Hot-rolled steel sheet and method of manufacturing the same

Abstract

The present invention provides a hot-rolled steel sheet having a large thickness, excellent scale adhesion, and small scale adhesion fluctuations in the width direction of the sheet, and a method for producing the same. A hot-rolled steel sheet having a prescribed composition and a scale on the surface of the sheet, the scale in the width direction of the sheet having a structure in which, in terms of area ratio, magnetite grains are 20 to 60%, eutectoid phase transformation structures of iron and magnetite are 30% or more, in which the magnetite is composed of the magnetite grains and magnetite contained in the eutectoid phase transformation structures, wustite is 15% or less, in terms of mass ratio, hematite is 5% or less, the average thickness of the scale in the width direction of the sheet is 5 to 20 μm, and the fluctuations in the thickness of the scale in the width direction of the sheet are 4 μm or less.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets with excellent oxide scale adhesion for use in construction machinery, automobiles, home appliances, building materials, etc., and a method for manufacturing the same. In particular, this invention relates to hot-rolled steel sheets suitable as raw materials for components of construction machinery subjected to laser cutting, with excellent oxide scale adhesion and minimal fluctuation in oxide scale adhesion along the width direction, 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 the surface. When hot-rolled steel sheets with this oxide scale (hereinafter referred to as black-scale hot-rolled steel sheets) undergo processing such as leveling, bending, stamping, and laser cutting, some of the oxide scale will peel off. This results in poor processing, contamination of the processing line, and surface defects in finished products. To avoid this, there is a strong demand for hot-rolled steel sheets with excellent oxide scale adhesion on the surface, and this requirement is becoming increasingly stringent.

[0003] Furthermore, the oxide scale on hot-rolled steel sheets exhibits the following tendency: the thicker the hot-rolled steel sheet, the greater the strain generated on the oxide scale during deformation, and the easier it is to peel off. As a result, it becomes more prone to peeling during forming processes with high processing degrees, such as bending and stamping. On the other hand, the demand for thicker black-scale hot-rolled steel sheets has been increasing in recent years. For example, for hot-rolled steel sheets with a thickness greater than 5.0 mm, there is a strong demand for improved oxide scale adhesion.

[0004] Furthermore, if the adhesion of the oxide scale in the width direction of the steel sheet fluctuates, or if there are areas with poor adhesion, it can cause processing defects. This is because, for example, during laser cutting, the absorption of thermal energy from the laser on the steel sheet surface becomes unstable. Therefore, it is necessary to remove the poorly adhered portions of the oxide scale before use. Thus, from the perspective of improving yield, workability, and appearance quality, hot-rolled steel sheets with uniform oxide scale thickness and excellent adhesion in the width direction are strongly required.

[0005] 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 in that, for steel raw material having a composition of C: 0.01-0.3%, Si: less than 0.20%, Mn: 0.01-2.0%, P: less than 0.10%, S: less than 0.10%, Al: less than 0.10%, Cr: 0.01-2.0% by mass, with the balance consisting of Fe and unavoidable impurities, rough rolling is performed, oxide scale is removed, and finishing rolling is performed at a finishing mill exit temperature of 800-950°C to meet the following ( The finishing mill of type 1) is cooled at an average cooling rate of 3°C / s or more and 80°C / s or less from the end of finishing milling to the start of coiling, and then coiled at a coiling temperature of 430 to 580°C. As a result, a magnetite layer is formed from the steel base side, and magnetite grains and / or a eutectoid phase transformation structure of iron and magnetite are formed on the upper layer of the magnetite layer. The average grain size of the magnetite grains and / or the average block size of the eutectoid phase transformation structure are 3 μm or more and 8 μm or less, and the mass fraction of ferroic ore contained in the oxide layer is 10% or less.

[0006] |T2-T1|≤50℃ and |T3-T2|≤50℃ …(1)

[0007] In the above formula (1),

[0008] T1: Temperature (°C) of the finished steel plate at a distance of 30m from the front end along the length direction and at the center along the width direction.

[0009] T2: Temperature (°C) at the center of the steel plate along both the length and width directions after precision rolling.

[0010] T3: Temperature (°C) of the finished steel plate at the center of the width direction, 30m from the end of the length direction.

[0011] Furthermore, Patent Document 2 proposes the following method: 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 the balance being Fe and unavoidable impurities, is heated at 1100°C or higher, hot-rolled at 800°C–950°C, and coiled at 400°C–650°C. This results in a hot-rolled steel sheet with excellent oxide scale density, characterized by a surface roughness of at least 300 times per inch of length for a height of 0.5 μm or more at the interface between the oxide scale and the steel substrate.

[0012] Furthermore, Patent Document 3 proposes a hot-rolled steel sheet with excellent oxide scale adhesion. This hot-rolled steel sheet has an oxide scale, characterized in that the oxide scale within 30 mm of the end face of the coil has a magnetite layer at the interface between the steel substrate and the oxide scale, with an area ratio of 90% or more in contact with the steel substrate. Above the magnetite layer in contact with the steel substrate is a co-eutectoid layer of iron and magnetite. Above the co-eutectoid layer of iron and magnetite is a magnetite layer. Above the magnetite layer is a hematite layer. The combined thickness of the magnetite layer above the co-eutectoid layer of iron and magnetite and the hematite layer is less than 30% of the overall thickness of the oxide scale. Furthermore, the difference between the thickness of the oxide scale at 30 mm from the end face of the coil and the thickness of the oxide scale at the center of the coil is less than 2 μm.

