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

Abstract

To provide a method for manufacturing a hot-rolled sheet steel enabling materialization of excellent acid cleaning property even with a thick direct-reduced iron layer.SOLUTION: A method 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 within a temperature range from the winding temperature of the coil to 500°C. The hot-rolling parameter defined by the following Equation (1) is more than 3.95. T1: rough rolling outlet side temperature (hereinafter, temperature units are (K)). T2: finish rolling inlet side temperature. T3: the finish rolling outlet side temperature. T4 : the cooling intermediate temperature. T5 : the coil winding temperature. t : sheet passing time (seconds) from the start of rough rolling to winding into a coil. Q :125140 [J / mol], and R : 8.314 [J / (K mol)].SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] There has been a demand for improved passenger 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 a tensile strength of 980 MPa or more are being put into practical use. To achieve a tensile strength of 980 MPa or more, it is necessary to add solid-solution strengthening elements to the steel, and examples of such elements include Si and Mn.

[0004] In the production of high-strength steel sheets containing solute Si, the cast slab is hot-rolled, pickled, cold-rolled, and annealed. A scale layer consisting of iron-based oxides is formed on the surface of the hot-rolled steel sheet obtained by hot-rolling. When the hot-rolled steel sheet is then coiled at high temperature, a hot-rolled steel sheet 1 having a layer structure as shown schematically in Figure 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 20 a of the steel sheet body 20 . The steel sheet body 20 is mainly composed of a steel sheet substrate 21 containing Si element in a solid solution state, and an internal oxidation layer 22 containing at least a portion of the Si element in the form of SiO2 (oxide state). With regard to the Si element in the internal oxidation layer 22, it is preferable that more than half of all the Si element contained in the internal oxidation layer 22 is in an oxide state, and it is particularly preferable that all the Si element is in an oxide state. However, Si in a solid solution state may be present in the internal oxidation layer 22. The internal oxidation layer 22 is formed on the surface 20a side of the steel sheet body 20, and the steel sheet substrate 21 is covered with the internal oxidation 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] This layer structure is due to the presence of Si, an easily oxidizable element, in solid solution 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 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 interior of the steel sheet. Because Si has a higher oxygen affinity than iron, the diffused oxygen combines with the Si solid-solubilized in the steel sheet to produce SiO2, thereby forming an internal oxide layer 22 containing SiO2. Meanwhile, the oxygen in the iron oxide layer 11 combines 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] Before cold rolling, the hot-rolled steel sheet 1 is pickled to remove the 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 any surface treatment that may be performed after cold rolling. In particular, the problem of the reduced iron layer 12 remaining in the scale layer 10 without being completely removed by the pickling treatment has been a problem. Therefore, a method for reliably removing the reduced iron layer 12 by the pickling treatment has been sought. For example, techniques are known that prevent the reduced iron layer 12 from remaining after the pickling treatment by reducing 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 solute Si on the surface of the cold-rolled steel sheet is large, the galvanizability decreases. In order to improve the galvanizability, it is effective for the surface of the cold-rolled steel sheet to be covered with an internal oxidation layer 22 (for example, Patent Document 2). At least a portion of the solute Si in the internal oxidation layer 22 is oxidized to SiO2. As a result, the concentration of solute Si in the internal oxidation layer 22 decreases, and the galvanizability of the cold-rolled steel sheet improves. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2017-222887 [Patent Document 2] Japanese Patent Application Publication No. 2022-136964 Summary of the Invention [Problem to be solved by the invention]

[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. In other words, the internal oxide layer 22 may be less in the edge portions in the coil width direction than in the central portion in the coil width direction of the hot-rolled steel sheet. 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] In order to eliminate uneven plating, it is only necessary to form an internal oxide layer 22 of sufficient thickness also on the edge portions in the coil width direction of the hot-rolled steel sheet. However, forming a thicker internal oxide layer 22 means that more oxygen is supplied from the iron oxide layer 11, which results in the generation of more reduced iron 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 property is reduced). In this specification, the degree of ease of removal of the scale layer 10 (particularly the reduced iron layer 12) by pickling treatment is referred to as "pickling property."

