Hot-rolled steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide: a hot-rolled steel sheet having excellent scale adhesion, particularly excellent sheet-like peeling resistance, even in a hot-rolled steel sheet having a larger sheet thickness; and a method for producing the hot-rolled steel sheet. A hot-rolled steel sheet having a prescribed component composition and having an oxide skin on the surface of the steel sheet, the oxide skin having an average thickness of 25 [mu] m or less, the oxide skin having a structure containing, in area ratio, 20% or more of magnetite, 30% or more of an eutectoid transformation structure of iron and magnetite, and 15% or less of Vickers, the total of the magnetite, the Vickers, and the eutectoid transformation structure of the iron and the magnetite is 90% or more (including 100%), and the arithmetic average roughness of the interface between the scale and the base iron is 0.3 [mu] m or more.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets with excellent oxide scale adhesion for use in automobiles, home appliances, building materials, etc., and a method for manufacturing the same. This invention also relates to hot-rolled steel sheets with excellent oxide scale adhesion, particularly suitable as raw materials for components of construction machinery subjected to temper rolling, bending, pressing, laser cutting, and other processing methods, and a method for manufacturing the same. Background Technology

[0002] Hot-rolled steel sheets are typically hot-rolled at high temperatures in an oxidizing atmosphere, inevitably resulting in the formation of oxide scale (iron oxides) on their surface. If these oxide-coated hot-rolled steel sheets undergo temper rolling, bending, pressing, laser cutting, or other processing, some of the oxide scale will peel off. This results in poor processing, contamination of the processing line, and surface defects in the finished product. To avoid this, hot-rolled steel sheets with excellent oxide scale adhesion are required, and this requirement is becoming increasingly stringent. In particular, the peeling of thin-sheet oxide scale is a major cause of processing line contamination, surface quality degradation, and even laser cutability degradation. Therefore, to improve oxide scale adhesion, it is necessary to suppress the peeling of thin-sheet oxide scale.

[0003] Furthermore, the greater the thickness of the hot-rolled steel sheet, the greater the strain generated within the oxide scale during deformation, and the easier it tends to peel off. Moreover, peeling is even more pronounced in forming processes with high processing degrees, such as bending and pressing. On the other hand, the demand for thicker black-scaled hot-rolled steel sheets has been increasing in recent years; for example, for hot-rolled steel sheets with a thickness exceeding 5.0 mm, there is a strong requirement to improve the adhesion of the oxide scale.

[0004] Previously, various solutions have been proposed to improve the adhesion of oxide scale. For example, Patent Document 1 proposes a hot-rolled steel sheet with excellent oxide scale adhesion, characterized by rough rolling of a steel billet containing, by mass%, 0.01-0.3% C, less than 0.20% Si, 0.01-2.0% Mn, less than 0.10% P, less than 0.10% S, less than 0.10% Al, less than 0.10% Cr, and the remainder consisting of Fe and unavoidable impurities, followed by oxide scale removal, and finishing at an exit temperature of 800-950°C. The finishing mill is subjected to the following formula (1), and after cooling at an average cooling rate of 3°C / s to 80°C / s from the finishing mill to the start of coiling, it is coiled at a coiling temperature of 430 to 580°C, thereby forming a magnetite layer on the iron side of the matrix and magnetite particles and / or a eutectoid phase transformation structure of iron and magnetite on the upper layer of the magnetite layer, wherein the average particle size of the magnetite particles and / or the average block size of the eutectoid phase transformation structure is 3 μm to 8 μm, and the mass fraction of Vittenite contained in the oxide layer is 10% or less.

[0005] |T2-T1|≤50℃ and |T3-T2|≤50℃・・・(1)

[0006] In the above formula (1),

[0007] T1: Temperature (°C) of the finished steel plate at a distance of 30m from the front end along the length direction and the center along the width direction.

[0008] T2: Temperature (°C) at the center of the length and the center of the width of the finished steel plate.

[0009] T3: Temperature (°C) of the finished steel plate at 30m from the end in the length direction and at the center in the width direction.

[0010] In addition, Patent Document 2 proposes a hot-rolled steel sheet with excellent dense oxide scale properties. Its characteristic is that a slab containing, by mass percent, 0.02–0.20% C, 0.1–2.0% Mn, less than 0.3% Si, less than 0.03% P, less than 0.03% S, less than 0.03% Ni, 0.03–0.3% Cu, 0.04–0.5% Cr, and 0.03–0.3% with the remainder consisting of Fe and unavoidable impurities is heated to above 1100°C, hot-rolled at a temperature range of 800°C–950°C, and coiled at 400°C–650°C. The resulting surface roughness, which serves as the interface between the oxide scale on the steel sheet and the iron substrate, has a roughness height of 0.5 μm or more per inch of length, with a number of roughness values ​​of 300 or more.

[0011] In addition, Patent Document 3 discloses a hot-rolled steel sheet with excellent blackness, characterized in that the surface of the hot-rolled steel sheet containing C: 0.001 to 0.20 mass%, Si: 0.001 to 0.50 mass%, Mn: 0.05 to 2.0 mass%, P: less than 0.05 mass%, S: less than 0.05 mass%, and sol.Al: 0.01 to 0.10 mass%, with the remainder being Fe and unavoidable impurities, has an oxide scale with a thickness of more than 4 μm. The oxide scale is composed of more than 50% Fe3O4 by volume, and does not contain precipitated Fe in the thickness direction from the oxide scale surface to a depth of at least 2 μm.