[0013] Existing technical documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Application Publication No. 2019-183267

[0016] Patent Document 2: Japanese Patent Application Publication No. 2004-027312

[0017] Patent Document 3: Japanese Patent Application Publication No. 2012-148286 Summary of the Invention

[0018] The problem that the invention aims to solve

[0019] In the technology described in Patent Document 1, steel raw material 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 magnetite grains in the upper layer of the magnetite layer on the steel base 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 uniformity and tightness of the oxide scale along the length direction are improved. However, a method for uniformly improving the tightness of the oxide scale along the width direction is not mentioned.

[0020] Patent Document 2 describes a hot-rolled steel sheet with excellent oxide scale density achieved by hot-rolling steel with added Ni, Cu, and Cr in specified amounts and controlling the surface roughness of the oxide scale interface between the steel sheet and the steel substrate within a specified range. However, while the adhesion between the oxide scale layer and the steel substrate is improved, there is a concern that the adhesion of the oxide scale may become insufficient when the thickness of the hot-rolled steel sheet increases. Furthermore, no method is mentioned for uniformly improving the adhesion of the oxide scale in the width direction.

[0021] Patent Document 3 discloses a method for manufacturing hot-rolled steel sheets by hot rolling and coiling steel. In this method, a rough-rolled steel sheet is finish-rolled at 850–1050°C. Then, the finish-rolled hot-rolled steel sheet is coiled at a coiling temperature of 500–650°C, while simultaneously cooling both ends of the hot-rolled steel sheet so that the end-face temperature reaches below 480°C within 5 minutes of coiling. The end-face temperature is then maintained below 480°C, and the sheet is slowly cooled in a coiled state from a point when the end-face temperature reaches 400–480°C. This results in hot-rolled steel sheets with excellent oxide scale adhesion, particularly at the edges of the hot-rolled coil. However, no suitable oxide scale structure is specified for improving adhesion in the central portion of the steel sheet in the width direction. As a result, there is a concern that the oxide scale adhesion may become insufficient when the thickness of the hot-rolled steel sheet increases. In addition, although the sealing at the outermost edge has improved, there is a concern that the sealing within 200mm of the edge has not been sufficiently improved.

[0022] The purpose of this invention is to solve the above-mentioned problems and provide a hot-rolled steel sheet with excellent oxide scale adhesion, even for thicker sheets, especially with small fluctuations in oxide scale adhesion in the width direction of the steel sheet, and improved laser cutability, as well as a method for manufacturing the same.

[0023] Methods for solving problems

[0024] The inventors first investigated why conventional hot-rolled steel sheets could not achieve uniform and excellent oxide scale adhesion in the width direction. During hot rolling, the oxide scale formed sequentially from the surface side of the oxide scale at high temperatures: hematite (Fe2O3), magnetite (Fe3O4), and aragonite (FeO). Among these, aragonite undergoes a eutectoid phase transformation during cooling after coiling, forming a eutectoid phase transformation structure (4FeO→Fe3O4+Fe) composed of magnetite and precipitated Fe. The eutectoid phase transformation structure composed of magnetite and precipitated Fe has particularly high integration with the surrounding magnetite grains and the steel matrix, thus contributing to improved oxide scale adhesion. In conventional techniques, oxide scale adhesion tends to be particularly poor, especially within 200 mm of the edge in the width direction. This is because the oxide scale is re-oxidized by air intruding from the edge after coiling, leading to an excessive increase in the amount of hematite on the oxide scale surface and the magnetite layer near the oxide scale surface, composed of columnar magnetite grains. This indicates that the proportion of eutectoid phase transformation structures, which contribute to improved oxide scale adhesion, decreases, while the oxide scale thickness at the edges increases. Furthermore, it shows that within 200 mm of the edge in the width direction, the oxide scale adhesion decreases at the edge and in the portion 200 mm from the edge due to different mechanisms. At the edge, the cooling rate is highest, resulting in residual aragonite in the final oxide scale structure, further reducing adhesion. On the other hand, in the portion 200 mm from the edge, the cooling rate decreases due to exposure to air intruding from the edge and reheating from the center in the width direction. Therefore, the formation of hematite and magnetite layers near the oxide scale surface is highest in the width direction, significantly reducing adhesion. In other words, this indicates that to obtain excellent oxide scale adhesion uniformly in the width direction, it is necessary to improve the oxide scale adhesion at the edge and in the portion 200 mm from the edge.

[0025] Furthermore, during manufacturing under normal conditions, especially with thick sheets, the phase transformation expansion after winding occurs unevenly along the length of the coil. This indicates the following: loosening of the hot-rolled coil occurs, and the central portion in the width direction is also re-oxidized due to air intrusion, resulting in poor sealing.

[0026] Therefore, in order to solve the above problems, the inventors conducted in-depth research on methods for obtaining hot-rolled steel sheets with excellent oxide scale adhesion even with larger sheet thicknesses, especially with small fluctuations in oxide scale adhesion in the width direction of the coil, and obtained the following insights.

[0027] (i) For steel raw materials with a specified composition, after hot rough rolling, the oxide scale is removed, and after finishing rolling at a finishing mill exit temperature of 800–950°C, the material is cooled to the coiling temperature at a specified cooling rate. This effectively controls the oxide scale thickness while suppressing the formation of cracks in the oxide scale that could lead to reduced adhesion.

[0028] (ii) From the start of winding, the temperature at the edge of the roll is cooled at an average cooling rate of 0.5°C / s to 6.0°C / s until the cooling stop temperature reaches 300°C to 450°C. Here, the edge of the roll refers to the area within 200 mm of the edge in the width direction of the roll. This improves the rigidity of the roll and prevents loosening. As a result, in the center of the width direction, re-oxidation is suppressed due to isolation from the oxidizing atmosphere, and a suitable eutectoid phase transformation structure for improved adhesion can be obtained. Furthermore, although it is difficult to completely isolate the edge of the roll in the width direction from the oxidizing atmosphere, re-oxidation is suppressed by cooling the edge, and the eutectoid phase transformation is carried out by reheating from the center of the width direction, ensuring excellent adhesion. In addition, this also suppresses fluctuations in the oxide scale thickness in the width direction, and the adhesion of the oxide scale is uniformly improved in the width direction, thereby improving laser cutability.