[0012] Thus, if the internal oxide layer 22 of the hot-rolled steel sheet is thickened to improve the galvanizability of the cold-rolled steel sheet, the reduced iron layer 12 becomes thicker, which reduces the pickling ability of the hot-rolled steel sheet. Therefore, a method is needed that can easily remove even a thick reduced iron layer 12 by pickling. However, conventional common technical knowledge has been that the means for improving the pickling properties of a hot-rolled steel sheet is to make the reduced iron layer 12 thinner (Patent Documents 1 and 2), and no consideration has been given to means for improving the pickling properties 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 producing the same. [Means for solving the problem]

[0014] Aspect 1 of the present invention is A step of hot rough rolling the slab to obtain a rough rolled steel plate; A step of hot finish rolling the rough rolled steel plate to obtain a rolled steel plate; a step of winding the rolled steel sheet into a coil; 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 to 500°C; Including, The present invention provides a method for producing a hot-rolled steel sheet, in which a hot-rolling parameter defined by the following formula (1) exceeds 3.95.

number

[0015] Aspect 2 of the present invention is In the method for producing a hot-rolled steel sheet according to aspect 1, the T4 temperature is in the range of 1073.15K to 1173.15K.

[0016] Aspect 3 of the present invention is In the method for producing a hot-rolled steel sheet according to aspect 1 or 2, the T1 is in the range of 1373.15K to 1473.15K.

[0017] A fourth aspect of the present invention is The chemical composition of the slab is C: 0.08% by mass or more, 0.30% by mass or less Si: more than 0.5 mass%, 3.0 mass% or less Mn: 1.5% by mass or more, 3.0% by mass or less Cr: more than 0 mass%, 1.0 mass% or less P: More than 0% by mass, 0.10% by mass or less S: More than 0% by mass, 0.05% by 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; The method for producing a hot-rolled steel sheet according to any one of Aspects 1 to 3, wherein the balance is Fe and unavoidable 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 the surface of the steel plate body, The steel plate body is an internal oxide layer located on the surface side, in which at least a portion of Si element exists in the state of SiO; a steel sheet substrate covered with the internal oxide layer and containing Si element in the form of solid solution Si; Including, The scale layer is an iron oxide layer in contact with the surface of the steel plate body; a reduced iron layer covering the iron oxide layer; and Including, In a cross-sectional view in the thickness direction, The average thickness of the internal oxide layer is 10.0 μm or more, The 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 chemical composition is C: 0.08% by mass or more, 0.30% by mass or less Si: more than 0.5 mass%, 3.0 mass% or less Mn: 1.5% by mass or more, 3.0% by mass or less Cr: more than 0 mass%, 1.0 mass% or less P: More than 0% by mass, 0.10% by mass or less S: More than 0% by mass, 0.05% by 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; Aspect 6. The hot-rolled steel sheet according to aspect 5, wherein the balance is Fe and unavoidable impurities. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic cross-sectional view of a hot-rolled steel sheet according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of a cross-sectional SEM image of the hot-rolled steel sheet produced in Test No. 5 of the example. [Figure 3] 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. DETAILED DESCRIPTION OF THE INVENTION

[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 improve galvanizability by thickening the internal oxide layer 22 provided on the surface side of the steel sheet body 20, while ensuring sufficient pickling properties even when the reduced iron layer 12 included in the scale layer 10 is thick. As a result, the inventors have found for the first time that the factor determining the pickling properties is the content of the reduced iron layer 12 in the scale layer 10 (reduced iron rate), and that the pickling properties can be improved by keeping the reduced iron rate low, which has led to the completion of 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 plate 1> 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 substrate 21 covered with 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 be 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 galvanizability 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 oxide layer 22 is not particularly limited from the viewpoint of plating properties. However, if the internal oxide layer 22 becomes thick, the reduced iron layer 12 also becomes thick, which may affect pickling properties. Therefore, the thickness of the internal oxide 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 measurement of the reduced iron rate is carried out in a cross section perpendicular to the rolling direction, near the center in the width direction of the hot-rolled steel sheet 1. The reduced iron rate will be described in detail later.