[0012] Prior art literature

[0013] Patent Literature

[0014] Patent Document 1: Japanese Patent No. 6760425

[0015] Patent Document 2: Japanese Patent No. 4153734

[0016] Patent Document 3: Japanese Patent No. 4061996 Summary of the Invention

[0017] In the technology described in Patent Document 1, a steel billet with a specified composition is used, and the finishing mill exit temperature, cooling rate after rolling, and coiling temperature are adjusted during hot rolling. This optimizes the average grain size of the magnetite particles in the upper layer of the magnetite layer on the iron side of the oxide scale and / or the average block size of the eutectoid phase transformation structure of iron and magnetite. Furthermore, by controlling the temperature along the length of the steel plate immediately after finishing rolling, the adhesion of the uniform oxide scale along the length is improved. However, no method for suppressing the peeling of thin, flaky oxide scale is mentioned.

[0018] Patent Document 2 describes a technique that involves hot-rolling steel with added Ni, Cu, and Cr in specified amounts, and controlling the surface roughness of the interface between the oxide scale and the base iron within a specified range to obtain a hot-rolled steel sheet with excellent tight oxide scale properties. However, while this improves the adhesion between the oxide scale layer and the base iron, there is a concern that oxide scale peeling may occur on the surface or inside the oxide scale, reducing adhesion. Furthermore, there is a concern that the adhesion of the oxide scale may decrease as the thickness of the hot-rolled steel sheet increases. Additionally, no method is mentioned for suppressing the peeling of thin, flaky oxide scale.

[0019] In the technology described in Patent Document 3, a steel billet with a specified composition is heated, then hot-rolled at a finishing mill exit temperature of 800°C or higher, and then cooled to below 650°C at a cooling rate of 50°C / s or higher, and coiled at 600°C or higher. This advantageously achieves blackening of the oxide scale while maintaining its adhesion. However, there is concern that the peeling of thin-film oxide scale may not be sufficiently suppressed. Furthermore, there is concern that the adhesion of the oxide scale may decrease as the thickness of the hot-rolled steel sheet increases.

[0020] The present invention addresses the aforementioned problems and aims to provide a hot-rolled steel sheet and a method thereof that exhibit excellent scale adhesion, particularly resistance to flaking, even when processed under harsh conditions, especially in the case of thicker hot-rolled steel sheets and straightening machine processing. It should be noted that the thickness of the hot-rolled steel sheet in this invention exceeds 2.0 mm and is less than 25 mm, preferably exceeding 5.0 mm and less than 25 mm.

[0021] The inventors first investigated the causes of flaky oxide scale peeling from conventional hot-rolled steel sheets. They found that the flaky oxide scale peels off from the interface between the oxide scale and the base iron. That is, to suppress flaky peeling, it is important to appropriately control the interfacial properties between the oxide scale and the base iron, increasing the contact area between them. Furthermore, since the composition of the base iron also affects the contact area, it is also necessary to control the composition of the base iron.

[0022] The oxide scale generated during hot rolling can be broadly categorized into primary oxide scale generated in the slab heating furnace, during rough rolling, and secondary oxide scale generated during finish rolling. In existing technologies for thin steel sheets, primary oxide scale is mostly completely removed by descaling before finish rolling; therefore, the characteristics of secondary oxide scale have a significant impact on the oxide scale adhesion of the final product. Based on these insights, the inventors have conducted in-depth research on methods for achieving excellent oxide scale adhesion, particularly resistance to flaking, even in hot-rolled steel sheets with greater thicknesses, and have obtained the following insights.

[0023] (i) A steel billet with a specified composition is heated to a temperature of 1150°C or higher, and after temporarily removing the oxide scale by descaling, it is subjected to hot rough rolling and descaling under water jetting at a pressure of 30 MPa or less. Thus, an uneven oxide scale / base iron interface suitable for improving the resistance to flaking in the final product is obtained without completely removing the primary oxide scale generated during rough rolling from the interface with the base iron.

[0024] (ii) After hot rough rolling, finish rolling is performed at a temperature of 800 to 950°C at the exit of the finish rolling mill, so as to appropriately control the oxide scale thickness in a state with an uneven interface between the primary oxide scale and the base iron, and suppress the generation of cracks in the oxide scale that cause a decrease in resistance to flaking.

[0025] (iii) After cooling at an average cooling rate of 5°C / s or more in the temperature range from the finishing mill exit temperature to 750°C, cooling is then performed at an average cooling rate of 1°C / s to 30°C / s in the temperature range from 750°C to the start of coiling. This suppresses the decrease in resistance to flaking caused by excessive oxide scale growth at high temperatures.

[0026] (iv) Winding is performed at a winding temperature of 500–630°C, and the temperature is maintained for at least 100 minutes in a temperature range above -50°C and below the winding temperature. During this process, after winding, the temperature is maintained for at least 100 minutes in a temperature range above -50°C and below the winding temperature, thereby allowing sufficient eutectoid phase transformation of Vittenite to occur, thus improving the resistance of the oxide scale to flaky peeling.

[0027] This invention is based on the above insights and specifically provides the following content.

[0028] [1] A hot-rolled steel sheet having the following composition: by mass % containing C: 0.01 to 0.30%, Si: less than 0.50%, Mn: 0.01 to 2.0%, P: less than 0.10%, S: less than 0.10%, sol.Al: less than 0.10%, N: less than 0.015%, with the remainder consisting of Fe and unavoidable impurities, having an oxide scale on the surface of the steel sheet, the average thickness of the oxide scale being less than 25 μm, the oxide scale having the following structure by area percentage containing magnetite: more than 20%, eutectoid phase transformation structure of iron and magnetite: more than 30%, visterite: less than 15%, the total of magnetite, visterite, and eutectoid phase transformation structure of iron and magnetite being more than 90% (inclusive), and the arithmetic mean roughness of the interface between the oxide scale and the base iron being more than 0.3 μm.