[0029] This invention is based on the above insights and specifically provides the following invention.

[0030] [1] A hot-rolled steel sheet having, by mass percent, a composition comprising 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 balance being Fe and unavoidable impurities, and having an oxide scale on the surface of the steel sheet, wherein the oxide scale in the width direction of the steel sheet has the following structure: by area percent The steel plate contains 20% to 60% magnetite grains, 30% or more of a co-eutectoid phase transformation structure of iron and magnetite, wherein the magnetite is composed of the magnetite grains and the magnetite contained in the co-eutectoid phase transformation structure, 15% or less aragonite, and 5% or less hematite by mass fraction. The average thickness of the oxide scale in the width direction of the steel plate is 5 μm or more and 20 μm or less, and the fluctuation of the thickness of the oxide scale in the width direction of the steel plate is 4 μm or less.

[0031] [2] The hot-rolled steel plate according to [1], wherein the 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%.

[0032] [3] The hot-rolled steel plate according to [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%.

[0033] [4] A method for manufacturing a hot-rolled steel sheet, wherein, for a steel raw material having the composition described in any one of [1] to [3], after hot rough rolling, the sheet is descaled, and then finished rolled at a finishing mill exit temperature of 800°C or higher and 950°C or lower. After cooling at an average cooling rate of 5°C / s or higher in a temperature range from the finishing mill exit temperature to 750°C, the sheet is cooled at an average cooling rate of 1°C / s or higher and 30°C / s or lower in a temperature range from 750°C to the start of coiling. The sheet is then coiled at a coiling temperature of 500°C or higher and 650°C or lower. From the start of coiling, during the period from the coiling temperature to the cooling stop temperature of 300°C or higher and 450°C or lower, the entire coil is cooled at an average cooling rate of 0.5°C / s or higher and 6.0°C / s or lower.

[0034] Invention Effects

[0035] According to the present invention, hot-rolled steel sheets with excellent oxide scale adhesion can be manufactured easily and inexpensively, resulting in particularly significant industrial benefits. Furthermore, the present invention reduces fluctuations in oxide scale adhesion along the width direction of the steel sheet, thereby greatly contributing to improved surface quality, enhanced laser cutability, and improved working environment of the finished product. Moreover, it also solves the problem of reduced oxide scale adhesion that occurs with increasing thickness of the hot-rolled steel sheet.

[0036] It should be noted that the thickness of the hot-rolled steel plate of the present invention is set to be greater than 2.0 mm and less than 25 mm, preferably greater than 5.0 mm and less than 25 mm. Detailed Implementation

[0037] The hot-rolled steel sheet and its manufacturing method of the present invention will now be described in detail. It should be noted that the present invention is not limited to the following embodiments. The following embodiments also include content that can be easily replaced or is substantially the same by those skilled in the art.

[0038] The hot-rolled steel sheet of the present invention comprises the following components. It should be noted that, unless otherwise specified, the unit "%" for the content of the components means "mass %".

[0039] C: 0.01~0.30%

[0040] Carbon (C) is an element useful for ensuring strength. When its content is less than 0.01%, its effect on ensuring strength is small; therefore, the C content is set to 0.01% or more. When C content is greater than 0.30%, CO gas is generated at the interface between the oxide scale and the steel substrate, causing delamination of the oxide scale at the interface during rolling, thus becoming a cause of oxide scale defects. Therefore, the C content is set to 0.30% or less. From the viewpoint of oxide scale adhesion, 0.20% or less is preferred.

[0041] Si: below 0.50%

[0042] Si is an element that functions as a deoxidizer. While it is not strictly necessary to contain Si, it is preferable to contain 0.01% or more to achieve this effect. However, when the Si content exceeds 0.50%, Si accumulates at the interface between the oxide layer and the steel substrate, forming a Si oxide layer. At the interface between this Si oxide layer and the oxide layer formed thereon, oxide layer peeling is likely to occur. Therefore, the Si content is set to 0.50% or less, preferably 0.20% or less.

[0043] Mn: 0.01~2.0%

[0044] Mn is an element that neutralizes dissolved sulfur (S) that causes embrittlement during hot working by forming MnS, and is also effective in improving strength. In particular, it has the effect of ensuring the strength of steel plates with large thicknesses, where strength is easily reduced after hot rolling. Its effect is minimal when its content is less than 0.01%. On the other hand, when it contains more than 2.0%, it leads to reduced toughness and the formation of Mn-based oxides at the interface between the oxide scale and the steel substrate, resulting in reduced oxide scale adhesion. Furthermore, the phase transformation after finishing rolling is delayed, and the phase transformation is not completed until coiling, resulting in localized and uneven phase transformation along the length direction after coiling. This leads to loosening of the hot-rolled coil after coiling, and re-oxidation of the central portion of the coil in the width direction due to contact between the steel plate surface and the oxidizing atmosphere, i.e., an increase in hematite and magnetite grains and a decrease in eutectoid phase transformation structures, leading to deterioration of oxide scale adhesion. Therefore, the Mn content is set to 0.01–2.0%. The preferred lower limit is 0.05% or more. The preferred upper limit is 1.5% or less.

[0045] P: below 0.10%

[0046] Phosphorus (P) has an adverse effect on grain boundary embrittlement, so it is desirable to use as little as possible. Furthermore, P forms a very brittle oxide layer at the interface between the oxide scale and the steel substrate, reducing the adhesion of the oxide scale. These adverse effects become more pronounced when the P content exceeds 0.10%, therefore it is set to 0.10% or less. Preferably, it is set to 0.05% or less. It is also possible to omit P, but from a manufacturing cost perspective, the lower limit is preferably 0.001% or more.