[0027] In this specification, the "reduced iron rate" refers to the ratio of the area of ​​the reduced iron layer 12 to the area of ​​the scale layer 10, 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 (%) = S1 / S0 × 100 (2) Here, S0 (μm 2 ) is the area of ​​the scale layer 10 in a cross-sectional view, and S1 (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 correlates with the thickness of the internal oxidation layer 22. Therefore, in order to make the internal oxidation 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 roughly proportional to the area S1 of the reduced iron layer 12 in a cross-sectional view. Therefore, to ensure a sufficient internal oxidation layer 22, the area S1 of the reduced iron layer 12 needs to be large to a certain extent. To keep the reduced iron rate below 29% while keeping the area S1 of the reduced iron layer 12 large to a certain extent, the area S0 of the scale layer 10 can be increased. The area S0 of the scale layer 10 corresponds to the sum of the areas of the reduced iron layer 12 and the iron oxide layer 11 in a cross-sectional view. Therefore, to keep the reduced iron rate low, 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] Here, the relationship between the reduced iron rate and the pickling property will be explained in detail. FIG. 3 is a graph showing the relationship between the reduced iron ratio and 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 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 subjected to pickling treatment 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 hot-rolled steel sheet 1 has good pickling property when the reduced iron ratio of the scale layer 10 is less than 29%.

[0030] The reason why the reduced iron ratio affects the pickling properties has not been clarified, but 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 rate) is less than 29%, the ratio of the area of ​​the remaining iron oxide layer 11 (referred to as the "iron oxide rate") will be 71% or more. The iron oxide rate 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 rate 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. 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 large 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.

[0032] The gap between the reduced iron layer 12 and the surface 20a of the steel plate body 20 depends on the thickness of the iron oxide layer 11 that has been present therebetween. It is presumed that 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 narrow, and as a result, 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 properties become poor. It is presumed that 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 large, and as a result, the film-like reduced iron layer 12 is likely to peel off from the surface 20a of the steel sheet body 20 and is unlikely to re-adhere. In other words, if the iron oxide layer 11 is thick, it is expected to have the effect of improving pickling properties.

[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 ratio is preferably 28% or less, more preferably 27% or less, and particularly preferably 25% or less. From the viewpoint of improving pickling performance, there is no particular lower limit for the reduced iron ratio, but the reduced iron layer 12 needs to be present to some extent in order to ensure the thickness of the internal oxidation layer 22. From the viewpoint of ensuring the thickness of the internal oxidation layer 22, the reduced iron ratio 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 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] 2 is a schematic diagram of a cross-sectional SEM image of a hot-rolled steel sheet 1 produced in Example Sample No. 5. 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 a 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 plate 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 area near 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 area near the surface 20a is the internal oxide layer 22. The internal oxide 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 sheet substrate 21 is slightly lighter gray than the internal oxidation layer 22, and has almost no crack-like lines (although several lines can be seen near the boundary with the internal oxidation layer 22). In addition, the steel sheet substrate 21 has almost no color unevenness.

[0038] As described above, the steel sheet substrate 21 contains Si elements in a solid solution state (solid solution Si), whereas at least a portion of the Si elements is contained in an oxide state (SiO2) in the internal oxidation layer 22. By identifying the SiO2 distribution, the solid solution Si distribution, and their content ratios using XPS or Auger spectroscopy, it is also possible to identify the range of the steel sheet substrate 21 and the range of the internal oxidation layer 22.