[0029] [2] The hot-rolled steel sheet according to [1], wherein the above composition further contains, by mass %, one or more of Cu: less than 1.0%, Ni: less than 0.50%, and Cr: less than 2.0%.

[0030] [3] The hot-rolled steel sheet according to [1] or [2], wherein the above composition further contains, by mass %, one or more of the following: Mo: less than 1.0%, Nb: less than 0.1%, V: less than 0.1%, Ti: less than 0.03%, B: less than 0.01%, and Sb: less than 0.03%.

[0031] [4] A method for manufacturing hot-rolled steel sheet, wherein a steel billet having the composition described in [1] or [2] is heated to a heating temperature of 1150°C or higher, descaled, and then subjected to hot rough rolling and descaling by water jetting at a pressure of 30 MPa or less, and then subjected to finishing rolling at a temperature of 800 to 950°C at the finishing mill exit side, and then cooled at an average cooling rate of 5°C / s or higher in a temperature range from the above-mentioned finishing mill exit side temperature to 750°C, and then cooled at an average cooling rate of 1°C / s to 30°C / s in a temperature range from 750°C to the start of coiling, and then coiled at a coiling temperature of 500 to 630°C, and then held at a temperature range of -50°C or higher and below the coiling temperature for 100 minutes or more.

[0032] [5] A method for manufacturing hot-rolled steel sheet, wherein a steel billet having the composition described in [3] is heated to a heating temperature of 1150°C or higher, descaled, hot roughing and descaling under water jet pressure of 30 MPa or lower are performed, finishing rolling is performed at a finishing mill exit temperature of 800 to 950°C, cooling is performed at an average cooling rate of 5°C / s or higher in a temperature range from the finishing mill exit temperature to 750°C, cooling is performed at an average cooling rate of 1°C / s to 30°C / s in a temperature range from 750°C to the start of coiling, coiling is performed at a coiling temperature of 500 to 630°C, and the temperature is maintained at a temperature range of -50°C or higher and below the coiling temperature for 100 minutes or more.

[0033] According to the present invention, hot-rolled steel sheets with excellent resistance to flaking of oxide scale can be easily and inexpensively manufactured, resulting in particularly significant industrial benefits. Furthermore, the present invention also has the effect of preventing the flaking of oxide scale, greatly contributing to improved product surface quality, prevention of product defects, and improvement of the working environment. In addition, it solves the problem of reduced resistance to flaking of oxide scale with increasing thickness of hot-rolled steel sheets, processing under harsh conditions, and especially processing by straightening machines. Detailed Implementation

[0034] The following provides a detailed description of the hot-rolled steel sheet and its manufacturing method according to the present invention. It should be noted that the present invention is not limited to the following embodiments. The following embodiments include embodiments that can be easily replaced by those skilled in the art or substantially the same embodiments.

[0035] The hot-rolled steel sheet of the present invention comprises the following components. It should be noted that, unless otherwise specified, the "%" used as the unit of measurement for the content of the components indicates "mass %".

[0036] C: 0.01~0.30%

[0037] Carbon (C) is a useful element for ensuring strength. When its content is less than 0.01%, its effect on ensuring strength is small; therefore, the C content is 0.01% or more. When C contains more than 0.30%, CO gas is generated at the interface between the oxide scale and the base iron, reducing the contact area between the oxide scale and the base iron, leading to oxide scale peeling during rolling and becoming a cause of oxide scale defects. Therefore, the C content is 0.30% or less. From the viewpoint of oxide scale resistance to flaking peeling, 0.20% or less is preferred.

[0038] Si: below 0.50%

[0039] Si is an element that acts as a deoxidizer, and to achieve this effect, it is preferable to contain 0.01% or more. However, when the Si content exceeds 0.50%, Si accumulates at the interface between the oxide layer and the base iron, forming a Si oxide layer. Oxide layer peeling easily occurs at the interface between this Si oxide layer and the oxide layer formed on it. Therefore, the Si content is 0.50% or less, preferably 0.20% or less.

[0040] Mn: 0.01~2.0%

[0041] Mn is an element that, in addition to neutralizing dissolved sulfur (S) which causes embrittlement during hot working by converting it to MnS, also improves strength. When its content is less than 0.01%, the effect is minimal; on the other hand, when it exceeds 2.0%, it leads to decreased toughness and the formation of Mn-based oxides at the interface between the oxide scale and the base iron, causing a decrease in the oxide scale's resistance to flaking. Therefore, the Mn content is 0.01% to 2.0%. The preferred lower limit is 0.05% or more. The preferred upper limit is 1.5% or less.

[0042] P: below 0.10%

[0043] Phosphorus (P) has an adverse effect on grain boundary embrittlement, and is therefore an element that should be minimized. Furthermore, P forms a very brittle oxide layer at the interface between the oxide scale and the base iron, reducing the oxide scale's resistance to flaking. These adverse effects become more pronounced when the P content exceeds 0.10%, therefore it is kept below 0.10%. Preferably, it is below 0.05%. It should be noted that P can be absent, but from a manufacturing cost perspective, the P content is preferably 0.001% or more.

[0044] S: below 0.10%

[0045] Sulfur (S) is an element that significantly deteriorates hot workability and toughness. Furthermore, S accumulates at the interface between the oxide scale and the base iron, reducing the oxide scale's resistance to flaking. These adverse effects become more pronounced when the S content exceeds 0.10%, therefore it is kept below 0.10%. Preferably, it is below 0.05%. It should be noted that S can be absent, but from a manufacturing cost perspective, the S content is preferably 0.0001% or higher.