[0047] S: below 0.10%

[0048] Sulfur (S) is an element that significantly deteriorates hot workability and toughness. Furthermore, S accumulates at the interface between the oxide scale and the steel substrate, reducing the adhesion of the oxide scale. These adverse effects become more pronounced when the S content exceeds 0.10%, therefore it is set to 0.10% or less. Preferably, it is set to 0.05% or less. It is also possible to eliminate S, but from a manufacturing cost perspective, the lower limit is preferably 0.0001% or more.

[0049] sol.Al: 0.10% or less

[0050] 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 set to 0.10% or less. Preferably, it is set to 0.06% or less.

[0051] N: below 0.015%

[0052] Nitrogen (N) is an element that forms nitrides such as BN, AlN, and TiN in steel. It reduces the steel's thermal ductility and surface quality. When the N content exceeds 0.015%, the surface quality deteriorates significantly. Therefore, the N content is set to 0.015% or less. Preferably, the N content is 0.010% or less. It should be noted that N can be absent, but from a manufacturing cost perspective, the N content is preferably set to 0.0001% or more. More preferably, the N content is 0.001% or more.

[0053] 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 also contain one or more of the following components as needed to improve various properties: Cu: less than 1.0%, Ni: less than 0.50%, and Cr: less than 2.0%.

[0054] Cu: below 1.0%

[0055] Cu is an element that, when enriched at the interface between the oxide scale and the steel substrate, promotes grain boundary oxidation and enhances the roughness of the interface, thereby improving the adhesion between the oxide scale and the steel substrate. To achieve this effect, a Cu content of 0.01% or more is preferred. However, when the content exceeds 1.0%, molten Cu penetrates the austenite grain boundaries of the steel substrate during heating, raising concerns about deterioration of surface properties due to thermal embrittlement. Therefore, when Cu is present, the content is set to 1.0% or less. Preferably, it is 0.8% or less.

[0056] Ni: below 0.50%

[0057] Like Cu, Ni is an element that accumulates at the interface between the oxide layer and the steel substrate, promoting grain boundary oxidation and enhancing the surface roughness of the interface, thereby improving the adhesion between the oxide layer and the steel substrate. 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, in the case of Ni content, it is set to 0.50% or less. Preferably, it is 0.40% or less.

[0058] Cr: less than 2.0%

[0059] Cr has the effect of improving strength, hardenability, and corrosion resistance. Furthermore, Cr is enriched at the interface between the oxide scale and the steel substrate, and by utilizing the unevenness of the interface, the oxide scale is allowed to penetrate the steel substrate, thus improving the adhesion of the oxide scale. To obtain such an effect, it is preferable to contain 0.01% or more Cr. On the other hand, when the content is greater than 2.0%, the above-mentioned effect saturates; therefore, in the case of Cr content, it is set to 2.0% or less. A more preferred lower limit is 0.07% or more, and even more preferably 0.12% or more. A more preferred upper limit is 1.0% or less, and even more preferably 0.8% or less.

[0060] In this invention, it may further contain one or more of the following as needed: 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.

[0061] Mo: 1.0% or less

[0062] 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 Mo. On the other hand, when the content is greater than 1.0%, the strength may increase excessively, while the toughness and formability may deteriorate. Therefore, when Mo is present, its content is set to 1.0% or less.

[0063] Nb: below 0.1%

[0064] 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 greater than 0.1% can sometimes lead to a decrease in toughness. Therefore, when Nb is present, its amount is set to 0.1% or less.

[0065] V: Below 0.1%

[0066] 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 greater than 0.1% can sometimes lead to a decrease in toughness. Therefore, when vitamin V is present, its amount is set to 0.1% or less.

[0067] Ti: below 0.03%

[0068] Ti is an element that enhances the strength and toughness of the base metal and is effective in ensuring the toughness of the weld heat-affected zone. To achieve these effects, it is preferable to contain 0.001% or more Ti. On the other hand, a content greater than 0.03% can sometimes lead to a decrease in toughness. Therefore, when Ti is present, its amount is set to 0.03% or less.

[0069] B: Below 0.01%

[0070] Boron (B) is an element that improves the hardenability of steel. This effect increases strength. To achieve this effect, a B content of 0.0005% or more is preferred. However, the effect saturates when the content exceeds 0.01%, therefore, when B is present, its amount is set to 0.01% or less.

[0071] Sb: below 0.03%

[0072] Sb accumulates on the surface of steel plates during heating, effectively suppressing the decrease in carbon content on the steel plate surface caused by decarburization during heating. To achieve this effect, an Sb content of 0.001% or more is preferred. On the other hand, when the Sb content exceeds 0.03%, it becomes a liquid metal during heating, eroding the original austenite grain boundaries and sometimes reducing the adhesion of the oxide scale. Therefore, in the case of Sb content, it is set to 0.03% or less.

[0073] The balance other than the above chemical composition consists of Fe and unavoidable impurities. As unavoidable impurities, O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less are permissible. Furthermore, the presence of any of the above optional elements at levels below the preferred lower limit is also considered an unavoidable impurity.

[0074] Next, the oxide scale structure in the width direction of the steel sheet of the present invention will be described. It should be noted that the oxide scale structure in the width direction of the steel sheet refers to the oxide scale structure within the central portion of the steel sheet in the width direction and within 200 mm from the edge in the width direction. In practice, the oxide scale structure at the central portion of the steel sheet in the width direction, and at positions 5 mm and 200 mm from the edge in the width direction, is measured separately. The structure at each position only needs to be within the following range. Furthermore, if the structure at each measurement position is within the following range, it is considered that the structure is uniform in the width direction.