[0039] Next, the thickness measurement of the internal oxide layer 22 will be described. In cross-sectional SEM observation, 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, assuming that the total length of the hot-rolled steel sheet 1 in the rolling direction is 100%. In this cross section, the surface vicinity 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 approximately 30 μm) is observed. SEM observation is performed, for example, at a magnification of 2000x and a field of view of 60 μm in the width direction and 40 μm in the vertical direction. Note that 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] The 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 obtained 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 S1 of the high-brightness region (corresponding to the reduced iron layer 12) is measured. The area S0 of the entire scale layer 10 is also measured. From these areas, the reduced iron rate is calculated using the above-mentioned equation (2). In addition, when a part of the reduced iron layer 12 is separated as shown in FIG. 2, the area of ​​the separated part is also added to the area S1 of the reduced iron layer 12.

[0042] (Chemical composition) Although the chemical composition of the hot-rolled steel sheet 1 is not particularly limited, 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. More precisely, the "chemical composition of the hot-rolled steel sheet 1" refers 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 is C: 0.08% by mass or more, 0.30% by mass or less Si: more than 0.5 mass%, 3.0 mass% or less Mn: 1.5% by mass or more, 3.0% by mass or less Cr: more than 0 mass%, 1.0 mass% or less P: More than 0% by mass, 0.10% by mass or less S: More than 0% by mass, 0.05% by 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; The balance is Fe and unavoidable impurities. Each element will be explained below.

[0044] [C: preferably 0.08% by mass or more and 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 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 3.0 mass% or less] Si is an inexpensive steel strengthening element that does not significantly affect the workability of cold-rolled steel sheets. Si also inhibits the decomposition of retained austenite, which is useful for improving the workability of cold-rolled steel sheets, and the formation of carbides. To effectively utilize this effect, the Si content is preferably greater than 0.5% by mass, more preferably 1.0% by mass or more, even more preferably 1.1% by mass or more, and particularly preferably 1.2% by mass or more. While there are no particular limitations on the upper limit of 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, manufacturing stability, the Si content is preferably 3.0% by mass or less, more preferably 2.7% by mass or less, and even more preferably 2.5% by mass or less.

[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 incorporated into 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 these effects, 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 possibly even 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 cold-rolled steel sheets. Furthermore, Cr improves the corrosion resistance of cold-rolled steel sheets and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheets. Similarly to B and Ti, Cr is also effective in improving the delayed fracture resistance of cold-rolled steel sheets. Therefore, Cr can be contained in an amount that does not affect the strength and workability, such as elongation, of the cold-rolled steel sheets. While the Cr content may be 0% by mass, to effectively exert these effects, the Cr content is preferably greater than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.01% by mass or more. On the other hand, excessive Cr content may deteriorate the workability, such as elongation, of the cold-rolled steel sheets. Therefore, the Cr content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.6% by 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. Excessive P content may deteriorate weldability. Therefore, the P content is preferably controlled 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. Usually, steel inevitably contains about 0.0005% by 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 controlled to 0.05% by mass or less, more preferably 0.010% by mass or less, and even more preferably 0.005% by 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 restricted to 0.010% by mass or less, more preferably 0.008% by mass or less, and even more preferably 0.005% by mass or less.