[0046] sol.Al: 0.10% or less

[0047] Sol.Al is an element that functions as a deoxidizer. The amount of sol.Al can be 0.00%, but to achieve this effect, it is preferable to contain 0.01% or more. On the other hand, when the content exceeds 0.10%, oxide inclusions increase, and the cleanliness decreases. Therefore, the amount of sol.Al is 0.10% or less, preferably 0.06% or less.

[0048] N: below 0.015%

[0049] Nitrogen (N) is an element that forms nitrides such as BN, AlN, and TiN in steel, and it reduces the thermal ductility and surface quality of steel. Furthermore, when the N content exceeds 0.015%, the surface quality deteriorates significantly. Therefore, the N content is 0.015% or less. The N content is preferably 0.010% or less. It should be noted that N can be absent, but from a manufacturing cost perspective, the N content is preferably 0.0001% or more. More preferably, the N content is 0.001% or more.

[0050] The above-mentioned chemical composition is an essential component of the hot-rolled steel sheet of the present invention. It should be noted that, in addition to the above-mentioned chemical composition, the hot-rolled steel sheet of the present invention may contain one or more of the following components, namely Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less, as needed to improve various properties.

[0051] Cu: below 1.0%

[0052] Cu is an element that accumulates at the interface between the oxide scale and the base iron, promoting grain boundary oxidation and enhancing the surface roughness of the interface, thereby improving the adhesion between the oxide scale and the base iron. To achieve this effect, a Cu content of 0.01% or more is preferred. However, when the Cu content exceeds 1.0%, molten Cu can penetrate the austenite grain boundaries of the base iron during heating, potentially leading to deterioration of surface properties due to thermal embrittlement. Therefore, the Cu content is 1.0% or less, preferably 0.8% or less.

[0053] Ni: below 0.50%

[0054] Like Cu, Ni is an element that accumulates at the interface between the oxide layer and the base iron, promoting grain boundary oxidation and enhancing the surface roughness of the interface, thus improving the adhesion between the oxide layer and the base iron. To achieve this effect, a content of 0.01% or more Ni is preferred. However, when the Ni content exceeds 0.50%, the above effect saturates, raising concerns about increased costs. Therefore, the Ni content is 0.50% or less, preferably 0.40% or less.

[0055] Cr: less than 2.0%

[0056] Cr has the effect of improving strength, hardenability, and corrosion resistance. Furthermore, Cr accumulates at the interface between the oxide scale and the base iron, and due to the unevenness of the interface, the oxide scale penetrates into the base iron, thus improving the adhesion between the oxide scale and the base iron. To obtain this effect, it is preferable to contain 0.01% or more Cr. On the other hand, when the content exceeds 2.0%, the above effect saturates; therefore, the Cr content is 2.0% or less. A more preferred lower limit is 0.07% or more. A more preferred upper limit is 1.0% or less. The most preferred lower limit is 0.12% or more. The most preferred upper limit is 0.8% or less.

[0057] In this invention, as needed, it may further contain one or more of the following: Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less.

[0058] Mo: 1.0% or less

[0059] Mo has the effect of improving strength and hardenability, and suppressing softening during tempering. To obtain such effects, it is preferable to contain 0.1% or more of Mo. On the other hand, when the content exceeds 1.0%, the strength is excessively increased, while the toughness and formability deteriorate. Therefore, when containing Mo, its amount is kept to be 1.0% or less.

[0060] Nb: below 0.1%

[0061] Nitrogen (Nb) is an element that improves the strength and toughness of the base material. To achieve this effect, it is preferable to contain 0.003% or more. On the other hand, a content exceeding 0.1% may lead to a decrease in toughness. Therefore, when containing Nb, its amount should be below 0.1%.

[0062] V: Below 0.1%

[0063] Vitamin V is an element that improves the strength and toughness of the base material. To achieve this effect, it is preferable to contain 0.003% or more. On the other hand, a content exceeding 0.1% may lead to a decrease in toughness. Therefore, when containing V, its amount should be below 0.1%.

[0064] Ti: below 0.03%

[0065] Ti is an element that improves the strength and toughness of the base material, and it also helps ensure the toughness of the heat-affected zone in welding. To achieve these effects, it is preferable to contain 0.001% or more Ti. On the other hand, a content exceeding 0.03% may lead to a decrease in toughness. Therefore, when Ti is present, its amount is kept below 0.03%.

[0066] B: Below 0.01%

[0067] Boron (B) is an element that improves the hardenability of steel, thereby increasing its strength. To achieve this effect, it is preferable to contain 0.0005% or more of B. On the other hand, the effect saturates when the content exceeds 0.01%, therefore, when containing B, its amount should be 0.01% or less.

[0068] Sb: below 0.03%

[0069] Sb accumulates on the surface of the billet during heating, thus suppressing the reduction of carbon (C) on the surface during heating. To achieve this effect, a Sb content of 0.001% or more is preferred. On the other hand, when the Sb content exceeds 0.03%, the billet becomes a liquid metal during heating, eroding to the original austenite grain boundaries and reducing the interfacial tightness between the oxide scale and the base iron. Therefore, the Sb content is 0.03% or less.

[0070] The remaining portion, excluding the aforementioned chemical components, consists of Fe and unavoidable impurities. As unavoidable impurities, the following are permissible amounts: O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less.

[0071] When any of the above-mentioned components are included at a value less than the preferred lower limit, the inclusion of any element at a value less than the lower limit does not impair the effects of the present invention. Therefore, when any of the above-mentioned elements are included at a value less than the preferred lower limit, the above-mentioned elements are included as unavoidable impurities.