[0075] Magnetite grain area ratio: 20% or more and 60% or less

[0076] The magnetite of this invention consists of magnetite grains and magnetite contained in a eutectoid phase transformation structure. These are structures that can be distinguished as follows: The magnetite grains in this invention include a magnetite layer near the oxide surface composed of columnar magnetite grains, and blocky proeutectoid magnetite grains formed inside or adjacent to the eutectoid phase transformation structure before the eutectoid phase transformation occurs. Additionally, a thin layer composed of fine magnetite grains formed at the oxide-steel substrate interface, i.e., a so-called magnetite ore layer, is included, distinct from the magnetite contained in the eutectoid phase transformation structure of iron and magnetite. Compared to aragonite and hematite, magnetite grains have high ductility at room temperature, which helps improve the adhesion of the oxide layer. In particular, the magnetite ore layer has high integration with the steel substrate; therefore, by suppressing peeling from the oxide-steel substrate interface, it helps improve the adhesion of the oxide layer. When the magnetite grain content is less than 20%, this effect cannot be sufficiently obtained; therefore, the area fraction of the magnetite grains is set to 20% or more, preferably 30% or more. On the other hand, when the magnetite grain content is greater than 60%, cracks are generated in the magnetite, and the oxide scale adhesion deteriorates. Therefore, the area fraction of magnetite grains is set to 60% or less, preferably 50% or less.

[0077] Area fraction of eutectoid phase transformation structure of iron and magnetite: over 30%

[0078] The eutectoid phase transformation structure of iron and magnetite contributes to improved oxide scale adhesion due to the high integration of magnetite and precipitated Fe with the steel matrix. This effect is insufficient when the area fraction is less than 30%, therefore, the area fraction of the eutectoid phase transformation structure of iron and magnetite is set to 30% or more. Preferably, it is 35% or more, more preferably 40% or more. While no specific upper limit is specified, to achieve a combined effect with improved oxide scale adhesion of magnetite grains, the eutectoid phase transformation structure of iron and magnetite is preferably 80% or less, more preferably 70% or less.

[0079] Area ratio of iron ore: below 15%

[0080] Hematite is a phase stable at high temperatures and largely disappears through a eutectoid phase transformation during cooling after winding. However, under high cooling rates, hematite sometimes remains at room temperature in an untransformed state. The cooling rate is particularly high near the edges in the width direction of the roll, making hematite more prone to remaining at room temperature. Hematite is more brittle than magnetite at room temperature, and its formation of cracks in the oxide scale impairs its adhesion. Therefore, the area fraction of hematite is set to 15% or less. Preferably, the upper limit of the area fraction of hematite is 10% or less, more preferably 7% or less. It should be noted that the area fraction of hematite can also be 0%.

[0081] Hematite mass fraction: below 5%

[0082] Besides magnetite grains, eutectoid phase transformation structures of iron and magnetite, and aragonite, hematite sometimes forms in layers on the surface of the oxide scale. Especially at the edges along the width of the roll, hematite is easily formed due to the re-oxidation of the oxide scale caused by air entering from the edges after winding. Hematite leads to deterioration of the oxide scale's adhesion and is a cause of surface defects such as red rust. Therefore, the mass fraction of hematite is set to 5% or less.

[0083] The average thickness of the oxide scale in the width direction of the steel plate is more than 5 μm and less than 20 μm, and the variation in the thickness of the oxide scale in the width direction of the steel plate is less than 4 μm.

[0084] When the average thickness of the oxide scale is less than 5 μm, it becomes a cause of processing failure. This is because, during laser cutting, the absorption of thermal energy from the laser on the steel plate surface becomes insufficient. On the other hand, when the average thickness of the oxide scale is greater than 20 μm, especially in the case of thick hot-rolled steel plates, the strain applied to the oxide scale surface during processing increases, causing cracks in the oxide scale and reducing its adhesion. Therefore, the average thickness of the oxide scale is set to 20 μm or less. Preferably, it is 18 μm or less, more preferably 15 μm or less. Furthermore, in order to improve the uniformity of the oxide scale adhesion in the width direction and obtain excellent laser cutability, the average thickness of the oxide scale in the width direction of the steel plate is controlled to be 5 μm or more and 20 μm or less, and the variation in the thickness of the oxide scale in the width direction of the steel plate is set to 4 μm or less. Preferably, the variation in the thickness of the oxide scale in the width direction of the steel plate is 3 μm or less. Here, the variation in the thickness of the oxide scale refers to the maximum difference between the average thickness of the oxide scale measured at two or more measurement locations. The methods for measuring the average thickness of the oxide layer at the two or more measurement locations, as well as the variation in the thickness of the oxide layer, will be described later.

[0085] Next, the method for measuring the oxide scale structure and oxide scale thickness of the hot-rolled steel sheet of the present invention will be described.

[0086] Regarding the magnetite grains, the eutectoid phase transformation structure of iron and magnetite, and the area ratio of aragonite, a section of plate thickness perpendicular to the steel plate surface and parallel to the rolling direction was cut, mirror-polished, and observed using a scanning electron microscope (SEM). The SEM field of view encompassed the entire thickness of the oxide scale, from its surface to the interface between the oxide scale and the steel plate. Therefore, measurements could be performed by observing the reflected electron image of the oxide scale section at a magnification that accommodates the entire oxide scale thickness. In the SEM reflected electron image, the magnetite grains appeared as the darkest areas, the steel substrate as the brightest areas, and the aragonite as areas with intermediate contrast. Furthermore, the eutectoid phase transformation structure of iron and magnetite was observed as a region where magnetite and iron formed in layers.