[0052] [Remainder] The balance is Fe and unavoidable impurities. A trace element (e.g., As, Sb, Sn, etc.) that is introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. is permitted as an unavoidable impurity. As mentioned above, P, S, and N are generally preferable as their contents are lower, so they can also be considered unavoidable impurities. However, these elements are specified as above because the present invention can achieve its effects by limiting their contents to a specific range. Therefore, 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 within the range that does 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 cold-rolled steel sheets and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheets, thereby improving the corrosion resistance of the cold-rolled steel sheets. 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 sheets, 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 cold-rolled steel sheets and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheets, thereby improving the corrosion resistance of the cold-rolled steel sheets. 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 sheets, 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 cold-rolled steel sheets and has the effect of suppressing hydrogen generation due to corrosion of the cold-rolled steel sheets, thereby improving the corrosion resistance of the cold-rolled steel sheets. Similarly to B and Cr, Ti is also an element that is effective in improving the delayed fracture resistance of cold-rolled steel sheets, so it 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 Ti content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.05% by mass or more. Furthermore, from the viewpoint of ensuring the workability of the cold-rolled steel sheets, the Ti content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and even more preferably 0.10% by 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 refines austenite grains after quenching, thereby improving the toughness of the cold-rolled steel sheets. To effectively exert these effects, the Nb content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.005% by mass or more. On the other hand, an excessive Nb content may produce 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% by mass or less, more preferably 0.12% by mass or less, and even more preferably 0.10% by mass or less.

[0058] [V: preferably more than 0 mass% and 0.15 mass% or less] Like Nb, V is also an element that is effective in improving the strength of cold-rolled steel sheets and refines 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 greater than 0% by mass, more preferably 0.0002% by mass or more, even more preferably 0.0003% by mass or more, and particularly preferably 0.0004% by 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% by mass or less, more preferably 0.004% by mass or less, and even more preferably 0.003% by mass or less.

[0060] <Method of manufacturing hot-rolled steel sheet 1> The method for manufacturing the hot-rolled steel sheet 1 is as follows: A process of roughly hot rolling the slab to obtain a rough-rolled steel plate (rough rolling process); A step of hot finish-rolling the rough-rolled steel plate to obtain a rolled steel plate (finish rolling step); a step of winding the rolled steel sheet into a coil (coil winding step); 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 coil winding temperature to 500 ° C. (coil cooling step); Includes. The temperatures T1 to T5 are controlled so that the hot rolling parameter defined by the following formula (1) exceeds 3.95.

[0061]

number

[0062] Each temperature (T1 to T5) in formula (1) is a temperature measured at the center of the steel sheet in the width direction at each measurement position using a radiation thermometer. The cooling intermediate temperature is generally the temperature at the center of the coil width direction measured by a radiation thermometer installed at a position 1 / 4 to 1 / 3 of the way 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 taken as 1. The cooling intermediate temperature T4 substituted into equation (1) is the cooling intermediate temperature measured at a position 1 / 3 of the way from the final stand of the finishing rolling mill.

[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 thicker the iron oxide layer 11 will be and the lower the reduced iron rate in the scale layer 10 will be. Therefore, we newly defined a "hot rolling parameter" as shown in formula (1) as an index for determining the thickness of the iron oxide layer 11 formed on the hot-rolled steel sheet 1, and confirmed 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 obtained hot-rolled steel sheet 1. As a result, we found that when the hot-rolling parameter exceeds 3.95, the reduced iron ratio is less than 29%. 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 when oxidation is occurring (A is a constant).

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[0065] To easily compare the relationship between the thickness of the iron oxide layer 11 and the reduced iron ratio, the inventors created equation (1) based on equation (3). In equation (1), Q is the activation energy of Fe diffusion in iron oxide (FeO) (125140 [J / mol]), and R is the gas constant (8.314 [J / (K mol)]).

[0066] On the left side of equation (1), the terms in the braces have the following meanings: · 1st term including T1 (rough rolling exit temperature): This is a term related 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. · 2nd term including T2 (finishing rolling entry temperature): This is a term related 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. · Item 3 including T3 (finishing rolling exit temperature): This is a term relating to the thickness of the iron oxide layer 11 formed during finish rolling.

[0067] · 4th term including T4 (cooling intermediate temperature): This is a term relating to the thickness of the iron oxide layer 11 formed between the finish rolling and the measurement position of the intermediate cooling temperature. · Item 5 including T5 (coil winding temperature): This is a term relating to the thickness of the iron oxide layer 11 that is formed from the measurement position of the intermediate cooling temperature until it is wound into a coil. After being wound into a coil, the surface of the hot-rolled steel sheet 1 does not come into contact with the atmosphere except for the steel sheet portion wound on the outermost side of the coil, so no further iron oxide layer 11 is formed.