[0072] Next, the oxide scale structure of the hot-rolled steel sheet of the present invention will be described.

[0073] The average thickness of the oxide layer is less than 25 μm.

[0074] When the average thickness of the oxide scale exceeds 25 μm, the strain applied to the outermost layer of the oxide scale during steel plate processing increases, causing cracks in the oxide scale and reducing the adhesion between the oxide scale and the base iron interface. Therefore, the average thickness of the oxide scale is 25 μm or less. Preferably, it is 20 μm or less, more preferably 18 μm or less. Furthermore, while there is no particular lower limit for the average thickness of the oxide scale, from the viewpoint of ensuring the stability of the cut surface quality during processing such as laser cutting, it is preferably 3 μm or more, more preferably 5 μm or more.

[0075] Magnetite: 20% or more

[0076] The high compatibility of magnetite with the iron matrix contributes to improved resistance to flaking of the oxide scale. This effect is insufficient when the magnetite content is less than 20%, therefore the magnetite area fraction is 20% or more, preferably 30% or more. While no specific upper limit is specified, when the magnetite area fraction exceeds 70%, the proportion of the eutectoid phase transformation structure of iron and magnetite that contributes to improving the oxide scale's resistance to flaking decreases, sometimes impairing resistance to flaking. Therefore, the magnetite area fraction is preferably 70% or less, more preferably 60% or less. It should be noted that this magnetite can be distinguished from the magnetite contained in the eutectoid phase transformation structure of iron and magnetite.

[0077] Eutectoid phase transformation structure of iron and magnetite: over 30%

[0078] The eutectoid phase transformation structure of iron and magnetite contributes to improved interfacial adhesion between the oxide scale and the matrix iron due to the high compatibility between magnetite and precipitated Fe with the matrix iron. This effect cannot be fully achieved when the area fraction is less than 30%, therefore the area fraction of the eutectoid phase transformation structure of iron and magnetite is 30% or more. Preferably, it is 40% or more, more preferably 45% or more.

[0079] Visteonite: below 15%

[0080] Besides the eutectoid phase transformation structure of magnetite and iron-magnetite, visterite sometimes remains in an untransformed state at room temperature. In addition to being more brittle than magnetite at room temperature, visterite has a lower compatibility with the matrix iron than the eutectoid phase transformation structure of magnetite and iron-magnetite, thus impairing the interfacial adhesion between the oxide scale and the matrix iron. Therefore, the area fraction of visterite is 15% or less. Preferably, it is 10% or less, more preferably 7% or less. It should be noted that the area fraction of visterite can be 0%.

[0081] In addition to visterite, a hematite layer sometimes forms on the outermost surface of the hot-rolled steel sheet. However, the formation of the hematite layer does not impair the effects of the present invention, and therefore it may be present. It should be noted that the hematite layer can lead to surface defects such as red oxide scale; therefore, the mass fraction of hematite is preferably 10% or less. It should be noted that the mass fraction of hematite can be considered as the area fraction.

[0082] The total proportion of eutectoid phase transformation structures of magnetite, visterite, and iron and magnetite is 90% or more (including 100%).

[0083] When a phase other than magnetite, visterite, and the eutectoid transformation structure of iron and magnetite is present, and the total fraction of the eutectoid transformation structure of magnetite, visterite, and iron and magnetite is less than 90%, it becomes a cause of surface defects and reduces resistance to flaking. From the viewpoint of ensuring surface quality and specified resistance to flaking, the total fraction of the eutectoid transformation structure of magnetite, visterite, and iron and magnetite should be 90% or more.

[0084] It should be noted that the remaining microstructure, besides magnetite, visterite, and the eutectoid phase transformation microstructure of iron and magnetite, includes non-ferrous oxides such as Si-based oxides and Cr-based oxides, in addition to hematite. If their combined area fraction is less than 10%, it does not impair the effectiveness of the present invention. Furthermore, these remaining microstructures can be determined by X-ray diffraction analysis using the same method as described later for determining hematite.

[0085] The arithmetic mean roughness of the interface between the oxide scale and the base iron is greater than 0.3 μm.

[0086] By ensuring that the arithmetic mean roughness of the interface between the oxide scale and the base iron is 0.3 μm or more, flake-like peeling caused by delamination at the interface can be suppressed. When the arithmetic mean roughness of the interface between the oxide scale and the base iron is less than 0.3 μm, this effect is insufficient, and resistance to flake-like peeling deteriorates. Therefore, the arithmetic mean roughness of the interface between the oxide scale and the base iron is 0.3 μm or more, preferably 0.4 μm or more. Furthermore, while no specific upper limit is specified, from the viewpoint of ensuring the stability of the cut surface quality during processing such as laser cutting, the arithmetic mean roughness of the interface between the oxide scale and the base iron is preferably 5 μm or less.

[0087] Next, the method for determining the oxide scale structure of the hot-rolled steel sheet of the present invention will be described.

[0088] The area ratios of magnetite, the eutectoid transformation structure of iron and magnetite, visterenite, and precipitated Fe were determined by cutting a thick section of the plate perpendicular to the steel plate surface and parallel to the rolling direction, followed by mirror polishing. Then, the area ratios were measured using a scanning electron microscope (SEM) at 3000x magnification, observing the reflected electron image of the oxide scale section. In the SEM reflected electron image, magnetite was the darkest, the matrix iron and precipitated Fe were the brightest, and visterenite showed a region with intermediate contrast. Furthermore, the average thickness of the oxide scale was calculated by averaging the oxide scale thickness measured at three arbitrary locations using SEM. Additionally, the area ratios of the eutectoid transformation structure of magnetite, visterenite, and iron and magnetite refer to the values ​​when the total area of ​​the oxide scale is set to 100%, excluding voids within the oxide scale from the area ratio calculation.