[0087] Hematite forms very thin layers on the surface of oxide scale and is easily detached during mirror polishing, making quantitative evaluation by area ratio on SEM difficult. Therefore, X-ray diffraction equipment and CoK diffraction techniques can be used. α The integral intensity of the diffraction peaks of each phase in the oxide scale is measured using a radiation source. The mass fraction is calculated using the following formula (2) by the ratio of the integral intensities of each phase in the standard sample and the test sample. As the standard sample, a sample obtained by mixing Fe, FeO (hemite), Fe2O3 (hematite), and Fe3O4 (magnetite) by weight is used. It should be noted that the mass fraction of hematite can be regarded as the area fraction.

[0088] Mass fraction of hematite (Fe2O3) = (I Fe2O3 / R Fe2O3 )×100 / ((I Fe / R Fe )+(I FeO / R FeO )+(I Fe2O3 / R Fe2O3 )+(I Fe3O4 / R Fe3O4 )) …(2)

[0089] In equation (2) above,

[0090] I A The integrated intensity of phase A in the tested sample.

[0091] R A The integrated intensity of phase A in the standard sample.

[0092] A: Fe, FeO, Fe2O3 or Fe3O4.

[0093] Furthermore, the average thickness of the oxide scale is determined according to the following steps. For example, a section perpendicular to the steel plate surface and parallel to the rolling direction is cut from the center of the hot-rolled steel plate in the width direction, a portion 200 mm from the edge of the hot-rolled steel plate in the width direction, and a portion 5 mm from the edge of the hot-rolled steel plate in the width direction, and mirror polished. Then, the oxide scale thickness at each of the three locations is measured using SEM, and the average is calculated to determine the oxide scale thickness at each width location. It should be noted that the average thickness of the oxide scale in the width direction of the present invention refers to an average thickness of 5 μm or more and 20 μm or less at each width location.

[0094] In addition, the fluctuation of oxide scale thickness in the width direction can be calculated by subtracting the minimum value from the maximum value of the average oxide scale thickness of the hot-rolled steel plate at the center in the width direction, the portion 5 mm from the edge in the width direction, and the portion 200 mm from the edge in the width direction.

[0095] Next, the method for manufacturing the hot-rolled steel sheet of the present invention will be described.

[0096] It should be noted that, in this invention, the specific temperature in each process refers 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 expressed as "(cooling start temperature - cooling stop temperature) / cooling time".

[0097] In this invention, the manufacturing method of the steel raw material composed of the above-mentioned components is not particularly limited, and commonly used methods 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 raw materials such as slabs by casting methods such as continuous casting. It should be noted that there is no problem using the ingot-slab rolling method. Typically, the steel raw material is hot-rolled after heating. This heating only needs to achieve sufficient solution treatment, preferably heating to point Ac3 or higher. Specifically, the typical slab heating temperature range of 1060°C to 1300°C is suitable. In the case of slabs manufactured by continuous casting, direct rolling or rolling after holding the slab to suppress temperature drop can be applied.

[0098] The hot rolling process consists of roughing and finishing rolling. For roughing, as long as a thin slab of the specified dimensions can be produced, the conditions for roughing are not particularly limited. Before roughing, it is preferable to remove the oxide scale generated during the heating of the slab by descaling. Alternatively, to perform finishing rolling at a specified temperature, the material to be rolled can be heated midway through the process using a thin slab heater or similar heating method. After roughing and before finishing, the oxide scale generated on the surface of the thin slab is removed by descaling at the mill inlet using high water pressure or similar methods.

[0099] Next, finish rolling is performed. When the temperature at the finish rolling entry side exceeds 1100°C, the thickness of the oxide scale sometimes increases, and the adhesion of the oxide scale decreases. On the other hand, when the temperature at the finish rolling entry side is below 950°C, it sometimes leads to a significant increase in rolling load and a decrease in productivity. In addition, as the thickness of the product plate increases, the thickness at the finish rolling entry side becomes thicker. For example, when the product plate thickness is greater than 5.0 mm, the time required until the start of finish rolling is longer, which sometimes leads to a decrease in productivity. Therefore, the finish rolling entry side temperature is preferably below 1100°C, more preferably below 1050°C. Furthermore, the lower limit of the finish rolling entry side temperature is preferably set to 950°C or higher.

[0100] Finishing mill exit temperature: above 800℃ and below 950℃

[0101] When the temperature at the finish mill exit side is below 800°C, the ductility of the oxide scale decreases, leading to crack formation. These cracks promote the re-oxidation of the oxide scale, forming hematite, which reduces the scale's adhesion. Furthermore, the oxide scale's microstructure becomes finer, increasing its hardness and further reducing its adhesion. On the other hand, when the temperature at the finish mill exit side exceeds 950°C, excessive growth of the oxide scale increases its thickness, reducing its adhesion. Additionally, the increased particle size of each phase within the oxide scale microstructure further reduces its adhesion. Therefore, the finish mill exit side temperature is set between 800°C and 950°C. A lower limit of 820°C or higher is preferred. An upper limit of 930°C or lower is preferred.

[0102] Cooling is performed 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.

[0103] Since scale grows faster in high-temperature zones, rapid cooling is necessary in the high-temperature zone immediately after finishing rolling to suppress the reduction in scale adhesion 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 reduced scale adhesion. Therefore, the average cooling rate from the finishing mill exit temperature to 750°C is set to 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 is greater than 80°C / s, the scale structure sometimes becomes finer, resulting in reduced scale adhesion. Furthermore, cracks sometimes form due to reduced scale ductility, and these cracks promote the re-oxidation of the scale, forming hematite, which also reduces scale adhesion. 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.

[0104] Cooling is performed at an average cooling rate of more than 1°C / s and less than 30°C / s within the temperature range from 750°C to the start of winding.