[0068] [About each manufacturing process] (Rough rolling process) Molten steel adjusted to a predetermined chemical composition is produced and cast into a slab by continuous casting, etc. The chemical composition of the slab is the same as that of the hot-rolled steel sheet to be produced. Rough rolling can be performed using a known rough rolling mill. The outlet temperature T1 (K) of the rough rolling mill is preferably 1373.15 K to 1473.15 K (1100° C. to 1200° C.), more preferably 1383.15 K to 1453.15 K (1110° C. to 1180° C.), and particularly preferably 1403.15 K to 1428.15 K (1130° C. to 1155° C.).

[0069] (Finishing rolling process) The roughly rolled steel sheet obtained in the rough rolling step is subjected to hot finish rolling. Finish rolling can be performed using a known finishing mill. The entry temperature T2 (K) of the finish rolling is preferably 1223.15 K to 1373.15 K (950°C to 1100°C), which can promote the formation of the iron oxide layer 11. The entry temperature T2 of the finish rolling is more preferably 1273.15 K to 1363.15 K (1000°C to 1090°C), and particularly preferably 1283.15 K to 1353.15 K (1010°C to 1080°C).

[0070] The finish rolling delivery temperature T3 (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] (Coil winding process) The rolled steel sheet obtained in the finish rolling process is wound into a coil. 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, thereby enabling appropriate control of the temperature of the rolled steel sheet during coil winding (coil winding temperature T5). To confirm and control the cooling rate of the rolled steel sheet, the intermediate cooling temperature T4 is measured at a predetermined position on the runout table (a position 1 / 3 of the way along the runout table from the final stand of the finish rolling mill). The intermediate cooling temperature T4 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. The temperature T4 is more preferably 1083.15 K to 1153.15 K (810° C. to 880° C.), and particularly preferably 1098.15 K to 1143.15 K (825° C. to 870° C.).

[0072] It is important to appropriately control the coil winding temperature T5 because it affects the thickness 22t of the internal oxide layer 22. The coil winding temperature T5 (K) is preferably 823.15 K to 1023.15 K (550°C to 750°C), more preferably 873.15 K to 973.15 K (600°C to 700°C), and particularly preferably 913.15 K to 963.15 K (640°C to 690°C).

[0073] (Coil cooling process) The wound coil is cooled at an average cooling rate of 0.5°C / min to 0.7°C / min in the temperature range from the coil winding temperature T5 (K) to 500°C (773.15K). During coil cooling, oxygen (O) diffuses from the iron oxide layer 11 into the steel sheet body 20, forming the internal oxide layer 22 and the 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 coil cooling rate exceeds 0.7°C / min, the coil cooling time is shortened, oxygen (O) diffusion into the steel sheet body 20 is insufficient, and the average thickness of the internal oxide layer 22 may be less than 10.0 μm. If the coil cooling rate is less than 0.5°C / min, the cooling time may be excessively long. The average cooling rate is calculated using the following formula: In the formula, t5 (min) is the time required to cool from the coil winding temperature T5 (K) to 500°C (773.15K). Average cooling rate (℃ / min)=(T5-773.15) / t5 [Example]