[0089] To determine the mass fraction of hematite, an X-ray diffraction apparatus was used, employing CoK... α The integral intensity of the diffraction peaks of each phase in the oxide scale is determined by the X-ray source. The integral intensity of each phase in the standard sample (a sample in which Fe, FeO (Virtue), Fe2O3 (Hematite), and Fe3O4 (Magnetite) are mixed in equal weights) and the sample under test can be calculated using the following formula (2).

[0090] Mass fraction of phase A = (I A / RA )×100 / ((I Fe / R Fe ) + (I FeO / R FeO ) + (I Fe2O3 / R Fe2O3 ) + (I Fe3O4 / R Fe3O4 )) ・・・(2)

[0091] In equation (2) above,

[0092] I A The integrated intensity of phase A in the tested sample

[0093] R A The integrated intensity of phase A in the standard sample

[0094] A: Fe, FeO, Fe2O3 or Fe3O4.

[0095] It should be noted that the mass fraction of hematite detected by X-ray diffraction is regarded as the area fraction. It is subtracted from the total area of ​​the oxide scale to 100%, and the area fractions of magnetite, eutectoid phase transformation structure and visterenite are calculated from the remaining area.

[0096] The arithmetic mean roughness of the interface between the oxide scale and the base iron was determined by removing the oxide scale from the steel plate surface through pickling, and then measuring the three-dimensional shape of the base iron surface using a laser microscope. Five roughness curves parallel to the rolling direction were extracted with a reference length of 100 μm. The roughness was then calculated by averaging the arithmetic mean roughness of these intervals as determined according to JIS B 0633:2001.

[0097] Next, the method for manufacturing the hot-rolled steel sheet of the present invention will be described.

[0098] It should be noted that the temperatures determined in each process of this invention refer to the surface temperature of the slab (steel billet) or steel plate, which can be measured using a radiation thermometer or the like. Furthermore, unless otherwise specified, the average cooling rate is ((cooling start temperature - cooling stop temperature) / cooling time).

[0099] In this invention, the manufacturing method of the steel billet composed of the above-mentioned components is not particularly limited, and any commonly used method can be applied. For example, it is preferable to melt molten steel having the above-mentioned components in a converter, electric furnace, or the like, and then produce steel billets such as slabs by casting methods such as continuous casting. It should be noted that there is no problem using an ingot-rolling method.

[0100] Heating temperature: above 1150℃

[0101] The steel billet is heated to a temperature of 1150°C or higher. This heating is performed to achieve sufficient solution treatment of the base steel and to roughen the interface between the primary oxide scale and the base iron. If the heating temperature is below 1150°C, sufficient interface roughening cannot be achieved; therefore, the heating temperature is 1150°C or higher. Preferably, it is 1180°C or higher. Further preferably, it is 1300°C or lower. It should be noted that in the case of slabs manufactured by continuous casting, direct rolling (i.e., direct rolling) or holding and rolling to suppress temperature drop can be applied.

[0102] The hot rolling process consists of roughing and finishing rolling. In roughing, as long as a thin slab of the specified dimensions can be produced, the conditions are not particularly limited. The oxide scale generated during the heating of the steel billet is removed by water jetting before roughing. This is because the oxide scale generated during heating is very thick, reducing the resistance of the final product to flaking and contributing to surface defects. In this case, the removal of the relatively thick oxide scale is sufficient, and conventional methods can be used. Alternatively, to perform finishing rolling at a specified temperature, the material being rolled can be heated midway through the process using heating methods such as thin slab heaters.

[0103] Pressure: Descaling under water jet below 30 MPa

[0104] Before finishing rolling, oxide scale is removed by water jetting at a pressure of 30 MPa or less at the entrance side of the finishing mill. This results in an uneven oxide scale-base iron interface suitable for improving the resistance to flaking of the final product, without completely removing the primary oxide scale generated during rough rolling from the interface with the base iron. When the water jetting pressure exceeds 30 MPa, the oxide scale is sometimes completely removed from the interface, leading to insufficient roughness at the oxide scale-base iron interface and reduced adhesion. Therefore, the water jetting pressure is 30 MPa or less, preferably 25 MPa or less. Furthermore, from the viewpoint of preventing excessive increase in oxide scale thickness, the water jetting pressure is preferably 5 MPa or more, more preferably 10 MPa or more.

[0105] Next, finish rolling is performed. When the temperature at the finish rolling inlet exceeds 1100°C, the thickness of the oxide scale sometimes increases, reducing the adhesion between the oxide scale and the base iron. On the other hand, when the temperature at the finish rolling inlet is below 950°C, the rolling load sometimes increases significantly, leading to a decrease in productivity. Furthermore, as the product plate thickness increases, the thickness at the finish rolling inlet also increases. Therefore, for example, when the product plate thickness exceeds 5.0 mm, the time required to start finish rolling is longer, sometimes resulting in a decrease in productivity. Therefore, the finish rolling inlet temperature is preferably below 1100°C, more preferably below 1050°C. Additionally, the lower limit of the finish rolling inlet temperature is preferably above 950°C.