[0105] Although the growth of oxide scale is slower in the temperature range from 750°C to the start of coiling compared to the high-temperature zone immediately after finishing rolling, it is still necessary to suppress the reduction in oxide scale adhesion caused by excessive oxide scale growth. When the average cooling rate in the temperature range from 750°C to the start of coiling is less than 1°C / s, excessive oxide scale growth occurs, leading to a decrease in oxide scale adhesion. Therefore, the average cooling rate in the temperature range from 750°C to the start of coiling is set to 1°C / s or more, preferably 3°C / s or more. On the other hand, when the average cooling rate in the temperature range from 750°C to the start of coiling is greater than 30°C / s, cracks are generated in the oxide scale due to the refinement of the oxide scale structure and the increased stress difference with the steel base. Since these cracks promote the re-oxidation of the oxide scale to form hematite, the adhesion of the oxide scale decreases. Therefore, the average cooling rate in the temperature range from 750°C to the start of coiling is set to 30°C / s or less, preferably 20°C / s or less.

[0106] Winding temperature: 500℃ or higher and 650℃ or lower

[0107] After the aforementioned cooling, the steel sheet is wound at a winding temperature of 500°C or higher and 650°C or lower. When the winding temperature is below 500°C, the eutectoid phase transformation from aragonite does not occur sufficiently after winding, resulting in excessive aragonite residue at room temperature. Consequently, the adhesion of the oxide scale decreases due to the brittle nature of aragonite at room temperature. When the winding temperature exceeds 650°C, excessive oxide scale growth occurs, further reducing its adhesion. Furthermore, in the central portion of the oxide scale in the width direction, isolated from the oxidizing atmosphere after winding, hematite and magnetite on the oxide scale surface are reduced to aragonite, resulting in insufficient magnetite grains. Therefore, the winding temperature is set to 500°C or higher and 650°C or lower. A lower limit of 530°C or higher is preferred. Furthermore, an upper limit of 630°C or lower is preferred.

[0108] From the start of winding, the temperature at the edge of the roll is cooled at an average rate of 0.5°C / s to 6.0°C / s until the cooling stop temperature reaches 300°C to 450°C. The entire roll is cooled in this manner.

[0109] From the start of winding, the temperature at the edges of the roll is cooled at an average rate of 0.5°C / s to 6.0°C / s until the cooling stop temperature reaches 300°C to 450°C. This cooling process cools the entire roll. This lowers the temperature at the edges of the roll, increases its rigidity, and prevents loosening. Here, "the entire roll" refers to both edges and the surface of the roll. The two edges refer to the area within 200mm of each edge in the width direction of the roll. The surface refers to the surface of the roll excluding the ends. As a result, in the central part of the roll in the width direction, re-oxidation is suppressed due to isolation from the oxidizing atmosphere, allowing for a sufficiently good eutectoid phase transformation structure that improves adhesion. Furthermore, while it is difficult to completely isolate the edges of the roll in the width direction from the oxidizing atmosphere, cooling the edges suppresses re-oxidation, and the eutectoid phase transformation occurs through reheating from the central part in the width direction ensures excellent adhesion. Furthermore, this also suppresses fluctuations in oxide scale thickness along the width direction, uniformly improving oxide scale adhesion and thus enhancing laser cutting performance. The edge temperature can be measured, for example, using a radiation thermometer. Regarding the surface temperature, since the surface temperature at a specific length position cannot be measured during winding, no specific temperature is specified; however, the aforementioned effect can be achieved by cooling the entire roll using the same cooling method as the edge. If the cooling stop temperature at the edge of the roll exceeds 450°C, the aforementioned effect cannot be sufficiently obtained. Furthermore, if the cooling stop temperature at the edge of the roll is below 300°C, the edge is overcooled, resulting in a large amount of ferroore residue remaining in the oxide scale at the edge, leading to reduced adhesion. Therefore, the cooling stop temperature at the edge of the roll is set to 300°C or higher and 450°C or lower. Preferably, it is 320°C or higher and 430°C or lower. Additionally, if the average cooling rate at the edge is less than 0.5°C / s, the aforementioned effect cannot be sufficiently obtained. When the average cooling rate at the edges exceeds 6.0°C / s, cracks form in the oxide scale due to the refinement of the oxide scale structure and the increased stress difference between it and the steel substrate. These cracks promote the re-oxidation of the oxide scale, leading to the formation of hematite, and reduce the adhesion of the oxide scale. Therefore, the average cooling rate at the edges is set to be 0.5°C / s or more and 6.0°C / s or less. Preferably, it is 1.0°C / s or more and 5.0°C / s or less. The cooling method for the coil is not particularly specified, but for example, it is preferable to use a cooling device that sprays water onto the surface and both edges of the coil while it is being wound inside the coiler.

[0110] It should be noted that it is preferable to promote the eutectoid phase transformation from argumentite and suppress oxidation of the outermost periphery and edges by loading the cooled roll into a hot rolling box or by covering the roll.

[0111] Furthermore, for hot-rolled steel sheets coiled into rolls, shape straightening can also be achieved by deforming the steel sheet using roller straighteners, tension straighteners, etc. For example, for a hot-rolled steel sheet with a thickness of 12mm, two φ250mm upper rollers and three lower rollers are configured to perform shape straightening under a pressing amount of 2mm.

[0112] Example

[0113] The embodiments of the present invention will be described below.

[0114] The steels with the compositions shown in Table 1 were smelted and cast to produce steel raw materials. These steel raw materials were then hot-rolled under the conditions shown in Table 2 to produce hot-rolled coils with a thickness of 3–23 mm. The obtained hot-rolled coils were straightened and then cut to specified lengths to produce hot-rolled sheets. Test pieces were cut from the center and edges of the obtained hot-rolled sheets in the width direction, and the oxide scale structure, oxide scale thickness, adhesion, and laser cutability were evaluated using the following methods. The evaluation results are shown in Table 3.

