or less; S: more than 0 mass% and 0.05 mass% or less; Al: more than 0 mass% and 1.0 mass% or less; and N: more than 0 mass% and 0.010 mass% or less, with the balance being Fe and unavoidable impurities.

5. A hot-rolled steel plate comprising a steel plate body and a scale layer covering the surface of the steel plate body, wherein the steel plate body is located on the surface side and at least a part of the Si element is SiO ２ and a steel plate base material covered with the internal oxide layer and in which Si element is present in a state of solid solution Si, wherein the scale layer comprises an iron oxide layer in contact with the surface of the steel plate body and a reduced iron layer covering the iron oxide layer, wherein, in a cross-sectional view in the thickness direction, the internal oxide layer has an average thickness of 10.0 μm or more, and a ratio of an area of ​​the reduced iron layer to an area of ​​the scale layer is less than 29%.

6. A hot-rolled steel sheet according to claim 5, having a chemical composition satisfying the following: C: 0.08% by mass or more and 0.30% by mass or less; Si: more than 0.5% by mass and 3.0% by mass or less; Mn: 1.5% by mass or more and 3.0% by mass or less; Cr: more than 0% by mass and 1.0% by mass or less; P: more than 0% by mass and 0.10% by mass or less; S: more than 0% by mass and 0.05% by mass or less; Al: more than 0% by mass and 1.0% by mass or less; and N: more than 0% by mass and 0.010% by mass or less, with the balance being Fe and inevitable impurities.

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