[0106] Finishing mill exit temperature: 800~950℃

[0107] When the exit temperature of the finishing mill is below 800°C, the ductility of the oxide scale decreases, leading to crack formation, which in turn reduces the scale's resistance to flaking. Furthermore, the oxide scale becomes finer, increasing its hardness and reducing the interfacial adhesion between the scale and the base iron. On the other hand, when the exit temperature exceeds 950°C, excessive scale growth increases its thickness, further reducing the interfacial adhesion between the scale and the base iron. Therefore, the exit temperature of the finishing mill is ideally between 800°C and 950°C. A lower preferred temperature is 820°C or higher, and a higher preferred temperature is 930°C or lower.

[0108] Cooling is carried out at an average cooling rate of 5°C / s or higher within a temperature range from the temperature at the finish mill exit side to 750°C.

[0109] Because scale grows faster in high-temperature regions, rapid cooling of the high-temperature region immediately after finishing rolling is necessary to suppress the reduction in scale resistance to flaking caused by excessive scale growth. When the average cooling rate from the finishing mill exit temperature to 750°C is less than 5°C / s, excessive scale growth occurs, leading to a decrease in the interfacial tightness between the scale and the base iron. Therefore, the average cooling rate from the finishing mill exit temperature to 750°C is 5°C / s or more, preferably 7°C / s or more. On the other hand, when the average cooling rate from the finishing mill exit temperature to 750°C exceeds 80°C / s, the scale structure sometimes becomes finer, thereby reducing the interfacial tightness between the scale and the base iron. Furthermore, cracks sometimes occur due to reduced scale ductility, also contributing to a decrease in the interfacial tightness between the scale and the base iron. Therefore, the average cooling rate from the finishing mill exit temperature to 750°C is preferably 80°C / s or less, more preferably 50°C / s or less.

[0110] Cooling is performed at an average cooling rate of 1°C / s to 30°C / s within the temperature range from 750°C to the start of winding.

[0111] From 750°C to the temperature range at which coiling begins, the growth of oxide scale is slower compared to the high-temperature region immediately after finishing rolling. However, it is necessary to suppress the reduction in interfacial adhesion between the oxide scale and the base iron caused by excessive oxide scale growth. When the average cooling rate from 750°C to the temperature range at which coiling begins is less than 1°C / s, excessive oxide scale growth occurs, contributing to the reduction in interfacial adhesion between the oxide scale and the base iron. Therefore, the average cooling rate from 750°C to the temperature range at which coiling begins is 1°C / s or more, preferably 3°C / s or more. On the other hand, when the average cooling rate from 750°C to the temperature range at which coiling begins exceeds 30°C / s, the oxide scale structure becomes finer, and the stress difference between the oxide scale and the base iron increases. Consequently, cracks may sometimes form in the oxide scale, reducing the interfacial adhesion between the oxide scale and the base iron. Therefore, the average cooling rate from the temperature at the finishing mill exit side to 750°C is 30°C / s or less, preferably 20°C / s or less.

[0112] Winding temperature: 500~630℃

[0113] After the above cooling process, the steel sheet is coiled at a coiling temperature of 500–630°C. When the coiling temperature is below 500°C, the eutectoid transformation of visterensis does not occur sufficiently, resulting in excessive visterensis remaining at room temperature. Consequently, due to the brittle nature of visterensis at room temperature, the resistance of the oxide scale to flaking peeling decreases. When the coiling temperature exceeds 630°C, excessive oxide scale growth occurs, leading to reduced interfacial adhesion between the oxide scale and the base iron. Therefore, a coiling temperature of 500°C–630°C is preferred, with a lower limit of 530°C or higher. Furthermore, a higher limit of 620°C or lower is preferred.

[0114] Maintain the temperature for more than 100 minutes within a temperature range between -50°C and below the winding temperature.

[0115] After winding, the temperature is maintained in a range of -50°C to below the winding temperature for at least 100 minutes to ensure sufficient eutectoid transformation of the visterenite. If the holding time in this temperature range is less than 100 minutes or less than -50°C, the eutectoid transformation of the visterenite will not occur sufficiently, resulting in excessive visterenite residue at room temperature. Consequently, due to the brittle nature of visterenite at room temperature, the resistance of the oxide scale to flaking peeling decreases. Therefore, the holding time in the temperature range of -50°C to below the winding temperature is at least 100 minutes, preferably at least 120 minutes. Furthermore, a longer holding time may lead to excessive internal oxidation at the oxide scale / base iron interface, which reduces the resistance of the oxide scale to flaking peeling. Therefore, the holding time in the temperature range of -50°C to below the winding temperature is preferably 300 minutes or less.

[0116] It should be noted that after winding, the roll material is preferably placed in a roll material box or covered to suppress oxidation of the outermost periphery and edges.

[0117] Furthermore, in hot-rolled steel sheets coiled into coils, shape straightening processes can be implemented by applying deformation to the steel sheet using roller straighteners, tension straighteners, etc. For example, in a 12mm thick hot-rolled steel sheet, two φ250mm upper rollers and three lower rollers are configured, and shape straightening is performed under a pressing amount of 2mm.

[0118] Example

[0119] The following describes embodiments of the present invention.

[0120] The steel with the composition shown in Table 1 was melted and cast to produce steel billets. These steel billets were hot-rolled under the conditions shown in Table 2 to produce hot-rolled coils with a thickness of 6–18 mm. The obtained hot-rolled coils were then shaped by a straightening machine and cut to specified lengths to produce hot-rolled sheets. Test pieces were taken from various parts of the obtained hot-rolled sheets, and the oxide scale structure and adhesion were evaluated using the following methods.