[0115]

[0116]

[0117]

[0118] Regarding the magnetite grains, the eutectoid phase transformation structure of iron and magnetite, and the area ratio of aragonite, a section of the plate thickness perpendicular to the steel plate surface and parallel to the rolling direction was cut. After mirror polishing, the reflected electron image of the oxide scale section was observed using SEM at a magnification of 3000x. In the SEM reflected electron image, the magnetite grains appeared as the darkest areas, the steel matrix as the brightest areas, and the aragonite as areas with intermediate contrast. The eutectoid phase transformation structure of iron and magnetite was characterized by the layered formation of magnetite and iron.

[0119] Regarding the mass fraction of hematite, X-ray diffraction equipment and CoK were used. α The integral intensity of the diffraction peaks of each phase in the oxide scale is determined by the X-ray source. The mass fraction is calculated by the ratio of the integral intensity of each phase in the standard sample (a sample obtained by mixing Fe, FeO (argumentite), Fe2O3 (hematite), Fe3O4 (magnetite) by weight) and the sample to be tested using the following formula (2).

[0120] Mass fraction of hematite (Fe2O3) = (I Fe2O3 / R Fe2O3 )×100 / ((I Fe / R Fe )+(IFeO / R FeO )+(I Fe2O3 / R Fe2O3 )+(I Fe3O4 / R Fe3O4 )) …(2)

[0121] In equation (2) above,

[0122] I A The integrated intensity of phase A in the tested sample.

[0123] R A The integrated intensity of phase A in the standard sample.

[0124] A: Fe, FeO, Fe2O3 or Fe3O4.

[0125] Regarding the average thickness of the oxide scale, sections perpendicular to the steel plate surface and parallel to the rolling direction were cut from the center of the obtained hot-rolled sheet in the width direction, 200 mm from the edge in the width direction, and 5 mm from the edge in the width direction. Then, after mirror polishing, the oxide scale thickness at three arbitrary locations was measured using SEM, and the average was calculated to determine the oxide scale thickness at each width location.

[0126] Regarding the adhesion of oxide scale, test pieces were cut from the center of the straightened hot-rolled sheet in the width direction, 200 mm from the edge in the width direction, and 5 mm from the edge in the width direction. Adhesive tape was applied to the surface of the steel sheet to remove the oxide scale. The presence or absence of exposed steel substrate on the steel sheet surface and the amount of oxide scale adhered to the tape were then evaluated. Specifically, adhesive tape was applied to the steel sheet surface, the removed tape was adhered to a transparent sheet, and the image was scanned and the amount of oxide scale removed was measured through image processing. When the area percentage of oxide scale adhering to the removed tape was less than 10%, the adhesion of the oxide scale was considered excellent, and was recorded as ○ in Table 3. On the other hand, when the area percentage of oxide scale adhering to the removed tape was 10% or more, the adhesion of the oxide scale was considered poor, and was recorded as × in Table 3.

[0127] Regarding laser cutability, for straightened hot-rolled thin plates, an ENSIS3015AJ laser cutting machine manufactured by Amada Machinery Co., Ltd., and a fiber laser oscillator were used for laser cutting in a straight line parallel to the width direction. Cases where cutting is impossible, or where any of the following are observed on the cut surface—slag, molten droplets, or notches—resulting in an unstable cut surface, are considered poor laser cutability and are marked with × in Table 3. Conversely, cases where none of the above occurs and a stable cut surface is obtained in the width direction are considered excellent laser cutability and are marked with ○ in Table 3. It should be noted that oxygen was used as the assist gas during laser cutting, the cutting speed was 1500 mm / min, the laser output power was 3 kW, and the focal point was set at a distance of 3.0 mm from the steel plate surface.

[0128] Regarding the examples of the present invention shown in Table 3, the oxide scale is uniform and exhibits excellent adhesion at any of the following locations in the width direction of the hot-rolled sheet: the central portion, the portion 5 mm from the edge in the width direction, and the portion 200 mm from the edge in the width direction. Laser cutability is also excellent in all of these locations. In contrast, the comparative examples show poor adhesion or laser cutability at a certain width position.

Claims

1. A hot-rolled steel sheet comprising, by mass%, 0.01-0.30% C, 0.50% or less Si, 0.01-2.0% Mn, 0.10% or less P, 0.10% or less S, 0.10% or less sol.Al, 0.10% or less N, and the balance being Fe and unavoidable impurities. The surface of the steel plate has an oxide scale. The oxide scale in the width direction of the steel plate has the following structure: In terms of area ratio, Magnetite grains comprise more than 20% and less than 60%. The eutectoid phase transformation structure of iron and magnetite accounts for more than 30%. in, Magnetite is composed of the magnetite grains and the magnetite contained in the eutectoid phase transformation structure. The content of iron ore is below 15%. Hematite comprises less than 5% by mass fraction. The average thickness of the oxide scale in the width direction of the steel plate is more than 5 μm and less than 20 μm. Furthermore, the thickness variation of the oxide scale in the width direction of the steel plate is less than 4 μm.

2. The hot-rolled steel plate according to claim 1, wherein, The composition, by mass percentage, also contains 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, by mass%, also contains 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 plate, wherein, For steel raw materials having the composition described in any one of claims 1 to 3, After hot rough rolling, the oxide scale is removed. Finish rolling is performed at a finishing roll exit temperature of 800℃ or higher but below 950℃. 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 is performed at an average cooling rate of more than 1°C / s and less than 30°C / s within the temperature range from 750°C to the start of winding. Winding is performed at a temperature above 500℃ and below 650℃. From the start of winding, the entire roll is cooled at an average cooling rate of 0.5°C / s to 6.0°C / s during the period from the winding temperature to the cooling stop temperature of 300°C to 450°C at the edge of the roll.

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