[0074] (Hot rolled steel plate (sample material)) Molten steel with a specified chemical composition was produced and then continuously cast into a slab. The slab was heated to 1200°C, then roughly rolled and finish rolled, and then coiled and cooled to produce a hot-rolled steel sheet (test material). The temperature conditions T1 to T5 and the sheet passing time t during the production of the hot-rolled steel sheets were 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 mentioned above. Test No. 1-9: The C content was 0.22 mass%, the Si content was 1.7 mass%, the Mn content was 2.0 mass%, the Cr content was 0.5 mass%, the Al content was 0.04 mass%, and the balance was Fe and unavoidable impurities. Test No.10~11: The C content was 0.13 mass%, the Si content was 1.0 mass%, the Mn content was 2.35 mass%, the Cr content was 0.25 mass%, the Al content was 0.02 mass%, and the balance was 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 rate) 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 area near the center of the width direction of the hot-rolled steel sheet (from the surface to a depth of approximately 30 μm) was observed using an SEM. The SEM observation was performed at a magnification of 2000x and a field of view of 60 μm in the width direction and 40 μm in the vertical direction. The cross-sectional SEM image of the scale layer 10 was binarized using image processing software, and the area S1 of the bright region (corresponding to the reduced iron layer 12) and the area S0 of the entire scale layer 10 were measured. The reduced iron rate was calculated from these areas using the above-mentioned formula (2).

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

[0079] [Table 1]

[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 an internal oxide layer thickness 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 Tests No. 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 test of these hot-rolled steel sheets confirmed that the minimum pickling temperature exceeded 70°C, and that their pickling properties were poor. In the hot-rolled steel sheet of Test No. 4, the cooling rate of the coil was fast, so the diffusion of oxygen (O) from the iron oxide layer 11 to the steel sheet body 20 was insufficient, and the thickness of the internal oxide layer was less than 10.0 μm. This is thought to be the reason for the poor galvanizability of the cold-rolled steel sheet after cold rolling. [Explanation of symbols]

[0082] 1 Hot rolled steel plate 10 Scale Layer 11 Iron oxide layer 12 Reduced iron layer 20 Steel plate body 20a Surface of steel plate body 21 Steel plate substrate 22 Internal oxide layer 22t Thickness of the internal oxide layer 22b Boundary surface

Claims

1. A step of hot rough rolling the slab to obtain a rough rolled steel plate; A step of hot finish rolling the rough rolled steel plate to obtain a rolled steel plate; a step of winding the rolled steel sheet into a coil; 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 to 500°C; Including, A method for producing a hot-rolled steel sheet, wherein the hot-rolling parameter defined by the following formula (1) is greater than 3.

95. [Equation 1] where: T １ : Rough pressing side temperature (K), T ２ : Temperature of upper pressure extension side (K), T ３ : Temperature of upper pressure extension side (K), T ４ : Cooling intermediate temperature (K), T ５ : Coil winding temperature (K), t: The time (seconds) from the start of the rough rolling to the time of winding 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. The chemical composition of the slab is C: 0.08% by mass or more, 0.30% by mass or less Si: more than 0.5 mass%, 3.0 mass% or less Mn: 1.5% by mass or more, 3.0% by mass or less Cr: more than 0 mass%, 1.0 mass% or less P: More than 0% by mass, 0.10% by mass or less S: More than 0% by mass, 0.05% by 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; The method for producing a hot-rolled steel sheet according to claim 1, wherein the balance is 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, The steel plate body is Located on the surface side, at least a part of the Si element is SiO ２ an internal oxide layer present in the state a steel sheet substrate covered with the internal oxide layer and containing Si element in the form of solid solution Si; Including, The scale layer is an iron oxide layer in contact with the surface of the steel plate body; a reduced iron layer covering the iron oxide layer; and Including, In a cross-sectional view in the thickness direction, The average thickness of the internal oxide layer is 10.0 μm or more, A hot-rolled steel sheet, wherein a ratio of an area of ​​the reduced iron layer to an area of ​​the scale layer is less than 29%.

6. The chemical composition is C: 0.08% by mass or more, 0.30% by mass or less Si: more than 0.5 mass%, 3.0 mass% or less Mn: 1.5% by mass or more, 3.0% by mass or less Cr: more than 0 mass%, 1.0 mass% or less P: More than 0% by mass, 0.10% by mass or less S: More than 0% by mass, 0.05% by 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; The hot-rolled steel sheet according to claim 5, wherein the balance is Fe and unavoidable impurities.

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