[0121]

[0122]

[0123] For the area ratio of magnetite, the eutectoid phase transformation structure of iron and magnetite, visterenite, and precipitated Fe, a plate-thickness section perpendicular to the steel plate surface and parallel to the rolling direction was cut and mirror-polished. Then, the oxide scale was determined by observing the reflected electron image of the cross-section at 3000x magnification using a scanning electron microscope (SEM). In the SEM reflected electron image, magnetite was the darkest, matrix iron and precipitated Fe were the brightest, and visterenite showed an area with intermediate contrast. Furthermore, the average thickness of the oxide scale was determined by averaging the oxide scale thickness measured at three arbitrary locations using SEM.

[0124] To determine the mass fraction of hematite, an X-ray diffraction apparatus was used, employing CoK... α The integral intensity of the diffraction peaks of each phase in the oxide scale is determined by the X-ray source. The integral intensity is calculated using the following formula (2) based on the ratio of the integral intensity of each phase in the standard sample (a sample in which Fe, FeO (Virtue), Fe2O3 (Hematite), and Fe3O4 (Magnetite) are mixed in equal weights) and the sample under test.

[0125] Mass fraction of phase A = (I A / R A )×100 / ((I Fe / R Fe ) + (I FeO / RFeO ) + (I Fe2O3 / R Fe2O3 ) + (I Fe3O4 / R Fe3O4 )) ・・・(2)

[0126] In equation (2) above,

[0127] I A The integrated intensity of phase A in the tested sample

[0128] R A The integrated intensity of phase A in the standard sample

[0129] A: Fe, FeO, Fe2O3 or Fe3O4.

[0130] For the arithmetic mean roughness of the interface between the oxide scale and the base iron, after removing the oxide scale from the steel plate surface by pickling, the three-dimensional shape of the base iron surface was measured using a laser microscope, and five roughness curves parallel to the rolling direction were extracted with a reference length of 100 μm. Then, the roughness was calculated by averaging the arithmetic mean roughness of these intervals.

[0131] The oxide scale adhesion evaluation in this invention is based on resistance to flaking peeling. Specifically, a 30mm × 100mm test piece is taken from a hot-rolled sheet processed by a straightening machine, with its length parallel to the rolling direction. This piece is then bent according to the bending method in JIS Z 2248. Adhesive tape is applied to the outer surface of the bent steel sheet, and the oxide scale is peeled off. The bending conditions are: the ratio of the radius r of the pressing tool tip to the thickness t of the test sheet (r / t) is 4, and the bending angle is 180°. An image of the steel sheet surface after the tape peeling is taken using a camera. The area ratio of the exposed base iron, i.e., the proportion of the area of ​​the exposed base iron to the area of ​​the peeled area, is determined from the obtained image. When the area ratio of the exposed base iron is less than 10%, the resistance to flaking peeling is rated as excellent, and is recorded as 0 in Table 2. On the other hand, when the area ratio of the exposed base iron is 10% or more, the resistance to flaking peeling is rated as poor, and is recorded as × in Table 2.

[0132] Table 2 shows that the oxide scales of the present invention exhibit excellent resistance to sheet peeling, while the oxide scales of the comparative examples exhibit poor resistance to sheet peeling.

Claims

1. A hot-rolled steel plate having the following composition: containing, by mass%, C: 0.01-0.30%, Si: less than 0.50%, Mn: 0.01-2.0%, P: less than 0.10%, S: less than 0.10%, sol.Al: less than 0.10%, N: less than 0.015%, with the remainder consisting of Fe and unavoidable impurities. The steel plate has an oxide scale on its surface. The average thickness of the oxide layer is less than 25 μm. The oxide scale has the following structure: by area percentage, it contains magnetite: more than 20%, eutectoid phase transformation structure of iron and magnetite: more than 30%, and visterite: less than 15%, with the total of magnetite, visterite, and eutectoid phase transformation structure of iron and magnetite being more than 90% (inclusive). The arithmetic mean roughness of the interface between the oxide scale and the base iron is greater than 0.3 μm.

2. The hot-rolled steel plate according to claim 1, wherein, The composition further contains, by mass%, one or more of the following: Cu: less than 1.0%, Ni: less than 0.50%, and Cr: less than 2.0%.

3. The hot-rolled steel plate according to claim 1 or 2, wherein, The composition further contains, by mass%, one or more of the following: Mo: less than 1.0%, Nb: less than 0.1%, V: less than 0.1%, Ti: less than 0.03%, B: less than 0.01%, and Sb: less than 0.03%.

4. A method for manufacturing hot-rolled steel sheet, comprising heating a steel billet having the composition described in claim 1 or 2 at a temperature above 1150°C to remove oxide scale, Descaling is performed by hot rough rolling and water jetting at a pressure below 30 MPa. Finish rolling is performed at a temperature of 800–950°C on the exit side of the finishing mill. After cooling at an average cooling rate of 5°C / s or more within a temperature range from the exit temperature of the finishing mill to 750°C. Cooling was performed at an average rate of 1°C / s to 30°C / s within the temperature range from 750°C to the start of winding. Winding is performed at a winding temperature of 500–630°C. Maintain the temperature for at least 100 minutes within a range between -50°C and below the winding temperature.

5. A method for manufacturing hot-rolled steel sheet, comprising heating a steel billet with the composition described in claim 3 at a temperature above 1150°C to remove oxide scale, Descaling is performed by hot rough rolling and water jetting at a pressure below 30 MPa. Finish rolling is performed at a temperature of 800–950°C on the exit side of the finishing mill. After cooling at an average cooling rate of 5°C / s or more within a temperature range from the exit temperature of the finishing mill to 750°C. Cooling was performed at an average rate of 1°C / s to 30°C / s within the temperature range from 750°C to the start of winding. Winding is performed at a temperature of 500–630℃. Maintain the temperature for at least 100 minutes within a range between -50°C and below the winding temperature.

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